Chromium Stable Isotope Fractionation During Reduction of Hexavalent Chromium by Zero-valent Iron and Biochar

Abstract

Chromium, particularly in its toxic hexavalent form, represents a severe environmental and public health hazard due to its high mobility, persistence in groundwater systems, and well-documented carcinogenicity. Industrial activities such as electroplating, leather tanning, and mining operations have led to widespread Cr(VI) contamination of aquatic systems, where it poses risks to both ecosystems and human populations through drinking water exposure. Unlike Cr(III), which is less soluble and a micro-nutrient, Cr(VI) readily migrates through subsurface environments, making its containment and remediation particularly challenging. A critical advancement in addressing Cr(VI) contamination lies in the application of chromium stable isotopes as powerful diagnostic tools for tracking remediation processes. Isotopic fractionation provides unique insights into the mechanisms and efficiency of Cr(VI) removal, distinguishing between physical adsorption, chemical reduction, and diffusion-limited processes. This approach enables researchers and practitioners to quantify reaction progress in real-world remediation systems and identify rate-limiting steps in contaminant transformation. The study investigates Cr(VI) removal by zero-valent iron (ZVI) under dynamic flow conditions, systematically evaluating the influence of ZVI quantity on reaction mechanisms and isotope fractionation. Reactive transport modeling (MIN3P) was employed to validate the conceptual framework of Cr(VI) reduction by ZVI, revealing a dual-stage removal process. Isotope fractionation adheres to a dual Rayleigh model, with distinct enrichment factors for each stage: the first dominated by direct Cr(VI) reduction by ZVI (ε = −1.2‰), and the second controlled by aqueous Fe(II)-mediated reduction (ε = −3.5‰). Kinetic parameters were optimized to unify the model across three ZVI scenarios, demonstrating how variable reactive surface areas influence removal efficiency and isotopic signatures. These results underscore the interplay between ZVI mass transport limitations and redox-driven fractionation in flow-through systems. This study also examines Cr(VI) removal under static conditions, demonstrating that oak-based biochar achieves near-complete (99%) elimination of Cr(VI) from solution at low pH. Advanced characterization techniques, including X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray absorption spectroscopy (XANES), confirm that Cr(VI) is both adsorbed onto the biochar surface and reduced to less toxic Cr(III). Fourier-transform infrared (FTIR) spectroscopy reveals that aliphatic and aromatic functional groups play a key role in the removal process. Chromium isotope analysis shows fractionation occurs during the removal process, with lighter isotopes preferentially removed, following a single Rayleigh model (ε = −1.33‰). These findings highlight the dual role of biochar as a sorbent and reductant, with isotope fractionation serving as a diagnostic tool for tracking reaction progress. The study then investigates Cr(VI) removal under dynamic, saturated flow conditions, simulating real-world scenarios such as transport through permeable reactive barriers. Unlike the batch system, the flow-through experiment reveals a two-stage removal process: initial sorption and diffusion (ε = −1.01‰) followed by reduction (ε = −3.19‰), as indicated by a dual Rayleigh model. XANES analysis confirms that Cr(III) dominates (75–85%) but residual Cr(VI) (15–25%) persists, indicating ongoing sorption with reduction. The study contrasts the isotope fractionation under flow conditions with the static batch system, emphasizing how hydrodynamic conditions influence removal mechanisms and isotope fractionation. These findings have critical implications for designing and monitoring remediation strategies. The dominance of reduction pathways in biochar systems suggests the potential utility in passive treatment systems. Isotope fractionation could track long-term performance. For ZVI-based technologies (e.g., permeable reactive barriers), the identification of dual removal mechanisms and intraparticle heterogeneity demonstrates the need to optimize reactive surface availability and residence times. The consistency of isotope fractionation patterns across systems is consistent with use as a diagnostic tool to distinguish between adsorption, diffusion-limited, and redox-driven removal in field applications. Future work could extend these insights to multicomponent contaminant systems or field-scale validation, bridging the gap between mechanistic studies and real-world implementation.