Chemical Reduction of Hexavalent Chromiumand Its Immobilisation Under Batch ConditionsUsing a Slurry Reactor

Débora V. Franco & Leonardo M. Da Silva &

Wilson F. Jardim

Received: 31 August 2008 /Accepted: 4 February 2009 /Published online: 26 February 2009# Springer Science + Business Media B.V. 2009

Abstract Chemical reduction of the hexavalent chro-mium, Cr(VI), present in contaminated soil andgroundwater was carried out in a slurry reactor underdynamic conditions (120 rpm and 25°C) usingdifferent reductants [ferrous sulphate (Fe(II))], sodiumbisulphite, sucrose, ascorbic acid and zerovalent iron(ZVI)] in order to evaluate the influence of thereductant on the redox process. Chemical analysis ofthe contaminated soil revealed a Cr(VI) concentrationof 528±31 mg kg−1. Batch studies under dynamicconditions (slurry reactor) using different [Cr(VI)]/[reductant] molar ratios revealed that only Fe(II) andZVI species can promote both reduction of Cr(VI)and immobilisation of Cr(III) (formation of aninsoluble hydroxide compound). It was verified that1.0 g of ZVI is capable of converting 104±5 mg of Cr(VI) in Cr(III). A kinetic redox study was carried outusing ZVI in different conditions. In all cases, it was

verified that Cr(VI) reduction follows a pseudo-first-order kinetic behaviour. The dependence of thepseudo-first-order kinetic rate constant, kobs, on[ZVI] indicates that the redox process taking placein the slurry reactor is rather complex. A phenome-nological kinetic equation for the redox processtaking place in the slurry reactor was presented inorder to describe the behaviour of kobs under non-ideal conditions.

Keywords Hexavalent chromium .

Reduction kinetics . Chromium Immobilisation .

Slurry reactor . Zerovalent iron

1 Introduction

As previously discussed by Franco et al. (2009),chromium can be considered a pollutant in itsdifferent oxidation states [Cr(0)–Cr(VI)]. The morestable forms of chromium in natural environments areavailable as soluble and insoluble compounds con-taining the Cr(VI) and Cr(III) species (Calder 1998;Rai et al. 1989; Palmer and Wittbrodt 1991).Hexavalent chromium [Cr(VI)] presents the higherstandard oxidation potential and exhibits toxic andcarcinogenic properties for different biological sys-tems (Calder 1998; Xu et al. 2004).

The anthropogenic occurrence of chromium indifferent polluted environments is regulated andcontrolled by the different environmental protection

Water Air Soil Pollut (2009) 203:305–315DOI 10.1007/s11270-009-0013-0

D. V. Franco (*) :W. F. JardimInstitute of Chemistry/LQA–UNICAMP,Cidade Universitária Zeferino Vaz,13083-970 Campinas, São Paulo, Brazile-mail: dvfranco@hotmail.com

W. F. Jardime-mail: wfjardim@iqm.unicamp.br

L. M. Da SilvaDepartment of Chemistry/FACESA–UFVJM,Laboratório de Eletroquímica e Química Ambiental,Rodovia MGT 367-km 583, N° 5000,39100-000 Diamantina, Minas Gerais, Brazile-mail: lsilvamorais@hotmail.com

agencies around the world. For instance, the USEnvironmental Protection Agency (USEPA) estab-lished a maximum concentration level (MCL) for totalchromium in potable water of 0.1 mg dm−3, whilst inBrazil, the National Sanitary Agency (ANVISA—Resolution MS no. 518) established a MCL for totalchromium in potable water of 0.05 mg dm−3.

According to the literature (Palmer and Wittbrodt1991; Kimbrough et al. 1999; Stanin 2005), mostcompounds containing the Cr(III) species present avery low solubility in water, and therefore, theirtransport in soil and groundwater is limited whencompared with the Cr(VI) species. Besides, the toxicityexhibited by compounds containing Cr(III) is consid-erably lower than that of compounds containing Cr(VI)(Calder 1998; Palmer and Wittbrodt 1991).

