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Environmental Application of Catalytic Processes: Heterogeneous LiquidPhase Oxidation of Phenol With Hydrogen PeroxideEkaterina V. Rokhinaa; Jurate Virkutytea

a Department of Natural and Environmental Sciences, University of Kuopio, Kuopio, Finland

Online publication date: 22 January 2011

To cite this Article Rokhina, Ekaterina V. and Virkutyte, Jurate(2011) 'Environmental Application of Catalytic Processes:Heterogeneous Liquid Phase Oxidation of Phenol With Hydrogen Peroxide', Critical Reviews in Environmental Scienceand Technology, 41: 2, 125 — 167To link to this Article: DOI: 10.1080/10643380802669018URL: http://dx.doi.org/10.1080/10643380802669018

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Critical Reviews in Environmental Science and Technology, 41:125–167, 2011Copyright © Taylor & Francis Group, LLCISSN: 1064-3389 print / 1547-6537 onlineDOI: 10.1080/10643380802669018

Environmental Application of CatalyticProcesses: Heterogeneous Liquid Phase

Oxidation of Phenol With Hydrogen Peroxide

EKATERINA V. ROKHINA and JURATE VIRKUTYTEDepartment of Natural and Environmental Sciences, University of Kuopio, Kuopio, Finland

The authors review the status of heterogeneous catalytic oxidationprocesses with hydrogen peroxide as an oxidant in a liquid phase.They focus on the priority organic pollutant–phenol as one of themost common persistent organic water contaminants, toxic even atlow concentrations. A wide range of heterogeneous catalysts is cov-ered, with a special emphasis on rapidly developing new catalyticsystems. Generally accepted mechanisms of the catalytic oxidationvia the formation of the most abundant reaction intermediatesand terminal products followed by the conceptual kinetic modelsdeveloped especially for the oxidation of phenol with hydrogen per-oxide are also discussed. Theoretical methods, widely used to gaina profound process understanding, such as factorial design andlife-cycle assessment, are summarized with popularization of theirmain principles, based on the most recent studies. The main ideais to identify and resume the main points of interest and problemsencountered, estimate the attribution of operation parameters forcatalyst selectivity and activity, elucidate the role of reactive oxi-dizing species in the process, and evaluate process potential for thefuture applications.

KEY WORDS: catalytic oxidation, experimental design, hydrogenperoxide, life cycle assessment, phenol

Address correspondence to Ekaterina V. Rokhina, Department of Natural and Environ-mental Sciences, University of Kuopio, Yliopistonranta 1 E, Snellmania, Kuopio, FI–70211,Finland. E-mail: katja.rokhina@uku.fi

125

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126 E. V. Rokhina and J. Virkutyte

1. INTRODUCTION

Catalysis is a very broad field of science that is closely interwoven with nu-merous other scientific disciplines (e.g., clean technologies). The main aimsof these technologies are to obtain great sustainability and improve environ-mental conditions. In the case of unavoidable pollutant emissions, catalyticclean-up technology helps to abate environmental pollution (Alnaizy andAkgerman, 2000). Clean technologies, involving catalytic processes, provideredesign of reaction chemistry by increasing the output of the reaction (viaenhanced selectivity) and decreasing the amount of stoichometric reagents.Moreover, catalysts are used in small amounts and can carry out a singlereaction many times (Hagen, 2006). Heterogeneous catalysis, in particular,addresses the goals of clean production by providing lower energy input, in-creased selectivity, and the ease of separation of the product and the catalyst,thereby eliminating the need for separation through distillation or extraction.In addition, environmentally benign catalysts such as clays and zeolites mayreplace more hazardous catalysts presently in use (Nguyen et al., 2001).

Heterogeneous catalytic oxidation falls into the category of advancedoxidation processes (AOPs), a giant group of chemical oxidation processesthat have been developed and characterized by using different oxidants (e.g.,ozone, hydrogen peroxide) as primary oxidants to generate hydroxyl radi-cals (• OH), an extraordinarily reactive species capable of attacking organicmolecules with rate constants usually in the order from 106 to 109 M−1 s−1

(Rey et al., 2008). Hydroxyl radicals not only are one of the most powerfuloxidation agents, but they also have a higher oxidation potential than otherchemicals commonly used in wastewater treatment (Molina et al., 2006).However, this high oxidizing power correlates to a relative lack of selectiv-ity due to the rapid evolution of hydroxyl radicals that tend to attack themolecules in close vicinity (Jones, 1999).

Most modern chemical oxidation processes, such as wet air oxidation(WAO) and ozonation, are expensive due to high operation costs or theneed for special equipment. One of the solutions to cope with this problemis the use of catalytically enhanced oxidation (Alnaizy and Akgerman, 2000;Perez et al., 2002). The use of an adaptable catalyst can reduce the energyconsumption of the treatment process, giving rise to the novel improvedsystem (e.g., catalytic wet air oxidation [CWAO]; Hagen, 2006). However,typical CWAO conditions are in the high temperature and pressure range,which leads to an increased cost of the treatment process. In this regard, theuse of AOPs utilizes free radicals as the oxidizing species, and thus providingreduced critical reaction conditions is an attractive option for the inexpensiveremoval of pollutants from aqueous media with high efficiency (Han et al.,2008; Liou et al., 2005). Contrary to the high energetic demand of chemicaloxidation processes, catalytic oxidation of phenol with hydrogen peroxide asan oxidant (WPO) exploits hydroxyl radicals under mild conditions (<373 K

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Environmental Application of Catalytic Processes 127

and atmospheric pressure). Esplugas et al. (2002), when comparing differentAOPs for phenol abatement, determined that Fenton’s reagent (iron-basedcatalysts and hydrogen peroxide) showed the fastest degradation rate (i.e.,40 times higher than UV process and photocatalytic oxidation and 5 timeshigher than ozonation) with one of the lowest expenditures, second to ozonein this respect. The major drawback of the WPO as a treatment process is baf-fling complexity to gain total mineralization of contaminant (Esplugas et al.,2002; Rey et al., 2008; Yube et al., 2007; Zazo et al., 2005). Nevertheless, it isprobably one of the most simple and effective techniques for partial oxida-tion of persistent organic pollutants into biologically amenable intermediates.Furthermore, the WPO system is easy to implement and control (Yube et al.,2007; Zazo et al., 2005). Also, another great advantage of this technique isthe ability of effortless integration with other treatment processes and units,such as different irradiations—ultraviolet (UV) (Arana et al., 2007), ultrasonic(US) (Kubo et al., 2005; Molina et al., 2006), microwave (MW) (Mei et al.,2004)—and other processes (pulse electrical discharge; Kusic et al., 2005),reverse osmosis (Goncharuk et al., 2002), to facilitate generation of hydroxylradicals and improve the utilization of the oxidant and catalyst in general.The application of modern tailor-made catalysts with high recycling capacitytranslates into the significant operating savings in chemicals consumptionand process maintenance, making a promotion for development of commer-cial WPO to treat highly contaminated industrial wastewaters in the mostbeneficial way in accordance with the clean technology concept.

Hydrogen peroxide, H2O2, is one of the most powerful oxidizing agentsknown, and has increasingly become an important chemical in manufactur-ing plants for environmental reasons due to the innocuous nature of itsbyproduct, water (Han et al., 2008; Liou et al., 2005; Molina et al., 2006;Nguyen et al., 2001; Perez et al., 2002). Although this makes it an envi-ronmentally appealing choice for many of the industrial applications, thecontinual decomposition of hydrogen peroxide does have significant disad-vantages: in addition to superior oxidizing power, one of the key charac-teristics of H2O2 is its instability (Jones, 1999). The effectiveness of H2O2 isclearly reduced as its concentration decreases, turning a valuable chemicalinto simple water and oxygen. Furthermore, heat is produced during thedecomposition reaction, and this can create a safety hazard in combinationwith the buildup of oxygen. Despite this, hydrogen peroxide is often theoxidant of choice for chemical oxidation because of its simplicity of op-eration (Esplugas et al., 2002). One of the desirable aspects of hydrogenperoxide’s instability is a tendency to liberate oxygen when in contact withactive surfaces, such as high-surface-area substances (e.g., activated carbon,a transition or heavy metal or its oxide), which tend to transfer electronsto the peroxide molecule (Puzari and Baruah, 2000). Thus, coupled with asufficient catalyst, hydrogen peroxide exhibits excellent reactivity and highutilization efficiency (Yube et al., 2007; Zazo et al., 2005).

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128 E. V. Rokhina and J. Virkutyte

In the present review only heterogeneous catalysts (heterocatalysts)serving the purpose of phenols oxidation are noted. Phenol is known asone of the most important representative of persistent organic pollutants be-cause it is toxic even at low concentrations (i.e., EC50 for water organisms isestimated to be 3.1 mg l−1 [48 h LC50 for Ceriodaphnia dubia]). Also its pres-ence in natural waters can lead to the formation of substituted compoundsduring chlorine disinfection processes (disinfection byproducts [DBPs]; Liouet al., 2005). Moreover, it is usually used as a simulating pollutant for ad-vanced wastewater treatment studies, such as being commonly present inindustrial effluent (e.g., from petrochemical, chemical, and pharmaceuticalindustries) as raw materials (Rey et al., 2008; Zazo et al., 2005). In addi-tion, phenol is considered to be an intermediate product in the oxidationpathway of higher molecular weight aromatic hydrocarbons (Jeong et al.,2005). An added advantage of the phenol oxidation is the formation of di-hydroxybenzenes (DHBs) as primary products of the reaction, which areextensively used in chemical industry (Rocha et al., 2003). Hydroquinone(HQ) is known to be a photographic developer, polymerization inhibitor,and antioxidant. It is also an important intermediate for the production ofnumerous dyes. Catechol (CL) is a starting material for a series of importantfine chemicals production used for pest control, pharmaceuticals, flavors,and aromas. Resorcinol (RC) is also used as an intermediate for dyes andfor ultraviolet stabilizers of polyolefins and pharmaceutical products (Jeonget al., 2005; Preethi et al., 2008; Rocha et al., 2003). It is important to pointout that the applicability of the CWPO is not only limited to phenols as themain principles of the technology but are also relevant to a wide range ofother aromatic and aliphatic contaminants present in industrial wastewatersfrom (a) dyeing and printing, (b) Kraft-pulp bleaching, (c) the petrochem-ical industry, (d) olive milling, (e) the H-acid manufacturing process, and(f) wood pyrolysis and cooking plants (Perathoner and Centi, 2005).

In the present work we discuss different aspects of heterocatalytic WPOtechnique for phenol degradation as an important environmental and indus-trial application with a great number of various types of catalyst and catalyticsystems.

