Pyrolysis temperature-driven structural evolution of wheat straw biochar and enhanced adsorption mechanisms for Cr(vi)

Wheat straw biochars pyrolyzed at 300 °C (WS300) and 500 °C (WS500) were compared for Cr(vi) removal. WS500 exhibited 1.94-fold higher adsorption capacity (23.39 mg g^-1) than WS300 (12.07 mg g^-1), attributed to its improved pore structure and abundant redox-active groups (e.g., phenolic -OH, C-O-C). Adsorption isotherms followed the Langmuir model, indicating monolayer adsorption. The kinetic process was better described by the pseudo-second-order model, which, together with other analyses, suggests that the adsorption process was likely governed by chemisorption. Integrated characterization (SEM-EDS, BET, XRD, FTIR, XPS) revealed the mechanism: (1) initial rapid uptake via surface adsorption and pore filling; (2) Cr(vi) reduction to Cr(iii) mediated by phenolic -OH (electron donor) and C-O-C (electron mediators), as evidenced by XPS Cr 2p (Cr(iii) at 577.4 eV) and the oxidation of C-OH to O-C[double bond, length as m-dash]O in C 1s spectra; (3) Cr(iii) complexation with oxygen groups, as validated by FTIR Cr-O vibrations (617 cm^-1) and XPS Cr(iii)-OH signals (532.7 eV). This work clarifies the critical role of pyrolysis temperature in tuning biochar's structure-function relationship for efficient Cr(vi) remediation.