In Situ Coupling of Reduction and Oxidation Processes with Alternating Current-Driven Bioelectrodes for Efficient Mineralization of Refractory Pollutants
Ye Yuan
,
Junjie Zhang
,
Wanxin Yin
,
Lulu Zhang
,
Lin Li
,
Tianming Chen
,
Cheng Ding
,
Wenzong Liu
,
Aijie Wang
,
Fan Chen
In Situ Coupling of Reduction and Oxidation Processes with Alternating Current-Driven Bioelectrodes for Efficient Mineralization of Refractory Pollutants
aSchool of Environmental Science and Engineering, Yancheng Institute of Technology, Yancheng 224051, China
bSchool of Ecology and Environment, Northwestern Polytechnical University, Xi’an 710129, China
cState Key Laboratory of Urban Water Resource and Environment, School of Civil and Environmental Engineering, Harbin Institute of Technology Shenzhen, Shenzhen 518055, China
The effective elimination of aromatic compounds from wastewater is imperative for safeguarding the ecological environment. Bioelectrochemical processes that combine cathodic reduction and anodic oxidation represent a promising approach for the biomineralization of aromatic compounds. However, conventional direct current bioelectrochemical methods have intrinsic limitations. In this study, a low-frequency and low-voltage alternating current (LFV-AC)-driven bioelectrode offering periodic in situ coupling of reduction and oxidation processes was developed for the biomineralization of aromatic compounds, as exemplified by the degradation of alizarin yellow R (AYR). LFV-AC stimulated biofilm demonstrated efficient bidirectional electron transfer and oxidation-reduction bifunctionality, considerably boosting AYR reduction (63.07% ± 1.91%) and subsequent mineralization of intermediate products (98.63% ± 0.37%). LFV-AC stimulation facilitated the assembly of a collaborative microbiome dedicated to AYR metabolism, characterized by an increased abundance of functional consortia proficient in azo dye reduction (Stenotrophomonas and Bradyrhizobium), aromatic intermediate oxidation (Sphingopyxis and Sphingomonas), and electron transfer (Geobacter and Pseudomonas). The collaborative microbiome demonstrated a notable enrichment of functional genes encoding azo- and nitro-reductases, catechol oxygenases, and redox mediator proteins. These findings highlight the effectiveness of LFV-AC stimulation in boosting azo dye biomineralization, offering a novel and sustainable approach for the efficient removal of refractory organic pollutants from wastewater.
Ye Yuan, Junjie Zhang, Wanxin Yin, Lulu Zhang, Lin Li, Tianming Chen, Cheng Ding, Wenzong Liu, Aijie Wang, Fan Chen.
In Situ Coupling of Reduction and Oxidation Processes with Alternating Current-Driven Bioelectrodes for Efficient Mineralization of Refractory Pollutants.
Engineering, 2024, 43(12): 131-144 DOI:10.1016/j.eng.2024.05.009
Aromatic compounds are extensively utilized in a wide range of industrial processes and products and are prevalent in wastewater, particularly industrial effluents [1]. Their introduction into natural ecosystems poses substantial environmental risks, including toxicity, carcinogenicity, persistence, and bioaccumulation [2]. Aromatic compounds typically exhibit stable aromatic ring structures with well-defined π-conjugated molecular configurations, rendering them resistant to direct oxidation and degradation in conventional biological wastewater treatment processes [3]. Therefore, implementing robust wastewater treatment protocols for the efficient removal of aromatic compounds is imperative.
Hydroxylation and ring cleavage are pivotal prerequisites for the biodegradation of aromatic compounds, and anaerobic bioreduction is the preferred method to achieve these essential transformations [4]. Recently, bioelectrochemical systems (BES) featuring biocatalyzed cathodes have attracted increasing interest in the reductive degradation of various aromatic compounds, including azo dyes, nitrobenzene, and halogenated organic compounds [5], [6]. Compared to conventional anaerobic bioreduction processes, biocathodes can enhance the reduction of aromatic compounds by consistently supplying electrons and establishing favorable redox conditions for functional bacteria. However, biocathodes exhibit limited potential for achieving complete mineralization of reduction products, thereby impeding their autonomous utilization as wastewater treatment units [7]. Therefore, consistent with the anaerobic reduction-aerobic oxidation process, bioelectrochemical processes can integrate cathodic reduction and anodic oxidation (CR-AO) to achieve the mineralization of aromatic compounds [7]. Various integrated BES reactors, such as the membrane-free up-flow biocatalyzed electrolysis reactor [8], hybrid anaerobic bioreactor with built-in BES [9], and BES-assisted up-flow reduction-oxidation reactor [10], have been demonstrated to couple the cathodic reduction and anodic oxidation processes proficiently. These integrated configurations confer the advantage of diminishing the spatial footprint and procedural intricacy and enhancing pollutant removal efficiency.
Direct current (DC) is the predominant form of the bioelectrochemical CR-AO process that maintains distinct cathode and anode reactions. However, DC has substantial impediments that limit its practical scalability. Cathodic and anodic biofilms differ in their electron transfer and film-forming properties, which require distinct culture conditions [10], [11]. Second, DC-driven processes consume high current densities and risk electrode losses while generating harmful byproducts (·OH, O3, H2O2, Cl2, and SO4·−) that can endanger microorganisms [12], [13], [14]. Finally, optimizing the distance between the cathode and the anode is critical for effective mass and electron transfer [15]. In recent developments, alternating current (AC), particularly low-frequency and low-voltage sinusoidal alternating current (LFV-AC), has emerged as a novel driving approach for BES, offering the capability of alternating oxidation and reduction processes on a single electrode [16]. In principle, AC-driven polarity inversion can facilitate the formation of bidirectional electron transfer biofilms [17], which can significantly enhance extracellular electron transfer (EET) between microbes and electrodes and promote the enrichment of electroactive bacteria (EAB) [18], [19]. Bidirectional EET biofilms have the potential to realize in situ reduction and oxidation of aromatic compounds. In addition, utilizing LFV-AC can enhance the metabolic activity of electro-biofilms through a polarisation process propelled by electrohydrodynamic forces [13], [20]. Additionally, AC mitigates the production of undesirable byproducts and corrosion of the electrode owing to the zero net charge for a complete cycle [18], [21]. Therefore, AC-driven electrochemical processes are expected to overcome the limitations associated with DC-driven processes, thereby enhancing the efficiency of aromatic pollutant mineralization. To the best of our knowledge, no prior studies exist regarding the application of LFV-AC on bioelectrodes to establish periodic reduction-oxidation cycles for the biomineralization of aromatic compounds. Furthermore, information on the effects of LFV-AC stimulation on microbial community composition and key functional genes is limited. Moreover, the metabolic characteristics, electron transfer pathways, and electrochemical attributes of the LFV-AC-stimulated biofilms remain poorly understood.
In this study, LFV-AC-driven bioelectrodes were developed to achieve in situ coupling of reduction and oxidation processes, aiming for the efficient mineralization of refractory pollutants (azo dye) in wastewater. This study aimed to ➀ Investigate the performance of LFV-AC-driven periodic reduction and oxidation processes on azo dye mineralization, ➁ demonstrate electron transfer capabilities and pathways utilizing methods such as electron balancing analysis, cyclic voltammetry, and electrochemical impedance spectroscopy, and ➂ reveal the composition and distribution of functional bacteria, key genes, and metabolic pathways through metagenomic sequencing. This study offers a novel pathway for the efficient mineralization of recalcitrant organic pollutants in wastewater.