Studies revealed that chemical reduction of hex-avalent chromium, Cr(VI), accompanied by formationof insoluble chromium compounds can be carried outusing several different organic and inorganic reduc-tant agents, for instance, sodium ditionate (Na2S2O4;Su and Ludwig 2005); sodium metabisulphite(NaS2O3; Beukes et al. 1999, 2000); ferrous sulphate(FeSO4; Seaman et al. 1999); calcium polysulphide(CaS5; Jacobs et al. 2001); citric and ascorbic acids(Xu et al. 2004; Su and Ludwig 2005); zerovalentiron particles (Cao and Zhang 2006, Ponder et al2000; Schrick et al. 2004; He et al. 2007; Li et al.2006; Xu and Zhao 2007; Franco et al. 2009), etc.

In light of these considerations, one can argue thatchemical reduction is a promising alternative technol-ogy for soil remediation where the permanentimmobilisation of chromium in soil is considerablydesired; the fixation of chromium species in soilmatrix avoids the undesirable transport of these metalspecies across the aquifer zone.

Among the different reductants currently availablefor chromium reduction in soil, those environmentallyfriendly are, obviously, the preferred ones [e.g. Fe(II),sulphites and some type of organic acids (e.g.ascorbic acid)] (Seaman et al. 1999; Beukes et al.2000; Xu et al. 2004; Su and Ludwig 2005).

Cr(VI) can be reduced by sulphur compounds,such as sulphites and sulphides (Haight et al. 1965;Beukes et al. 1999; Thornton and Amonette 1999;Kim et al. 2001). Sulphite is an effective reductant forCr(VI) in acid medium (pH 1.0–5.5) under optimisedconditions (Beukes et al. 2000). Analysis of the half-life values obtained for Cr(VI) reduction from the

results reported by Beukes et al. (1999) for S(IV)indicates that S(IV) reduces the species Cr(VI) morerapidly than Fe(II) in pH range of 2–5. However,rationalisation of these findings revealed the redoxkinetic process using Fe(II) is faster for pH>5.

Other interesting reductants for Cr(VI) are basedon organic acids (e.g. sucrose and ascorbic acid). Inthis case, the redox process is accompanied byconsumption of hydrogen ions and, as a result, theprocess is favoured in acid conditions and mildalkaline conditions (Palmer and Wittbrodt 1991; Xuet al. 2004). According to the literature (Palmer andWittbrodt 1991; Xu et al. 2004; Su and Ludwig2005), the chemical reduction of Cr(VI) using organiccompounds (ascorbic acid, sucrose and citric acid) arepromising reductants since these species are ubiquitousfor humans and animals.

Interest has grown during the last years concerningthe application of the zerovalent iron nanoparticles,ZVIn, on the treatment of groundwater; the suspen-sion containing ZVIn can be applied directly in thecontaminated soil using an optimised flow rate (Elliotand Zhang 2001; Schrick et al. 2004; Li et al. 2006;He et al. 2007; Franco et al. 2009). In principle, theZVI nanoparticles are freely transported across theporous soil structure comprising the aquifer zone,whilst in the case of zerovalent iron (ZVI), thereductant can only be used as permeable reactivebarrier (Elliot and Zhang 2001; Schrick et al. 2004).According to the literature, ZVIn can also be appliedon soil treatment using slurry reactors under dynamicconditions (Xu and Zhang 2000).

According to the literature (Ponder et al. 2000; Xuand Zhang 2000; Li et al. 2006), ZVIn can indeedprovide a higher reaction rate when compared to ZVI,since the nanoparticles can present a very highspecific surface area and a very high surface energyfor the redox process.

Studies carried out using ZVIn revealed that thetypical product of this redox treatment is chromiumhydroxide, Cr(OH)3, with some substitution of Crby Fe; a mixed insoluble compound described by(Cr1−xFex)(OH)3, where “x” is typically around 0.33,is the most probable final product (Ponder et al.2000).

The objective of this work was to present acomparative study concerning reduction of the hex-avalent chromium present in soil (extracted from acontaminated landfill) using different reductants and

306 Water Air Soil Pollut (2009) 203:305–315

to provide a kinetic treatment specially proposed inorder to represent the redox process taking place inthe slurry reactor under agitation conditions. Theinfluence of the reductant on both reduction andimmobilisation processes is also evaluated.

