Actual decay of tetrachloroethene (PCE) and trichloroethene (TCE) in a highly contaminated shallow groundwater system

Abstract

The Quaternary shallow aquifer that served as the subject of research in this work is under the influence of strong anthropopressure manifested by extreme values of contamination with both organic and inorganic substances (max. TOC 1.6 g/l and Cl− 10 g/l) from chemical production. The main attention was paid to the dehalogenation of organochlorine compounds. The research included chlorinated ethane's and chlorinated methane's. In the groundwater samples, the most common components from the group of chlorinated ethenes were determined, i.e. tetrachloroethene (PCE), trichlorethylene (TCE), dichloroethene (DCE) and vinyl chloride (VC) as well as typical components from the group of chlorinated methane's, i.e. tetrachloromethane (PCM), trichloromethane (TCM) and dichloromethane (DCM), were also made. The determinations were made by gas chromatography with electron capture detection (GC-ECD), after liquid-liquid extraction (vinyl chloride: gas chromatography with mass detection (GC-MS), after liquid-liquid extraction).PCE initially undergoes a classical decomposition as a result of dehalogenated reduction to TCE and the Cl− ion (Ai et al., 2019) under aerobic conditions around Eh +100 (+50) to 0 mV. While maintaining the double bond between the carbon atoms, TCE decays under slightly reducing conditions of around Eh -50 to -100 mV to dichloroethene (DCE) and Cl−. Therefore, in the event that the π bond between the carbon atoms is not retained and broken, the TCE breaks down into two functional groups CHCl= and CCl2=. When hydrogen and chlorine are added to the CHCl= functional group, trichloromethane (TCM) is formed, and in the case of two hydrogen atoms it is respectively dichloromethane (DCM). Additionally TCM undergoes a classical decomposition under aerobic conditions by dehalogenation to DCM and the Cl− ion. Chlorine and hydrogen atoms can analogously join the CCl2 = function group to form DCM. Commonly in Quaternary aquifers, often shallow ones, there are favorable conditions for the decomposition of these unstable substances by oxidation, both under the conditions of availability and lack of oxygen (with the presence of e.g. Fe2+ and Mn2+) and immediate, complete mineralization to CO2, H+ and Cl− ions.