Indian Journal of Engineering & Material s Sciences Vol. 8, April 2001, pp. 104-109

Kinetics of photocatalytic degradation of textile dye reactive red 2

M V Shankar, B Neppolian, S Sakthivel, Banumathi Arabindoo, M Palanichamy & V Murugesan*

Department of Chemistry, Anna University, Chennai 600025, India

Received 20 Jun e 2000; accepted 3 January 200]

The photocatalytic degradation of the dye, reactive red 2 has been investigated in aqueous heterogenous solution containing Ti02 as photocatalyst in a batch reactor. The di sappearance of the dye follows approximately pseudo-first order kinetics according to the Langmuir-Hinshelwood model. The addition of optimal amount o f hydrogen peroxide and potassium persulphate enhances the degradation rate of the dye. The presence of sodium carbonate and sodium chloride hinders the photocatalytic degradation. Ti02 has been experimentally found to be highly efficient photocatalyst in the degradation of textile dyes and hence there is a great potential in the treatment of effluents from textile industries by photocatalytic degradation technique.

The textile industry occupies a unique place in the economy of India by virtue of its contribution to the industrial output, employment generation and foreign exchange earnings. The industry has witnessed a phenomenal growth during the last decade l

. A great variety of organic dyes are used in dyeing the fabrics. Azo dye stuffs are among the largest group of colourants manufactured and used worldwide. The wide variety and the ease of manufacture of such dyes have made them useful in a variety of colourant applications2.3. The textile wastewater discharged from dyeing process causes serious environmental problems. Textile dyeing and finishing operations are such that the dye stuffs used in a mill vary from day to day and sometimes even several times a day mainly because of batchwise nature of the dyeing process. Frequent changes in dyes and chemicals in the dyeing process cause considerable vanatlOn in the wastewater characteristics like pH, colour, BOD and COD values4

• Wastewater discharged from the textile mill contains a large number of compounds such as raw materials, intermediate products and even the dye itself.

The major problem with wastewater from textile mills is the removal of dyes from the effluents. The dye concentration may be less than many other chemicals found in wastewater, but even at low concentration it imparts deep colouration. The processes such as adsorption, precipitation, chemical coagulation etc. are not effective in the degradation of

' For correspondence

dyes and they have many drawbacks. In the study reported herein, the illumination of semiconductor particles produces excited high energy states of electron and hole pairs (e-fh+) that migrate to the surface of the particle and initiate a wide range of redox reactions. The main advantage of the photocatalytic process is its mild operation conditions. A variety of semiconductor particles are being used as photocatalysts. Ti02 (Degussa P25) has been chosen in the present study for the photocatalytic degradation of reactive red 2, a commonly used dye.

Reactive Red 2

Experimental Procedure Degussa P25 Ti02 was used as the photocatalyst.

The titania particles are a mixture of both anatase and rutile crystalline phase (mostly anatase) with an average particle size of 30 nm and surface area of 50 m2/g. The textile dye reactive red 2 obtained from Vanavil Limited was used as such. Solutions were prepared by dissolving the dye in distilled water. The photocatalytic degradation study was carried out in a

SHANKAR et at.: PHOTOCATALYTIC DEGRADATION OF REACTIVE RED 2 DYE 105

batch reactor containing a cylindrical quartz reactor of 200 mL capacity with ports at the top for air sparger. The reactor was surrounded by 6x8 W low pressure mercury vapour lamp of wavelength 254 nm.

The reactor assembly was placed on a magnetic stirring plate to enhance the agitation. The slurry composed of dye solution and catalyst was placed in the reactor and stirred magnetically with simultaneous exposure to UV lamp. Samples were withdrawn at periodic intervals from the reactor to assess the extent of decolourisation and degradation using spectrophotometer (model Systronics 106) and COD digester respectively. The COD tests were performed according to the standard methods5

. High performance liquid chromatography (HPLC Shimadzu model LC-lOAT VP series, ODS column) was used to assess the course of mineralisation.

Results and Discussion

Effect of dye concentration

The influence of the initial concentration of the solute in the photocatalytic degradation rate of most organic compounds is described by pseudo-first order kinetics. This is rationalised in terms of the Langmuir­Hinshelwood model modified to accommodate reactions occurring at a solid-liquid interface6 and the final form of the expression can be represented as

In[CoIC] = k,Kt = k' t

tJ12 = 0.693/k'

where t = time in min. required for the initial concentration of solute Co to decrease to C, K = equilibrium constant for adsorption of the organic substrate onto Ti02 particles, (mg '!L) k, = limiting rate of the reaction after maximum coverage under the experimental conditions, (mg L'! min'!) k' = apparent rate constant of the photocatalytic degradation (min-I).

