Journal of Photochemistry and Photobiology A: Chemistry 170 (2005) 189194

Photocatalytic reduction of hexavalentsolution over sulphate modi

antant, Bhub

ly 2004

Abstract

Samples o d TiO2of sulphate i ower ppercentage o t low pof sacrificia ction wreduction of particand rutile ph than th 2004 Else

Keywords: H 2 ption

1. Introduction

The preknown toheavy mettrial wastewchromiummaterials, s

Chromihexavalentdepends strhighly toxironment friof 0.10.3the amountform of chrmetabolism

Corresponfax: +91 674

E-mail ad(K. Parida).

man, animals and plants [4]. In the environment Cr(VI) salts

1010-6030/$doi:10.1016/jsence of heavy metals in aquatic bodies has beencause pollution problems. The major source ofals is the improper discharge of various indus-

ater [1]. Among them the potential sources ofinclude leather tanning, paints, dyes, photographicteel alloy cement industries, mining, etc. [2].

um exhibits variable valencies of which tri andforms are common. The behaviour of Cr-speciesongly on its oxidation state. Cr(VI) is mobile andc where as Cr(III) is mostly immobile and envi-endly [3]. Chromium is necessary for life, a dosemg/day is required for normal development andcomes from various foods and drinks. Trivalent

omium plays an essential role in plant and animal, while hexavalent chromium is directly toxic to

ding author. Tel.: +91 674 2581 636x305/425;2581 637.dresses: kmparida@yahoo.com, kmparida@rrlbhu.res.in

do not readily precipitate or become bound to components ofsoil, therefore Cr(VI) can move throughout aquifers to con-taminate ground water and other sources of drinking wateras well as present a toxic hazard to livestock and wildlife.Also as the hexavalent form is more stable than trivalent, itcan easily penetrate through the cell membrane in case of an-imals and subsequently retains inside the cell forming stablecomplexes. Hexavalent form of chromium is 100 times toxicthan trivalent [4]. Therefore, increasing in the concentrationof chromium and consequent increase in intake manifest in avariety of metabolic diseases such as dermatitis, perforationof nasal septum and abortion, DNA lesim and finally leads tocancer.

The methods employed for the removal of hexavalentchromium are chemical precipitation, reverse osmosis, ionexchange, foam flotation, electrolysis, photocatalytic reduc-tion, adsorption, etc. However, most of these methods requireeither high energy or large quantities of chemicals whereasthe photocatalytic process is found to be superior to all.

Photocatalytic process in aqueous suspension of semi-conductor has received considerable attention in view of

see front matter 2004 Elsevier B.V. All rights reserved..jphotochem.2004.08.012Priyabrat Mohapatra, Santosh Kumar SamRegional Research Laboratory (CSIR), EM & IC Departme

Received 21 April 2004; received in revised form 23 Ju

f different crystal forms and sizes of unmodified and sulphate modifieon and by activating at different temperatures. Samples prepared at lf photocatalytic reduction of Cr(VI) under solar radiation is higher al electron donor such as EDTA enhances the photocatalytic reduhexavalent chromium depends on surface area, the crystal form andases exhibit highest activity for reduction of hexavalent chromiumvier B.V. All rights reserved.

exavalent chromium; Sulphated TiO ; pH; Photocatalytic reduction; Adsorchromium in aqueousfied titaniaray, Kulamani Parida

aneswar 751 013, Orissa, India

; accepted 13 August 2004

were prepared by varying pH, source and concentrationH exhibit more surface area than that of higher pH. TheH and decreases with rise in pH of suspension. Presencehereas presence of oxygen reduces it. Photocatalytic

le size of TiO2. Samples containing mixtures of anataseat of single phases.

