Indian Journal of Chemical TechnologyVol. 4, January)997, pp. 1-6

Ti02-catalysed photodegradation of reactive orange 84 and alizarin red Sbiological stain

N N Rao" & Sangccra DubcCentral Salt & Marine Chemicals Research Institute, Gijubhai Badheka Marg, Bhavnagar 364 002. India

Received 30 April 1996; accepted 26 August 1996

Photocatalytic (TiO/UV) degradation of Reactive Orange H4 azo dye effluent (RO tl41 and Ali-zarin Red S (ARS) has been examined using TiO. catalyst either as slurry in aqueous dye solutionor supported on Ti sheet or polyester fabric. The supported forms of catalysts have been used fortreating larger volume of RO 84 effluent. Spectrophotometric data on photo reacted dye solutionshave suggested that azo group of RO 84 and quinoid moiety in ARS are affected by photocatalyticprocess at the onset. Complete colour bleaching occurred within 30 min to 2 h depending upon theinitial concentration of the dyes, and ~ <}O'Yo chemical oxygen demand (COD) removal efficiencycould be attained using suspended TiOe in 4-6 h of exposure to UV light. The rates of photodegrad-ation of these dyes have shown pseudo-first-order dependence on the concentration of these dyes(~ 100 ppm). The rate of photodegradationof RO 84 with the supported forms of TiOe catalysts isabout an order of magnitude lower. The photodegradation of 'ARS dye has exhibited remarkahledependence on pH while that of RO H4 remained unaffected. This work suggests that TiO/UVtechnique can he employed for removing colour and COD of dye-house effluents.

The use of UV light in conjunction with semicon-ductor catalysts (TiOz, ZnO etc.,) for destructionof a variety of organic pollutants is endowed withtechnological promisel ?'. The semiconductor/UVtechnique has been shown to be useful for colourremoval and TOC/COD reduction originatingfrom dyestuffs belonging to triarylmethyl, azo, he-terocyclic, anthraquinoid and pthalein classes also.Some textile azodyes are subjected to reductivedecolorization using TiOz and W03 catalysts",while the sensitized oxidation of Rose Bengal atTiOz and ZnO photo catalysts is retarded at high-er dye concentrations", The TiO/solar light de-gradation of methylene blue followed first-orderkinetics with 99% removal efficiency". Reaves etal.x used highly concentrated sunlight and Ti02catalyst for destroying several classes of dyes andstains from wastewater, most dyes underwent de-gradation and reached < 1 ppm levels (from in-itial 30 ppm) in less than 20 min. On the otherhand, Ruppert et al.9 compared the efficiencies ofUV/03, UV/HzOz, UVlTiOz and UVIH20/Fe2+to decolourize wastewater containing ReactiveRed 218 and Reactive orange 16 and found thatUV/Ti02 as well as UV/Hz02 were less effectiveas the direct excitation of Ti02 and H202 is

"Author to whom correspondence should be addressed.

strongly inhibited by these intensely coloureddyes. In the present paper, Ti02 catalysed photo-degradation of Reactive Orange 84 azo-dye efflu-ent (RO 84) and Alizarin Red S stain (ARS) hasbeen reported. The study included examination ofvarious control experiments, rate parameter eval-uation, pH effects and applicability of Ti02 sup-ports for treating dye waste (RO 84). The resultssuggest that water containing the azodye effluentsand Alizarin Red S stain can be efficiently treatedusing photocatalysts.

Experimental ProcedureMaterials -Powder titanium dioxide from De-

gussa (P-25, predominantly anatase, surface area,50 ± 15 m2g-1 and particle size, 21 nm) wasused in the present study. In addition, flame-oxid-ized titania sheet (TiITi02) and Ti02 coated pol-yester support (PE-PVA-Ti02) were used. Thepreparation and use of Ti/Ti02 and PE-PVA-Ti02 catalysts has been reported earlier 10. Azo-dye effluent (Atlas Dye Chern. Ltd, Ahmedabad)was used in the form of concentrated effluent('filtrate' from filter press). -This was reported tocontain Reactive Orange 84 dye MW = 1672(structure 'a'). This is a benzidine based chlorotri-azine bis azo dye. The dye effluent containedsignificant quantities of inorganic salts {NaCl and

2 INDlAN J. CHEM. TECHNOL.,JANUARY 1997

CI J!.A r::;:(.SIlf' ~ "0"

O- ••• -tc;:L .•.~"'~N:A. •••_O

so:!, OH

o • ©l9TN•N18@ ...

s03H Stl3H 2

A.actlve Orange " I RO *' I (0)

~ o OH"Izorin Red 5 IAIIS) (b)

STRUCTURE

KC1). The as-received dye effluent was used forpreparing a stock solution of known COD whichwas inturn used for preparing dye solutions ofdifferent initial COD levels. Alizarin Red S, a bi-ological stain was, supplied by SD's Lab-ChernIndustry, Bombay (structure 'b').

