ELECTROCHEMICAL TREATMENT OF TEXTILE DYE WASTE

Abstract

Textile factories such as Rift Valley Textile Limited (RIVATEX) in Eldoret Kenya produce large volumes of wastewater which is alkaline, highly coloured and has high chemical oxygen demand (COD) and can adversely affect human health and aquatic life. Two electrochemical oxidation methods using boron doped diamond (BDD) and stainless steel (SS) anode were investigated for the treatment of such wastewaters. Two dyes widely used at RIVATEX namely; C.I. Reactive Red 76 (RR) and C.I. Disperse Blue 79 (DB) were used as model dyes and the parameters studied were:- applied voltage, dye concentration, supporting electrolyte, current density, surface area to volume ratio, inter-electrode distance, pH and temperature. Treated water was then analyzed for pH, colour, electrical conductivity and chemical oxygen demand. Scanning electron microscopy-energy dispersive x-ray spectroscopy (SEM/EDX) was carried out to characterize SS electrode while UV-VIS was used to study the degradation of dye molecules. The optimum conditions obtained using SS were; potential difference of 10 V, S/V ratio of 16 m2/m3 in RR and 20 m2/m3 in DB, NaCl supporting electrolyte 4g/l and 6g/l in RR and DB respectively, electrode gap of 12 mm and a pH of 6.5. It was found that increasing current density led to a reduction in treatment time and a reduction in COD while increasing temperatures led to reduction in specific energy consumption as well as electrocoagulation time. It was found that DB had a lower specific energy consumption compared to RR. Electrocoagulation with SS anode produced Fe (III) hydroxide coagulant resulting from oxidation of Fe. Dye molecules were destabilized and formed “flocs” which were adsorbed on the surface of coagulant. The optimum conditions for BDD were; pH 8, temperatures below 40 °C, dye concentrations below 10 mg/L, electrode gap of 12 mm, current density of 300 Am-2, 4g/l NaCl (RR) and 2g/l Na2SO4 (DB). Addition of NaCl reduced both degradation time and specific energy more than Na2SO4. Increasing current density enhanced degradation and reduced treatment time by 50 % while high temperatures reduced degradation efficiency. Increasing dye concentration led to increased specific energy consumption and treatment time. RR required lower specific energy and less time for complete mineralization than DB. SEM/EDX studies indicated that the SS electrode was chromic steel that underwent surface corrosion during electrocoagulation process. UV-VIS spectroscopy indicated that decolourization was achieved completely with SS and BDD electrodes. Hydroxyl radicals were developed on the BDD surface and degraded the dye molecule by oxidizing the organic molecule and shifting the absorption spectra from the VIS region. NaCl electrolyte was found to be superior to Na2SO4 in degradation efficiency as high COD removal of 84 – 94 % was obtained at 30 – 60 Am-2 current densities. Treatment of RIVATEX wastewaters required 180 min for 92 % COD reduction; however colour removal was achieved in 20 min. The key parameters in degradation were; pH, supporting electrolyte, current density and temperature. It was concluded that RIVATEX can adopt and use combined textile wastewater treatment technologies for colour removal followed by organic degradation. iv