Journal of Scientific & Industrial Research

Vol. 59)uly 2000, pp 556-562

TiOz Catalysed Photodegradation of Leather Dye, Acid Green 16

S Sakthi vel, B Neppolian, Banumathi Arabindoo, M Palanichamy and V Murugesan

Department or Chemistry, Anna University, Chennai 600 025 , India

Received: 23 Jul y 1999; accepted: 25 February 2000

Photocatalyti c degradati on o r Acid Green 16 has been inves tigated in aqueous heterogeneous so luti ons containin~ TiO, as photocatalyst. The innuence o r irradiation period , dye co ncentration , catalyst loading, li ght intensity, H20 2, FeCI~ and Fenton 's reagent has been systematically studi ed. Chemica l Oxygen Demand (COD) dete rminatio n and OD m asurement (spectroscopy) have been used to study the degrad ation and deco lori sati on, respecti ve ly. A so luti on containing 9 x 1 o·" M or the dye could be compl etely decolourised in 30 min and 90 per cent degradati on could be achi eved in 3 h

Introduction

Dye stuffs are widely used in textile, leather and printing industries. But dyes and other commercia l co lourants in wastewater have emerged as a focus of environmenta l remediation efforts

1··'. These effo rts

have large ly been targeted towards remov ing co lourants form wastewate r effluents . Among all the categories of dye stuffs, leathe r dyes constitute a significant portion and probably have the least des irable consequences in terms of the su rrounding ecosystem. Techniques like di sso lved air f lota ti on, coagu lat ion , ion exchange, reverse osmosis, adsorpti on and ox idation with peroxide (o r) ozone are usua ll y app lied for the removal/destruction o f dyes in was tewater~. Recently, some reports on Ti02/UV based degradation of various organic pollutants present in wastewater have appeared

5'6

. Titanium dioxide has a su itab le band gap energy, is stable and has hi gh photoacti vit/. The characteristics of photocatalyti c reaction using semiconductor powders as photocatalys t are similar to those of electrochemical oxidati on and reduction reactions

8-10

. The decomposition of organic pollutants in titanium dioxide suspension is due to the photogeneration of electrons and holes in this material 11

• Upon illumination, the system promotes e lectrons from the valence band to the conduction band and holes migrate to the surface. The holes react with the e lectron donors in the electrolyte to produce powerful oxidising free radicals such as "OH. The

* i\ uthor for correspondence

organic compounds adsorbed on the surface o f the photocata lys t are oxidi sed by the free radical s.

The photocatalytic decompos iti on of pollutants in TiOz suspension system has been explored for more than a decade

12. However, most of t~1e investigators

reported the decompos ition of pherwi'-'· 1 ~, halogenated . I I . 'd " 16 0 I f organtc compounc s atiC pesttct es · · . n y a ew

reports are available in the area of photodecomposition of dyes which arc serious pollutants in wastewater. fn the present study, the photodecomposition of leathe r dye, Acid Green 16 (Figure I) us ing Ti02 has been investi gated . Acid Green 16 is a typica l leathe r dye and its co lour and struc ture strong ly depend on pH .

Materials and Methods

The commercial sample of the leather dye, Acid Green 16 obtained from C lariant Chemical Company was used as such . Acid Green 16 shows intense absorbance at 640 nm. Degussa - P25 grade Ti02

Figure I -Acid Green 16

SAKTHIVEL eta/.: Ti02 CATALYSED PHOTODEGRADATION OF LEATHER DYE, ACID GREEN 16 557

(70 per cent anatase and 30 per cent rutile, particle size ?

30 nm and surface area 50 m-Ig) was used as the photocatalyst. The catalysts Ti02 (anatase) and a-Fe20 3 were prepared in the laboratory. Sn02 and Zr02 were obtained from BDH and SD fine chemicals, respectively. ZnO was obtained from Merck and CdS was obtained from CDH. All other chemicals were extra pure grade and were used as received. The dye solution was prepared by mixing the desired amounts of Acid Green 16 in double distilled water.