190 P. Mohapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 170 (2005) 189194

solar energy conversion [5,6]. This photocatalytic processwas achieved for rapid efficient destruction of environmentalpollutants. Upon illumination of semiconductorelectrolyteinterface wband gap,conductionrespectivelsemiconduspecies in sof Cr(VI) toand toxicitCr(VI) with[9] and TiOAmong thowing to it

So in themodified anreduction oalso madetion proces

2. Experim

2.1. Sampl

Hydratedilute ammtetrachloridtered and wtest), driedsize and ke

One serweight percas the sourcand the otwetness im= 1.0 M). Ta hot plateair oven atat 573, 673283 K/min

2.2. Chara

The XRwere recor

IR spectraElmer (moof 400040morphologH-600 tranarea (BET)method at(Quantachr

2.3. Photocatalytic reduction and adsorption

The photocatalytic reduction of hexavalent chromium wasrmed) in 1f thecid (Bom teetic srema

xperimhalf o

p.m.n duexperimg dichxtentue to

the sused qm usi

esults

Sampl

om that pHf sizely dec

amplephase

es. It ise of Tiarea m

nt ofus netts are

Photo

om thyst unxavalFig. 1ntagehen ittion tion takncludinatiog. 2 shge ofs we iith light energy greater than the semiconductorelectronhole pairs (eh+) are formed in theand the valence band of the semiconductor,

y [7]. These charge carriers, which migrate to thector surface, are capable of reducing or oxidizingolution having suitable redox potential. Reduction

Cr(III) leads to drastic decrease in bioavailabiltyy of this element. The photocatalytic reduction of

semiconductor such as CdS [8], ZnS, WO3, ZnO2 [10] in UV light has been reported in literature.

em TiO2 is found to be suitable photocatalysts low cost of production and high activity.

present work, we have studied the activity of un-d sulphate modified TiO2 towards photocatalyticf hexavalent chromium under solar radiation and

a comparison between photocatalytic and adsorp-s.

ental

e preparation

d titania was prepared at pH 3, 5, 7 and 9 by addingonia to the stirred aqueous solution of titaniume (Spectrochem, Bombay). Obtained gel was fil-ashed repeatedly to remove Cl (negative AgNO3at 383 K for 10 h, powdered to 4575m meshpt for anion impregnation.ies of sulphated titania samples with varying theentage of SO42 was prepared using (NH4)2SO4e of sulphate ions by solid-solid kneading method

her series of samples was prepared by aqueouspregnation method using dilute H2SO4 (strengthhe suspended mass was evaporated to dryness onwhile stirring. The samples were then dried in an383 K and subsequent activation (by calcination), 773, 873, 973 and 1073 K at the heating rate ofin a muffle furnace for 3 h.

cterisation

D patterns of pure and modified TiO2 samplesded on a Philips X-ray diffractometer. The FT-of the samples were recorded with a Perkin-

del: Paragon 500) FT-IR spectrometer in the range0 cm1 on KBr (spectrophotometric grade). Theies of the samples were examined using a Hitachismission electron microscope (TEM). Surfacewas determined by the N2 adsorptiondesorptionliquid nitrogen temperature using Quantasorbome, USA).

perfoBDHpH oric aat romagnalystthe eond14.00tratioilar etakinthe etion dtionanaly348 n[11].

3. R

3.1.

Frparedcles osharpthe srutilephasphasfaceamou

pororesul

3.2.

Frcatalof heseen

perceand treducsorptbe coillum

Ficentathat ataking 25 ml of 20 ppm Cr(VI) solution (K2Cr2O7,00 ml pyrex flask and 0.6 g/L of catalyst. Thedispersion was adjusted by addition of sulfu-DH). The solutions were exposed to sunlight

mperature (no extra cooling) and agitated withtirrer so that no appreciable amount of the cat-ined on the bottom of the reaction vessel. Allents were performed in triplicate during the sec-

f March 2003 (sunny days), from 10.00 a.m. toThe reduction in hexavalent chromium concen-to adsorption is measured by carrying out sim-ent in dark. Blank experiment was carried out

