Removal of chemical oxygen demand and ammonia nitrogen from lead smelting wastewater with high salts content using electrochemical oxidation combined with coagulation–flocculation treatment

Electrochemical oxidation (EO) is a versatile treatment method; however, it has not been used yet for the removal of chemical oxygen demand (COD) and ammonia nitrogen (NH4+-N) wastewater produced from the lead smelting or other smelting operations. In current research work, an attempt was made to apply EO combined with coagulation–flocculation (CF) mechanism for the removal of COD and NH4+-N from lead smelting wastewater containing high salts by using low-cost graphite anode. Systematic studies based on laboratory experiments were conducted to evaluate COD and NH4+-N removal, and the removal mechanism was discussed. Results showed that, current density was the most influential factor among the three factors studied including current density, initial pH and Cl− concentration. Both the COD and NH4+-N were removed completely from the wastewater with increasing current density and electrolysis time. The morphology of graphene stripped out after electrolysis was also studied. Total organic carbon and excitation–emission matrix fluorescence analysis proved that the reduction of COD was mainly due to the degradation of most of the organic matter in wastewater to carbon dioxide. The total nitrogen analysis means that the reduction of NH4+-N was mainly due to the degradation of NH4+-N to nitrogen. From overall results, it was concluded that COD reduction was the result of the synergistic effect of the direct oxidation of organic matter on the anode plate and the indirect oxidation of active chlorine in solution, while the reduction of NH4+-N was the indirect oxidation of the active chlorine in solution. As the low-cost graphite electrode was used as anode, thus the current technology avoided the use of costly boron-doped diamond and metal oxide (eg. RuO2, IrO2 and PbO2) electrodes and achieved the fixed investment cost of electrode plates lower and more advantageous in industrial applications. Laboratory-scale test treatment cost was 9.28 US$/m3.