Ind ian Journal of Engi neering & Mate ri als Sciences Vol. 10. February 2003, pp. 69-74

Solar photocatalytic degradation of metal complex azo dyes and treatment of dye house waste

Ami! Kumar Saha & Malay Chaudhuri Department of C ivi l Engi neering, Indi an In stitute of Technology, Kanpur 20S 016, Indi a

Received /6 April 200 /; revised received 29 August 2002

In a laboratory study, the effec ti veness of zinc oxide-med iated solar photocata lytic degrada ti on o f three metal compkx azo dyes (Ac idol Ye llow, Ac idol Grey and Acido l Scarlet) and treatment of a woollen tex tile dye house waste were exam­ined. A 53-S6% degradati on of the dyes was achieved by 15 min illumination wi th a sunli ght in tensity of 0 .5-0.S kW/ Il12

and zinc ox ide dose of 1.5 giL. A 67-93% decrease in degrada tion rate was observed for degradatio n of the dyes in mi xture: (simultaneous degradati on). It was, however, observed that pre-adsorption was not a necessary prerequi s ite for de:g r<.Jdatio n. Treatment of the dye hOllse waste containing the three dyes, with zinc ox ide (3.0 giL), reduced colour (S9 %) and COD (63 %) in 2.75 h illuminati on. BOD51COD ratio increased from 0.43 to 0.66. The study has de monstrated that zinc ox ide­mediated solar photocatalyt ic degradation is a potentially lIseful method of treatment of dye ho use waste conta ining metal complex azo dyes.

Dye ing is one of the important processes in a textile industry which produces the spent dye bath as waste. The dye house waste contains small amounts of dye with compl ex composition, which is aesthetically ob­jectionable and often tox ic to the aquatic life. There is no uni versal method for the removal of colour from dye waste. As the charac teri stics of the dye waste are variable, different phys ical, chemical and biological methods have been employed for its treatment. Re­cently , it has been demonstrated that semiconductor photocatalytic degradation of organic substances can be an alternati ve to conventional methods for the re­mova l of organi c pollutants from water and airl. An add itional advantage of the photocatalytic process is its mild operating conditions and the fact that the semiconductor can be acti vated by sunlight (near UV), thus reducing significantly the electric power requirement and hence the operating cost2

.

A simpli fied mechani sm for semiconductor photo­catalytic degradation of organic substances is out­lined ' . Certain semiconductors, notably zinc ox ide and titanium dioxide are known to be photosensiti sers or photocatalysts. Illumination of these oxides by photons having an energy level that exceeds their band gap energy level, exc ites electrons from the va­lence band to the conduction band and holes are pro­duced in the valence band. The photogene rated va­lence band holes react with ei ther water or hydroxyl iO lls wh ich are adsorbed on the catalyst surface to

generate hydroxy l radicals which are stro ng ox idants. The photogenerated electrons in the conducti on band may react with oxygen to form superox ide ions. The superoxide ions can then react with water to provide hydrogen perox ide and hydroxy l ions. Cleavag.e or hydrogen peroxide by the conduction band electrons yields further hydroxy l radicals and hyd roxy l ions. The hydroxy l ions can then react with the va lence band holes to form additi onal hydroxy l radica ls. Deg­radation of organic substances can be achieved by their reaction with the hydroxy l radicals or by their direct attack from the valence band holes . Recombi­nation of the photogenerated electrons and holes may also occur and indeed it has been suggested that prc­adsorption of substrate (organic substance) onto the photocatalyst is a prerequi site for hi ghl y efficient deg­radation.

Titanium dioxide-mediated photocatalys is or dye I · 4-8 II '1 d ~ - P I so ut!ons as we as text! e ye waste - were stue-

ied. A recent study l3 has indicated that long illumina­tion time (ca. 12 h) is necessary for deco louri sati on of a dye house waste containing metal complex azo dyes when titanium di ox ide is used as the semi co nductor and sunli ght as the source of photons. Korman n el

al.14 and Hoffman e l 01.1 5 observed hi gher quantum yields of hydrogen perox ide fo rmation for zinc oxide versus titanium dioxide and showed that lower steady state concentration of perox ide for titanium clioxide ari ses from a decreased quantum yield of peroxide

70 INDIAN J. ENG. M ATER. SCI., FEBRUARY 2003

formation combined with an enhanced degradati ve quantum yield. Poulios and Aetopoulou l6 showed that zinc ox ide degraded the azo dye, Reactive Orange 16 within 15 min when exposed to UV radiati on. In this context, it appeared appropriate to examine the effec­ti veness of zinc oxide as a photocatalyst for decol­ourisation and treatment of dye house waste contain­ing metal complex azo dyes . The present study was undertaken to examine zinc ox ide-mediated solar photocatalytic degradation of three metal complex azo dyes and its effectiveness in the treatment of a wool­len textile dye house waste containing the dyes.

