Indian Journal of Chemica l Technology Vol. 10, January 2003, pp. 53-59

Articles

Decolourisation of industrial wastewaters of textile dyeing industry by photocatalysis

S Kanmani* & K Thanasekaran Centre for Environmental Studies, Anna University, Chennai 600 025, India

Received 2 April 2002; revised received 9 September 2002; accepted 14 October 2002

The feasibility of photocatalytic decolourisation of real textile dyeing rinse wastewaters (RWWs) collected from the low salt cotton textile dyeing industry was studied, using two grades of titanium dioxide (Ti02) under ultraviolet (UV) and solar light sources. The effects of pH in the range of 6- 10, catalyst concentration in the range of 0.05 - 0.5 g/L for indoor UV studies and 0.25- 2.0 giL for outdoor solar studies and catalyst reuse for twenty cycles were studied on photocatalyti c decolourisation of four batches of industrial RWWs. Since the RWWs contained more than one dye, their colour measure­ments were done at multiple wavelengths of 436, 525 and 620 nm. In order to compare the effect of the operating vari ables on rinse wastewaters, the reaction time at 436 nm was taken into consideration since the reaction time necessary was the maximum at 436 nm. It is concluded that the decolouri sation of RWWs could be carried out at the natural pH itself. A catalyst concentration of I giL was found to be necessary in solar studies, whereas only one tenth of I g/L was needed for UV stud ies. The titanium dioxides were found to maintain their photoactivity during reuse for 20 cycles.

Dyeing is a process of colouring fabric with the dye so as to improve the aesthetic and functional values of the fabric. Dyeing process is of primary environ­mental concern because dyeing is a water intensive process, as it requires I 00- 200 L of water for dyeing one kg of fabric. The wastewater of the dyeing proc­ess is highl y coloured by the release of the unfixed dye. The coloured wastewaters of the dyeing process are aesthetically objectionable and prevent reoxy­genation in receiving water by cutting off light pene­tration. Extremely high doses of colour can interrupt photosynthesis and lower the dissolved oxygen con­tent of receiving waters, which may lead to fish kills . The textile dyeing industries are placed under the category of highly polluting industries and the re­moval of colour from the tex tile dyeing wastewater has been brought under the purview of legal and

I . I regu atory requtrements .

Due to stringent environmental regulations, the textile dyeing industries are switching over from the traditional "end of pipe treatment" to " Pollution Pre­vention" measures such as process modification and improved operating practices. These measures aim at the maximum reduction of pollutants at the source itse lf. One such measure is the introduction of cleaner

*For correspondence (E- mail: skanmani @hotmail.com; Fax: 044-23547 17)

technology namely the use of ' Low salt/High fixati on dyeing' 2

. The low salt dyes are used in the dyeing process in the textile industries in Tirupur, Tamil­nadu3. The low salt dyes require less salt for dye fixa­tion4. Also, an improved operating practice of 'sepa­ration of wastewater into two streams, namely dye bath wastewater (high salt content) and rinse was te­water (less salt ~ontent) is practiced in the industries .

The removal of colour from the textile wastewater is achieved either by separation or oxidation tech­niques . The separation techniques viz ., chemical co­agulation and adsorption are not environment friendly , since they generate large amount of sludge that require disposal. The oxidative decolourisation techniques also have several limitations e.g. biologi­cal oxidation is not effective, oxidation with hydrogen peroxide requires long reaction time and oxidation with ozone is costly. Hence there is a need for a more suitable technology for decolouri sation of textile dyeing wastewaters .

