Photocatalytic degradation of polycarboxylic benzoic acids in UV-irradiated aqueous suspensions of...

Applied Catalysis B: Environmental 24 (2000) 71–87

Photocatalytic degradation of polycarboxylic benzoic acids inUV-irradiated aqueous suspensions of titania.

Identification of intermediates and reaction pathway of thephotomineralization of trimellitic acid (1,2,4-benzene tricarboxylic acid)

Ali Assabanea, Yahia Ait Ichoua, Halima Tahiria,b, Chantal Guillardb, Jean-Marie Herrmannb,∗a Laboratoire de Chimie-Physique, Université Ibnou Zohr, Agadir, Morocco

b Laboratoire de Photocatalyse, Catalyse et Environnement LPCE, (IFOS UMR CNRS n◦ 5621); Ecole Centrale de Lyon, B.P. 163, 69131Ecully-cedex, France

Received 20 June 1999; received in revised form 20 July 1999; accepted 20 July 1999

Abstract

Three polycarboxylic (hemimellitic (Hem), trimellitic (Tri) and pyromellitic (Pyro)) acids, representative of industrialpollutants and of compounds from biomass, were degraded by heterogeneous photocatalysis. The three molecule disappearancerates followed the order Pyro > Hem > Tri. They obeyed a Langmuir–Hinshelwood mechanism. The two competitive initialsteps of attack of the molecules corresponded (i) to a hydroxylation reaction induced by photogenerated OH• radicals and (ii)by a decarboxylation (photo-Kolbe) reaction resulting from the direct attack of one carboxylic group by a positive photo-hole.A very careful analysis of the degradation intermediates was performed using high performance liquid chromatography(HPLC) and gas chromatograph/mass spectrometer (GC/MS), especially in the case of Tri. The loss of several carboxylgroups leading to benzoic acid formation was observed before the aromatic ring opening. Several aliphatic acidic fragmentswere detected, such as malonic and succinic acids. Interestingly, a condensation product was detected, which indicated thatsome carboxylic radicals could attack a Tri molecule and form a Pyr molecule. However, all these acid intermediates couldbe photodecomposed — in agreement with previous results from the laboratory — into CO2 with a complete carbon massbalance. A detailed degradation pathway could be proposed.

Such a reaction demonstrated that complex aromatic water pollutants, originating either from industry or from biomass,could be totally mineralized and that they could produce clean water. ©2000 Elsevier Science B.V. All rights reserved.

Keywords:Titania; Photocatalysis; Photocatalytic degradation; Trimellitic; Hemimellitic; Pyromellitic; Polycarboxylic acids;Benzene-tricarboxylic acids; Biomass; Water purification; Water detoxification

∗ Corresponding author. Tel.: +33-472-1864-93;fax: +33-478-3303-37E-mail address: [email protected] (J.-M. Her-rmann)

1. Introduction

Waste waters contain numerous organic micro-pol-lutants of anthropogenic origin, whose behavior andfate are of primordial importance for the protectionof the environment. New methods of water treatment

0926-3373/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved.PII: S0926-3373(99)00094-6

72 A. Assabane et al. / Applied Catalysis B: Environmental 24 (2000) 71–87

Scheme 1. Developed formulae of the PCBA studied.

as well as improvements of the existing processes arerequired because of more severe regulations resultingfrom the increasing awareness of the urgent need todecrease or eliminate the presence of these undesirabletoxic products in natural waters. Many processes areavailable among which biodegradation is probably themost frequently used (see for instance [1,2]).

Among the new oxidation methods under develop-ment denoted ‘advanced oxidation processes’ (AOP),heterogeneous photocatalysis appears as an emergingnon-biological technology for destroying and totallymineralizing most of the organic pollutants [3–8]. Ageneral list of various families of organic pollutantswhich can be treated by photocatalysis has been givenin [9].

In our laboratory, various types of contaminantswere studied. The different isomers of mono-, di-and trichlorophenols were totally converted into CO2and C1− with illuminated suspensions of titania[10,11]. Other aromatic pollutants containing otherheteroatoms such as oxygen in dimethoxybenzene[12] or sulfur and phosphorus in pesticides [13–15]have also been successfully mineralized into inoccu-ous products (CO2, H2PO4

− and SO42−).

