Indian Journal of Engineering & Materials Sciences Vol. 11, December 2004, pp. 499-504

Decolourisation of metal complex azo dyes and treatment of a dyehouse waste by modified photo-Fenton (UV-vis/ferrioxalate/H2O2) process Pooja Tripathi & Malay Chaudhuri Department of Civil Engineering, Indian Institute of Technology, Kanpur 208 016, India Received 1 March 2004; accepted 5 August 2004 In a laboratory study, decolourisation of three metal complex azo dyes (Acidol Yellow, Acidol Grey and Acidol Scarlet) in mixture by the modified photo-Fenton (UV-vis/ferrioxalate/H2O2) process, and treatment of a dyehouse waste containing the three dyes were examined. From decolourisation of the dye mixture, process conditions for decolourisation were selected – pH 4, Fe(III) dose 12.5 mg/L, oxalic acid dose 50 mg/L, hydrogen peroxide dose 60 mg/L and irradiation time 90 min. Ninety-two percent decolourisation of the dye mixture (colour 120 SU) occurred in 90 min under irradiation with sunlight intensity in the range 0.5-0.8 kW/m2. The process was found effective in decolourisation of highly coloured solutions. Modified photo-Fenton decolourisation of the synthetic dyehouse waste (pH 7.5, colour 120 SU, COD 260 mg O2/L, BOD5 125 mg/L, turbidity 1.5 NTU, conductivity 1265 μmho/cm and total solids 875 mg/L) under selected process conditions and irradiation with sunlight or incandescent lamp, was also studied. Ninety-two percent decolourisation (residual colour ca 10 SU) was achieved under 90 min irradiation with sunlight (0.5-0.8 kW/m2) or incandescent lamp (6.5 kW/m2). A depth of 30 cm was found to be the maximum depth for decolourisation. The study has demonstrated that modified photo-Fenton is an effective process for decolourisation of dyehouse waste containing metal complex azo dyes. A scheme for oxidative treatment of the dyehouse waste was formulated and the treatment efficiency was evaluated – 90-92% or 90-92%, 69-70% or 63-73%, and 52-65% or 50-60% removal of colour, COD and BOD5 were achieved under irradiation with sunlight or incandescent lamp. IPC Code: Int. Cl.7 C09B 57/10

The textile dyehouse waste is usually very complex and variable in characteristics, and hence most conventional methods of treatment are inadequate. Advanced oxidation processes (AOPs) are considered to be more effective than conventional processes in the treatment of textile dyehouse waste. There have been a number of studies of decolourisation/degradation of dyes and treatment of dyehouse waste and textile mill effluent using AOPs – decolourisation/degradation of dyes by vis/TiO21, vis/Fe3+/H2O22 and UV/H2O2/ ultrasonication3; decolourisation/degradation of textile dyes by sunlight/TiO2 or sunlight/ZnO4, UV/Fe3+/2+ or UV/H2O2 or UV-vis/TiO25 and UV/ferrioxalate/ H2O26,7; decolourisation/degradation of textile azo dyes by UV/SnO2/TiO28, vis/TiO29-11, sunlight/TiO211, UV/TiO210-13, vis/ZnO11, UV/ZnO11,13, UV/H2O214 and sonolysis-photocatalysis (UV/TiO2)15; and treatment of dyehouse waste and textile mill effluent by UV/H2O216, UV/ZnO17, UV/TiO217,18 and UV/TiO2/ Fe3+/H2O219. Decolourisation/degradation of a metal complex azo dyehouse waste by sunlight/TiO2 or sunlight/ZnO has been recently investigated20,21. However, rather long time (six and three hours) were

