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 Pb 2+ and Cd 2+ . 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 Fe 2+ /Fe 3+ and S 2- salts mixture (Fe 3 O 4 -Fe 3 S 4 /BC), that offers a cost-effective, sustainable, and efficient material for Pb 2+ and Cd 2+ removal. BC, Fe 3 O 4 -Fe 3 S 4 /BC, and Pb 2+ and Cd 2+ adsorbed Fe 3 O 4 -Fe 3 S 4 /BC materials were characterized to differentiate surface morphologies, elemental compositions, and surface chemical states. The Fe 3 O 4 -Fe 3 S 4 /BC composite exhibited an average nanoparticle diameter of ∼20 nm. Fe 3 O 4 -Fe 3 S 4 /BC showed Langmuir adsorption capacities of 138.9 and 49.5 mg g -1 for Pb 2+ and Cd 2+ at pH 5 (25 °C). Pb 2+ and Cd 2+ 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 Fe 3 O 4 -Fe 3 S 4 /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 Pb 2+ and Cd 2+ adsorption, this research bridges the gap by demonstrating the synergistic effect for Pb 2+ and Cd 2+ adsorption from hybrid magnetite-greigite nanoparticles. • Greigite and magnetite nanoparticles were deposited using the co-precipitation method. • XPS aided in a detailed mechanistic understanding of Pb 2+ and Cd 2+ chemisorption on Fe 3 O 4 -Fe 3 S 4 /BC. • Pb 2+ and Cd 2+ adsorption capacities were 138.9 and 49.5 mg g -1 , respectively, at 25 °C (pH 5). • The competitiveness of Pb 2+ and Cd 2+ was investigated at different initial concentrations. • Solution effects and scale-up studies were performed during the Pb 2+ and Cd 2+ removal.