Electrostimulated bio-dechlorination of trichloroethene by potential regulation: Kinetics, microbial community structure and function
Graphical abstract
Introduction
Trichloroethene (TCE), a typical representative of chlorinated aliphatic hydrocarbons (CAHs), is among the most ubiquitous and persistent organic pollutants found in groundwater and aquifers [1]. TCE is carcinogenic to humans by all routes of exposure and poses a potential human health hazard for noncancer toxicity [2]. Bioremediation via anaerobic dechlorination is regarded as an economic and environmental-friendly approach for the treatment of groundwater contaminated CAHs [3]. The approach relies on organohalide-respiring bacteria, which use CAHs as a terminal electron acceptor for respiration and metabolism, with less/none halogenated hydrocarbons sequentially generated as end products [4]. For either ex situ bioreactors or in situ bioremediation sites, the mostly applied electron donors are organic substrates which release H2 upon fermentation [5], [6]. However, a number of drawbacks were frequently emerged, including the competition of electrons between dechlorination and competing metabolisms, accumulation of undesirable toxic by-products, and the poor control of dechlorination process [7].
Recently, bioelectrochemical systems (BESs) have been developed for the accelerated reduction of various refractory organic pollutants including nitroaromatics, halogenated aromatics, azo dyes and etc. [8], [9], [10], [11]. As far, several biocathode BESs have been established for CAHs dechlorination by acclimating Geobacter, Dehalococcoides or the highly enriched dechlorination consortia [12], [13], [14], [15], [16], [17], [18]. In view of application perspective, it is very difficult to consider dechlorinating isolates or consortia since most of them possess either narrow ecological niches or strict nutrients/redox potential demands [4], [19], [20]. In principle, biocathode systems can manipulate the redox potential to create the favorable environment for biodechlorination either through the direct electron transfer from electrode surface or via the intermediate electrolytic generation of H2 [21]. It has been demonstrated that electron transfer pathway between electrode and microorganism partially relied on the applied cathode potential [22], [23], [24]. The rate and extent of CAH dechlorination, as well as the competition for available electrons, are also highly dependent on the set cathode potential [17], [22], [25]. Aulenta et al. revealed about 3.7 times higher TCE dechlorination rate was obtained as cathode potential decreased from −0.25 to −0.45 V, however, over 60% of electric current would be consumed for the other microbial metabolisms (like methanogenesis) with the potential of −0.45 V [22]. Sáez et al. found a current density under 4 mA cm−2 was sufficient enough for the reasonable PCE conversion rate (>85%) and degradation efficiency (>65%) on the lead-made cathode [25]. These studies demonstrated that the regulation of cathode potential was as an essential element for optimizing the electrostimulated CAHs dechlorination. It is known that the formation of an effective cathode biofilm is vital for the dechlorination performance of a biocathode [14], [23], [26]. By supplying the different negative potentials during the acclimation of anaerobic sludge, the microbial community structure and composition in the cathodic biofilm would vary and further influence the dechlorination capacities and characteristics. However, as far, the microbial community structure variation laws and the cooperative relationship of functional populations in response to the different cathode potential settings, as well as the related dechlorination functions and characteristics (efficiency, kinetics and metabolites) remain poorly understood.
In this study, biocathode systems were established for TCE dechlorination through acclimatization with anaerobic sludge given four different constant cathode potential settings (−0.06 V, −0.26 V, −0.46 V and −0.66 V vs standard hydrogen electrode, SHE). The dechlorination feasibility, characteristics and functional communities were investigated thoroughly under the different cathode potentials. Objectives of this study were to reveal whether and how potential regulation affects microbial community structure and function in cathodic biofilm as well as the influence law on TCE dechlorination rate and metabolic pathway, through a comprehensive microbial analysis conducted at molecular and genetic expression levels.
Section snippets
System construction
A schematic diagram of the bioelectrochemical reactors employed in this study is shown in Fig. S1. The reactors consisted of two equally sized cylindrical borosilicate glass chambers (4 cm in diameter and 8 cm in length with an effective volume of 80 mL) and separated by cation exchange membranes (CEM, Ultrex CMI-7000, Membranes International, Ringwood, NJ, U.S.). The two chambers were fixed together with an aluminum clamp and rubber seal, ensuring airtightness. Graphite fiber brushes (both
Cathode potential regulated dechlorination activities
After inoculation with anaerobic sludge for approximately 60 days, taking conditions with a set potential of −0.26 V as an example, the E24h for TCE approached over 90% in the seven successive cycles (Fig. S2); correspondingly, currents underwent the similar trends in different cycles, starting at the relatively high levels (0.16–0.16 mA) and dropping to less than 0.05 mA after 24 h. The similar current variation trend and the efficient TCE dechlorination capacity were observed under the
Conclusions
The variation of cathodic potential settings gave rise to the significant change in bacterial community structure and trichloroethene dechlorination function in constructed biocathodes. The optimal trichloroethene dechlorination capacity was found at a cathode potential of −0.26 V (dechlorination rate of 1.01 mmol/L·d), which was about 4.3 times higher than the opened circuit. The higher (−0.06 V) or lower potentials (−0.46 and −0.66 V) resulted in the decreased dechlorination capacities, but
Acknowledgments
This research was supported by National Natural Science Foundation of China (No. 31400104, No. 21577162), by Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. ES201806), by Special Financial Grant from China Postdoctoral Science Foundation (2015T80359), by Natural Science Foundation of Heilongjiang Province China (No. C2018035).
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