2. Material and methods
2.1. Construction of LFV-AC-driven bioelectrodes
A schematic diagram of the LFV-AC-driven bioelectrode is shown in Fig. S1 in Appendix A. The construction details of the electrochemical reactor are provided in Appendix A. The working electrode was constructed as an LFV-AC-driven bioelectrode based on the basic framework of a dual-chamber reactor. A function generator (model-SDG2000X; Shenzhen Dingyang Technology Co., Ltd., China) provided stable alternating currents. A DC power supply (model-MS152D; Dongguan Maihao Electronic Technology Co., Ltd., China) provided a constant current. The working electrode, counter electrode, reference electrode, and external resistor (10 Ω) were connected with a data logger (Keithley 2700; Keithley Co., Ltd., US) to record the voltages across the resistor and electrode potentials every 10 min. A sealed aluminum foil bag (14 cm wide × 20 cm long; Yancheng Puriqi Experimental Instrument Co., Ltd., China) was fixed at the top of the working chamber to balance the gas pressure in the working chamber. All potentials are reported for a saturated calomel electrode (SCE).
2.2. Acclimation and operation of LFV-AC-driven bioelectrodes
The seed sludge for electrode biofilm inoculation was obtained from an activated sludge thickener (Cheng Dong wastewater treatment plant, Yancheng, China). Before inoculation, the sludge was screened using a Tyler mesh (0.2 mm) to remove inorganic particles and then sparged with pure N2 overnight. Alizarin yellow R (AYR) is a model aromatic compound that is a common azo dye extensively employed in the printing and dyeing industry [22]. The applied AYR concentration was set to 100 mg·L−1, aligned with the concentrations found in actual wastewater and previous relevant studies [10]. In the working chamber, an electrolyte solution comprising AYR (9 mg), KCl (11.7 mg), NH4Cl (27.9 mg), NaH2PO4·2H2O (249.3 mg), Na2HPO4·12H2O (1039.5 mg), Wolf’s vitamin solution (0.09 mL), and Wolf trace element solution (0.09 mL) was added to a volume of 90 mL. The initial electrolyte pH was maintained at (7.0 ± 0.1) using 1 mol·L−1 HCl or NaOH solution. The compositions of Wolf’s vitamin and trace element solutions are presented in our previous study [10]. In the counter chamber, an electrolyte solution comprising K3Fe(CN)6 (2962.8 mg), K4Fe(CN)6 (3314.7 mg), NaH2PO4·2H2O (249.3 mg), and Na2HPO4·12H2O (1039.5 mg) was added to a volume of 90 mL. Before filling the chamber, the electrolyte solutions were purged with N2 for 10 min to eliminate dissolved oxygen. During the reduction phase, the Fe2+ in K4Fe(CN)6 functioned as an electron donor, providing electrons to the working electrode for AYR reduction. Conversely, in the oxidation phase, the Fe3+ in K3Fe(CN)6 serves as an electron acceptor, accepting electrons from the working electrode to facilitate the mineralization of intermediates.
Four groups of BES were operated under the parameters and conditions shown in Fig. 1(a) and Table S1 in Appendix A. Three BES reactors ran in parallel with sine wave AC stimulation (peak voltage (Vpeak) = 0.7 and −0.7 V; frequency = 0.83 mHz, with polarity changing every 10 min), whereas another set of three parallel BES reactors utilized square wave AC stimulation (voltage = 0.5 and −0.5 V; frequency = 0.83 mHz, with polarity changing every 10 min). Additionally, three parallel BES biocathode reactors were operated with DC stimulation at a voltage of −0.5 V, and three parallel BES bioanode reactors were operated with DC stimulation at a voltage of 0.5 V. The effective value (root-mean-square voltage, Vrms) of sine wave AC voltage was 0.5 V () [23], generating equivalent power to a DC voltage of 0.5 V. DC voltages of −0.5 and 0.5 V were selected according to the potentials of AYR reduction and intermediate product oxidation [22], [24]. All reactors underwent parallel operations with consistent acclimation procedures to maintain constant sludge inoculation. The working chamber was initially incubated with 100 mL of fresh inoculum (seed sludge, 10 mL; electrolyte solution, 90 mL) mixed with a magnetic stir bar (250 r·min−1) for 5 d. Subsequently, the electrolyte solution was replaced with fresh inoculum through three repeated inoculations to continually develop the electrode biofilm. Subsequently, with minimal planktonic biomass expected after sufficient liquid exchange, the biofilm attached to the electrode stabilized. The BES reactors were acclimated to the corresponding voltage modes, and the electrolyte solutions were replaced every five days. Successful start-up was confirmed by observing stable AYR and total organic carbon (TOC) removal after several batch cycles. To further understand the mechanisms facilitating enhanced AYR degradation, the following control groups were used: ➀ An abiotic DC stimulation reactor where the DC was applied in parallel without biofilm inoculation; ➁ an abiotic AC stimulation reactor where the sine wave AC was applied in parallel without biofilm inoculation; and ➂ an open-circuit BES reactor in which no voltage was applied while the seed sludge was inoculated in parallel. All prepared reactors under different conditions were performed in triplicate at an ambient temperature of (25 ± 2) °C.
2.3. Analytical methods and calculations
The water samples collected from the working chambers of all reactors were filtered by a filter (0.22 µm; Shanghai Xingya Material Co., Ltd., China) and diluted three times for the following determination. Detailed measurement methods for AYR and intermediate products (p-phenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA)) are presented in our previous study [10]. The TOC concentration was determined using a TOC analyzer (MultiN/C2100; Analytik Jena AG, Germany). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to examine the electrochemical characteristics of the enriched biofilms using an electrochemical workstation (CHI660D; Chenhua Instruments Co., Ltd., China; V13.12 Electrochemical Software). CV and EIS tests were conducted in a three-electrode configuration with SCE reference electrodes as the working and counter electrodes. The voltage range for the CV tests of the working bioelectrode was −0.8 to 0.6 V. Cyclic voltammograms were recorded at various scanning rates (1-10 mV·s−1) to identify the main mechanism of electron transfer between the biofilm and electrode [17]. The EIS tests were presented as Nyquist plots and analyzed using a Randles equivalent electrical circuit. Detailed information on the EIS analysis is provided in Appendix A.
The AYR removal efficiency (), accumulation ratio of intermediate products (, %), degradation efficiency of intermediate products (), and TOC removal efficiency () were calculated using Eqs. (1), (2), (3), (4), respectively: The AYR and TOC removal kinetics were fitted using the apparent first-order reaction model in Eq. (5).where represents the reaction time (h or d), represents the initial AYR concentration (mg·L−1), represents the AYR concentration (mg·L−1) at time (h), represents the concentration (mg·L−1) of PPD at time (h), represents the concentration (mg·L−1) of 5-ASA at time (h), represents the initial TOC concentration (mg·L−1), represents the TOC concentration (mg·L−1) at time (h), represents the AYR/TOC concentrations (mg·L−1) at time t (d), represents the initial AYR/TOC concentrations (mg·L−1), and represents the rate constant (d−1). All data and standard deviations were obtained based on the average of duplicate reactors.
The effective AC value (, mA) was calculated using Eq. (6) [25].where is the time cycle (h) and is the input current at time (h).
The calculation of electron utilization efficiency (, %) is available in Appendix A.
2.4. Microbial community structure and function analysis
The microbial community structure and function in sine-wave AC stimulation, square-wave AC stimulation, biocathodes, and bioanodes were investigated using 12 samples from three biologically replicated reactors. Samples were collected at the end of the experiment for cryopreservation and metagenomic sequencing. Details of the DNA extraction, PCR amplification, sequencing, and taxonomic classification are available in Appendix A. Differences in microbial community composition were explored and visualized using non-metric multidimensional scaling (NMDS) and heatmap based on the “Bray-Curtis” distance, which was completed using the Lingbo MicroClass†(†http://www.cloud.biomicroclass.com). Metagenomic sequencing was performed to analyze the microbial community structure, relative abundance of functional genes, and microbial metabolic pathways. Normalized abundance is presented in terms of transcripts per million (TPM).
