Sodium citrate-modification enhanced Fe₃S₄ for Cr(Ⅵ) removal from aqueous solution and soil

Fe₃S₄ has been widely employed to remove Cr(Ⅵ) from wastewater, however, its practical effectiveness is often limited by agglomeration and passivation. This study introduces sodium citrate (SC) as a ligand to synthesize an Fe₃S₄-SC magnetic micro-crystal for Cr(Ⅵ) removal from aqueous solutions and contaminated soils. Experimental results show that Fe₃S₄-SC exhibits superior Cr(Ⅵ) removal efficiency, especially in acidic environments, with a maximum adsorption capacity of 449.12 mg/g. When Fe₃S₄-SC was used to remediate Cr(Ⅵ)-contaminated soil with a Cr(Ⅵ) content of 664.98 mg/kg and a TCLP-Cr(Ⅵ) concentration of 26.57 mg/L, the removal efficiencies of Cr(Ⅵ) and TCLP-Cr(Ⅵ) were 99.29% and 98.52% after 60 days. Cr speciation shifted from exchangeable fraction and weak acid-soluble fraction to more stable species bound to Fe-Mn oxides and residual fraction. Cr(Ⅵ) removal was primarily facilitated by surface Fe(Ⅱ), dissolved Fe(Ⅱ), and surface S(-Ⅱ). Surface S(-Ⅱ) provided electrons to Fe(Ⅲ), facilitating Fe(Ⅱ) regeneration for the continuous reduction of Cr(Ⅵ). The SC ligand enhanced material dispersion and stability, promoted Fe(Ⅱ) dissolution, reduced passivation layer formation, and improved electron transfer efficiency, thus increasing the efficacy of Fe₃S₄-SC in Cr(Ⅵ) removal. These findings provide a valuable reference for effectively remediating Cr(Ⅵ) contamination in wastewater and soil.