Dimercaptosuccinic acid modified lignin rich biochar-hydroxyapatite composite for selective and efficient mercury adsorption: Mechanistic and RSM modeling approach

Industrial hemp stalks (Cannabis sativa), an underutilised agricultural residue, were used to prepare lignin-rich hemp hurd biochar (HHB), which was esterified with dimercaptosuccinic acid (DMSA) to form dithiol-modified biochar (HS-HHB-SH). The aromatic and oxygen-containing groups from lignin enhanced surface reactivity and enabled thiol functionalisation. Hydroxyapatite (HAPs) was grown on HS-HHB-SH to form a composite (HAPs@HS-HHB-SH), where DMSA's hydrogen bonding with HAPs helped prevent phosphate leaching. The composite was characterised using FTIR, XRD, FESEM, BET, and TGA. HAPs@HS-HHB-SH showed outstanding Hg(II) ion adsorption with a distribution coefficient (KD) of 2 × 10⁶ mg L^-1. Optimized via Box-Behnken Response Surface Methodology (RSM-BBD), the composite achieved 99.8 % removal of Hg(II) ions at pH 6, 465 mg L^-1 concentration, and 9-min contact time. Kinetic studies followed pseudo-second-order behavior, while equilibrium data fit the Langmuir isotherm model, indicating monolayer adsorption with a maximum capacity of 769.23 mg g^-1, closely matching the experimental value of 755.96 mg g^-1. The adsorbent retained 84 % efficiency after seven reuse cycles, highlighting its excellent stability and potential for practical wastewater treatment applications.