3. Results and discussion
3.1. Performance of AC-driven periodic reduction and oxidation process
3.1.1. AYR removal and intermediates mineralization
The stable removal of AYR and TOC was observed after seven batch cycles (Fig. S2 in Appendix A), indicating the successful initiation and stable maintenance of all the bioreactors under various electrical modes. As shown in Fig. 1(b), AYR exhibited notable removal under sine and square wave AC stimulations, achieving the highest efficiency of (63.07% ± 1.91%) and (56.89% ± 0.71%), surpassing the removal efficiency observed at the biocathode (53.13% ± 1.15%) and the bioanode (40.61% ± 0.15%). These results suggest that the periodic reduction-oxidation process is more effective for AYR removal than independent reduction or oxidation processes. This outcome aligns with the prevailing consensus that the integration of the reduction and oxidation processes constitutes a more effective pathway for the biomineralization of AYR [7], [10]. Furthermore, AYR removal in sine wave AC stimulation exceeded that in square wave AC stimulation, which is consistent with a previous study that sinusoidal waveforms were more effective than square waves in enhancing the efficiency of the heterotrophic-autotrophic denitrification process [18]. This result was attributed to the fact that continuous gradient voltage changes enhance the metabolic activity of microorganisms more effectively than direct polarity reversal [26]. Gradient voltage changes can promote the EET between microorganisms and electrodes. By gradually changing the voltage, the bioelectrode can determine the optimal points for electron donation and acceptance, facilitating more efficient microbial metabolism. The biocathode exhibited superior AYR removal efficiency compared to the bioanode, affirming the ease of cathodic reduction over direct anodic oxidation of azo dyes [27], [28]. Azo dyes, which are electron-deficient xenobiotics, are resistant to degradation via azo oxidation [29]. Biocathodes facilitate reductive degradation by providing electrons to the electron-deficient components. In the open circuit condition, the AYR removal efficiency after 120 h was (19.82% ± 0.35%), which was significantly lower than that observed in other bioelectrochemical modes. This observation strongly suggests that electrical stimulation can enhance the microbial degradation capability. Under abiotic DC/AC stimulation, the AYR removal efficiency was less than 7% within 120 h, which was significantly lower than that observed in the BES mode. This outcome underscores the crucial role of microbial catalysis in promoting AYR removal, consistent with previous findings that electroactive biofilms capture electrons from the cathode for the bioreduction of refractory pollutants [11], [30]. Considering reaction volume, duration, and initial concentration, the AYR removal efficiency under sine wave AC stimulation was (62.41 ± 1.81) mg·L−1·d−1), closely aligning with the (66.51 ± 0.12) mg·L−1·d−1 removal efficiency achieved in a DC-stimulated BES system with the addition of 500 mg·L−1 sodium acetate as co-substrate [31]. As shown in Table 1 and Fig. 1(b), the AYR removal in all operation modes fitted well with the first-order kinetic model (0.930 ≤ R2 ≤ 0.988). k of the sine wave AC stimulation was 1.19-14.57 times that of other electrical modes, highlighting the excellent AYR removal capacity of sine wave AC stimulation.
The accumulation of key intermediate products of AYR (5-ASA and PPD) is governed by the rate differential between the initial reduction and subsequent oxidation reactions. As shown in Fig. 1(c), the intermediate product accumulation ratio, represented by the accumulation of 5-ASA and PPD to the AYR removal concentration, was less than 10% at 120 h for both the sine- and square-wave AC stimulations. Notably, these values were significantly lower than those observed for the bioanode (> 30%) and biocathode (> 40%). Additionally, the TOC removal efficiency in the sine- and square-wave AC stimulation was much higher than that of the bioanode and biocathode. These findings confirm that, compared to the individual oxidation or reduction processes of the biocathode and bioanode, the periodic switching of the reduction and oxidation processes in the AC stimulation mode efficiently facilitates the deep degradation of AYR, ultimately leading to mineralization. Despite the similarity in the intermediate product accumulation ratios between sine- and square-wave AC stimulations, the total amount of AYR removal in sine wave AC stimulation was significantly higher, indicating that sine wave AC stimulation exhibited an enhanced capability to degrade the initial substrate and intermediate products. The LFV-AC stimulated process exhibited a subsequent mineralization rate of (98.63% ± 0.37%) for intermediate products, surpassing the previously reported high value of (89.01% ± 1.32%) in the intermittent electric field-stimulated reduction-oxidation coupled process [10]. While many studies have used carbon sources to enhance AYR degradation, AC-driven bioelectrodes effectively couple reduction degradation and mineralization of intermediate products without the need for additional carbon sources. As shown in Table 1 and Fig. 1(d), TOC removal in all operation modes fitted well with the first-order kinetic model (0.971 ≤ R2 ≤ 0.996). The k value for sine wave AC stimulation was 1.10-32.00 times that of the other electrical modes, showing a superior capacity for intermediate product degradation in sine wave AC stimulation. These results can be attributed to the efficient coupling of the biocathode reduction of AYR to 5-ASA and PPD, along with the oxidation of 5-ASA and PPD to CO2 in the AC-stimulated mode [10], [24].
3.1.2. Current variation and electron utilization efficiency
Current, a crucial parameter characterizing electron transfer capacity was employed to assess sine- and square-wave AC stimulations (Figs. 2(a) and (b)). The current in both modes exhibited periodic alternation between negative and positive values, indicating stable switching between the reduction and oxidation processes. Notably, the effective value of an AC generates the same heating effect as an equivalent DC [25]. The effective value of the sine wave AC stimulation (IEVA = ± 1.05 mA) was 1.36-fold higher than that of square wave AC stimulation (IEVB = ± 0.77 mA). Square-wave AC stimulation induces instantaneous electrode polarity reversals, potentially leading to ohmic losses and ion transmission [32]. Consequently, this can result in a reduction in the current amplitude. Electroactive biofilms are specially adapted to flourish in electrochemically active environments where high current conditions offer a substantial energy source, resulting in increased growth and activity [33]. Generally, the higher current observed in the sine wave AC mode offers a favorable environment for enhancing the enrichment of EAB and boosting the catalytic capacity of biofilms [34].
However, the electron utilization efficiency (EUE) in sine wave AC stimulation (22.26% ± 1.67%) was 1.83 times higher than that in square wave AC stimulation (12.16% ± 0.23%) at 120 h (Figs. 2(c) and (d)). The enhancement of the EUE suggests that the electrode exhibited enhanced electron transfer capability during the AYR degradation process [35]. Furthermore, during the reduction period (Figs. 2(e) and (f)), the EUE of sine wave AC stimulation averaged (8.34 ± 1.86%), significantly exceeding that of square wave AC stimulation (4.54% ± 0.46%). Meanwhile, the total electron fluxes in the sine-wave AC stimulation were less than those in the square-wave AC stimulation, but the electron fluxes used for AYR reduction in the two modes were almost equivalent (1.62 vs 1.54 mmol·L−1). This indicates that sine-wave AC-stimulated biofilms have a higher efficiency in providing electrons for AYR reduction. During the oxidation period (Figs. 2(g) and (h)), the EUE of sine wave AC stimulation (37.07% ± 0.31%) was evidently higher than that of square wave AC stimulation (19.57% ± 1.21%). Moreover, compared to square-wave AC stimulation, the total electron fluxes in sine wave AC were significantly lower, but the electron fluxes available for PPD and 5-ASA oxidation in the two modes were almost equivalent (7.87 vs 6.93 mmol·L−1). This suggests that sine wave AC-stimulated biofilms obtain more electrons from intermediate (5-ASA and PPD) oxidation. Electroactive biofilms are involved in intricate electron transfer and storage mechanisms and are characterized by transient electron storage proficiency. Consequently, these biofilms exhibit pseudocapacitive characteristics during oxidative and reductive degradation [30]. The pseudocapacitive property represents dynamically controlled electron transfer and plays a crucial role in the current generation and substrate degradation processes [17]. Sine-wave AC stimulation exhibited excellent pseudocapacitive behavior, efficiently regulated biological reactions, and facilitated the reduction of AYR and oxidation of intermediate products.
