Response surface methodology investigation into optimization of the removal condition and mechanism of Cr(Ⅵ) by Na₂SO₃/CaO

The removal of Cr(Ⅵ) by chemical reduction-precipitation is widely applied in wastewater treatment plants. Nevertheless, the formation of Cr(OH)₃ with gel properties has weak settlement performance, making it necessary to add a coagulant aid to reduce the settling time and improve the settling effect. In this investigation, a high concentration of Cr(Ⅵ) was removed using Na₂SO₃ as a reducing agent and CaO as a coagulant. An improved reduction and precipitation experiment was modeled by applying a three-factor central composite experimental design (CCD). To reveal as many mechanisms as possible for Cr_T removal, other verification experiments were performed. The Cr_T removal efficiency decreased, which can be explained by the following three reasons: dissolution of Cr(Ⅲ), competition for adsorption between Ca²⁺ and Cr(Ⅲ) at different coagulation times, and formation of a solubility complex with Cr(Ⅲ) due to the surplus SO₃²⁻ in solution. The increasing Cr_T removal efficiency can be explained by the following two reasons: dissolved Ca²⁺ from CaO can neutralize CrO₂⁻ that is produced by the dissolution of Cr(OH)₃ in alkaline solution and can broaden the optimal final pH range of coagulation. Ca²⁺ could also strengthen the Cr_T removal through adsorption bridging and co-precipitation with CaO as the core of flocs.