2. HETEROGENEOUS CATALYTIC OXIDATION OF PHENOL WITHHYDROGEN PEROXIDE

2.1. Catalysts

Presently, a wide range of heterocatalysts has been applied for the oxida-tion of phenol in the presence of hydrogen peroxide. They can be clas-sified into three groups: (a) transition metals and their compounds (e.g.,Fenton-like reagent, Chakinala et al., 2008; CuO, Drijvers et al., 1999; Cu-Bi-V-O complex, Sun et al., 2000), metallophthalocyanine (MPc) complexes

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Environmental Application of Catalytic Processes 129

(e.g., cobalt tetrasulfophthalocyanine), and chemical conjunctions of Ni, V(complex oxides; Chunrong et al., 2000), ZnO-based (Harmankaya et al.,1998) Pd/Mg and Pd/Fe bimetallic systems (Morales et al., 2002), and ruthe-nium iodide (Rokhina et al., 2009); (b) solid acid catalysts (e.g., Lewis andBrønsted), such as molybdovanadophosphoric acid modified zirconia (Paridaand Mallick, 2008), tetravalent metal phosphates (Rocha et al., 2002), ti-tanosilicalites (TS-1) (Klaewkla et al., 2007; Yube et al., 2007), tin silicalite-1s(Klaewkla et al., 2007), heteropolyacids (HPA) and tetrabuthylammonium(TBA) salts (Kuznetsova et al., 2007), especially Keggin type of HPA (Wanget al., 2007) and perovskites (Sotelo et al., 2004); and (c) their various com-binations: metals encapsulated in zeolites (Atoguchi and Kanougi, 2004;Maurya et al., 2006; Maurya et al., 2003) and aluminophosphate molecu-lar sieves (Xingyi et al., 2004), and metal ions incorporated into mesoporoussilicates of different type (Liu et al., 2001).

Undivided attention should be allocated to Fenton and Fenton-likereagents. These advantageous systems are versatile, can be used very easily,and provide very high (even up to the total mineralization) degradation effi-ciency at relatively low cost (Klaewkla et al., 2007; Molina et al., 2006). Theuse of Fenton’s reagent as an oxidant for wastewater treatment is attractivedue to the fact that iron is widely available and a nontoxic element, andhydrogen peroxide is easy to handle and the excess decomposes to environ-mentally safe products (Zazo et al., 2005). Among the advantages of Fenton’sprocess, unlike other AOPs techniques, are the simplicity of equipment andthe mild operation conditions (atmospheric pressure and low temperature).Thus, mainly for these reasons Fenton’s process has been regarded as themost economical alternative for a wide array of applications (Perez et al.,2002). Because of the remarkable success of Fenton reagent for phenol ox-idation, extensive research is now focused on the exploration of differenttypes of Fenton-like or modified systems.

Fe(III) on the resin (Liou et al., 2005), Fe-ZSM-5 zeolites (Nguyenet al., 2001), Fe2O3/silicalite with Si/Fe ratios varying from 40 to 200 (Liuet al., 2001), synthetic chloride green rust GR (Cl−; Matta et al., 2008), iron-containing mesoporous mesophase materials Fe-MMM-2 (Timofeeva et al.,2007), zero-valent iron (advanced fenton process [AFP]); Bremner et al.,2006), and Fe-SBA-15 (Molina et al., 2006) have been developed and suc-cessfully applied for the total mineralization of phenol. New Fenton-likereagents have been able to deal with highly toxic streams showing stabil-ity with increasing concentrations of phenol (Liou et al., 2005). The prod-ucts obtained from such decomposition are relatively the same; however, insome cases they may be formed over a much longer time scale (Sun et al.,2000) or with considerably higher (5–10 gl−1) catalyst concentration (Chun-rong et al., 2000; Matta et al., 2008) in comparison with regular Fenton (0.1–0.5 gl−1) process (Zazo et al., 2005). In general, all Fenton-like systems are re-ported to be applicable at low pH (the desirable pH lies in the range of 3–4).

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130 E. V. Rokhina and J. Virkutyte

Although, such working range of pH and increasing temperature favor ironleaching from the solid catalyst and may lead to a decrease in catalytic activ-ity regarding phenol oxidation with H2O2 (Timofeeva et al., 2007). However,new findings by Huang and Huang (2008), who studied the use of highlyordered or crystalline iron oxides for phenol oxidation at pH 4 and 30◦C,reported the absence of iron leaching for this kind of structure. Moreover,Fenton-like systems support the oxidation at neutral or nearly neutral pH(Matta et al., 2008; Nguyen et al., 2001). Timofeeva et al. (2007), who syn-thesized Fe-MMM2 under weak acidic conditions, reported the resistanceto iron leaching due to coordination environment of Fe atoms in a meso-porous silicate. Application of humic acid (HA) has been studied by Vioneet al. (2004) and Georgi et al. (2007), who claimed the application of HAas a promising means for extending the optimum pH of the Fenton processtoward neutral conditions, increasing the robustness of the catalytic systemagainst neutralization of the reaction medium for HA.

Different transition metals, their oxides, and possible combinations withvarious supports produced a great variety of new catalytic systems developedfor oxidation of phenol with hydrogen peroxide at mild conditions. Complexoxide HxV2Zr2O9xH2O was hydrothermally synthesized by Yu et al. (1997);however, exploring different crystal size the highest efficiency of phenoldecay did not exceed 20%. Bimetallic (biomimetic porphyrines) catalystshave also gained more interest over the last several years. Such immobilizedorganometallic systems as mono- and bimetallic polymeric porphyrinic struc-tures resulted in a more efficient catalyst than the individual components,demonstrating a synergistic effect between the metal centers (Carballo et al.,2008). Multilayered heterobimetallic porphyrinic structures, either polyFePPbased (when a second polymer is deposited over a film of polyFePP, polyFe-CuPP, and polyFeNiPP) or polyCoPP based (polyCoCuPP and polyCoNiPP)over Au surface prepared by the electropolymerization technique, showedup to 3.5 times higher efficiency than the corresponding monometallic struc-tures. TiO2 species such as rutile (Barakat et al., 2005) and commercialDegussa P25 (Chiou et al., 2008) were explored in photodegradation andanatase (Kubo et al., 2005) in sonodegradation of phenol. However, onlythe combination of TiO2, H2O2, and UV light and ultrasound irradiationsystem demonstrated significant degradation of phenol, whereas the catalystalone was not able to provide satisfactory treatment efficiency (Barakat et al.,2005; Chiou et al., 2008).

There are some practical disadvantages associated with heterocatalysts.Several traditional catalysts demonstrated relatively limited generation of rad-icals without coupling with UV–US irradiation or other similar methods (Huaet al., 2001) others had extremely low (less than 10 m2g−1) BET surfacearea (Liou et al., 2005, Matta et al., 2008). A serious drawback of variousdeveloped catalytic systems was reported to be high catalyst concentrationdemand, reaching up to 10 g l−1 (Drijvers et al., 1999; Matta et al., 2008).

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Environmental Application of Catalytic Processes 131

Various catalysts and the studied parameters of phenol oxidation are pre-sented in Table 1.

2.2. Activity and Stability of the Catalyst

The activity and stability of the catalyst is usually evaluated in terms of per-formance at various operational conditions and strongly depends on thepreparation method. In CWPO processes, the oxidation–reduction proper-ties of transition metals are used to generate hydroxyl radicals under mildreaction conditions in the presence of hydrogen peroxide. Therefore, thesecatalysts are very sensitive to pH changes. Several attempts to immobilizetransition metals (especially iron species) over various supports have beendescribed in literature, searching for the active and hydrothermally stablematerials applicable for a wide pH range (Calleja et al., 2005).

2.2.1. SUPPORT OF THE CATALYST

It is possible to stabilize the reduced oxidation states of the transition metalsby supporting well-dispersed metal ions on a suitable support or anchor-ing them to the exchangeable sites of microporous materials (Perathonerand Centi, 2005). Catalyst support not only tunes the stability but also theactivity of the catalyst as well. The percent conversion of phenol with un-supported catalysts was reported to be low in comparison to the supportedcatalyst, which can be attributed to the high activation energy needed foroxidation of phenol. For instance, N, N’bis (o-hydroxy acetophenone) andethylenediamine Schiff base (HPED) and its amino derivative were synthe-sized and anchored on the cross-linked polystyrene beads. The unsupportedand polymer-supported HPED Schiff base was loaded with iron (III), cop-per (II) and zinc (II) ions. The unsupported Schiff base complexes of thesemetal ions showed similar trends in phenol oxidation, but the rate (Rp) andturnover number (TON) of phenol oxidation was low in comparison to thepolymer-supported Schiff base complexes of metal ions (Gupta and Sutar,2008a).

In the past few years, activated carbon (AC) received more attentiondue to the synergy between the physicochemical properties of carbon ma-terial and the catalyst efficiency (Britto et al., 2008; Rey et al., 2008; Rokhinaet al., 2009; Zazo et al., 2006). The large surface area, well-developed porousstructure, and variable surface compositions, which determine the importantdifferences in the reactivity of activated carbons allowing to study the ef-fects of these features on the catalytic behavior, are among the excellentproperties of activated carbons (Rey et al., 2008). Zazo et al. (2006) pro-vided a comparison of the results obtained for iron-catalyzed oxidation ofphenol with various supports such as AC, silica, alumina, zeolites, resin, andpillared clays, and concluded that the Fe–AC catalyst exhibits excellent ox-idative ability that is comparable with other Fe supports. Moreover, Britto

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Environmental Application of Catalytic Processes 133

et al. (2008), who studied the carbon-supported copper catalyst, obtainedfrom sulfonated styrene–divinylbenzene resin, reported it to be an efficientalternative to remove phenol from industrial wastewater, in the presence ofhydrogen peroxide; however, copper leaching was observed at high concen-trations of phenol. Contrary to that ruthenium on carbon demonstrated anoutstanding stability and the resistance to leaching (Rokhina et al., 2008b).