romate solution without photocatalyst to knowof reduction of hexavalent chromium concentra-solar radiation. After irradiation and also adsorp-

spension was filtered and the Cr(VI) content wasuantitatively by measuring the absorption band atng Cary-1E (Varian, Australia) spectrophotometer

and discussion

e characterisation

e TEM images it is found that TiO2 sample pre-3 possesses uniform, finely distributed TiO2 parti-=12 nm and with sulphate loading the particle sizereases to 23 nm. The XRD patterns revealed thats prepared at pH 3 possess mixture of anatase ands whereas at pH 5, 7 and 9 possess only anatasealso found that sulphate ion stabilises the anatase

O2 up to 973 K activation temperature. From sur-easurement, it is found that the presence of lowsulphate ion is responsible for the formation of

work. The details of the sample characterisationpublished elsewhere [12].

catalytic reduction and adsorption

e experiments carried out without the presence ofder solar radiation we found that the percentageent chromium reduction is very negligible. It isthat with increase in reaction time up to 3 h, theof reduction and as well as adsorption increasesremains almost constant. While 80% of Cr(VI)

ake place during photocatalysis only 23% of ad-e place under similar conditions. From this it can

ed that equilibrium is achieved only after 3 h ofn.ows the effect of catalyst concentration on the per-reduction of hexavalent chromium. It is observedncrease the catalyst concentration up to 0.8 g/L the

P. Mohapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 170 (2005) 189194 191

Fig. 1. Effect of time on photocatalytic reduction and adsorption of Cr(VI).[Cr(VI)] = 20 ppm, catalyst: 2.5 wt.% SO42/TiO2 (0.6 g/L).

percentage of reduction increases and there after it remainsconstant. This may be due to that with increase in catalystconcentration the number of photons absorbed by TiO2 par-ticles and nsurface are

Fig. 2. Effectadsorption oftime = 3 h.

Fig. 3. Effectadsorption of(0.6 g/L), time

is no furthehence additivity. Fromincrease in

of theuctionases thon ofductioumbers of reacting molecules adsorbed on TiO2increased. But after certain concentration there

area

toredincreetratiof reof catalyst concentration on the photocatalytic reduction andCr(VI). [Cr(VI)] = 20 ppm, catalyst: 2.5 wt.% SO42/TiO2,

pH is onthe adsorpting place oresents theon the adsochromium.centage of rdecreases gavalent chrobtained atnor reductiported by obe due to tincreasingform CrO4surface becThe pH affalso the stasorption. Staking diffevated red mof H+ ionsconductor sof pH of the solution on the photocatalytic reduction andCr(VI). [Cr(VI)] = 20 ppm, catalyst: 2.5 wt.% SO42/TiO2= 3 h.

r reacting molecules are available for adsorptiontional catalyst are not involved in the catalytic ac-

this observation it can also be presumed that withcatalyst loading there is an increase in the surfacecatalyst available for adsorption and hence pho-, however further increase in the catalyst loadinge solution opacity leading to decrease in the pen-the photon flux thereby affecting the percentagen.e of the most important controlling parameter inion and photocatalytic reduction of metal ion tak-

n semiconductor metal oxides [15,13]. Fig. 3 rep-effect of pH (adjusted by H2SO4) of the solutionrption and photocatalytic reduction of hexavalentWith increasing pH of the solution up to 2, the per-eduction remains almost constant and thereafter itradually. The same trend is also observed for hex-omium adsorption. The highest reduction rate wasthe lower pH. At pH above 7 neither adsorption

on takes place; this type of observation is also re-ther researchers [16,14]. The reason probably mayhe dominant form of Cr(VI) at pH 2 is HCrO4,the pH will shift the concentration of HCrO4 to and Cr2O72. At pH above 7 the photocatalystomes negative and repels dichromate ion [14,15].ects the surface charge on the photocatalyst andte of ionization of the substrate and hence its ad-imilar observations were also made by others byrent materials such as rice husk carbon [16], acti-ode [2], hydrotalcite [17]. The specific adsorptionfavours the approach of Cr(VI) species to the semi-urface, whereas for higher pH excess of absorbed

