Chemical Treatment of a Contaminated Aquifer: Impact of Different Oxidants and Reductants on CAHs and Post-Chemical Biodegradation

摘要

Here we report on a lab-scale experiment where a homogenized contaminated aquifer slurry from a Flemish site was subjected to three reducing (nano-, micro-, and granular zero-valent iron) and 3 oxidizing agents (permanganate, cold and activated persulphate), each in three different concentrations. Over time the evolution of the contaminants and breakdown products (PCE, TCE, cDCE, vinylchloride, dichloromethane, 1,2-dichloroethane, toluene, ethylbenzene, ethene, ethane, acetylene) was followed, and samples for characterisation of the soil matrix have been taken. The results showed that, for the tested aquifer, chemical reduction had a much higher impact on the chlorinated contaminant concentration than chemical oxidants. Even repeated applications of the chemical oxidants did not lead to a total removal of the pollution, while a single application of zerovalent iron did remove most of the chlorinated contaminants. The very high matrix demand that was observed may be part of the explanation here. This emphasizes the importance of considering the redox state of the aquifer during the selection of the chemical reagent. BTEX-compounds were not degraded by chemical reduction as expected. After three months of incubation, when the chemical oxidants were no longer reactive, lactate was added to evaluate the remaining biodegradation potential of soils after chemical treatments. In the original aquifer material, without chemical treatment, an anaerobic biodegradation potential for a number of the chlorinated compounds was observed. The chemical treatments, however, did impact the biodegradation in most of the chemical oxidation cases. While in the test conditions with the lowest oxidant concentration biodegradation activity was observed after a lag phase, no activity was seen in the test conditions where higher doses were applied. The highest negative impact was observed with permanganate. The redox potential seems to be a crucial parameter. In the presence of chemical reductants, elevated concentrations of vinyl chloride (only a major degradation product of biodegradation and not of chemical reduction) suggests simultaneous occurrence of biodegradation and chemical reduction.