Optimizing Arsenic Removal in a Groundwater Treatment Cell
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The goal of this research is to investigate geochemical factors on arsenic removal from groundwater in a groundwater treatment cell. The treatment cell is near Metaline Falls, WA, and is operated by Geosyntec Consultants. Using PHREEQ-C, a program developed by the USGS for chemical modeling, I determined which adsorptive media will remove the most arsenic under site conditions, which ions inhibit or encourage arsenic adsorption, and which ions have the potential to remove arsenic through co-precipitation. The groundwater samples collected from the site were taken to the UW SEFS Analytical Lab for inductively coupled plasma mass spectrometry (ICP-MS) to determine the concentrations of each ion present in the groundwater. These lab results were used for the model validation initial conditions. Historical data collected at the site by Geosyntec was compiled into a database for statistical analysis, which identified conductivity, a proxy for ionic strength, and manganese in solution as two main factors influencing arsenic removal. These factors were further studied using PHREEQ-C. From the modeling results I found that titanium oxide media, particularly MetasorbG manufactured by Graver Technologies, to be the most efficient media at removing arsenic via adsorption. I found that phosphate in the groundwater plays the largest role in inhibiting arsenic adsorption by directly competing with arsenate for surface sites. Conductivity, or ionic strength, will reduce adsorption rates upgradient of the treatment cell and lead to higher arsenic concentrations being delivered to the cell. And, finally, I determined that arsenic is oxidized by manganese oxides and will readily precipitate with manganese ions in high pH conditions, which are typically found upgradient of the treatment cell. For further study, I recommend monitoring phosphate levels in the treatment cell, determining residence time of the water in the treatment cell, performing XRD or FTIR analysis on gravel inside the treatment cell to determine what minerals are precipitating, and starting two batch-scale trials with titanium oxide and manganese oxide media.