required for decolourisation/degradation. Titanium dioxide (Ebg = 3.2 eV) and zinc oxide (Ebg = 3.2 eV) absorb light up to λ ≅ 385 nm22, whereas ferrioxalate absorbs light strongly at longer wavelengths (up to λ ≅ 550 nm) and generates hydroxyl radicals with high quantum yield23. Therefore, efficiency of oxidative decolourisation/degradation mediated by ferrioxalate under irradiation with sunlight (λ ≥ 320 nm) ⎯ the modified photo-Fenton (UV-vis/ferrioxalate/H2O2) process ⎯ is expectantly higher than that by titanium dioxide or zinc oxide under irradiation with sunlight. A simplified mechanism for the modified photoFenton (UV-vis/ferrioxalate/H2O2) process is outlined24. The ferrioxalate complex, FeIII(C2O4)33-, is highly photosensitive and reduction of Fe(III) to Fe(II), through a photoinduced ligand to metal charge transfer, can occur over the ultraviolet and into the visible (out to λ ≅ 550 nm): FeIII(C2O4)33- + hν → Fe2+ + 2C2O42- + C2O4•C2O4•- → CO2•- + CO2 CO2•- + FeIII(C2O4)33- → Fe2 + CO2 + 3C2O42-

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The reactions can be collapsed into one reaction, since the short lifetime of the oxyl radical, C2O4•-, should preclude it from participation in other reactions, and its decarboxylation product, CO2•-, is not involved in any other significant reactions: FeIII(C2O4)33- + hν → Fe2+ + CO2 + 2.5C2O42There are no other significant photochemical reactions (e.g. H2O2 photolysis) because the molar extinction coefficients of the reactants are such that ferrioxalate is the predominant absorber. The Fe2+ produced then generates hydroxyl radical, •OH, via the Fenton reaction: 2

Fe + H2O2 +

3C2O42-

•

→ Fe (C2O4)3 + OH + OH III

3-

-

In the presence of a sufficient excess of oxalate, Fe(III) will coordinate with either two or three oxalate ligands. As with the photo-Fenton reaction, iron cycles between oxidation states and so the production of hydroxyl radicals is limited only by the availability of light, H2O2 and oxalate, the latter two of which are depleted during the reaction. The present study examined decolourisation of three metal complex azo dyes in mixture and oxidative treatment of a synthetic dyehouse waste containing the three dyes, by modified photo-Fenton (UVvis/ferrioxalate/H2O2) process under irradiation with sunlight (λ ≥ 320 nm) or incandescent lamp (λ = 350750 nm). Materials and Methods Dyes, dye mixture and dyehouse waste

Three chromium complex azo dyes, Acidol Yellow FB M-3RL, Acidol Grey FB M-BRL and Acidol Scarlet FB M-L, used in a local woollen textile mill for khaki shade, and Uniperol FB SE used as a levelling agent, were obtained from M/s Supreme Dyes and Chemicals, Kanpur. The dye bath contained a mixture of the three dyes and three levelling agents (Uniperol FB SE, ammonium sulphate and acetic acid). Dye mixture was prepared by mixing the dyes with tap water in proportion to their amounts in the dye bath (Acidol Yellow:Acidol Grey:Acidol Scarlet = 9.6:7.3:1.0) and diluted with tap water to match the colour of the spent dye bath (dyehouse waste). Logistics determined the use of a synthetic dyehouse waste based on the characteristics of the actual dyehouse waste produced in the woollen textile mill. The dye

bath was prepared as per the method followed in the woollen textile mill and diluted with tap water to match the colour of the spent dye bath (dyehouse waste). The physical and chemical characteristics of the synthetic dyehouse waste were determined by methods outlined in the Standard Methods25, except for colour which was measured by the method outlined below. The characteristics of the synthetic dyehouse waste were: pH 7.5, colour 120 SU, chemical oxygen demand (COD) 260 mg O2/L, 5-day biochemical oxygen demand (BOD5) 125 mg/L; turbidity 1.5 NTU, conductivity 1265 μmho/cm and total solids 875 mg/L. Measurement of colour

Colour of the dye mixture and dyehouse waste were measured in space units (SU)26. The absorbance of the dye mixture or dyehouse waste (filtered through a 0.45-μm membrane filter) was measured in the wavelength range 400-700 nm with an interval of 1 nm and the area under the spectrum was calculated by the trapezoidal method. The area represented the colour of the sample in SU. Irradiation source

