Chromium isotope fractionation during the removal of hexavalent chromium by oak-based biochar

Chromium, especially in its hexavalent form (Cr(VI)), poses significant health risks due to its carcinogenic properties. Emerging research suggests that biochar, a carbon-rich material derived from biomass pyrolysis, holds promise as an effective and sustainable solution for Cr(VI) remediation. Biochar's unique physicochemical properties, such as its high surface area, porous structure, and functional groups, contribute to its exceptional adsorption capacity for metals. In a series of batch experiments with varying durations, an oak-based biochar was exposed to a 48 mg L⁻¹ Cr(VI) solution. The results demonstrate that the biochar effectively removed 99% of the Cr(VI) after 100 h, with a removal capacity greater than 5 mg g⁻¹. Fourier transform infrared (FTIR) spectroscopy suggests that the removal of Cr(VI) involved aliphatic and aromatic C-H bonds. X-ray photoelectron spectroscopy (XPS) also indicated the role of aliphatic groups in the removal process and suggested the involvement of carbonyl groups. XPS analysis also detected both Cr(III) and Cr(VI) on the surface of the biochar, indicating the occurrence of reduction and sorption processes. The presence of Cr(III) and Cr(VI) was further confirmed at the Canadian Light Source (CLS) synchrotron facility using X-ray absorption near edge structure (XANES) spectroscopy. Cr isotope analysis showed an increase in δ⁵³Cr as the concentration of Cr in solution decreased, indicating Cr(VI) reduction. The isotope data followed a Rayleigh curve with a kinetic fractionation ε⁵³Cr of -1.33‰, highlighting that Cr stable isotopes can effectively be used as an indicator of biochar-Cr reactions.