Combining multiple high-resolution in-situ techniques to understand the metals mobilization at anoxic-oxic interfaces...

Industrial sites in coastal cities experience fluctuating redox conditions driven by climate change, forming anoxic-oxic interfaces and influencing the availability of heavy metals (HMs). What remains elusive is how oxygen contents influence the formation of anoxic-oxic interfaces and HMs mobilization in industrial soils with varying physicochemical properties. Here we investigated the mobilization mechanisms of HMs (Fe, Mn, Cu, Ni, and Zn) at anoxic-oxic interfaces in different depths soils at a historical industrial site using high-resolution in-situ techniques, including planar optodes (PO) and diffusive gradients in thin-films (DGT). The PO accurately captured O₂ and pH distributions, revealing greater O₂ diffusion in surface silty loam soil compared to deep silty clay loam soil. Results showed distinct redox-mediated HMs mobilization patterns between surface and deep soils. Surface soils exhibited rapid Fe(II)/Mn(II) oxidation, yet Cu, Ni, and Zn remained relatively stable due to higher contamination and DOC content. Conversely, deep soils displayed significant Cu, Ni, and Zn mobilization at anoxic-oxic interfaces, correlating to the changes of O₂ and pH. These findings demonstrate the critical role of soil properties and microscale redox dynamics in controlling HMs behavior, offering valuable insights for managing contaminated industrial sites.