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

- Central composite design is helpful to find coagulation-precipitation mechanisms.
- The three roles of dissolved Ca in CaO for CrT removal were discovered.
- Ca2+ can neutralize negative charge, weakening the solubility of Cr(OH)3 in alkali.
- Soluble complex formed by surplus SO32−-Cr(Ⅲ) has a negative effect on CrT removal.

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 CrT removal, other verification experiments were performed. The CrT 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 CrT 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 CrT removal through adsorption bridging and co-precipitation with CaO as the core of flocs.