Colloids (submicron-sized mineral particles or organic matter that remain in suspension and move with the aqueous phase) are suspected of mediating the mobility of metals, actinides and nutrients in sediments that experience strong wet-dry cycling. Significantly, however, there are large gaps in our understanding of the nature of these colloids and parameters controlling their generation and longevity in suspension. In this study, we hypothesized that, during hydrologically induced redox transitions, oxidation of dissolved sulfides (onset of oxidizing conditions) or reductive dissolution of FeIII-(oxyhydr)oxide (reducing conditions), releases organic carbon, metals, and anions, providing conditions ideal to generate colloids that can bind and mobilize metal(loid)s, such as U, Mo, Zn, Pb. Improved knowledge of colloid formation and metal(loid) binding is needed to improve our understanding of how water quality will respond to hydrological changes, such as drought and flooding.<br/>Here, we examined the influence of sulfidation of FeIII-(oxyhydr)oxide nanoparticles associated with organic ligands on the generation, stability, and nature of colloids. We observed that reductive dissolution of ferrihydrite generated nanoclusters of mackinawite (FeS). Their subsequent aggregation, which promotes settling of mackinawite into the solid phase, was directly correlated to the concentration of sulfides relative to iron oxides  (S/Fe ratio). At low sulfide concentration (S/Fe ratio ≤0.5), the aggregation of mackinawite nanoclusters took between 14 days to several months, indicating that colloids generated from sulfidation are able to bind and transport metals in groundwater for at least 14 days after their generation. Inversely, at high sulfide concentration (S/Fe ratio &gt;0.5) the sulfidation rate was rapid and the aggregation of mackinawite nanoclusters was accelerated. This inhibited the generation of suspended FeS colloids. However, the S/Fe ratio also controlled the concentration of DOC, which dramatically increased at high sulfide concentrations, thus promoting the mobility of organic-bound metals at conditions when FeS colloids were insignificant. Finally, the presence of organic ligands increased the time of suspension of mackinawite nanoclusters, whereas an increased ionic strength of the groundwater inhibited the generation of FeS colloids. This investigation provides, for the first time, a general conceptual model to predict the formation of sulfidic nanoparticles.

Colloid formation driven by redox processes: Impact on groundwater quality in shallow alluvial aquifers

2019 Fall National Meeting

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