Among the numerous water pollutants, whichcan be treated by heterogeneous photocataly-sis, organic acids constitute an important group.The photo-assisted degradation of various organicacid molecules has been reported. They were ei-ther aliphatic, such as formic acid [16,17], aceticacid [18–21], propionic and butyric acids [20,21],poly-chloroacetic acids [21–24], oxalic acid [25],or aromatic such as benzoic acid [26–30] and itsmono- and polysubstituted derivatives [26–28] suchas salicylic [27,31–33] and chlorosalicylic acids [34].More complex chlorinated aromatic acids could be

mineralized in TiO2 aqueous suspensions. They were2,4-dichlorophenoxyacetic acid (2,4-D) [35,36] andtrichlorophenoxyacetic acid (2,4,5-T) [37], both com-pounds being known as strong herbicides.

After the previous results on the photocatalytic de-composition of the three isomers of chlorobenzoic acid(CBA) in relation with the nature and the position ofboth substituents on the aromatic ring [38,39], it wasattempted to extend this detoxification process to poly-carboxylated aromatic acids (PCBA) [40] and to iden-tify the maximum of intermediate products formedand to follow their disappearance. We chose trimelliticacid (Tri) and its homologues: Hem (1,2,3-benzene tri-carboxylic) and Pyr (1,2,4,5 benzene tetracarboxylic),all described in Scheme 1.

Additionally, the elimination of such compoundsfrom water can be of interest since (i) they are pro-duced and used in industry as intermediate reactants inthe manufacture of resins, plasticizers, dyes, printinginks and polymers, and (ii) they can constitute modelmolecules for pollutants originating from biomass(motifs of humic and fulvic acids) and from fossilfuels, mainly coal.

2. Experimental

2.1. Materials

The polycarboxylic benzoic acids (PCBA) andthe commercial intermediate products of degradationmentioned hereafter were purchased from Aldrich(purity better than 96%) and used as received. Thephotocatalyst was TiO2 Degussa P-25, mainly anatase,non-porous with a specific area of 50 m2/g, corre-sponding to a mean particle size of 30 nm.

A. Assabane et al. / Applied Catalysis B: Environmental 24 (2000) 71–87 73

Fig. 1. Disappearance of Pyro. Curve A: UV-irradiation alone (λ ≥ 290 nm), without titania; curve B: titania in the dark; curve C: UV-lightwith titania.

2.2. Photoreactor

The reaction was carried out in water at room tem-perature in a static batch photoreactor of ca. 2 l, con-sisting of a Pyrex cylindrical flask, open to air, withseveral apertures to ensure the measurement of pH, andof temperature as well as the introduction of oxygen.UV-light was delivered by a high-pressure mercurylamp (Philips HPK 125 W), which was immersed inthe central water-cooled jacket. UV-light was filteredat λ ≥ 290 nm by the inner wall made of Pyrex. Theradiant flux was measured with a powermeter (UDT,model 21 A) calibrated against a microcalorimeter.For the identification of intermediate products presentas traces, we also used a microphotoreactor, workingwith 50 ml and 70 mg TiO2, as in [38,39].

2.3. Initial conditions

The volume of the aqueous solution introduced intothe photoreactor was 1 l and the optimum concentra-tion of catalyst was 1 g/l. The mass employed corre-sponded to a total absorption of light by the suspendedcatalytic bed. The initial concentration of PCBA wasvaried between 0.2 and 1 mmol/l. Before each photo-catalytic test, the suspension was stirred magnetically

for 2 h to reach the adsorption equilibrium in the darkprior to illumination.

2.4. Analyses

Centrifugation or millipores discs (0.45mm) wereused to separate titania from the solution before analy-sis. The organics (PCBA and aromatic intermediates)were analyzed by high performance liquid chromatog-raphy (HPLC) using a Varian Polychrom 9065 pho-todiode array detector and a Varian 9010 pump. Agood separation of the products was achieved usingthe reverse-phase column ODS2 (length 25 cm, innerdiameter 4.6 mm). The mobile phase was composedof the following eluent: 20% methanol/80% water atpH = 2.8 with H3PO4 and at a flow rate of 1 ml/min.The aliphatic acids were analyzed by using a Car-Hcolumn (length: 30 cm; inner diameter: 7.8 mm). Themobile phase was composed of H2SO4, 5× 10−3 Mand the flow rate was 0.6 ml/min.