Materials and Methods

Dyes, levelling agent and dye house waste

Three chromium complex azo dyes, Acidol Yel­low, Ac idol Grey and Acidol Scarlet, used in a local woo ll en textile mill for khaki shade, and Uniperol used as a levelling agent were obtai ned from Mis Su­preme Dyes and Chemicals, Kanpur. The dye bath contained a mixture of the three dyes and three dyeing additives (levelling agents). Composition of the dye bath is shown in Table 1. Logistics determined the use of a sy ntheti c dye house waste based on the charac­teristics of the actual dye house waste produced in the woollen textil e mill. The dye bath was prepared as per the method fo llowed in the woollen text ile mill and diluted with tap water to match the colour of the spent dye bath (dye house waste). The physical and chemi­cal characteristics of the synthetic dye house waste were determined by methods outlined in the Standard Methods l 7 except for colour which was measured by the method outlined below. The characteri sti cs of the synthetic dye house waste are shown in Table 2.

Zinc oxide

Laboratory reagent grade zinc ox ide was obtained from the Ranbaxy Laboratori es Limited, Chandigarh.

The particle size is 0.5-1.0 /lm and the BET surface area is 5.35 m2/g (determined by N2 adsorption method) .

Measurement of dye concentration and colour or dye house waste

Concentration of dyes in pure state and in mixture or concentrat ion of the dye mi xture were determined by measuring the absorbance at the wave leng th of maximum absorbance (Acidol Yellow: 275 11m; Aci­dol Grey: 575 nm ; Acidol Scarlet: 495 nm; dye mi x­ture: 445 nm) by an UV-visible spectrophotometer (Cary 50 Conc, Varian). Colour of the dye house waste was measured in space units (SU)I S. The waste was filtered through a 0.45 /lm membrane fi Iter before measuring the absorbance by an UV-v isibl e spectro­photometer. The absorbance of the fi ltrate was meas­ured in the wavelength range 200-700 11m wit h an interval of I nm and the area under the spectrum was calculated by the trapezoidal method. The area repre­sented the colour of the sample in SUo

Sunlight intensi ty

Sunlight intensity was measured in kW/m2 by a Windmonitor WM 250 (Envirotech Instruments Pv\. Ltd ., New Delhi ).

Table 2-Characterist ics o f syntheti c dye ilnuse waste

Parameter

Total solids

Tota l dissolved solids

Total suspended sol ids

Conducti vity

pH

Turbidity

Chemical oxygen demand (COD)

5-day biochemica l oxygen demand (BOD5)

Colour

Va lue

795 mg/L

790 mg/L

51llg/L

1120 pilliln/cill

7.37

0.05 NTU

262 Illg O,/L

1121llg/L

340 SU

Table I-Composi tion of dye bath

Commercia l name

Acido l Yellow FB M -3RL

Ac idol Grey FB M -BRL

Acidol Scarlet FB M -L

Wool: water - I : 15.

Dye

Co lour index code

Acid Ye llow 194

Acid Green 104 Ac id Blue 193

Acid Red 357

Amount for 100 kg of woo l

0.5 kg

0.38 kg

0.052 kg

Levelling agent

Agent Amount for 100 kg of wno l

Uniperol FB SE 0.75 kg

A mmonium sulphate 5.0 kg

Acetic acid 1.0 kg

SAHA & CHAUDHURI: SOLAR PHOTOCATALYTIC DEGRADATION OF METAL COMPLEX AZO DY ES 71

Degradation of metal complex azo dyes

To study zi nc oxide-mediated photocatalytic deg­radation , concentration of a dye or dye mixture was chosen in the range as it appeared in the dye house waste. In each case, one ' light control' (without zi nc oxide) was used. Two hundred millilitres of a 10-50 mg/L dye solution or dye mixture were taken in a 500 mL Borosil glass beaker to which zi nc oxide was added and kept in suspension by constant stilTing and exposed to sunlight. A 20 mL aliquot was withdrawn at specifi ed time intervals, filtered through a 0.45 /1m membrane filter and the concentration of the dye or the dye mixture in the filtrate was measured.

