Ozone-mediated carboxylation: a strategy for enhanced organic matter removal via in-situ crystallization of hardness ion

Under increasingly stringent environmental regulations, the high concentrations of inorganic ions and organic contaminants in reverse osmosis concentrate pose a serious challenge to achieving zero discharge of wastewater. To address this issue, a hybrid crystallization and ozonation (HCO) process was developed for simultaneous removal of hardness and organics from reverse osmosis concentrate. The positive interplay between ozone and Sodium hydroxide (NaOH) dosage exhibited remarkable performance, achieving removal efficiencies of 59.7 % for chemical oxygen demand (COD), 60.6 % for UV₂₅₄ absorbance, 80.2 % for calcium (Ca), and 99.8 % for magnesium (Mg). The reduction in fluorescent and functional groups indicated effective removal of aromatic substances and oxygen-containing groups. Furthermore, molecular-level transformation of organic compounds showed saturation of compounds and diminution of oxidizing carbon by HCO. Visualization of molecular formula distribution indicated higher oxidation capacity of HCO compared to ozonation as evidenced by lower double-bond equivalents (DBE) value and increased carboxyl groups in the products. Crystals analyses identified calcite (CaCO₃) and brucite (Mg(OH)₂) as dominant phases, with surface-adsorbed organic carbon qualified via carbon species analysis, demonstrating effective co-precipitation and adsorption of organic matters. Consequently, the beneficial role of increased carboxyl groups and the significant influence of Mg crystals for organics removal were demonstrated. The presence of hydroxyl and chlorine radicals (•OH, •O₂⁻, •Cl, •Cl₂⁻) were further confirmed, contributing to the advanced oxidation of organic substances. These findings elucidated the mechanism by which HCO facilitates the simultaneous removal of organics and hardness ions through a synergistic effect involving ozone-mediated carboxylation and ion crystallization. This study offers valuable insights for achieving efficient co-removal of organics and hardness within a streamlined treatment process.