Different clays (e.g., alumosilicates) such as montmorillonite (Kurian andSugunan, 2006), beidellite (Catrinescu et al., 2003), smectite (Zhou et al.,2006), and bentonite and boehmite (Carriazo et al., 2008); zeolites, suchas ZSM-5 (Maduna Valkaj et al., 2007), grafted betazeolite (Dimitrova andSpassova, 2007; Timofeeva et al., 2007), HY-5 zeolite (Zrncevic and Gomzi,2005), and MCM-41 (Hua et al., 2001); hydrotalcites, such as double-layeredhydroxides (Dubey et al., 2002) and silica oxide (SiO2; Liu et al., 2001), aswell as various silica matrixes (Tatibouet et al., 2005); and aluminophos-phate molecular sieves (Xingyi et al., 2004) have been extensively used asporous support in order to increase the active surface area accompanied bythe stability improvement. Pillar interlayered clays constitute a novel class ofmicroporous materials with good thermal stability and pronounced Brønstedand Lewis acidity (Barrault et al., 2000; Guelou et al., 2003; Guo and Al-Dahhan, 2003; Zhang et al., 2004). The particular coordination environmentof active metal cations inside the porous structure, where the strong electro-static fields are present, causes high activity of the catalyst even at neutral pHand provides resistance to leaching (Alnaizy and Akgerman, 2000; Timofeevaet al., 2007; Zhang et al., 2004). Moreover, support constitutes surface acidityneeded for generation of free radicals from hydrogen peroxide (Timofeevaet al., 2007). Therefore, transition metal exchange decreases the externalsurface area, creating a three-dimensional porous network via pillaring, thusresulting in a significant increase in microporous surface area and porousvolume (Tatibouet et al., 2005). However, a common disadvantage of thesenatural structures is their resistance to modification. Therefore, the use ofsynthetic inorganic polymers provides a good alternative to natural clays(Rocha et al., 2002). Preparation methods are an effective tool to improvethe catalyst characteristics toward stability. The addition of silica gel into themetal salt solution before coprecipitation with ammonia is an effective wayto prepare metal oxide or complex oxide catalysts with small crystallite sizeand high surface area. The dispersion in a silica matrix prevents the sinteringof the catalysts upon the high temperature treatment (Chunrong et al., 2000).Catalysts with more uniform distribution generally demonstrate higher activ-ity (Rey et al., 2008). Incorporating specific cations into pillared clays hasbeen found to be a useful technique for improving catalytic and adsorptionproperties (Alnaizy and Akgerman, 2000). It is well known that the calcinatedpillared clays lose most of the cation exchange capacity (CEC) due to themigration of protons into the silicate layers. Calleja et al. (2005) studied theiron-containing amorphous, zeolitic, and mesostructured materials for wet

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134 E. V. Rokhina and J. Virkutyte

peroxide oxidation of phenol. They reported that nature and the local envi-ronment of iron species highly depends on the synthetic route, which dra-matically influences their catalytic performance (Galleja et al., 2005). Ageingof iron-containing SBA-15 materials at high pH promotes the appearance ofcrystalline oxide entities with a gradual disappearance of mesoscopic orderand decreases the specific surface area, whereas crystallization of the amor-phous Fe2O3-SiO2 xerogels into a zeolitic framework enhances the stabilityof the active metal species. In particular, partially crystalline samples havinga dual pore system structure allow a faster removal of aromatic compoundsas compared with the raw xerogel and 100% crystalline zeolite (Melero et al.,2004).

2.2.2. PHYSICOCHEMICAL PROPERTIES OF THE CATALYST

As mentioned previously, the physicochemical properties of the catalyst re-sults from the catalyst synthesis. To produce stable and selective catalysts,the parameters of the synthesis should be carefully adjusted. BET surfacearea, pore size, crystallinity, content of the active parts, and the particle sizeof the catalyst are the key parameters of its stability and activity. Duringthe oxidation process, the formation of some byproducts such as tar leadsto the significant decrease in BET surface area and the pore volume. In-deed, Klaewkla et al. (2007) reported large differences in fresh TS-1 (BETarea 403 m2 g−1, total pore volume 0.32 cm3 g−1) and spent TS-1 (BET area194 m2 g−1, total pore volume 0.22 cm3 g−1) properties. The physicochemicalproperties are interrelated (e.g., the variation in the particle size leads to theincrease in BET surface area and these variations are remarkable especiallyfor ultrasound-assisted processes; Rokhina et al., 2008a). The accurate con-trol of the catalyst particle size plays an important role in their applications(Zhou et al., 2006). Nanoparticles of inorganic materials, such as metal ox-ides and semiconductors, have already generated a considerable attentiondue to their novel properties compared with bulk materials (Zelmanov andSemiat, 2008). Studies performed employing catalytic nanocomposite mate-rials determined that the size of catalytic species has a considerable effecton the catalyst performance (Zhou et al., 2006). The advantages of usinga nanocatalyst are profound: The particle size creates a very high surfaceto volume ratio and nanocatalysts are able to be placed where traditionalcatalysts will not fit.

However, owing to the high surface reactivity of nanoparticles, the sta-bility maintenance of nanosized particles, method of manufacture, and theprice still represents a challenge in the efficient development of these cata-lysts (Zhou et al., 2006). The possible way to overcome such a drawback isto develop supported nanoparticles. Han et al. (2008) studied gold nanopar-ticles (average size = 4.9 nm) dispersed onto different supports, such ashydroxyapatite (HAp), ferrous (III) oxide, TiO2, and carbon. They argued

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Environmental Application of Catalytic Processes 135

that gold nanoparticles are a primary source of active sites. High selectivityand stability (no metal leaching even at pH 2 was detected) was exhibitedby Au–HAp (2.4 wt%) compatible with Fe-ZSM-5, resulting in up to 80% ofphenol degradation within half an hour of the reaction time.

Zhou et al. (2006) revealed that highly isolated iron nanoparticles dis-persed in clays could lead to deep catalytic oxidation. These novel nanocom-posites with iron particles imbedded in the montmorillonite matrix offernovel controllable catalytic properties and thus present new insights in cat-alytic engineering applications for catalysis and related industrial processes,by degrading 49.5% of phenol with a molar ratio of (CO3)2−/Fe2+ = 0.5(Zhou et al., 2006). Also, Zelmanov and Semiat (2007), who investigated thecatalytic activity of colloidal iron-based nanoparticles prepared by hydroly-sis, found that phenol destruction rate with iron oxide-based nanoparticlesis about 35 times higher than the values reported in the literature for Fen-ton reagents with no nanocatalyst aging being observed. The half-life forsuch a catalyst is 0.05–1 min for concentrations ranging from 15 to 60 ppm.Thus, metal and metallic oxide nanoparticles with controllable structure andappropriate particle size exhibit unique chemical and physical properties,which are thermally stable materials and can be recycled many times withoutany loss in the catalytic activity (Zelmanov and Semiat, 2008). In particular,nanostructured composite materials made of nanometric metal iron and itsoxide particles embedded in various matrices present a variety of interestingmagnetic, electric, and also novel controllable catalytic properties and, thus,show potential for engineering applications of catalysis and related industrialprocesses (Zhou et al., 2006).

2.2.3. DEACTIVATION OF THE CATALYST

The change in physicochemical properties usually leads to the catalyst de-activation. The deactivation phenomenon is attributed to several factors:(a) formation of carbonaceous deposits that can reduce the specific surfacearea of the catalyst; (b) poisoning of the catalyst by complexation of activesites with acidic organic compounds, thus preventing its reactivity with otherreactants; and (c) leaching of iron species within the liquid phase (Martınezet al., 2007). Special interest has been focused on the resistance to leach-ing of active species during the CWPO. Leaching of active species that isa complex problem and is closely related to the method and conditions ofpreparation, local environment for the complex catalysts, and the reactionconditions presents one of the major drawbacks of the method. The lowleaching was observed for (Al-Fe) pillared clay catalyst because iron specieswere highly stabilized by the clay matrix and iron itself was strongly boundto the aluminum pillars (Barrault et al., 2000). Melero et al. (2007) sup-ported a nanocomposite solid catalyst that contained mixtures of crystallineiron oxides over a silica SBA-15 matrix as highly stable active materials that

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136 E. V. Rokhina and J. Virkutyte

are effective over a wide pH range. The amorphous structure of iron oxidesbond weakly with the support and tends to dissolute in the presence of someorganic compounds produced by phenol degradation (Huang and Huang,2008).

Generally, the extreme reaction conditions lead to an increased leachingof achieved species from the catalyst. The leaching of the active compoundis not capable of destroying the organic pollutant and it may generate asecondary pollution, unless it becomes reoxidized by the excess hydrogenperoxide (Hagen, 2006; Martınez et al., 2005). Thus, the acid pH (Calleja et al.,2005; Rey et al., 2008), high temperatures (e.g., Cu leaching is significantabove 310 K; Sotelo et al., 2004), the excess of the catalyst (Catrinescu et al.,2003), the ratio between the catalyst and hydrogen peroxide (Martınez et al.,2005), and the presence of oxidation products (organic acids) in the reactionmedia (Britto et al., 2008; Huang and Huang, 2008; Rey et al., 2008) contributeto the leaching of active compounds from the catalyst. For instance, hydrogenions dissociate from the phenol degradation intermediates, such as aliphaticacids and carboxylic acids, decreasing the pH and increasing the solubility ofiron (Huang and Huang, 2008). Oxalic acid generated as a product of phenoldecomposition forms a soluble complex with iron, thus also contributing toiron leaching (Huang and Huang, 2008; Rey et al., 2008).

However, the leaching of active species is not only promoted by asimple dissolution mediated by the acidic nature or high temperature ofaqueous solutions. It has a more complex mechanism in which the peroxidecontent to active species ratio within the course of the catalytic run plays anessential role (Sotelo et al., 2004). Two main hypotheses were reported byMelero et al. (2008) that explain the instability of the iron species supportedover Fenton-like catalysts. One of them regards the oxidizing conditions inwhich the reaction takes place and the other one relates to the reactionsoccurring on the catalyst surface between the active sites of iron catalyst andthe oxidizable organic compounds (Melero et al., 2008).

Another deactivation mechanism occurring during heterogeneous cat-alytic process is the poisoning of the active catalytic sites and sintering of thecatalyst particles. The oxidation of the organic species adsorbed on the ac-tive sites of the catalyst may be associated with the delays observed for shortreaction times. Therefore, when the catalyst was used after an intermediatecalcination step, its catalytic activity was restored, indicating the absence ofthe significant deactivation due to the loss of iron (Catrinescu et al., 2003).The sintering usually takes place with the increase in temperature. However,these types of deactivations are less problematic for CWPO because of therelatively mild operational conditions engaged. Nevertheless, when there isunavoidable need for high temperatures, the use of ultrasound irradiationmay be able to overcome the deactivation. The turbulence induced by theultrasound removes all the possible deposits from the catalyst surface andprevents the sintering of the particles (Rokhina et al., 2009).

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2.3. Reaction Pathway of Phenol Oxidation

The overall behavior of phenol during the oxidation process is determinedby the underlying mechanism, consisting of a sequence of elementary steps,divided by the inflection point: induction and steady state period (Guo andAl-Dahhan, 2003). There is a huge variety of reaction mechanisms available,and the schemes of reaction are generally complex. Also, there are manydifferent short- and long-lived intermediates formed over the course of thereaction, but often the concentration of one of the intermediates is muchlarger than the concentration of all other intermediates. This intermediate isthen called the most abundant reaction intermediate (MARI). Resulting reac-tion pathway is introduced via the formation of MARI to terminal products:carbon dioxide and water. It is worthy to note that the reaction of full phenolmineralization consists of few necessary steps, which can be represented bya simplified scheme:

Ph, TOC, COD + H2O2 + cat → MARI (1)

MARI + H2O2 → CO2 + H2O + inorganic salts (2)

Total organic carbon (TOC) and chemical oxygen demand (COD) analysesare sometimes used to characterize intermediate residues–organic acids thatare byproducts of the aromatic ring cleavage that can accumulate in the so-lution, and may serve the purpose to assess the degree of organic carbonmineralization in the liquid phase (Matta et al., 2008). The TOC removalrate is usually less than the phenol elimination rate, clearly showing thatphenol oxidation takes place in multiple steps and results in several byprod-ucts rather than CO2-only formation (Barrault et al., 2000). In practice, totalmineralization of phenol and its derivatives is difficult to achieve due to theincreased reaction time or consequent excessive chemicals consumption. Forthis reason, it is necessary to study the reaction pathway in depth, as the tox-icity of some intermediates can be higher than that of the initial compound(Zazo et al., 2005).

