Keywords

6.1 Introduction

The decline of valuable metal resources, together with the increased future valuable metals demand, is likely to provide future impetus for increased metal recovery from wastes such as fly ash, sewage sludge, spent batteries, and electronic scrap materials, as well as hydroprocessing catalysts. The recovery and reuse of these wastes usually require the conversion from an insoluble to a soluble form. While a number of pyrometallurgical methods have been employed to achieve dissolution of the metal oxides, the emission of toxic gases into the environment, high energy costs, and associated expensive capital equipment costs decrease its desirable attraction. The hydrometallurgical process is thus more favorable from an environment conservation viewpoint. However, this process requires large amounts of reagents and thus augments the operational costs. In addition, it also results in the co-dissolution of other metals, increasing the complexity and cost of recovering value-added metals and treatment of unwanted elements. A biohydrometallurgical process or bioleaching offers attractive features for the extraction of metals from solid materials due to lower cost and energy requirements, environmental safety, and operational flexibility [1]. However, there are additional remaining challenges for using this approach, such as increasing leaching rates and reducing sludge generation. Electrochemical reduction is regarded as a potential strategy for the separation of the dissolved metals from solutions owing to multiple merits such as effectiveness, selectivity, robustness, versatility, controllability, less sludge production, easy operation, short retention time, reusability of the effluent, and amenability to automation and control [2]. However, electrochemical processes have high energy requirements and can require expensive catalysts to decrease electrode overpotentials. Development of more environmentally benign and less energy-demanding technologies would therefore be useful for treating these metal wastes and wastewaters with simultaneous value-added metal recovery.

Bioelectrochemical systems (BESs) is a newly developed technology for wastes and wastewaters treatment based on the integration of biological processes, electrochemical reduction, material science, engineering, and many related area together. BESs have recently attracted much attention owing to its high efficiency, low cost, environmental sustainability, and ambient operating temperatures with biologically compatible materials [3, 4]. BESs present potential opportunities for the microbially catalyzed conversion of electrical current into attractive value-added products, providing significant environmental benefits through the displacement of chemical production by conventional means [3,4,5,6,7,8]. Following this exploration, an emerging research field recovering metals from wastes using BESs, namely, metallurgical BESs, is being developed in an early stage and shows the most promising prospects due to its beneficial for both limited resource and environmental ecosystem. There are a few reviews about BES technologies for metal recovery [9,10,11,12,13]. In an effort to minimize overlap, this review gives a condensed overview of our current knowledge of metal recovery from wastes using these next-generation technologies, highlighting recent discoveries of the so-called self-driven BES processes for mixed metal recovery and discussing critically the influence of different processes and design parameters for recovery efficiencies.

6.2 Bioelectrochemical Systems (BESs)

A BES is called a microbial fuel cell (MFC) if electricity is generated and the overall reaction is exothermic. When the overall reaction is endothermic, power is needed to drive the non-spontaneous reaction, and this BES is regarded as a microbial electrolysis cell (MEC) [7]. It is reasonably believed that microbial electrosynthesis is being emerged as an alternative option to provide reducing/oxidizing power for biochemical production via electricity [4]. In terms of metal recovery, the specific cathodic condition in BESs provides preferable situation for metal reduction, and this metallurgical BES technology has thus widened the application range of BESs [14]. In the following sections, latest experimental results on bio-electroreduction for heavy metals and the developments of two aspects, namely, abiotic cathodes and biocathodes, will be briefly summarized. The newly developed MFC-MEC self-driven systems for multiple metal recovery will be emphatically discussed. Influencing factors and electron transfer mechanisms in these systems, as well as the scientific and technical challenges that have yet to be faced in the future, will be reviewed in detail.

6.3 Abiotic Cathodes

The reducing environment in the BES cathode, which is a sink for electrons originally coming from organic compounds in the anode, holds an advantage for the treatment of oxidized metal pollutants. In most cases, the oxidative electron acceptors contact with the electrode surface directly and receive the electrons released from the cathode. In addition, cathodic electrons can be also indirectly transferred through mediators such as anthraquinone analogues, riboflavin, Fe(III), and O2 (Fig. 6.1 and Table 6.1) [15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52]. These direct and indirect electron transfer processes generally occur on the cathodes due to the high redox potentials of oxidative metal electron acceptors. Take the extensively explored Cr(VI) reduction in MFCs, for example (Table 6.1). Cr(VI) can be directly reduced to the less toxic Cr(OH)2+ and Cr(OH)2+ in addition to Cr(OH)3 on the abiotic cathodes of MFCs [15, 22]. Alternatively, Cr(VI) also indirectly accepts electrons through the in situ generated hydrogen peroxide from oxygen oxidation [15] or the external added riboflavin or Fe(III) [17, 18], which receives electrons either directly from the abiotic cathodes or via the mediator of anthraquinone-2,6-disulfonate. These mediated electron transfers explain the accelerated Cr(VI) reduction on the abiotic cathodes.