Treatment of dye house waste

Two hundred millilitres of dye house waste were taken in a 500 mL Borosil glass beaker to which zinc oxide was added and kept in suspension by constant stirring, and exposed to sunli ght. A 20 mL aliquot was withdrawn at specified time intervals and filtered through a 0.45 /1m membrane filter. The colour of the filtrate was measured, and the COD and BODs of the filtrate were determined. To study the reusability of zinc ox ide, 100 mL of dye house waste were taken in a 500 mL Borosil glass beaker wi th a predetermined dose of zinc oxide and exposed to sunli ght for a speci­fied duration with constant st irring. The suspension was fi ltered through a 0.45 /1m membrane filter ancl the colour of the filtrate was measured. The residual zinc oxide on the filter was reused by washing the filter with another 100 mL of dye house waste and exposed to sun light. This cycle was repeated for six times.

Results and Discussion

Degradation of dyes in pure state and in mixture

Many previous researchers used a dark adsorption peri od of 15-90 min before illumi nation for photo­catal ytic degradation of dyes6.7 .1 6.19 and chloroorgan­ics20

.23 to achi eve sufficient solid-phase concentration

of substrate for preventing the recombination reaction. In the present study, in all init ial experiments on photocatalytic degradation of the metal complex azo dyes, 30 min clark adsorption was used before illumi­nat i ng the semi conductor (zi nc ox ide)-dye suspension with sunli ght.

In a background experiment, degradation of Acidol Yellow under varying zinc oxide dose (0.2-3.0 giL) was examined. An initi al dye concentrati on of 20 mg/L was used (typical concentration of the dye in the dye house waste) . Maxim um dye degradat ion was

achieved with a zinc ox ide dose of 1.5 gi L in IS min. A marginal degradation (ca . 6%) of dye occurred by sunli ght exposure only. A zinc oxide dose of 1.5 giL was used in degradation of the three dyes in the sub­sequen t experiments. Degradation of Ac idol Yell ow. Acidol Grey and Acidol Scarlet (initial dye co ncen­tration 20 mg/L) are shown in Fig. I. Initi al and final pH of the dye solutions were 6.6, 6.3 and 6.5, and 6.8. 6.7 and 6.9, respectively . Parallel experiments were conducted with initi al dye concentration in the range 10-50 mg/L (data not shown). A 63-86% degradati on of Acidol Yellow, 53-85 % degradation of Acido l Grey and 61-85% degradation of Acido l Scarlet oc­curred in IS min. The importance of substrate (dye) pre-adsorption on a gi ven photocatalyst can be probed by the use of a Langmuir-Hinshelwood (L-H ) kineti c model24 modified to accommodate reactions occurri ng at the solid-liquid interface. This model assumes that (i) at eq uilibrium, the number of surface adsorpti on sites are fixed; (ii) on ly one substrate may bind at each surface site; (i ii ) the heat of adsorpti on by the substrate is identical for each site and is independent

20 ~----,------------------------------

16 :::: 00 E C 12

.~ E C 8 ::: g u ~

Sunlight intensity: 0.5-0.8 kW/m2

• Aeidol Ye llow

o Acitiol Grry

• Aridul SC:lrlct

O)'e cOile: 20 mglL Zinc ox ide: I.S giL

•

o L---~------~------~----~------_

OJ>

~ .: E

" ..::

2.0

1.5

1.0

U.S

U.O

o

r--rl 30-m in dark adsorp tion

e- e

U.OO 0.02

8

Time, min

~.

