A Review of Stand-Alone and Hybrid Microbial Electrochemical Systems for Antibiotics Removal from Wastewater

: The growing concern about residual antibiotics in the water environment pushes for innovative and cost-effective technologies for antibiotics removal from wastewater. In this context, various microbial electrochemical systems have been investigated as an alternative to conventional wastewater technologies that are usually ineffective for the adequate removal of antibiotics. This review article details the development of stand-alone and hybrid or integrated microbial electrochemical systems for antibiotics removal from wastewater. First, technical features, antibiotics removal efﬁciencies, process optimization, and technological bottlenecks of these systems are discussed. Sec-ond, a comparative summary based on the existing reports was established to provide insights into the selection between stand-alone and hybrid systems. Finally, research gaps, the relevance of recent progress in complementary areas, and future research needs have been discussed.


Introduction
Antibiotics are widely used in humans and livestock due to their effectiveness in inhibiting bacterial infection [1,2]. However, only a small portion of consumed antibiotics can be metabolized by the human or animal body [3,4]. The rest is discharged to the environment via domestic and animal wastewater [3,4]. Several studies reported mg/L levels of antibiotics in different wastewater streams, including domestic sewage, livestock wastewater, pharmaceutical industry wastewater, hospital effluents, and so on [3,5]. Such unprecedented levels of antibiotics in wastewater have raised significant public health concerns [1]. In particular, residual antibiotics in the aquatic environment exert selective pressure to propagate antibiotic resistance genes and spread antibiotic-resistant bacteria [1,3]. The overuse and discharge of antibiotics in wastewater have increased the emergence of antibiotic-resistant bacteria at an alarming rate and have caused the death of humans infected with antibiotic-resistant bacteria [6]. Therefore, antibiotic resistance is currently considered the most critical public health threat [7]. As a result, understanding the fate and removal of antibiotics during wastewater treatment, which is regarded as a hotspot for disseminating antibiotic-resistant bacteria, has received significant attention [3,5,8].
To minimize the dissemination and risks of antimicrobial resistance, it is critical to remove antibiotics before discharging treated wastewater to the environment. However, traditional wastewater treatment processes, such as the aerobic activated sludge process, cannot effectively remove antibiotics [9]. To date, different biological and physicochemical treatment processes have been investigated for removing antibiotics from wastewater [2,[9][10][11][12][13][14]. Despite the effective removal of a broad spectrum of antibiotics, most of these methods require external energy input and/or extensive chemical usage or replenishment for their implementation [15]. For instance, advanced oxidation processes (AOPs) are highly effective for antibiotics removal via oxidation to simple molecules or mineralization; however, they require expensive chemicals or catalysts [8,9]. Therefore, there is a growing need for innovative and low-cost wastewater technologies to effectively remove antibiotics. In this context, microbial electrochemical systems have shown great potential.
Despite different design configurations that have evolved over the years, microbial electrochemical systems usually consist of anode and cathode electrodes, often physically separated by a membrane [16]. Electroactive bacteria on the anode biofilm can capture electrons from the organic substrates in wastewater and transfer them to the electrode surface via a unique extracellular electron transfer (EET) process [17,18]. The transferred electron flows through the external circuit to the cathode, which can be abiotic or biotic depending on the target application. By manipulating the terminal electron acceptors in the cathode, the system can be engineered for high value-added applications, such as bioenergy (e.g., bioelectricity, hydrogen, methane) production or the synthesis of industrial chemicals (e.g., hydrogen peroxide) [19,20]. Thus, microbial electrochemical systems provide simultaneous wastewater treatment and resource recovery. Furthermore, microbial electrochemical systems have been investigated for nutrient recovery [21,22], water desalination [23,24], and biosensing applications [25,26].
Several studies have recently investigated antibiotics removal in microbial electrochemical systems as a stand-alone process [27][28][29][30][31] as well as their integration with constructed wetlands [32][33][34] and other physicochemical processes, such as advanced oxidation processes [35][36][37], membrane bioreactors [38], and electrosorption [39]. However, a critical appraisal of these processes is vital to guide their selection to meet target antibiotics removal efficiencies and future development needs. Therefore, this article presents a systematic critical overview of stand-alone and hybrid microbial electrochemical systems for antibiotics removal. Furthermore, the research gaps and future outlook are discussed.

