Redox-salinity coupling regulates arsenic release and hysteresis in coastal wetland soils under sea-level rise

Sea level rise and saltwater intrusion can reactivate legacy-contaminated coastal soils, changing them from contaminant sinks to episodic sources of As. However, the coupled controls of redox potential (Eh) and salinity on Fe-As-S transformations during inundation and flooding recovery are poorly constrained. Here, we combine controlled redox-salinity experiments with synchrotron micro-X-ray fluorescence (μ-XRF) and micro-X-ray absorption near-edge structure (μ-XANES) spectroscopy, and mechanistic kinetic modeling to quantify the impact of seawater inundation on As cycling in contaminated coastal wetlands. Iron(III) reductive dissolution is the dominant driver of As release, but seawater amplified both the magnitude and persistence of dissolved As by weakening sorption via ionic competition and stabilizing reduced As by sulfate reduction and likely chloride complexation. The μ-XANES shows a sequential transformation of Fe-bound As(V) to As(III) and As-Fe-S phases, followed by a geochemically significant incomplete re-oxidation that produced persistent As(III) and chemical hysteresis. The mechanistic model successfully quantified the coupled controls defined thresholds: where 50% of Fe(III) sites are active of -150 ± 15 mV, sulfide inhibition onset near -260 ± 20 mV, and a salinity amplification factor of 0.03 ± 0.008 L g^-1. Redox-salinity coupling sustains As mobilization and persistence due to chemical hysteresis, which allows for elevated dissolved As even after re-oxidation following inundation. These results provide a quantitative framework for forecasting As mobilization risk in wetlands experiencing salinization globally.