Ph.otoreactor

The cylindrical photochemical reactor having dimensions 30 em x 3 em (height x diameter) provided with a water circulation arrangement in order to maintain the temperature in the range of 25-30°C was used in all the experiments. The cover of the reactor had ports for sampling gas purger and outlet. The irradiation was carried out using 8 x 8 W low pressure mercury arc lamp built into a lamp housing with polished anodised aluminium reflectors and was placed 6.5 em away from the reactor. In all the cases, during the photolysis experiments, the dye solution containing the appropriate quantity of the semiconductor powder was magnetically stirred before and during illumination. The reactor set up was covered by aluminium foil followed by cloth to prevent UV leakage. At specific time intervals suitable aliquots of the sample were withdrawn and analysed after centrifugation.

Actinometer

The near UV intensity was measured by potassium ferric oxalate actinometer solution. A value of 9.12 x 10 14 quanta/s was estimated for photon absorption rate in the vessel. Scattering losses resulting from suspended solids were not accounted.

Procedure

The decolorisation of Acid Green 16 was measured with UV-Yis spectrophotometer at 640 nm.

The Chemical Oxygen Demand (COD) was measured by the closed reflux method. The formation of C02, SO/, NOJ- and NH/ was identified by Crypton gas analyser and spectrophotometric methods. The photodegradation efficiency for each sample was calculated from the expression (I).

TJ=

lcODo-CODJ

l CODo X ]QQ ... ( I )

where 11 is the photodegradation efficiency; COD0 is the COD of dye solution before illumination , and COOt is the COD of dye solution after time t.

Results and Discussion

The photocatalytic nature of Ti02 in the degradation of organic dye and the effects of irradiation period, initial concentration of the dye, catalyst loading, particle size and light intensity were examined. Photodegradation efficiency in the presence of other photocatalysts, pH, bubbling oxygen and nitrogen, H20 2, FeCh and Fenton's reagent were also investigated.

Effect of Irradiation Period

The results of photodegradation of 9 x I o-~M Acid Green 16 containing 250 mg/1 00 mL Ti02 are presented in Table I . Under the experimental conditions the complete docolorisation of the dye occurred within 30 min of irradiation and the photodegradation efficiency was 60 per cent after I h of irradiation, while after 6 h of degradation, it was complete. It is proposed that the photocatalytic degradation reaction of the dye occurs on the surface of Ti02, primarily in trapped holes. 'OH and ot · are

d l617 . . . d 0 d propose · as pnmary reactive species an 2 an H20 are necessary for photocatalytic degradation 18.

Table I - Photodegradation efti ciency of Acid Green 16 with irradiation period

Initial concentration of the dye: 9 x I o-4M Amount of Ti02 : 250 mg I I 00 mL

Photodegradation efficienc , (%)

1/2

44.48 I

60.65

2

74.77

Irradiation period , h

3

81.87

4

91.79

5

97.78

6

100

558 1 SCI IND RES VOL 59 JULY 2000

Oxygen adsorbed on the TiOz, surface prevents the recombination of electron-hole pairs by trapping electrons and superoxide ions are thus formed . 'OH radicals are formed from holes racting with either H20 or OH" adsorbed on the TiOz surface. ·oH and 0 2

2 .• are

also formed from Hz016

'19

•

In this process, HzOz, Oz and HOz' are suitable for trapping electrons and 'OH and Oz.. are the most important oxidants. The oxidising powder of the 'OH radicals is strong enough to break bonds of the Acid Green 16 dye molecule adsorbed on the surface of the Ti02 leading to the formation of COz and inorganic ions. When the intensity of light was constant, the number of 'OH and o/· increased with increasing irradiation period. As long as the irradiation period was long enough, the dye completely degraded.

Effect of Initial Concentration of Dye

The photodegradation efficiency decreased with increasing the initial concentration of the dye as is evident from the results reported in Table 2. At 5x I o·4

M of dye, the photodegradation efficiency was 97.45 per cent and at I xI o·' M of dye, the photodegradation efficiency decreased to 79.45 per cent for 250 mg/1 00 mL catalyst in 3 h irradiation. The reason for this behaviour is that as the initial concentration of the dye increases, more and more organic substances are adsorbed on the surface of TiOz but the intensity of light and illumination period are constant. ·oH and O/ · formed on the surface to TiOz are also constant. Hence, the relative number of ·oH and o / · attacking the dye molecules decreases and the photodegradation efficiency also decreases.