Process Schemes and Antibiotics Removal Efficiencies
To date, several studies have investigated microbial electrochemical systems as a stand-alone process for antibiotics removal from synthetic wastewater containing acetate, glucose, sucrose, and lactate as carbon sources (Table 1). Most studies used microbial fuel cells (MFCs) (without an external power supply), except for Liang et al. [30], who used a microbial electrolysis cell (MEC) with an external power supply of 0.5 V. Several studies considered comparing their results with open-circuit biotic and/or abiotic reactor having electrodes as control [27][28][29][30][31]. For instance, Wu et al. [29] reported an almost doubled sulfamethoxazole removal in an MFC compared to an open-circuit MFC (i.e., control). Moreover, Yan et al. [27] found negligible removal of oxytetracycline in an abiotic control reactor having electrodes. Thus, these findings highlighted that bio-electrochemical degradation played a major role in removing antibiotics in stand-alone microbial electrochemical systems. Furthermore, previous studies have revealed that syntrophic interactions between electroactive bacteria and antibiotic-degrading bacteria would play a critical role in effective antibiotics removal in microbial electrochemical systems [40,41]. Figure 1 summarizes different process schemes studied for antibiotics removal with stand-alone MFCs. Both single-and dual-chamber MFCs have been used in previous studies (Table 1), while most were dual-chamber configurations. The maximum removal efficiencies for different antibiotic compounds ranged from 59.4% to 100%. To the best of the authors' knowledge, no studies systematically compared single-and dual-chamber designs to date. However, depending on the type of antibiotics, both single-and dualchamber configurations could provide up to 99-100% removal (see Table 1), suggesting that MFCs as a stand-alone process could effectively remove certain antibiotics from wastewater. Single-chamber configuration would reduce the reactor fabrication cost and maintenance requirement due to the exclusion of membrane [42,43]; thus, it would be preferred over the dual-chamber design. However, further confirmation of the effectiveness of single-chamber configuration for removing a broad spectrum of antibiotics is still warranted due to a limited number of studies. In dual-chamber configurations, antibiotics removal primarily occurs in the bioanode, while a few studies investigated treatment options with a biocathode or a sequential anode-cathode operation [15,30,31,41,44]. The major benefit of biocathode-based treatment is faster antibiotic degradation than bioanode or abiotic cathode-based treatment [15,30,41]. For instance, Sun et al. [15] found that biocathode-based treatment could provide 3.2 times faster degradation of chloramphenicol than the abiotic cathode. Chen et al. [44] found that a biocathode could provide a slightly higher removal of oxytetracycline than a bioanode (95% vs. 91.8%). Furthermore, Cheng et al. [31] recently reported that the sequential anode-cathode mode (see Figure 1c) could effectively remove residual antibiotics from anodic effluent. Therefore, future studies should systematically compare different process schemes to select an optimum approach for antibiotics removal.

Process Parameters
Previous studies have primarily focused on investigating the removal efficiencies of different antibiotic compounds and the effects of their initial concentrations (Table 1). However, a few studies have also focused on other process parameters, such as pH, temperature, and biofilm acclimatization [27,28,40]. Among 10+ antibiotic compounds researched in different studies (see Table 1), only a few of them (e.g., sulfamethoxazole, oxytetracycline, chlortetracycline, chloramphenicol, sulfadiazine, and tetracycline) have In dual-chamber configurations, antibiotics removal primarily occurs in the bioanode, while a few studies investigated treatment options with a biocathode or a sequential anode-cathode operation [15,30,31,41,44]. The major benefit of biocathode-based treatment is faster antibiotic degradation than bioanode or abiotic cathode-based treatment [15,30,41]. For instance, Sun et al. [15] found that biocathode-based treatment could provide 3.2 times faster degradation of chloramphenicol than the abiotic cathode. Chen et al. [44] found that a biocathode could provide a slightly higher removal of oxytetracycline than a bioanode (95% vs. 91.8%). Furthermore, Cheng et al. [31] recently reported that the sequential anode-cathode mode (see Figure 1c) could effectively remove residual antibiotics from anodic effluent. Therefore, future studies should systematically compare different process schemes to select an optimum approach for antibiotics removal.

Process Parameters
Previous studies have primarily focused on investigating the removal efficiencies of different antibiotic compounds and the effects of their initial concentrations (Table 1). However, a few studies have also focused on other process parameters, such as pH, temperature, and biofilm acclimatization [27,28,40]. Among 10+ antibiotic compounds researched in different studies (see Table 1), only a few of them (e.g., sulfamethoxazole, oxytetracycline, chlortetracycline, chloramphenicol, sulfadiazine, and tetracycline) have been studied multiple times. While most investigated removal of a single antibiotic compound, a few studies compared removal efficiencies of multiple antibiotics [31,[45][46][47].
