- Email: [email protected]
Comparison of Co(II) reduction on three different cathodes of microbial electrolysis cells driven by Cu(II)-reduced microbial fuel cells under various cathode volume conditions
Chemical Engineering Journal 266 (2015) 121–132
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Comparison of Co(II) reduction on three different cathodes of microbial electrolysis cells driven by Cu(II)-reduced microbial fuel cells under various cathode volume conditions Dan Wu a, Yuzhen Pan b, Liping Huang a,⇑, Xie Quan a, Jinhui Yang b a Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China b Experiment Center of Chemistry, Dalian University of Technology, Dalian 116024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Co(II)-reduced MECs were
successfully driven by Cu(II)-reduced MFCs. Cathode materials in Co(II)-reduced MECs substantially affected system performance. Titanium sheet cathode was more favorable for Co(II) and Cu(II) reduction. Stainless steel mesh was more costeffective for Co(II) and Cu(II) reduction. Cathode volume affected performance depending on material of Co(II)-reduced MECs.
a r t i c l e
i n f o
Article history: Received 2 October 2014 Received in revised form 15 December 2014 Accepted 20 December 2014 Available online 27 December 2014 Keywords: Microbial electrolysis cell Microbial fuel cell Cathode Metal reduction
a b s t r a c t Reduction of aqueous Cu(II) and Co(II) is one critical step for simultaneous recovery of copper and cobalt, and recycle of spent lithium ion batteries, but suffers from consumption of large amount of energy and chemicals. Here we report Co(II)-reduced microbial electrolysis cells (MECCo) can be driven by Cu(II)reduced microbial fuel cells (MFCCu) for simultaneous Cu(II) and Co(II) recovery with no external energy consumption, and system performance was heavily dependent on cathode material of MECCo and cathode volumes in both MECCo and MFCCu. Either titanium sheet (TS) or stainless steel mesh (SSM) cathode achieved efficient Co(II) reduction whereas carbon rod (CR) cannot proceed this occurrence. While smaller cathode volumes in MFCCu led to appreciable Co(II) reduction (41.4 ± 3.8%) on the CR cathode, the highest Co(II) reduction using TS (45.0 ± 0.3%) or SSM (39.7 ± 3.6%) was obtained under smaller cathode volumes in both MFCCu and MECCo. Moreover, when a mixed Cu(II) and Co(II) catholyte was deliberately used as the influent of MFCCu and the effluent of MFCCu was subsequently imported into the connected MECCo for tentatively simultaneous Cu(II) and Co(II) recovery from simulated mixed wastes, this so-called sequential MFCCu and MECCo operation achieved Cu(II) reduction of 100% and Co(II) reduction of 65.3–72.0% using either TS or SSM cathodes. These results illustrate cathode material of MECCo and cathode volumes in both MECCo and MFCCu were critical for efficient Co(II) reduction in MECCo driven by MFCCu with achievements of simultaneous copper and cobalt recovery as well as no external energy consumption. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel./fax: +86 411 84708546. E-mail address: [email protected] (L. Huang). http://dx.doi.org/10.1016/j.cej.2014.12.078 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
122
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Nomenclature CR TS SSM SHE OCCs SC MFC MEC MFCCu MECCo YMFC,Cu
carbon rod titanium sheet stainless steel mesh standard hydrogen electrode open circuit conditions solution conductivity microbial fuel cell microbial electrolysis cell Cu(II)-reduced MFC Co(II)-reduced MEC yield of copper (mol/mol COD)
1. Introduction Copper and cobalt are elements with excellent ferromagnetic properties, relatively stable against corrosion and easily to handle, and thus useful in microelectronic fields such as lithium ion batteries. Recovery of copper and cobalt from spent lithium ion batteries is in consequence one of the primary objectives in the recycling of these wastes due to the shortage of natural ores and environmental/ecological considerations [1]. Conventional pyrometallurgical or hydrometallurgical processes, bioleaching, and electrochemical processes can recover Cu(II) and Co(II) after the spent battery dismantling occurs [1,2]. The great energy consumption, high cost and serious second pollution urge a turn to environmental friendly and cost-effective strategies. The newly developed Cu(II)-reduced microbial fuel cells (MFCCu) [3–6] and Co(II)-reduced microbial electrolysis cells (MECCo) [7,8] provide new approaches for individual copper and cobalt recovery, where Cu(II) and Co(II) can accept the cathodic electrons and be reductively formed as Cu and Co on the cathodes of MFCCu and MECCo, respectively based on their halfcell reactions in the cathodes and acetate oxidation in the anodes. The remaining challenges include simultaneously recovering Cu(II) and Co(II) from mixed wastes, in situ utilization of power from MFCCu, and minimizing or requiring no external power supply for MECCo. Despite exploration of MFCs integrated with either conventional processes or MECs for wastewater treatment with simultaneous production of hydrogen, alkali, cobalt, zinc, methane and formic acid [9–14], to date, no essential attention has been paid to the MECCo driven by MFCCu system as a promising technology for simultaneously Cu(II) and Co(II) recovery from wastes with no external energy consumption. Cathode potential is one important parameter in MECs where different chemicals are produced [15–17]. It is well known various material cathodes of MECs exhibit different overpotentials and considerably affect hydrogen production [15–20]. In the case of MECCo driven by MFCCu system, Co(II) reduction may occur concomitantly with hydrogen evolution due to their similar half-reaction potentials [7,8], particularly at much low metal concentrations with slow mass transfer (eg. 200 mg/L) [4,5]. We thus guess that various material cathodes of MECs may indirectly affect Co(II) reduction via hydrogen evolution in this MFCCu–MECCo system. Considering the usage of carbon rod (CR), titanium sheet (TS) and stainless steel mesh (SSM) as the cathodes of individual MECs/ MFCs for hydrogen evolution [15–19], these materials were chosen here as the cathodes of MECCo driven by MFCCu for suitably efficient Co(II) and Cu(II) reduction. A change in electrode distance in either MFCCu or MECCo is well known to change the internal resistances [21,22]. In the case of MECCo driven by MFCCu system, a close electrode distance through decrease in cathode volume in either MFCCu or both MFCCu and MECCo may improve system performance [13]. Together with
YMEC,Co yield of cobalt (mol/mol COD) Y MEC;H2 yield of hydrogen (mol/mol COD) CE coulombic efficiency (%) CEMFC,an anodic CE in MFCCu (%) CEMEC,an anodic CE in MECCo (%) CEMFC,ca cathodic CE in MFCCu (%) CEMEC,ca,Co cathodic CE in MECCo based on reduced Co(II) (%) CEMEC;ca;H2 cathodic CE in MECCo based on evolved hydrogen (%) gsys overall system efficiency (%) COD chemical oxygen demand (g/L)
considering various material cathodes of MECCo, smaller cathode volumes in MFCCu only or both MFCCu and MECCo are expected to further enhance reduction of Cu(II) in MFCCu and Co(II) in MECCo. In this study, a truly environmental friendly MECCo driven by MFCCu system was introduced, which not only extracted energy from wastewaters through the anodes of MFCCu and MECCo, and in situ utilized the energy of MFCCu to power MECCo, and thus avoided the need for external energy input in MECCo, but also achieved simultaneous copper and cobalt recovery. CR, SSM and TS were alternatively tested as the cathode of MECCo for suitably efficient system performance. Moreover, smaller cathode volumes in MFCCu only or both MFCCu and MECCo were explored for enhanced Cu(II) and Co(II) reduction. Following this investigation, a so-called sequential MFCCu and MECCo operation was studied, in which a mixed Cu(II) and Co(II) catholyte was deliberately used as the influent of MFCCu and the effluent of MFCCu was subsequently imported into the connected MECCo for tentatively simultaneous Cu(II) and Co(II) recovery from simulated mixed wastes. Multiple parameters including Cu(II) and Co(II) reduction, yields of copper, cobalt and hydrogen, anodic coulombic efficiency (CE), cathodic CE, and overall system efficiency were extensively used to evaluate system performance. Deeper insight into these aspects will enhance the system performance for simultaneous copper and cobalt recovery, and thus recycle of spent lithium ion batteries with no external energy consumption as discussed subsequently.
