14.4.4: Chlorinated solvents

Chlorinated solvents are relatively common contaminants that can cause significant adverse health effects. There are numerous compounds that fall within the category of chlorinated solvents. Here we focus on two of the most common, perchloroethylene (PCE) and trichloroethylen (TCE), as well as their degradation products (Table \(14.1\)). These solvents have numerous commercial and industrial uses, including use in dry cleaning and as degreasing agents and can be added to soil and groundwater from point sources, including improper disposals, accidental spills, and leaks from storage reservoirs (DeSimone et al., 2014). Groundwater contaminated with chlorinated solvents most commonly occurs at shallow depths under urban areas, reflecting their uses (DeSimone et al., 2014). Consumption of water contaminated with these solvents has been linked to liver, kidney, nervous system, and circulatory problems and increased risks of cancer. As such, US EPA maximum contaminant levels for these solvents are set to very low concentrations (see Table \(14.1\)).

Table \(14.1\): Composition and maximum contaminant levels for some common chlorinated solvents

Chlorinated solvent	Acronym	Formula	MCL* \((\mu \text{g/L})\)
Tetrachloroethylene	PCE	\(\text{C}_{2} \text{Cl}_{4}\)	5
Trichloroethylene	TCE	\(\text{C}_{2} \text{HCl}_{3}\)	5
1,1-Dichloroethylene	1,1-DCE	\(\text{C}_{2} \text{H}_{2} \text{Cl}_{2}\)	7
cis-1,2-Dichloroethylene	cis-DCE	\(\text{C}_{2} \text{H}_{2} \text{Cl}_{2}\)	70
trans-1,2-Dichloroethylene	trans-DCE	\(\text{C}_{2} \text{H}_{2} \text{Cl}_{2}\)	100
Vinyl chloride	VC	\(\text{C}_{2} \text{H}_{3} \text{Cl}\)	2
*US EPA maximum contaminant levels (MCLs) for public water systems available at https://www.epa.gov/ground-water-and-drinking-water/national-primary-drinking-water-regulations#Organic

Chlorinated solvents exist as dense non-aqueous phase liquids (DNAPLS) that sink through the subsurface until they encounter low-permeability sediment or bedrock (Fig. \(14.9\)). Dissolution of the DNAPL components into the adjacent groundwater creates an aqueous phase contaminant plume. Some of the DNAPL can also volatilize, forming a vapor plume in the unsaturated pore space overlying the water table.

Figure \(14.9\): Schematic illustration of the spreading of light non-aqueous phase liquid (LNAPL) and dense non-aqueous phase liquid (DNAPL) in the subsurface. LNAPLs are less dense than water and thus can accumulate at the water table. In contrast, DNAPLs are denser than water and thus can sink below the water table until they encounter a low permeability (k) surface. Both LNAPLs and DNAPLs volatilize and dissolve, creating vapor and groundwater plumes, respectively.
https://commons.wikimedia.org/wiki/File:NAPLs.jpg

PCE and TCE are resistant to degradation under oxic conditions. Under anoxic conditions, However, microorganisms can degrade chlorinated solvents through what is referred to as organohalide respiration (Adrian and Löffler, 2016). Although we often think of organic compounds as potential electron donors, some can also serve as terminal electron acceptors, and that is the case for chlorinated solvents. Microorganisms can oxidize organic electron donors or dihydrogen and transfer the electrons to chlorinated solvents (Debruin et al., 1992). As an example, the following reaction describes dihydrogen oxidation coupled with reduction of PCE to TCE: \[\text{C}_{2} \text{Cl}_{4} + \text{H}_{2} \longleftrightarrow \text{C}_{2} \text{HCl}_{3} + \text{H}^{+} + \text{Cl}^{-}\]

Under ideal conditions, microorganisms remove the chlorine atoms from organohalides in stepwise reactions that ultimately produce ethylene \(\left(\text{C}_{2} \text{H}_{4}\right)\), which is nontoxic (Lee et al., 1998) (Fig. \(14.10\)). However, the reactions can also stall at harmful intermediates such as cis-DCE or VC (Table \(14.1\)), allowing their concentrations to increase.

Organohalide respiring bacteria are widespread, possibly reflecting the fact that some organohalides occur naturally at low concentrations in diverse environments (Adrian and Löffler, 2016). However, bacterial genus Dehalococcoides is the only group known to contain species capable of completing the final steps of degradation, that is reduction of cis-DCE to VC and then to ethylene (Atashgahi et al., 2016). Thus, factors that influence growth of members of this genus can in turn determine the extent of degradation. Among those factors, the availability of dihydrogen seems to be particularly important, given that all isolated obligate organohalide reducers and many facultative organohalide reducers use dihydrogen as an electron donor (Richardson, 2016). Moreover, acetate availability is also important because it is required as a carbon source by some organohalide reducers including Dehalococcoides (Richardson, 2016). Both dihydrogen and acetate are produced by fermenting microorganisms in anoxic environments, and thus their roles in organohalide degradation are essential.

Figure \(4.10\): Sequential removal of chlorine during organohalide respiration with dihydrogen as the electron donor. Acronyms are defined in Table \(14.1\). At each step, two electrons from dihydrogen oxidation are added to the organohalide, reducing it and causing its chlorine atoms to be replaced by hydrogens.
https://commons.wikimedia.org/wiki/File:Organohalide_respiration.jpg