Fig. 6.1
figure 1

Electron transfer pathways in the abiotic cathodes of BESs

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Table 6.1 Metals removed/reduced in the abiotic cathodes of BESs
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6.3.1 Individual Metal Recovery

By controlling operating conditions, some desirable metals or products can be generated from the cathode chamber. BESs thus could be used as not only an environmental remediation technology, but also a tool to produce metals from low-grade ores in hydrometallurgical processes. Great attention has been paid to the finding of metals possibly used as cathodic electron acceptors in BESs. Diverse aqueous metals including Cr(VI) [18, 22, 23], V(V) [34, 35, 44], Mn(VII) [33], Hg(II) [32], Ni(II) [42], Cu(II) [14, 25,26,27,28,29,30,31], Ag(I) [19, 20], Au(III) [21], and Co(II) [39, 40] have been individually reduced, whereas Cd(II) was removed through biosorption, and Zn(II) was formed as sulfides precipitation or separated through supported liquid membrane extraction in one-chamber air-cathode BESs [43, 47] (Table 6.1). This list does not seem to have an end so far. Besides aqueous metal ions, metals in dissoluble particles such as Co(III) in particles LiCoO2, major component of the extensively applied lithium-ion batteries, can be also reduced on the cathodes of both MFCs and MECs [36, 37]. Cathodic electrons play a synergetic interaction with HCl for cobalt leaching, leading to the decrease of apparent activation energy of cobalt leaching in both MFCs (30.6 kJ/mol) [37] and MECs (16.6 kJ/mol) [36], in comparison with the 30.8–98.7 kJ/mol in open circuit controls (OCC). The presence of Cu(II) catalyst further decreases the apparent activation energy of cobalt leaching in MFCs to 11.8 kJ/mol [38]. These results demonstrate the more efficiency of BES technologies than conventional chemical processes, and thus provide new efficient approaches for recovery of metals in solid wastes and broaden the applicable BESs for recycling spent lithium-ion batteries. In terms of net energy production/consumption, BES technologies show appreciable advantages over conventional electrochemical processes due to the always free fuels in the anodes [4]. Taking silver metal, for example, an abiotic cathode MFC can achieve recovery of pure silver metal and electrical production at a rate of 0.0143 kWh per kg of silver (69.9 kg silver per kWh energy output) in comparison with an electricity spending of 3.81 kWh per kg of silver at an optimum condition in a conventional electrowinning [19]. Thus the use of abiotic cathode MFCs for metal recovery would be to use the “green” electricity produced in the MFC to supply power for electrowinning. This process has the advantage to keep the reactions take place in only one system and thus reduce the overall energy losses. Besides, abiotic cathode BESs can also achieve higher metal removal efficiency and product purity than conventional electrolysis reactors [20, 26, 27]. In terms of endurance to high metal concentrations and acidic environments, abiotic cathodes show advantages over biological processes, in which microorganisms can only endure to a certain metal concentration at neutral or close to neutral pHs, after which inhibition of the biological processes takes over [39, 53]. Another striking feature is that abiotic cathodes can work well at a wide range of metal concentration compared to either a maximal metal concentration for conventional biological processes or a minimal metal concentration required for conventional electrolysis process [20, 26, 27]. All of these aforementioned above demonstrate the advantages of BESs over conventional technologies for individual metal leaching and/or subsequent recovery from aqueous phase to solid phase.

6.3.2 Multiple Metal Recovery and Self-Driven BESs

While numerous initiatives have attempted to develop abiotic cathodes for individual metal recovery, there is a trend of switch to recover multiple metals, making BES a more practical application (Table 6.1). Species of V(V) and Cr(VI), co-present in wastewaters from vanadium mining and vanadium pentoxide manufacture, are recently proved to be, respectively, reduced on the abiotic cathodes of MFCs [44]. Cr(VI) is firstly reduced as an electron acceptor due to its higher electrochemical redox potential than V(V), which leads to Cr(VI) decreasing and Cr(III) depositing, and the electrochemical redox potential of V(V) then exceeds that of Cr(VI) and begins to act as an electron acceptor to be converted into soluble V(IV). This repeatable and alternative reduction of Cr(VI) and V(V) provides an applicable abiotic cathode MFCs for separating Cr(VI) from V(V) in practical wastewaters. Closely following this report and by varying the cathode potentials of MECs, multiple metals of Cu, Pb, Cd, and Zn are selectively and sequentially separated from a simulated municipal solid waste incineration ash leachate, providing an approach for cathodic recovery of metals from municipal solid waste incineration ash leachate [5]. Similarly, simulating fly ash leachate containing multiple metals of Zn(II), Pb(II), and Cu(II) can be also successfully recovered with Zn(0) and Pb(0) in electrolysis cells and Cu(0) in MFCs [54]. While Cu(0) and Ni(0) are deposited on the same cathodes of MECs at an applied voltage of 1.0 V [45], the Cu(0) deposited in MFC mode substantially enhances the subsequent Cd(II) reduction on the same cathode but in MEC mode [52], stressing the critical catalysis role of previously deposited copper in Cd(II) reduction. Cu(0) deposited on the cathodes of titanium sheet or stainless steel woven mesh has also been observed to improve electricity generation and Cu(II) removal from cathoyte of MFCs over prolonged time [31]. Obviously, competition of electrons among protons, Cu(II), Ni(II), and Fe(II) on the cathodes of MECs was also observed, explaining the delay of each metal ion reduction in comparison with individual Cu(II), Ni(II), or Fe(II) reduction on the same cathodes [45]. While MFCs or MECs as wastes treatment methods could be potentially used for treating ash leachates, metallurgical wastewaters, and landfill leachates, the products with multiple metals require the subsequent separation of these mixed metals unless otherwise specially used. In addition, these MFCs, MECs, and electrolysis cells were separately operated, in which not only electricity generated from MFCs was not utilized but also external applied voltages of 1.0–6.0 V were required for MECs and electrolysis cells [45, 54]. In view of this point and enlightened from MFC-MEC coupled system for hydrogen production [55], a self-driven MFC-MEC system successfully carried out the two processes of Co(II) firstly released from particles LiCoO2 on the cathodes of MFCs and subsequently reduced on the cathodes of the connected MECs, which are completely powered by the cobalt leaching MFCs [41]. This self-driven system thus provides a new process of linking MFCs to MECs for complete recovery of cobalt and recycle of spent lithium-ion batteries with no any external energy consumption. To develop the concept of self-driven system, Cr(VI)-reduced MFCs and Cu(II)-reduced MFCs are connected in parallel or series to successfully power Cd(II)-reduced MECs with simultaneous Cr(VI), Cu(II), and Cd(II) recovery, despite the individual metal influents in each reactor units [48]. Appropriately adjusting the composite of mixed metals of Cr(VI), Cu(II), and Cd(II) under continuous operating condition can achieve complete separation of Cr(VI), Cu(II), and Cd(II) from the mixed influents using this self-driven MFC-MEC systems [49]. For W and Mo deposition, stacked MFC-MEC made of one MEC unit serially connected with three parallel-connected MFC units outperformed other modules, achieving depositions of 27.6% (W) and 75.4% (Mo) with a separation factor of 8.1 and hydrogen production of 0.34 m3/m3/day in the MEC unit, compared to 12.3% (W), 52.6% (Mo), and 7.9 (separation factor) in the MFC unit [56]. In the controls of either MEC or MFC unit only, only 15.3% (W) and 60.1% (Mo) (MFC only) and 12.9% (W) and 56.1% (Mo) (MEC only) were deposited from a mixture of W(VI) and Mo(VI). Thus, this process provides a truly sustainable strategy for applicable recovery of multiple metals from electroplating wastewater and ore dressing wastewater used during W and Mo extraction processes with no need for external energy input. Ingenious designs of self-driven MFC-MEC coupled systems together with appropriate influent composites, solution chemistry, and operation modes provide guarantee for sequential metal recovery and complete separation from mixed influents using these zero energy consumption technologies. While metals deposited on the electrodes may need to be peeled from the electrode to achieve their final recovery, the in situ utilization of these deposits for photocatalytic processes may become an attractive strategy for reuse, since many metal oxides exhibit excellent photocatalytic properties [56, 57]. Multiple parameters including initial metal concentration, initial pH, electrode material, electrode distance, exoelectrogenic activities, and the copresence of multiple electron acceptors can particularly affect system performance as well as final products. It is thus essential to discuss these parameters in the following sections.