2. Materials and methods 2.1. Reactor configuration Identical two-chamber MFCCu and MECCo were used in all experiments, with the chambers separated by a cation exchange membrane (CMI-7000 Membranes International, Glen Rock, NJ). Graphite felt (Sanye Co., Beijing, China) was served as the anodes of both MFCCu and MECCo. The anodic working volume in either MFCCu or MECCo was 20 mL. Carbon rod (CR, Chijiu Duratight Carbon Co., China) was always used as the cathode of MFCCu. CR, TA1 titanium sheet (TS) and SS 304 woven mesh (SSM) (99.9%, Qingyuan Co., China) were alternatively served as the cathode of MECCo with a same projected surface area of 12 cm2. These materials were cleaned before tests using 0.5 M H2SO4. 2.2. Inoculation and operation The anodes of both MFCCu and MECCo were inoculated from the anodes of operating MFCs running on acetate. The anolyte was composed of (g/L) sodium acetate, 1.0; KH2PO4, 4.4; K2HPO4, 3.4; NH4Cl, 1.3; KCl, 0.78; MgCl2, 0.2; CaCl2, 0.0146; NaCl, 0.5; trace vitamins and minerals [7].
123
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Prior to add the solutions into MFCCu and MECCo, the anolyte was sparged with ultrapure N2 gas for 15 min. Both MFCCu and MECCo were initially acclimated using deionized water as catholyte with an external resistor of 1000 X. After six-cycle refreshments, the catholyte in MFCCu and MECCo was replaced by aqueous CuCl2 and CoCl2, respectively in addition to an applied voltage of 0.2 V to MECCo (DC Power Supply PS-1502DD, Yihua, Guangzhou, China). After another two or three-refreshments with repeatable and stable electrode potential observation in both MFCCu and MECCo, the acclimation of MECCo and MFCCu was finished. The external power source in MECCo was replaced by the well-developed MFCCu and a low resistance of 10 X was used in the circuit in order to measure the circuit current, although this including a resistor resulted in an additional voltage loss in the system and thus the actual applied voltage over the anode and cathode was smaller than the power source of MFCCu. During the following entire experiments, MECCo was always powered by the MFCCu, thus exhibiting no consumption of external energy. For the initial investigations, aqueous CuCl2 (50 mg/L) was always added in the cathode of MFCCu with simultaneous supplementation of CoCl2 (50 mg/L) in the cathode of MECCo. Different cathode volumes (MFCCu25 mL–MECCo25 mL, MFCCu13 mL– MECCo25 mL, MFCCu13 mL–MECCo13 mL) were explored for efficient reduction of Cu(II) in MFCCu and Co(II) in MECCo under various material cathodes of CR, TS or SS in MECCo. In order to abate electron consumption for hydrogen evolution and benefit more for Co(II) reduction, slight acidic catholyte of pH 5.8 with solution conductivity (SC) 5.5 mS/cm was firstly set in MECCo [7,8] whereas generally reported catholyte of pH 2.0 with solution conductivity (SC) 8.7 mS/cm [3–6] was employed in MFCCu. Identical catholyte of pH 2.0 with SC 8.7 mS/cm in MFCCu and MECCo was then used to eliminate the medium differences except ions of Cu(II) in MFCCu and Co(II) in MECCo. Following this exploration, a synthetic mixed Cu(II) and Co(II) catholyte at a same concentration of 50 mg/L was deliberately used as the influent of MFCCu and the effluent of MFCCu was then imported to the MECCo, which was connected and powered by the MFCCu. This so-called sequential MFCCu and MECCo operation was tentatively explored to recover Cu(II) and Co(II) from simulated mixed wastes with no consumption of external energy. Two controls were also operated: one was run in open circuit conditions (OCCs) to examine changes of Cu(II) and Co(II) in the absence of current generation. In the second control, new bare cathodes lacking either Cu(II) or Co(II) were accordingly analyzed for redox behavior to exclude the roles of Cu(II) or Co(II) on electrode reactions. Unless otherwise stated, new cathodes were used for each operation cycle. All reactors were operated in fed-batch mode and all experiments were run in duplicate and maintained at room temperature (20 ± 3 °C). All of the inoculation and solution replacements were performed in an anaerobic glove box (YQX-II, Xinmiao, Shanghai).
