New Insight into the Natural Detoxification of Cr(VI) in Fe-Rich Surface Soil: Crucial Role of Photogenerated Silicate-Bound Fe(II)

Photoexcitation of natural semiconductor Fe-(III) minerals has been proven to generate Fe-(II), but the photogeneration of Fe-(II) in Fe-rich surface soil as well as its role in the redox biogeochemistry of Cr-(VI) remains poorly understood. In this work, we confirmed the generation of Fe-(II) in soil by solar irradiation and proposed a new mechanism for the natural reductive detoxification of Cr-(VI) to Cr-(III) in surface soil. The kinetic results showed that solar irradiation promoted the reduction of Cr-(VI) in Fe-rich soils, while a negligible Cr-(VI) reduction was observed in the dark. Fe-(II), mainly in the form of silicate-bound Fe-(II), was generated under solar irradiation and responsible for the reduction of Cr-(VI) in soils, which was evidenced by sequential extraction, transmission electron microscopy with electron energy loss spectroscopy, and electron transfer calculation. Photogenerated silicate-bound Fe-(II) resulted from the massive clay-iron (hydr)-oxide associations, consisting of iron (hydr)-oxides (e.g., hematite and goethite) and kaolinite. These associations could generate Fe-(II) under solar irradiation either via intrinsic excitation to produce photoelectrons or via the ligand-to-metal charge transfer process after the formation of clay-iron (hydr)-oxide-organic matter complexes, which was proven by photoluminescence spectroscopy and X-ray photoelectron spectroscopy. These findings highlight the important role of photogenerated Fe-(II) in Cr-(VI) reduction in surface soil, which advances a fundamental understanding of the natural detoxification of Cr-(VI) as well as the redox biogeochemistry of Cr-(VI) in soil.