The potential for reductive dechlorination under microwave extraction conditions
Microwave-enhanced extraction is gaining in popularity because it allows for faster extraction, reduced solvent use, and high recovery efficiency compared to traditional methods such as Soxhlet extractions. The elevated temperatures and pressures applied to samples during microwave-enhanced extraction have the potential to accelerate abiotic degradative reactions producing artifactual apparent breakdown products absent from the original samples. In addition, humic substances have been shown to shuttle electrons between donors and acceptors. Because chlorinated organic compounds have proven to be good electron acceptors and because ferrous iron is often present in anoxic soils, the possibility appeared to exist that reductive dechlorination of chloroorganic compounds in samples of anaerobic soils and that this degradative reaction could be accelerated by the conditions applied during microwave-enhanced extraction.
Aqueous solutions of between 10 and 40 μg of PCP per ml were subjected to wide ranges of temperatures and pressures in the presence of 0.4 mg/ml of humic acid extracted from a commercial peat, in the presence of 400 μg/ml of FeSO4·7H2O, in the presence of both humic acid and ferrous iron, and in the presence of neither. These four treatments were tested for four days at room temperature. These treatments were also microwaved for 30 min. at 121, 145, and 170°C, for four hours at 160, and 170°C, and for one hour at 190°C, which was the operational upper limit of the microwave-extraction, pressure vessels. In none of the four treatments, following none of the time and temperature programs, were any additional apparent dechlorination products detected. Changing the solvent from water to ethyl acetate before microwaving 40 μg/ml PCP for 30 min. also failed to promote dechlorination. Microwaving a 20-μg/ml, aqueous solution of 2,3,4-trichlorophenol with the same combinations of humic acid and ferrous iron also failed to produce any evidence of reductive dechlorination. Four hours of microwaving at 160°C also failed to effect dechlorination of either 20 μg/g of PCP in any of four different soils or of 20 μg/ml of PCP in the usual four treatments in which the peat humic acid was replaced by Aldrich humic acid.
0542: Chemical engineering