6.3.3 Critical Factors Influencing System Performance

6.3.3.1 Initial Metal Concentration

A decrease in initial metal concentration resulted in a decrease in cathode potential and an increase in internal resistance of BESs. As a result, cell voltage, current density, and cathodic efficiency decreased as well [14, 19, 20, 25,26,27]. Thus a high initial metal concentration will generally benefit for BES system performance [14, 29, 30]. However, in view of reduction products, this high initial metal concentration can lead to the deficient cathodic reducibility, which may change the products formed. For example, high initial Cu(II) concentrations of 500–6400 mg/L have led to the formation of non-reductive product of Cu4(OH)6SO4 compared to the reductive products of Cu2O and Cu at a low initial Cu(II) concentration of 200 mg/L [25,26,27]. It is thus essential to control initial metal concentration in order for the formation of desirable products and in particular the preferable low metal concentrations for pure reductive metals. However, even at the same initial concentration and the identical metal ion, the variety of metal compounds also affect metal reduction rate, power production, as well as product purity. For example, at identical initial Ag(I) concentrations in the same MFC reactors, species of Ag(I) ions achieved apparent higher reduction rate and power production than Ag(I) thiosulfate complex (AgS2O3)− in addition to the pure Ag in the former and trace Ag2O in the latter (Table 6.1) [20], stressing the complexity of metal reduction on the cathodes as well as the importance of various metal compounds on system performance.

6.3.3.2 Initial pH

A comparatively high cathode pH is in favor of the reduction of oxidized contaminants that require higher pH, while a low cathode pH benefits to the reduction of oxidized substrates in need of more acidic conditions. In most cases, metal reduction in the abiotic cathode requires an acidic pH such as 2.0–3.0 (Table 6.1). For example, Cr(VI) was reduced to Cr(III) in the abiotic cathode MFCs, during which a low pH substantially improved reduction rate according to Eq. 6.1 [22].

$$ {\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2-}+14{\mathrm{H}}^{+}+6{e}^{-}\to 2{\mathrm{Cr}}^{3+}+7{\mathrm{H}}_2\mathrm{O} $$
(6.1)

However, in the case of Ag(I) reduction, the pH effect was dependent on the original form of Ag(I) electron acceptor, in which a higher pH of 10 was favorable for the reduction of Ag(I) thiosulfate complex than the pH 4.0 for ion Ag(I) [20]. Different from this, Co(II) reduction in MECs was improved at a range of 85–97% with an increase in initial pHs from 3.8 to 6.2, mainly due to the beneficial acidic environment for hydrogen-producing process and reasonably disadvantage to its electron competitor of Co(II) reduction [39]. These results in concert imply the complex interrelated effects of initial pH, original form of metal, and hydrogen evolution on reducing metals to the same final products. Besides the aforementioned above, the formations of reductive products are also influenced by initial pH. At the tested range of low pHs, pure crystals of copper [14, 25,26,27], vanadium [34, 35], and mercury [32] with no trace of other corresponding oxides and hydrates were formed on the cathodes because a high pH made these metal ions precipitate as metal oxide and was unavailable for reduction. Considering the fact of low pHs in these metal containing waste streams, the chemical conditions of such wastewaters are suitable for them to act as electron acceptors in the abiotic cathodes, which prefer low pHs and directly reduce metals from wastes with no pH adjustment. However, a bioanode covered by exoelectrogens was preferably operated at near neutral pH to achieve higher power generation from MFCs [4]. A bipolar membrane was therefore more effective to prevent the pH in the catholyte from increasing and the anolyte pH from dropping although part of the energy was lost for maintaining the pH difference [58].