3.1.3. Electrochemical characterizations of biofilms
To study the interactions between the electrode biofilm and AYR under different electrical modes, we conducted further electrochemical characterizations. The CV spectra of sine and square wave AC stimulations exhibited two distinct pairs of redox peaks centered at E1a = −115 mV and E1b = 895 mV and E2a = −136 mV and E2b = 665 mV, respectively (Figs. 3(a) and (b)). The centers of the reduction and oxidation peaks stimulated by AC were close to the biocathode (E3 = −98 mV) and bioanode (E4 = 634 mV), respectively (Figs. 3(c) and (d)), indicating that the AC-stimulated biofilm integrates efficient reduction and oxidation functions during the degradation process of AYR. Moreover, the two redox peak centers (E1a and E1b) of sine wave AC stimulation exhibited positive shifts of 21 and 230 mV, respectively, compared to square-wave AC stimulation (E2a and E2b). This result implies that compared with square-wave AC stimulation, sine-wave AC stimulation can improve the catalytic activity of the electrode biofilm and increase the electron transfer rate during the degradation process of AYR [11]. The sine and square wave AC stimulations considerably improved AYR removal efficiency compared to biocathode and bioanode, as evidenced by extraction rates of (63.07% ± 1.91%), (56.89% ± 0.71%), (53.13% ± 1.15%), and (40.61% ± 0.15%) at 120 h, respectively (Fig. 1(b)). This corresponded to specific AYR removal rates per unit electrode area of (249.04 ± 7.64), (242.00 ± 2.84), (210.08 ± 4.60), and (159.08 ± 0.60) mg·L−1·m−2. As the scanning rate increased from 1 to 10 mV·s−1, the reduction and oxidation peak currents increased linearly with the square root of the scanning rate (Figs. 3(a)-(d)), suggesting that the reduction and oxidation periods are diffusion-controlled reactions [36].
Moreover, EIS was employed to assess variations in the internal resistance composition across distinct electrical modes (Fig. 3(e)). The three primary sources of electrode voltage loss are the solution resistance (Rs), charge transfer resistance (Rct), and finite diffusion resistance (Rd), which can be identified and quantified through EIS analysis [37]. Correspondingly, with the fitted curve of EIS and the equivalent circuit, the Rs, Rct, and Rd of the sine wave AC stimulation achieved 0.28, 0.24, and 0.06 Ω (Fig. 3(f)), respectively, apparently lower than that of square wave AC, biocathode, and bioanode. The lower Rs values indicate that the sine wave AC-stimulated biofilm has better electron conductivity [38], which is consistent with better AYR removal and higher current in the sine wave AC mode. Rct can be affected by two parameters: ➀ electron transfer between the electrode and the electron acceptor (AYR) during the reductive period and ➁ the electrode and electron donor (PPD and 5-ASA) during the oxidative period [24]. The lower Rct indicated that sine wave AC-stimulated biofilms exhibited better pseudocapacitive behavior throughout the reductive and oxidative degradation processes. The capacitive arc in the low-frequency range can be attributed to the finite diffusion process caused by mass transport within the solution diffusion layers and electrodes. The lower Rd demonstrated that sine-wave AC stimulation might promote diffusion across the biofilm layers and bioelectrodes [38]. Consequently, AYR degradation under sinusoidal AC stimulation has the potential to maximize the efficiency of electronic utilization and minimize apparent resistance.
3.2. Microbial community composition and function
3.2.1. Microbial community diversity and differentiation
The Shannon indices of the biosamples across the various electrical modes exhibited comparable values (Table S2 in Appendix A), reflecting a statistically insignificant level of microbial community diversity. Additionally, the low Simpson indices for all biosamples suggested a generally low diversity. This observation may be attributed to the enrichment of specific functional microbial species in the selective environments of electrical stimulation and pollutant exposure. The heatmap generated using the Bray-Curtis distance metric revealed substantial variations in the microbial community among samples subjected to different electrical modes. Specifically, sine AC exhibited a high degree of similarity to square AC (Fig. 4(a)) while showing significant differences from bioanode (BA) and biocathode (BC).
Moreover, non-metric multidimensional scale (NMDS) analysis showed that the electrical stimulation mode significantly shaped the microbial community structure, leading to the formation of four distinct clusters (Fig. 4(b)). Along the NMDS 1 dimension, sine/square AC (Cluster 1/Cluster 2) was located far from BC/BA (Cluster 3/Cluster 4), indicating that the microbial communities stimulated by AC were significantly different from those on the bioanode and biocathode driven by DC. Analysis along the NMDS2 dimension revealed distinct differentiation within the microbial clusters. Notably, BC and BA exhibited a more dispersed distribution, whereas sine- and square-wave AC formed tighter and more cohesive clusters. This observation implies that AC demonstrated a greater capacity to promote a more stable and homogeneous microbial community structure than DC. Additionally, stochastic events, including microbial migration, mutations, and environmental interactions, likely occurred independently in each reactor, shaping their unique microbial landscapes. These stochastic events could contribute to nondeterministic outcomes, even under comparable operational conditions, explaining the observed variations in the NMDS dimensions [39]. Future research can elaborate extensively on the specific factors underlying these differences by comparing operational histories, microbial community compositions, and environmental variables across reactors.
3.2.2. Microbial community structure and potential functions
The microbial community structures of the electrode biofilms at the phylum and class levels under different electrical modes are shown in Fig. S3 in Appendix A. Proteobacteria, Chlorofexi, and Bacteroidetes were the dominant phyla under all electrical stimulation conditions, with a substantial presence of electroactive and AYR-degrading bacteria within these phyla [40]. Moreover, differences in electrical stimulation modes resulted in significant variations at the phylum level. Notably, the abundance ranking of the Proteobacteria phylum was as follows: sine wave AC stimulation (68.75% ± 3.51%) > square wave AC stimulation (63.17% ± 0.42%) > biocathode (53.10% ± 5.52%) > bioanode (52.81% ± 0.32%) (Fig. S3(a)). At the class level, Alphaproteobacteria, a prominent subclass within the Proteobacteria phylum, exhibited dominance across various electrical stimulation modes, with abundance rankings as follows: sine wave AC stimulation (46.53% ± 4.30%) > square wave AC stimulation (41.4% ± 3.70%) > bioanode (31.01% ± 5.70%) > biocathode (29.97% ± 10.00%) [41] (Fig. S3(b)).