Previous studies have suggested that removal efficiencies would be different depending on the type of antibiotics. Zhou et al. [47] studied the removal efficiencies of four antibiotics (aureomycin, sulfadimidine, roxithromycin, and norfloxacin) belonging to dif-ferent chemical classes in a single-chamber MFC. The authors found that all antibiotics could be effectively degraded (99.9-100% removal). Cheng et al. [31] studied the removal efficiencies of three antibiotics (sulfamethoxazole, sulfamethazine, and sulfadiazine) that belong to the same class (sulfonamide). Their results suggested that sulfamethoxazole could be degraded more easily than sulfamethazine and sulfadiazine. The removal efficiency of sulfamethoxazole was as high as 59.4%, while the maximum removal efficiencies of sulfamethazine and sulfadiazine were <20%. Thus, antibiotics under the same classes could exhibit different removal efficiencies. Wang et al. [48] also suggested slight differences in removal efficiencies for two tetracycline antibiotics (chlortetracycline and oxytetracycline).
The initial concentrations of antibiotics in wastewater have shown considerable impact in several studies [27,41,[49][50][51]. In general, removal efficiencies decreased after increasing initial concentrations of antibiotics to certain levels. For instance, Catal et al. [49] reported 88% removal of neomycin sulfate for an initial concentration of 20 mg/L, while removal efficiency considerably decreased after increasing the initial concentration to 75 mg/L. Furthermore, several studies reported a negative impact of higher initial antibiotics concentrations on MFC performance (i.e., energy output in terms of voltage or current) [46,52]. Thus, high antibiotic concentrations in wastewater may impede MFC performance in terms of antibiotics removal and energy output.
However, Yan et al. [27] suggested that long-term acclimation with gradually increasing antibiotic concentration could establish a more resilient and kinetically efficient anode biofilm in MFCs for antibiotics removal. The authors slowly increased oxytetracycline concentrations in the influent from 0.5 to 10 mg/L over ten months, achieving up to 99% removal at an initial oxytetracycline concentration of 10 mg/L. Notably, such long-term acclimation enriched Eubacterium species, a functional bacterial genus responsible for oxytetracycline degradation. Miran et al. [28] also found that acclimatized cultures in MFC could provide higher sulfamethoxazole removal than unacclimatized cultures. Furthermore, voltage generation was unaffected despite increasing sulfamethoxazole concentration from 0.04 to 0.20 mM. MFC inoculated with acclimatized cultures showed a higher abundance of Thauera species that have been reported to be capable of antibiotic degradation and adaptive to the antibiotic pressure in the literature [3,53]. Table 1. Summary of studies using microbial electrochemical systems as a stand-alone process for antibiotics removal from wastewater. The removal efficiencies of sulfadiazine (16.8-19.5%) and sulfamethazine (14.0-16.3%) were relatively lower than sulfamethoxazole. Moreover, the sequential anode-cathode operation was more efficient than the single (anode) continuous mode. Dual graphene-modified bioelectrode was developed for more efficient oxytetracycline removal. The removal efficiencies of sulfadiazine (13.39-66.91%) and sulfamethazine (32.84-67.21%) were relatively low.

Antibiotic (Initial
[45] Single chamber MFC -Anode biofilm was resilient against antibiotics in µg/L level but was sensitive to g/L level (inhibition ratio increased with increasing concentrations). [52] Neomycin sulfate (20-

Constructed Wetland-Microbial Fuel Cell (CW-MFC)
Constructed wetlands (CWs) are nature-based wastewater treatment processes that can provide cost-effective and adequate removal of various environmental pollutants [57][58][59]. CWs simultaneously combine multiple mechanisms for pollutants removal, including photodegradation, biodegradation, plant uptake, filtration, sorption, and phytoremediation [57]. CWs are often considered as a low-cost stand-alone alternative to conventional treatment processes, particularly in areas not limited by land availability [57,59]. For instance, CWs can be a feasible option for wastewater generated from livestock farms as well as agro-industrial sources in remote locations [59]. Both lab and field-scale studies demonstrated effective antibiotics removal using conventional CWs [10,57,60]. Notably, the literature has highlighted that CWs can provide more effective removal of antibiotics than conventional wastewater treatment processes [57]. Several studies have demonstrated that MFCs could be coupled with CWs to further improve their effectiveness for pollutants removal along with energy recovery opportunities [61,62]. Design-wise, CWs consist of upper aerobic and lower anaerobic zones, similar to typical aerobic and anaerobic chambers of MFCs [61]. Systems combining CWs and MFCs are called constructed wetland-microbial fuel cell (CW-MFC), where anode and cathode electrodes of an MFC are embedded within CW [61]. Many studies demonstrated the effectiveness of CW-MFCs for removing various recalcitrant and hazardous pollutants from wastewater, such as dyes, salts, antibiotics, personnel care products, pesticides, etc. [61,62]. Furthermore, the integration of MFCs with CWs was reported as a promising approach for mitigating methane emissions from CWs [63,64].