6.3.3.3 Electrode Material

Cathode electrode materials and their design were the most challenging aspects of BESs using air as a final electron acceptor [4]. In this case, cathodic reactions took place on the three-phase surface of solid electrode, liquid catholyte, and gaseous oxygen. Increasing cathode surface area and retaining a small anode relative to the cathode area can keep cathodic reactions from limiting rates of electron transfer at the bioanode and therefore improve power production from MFCs [4]. For soluble metal reduction on the abiotic cathodes, a certain concentration of highly soluble metals such as Cu(II) >200 mg/L at acidic conditions can preserve faster mass transfer in comparison with the occurring of mass transfer limitations of oxygen as a result of low oxygen solubility in air-cathode MFCs [14]. The overpotential for soluble metal reduction is thus much lower than that of oxygen reduction reaction. Consequently, much more porous electrode materials commonly used in aqueous air-cathodes such as granule graphite and graphite felt are not always necessary for abiotic cathodes for recovery of metals at high concentrations. Instead, carbon-based cathodes with equally apparent sizes of anodes like graphite plate and graphite foil are usually accepted [14, 20, 25,26,27]. However, under the mediation of dissolved oxygen, which is heavily dependent on electrode materials, reduction of metal ions such as Cr(VI) is reasonably related with cathode materials [15]. In addition, metal ions at low concentrations exhibit high overpotentials, resulting in the occurrence of electron competition with other species. For example, hydrogen is well known to be evolved in MECs, and the efficiency is heavily dependent on electrode materials [59, 60]. As a consequence, the reduction of Co(II) as low as 50 mg/L in MECs is indirectly related with electrode material via competition with hydrogen evolution [40]. In view of these considerations, species in the catholyte such as dissolved O2 or hydrogen evolution should be carefully investigated to ensure efficient metal reduction.

In the case of self-driven MFC-MEC system for multiple metal recovery and separation, cathode material in MEC is crucial for efficient metal recovery, morphology, and crystal form of final products due to its substantial effects on electrode potential and circuit current [48, 50, 51]. Carbon rod as the cathodes of MECs cannot lead to Cd(II) or Co(II) reduction inside regardless of the serial or parallel-connected Cr(VI)-reduced MFCs and/or Cu(II)-reduced MFCs, mainly ascribed to the unsatisfied low voltage output from the MFCs and the consequent high cathode potentials unfavorable for Cd(II) reduction in MECs [50]. Conversely, titanium sheet or stainless steel mesh is a suitable cathode material used successfully for proceeding Cd(II) or Co(II) reduction in MECs with simultaneous Cr(VI) and/or Cu(II) reduction in the serially or parallel-connected MFCs [50]. Even for the same material of stainless steel mesh, Mesh #60 instead of #20 and #120 can achieve the best and complete separation of Cu(II), Co(II), and Li(I) [51]. In addition, the morphology and crystal form of final Co(II)-reduced products are substantially different and heavily dependent on the MEC cathode materials of carbon rod, titanium sheet, and stainless steel mesh [50]. These results in concert stress the importance of MEC cathode materials for multiple metal recovery and separation in the self-driven MFC-MEC systems, which should be conditionally considered as the aforementioned.

6.3.3.4 Initial Concentration and Ratio of Different Metals

The ratios of different metals in the influent of MFC-MEC coupled system play critical roles in the separation of these metals from mixed influents. Mixed Cu(II) and Co(II) at a same concentration of 50 mg/L was firstly fed in the cathodes of MFCs, followed by the cathodes of the connected MECs. This sequential MFC-MEC cannot achieve the complete separation of Cu(II) and Co(II), leading to the mixed reduced products of Cu(0) and Co(0) on the same cathodes of MECs [50]. Similarly, metals of Cr(VI), Cu(II), and Cd(II) with each of 5 mg/L cannot be completely removed using the self-driven MFC-MEC system, whereas a composite of either 5 mg/L Cr(VI), 1 mg/L Cu(VI), and 5 mg/L Cd(II) or 1 mg/L Cr(VI), 5 mg/L Cu(II), and 5 mg/L Cd(II) can be completely and sequentially recovered from the mixed metals, illustrating the importance of metal composite and ratios for complete metal recovery and separation [49].

6.3.3.5 Electrode Distance

A properly closed anode and cathode distance can decrease internal resistance and thus improve electron transportation from anode to the cathode, and consequently benefit to completely metal reduction. For example, in a pilot and membrane-free MFC using Cu(II) as an electron acceptor, the internal resistance can be decreased from 1694 Ω at a distance of 65 cm to 304 Ω at 35 cm [25]. It was thus concluded that a close anode and cathode created a high circuit current and provided more sufficient electrons for Cu(II) reduction for pure copper, whereas the limited electrons or lower currents at a far anode and cathode distance resulted in the less reduced copper species such as partial Cu(II) reduction to Cu2O or CuCl [25]. In terms of reactor size, however, a far anode and cathode distance is generally observed in large reactors and results in the consequent low system performances. For example, a large volume up to 16 L in pilot-scale membrane-free MFC substantially decreased system performance for both Cu(II) reduction and power generation compared to other smaller volume MFCs (Table 6.1) [25]. In view of practical application, scale-up reactors with large volumes will satisfy the requirement of large amount wastewater treatment. Based on these considerations, performance in stack cells where many small reactors are connected in parallel or in series may be an alternative choice. However, the variability in the capacity for individual reactor in the stack may lead to voltage reversal in some reactors [61]. In view of this point, a same hydraulic condition and a same substrate concentration are beneficial for less voltage reversal [62]. In addition, various types of control circuit for each cell in the stack system may also avoid this phenomenon [63]. Much effort is still in great need along this direction for more efficient and practically applied metal recovery from wastes.

