Experimental Study and DFT Calculation Analysis of Pb(II) and Cd(II) Adsorption from Water by MgCl2-Modified Kiwifruit Pomace Biochar

MgCl2-modified kiwifruit pomace biochar (MgBC) was synthesized and applied for Pb(II) and Cd(II) removal from water. The material exhibited a mesoporous structure (71.88 m2/g) and achieved high adsorption capacities of 604.70 mg/g for Pb(II) and 304.98 mg/g for Cd(II), with Langmuir maxima of 763.82 and 532.80 mg/g, respectively. In binary-metal systems, MgBC showed preferential Pb(II) uptake, and Cd(II) coexistence unexpectedly enhanced Pb(II) adsorption. Density functional theory (DFT) and molecular dynamics (MD) simulations revealed three adsorption configurations: direct surface bonding (MgBC-Pb/Cd), hydroxyl-mediated interactions (MgBC-OPb/OCd) and defect-site occupation (MgBCPb/Cd). Adsorption energies followed MgBC-Pb (−2.93 kcal/mol) < MgBC-OPb (−2.57 kcal/mol) < MgBC-Cd (−1.09 kcal/mol) < MgBC-OCd (−0.08 kcal/mol) ≪ MgBCCd (298 kcal/mol) and MgBCPb (408 kcal/mol), which confirms chemisorption by direct and hydroxyl-assisted pathways as the dominant mechanisms with Pb(II) being thermodynamically more favorable. Projected density of states (PDOS) analyses corroborated the adsorption capacity: MgBC-Pb > MgBC-OPb > MgBC-Cd > MgBC-OCd ≫ MgBCPb and MgBCCd. This work provides atomic-scale mechanistic insights and a sustainable strategy for converting agricultural waste into an effective biochar adsorbent for heavy metal removal.