Sunlight and a 200 W incandescent bulb were used as irradiation sources. Light intensity during the experiment was measured with a radiometer (Model PSP, Eppley Laboratory Inc., Newport, RI, USA). Decolourisation of dye mixture

Two hundred and fifty millilitres of the dye mixture were taken in a 500 mL borosilicate glass beaker, its pH adjusted with sulphuric acid or sodium hydroxide to a predetermined value, and different preselected dosages of Fe(III), oxalic acid and hydrogen peroxide were added. The dye mixture was kept stirred by a magnetic stirrer to ensure a homogenous mixture and aeration, and kept under the irradiation source. Aliquots were withdrawn at 30 min intervals and filtered through a 0.45 μm membrane filter for measurement of colour. Based on the pattern of decolourisation, optimum process conditions (pH, dosages of Fe(III), oxalic acid and hydrogen peroxide, and irradiation time) for decolourisation were selected. Decolourisation of dyehouse waste

Two hundred and fifty millilitres of the synthetic dyehouse waste were taken in a 500 mL borosilicate glass beaker and kept stirred by a magnetic stirrer to

TRIPATHI & CHAUDHURI: DECOLOURISATION OF METAL COMPLEX AZO DYES

ensure a homogenous mixture and aeration, and subjected to decolourisation under selected process conditions and irradiation with sunlight or incandescent lamp. Aliquots were withdrawn at 30 min intervals and filtered through a 0.45 μm membrane filter for measurement of colour. To study the effect of depth on decolourisation, sufficient volume of the synthetic dyehouse waste was placed in 80 mm ID cylindrical borosilicate glass vessels to achieve different depths (15, 30 or 50 cm), and subjected to decolourisation under selected process conditions and irradiation with sunlight or incandescent lamp. Results and Discussion Decolourisation of dye mixture

To determine the optimum pH and irradiation time for decolourisation of the dye mixture, a test was conducted with excess dosages of Fe(III) [40 mg/L], oxalic acid (H2C2O4) [275 mg/L] and hydrogen peroxide (H2O2) [1500 mg/L], and at different pH (2-6). Fig. 1 shows that rapid decolourisation occurred in about 30 min and decolourisation remained constant after 90 min. The residual colour at pH 2, 3 and 4 were almost similar, but the residual colour at pH 5 and 6 were higher. At pH 2-4, the maximum colour removal was 92% in 90 min. All subsequent tests were conducted at pH 4 and with 90 min irradiation. The dependence of decolourisation on Fe(III) dosage was studied by employing different dosages of Fe(III) [5.0, 6.5, 8.0, 10.5, 12.5 and 40.0 mg/L] with 275 mg/L H2C2O4 and 1500 mg/L H2O2 (Fig. 2). Decolourisation rate increased with increasing Fe(III) dose and maximum decolourisation (90-92%) oc-

Fig. 1⎯ Effect of pH and irradiation time on decolourisation of dye mixture

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curred with Fe(III) dosages of 10.5, 12.5 and 40 mg/L. To determine the effect of oxalic acid dosage on decolourisation, Fe(III) and H2O2 dosages were held at 12.5 and 1500 mg/L, and H2C2O4 dose was varied (20-275 mg/L). The results presented in Fig. 3 show that residual colour profiles were similar for H2C2O4 dosages of 50-275 mg/L. Nansheng et al.27 have shown that photolysis of Fe(III)-OH complexes (photo-Fenton reaction) can promote degradation of dye molecules and decolourisation of dye solutions. In order to distinguish the effect of Fe(III)-OH complexes on decolourisation of the dye mixture, a control test was conducted without H2C2O4 and only 40% decolourisation occurred in 90 min (Fig. 3), indicating the effectiveness of ferrioxalate complexes in decolourisation of the dye mixture. Fig. 4 shows decolourisation of the dye mixture at

Fig. 2⎯Effect of Fe(III) dosage on decolourisation of dye mixture

Fig. 3⎯Effect of oxalic acid dosage on decolourisation of dye mixture

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To examine the effect of dye concentration on decolourisation, dye mixture of different concentrations (42, 85 and 170 mg/L) were irradiated under selected process conditions (Fig. 5). Ninety-one percent decolourisation (residual colour 10 SU) occurred in 90 min for all dye concentrations, indicating the effectiveness of ferrioxalate complexes in decolourisation of even strongly coloured solutions.