Despite that only limited studies have focused on the application CW-MFCs from antibiotics removal from wastewater (see Table 2), it has still been the most examined hybrid microbial electrochemical system studied for antibiotics removal. In addition to demonstrating the superior removal efficiencies for various antibiotics with hybrid CW-MFC systems over conventional CWs, previous studies investigated the effective electrode materials [65], and the significance of various process parameters [32][33][34]. To date, CW-MFCs have been assessed to remove sulfamethoxazole, sulfadiazine, ciprofloxacin, and tetracycline from synthetic wastewater (see Table 2). Most studies used vertical up-flow CW-MFCs ( Figure 2). To the best of our knowledge, Zhang et al. [66] first demonstrated the feasibility of using a CW-MFC system for tetracycline and sulfamethoxazole removal from synthetic wastewater over a period of 6 months. Their results demonstrated that tetracycline could be more effectively degraded than sulfamethoxazole; however, effluent antibiotic concentrations remained below <2.5 µg/L for a wide range of influent antibiotic concentrations (400-1600 µg/L). Granular activated carbon (GAC) particles were used as electrode materials in their study, while antibiotic adsorption onto GAC has been reported in the literature [67]. However, their study did not provide any control test results for systematic comparison between CW and CW-MFC systems. Recently, Dai et al. [68] compared the performance of CW and CW-MFCs for sulfamethoxazole (4 mg/L) removal from synthetic wastewater. Based on their results, CW-MFC could provide higher removal of sulfamethoxazole (82.37 vs. 71.32%) than a control CW. Furthermore, Song et al. [34] compared sulfadiazine removal efficiencies of CW-MFCs under open and closed circuit operation. Their results showed that sulfadiazine concentrations in the effluent of CW-MFCs operating under an open-circuit condition were relatively higher than the closed-circuit operation, demonstrating the significant contribution of anodic microbial oxidation in antibiotics removal. However, some antibiotics could still be adsorbed by the electrodes under an open-circuit condition [67]. Notably, understanding the relative contribution of anodic microbial oxidation, adsorption by electrodes, and other mechanisms associated with CWs would be necessary for further system optimization from an energy recovery perspective. Furthermore, antibiotics adsorbed onto the electrodes may contribute to selective pressure for ARG proliferation and eventually increase ARG transmission risks [69]. Table 2. Summary of studies related to constructed wetland coupled with microbial fuel cell (CW-MFC) for antibiotics removal from wastewater.

Antibiotic (Initial Concentration) Carbon Source Key Findings
Ref.

Antibiotic (Initial Concentration) Carbon Source Key Findings
Ref.   Li et al. [65] compared graphite and manganese (Mn) ore media as electrodes for CW MFCs for antibiotics removal. As Mn ores are composed of manganite and ferric oxid [73], the authors hypothesized that such electrode material could enhance anodic oxid tion by electroactive bacteria. Furthermore, manganese oxide (MnO2), a powerful oxida with adsorptive capability [74], can contribute to antibiotics removal. Conversely, grap and closed-circuit operation contributed to removing sulfamethoxazole (5-100 µg/L), tetracycline (5-50 µg/L). At an HRT of 1 day, removal efficiencies were >99% for both antibiotics. However, the closed-circuit operation had a more significant impact on antibiotics removal than the plants. The authors suggested that the plant might increase oxygen levels on the cathode, thereby improving performance. Wen et al. [70] also found that external aeration in the cathode can enhance antibiotics removal in CW-MFCs. However, providing external aeration can be expensive. Therefore, Wen et al. [69] proposed adding calcium peroxide in the cathode of CW-MFCs for improving dissolved oxygen (DO) levels to enhance antibiotics removal. Calcium peroxide is known for oxygen releasing capacity and has been widely used in recalcitrant pollutants removal in water and soil [71]. Therefore, such an approach could serve as an alternative to the conventional energy-intensive aeration method.

Sulfamethoxazole
A few studies have also focused on developing or modifying electrodes to improve antibiotics removal in CW-MFCs [65,69]. Wen et al. [65] found that the amendment of a GAC-based anode electrode with sponge iron (s-Fe 0 ) can further intensify the antibiotics removal efficiencies of CW-MFCs. Based on the literature, sponge iron (also known as direct reduced iron) can facilitate the electrochemical synthesis of ferrate(VI), which can oxidize antibiotics [72]. Wen et al. [65] also found that adding sponge iron could reduce antibiotics (sulfamethoxazole and tetracycline) accumulation on the cathode, consequently decreasing ARG abundance. Sponge iron is a cheap by-product found in steel plants [72]; thus, such an approach can provide an affordable solution to enhance CW-MFC performance for antibiotics removal.