Photocatalytic experiments-Aqueous aeratedsolutions of dye (500 mL of RO 84 azo-dye efflu-ent or Alizarin Red S) containing 0.25 g P-25Ti02 catalyst were irradiated with UV light (an-nular immersion well reactor, SAIC, India; 400Wor 125W medium pressure Hg lamps, 365 nmpeak emission). The suspensions were stirredmagnetically and equilibrated with air using anaerator pump. An initial one hour equilibrationtime was allotted for attaining saturation adsorp-tion of dyes onto catalysts before commencingUV irradiation. For experiments with stationaryTi02 supports (TiITi02 and PE-PVA-Ti02), 10 or20 L RO 84 azo-dye effluent was recirculated(flow rate 50 mL min - J) through a double-stagephotoreactor (houses two Hg lamps one in eachreactor) using polypropylene water circulation

IGHT

UII#i30'" - - - - -.,-- -....!luV+O~~t>- I • 2~I -':, 0 TI02~2+S3Onm

g20' --~

10

o 10 120 \10 240TIIMof .-nation ,,,*,

Fig. 1-COD as a function of illumination time under variouscontrol experiments carried out on RO 84 azo dye effluent.

pump. A bye-pass valve in the loop facilitatedcontrol of flowrate. Air was not needed to bepumped separately in this case as the dye effluentin open tank was well mixed with ambient air.These recirculation experiments were conductedfor a total irradiation period of 26 h spread over4 or 5 days. The lamps were switched off duringnight. Several of the control experiments conduct-ed include : use of mere catalyst in dark, use ofUV light without catalyst, use of O2 and UV light,use of N2 and UV light, and finally use of catal-yst, O2 and visible light (533 nm). Oxygen was ex-cluded from dye solutions by vigorous purgingwith lolar grade N2 gas in order to assess the roleof oxygen.

Rate measurements were made at different in-itial concentrations of dye solutions by estimatingchemical oxygen demand (COD) as a function ofirradiation time. Duplicate COD measurementswere made susing acidic dichromate method (er-ror, ± 3-5%). The powder catalyst was removedeither by microfiltration (0.2 f.1 membrane filter)or centrifugation and the catalyst-free aqueoussamples were processed for further analysis. In in-dependent experiments, the CO2 evolved waspassed through alkali scrubber and estimated bytitration against acid. The other degradation pro-ducts such as SO~-, NO; and NH: were detect-ed by known spectrophotometric and ion selec-tive electrode methods.

Results and DiscussionAbout 15-20% of RO 84 from the azo dye ef-

fluent and 5% ARS dye are adsorbed over theP-25 Ti02 powder catalyst. Adsorption attainedsaturation within Ih under stirred conditions. Therecovered Ti02 powder catalyst acquired brown

2.5r.-.-.-......... 2.sll2.~ " 2.0•

itS, \ \110

~ ~ 05a .•~ 1.0

•\O~\•214M!

300 400 500Wavelength

ROI4 150 ppm)

P-2S TI02125W lamp.0.5

0' ! I I ,533... I

~ ~ ~ aoTIIMof .-nation,"*'

Fig. 2-Decrease in absorbance at 533 and 284 nm peaks ofRO 84 azo dye effluent as a function of irradiation time. In-set, stack plot of UV-vis spectra of RO 84 at different inter-

vals of irradiation time

RAO & millE: PHOTOCATALYTIC DEGRADATION OF R084- AND ARS

colour due to adsorbed dye. Exposure of suchaerated aqueous Ti02 suspensions containingdyes to UV light or sunlight in open caused grad­ual disappearance of colou.r. The dye adsorbedTi02 powder catalyst also sheds its colour slowlyunder prolonged illumination.

The results of various control experiments aregiven in Fig. 1. It is evident that the combinationof UV light, O2 and Ti02 catalyst showed re­markable colour disappearance from dye solutionas well as caused faster COD reduction.

