Developing persulfate-activator soft solid (PASS) as slow release oxidant to remediate phenol-contaminated groundwater

The research objective was to develop a persulfate-activator soft solid (PASS) as a biodegradable slow-release oxidant to treat phenol-contaminated groundwater. PASS was prepared by graft copolymerization of acrylic acid (AA) and acrylamide (AM) onto 1% (w/v) sodium alginate mixed with 500 mg L−1 sodium persulfate and 5 mg L−1 ferrous sulfate. The physical and chemical properties of PASS were characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, differential scanning calorimetry, the water content and swelling ratio. Various variables, including the ratio of AA/AM, pH, temperature and the type of groundwater cations affecting PS release, were investigated. The maximum PS release in DI water was 98% in the ratio of PASS 1 (AA/AM, 75/25), 96% at pH 3, 83% at 25 °C, and 80% with Na+. The major factors controlling PS release were the AA/AM ratio and pH. PASS 1 can be stable in size and shape for 6–8 days and completely degraded within 34 days. The degradation rates of 10 mgL−1 phenol using PASS produced the highest kobs values for each variable at a ratio of PASS 1 (k = 0.1408 h−1), pH 7 (k = 0.1338 h−1), 25 °C (k = 0.1939 h−1), and Ca2+ (k = 0.1336 h−1). The temperature of the groundwater was key to driving the reaction between PS and phenol. PASS 1 was applied in simulated phenol-contaminated groundwater via horizontal tanks containing Ottawa sand. The results indicated 93.2% phenol removal within 72 h in a narrow horizontal flow tank and 41.7% phenol removal in a wide horizontal flow tank with aeration

Developing slow-release persulfate candles to treat BTEX-contaminated groundwater

Leaking underground storage tanks (UST) are one of the leading causes of groundwater contamination in the United States. Despite significant progress in remediating sites impacted by UST over the last 25 years, new technologies are still being sought to deal with petroleum products released into difficult formation, such as low permeable aquifers. One technology gaining momentum is the use of slow-release chemical oxidants to create in situ oxidant barriers in low-permeable zones. Our objective was to develop slow-release persulfate candles to treat BTEX-contaminated groundwater. This was accomplished by manufacturing various blends of persulfate, with and without activators, in a paraffin matrix. Laboratory-scale candles were prepared by heating and mixing Na2S2O8 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 FeSO4 or zerovalent iron (ZVI) and wax. By treating benzoic acid and BTEX compounds with slow-release persulfate and ZVI candles under batch conditions, we observed rapid transformation of all contaminants. By using 14C-labeled benzoic acid and benzene, we also confirmed mineralization occurred upon exposure to the candles (i.e., conversion to CO2). 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 (14x14x2.5 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 (Co =1 mM) decreased by \u3e90% 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. Advisor: Steve Comfort This thesis is embargoed until December 2013

Using Slow-Release Permanganate Candles to Remove TCE from a Low Permeable Aquifer at a Former Landfill

Past disposal of industrial solvents into unregulated landfills is a significant source of groundwater contamination. In 2009, we began investigating a former unregulated landfill with known trichloroethene (TCE) contamination. Our objective was to pinpoint the location of the plume and treat the TCE using in situ chemical oxidation (ISCO). We accomplished this by using electrical resistivity imaging (ERI) to survey the landfill and map the subsurface lithology. We then used the ERI survey maps to guide direct push groundwater sampling. A TCE plume (100-600 µg L-1) was identified in a low permeable silty-clay aquifer (Kh = 0.5 m d-1) that was within 6 m of ground surface. To treat the TCE, we manufactured slow-release potassium permanganate candles (SRPCs) that were 91.4 cm long and either 5.1 cm or 7.6 cm in dia. For comparison, we inserted equal masses of SRPCs (7.6-cm vs 5.1-cm dia) into the low permeable aquifer in staggered rows that intersected the TCE plume. The 5.1-cm dia candles were inserted using direct push rods while the 7.6-cm SRPCs were placed in 10 permanent wells. Pneumatic circulators that emitted small air bubbles were placed below the 7.6-cm SRPCs in the second year. Results 15 months after installation showed significant TCE reductions in the 7.6-cm candle treatment zone (67-85%) and between 10 to 66% decrease in wells impacted by the direct push candles. These results support using slow-release permanganate candles as a means of treating chlorinated solvents in low permeable aquifers. Includes Supplementary Materials

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

Developing slow-release persulfate candles to treat BTEX contaminated groundwater

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 Na2S2O8 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 FeSO4 or zero-valent 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 14C-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 (Co = 1 mM) decreased by \u3e90% 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. Includes Supplementary Materials

Modeling the release and spreading of permanganate from aerated slow-release oxidants in a laboratory flow tank

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

Remediating 1,4-dioxane-contaminated water with slow-release persulfate and zerovalent iron

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+, 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

A five-year performance review of field-scale, slow-release permanganate candles with recommendations for second-generation improvements

In 2009, we identified a TCE plume at an abandoned landfill that was located in a low permeable siltyclay 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. Includes supplementary materials

Remediating Contaminated Groundwater with an Aerated, Direct-Push, Oxidant Delivery System

One of the biggest challenges to treating contaminated aquifers with chemical oxidants is achieving uniform coverage of the target zone. In an effort to maximize coverage, we report the design and installation of a novel aerated, slow-release oxidant delivery system that can be installed by direct-push equipment. By continuously bubbling air beneath a slow-release oxidant in situ, an airlift pump is created that causes water and oxidant to be dispersed from the top of the outer screen and drawn in at the bottom. This continuous circulation pattern around each drive point greatly facilitates the spreading of the oxidant as it slowly dissolves from the wax matrix (i.e., oxidant candle). Given that the aeration rate controls the outward flow of oxidant from the outer screen in all directions, the radius of influence around each drive point is largely a function of the outward velocity of the oxidant exiting the screen and the advection rate opposing the upgradient and lateral spreading. Temporal sampling from three field sites treated with the aerated oxidant system are presented and results show that contaminant concentrations typically decreased 50–99% within 6–9 months after installation. Supporting flow tank experiments that demonstrate oxidant flow patterns and treatment efficacy are also presented