Mechanistic insights into the role of lignin units in chromium adsorption on low-temperature pyrolyzed biochar.

Biochar synthesis typically demands high pyrolysis temperatures, incurring substantial energy penalties. Therefore, elucidating how pyrolysis temperature drives structural evolution and dictates adsorption performance is highly desirable. In this work, we demonstrated that a low-temperature pyrolysis regime (300°C) coupled with sodium hydroxide pre-treatment successfully fabricates a superior bamboo-derived biochar, yielding a competitive Cr(VI) adsorption capacity of 39.68 mg/g. Comprehensive mechanistic insights via kinetic modeling and multi-spectroscopic characterizations revealed that the remediation process is dictated by a multi-step chemisorption pathway, including electrostatic interactions, surface complexation, and chemical reduction. Uniquely, GC-MS and FTIR analyses unveiled the decisive role of specific lignin units-particularly Syringyl (S-type) monomers selectively retained under this low-temperature regime-as the key functional active sites. This finding was further validated by Density Functional Theory (DFT) calculations, which demonstrated that the oxygen-containing functional groups intrinsic to these lignin units provide a highly favorable thermodynamic pathway for anchoring HCrO4- through stable Cr - O coordination bonds and robust hydrogen-bonding networks. These insights provide a molecular-level understanding of the role of ancestral lignin units during low-temperature biochar activation, validating a commercially viable and low-carbon roadmap for industrial heavy metal decontamination.