The semiconductor photocatalysis has proved to be a potential oxidation technique. Photocatalysis is ba­sically the acceleration of a photochemical reaction by the presence of a photocatalyst. It involves generation of hydroxyl radicals (powerful oxidants) by photoac­tivation of a semiconductor by the ultrav iolet light radiation. Photoexcitation in a semiconductor occurs upon absorption of li ght of a suitable wavelength. The

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necessary condition for the absorption of the light photons by the semiconductor is that the energy of the photon should exceed the energy of the band gap (Ebg) of the semiconductor5

·6

. In photoexcitation of the semiconductor, an electron (e·) is excited from va­lence band to the conduction band. There is corre­spondingly an electron vacancy or hole (lz+) that re­mains in the valence band . The holes (It) in the va­lence band have an affinity for e lectrons (e·) and hence they ox idi se the adsorbed water molecules and generate hydroxyl radicals (OH"). Both the holes and hydroxyl radical s are very powerful oxidants, which can be used to oxidise the most organic materials 7.8 _

Extensive sc ientific research on semiconductor as photocatalyst for the photocatalytic oxidation (PCO) of organic pollutants has been carried out in the last three decades. Several rev iews have been publi shed di scussing the underlying reaction mechanisms9

-16

.

Scientific research in the last 30 years has shown great potential for using solar photocatalysis in treat­ing industrial wastewaters 17

. Blake18 published a bib­liography in which he compiled a li st of 300 com­pounds, which can be degraded by photocatalysis .

The solar PCO systems have been developed to a point where they can be applied for the treatment of industrial wastewaters. Goswami et a/. 19 have de­scribed a methodology, which can be used for the de­. ign of a si mple solar PCO system. It consisted of the determ ination of reaction rate constant, catalyst con­centration and pH control by a laboratory study 17 and determination of treatment cos ts based on the system, power and operation and maintenance related costs. The operational cost of solar PCO system is greatly influenced by the catalyst cost and photoactivity of the catalyst during reuse. In the present study, the fea­sibil ity of photocatalytic decolouri sation of real texti le dyeing rinse wastewaters (RWWs) collected from the low salt cotton textile dyeing industry was studied, using two types of titanium dioxides under UV and solar light sources . The effects of pH, catalyst con­centration and catalyst reuse were studied on photo­catalytic decolouri sation of industrial RWWs so as to evaluate the practical applications of solar PCO tech­nique to the low salt cotton textile dyeing industry.

Experimental Procedure

Titanium dioxide Two grades of titanium dioxide viz., Degussa P25

procured from Germany and indigenous IS grade pro­cured fro m Travancore Titanium Products Ltd ., India

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Indi an J. Chem. Techno!. , January 20o:l

were used. Degussa P25 is the most commonly used commercial titanium dioxide. The photoact ivity of the indigenous Ti02 was compared with Degussa P25 Ti02. The band gap energy of Degussa P25 Ti02 as stated in literature was 3.2 eV. For the indigenous catalyst, the band gap as recorded from the di ffuse reflectance spectrum was also calculated to be 3.2 e V. The mean particle size and the surface area as given by the manufacturers are reported to be 0.03 micron and 56 m2/g for the Degussa P25 Ti02 and 0.4 micron and 12- I 3 m2/g for the indi genous catalyst.

Instruments A MeteoLOGTDL 14 Data Logger was used fo r

measuring the meteorological parameters and pyra­nometer CM3 in sensor 6 of the data logger was used for measuring the solar radi at ion. The correlat ion between global solar radi ation and global solar UV radiation was estimated by means of Dr. Honle UV meter. A spekol UV- Vi s spectrophotometer was used to measure the absorbance of the liquid sampl es at appropri ate wavelengths. Heracus - Sepatech Labo­fuge 200 Centrifuge was used for the centri fuging of samples at 5000 rpm for 30 min du rat ion fo r remov­ing turbidity before colour measurement.

Kinetics of photocatalysis The graphical method was used for finding the ki ­

netic rate constant (k) of the photocatalytic decolouri­sation of textile dyeing wastewaters. In all the decol­ouri sation studies, a plot of ala0 (absorbance at time t/initial absorbance) versus time (r) was made so as to find the rate of decolouri sation . By regression model­ling, exponential curves were found to be the best fi t fo r the data with regression coefficient (R2

) values in the range of 0 .90-0.99. The values of pre-exponentia l fac tor (A) and rate const~nt (k) were direct ly obtained fro m the ex ponenti al equat ion fo r each set of ob­served data. Using the values of A and k , the dye re­moval rate at 50% decol ouri sation (!los) and the reac­tion time (i.e. time required for 90% decolouri sation) were derived. They were used to compare the effects of operating variables on photocatalytic decolouri sa­tion of wastewaters.