2 Experimental

2.1 Materials and Methods

Pure grade reagents were used throughout. Potassiumdichromate (K2Cr2O7—Synth); ferrous sulphate(FeSO4×7H2O—Synth); ascorbic acid (C6H8O6—Synth); sodium bisulphite (NaHSO3—Synth); diphe-nylcarbazide (C13H14N4O—Vetec); sodium borohy-dride (NaBH4—Acros Organics); sodium hydroxide(NaOH—Synth); sodium carbonate (Na2CO3—Vetec);sulphuric acid (H2SO4—Synth), chloridric acid (HCl—Synth); acetone (CH3COCH3—Mallinkrodt); ethanol(CH3CH2OH—Mallinkrodt). All solutions were pre-pared using deionised water (18.2 MΩ cm).

Soil and groundwater used in the present work wereobtained from a contaminated site located in Brazil(industrial waste landfill from a deactivated industry).Chemical analysis revealed an average total chromi-um concentration in groundwater and soil (frank claysandy texture) of 500±23 mg dm−3 and 528±31 mgkg−1, respectively. Determination of the Cr(VI)concentration present in aqueous samples was carriedout using the 7196A colorimetric method fromUSEPA, whilst total chromium concentration wasobtained in accordance with the 3050B method fromUSEPA. Extraction of hexavalent chromium from soilsamples was carried out according to the 3060Amethod described by USEPA, whilst the chemicalanalyses were provided using the 7196A method(USEPA) mentioned above.

2.2 Synthesis and Ex Situ Characterisation of ZVINanoparticle Clusters (ZVInc)

Taking into account the literature’s procedure (Ponderet al. 2000), 10 g of FeSO4×7H2O was dissolved in200 mL of ethanol (30%, v/v). The solution pH wasadjusted to 6.2 using a 4.0 mol L−1 NaOH solution.After this procedure, 3.5 g of NaBH4 was slowlyadded into a beaker (500 mL) containing the Fe(II)solution. The solid material obtained after filtration

using an acetate membrane (∅=0.45 μm) was washedusing ethanol solutions.

ZVI nanoparticle clusters (ZVInc) were character-ised by means of the X-ray diffraction (XRD) andtransmission electron microscopy (TEM) using amodel D500 diffractometer (Siemens). TEM imageswere obtained at 80 kV using a CEM 902 processorcoupled to a model HSC2 CCD pro-camera andrecorded using the software iTEM®.

2.3 Reduction of Cr(VI) Under Batch Conditions

Figure 1 shows the scheme describing the experimen-tal setup and the major unit operations used in batchstudies. In all cases, a 3.0 g of previously dried andsieved contaminated soil sample was treated using24 cm3 of the reducing solution. All suspensionsobtained in this study were stirred (120 rpm and24 h), centrifuged (3,600 rpm and 10 min) and filteredusing a cellulose acetate membrane (∅=0.45 μm).The supernatant was characterised by: pH; EH, [Cr(VI)] and [Cr(total)]. The quantitative analysis of Cr(VI) present in the supernatant (originated from soil)was carried out just after 24 h. Since in this case theCr(VI) specie comes from the soil matrix during theextraction procedure, it was verified that [Cr(VI)]found in the supernatant indeed depends on strengthof the extractor used.

Distilled water and a 0.5 mol L−1 KH2PO4/0.5 molL−1 K2HPO4 buffer solution were used in the batchstudy to extract the labile and exchangeable fractionsof Cr(VI) present in the contaminated sample, respec-tively. The extraction carried out using distilled water(pH 5.7) permitted to obtain the so-called labile Cr(VI)fraction present in soil [the weakly attached fraction ofCr(VI) present in soil]. The phosphate buffer (pH 7.0)was used in order to furnish the amount of Cr(VI)denoted as the exchangeable form [the more stronglyattached fraction of Cr(VI) present in soil] (James et al.1995).

The comparative reduction study was carried outusing different reductants: (1) ferrous sulphate(FeSO4); (2) ZVI nanoparticle clusters (ZVInc); (3)sodium bisulphite (NaHSO3); (4) sucrose (C12H22O11)and (5) ascorbic acid (C6H8O6). Reduction of Cr(VI)with these species was conducted using different [Cr(VI)]/[reductant] molar ratios.

In order to evaluate the effectiveness of thesereductants, 13 different reducing solutions were used

Water Air Soil Pollut (2009) 203:305–315 307

in the redox study (see Table 1). All solutions werejust prepared before their use in order to avoidundesirable oxidation by air.