and the crystal products. The potentials of cathode and anode were collected by a data logger using an automatic data acquisition system (PISO-813, Hongge Co., Taiwan). The cathode redox behavior in MECCo was studied using cyclic voltammetry (CV, CHI 650, Chenhua, Shanghai) at a scan rate of 1.0 mV/s. Linear sweep voltammetry (LSV) was performed using a potentiostat (CHI 650, Chenhua, Shanghai) at a scan rate of 0.1 mV/s and power density was calculated based on per cathode working volume (W/m3). The three electrode system for both LSV and CV consisted of a working electrode (cathode electrode), counter electrode (platinum plate), and Ag/AgCl reference electrode (195 mV versus a standard hydrogen electrode, SHE). All potentials shown were corrected to a SHE. 2.4. Calculations Reduction of Cu(II) in MFCCu (%) and Co(II) in MECCo (%), yields of copper in MFCCu (YMFC,Cu, mol/mol COD), cobalt in MECCo (YMEC,Co, mol/mol COD) and hydrogen in MECCo (Y MEC;H2 , mol/mol COD), anodic coulombic efficiencies (CEs) in MFCCu (CEMFC,an, %) and MECCo (CEMEC,an, %), cathodic CEs in MFCCu (CEMFC,ca, %) and MECCo (CEMEC,ca,Co, % and CEMEC;ca;H2 , %), and overall system efficiency (gsys, %) were modified and calculated as Eqs. (1)–(11):
CuðIIÞreduction ð%Þ ¼
Cui Cut 100% Cui
ð1Þ
CoðIIÞreduction ð%Þ ¼
Coi Cot 100% Coi
ð2Þ
Y MFC;Cu ¼
Y MEC;Co ¼
Y MEC;H2 ¼
Cui Cut 64
V MFC;ca;l
ð3Þ
MFC V MFC;an DCOD 32
Coi Cot 59
V MEC;ca;l
ð4Þ
MEC V MEC;an DCOD 32
nH2 ;t V MEC;ca;g
ð5Þ
MEC V MEC;an DCOD 32
R CEMFC;an ¼
96; 485
R CEMEC;an ¼
CEMFC;ca ¼
96; 485
Idt
4DCODMFC V MFC;an 32
Idt
4DCODMEC V MEC;an 32
100%
ð6Þ
100%
ð7Þ
a1 V MFC;ca;l ðCui Cut Þ 96; 485 R 100% Idt
CEMEC;ca;Co ¼
a2 V MEC;ca;l ðCoi Cot Þ 96; 485 R 100% Idt
CEMEC;ca;H2 ¼
a3 V MEC;ca;g nH2 ;t 96; 485 R 100% Idt
ð8Þ
ð9Þ
2.3. Measurements and analyses Total chemical oxygen demand (COD) in the anolyte was measured using standard methods. The concentrations of Cu(II) and Co(II) in catholyte were measured by an atomic absorption spectrophotometer (AAnalyst 700, PerkinElmer). Hydrogen produced in the headspace of MECCo cathode was analyzed using a gas chromatograph (GC7900, Tianmei, Shanghai) and a molecular sieve column (TDX-01, 60–80, 4 mm 2 m) with argon as the carrier gas. Scanning electronic microscopy (SEM) (QUANTA450, FEI company, USA) equipped with an energy dispersive X-ray spectrometer (EDS) (X-MAX 20 mm2/50 mm2, Oxford Instruments, UK), and X-ray diffraction (XRD-6000, Shimadzu LabX, Japan) were used to examine the morphologies of the electrode after copper and cobalt recovery,
gsys ¼
ð10Þ
ðCui Cut Þ V MFC;ca;l a1 þ ðCoi Cot Þ V MEC;ca;l a2 þ nH2 ;t V MEC;ca;g a3 4 ðV MFC;an DCODMFC þ V MEC;an DCODMEC Þ 32 100% ð11Þ
Cui and Coi are the initial Cu(II) and Co(II) concentrations in the catholytes whereas Cut and Cot are the Cu(II) and Co(II) concentrations at an operation time of t (mol/L); nH2 ;t is hydrogen concentration in headspace of the cathodes at t (mol/L); VMFC,an and VMEC,an are anolyte volumes in MFCCu and MECCo, respectively (L); VMFC,ca,l
124
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
and VMEC,ca,l are catholyte volumes in MFCCu and MECCo, respectively (L); VMEC,ca,g is volume of headspace in the cathodes of MECCo (L); a1 (2 mol/mol), a2 (2 mol/mol) and a3 (2 mol/mol) are the number of electrons required for Cu(II) and Co(II) reduction as well as hydrogen evolution, respectively. DCODMFC and DCODMEC are the cumulative COD consumptions over a set period of t in the anodes of MFCCu and MECCo, respectively (g/L); I is circuit current (A); 64, 59, and 32 are the atom/molecule weights of Cu, Co and O2, respectively (g/mol); 4 is the molar number of electrons required for oxygen reduction (mol/mol); 96,485 is Faraday constant (C/mol e).
3. Results and discussion 3.1. Reactor setup Following inoculation, anode potentials in both MFCCu (Fig. 1A) and MFCCo (Fig. 1C) experienced gradual decrease. The initial five or six-cycle refreshments using deionized water as catholyte resulted in the decrease of anode potentials in both MFCCu and MECCo to 0.23 to 0.25 V, values frequently observed using acetate as an anodic fuel. These results demonstrate the gradual development of anodic biofilms and the finishing of biofilms acclimation at five or six-cycle refreshments. A shift from deionized water to 50 mg/L CuCl2 solution in the catholyte of MFCCu substantially decreased the cathode potentials (Fig. 1A), mainly ascribed to the lower half cell potential of Cu(II) reduction than O2 reduction [3]. In the case of MECCo acclimation and after the replacement of deionized water by 50 mg/L CoCl2 in the cathode, an applied voltage of 0.2 V resulted in higher anode than cathode potentials (Fig. 1C), always observed in MECs. This trend appeared stably and repeatedly in the subsequent three-cycle refreshments (Fig. 1C), implying the suitability of this MECCo for the following experiments. In addition, we cannot exclude the occurrence of cat-
alytic water hydrolysis here due to the applied voltage of 0.2 V slightly lower than the 0.3 V typically used even with Pt catalysts [23]. There was a general increase trend in open circuit voltage (OCV) and power production from MFCCu with the prolonged acclimation period, achieving similar values of 0.64 V and 4.3 W/m3 at an acclimation period of 9 d, and 0.65 V and 4.6 W/m3 at 12 d (Fig. 1B). This power of approximate 0.11 mW was either nearly 2-fold as the reported at a similar Cu(II) concentration and with a large working volume of 1.0 L [4], or lower than the 0.62 mW at a much higher Cu(II) concentration of 800 mg/L and with a small working volume of 14 mL [6]. Power overshoot, which the cell voltage and current dropped very quickly and resulted in a doubling back of the power density curve, was observed at an acclimation time of 12 d (Fig. 1B). Shorter acclimation periods of 6 d and 9 d led to the disappearance of power overshoot, reflecting electron transfer limitation at acclimation time of 12 d in MFCCu (Fig. 1B). The limitation was most likely related to a slow response from the acclimated anodic biofilms to adjust to the external resistance. Cu(II) removal in MFCCu was attributable to both cathodic reduction and chemical adsorption, in which cathodic reduction occupied 30.6 ± 3.3% and chemical adsorption possessed 7.1 ± 1.0% (Fig. 1D). Different from Cu(II) removal, no Co(II) reduction was observed on the CR cathode and a Co(II) removal of 16.0 ± 0.7% was completely ascribed to Co(II) adsorption (Fig. 1D). These results clarify the contribution of cathodic reduction and chemical adsorption to removals of Cu(II) in MFCCu and Co(II) in MECCo. 3.2. Comparison of CR, TS and SSM performances After a 12-d acclimation, both MFCCu and MECCo were well developed (Fig. 1) and used for Co(II) and Cu(II) recovery. As shown in Fig. 2A, no Co(II) reduction was observed on the CR cathode during an operation period of 6 h whereas appreciable Cu(II) reduction
Fig. 1. (A) MFCCu acclimation, (B) polarization curve of MFCCu periodically during acclimation, (C) MECCo acclimation, and (D) removals of Cu(II) in MFCCu and Co(II) in MECCo (cathode volume in both MFCCu and MECCo: 25 mL; CR as the cathode of MECCo; MECCo at an applied voltage of 0.2 V; arrows indicate when anodes were fed with fresh medium).