Experimental set-up The indoor UY photocatalytic decolouri sati on

studies were carried out in an annu lar photoreactor viz. , Heraeus UV photoreactor (TQ 150 model) . It consisted of a medium pressure mercury vapour UV lamp (150 W), a cooling jacket made of quartz g lass

Kanmani & Thanasekaran: Decolourisation of industria l wastewaters of textile dyeing industry by photocatalysis Articles

and a reaction vessel of 500 mL capacity. The UY immersion lamp with cooling jacket was placed in the

reaction vessel (Fig. I ). Cooling water at 27°C was circulated through the reactor with the help of the cooling system so as to prevent the overheating of the lamps. Through an opening provided at the bottom of the reactor, air supply was done using an aquarium pump. The outdoor solar photocatalytic de­colourisati on studies were carried out using multiple borosilicate glass trays (each of size 200 x 200 x 50 mm) as reactors. Aquarium pumps were used for air supply and mixing of the catalyst. The indoor and outdoor experiments were conducted on 500 mL wa tewater samples (in triplicate) at its natural pH . The time durations of experiments were one hour and two hours for indoor and outdoor experiments respec­tively . Samples were collected at 10 min (in indoor studies) and 15 min (in outdoor studies) for analysis of colour.

Characterisation of RWWs

Twenty-six samples of rinse wastewaters were co llected from a low salt cotton textile dyeing indus­try at different times in a year and their characteristics were determined as per the standard methods . The pH of the industrial rinse wastewaters varied from 6- I 0, the total di ssolved solids varied from I ,864 - 2, 182 mg/L and the chemical oxygen demand varied from 96- 3 16 mg/L. Since the RWWs contained more than one dye and no single peak was obtai ned. Hence, their co lour measurements were done at multiple wave­lengths of 436, 525 and 620 nm. In all the studies, first order kinetics was observed for all the three wavelengths. The rate of decolourisation was com­paratively less at 436 nm than at 525 and 620 nm and hence the reaction time necessary was the maximum at 436 nm. Hence, in order to compare the effect of the operating vari ables on rinse wastewaters, the re­acti on time at 436 nm was taken into consideration. Goswami et a /. 20 also suggested that the largest reac­tion time should be taken into consideration , since the lowest value of rate constant was used in the design of solar photoreactors .

Effect of operating variables on rinse wastewaters

The effect of operating va ri ables viz., pH, catalyst concentration and catalyst reuse were studied on the decolouri sation of industri al rinse wastewaters using both the catalysts under the two light sources . Catalyst concentrations of 0.1 g/L fo r indoor studies and 1.0

2 --.-

~3 t t ~ -4 -5

----... i---7 ---- f---8 9 ----- --- --

I== ---- F=

1 = 1!6 ---- 1 -- --

Fig. !-Schematic diagram of UY photoreactor

giL for outdoor studies were adopted and the studies were conducted without any pH change. As the pH of the industrial rinse wastewaters varied from 6-10. the e ffect of pH was studied in the range of 6- 10. The effect of catalyst concentration was studi ed in the range of 0.05-0.5 g/L for indoor studi es and 0.25-2.0 g/L for outdoor studies. The effect of catalyst reuse was studied fo r twenty cycles. The catalyst dosage of 1.0 g/L was adopted for the first cycle. At the end of each cycle, the treated wastewater containing the spent catalyst was settled during the night. The spent catalyst settled at the bottom of the container was re­covered by carefully decanting the supernatant and used for the subsequent cycles without any further treatment.

Results and Discussion

EffectofpH The effect of pH on PCO reaction kinetics is still

very much unresolved . In the literature some type of pH dependence, although, it is usually slight, is noted with almost every organic substrate. However, it has not been possi ble to draw any general conclusions with respect to pH about PCO kinetics. Barbeni 1'/

a /.2 1 and Palmisano et a/. 22 studied the effect of pH on the initial reaction rates of phenol and 2- , 3- and 4-nitrophenols. Photocatalytic ox idation of 3-nitro­phenol was less pH-dependent than that of 2 and 4 isomers.

