Biochar Regulates Contrasting Pb2+ and Cd2+ Transport Regimes Through Water-Dependent Partitioning Transitions Under Unsaturated Conditions.

Biochar has been widely applied for immobilization of potentially toxic elements (PTEs) in contaminated soils; however, its effects on metal transport under transient unsaturated conditions remain insufficiently understood. This study investigated the adsorption, desorption, and transport behavior of Pb2+ and Cd2+ in biochar-amended soils by combining batch experiments, unsaturated conditions column experiments, and transport modeling. The adsorption kinetics, isotherms, and desorption characteristics of Pb2+ and Cd2+ were evaluated under different biochar levels, and their redistribution during unsaturated infiltration was further analyzed using soil-water-dependent models. Biochar increased the apparent adsorption capacity of both metals, with a stronger response observed for Pb2+ than Cd2+. The Elovich and two-constant models provided better statistical descriptions of adsorption kinetics than the pseudo-second-order model, indicating that adsorption rate behavior involved heterogeneous and multi-rate processes. Freundlich fitting showed that Pb2+ and Cd2+ exhibited non-ideal adsorption behavior, while desorption experiments demonstrated a stronger reduction in apparent Pb2+ release compared with Cd2+ under biochar. Under transient unsaturated infiltration, Pb2+ and Cd2+ exhibited distinct transport patterns. Pb2+ redistribution was adequately described by a linear partitioning relationship, with R2 values ranging from 0.988 to 0.990, indicating relatively stable solid-liquid partitioning within the tested conditions. In contrast, Cd2+ showed stronger dependence on soil-water conditions, and an empirical nonlinear water-dependent model improved model performance, increasing R2 values from 0.533-0.652 for the linear model to 0.903-0.973. The results suggest that biochar effects on potentially toxic element migration under unsaturated conditions are element-specific, involving both apparent sorption enhancement and water-dependent redistribution processes. This study highlights that biochar performance in potentially toxic elements remediation should not be evaluated solely based on equilibrium adsorption capacity. Under field-relevant unsaturated conditions, amendment effectiveness may also depend on interactions between metal-specific retention behavior and soil hydraulic variability. Further studies integrating structural characterization, long-term aging, and reactive transport modeling are required to validate these processes under heterogeneous field conditions.