Some of the intermediates were also identified by agas chromatograph/mass spectrometer (GC/MS) afterextracting the aqueous solutions with proper solvents.The solution was first concentrated in a rotavapor andthe extraction was performed in ether at pH = 1 byadding H2SO4. GC/MS analyses were performed


Fig. 2. Influence of suspended titania concentration on the initial disappearance rate of Tri.

before and after derivatization with diazomethaneusing a gas chromatograph (HP 5890, series II)equipped with a Chrompack CP-SIL 5-CB column(length 25 m, inner diameter 0.25 mm, film thick-ness 1.2mm), connected to a mass spectrometer (HP5971A) operating in the electron impact mode. Theinjections were made in the splitless mode. The col-umn temperature program was 363–553 K (heatingrate: 10 K/min; hold time: 20 min).

The kinetics of the evolution of CO2 formed wasfollowed by using the method of Chemssedine andBoehm [23], which consists of flushing the CO2produced by oxygen into a flask containing 250 mlof baryum hydroxide (10−2 mol/l) and to follow theconductivity of the solution with a conductimeterWTWLF model 521. CO2 precipitates as BaCO3,thus decreasing the ionic conductivity in water.

3. Results and discussion

3.1. Comparison between photocatalysis and directphotolysis

To obtain relevant information about the photocat-alytic degradation, it was necessary to carry out ex-periments from which direct photolysis was excluded.Experiments were made with cut-off filters at 290 and340 nm in the absence of TiO2. In both cases, no dis-appearance of PCBA’s was observed. With titania inthe dark, only adsorption was observed (Fig. 1). Acomplete disappearance of PCBA’s was observed onlywith the simultaneous presence of titania and UV-lightas exemplified in Fig. 1 for pyromellitic acid (Pyro).This indicates that the system is working in a purephotocatalytic regime.


Fig. 3. Kinetics of Tri disappearance at room temperature for various initial concentrations.

Fig. 4. Kinetics of the photodegradation of degradation of three PCBA’s under identical conditions (initial concentration = 6× 10−4 mol/l).

3.2. Determination of the optimum concentration oftitania

In every photoreactor used, the optimal concentra-tion of titania has to be determined since it strongly

depends on the geometry of the photoreactor and ofthe incident radiant flux as well on the mean opticalpathway within the suspension.

Presently, the influence of the concentration of tita-nia on the reaction rater has been reported in Fig. 2.


Fig. 5. (A) Variations of the initial degradation rates of the three PCBA’s as a function of the initial concentrations. (B) Linear transforms1/r0 = f(1/C0) of the kinetic curves in Fig. 4.

Table 1Rate and adsorption constants of the disappearance of PCBA’s

Acid k (mmol/l/min) K (l/mol) × 10−3

Hem 12.1 4.8Tri 6.7 9.6Pyro 14.1 1.9

As expected from the literature [41], the curve exhibitsa primary linear variation of the rate versus [TiO2], in-dicating that the rate is proportional to the total surfaceexposed, i.e. to the total number of active sites present.

As [TiO2] increases, the curve levels off, indicating aprogressive saturation of the photonic absorption for agiven incident radiant flux. The plateau indicates thatsome extra titania particles remain in the shade of theothers and are quite photo-inactive. The optimum con-centration of catalyst corresponds to 1 g titania/l. Thisrepresents a 2.5 times smaller value than that used inthe small photoreactor [38,39], but a five times largervalue than that used in the recirculation solar photore-actor in Almeria (Spain) [35,36]. In what follows, thisconcentration will be used systematically.


Fig. 6. Kinetics of CO2 appearance during the photocatalytic degradation of PCBA’s (C0 = 2× 10−4 mol/l).

Fig. 7. HPLC chromatogram representing the three hydroxy-Tri isomers.

3.3. Photocatalytic degradation of Tri

The kinetics of Tri degradation performed under theconditions indicated above was followed by measuringthe quantity of Tri remaining in the solution as a func-

tion of UV-irradiation time. The results are presentedin Fig. 3 with normalized concentrations, the initialconcentration varying between 0.2 and 1 mmol/l.