In thi s study, the effect of pH on decolourisation of RWWs was studied in the range of 6 to I 0 using both the catalysts under the two li ght sources. In all four

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250 --1

200

~ 150

0 100

~ ~ ---- 1'25 Solar

<:<: 50

-+-IS Solar --1'25 uv ~ISUV

0 L-----------.-----------~----~----~

.100

250

c 200

"J 150 c

lOll

" ;_) X

:\0

()

6 7 ') 10

pH

Fig. 2-Effect of pH on decolouri sati on of R WW I

6 7

pll

---- 1'25 So lar -+-IS Solar __..._ 1'25 uv ~IS UV

') 10

Fig . 3- Effect of pH on decolouri sation of RWW2

II

II

batches of rinse wastewaters studied (RWWs 1, 2, 3 and 4), the rate of decolourisation was found to be maximum at their natural pH for all batches except batch I . A similar study was conducted using IS TiOz under solar and UV light sources. The effect of pH was found to be si milar to P25 Ti02 study. The plots of pH versus reaction time were made for all the four batches of RWWs and the same are illustrated in Figs 2-5. It was observed that the reaction time re­quired was always higher at 436 nm than at 525 and 620 nm.

The rate of decolourisation under UV light was higher than that of solar light . In solar photocatalysis, the photoactivity of IS Ti02 was better than P25 TiOz for RWWs I and 4. However, for RWWs 2 and 3, the photoactivity of P25 Ti02 was better than IS Ti02. In UV photocatalysis, the photoactivity of P25 Ti02 was always better than IS Ti02. Since the rate of decol­ouri sat ion was the maximum at their natural pH for all batches except batch I , it is concluded that the decol­ourisation of RWWs could be carried out at the natu­ral pH itself.

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Indian J. Chem. Techno!. , January 200:1

525

450

" .100 -E

c: 225 -~ u 2 150

0::

75 -

----1'25 Solar -.-IS Solar --+-- 1'2 .~ uv -+<- IS UV

() +-----~---------,--------------------- -----6 10 II

pH

Fig. 4-Effect of pH on decolouri sati on of RWW:l

260 -

230

-~ 200

<U 170 -.§ .3 140

u " 110 '-'

<:<:

so __..._ P25 UV

-><-IS UV

50

6 7 s 9 10 I I pll

Fig. 5-Effect of pH on decolouri sation of RWW4

Effect of catalyst concentration

The effect of catalyst concentration on the rate of decolourisation was studied on four batches of RWWs using both the catalysts under the two light sources . The catalyst concentration was varied in the range of 0 .25 to 2 g/L for outdoor solar studies and 0.05 to 0.5 g/L for indoor UV studies. For all the four batches of RWWs, a catalyst concentration of l g/L was found to be necessary in solar studies using both the catalysts. The plots of catalyst concentration versus reaction time for all the experimental conditions of solar stud­ies are illustrated in Figs 6 and 7. At a catalyst con­centration of 1 g/L, the reaction time for the four batches of RWWs was in the range of 129 to 194 min and 135 to 224 min for P25 and IS Ti02 catalysts re­spectively at 436 nm. The differences in reacti on times for different RWWs might be due to differences in their composition and also due to varying solar light intensities. At catalyst concentration of I g/L the ratio of reaction time of IS Ti02 to th at of P25 Ti02 for four batches of RWWs were found to be 0.85. 1.05, 1.05 and 1.54 i.e. the decolourisation by IS

Kanmani & Thanasekaran: Decolourisation of industrial wastewaters of textile dyeing industry by photocatalysi s Articles

700 r-----·----- ----------, Solar light --w- RWWI

600 -+-R\VW2 -- RWWJ

500 _,._R\V\V4

. ~ J(XI

~ ~ 200

c::

I ()()

