Abstract
An innovative bioelectrochemical reductive/oxidative sequential process was developed and tested on a laboratory scale to obtain the complete mineralization of perchloroethylene (PCE) in a synthetic medium. The sequential bioelectrochemical process consisted of two separate tubular bioelectrochemical reactors that adopted a novel reactor configuration, avoiding the use of an ion exchange membrane to separate the anodic and cathodic chamber and reducing the cost of the reactor. In the reductive reactor, a dechlorinating mixed inoculum received reducing power to perform the reductive dechlorination of perchloroethylene (PCE) through a cathode chamber, while the less chlorinated daughter products were removed in the oxidative reactor, which supported an aerobic dechlorinating culture through in situ electrochemical oxygen evolution. Preliminary fluid dynamics and electrochemical tests were performed to characterize both the reductive and oxidative reactors, which were electrically independent of each other, with each having its own counterelectrode. The first continuous-flow potentiostatic run with the reductive reactor (polarized at −450 mV vs SHE) resulted in obtaining 100% ± 1% removal efficiency of the influent PCE, while the oxidative reactor (polarized at +1.4 V vs SHE) oxidized the vinyl chloride and ethylene from the reductive reactor, with removal efficiencies of 100% ± 2% and 92% ± 1%, respectively.
Highlights
Chlorinated aliphatic hydrocarbons (CAHs) are among the most frequent groundwater contaminants due to their intense use as solvents or degreasing agents in the mechanical industry coupled with unregulated and inappropriate disposal procedures [1]
The highly chlorinated CAHs can be removed by a reductive dechlorination (RD) reaction through sequential steps in which the chlorinated molecule loses a chlorine atom at each step until the complete dechlorination of the molecule [7]: For this reaction, the microorganisms usually need an electron donor such as hydrogen, which can be produced by fermenting organic substrates that provide for the slow release of hydrogen [8]
The innovative reactor configurations permitted an effective sequential reductive/oxidative bioelectrochemical process that permitted the complete mineralization of perchloroethylene into nonharmful compounds, driving microbial dechlorinating metabolism through the use of electric currents
Summary
Chlorinated aliphatic hydrocarbons (CAHs) are among the most frequent groundwater contaminants due to their intense use as solvents or degreasing agents in the mechanical industry coupled with unregulated and inappropriate disposal procedures [1]. CAHs, due to their low solubility, are persistent in the environment; their low solubility is sufficiently high to overcome the concentration limits established for human health [2]. Engineered bioremediation and natural attenuation (NA) processes permit CAH removal from contaminated groundwater directly in situ, which usually results in an environmentally friendly and cost-effective treatment with respect to conventional technologies such as pump-and-treat [3,4,5]. The engineered bioremediation consists of stimulating the microbial activity that is naturally present in the soil/groundwater matrix to remove pollutants through their metabolic activity [6]. In terms of chlorinated ethenes, such as tetrachloroethene (PCE), the main limitation and risk of RD is represented by its last
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