The decrease in [Tri] could be accounted for by aLangmuir–Hinshelwood kinetic model (see later), the


Fig. 8. UV-spectra of Tri and of hydroxy-trimellitic isomers.

reaction rater being proportional to the coverageθ :

r = −d[Tri]

dt= k θ = k KC

1 + KC

wherek is the true rate constant of Tri disappearanceand K is the adsorption constant on TiO2 in aque-ous suspension. The linear transform 1/r = f(1/C) ofEq. (1) gives a straight line, whose slope and ordi-nate at the origin enable one to determinek and K.The two constants were found equal to, respectively,6.7× 10−6 mol/l/min and 9.6× 103 l/mol.

This kinetic model is in line with the fraction ofadsorbed molecules in the dark prior to illumination:

the nominal concentrationC0 of 6× 10−4 mol/l de-creased by 16% at pH = 3, which corresponded to acoverage of 1.15 molecules adsorbed per nm2. If theadsorption sites are considered as the amphoteric OHsurface groups of titania, whose maximum concentra-tion has been estimated to be equal to 5× 1018/m2

[42], this corresponds to a coverage of 23%. The ad-sorption can be described with an orientation of themolecules perpendicularly to the surface of titania withone carboxylic group linked to one hydroxyl surfacesite. Such a model has already been proposed for theadsorption of benzoic and terephtalic acids on the sur-face of alumina [43].


Fig. 9. GC/MS chromatogram of various acids without derivatization identified by their retention times compared to those of their injectedcommercial standards.

3.4. Comparison of the photocatalytic degradabilityof the three PCBA’s

In Fig. 4, the kinetics of the disappearance ofPCBA’s are given with a nominal initial concentra-tion C0 of 6× 10−4 mol/l. They provide a degradationreactivity pattern for the three acids in the followingorder:

Tri < Hem< Pyro

The same reactivity order was confirmed when follow-ing the variations of the initial rates as a function ofthe initial concentrations (Fig. 5A). The linear trans-forms of the curves in Fig. 5A are presented in Fig.5B and they give access to the respective values ofkand ofK for the three PCBA’s.

These constants confirm Cunningham’s paradox,which indicates that the less adsorbed molecules arethose which are more quickly photodecomposed [27].It can be observed in Table 1 that the adsorption con-


Scheme 2. MS fragmentation of malonic acid.

stants increase with the decreasing degree of sterichindrance of the molecules described in Scheme 1.The observation of Cunningham’s paradox indicatesthat a common Langmuir–Hinshelwood mechanism isperhaps not the only way to account for the observedkinetics, which can also be treated by the steady stateprinciple [49].

The same reactivity order was found for the totalmineralization followed by CO2 evolution. The kinet-ics of CO2 evolution (Fig. 6) from a solution con-taining an initial concentration of 2× 10−4 mol/l in-dicates that the final level is reached within 2 h underthe conditions chosen and corresponds quantitativelyto the total mineralization of PCBA’s according to theequation

C6H3–(COOH)3 + 152 O2 → 9CO2 + 3H2O (1)

The stoichiometry of Eq. (1) was checked by establish-ing the carbon balance for the three different PCBA’s.The period of time necessary to reach the total min-eralization is thus ca. two times longer than that nec-essary for the disappearance of each acid. Thisimplies that total degradation requires a longer time for

degrading all the possible reaction intermediate prod-ucts involved. A thorough identification of these in-termediates has been described in the next paragraphin the case of Tri. The higher rate of disappearance ofPyro can be related to the higher number of carboxylicgroups which readily decompose as CO2 accordingto the photo-Kolbe rection, initially proposed by Bard[18]:

R–COO− + p+ → R–COO• → R• + CO2 (2)

For a potential application, one has to consider thestage where the treatment has to be stopped as a func-tion (i) of the cost of the treatment, (ii) of the regula-tions and (iii) mainly of the degree of detoxificationof the water which can later be submitted to biologi-cal treatments which remain the less expensive meansof purifying used waters.