2.4 Kinetic Study Concerning Removal of Cr(VI)Present in Soil and Groundwater Using ZVInc

A soil mass of 10 g was added into a vesselcontaining 40 mL of contaminated groundwatercontaining Cr(VI) and the mixture was put to restduring 2 h. After this, different masses of ZVInc (0.2,0.4 and 0.8 g) were added in the mixture in order toprovide reduction of Cr(VI). The reacting mixturewas stirred (120 rpm and 25 h) using a shaker table(see Fig. 1). Analytical studies were carried outperiodically in order to determine the residual fractionof Cr(VI) present in aqueous phase. In all cases 1 mLof the sample was withdrawn using a syringe coupledto a membrane filter (∅=0.45 μm). The concentration

Fig. 1 Scheme describingthe experimental setup andthe major unit operationsused in batch studies

Table 1 Molar ratios used in the redox studies carried outusing a slurry reactor under dynamic conditions (120 rpm and25°C)

Reagent Molar ratio[Cr(VI)]/[reductant]

FeSO4 0.010 M 1:10

FeSO4 0.025 M 1:20

SBS 0.010 M 1:10

SBS 0.025 M 1:20

SCR 0.050 M 1:50

AA 0.050 M 1:50

FeSO4 0.010 M + SBS 0.010 M 1:(10:10)

FeSO4 0.010 M + SBS 0.010 M +SCR 0.050 M

1:(10:10:50)

ZVInc 0.010 M 1:10

ZVInc 0.020 M 1:15

ZVInc 0.060 M 1:40

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of Cr(VI) was determined using the 7196A colori-metric method from USEPA.

3 Results and Discussion

3.1 Ex Situ Characterisation of ZVInc: XRDand TEM Analysis

Figure 2 shows XRD diffractograms obtained forZVInc synthesised under different experimental con-ditions. This study revealed that most of the ironspecies are present as zerovalent since only anegligible amount of Fe2O3 was detected.

Figure 3 presents TEM images obtained forclusters containing the ZVI nanoparticles. Histogramanalysis obtained from TEM data permitted toevaluate that ZVI clusters are mainly constitutedparticles presenting an average size lower than 100 nm.

3.2 Reduction of Cr(VI) Using a Slurry ReactorUnder Agitation Conditions

pH and EH Table 2 shows the pH and EH valuesobtained during Cr(VI) reduction under batchconditions.

Analysis of Table 2 reveals that the supernatantpresents a pH in the 2.57 to 8.64 range. We can proposebased on these findings that these low pH values areconsequence of the oxidation and hydrolysisconcerning both Fe(III) and Cr(III) species, accompa-nied by a low buffering capacity presented by the

contaminated soil (Seaman et al. 1999). In this case,the acid conditions do not avoid the reduction ofchromium [Cr(VI)→Cr(III)]; instead, it can favoursolubilisation of Cr(III) already present in contami-nated soil. This apparent experimental drawback canbe circumvented by adjusting the soil pH usingsodium carbonate.

In the special case involving application of ZVInc,it was observed that there was no significant changein pH during the treatment. This finding can beunderstood considering liberation of OH− ions duringCr(VI) reduction and the precipitation processinvolves formation of an insoluble mixed hydroxide(see Eqs. 1 and 2; Melitas et al. 2001). As aconsequence, this redox process acts influencing thepH of the reacting medium:

ZVInc sð ÞþCrO2�4 aq:ð Þþ4H2O 1ð Þ¼ Cr OHð Þ3 sð Þ

þFe OHð Þ3 sð ÞþOH�aq:ð Þ

ð1Þ

xFe OHð Þ3 sð Þ þ 1� xð ÞCr OHð Þ3 sð Þ ¼ FexCr1�xð Þ OHð Þ3ðsÞ:ð2Þ

Table 2 also shows that EH changes from more toless positive values when the extraction procedureusing distilled water and phosphate buffer are used,Fig. 2 XRD spectrum obtained for ZVInc nanoparticle cluster

Fig. 3 TEM images obtained for ZVInc nanoparticle cluster

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respectively. This behaviour indicates that the oxidativeproperties of the medium are lowered during thetreatment and, therefore, the presence of an excess ofthe reductant in the slurry after the redox treatment.