At the genus level, the combined influence of electrical stimulation and pollutant selection led to the enrichment of a substantial population of azo dye-reducing bacteria, aromatic-oxidizing bacteria, and EAB within the electrode biofilms. However, the abundance of these functional bacteria varied significantly owing to differences in the electrical stimulation modes (Figs. 5(a) and (b)). Sphingopyxis was the dominant genus under all electrical stimulation modes, with the following abundance: sine AC wave stimulation (17.99% ± 2.01%) > square wave AC stimulation (17.12% ± 2.50%) > bioanode (9.55% ± 4.21%) > biocathode (8.30% ± 3.30%). Sphingopyxis is a typical aromatic pollutant-degrading bacterium with robust abilities in hydroxylation, ring cleavage, and secondary metabolite degradation, which shows excellent versatility in the degradation of azo dyes [42], [43]. Notably, Geobacter, a typical EAB with bidirectional EET and robust pollutant transformation abilities [40], was significantly more abundant under AC stimulation than under DC stimulation. Specifically, the abundance was as follows: sine wave AC stimulation (3.30% ± 0.94%) > square wave AC stimulation (2.79% ± 1.10%) > bioanode (1.69% ± 1.34%) > biocathode (1.37% ± 0.78%). Stenotrophomonas, a typical azo dye-reducing bacterium known for its proficiency in azo bond destruction and nitro-group reduction [44], demonstrated varying abundances under different electrical stimulation modes. Specifically, under sine wave AC stimulation, the abundance of Stenotrophomonas (3.00% ± 0.33%) was 2.03 times that of square wave AC stimulation (1.48% ± 0.34%), 3.26 times that of the biocathode (0.92% ± 0.03%), and 4.11 times that of the bioanode (0.73% ± 0.01%).
Based on the reported functionalities of the genera, the predominant genera in the electrode biofilms were classified into three clusters: azo dye-reducing bacteria, aromatic-oxidizing bacteria, and EAB (Fig. 5(c)). The abundance of azo dye-reducing bacteria, including Bradyrhizobium [41], Stenotrophomonas [44], and Chlorella [45], under sine wave AC stimulation was (5.24% ± 0.34%), which was 1.78 fold higher than that under square wave AC stimulation (2.94% ± 0.33%), 2.17 fold higher than that of the biocathode (2.41% ± 0.01%), and 2.59 fold higher than that of the bioanode (2.02% ± 0.02%). Besides, AC, particularly sinusoidal AC, promoted the enrichment of aromatic-oxidizing bacteria, including Sphingopyxis, Sphingomonas, Sphingobium, and Afipia [43], [46], [47], which was confirmed by respective abundances under sine wave AC stimulation (24.52% ± 2.71%), square wave AC stimulation (22.51% ± 2.63%), biocathode (12.25% ± 1.23%), and bioanode (14.72% ± 1.21%). The enrichment of EAB, including Geobacter [40], Pseudomonas [22], and Shinella [44], exhibited a trend similar to that of azo dye-reducing and aromatic-oxidizing bacteria, with the order of sine wave AC stimulation (6.39% ± 0.20%) > square wave AC stimulation (5.24% ± 0.11%) > bioanode (2.71% ± 0.11%) > biocathode (2.61% ± 0.72%). Moreover, Pseudomonas demonstrated versatile reduction capabilities beyond AYR, including the reduction of various heavy metals, such as vanadium, owing to its notable EET proficiency [48], [49]. These results indicated that AC stimulation promoted the enrichment of azo dye-reducing bacteria, aromatic-oxidizing bacteria, and EAB at the bioelectrode. Furthermore, the abundance of azo dye-reducing bacteria, aromatic-oxidizing bacteria, and EAB were significantly and positively correlated with the removal of AYR, oxidation of intermediate products, and EUE, respectively. Notably, ammonia-oxidizing bacteria, such as Nitrosomonas, were significantly enriched under sine wave AC stimulation (Fig. 5(a)). This observation may be attributed to the removal of ammonia released during azo dye degradation. This indicates that the efficient composition of functional bacterial clusters in the electrode biofilm under the AC stimulation mode served as the biological foundation supporting the in situ high-efficiency mineralization of the AYR.
3.3. Key functional genes and metabolic pathways
Metagenomic sequencing was performed to determine the genomic potential of the electrode microbiomes subjected to various stimulation modes (Fig. 6). Azoreductase can catalyze the reductive cleavage of azo bonds into the corresponding aromatic amines by utilizing nicotinamide adenine dinucleotide (NADH) and/or nicotinamide adenine dinucleotide phosphate (NAD(P)H) as electron donors [50]. Under various electrical stimulation modes, sinusoidal AC stimulation exhibited the highest enrichment of azoreductase genes (K01118) and NAD(P)H dehydrogenase genes (K00355, K03809, and K19267) (Fig. 6(a)). Meanwhile, the nitroreductase gene (K10679) showed a relatively high TPM in sine-wave AC stimulation, which can utilize NAD(P)H as an electron donor to catalyze nitro group reduction [51]. Azoreductases and nitroreductases, which possibly originate from common genomic reductases, were involved in the bioreduction of azo dyes and nitro groups. Catechol oxygenases, which are implicated in the oxidative cleavage of aromatic rings during the degradation of aromatic compounds [52], [53], potentially play a crucial role in the mineralization of AYR reduction intermediates. Notably, the total TPM of catechol oxygenases (K00446, K03381, and K21726) under sine wave AC stimulation surpassed those of the other modes. Correspondingly, the abundance of typical host bacteria for azoreductase (Stenotrophomonas) [54], nitroreductase (Bradyrhizobium) [55], and catechol oxygenase genes (Sphingopyxis, Sphingomonas, and Sphingobium) [56] was significantly higher under sine wave AC stimulation compared to other modes (Fig. 6(a)). These results confirm that sine wave AC stimulation can significantly enrich key genes and functional bacteria involved in AYR reduction and oxidation of intermediate products.
The EET between the EAB and electrode plays a crucial role in AYR degradation. Direct EET occurs through interactions between electrodes and microbes via C-type cytochromes and e-pili, whereas indirect EET involves electron transfer facilitated by redox mediators such as riboflavin and phenazine. No significant variations were observed in the abundance of genes encoding riboflavin (K00793), phenazine (K20940), and e-pili (K02651) across diverse experimental conditions (Fig. 6(b)) [36], [57], [58]. However, sine wave AC stimulation specifically led to a higher enrichment of genes encoding cytochrome C than square-wave AC stimulation, biocathode, and bioanode conditions. The noticeable increase in cytochromes with sine wave AC stimulation compared with square wave AC stimulation, biocathode, and bioanode suggested that sine wave AC prominently enhanced the direct EET pathways between microbes and electrodes. Functional bacterial genera (Geobacter and Pseudomonas) harboring these genes exhibited a similar enrichment trend, supporting the notion that biofilms cultivated under sine wave AC stimulation show increased electroactivity efficiency.
Analysis of microbial KEGG metabolic pathways showed that the relative abundance of functional genes associated with KEGG (Level 2) categories, including signal transduction, amino acid metabolism, and global and overview maps, was higher under sine wave AC stimulation than under other modes (Fig. 6(c)). Specifically, at KEGG Level 3, the two-component system was a predominant functional pathway in sine wave AC stimulation, associated with signal transduction, and played a crucial role in enhancing bacterial survival and azo dye degradation [59]. Simultaneously, the increased relative abundance of pathways such as lysine degradation; valine, leucine, and isoleucine biosynthesis; lysine biosynthesis; and arginine and proline metabolism associated with amino acid metabolism in sine wave AC stimulation suggested that bacteria adapted to azo dye degradation products, and their intracellular metabolic activity was more pronounced compared to other modes [60], [61]. The elevated abundance of fatty acid metabolism in sine wave AC stimulation indicated enhanced energy and carbon provisioning through fatty acid degradation during azo dye degradation [62].