Usually, mechanism of phenol degradation is based on the MARI, as-suming the full mineralization by the end of the route, as shown in Figure 1.During the first stage (induction period), formation of DHBs with two hy-droxyl groups substituted in benzene rings upon hydroxylation of aromaticring is observed. This is achieved via the formation of dihydroxycyclohexa-dienyl (DHCD) radical (• C6H5(OH)2) with possible decays to form phenoxylradical (• C6H5O). It further reacts with the • OH radical formed during hy-drogen peroxide decomposition to form DHBs. Free radicals, formed dur-ing hydrogen peroxide decomposition, may attack the phenol moleculesdirectly and facilitate the formation of phenoxy radical (Kurian and Sug-unan, 2006). Norena-Franco et al. (2002) stated that, thermodynamically,o- and p-substitutions are equally feasible; whereas kinetically substitution

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138 E. V. Rokhina and J. Virkutyte

Phenol

Condensation products

Hydroquinone

p-BenzoquinoneCatechol

o-Benzoquinone

Resorcinol

Muconic acid

Fumaric acid

Oxalic acid

Oxalic acid

Acetic acid

Formic acid

Malonic acid

Maleic acid

OH

O

OO

O

OH

OH

OHOH

OHOH

OH

OH

O

O

OH

OHO

OO

O

OH

OH

O

CH OH

OH

O

OH

OHO

O OH O OH

O O

OH

OHO

O

OH2+CO2

FIGURE 1. Reaction pathway of the full phenol mineralization via the formation of generallyaccepted MARI.

at o-position is more favored. This suggests a steric effect on the reaction,which can be explained in terms of a transition stage involving a complexwith the OH of phenol and OH radical of the peroxide (Norena-Franko et al.,2002). Orto position is preferable when both, hydrogen peroxide and phe-nol adsorb onto the same active site (Preethi et al., 2008). Also, Preethi et al.(2008) suggested that nature of the catalyst can greatly influence the processselectivity during hydroxylation. Hydrophilic properties of the catalytic sub-stance switch off the selectivity toward the p-substitution, which takes placein a case of preferential adsorption on active sites, leaving more phenol inunadsorbed state. Owing to the polarity factor, the more polar and relativelysmall-sized hydroxyl radical is preferentially chemisorbed and the reaction

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Environmental Application of Catalytic Processes 139

OH OH

OH

Catechol

Attack at o-position

OHo

OH OH

OH

Hydroquinone

Attack at p-position

OHo

FIGURE 2. Electrophilic substitution of phenol at orto and para positions (adapted fromDubey et al., 2002).

between free phenol and hydroxyl radical may yield a more selective hy-droquinone (Preethi et al., 2008). Here, an undesired reaction such as theformation of tarry products due to strong adsorption of phenol and dihy-droxy benzenes on Lewis acid site can occur (Klaewkla et al., 2007). Anelectrophilic attack of the hydroxyl radical is shown in Figure 2. Withinthe next stage of oxidation, the rings open to form carboxyl acids withlower carbon numbers as oxidation lapses. Ring opening of CL gives riseto muconic acid, which is further oxidized to maleic and fumaric acids.However, it was observed that muconic acid was not formed during hy-droquinone and p-benzoquinone oxidation, where maleic acid was the pri-mary product from ring cleavage (Zazo et al., 2005). When high catalystand hydrogen peroxide concentrations were used, all of the intermediateswere finally oxidized to formic and oxalic acids. Following the proposedscheme, formic acid was further oxidized to water and carbon dioxide,whereas oxalic acid showed quite refractory behavior and remained in thesolution (Rey et al., 2008). Thus, the stages of phenol oxidation may besimplified as the following: (a) initial formation of a dihydroxy compound(HQ and CL) upon hydroxylation of aromatic ring; (b) aromatic ring cleav-age, producing ring-opened products—low weight organic acids (e.g., va-leric acid, propionic acid, propanoic acid, tartaric acid, glyoxylic acid, mu-conic acid, oxalic acid), that may be further oxidized to water and carbondioxide according to the reaction conditions (Drijvers et al., 1999; Guelou

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140 E. V. Rokhina and J. Virkutyte

et al., 2003; Guo and Al-Dahhan, 2003; Klaewkla et al., 2007; Zazo et al.,2005).

Consequently, all the discussed reactions may be summarized into threemajor groups according to the principle, proposed for Fenton-like reactions:(a) principal inorganic reactions that represent the interactions among vari-ous inorganic species including • OH, • HO2, • O−

2 , and H2O2; (b) the inter-actions among organic species (parent compounds and their intermediatesor products) and the reactive inorganic species except the catalyst species;(c) the reactions between catalysts and the organic species that influencethe catalyst–redox cycle (Kang et al., 2002). The possible interactions in thethird group, particularly concerning the roles of organic intermediates orfinal products in the catalyst–redox cycle, would merit further investigation.

Monitoring of the reaction intermediates is the matter of great impor-tance due to undesirable effects produces by some of the oxidation products,such as toxicity and enhanced leaching of the active species from the catalystreported in details in sections 2.5 and 2.2.3, respectively.

2.4. Kinetic Models for Phenol Oxidation

Kinetic models, developed for catalytic oxidation of phenol in the pres-ence of hydrogen peroxide, have been mainly expressed as the associatedset of differential equations that describe the reaction rate. The vast major-ity of kinetic investigations were performed by using model solutions withconcentrations of phenol ranging from 100 to 2000 ppm. In nearly all thecases, the reaction order regarding phenol degradation followed the first orpseudo-first-order kinetics (Maduna Valkaj et al., 2007).

Zelmanov and Semiat (2008) reported an exponential decrease in phe-nol concentration (first-order reaction kinetics) during catalytic aqueous ox-idation of phenol with H2O2 over iron nanopacticles carried out in a dark-ened vessel at ambient temperature. The exponential decrease determinedthe character of the reaction, and kinetics of the process was described withthe following expression:

− (t/ ln(C/C0)) = α + βt (3)

where α (min) and β (dimensionless) are the two constants. The constantscan be calculated for different initial concentrations of hydrogen peroxideand iron oxide-based nanocatalysts.

The kinetic model for linear disappearance of phenol via conversion(X) was developed by Tatibouet et al. (2005). An Al-Fe pillared clay catalystwas studied in an isothermal, continuously operated flow reactor with theperistaltic pump to avoid external mass transfer limitations by increasing theflow rate throughout the catalytic bed. The rate of phenol conversion was

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expressed as a first-order kinetic reaction using the rate law:

1/X = 1 + 1/kτ (4)

where X is phenol conversion (dimensionless), k is the rate constant (mLmin−1 g−1), and τ is the catalyst weight and total flow rate (min g−1 mL−1).The rate constant k was determined as the slope of the straight line obtainedby drawing 1/X as a function of 1/τ .

The competition for adsorption sites makes surface reactions more com-plicated than reactions in the solution or in the gas phase. Thus, surface re-action mechanisms with a few steps have a very complicated kinetics causedby the effects of heat and mass transmissions. Maduna Valkaj et al. (2007)argued that when external mass transfer controls the reaction rate, the mostimportant factor is the speed of stirring with further assumption that at thestirrer speed higher than 150 rpm, the external mass transfer is negligibleand the internal mass transfer is also not a limiting step. This is due to theproportionality between the rate constant of phenol oxidation, the catalyst(Cu-ZSM-5) loading and the concentration of the catalytic active materialobtained (Maduna Valkaj et al., 2007). Such a kinetic model is following:

− (dcPh/dt) = kPhcnPhcHP (5)

−(dcHP/dt) = kHPcHP + kPhcnPhcHP (6)

In addition, Klaewkla et al. (2007) developed a kinetic model using Pseudo-Steady-State Hypothesis (PSSH) with the Langmuir–Hishelwood approach.They studied hydroxylation of phenol using titanium and tin silicalite-1s (Ti-Sn-S-1) in a glass reactor fitted with a condenser and a mechanical stirrer.Figure 3 presents a parallel–sequential reaction scheme, where the forma-tion of DHBs (HQ, BQ, and CL) subsequently changes under hydroxylationconditions (e.g., BQ and CL are conversed to TA, HQ is conversed to BQ).In a case of Ti-Sn-S-1, the reaction mechanism is almost the same as withTS-1, except for the adsorption of the reactants on tin active sites and theirreversible reactions of these adsorbed active sites on the catalyst surface.The two hypotheses were introduced to evaluate the kinetic equations ona theoretical basis: (a) the existence of a rate-limiting step and (b) the as-sumption of a stationary state approximation for the unstable intermediatesobserved during PSSH application (Klaewkla et al., 2007). The adsorption orthe surface reaction is regarded as the rate-limiting step because the rate ofdesorption of the product is very rapid. Using a set of equations represent-ing the concentration of components in the system at any reaction time andutilizing the conversions of phenol, CL, BQ, HQ, and hydrogen peroxide,according to the proposed scheme (Figure 3), the differential equation is as

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142 E. V. Rokhina and J. Virkutyte

O

O

OH

OH

OH

OH

OH

Phenol

Catechol

Hydroquinone

p-Benzoquinone

Tar

FIGURE 3. Proposed reaction scheme for oxidation of phenol for kinetic model upon PSSHwith Langmuir–Hishelwood approach (adapted from Klaewkla et al., 2007).

follows:

dCi/dt = −ri ∗ W ∗ SBET (7)

where Ci is the concentration of the component i at any time (mol l−1), ri

is the reaction rate of the component i (mol l−1 m−2 s−1), W is the catalystweight (g), and SBET is the catalyst surface area (m2 g−1).

All the studies describe the disappearance rate of pure phenol frommodel solutions; however, a comprehensive study predicting a completeconversion of all the organic species present in the wastewater, regardlessof whether they are initially present or formed as intermediate products, isneeded.

Hence, TOC is a parameter, which accounts all organic species presentin the wastewater and draws the general picture of oxidative mechanisms.Guo and Al-Dahhan (2003) proposed a unique generalized lumped kineticmodel by assuming that the homogeneous and heterogeneous reaction ratesare additive. This model captures the two parallel reaction network schemes,reflecting the homogeneous noncatalytic oxidation based on a set of second-order reactions and the heterogeneous catalytic reaction pathway in theframework of a Langmuir–Hishelwood scheme (Guo and Al-Dahhan, 2003).

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Environmental Application of Catalytic Processes 143

O

O

O

O

OH

OH

OH

OH

OH

HOOC COOH

COOHH3CPhenol

Catechol o-Benzoquinone

Oxalic acid

Acetic acid

Hydroquinone p-Benzoquinone

CO2+H2O

FIGURE 4. Proposed reaction scheme for oxidation of phenol for homogeneous–heterogeneous kinetic model (adapted from Guo et al., 2003).

