Treatment of tannery effluent by chemical coagulation combined with batch-recirculated electro-oxidation at different anode materials.

The aim of this work was to evaluate the pollutant load from tannery effluents treated by chemical coagulation (CC) followed by electro-oxidation (EO), performed in two different experimental batch-recirculated setups, one with a BDD anode and the other with Ti/Pt/PbO2 and Ti/Pt/SnO2-Sb2O4 anodes (PS). Results were compared with those obtained from EO of the raw sample. CC was performed with a Fe3+ concentration of 0.25 g L-1, and the applied current densities for EO in each setup were 60 mA cm-2 for BDD and, in the PS setup, 20 and 40 mA cm-2 for Ti/Pt/SnO2-Sb2O4 and Ti/Pt/PbO2, respectively. During CC, removals of 27% in chemical oxygen demand (COD), 14% in total nitrogen, 100% in sulfide, and 73% in Cr(VI) were observed. COD removal in the EO of the raw sample was higher than that obtained for the combined CC + EO, for both setups, showing that the organic compounds removed by CC are mainly those that would be more easily removed by EO. For most of the other parameters related with carbon and nitrogen, the removals for CC + EO were higher than for EO alone. During EO, sulfide is converted to sulfate, especially with BDD. Concerning Cr(VI) concentration, it increases during EO, in particular for PS setup. Combined treatment, with both setups, proved to be an effective choice to treat tannery effluents.