RO 84 has shown three distinct absorptions inits UV-vis spectra : broad absorption band be­tween 600 and 450 nm with maximum at 533nm, 350-250 nm (A.maxat 284 nm) and at 234 nm.The photocatalytic degradation of RO 84 is mon­itored at A. = 533 nm (for colour bleaching) and284 nm (Fig. 2). The decrease in absorption ateach wavelength has shown two different ratezones. Initially, the absorption at 533 nm dec­reased at a rate of 1.90 absorption units (au) perhour, but during second hour of irradiation, therate decreased to 0.48 au h -I. A reverse trend isobserved for the 284 nm peak. Here, the initialrate is 0.38 au h-I which increased to 1.41 au

h - 1 during the second hour of irradiation. Thus,the absorptions at 533 and 284 nm are complete­ly eliminated within 2 h of UV irradiation. Thechromophore responsible for 234 nm, althoughless reactive, is nevertheless degraded during thecourse of UV irradiation.

Similarly, the photo degradation of ARS dyewhich showed absorption maxima at 518, 330and 357 nm is also monitored as a function of ir­

radiation time (Fig. 3). While the absorption at518 and 330 nm completely disappeared within60 min, the absorption at 257 nm persisted evenupto 2 h of irradiation. The rate of decrease ofabsorption at 257 nm is faster (0.7 au h -I) thanthat at 518 nm (0.3 au h-I). Further, the absorp­tion peak in the visible region progressively shift­ed towards blue (from 518 to 490 nm) as the irra­diation time increased. However, the position of257 nm peak is relatively unaffected.

The photodegradation of RO 84 effluent andARS dye in terms of the decrease in COD levelsand its dependence on their initial COD levels isdisplayed in Fig. 4. Complete colour removal aswell as 86% reduction in COD (relative to initial

Table I-Rates, rate constants and half-value periods for photocatalytic degradation of RO 84 and ARS dyes

COD,ppm

Rate*, ppm min - I Rate constant k, 102min-I

Half-value period + , tillmin

R084ARsR084ARSR084ARS28.3

0.50-2.90-24.0

30.0

-0.,33-1.76-39.0

90.5

-0.~6-1.80-38.5

1001.33-2.80-25.0

2001.66-0.86-80.5

2750.501.5-0.88-79.0

*Data points upto 1 h (Fig. 4) have been used for rate estimations. **t1l2 = 0.693/K

360300

•••••.•••• ROt40----<> ARS

60 . 120 180 240T•••• of ••••••••• tion. min

,,,I1

I

--. "t\,

'\:~0--~.-

2ARK :LIGHT,,

,1)0

o 60 120 180 240 JOOTime of llumination, "*'

Fig. 4-Effect of initial COD levels of RO 84 and ARS dyeson the rate of COD removal

j 1.5,'. J 1.1

~ ~01oo~~ Wavelength

X .251 nm.

AlIZARIN RED S (50ppm1

P-25 TIllz125W lamp

2.0

2.5

~ .518-60-- 90 120

TiN of -.....atlon

Fig. 3-Decrease in absorbance at 518 and 257 nm pealq; ofAlizarin Red S as a function of irradiation time. Inset, stackplot of UV-vis spectra of ARS dye at different intervals of ir-

radiation time

4 INDIAN J. CHEM. TECHNOL., JANUARY 1997

COD of 30 ppm} became possible in the firstone hour of irradiation itself (initial rate = 0.5ppm min: I). The rate constants were evaluatedfrom -log (COD) vs time of irradiation plots(Table I). The rate constants are found to be ap-proximately independent of the initial concentr-ations of dyes (up to 100 ppm). The rate constantfor photodegradation of both RO 84 and ARSdyes (at their highest initial COD level of 200 and275 ppm respectively) is about 2-3 times lowcompared to corresponding values at the lowerinitial COD levels. Thus, it may be noted that thephotodegradation of both RO 84 and ARS dyesfollows pseudo-first-order kinetics at initial CODlevels ::s;; 100 ppm. Rate constants of this order(10- 2 min - I) are generally found for photode-gradation of a variety organic compounds II. Thesignificant decrease in the rate constant at higherinitial COD levels (~ 200 ppm) may be attribut-ed to strongly inhibited direct excitation of Ti02semiconductor due to diminished penetrationdepth of UV light in these highly coloured solu-tions. The analysis of degradation products con-firmed that these dye molecules are converted in-to CO2 (the per cent COD removals and per centcarbondioxide yields agreed in general within5-10% error limit); heteroatoms such as N, Shavebeen converted into NH:,. NO; and SO~- re-spectively. In the case of ARS dye only S042- re-sulted as a product of degradation apart fromCOco

