Soil Colloid Release Affected by Dissolved Organic Matter and Redox Conditions

Mobilization of soil colloids has attracted much research attention because of colloids' potential to facilitate transport of strongly sorbing constituents (e.g., contaminants, pesticides, and nutrients). In redox-dynamic soils or sediments, colloid mobility is largely controlled by Fe3+ dissolution, shifts in pH and ionic strength, and interactions with organic matter (OM). We investigated the complex interplay of colloid release, OM content, and redox conditions via batch experiments to quantify and characterize soil colloid release after the addition of dissolved OM (DOM). Under aerobic conditions, the concentration of released colloids increased from similar to 1000 to similar to 3000 mg L-1 as the DOM concentration increased from 0 to 20 mg C L-1, respectively. The initial concentrations of released colloids were higher under anaerobic conditions, but they decreased by similar to 80 to 90% as incubation continued. In addition, unlike in the aerobic samples, the effects of the added DOM on colloid release in the anaerobic samples were minimal. Iron dissolution under anaerobic conditions coincided with the initial high level of colloid release as well as the release of indigenous DOM (DOMin), which may have led to subsequent colloid aggregation due to bridging. These results demonstrate that (i) DOM stabilized the colloids released under aerobic conditions but had minimal effects on colloids released under anaerobic conditions and (ii) high concentrations of DOMin released under anaerobic conditions decreased subsequent colloid release due to DOMin-colloid association. The findings have implications for colloid release and colloid-associated nutrient and contaminant mobilization and transport in redox-dynamic environments such as wetlands.