Electro-decolorization for whitening and purification of crystallized salts from high-salinity industrial concentrates

Within the context of near-zero liquid discharge (NZLD) of industrial wastewater, wide application of membrane separation technologies leads to inevitable production of high-salinity concentrates. During the subsequent evaporative crystallization process, recalcitrant colored organics exert adverse impact to the quality of resulting crystallized salts. To address this issue, we proposed electro-decolorization (EDC) for whitening and purification of crystallized salts from high-salinity industrial concentrates, practically those derived from high-pressure reverse osmosis concentrate (HPROC) and nanofiltration concentrate (NFC). The EDC based on Ti/RuO₂-IrO₂ anode exhibited remarkable COD removal of 78.64% for HPROC and 53.72% for NFC, thereby enhancing the whitening and purification of crystallized salts. The economic analysis revealed the viability of EDC with operational costs of $4.96/m³ for HPROC and $10.84/m³ for NFC, accounting for cost saving of 72.3% and 41.6%, respectively. The organic transformation at the molecular level showed a significant reduction in specific aromatic compounds, humic acid-like substances, dissolved microbial by-products, lignins, and tannins, especially the chromophoric organics mainly characterized by π-conjugated structures. The chlorine radical-mediated oxidation had major contribution to degradation of chromophoric components, leading to enhanced decolorization of concentrates, and subsequent recovery of whiter and purer crystallized salts. This study provides a practically feasible EDC strategy for treatment of high-salinity concentrates and reclamation of high value-added crystallized salts, thereby enhancing the effectiveness, cost-efficiency, and sustainability of industrial wastewater treatment and resource utilization.