Li et al. [65] compared graphite and manganese (Mn) ore media as electrodes for CW-MFCs for antibiotics removal. As Mn ores are composed of manganite and ferric oxides [73], the authors hypothesized that such electrode material could enhance anodic oxidation by electroactive bacteria. Furthermore, manganese oxide (MnO 2 ), a powerful oxidant with adsorptive capability [74], can contribute to antibiotics removal. Conversely, graphite was selected for its high adsorptive capacities and electrical conductivity. The authors found that a graphite-based electrode could provide superior sulfadiazine removal due to its high adsorption capacity, while both systems efficiently removed ciprofloxacin. In contrast, Mn ore electrode exhibited higher voltage generation than graphite. Moreover, the estimated cost of Mn ore electrode was~18 times lower than graphite (based on per m 3 anodic chamber). Thus, the type of antibiotics in incoming wastewater should be carefully considered in selecting optimum electrode materials for CW-MFCs.

Microbial Electrochemical Systems Coupled with Other Physicochemical Processes
In addition to slow biodegradation kinetics, some antibiotics may be highly recalcitrant to biodegradation. Furthermore, biodegradation may often lead to the formation of refractory intermediates. As shown in Table 3, several studies proposed the integration of microbial electrochemical technologies with advanced physicochemical processes, such as advanced oxidation process (AOP), membrane, and electrosorption, to achieve high antibiotics removal efficiencies (88.73-98.8%).
Particularly, integrating the electro-Fenton process as an AOP with microbial electrochemical systems has been investigated by multiple studies for the degradation and mineralization of antibiotics [35][36][37]. Briefly, the Fenton process uses a mixture of hydrogen peroxide (H 2 O 2 ) and ferrous iron (Fe 2+ ) to produce hydroxyl radicals (·OH) that can oxidize recalcitrant environmental pollutants, including antibiotics [75]. However, storage and transportation of H 2 O 2 can often be challenging. The electro-Fenton process can be used for the on-site synthesis of H 2 O 2 from the cathodic two-electrode reduction of dissolved oxygen in electrochemical cells [20,76]. Combining microbial electrochemical systems can provide a sustainable option for powering the electro-Fenton process. Such hybrid systems are called the bio-electro-Fenton (BEF) process (see Figure 3) [20,35]. Wang et al. [35] demonstrated a BEF process with a γ-FeOOH graphene polyacrylamide carbonized aerogel cathode for in situ removal of sulfamethoxazole and norfloxacin in wastewater sludge. Their study showed that both antibiotics could be effectively removed under neutral pH from both aqueous and solid phases, despite differences in their different absorbability. Sulfamethoxazole has moderate sorption ability to sludge, while norfloxacin can be highly adsorbed to sludge. The authors found that the BEF process could accelerate the desorption of antibiotics from sludge to the aqueous phase; thus, most antibiotics (79.9-86.6%) could be removed primarily by ·OH radicals. Li et al. [36] reported that the BEF process could provide effective removal of a low concentration of erythromycin (50 µg/L), and associated resistance genes (erm).
Processes 2022, 10, x FOR PEER REVIEW 10 of 19 wastewater sludge. Their study showed that both antibiotics could be effectively removed under neutral pH from both aqueous and solid phases, despite differences in their different absorbability. Sulfamethoxazole has moderate sorption ability to sludge, while norfloxacin can be highly adsorbed to sludge. The authors found that the BEF process could accelerate the desorption of antibiotics from sludge to the aqueous phase; thus, most antibiotics (79.9-86.6%) could be removed primarily by ⋅OH radicals. Li et al. [36] reported that the BEF process could provide effective removal of a low concentration of erythromycin (50 µg/L), and associated resistance genes (erm). Li et al. [38] developed a hybrid membrane bioreactor (MBR) integrated with MFC having a conductive membrane cathode, and demonstrating up to 90% removal of tetracycline. The membrane cathode was fabricated with polyvinylidene fluoride (PVDF) coated carbon fiber cloth and was further expanded with a dynamic granular activated carbon (GAC) cathode doped with FeOOH/TiO2, which could catalyze in situ electro-generation of H2O2 for Fenton-like oxidation. The vertical design of their reactor (anode at the top and cathode at the bottom) enabled permeate generation through the conductive cathode membrane by gravity, indicating an attractive energy-saving option (i.e., no requirement of pumping). Furthermore, over 60 days of operation membrane showed negligible fouling, possibly due to the generation of H2O2 [77] as well as electric field [78]. However, the authors operated the system with synthetic wastewater (glucose) containing tetracycline. Therefore, further investigation with real wastewater containing antibiotics is warranted for such promising designs.