192 P. Mohapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 170 (2005) 189194

Fig. 4. Effect= 20 ppm, cat

OH ionsCr(VI) ads

The prespecies maions [13]. Inhave carrieethanol andthe significof others. TEDTA thathance the areduction o

The rolhexavalentthe reductiand nitrogefor suspenwith photowhich resuThere aresome authoof oxygenresults obtamay be duwere perfo

Fig. 6 rdifferent pchromium.increase insurface areof hydroxy

. Effect of nitrogen, oxygen and air on the photocatalytic reduction of). [Cr(VI)] = 20 ppm, catalyst: different wt.% of sulphate impregnatedrepared at pH 3 (0.6 g/L), time = 3 h.of EDTA on the photocatalytic reduction of Cr(VI). [Cr(VI)]alyst: 2.5 wt.% SO42/TiO2 (0.6 g/L).

has the opposite effect, leading to low levels oforption.sence of sacrificial electron donor such as organicy accelerate the photocatalytic reduction of metal

order to investigate the role of organic species wed out the experiments in the presence of methanol,

EDTA (Fig. 4). It was observed that EDTA hasant effect on the photocatalytic reduction than thathis may be due to the strong chelating action of

Fig. 5Cr(VITiO2 pforms stable complex with Cr(VI). This can en-dsorption of Cr(VI) and hence the photocatalyticf hexavalent chromium.e of oxygen in the photocatalytic reduction ofchromium is controversial. We have carried outon by bubbling the suspension with oxygen, airn. Fig. 5 shows that the rate of reduction is highersion bubbled with nitrogen. Oxygen competesgenerated electron on the surface of photocatalyst,lts in decrease in the percentage of reduction.controversial results found by different authors;rs found the negative effect [14] and no effect

is also reported [18]. Our results agreed with theined earlier [19]. The discrepancies in the result

e to varying conditions under which experimentsrmed.epresents the effect of TiO2 samples prepared atH on the photocatalytic reduction of hexavalentThe percentage of reduction decreases with thepH of precipitation. This may be due to the high

a, lower porosity, crystallite size, and more numberl groups present on the TiO2 sample prepared at

Fig. 6. Effect of pH of precipitation on the photocatalytic reduction ofCr(VI). [Cr(VI)] = 20 ppm, catalyst: 2.5 wt.% SO42/TiO2 prepared at dif-ferent pH (0.6 g/L), time = 3 h.

P. Mohapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 170 (2005) 189194 193

Fig. 7. Effect[Cr(VI)] = 20prepared at pH

low pH [12groups boufor adsorptsurface of tand enhancprepared aphase and sfor highersubstrate.

Fig. 7 shon the phoSample loacentage ofsulphate coreduction. Tof sulphatewhich facilmay be dueA blue shifthe size of sever, furthenot affect tage of redureduction o

Fig. 8 remodified areduction oduction gra

Effecttion of

prepare

ure up. But tivatioulphatre of

ductior thanof wt.% of sulphate on the photocatalytic reduction of Cr(VI).ppm, catalyst: different wt.% of sulphate impregnated TiO23 (0.6 g/L), time = 3 h.

]. As reported earlier [20] the surface hydroxylnd to titanium atoms constitute the active sitesion. The large amount of hydroxyl groups on the

Fig. 8.adsorpTiO23 h.

peratTiO2in actfor smixtuof rehigheitania might have trap the holes in the valence banded the reduction in the conduction band. Samplet pH 3 contains a mixture of anatase and rutilehows higher surface area. This may be responsiblerate of reduction by effective adsorption of the

ows the effect of wt.% of sulphate loading in TiO2tocatalytic reduction of hexavalent chromium.ded with 2.5 wt.% of sulphate shows higher per-reduction. However, further addition of higherntent does not affect more to the percentage ofhis may be due to the addition of a small amountthat decreases the crystallite size of titania [12],

itates higher percentage of reduction. The reasonto the so-called particle size quantization effect.