6.3.3.6 Exoelectrogenic Activities

Cathodic electrons originally come from organic compounds oxidized by exoelectrogens on the anodes. Exoelectrogenic activities reasonably affect metal reduction on the abiotic cathodes. For example, bioanodes catalyzed by either Shewanella decolorationis S12 or Klebsiella pneumoniae L17 exhibited slower Cr(VI) reduction than anaerobic activated sludge, mainly ascribed to their different exoelectrogenic activities [15]. In the case of Co(III) reduction on the abiotic cathodes, exoelectrogenic activities were substantially different from those using pentachlorophenol as an electron acceptor in the cathodes in spite of their similar microbial community compositions [37, 64], stressing the changes of exoelectrogenic activities with cathodic electron acceptors. While bacterial community collaboration may occur among many other bacteria and exoelectrogens on the anodes [4, 65], exoelectrogenic activities in linkage with cathodic metal acceptors have attracted less attention. Further investigation of the exoelectrogenic activities of bacteria with diverse metal reductions on the cathodes is still needed.

6.3.3.7 Other Electron Acceptors

Other electron acceptors such as oxygen can heavily affect system performance due to its higher redox potential and competitive ability than the metals present in the cathode. In the case of Cu(II) or Co(II) reduction, the presence of oxygen also consumed electrons and consequently resulted in adverse effects on Cu(II) or Co(II) reduction as well as low cathodic efficiencies [14, 40]. For W(VI) and Mo(VI) deposition in MFCs, however, the presence of oxygen can enhance W and Mo deposition through the in situ produced H2O2 and the consequent predominant peroxo-tungstate and peroxo-polymolybdate despite the always occurrence of competition between oxygen reduction and metal deposition for H+ ions [66]. The purity of reduced products was also dependent on aerobic and anaerobic environments, where pure copper crystals were attributable to the anaerobic condition, and CuO and Cu2O other than Cu(0) were formed under an aerobic environment [25,26,27]. In terms of power production, it is understandable that the multiple electron acceptors of oxygen and Cu(II) had higher current densities than the Cu(II) individually due to a high redox potential of 0.8 V for oxygen [13, 14]. In fact, in view of oxygen reduction, copper here may also function as a catalyst, although the catalysis mechanism was still unclear [14]. Quantitative competition between metal ions and other electron acceptors for electrons transferred from the anode may need to be further reinforced to stress the greater efficiencies and advantages of abiotic cathodes compared to conventional processes for metal recovery.

6.4 Biocathodes

6.4.1 Recovered Metal

While an abiotic cathode employed as a direct electron donor in the reduction of metals has been proposed, development of microbially catalyzed cathodes (microbial cathodes or biocathodes) revealed that certain electrochemically active bacteria (electrotrophs) are capable of “picking” electrons from the surface of cathodic electrodes and using them to metabolically reduce the oxidative metals in the catholytes. The use of bacteria can avoid some of the drawbacks such as much acidic condition and low sustainability in abiotic cathodes [67, 68]. Metal reduction on the biocathodes can be dated back to 2005, in which Gregory and Lovley [69] demonstrated the occurrence of U(VI) reduction on a graphite plate cathode at a poised potential of −0.3 V (vs SHE) under the catalysis of either Geobacter sulfurreducens or enrichment culture (Table 6.2). A substantially higher U(VI) reduction rate of 0.58–0.77 mg/L/h with the presence of G. sulfurreducens implies the preferable G. sulfurreducens instead of enrichment culture to U(VI) reduction. The pure culture of G. sulfurreducens can get energy from reducing or adding electrons to U(VI) and reduce uranium dissolved in groundwater and thus make this metal much less soluble and abate the spread of its contamination. Similar to U(VI) reduction, Shewanella species was recently proved to use electrode as electron donor for Cr(VI) reduction [17, 70]. Instead of pure culture, Tandukar et al. [71] constructed a complete biological MFC with mixed culture at both the anode and the cathode and achieved a Cr(VI) reduction rate of 0.17–0.42 mg/L/h on the cathode with spontaneous electricity production of 0.9 W/m3 (Table 6.2). The Cr(VI)-reducing biocathode was further demonstrated with preferable electrode materials for electrotrophic attachment [67], modifications to reactor architecture [72], and minimization of start-up period and enhancement of system performance [73]. The newly established biocathode MECs dominantly composed of G. psychrophilus, Acidovorax ebreus, Diaphorobacter oryzae, Pedobacter duraquae, and Prolixibacter bellariivorans provide a new approach for aqueous Co(II) recovery concomitant with production of other biomaterials such as gaseous methane and liquefied acetate [53]. Besides metal recovery and other biomaterials production with simultaneous wastes treatment and environmental remediation, another potentially applicable field for biocathodes is metal nanoparticles synthesis, which is a very exciting field because of its potential application in bioenergy, catalysis, electronics, optics, medicine, and environmental remediation. While a large number of bacteria including Shewanella oneidensis have been illustrated to act as nanofactories, showing advantages over chemical methods due to the consumption of strong reducing agents and large quantities of chemicals that can contaminate the nanoparticles [74], biocathodes are expected to develop microbial consortia or pure culture exhibiting both electrotrophic activities and synthesizing metal nanoparticle abilities [75]. However, this concept is still not extensively proved in BESs, and metal-reducing biocathodes are demonstrated in very limited literature (Table 6.2), in which only metals of U(V), Cr(VI), Se(VI), Co(II), Cu(II), and Cd(II) together with a narrow range of operating conditions including initial metal concentration, initial pH, anodic acetate dose, cathodic electrode material, and optimal start-up time were reported [69,70,71,72,73, 76,77,78,79,80,81,82,83,84,85]. In addition, OH− generated from oxygen-reducing biocathode MFCs in situ reacted with Co(II) to form precipitated Co(OH)2, providing a new clean approach for the production of cobalt dihydroxide with simultaneous electricity generation (Table 6.2) [81]. It is very recent that a directed production of selenium-containing nanoparticles in S. oneidensis MR-1 cells, with fine-tuned composition and subcellular synthetic location, was achieved by modifying the extracellular electron transfer chain, leading to the development of fine-controllable nanoparticles biosynthesis technologies [75]. Much work is still needed to be paid on this emerging alternative and inexpensive technology for devising new microbial cathode systems for efficient metal reduction and broadening applicable fields of BESs as well. On the other hand, the recovery of metals by biocathodes will likely not displace existing methods of electrochemical or chemical-physical processes, especially for high-strength metal recovery, because of detrimental effects of high concentration of metals on electrotrophic activities. Biocathodes will likely be more appropriate for treatment of relatively low-strength or dilute metal effluents [53, 81, 85]. The overall advantages of biocathodes for recovery of metals from wastes could make them an important method for metal reduction in the near future. Factors including bacterial origin and evolution, initial pH, and metal concentration can particularly influence biocathode performance since environmental conditions can shape microbial consortia in terms of various bacterial roughness, biocompatibilities, electron transfer efficiencies, and stimulus to microbial consortia [53]. In addition, electron transfer mechanisms on the biocathodes, properly different from the bioanodes, are still debatable [68, 86]. In the following sections, these aspects in linkage with metal recovery will be in particular addressed.