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
125
Fig. 2. (A) Co(II) and (B) Cu(II) reduction, (C) applied voltage and (D) cathode potential in MECCo, (E) circuit current, and (F) cyclic voltammetry tests with CR, TS or SSM (cathode volume in MFCCu or MECCo: 25 mL; catholyte in MFCCu: pH 2.0 with SC 8.7 mS/cm; catholyte in MECCo: pH 5.8 with SC 5.5 mS/cm).
of 77.3 ± 0.9% was achieved in MFCCu at the same 6 h (Fig. 2B). The no occurrence of Co(II) reduction using CR was mainly ascribed to lower applied voltages of 0.03–0.18 V (Fig. 2C) and higher cathode potentials of 0.34 to 0.17 V (Fig. 2D), both of which were insufficient for proceeding Co(II) reduction. Under the present experimental condition, the potential required for Co(II) reduction was more negative than 0.37 V. A high circuit current of 13.7 ± 1.5 A/m3 with CR (Fig. 2E) may have contributed to the high Cu(II) reduction (Fig. 2B) and the appreciable CEMFC,ca (Table 1). Different from CR, both TS and SSM successfully proceeded Co(II) reduction, reaching 28.3 ± 1.1% (Fig. 2A) with YMEC,Co of 0.30 ± 0.02 mol/mol COD (Table 1) (TS), and 23.6 ± 0.5% (Fig. 2A) with 0.22 ± 0.00 mol/mol COD (Table 1) (SSM) at 6 h. In addition, a higher CEMEC;ca;H2 of 14.2 ± 0.2% with TS than 3.9 ± 0.5% using SSM (Table 1), was in agreement with other observations that TS was one favorable electrode for hydrogen evolution in individual MECs [19]. It cannot be excluded that the more cobalt produced on the cathode may have been more beneficial for hydrogen
evolution, shown as the positive correlation of hydrogen production and Co(II) reduction with TS or SSM (Fig. 2A and Table 1). It was also understandable the higher applied voltages of 0.26– 0.35 V (Fig. 2C) and more negative cathode potentials of 0.52 to 0.59 V (Fig. 2D) led to higher Co(II) reduction and hydrogen production. Concomitantly, similar Cu(II) reduction of 37.6% was observed with TS and SSM (Fig. 2B). These results stress the importance of cathode of MECCo on efficient Cu(II) and Co(II) reduction in this system. Compared to the no occurrence of Co(II) reduction on the CR cathode (Fig. 2A), Co(II) reduction with either TS or SSM (Fig. 2A) somewhat consumed circuit electrons, in consistent with lower circuit currents of 0.17–0.18 mA (Fig. 2E). In the open circuit controls (OCCs), only 15.8 ± 0.7% of Cu(II) was removed (Fig. 2B), reflecting the importance of cathodic electrons on Cu(II) reduction. Cyclic voltammetry analysis on the CR electrode showed the presence of one reduction peak of 0.31 V with the presence of Co(II) (Fig. 2F), which was more negative than the 0.23 V with SSM and 0.12 V with TS. These results demonstrate the more benefi-
126
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Table 1 Comparison of yields of copper, cobalt and hydrogen, and operational efficiencies in the MECCo driven by MFCCu system under various material cathodes of MECCo and operational conditions.
gsys (%)
Operational conditions
Cathode of MECCo
Anodic CEs CEMFC,an (%)
CEMEC,an (%)
CEMFC,ca (%)
Cathodic CEs CEMEC,ca,Co (%)
CEMEC;ca;H2 ð%Þ
MFCCu25– MECCo25
CR TS SSM
42.2 ± 7.2 35.8 ± 0.0 45.0 ± 1.5
46.3 ± 7.9 53.1 ± 0.0 47.7 ± 1.6
35.2 ± 6.4 36.6 ± 0.7 35.8 ± 3.9
0.7 ± 2.5 28.4 ± 1.6 22.7 ± 0.2
0.0 ± 0.0 14.2 ± 0.2 3.9 ± 0.5
MFCCu13– MECCo25
CR TS SSM
43.3 ± 1.2 32.8 ± 7.6 45.0 ± 1.1
60.5 ± 1.6 63.0 ± 7.2 86.4 ± 2.1
13.6 ± 1.0 23.6 ± 7.9 18.7 ± 1.7
18.1 ± 1.9 34.4 ± 0.3 16.3 ± 0.1
MFCCu13– MECCo13
CR TS SSM
55.8 ± 3.5 32.8 ± 1.1 54.2 ± 3.2
77.9 ± 4.9 63.0 ± 2.1 104 ± 6.2
12.4 ± 1.5 27.8 ± 3.3 19.6 ± 2.1
pH: 2.0, SC: 8.7 mS/cm
CR TS SSM
55.8 ± 3.5 34.3 ± 1.1 75.5 ± 1.1
77.9 ± 4.9 65.9 ± 2.1 145 ± 2.0
Sequential MFCCu and MECCo
CR TS SSM
58.7 ± 5.3 19.8 ± 2.7 44.1 ± 0.0
82.0 ± 7.4 27.6 ± 3.7 61.6 ± 0.0
Yield YMFC,Cu (mol/ mol COD)
YMEC,Co (mol/ mol COD)
Y MEC;H2 (mol/ mol COD)
8.7 ± 0.6 18.6 ± 0.6 16.6 ± 0.9
0.29 ± 0.00 0.26 ± 0.00 0.32 ± 0.02
0.00 ± 0.02 0.30 ± 0.02 0.22 ± 0.00
0.00 0.15 ± 0.01 0.04 ± 0.01
0.4 ± 0.2 21.9 ± 5.4 6.4 ± 0.1
7.6 ± 0.5 16.9 ± 2.6 12.2 ± 0.6
0.12 ± 0.01 0.15 ± 0.02 0.17 ± 0.01
0.20 ± 0.01 0.43 ± 0.10 0.28 ± 0.01
0.00 ± 0.00 0.27 ± 0.04 0.11 ± 0.03
0.0 ± 0.0 22.5 ± 0.8 12.1 ± 1.9
0.0 ± 0.0 22.1 ± 0.2 7.2 ± 1.2
4.0 ± 0.2 15.6 ± 0.9 13.8 ± 0.8
0.14 ± 0.00 0.18 ± 0.02 0.21 ± 0.01
0.00 ± 0.00 0.28 ± 0.00 0.25 ± 0.03
0.00 ± 0.00 0.28 ± 0.01 0.15 ± 0.02
13.9 ± 0.0 32.8 ± 1.6 17.9 ± 0.9
0.0 ± 0.0 14.6 ± 1.1 9.0 ± 0.2
0.0 ± 0.0 23.7 ± 0.4 0.0 ± 0.0
4.5 ± 1.8 16.1 ± 0.5 13.4 ± 0.7
0.15 ± 0.01 0.23 ± 0.02 0.27 ± 0.01
0.00 ± 0.00 0.19 ± 0.01 0.25 ± 0.00
0.00 ± 0.00 0.31 ± 0.02 0.00 ± 0.00
16.9 ± 1.6 45.9 ± 6.2 20.4 ± 0.0
0.0 ± 0.0 27.2 ± 2.2 13.6 ± 0.0
0.0 ± 0.0 17.4 ± 2.8 1.1 ± 0.1
5.8 ± 0.0 10.3 ± 0.9 9.0 ± 0.5
0.18 ± 0.00 0.18 ± 0.00 0.18 ± 0.00
0.00 ± 0.00 0.15 ± 0.01 0.17 ± 0.00
0.00 ± 0.00 0.09 ± 0.00 0.01 ± 0.00