Effect of Catalyst Loading

The influence of catalyst loading on the rate of degradation is shown in Figure 2. The data show that the photodegradation efficiency increases rapidly with increasing amounts of TiOz from I 00 mg to 250 mg and then decreases on further increase of TiOz for dye concentration of 9 x 10·4 M in 3 h irradiation. The photodegradation efficiency decreased from 83 to 77 per cent with increase in TiOz loading from 250 mg to 300 mg. The general features of these resu lts can be rationalised in terms of availability of active sites on the TiOz surface and the penetration of photoactivating ligbt into the suspension. The availability of active sites increases with the suspension loading but the light penetration and hence the photoactivated volume of the

;:... g 80 Ql

:§ --Gl 60 c 0

:;:: .g 40 0 ... 0> Gl

-g 20 0 .s::. 0..

0~--~r----r----,-----r----~ ~0 100 150 200 250 :300

Amount of Ti02 , mg

Figure 2 - Effect of catalyst loading on the photodegradation efficiency of Acid Green 16

Table 2- Effect of initi al concentrat ion of the dye

liTadi ation period : 3 h Amol)nt ofTi02 : 250 mg I I 00 mL

Dye Concentration

(x i0 4 M) 5 6 8 9 10

Photodegradat ion efficiency, %

97.45 86.93 82.94 81.87 79.45

suspension shrinks. The trade-off between these two effects is that at low solute concentration, when there is an excess of active sites, the balance between the opposing effects is evenly poised and change in suspension loading makes little difference to rate. At high solute concentrations, the availability of excess active sites outweighs the diminish ing photoactivated volume and significantly greater degradation efficiency was achieved at increased TiOz loading20

. The results reveal that the incident photons are not completely absorbed by TiOz in dilute solution. Increasing loading of TiOz increased the quantity of photons absorbed. Consequently, the decolorisation and degradation efficiencies increased. Further increase in catalyst loading beyond 250 mg may result in the deactivation of activated molecules due to collision with ground state molecules. Shielding by TiOz may also take place.

Effect of Light Intensity Variation

In order to study the effect of light intensity dye solution (9 x 10-4 M) with 250 mg/1 00 mL catalyst has

SAKTHIYEL et at.: Ti02 CATAL YSED PHOTODEGRADATION OF LEATHER DYE, ACID GREEN 16 559

been irradiated for 2 h using light source of various intensities. In Table 3 are reported the effect of light intensity on the photodegradation efficiency. It can be seen that degradation efficiency increased as the light intensity increased up to 9.12 x 10 14

quanta/s. With further increase in light intensity ( 12.6 x 1014 quanta/s) , no change m photodegradation efficiency was observed.

Effect of pH

The pH variation studies were conducted with dye solution of 9 x 1 o·4 M and 250 mg/ I 00 mL catalyst irradiated for 3 h in the pH range 3-11. The results (Figure 3) indicate that pH plays an important role in the photodegradation of Acid Green 16 in TiOz suspension . It is observed that photodegradation efficiency was more in acidic pH range of the dye solution. Changes both in the behaviour of the dye molecule and in the surface behaviour of TiOz powder may be responsible for this rate enhancement

21•

Photodegradation Efficiency in Presence of Other Photocatalysts

Experiments were performed (2 x 10-4 M dye solution and I 00 mg/mL of catalyst in I h irradiation) with other commercial photocatalysts and the results were compared with those of Ti02- P25. In Figure 4, the photodegradation efficiency of Acid Green 16 in the presence of various catalysts is shown. Generally, semiconductors having large band gaps have strong photocatalytic activities. In the present case, Ti02 and ZnO have band gaps larger than 3 e V and show strong activities. CdS and a-Fe20 , having a smaller band gap show less activity since their conduction bands are much lower than that of Ti02. Electron (CB) in these semiconductors cannot move into the electron acceptor in the solution rapidly. It should be noted that the

