Green strategy with high iron utilization for Cr(VI) removal via sodium polyacrylate-based hydrogel

A green strategy involving photocatalysis unit (PU) and adsorption unit (AU) using sodium polyacrylate hydrogel (PAAS) was proposed to achieve high iron utilization in efficient Cr(VI) removal. In PU, the novel process of light-controlled sustained release (LCSR) led to the increased utilization of Fe(II). Meanwhile in AU, the highly selective recovery of Fe(III) was achieved due to the larger complexation rate. Specifically, the LCSR process in PU was implemented by PAAS loaded with Fe(III) (PAAS-Fe), which generated Fe(II) to reduce Cr(VI) via ligand–metal charge transfer (LMCT) reaction. And the utilization of Fe(II) was up to 95.30%, meanwhile 45.85% of generated Fe(III) participated in the coordination with PAAS to maintain LMCT reaction. The treatment capacity via LCSR process in fluidizer reached about 14,400 and 13000 mL/g for simulated and real effluent, respectively. In AU, the selective separation factor of Fe(III) was 42.75 and iron recovery ratio was 98.01%. Furthermore, PAAS in AU loaded with Fe(III) could be reused as photocatalyst by switching the light. The chemicals cost of the strategy was reduced by 93.90%-98.25% compared with other reductants. In general, PU-AU strategy is efficient and eco-friendly for Cr(VI) removal, which seeks a sustainable way with lower chemical consumption.