Table 6.2 Metals removed/reduced in the biocathodes of BESs
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6.4.2 Bacterial Origin and Evolution

Microbial consortia inoculated from different sites exhibit various Cr(VI) reduction rates, in which bacteria from a wastewater treatment plant achieved a specific Cr(VI) reduction rate of 0.30 mg/g biomass/h [71] compared to 2.4 mg/g biomass/h obtained from a Cr(VI) contaminated site [72]. Although other factors including reactor architecture and electrode material may also contribute to these differences in Cr(VI) reduction rate, microbial consortia well developed at a Cr(VI) contaminated site is presumably more adaptive and favorable for the Cr(VI) environment in the biocathodes and thus attribute to more efficient Cr(VI) reduction [72]. Further exploration should use the same reactor architecture with identical electrode material to compare effects of different bacterial origins on metal reduction in order to deeply understand relations between microbial consortia and metal reduction.

Another important issue about the catalysts of microbial consortia is the efficient evolution strategies for specific microbial consortia. It has long been recognized that mixed species biofilm of Klebsiella pneumoniae, Pseudomonas fluorescens, and Pseudomonas aeruginosa grown in a flow cell fitted with two platinum wire electrodes remained changeable with the alternative anode and cathode. The biofilm expanded by approximately 4% when the wire was cathodic but was reduced to 74% of the original thickness when the wire was anodic, explained by electrostatic interactions between negatively charged groups in the biofilm and the charged wire which caused biofilm expansion when the wire was cathodic and contraction when the wire was anodic [87]. It is thus reasonably feasible to apply an optimal selected cathode potential for shortened start-up period and enhanced Cr(VI) reduction on the biocathodes of MFCs [73] based on the roles of applied electrode potential on microbial physiology, which include changing the cell surface properties, increasing the enzyme activity, as well as shortening the doubling time of the bacteria [88]. Similarly and in the case of Co(II) reduction on the biocathodes of MECs, applied voltages of 0.1–0.7 V achieved different cathode potentials, electric currents, and cathodic distributions of charges for Co(II) reduction, hydrogen evolution, methane and acetate production, as well as bacterial growth [53], reasonably resulting in diverse microbial community compositions. However, at the same applied voltage of 0.2 V, the composition of bacterial community developed for 1 month exhibited a somewhat shift from that evolved for 3 months in spite of similar Co(II) reduction [53]. Different from the strategy of applied voltage for bacterial community, carbon sources of acetate or NaHCO3 at long-term bacterial community acclimation (6 months) and elevated Cd(II) concentrations (20–50 mg/L) can also enhance Cd(II) removal with simultaneous hydrogen production [83]. Cd(II) removal of 7.33 mg/L/h (acetate) and 6.56 mg/L/h (NaHCO3) and hydrogen production of 0.301 m3/m3/day (acetate) and 0.127 m3/m3/day (NaHCO3) were achieved at an initial Cd(II) of 50 mg/L with the observation of the same predominant species but in different proportions in the acetate or NaHCO3 biofilms. Deeper understanding of the microbial consortia effects on biocathode performance is thus critical to maintain a healthy operation, and proper control of the composition of microbial consortia will also be necessary.