The pH dependence of the photocatalytic de-gradation rate of the dye evaluated for RO 84 (atpH 3.7 and 8.2) and ARS dye (at pH 4.7 and 13)is depicted in Fig. 5. Higher initial COD levels

200

160

'Q101208

so

40

(200 ppm RO 84 and 1000 ppm ARS dye) werechosen in order to check whether pH changes cancause rapid degradation of these dyes at higherinitial COD levels with favourable rates. It isfound that the overall rate of photodegradation ofRO 84 (0.68 ppm min -I) is unaffected by pHchange, but that of ARS dye showed remarkablepH dependence. The rate of photodegradation ofARS dye (0.38 ppm min-I at pH 4.7) is six timesenhanced at pH 13 (2.37 ppm min-I). Changesboth in the chemistry of the dye molecule ano inthe surface chemistry of Ti02 powder catalystmay be responsible for this rate enhancement.

The application of stationary supported Ti02catalysts (Tiffi02 and PE-PVA-TiOz) for photo-degradation of RO 84 (10 or 20 L containing thisdye in < 100 ppm levels) in a double-stagephotoreactor is illustrated in Fig. 6. Due to eva-poration of significant amount of water from opentank during 4-5 days duration of experiment, thecolour changes were slowly perceptible and CODlevels of intermediate samples indicated lesservariations as a function of time. Hence, the CODof final sample was determined after making upfor evaporation losses. The rate of photodegrada-tion of the dye in 10 or 20 L of tap water (TW)using Ti/Ti02 and PE-PVA-Ti02 catalysts is com-parable (3.6 and 4.0 ppm h -I respectively, omitt-ing evaporation losses). This amounted to 67 and79% COD reductions respectively, in 26 h. How-ever, use of double-distilled water (DW) in placeof tap water, increased the rate of degradation(5.2 ppm h-I) of RO 84 using PE-PVA-Ti02• Inthis case, about 87% COD reduction was attainedin 26h. These rates of photo degradation of RO

II (b)I

I1000 I

900 \-..,"__ pH:I..7800 I, ----.-----I ,

I ~

I \I \I '\I ,. '.,,,,-,,,,

• ". pH,.ll' .....••---

ALIZARIN REO S

700

600

soo.z 400

~300200

100

o 2 l 4 2/6810121416Tine of lIumnatlon, h Tine of llun*'ation, h

Fig. 5-Effect of pH on the rate of decrease of COD: RO 84(a) ARS and (b) dyes

RAO & DUBE: PHOTOCATALYTIC DEGRADATION OF R084 AND ARS 5

RO&4- TVTI01. IOl.TW--- PE-PVA-Ti02.1OLTW•••...• PE-PVA-TIOz·lOlDW

100

20

~-2t--~4----:6~-1'-"""*1O--+'12:--~14:--~

Time of Alumklatlon. h

Fig. 6-Photocatalytic degradation of RO 84 in 10 or 20 L oftap water (TW) or double distilled water (DW) using TilTi02

and PE-PVA-Ti02 supported catalysts. The arrow indicatesthe COD values in 26 h after making up for evaporation

losses

84 are about an order of magnitude lower com-pared to those obtained with Ti02 suspended ca-talysts in a-single-stage stirred photoreactor (Table1). This difference can mainly arise fromreducedmass-transfer effects in suspended catalyst system,although the weight-to-volume ratio of catalystand dye solution volume (hence surface areaavailable for the reaction) could also cause thisdifference. The weight of Ti02 in the form ofcoatings on Ti sheet or PE-PVA sheet is substan-tially lower than 0.25 g P-25 Ti02 catalyst (sur-face area = 50 ± 15 m2g- 1) suspended in 500mL of dye solution. The catalysts (Tiffi02 andPE-PVA-Ti02) may be used for slow degradation.of RO 84 from larger volumes of the dye efflu-ents.

The foregoing data concerning colour removal,COD reduction together with detection of CO2,NH:, NO; and SO~- confirm that RO 84 azodye and ARS dye are degraded photocatalytically.Three components are essential for this reaction:Ti02 catalyst UV-light and oxygen. The band-gapillumination of Ti02 using artificial UV sourcesor sunlight (UV component) is effective. Theother possible route for dye degradation viaphotosensitization! of Ti02 by the visible light ab-sorbing RO 84 and ARS dyes appears to be inef-fective in the present case. Thus, the direct parti-cipation of semiconductor surface and the accom-panying charge transfer process involving surfaceadsorbed O2 and hydroxyls (OH) may be envi-saged as a cause> for photodegradation of thedyes.

