Humic Substances Derived from Weathered Coal for Selective Washing of Cd and As Species from Soil

Humic substances (HSs) possess abundant oxygen-containing functional groups that theoretically confer exceptional complexation capacity for heavy metal (HM) remediation; however, their speciation-dependent extractive behaviors across multi-metal contaminated soils remain insufficiently quantified. This study systematically evaluated the washing performance of six humic-based agents derived from weathered coal and lignite on soils contaminated with representative cadmium and arsenic species. Batch sequential washing experiments demonstrated that a four-cycle extraction at a liquid-to-solid ratio of 30:1 achieved high average cumulative removal efficiencies of 92.9% for Cd and 83.0% for As, with weathered coal-derived humic acid (HAW) exhibiting the highest removal efficiency. Microstructural and spectroscopic characterizations indicated that abundant oxygen-containing functional groups and highly condensed aromatic structures served as the core drivers for HM complexation. Furthermore, the XGBoost-SHAP model accurately predicted Cd leaching efficiency, identifying washing times as the most influential variable, followed by contaminant speciation, leaching agent type, and L/S ratio. The substantially lower performance for As suggests that arsenic leaching is governed by additional physicochemical factors beyond the four operational variables, highlighting the need for speciation-related descriptors in future modeling efforts. Overall, this work demonstrates the technical feasibility of converting low-rank coal resources into high-value, sustainable soil washing agents, offering robust data-driven insights for the targeted design of site remediation strategies.