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 (MnO(4)(-)) consumption and PCE oxidation as a function of the MnO(4)(-) 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 MnO(4)(-) by reaction with the sediment, termed the natural oxidant demand (NOD), is the primary reaction with regards to quantification of MnO(4)(-) consumption. Dissolved PCE in concentrations up to 100 mg/l in the sediments investigated is not a significant factor in the total MnO(4)(-) consumption. Consumption of MnO(4)(-) increases with an increasing initial MnO(4)(-) concentration. The sediment type is also important as NOD is (generally) higher in clayey than in sandy sediments for a given MnO(4)(-) concentration. For the different sediment types the typical NOD values are 0.5-2 g MnO(4)(-)/kg dry weight (dw) for glacial meltwater sand, 1-8 g MnO(4)(-)/kg dw for sandy till and 5-20 g MnO(4)(-)/kg dw for clayey till. The long term consumption of MnO(4)(-) and oxidation of PCE can not be described with a single rate constant, as the total MnO(4)(-) 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 MnO(4)(-) 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 MnO(4)(-). The consumption of MnO(4)(-) by reaction with the reactive species in the sediment is the result of several parallel reactions, during which the reaction between the contaminant and MnO(4)(-) also takes place. Hence, application of low MnO(4)(-) 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.

Role of diffusion in chemical oxidation of PCE in a dual permeability system.

The effect of in situ chemical oxidation (ISCO) with permanganate (MnO4-) on tetrachloroethene (PCE) in a dual permeability system consisting of low permeability clay with high permeability sand lenses was investigated by two-dimensional laboratory experiments. The experiments imitate a field remediation at a former dry cleaning facility in Denmark. Results from laboratory experiments and field observations both show that after an application of MnO4- in the sand layer, the diffusion rate into the matrix is decreased due to reaction with PCE and the natural oxidant demand (NOD) related to the clayey till. A narrow but very efficient reaction zone is created in the clayey till. Initiallythe zone developed rapidlyfollowed by a slower expansion with time. PCE will counter diffuse into the reaction zone, where it will be degraded as long as MnO4- is present. A mass balance for the laboratory experiment revealed that the reaction between MnO4- and the clayey till was responsible for up to 90% of the total MnO4- consumption. Based on laboratory experiments, the high MnO4- consumption from reaction with clayey till appears to have been the limiting factor for the oxidation of PCE at the field site and is thus impairing the efficiency of ISCO as a remedy.

Sorption and degradation of the herbicide 2-methyl-4,6-dinitrophenol under aerobic conditions in a sandy aquifer in Vejen, Denmark.

A pulse (7 days) and a continuous (216 days), natural gradient field injection experiment with herbicides, including 2-methyl-4,6-dinitrophenol (4,6-dinitro-o-cresol, abbreviated DNOC), and a bromide tracer were conducted in a shallow, aerobic aquifer near Vejen, Denmark. The pulse and continuous plume were monitored in a dense, three-dimensional monitoring network installed in the aquifer downgradient of the injection. The sorption and degradation of DNOC were evaluated based on moment analysis of breakthrough curves, cross sections, and snapshots of the DNOC plume and supported by results from laboratory experiments. Significant and spatially variable sorption of DNOC (Kd range, 0.10-0.98 L/kg) was observed due to a specific binding of DNOC to clay minerals. The spatial variation was mainly a result of variation in pH, with stronger sorption at lower pH, whereas other factors such as cation composition on the solid matrix appeared to be negligible. Significant degradation of DNOC in the aquifer was revealed by moment analysis of data from the continuous field injection experiment. Degradation of DNOC in the field was slow and/or subject to long lag phases, and the data suggested spatially varying degradation potentials. This was supported by the laboratory experiments. The potential for natural attenuation of DNOC in aerobic aquifers appears promising.