t in absorption spectra is usually observed whenemiconductor particles becomes small [21]. How-r increasing the wt.% of sulphate on TiO2 doeshe crystallite size resulting almost same percent-ction. So, crystallite size is an important factor forf hexavalent chromium.presents the effect of activation temperature of un-nd sulphate modified TiO2 on the photocatalyticf Cr(VI). It is observed that the percentage of re-dually increases with increase in activation tem-

ature. Simiof sample ftivity at higporous strucles due toanatase pha

From threduction olution contenergy greaDuring phowas dispers[13]. Thissence of ligoccurs beccreated insof these sperated elecwater. Thetion of hexTiO2 TiCr2O72 +H2O+ h+Which is aof calcination temperature on the photocatalytic reduction andCr(VI). [Cr(VI)] = 20 ppm, catalyst: 2.5 wt.% SO42/TiO2 andd at pH 3 calcined at different temperatures (0.6 g/L), time =

to 773 K and thereafter decreases for unmodifiedhe percentage of reduction increases with increasen temperature up to 973 K and thereafter decreasese modified TiO2. This is due to the presence ofanatase and rutile phase of TiO2. The percentagen of Cr(VI) in case of sulphate modified TiO2 isthat of unmodified TiO2 at all activation temper-

lar trend is also observed in case of both the seriesor adsorption. The decrease in photocatalytic ac-her activation temperature is perhaps due to loss ofcture of nano-particles, formation of denser parti-sintering as well as transformation into rutile from

se.e above studies it is observed that photocatalyticf Cr(VI) to Cr(III) takes place when Cr(VI) so-

aining TiO2 is illuminated in light having photonter than the band gap energy of the semiconductor.tocatalysis, adsorption occurred first when TiO2ed in the aqueous solution containing metal ions

process is reversible and even takes place in ab-ht illumination. The reduction of Cr(VI) to Cr(III)ause under illumination electronhole pairs areide the semiconductor particles. After migrationecies to the surface of the particle, the photogen-trons reduce Cr(VI) to Cr(III) and holes oxidizepossible mechanism of the photocatalytic reduc-avalent chromium is as follows:O2(h+ + e)H+ + e Cr3+ + H2O O2 + H+lso supported by Munoz and Domenech [15].

194 P. Mohapatra et al. / Journal of Photochemistry and Photobiology A: Chemistry 170 (2005) 189194

4. Conclusion

1. Complete reduction of hexavalent chromium (20 ppm) inaqueous solution takes place using 0.8 g/L (2.5 wt.%) sul-phated TiO2 prepared at pH 3 in 3 h.

2. Photocatalytic reduction of hexavalent chromium dependson surface area, phase, composition and crystallite size oftitania.

3. sulphated TiO2 can be effectively used for reduction ofhexavalent chromium under solar radiation.

4. The photocatalytic reduction of hexavalent chromium canbe enhanced in the presence of complexing agent such asEDTA.

5. Adsorption and photocatalytic reduction proceeds side byside; photocatalytic process increases with increase in ad-sorption.

Acknowledgements

The authors are thankful to Dr. S.N. Das, Head, EM& IC Department, for his co-operation and to Dr. VibhutiN. Misra, Director, Regional Research Laboratory (CSIR),Bhubaneswar, for giving permission to publish this paper.The authors PM and SKS are obliged to CSIR, New Delhi,for senior research fellowship and research associateship, re-spectively.

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Photocatalytic reduction of hexavalent chromium in aqueous solution over sulphate modified titaniaIntroductionExperimentalSample preparationCharacterisationPhotocatalytic reduction and adsorption

Results and discussionSample characterisationPhotocatalytic reduction and adsorption

ConclusionAcknowledgementsReferences