Unexpected bias of freeze-drying on the performance assessment of chemical oxidation of soils contaminated by polychlorinated biphenyls
Résumé
Processes based on chemical oxidation are widely used for environmental remediation. Analysis of environmental samples such as soils often requires a freeze-drying step prior to the measurement of the concentration of contaminants. However, the effect of freezing on the quantification of the removal efficiency is poorly known. Here we studied the removal of polychlorinated biphenyls (PCBs) in contaminated soils using H 2 O 2 , persulfate, permanganate and ferrate(VI), firstly at room temperature. To assess the chemical oxidation performance, soil samples were freeze-dried and extracted with hexane and PCBs were quantified in the extract by gas chromatography–mass spectrometry. Unexpectedly, in batch samples which were freeze-dried after 5 min of ferrate(VI) oxidation, the removal of PCBs was about two times higher than in samples lyophilized after 3 h of reaction time. Similar oxidation treatments performed at − 20 °C, instead of room temperature, showed that the enhanced degradation in soil is related to freezing conditions. This phenomenon is explained by the freezing concentration effect, i.e., the increased concentration of reactants in the liquid brine. Although this freezing effect is already known in aqueous media, this is the first time it is demonstrated in a heterogeneous soil system, particularly for the remediation by chemical oxidation. The non-consideration of this unforeseen effect during the soil preparation in laboratory studies may lead to an incorrect assessment of oxidation performance. © 2019, Springer Nature Switzerland AG.
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