Date of this Version
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 >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.
Advisor: Steve Comfort
This thesis is embargoed until December 2013.