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Treating 1,4-dioxane with Slow-Release Persulfate and Zerovalent Iron and Modeling the Radius of Influence of Aerated Oxidant Candles
Aerated, slow-release oxidant candles are a relatively new technology for treating aquifers contaminated with petroleum products and chlorinated solvents. The chemistry of metal-activated persulfate can degrade many of these contaminants but reaction kinetics have typically been characterized by a rapid decrease during the first 30 min followed by either a slower decrease or no further change (i.e., plateau). By using 1,4-dioxane (dioxane) as a model contaminant, this research determined that excess ferrous iron produced from the Fe0-persulfate reaction scavenges sulfate radicals and prevents further dioxane degradation. By limiting the availability of Fe0 in a slow-release formulation, we found degradation plateaus were avoided and 100% removal of dioxane was achieved. While the chemistry provided by slow-release oxidants is effective, a critical need for advancing this technology is developing a reliable method for predicting the radius of influence (ROI). We performed a series of laboratory flow tank experiments and numerical modeling efforts to predict the release and spreading of permanganate from aerated oxidant candles. Aeration is used to minimize downward density-flow of the oxidant and we simulated this action by coupling a two-phase bubbly flow model with the Darcy flow model in porous media. To mimic the design of the oxidant candles used in the field, a double screen was used where the oxidant candle was placed inside the inner screen and air was bubbled upward between the inner and outer screens. This airflow pattern caused water and oxidant to be dispersed from the top of the outer screen and drawn in at the bottom. Using this design, we observed that permanganate spreading and ROI increased with aeration and decreased with advection. Our coupled bubble flow and transport model was able to reproduce observed results by mimicking the upward shape and spreading of permanganate under various aeration and advection rates. Given that the aeration rate controls the outward flow of oxidant from outer screen in all directions, we predict that the ROI is largely a function of the outward velocity of the oxidant exiting the outer screen and the groundwater advection rate, which opposes the up gradient and lateral spreading of the oxidant.
Kambhu Na Ayudhya, Ann, "Treating 1,4-dioxane with Slow-Release Persulfate and Zerovalent Iron and Modeling the Radius of Influence of Aerated Oxidant Candles" (2019). ETD collection for University of Nebraska - Lincoln. AAI13426658.