Kinetics of Permanganate Consumption by Natural Oxidant Demand in Aquifer Solids

Effectiveness of permanganate (MnO4-) injection for in situ chemical oxidation is often controlled by the natural oxidant demand (NOD) of the aquifer solids. In this work, a simple procedure was developed and applied to generate a database of NOD kinetic parameters for six different models for 50 different aquifer materials. Representing oxidant consumption as an initial instantaneous reaction with a portion of the total NOD and as a second order reaction between MnO4- and the remainder of the NOD provided a good match with experimental results from batch studies, without imposing an unnecessary computational burden. Wide variations in NOD parameters were observed including total NOD, fraction fast/instantaneous, and second order rate coefficients. Approximately 80% of the samples had a total NOD between 0.002 and 0.158 mmol/g with a median value of 0.028 mmol/g. Most of the NOD present was slow reacting, so MnO4- could persist for weeks to months once the fast reacting fraction is depleted. Total NOD was not correlated with fraction fast/instantaneous or the reaction rate coefficients, thus indicating that NOD reactivity is independent of the total amount of NOD. Results from 48-h NOD measurements were also shown to be poor predictors of total NOD and should not be used to estimate long-term MnO4- consumption.