Synergistic effect of magnetic magnetite and greigite nanoparticles dispersed pinewood biochar for aqueous lead(II) and cadmium(II) adsorption

Heavy metals such as lead and cadmium cause adverse effects on all living organisms. Their remediation is complex in the aqueous phase. Biochar is a low-cost, environmentally friendly adsorbent material that exhibited a limited ability to adsorb Pb²⁺ and Cd²⁺. Iron-based magnetite and greigite nanoparticles have proven high adsorption capacity due to high amounts of oxygen and sulfur-contained functional groups per unit volume. In this study, a mixture of greigite and magnetite nanoparticles was simultaneously synthesized on pinewood biochar (BC), aiding co-precipitation from a Fe²⁺/Fe³⁺ and S^2- salts mixture (Fe₃O₄-Fe₃S₄/BC), that offers a cost-effective, sustainable, and efficient material for Pb²⁺ and Cd²⁺ removal. BC, Fe₃O₄-Fe₃S₄/BC, and Pb²⁺ and Cd²⁺ adsorbed Fe₃O₄-Fe₃S₄/BC materials were characterized to differentiate surface morphologies, elemental compositions, and surface chemical states. The Fe₃O₄-Fe₃S₄/BC composite exhibited an average nanoparticle diameter of ∼20 nm. Fe₃O₄-Fe₃S₄/BC showed Langmuir adsorption capacities of 138.9 and 49.5 mg g^-1 for Pb²⁺ and Cd²⁺ at pH 5 (25 °C). Pb²⁺ and Cd²⁺ followed pseudo-second-order kinetics, and the equilibriums were achieved after ∼2 h and ∼30 min, respectively, for 125 and 1250 μmol L^-1 concentrations, respectively, where the fast adsorption rates make Fe₃O₄-Fe₃S₄/BC hybrid system a practical option for real-time treatment. Further, adsorption performances were influenced by ionic strength, dose optimization, and the presence of competing ions, suggesting the potential for fine-tuning adsorbent conditions in practical applications. This study enhanced the understanding of adsorption characteristics for a treatment facility that can effectively remediate lead and cadmium-contaminated wastewater. While there have been studies on using biochar and nanoparticles separately for Pb²⁺ and Cd²⁺ adsorption, this research bridges the gap by demonstrating the synergistic effect for Pb²⁺ and Cd²⁺ adsorption from hybrid magnetite-greigite nanoparticles.