To study the effectiveness of sunlight (λ ≥ 320 nm) vis-à-vis incandescent lamp (λ = 350-750 nm) in decolourisation by the modified photo-Fenton process, decolourisation of the synthetic dyehouse waste was studied under selected process conditions and irradiation with sunlight or incandescent lamp (Fig. 6). Residual colour profiles were similar and 92% decolourisation (residual colour 10 SU) occurred under irradiation with either sunlight or incandescent lamp. Figs 7 and 8 show the effect of depth on decolourisation. Under sunlight irradiation, 92%, 92% and 83% decolourisation occurred for 15, 30 and 50 cm depth, respectively, whereas under incandescent lamp irradiation, 91%, 91% and 65% decolourisation occurred for 15, 30 and 50 cm depth, respectively. Therefore, 30 cm was the maximum depth for decolourisation of the dyehouse waste by the modified photo-Fenton process.

Decolourisation of dyehouse waste

Treatment scheme and evaluation of treatment efficiency

Fig. 4⎯Effect of hydrogen peroxide dosage on decolourisation of dye mixture

Fig. 6⎯Decolourisation of dyehouse waste under irradiation with sunlight or incandescent lamp

Fig. 5⎯Effect of dye concentration on decolourisation

Fig. 7⎯Effect of depth on decolourisaton of dyehouse waste under irradiation with sunlight

pH 4 with Fe(III) and H2C2O4 dosage of 12.5 and 50 mg/L, and H2O2 dosages of 30-1500 mg/L. Ninetytwo percent decolourisation occurred in 90 min with H2O2 dosages of 60-1500 mg/L. The selected process conditions for decolourisation of the dye mixture were pH 4, Fe(III) dose 12.5 mg/L, H2C2O4 dose 50 mg/L, H2O2 dose 60 mg/L and irradiation time 90 min.

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dyehouse waste was subjected to batch treatment in triplicate according to the scheme in a laboratory model under selected process conditions and irradiation with sunlight or incandescent lamp. The effluent quality was: colour 10-12 SU (90-92% removal), COD 77-80 mg O2/L (69-70% removal) and BOD5 44-60 mg/L (52-65% removal) for sunlight irradiation, and colour 10-12 SU (90-92% removal), COD 70-95 mg O2/L (63-73% removal) and BOD5 50-62 mg/L (50-60% removal) for incandescent lamp irradiation (Table 1).

A scheme for oxidative treatment of the dyehouse waste is shown in Fig. 9. The selected process conditions were: pH (stirred reaction tank) 4; Fe(III) dose 12.5 mg/L; oxalic acid dose 50 mg/L; hydrogen peroxide dose 60 mg/L; irradiation time 90 min; pH (aeration tank) 7.5-8.0; aeration time 15 min; and settling time 2 h.

Conclusions Modified photo-Fenton (UV-vis/ferrioxalate/H2O2) is an effective process for decolourisation of metal complex azo dyes and treatment of dyehouse waste. High (90-92%) decolourisation is achieved under irradiation with sunlight or incandescent lamp. The process is useful for treatment of dyehouse waste in tropical and equatorial regions where sunlight is abundant. In the absence of sunlight, irradiation with incandescent lamp is equally effective. References

Fig. 8⎯Effect of depth on decolourisation of dyehouse waste under irradiation with incandescent lamp

To evaluate the treatment efficiency, the synthetic

Fig. 9⎯Scheme for oxidative treatment of dyehouse waste Table 1—Evaluation of treatment efficiency Irradiation source

Sunlight

Incandescent lamp

Parameter Colour, SU COD, mg O2/L BOD5, mg/L Colour, SU COD, mg O2/L BOD5, mg/L

Influent

Effluent

Removal, %

120 260 125 120 260 125

10-12 77-80 44-60 10-12 70-95 50-62

90-92 69-70 52-65 90-92 63-73 50-60

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