The approach is based on three basic lumps, which are expressed in carbonconcentration terms for oxidation kinetics: (a) phenol, denoted as lump A,is the parent pollutant to be removed; (b) intermediate organic carbon, de-noted as lump B, is the phenol oxidation byproduct; and (c) total inorganiccarbon, denoted as lump C, is fully mineralized end product. On the basis ofthe carbon scale, (B) = TOC −(A) and (C) = (A)0 −TOC. Accordingly, thedevelopment of the apparent kinetic models involves the following steps:(a) development of the homogeneous kinetic model (with rate rh and con-stant kh) using the homogeneous experimental data, and (b) developmentof the heterogeneous kinetic model (with rate rH and constant kH) using theheterogeneous experimental data and the developed homogeneous modelwith its fitted parameters (Guo and Al-Dahhan, 2003). The elementary stepsfor the homogeneous contribution (i.e., the noncatalytic oxidation route withrate rh, a sequential–parallel reaction scheme) is proposed where the pro-duction of lump C is generated by irreversible oxidation of lumps A and B.Lump B results from the irreversible oxidation of lump A. The elementarysteps are shown in Equations 8–10, which is consistent with the proposedreaction pathway for phenol oxidation, as presented in Figure 4.

H2O2 + A → B (8)

H2O2 + A → C (9)

H2O2 + B → C (10)

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144 E. V. Rokhina and J. Virkutyte

The heterogeneous elementary steps are characterized by the adsorption ofthe reactants on the active sites of the catalyst (represented by ∗), followedby the surface reaction and desorption of the products back into the liquid:

A+∗ ↔ A∗ (11)

H2O2 + A∗ → B∗ (12)

H2O2 + B∗ → C +∗ (13)

B∗ ↔ B+∗ (14)

However, hydrogen peroxide is assumed to remain unadsorbed on the activesites and to react directly with chemisorbed phenol. In addition, the stepsof adsorption and desorption are assumed to be instantaneous as comparedto surface reactions of chemisorbed phenol and its intermediate products,which are the rate-controlling steps. The final model, presented by the setof two differential equations, describes the depletion rates of phenol andintermediate products:

− d(A)/dt = rh,A + r∗H,ACpcat (15)

d(B)/dt = rh,B + r∗H,BCpcat (16)

where the initial conditions are (A) = (A)0 and (B) = 0 at t = 0. In everypart of Equations 15 and 16, the first term on the right-hand side gives thehomogeneous and the second term gives the heterogeneous contribution tothe rate. When analyzing this model, Perathoner and Centi (2005) arguedthat the kinetic model does not account the reaction mechanism of theseradical-type reactions (including the rate of generation and termination ofthe hydroxyl radicals). They also indicated that the kinetics of homogeneousconversion of phenol with Fe2+/H2O2 is much more complex following zero-order kinetics during the major part of the reaction, and changing at the enddue to an autocatalytic effect of phenol (Perathoner and Centi, 2005).

Moreover, the findings of Melero et al. (2007), who studied the kineticsof the oxidation process in the presence of nanocomposite Fe2O3/SBA-15,revealed that stable metallic organic complexes with the supported ironspecies experienced autoextraction from the catalyst surface when oxida-tion byproducts were desorbed. Therefore, during the CWPO processes, thismechanism is responsible for the partial iron dissolution, which can act ashomogeneous source of iron ions to enhance the overall catalytic perfor-mance (Melero et al., 2007). Parallel reduction–oxidation reactions of immo-bilized iron species can be partially responsible for the activity and stabilityof the heterogeneous catalyst, even in the absence of hydrogen peroxide. Itis remarkable to note that iron (III) ions have been reported to be effectiveoxidants for the direct degradation of phenol or intermediates of the reaction

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(Melero et al., 2007), whereas Guo and Al-Dahhan (2003) attributed all theoxidation activity to H2O2.

2.5. Influence of the Reaction Parameters

The evolution of phenol oxidation depends on the operation parameters andthe catalyst nature as well. Extensive variations in the reaction conditionsmay lead to a change in the rate-limiting step due to the competition of thereactants for sites (Lynggaard et al., 2004). As a result, heterocatalyst andthe reaction conditions along with the functional group in the aromatic ringdetermine the composition of the oxidation products (Klaewkla et al., 2007).Thus, it is vitally important to evaluate the impact of the reaction conditionson the process and to discuss the actual influence of the reaction conditionsto extend the WPO applications. Actual significance of each parameter suchas pH, catalyst and hydrogen peroxide concentrations, temperature, andinitial phenol concentration are carefully investigated in the articles reviewedsubsequently (Table 1). Main observations are given for every significantparameter in order to gain a better understanding of the process validity andniche for future applications.

2.5.1. INFLUENCE OF PRIMARY REACTION PARAMETERS: PH, CATALYST,AND HYDROGEN PEROXIDE CONCENTRATIONS AND TEMPERATURE

ON PHENOL OXIDATION

From the practical point of view, pH is one of the most important parametersinfluencing heterogeneous liquid phase catalytic oxidation of phenol. Phenolitself is incapable to vary the pH due to relatively weak acidity that confersslight acidic properties to the aqueous solution. Without the pH adjustment,the initial solution of phenol and deionized water exhibits pH in the rangeof 5.8–6.0, whereas the most appropriate pH for phenol degradation withhydrogen peroxide is acidic (Guo and Al-Dahhan, 2003). It can be explainedthat at pH values above 4, the rapid H2O2 decomposition produces molec-ular oxygen without the actual formation of hydroxyl radicals (Jones, 1999).Accordingly, for Fenton process the most benign pH range is 3–3.5, althoughsome authors have reported a wider 2–4 pH range (Liu et al., 2001). At pH3, the concentration of Fe3+ active species and the lowest rate of parasiticdecomposition of hydrogen peroxide is observed (Liou et al., 2005). How-ever, it has been stated that at pH below 3, intensive Fe leaching is detected(Zhanga et al., 2003). Also, for Fenton and Fenton-like processes, the effectof pH mainly relates to the oxidation state of iron, which influences theproduction of hydroxyl radicals. At higher pH, Fe (III) tends to precipitateas Fe(OH)3, which decomposes hydrogen peroxide into O2 and H2O (Wanget al., 2007). According Guo and to Al-Dahhan (2003), the optimum pH forthe maximum reaction rate is about 4.0 for both, the induction period andthe steady-state regime. As a matter of fact, high pH favors the formation of

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146 E. V. Rokhina and J. Virkutyte

carbonate ions, which are effective scavengers of hydroxyl ions and there-fore can reduce the efficiency of the overall degradation process (Jones,1999). Moreover, hydrogen peroxide is unstable in basic solutions and maydeliberately decompose to O2 and H2O, losing some of its oxidation ability(Alnaizy and Akgerman, 2000). The reaction as well as adsorption of thereactant on the catalyst surface at high pH is slower, as the catalytic surfacebecomes negatively charged and phenolate becomes dominate organic form(Wang et al., 2007). Also, the low activity of catalyst at alkaline pH couldbe due to the generation of OH− species, which would approach the activecenter thereby inhibiting the adsorption of hydrogen peroxide, and in turnnegatively affecting the generation of hydroxyl radicals (Dubey et al., 2002).

However, some experimental studies conducted at neutral or circumneutral pH revealed that at this particular pH, active species are still presentand they are able to establish an effective redox system with H2O2 (Alnaizyand Akgerman, 2000; Rokhina et al., 2008a). Good yield at neutral pH fordifferent heterocatalysts including Fenton-like catalysts were reported forthe synthetic and real wastewaters (Dubey et al., 2002, Matta et al., 2008;Namkung et al., 2008).

Correct dosage of chemicals is essential due to detrimental effects ofexcess of chemicals because the adduct species can react with intermediates,such as hydroxyl radical, responsible for the direct oxidation of phenol. Thetotal amount of hydrogen peroxide needed for the complete mineralizationcorresponds to the following reaction:

C6H5OH + 14H2O2 → 6 CO2 + 37 H2O (17)

The stoichometric coefficient is approximately 14 mol of organic compoundper 1 mol of hydrogen peroxide. A typical range for Fenton reactants is about1 part of iron per 5–25 parts (% wt/wt) of hydrogen peroxide (Zazo et al.,2005). However, it is also known that an excess of hydrogen peroxide actsas • OH scavenger, hence an increase in its concentration leads to a decreasein the concentration of free hydroxyl radicals, which causes a decrease inthe rate of phenol degradation (Molina et al., 2006; Zelmanov and Semiat,2008). Nevertheless, insufficient amount of hydrogen peroxide also slowsdown the reaction rate (Gupta and Sutar, 2008a).

The amount of catalyst substantially affects the reaction rate. In mostcases, initially phenol oxidation increases with increasing in catalyst con-centration and then the rate per weight of catalyst levels off and becomesconstant (Hagen, 2006). The increase in catalyst concentration acceleratesdecomposition of hydrogen peroxide (Jones, 1999). However, too high ofan amount of the catalyst catalyzes the surface chemical reaction so rapidlythat reactants become depleted in the liquid phase and thus the diffusionof reactants controls the rate of the reaction (Hagen, 2006). On the other

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hand, too low of a concentration prolongs the reaction time up to 12times due to that oxidation products (e.g., organic acids) block the cata-lyst via the formation of complexes, thus impairing the whole catalytic cycle(Jones, 1999). Moreover, the increased amount of active sites in concur-rence with an increase in catalyst mass can also lead to the readsorptionof products to active sites (Hagen, 2006). Additionally, several researchersconsider catalyst efficiency to be also affected by the peroxide to active cen-ters ratio (Guo and Al-Dahhan, 2003; Hagen, 2006; Jones, 1999; Sotelo et al.,2004).

The efficiency of catalyst ultimately depends on interactions of reactantswith the catalyst and is controlled by the reaction temperature (Gupta andSutar, 2008a). The influence of temperature on the reaction rate gives aninsight to which of the reaction steps are the slowest steps of the total catalyticprocess (Hagen, 2006). The term of wet peroxide oxidation implies thatstudied range of temperatures for phenol oxidation does not exceed 100◦C.Playing an important role in the process, an increase in temperature maycause two opposite effects: The phenol oxidation rate increases accordingto the Arrhenius equation, but the destruction rate of H2O2 to O2 and H2Oalso increases at high temperatures (Jones, 1999). Therefore, the removal ofphenol (its oxidation rate) should be a function of the competition between•OH radical formation and thermal degradation of hydrogen peroxide (Liouet al., 2005). High temperature can also cause disappearance of phenol dueto its semivolatile nature. A majority of scientists claim the optimal range oftemperatures for the highest conversion efficiency and less retention time tobe in the range of 50–80◦C (Guo and Al-Dahhan, 2003; Rocha et al., 2002);however, this effect is significant only at shorter retention times (Alnaizy andAkgerman, 2000). The decrease in phenol conversion at higher temperatureis due to that formed DHBs may be further oxidized to tar, which is themajor poison for surface sites (Klaewkla et al., 2007).

