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

Low-temperature pyrolysis (300°C) optimizes the surface reconstruction of bamboo biochar. • ATBC-300 exhibits superior Cr(VI) removal via enriched oxygen groups. • S-type lignin monomers are identified as key drivers for Cr(VI) adsorption. • DFT calculations reveal the synergy of Cr − O bonding and redox reactions. 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, a low-temperature pyrolysis regime (300°C) coupled with sodium hydroxide pre-treatment was demonstrated to successfully fabricates a superior bamboo-derived biochar. Comprehensive mechanistic insights via kinetic modeling and multi-spectroscopic characterizations revealed that the remediation process was dictated by a multi-step chemisorption pathway, including electrostatic interactions, surface complexation, and chemical reduction. GC–MS and FTIR analyses suggested that lignin-derived structural units, particularly Syringyl (S-type) monomers selectively retained under the low-temperature regime—made an important contribution to the formation of active adsorption sites. Density Functional Theory (DFT) calculations further demonstrated that oxygen-containing functional groups associated with these lignin-derived structures provided favorable interaction sites for HCrO 4 − through stable Cr − O coordination and hydrogen-bonding interactions. Moreover, ATBC-300 retained approximately 75% of its initial Cr(VI) removal performance after five adsorption–desorption cycles, indicating acceptable regeneration stability. These findings provide molecular-level insights into the role of native lignin structural units during low-temperature biochar activation and demonstrate the potential of low-temperature pyrolysis as a strategy for Cr(VI) remediation. Download: Download high-res image (148KB) Download: Download full-size image