Removal of high-risk organic recalcitrants in reverse osmosis concentrate by solar/ZnO: performance, mechanism, and application

As reverse osmosis (RO) is widely applied in wastewater reclamation, developing green and efficient technologies for persistent high-risk organic contaminants in RO concentrate (ROC) is crucial for aquatic ecosystem safety and water supply security. Solar-driven ZnO catalytic oxidation was demonstrated to form highly reactive hydroxyl radical (HO • ), selective superoxide radical (O 2 •- ), and valence band holes (h vb + ) for depollution. Among six typical organic contaminants frequently detected in ROC, obvious degradation was observed for sulfamethoxazole (SMX), atrazine (ATZ), imidacloprid (IMD), and methylisothiazolinone (MIT), but the removal of halogenated iopamidol (IPA) and sodium diatrizoate (DTZ) was retarded during solar/ZnO treatment. Compared with traditional TiO 2 photocatalyst, abundant reactive species were readily generated on the ZnO surface and gradually diffused into surrounding solution. The degradation rates of target contaminants exhibited a positive correlation with ZnO dosage, and were affected by solution pH in multiple aspects. Specifically, strong acidic conditions were detrimental to ZnO material stability, while alkaline environments hindered sufficient particle dispersion in water. Common inorganic anions (e.g., HCO 3 − and NO 2 − ) and humic acid usually suppressed the solar/ZnO photocatalytic oxidation of above organic contaminants to varying degrees through competing for reactive species and/or adsorption sites. The sequential solar/ZnO oxidation-ZVAl reduction process was effective for actual ROC during chemical oxygen demand, total nitrogen, total phosphorus loadings and persistent high-risk organic contaminants burdens control. Transformation pathways of six typical contaminants were proposed and acute/chronic ecotoxicity variation were evaluated. This work offers a feasible technology and application guidance for ROC treatment, though appropriate dilution may still be required before discharge.