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

Abstract

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–CO 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.