6.4.3 Initial pH and Metal Concentration

Initial pH and metal concentration extensively stressed in abiotic cathodes also affect the performance of biocathodes [53, 67, 71, 85] since initial pH and metal concentration are primarily responsible for structuring whole communities, and the diverse microbial taxa response differently to various environmental conditions [89]. It is generally recognized electrotrophs can only endure an appropriate metal concentration, after which inhibition of the electrotrophic activities takes over [53, 67, 85]. Take Cr(VI), for example. The presently reported Cr(VI) concentrations in the biocathodes ranged from 2.5 mg/L with pure culture of Shewanella to 40 mg/L with enrichment culture (Table 6.2) [70, 73], reflecting the applicable biocathodes for reducing Cr(VI) at these concentration levels. In terms of microbial characters, the pH changes may have affected the surface properties of the cells, including cell surface hydrophobicity, net surface electrostatic charge, cell surface shape and polymers, cell morphology, cell size at cell division, time to division, as well as biofilm structure [87, 88], and consequently influenced the bio-catalytic activity on electron transfer from cathode to bacteria and the subsequent metal reduction. A neutral condition is more beneficial for electrotrophic activities, whereas a more alkaline environment is inclined to form metal precipitates and not only influences electrotrophic activities but also augments metal reduction overpotential. A more acidic condition, however, favors for hydrogen evolution and detrimental to electrotrophic activities. Optimal pHs and initial metal concentrations thus benefit to both electrotrophic activities and metal reduction via electrochemical and biological reactions [53, 67, 70, 85]. Investigation is necessary to better clarify the nature of the competitive processes on the biocathodes and achieve efficient system performance for metal recovery.

6.4.4 Electron Transfer Mechanism

In contrast to electron transfer mechanisms in the bioanodes, the exact mechanisms of electron transfer from the cathode, through the bacteria, and finally, to the terminal electron acceptors in biocathodes have not yet been studied in detail. There are actually close interactions between microorganisms and the cathodic electrodes. Gene expression and deletion analysis demonstrate that the mechanisms for electron transfer from electrodes to G. sulfurreducens differed significantly from the mechanisms for electron transfer to electrodes [90]. To date, two main mechanisms, namely, direct and indirect electron transfers, have been reported (Fig. 6.2), which are more complex than those in abiotic cathodes (Fig. 6.1). Direct electron transfer on the biocathodes requires a physical contact between the bacterial cell membrane and the cathode electrode surface, and electrons from the electrode are directly received by the outer membrane redox macromolecules such as cytochromes (Fig. 6.2). G. sulfurreducens is one of the few microorganisms available in pure culture known to directly accept electrons from a negative poised electrode. It is believed that c-type cytochromes inside bacteria are essential electron-transferring proteins, and outer membrane cytochromes have the ability to catalyze the last step of the respiratory chains. Alternatively, a versatile bacterium of S. putrefaciens in anodic electron transfer through excreted flavins and menaquinone-related redox mediators as well as outer membrane cytochromes can utilize an outer membrane-bound redox compound for electron transfer in microbially cathodic oxygen reduction although this compound was still unidentified. In both cases, c-type cytochromes are essential electron-transferring proteins. They make the journey of respiratory electrons from the cytoplasmic membrane through periplasm and over the outer membrane possible [91]. Similarly, the absence of ferrous iron repressed the transcription of genes encoding outer membrane cytochromes necessary for the reduction of metals such as MnO2, reflecting the importance of outer membrane cytochromes in S. oneidensis MR-1 for MnO2 reduction [92]. With the presence of lactate and electrode, S. oneidensis MR-1 can use both as the electron donor for accelerated Cr(VI) reduction because (i) the forming chelates of Cr(III)-lactate interaction delayed the electrode deactivation by Cr(OH)3 precipitate, (ii) electron mediators produced mediated electrons from the electrode to Cr(VI) and promoted indirect Cr(VI) reduction, and (iii) the presence of lactate and redox mediators produced enabled S. oneidensis MR-1 to be actively involved in the electrode oxidation process and drive direct or indirect Cr(VI) reduction [17]. With the help of noninvasive imaging technique of a naphthalimide-rhodamine-based Cr(III) fluorescent probe [93], four Gram-negative electrotrophs Stenotrophomonas sp. YS1, Stenotrophomonas maltophilia YS2, Serratia marcescens YS3, and Achromobacter xylosoxidans YS8 isolated from previously well-developed mixed culture biocathodes for Cr(VI) reduction [85] were imaginably and quantitatively mapped for intracellular Cr(III) ions [76]. These electrotrophs were intracellularly accumulated by chromium, shown as a total of 45.1–60.5% with a composite of Cr(III) ions (23.7–27.3%) and other forms of chromium complex (18.7–32.2%), compared to 10.2–11.7% (Cr(III) ions: 8.2–9.5%; other forms: 0.2–0.3%) in the controls in the absence of cathodic electrons, implying the direction of cathodic electrons for more intracellular chromium. In parallel, another four indigenous Gram-negative electrotrophs Stenotrophomonas maltophilia JY1, Citrobacter sp. JY3, Pseudomonas aeruginosa JY5, and Stenotrophomonas sp. JY6 isolated from well-adapted mixed cultures on the MFC cathodes for Cu(II) reduction [85] were proved to play diverse functions between cellular electron transfer processes and either Cu(II) reduction or circuital current [77]. Strains JY1 and JY5 exhibited a weak correlation between circuital current and Cu(II) reduction, whereas a much stronger correlation was observed for strain JY3 followed by strain JY6. In the presence of electron transfer inhibitor of 2,4-dinitrophenol or rotenone, significant inhibition on strain JY6 activity and a weak effect on strains JY1, JY3, and JY5 were observed, confirming a strong correlation between cellular electron transfer processes and either Cu(II) reduction or circuital current. With the help of a rhodamine-based Cu(II) fluorescent probe [94], Cu(II) ions were imaginably and quantitatively tracked in these electrotrophic subcellular sites [78]. Similar to the imaginable Cr(III) ions in the corresponding electrotrophs [76], cathodic electrons also led to more Cu(II) ions in the intracellular site compared to the prolonged appearance of more Cu(II) ions in the controls in the absence of cathodic electrons. For Cd(II) removal on the biocathodes of MECs and with the help of a quinoline-based Cd(II) fluorescent probe [95], four indigenous electrotrophs of Ochrobactrum sp. X1, Pseudomonas sp. X3, Pseudomonas delhiensis X5, and Ochrobactrum anthropi X7 isolated from mixed culture for Cd(II) removal [85] imaginably exhibited diverse distributions of Cd(II) ions at the subcellular level with heavy dependence on current and electron transfer inhibitor of 2,4-dinitrophenol (2,4-DNP) [79]. These results in concert may provide evidence for explaining the previous always observation of more efficient biocathodes for heavy metals removal at the subcellular level [53, 67, 70, 85].

