Study of use of N-sodium silicate for sequestering manganese dioxide produced during in-situ chemical oxidation of DNAPLs with potassium permanganate

In situ chemical oxidation using potassium permanganate (KMnO4) is a popular method for removing dense non-aqueous phase liquids (DNAPLS) from contaminated groundwater aquifers. Manganese dioxide (MnO2) precipitation and carbon dioxide (CO2) production during in situ chemical oxidation reduces the treatment efficiency by hindering mass transfer of potassium permanganate to the DNAPL zone. Experiments were designed to study the effect of reduced MnO2 precipitation and CO2 degassing on overall mass transfer rate. A solution of potassium permanganate and N-sodium silicate (Na4SiO4) was injected into a saturated silica bed, contaminated by trichloroethylene (TCE). Na4SiO 4 was added for sequestering MnO2 precipitates and enhancing CO2 dissolution, both produced during in situ chemical oxidation of TCE with KMnO4.;Experiments performed in the vials demonstrate that if added in adequate amount, Na4SiO4 may hinder MnO2 precipitation by sequestration. In comparison, during two-dimensional tank experiment, which simulated site treatment conditions in the laboratory, Na4SiO 4 addition was not as effective in sequestering MnO2 although it reduced CO2 degassing and hence enhanced mass transfer between DNAPL and aqueous phases. Na4SiO4 also assisted in maintaining high pH, which increased the solubility of CO2 and reduced bubble formation to considerable extent. A series of reaction rate experiments showed sodium silicate did not affect the rate and activation energy of oxidation of TCE.;Contrary to the information available in the literature, this study demonstrated that chloride (Cl-), chlorite (ClO2 -) and chlorate (ClO3-) were produced during TCE oxidation. A mass balance calculations were performed on the basis of production of equivalent chloride ion (Cl-) produced during TCE oxidation with KMnO4. This study was limited to performing a total mass balance and no further investigation was conducted to determine the mechanism for ClO2- and ClO3- formation.