Highly efficient removal of Cr(VI) from wastewater using electronegative SA/EGCG@Ti/SA/PVDF sandwich membrane

The use of green, non-toxic raw materials is of great significance to the sustainable development of the environment, among which epigallocatechin gallate (EGCG) is a renewable carbon source from plants. At present, there is a lack of research on the metal-polyphenol nanomaterials their use in water decontamination. In this study, a novel SA/EGCG@Ti/SA/PVDF (SESP) sandwich membrane was prepared to effectively solve the problems of difficult recovery of nanomaterials and the leaching of metal ions. The membrane was made by scraping SA on the surface of the PVDF substrate as the bottom protective layer, depositing EGCG@Ti NPs as the functional layer, then coating SA as the surface isolation layer, and finally cross-linking with anhydrous calcium chloride. Results showed that EGCG@Ti NPs dispersed well on the surface of the SA/PVDF basement membrane. SESP sandwich membrane had good hydrothermal and acid-base stability, and it can be applied to wastewater with multiple co-existing heavy metals (e.g., Cu, Pb, Cd, and Ni). The contact angle and pure water flux of the SESP sandwich membrane with a negatively charged surface were 14.0-15.6° and 171.40 L/m² h, respectively. The pure water flux of the regenerated membrane after BSA pollution recovered to 98.68 L/m² h, and the interception efficiency and the interception flux of Cr(VI) were 100 % and 72.92 L/m² h at 40 min of interception, respectively. Additionally, the removal efficiency of Cr(VI) by SESP sandwich membrane was maintained above 83 % for simulated wastewater and 100 % for actual wastewater after five adsorption-desorption cycles. Cr(VI) and Cr(III) can be removed simultaneously with the negatively charged SESP sandwich membrane. EDS and XPS analysis showed that the removal of Cr(VI) was controlled by the Donnan effect, anion exchange, chelation/complexation, and reduction mechanism. In contrast, Cr(III) was mainly influenced by electrostatic attraction and chelation/complexation mechanisms. In conclusion, the newly prepared sandwich membrane has good application potential in treating Cr(VI) wastewater.