Recent advances in the use of activated peroxygens (hydrogen peroxide and persulfate) for in situ chemical oxidation (ISCO) hold promise for the rapid remediation of dissolved, sorbed, and dense nonaqueous phase liquid (DNAPL) contaminants. Although peroxygen chemistry is highly complex, most ISCO peroxygen vendors use one set of process conditions, not based on fundamental chemistry, for all sites. The original objectives of this project were to (1) apply rational process chemistry to improve the design and implementation of peroxygen ISCO at the demonstration level; (2) validate the effectiveness of peroxygen ISCO in the field by detailed assessment of contaminant loss, fate, and product formation; and (3) implement an ISCO optimization approach that involved multiple iterations between oxidant applications and performance monitoring. Difficulties with site selection and characterization resulted in an inability to complete the demonstration as originally planned; however, guidance for field application of catalyzed H2O2 propagations (CHP) stabilization was developed based on existing case studies.
Peroxygen ISCO processes include CHP (i.e., modified Fenton's reagent) and activated persulfate. In CHP, hydrogen peroxide decomposition is catalyzed by soluble iron or naturally occurring subsurface minerals and results in the generation of a suite of oxidants, reductants, and nucleophiles, providing a universal treatment mixture capable of degrading oxidized, sorbed, and DNAPL contaminants. The primary disadvantage is low hydrogen peroxide stability. However, results from SERDP project ER-1288 demonstrated that hydrogen peroxide can be stabilized through the addition of salts of some organic acids, such as phytate, allowing the oxidant source to move greater distances downgradient from injection wells.
Recent advances have been made in stabilizing hydrogen peroxide in the presence of subsurface solids. The addition of sodium citrate, sodium malonate, and sodium phytate can potentially slow hydrogen peroxide decomposition rates by up to 50 fold. The optimal implementation of these stabilizers for use in CHP field applications is detailed in the guidance document. Multi-tiered treatability studies are described. The first step in treatability studies is the evaluation of stabilized and unstabilized hydrogen peroxide decomposition rates. The optimum hydrogen peroxide concentration and stabilizer concentration are then used in field implementation. This guidance document then outlines field development of stabilized CHP. Field conditions, site conditions, and health and safety issues are addressed. The guidance document concludes with detailed descriptions of two case histories.
CHP is the ISCO process with the most robust chemistry and potential for contaminant destruction. Because it generates high fluxes of hydroxyl radical, superoxide radical, and hydroperoxide anion, CHP can destroy nearly all environmental contaminants of concern and provide enhanced treatment of sorbed contaminants and NAPLs. However, hydrogen peroxide is unstable in the subsurface and as a result, CHP use has decreased in favor of activated persulfate as an ISCO reagent.