Laboratory studies of electrochemical treatment of industrial azo dye effluent

Removal of color and reduction of chemical oxygen demand (COD) in an industrial azo dye effluent containing chiefly reactive dyes were investigated under single-pass conditions at a dimensionally stable anode (DSA) in a thin electrochemical flow reactor at different current densities, flow rates, and dilutions. With 50% diluted effluent, decolorization was achieved up to 85-99% at 10-40 mA/cm(2) at 5 mL/min flow rate and 50-88% at 30-40 mA/cm(2) at high (10-15 mL/min) flow rates. The COD reduction wasmaximum (81%) at39.9 mA/cm(2) or above when solution-electrode contact time (C-t) was as high as 21.7 s/cm(2) and decreased as C-t declined at a given current density. Cyclic voltammetric studies suggesting an indirect oxidation of dye molecules over the anode surface were carried out at a glassy carbon electrode. The effect of pH on decolorization and COD reduction was determined. An electrochemical mechanism mediated by OCl- operating in the decolorization and COD reduction processes was suggested. The effluent was further treated with NaOCl. The oxidized products from the treated effluents were isolated and confirmed to be free from chlorine-substituted products by IR spectroscopy. From the apparent pseudo-first-order rate data, the second-order rate coefficients were evaluated to be 2.9 M-1 s(-1) at 5 mL/min, 76.2 M-1 s(-1) at 10 mL/min, and 156.1 M-1 s(-1) at 15 mL/min min for color removal, and 1.19 M-1 s(-1) at 5 mL/min, 1.79 M-1 s(-1) at 10 mL/min, and 3.57 M-1 s(-1) at 15 mL/min for COD reduction. Field studies were also carried out with a pilot-scale cell at the source of effluent generation of different plants corresponding to the industry. Decolorization was achieved to about 94-99% with azo dye effluents at 0.7-1.0 L/min flow costing around Indian Rupees 0.02-0.04 per liter, and to about 54-75% in other related effluents at 0.3-1.0 L/min flow under single-pass conditions.