cial TS than SSM, and the most unfavorable CR for Co(II) reduction in this system. Current densities at potentials more negative than 0.78 V exhibited a similar more negative trend on the three various material electrodes (Fig. 2F), mainly ascribed to the acute occurrence of hydrogen evolution [16,18]. In the controls with the absence of Co(II), very weak peaks for both Co(II) reduction and hydrogen evolution were observed (Fig. 2F), reflecting the critical roles of Co(II) in these reactions on the electrode. 3.3. System performance under smaller cathode volumes in either MFCCu only or both MFCCu and MECCo For CR, a smaller cathode volume of 13 mL in MFCCu achieved Co(II) reduction of 41.4 ± 3.8% (Fig. 3A) compared to no occurrence with a 25 mL cathode volume (Fig. 2A). This was mainly ascribed to higher applied voltages (Fig. 3D), more negative cathode potentials in MECCo (Fig. 3F) and higher circuit currents (Fig. 3E), resulted from higher voltage output and power generation (Fig. 3B) from MFCCu with this smaller cathode volume [21,22]. Compared to the 25 mL cathode volume (Fig. 2A), more electrons consumed for Co(II) reduction with this 13 mL cathode volume reasonably led to lower Cu(II) reduction (67.5 ± 1.4%) (Fig. 3B). However, the smaller cathode volumes in both MFCCu and MECCo completely impeded Co(II) reduction on the CR cathode (Fig. 3A), explained by lower applied voltages (Fig. 3D) and higher cathode potentials (Fig. 3F). This no occurrence of Co(II) reduction may have saved more electrons and thus led to higher Cu(II) reduction (79 ± 0.3%) than those of other materials (Fig. 3B). For TS and SSM, smaller cathode volumes in either MFCCu only or both MFCCu and MECCo generally improved reduction of Co(II) in MECCo and Cu(II) in MFCCu. TS had Co(II) reduction of 36.5 ± 9.2% (Fig. 3A) and Cu(II) reduction of 46.9 ± 6.1% (Fig. 3B) in MFCCu13– MECCo25, and 45.0 ± 0.3% (Fig. 3A) and 54.0 ± 1.8% (Fig. 3B) in MFCCu13–MECCo13, higher than 28.3 ± 1.1% (Fig. 2A) and 37.5 ± 0.6% (Fig. 2B) in MFCCu25–MECCo25. While gsys was heavily dependent on cathode materials and appeared lower with a 13 mL cathode volume in MFCCu (Table 1), this smaller cathode volume substantially improved operational efficiencies in MECCo with TS including CEMEC,an, CEMEC,ca,Co, CEMEC;ca;H2 , YMEC,Co and Y MEC;H2 (Table 1). Comparatively, SSM exhibited Co(II) reduction of 23.4 ± 0.7% (Fig. 3A) and Cu(II) reduction of 39.7 ± 3.6% (Fig. 3B) in MFCCu13–MECCo25, and 39.7 ± 3.6% (Fig. 3A) and 66.0 ± 5.2% (Fig. 3B) in MFCCu13–MECCo13, compared to 23.6 ± 0.5% (Fig. 2A) and 37.5 ± 0.6% (Fig. 2B) in MFCCu25–MECCo25. Smaller cathode
volumes in MFCCu reasonably increased voltage output and power production (Fig. 3C) [21,22], and thus higher applied voltages (Fig. 3D) and more negative cathode potentials in MECCo (Fig. 3F), both of which favored Co(II) reduction with either TS or SSM. The fact that some CEMEC,an exceeded 100% (Table 1) was in good agreement with previously reported [7,9,13,18]. The smaller cathode volumes in either MFCCu only or both MFCCu and MECCo improved circuit current with CR or SSM (Fig. 3E). Circuit current with TS, however, was invariable to the change of cathode volume (Figs. 3E and 2E). This result was mainly ascribed to the higher resistance of TS than SSM and CR [19,24], resulting in the ignorance of resistance change in response to cathode volume change in either MFCCu only or both MFCCu and MECCo. Considering the more benefit of a 13 mL cathode volume in either MFCCu using CR, or both MFCCu and MECCo with TS and SSM (Fig. 3A), CR was thus performed in MFCCu13–MECCo25 whereas TS and SSM were operated in MFCCu13–MECCo13 in the subsequent experiments. 3.4. System performance under identical catholytes in MFCCu and MECCo Practical application of the self-driven MFCCu and MECCo system requires an identical catholyte in both MFCCu and MECCo due to the coexistence of Cu(II) and Co(II) in the same wastes. Identical catholyte of pH 2.0 with SC 8.7 mS/cm in MECCo and MFCCu was thus explored, compared to the catholyte of pH 5.8 with SC 5.5 mS/cm in MECCo and pH 2.0 with SC 8.7 mS/cm in MFCCu mentioned above. For all the three materials, catholyte of pH 2.0 with SC 8.7 mS/cm possessed a low resistance and reasonably resulted in higher circuit currents (Fig. 4A), lower applied voltages (Fig. 4B) and higher cathode potentials of MECCo (Fig. 4C) than that of pH 5.8 with SC 5.5 mS/cm. Cyclic voltammetry showed an identical reductive peak at 0.42 to 0.40 V on both the CR and SSM electrodes (Fig. 4D). Comparatively, TS exhibited a slightly positive shift to 0.34 V with a high peak current of 2.80 A/m2 (Fig. 4D), implying the more efficient Co(II) reduction on the TS electrode than the others. All the reductive peaks of 0.40 V (CR), 0.34 V (TS) and 0.42 V (SSM) (Fig. 4D) were more negative than those of 0.31 V (CR), 0.12 V (TS) and 0.23 (SSM) in pH 5.8 with SC 5.5 mS/cm (Fig. 2F), implying more favorable of pH 5.8 with SC 5.5 mS/cm for Co(II) reduction. Either no occurrence of Co(II) reduction with CR or decreased Co(II) reduction (29.8 ± 1.1%) and YMEC,Co (0.19 ± 0.01 mol/mol COD) using TS were observed at pH 2.0 with SC 8.7 mS/cm
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
127
Fig. 3. (A) Co(II) and (B) Cu(II) reduction, (C) applied voltage and (D) cathode potential in MECCo, (E) polarization curve of MFCCu, and (F) circuit current with small cathode volumes in either MFCCu only or both MFCCu and MECCo (catholyte in MFCCu: pH 2.0 with SC 8.7 mS/cm; catholyte in MECCo: pH 5.8 with SC 5.5 mS/cm).