The plot of natural logarithm of the normalised concentration of the dye versus irradiation time shows a good approximation over the range of 80 mg/L to 120 mg/L dye concentration (Fig. O. k' values obtained from the slopes of the plots (Fig. 1 ) and t 112

values obtained from the equation are presented in Table 1. k' values decrease with increasing initial concentration of the dye. This suggests that as the initial concentration of the dye increases, the

requirement of catalyst surface needed for the degradation increase. Since irradiation time and amount of catalyst are constant, the O2- ·and ·OH radicals formed on the surface of Ti02 are also constant. Hence the relative number of ·OH and O2- .

attacking the dye molecules decreases with increasing initial c.oncentration of the dye7

. Hence the rate of degradation decreases considerably with increase in concentration of the dye.

Effect of Ti02 10ading Effect of Ti02 10ading was investigated in the range

of 25-150 mg/lOO mL of the catalyst for a dye

Table I-Effect of dye concentration [Amount ofTi02 = 125 mg/loo mL; Irradiation period = 1 h]

Initial concentration k' t l/2 (mglL) (min-') (min)

80 0.0485 14 90 0.0445 16 100 0.0325 21 110 0.0314 22 120 0.0304 23

Table 2-Effect ofTi02 1oading [Initial concentration of dye =100 mglL; Irradiation period = I h]

Amount of TiOz k' tl/2

(mg) (min-') (min)

25 0.0187 37

50 0.0236 29

75 0.0251 28

100 0.0302 23

125 0.0325 21

150 0.0320 22

4

80PPM

3

20 40 60 80 Irr(Jdlotion Time, min

Fig. I-Plot In[CJc] versus. irradiation time for various initial concentration of dye with Ti02

106 INDIAN J ENG MATER SCI, APRlL 200 1

concentration of 100 mg/L and irradiation period of Ih (Table 2). The increase in the Ti02 10ading from 25 to 125 mg/IOO mL has increased the apparent rate constant from 0 .0187 to 0.0325. Experimental studies have revealed that a catalyst loading of 125 mgllOO mL is the optimal dose for the degradation of 100 mg/L of the dye in 1 h irradiation period. Beyond the optimal dose k' value decreases and t l/2 value increases . Thi s can be rationalised in terms of availabi lity of active sites on Ti02 surface and the penetration of photoactivating light in to the suspension. The avai lability of acti ve sites increases wi th catalyst loading, but the light penetration and hence the photoactivated volume of the suspension shrinks. The trade-off between these two effects is that at low solute concentration , when there are excess active sites, the balance between the opposi ng effect is evenly poised and change in suspension loading makes little difference on the rate of degradation. At high catalyst concentration availabi lity of excess active si tes outweighs the diminishing photoactivated volume and significantly greater rate of degradation is achieved at increased Ti02 loading8

• The decreased k' value at higher catalyst loading may be due to the deactivation of activated molecu les by collision with ground state molecules9

•

Effect of irradiation time

Table 3 presents the % degradat ion of the dye at different irradiation period at optimum catalyst loading and dye concentration . Under the experimental conditions complete degradation of the dye occurred within 180 min of irradiation. The photocatalytic degradation of the dye occurs on the surface of Ti02 where ·OH and O2- • radicals are trapped in the holes of reacti ve species. Oxygen and water are essential for photocatalytic degradation . The ·OH radicals are strong enough to break the bonds in the dye molecules adsorbed on the surface of Ti02.

When the intensity of light and concentration of dye

Table 3-Effect of irradiation period [Ini tial concentration of dye = 100 mglL: Amount ofTi02 =

125 mg/ 100 mLl

Irradi ation COD % Degradation period (mg/L)

(mi n) 0 45 0 30 18.4 59 60 4.6 90 90 3.0 93 120 1.0 98 180 0.0 100

are constant, the number of ·OH and O2- • radicals increase with increase in irradiation period and hence the dye molecules are completely degraded into smaller fragments 10.

The chromatograms (HPLC) of the sample at various irradiation periods are given in Fig. 2. As the irradiation time increases, several intermediate products are formed due to fragmentation of the dye molecules. These intermediates undergo degradat ion on further irradiation. During the course of photochemical reaction sui ph ... ~ and nitrate ions are formed which is a good evidence for the fragmentation of azo groups in the dye structure leading to complete degradation.

