No abstract

The development of slow-release chemical oxidants for sub-surface remediation is a relatively new technology. Our objective was to develop slow-release persulfate-paraffin candles to treat BTEX-contaminated groundwater. Laboratory-scale candles were prepared by heating and mixing Na(2)S(2)O(8) with paraffin in a 2.25 to 1 ratio (w/w), and then pouring the heated mixture into circular molds that were 2.38 cm long and either 0.71 or 1.27 cm in diameter. Activator candles were prepared with FeSO(4) or zerovalent iron (ZVI) and wax. By treating benzoic acid and BTEX compounds with slow-release persulfate and ZVI candles, we observed rapid transformation of all contaminants. By using (14)C-labeled benzoic acid and benzene, we also confirmed mineralization (conversion to CO2) upon exposure to the candles. As the candles aged and were repeatedly exposed to fresh solutions, contaminant transformation rates slowed and removal rates became more linear (zero-order); this change in transformation kinetics mimicked the observed dissolution rates of the candles. By stacking persulfate and ZVI candles on top of each other in a saturated sand tank (14×14×2.5 cm) and spatially sampling around the candles with time, the dissolution patterns of the candles and zone of influence were determined. Results showed that as the candles dissolved and persulfate and iron diffused out into the sand matrix, benzoic acid or benzene concentrations (C(o)=1 mM) decreased by >90% within 7 d. These results support the use of slow-release persulfate and ZVI candles as a means of treating BTEX compounds in contaminated groundwater.

1,4-dioxane is an emerging contaminant that was used as a corrosion inhibitor with chlorinated solvents. Metal-activated persulfate can degrade dioxane 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). Our objective was to identify the factors responsible for this plateau and then determine if slow-release formulations of sodium persulfate and Fe0 could provide a more sustainable degradation treatment. We accomplished this by conducting batch experiments where Fe0-activated persulfate was used to treat dioxane. Treatment variables included the timing at which the dioxane was added to the Fe0-persulfate reaction (T = 0 and 30 min) and including various products of the Fe0-persulfate reaction at T=0 min (Fe2+, Fe3+, and SO42−). Results showed that when dioxane was spiked into the reaction at 30 min, no degradation occurred; this is in stark contrast to the 60% decrease observed when added at T = 0 min. Adding Fe2+ at the onset (T= 0 min) also severely halted the reaction and caused a plateau. This indicates that excess ferrous iron produced from the Fe0-persulfate reaction scavenges sulfate radicals and prevents further dioxane degradation. By limiting the release of Fe0 in a slow-release wax formulation, degradation plateaus were avoided and 100% removal of dioxane observed. By using 14C-labeled dioxane, we show that ~40% of the dioxane carbon is mineralized within 6 d. These data support the use of slow-release persulfate and zerovalent iron to treat dioxane-contaminated water.

In 2009, we identified a TCE plume at an abandoned landfill that was located in a low permeable silty-clay aquifer. To treat the TCE, we manufactured slow-release potassium permanganate cylinders (oxidant candles) that had diameters of either 5.1 or 7.6 cm and were 91.4 cm long. In 2010, we compared two methods of candle installation by inserting equal masses of the oxidant candles (7.6-cm vs 5.1-cm dia). The 5.1-cm dia candles were inserted with direct-push rods while the 7.6-cm candles were housed in screens and lowered into 10 permanent wells. Since installation, the 7.6-cm oxidant candles have been refurbished approximately once per year by gently scraping off surface oxides. In 2012, we reported initial results; in this paper, we provide a 5-yr performance review since installation. Temporal sampling shows oxidant candles placed in wells have steadily reduced migrating TCE concentrations. Moreover, these candles still maintain an inner core of oxidant that has yet to contribute to the dissolution front and should provide several more years of service. Oxidant candles inserted by direct-push have stopped reducing TCE concentrations because a MnO2 scale developed on the outside of the candles. To counteract oxide scaling, we fabricated a second generation of oxidant candles that contain sodium hexametaphosphate. Laboratory experiments (batch and flow-through) show that these second-generation permanganate candles have better release characteristics and are less prone to oxide scaling. This improvement should reduce the need to perform maintenance on candles placed in wells and provide greater longevity for candles inserted by direct-push.

Aerated, slow-release oxidants are a relatively new technology for treating contaminated aquifers. A critical need for advancing this technology is developing a reliable method for predicting the radius of influence (ROI) around each drive point. In this work, we report a series of laboratory flow tank experiments and numerical modeling efforts designed to predict the release and spreading of permanganate from aerated oxidant candles (oxidant-wax composites). To mimic the design of the oxidant delivery system used in the field, a double screen was used in a series of flow tank experiments where the oxidant was placed inside the inner screen and air was bubbled upward in the gap between the screens. This airflow pattern creates an airlift pump that causes 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. A coupled bubble flow and transport model was able to successfully reproduce observed results by mimicking the upward shape and spreading of permanganate under various aeration and advection rates.