(Fig. 4E and Table 1), compared to 41.4 ± 3.8% and 0.20 ± 0.01 mol/ mol COD (CR) and 45.0 ± 0.3% and 0.28 ± 0.00 mol/mol COD (TS) at pH 5.8 with SC 5.5 mS/cm (Fig. 4E and Table 1). In addition, hydrogen evolution termed as CEMEC;ca;H2 and Y MEC;H2 in pH 2.0 with SC 8.7 mS/cm was completely impeded on the CR and SSM cathodes (Table 1). Lower resistances with CR and SSM than the other materials [19,20,24] presumably resulted in the lower applied voltages (Fig. 4B) and higher cathode potentials (Fig. 4C), both of which were unsatisfactory for hydrogen reduction [15–17]. On the other hand and for SSM, the negative effects of lower applied voltages (Fig. 4B) and higher cathode potentials (Fig. 4C) due to catholyte of pH 2.0 with SC 8.7 mS/cm may have been compensated by the higher circuit currents (Fig. 4A), explaining the similar Co(II) reduction in these two catholytes (Fig. 4E). For all the three materials, Cu(II) reduction (Fig. 4F) and YMFC,Cu (Table 1) in pH 2.0 with SC 8.7 mS/cm were reasonably higher, mainly attributable to the higher circuit currents (Fig. 4A). Despite the more beneficial pH 5.8 with SC 5.5 mS/cm for Co(II) reduction on the CR and TS cathodes of MECCo (Fig. 4E), Cu(II)
would be undesirably precipitated as Cu(OH)2 if the same catholyte was also used in MFCCu [4,5]. Based on this consideration, catholyte used for subsequent sequential MFCCu and MECCo operation, and morphology and product analysis preparation was uniformly maintained at pH 2.0 with SC 8.7 mS/cm. 3.5. Performance of sequential MFCCu and MECCo Compared to single Cu(II), a mixed Co(II) and Cu(II) influent to MFCCu decreased open circuit potential and power production (Fig. 5A), and thus abated applied voltages (Fig. 5B) and increased cathode potentials in MECCo (Fig. 5C) with all the three materials. There was no appreciable difference in circuit current between single Cu(II)/Co(II), and a mixed Co(II) and Cu(II) influent (Fig. 5D). TS exhibited the highest applied voltage (Fig. 5B), and the lowest cathode potential (Fig. 5C) and circuit current (Fig. 5D), followed by SSM. Cu(II) and Co(II) were considerably reduced in this sequential MFCCu and MECCo, achieving 100% and 65.3–72.0%, respectively using either TS or SSM, among which amounting to 70.0–85.5%
128
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Fig. 4. (A) applied voltage and (B) cathode potential in MECCo, (C) circuit current, and reduction of (E) Co(II) and (F) Cu(II) in an identical catholyte of pH 2.0 with SC 8.7 mS/cm in both MECCo and MFCCu, compared to those of pH 5.8 with SC 5.5 mS/cm in MECCo and pH 2.0 with SC 8.7 mS/cm in MFCCu. (D) Cyclic voltammetry tests in pH 2.0 with SC 8.7 mS/cm (CR: MFCCu13–MECCo25; TS and SSM: MFCCu13–MECCo13).
of total Cu(II) was reduced in MFCCu (Fig. 5E) and 61.3–68.7% of total Co(II) was reduced in MECCo (Fig. 5F). These results were higher than those in single Cu(II) and Co(II) influent at identical catholyte (Fig. 4E and F), reflecting the catalysis roles of either Co(II) ion in Cu(II) reduction or Cu(II) ion in Co(II) reduction, consistent with previously reported [2,25,26]. TS had similar YMFCs,Cu (0.18 ± 0.00 mol/mol COD) and YMECs,Co (0.16 ± 0.01 mol/mol COD) to SSM (Table 1). While the Cu(II) and Co(II) reduction rates could not be compared with conventional electrochemical processes due to greatly different operational conditions and reactor architectures [1,2], the Cu(II) reduction rate here was 4.8 times of individual MFCs at a higher Cu(II) concentration of 200 mg/L and a higher pH of 4.7 [3–5], and 1.4 times of that at a Cu(II) of 100 mg/L and a more acidic pH of 3.0 [27] whereas the Co(II) reduction rate was
equivalent to previous individual MECs with an energy consumption of 1.6–4.0 kWh/kg Co [7,8]. Despite a high circuit current using CR (Fig. 5D), the lowest applied voltage (Fig. 5B) and the highest cathode potential in MECCo (Fig. 5C) led to the no occurrence of Co(II) reduction (Fig. 5F), similar to the single Cu(II)/Co(II) influent (Fig. 4A–C and E). For all the three materials, an identical small amount of Co(II) (10.5%) was removed in MFCCu (Fig. 5E), mainly ascribed to chemical adsorption on the cathode. Comparatively, much more Co(II) (61.3–68.7%) was removed on the TS or SSM cathode of MECCo (Fig. 5F), reflecting the selectivity of MFCCu and MECCo for cobalt recovery. While Cu(II) was mainly reduced in MFCCu (Fig. 5E), the residual Cu(II) in the effluent of MFCCu (10.5–30.0%) was completely removed in the subsequent MECCo with all the three mate-
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
129
Fig. 5. (A) voltage output and power density in MFCCu, (B) applied voltage and (C) cathode potential in MECCo, (D) circuit current, changes of Cu(II) and Co(II) concentrations in sequential (E) MFCCu and (F) MECCo operation at an identical catholyte of pH 2.0 with SC 8.7 mS/cm in both MFCCu and MECCo (CR: MFCCu13–MECCo25; TS and SSM: MFCCu13–MECCo13).
rials (Fig. 5F), illustrating the more efficiency of sequential MFCCu and MECCo operation for Cu(II) removal. In view of purity of final products on the cathodes of sequential MFCCu and MECCo, however, mixed copper and cobalt were somewhat unavoidably formed, mainly ascribed to the more or less chemical adsorption of Co(II) in MFCCu (Fig. 5E) and the Cu(II) reduced in the subsequent MECCo (Fig. 5F). In view of material cost, SSM (73 $/m2) was much lower than TS (450 $/m2). Taken together with the appreciable Co(II) reduction on the SSM cathode with simultaneous Cu(II) reduction in MFCCu (Fig. 5E and F), the substantially low cost of SSM as well as its easily produced and efficiently performed significantly enhanced the economic feasibility of this self-driven MFCCu–MECCo system for simultaneous Cu(II) and Co(II) recovery.
