Visualization of the influence of sodium citrate on iron and uranium precipitation in a 2D laboratory scale tank

Wetlands are complex biogeochemical systems that are essential to maintaining groundwater and surface water quality. Particulate iron and natural organic matter (NOM) solids and can impact the transport of trace metals and contaminants in surface waters and across the hyporheic zone. The formation of Fe-NOM solids in the subsurface can attenuate contaminants, while the formation of Fe-NOM colloids in surface waters can serve as vehicles for trace metal and/or contaminant transport. To better understand these systems, this study constructed a bench-top 2D tank system capable of recreating the conditions present in a wetland hyporheic zone under gaining stream conditions. The system was able to visualize and quantify iron precipitate formation using digital processing of optical images. The ability of researchers to be able to visually observe where oxidation and subsequent precipitation of iron hydroxides occurs allows them to clearly delineate sharp redox gradients within hyporheic zones which appear to control trace metal mobility. Sodium citrate was used to assess the impact of varying ratios of NOM to Fe in influent solutions on the distribution of Fe and uranium (U) in the system. When no sodium citrate was added to the system, Fe (hydr) oxides formed in the sand layer, and visible Fe (hydr)oxide colloids formed in the overlying waters. However, increasing the ratio of sodium citrate to Fe, decreased the formation of detectable Fe (hydr)oxides in the sand layer and increased the concentration of Fe and U in the overlying waters. Additionally, increased ratios of sodium citrate to Fe maintained elevated concentrations of Fe and U in the overlying waters, while when no sodium citrate was added to the system Fe suspension in the overlying waters ceased after less than one day.