THE EFFECT OF IN SITU CHEMICAL OXIDATION (ISCO) OF SOLVENTS IN GROUND WATER ON DISSOLVED METALS: FENTON’S REAGENT COMPARED TO PERMANGANATE

Groundwater chemistry data from field trials at Fort Lauderdale, FL, Kitchener, ON, and North Haven, CT using Fenton’s Reagent and KMnO4 treatments are used to demonstrate the influences of ISCO on metal concentrations in ground water. At Fort Lauderdale, the injection of Fenton’s Reagent created a low pH zone in the upper aquifer where there is little carbonate-mineral neutralization capacity. Impurities in the injection solution and mineral-water reactions caused elevated concentrations of Ca, Mg, Na, Fe, Cd, Pb, Ni, Mo and V. The increase in Fe is attributed to the Fe(II)-containing Fenton’s Reagent; increases in the concentrations of Ca, Mg and Na are related to pH buffering by carbonate dissolution and subsequent exchange of Ca and Mg for Na at ion exchange sites. The remaining metals probably originate as impurities in the Fenton’s Reagent injection solution.

The neutralization capacity at the Kitchener site is high, and neutralization reactions following the injection of KMnO4 resulted in elevated concentrations of Ca, Mg and Na in the ground water. However, heavy metals that persist in the ground water are limited to Cr and Zn. The neutralization capacity at North Haven is very low and reactions between KMnO4 and TCE result in a significant decrease in pH (as low as 2.45). This leads to increased concentrations of Al, Fe and Ni. The increase in Al and Fe concentrations results from enhanced Al and Fe(III) solubility and accelerated rates of Al- and Fe-mineral dissolution at low pH. The cause of the elevated Ni concentrations is uncertain but probably results either from the dissolution of naturally occurring Ni-bearing silicate minerals, or from desorption of Ni from dissolving Fe-oxy-hydroxide minerals.

The secondary-mineral coatings that form during the KMnO4 ISCO reactions have been studied with a variety of analytical techniques. The coatings are dominated by MnO2, however Al and Si occur in solid solution with MnO2. Where the coatings form on Fe(II)-bearing primary minerals, oxidation of the Fe forms discrete Fe(III) phases intermixed with the MnO2. Preliminary data suggest that trace metals immobilized from the groundwater partition between the Mn and Fe oxides, suggesting that small quantities of Fe(II)-bearing primary minerals in the aquifer may be favourable for the immobilization of trace metals.