Quantification of potassium permanganate consumption and PCE oxidation in subsurface materials

A series of laboratory scale batch slurry experiments were conducted in order to establish a data set for oxidant demand by sandy and clayey subsurface materials as well as to identify the reaction kinetic rates of permanganate (MnO4−) consumption and PCE oxidation as a function of the MnO4− concentration. The laboratory experiments were carried out with 31 sandy and clayey subsurface sediments from 12 Danish sites. The results show that the consumption of MnO4− by reaction with the sediment, termed the natural oxidant demand (NOD), is the primary reaction with regards to quantification of MnO4− consumption. Dissolved PCE in concentrations up to 100 mg/l in the sediments investigated is not a significant factor in the total MnO4− consumption. Consumption of MnO4− increases with an increasing initial MnO4− concentration. The sediment type is also important as NOD is (generally) higher in clayey than in sandy sediments for a given MnO4− concentration. For the different sediment types the typical NOD values are 0.5–2 g MnO4−/kg dry weight (dw) for glacial meltwater sand, 1–8 g MnO4−/kg dw for sandy till and 5–20 g MnO4−/kg dw for clayey till. The long term consumption of MnO4− and oxidation of PCE can not be described with a single rate constant, as the total MnO4− reduction is comprised of several different reactions with individual rates. During the initial hours of reaction, first order kinetics can be applied, where the short term first order rate constants for consumption of MnO4− and oxidation of PCE are 0.05–0.5 h− 1 and 0.5–4.5 h− 1, respectively. The sediment does not act as an instantaneous sink for MnO4−. The consumption of MnO4− by reaction with the reactive species in the sediment is the result of several parallel reactions, during which the reaction between the contaminant and MnO4− also takes place. Hence, application of low MnO4− concentrations can cause partly oxidation of PCE, as the oxidant demand of the sediment does not need to be met fully before PCE is oxidised.