Recent studies on WPO of phenol at ambient temperature demonstratedthe ability of phenol to degrade with the high conversion rate at a reasonabletime. For instance, Namkung et al. (2008) proved the ability of AFR systemto be very effective at 25◦C (±1◦C), removing up to 60% of TOC during60 min. Also, Zelmanov and Semiat (2008), who used iron nanoparticles, andKuznetsova et al. (2007), who used HPA, reported high oxidation efficienciesof these catalytic systems at ambient temperatures even up to a completemineralization of phenol. Meanwhile, Guo and Al-Dahhan (2003) reportedthat the influence of pH and temperature is markedly higher at initial stageof oxidation, whereas at steady state the oxidation rate of phenol shows aweaker function of these reaction parameters. At shorter reaction times, theinitial pH strongly affects the phenol removal. As the reaction proceeds, thefragmentation of organic material into carboxylic acids leads to the decreasein pH and in acceleration of the process (Catrinescu et al., 2003).

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2.5.2. INFLUENCE OF SECONDARY REACTION PARAMETERS: INITIAL PHENOL

CONCENTRATION, ROTATION SPEED, AND THE PRESENCE OF AIR

IN THE REACTION MIXTURE

It has been reported that high concentrations of organic pollutants usuallyinhibit free radical formation that is responsible for the oxidation of the re-action intermediates in the WPO process (Kurian and Sugunan, 2006). Liouet al. (2005) reported that the relative activity does not decrease with increas-ing in the pollutant concentration during oxidation and that the inhibitionof free radicals observed on the surface of the catalyst is not important ifthe pollutant concentration does not exceed 2000 ppm. However, it wasreported that amount of phenol oxidized was directly dependant on theamount of phenol originally present in the reaction mixture, concludingthat a minimum concentration of phenol was required for good conversions(Hagen, 2006). Likewise, many scientists who studied degradation of phenolin the similar concentration range (100–2000 ppm) reported that a higherinitial concentration of phenol led to the lower conversion under otherwiseidentical conditions (Guo and Al-Dahhan, 2003). Consequently, most authorsclaimed that decrease in phenol conversion was observed for high phenol:hydrogen peroxide (at least more than 1:3) molar ratios (Jones, 1999).

Rotation speed of the reaction mixture is an important parameter, whichserves two purposes: to provide a complete mixing for uniform distribution(full suspension of catalysts particles in the aqueous solution) and to mini-mize the external mass transfer during catalytic oxidation. The resistance forthe species to transfer to the surface of the catalyst, and all external masstransfer limitations can be removed by proper mixing (Hagen, 2006). Whileexploring the effect of the impeller rotation speed to identify the rate (inrotations per minute [rpm]), above which the apparent reaction rate is notchanging, authors received controversial data. Maduna Valkaj et al. (2007)argued that a stirrer speed higher than 150 rpm was enough to neglectthe external mass-transfer resistance, whereas Guo and Al-Dahhan (2003)claimed that only a rotation speed above 400 rpm and higher served tothat purposes. However, all the observations supported the hypothesis thatrotational velocities in excess of at least 750 rpm ensured very good masstransfer between the catalyst and the liquid due to an internal circulation,which was generated in the reactor, as proposed by Catrinescu et al. (2003).Klaewkla et al. (2007) reported that the resistance to mass transfer from theliquid phase to the solid surface was absent at 800–1000 rpm. Beyond thestirring rate of 1000 rpm, a vortex flow pattern occurred and impaired theoverall process (Klaewkla et al., 2007).

The presence of air is an important parameter for the radical formationreaction. Air itself is used as an oxidant, and therefore its combination withhydrogen peroxide should be beneficial. The air flow passing through thecatalyst bed may be applied to (a) maintain an oxygen saturation of the

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water solution and (b) gently rearrange catalyst particles to allow gaseousCO2 formed during the reaction to be efficiently removed from the catalystbed (Tatibouet et al., 2005). Oxygen-saturated solution is able to producenew radicals via a wide range of recombination reactions. In the CWPOsystems, additional • OH radicals may be generated from molecular oxygenreductively activated with hydrogen (Klaewkla et al., 2007). Consequently,the presence of oxygen could then increase the propagation steps of theradical reactions leading to the phenol oxidation (Barrault et al., 2000). Kuboet al. (2005) determined that when the dissolved oxygen concentration wasincreased by purging pure oxygen into the solution, phenol degradationrates significantly increased. However, not all the gases may be beneficialfor the reaction. Barrault et al. (2000) reported that phenol conversion andTOC abatement remained low under the nitrogen atmosphere, whereas fullphenol conversion was observed under the air atmosphere for the samereaction conditions.

2.6. Evolution of Ecotoxicity During Oxidation of Phenol

As it was already indicated, phenol itself is rather toxic and consequentlymust be removed from water and wastewater streams. However, one pe-culiarity of phenol oxidation is that intermediate species generated duringthe process can be more toxic and less biodegradable than phenol itself(Mijangos et al., 2006). Thus, careful attention should be paid to the pro-cess parameters to adjust them in an effective manner, which allows theavoidance of toxic byproducts’ accumulation in the solution. For this reason,it is vitally important to define the most toxic products of phenol decom-position. The acids formed during the final stages of phenol degradationare biodegradable, so their environmental impact is relatively low. Hydro-quinone and p-benzoquinone are by far the most toxic species in the ox-idation route of phenol (Rey et al., 2008). Zazo et al. (2007) studied thetoxicity evolution during Fenton process and revealed that hydroquinoneand p-benzoquinone yield ecotoxicity four orders of magnitude higher thanthat of phenol, whereas the organic acids are at least one order of magni-tude less toxic than phenol. The toxicity increase during the initial stagesof the oxidation process is due to the rapid transformation of phenol intop-benzoquinone and hydroquinone. The subsequent decrease in toxicity cor-responds to a disappearance of aromatic intermediates giving rise to organicacids of low toxicity (Zazo et al., 2007). Zazo et al. (2007) also indicated thattoxicity values are directly related to the residual H2O2 concentration in thereaction medium. However, in alkaline environments H2O2 tends to decom-pose with the release of oxygen and water. Therefore, the neutralization oftoxic intermediates and the residual hydrogen peroxide concentration wouldbe advantageous final stage of Fenton and Fenton-like oxidation processes.

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2.7. Experimental Design

The use of a multivariate design of experiments has proven to be an im-portant tool for obtaining valuable and statistically significant models ofoxidation process by performing a well-planned set of experiments. Thepreference of the experimental design is that instead of conducting a seriesof independent studies, these studies are effectively combined into a singleone (Rokhina et al., 2008b). Using this technique, it is possible to assessthe importance of each individual variable and the interaction effects be-tween them at different level (usually two or three), thereby yielding themost desirable response (Nguyen et al., 2001). One of the most used toolsis the factorial design (including fractional factorial design and full factorialdesign)—the construction of factorial design matrix, which investigates theselection of the optimum experimental conditions for phenol degradation,such as temperature, pH, phenol to hydrogen peroxide molar ratio, and theconcentration of catalyst. Studied parameters, named independent variables,are coded at different levels (−1, 0, and 1 for low, medium, and high levels,respectively), providing a wide range of operation parameters (from low tohigh concentrations of catalyst, hydrogen peroxide). To build up the de-sign matrix, all experimental results (expressed in % conversions of phenol,TOC, or COD) are represented along with the real values for the indepen-dent variables. The experimental values acquired allow the generation ofa matrix with the response variables obtained for all the reaction condi-tions (Namkung et al., 2008). Predictive equations are usually solved basedon the final matrix according to the chosen model with statistical software(e.g., STATISTICA, MINITAB) and the surface response curves are build up(Nguyen et al., 2001; Rokhina et al., 2009). The model function is written asthe following:

Y = b0 +∑

bixi +∑

bijxixj + · · · + b 12...kx1 ... xk (18)

where bs are the interaction coefficients, which values show the synergeticeffect of variables interaction (if positive) or antagonistic (if negative). Thequality of a model is usually confirmed by correlation coefficients (R2) and astandard deviation value. The adequacy of the models is possible to justifywith an analysis of variance (ANOVA) at 95% confidence level. For example,Molina et al. (2006) described 32 factorial design that aimed to minimizethe use of hydrogen peroxide concentration, yet to maximize the degra-dation of phenol and to determine the influence of the variables (catalystand hydrogen peroxide concentrations) on the stability of the heterogeneoussono-Fenton system. They assumed a second-order polynomial model anda Levenberg–Marguard algorithm for the nonlinear regression. The 3D re-sponse surfaces have been designed for the TOC conversion at different

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time intervals (90, 180, and 270 min) and leaching of iron at 270 min, de-pending on catalyst and hydrogen peroxide concentrations. Furthermore,Chakinala et al. (2008) investigated AFP combined with the hydrodynamiccavitation, accomplished the 32 design with hydrogen peroxide dosage anddifferent amounts of zero-valent iron metal pieces acting as catalyst in theselected process. The response variables were TOC conversion at 15, 60,and 105 min of treatment time, with the objective to study the influence ofthe independent variables on the activity and the amount of iron dissolvedfrom the catalyst during the process and to determine the influence of theindependent variables on the modified AFP. They also assumed a second-order polynomial model and a Levenberg–Marquard algorithm for nonlinearregression to construct the design matrix. Another approach was used byNamkung et al. (2008), who applied the Taguchi method to study ultrasoni-cally enhanced AFM in a flow-through system. Five main effects (hydrogenperoxide concentration, pH, ultrasound irratiation, initial concentration ofphenol, retention time) and all 10 two-factor interactions, leading to 15 ef-fects of interest, were taken into consideration. Validity of the method wasverified with the F test and the Anderson–Darling test showing no departurefrom normality (Namkung et al., 2008).

It should be noted that all the predictions are valid only for the cer-tain measurement range. The study of all meaningful effects identifies rolesplayed by various operational parameters investigated and gives rise to essen-tial opportunities for development of the process on the basis of performedexperiments. However, a screening of all the effects should be accomplishedin order to determine whether the smallest effects are at all meaningful andsignificant for the process.

2.8. Free Radical Species, Their Determination, and the OxidativeRole

A heterogeneous free radical reaction mechanism has been generally ac-cepted to explain chain reactions occurring in the presence of solid catalystand its interaction with hydrogen peroxide resulting in the free radicals for-mation. The heterogeneous–homogeneous reaction mechanism in a liquidphase oxidation over solid catalysts was initially proposed by Meyer in 1965.In the case of phenol hydroxylation with hydrogen peroxide as the oxidant,free radicals can be generated on the solid catalyst surface in two possibleways: (a) the catalyst accelerates the decomposition of hydrogen peroxideinto radicals, or (b) the catalyst activates the phenol molecules directly andthus facilitates the formation of phenoxy radicals (Chunrong et al., 2000). Thebehavior of radicals was postulated by Kim and Metcalfe (2007): (a) gener-ation of • OH radicals mainly occurs on the active surface of the catalystbecause most of radicals are produced as a result of catalytic decompositionof H2O2 and (b) most of the radical reactions occur near the surface of the

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152 E. V. Rokhina and J. Virkutyte

catalyst because • OH are very reactive and have a short lifetime. The freeradical mechanism of the widely researched Fenton reaction has been ques-tioned from time to time, and alternatives have been proposed that involvehypothetical transients other than free • OH radicals (Pignatello et al., 1999).Although some authors have offered kinetic evidence that • OH is the prin-cipal active oxidant in the oxidation of phenol with hydrogen peroxide andheterogeneous catalyst, it must be mentioned that a transient species formedalong with • OH may be difficult to detect if they exhibit a similar pattern ofreactivity as • OH, or if they react slower than • OH with the target molecule(Pignatello et al., 1999).

