Enhanced bioelectroremediation of a complexly contaminated river sediment through stimulating electroactive degraders with methanol supply
Introduction
As an important component of aquatic environment, river sediments accommodate plentiful inorganic/organic matters and microbial communities. Microbial consortium in sediment is an important element for the self-sustaining stability of the entire aquatic ecosystem [1]. Currently, rapid industrialization and urbanization have led to the release of toxic pollutants, such as persistent organic pollutants, pesticides, and emerging pollutants (e.g. antimicrobial agents) into the aquatic environments and mainly accumulated in the aquatic sediments [[2], [3], [4], [5]]. The contaminated sediments will further affect the overlying water (o-water) quality for a long time, and even could potentially contaminate groundwater [6]. This is an important secondary pollution source for water bodies and poses a serious threat to environmental safety and human health [7].
Unfortunately, the natural self-purification of the contaminated aquatic environment is a very slow process under both aerobic and anaerobic conditions and needs a long recovery time to its natural state, even though there are no new pollutants discharged into the river [8]. The restoration of complexly contaminated sediments is a major social requirement to ensure the safety of aquatic ecological environment [9,10]. Therefore, in order to reduce the pollution levels, improve the water quality, and protect the water resources [11], it is very urgent to develop efficient, environment friendly, and sustainable remediation technologies for the cleaning up of complexly contaminated sediments.
Bioremediation is a potential and economical method compared with the physical and chemical remediation processes [12], usually commercially applied at large scale. Bioaugmentation with biodegradative bacteria is one of the bioremediation strategies for the remediation of environmental pollutants [13,14]. However, the colonization and maintenance of the biodegradative bacteria in the complexly contaminated environments, such as in anoxic sediments, is an important prerequisite for efficient bioremediation [4]. In situ biostimulation (by supply of a slow-release carbon substrate, low carbon substrate or electron acceptor such as nitrate) is another important bioremediation strategy which has been extensively employed in the contaminated groundwater and sediment sites [[15], [16], [17], [18], [19]]. Sediment microbial fuel cells (SMFCs) have attracted growing attention for their merits including accelerated degradation, sustainability, relatively controllability, and environmental benignity [20]. Concretely, SMFC employs electrochemically active microorganisms within sediments to catalyze organic pollutants oxidative reaction in the anode, and the cathode is generally placed in the o-water layer (commonly employed oxygen as electron acceptor). Various organic pollutants (e.g. benzene, toluene, naphthalene, phenanthrene, pyrene, and benzo[a]pyrene) could be oxidized by inserting an anode with electricity generated simultaneously [8,[20], [21], [22], [23], [24]]. However, it is worth noting that total organic carbon (TOC) content in the sediment could obviously influence the SMFC performance. SMFC with low TOC showed low power output. The low anode respiration activity may be related to the available substrates limitation in the sediment [25,26]. Therefore, it is possible to selectively enrich electroactive degraders and further stimulate the anodic biofilm respiration and total degradation activity by injecting low-molecular-weight carbon substrate (e.g. methanol) into the low TOC containing contaminated sediments. However, the enhanced bioremediation of such contaminated river sediments by joint electrical and exogenous methanol stimulation has not yet been demonstrated.
The objectives of this study were to test (i) whether the electrical stimulation could enhance the bioremediation efficiency of a low TOC containing contaminated river sediment by a cylindric SMFC; (ii) whether the methanol stimulation could further enhance the bioelectroremediation efficiency and (iii) whether the joint stimulation (electricity and methanol) could significantly alter the overall microbial community structure and selectively enrich specific functional genera within a 200 d remediation course. This study highlights a new strategy to enhance the bioremediation of the complexly contaminated sediments and offers a new insight into the response of electroactive degraders to the joint stimulation process.
Section snippets
Sediment and chemicals
Sediment samples were collected from Ashi River, Harbin, China (126°46′N, 45°48′E). The o-water was also collected. A chemical plant and Garage place in the upper reaches of the river. The sediment was collected from 10 to 50 cm below the water/sediment interface, and transported into laboratory on ice. After being transported to the laboratory, the sediment was further sieved to remove the solid larger than 2 mm diameter. Sediments were homogenized under a stream of N2 for 30 min.
Total
Current production with methanol addition
Electric current increased gradually in the reactors after the connection of the external resistance and the electrodes. The current reached 0.2 mA within 20–25 days and then showed some fluctuations. On 43rd d, 100 mM methanol was added to the reactor Met-2. The current profile showed a sharply decline. On 70th d, the current began to reinstate. During the period of 70–110th d, the currents of the reactor MFC and Met-1 showed some fluctuations. On 110th d, additional 100 mM methanol was added
Conclusions
The joint stimulation obviously enhanced the organic pollutants degradation efficiency in the electrode district sediment compared with those in the non-electrode district sediment without joint stimulation. The electrode and non-electrode district microbial communities were significantly different and the overall sediment communities were shaped by the variables of characteristic and total pollutants degradation efficiencies. Key electroactive degraders (Geobacter and Desulfobulbus) dominated
Acknowledgments
This study was supported by the National Natural Science Foundation of China (No. 31500084), and the State Key Laboratory of Urban Water Resource and Environment of Harbin Institute of Technology (No. 2016DX03).
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