Sulfidation-engineered magnetic LDHs/Biotite composite: Synergistic adsorption and magnetic recovery for heavy metal decontamination in soil

Abstract

Soil contamination with heavy metals poses a serious threat to ecosystem safety and human health, highlighting the need for efficient adsorbents for remediation. A novel magnetic composite adsorbent was developed in this study through the in-situ growth of layered double hydroxides (LDHs) on biotite followed by sodium dimethyldithiocarbamate (DTC) intercalated (DTC-LDH/Biotite), designed for the efficient removal of Cu(II), Pb(II), As(V), and Cr(VI) from contaminated soil. The successful insertion was confirmed by characterization of C₃H₆NS₂⁻ into the interlayers of LDHs. A three-dimensional porous structure was revealed by SEM with uniformly dispersed LDHs on Biotite surface. Adsorption kinetics followed both the pseudo-second-order model and intraparticle diffusion model, indicating the adsorption process was predominantly controlled by active sites. Isotherm studies demonstrated that Cu(II)/Pb(II) adsorption followed the Freundlich model, indicative of multilayer sorption, while As(V)/Cr(VI) adsorption aligned with the Langmuir model, consistent with monolayer sorption. The maximum adsorption capacities reached 198.7 mg/g (Cu), 211.5 mg/g (Pb), 175.2 mg/g (As), and 163.8 mg/g (Cr). It was revealed by mechanistic analyses that Cu/Pb were immobilized through complexation with C₃H₆NS₂⁻ and with hydroxyl groups to form precipitates, whereas As/Cr were fixed through anion exchange and redox reactions. Soil column tests demonstrated that DTC-LDHs/Biotite effectively intercepted heavy metals within the 10–20 cm soil layer, minimizing vertical migration. The composite enabled efficient magnetic separation, with removal rates for As(V) and Cr(VI) remaining above 82 % after five cycles. This work presents a sustainable strategy for heavy metal remediation, integrating high adsorption efficiency, and recyclability, offering promising applications in soil pollution control.