3.4. A concept model for the working mechanism of AC-driven bioelectrodes
By leveraging the AYR mineralization efficiency, degradation pathways, and associated microbial information, the working mechanism of AC-driven bioelectrodes for AYR biomineralization was proposed, as shown in Fig. 7. Alternating reduction and oxidation stages were observed during the AC-stimulated AYR degradation cycle. During reduction, electrons from the electrode facilitated the destruction of azo bonds and the reduction of nitro-groups. Correspondingly, AYR removal in sine-wave AC stimulation exceeded that in the other modes by 1.12-1.57 times. Selective enrichment of azo dye-reducing bacteria occurred in the sine-wave AC stimulation biofilm, with abundances 1.78-2.59 times higher than other modes. The increased presence of azoreductase and nitroreductase genes in the sine-wave AC stimulation biofilm substantiated its superior AYR removal efficiency. In the oxidation phase, the primary intermediate products of the AYR reduction, PPD, and 5-ASA, underwent further degradation. In sine wave AC stimulation, the removal of PPD and 5-ASA surpassed other modes by 1.03-4.15 times based on accumulated intermediate product ratios. Based on the chemical structures of the metabolites (Fig. S4 and Table S3 in Appendix A), PPD and 5-ASA exhibited more efficient catabolism into simpler and less toxic metabolites (3-amino-catechol, 3-oxoadipate, and succinate) under sine-wave AC stimulation. The sine wave AC stimulation biofilms exhibited significant enrichment of aromatic-oxidizing bacteria and the corresponding catechol oxygenase genes, facilitating the degradation of PPD and 5-ASA. Notably, EAB abundance (Geobacter and Pseudomonas) with bidirectional electron transfer functionality selectively increased in the sine-wave AC stimulation biofilm, surpassing other modes by 1.26-2.90 times. This augmented the presence of genes associated with EAB, including cytochromes, phenazine biosynthesis, riboflavin, and e-pili biosynthesis genes in the sine-wave AC stimulation biofilm. Therefore, the synergistic effects of multi-pathway interactions involving reduction, oxidation, and bidirectional electron transfer under sinusoidal AC stimulation facilitated AYR biomineralization. The essential mechanism for achieving high TOC removal during AC stimulation is the efficient in situ coupling of the reduction and oxidation processes on a single bioelectrode.
This study facilitates promising opportunities for the efficient biomineralization of aromatic compounds using an autonomous bioelectrode. Intermediate product degradation generates small organic molecules that fulfill the metabolic requirements of the electrode biofilm [63], eliminating the need for external carbon sources in AC-stimulated bioelectrodes. AC stimulation enhanced the enrichment of functional bacteria and genes while regulating interspecific cooperation within the electrode biofilm. The configuration of the reactor and operational parameters, including the applied voltage, electrode materials, and AC frequency, exerted a pivotal influence on the efficacy of AYR removal. Further fine-tuning of these parameters holds promise for enhanced performance in AYR elimination. Frequency is a crucial parameter for AC-driven bioelectrodes, as it determines the number and duration of polarity reversals, thereby further affecting the coupling efficacy between the reduction and oxidation processes. High frequencies can harm microbial metabolism and cell membranes, whereas overly low frequencies can reduce the efficacy of microbial group selection via electrical stimulation [20]. In addition, research on electrode spatial distribution, modification, module design, power management systems, and reactor configuration is crucial. Further exploration of the efficacy of AC stimulation in the treatment of other aromatic compounds (nitrobenzene and antibiotics) is warranted. During the alternating reduction and oxidation biodegradation processes of AYR, nitrogen may undergo a complex transformation process. This process begins with the cleavage of the azo bonds in the AYR molecule and the release of nitrogen-containing intermediates (aromatic amines). These intermediates were reduced to simpler compounds like NH3 during the bio-reduction phase, while NH3 was oxidized to NO2− and NO3− during the bio-oxidation phase. Further, NO2− and NO3− could be reduced to N2 via denitrification. Notably, nitrogen transformation during AYR biodegradation involves additional intermediate reactions depending on the conditions and microbial communities. Further research is crucial to comprehensively understand this process and its environmental implications. The economic viability of the AC stimulation process is notable because it avoids the need for additional treatment units and organic carbon supplementation.
4. Conclusions
The LFV-AC-driven bioelectrodes offering periodic reduction-oxidation cycles achieved efficient biomineralization of aromatic compounds (azo dye AYR), as evidenced by the enhanced AYR reduction and intermediate product oxidation compared to the biocathode, bioanode, and DC polarity reversal. The increased mineralization efficiency, enhanced production of redox-active mediators, and improved electron utilization and transfer towards the LFV-AC-stimulated biofilm resulted in integrated biocatalytic oxidation and reduction functions, bidirectional electron transfer characteristics, and pseudocapacitive properties. LFV-AC stimulation facilitated the assembly of a collaborative microbiome dedicated to AYR metabolism, characterized by an increased abundance of functional consortia proficient in azo dye reduction (Stenotrophomonas and Bradyrhizobium), aromatic intermediate oxidation (Sphingopyxis and Sphingomonas), and electron transfer (Geobacter and Pseudomonas). The collaborative microbiome demonstrated a notable enrichment of functional genes encoding azo- and nitro-reductases, catechol oxygenases, and redox mediator proteins. Addressing the efficiency constraints of AC-driven bioelectrodes and elucidating the transformation pathways of accompanying components are crucial steps towards advancing their practical applications.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (52170054, 51608467, and 52200202) and the “Qing Lan Project” of Colleges and Universities in Jiangsu Province.
Compliance with ethics guidelines
Ye Yuan, Junjie Zhang, Wanxin Yin, Lulu Zhang, Lin Li, Tianming Chen, Cheng Ding, Wenzong Liu, Aijie Wang, and Fan Chen declare that they have no conflict of interest or financial conflicts to disclose.
K.Kuroda, T.Narihiro, F.Shinshima, M.Yoshida, H.Yamaguchi, H.Kurashita, et al. High-rate cotreatment of purified terephthalate and dimethyl terephthalate manufacturing wastewater by a mesophilic up-flow anaerobic sludge blanket reactor and the microbial ecology relevant to aromatic compound degradation. Water Res, 219 (2022), Article 118581.
[2]
J.Luo, S.Y.Miao, R.Koju, T.P.Joshi, R.P.Liu, H.J.Liu, et al. Simultaneous removal of aromatic pollutants and nitrate at high concentrations by hypersaline denitrification: long-term continuous experiments investigation. Water Res, 216 (2022), Article 118292.
[3]
J.X.Lu, J.Zhang, H.J.Xie, H.M.Wu, Y.M.Jing, M.D.Ji, et al. Transformation and toxicity dynamics of polycyclic aromatic hydrocarbons in a novel biological-constructed wetland-microalgal wastewater treatment process. Water Res, 223 (2022), Article 119023.
[4]
L.Yang, Y.J.Liu, C.Li, Z.Liu, X.S.Liu, C.X.Wei, et al. Biodegradation time series characteristics and metabolic fate of different aromatic compounds in the biochemical treatment process of coal chemical wastewater. Bioresour Technol, 361 (2022), Article 127688.
[5]
S.Z.Pei, Y.Wang, S.J.You, Z.G.Li, N.Q.Ren. Electrochemical removal of chlorophenol pollutants by reactive electrode membranes: scale-up strategy for engineered applications. Engineering, 9 (2022), pp. 77-84.
[6]
A.Jain, Z.He. Cathode-enhanced wastewater treatment in bioelectrochemical systems. NPJ Clean Water, 1 (2018), p. 23.
[7]
K.C.Yang, Y.X.Zhao, X.Zhou, Q.Wang, T.H.Pedersen, Z.C.Jia, et al. “Self-degradation” of 2-chlorophenol in a sequential cathode-anode cascade mode bioelectrochemical system. Water Res, 206 (2021), Article 117740.
