Evaluating chromium isotope fractionation during the removal of hexavalent chromium by oak-based biochar under saturated flow conditions

• Saturated flow conditions influence Cr isotope fractionation • Cr isotope fractionation is best modelled with two Rayleigh curves • An initial ε 1 of −1.01‰, is attributed to Cr(VI) sorption and diffusion • A later ε 2 of −3.19‰, is attributed to the reduction of Cr(VI) • Cr(VI) is sorbed onto the biochar and subsequently reduced to Cr(III) Biochar is increasingly being used to treat surface water and groundwater contaminated with metals, including Cr. Chromium isotope fractionation occurs during the reduction of Cr(VI) to Cr(III). The degree of fractionation is dependent on the reaction mechanism. In this study, a flow-through cell experiment was conducted to evaluate the effect of Cr(VI) reduction by oak-based biochar under saturated flow conditions on Cr isotope fractionation. Chromium isotope ratio and concentration measurements were made on effluent water samples throughout the experiment. Following the experiment, the residual solid phase was analysed by X-ray absorption spectroscopy, which confirmed the presence of Cr(III) and Cr(VI) on the biochar grains, indicating both sorption and reduction were occurring. An increase in δ 53 Cr values was associated with a decrease in Cr(VI) concentration. The isotope data were better fit with two Rayleigh curves, supported by a Chow test (p = 1.5 × 10 -9 ), in contrast to a previous biochar-Cr isotope study conducted under static conditions that employed one Rayleigh curve. The first Rayleigh curve (ε = −1.01‰) was associated with Cr(VI) sorption onto and diffusion into the biochar particles, and the second Rayleigh curve (ε = −3.19‰) was associated with Cr(VI) reduction by the biochar.