Biogeochemical modelling of in situ biodegradation and stable isotope fractionation of intermediate chloroethenes in a horizontal subsurface flow wetland

The use of wetlands as a bioremediation treatment technology for halogenated hydrocarbons has recently gained interest. However, the impact of biogeochemical dynamics over a wetland's life-span on contaminant removal remains poorly understood. Based on the results of a constructed wetland experiment, we established a reactive transport model (RTM) that includes the transition from oxidation to reductive dehalogenation of cis-1,2-dichloroethene (cDCE) and corresponding carbon isotope signatures to quantify removal pathways in wetland environments. cDCE was removed during the entire life-span of the wetland despite significant changes in its biogeochemistry. Model results showed that during the oxic phase, 88% of cDCE throughput was removed, with oxidative cDCE mineralization accounting for 53.3 mg m−2 d−1 (45%) and carbon isotope enrichment factors ranging from ɛDCE-Oxid =−1 to − 4.0 ‰. During the anoxic phase, the wetland removed 85% of cDCE mainly by volatilization and reductive dehalogenation (εDCE−Dehal=−32   to   −36‰)εDCE−Dehal=−32   to   −36‰, with the latter degrading 21.3 mg m−2 d−1 of cDCE (28%) and producing 13.7 mg m−2 d−1 of vinyl chloride (VC). cDCE oxidation (εDCE-Oxid=−9.4   to   −11.5‰)εDCE-Oxid=−9.4   to   −11.5‰ accounted for only 9.1 mg m−2 d−1 (12%). 4.4 mg m−2 d−1 of the VC produced in the wetland was biodegraded via oxidation (32%), while 2.8 mg m−2 d−1 (20%) was converted into non-toxic ethene via dehalogenation. Plant uptake of cDCE accounted for up to 23% of the mass removal during the oxic phase and 16% during the anoxic phase (6% for VC). Altogether, the RTM enables a mechanistic representation and quantitative prediction of the fate of intermediate chloroethenes in redox-dynamic environments such as wetlands.

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