3.5. Identification of the intermediate products inTri degradation

On account of the structural similarities betweenthe three acids, the identification of the intermedi-


Scheme 3. MS fragmentation of hydroxy-trimetallic acid isomers.

ate products was performed only on Tri. The iden-tification required several analytical methods furthersequentially presented according to the increasing or-der of analytical difficulties and of the sophisticationdegree of the analytical procedures used. To better in-crease the transient concentrations of the possible in-termediates formed, the reaction was performed in astatic microphotoreactor using 50 ml of a more con-centrated (2 mmol) solution of Tri, which contained70 mg TiO2 in suspension and was irradiated for 3 h.The corresponding conversion was about 10%. Thefiltrated solution was subsequently analyzed by HPLCbefore and after concentration by a factor of 11 in a ro-tavapor. This concentrated solution was then analyzedby GC/MS before and after being derivatized withdiazomethane.

3.5.1. Analysis by HPLC without concentrationAfter 3 h of UV-irradiation, three peaks appeared

with retention times equal to 3.40, 4.79 and 5.4 min, re-spectively (Fig. 7). Tri eluted attR = 7.1 min, whereasthe peak attR = 2.63 min corresponds to the dead vol-ume, i.e. to all the compounds, which are not retainedby the column. Since these intermediates are less re-tained than Tri on the ODS2 column, this means thatthey are more polar than Tri. In addition, their identicalUV-spectra are characteristic of substituted aromaticcompounds with a red shift to longer wavelengths(Fig. 8). Since all three spectra are very similar, it canbe inferred that they correspond to the three isomersof hydroxy-tricarboxybenzene, OH–C6H2–(COOH)3,which are not commercial products. The red shift canbe accounted for by an extension of the chromophore


Fig. 10. GC/MS chromatogram of various acids after derivatization with diazomethane, identified by their retention times compared tothose of their injected commercial standards.


Scheme 4. Mechanism of the aromatic ring opening (d: detected).

from the electron donating group (–OH) to the elec-tron withdrawing group (–COOH) through the ben-zene ring, in agreement with [44].They can result fromthe attack on the molecule by a photogenerated OH•

radical originating from the neutralization of OH− sur-face groups of titania by a photohole p+:

(H2O ⇔ H+ + OH−) + p+ → H+ + OH• (3)


C6H3–(COOH)3 + OH•

→ H• + OH–C6H2–(COOH)3 (4)

3.5.2. Analysis by HPLC after concentrationThe analysis by HPLC after concentration enabled

us to detect the presence of phtalic, isophtalic, tereph-talic, hydroxy-terephtalic and hydroxy-isophtalicacids. These acids could be identified by compari-son of their retention times and of their UV-spectrawith those of the injected commercial standards. Ithas to be mentioned that they represent only 1–2%of initial Tri. The solution was also analyzed byHPLC using a CAR-H column specially devoted tothe detection of small linear carboxylic acids. Somealiphatic acids such as oxalic and malonic acids(COOH–CH2–COOH) were detected. The absenceof acetic and formic acids could be due (i) to theirsimultaneous elution with Tri, (ii) to their small con-centrations and (iii) to their very small absorptivity.The aliphatics detected represent fragments resultingfrom the aromatic ring opening.

3.5.3. Analysis by GC/MS without derivatizationThe chromatogram obtained is presented in Fig.

9. Phtalic and isophtalic acids, already detectedby HPLC, were confirmed, whereas benzoic acidwas presently put into evidence by this technique.The presence of these acids has to be ascribed thephoto-Kolbe reaction, initially proposed by Bard [18]and observed in the degradation of chlorobenzoic iso-mers [39] and of malic acid [45,46]. It consists of thedirect reaction of photoholes with carboxylate ions(Eq. (2)).

An important peak is identified as the anhydride ofPyro. Actually, the improbable presence of an anhy-dride in an aqueous medium is explained by the wellknown dehydration ofa-b dicarboxylic acids in themass detector during analysis. The detection of Pyro,which is a tetracarboxylic aromatic acid, observed asits anhydride in the mass detector, can be accounted forby a photocatalytic coupling reaction between Tri anda •COOH radical. Such a radical can originate fromthe photo-Kolbe reactions, involving mainly the initialacid or others, even carbonic acid O=C=(OH)2. Cou-pling reactions can be encountered in favorable caseswith sensitive analytical tools as already observed be-tween phenol and acetate ions [47]. Such a•COOH

radical has already been involved in radiolytic reac-tions [48].