Table 3- Effect of intensity of light

Initial concentration of the dye: 9 x 10'4M

Irradiation period : 2 h Amount of Ti02 : 250 mg I I 00 mL

Intensity of light ( X I 014 quanta/s)

3.04 4.56

6.08 7.60 9.12 12.16

Photodegradation efficiency,%

64.50 66.38 68.86

74.98 76.48 75.91

0:100 » 1.)

c 90 • ·;:; :: 80 .. .~ .70 ~ -

E! 60 0 · .. ~ ~0 0 s::. 3 4 ~ 6 7 8 9 10 II a..

pH

Figure 3 -Effect of pH on photodegradation efficiency of Acid Green 16

~ 0

>.

~ 80 • ·u ;;: .._ .. ·~ 60. :;:: 0 "0 0 .... go 40 "0 0 -0

s::. a..

20

0~~~====~=---~~~ 0 0.~ 1.0

Time, h

1.~ 2.0

Figure 4- Photodegradation efficiency of Acid Green 16 with various photocatalysis

activity of the photocatalyst is also affected by the particle size, crystallinity and concentration of impurities included in the catalysts. All other photocatalysts cause partial decomposition of the organic molecules in comparable times. However, Ti02

- P25 appears to be the most efficient photocatalyst. The superiority of Ti02 - P25 may be attributed to the morphology of crystallites which was proposed to be one of the most critical properties for the photocatalytic efficiency of P25 among various grades of Ti02 .

Crystallographic study shows that multiples of amorphous, anatase and rutile forms exist. The close proximity of these phases and in some cases the overlapping of forms makes it difficult to differentiate

5fi0 J SC I IND RES VOL 59 JULY 2000

and has been cited to be the reason for long lasting excitation of electrons from the valence band to the conduction band, allowing for efficient and effective degradation of organic compounds22

•

E./feet of Bubbling Oxygen and Nitrogen

The effects of bubbling oxygen and nitrogen through the aqueous suspension containing 9 x I 0-4 M dye solution with 250 mg/ I 00 mL catalyst in I h irn~diation are presented in Table 4. It is observed that photodegradation efficiency severely retarded by bubbling pure nitrogen but the photodegradation efficiency increased rapidly with increasing oxygen flow. Dissolved oxygen in the reaction solution played an important role by trapping the conduction band electrons forming superoxide ions con and thus delayed the electron-hole pairs recombination (02 + e --7 Oz') and at the same time Hz0 2 was formed fro m Oz .. (Ref. 15).

E./feet of H20 2

The effect of H?O? concentration on the photodegradation efficiency of 9 X 10-4 M dye solutioin with 250 mg/100 mL cata lyst in I h irradiat ion is shown in Figure 5. Small amounts of H20 2 (up to 4.4 x I o·2 M) increased the photodegradation efficiency rapidly but when Hz0 2 concentration was larger than 4.4 X 10-2 M, the photodegradation effic iency decreased gradually. H20 2 has been found suitab le for trapping

16-19 b . I b. . f electrons y preventmg t 1e recom tnatton o electron-hole pairs, thus increasing the chances of the formation of ·oH and ot on the surface of TiOz, but when the H20 2 concentration was higher, the amount of ·oH formed on the surface of the TiOz increased rapidly and hence the ann ihilation rates of ·oH and

Table 4- Effect of bubbling oxygen and nitrogen on the

photodegradation efficiency of Acid Green 16

Initial concentration of the dye : 9 x I o·4M Irradiation period: I h Amount ofTi02 : 250 mg I I 00 mL

Gas !low rate (mUmin)

Oxygen Nitrogen

20 30 40

120

40

Photodegradation efficiency, %

60.65 53.72 70.92 89.73

97.60 99.32

·oH were faster than the reaction rates of ·oH and organic contaminants (OH + ·oH --7 1-hOz) . The ·oH radicals annihi lated before the reaction of ·oH with

. . 15 orgamc contammants · .