3.6. Morphology of the cathode and product confirmation In order to confirm products on the cathodes, totally 6 repeated batch cycles were particularly performed under the optimal conditions of CR cathode in MFCCu13–MECCo25, and TS and SSM cathodes in MFCCu13–MECCo13. Together with considering products on the CR cathode of the connected MFCCu, totally 6 samples were accordingly obtained for SEM, EDS and XRD analysis. Since morphology and product on CR cathode of MFCCu under conditions of CR, TS and SSM cathode of MECCo were very similar and for simplicity, only one sample results about CR cathode of MFCCu were presented here. As shown in Fig. 6A, rose-like crystals were clearly observed on the CR cathode of MFCCu. Cu signals at 0.98, 8.06 and 8.96 keV s were detected using EDS analysis (Fig. 6B), demonstrat-
130
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Fig. 6. SEM observation on (A) the CR cathode of MFCCu, and (D) CR, (G) TS, and (J) SSM cathode of MECCo after reduction of (A) Cu(II) and (D, G and J) Co(II). EDS (B, E, H and K) and XRD (C, F, I and L) analyses on products of (A) Cu(II) and (D, G and J) Co(II) reduction. (CR: MFCCu13–MECCo25; TS and SSM: MFCCu13–MECCo13.)
ing the formation of copper product. The ratio of Cu and O was approximate 11, much higher than the 2 as a final product of Cu2O, implying most Cu(II) was reduced to pure copper [4–6]. XRD spectra showed copper were clearly present in the powder collected from the cathode of MFCCu with a stronger Cu(1 1 1) relative peak intensity at 43.2° than others of Cu(2 0 0) at 50.3° and Cu(2 2 0) at 77.1° (Fig. 6C), all of which closely matched with the reported pure copper [4–6]. These results confirm the occurrence of reduction reaction from Cu(II) to Cu(0). For the CR cathode of MECCo, particles were distributed sparsely over the surface (Fig. 6D). The binding energy peaks at 0.79, 6.95 and 7.69 keV s by EDS analysis represented that cobalt was a main
cathode product, which was also reflected by a high ratio of Co and O of 7 (Fig. 6E). The XRD patterns closely matched that of metal Co (Co0) with standard peaks at 1 1 1, 2 0 0, and 2 2 0 degree in 2h (Fig. 6F). Compared to CR, the surface of TS was clearly covered by uniform particles (Fig. 6G) while similar binding energy peaks by EDS analysis (Fig. 6H) and characteristic peaks of Co(0) by XRD analysis (Co(1 1 1), Co(2 0 0) and Co(2 2 0)) (Fig. 6I) were both observed. Comparatively, larger particles were formed on the SSM cathode (Fig. 6J) in addition to weaker binding energy peaks in EDS (Fig. 6K) and the absence of Co(2 0 0) at 53.1° in XRD analysis (Fig. 6L). These results demonstrate the somewhat dependence of crystal forms of final products on the cathode materials of
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
131
the complexity of the treatment process, hence creating more challenges for process optimization and maintenance. While copper and cobalt recovered from the sequential MFCCu and MECCo operation need to be further separated and purified, Co(II) remaining in the effluent of this system still need to be recovered in order for the requirement of national wastewaters release standard criterion. In addition, considering the more complexity of actual wastewaters than acetate used in the present anodes as well as the lower solution conductivity of 1.0 mS/cm in actual wastewaters than the 5.5 mS/cm in the present anodes, results obtained here will be different from those using actual wastewaters and most likely higher than the latter. Enhanced Cu(II) and Co(II) reduction with development of continuous flow systems together with considering actual wastewaters are thus needed to advance this new technology. Further investigations in this direction are certainly warranted.
4. Conclusions
Fig. 7. Theoretical cathode potentials for half-reactions of (A) Cu(II) to Cu(I), Cu(II) to Cu and Cu(I) to Cu, and (B) Co(III) to Co(II), Co(III) to Co3O4 and Co(II) to Co, at a same Cu(II) or Co(II) concentration of 50 mg/L and different pH values.
