Xianning Li

Enhanced degradation of azo dye by a stacked microbial fuel cell-biofilm electrode reactor coupled system

In this study, a microbial fuel cell (MFC)-biofilm electrode reactor (BER) coupled system was established for degradation of the azo dye Reactive Brilliant Red X-3B. In this system, electrical energy generated by the MFC degrades the azo dye in the BER without the need for an external power supply, and the effluent from the BER was used as the inflow for the MFC, with further degradation. The results indicated that the X-3B removal efficiency was 29.87% higher using this coupled system than in a control group. Moreover, a method was developed to prevent voltage reversal in stacked MFCs. Current was the key factor influencing removal efficiency in the BER. The X-3B degradation pathway and the types and transfer processes of intermediate products were further explored in our system coupled with gas chromatography-mass spectrometry.

Bioelectrochemical approach for control of methane emission from wetlands

To harvest electricity and mitigate methane emissions from wetlands, a novel microbial fuel cell coupled constructed wetland (MFC-CW) was assembled with an anode placing in the rhizosphere and a cathode on the water surface. Plant-mediated methane accounted for 71-82% of the total methane fluxes. The bioanode served as an inexhaustible source of electron acceptors and resulted in reduced substantial methane emissions owing to electricigens outcompeting methanogens for carbon and electrons when substrate was deficient. However, when supplying sufficient organic carbon, both electricity and methane increased, indicating that electrogenesis and methanogenesis could co-exist in harmony. Direct methane emission (diffusion/ebullition) and plant-mediated methane emission were affected by operating conditions. Methanogenesis was significantly suppressed (∼98%) at HRT of 96h and with external resistance of 200Ω, accompanied with improved coulombic efficiency of 14.9% and current density of 187mA/m2. Contrarily, change of electrode polarity in the rhizosphere led to more methane efflux.

Elimination of estrogens and estrogenic activity from sewage treatment works effluents in subsurface and surface flow constructed wetlands

The effluents discharged from sewage treatment works (STWs) are major sources of environmental estrogens, which poses an urgent need to explore appropriate techniques for effluent-polishing. In light of the debate concerning the effectiveness of constructed wetlands (CW) for the elimination of estrogens, the present study evaluated and compared the performance of two basic types of CW, free water surface (FWS) and subsurface flow (SSF) systems. Two FWS and two SSF field CW mesocosms were fed continuously with an STW effluent. All the mesocosms provided an effective elimination of estrogens and estrogenic activity. Unexpectedly, the performance of FWS mesocosms was not inferior to that of SSF mesocosms. Additional shading experiments demonstrated that the presence of filamentous green algae along with the sunlight enhanced the removal of estrogens and estrogenic activity in FWS mesocosms, enabling FWS mesocosms to perform comparably to SSF mesocosms. Microbial inhibition tests further indicated that Spirogyra sp. itself rather than algae-attached bacteria played an important role in the removal of estrogen and estrogenic activity.

Reductive dechlorination of hexachlorobenzene subjected to several conditions in a bioelectrochemical system

A microbial fuel cell (MFC) is a very promising way to remove organic pollutants. Hexachlorobenzene (HCB) is a widely used agricultural pesticide. In this study, single-chamber and membrane-less soil MFCs were constructed. The HCB was degraded to pentachlorobenzene (PeCB), tetrachlorobenzene (TeCB), and trichlorobenzene (TCB) in sequence by a reductive dechlorination process in soil MFCs. The influences of the external resistance, concentration of phosphate buffer, and electrode spacing in soil MFCs on the degradation rate and removal efficiency of HCB were analyzed. The results showed that the degradation rate and removal efficiency of HCB were increased when the external resistance decreased from 2000 to 20Ω, and also when the concentration of phosphate buffer increased. The anode area played a significant role in dechlorination of HCB. Altering the spacing of the reducing electrode resulted in a lower ohmic resistance in the soil MFCs. The ohmic resistance was negatively correlated with the removal efficiency and degradation rate (P<0.05). In conclusion, HCB removal efficiency could be enhanced by soil MFCs, the performance of which was improved by a decrease in external resistance and internal resistance, and an increase in phosphate buffer concentration, rather than just by shortening the electrode spacing.