Cr(VI) and Cr(total) Table 2 also shows the behav-iour of Cr(VI) and Cr(total) during the redoxtreatment carried out in the slurry reactor underdynamic conditions. It is worth emphasising thatinitial concentration values indeed depend on strengthof the extractor used. Analysis of the experimentalfindings revealed that the extraction procedure usingdistilled water furnishes a liquid phase containing51.5 mg dm−3 of Cr(VI). In the case of the phosphatebuffer, which furnished a Cr(VI) exchangeable frac-tion of 58.1 mg dm−3, the formation of a moreconcentrated Cr(VI) solution was verified (after 24 hof extraction). These findings indicate that availabilityof Cr(VI) for the environment will depend on thenature of the matrix where the contaminant isconfined. From these considerations, it can be arguedthat only certain fractions of the contaminant presentin soil will interact with the environment in a greatextension (natural availability of the contaminant).

Figure 4 shows findings concerning the determi-nation of the Cr(VI) and Cr(total) chromium species.

Analysis of Fig. 4 reveals that reduction of Cr(VI)considerably depends on the reductant used. It wasfound that reduction of Cr(VI) using sucrose (SCR)does not constitute an efficient process for thetreatment time of 24 h established in the presentwork. However, one can verify promising findings forthe other redox treatments involving application of Fe(II), ZVInc and ascorbic acid (AA). A moderate redoxbehaviour concerning reduction of Cr(VI) was pre-sented by the reductant BSS.

It was also verified that removal of total chromium(Cr(total) = Cr(VI) + Cr(III)) is led to a considerableextent when the iron species were used (mainly in thecase where the pH adjustment was carried out).Studies concerning application of the other reductantsrevealed that these species are not efficient in order topromote immobilisation of the reduced chromium.

Rationalisation of findings gathered in Fig. 4reveals that both Fe(II) and ZVInc species areefficient in order to promote reduction of Cr(VI).However, the major difference involving applicationof these species resides in the pHfinal (see Table 2),

Table 2 Experimental findings obtained under batch conditions using different extractors and reductant agents

Exp. no. Reagent pH finala EH final (mV) [Cr(VI)] (mg L-1) [Cr(total)] (mg L-1)

1 Distilled water 8.64±0.06 170±12 51.5±6.5 70.0

2 KH2PO4 0.5 M/KHPO4 0.5 M 6.68±0.00 331±10 58.1±3.5 83.0

3 FeSO4 0.010 M 4.58±0.12 134±15 <0.01 0.10

4 FeSO4 0.025 M 4.01±0.10 123±21 <0.01 0.24

5 FeSO4 0.010 M (pHfinal 7)b 6.50±0.02 129±18 <0.01 <0.02

6 FeSO4 0.025 M (pHfinal 7)b 5.00±0.02 118±28 <0.01 <0.02

7 SBS 0.010 M 6.41±0.06 115±17 14.3±1.3 15.2

8 SBS 0.025 M 3.51±0.06 104±31 0.46±0.10 28.0

9 SCR 0.050 M 8.15±0.03 329±35 50.50±0.42 65.0

10 AA 0.050 M 5.52±1.78 116±16 <0.01 48.0

11 FeSO4 0.010 M + SBS 0.010 M 2.57±0.45 26±5 <0.01 38.0

12 FeSO4 0.010 M + SBS 0.010 M + SCR 0.050 M 2.95±0.15 34±8 <0.01 17.0

13 0.015 g ZVInc 8.64±0.03 112±14 5.2±1.7 6.8c

14 0.030 g ZVInc 8.22±0.21 72±21 0.10±0.01 0.12c

15 0.080 g ZVInc 7.87±0.02 24±13 <0.01 <0.02

Treatment time, 24 h (120 rpm and 25°C)a Supernatant pH measured after centrifugationb pH adjusted to 7 using 1.0 mol L−1 Na2CO3 after 1 h of reaction (before centrifugation)c Chromium present as Cr(VI)

310 Water Air Soil Pollut (2009) 203:305–315

which in turn affects the [Cr(total)] in the slurry.Hydrolyses process during the redox reaction involv-ing the Cr(III) and Fe(III) species leads to a pHfinal

of ~4. Thus, one can argue that the redox treatmentusing ZVInc presents interesting findings, since inthis case, the pH of the reacting medium (slurry) wasfound to be more stable in more amenable conditions(pH range of 7 to 8). Therefore, application of ZVIncprovides an experimental scenario where precipitationof both Cr(III) and Fe(III) species can be carried outin the slurry without a further pH control.

