Anode modification with palladium nanoparticles enhanced Evans Blue removal and power generation in microbial fuel cells
Introduction
Bioelectrochemical system (BES), as a promising technology, has drawn increasing interests recently for simultaneous pollutants removal and electricity generation (Logan, 2009). Microbes, as the biocatalyst in BES, can transform chemical energy stored in organic compounds to electrical energy. Azo dyes, characterized with a structure of aromatic ring and double bonds, widely exist in the effluent from textile industries. Azo dyes are toxic and hard to be degraded by microorganisms. BES has been explored for azo dyes degradation recently (Fernando et al., 2014, Gao et al., 2016). Although some anode exoelectrogens can transform azo dyes via co-metabolism, their microbial activity may be inhibited and power generation ability may be weakened due to the toxicity of azo dyes. Therefore, a long term pre-acclimation is generally required for the anode bacteria to adapt to azo dyes and degrade them for power generation (Wang et al., 2013).
Electro-catalysts loaded on anode electrodes could help to oxidize some organic molecules. For example, palladium nanoparticles (PdNPs) deposited on anode electrode could catalyze formate, lactate and acetate directly (Quan et al., 2015a). Pd nanoparticles, which were produced by microbial reduction, were also used for azo dye reduction in the presence of different electron donors, such as hydrogen, formate, acetate, ethanol and glucose (Quan et al., 2015b). Whether is it possible to accelerate azo dye reduction and power generation in the anode chamber of BES through anode decoration with Pd catalyst? This question has not been answered and deserves further study.
PdNPs based catalysts have attracted great attention due to large specific surface and more active sites for catalyzing (Erikson et al., 2011). PdNPs are generally prepared using chemical methods with many toxic chemical agents under harsh conditions (Moon et al., 2014, Watts et al., 2017). Recent studies have found that some microorganisms, such as Paracoccus denitrificans (Bunge et al., 2010) and Shewanella oneidensis (S. oneidensis) (Hennebel et al., 2009), can reduce soluble Pd (II) and form biogenic nano-palladium in periplasms or cell walls. This microbial reduction method occurred under benign and gentle conditions with much less chemical agents (Hennebel et al., 2009). Biogenic Pd also shows the advantages of high catalytic activity and good biocompatibility (Yates and Logan, 2014).
In this study, Pd nanoparticles were prepared through microbial reduction by a strain S. oneidensis. Anode electrode was modified using Pd catalyst and applied in microbial fuel cells (MFCs) aimed to improve pollutant removal and power generation from azo dye containing wastewater. Evans Blue (EB) was selected as the studied azo dye, because it is widely used as chemical and biochemical analysis agent, and the removal of Evans Blue in BES has seldom been reported (Antonin et al., 2015). The electrochemical behavior and morphological property of the Pd electrode was characterized using Cyclic Voltammograms (CVs) and Scanning Electron Microscopy (SEM), respectively. The performance of the MFCs with Pd anodes was evaluated from pollutant removal rate and power generation capacity. Effects of Pd decoration on microbial community changes of anode biofilm were investigated through high-throughput sequencing based on Illumina Miseq platform.
Section snippets
Microbial culture and Pd NPs preparation
S. oneidensis MR-1was used as the bacteria to form PdNPs. This strain was from a lab from Tsinghua University (Beijing, China). The strain S. oneidensis MR-1 was capable of forming Pd NPs in cells via reducing Pd (II) ion (Windt et al., 2005). S. oneidensis MR-1 was first cultured through incubation in Luria-Bertani (LB) medium for 24 h at 28 °C. The microbial culture was then collected and production of Pd proceeded in M9 medium according to the method described previously (Windt et al., 2005
Electrochemical characteristics of Pd modified electrode
Cyclic voltammetry was performed to evaluate the electrochemical activity of different electrodes in 5.0 mM Fe(CN)6 3-/4- and 0.2 M Na2SO4 solutions (Fig. 1). As shown in Fig. 1(a), the CV graph of the Pd electrode displayed apparent peaks of current density at the potential of 293 mV (reductive peak) and 490 mV (oxidative peak), resulting from redox reaction of potassium hexacyanoferrate. The bare electrode showed much smaller redox peaks compared to the Pd electrode, suggesting higher
Conclusions
This study presents a promising strategy to update MFCs for azo dye wastewater treatment and electricity recovery based on anode decoration with PdNPs. The MFCs with the Pd anode showed a higher Evans Blue decolorization rate, maximum power density, CE and lower electron transfer resistance compared to that with the control anode. Pd catalyst on the anode electrode endowed the electrode with direct Pd catalytic ability and therefore accelerated azo dye reduction in the presence of suitable
Acknowledgements
This work of supported by Natural Science Foundation of China (51678055).
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