0~---------~---.---~

0 0.5 1.5 2.5

T i0 2 concenlralion (g/L)

Fi g. 6- Effect o f catalyst co ncentration (P25 Ti02 & Solar light)

1050 So la r lig ht --w- R\VW I

'l<XI -+- RW\V2 -- RW\V3

§ 750 . _,._RW\V4

'lJ

E 600

c 0 450 u ~

c:: .100

150

0~------.---~---~--~

0 0.5 I 1.5 2.5 Ti 0 2 concentration (g/L)

Fig. 7- Effect o f ca talyst concentration (IS Ti02 & Solar li ght)

Ti02 was better than P25 Ti02 in RWW 1 and the P25 Ti02 was found to be slightly better than IS Ti02 in RWWs 2, 3 and 4 .

In the decolouri sation of RWWs under UY light, the rate of decolouri sation was found to decrease when the catalyst concentration was higher than 0.1 g/L. At 0.1 g/L, the reaction time was found to vary from 59 to 95 min and from 64 to 98 min for P25 and IS Ti02 catalysts respectively. The plots of catalyst concentration versus reaction time for all the cond i­tions are illustrated in Figs 8 and 9 . At catalyst con­centration of 0.1 g/L, the ratios of the reaction time of IS Ti02 to that of P25 Ti02 were 1.25. 1.27, 1.09 and 1.03 for RWWs I, 2, 3 and 4 respectively . Hence the photoactivity of P25 Ti02 was found to be slightly better than IS Ti02 under UV light.

From the above study. it was observed that the rate of decolouri sation increased with increase in concen­tration of the catalyst up to 1.0 g/L in solar photoca­talysis and 0.1 g/L in UV photocatalysis, after which , the rate constant either increased marginally or de­creased. Beyond the above Ti02 concentrations, the

300

uv li g ht --RWW I -6-RWW2

250 --1< \VW.\

" g --RWW4

~ 200 c:

c: .s

150 u "' "-' c::

100

50 .

0 0.1 0.2 0 .3 0.4 0.5

Ti02 concentrati on (g/LJ

Fig. 8-Effect of catalyst co ncentration (P25 TiO~ & UV light )

460

~ 4 10

" .160 .E

"' 310

.§ 260

.9 ~ u 210

"' " c:: 160

110

60

0

~

0. 1

uv li g ht

0.2 0.3

--w- RWWI -1r- RW\V 2 ...._ RWW3 -><- KWW4

0.4 0.5 Ti02 concenlratio n (g/L)

0 .6

0. 6

Fig. 9- Effect o f catal yst concentration (IS Ti02 & UV li ght)

rate of decolourisation either Increased slightly or re­mained constant. This suggests that upper level for catalyst effectiveness exists and similar trends were reported previously in other photocatalytic reac-

. 23-25 A h. h I . h ttons . t 1g er cata yst concentrations, t e wastewater was so opaque th at light was unable to penetrate. Reeves et al. 26 also reported simi Jar obser­vations in the photocatalytic degradation of several classes of organic dyes. Matthews27 found that the rate of sa li cy late formation in the oxidation of sodium benzoate increased with the quantity of Ti02 upto 2 g/L, but decreased slightly at higher loading. Augug­liaro et a/. 28 observed the same phenomenon with a maximum phenol PCO rate at I g/L Ti02. The rate o r decolouri sation was low at lower catalyst loading and this can be attributed to the fact that more li ght is transmitted though the reactor and the transmitted light is not utili sed for photocatal ys is29

.

Effect of catalyst reuse

The effect of catalyst reuse was studi ed for twenty cycles using both the catalysts under solar light. The

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12 16 20

Number o f recyc le

Fig. I 0-Effect o f CJ talys t reuse under so l:1r li ght (ReJction time ve rsus umber of recycle)

rate of decolouri sation was varying throughout the studi es and it was observed that the rate of decolouri­sati on was the minimum during the ninth cyc le, wherein the li ght intensity was also found to be the minimum at 17.6 watt/m2

. The rate of decolouri sati on was the maximum during the initial run, wherein the light intensity was al so fo und to be the maximum at 36.0 watt/m2

.