The presence of succinic anhydride is due to the de-hydration in the mass detector of succinic acid, whichalso corresponds to an acidic fragment of the aromaticring opening.

Other compounds are also present, but the ab-sence or the small amplitude of their mass fragmentsmakes their identification more difficult. For instance,at tR = 7.4 min, the mass fragments 42/43/44/45/60and 87 are correctly detected, whereas the expectedmass of 104 is included in the base line noise. Thiscompound probably corresponds to malonic acid, inagreement with its fragmentation (Scheme 2) and toits previous detection by HPLC (see Section 3.5.2).However, the detection of short chain acids as well asthe confirmation of hydroxy-Tri (whose fragmentationpathway is proposed in Scheme 3) require more ana-lytical investigations, in particular, by derivatization.

3.5.4. Analysis by GC/MS with derivatizationAfter a 3 h photocatalytic treatment, the solution

was dried off and 200ml of propanol and 200ml of di-azomethane were added to the yellow powdered solidresidue. After several minutes of stirring, the samplewas introduced in the GC/MS. The chromatogram ob-tained is given in two parts in Fig. 10. Diazomethaneproduces a methylation of the carboxylic groupsaccording to the reaction

R–COOH+ CH2=N2 → R–COO–CH3 + N2

However, the methylation was not complete as ob-served in the products found, which are presented inFig. 10.

The derivatization of the acidic intermediates en-abled one to detect malonic acid, hydroxy-Tri’s as wellas the other benzene carboxylic acids, already detectedby the other analytical methods. The different stepsof hydroxylation, of decarboxylations and of the aro-matic ring opening were clearly put in evidence for atentative degradation pathway proposal.

3.6. Reaction mechanism

According to the various results concerning (i) thekinetics of the reaction, (ii) the influence of the var-ious parameters and (iii) the identification of various


Scheme 5. Degradation pathway of trimellitic acid (all compounds presented were actually detected by the different analytical methodsmentioned in the text.

intermediates, the following reaction mechanism canbe proposed:1. Adsorption (in the dark) of PCBA isomers with

the carboxylic group orientated to the surface [39].2. Initial photoinduced hydroxylation of the aro-

matic ring and/or decarboxylation of PCBA(photo-Kolbe reaction).

3. Opening of the aromatic ring, and subsequentdegradation into acidic intermediates leading tothe final evolution of CO2 as detailed in Scheme4.

The first point is supported by the adsorption inthe dark with a certain density of adsorbed PCBAmolecules. The carboxylic group is in ionic interactionwith the OH groups of titania, which induces a pref-erential orientation of the molecule, perpendicularlyto the surface as proposed by Ogawa [43]. The sec-ond point (initial decarboxylation) is in line with thepreferential mode of the adsorption of PCBA’s. Sincethe carboxylic group is the only part of the moleculein direct interaction with the surface, this means thatthis substituent will undergo the initial degrading


attack by positive holes and/or photoinduced OH•radicals. The subsequent hydroxylations and decar-boxylations of the intermediates resulting from thearomatic ring opening are in conformity with previousstudies devoted to the photocatalytic degradation ofaliphatic and aromatic acidic fragments [38,39,45,46].A general pathway is represented in Scheme 5.

4. Conclusions

Three PCBA acids were easily destroyed by photo-catalysis. Total mineralization could be achieved forall of them, but with slightly different reaction timesdepending on their nature. The aromatic intermediateproducts detected progressively disappeared during il-lumination, indicating that this method does not gen-erate stable by-products that could be more toxic thanthe initial pollutant. Heterogeneous photocatalysis ap-pears as a true catalytic system, where thermal acti-vation is replaced by photonic activation of the solid.From the energetic point of view, besides the heating ofthe lamp, there is no additional waste of energy spentin heating water, which is characterized by an elevatedheat capacity. This study confirms the potentialities ofheterogeneous photocatalysis to decontaminate wastewaters containing polycarboxylic aromatic pollutants,which can be found either in industrial waters or innatural waters containing biomass residues.

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