() . U~ 0.06

IIC, Llmg

12 1(,

• Al.:ilJul Ydlu', (.. Acidul G IT)

O.OM 0. 10 11.12

Fi g. I-Degradalion of Ac ido l Yellow. Acidol (i rt!)' and r\ cid,.1 Searlel

72 INDIAN J. ENG. MAT ER. SCI. , FEBRUARY 2003

of surface coverage; (i v) there is no interaction be­tween adjacent adsorbed molecules; (v) the rate of surface adsorption of the substrate is greater than any subsequent reac tions; and (vi ) no irreversible blocking of ac tive sites. With these assumptions, the surface coverage (8) is related to the initi al concentration of substrate (e) and to the apparent adsorption equilib­riu m constant (K) by 8 = Kc/( I + KC) and the rate of product formation can be written as a single­component L-H kinetic rate expression ro = -dC/dt = k,Ke /( I + Ke), where kr is the apparent reaction rate constant occurring at the acti ve site on the photocata­lyst surface. The linearity of a plot of lira versus lie tests the validity of the L-H model, where Ilk, is the y intercept and I/krK is the slope. A plot of lira versus I Ie (Fig. I) shows a linear relati onship, indicating degradation in accordance with the L-H model. Two ex treme situations may arise in definin g surface cov­erage (8) of the photocatalys t parti cle (zinc oxide): (i) both the substrate (dye) and the solvent (water) com­pete fo r the same acti ve sites, and (i i) both the sub­strate and the solvent are adsorbed on the surface

..J

'"" E

is

" .... z; <J

= 0 v

12

10

8

6

.j

I

30-lIIill d a rk adsorptio n

Sunlight intensity : 0.8-0.9 kW/m' 0 Acidol Scarlet

Dye mu ture : 20 mglL Zinc oxide: I.S giL

4 8

Time, min

o Acidol Grey

• Acidol Yellow

o

12 16

25 ,-----------------------------------.

o Acidol Scarlet o Acidol Grey • Acidol Yellow

0.0 0.2 0.4 0.6 0.8 1.0

l ie, Umg

Fig. 2-Degrada ti on of dye in mi xture

without competing for the same acti ve sites. The L-H model cannot discriminate between these two ex tre me situations25

. Accordingly, no attempt was made to determine the value of K and k, as they afford no clue as to which situation prevail s in the so lid-liqu id reac­tions without characterization of the adsorpti on iso­therm of the system and without the know ledge of the value of the adsorption coefficient of the solve nt (water) .

Degradati on of dye in mi xture (Ac ido l Yel low: Acidol Grey: Acidol Scarlet = 9.6 : 7.3 : I, the pro­portion in the dye bath) , also indicated that the degra­dati on was according to the L-H model (Fig. 2). For comparing the rates of degradati on in pure state and in mi xture, a concentration of 20 mg/L of each dye and dye mi xture was considered (Table 3). It is clear that initi al degradation rate decreased by 67-93 % in the mi xture (simultaneous degradati on). [t is also ob­served that degradation of Acidol Yellow was most rapid foll owed by Acidol Grey and Acidol Scarl et. To ascertain the effec t of sunlight intensity, an experi­ment was conducted in a cloudy day (sunlight inten­sity 0.07 kW/m2). A 45 min illumination was neces­sary to achieve degradation similar to that achi eved by 15 min illumination with sunli ght intensi ty of 0.5-0 .8 kW/m2 .

It has been argued that it is not possible to disti n­gui sh between the di stinctly possib le pathways or degradation reac tion between the substrate and photo­generated oxidants occurs whi Ie both the spec ies are adsorbed, with an adsorbed substrate and a free oxi­dant, with a bound ox idant and a free substrate, or with both the oxidant and substrate free ly d i sso l ved ~(' . Plausibly, photocatalytic degradati on of dyes occurs through a combination of these pathways. To ascer­tain whether pre-adsorption of the dye was a pre­requi site for photocatalyti c degradation of the dyes, degradation of the dye mi xture by direct illuminati on and by illumination after 30 min dark adsorpti on were studied (Fig. 3). There was no significant change In

Table 3-Initi al degrada tion rate or dyes

Initi al degradati on rate (r.,) , mg/L.m in Dye Pure" In mi xture h

Acidol Ye llow 1.31 88 0.4398

Ac idol Grey 0.4078 0. 1204

Acidol Scarlet 0. 6049 0.0427

"Initia l co ncentra ti on 20 mg/L. b Initi al conce nt rat ion 20 Ill )!/ L

(Ac idol Ye llow: Acido l Gre y : Ac ido l Scarle t = 9.6 : 7.3: I ).

