Treatment of cellulose bleaching effluents and their filtration permeates by anodic oxidation with H2O2 production

BACKGROUND: Electrochemical advanced oxidation processes (EAOPs), particularly those based on either cathodic electrogeneration of H2O2 or anodic oxidation (AO) via (OH)-O-center dot, have become attractive technological options for the complete detoxification of wastewaters. Their integration with separation pre-treatment such as ultrafiltration (UF), nanofiltration (NF) or reverse osmosis (RO) may be a plausible way to reduce processing time and costs. RESULTS: Raw effluents from the acid and alkaline elemental chlorine free bleaching stages of a hardwood-based kraft pulp mill, as well as their UF, NF and RO permeates, have been characterized and then treated by AO-H2O2 in a tank reactor with an air-diffusion cathode, which allowed efficient reduction of O-2,O- and a DSA-RuO2 or BDD anode at constant cell voltage (2-12V). Due to the complexity of the matrix, a larger H2O2 accumulation in the acid effluents was observed. DSA favoured the accumulation of ClO2- ions, whereas BDD allowed their further transformation into ClO3- owing to the larger oxidizing power of its physisorbed (OH)-O-center dot. The contribution of this species, along with H2O2 and active chlorine, accounted for the significant TOC abatement reached in the different individual and coupled treatments. CONCLUSIONS: AO-H2O2 and UF/AO-H2O2 yielded similar mineralization levels (65-68%) for acid effluents, but lower energy consumption (EC) resulted in the latter process due to the lower conductivity of the permeates. NF/AO-H2O2 coupling yielded the largest mineralization of alkaline effluents (96% instead of 75% obtained by AO-H2O2) with low EC. (c) 2014 Society of Chemical Industry