It was also found that the reduction process usingAA is more effective for the reduction of Cr(VI) whencompared to the redox treatment using sucrose.However, it was observed in this case that the majorfraction of Cr(total) present in water was available aslabile chromium (t=24 h). Therefore, it can beconcluded that application of AA does not lead tochromium immobilisation in the slurry.

3.3 Reduction of Cr(VI) Via Redox ProcessUsing ZVInc

Due to the promising results presented by ZVInc inthe present work, more detailed studies regardingchemical reduction of Cr(VI) were carried out in orderto investigate the influence of ZVInc on the extent ofthe reduction process taking place in the slurryreactor. Figure 5 shows the dependence of [Cr(VI)]

as functions of the treatment time when 10.0 g of soiland 40 mL of groundwater and different amounts ofZVInc are used.

Analysis of Fig. 5 reveals that application of 0.2 gof ZVInc leads to a Cr(VI) reduction of 95% after atreatment time of ~25 h. It was also verified that anincrease in the ZVInc amount from 0.4 to 0.8 g leadsto a Cr(VI) reduction of 99% for a treatment time of4.0 and 1.3 h, respectively. It was found thatapplication of ~1.0 g of ZVInc is necessary in order

Fig. 5 Cr(VI) reduction and Cr(III) immobilisation as functionof the mass of ZVInc. Sample conditions 10.0 g of contami-nated soil ([Cr(VI)]soil=528±31 mg kg−1) + 40 mL ofgroundwater ([Cr(VI)]groundwater=500±23 mg dm−3) and differentamount of ZVInc

Fig. 4 Experimental find-ings obtained for the redoxstudies carried out using aslurry reactor under constantagitation using different ex-per imenta l condi t ions .Treatment t ime, 24 h(120 rpm and 25°C). Thevertical bars are presentedusing the logarithmic scale(mg L−1). TPH phosphatebuffer, SBS sodium bisul-phite, AA ascorbic acid

Water Air Soil Pollut (2009) 203:305–315 311

to provide the reduction of 104±5 mg of Cr(VI)present in the slurry. The conversion efficiencyobserved for the Cr(VI)→Cr(III) redox process washigher when compared to the literature (Cao andZhang 2006; Ponder et al. 2000).

It is important to emphasise that the presence of anelectron donor specie(s) in the reacting mediumcontaining ZVInc can lead to occurrence of undesir-able side reaction(s), thus decreasing the amount ofZVInc available for the redox process concerning Cr(VI). In light of these considerations, one can proposethat ZVInc must be applied using an excess of thisreagent in order to compensate these side reactions.The major undesirable redox processes leading toconsumption of ZVInc are presented in Eqs. 3 and 4(Xu and Zhao 2007; Ponder et al. 2000):

ZVInc sð Þ þ 2H2O 1ð Þ ! Fe2þaq:ð Þ þ H2 gð Þ þ 2OH�aq:ð Þ

ð3Þ

ZVInc sð Þ þ O2 gð Þ þ 2H2O 1ð Þ ! 2Fe2þaq:ð Þ þ 4OH�aq:ð Þ:

ð4Þ

3.4 Kinetic Study of the Redox Processes TakingPlace in the Slurry Reactor

Figure 6 shows representative kinetic profiles ac-counting for Cr(VI) reduction using ZVInc in theslurry reactor under agitation conditions (120 rpm and25°C). Good correlation coefficients (r≥0.965) wereobtained from the linear regression analysis, thusrevealing that Cr(VI) reduction using ZVInc obeys apseudo-first-order kinetic model. Data obtained in thisstudy are gathered in Table 3.

Analysis of the experimental findings revealed thatfurther addition of 0.8 g of the ZVInc in the slurrycontaining Cr(VI) leads to an increase of the pseudo-first-order kinetic rate constant, kobs, from 0.10 up to7.2 h−1. This behaviour revealed that kobs is consid-erably affected by the amount of ZVInc presentalways in excess in the reacting medium (pseudo-first-order conditions), thus showing that Cr(VI)reduction in the slurry constitutes a rather complexprocess.