There are two approaches for observation and detection of radicalsavailable: qualitative and quantitative. The detection and the detailed studyof radicals formation needs to use highly sensitive techniques of spin trap-ping due to their very short lifetime in the nanoseconds range, such aselectron spin resonance (ESR) and the nuclear magnetic resonance (NMR)technique. There are some popular nitrone spin trapping agents with theexcellent ability to form an adduct with the oxygen centered radicals:5, 5-dimethyl-1-pyrroline-1-oxide (DMPO), and its derivatives (DEPMPO,DIPPMPO, DPPMPO, DBPMPO, DEHPMPO) with a more complex structurethat causes the improved stability of the radical adduct (Bacic et al., 2008;Kim and Metcalfe, 2007; Tatibouet et al., 2005). The quantitative method isbased on the addition of radical scavengers (e.g., 2-propanol, alcohols, t-butanol, humic acids, chloride and sulfate ions), which inhibit the oxidationby quenching radicals, thereby confirming the predominant role of radicalsin the process (Matta et al., 2008). It is interesting to note that major productsof the phenol–hydrogen peroxide reaction are generally the same regardlessdifferent conditions and catalysts used.

The established reaction scheme for the oxidation of phenol proceedsvia intermediates route, where the significant interconnection between thepresence of DBHs and responsible reactive species can be observed (Mattaet al., 2008). Because hydroxyl radicals and hydroperoxyl radicals are elec-trophilic, they allow electrophilic attack at the ortho and para positions ofphenol to form CL and HQ (Sun et al., 2000). Thus, the similarity in prod-uct distribution is consistent with the radical mechanism of hydroxylation ofphenol (e.g., preferential substitution to the o- and p-positions showed inFigure 2; Gupta and Sutar, 2008a; Namkung et al., 2008). The Weiss mech-anism is proposed earlier for metal-catalyzed decomposition of H2O2 (Linand Gurol, 1998):

S + H2O2 → S+ + OH− + • OH (19)

• OH + H2O2 → H2O + • HO2 (20)

S+ + • O−2 → S + O2 (21)

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Environmental Application of Catalytic Processes 153

S + HO−2 → S+ + HO−

2 (22)

S+ + HO−2 S → + • HO2 (23)

Here, S and S+ represent the uncharged and charged parts of the metalsurface. In this mechanism, the principal role of H2O2 is the oxidation of themetal surface, resulting in the formation of hydroxyl radicals (Matta et al.,2008). The primary radicals initiate a complex chain reaction, developing viathe reactions of radicals’ recombination during the propagation and termina-tion periods of the reaction:

• OH + H2O2 → H2O + • HO2 (24)

• HO2 + H2O2 → • OH + H2O + O2 (25)

• HO2 + • HO2 → H2O2 + O2 (26)

• OH + • OH → H2O2 (27)

• OH + • OH → H2O + O (28)

• OH + • OH → H2 + O2 (29)

Being a classic process known for more than a century, Fenton andFenton-like processes have been the center of attention for many scientists.Therefore, the largest amount of interpretations for radical formation andpropagation reactions were found for these processes. The outersphereelectron-transfer reaction between Fe(II) and H2O2, according to the classicinterpretation of Haber and Weiss cycle is the following:

Fe(II) + H2O2 → Fe(III) + OH− + • OH (30)

Fe(III) + H2O2 → Fe(II) + • O−2 /• HO−

2 (31)

Fe(III) + • O−2 /• HO−

2 → Fe(II) + O2 (32)

Fe(II) + • O−2 /• HO−

2 → Fe(III) + H2O2 (33)

According to the Pourbaix diagram, at pH 2.5–4.5 the main iron species inwater are (FeIII(OH))2+ and (FeII)2+. Tatibouet et al. (2005) suggested thatthe following reactions may then occur in the system:

(FeIII(OH))2+ + H2O2 → (FeII)2+ + • HO−2 + H2O (34)

(FeII)2+ + H2O2 → (FeIII(OH))2+ + • OH (35)

Kim and Metcalfe (2007) also established that the main active species arehydroxyl (• OH) or hydroperoxyl (• HO2) radicals formed as a result ofhydrogen peroxide reaction with iron species probably located in the poresof the catalyst surface. The experiments revealed that the • OH productionfollowed the same profile as the catalytic activity in phenol oxidation, as

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154 E. V. Rokhina and J. Virkutyte

a function of pH. However, some surface species such as iron peroxo orhydroperoxo species can also exist and play a role in the catalytic oxidationof phenol (Tatibouet et al., 2005). These results are in a good agreement withfindings of other scientists, such as Guo and Al-Dahhan (2003), who claimedthat (FeIII(OH))2+ is the dominant species at pH 3–4 and the mechanismof wet oxidation in the presence of H2O2 can be considered as a radicalreaction with • OH, • HO2, or ROO• radicals being the main oxidants.

Although the free radical mechanism is widely used, some scientistsquestion the exclusive role of • OH in the reaction—whether • OH produc-tion is not too slow to compete with the direct electron transfer betweenthe substrate and hydrated higher valent iron species as long as differentradical species formed within the reaction are more reactive in the oxidationof target compound as compared to free hydroxyl radical (Bossman et al.,1998; Kim and Metcalfe, 2007). Gupta and Sutar (2008b) argued that phe-nol degradation intermediates (e.g., catechol and hydroquinone) were likelyto reduce Fe (III) to Fe (II) more quickly than H2O2/• HO2/• O−

2 . Still, thedominant role of the hydroxyl radical as an initiator of the oxidative chainreaction has been secured for catalyzed oxidation of phenol in the presenceof hydrogen peroxide.

2.9. Life Cycle of Heterocatalysts

The essential factors that have to be taken into account before the full-scale implementation of heterogeneous catalytic processes in the industrialsector are environmental control, the decrease in manufacturing costs andthe tendency toward long lifetime, and reusability of the catalyst. Detailedstudy of all aforementioned factors are included in Life Cycle Assessment(LCA), which is an environmental management tool used to predict andcompare the environmental impacts of a product or service (Eijsbouts et al.,2008).

WPO is reported to be itself a process that directly contributes to acleaner environment. Nevertheless, it has relatively high importance for het-erocatalysts utilized in this process to be handled according to the stingiestenvironmental standards because during their life cycle catalysts are rapidlyrecycled and transported by different users and service providers.

Depending on the application, different deactivation mechanisms areconsidered to be important (e.g., active phase leaching, sintering, coke for-mation) during oxidation of phenol (Hagen, 2006; Martınez et al., 2007).Except for extremely contaminated ones, the vast majority of catalysts areregenerated and reused. During their life cycle, heterocatalysts undergo sev-eral transformations from the low to high oxidation state and vice versa.Thus, their life cycle is, therefore, very complex and involves many dif-ferent steps and aspects. The leading catalyst manufacturers together withspecialized firms offer total catalyst management during the entire catalyst

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Fresh catalyst

Catalyst handling

Catalyst activation

End of cycle

Catalyst Analysis

Regeneration

Reuse

Reclaiming

Use

Production

Recycle

FIGURE 5. Total heterocatalyst management during laboratory life cycle (adapted fromEijsbouts et al., 2008).

life cycle, starting with the purchase of the fresh catalyst and ending withits final recycling or disposal. Total catalyst management includes a broadrange of services, ensuring optimal timing during the change-out process;reliable, smooth, and safe operation; minimal downtime and the maximumcatalyst; and unit performance. Professional catalyst life cycle managementregards the catalyst through its whole life, from procurement to the finalrecycling (Eijsbouts et al., 2008). For laboratory scale application, LCA is in-troduced in the simplified scheme in Figure 5. The catalyst life cycle typicallyinvolves a long chain of operations: fresh catalyst is activated, then used forthe process, after which it may be regenerated or reclaimed. The reclaimedchemicals (e.g., metals) are further disposed or reused for further catalystpreparation.

Technically, the cycle is promoted via oxidation tests, carried out bycontacting fresh phenol solutions with the aged catalyst taken from previousruns, so that the capability of the heterocatalyst to destroy phenol could beevaluated (Guo and Al-Dahhan, 2003). After the process is over, the catalystsimply resides in the aqueous phase, and catalyst separation from the water-insoluble product is achieved by the phase separation. Additionally, catalystrecycling requires that the catalyst remains intact after each run (Sun et al.,2000). After that, the catalyst deactivation is evaluated in terms of activity andselectivity loss and probable reason for that as indicated by main principles ofLCA. The simplified LCA for laboratory LCA of catalyst are widely performed,mainly concentrating on the recycling of the catalyst and the study of loss inits activity (Alnaizy and Akgerman, 2000; Guo and Al-Dahhan, 2003; Martinez

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TABLE 2. Conpresent heterogeneous wet peroxide oxidation processes

Reference Catalyst Conpresent technology

Molina et al., 2006 Fe-SBA -15 USAzabou et al., 2007 (Al-Fe)PILC UVKubo et al., 2005 Ti2O USMei et al., 2004 Fe-Ti-PILC MWKusic et al., 2005 FeZSM5 gas/liquid electrical dischargeGoncharuk et al., 2002 FeCl2 reverse osmosisChakinala et al., 2008 Zero-valent iron (AFP) US 3.6 kWDrijvers et al., 1999 CuO US 520 kHzHua et al., 2001 Cu-MCM-41 UVRokhina et al., 2009 RuI3 USBi et al., 2008 CuOn-La2O3/γ -Al2O3 MW

et al., 2007; Timofeeva et al., 2007; Zelmanov and Semiat, 2008). Therefore,it is anticipated that a detailed LCA for every heterocatalyst used for phenoloxidation would soon be the routine procedure because WPO is a rapidlydeveloping field of clean technologies.

2.10. Improvement of Phenol Oxidation by Conpresent WorkingProcesses

The efficiency of the WPO system due to its flexibility and simplicity canbe sufficiently accelerated by the synergistic combination of various irradi-ations and other treatment processes. Intensification of WPO with UV, US,MW, and their combinations have been recently reported with a growinginterest to extend the application of conventional wet peroxide oxidation bythese hybrid techniques. The most important catalysts used in the conpresentprocesses are summarized in Table 2.

