Transport of Heat Activated Persulfate and Its Application for In-situ Chemical Oxidation of Residual Trichloroethylene

In situ chemical oxidation is a promising technology for the remediation of persistent subsurface contamination. Increasingly, the persulfate ion is being studied for use in these systems, both on its own as a strong oxidant and as the precursor to the even more reactive sulfate radical. Persulfate has been shown to treat a wide range of contaminants, from traditional Superfund contaminants such as chlorinated solvents to emerging pharmaceutical contaminants. Additionally, persulfate ISCO can be tailored to site and pollutant specific characteristics based on the method of persulfate activation (e.g., energy and catalysis activation) to the sulfate radical. Thermal activation of persulfate is particularly promising because it can be easily controlled, requires no additional reagents, and commonly creates only non-toxic end products. While persulfate in-situ chemical oxidation technology is being commercially used, a mechanistic study of the physical and chemical processes controlling the effectiveness of this remedial approach is not well documented in the literature. Published work characterizing persulfate ISCO largely focuses on reactions in aqueous, batch systems, which fail to provide crucial design data when working with ever transient, multi-phase groundwater systems. The purpose of this research was twofold. Initial studies characterized the overall transport behavior of unactivated and thermally-activated persulfate (20, 60, and 90°C) in one-dimensional soil column systems packed with a natural sandy porous media. This necessitated the development of a flow-through, temperature-controlled, continuous-injection system for the delivery of heat-activated persulfate. Finally, as a proof of concept, experiments were conducted to investigate persulfate ISCO as a remedial approach for residual-phase trichloroethylene (TCE), a commonly detected, persistent subsurface contaminant. At all activation temperatures investigated, persulfate exhibited ideal transport behavior with negligible differences in the observed breakthrough curves of persulfate ion and nonreactive tracers in miscible displacement experiments. Additionally, moment analysis of the breakthrough curves measured for persulfate ion in solution indicated negligible interaction of persulfate with the sandy material under steady-state flow (average retardation factor equaled 1.00 ± 0.021). Persulfate ISCO for residual-phase trichloroethylene (TCE) was characterized at two flow rates, 0.2 mL/min and 0.5 mL/min, resulting in two degrees of apparent persulfate activation, 39.5% and 24.6%, respectively. Both ISCO soil column systems showed an initial, long-term plateau in effluent concentrations measured for TCE indicating steady-state dissolution of pure phase TCE. Effluent concentrations of TCE began decreasing after 75 and 100 pore volumes (normalized for the residual fraction of TCE in individual soil columns) in the 39.5% and 24.6%