Quantum chemical DFT-based adsorption mechanism of Pb(II) on a modified biochar

Heavy metals possess biological accumulation and toxicity, significantly endangering human health. Modified biochar adsorption is considered one of the most effective heavy metal removal techniques. In previous studies, explaining the microscopic mechanisms through macroscopic experiments was often difficult. This study employed multiple characterization methods to investigate the elemental composition, graphitization degree, and carbon chain structure of ZnCl₂-modified biochar materials at a microscopic scale, analyzing the structural features of biochar and constructing a three-dimensional molecular structure monomer model of modified biochar. The quantum chemical density functional theory was introduced into the adsorption reaction between Pb²⁺ and biochar, calculating and analyzing the weak interactions during adsorption. The characterization results indicate that the molecular structure of ZnCl₂-modified biochar is predominantly composed of polycyclic aromatic carbons with a molecular formula of C₄₇H₂₃NO₁₂. Activated biochar forms abundant pore structures and contains abundant oxygen-containing functional groups on its pore surfaces. By calculation, it can be obtained that the adsorption reaction mainly occurs near oxygen-containing functional groups with high electron cloud density. Sites with negative charges and high charge numbers drive the adsorption of Pb²⁺ on the biochar surface, where weak ionic bonds, van der Waals forces, and cation -π interactions collectively promote Pb²⁺ adsorption on the biochar surface. Through analyzing simulated data, it was found that weak ionic bonding of aldehyde groups and van der Waals interactions of aromatic rings can synergistically enhance adsorption performance; the maximum adsorption energy under synergistic action is 119.25 kJ/mol. This study provides theoretical support for the practical application of modified biochar in removing heavy metal ions from soil.