Photolysis of hydrogen peroxide directly yields hydroxyl radicals with aquantum yield of two • OH radicals per quantum of the absorbed radiation:

H2O2 + hυ → 2 • OH (36)

Titanium dioxide (TiO2), Cu-MCM-41, and Fenton-like reagents were stud-ied extensively in the photocatalytic degradation of phenol and phenoliccompounds (Arana et al., 2007; Chiou et al., 2008; Esplugas et al., 2000; Huaet al., 2001; Martinez et al., 2005; Pignatello et al., 1999). It has been reportedthat the combined use of photocatalyst, H2O2, and UV light could greatlyenhance the efficiency of phenol degradation, overcoming the RLS by addi-tional radical production via H2O2 photodissociation (Kormann et al., 1988).Being an inexpensive catalyst with the high reusability, TiO2 is applied usingmild reaction parameters, yielding high phenol conversion (up to 94%) withincompatible reaction time. The kinetic model of photochemical phenol degra-dation is usually supported by the Langmuir–Hinshelwood model, assuming

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the first- or second-order kinetics (Chiou et al., 2008). The main disadvan-tage of catalytic photooxidation in the presence of hydrogen peroxide is thatthe catalyst does not significantly absorb the UV light beyond 300 nm andweakly absorbs in the range 200–300 nm. Therefore, the UV–H2O2 processis not suitable for the treatment of polluted aqueous solutions with a highUV absorbance or a high total organic carbon background concentration(Alnaizy and Akgerman, 2000). Barakat et al. (2005) also studied the effectof O2 and N2 atmospheres on the efficiency of UV processes in the presenceof hydrogen peroxide and the catalyst. The influence of oxygen or nitrogenatmosphere was found to be negligible at high peroxide concentrations. Az-abou et al. (2007) studied the photodegradation of phenolic olive oil millwastewater, catalyzed by aluminium–iron pillared montmorillonite (Al-Fe)PILC and reported that the process was quite efficient for the model and realOMW effluent and the observed decrease in toxicity was up to 74%. Iron-containing SBA-15 catalyst consisting of crystalline hematite particle oxidessupported onto a mesostructured silica matrix was found to be a promisingcatalyst for the treatment of phenolic solutions in photo-Fenton processes.The outstanding stability of iron-containing SBA-15 catalyst defined by thesignificant decrease in iron leaching for high oxidant concentrations was at-tributed to the shielding effect of surrounding metal–H2O2 complex specieswhich prevented photo-induced leaching phenomenon by UV-vis (higherthan 313 nm) irradiation (Martınez et al., 2005).

US refers to sound waves with a frequency greater than 20 kHz (range of20–100 kHz is generally used in sonochemistry). The phenomenon is basedon the formation, growth, and implosive collapse of acoustic cavity bubblesthat entrap molecules of gases and water vapor from the surrounding liquid(Gogate, 2008). During collapse, gas molecules are thermally fragmented togenerate a variety of reactive species, including hydroxyl radicals. The radi-cals either react and recombine in the gas and gas–liquid–solid interface orescape into the aqueous phase, where they readily attack organic moleculesfor oxidative destruction (Gogate, 2008). US has potentially important appli-cation in heterogeneous catalytic systems due to the fact that the presenceof additional liquid–solid interface in the liquid bulk may promote the cav-itation. Three potential effects of ultrasound on the heterogeneous catalyticoxidation of phenol may be listed as the following: (a) an increase in theactive catalyst surface area via fragmentation of the catalyst particles, (b) thepromotion of cavitation bubbles formation, and (c) the removal of impuritiesdeposited on the catalyst, thereby preventing the deactivation of catalyst,and at the same time producing free radical according to Reaction (37):

H2O)))−→• OH + • H (37)

Hydrodynamic cavitation is effective for the production of highly reactivefree radicals due to the creation of high temperature and pressure zones

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and the generation of intense turbulence and liquid circulation presents,which can be effectively harnessed for wastewater treatment applications(Chiou et al., 2008). It is reported that irradiation of water with ultrasoundproduces hydrogen peroxide. However, Rokhina et al. (2008a) observedthat the amount of hydrogen peroxide produced during irradiation of waterwith ultrasound (24 kHz) is very small to be responsible for the continuousoxidation process. Sole US application for phenol degradation has beenrecently reviewed by Kidak and Ince (2006); however, successful applicationof ultrasound for full phenol degradation has been reported only for thenegligible (several ppm) initial concentrations. Sonochemical degradation ofphenol has been successfully applied in the presence of CuO, Cu/Al/Al2O3

(Cu/Al), CuO• ZnO/Al2O3 (Cu/Zn), and Fe-SBA-15 catalysts (Drijvers et al.,1999; Gogate, 2008; Kidak and Ince, 2006; Mei et al., 2004; Nguyen et al.,2001), and attracts attention as the effective, inexpensive, and reliable methodto decompose phenol in aqueous solutions.

Unfortunately, US can increase leaching of the active compound fromthe catalyst and as a consequence may lead to the enhanced corrosion ofmetal surfaces. The main reason for that is the removal or destruction ofpassivation films on the metal surface by cavitation effects as reported byNamkung et al. (2008).

MW irradiation was found to have a promotion effect on the wet per-oxide oxidation of phenol as a result of significant increase in water tem-perature. Due to the thermal decomposition, the conversion rate of H2O2 to• ROO, • OH, • HO2 radicals. is accelerated. Moreover, MW irradiation to-gether with the catalyst may initiate more efficient formation of free radicalsin the presence of H2O2. Bi et al. (2008) discovered that microwave irra-diation in MW-transparent media affected electronically excited moleculesor short-lived transition states. However, the lifetime of the species mustbe long enough to provide a sufficient time for the interaction with such alow-frequency radiation. The introduction of microwave irradiation may beused to greatly shorten the reaction time (from 90 to 8 min) as reported forthe complete mineralization of phenol (Mei et al., 2004).

The synergistic effect of UV, US, and MW with different catalysts hasbeen investigated for catalyst/H2O2/UV/MW and catalyst/H2O2/UV/US sys-tems (Gogate, 2008; Kusic et al., 2005; Mei et al., 2004). Thus, individualeffects of every irradiation simultaneously influenced a single chemical step;therefore, more free radical species were available in the system, thereby es-sentially increasing the degradation rate. Naturally, if the intermediates reactwith different rate constants, the total observed rate constant of the reactionis proportional to the sum of all individual rate constants.

The efficiency of photoreaction can be influenced by a microwave irra-diation when the excitation energy of the molecule is temperature dependent(Bi et al., 2008). The indirect reaction of phenol with • OH radicals may bethe dominant in the absence of microwave irradiation, and meanwhile a

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direct reaction of phenol with hydrogen peroxide may be the dominantin the presence of microwave irradiation, except for low concentrations ofhydrogen peroxide (Mei et al., 2004). Nevertheless, the industrial scale ap-plication of such a system is still under the question due to the technicalcomplexity and also because the galvanic heating is likely too difficult toapply in a large-scale wastewater treatment facilities because it is unrealisticto raise the temperature of a large volume so high as the content becomesdegradable (Kusic et al., 2005).

The use of sonophotocatalytic oxidation is based on the ability of cavi-tation to keep the surface area of heterocatalysts active by removing all thedeposits and preventing the adsorption of contaminants as well as block-ing the UV-activated sites. Also, the number of free radicals generated insimultaneous irradiation greatly increases the rate of degradation. Moreover,the turbulence induced by the cavitation phenomena assists in the severemass-transfer limitations related to photocatalytic oxidation. The vitally im-portant factors should be taken into consideration during the performance ofthe system: only simultaneous operation of US and UV due to the cleaningaction of US must be performed and the special attention should be paidto the stability of the photocatalyst to avoid damage of its crystal structure(Gogate, 2008).

An interesting alternative, such as pulsed electrical discharge, has beenrecently evaluated for the generation of highly reactive species to degradephenol in wastewater. Hybrid gas–liquid electrical discharge reactors involvesimultaneous high-voltage electrical discharges in the liquid phase and in thegas phase above the liquid surface (Shen et al., 2008). The chemical effectsof the electrical discharge plasma are attributed to the direct photolysis (theelectrical discharge plasma only), indirect photolysis (combination of chem-ical additives and the electrical discharge plasma), and pyrolytic destructionin plasma channels (Kusic et al., 2005).

The combination of electrical discharge plasma and photocatalyticaltreatment benefits additional effect of ultraviolet radiation from the plasma,resulting in the photocatalytic formation of • OH radicals on the surface ofphotocatalyst (TiO2) particles, thus the increasing the • OH radicals’ yield,available for phenol degradation (Shen et al., 2008).

The hybrid techniques offer a viable treatment scheme for phenol degra-dation with hydrogen peroxide, capable to overcome the drawbacks of con-ventional WPO technique. The synergism of the coupled methods originatesfrom the observation that total rate constant is proportional to the sum ofall rate constants of individual processes (Gogate, 2008; Shen et al., 2008).Nevertheless, it should be noted that simple increase in the radical formationdoes not necessary lead to the process improvement. The final efficiencyof the process strongly depends on the system implementation, which pro-motes better utilization of formed species with impurities in self-regeneratingmode. The major oxidation rate-controlling factor is the stability of the

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catalyst under the different kinds of irradiations and efforts should be directedto combine the units in a more beneficial way with an overall reasonablecost.

3. FUTURE RESEARCH NEEDS, CHALLENGES,AND OPPORTUNITIES

The use of heterocatalysts for the development of novel processes and tech-nologies is a promising approach for a great variety of future applications.From the first glance, in the present paper we review a narrow topic focus-ing entirely on the oxidation of phenol by means of wet peroxide oxidation.However, it covers various aspects of the process, which is valuable for thestudy of phenol degradation and WPO of other persistent organic pollutantsas well. The need for catalytic processes is mainly driven by the necessity forclean, sustainable, and low-cost green process combined with the high effi-ciency. The great advantage of the reported systems is the flexibility and theplace for further development and innovation. According to the presenteddata, it is evident that WPO is a compact, versatile, and reliable system withunlimited possibilities for improvement.

Different types of catalysts have been extensively used for the oxida-tion of phenol. Their complexity, stability, activity, selectivity and reusabilityhave been discussed in terms of scientific and practical application. Tailoringof the catalyst is still a challenging task due to inability of a single catalystto fulfill all the needs and requirements of a target process. Apparently, fu-ture development in manufacturing of the heterocatalysts should be directedtoward the ability of the catalysts to carry out the oxidation reaction at de-sirable conditions. It is greatly anticipated that the future of WPO lies inits ability to be applied without alteration of the initial effluent conditions.The present survey revealed the enormous opportunities for designing theprocess for any kind of wastewater treatment or chemical synthesis of DHBs,with the help of single or hybrid techniques that can be applied separatelyfor the catalyst activation or totally incorporated in the process. Moreover,the reaction engineering aspect should always be taken into considerationespecially for hybrid treatment facilities.

The use of novel theoretical approaches, such as experimental designand LCA, provides an in-depth understanding of the WPO, showing howall the major parts of the process—reaction parameters, reactants, treatedcompound, and the heterocatalyst—work and interact during the oxidationof the target compound. The resulting detailed study is comprehensive andreflects the needs and goals of clean technologies with high future potentialto become a routine procedure of every research.

Further investigation with a variety of industrial effluents is needed inorder to demonstrate the capability of WPO process to considerably improve

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the ultimate (bio)degrability, while reducing the toxicity of streams with highcontent of organic contaminants.

ACKNOWLEDGMENTS

Financial support from MVTT Foundation and Academy of Finland (No.212649) is gratefully acknowledged.

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