A plot of number of recycle versus reaction time is presented in Fig . 10. The reaction time was found to vary from 145 to 375 min and from 130 to 322 min fo r P25 and IS Ti02 catalysts respecti vely at 436 nm. The react ion time increased with the decrease of light intensity (Fi g. II ). Hence, it is evident that the vari a­ti on in reacti on time during reuse was mainly due to lluctuation in li ght intensity and both the catalys ts are reusable for at least 20 cycles for treating RWWs.

Conclusion A very de tailed laboratory scale feasibility study

was carried out on photocatalyti c decolouri sation of four batches of industrial rin se wastewaters (RWWs l , 2. 3 and 4) and the photoacti vity of two catalys ts viz., P25 Ti02 and IS Ti02 was studied under UV and solar li ght sources. In thi s study, the effects of pH. catal yst concentrati on and catalyst reuse were studied. The studi es had shown the feasibility of decolouri sa­ti on of RWWs. The photocatal ytic decolouri sation of tex til e dyeing wastewaters was found to be of first nrder. Since. the rate of decolourisation was the max imum at their natural pH for all batches except batch I, it is concluded that the decolouri sation of RWWs could be carried out at the natural pH itself. A catalyst concemrat ion of I g/L was found to be neces­sary for solar decolou risati on studies, whereas only

SR

Indian J. Chern . Techno!. , January 200]

-1 400

351)

·= E 300

"' E

g 250

u "' 200 -" 0::

150

100

15 19 31 y; ~I)

lj uv (W /sq .Ill .)

Fi g. I ! - Effect of cJ tJiyst reuse under so!Jr l ight (ReJc ti on time versus quvl

one tenth of I g/L was neec!ed for UV deco louri sa ti on studies. In solar photocatalys is, at a catalys t concen­tration of I g/L, the reacti on time fo r the four batches of RWWs varied in the range of 129 to 194 min and 135 to 224 min for P25 and IS Ti02 catalysts respec­tively at 436 nm. The differences in reac ti on times for different RWWs might be due to di ffe rences in their composition and al so due to varying solar li ght int en­sities. In UV photocatalysis, at 0. 1 g/L. the reac tion time was found to vary from 59 to 95 min and from 64 to 98 min fo r P25 and IS Ti02 catalys ts respec­tively. The photoactivity of both P25 and IS Ti0 2

catalysts were not affected by the reuse of the cata­lysts for 20 cyc les. The vari ati on in the rate of deco l­ouri sation observed during the st udy ~ as mainly due to the variation in the solar light in ten~ i t y .

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(CentrJI Pollution Control Board (CPC B) publi cation. Se ri c> COINDS/59/1999-2000), 2000.

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Kanmani & Thanasekaran : Decolouri sation of industrial wastewaters of tex tile dye ing industry by photocatalys is Articles

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( 1994) 11 7. 20 Goswami D Y. Sharma S K. Mathur G D & Jotshi C K.

J Solar Energy Engg, 11 9 ( 1997) I 08 . 2 1 Barbeni M, Parmauro E. Peli zzetti E. Borgarello E & Ser­

pone N, Chemosphere, 14 (1985) 195 . 22 Palmisano L, Augugli aro V, Schiavello M & Sclafani A.

J Mol Carat , 56 ( 1989) 284. 23 Okamoto K, Yamamoto Y. Tanaka H. Tanaka M & ltayu A.

Bull Chem Soc Japan , 58 (1985) 20 15. 24 Auguli aro V, Palmisano L & Schiavello M, AIChE. 37

( 199 1) 1096. 25 Davis R J, Gai ner J L, Neal G 0 & 1-Wenwu. Water Elll ·i­

romnent Res. 66 ( I) ( 1994) 50. 26 Reeves P, Ohlhausen R, Solan D, Da mplin K & Slogg ins T.

Solar Energy, 48(6) ( 1992) 4 13. 27 Matthews R W. J Chem Soc. Faradav Trw1s /, 80 ( 1984)

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