Fig. 6.2
figure 2

Electron transfer pathways in the biocathodes of BESs

Full size image

In comparison with Gram-negative bacteria, little is known about Gram-positive bacteria for dissimilatory metal reduction. Thermincola potens, isolated from a MFC and reserving unusual abundance of multiheme c-type cytochromes localized to the cell wall or cell surface, can couple acetate oxidation to the reduction of hydrous ferric oxides or anthraquinone-2,6-disulfonate [96]. This result provides direct evidence for cell wall-associated cytochromes and supports multiheme c-type cytochromes involvement in conducting electrons across the cell envelope of a Gram-positive bacterium. In addition, a wide variety of microbially induced extracellular mechanisms have been used to explain the role of microorganisms in the increase of surface potential on passive metals, such as the generation of protons and hydrogen peroxide near the surface and the production of organometallic catalysts of metal reduction, specific enzymes, and passivating siderophores [15, 88, 89]. All the aforementioned enriches the electron transfer mechanisms in the biocathodes.

Compared with the increasing attention being paid on the electron transfer mechanisms between cathodic electrodes and microorganisms, present information about the subsequent link between the electrons derived from the electrodes and the terminal electron acceptors of metals is minimal and debatable (Fig. 6.2). Even for the extensively investigated electron acceptor of oxygen, it has not yet been demonstrated that the electron transfer is a respiratory mechanism in which electrons derived from the cathode serve as an energy-yielding electron donor for oxygen reduction, and there are a variety of other possible mechanisms by which cells might catalyze enhanced oxygen reduction [68]. Riboflavin, an electron mediator naturally produced by S. oneidensis MR-1, was found to have a positive impact in potentiostatically controlled cathodes [17], implying its function as a mediator for electron transfer between S. oneidensis MR-1 and Cr(VI). While Gram-negative bacteria of Shewanella and Geobacter are model organisms enabling the dissimilatory reduction of extracellular electron acceptors, it is recently found that G. sulfurreducens can donate electrons through pili, a type of metal-like conductive nanofilaments or nanowires and made from protein produced by themselves, photosynthetic cyanobacteria, and thermophilic methanogens, to the external electron acceptor of uranium [97]. The bacterial pili can move charges over thousands of times the bacterium’s length. Compared to the no-pili controls, in which G. sulfurreducens reduced uranium within the cell envelope and thus poisoned the cell in the process, the great surface area of pili had provided more occurrence of the precipitation around the pili and thus greatly increased the amount of uranium that G. sulfurreducens was able to remove [98]. While this result provides evidence for long-range electron transfer along the pili, G. sulfurreducens analogous to S. oneidensis [99] can reduce U(VI) much as it reduced the soluble, extracellular electron acceptors of anthraquinone-2,6-disulfonate and Fe(III) citrate without the requirement of pili, and a number of outer-surface c-type cytochromes contribute to U(VI) reduction. These results support the conclusion that pili were necessarily required for long-range electron transport to insoluble electron acceptor such as Fe(III) oxides in the Geobacter species [100, 101] and electron exchange between syntrophic partners [102], as well as electron conduction through current-producing biofilms [103]. Based on this observation and after fine-tuning the properties of the pili or adding different functional groups on the pili, these amended pili may be also used to precipitate other metal elements. In view of this point, the discovery of conductive pili is not only an important new principle in biology but also in materials science.

While biocathodes are presently limited to reduce U(VI), Cr(VI), Se(IV), Cu(II), Cd(II), and Co(II), few attempts have been made to elucidate the basic aspects of microbial activities such as interaction of substrate metabolism and electron transfers in the biocathodes. Although gene expression and deletion analysis are usually used for clarifying electron transfer mechanisms in U(VI)-reducing and pure-culture biocathodes [100,101,102,103], whether the cathodic electrons are the only energy source for the organisms forming the biofilm, which would make these microorganisms electrochemical lithotrophs, and what function this property plays in nature remain to be elucidated. Development of novel noninvasive imaging techniques to characterize the structure and biochemical composition of the electrotrophic biofilm is of particular importance. That a number of highly selective metal ion-sensitive fluorescence probes are synthesized and combined with confocal laser scanning microscopy for metal detection in cell biology [104] will potentially provide critical insights into metal distribution and electron transfer within the electrotrophs, as well as tools to characterize the mechanisms of electron transfer, leading to a better understanding of the electrotrophic roles in electron transfer mechanisms [76, 78, 79].

6.5 Conclusion

Metallurgical BES processes have been proved in labs and will be well established. However, these technologies are still far from finding real applications in wastes treatment. In addition, much is known about recovering single metals from individual abiotic cathodes; more attention should be paid to MFC-MEC coupled systems and/or BES-other technology combined processes for sequential metal recovery from wastes. Electron transfer mechanisms on the biocathodes are ultimately required to be elucidated in order to understand their limitations and hence maximize metal recovery in the near future.