[8]
A.J.Wang, D.Cui, H.Y.Cheng, Y.Q.Guo, F.Y.Kong, N.Q.Ren, et al. A membrane-free, continuously feeding, single chamber up-flow biocatalyzed electrolysis reactor for nitrobenzene reduction. J Hazard Mater, 199-200 (2012), pp. 401-409.
Y.Yuan, W.X.Yin, Y.T.Huang, A.Q.Feng, T.M.Chen, L.Qiao, et al. Intermittent electric field stimulated reduction-oxidation coupled process for enhanced azo dye biodegradation. Chem Eng J, 451 (2023), Article 138732.
[11]
H.Yun, B.Liang, D.Y.Kong, H.Y.Cheng, Z.L.Li, Y.B.Gu, et al. Polarity inversion of bioanode for biocathodic reduction of aromatic pollutants. J Hazard Mater, 331 (2017), pp. 280-288.
[12]
S.Qian, Y.F.Cheng. Accelerated corrosion of pipeline steel and reduced cathodic protection effectiveness under direct current interference. Constr Build Mater, 148 (2017), pp. 675-685.
[13]
E.Hoseinzadeh, A.Rezaee, M.Farzadkia. Nitrate removal from pharmaceutical wastewater using microbial electrochemical system supplied through low frequency-low voltage alternating electric current. Bioelectrochemistry, 120 (2018), pp. 49-56.
[14]
W.T.Zheng, S.J.You, Y.Yao, N.Q.Ren, B.Ding, F.Li, et al. Sustainable generation of sulfate radicals and decontamination of micropollutants via sequential electrochemistry. Engineering, 30 (2023), pp. 144-152.
[15]
L.L.Tian, C.M.Liao, X.J.Yan, Q.Zhao, Z.Y.Wang, T.Li, et al. Endogenous electric field accelerates phenol degradation in bioelectrochemical systems with reduced electrode spacing. J Hazard Mater, 442 (2023), Article 130043.
[16]
L.Lu, Y.H.Xie, Z.Yang, B.L.Chen. Sustainable decontamination of heavy metal in wastewater and soil with novel rectangular wave asymmetrical alternative current electrochemistry. J Hazard Mater, 442 (2023), Article 130021.
[17]
D.D.Liang, Z.Li, G.H.Liu, C.Li, W.H.He, J.N.Li, et al. Construction of bidirectional electron transfer biofilms via periodic polarity reversal. Chem Eng J, 452 (2023), Article 139145.
[18]
S.Dehghani, A.Rezaee, S.Hosseinkhani. Biostimulation of heterotrophic-autotrophic denitrification in a microbial electrochemical system using alternating electrical current. J Clean Prod, 200 (2018), pp. 1100-1110.
[19]
Z.Moghiseh, A.Rezaee. Removal of aspirin from aqueous solution using electroactive bacteria induced by alternating current. Environ Sci Pollut Res Int, 28 (20) (2021), pp. 25327-25338.
[20]
X.Wang, L.Zhou, L.Lu, F.L.Lobo, N.Li, H.M.Wang, et al. Alternating current influences anaerobic electroactive biofilm activity. Environ Sci Technol, 50 (17) (2016), pp. 9169-9176.
[21]
Z.Moghiseh, A.Rezaee, S.Dehghani. Minimization of hazardous sludge production using a bioelectrochemical system supplied by an alternating current electric field. Bioelectrochemistry, 132 (2020), Article 107446.
[22]
Q.Sun, Z.L.Li, Y.Z.Wang, C.X.Yang, J.S.Chung, A.J.Wang. Cathodic bacterial community structure applying the different co-substrates for reductive decolorization of alizarin yellow R. Bioresour Technol, 208 (2016), pp. 64-72.
[23]
J.Gilmore, M.Islam, J.Duncan, R.Natu, R.Martinez-Duarte. Assessing the importance of the root mean square (RMS) value of different waveforms to determine the strength of a dielectrophoresis trapping force. Electrophoresis, 38 (20) (2017), pp. 2561-3254.
[24]
D.Cui, Y.Q.Guo, H.S.Lee, H.Y.Cheng, B.Liang, F.Y.Kong, et al. Efficient azo dye removal in bioelectrochemical system and post-aerobic bioreactor: optimization and characterization. Chem Eng J, 243 (2014), pp. 355-363.
[25]
J.W.Wang, S.Rathi, B.Singh, I.Lee, H.I.Joh, G.H.Kim. Alternating current dielectrophoresis optimization of Pt-decorated graphene oxide nanostructures for proficient hydrogen gas sensor. ACS Appl Mater Interfaces, 7 (25) (2015), pp. 13768-13775.
[26]
J.He, Q.Zhang, M.Li, T.Z.Ming, J.P.Feng, H.J.Peng, et al. Evaluating the influence of gradient applied voltages on electro-enhanced sequence batch reactor treating aniline wastewater: system performance, microbial community and functional genes. J Clean Prod, 389 (2023), Article 136077.
[27]
W.T.Huang, J.F.Chen, Y.Y.Hu, L.H.Zhang. Enhancement of Congo red decolorization by membrane-free structure and bio-cathode in a microbial electrolysis cell. Electrochim Acta, 260 (2018), pp. 196-203.
[28]
P.Mani, V.T.Fidal, K.Bowman, M.Breheny, T.S.Chandra, T.Keshavarz, et al. Degradation of azo dye (acid orange 7) in a microbial fuel cell: comparison between anodic microbial-mediated reduction and cathodic laccase-mediated oxidation. Front Energy Res, 7 (2019), p. 101.
[29]
F.Y.Kong, H.Y.Ren, D.Liu, Z.L.Wang, J.Nan, N.Q.Ren, et al. Improved decolorization and mineralization of azo dye in an integrated system of anaerobic bioelectrochemical modules and aerobic moving bed biofilm reactor. Bioresour Technol, 353 (2022), Article 127147.
[30]
C.Li, S.G.Hu, C.C.Ji, K.X.Yi, W.L.Yang. Insight into the pseudocapacitive behavior of electroactive biofilms in response to dynamic-controlled electron transfer and metabolism kinetics for current generation in water treatment. Environ Sci Technol, 57 (48) (2023), pp. 19891-19901.
[31]
D.Cui, M.H.Cui, B.Liang, W.Z.Liu, Z.E.Tang, A.J.Wang. Mutual effect between electrochemically active bacteria (EAB) and azo dye in bio-electrochemical system (BES). Chemosphere, 239 (2020), Article 124787.
S.Ishii, S.Suzuki, Y.Yamanaka, A.Wu, K.H.Nealson, O.Bretschger. Population dynamics of electrogenic microbial communities in microbial fuel cells started with three different inoculum sources. Bioelectrochemistry, 117 (2017), pp. 74-82.
[34]
X.Zhang, A.Prevoteau, R.O.Louro, C.M.Paquete, K.Rabaey. Periodic polarization of electroactive biofilms increases current density and charge carriers concentration while modifying biofilm structure. Biosens Bioelectron, 121 (2018), pp. 183-191.
[35]
W.W.Cai, Z.J.Zhang, G.Ren, Q.X.Shen, Y.A.Hou, A.Z.Ma, et al. Quorum sensing alters the microbial community of electrode-respiring bacteria and hydrogen scavengers toward improving hydrogen yield in microbial electrolysis cells. Appl Energy, 183 (2016), pp. 1133-1141.
Y.Yin, G.T.Huang, Y.R.Tong, Y.D.Liu, L.H.Zhang. Electricity production and electrochemical impedance modeling of microbial fuel cells under static magnetic field. J Power Sources, 237 (2013), pp. 58-63.