Effect of FeCIJ

The effect of FeCI , (5x I o·"M) on the photodegradation efficiency of 9 X I o·4 M dye solution with 250 mg/ I 00 mL catalyst in 1 h irradiation is shown in Figure 6. It is found that Fe:~+ behaves as an electron scavenger (Fe3+ + e· --7 Fe2+), thus preventing the recombination of electron - hole pairs. Under the experimental condit ions, the reacti ons (2) and (3) become very significant23

.

cfl-100 >: u

80 c Gl ·;:; ;;: .... 60 Gl c 0 :;::

40 0 , 0 ... ~ 20 , 0 -0 J: 0 Cl.

0 2

... (2)

4 6 8 10 12 14 -2

Concentration of H2o2, X 10 M

Figure 5- Effect of H20 2 on the photodegradation effi ciency of Acid Green 16

0 0'.. 100....---------->. u c

-~ 80 --Ill 60 c:: -~ -0 40 -o 0 .... 01 Ill 20 , 0 -0 J: 0 Cl.

~---·

~--r-~-~~-~----r---r--~~

0 2 4 6 8 10 12 14 -3

Concentration of Fe Cl3, X 10 M

Figure 6- Effect of FeCI3 on the photodegradation efficiency of Ar. id r.r~~n I ()

SAKTHIYEL e1 a /.: Ti02 CATALYSED PHOTODEG RADATION OF LEATHER DY E, AC ID GREEN 16 56 1

Table 5- Effect of Fenton's reagent

Initi al concentration of the dye : 9 X I o·~M Irrad iation period: I h Amount of Ti0 2 : 250 mg I I 00 mL

Concentrati on of Fenton's reagent

il

5x I o·' M FeS04 + 4.4 x I 0'2 M H20 2

1 x 1 o·2 M Feso~ + 8.8 x 1 o·2 M H20 2

1.5x I 0'2 M FeSO~ + 13.2 x I 0-2 M H20 2

Photodegradation

efficiency, % 60.65

100

99.53

94.32

... (3)

These reacti ons increase the formati on of ·o H and H20 2, thus improving the effi c iency of the photocatalyti c process. When Fe3

+ concentrati on was in excess of 5x I o·' M, the photodegradation effi c iency decreased graduall y due to the depos ition of iron on semiconductor parti c les. Probabl y the acti ve sites of the cata lyst are covered and hence the photon absorpti on by the cata lyst decreases.

Effect of Fenton's Reagent

T he photodegradati on effic iency was fo und to increase ( 9 X I 0'

4 M dye so luti on with 250 mg/ I 00 mL in I h irradi ation) rap idly with add iti on of small

amount of Fenton's reagent (5x I o·' M FeS04 + 4.4 x 10-2 M H20 2) (Table 5). The mechani sm of Fenton chemistry can be described by Eq . (4).

.. . (4)

Walling24

simplified the overall Fenton chemi stry by taking into considerati on the di ssoc iation of water [Eq. (5) ].

. . . (5)

Equation (5) indicates the need for an ac id ic environment to produce the max imum amount of hydroxy l radicals25

. When suitable amounts o f Fe2+ and

H20 2 are present the foll owing reacti ons [Eqs (6) and (7) ] become very signi ficant.

. . . (6)

. . . (7)

The ·oH concentration increases at ac idic pH (pH 3.5), thus improving the effici ency of the

h I . 26 21 Wh F , p otocata yttc process · . en en ton s reagent concentration is in excess o f (5x 10-3 M FeS04 + 4.4 x I o·2

M H20 2), the photodegradation effi c iency slow ly decreases as in the case of FeC i) for the same reason.

Conclusion

The study shows effici ent photodegradation of the leather dye Acid Green 16 in the wastewater usi ng Ti02 in aqueous suspension. The complete decolori sation and minerali sation of the dye occur in 30 min and 6 h, respecti vely. The addition of optimum of oxygen, H20 2, FeC i) and Fenton's reagent improve the degradation effi c iency significantly. Photodegrada­tion of the dye is found to be effecti ve in the ac idic pH .

Acknowledgement

T he authors gratefully acknowledge the f inancial support from the All India Council fo r Tec hnica l Education, New De lhi , India, fo r thi s project.

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