Transformation of surface chemistry and microstructure in hydrochar under alkaline soil conditions: Impact on Pb(II) adsorption

The increasing application of hydrochar (HC) for soil amendment and contaminant immobilization necessitates a comprehensive understanding of its long‑term aging behavior under natural conditions. In this study, the transformation of HC in weakly alkaline soil was investigated over a 12‑month period. The aged HC exhibited substantial structural changes, with specific surface area, pore volume, and pore size increasing by 288%, 28%, and 122%, respectively. These modifications were mainly attributed to the leaching of labile organic matter, minerals, and surface debris, which was consistent with observed declines in electrical conductivity and hemicellulose content. Boehm titration, X‑ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) analyses revealed significant changes in oxygen‑containing functional groups (OCGs), including a decrease in carboxyl group acidity from 1.38 mmol·g⁻¹ to 0.46–0.49 mmol·g⁻¹, an increase in lactone acidity from 0.24 mmol·g⁻¹ to 0.59–0.60 mmol·g⁻¹, and a consequent reduction in total acidity from 1.92 mmol·g⁻¹ to 1.36–1.39 mmol·g⁻¹. Aging time was identified as the dominant driver of these physicochemical transformations. Despite the decline in total acidity, the enhanced porosity resulted in an approximately twofold increase in the Pb(II) adsorption capacity of HC. These findings provide valuable insights for the application of HC in sustainable soil remediation.