Effect of pH pH of the slurry containing 100 mg/L dye soluti on

was 4.65. At this pH, k' value was found to be 0.0325 min·1 and corresponding t ll2 value was 21.32 for an irradiation period of 1 h (Table 4) . The photodegradation of the dye was inves tigated in the pH range 4-12. It is seen that pH range of 4-6 is more advantageous than alkaline pH . The inhibitory effect is more pronounced in alkaline pH range. At high pH values the hydroxyl rad icals are so rapidly scavenged that they do not have opportunity to react with dyes ll

. Hence, k' values decrease with increas ing pH values.

e (0 ) e (b)

~ • u, 6 4h 6 Ih

:> =e. 4 e 4

2 2

0 U 0

0 ~ 10 0 ~ 10 min min

6 6 (el (d)

e Ie. e IIh

:> :>

e " E4

2 2

0 k.! A 0

:> II 10 0 II 10

min min

Fig. 2-HPLC chromatograms as a function of irradiation time

SHANKAR et al.: PHOTOCAT AL YTIC DEGRADATION OF REACTIVE RED 2 DYE 107

Table 4--Effect of pH [Initi al concentration = 100 mg/L; Amount ofTi02 = 125 mg/lOO

mL: Irradiati on period = I h]

pH k' tl 12

(min·l) (min)

4 0.0328 2 1

4.65 0.0325 2 1 5 0.0320 22 6 0.03 17 22 7 0.031 7 22 8 0.03 10 22 9 0.0260 27 10 0.0227 3 1 II 0.0147 47 12 0.01 40 50

Table 5-Effect of hydrogen perox ide [Initial concentration of dye = 100 mg/L; Amount of Ti02 = 125

mg/l 00 mL; Irradiation period = I h]

Concentrati on of k' t 112

H20 2 (min ' I ) (min) (M )

0 0.0325 2 1 4.4 lx I 0" 0.0540 13 8.82x 10') 0.05 12 14 1.32x I 0·2 0.0490 14 1.76x 10.2 0.0470 15

Effect of hydrogen peroxide Since reactive hydroxyl radicals are easily

generated by the break down of hydrogen peroxide, the presence of hydrogen peroxide in the reaction mixture plays a key role in the photocatalytic degradation process. In order to understand this effect, optimum amount of hydrogen peroxide in the concentration range 4.41 x 1O·3 -1.76x 1 0.2 M was added in the dye solution (Table 5). k' value has been observed to be high in the presence of appropriate amount of hydrogen peroxide. It has been reported that the addition of small amount of hydrogen peroxide greatly enhances the oxidation of organic pollutants mediated by Ti02 catalyst l2

. The enhanced k' values in the presence of hydrogen peroxide may be either directly via conduction band electrons or indirectly via superoxide which produces hydroxyl radicals. According to Mugglestone and Hughes l3 and Fujihira t4 hydroxyl radical has 2.05 times more oxidising power than chlorine, 1.58 times more than H20 2 and 1.35 times more than ozone. The enhancement in k' value in the presence of H20 2 may be understood from the equations gi ven below.

Table 6-Effect of potass ium persulphate [k itia l concentration of dye = 100 mg/L; Amollnt ofTi02 =

125 mg/l 00 mL; Irradiation period = I h]

Amount of k' K2S2OS (min· l)

(mg)

0 0.0325 10 0.0495 20 0.0623 25 0.0680 50 0.0660 100 0.0493 200 0.028 1

H20 2+hV-72"OH H20 2+02-"-7"OH+OW+02 H20 2+e--7"OH+OW H20+h+-7"OH+H+ OW+h+-7"OH

tl 12

(min)

2 1 16 12 10 II 14 25

The decrease in k' value beyond the optimal amount of H20 2 is due to the annihilation effed 5

.

Effect of persulphate ion The effect of persulphate ion (electron scavenger)

on the photocatalytic degradation of the dye was investigated by varying its amount from 10 to 200 mg/l00 mL for the dye solution of concentration 100 mg/L. The data are presented in Table 6. k' value increases with increasing amount of persulphate ion and attained an optimum value for 25 mg persulphate. The enhanced k' value in the presence of persulphate ion may be accounted by the following facts: It is a beneficial oxidising agent in photocatalytic detoxification because S04' " is formed from the oxidant by reaction with semiconductor generated electron (e-cb)

S20 g 2-+e-cb-7S04 -" +S04 2-

The sulphate radical anion (S04 - ") is a strong oxidant (P = 2.6 eV) and engages in the following three possible modes of reactions with organic compounds (i) by abstracting a hydrogen atom from saturated carbon, (ii) by adding to unsaturated or aromatic carbon and (iii) by removing one electron from the carboxylate anions and from certain neutral

I I 161 7 I dd' .. h mo ecu es ' . n a ItlOn, It can trap t e photogenerated electron and/or generate hydroxyl radicals.

