Dynamic redox-regulation of layered Fe-Cr(III) hydroxides for heavy metals capture: Exploring sustainable utilization of Fe-Cr residues

[Display omitted] •GROX-Cr(III) show stable structure under dynamic Fe(II)/Fe(III) cycling.•Enhanced Cd(II) and Zn(II) adsorption was achieved via Fe redox manipulation.•The heavy metals adsorption is tunable via redox regulation.•Dynamic redox regulation is a feasible strategy for Fe-rich waste treatment. In this study, unique Fe(III)-Cr(III) hydroxides with layered structure (denoted as GROX-Cr(III)) was formed after the reaction between Cr(VI) and green rusts, exhibiting intriguing redox properties under typical environmental conditions. The redox state of iron in GROX-Cr(III) was modulated using sodium dithionite to enhance the capture of Cd(II) and Zn(II). The adsorption kinetics were rapid, with both Cd(II) and Zn(II) reaching equilibrium within just 5 min. The adsorption capacity, as determined by Langmuir fitting, escalated from 76.92 mg/g to 298.51 mg/g for Cd(II) and from 14.55 mg/g to 167.22 mg/g for Zn(II) as the Fe(II)/Fetotal ratio in GROX-Cr(III) rose from 0 % to 100 %. The performance of the fully reduced GROX-Cr(III)-100 % (Fe(II)/Fetotal = 100 %) was found to be stable across a broad pH range and in the presence of multiple heavy metals. It is noteworthy that desorption of Cd(II) and Zn(II) (> 95 %) could be regulated by pH. Solid-state characterization and measurements of Fe(II) species have elucidated that GROX-Cr(III) provides a stable layered structure conducive to the regulation and cycling of Fe(II)/Fe(III) redox processes. These interesting findings may serve as a valuable reference for devising innovative waste-treating-waste strategies for the immobilization or recovery of heavy metals under fluctuating redox conditions.