In addition to the direct removal of antibiotics, MFCs can also be used to serve as energy suppliers for the electrosorption (ES) process for antibiotics removal (see Figure  4). Electrosorption is considered as an efficient adsorption method for removing heavy metals, salinity, organic pollutants, pharmaceuticals, personal care products, and (nano)microplastics [79][80][81]. The major benefits of electrosorption include a lack of secondary pollutants generation and flexible electrode regeneration by applying reverse voltage or removing voltage [39,82]. Due to the application of a low voltage between the anode and the cathode, charged ions or polar molecules (i.e., target pollutants) can migrate to the counter-charged electrode, accumulating an electric double layer on the electrode surface [80,82]. However, the operation of electrosorption requires electrical energy input for Li et al. [38] developed a hybrid membrane bioreactor (MBR) integrated with MFC having a conductive membrane cathode, and demonstrating up to 90% removal of tetracycline. The membrane cathode was fabricated with polyvinylidene fluoride (PVDF) coated carbon fiber cloth and was further expanded with a dynamic granular activated carbon (GAC) cathode doped with FeOOH/TiO 2 , which could catalyze in situ electro-generation of H 2 O 2 for Fenton-like oxidation. The vertical design of their reactor (anode at the top and cathode at the bottom) enabled permeate generation through the conductive cathode membrane by gravity, indicating an attractive energy-saving option (i.e., no requirement of pumping). Furthermore, over 60 days of operation membrane showed negligible fouling, possibly due to the generation of H 2 O 2 [77] as well as electric field [78]. However, the authors operated the system with synthetic wastewater (glucose) containing tetracycline. Therefore, further investigation with real wastewater containing antibiotics is warranted for such promising designs.
In addition to the direct removal of antibiotics, MFCs can also be used to serve as energy suppliers for the electrosorption (ES) process for antibiotics removal (see Figure 4). Electrosorption is considered as an efficient adsorption method for removing heavy metals, salinity, organic pollutants, pharmaceuticals, personal care products, and (nano)microplastics [79][80][81]. The major benefits of electrosorption include a lack of secondary pollutants generation and flexible electrode regeneration by applying reverse voltage or removing voltage [39,82]. Due to the application of a low voltage between the anode and the cathode, charged ions or polar molecules (i.e., target pollutants) can migrate to the counter-charged electrode, accumulating an electric double layer on the electrode surface [80,82]. However, the operation of electrosorption requires electrical energy input for its operation. To the best of the authors' knowledge, Yang et al. [39] first demonstrated that MFCs could generate adequate energy output to enable the operation of continuous-flow electrosorption for tetracycline removal. The authors found that connecting multiple MFCs would be necessary to attain high adsorption capacity. For instance, compared to a single MFC, connecting three MFCs in series could considerably increase tetracycline adsorption capacity from 14.16 to 23.12 mg/g. However, adsorption capacity did not increase linearly with an increasing number of MFCs, which could be attributed to voltage reversal during the operation of MFCs in series. Zhao et al. [82] also demonstrated that electrical energy generated by MFCs could be utilized for running continuous-flow electrosorption for oxytetracycline removal. Furthermore, effluent from the electrosorption process could be further treated with MFCs to remove residual oxytetracycline (see Figure 4), providing up to 98.8% removal for an initial antibiotic concentration of 2 mg/L. However, at a higher initial antibiotic concentration of 10 mg/L, removal efficiency decreased to 76.3%, which was primarily caused by the decline in removal efficiency in the electrosorption unit due to enhanced competition between antibiotics molecules for active sites at higher concentrations. In contrast, the performance of MFCs was unaffected by the increase in oxytetracycline concentration from 2 to 10 mg/L. Thus, in addition to being an energy supplier, MFCs could provide additional benefits by treating electrosorption effluent. Nonetheless, Yang et al. [39] emphasized that pH would be a critical factor for the electrosorption process, which could pose concerns for the operation of MFCs. Depending on the pK a values of the target antibiotic, acidic pH may be necessary to achieve efficient electrosorption. For instance, for pH < 3.3, tetracycline would exist in the form of protonated tetracycline (TCH 3 + ), which can easily be combined with the negative charge on the electrode surface, facilitating cation exchange for adsorption [39]. In contrast, neutral or alkaline pH may decrease the electrosorption of tetracycline. Therefore, adjusting pH to the acidic condition may be required for effective electrosorption. However, treating acidic effluent from the electrosorption process using MFC would be challenging, as acidic pH may inhibit the activity of electroactive bacteria in an anode biofilm [83].
