Influence of iron mass on Cr isotope fractionation associated with Cr(VI) reduction by zero-valent iron

Chromium contamination in groundwater remains a significant environmental concern, primarily due to the toxicity and mobility of its hexavalent form (Cr(VI)). The effect of ZVI mass on Cr(VI) reduction and isotope fractionation was investigated under dynamic flow conditions. Flow-through cell (FTC) experiments were conducted to simulate permeable reactive barriers, with stable Cr isotope fractionation (δ⁵³Cr) used to differentiate direct ZVI-mediated and indirect Fe(II)-mediated Cr(VI) reduction pathways. Results showed a dual-phase removal mechanism: an initial rapid surface-mediated reduction followed by a slower aqueous-phase reduction. Isotope fractionation increased as Cr(VI) concentrations declined. Cr(VI) removal efficiencies of 1.6 mg g^-1 were observed in FTCs containing 2 g or 4 g of ZVI. A transition from a rapid surface-mediated mechanism to a slower aqueous-phase reduction occurred at 85% and 75% breakthrough in the 2 g and 4 g FTCs, respectively. Isotopic enrichment was greater with increased ZVI mass, with maximum δ⁵³Cr values reaching 0.23‰ (2 g ZVI) and 0.30‰ (4 g ZVI), compared to 0.47‰ for 6 g systems observed in previous studies. An updated MIN3P reactive transport model, incorporating mineral passivation and refined kinetic parameters, captures these trends. The optimized model assigns an isotope fractionation factor (ε) of -0.07‰ for the rapid, surface-mediated reduction reaction and -3.5‰ for the slower Fe(II)-mediated reduction. These values align with fractionation factors reported for analogous geochemical processes. This study isolates the influence of reactive surface area on Cr(VI) removal efficiency and isotopic behavior, enhancing mechanistic understanding and model reliability.