Low-Dose CaO₂ Enhanced Arsenite Coagulation via Elevating Solution pH and Persistently Oxidizing As(III) into As(V)

As a low-cost and commonly used technology, coagulation usually encounters two difficulties in disposing arsenite (As(III)): (1) the removal efficiency is much lower than that of arsenate (As(V)), and (2) acidic conditions are quite detrimental to As(III) removal. Calcium peroxide (CaO2) possesses dual functions of oxidizing pollutant and elevating solution pH. A CaO2-enhanced coagulation method was thus constructed in this study, aiming to develop a versatile coagulation strategy to treat As(III) pollution. Results showed that low-dose CaO2 (20 mg/L) significantly improved As(III) coagulation, and cocoagulation with Ti(SO4)(2) (10 mg Ti/L) achieved above 90% removal (initial As(III) was 1.0 mg/L) at pH 4.0-7.0. The oxidation function of CaO2 played the dominant role, whereas the pH-elevation mechanism worked a more prominent role with the increase of the CaO2 dose. The CaO2 with long-lasting oxidizing ability persistently converted As(III) into As(V), thus prompting the quick sequestration of resultant As(V) by coagulation reaction. The dried flocs could be further reused as As(III) adsorbent and reached a maximum adsorption capacity of 345.3 mg/g Ti (pH 5.0). CaO2-enhanced coagulation was a universal As(III) pollution control method with high efficiency and operational simplicity. For practical wastewater treatment, the dose of coagulant and CaO2 should be further optimized according to As(III) concentration, coagulant type, and water quality parameters.