Final Report - Strategic Environmental Research and Development ...

5. BACKGROUNDThe contamination of soils and groundwater by hazardous organic chemicals has been asignificant concern for over thirty years. Even though many contaminated sites have beencleaned up, thousands more remain to be treated (Freeze, 2000) and thousands of hazardousmaterials spills occur each year (Hackman et al., 2001). Furthermore, despite over two decadesof development of innovative technologies, some sites are still not effectively treated. A primaryreason for the difficulty in cleaning up many contaminated sites is the difficulty of bringingreactants into contact with contaminants, particularly when the contaminants are found in lowpermeability matrices (Bedient et al., 1994; Watts, 1998).Difficulties in reagent transport and contact with contaminants are inherent in mostremedial technologies, including the rapidly growing area of in situ chemical oxidation (ISCO).Catalyzed H 2 O 2 propagations (CHP—modified Fenton’s reagent), permanganate, and ozonehave been the most common ISCO processes through the year 2000 (Watts and Teel, 2006).Each of these ISCO reagents has different reactivities with contaminants of concern. CHP hasthe potential to degrade a wide range of organic contaminants, and can effectively treat sorbedcontaminants (Watts and Teel, 2005; Corbin et al., 2007). Furthermore, if CHP processconditions favor the generation of superoxide, the process has the potential to degrade densenonaqueous phase liquids (DNAPLs) more rapidly than their rate of dissolution (Teel et al.,2002; Smith et al., 2006). Ozone, when decomposed to other reactive oxygen species, is alsohighly reactive with most contaminants. In contrast, permanganate is stable in the subsurface(Siegrist et al., 2001), but is reactive with a narrow range of contaminants (Tratnyek andWaldemer, 2006).The use of these reagents is restricted by limitations in the stability, transport, anddelivery of the oxidants. Transport of hydrogen peroxide in the subsurface is limited by its lowstability, which rarely has a half-life greater than 48 hours (Kakarla and Watts, 1996); depletionof hydrogen peroxide is sometimes found within 1–2 m of the injection well. Ozone is limited byits mass transfer and solubility in groundwater as well as by its often variable reactivity, which isinfluenced by the subsurface mineralogy (Wong et al., 1997). Permanganate is limited by itsreactivity with natural organic material (NOM); in soils with high NOM, permanganate is rapidlyconsumed, minimizing its transport and delivery to contaminated regions. As a result, bothprocesses may require large oxidant dosages and close spacing of injection wells. The use ofpermanganate is further limited by its inability to treat several key groundwater contaminants,such as benzene and carbon tetrachloride, and by the formation of manganese oxide precipitates,which may clog subsurface pores (Siegrist et al., 2001). Therefore, the current state ofdevelopment of ISCO presents a dilemma. The most widely-reactive oxidant, CHP, while able tooxidize nearly all contaminants of concern, is the least stable and therefore most difficult toapply effectively. The most stable oxidant, permanganate, is limited by both the type ofcontaminant it can oxidize and by the geological matrix.Sodium persulfate is the newest oxidant source to be used for ISCO. The use of sodiumpersulfate as an ISCO reagent has the potential to overcome many of the mass transferlimitations characteristic of modified Fenton’s reagent and permanganate. Sodium persulfate is a2