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

Electrochemical advanced oxidation processes (EAOPs), particularly those based on either cathodic electrogeneration of H₂O₂ or anodic oxidation (AO) via ^•OH, 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.

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, and a DSA-RuO₂ or BDD anode at constant cell voltage (2–12 V). Due to the complexity of the matrix, a larger H₂O₂accumulation in the acid effluents was observed. DSA favoured the accumulation of CIO₂^– ions, whereas BDD allowed their further transformation into CIO₃^– owing to the larger oxidizing power of its physisorbed ^•OH. 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.

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. © 2014 Society of Chemical Industry