Presentation Description
Recent Advances in Peroxygen Activation and Contaminant Degradation Pathways
The use of modified Fenton’s reagent and activated persulfate has become increasingly popular for the in situ and ex situ treatment of surface soils and the in situ remediation of the subsurface. These processes are based on the catalyzed decomposition of the peroxygens; hydrogen peroxide decomposition is mediated by soluble iron, iron chelates, and iron minerals, and persulfate is activated by iron species, base, and heat. Activation results in the generation of the strong oxidants hydroxyl radical or sulfate radical as well as other reactive oxygen species, such as perhydroxyl radical, superoxide radical anion, and hydroperoxide anion.

A dominant reactive oxygen species formed in modified Fenton’s systems is superoxide, which is responsible for the enhanced treatment of sorbed and nonaqueous phase liquid (NAPL) contaminants. Although superoxide is not reactive with highly chlorinated contaminants in deionized water, recent findings have documented that the hydrogen peroxide of modified Fenton’s systems increases the reactivity of superoxide, which results in significant contaminant degradation. Furthermore, solid surfaces increase the reactivity of superoxide, so subsurface solids actually enhance contaminant destruction by superoxide.

Although modified Fenton’s reagent has the potential to destroy nearly all contaminants in an ideal system, its use for subsurface remediation is limited by the rapid decomposition of hydrogen peroxide. Therefore, recent investigations have focused on stabilizing hydrogen peroxide in the presence of subsurface minerals using organic ligands. Citrate, malonate, and phytate have been shown to increase hydrogen peroxide longevity by >1000% in the presence of subsurface solids.

The reactivity of activated persulfate formulations has been found to be varied, depending on the method of activation. For example, base-activated persulfate reactivity increases with increasing molar ratios of base:persulfate. Iron chelate-activated persulfate provides relatively specific activity compared to base-activated persulfate formulations. The activation of persulfate by both dominant and trace minerals varied substantially; most minerals have not been found to be activators of persulfate, but iron pyrite rapidly activates persulfate.
Recent findings demonstrate that the chemistry of peroxygen ISCO systems is highly complex. Both of these peroxygens have high potential for destroying organic contaminants in the subsurface because of the complex mix of free radicals generated during their catalytic decomposition.