SAHA & C HAUDHURI: SOLAR PHOTOCATALYTIC DEGRADATION OF METAL COMPLEX AZO DYES n

20 ~.----------------------------------~

....l 15 OL E c

.S E 10

~ c

d 5

Sunlight intensit y: 0.8-0.9 kW/m'

Dye mixture : 20 mg/L Zinc oxide: 1.5 giL

• Illumination after 30-min dark adsorption o Direct illumination

o I----~--~----~--~--~----~--~--~

o 4 !l 12 16 20 24 28 32

Time, min

Fig. 3--Degradati on o f dye mi xture

350

300

250 ::> (Jj

200 .: " 0 150 :; U

100

50

0 I

0 50 100 150 200 250 300

Time, min

Sunlight intensity: O.7~O. 9 kW/ml

Zillc oxide

• 1.0 giL o 2.0 gIL • 3.0 giL v S.O giL

280

240

::: 200 N

0 160 "" E

Q 120 0

80 u

~o

0 --~ ----0 50 100 150 200 250 300

Time, min

Fig. 4--Solar pholocatalytic treatment of dye house waste

the rate of degradation of the dye mixture. So, for so­lar photocatalytic treatment of the dye house waste, the pre-adsorpti on (dark adsorption) step was elimi­nated.

Treatment of dye house waste

Treatment of the dye house waste with zinc oxide dose of 1.0-5.0 giL is shown in Fig. 4. It shows that

T able 4--Repetiti ve use of zinc ox ide

Cycle Colour reducti on. 'ii

I 90

2 g8

3 84

4 77

5 70

6 68

82% colour reduction occurred in 2.75 h with zinc oxide dose of 3.0 giL. The final colour of the dye waste was 60 SUo The tap water showed a 25 SU background and hence with reference to the tap water. the colour reduction would be 89%. In terms ur ADMI colour value ' 7, the colour reduction was l)H.5 <k - initial and final colour of the dye house waste 1600 and 25, respectively. The final COD of the dye house waste was 98 mg 0 2/L - a 63 % reduction. Presuma­bly , partial minerali sation of the dyes and other chemicals occurred. The BOD5 of the treated dye house waste was 65 mglL , indicating an increase in the BODslCOD ratio from 0.43 to 0.66 and that pho­tocatalytic degradati on products were more biode­gradable.

The dye house waste contained a leve lling age nt. Uniperol, a detergent. The detergent may form mi ­celle around zinc oxide particles, preventing photons to strike zinc oxide and, as a result , production of hy­droxyl radicals would decrease. Uniperol also absorbs UY radiation. Further, it produces foam which may hinder diffusion of atmospheric oxygen, causing less production of superox ide ions. These factors together with the simultaneous presence of the three dyes. were possibly responsible for a much slower degrada­tion of the dye house waste.

Table 4 shows the results of repetitive use of zinc oxide (six cycles) for colour reducti on of the dye house waste. A 24% reduction in the effi ciency of zinc oxide occUlTed when it was used six times. but 84% colour reduction was achieved in the third cyc le.

A scheme for batch treatment of the dye house waste is proposed. The dye house waste from 2 days of operation is collected in an equali sati on tank and subjected to solar photocatalyti c treatment for :1 h us­ing zinc oxide dose of 3.0 giL. The tank is provided with suitable mixing arrangement to keep zinc oxide in suspension. Thereafter, zinc ox ide particles are al­lowed to settle for 2 h and the supernatant is di sposed of. The settled zi nc ox ide is reused for photocatalyt ic treatment of the nex t batch of the dye house waste.

74 INDIAN J. ENG. MATER. SCI., FEBRUARY 2003

Conclusions

The study has demonstrated that zinc oxide­mediated so lar photocatalytic degradation is effective in the decolouri sation of metal complex azo dyes and is a potentially useful method of treatment of dye house waste containing metal complex azo dyes pro­ducing a low-colour and low-COD effluent. A study with zinc oxide and hydrogen peroxide (so­larlZnO/H10 2) should be conducted so as to reduce the zinc ox ide requirement and/or for more efficient degradation of the dye house waste.