[38]
S.S.Lim, J.M.Fontmorin, P.Izadi, W.R. WanDaud, K.Scott, E.H.Yu. Impact of applied cell voltage on the performance of a microbial electrolysis cell fully catalysed by microorganisms. Int J Hydrogen Energy, 45 (4) (2020), pp. 2557-2568.
[39]
I.Hayashi, H.Fujita, H.Toju. Deterministic and stochastic processes generating alternative states of microbiomes. ISME Communications, 4 (1) (2024), Article ycae007.
[40]
Y.Mittal, S.Dash, P.Srivastava, P.M.Mishra, T.M.Aminabhavi, A.K.Yadav. Azo dye containing wastewater treatment in earthen membrane based unplanted two chambered constructed wetlands-microbial fuel cells: a new design for enhanced performance. Chem Eng J, 427 (2022), Article 131856.
[41]
C.J.Zhang, Q.Peng, R.H.Xuan, Y.M.Zhang, G.Xue, W.H.Sun. Accelerating biodegradation of an azo dye by using its inorganic intermediates. J Chem Technol Biotechnol, 96 (8) (2021), pp. 2336-2342.
[42]
D. de losCobos-Vasconcelos, N.Ruiz-Ordaz, J.Galindez-Mayer, H.Poggi-Varaldo, C.Juarez-Ramirez, L.M.Aaron. Aerobic biodegradation of a mixture of sulfonated azo dyes by a bacterial consortium immobilized in a two-stage sparged packed-bed biofilm reactor. Eng Life Sci, 12 (1) (2012), pp. 39-48.
[43]
H.Verma, G.G.Dhingra, M.Sharma, V.Gupta, R.K.Negi, Y.Singh, et al. Comparative genomics of Sphingopyxis spp. unravelled functional attributes. Genomics, 112 (2) (2020), pp. 1956-1969.
[44]
Q.Dai, S.Zhang, H.Liu, J.Huang, L.Li. Sulfide-mediated azo dye degradation and microbial community analysis in a single-chamber air cathode microbial fuel cell. Bioelectrochemistry, 131 (2020), Article 107349.
[45]
V.Tarbajova, M.Kolackova, P.Chaloupsky, M.Dobesova, P.Capal, Z.Pilat, et al. Physiological and transcriptome profiling of Chlorella sorokiniana: a study on azo dye wastewater decolorization. J Hazard Mater, 460 (2023), Article 132450.
[46]
Y.Miao, N.W.Johnson, P.B.Gedalanga, D.Adamson, C.Newell, S.Mahendra. Response and recovery of microbial communities subjected to oxidative and biological treatments of 1,4-dioxane and co-contaminants. Water Res, 149 (2019), pp. 74-85.
[47]
J.Y.Zhang, K.Y.Zhou, F.L.Guo, H.X.Lei, R.X.Zhao, L.Lin, et al. The successional pattern of microbial communities and critical genes of consortia subsisting on chloramphenicol and its metabolites through long-term domestication. Engineering, 31 (2023), pp. 59-69.
[48]
Y.M.Fei, B.G.Zhang, D.D.Chen, T.X.Liu, H.L.Dong. The overlooked role of denitrifying bacteria in mediating vanadate reduction. Geochim Cosmochim Acta, 361 (2023), pp. 67-81.
[49]
S.X.Wang, B.G.Zhang, Y.M.Fei, H.Liu, Y.Zhao, H.M.Guo. Elucidating multiple electron-transfer pathways for metavanadate bioreduction by actinomycetic streptomyces microflavus. Environ Sci Technol, 57 (48) (2023), pp. 19921-19931.
[50]
M.Imran, M.Arshad, F.Negm, A.Khalid, B.Shaharoona, S.Hussain, et al. Yeast extract promotes decolorization of azo dyes by stimulating azoreductase activity in Shewanella sp. strain IFN4. Ecotoxicol Environ Saf, 124 (2016), pp. 42-49.
[51]
B.Liang, H.Y.Cheng, J.D. Van Nostrand, J.C. Ma, H. Yu, D.Y. Kong, et al. Microbial community structure and function of nitrobenzene reduction biocathode in response to carbon source switchover. Water Res, 54 (2014), pp. 137-148.
[52]
P.Semana, J.Powlowski. Four aromatic intradiol ring cleavage dioxygenases from Aspergillus niger. Appl Environ Microbiol, 85 (23) (2019), pp. e01786-e01819.
[53]
Y.J.Zhang, Q.Zhang, H.J.Peng, W.L.Zhang, M.Li, J.P.Feng, et al. The changing C/N of aggressive aniline: metagenomic analysis of pollutant removal, metabolic pathways and functional genes. Chemosphere, 309 (2022), Article 136598.
[54]
J.A.Vilchis-Carmona, I.C.Rodriguez-Luna, T.O.Elufisan, A.Sanchez-Varela, M.Bibbins-Martinez, G.Rivera, et al. The decolorization and degradation of azo dyes by two Stenotrophomonas strains isolated from textile effluent (Tepetitla, Mexico). Braz J Microbiol, 52 (4) (2021), pp. 1755-1767.
[55]
J.Hritz, G.Zoldák, E.Sedlák. Cofactor assisted gating mechanism in the active site of NADH oxidase from Thermus thermophilus. Proteins, 64 (2) (2006), pp. 465-476.
[56]
M.G.Waigi, F.X.Kang, C.Goikavi, W.T.Ling, Y.Z.Gao. Phenanthrene biodegradation by Sphingomonads and its application in the contaminated soils and sediments: a review. Int Biodeterior Biodegradation, 104 (2015), pp. 333-349.
[57]
R.Ilamathi, A. MerlineSheela, N.N.Gandhi. Comparative evaluation of Pseudomonas species in single chamber microbial fuel cell with manganese coated cathode for reactive azo dye removal. Int Biodeterior Biodegradation, 144 (2019), Article 104744.
[58]
K.Shi, H.Y.Cheng, C.R.Cornell, H.W.Wu, S.H.Gao, J.D.Jiang, et al. Micro-aeration assisted with electrogenic respiration enhanced the microbial catabolism and ammonification of aromatic amines in industrial wastewater. J Hazard Mater, 448 (2023), Article 130943.
[59]
R.Wang, H.N.Li, Y.F.Liu, J.H.Chen, F.Peng, Z.B.Jiang, et al. Efficient removal of azo dyes by Enterococcus faecalis R1107 and its application in simulated textile effluent treatment. Ecotoxicol Environ Saf, 238 (2022), Article 113577.
[60]
G.T.Chen, X.J.An, L.L.Feng, X.Xia, Q.H.Zhang. Genome and transcriptome analysis of a newly isolated azo dye degrading thermophilic strain Anoxybacillus sp. Ecotoxicol Environ Saf, 203 (2020), Article 111047.
[61]
Y.N.Wang, F.Ma, J.X.Yang, H.J.Guo, D.L.Su, L.Yu. Adaption and degradation strategies of methylotrophic 1,4-dioxane degrading strain Xanthobacter sp. YN2 revealed by transcriptome-scale analysis. Int J Mol Sci, 22 (19) (2021), p. 10435.
[62]
W.Z.Gao, Y.Q.Guan, Y.M.Li, X.W.Zhang, Z.X.Fu, Z.Zhang. Treatment of nitrogen and phosphorus in wastewater by heterotrophic N- and P-starved microalgal cell. Appl Microbiol Biotechnol, 107 (4) (2023), pp. 1477-1490.
[63]
H.Y.Cheng, B.Liang, Y.Mu, M.H.Cui, K.Li, W.M.Wu, et al. Stimulation of oxygen to bioanode for energy recovery from recalcitrant organic matter aniline in microbial fuel cells (MFCs). Water Res, 81 (2015), pp. 72-83.
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