Reaction steps (1-4) are considered more fre-quently to explain Ti02 mediated photocatalytic

degradation of organic compounds 1 - 4.12.Ti02 + hv - Ti02(e- ..... h ") - eCB + h-W

... (1)..: (2it)... (2b)

... (3)

OH~ds + h\.s ....•OHadsH20ads + h\.s ....•OHads +H+[02lads+ eCB ....•[Oil.ds + H+ ....•[OOH]adsSubstrate + OH).d/(OOH)ads ....•

degradation products ... (4)The ability of UV irradiated Ti02 surface to

generate OH and OOH radicals is wel-knowri'<". The control experiments performed inthis study suggested the participation of TiOz andO2 in conformity with the above reaction steps (1-3). The 'OHl02H radicals attack the frameworkof the dye molecules initially at vulnerable sites(such as at unsaturation, carbon positions a tosubstituents etc.). It appears that in several suchsequential attacks, the hydrocarbon moieties aretransformed into carbondioxide, and hetero atomsare mineralized as their oxo anions. Concerningthe degradation of nitrogen containing com-pounds at Ti02/H20 interface" it is reported thatazo groups are expelled as N2 gas, amino groupsare converted into ammonium ions, nitro groupsgo as NO; and heterocyclic nitrogens are trans-formed into either or both NH: and NO;.

The UV-vis spectral changes (Figs 2 and 3) ob-served as a function of irradiation time may beregarded as representative of reaction steps priorto bond-cleavage events. The absorption peaks at533 and 284 run of RO 84 can be traced to n-31:*and 31:-31:*transitions respectively of the azo groupin the dye molecule. The peak at 234 run appearsto arise from conjugated double bond ( - C = N - )of triazine groups. Similarly, the 518 and 335 runabsorptions in ARS dye correspond to n-31:*and31:-31:*transitions respectively of quinoid group inARS dye. The phenyl ring 31:-31:*transition of ARSdye is at 257 run.

The rapid disappearance of absorptions due ton-31:*and 31:-31:*transitions of azo and quinoid.groups (in RO 84 and ARS respectively) impliesthat these groups are affected by photocatalyticprocess. Two distinct rate zones observed for thedecrease in absorptions at 533 and 284 run (ofRO 84) further suggest that the attack ofOHl02H radicals on the dye molecule is selec-tive. The initial faster rate of decrease of absorp-tion at 533 run compared to that of 284 run im-plies that the n-31:*transition is impaired first.When the transition is substantially annulled, theabsorption due to 31:-31:*at 284 run begins t.o dec-rease faster. Similarly, the relatively faster rate ofdecrease in absorption at 257 run (of ARS dye)may imply that the conjugation associated with

6 INDIAN J. CHEM. TECHNOL., JANUARY 1997

phenyl rings is disturbed initially. As a consequ-ence of this, the energy requirement for quinoidbased n-n" transition is expected to be altered.And indeed, the n-n" transition at 518 nm ofARS dye showed hypsochromic shift towards 491nm. Free-radical (OH) substitution on phenyl ringmay be regarded as responsible for this shift.

ConclusionsDirect UV irradiation of Ti02 catalysts (su-

spended or supported) in aerated aqueous solu-tions is capable of destroying RO 84 azodye indye-house effluent and Alizarin Red S biologicalstain in water. The rate of photodegradation ofthese dyes (at s 100 ppm) follows pseudo-first-order kinetics. The rate of degradation of ARSdye can be accelerated in strongly alkaline medi-um. These dyes photodegrade into carbondioxideand inorganic cations (such as NHt) and oxo an-ions (NO), SO~- etc.) of the heteroatoms presentin the dye molecules. The photodegradation ofthese dyes appears to set in by affecting the fund-amental transitions (n-rt" and n-rt") of azo andquinoid groups which are eventually cleaved fromthe parent dye molecules.

AcknowledgementsThe authors thank Prof. P. Natarajan, Director,

CSMCRI, Bhavnagar for encouraging to performthis work as well as for useful discussions. Theauthors also thank Mr. R R Shah (Technical Di-

rector, Ms. Atlas Dye Chern. Ltd., Ahmedabad)for providing much needed information on thestructure of RO 84.

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