S04-"+e-cb-7S0/­S04-"+H20-7"OH+SO/-+H+

108 INDIAN J ENG MATER SCI, APRIL 2001

Table 7-Effect of sodium carbonate [Initial concentration of dye = 100 mglL; Amount of Ti02 = 125

mg/iOO mL; Irradiation period = I h]

Amount of Na2C03 k' 11/2

(mg) (min-I) (min)

0 0.0325 21 25 0.0146 47 50 0.0115 60 100 0.0113 61 200 0.0101 69

The hydroxyl radical and sulphate radical anion being powerful oxidants degrade the dye molecule at a faster rate. The S04- ° has the unique nature of attacking the dye molecule at various positions and hence the fragmentation of the dye molecules is rapid.

S04 - 0 +Dye~S04 2-+Dye (intermediate)

S04 -ODye (intermediate)~S04 2-+C02+N02+ other inorganics.

Further increase in persulphate concentration has decreased the degradation rate owing to the adsorption of sulphate ions formed during the reaction on the surface of Ti02 deactivating a section of the catalyst.

Effect of sodium carbonate Sodium carbonate is mainly used in dyeing bath in

order to adjust the pH as it plays an important role in the fixing of dye on the fabrics and in the fastness of the colour. Therefore, the wastewater from the dyeing operation contains considerable amount of carbonate ion. Hence, it is important to study the influence of carbonate ion in the photodegradation efficiency. Experiments were performed with sodium carbonate in the range 25-200 mg for the dye solution of 100 mglL. It is observed that k' value gradually decreases with increasing amount of carbonate ion (Table 7). The decrease in the rate of degradation in the presence of carbonate ion is due to the hydroxyl scavenging property of carbonate ions l8 as seen from the following reactions:

°OH+C032

-~Olr +C03°­

°OH+HC03-~H20+C03°-

Hence, auxiliary chemicals like sodium carbonate may hinder the photocatalytic degradation of textile dyes.

Table 8-Effect of sodium chloride [Initial concentration of dye = 100 mglL; Amount of Ti02 = 125

mgllOO mL; Irradiation period = I h]

Amount of NaCI k' 11/2

(mg) (min-I) (min)

0 0.0325 21 25 0.0263 26 50 0.0260 27 100 0.0227 31 200 0.0197 35

Effect of sodium chloride The photocatalytic degradation efficiency is

considerably decreased upon the addition of inert salts like sodium chloride, sodium sulphate and sodium phosphate. Hence, in the present investigation the effect of another interfering inorganic ion, chloride on the photocatalytic degradation has also been attempted. Sodium chloride usually comes out in the effluent along with sectional wastes of textile mills. Photodegradation studies were carried out with sodium chloride in the range 25-200 mg/lOO mL dye solution of 100 mglL. k' value of the degradation process decreases gradually with increase in the amount of chloride ion . The experimental data are presented in Table 8. The decrease in the % degradation of the dye in the presence of chloride iOlls is due to the hole scavenging properties of these ions as shown in the following reaction sequence l9

. This is a typical example for competitive inhibition.

Ti02+hv~Ti02 (h\b+e-cb) Cr+h\b~Clo+H+ Clo +CI-~CIz 0 -

However, this kind of inhibitions can be reversed by washing the catalyst with pure water to fully restore its photocatalytic activity when chloride is present.

Conclusions Results obtained in this study demonstrate that

photoassisted Ti02 mediated degradation In

combination with H20 2 and persulphate ion is an effective treatment technology for textile dye effluents. The presence of inorganic salts such as sodium carbonate and sodium chloride hinders the photocatalytic degradation of textile dyes. Complete mineralisation of the textile dyes may be possible in a short irradiation period if the concentration of [he dye, catalyst loading, pH, amount of H20 2 and persul phate are optimised properly.

SHANKAR et al. : PHOTOCATALYTIC DEGRADATION OF REACTIVE RED 2 DYE 109

Acknowledgement The authors gratefully acknowledge the financial

support in the form of R&D project from the Ministry of Environment and Forests, New Delhi.

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