Feedback of threshold via estimating sources and composition of sedimentary organic matter across trophic gradients in freshwater lakes

The quantity and quality of sedimentary organic matter (SOM) in relation to material and energy flows are crucial for understanding the current state and future development of lake systems, yet, characterization of organic matter sources and assessment of their relative contributions in different trophic-state lakes caused by anthropogenic impacts are scarcely known. In this study, for obtaining information concerning the source of SOM and its compositional diversity along different trophic gradients, a total of thirty-one sampling sites from four freshwater lakes located in China and Japan were performed by the molecular level analysis using source-specific fatty acid biomarkers. Results indicated that SOM in these lakes was composed of microalgae-, aquatic plant-, terrestrial plant- and bacteria-derived organic matters based on their fatty acid profiles. The scatter plot matrix exhibited correlations between these sources, however, only terrestrial plant-derived organic carbon was a well predictor for sediment TOC with strong, spatiotemporal dynamics. The source and composition of SOM were evidently influenced by lake trophic state with redundancy analysis. Moreover, increase of lake trophic state led to the relatively higher contribution of aquatic organic matter sources to SOM pool compared with terrigenous sources, as evidenced by significant correlations between the trophic state index [TSI (TP)] and the ratio of terrigenous to aquatic fatty acids (TARFA ratio). Yet, this changing trend became more gradual with higher trophic state and prevented the occurrence of regime shift from allochthonous to autochthonous dominant state by a threshold (0.683) of TARFA ratio. Together, a conceptual diagram was proposed, which highlighted the prevailing state of allochthonous source and implicated sedimentary organics in biogeochemistry cycle within freshwater lakes.

Electrode and azo dye decolorization performance in microbial-fuel-cell-coupled constructed wetlands with different electrode size during long-term wastewater treatment

Microbial-fuel-cell-coupled constructed wetlands (CW-MFCs) with various cathode layers were used for long-term azo dye wastewater treatment. Their performance was assessed using cathode diameters ranging from 20 to 27.5cm and the influence of plants at the cathode was also examined. Bioelectricity generation, ABRX3 decolorization, and chemical oxygen demand (COD) removal performances first increased and then decreased with increasing cathode diameter. The CW-MFCs with larger cathodes had an anoxic region at the cathode where ABRX3 was decolorized. This phenomenon has not been reported in previous research on MFCs using traditional air cathodes. Anode performance was influenced by the cathode. The CW-MFC with a cathode diameter of 25cm showed the best electrode performance, and the highest voltage and power density were 560mV and 0.88W/m3, respectively. The highest ABRX3 decolorization and COD removal volumes were 271.53mg/L and 312.17mg/L, respectively.

The performance of the microbial fuel cell-coupled constructed wetland system and the influence of the anode bacterial community

ABSTRACT In order to analyse the influences of substrate and electrode on the performance of microbial fuel cell-coupled constructed wetland (CW-MFC), the electrical generation efficiencies, the decolourization mechanism of reactive brilliant red X-3B, and the microbial communities in the anode were investigated. The closed circuit reactor fed with a mixture of X-3B and glucose (166.7 mg/L X-3B and 140 mg/L glucose) (the mixture CC reactor) got a decolourization rate of 92.79%, which was higher than the open circuit reactor (the mixture OC reactor) and the reactor fed with X-3B (the X-3B reactor). The mixture CC reactor got a maximum power density of 0.200 W/m3, which was much higher than the X-3B reactor. The intermediates produced by X-3B decolourization were further degraded in CW-MCs. The PCR-denatured gradient gel electrophoresis analysis indicated the dominance of Proteobacteria-like 16S rRNA gnen sequences. The brightest band was detected to be dominant by a Lactobacillus kefiranofaciens-like sequence. The electrogenic bacteria-associated sequences, such as Geobacter metallireducens and Desulfobulbaceae, both existed in the closed circuit and the open circuit reactors, accompanied with Desulfobacterium sp., Klebsiella sp., Aminobacter sp., Flavobacterium sp., Thauera aromatic, and Sphingomonas sp. The abundances of Geobacter sulfurreducens and Betaproteobacteria in the mixture CC reactor were 32.2% and 7.2%, respectively, and were higher than those in the mixture OC reactor. In summary, substrate and electrode can promote the performance of the CW-MFC and have effects on the microbial community in the anode of the CW-MFC.