its operation. To the best of the authors' knowledge, Yang et al. [39] first demonstrated that MFCs could generate adequate energy output to enable the operation of continuousflow electrosorption for tetracycline removal. The authors found that connecting multiple MFCs would be necessary to attain high adsorption capacity. For instance, compared to a single MFC, connecting three MFCs in series could considerably increase tetracycline adsorption capacity from 14.16 to 23.12 mg/g. However, adsorption capacity did not increase linearly with an increasing number of MFCs, which could be attributed to voltage reversal during the operation of MFCs in series. Zhao et al. [82] also demonstrated that electrical energy generated by MFCs could be utilized for running continuous-flow electrosorption for oxytetracycline removal. Furthermore, effluent from the electrosorption process could be further treated with MFCs to remove residual oxytetracycline (see Figure 4), providing up to 98.8% removal for an initial antibiotic concentration of 2 mg/L. However, at a higher initial antibiotic concentration of 10 mg/L, removal efficiency decreased to 76.3%, which was primarily caused by the decline in removal efficiency in the electrosorption unit due to enhanced competition between antibiotics molecules for active sites at higher concentrations. In contrast, the performance of MFCs was unaffected by the increase in oxytetracycline concentration from 2 to 10 mg/L. Thus, in addition to being an energy supplier, MFCs could provide additional benefits by treating electrosorption effluent. Nonetheless, Yang et al. [39] emphasized that pH would be a critical factor for the electrosorption process, which could pose concerns for the operation of MFCs. Depending on the pKa values of the target antibiotic, acidic pH may be necessary to achieve efficient electrosorption. For instance, for pH < 3.3, tetracycline would exist in the form of protonated tetracycline (TCH3 + ), which can easily be combined with the negative charge on the electrode surface, facilitating cation exchange for adsorption [39]. In contrast, neutral or alkaline pH may decrease the electrosorption of tetracycline. Therefore, adjusting pH to the acidic condition may be required for effective electrosorption. However, treating acidic effluent from the electrosorption process using MFC would be challenging, as acidic pH may inhibit the activity of electroactive bacteria in an anode biofilm [83].

Summary and Outlook
To date, microbial electrochemical systems have been studied to remove around 17 antibiotic compounds ( Figure 5). The most widely investigated compounds were sulfamethoxazole, tetracycline, sulfadiazine, and oxytetracycline. About half of the compounds were only studied in stand-alone single or dual-chamber MFCs. In contrast, a few compounds (sulfadiazine, ciprofloxacin, erythromycin, and metronidazole) were examined in hybrid systems. Previous research has studied only a handful of these compounds (sulfamethoxazole, tetracycline, oxytetracycline, and norfloxacin) in both stand-alone and hybrid systems. Thus, limited information is available in the literature to establish a systematic comparison between stand-alone and hybrid microbial electrochemical systems for antibiotics removal. It was apparent that most of the antibiotic compounds could be efficiently degraded in stand-alone MFCs ( Figure 6). Some antibiotics (ciprofloxacin, erythromycin, and metronidazole) were effectively degraded in hybrid systems (e.g., CW-MFCs, BEF), while they have not been studied in stand-alone MFCs ( Figure 5). Thus, further studies will clearly be needed to identify if these antibiotics can be adequately degraded in stand-alone MFCs. Moreover, among stand-alone MFCs, systematic studies to compare single-and dual-chamber MFCs are also required. Processes 2022, 10, x FOR PEER REVIEW 13 of 19 Initial antibiotics concentrations widely varied in previous studies (see Tables 1-3), which would be a critical factor influencing their removal efficiencies [27,41,[49][50][51]. Higher initial antibiotics concentrations could evidently deteriorate the performance of microbial electrochemical systems and antibiotics removal efficiencies [46,49,52]. Nonetheless, most antibiotics were efficiently degraded (≥90%) in microbial electrochemical systems except for chlortetracycline and sulfamethazine ( Figure 6). Chlortetracycline and sulfamethazine were only studied in dual-chamber MFCs [31,45,46,48]. It has been previously reported that these antibiotic compounds are highly persistent and difficult to biodegrade [84][85][86]. Among various tetracycline derivatives, chlortetracycline has been identified to be the most toxic [84]. Notably, its toxicity can be 100 times higher than sulfamethazine [87]. Topcu et al. [46] also reported that chlortetracycline showed the highest toxicity to anode biofilms among the three antibiotics (tetracycline, chlortetracycline, and oxytetracycline) tested in their study. Thus, stand-alone MFCs may not be highly effective for removing persistent and toxic antibiotics. Previous studies have suggested that Fenton-based AOPs might be required to effectively remove persistent antibiotics such as chlortetracycline and sulfamethazine [86,88,89]. Therefore, hybrid microbial electrochemical systems such as BEF should be investigated to remove such persistent antibiotics (e.g., chlortetracycline, sulfamethazine, etc.). Overall, it can be summarized that both standalone and hybrid microbial electrochemical systems can be technically viable for removing antibiotics from wastewater. However, the selection between stand-alone and hybrid processes should be between the type of antibiotics present in the wastewater. Initial antibiotics concentrations widely varied in previous studies (see Tables 