From a theoretical point of view, the redox processtaking place in a non-isotropic (suspended soilmicroparticles) and heterogeneous (Fe0/liquid inter-

face) medium can indeed result in a rather complexbehaviour (Levenspiel 1999, 2002). Considering thatthe redox process obeys a pseudo-first-order kineticmodel, the reduction of Cr(VI) taking place in theslurry reactor can be described by the followingrelation (Ponder et al. 2000; Cao and Zhang 2006; Xuand Zhao 2007).

ln½CrðVIÞ�½CrðVIÞ�0

� �¼ �k*obst; ð5Þ

where [Cr(VI)] is the instantaneous concentration ofCr(VI) (mg L−1), t is the treatment time (h) and k*obs isthe overall kinetic rate constant for the heterogeneousredox process (h−1).

Taking into account the heterogeneous nature ofthe redox process carried out under constant andintense agitation and the collisions comprising thesuspended soil microparticles (SSM), as well thecollisions of these particles with the ZVInc, one can

Fig. 6 Pseudo-first-order kinetic profiles obtained for Cr(VI)reduction using ZVInc in the slurry reactor (120 rpm and25°C). Sample conditions: 10.0 g of contaminated soil ([Cr(VI)]soil=528±31 mg kg−1) + 40 mL of groundwater ([Cr(VI)]groundwater=500±23 mg dm−3) and different amount ofZVInc

Table 3 Influence of the ZVInc mass and the molar ratio onkinetic for reduction of Cr(VI) present in soil and groundwater

Mass of ZVInc (g) k*obs (h−1) t1/2 (h) Molar ratio

[Cr(VI)]/[ZVInc]

0.20 0.10 69 1:15

0.40 2.5 0.28 1:30

0.80 7.8 0.090 1:60

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propose that kobs* cannot be represented by a simpleconstant.

Taking into account the specific characteristicspresented by the redox process taking place in theslurry, it is proposed in the present work that the nextphenomenological relation in order to represent kobs*for reduction of Cr(VI) using ZVInc:

k*obs ¼ kh Cr VIð Þ½ �0 ZVInc½ �0� �Y

; ð6Þwhere [Cr(VI)]0 and [ZVInc]0 are the initial concen-trations of Cr(VI) and ZVInc, respectively, and kh and Yare empirical constants for the particular redox process.

The parameter kh(=ksaasZhθA(SSM)) is the heteroge-neous kinetic rate constant representing the redoxprocess taking place at the surface of the ZVInc underperfect mixing conditions, in contact with SSM, where:(1) Zh is the heterogeneous collision frequency factor;(2) ksa is the specific kinetic rate constant related to theactive surface area of the ZVInc (dm3 h−1 m−2; He etal. 2007); (3) as is the average specific surface areapresented by ZVInc (m2 g−1; He et al. 2007) andθA(SSM), with 0≤θ≤1, is the average active surface areaof the SSM where θ is the fraction of the SSM surfacecontaining Cr(VI).

Obviously, in the absence of SSM containing Cr(VI), the heterogeneous kinetic rate constant isdescribed by k*h ¼ ksaasZh.

From the above considerations, the kinetic equa-tion describing the overall redox process can bedenoted by Eq. 7:

lnCr VIð Þ½ �Cr VIð Þ½ �0

� �¼ �ksa asZhqA SSMð Þ

� �

Cr VIð Þ½ �0 ZVInc½ �0� �Y � t:

ð7Þ

Analysis of Eq. 7 clearly reveals that experimentalchanges in k*obs can be promptly correlated with thenext factors: (1) influence of the intrinsic propertiespresented by the suspended ZVInc; (2) heterogeneouscollision frequency between SSM and ZVInc and (3)the average specific surface area for the redox processpresented by both ZVInc and SSM.

As predicted by Eq. 7, the increase in the activesurface areas (ZVInc and SSM) and the collisionfrequency, which is a function of the agitationconditions, as well the intrinsic electronic propertiesof the ZVInc incorporated in ksa, can lead to aconcomitant increase in the rate for the redox reactionin the slurry reactor. Besides, Eq. 7 also shows that k*obs

increases on increasing the fraction of the SSM surfacecontaining Cr(VI).

