Genesis and Effects of Particles Produced during In Situ Chemical Oxidation Using Permanganate

A research effort was undertaken to investigate the genesis of particles produced during in situ chemical oxidation (ISCO) of trichloroethene (TCE) with permanganate $\left({\mathrm{MnO}}_4^{-}\right)$ and to explore the effects of those particles on system permeability and metal mobility. The experimental approach included characterization of soil and groundwater samples from an ISCO field site, batch experiments with a replicated $2^5$ factorial design, and flow-through column experiments. Analyses of intact soil cores from an ISCO field site revealed that ${\mathrm{MnO}}_2$ solids were present in the subsurface near an injection well for ${\mathrm{NaMnO}}_4$ but at low levels (2.3–2.5 mg/g dry wt media) calculated to fill <1% v/v of the aquifer porosity. Batch tests revealed that the mass of filterable solids (>0.45 μm) produced during chemical oxidation with ${\mathrm{MnO}}_4^{-}$ was increased at higher TCE concentrations (54 versus 7 mg/L) and in the presence of ambient silt/clay-sized particles in the groundwater (750 versus 7.5 mg/L). Under otherwise comparable conditions, increasing the ${\mathrm{MnO}}_4^{-}$ dose markedly increases the oxidant consumption and also increases the solids production. The oxidant form $({\mathrm{NaMnO}}_4$ versus ${\mathrm{KMnO}}_4)$ or reaction time (15 versus 300 min) had little effect on oxidant consumption or filterable solids production. During ${\mathrm{MnO}}_4^{-}$ oxidation of higher levels of TCE in a groundwater with ambient silt/clay particles present, there can be substantial increases in filterable solids generated, which are <1 μm in size and consist of ${\mathrm{MnO}}_2\text{,}$ commingled with other mineral matter. Conceivably, low volumetric fillings of these solids could cause permeability loss. Flow-through column experiments revealed that permeability loss was possible during ISCO but only under conditions with very high ${\mathrm{MnO}}_2$ solids production. On the positive side, the ${\mathrm{MnO}}_2$ solids produced can increase the sorption potential for metals such as cadmium and can represent a mode of immobilization. This research demonstrated that ISCO with permanganate has the potential to yield system permeability loss under some conditions as well as to affect metal mobility. The magnitude of these effects is related to the subsurface conditions, target organic chemical mass, and permanganate dose and delivery method. The production of solids during ISCO needs to be carefully considered during process design and operation to avoid solids-related performance problems while exploiting potential benefits.