References I Olli s D F, Peli zzetti E & Serpone N, £lIl'iroll Sci Techllol, 25

( 199 1) 1523. 2 Goswami D Y, in Advallces ill solar ellergy, ed ited by Bower

K W (A meri can Solar Energy Inc., Boulder, Colorado), 1995, 165.

3 A I-Ekabi H, Bulters B, Delany D, Holden W, Powell T & Story J, in Pholocalalylic purificalioll alld Irealll1el1l of waleI' alld air, edited by Olli s D F & AI-Ekabi H (El sevier Science Publi shers, Amsterdam), 1990, 719.

4 Liu G, Wu T & Zhao Y G, £lIviroll Sci Techlloi, 33 ( 1999) 2081.

5 Vinodgopal K, Wynkoop D E & Kamat P V, £l1viroll Sci Techllol, 30 ( 1996) 1660.

6 Poulios I & Tsachpini s I , J Chelll Techllol Biolechllol , 74 ( 1999) 349.

7 POlllios I, Avranas A, Rekliti E & Zouboulis A , J Chelll Techlloi Biolechlloi , 75 (2000) 205.

8 Tanaka K, Padennpole K & Hi sanaga T , WaleI' Res, 34 (2000) 425.

9 Davis R J, Gainer] L, O'Neal G & Wu I. W(I{l'r EIII 'in"l Res, 66 ( 1994) SO.

10 Balcioglll G R & Arslan I, £ lI vi ro ll Techllol , 18 ( 1<)<) 7) 1053. II Pera lta-Zamora P, M oraes S G. Pelgrini R. Friere M Jr ..

Reyes J, M ansilla H & Duran N, Chelllo.l'ph('/'I'. 36 ( 1l)<)X ) 2119.

12 Li X Z & Zhao Y G, Water Sci Tl'cI/lllJl . 39 ( 19<)<) ) 2~l).

13 Mishra S R, All ill veslig(l{ioll of .1·l'llI icll llt/UCliJ r phll/()('{/Ill­iylic oxidarioll for Ihe Ire(l{llIelll of lI 'oailell le.rliie d."('\ \'(/.1'1 1' .

M. Tech. Thesis, Indian Institute or Techllology. Ka IlJllll·. 2000.

14 KormanIl C, Bahnemann D W & Hoffm~lI1ll M R. EIII 'il'llll Sci Techlloi , 22 ( 1988) 798.

IS Hoffmann A J, CarrJway E R & HolTman n MR. EIII 'iJ'{lIl Sci Techlloi, 28 ( 1994) 776.

16 Poulios I & Aetopoulou I, EIIVi/'01I Techlllli. 20 ( IY<)l) .:17<). 17 APHA, A WWA & WPCF, Slallt/ard Il leli!oi/.I./(Jl' !il l' 1'.l'll/ lli ­

lIalioll of \V(I{er alld Il'a.l' lew(l{er (American Publ ic Hea lth Assoc iation, Washington DC), 1995 .

18 Gregor K H, in Chell/ieal oxidalioll lechll ll log iesj(JI' lIill e/in . edited by Eckenfielder W W, Bower A R & Roth J A (Tech­nomic Publi shing Co. , Inc .. Pennsy lvenia). 1992. 161.

19 Wu T , Liu T , Zhao J, Hidaka H & Serpone N. ElIl'iJ'{lIl Sci Techlloi, 33 ( 1999) 1379.

20 D'Oliveira J-C, AI -Sayyed G & Pichat P. Elll'il'llll Sci Tn/I-110 1, 24 (1990) 990.

21 Kyriacou G, Tzaanas K & Poulios I. J EilI'imll Sci Heailh A. 32 (1997) 963.

22 Poulios I, Korsitzi M & Kouras A. J Pho/()chelll Ph%/Jiol A: Chem, 115 ( 1998) 175.

23 EI-Morsi T M , Budakowsk i W R, Abd-EI-A ziz A S & Fri e-sen K J, EIIVil'01I Sci Teellllol , 34 (2000) W i g.

24 Fox M A & Dulay M T, Chelll Rev, 93 ( 1993) 34 1. 25 AI-Ekabi H & Serpone N, J Phys Chelli . 92 ( 1988) 57'26. 26 Turchi C S & Ollis D F, J Calai, 122 ( 1990) 178.