Experimental insights into iron and manganese transformation in soil during groundwater fluctuations.

Iron (Fe) and manganese (Mn) are key redox-sensitive elements in soil-groundwater systems, and their primary environmental risk lies in their ability to influence the mobilization and release of co-occurring hazardous contaminants, such as arsenic. Previous studies have primarily focused on variations in aqueous-phase concentrations of Fe/Mn. However, under fluctuating groundwater conditions, the dynamic transformation mechanisms of different forms of Fe/Mn in soil remain underexplored. This study addressed this gap by employing dynamic groundwater table simulation experiments. The responses of various forms of Fe and Mn in the soil to fluctuations in groundwater levels were investigated and quantitatively evaluated the contribution ratios of different Fe/Mn forms to geochemical processes. The results show that a decrease in the groundwater table increases the redox potential (Eh), creating an oxidative environment that promotes the enrichment of oxide-bound iron/manganese (Ox-Fe/Mn) and a reduction in organic matter-bound iron/manganese (Om-Fe/Mn). For Fe, only the Ox-Fe and Om-Fe forms are affected by groundwater level fluctuations, while the exchangeable form (EXC-Fe) and carbonate-bound form (Carb-Fe) remain at a low level with insignificant changes. However, all four forms of Mn change significantly with the groundwater level fluctuations. Quantitative analysis revealed that redox processes involving Ox-Fe/Mn are the primary drivers of Fe/Mn cycling in the soil-groundwater system, contributing more than 50 % on average. This research provides key insights into the fate and transformation of Fe and Mn in soil and aims to assess their potential environmental risks.