Although Eq. 7 was indeed proposed in order toextract qualitative information concerning the redoxsystem under investigation, it is apparent that a set ofexperiments carried out changing the different variableswill permit to evaluate the influence of the individualcontributions of the different parameters on the kineticbehaviour.

Considering that Cr(VI) can be found both in wateras Cr(VI)(aq.) or in SSM (adsorbed or absorbed), onehas the major processes leading to reduction of the Cr(VI) species free in solution and adsorbed on SSM inthe slurry reactor can be described by two redoxprocesses running in parallel. Thus, the overall redoxprocess can be lumped in the next reaction scheme:

ZVInc sð Þ þ Cr VIð Þ aq:ð Þ þ 4H2O 1ð Þ ���!k*obs�1 FexCr1�xð Þ OHð Þ3ð8aÞ

ZVInc sð Þ þ Cr VIð Þ � SSMð Þ sð Þ

þ 4H2O 1ð Þ���!k*obs�2 FexCr1�xð Þ OHð Þ3

ð8bÞ

where Cr(VI)–(SSM)S is an active site at the surfaceof the SSM containing Cr(VI).

In principle, one can suppose as a first approxima-tion that the collision frequency between Cr(VI)(aq.)and ZVInc(s) overcomes the collision frequencybetween SSM and ZVInc(s) k*obs�1 >> k*obs�2

� �for the

case where water is heavily contaminated (initially orafter a considerable fragmentation of the SSM in theslurry reactor leading to chromium release). It isworth mentioning that constant fragmentation of SSMcan lead to an increase in [Cr(VI)](aq.), thus increasingthe effective collision between the ZVInc(s) and Cr(VI)(aq.).

Therefore, in the case where [Cr(VI)](aq.) >> [Cr(VI)](adsorbed), the kinetic equation for the heterogeneousredox process can be simplified considering that kh mustbe replaced by k*h , thus resulting in the next equation:

lnCr VIð Þ½ �Cr VIð Þ½ �0

� �¼ �ksa asZhf g Cr VIð Þ½ �0 ZVI½ �0

� �Y � t:

ð9Þ

For a given set of conditions concerning the slurryreactor (e.g. agitation), the behaviour of the kinetic

Water Air Soil Pollut (2009) 203:305–315 313

process described by the limit case denoted by Eq. 9will depend solely on the intrinsic characteristicsexhibited by the ZVInc (specific surface area andredox potential).

Also, one can argue that collisions provided byconstant mechanical agitation can lead to a concom-itant increase in the effective surface area of the SSMdue to fragmentation. Therefore, depending on [Cr(VI)] present in soil, a dependence of k�obson treatmenttime can be found. Considering that this behaviourwas not evidenced in the present work, one can arguethat this case does not take place in the slurry reactorunder the prevailing experimental conditions.

4 Conclusions

Studies concerning the reduction of hexavalentchromium present in contaminated soil were carriedout in a slurry reactor under constant agitation usingdifferent reductants. The parameters pH, EH, [Cr(VI)]and [Cr(total)] were monitored during the redoxtreatment of soil. The redox treatment study revealedthat application of both inorganic and organicreductant agents on reduction of Cr(VI) affects thepH of the slurry.

Analysis of the experimental findings revealed thatamong the five reductants used in this study, only theiron species (Fe(II) and ZVInc) and ascorbic acid areable to reduce Cr(VI). ZVInc presented the best results,since both chemical reduction of Cr(VI) and immobili-sation of Cr(III) were achieved in the present case[formation of an insoluble compound containing Cr(III)].

The kinetic study concerning the redox process inthe slurry reactor using ZVInc revealed that theoverall pseudo-first-order kinetic rate constant, k�obs,depends on the oxidant/reductant molar ratio. Maxi-mum k�obsvalue was found for the 1(oxidant):15(reductant) molar ratio. A phenomenological ratelaw for the redox process taking place in the slurrywas proposed in order to represent the behaviourpresented by k�obsunder different experimental con-ditions. These studies revealed that treatment ofcontaminated soil containing Cr(VI) via applicationof ZVInc can present a promising technological base.

Acknowledgements The authors wish to thank the NationalScientific Council for Research and Development—CNPq(process no. 141024/2005-4).

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