Field study of chlorinated aliphatic hydrocarbon degradation in contaminated groundwater via micron zero-valent iron coupled with biostimulation

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Chlorinated aliphatic hydrocarbons (CAHs) pollution poses a severe threat to human health and is persistent in groundwater. Although chemical reduction via bioremediation is effective, it is still difficult to achieve complete and rapid dechlorination. Recently, the combination of zero-valent iron (ZVI) and biostimulation has been considered to be one of the most promising strategies, but field studies of this technology are scarce. In a typical site contaminated by various types of CAHs, basic physicochemical parameters of groundwater, CAHs and their product concentrations, and microbial abundance and diversity were monitored after a remediation slurry containing both micron ZVI (mZVI) and biostimulation components was directly injected into an aquifer. Results showed that groundwater could maintain a low oxidation-reduction potential (ORP), a neutral pH, and anoxic conditions after different degrees of fluctuations immediately following treatment with the remediation slurry, which are conditions that support the reductive dechlorination of CAHs. The injection also caused sulfate reduction and an increase in the total organic carbon concentration. After 253 days post-injection, approximately 97.5% of chlorinated ethylene and 80.2% of chlorinated ethane were removed. An increase in ethylene at day 67 post-injection reflected the complete dechlorination of CAHs. The occurrence of chloroethane suggested that hydrogenolysis dechlorination was one of the main degradation pathways for chlorinated ethane, and hints at the activation of biological dechlorination. Additionally, the total bacterial counts increased by 2–3 orders of magnitude after 253 days post-injection, the microbial species richness decreased and the activity of a variety of potential degrading bacteria was stimulated. This work demonstrates that mZVI and biostimulation can be combined to achieve the efficient removal of various CAHs from contaminated groundwater sources.