Effects of electrode gap and wastewater condition on the performance of microbial fuel cell coupled constructed wetland.

ABSTRACT The effects of electrode gap, PB solution concentration and azo dye on the wastewater treatment and electricity generation of microbial fuel cell coupled constructed wetland (CW-MFC) were studied. The electrode gap had obvious influence on the decolorization, while the influence of PB concentration on the decolorization was not obvious. The best decolorization efficiency was 91.05% and was gained when the electrode gap was 13.2 cm. The smaller the electrode gap, the smaller the ohmic resistance. However, a too small electrode gap would reduce the electricity generation. The best PB concentration in this study was 50 mM. In the glucose group, when the PB concentration was 50 mM, the power density was enhanced to 0.38 W/m3, while the PB concentration was 5 mM, the power density was only 0.14 W/m3. In the ABRX3 group, when the PB concentration was 50 mM, the power density was 0.18 W/m3, while when the PB concentration was 5 mM, the power density was 0.12 W/m3. The electricity generation performance of the CW-MFC was enhanced with an increase in running time. Long-time running CW-MFC got a higher cathode potential and a smaller internal resistance.

Augmenting atrazine and hexachlorobenzene degradation under different soil redox conditions in a bioelectrochemistry system and an analysis of the relevant microorganisms

Soil microbial fuel cells (MFCs) are a sustainable technology that degrades organic pollutants while generating electricity. However, there have been no detailed studies of the mechanisms of pollutant degradation in soil MFCs. In this study, the effects of external resistance and electrode effectiveness on atrazine and hexachlorobenzene (HCB) degradation were evaluated, the performance of soil MFCs in the degradation of these pollutants under different soil redox conditions was assessed, and the associated microorganisms in the anode were investigated. With an external resistance of 20Ω, the degradation efficiencies of atrazine and HCB were 95% and 78%, respectively. The degradation efficiency, degradation rate increased with decreasing external resistance, while the half-life decreased. There were different degradation trends for different pollutants under different soil redox conditions. The fastest degradation rate of atrazine was in the upper MFC section (aerobic), whereas that of HCB was in the lower MFC section (anaerobic). The results showed that electrode effectiveness played a significant role in pollution degradation. In addition, the microbial community analysis demonstrated that Proteobacteria, especially Deltaproteobacteria involved in current generation was extremely abundant (27.49%) on soil MFC anodes, although the percentage abundances of atrazine degrading Rhodocyclaceae (8.77%), Desulfitobacterium (0.64%), and HCB degrading Desulfuromonas (0.73%), were considerably lower. The results of the study suggested that soil MFCs can enhance the degradation of atrazine and HCB, and bioelectrochemical reduction was the main mechanism for the pollutants degradation.

The degradation of azo dye with different cathode and anode structures in biofilm electrode reactors

In this study, biofilm electrode reactors (BERs) were constructed to degrade the azo dye Reactive Brilliant Red (RBR) X-3B. Three different BERs, namely, cathodes with differently structured reactors, cathodes filled with a granular activated carbon (GAC) reactor, and a dimensionally stable anode (DSA) electrode anode reactor, were individually studied to investigate their influence on the removal of both X-3B and chemical oxygen demand (COD). Experimental results showed that it was the internal resistance rather than the total surface area of the cathode that influenced the X-3B removal efficiency in the different cathode reactors. The smaller the internal resistance, the larger the current in the reactor. The larger the current in the whole system, the more electrons that could be utilized for the reduction of X-3B, resulting in higher removal efficiency. The cathode filled with a GAC-BER did not improve the COD removal efficiency, while the X-3B removal efficiency actually declined. Using the DSA electrode as an anode increased the current in the system and improved the removal efficiency. Scanning electron microscopy results showed that the DSA electrode anode had a longer service life than a graphite anode. UV-vis spectral analysis determined that the conjugate system in X-3B was destroyed.