1-3), which would be a critical factor influencing their removal efficiencies [27,41,[49][50][51]. Higher initial antibiotics concentrations could evidently deteriorate the performance of microbial electrochemical systems and antibiotics removal efficiencies [46,49,52]. Nonetheless, most antibiotics were efficiently degraded (≥90%) in microbial electrochemical systems except for chlortetracycline and sulfamethazine ( Figure 6). Chlortetracycline and sulfamethazine were only studied in dual-chamber MFCs [31,45,46,48]. It has been previously reported that these antibiotic compounds are highly persistent and difficult to biodegrade [84][85][86]. Among various tetracycline derivatives, chlortetracycline has been identified to be the most toxic [84]. Notably, its toxicity can be 100 times higher than sulfamethazine [87]. Topcu et al. [46] also reported that chlortetracycline showed the highest toxicity to anode biofilms among the three antibiotics (tetracycline, chlortetracycline, and oxytetracycline) tested in their study. Thus, stand-alone MFCs may not be highly effective for removing persistent and toxic antibiotics. Previous studies have suggested that Fenton-based AOPs might be required to effectively remove persistent antibiotics such as chlortetracycline and sulfamethazine [86,88,89]. Therefore, hybrid microbial electrochemical systems such as BEF should be investigated to remove such persistent antibiotics (e.g., chlortetracycline, sulfamethazine, etc.). Overall, it can be summarized that both stand-alone and hybrid microbial electrochemical systems can be technically viable for removing antibiotics from wastewater. However, the selection between stand-alone and hybrid processes should be between the type of antibiotics present in the wastewater.
The key features highlighted for microbial electrochemical systems often lie in energy and resource recovery. Despite demonstrating the technical feasibility of various microbial electrochemical systems, results from lab-scale studies may not provide a clear picture of the energy/resource recovery potential [90]. Until now, almost all studies have used synthetic wastewater containing antibiotics (see Tables 1-3), while future research should focus on testing antibiotics removal from real wastewater followed by in-depth techno-economic analysis. Moreover, scale-up remains an ongoing challenge towards the commercialization of microbial electrochemical technologies [91][92][93][94]. The field-pilot of different microbial electrochemical technologies has been limited [90,94]. However, there have been some encouraging reports. A report was recently published on the successful demonstration of a pilot-scale (1400 L) single-chamber MFC [94]. The integration of MFC with a biofilter could reduce energy consumption by more than 50% compared to conventional wastewater treatment [94]. Another study demonstrated a pilot-scale (100 L) microbial electrochemical system for cathodic H 2 O 2 generation [95]. Although the system could not generate concentrated H 2 O 2 , the results suggested that the concentration should be adequate for in situ oxidation of pollutants, such as applications as the cathodic Fenton process (i.e., BEF). A process quite similar to CW-MFCs (called METland ® ) has recently been demonstrated in Spain and Denmark to treat wastewater generated from small communities [96,97]. METland ® could effectively remove (>90%) sulfamethoxazole from wastewater [98]. Given these promising developments, the possible implementation of microbial electrochemical systems for antibiotics removal is likely for years to come. The key features highlighted for microbial electrochemical systems often lie in energy and resource recovery. Despite demonstrating the technical feasibility of various microbial electrochemical systems, results from lab-scale studies may not provide a clear picture of the energy/resource recovery potential [90]. Until now, almost all studies have used synthetic wastewater containing antibiotics (see Tables 1-3), while future research should focus on testing antibiotics removal from real wastewater followed by in-depth technoeconomic analysis. Moreover, scale-up remains an ongoing challenge towards the commercialization of microbial electrochemical technologies [91][92][93][94]. The field-pilot of different microbial electrochemical technologies has been limited [90,94]. However, there have been some encouraging reports. A report was recently published on the successful demonstration of a pilot-scale (1400 L) single-chamber MFC [94]. The integration of MFC with a biofilter could reduce energy consumption by more than 50% compared to conventional wastewater treatment [94]. Another study demonstrated a pilot-scale (100 L) micro-

Conclusions
This review summarized the application of microbial electrochemical systems for antibiotics removal from wastewater. Both stand-alone and hybrid microbial electrochemical systems hold great potential for antibiotics removal; however, significant research gaps appear as the unavailability of their systematic comparison. Nonetheless, it seemed that most antibiotics could be effectively removed in stand-alone MFCs, while persistent and toxic antibiotics may require hybrid or integrated systems. However, only a limited number of antibiotic compounds have been investigated thus far; therefore, further investigation needs to be expanded to more persistent and toxic antibiotics. In addition, the studies primarily used synthetic wastewater, while a complex matrix of actual wastewater may influence removal efficiencies and energy output. Therefore, comprehensive environmental and cost-benefit analysis and efficient system design are needed before field implementation of microbial electrochemical systems for antibiotics removal from wastewater.