MECCo. However, a similar ratio of Co and O of 7 under conditions of CR, TS or SSM cathode implies the invariable products on these three material cathodes. The products formed on the cathodes mainly depend on substrate concentration and pH value [4–6]. Under the present experimental condition, cathode theoretical potentials at different pHs were calculated based on the Nernst equations (Fig. 7). Considering the slight change of 0.1–0.2 pH units in the effluent of both MFCCu and MECCo at an identical initial pH of 2.0, Cu(II) and Co(II) were reductively changed to Cu(0) (Fig. 7A) and Co(0) (Fig. 7B), respectively, further confirming the final products in MFCCu and MECCo. Cu(0) and Co(0) crystals can be also attributable to the cathodic strict anaerobic environment. Under an aerobic/facultative environment, dissolved oxygen can compete with species of Cu(II) and phenols for cathodic electrons, resulting in the formation of partial reduction products [3–7,28]. The primary functions of this system are not only its environmental benefit to remove Cu(II) and Co(II) from the spent lithium ion batteries but also economic benefit to recover Cu(II) and Co(II) with the present market values of $3.3 and $16.8 per pound, respectively and no consumption of external energy. The in-site utilization of electricity from MFCs may also provide more environmental benefits compared to direct harvest electricity from MFCs [20]. Therefore, this self-driven MFCCu–MECCo system is an environmental friendly method to treat wastewaters from spent lithium ion batteries with the potential economic benefit. While this MECCo driven by MFCCu system represents a true sustainable process and is new to simultaneous recovery of Cu(II) and Co(II) from wastes, there are still many challenges to enable practical applications. The MFCCu coupled with MECCo might add up to
The MECCo driven by MFCCu system exhibited concomitant Cu(II) and Co(II) reduction with no consumption of external energy. System performance was heavily dependent on the cathode material of MECCo and cathode volumes in both MECCo and MFCCu. Co(II) was efficiently reduced using either TS or SSM, and CR cannot proceed this occurrence. While smaller cathode volumes in MFCCu achieved appreciable Co(II) reduction of 41.4 ± 3.8% on the CR cathode, the highest Co(II) reduction using TS (45.0 ± 0.3%) and SSM (39.7 ± 3.6%) was obtained under smaller cathode volumes in both MFCCu and MECCo. Sequential MFCCu and MECCo operation efficiently reduced Cu(II) and Co(II), achieving 100% and 65.3–72.0%, respectively with either TS or SSM. These results demonstrate cathode material of MECCo and cathode volumes in both MECCo and MFCCu were critical for efficient Co(II) reduction in MECCo driven by MFCCu with achievements of copper and cobalt recovery as well as no external energy consumption. Acknowledgments The authors gratefully acknowledge financial support from the Natural Science Foundation of China (Nos. 51178077 and 21377019), the National Basic Research Program of China (No. 2011CB936002), Specialized Research Fund for the Doctoral Program of Higher Education ‘‘SRFDP’’ (No. 20120041110026), and Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05). References [1] J. Xu, H.R. Thomas, R.W. Francis, K.R. Lum, J. Wang, B. Liang, A review of processes and technologies for the recycling of lithium-ion secondary batteries, J. Power Sources 177 (2008) 512–527. [2] M.B.J.G. Freitas, V.G. Celante, M.K. Pietre, Electrochemical recovery of cobalt and copper from spent Li-ion batteries as multilayer deposits, J. Power Sources 195 (2010) 3309–3315. [3] A. Ter Heijne, F. Liu, R. van der Weijden, J. Weijma, C.J.N. Buisman, H.V.M. Hamelers, Copper recovery combined with electricity production in a microbial fuel cell, Environ. Sci. Technol. 44 (2010) 4376–4381. [4] H. Tao, M. Liang, W. Li, L. Zhang, J. Ni, W. Wu, Removal of copper from aqueous solution by electrodeposition in cathode chamber of microbial fuel cell, J. Hazard. Mater. 189 (2011) 186–192. [5] H. Tao, L. Zhang, Z. Gao, W. Wu, Copper reduction in a pilot-scale membranefree bioelectrochemical reactor, Bioresour. Technol. 102 (2011) 10334–10339. [6] S. Cheng, B. Wang, Y. Wang, Increasing efficiencies of microbial fuel cells for collaborative treatment of copper and organic wastewater by designing reactor and selecting operating parameters, Bioresour. Technol. 147 (2013) 332–337. [7] L. Jiang, L. Huang, Y. Sun, Recovery of flakey cobalt from aqueous Co(II) with simultaneous hydrogen production in microbial electrolysis cells, Int. J. Hydrogen Energy 39 (2014) 654–663.
132
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
[8] L. Huang, L. Jiang, Q. Wang, X. Quan, J. Yang, L. Chen, Cobalt recovery with simultaneous methane and acetate production in biocathode microbial electrolysis cells, Chem. Eng. J. 253 (2014) 281–290. [9] M. Sun, G. Sheng, L. Zhang, C. Xia, Z. Mu, X. Liu, H. Wang, H. Yu, R. Qi, T. Yu, M. Yang, An MEC–MR-coupled system for biohydrogen production from acetate, Environ. Sci. Technol. 42 (2008) 8095–8100. [10] Y. Wang, X. Liu, W. Li, F. Li, Y. Wang, G. Sheng, R. Zeng, H. Yu, A microbial fuel cell-membrane bioreactor integrated system for cost-effective wastewater treatment, Appl. Energy 98 (2012) 230–235. [11] Y. He, F. Yan, H. Yu, S. Yuan, Z. Tong, G. Sheng, Hydrogen production in a lightdriven photoelectrochemical cell, Appl. Energy 113 (2014) 164–168. [12] K.R. Fradler, I. Michie, R.M. Dinsdale, A.J. Guwy, G.C. Premier, Augmenting microbial fuel cell power by coupling with supported liquid membrane permeation for zinc recovery, Water Res. 55 (2014) 115–125. [13] L. Huang, B. Yao, D. Wu, X. Quan, Complete cobalt recovery from lithium cobalt oxide in self-driven microbial fuel cell – microbial electrolysis cell systems, J. Power Sources 259 (2014) 54–64. [14] D. Cui, Y. Guo, H. Lee, H. Cheng, B. Liang, F. Kong, Y. Wang, L. Huang, M. Xu, A. Wang, Efficient azo dye removal in bioelectrochemical system and postaerobic bioreactor: optimization and characterization, Chem. Eng. J. 243 (2014) 355–363. [15] A. Kundu, J.N. Sahu, G. Redzwan, M.A. Hashim, An overview of cathode material and catalysts suitable for generating hydrogen in microbial electrolysis cell, Int. J. Hydrogen Energy 38 (2013) 1745–1757. [16] E. Ribot-Llobet, J.Y. Nam, J.C. Tokash, A. Guisasola, B.E. Logan, Assessment of four different cathode materials at different initial pHs using unbuffered catholytes in microbial electrolysis cells, Int. J. Hydrogen Energy 38 (2013) 2951–2956. [17] Q. Wang, L. Huang, H. Yu, X. Quan, G. Chen, Recent developments of graphene electrodes in bioelectrochemical systems, Acta Phys. Chim. Sin. 29 (2013) 889– 896.
[18] Y.M. Zhang, M.D. Merrill, B.E. Logan, The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells, Int. J. Hydrogen Energy 35 (2010) 12020–12028. [19] Q. Chen, J. Liu, Y. Liu, Y. Wang, Hydrogen production on TiO2 nanorod arrays cathode coupling with bio-anode with additional electricity generation, J. Power Sources 238 (2013) 345–349. [20] W. Li, H. Yu, Z. He, Towards sustainable wastewater treatment by using microbial fuel cells-centered technologies, Energy Environ. Sci. 7 (2014) 911– 924. [21] Y. Fan, S. Han, H. Liu, Improved performance of CEA microbial fuel cells with increased reactor size, Energy Environ. Sci. 5 (2012) 8273–8280. [22] L. Zhang, J. Li, X. Zhu, D. Ye, Q. Liao, Anodic current distribution in a liter-scale microbial fuel cell with electrode arrays, Chem. Eng. J. 223 (2013) 623–631. [23] B.E. Logan, K. Rabaey, Conversion of wastes into bioelectricity and chemicals by using microbial electrochemical technologies, Science 337 (2012) 686–690. [24] A.M. Couper, D. Pletcher, F.C. Walsh, Electrode materials for electrosynthesis, Chem. Rev. 90 (1990) 837–865. [25] Y. Liu, J. Shen, L. Huang, D. Wu, Copper catalysis for enhancement of cobalt leaching and acid utilization efficiency in microbial fuel cells, J. Hazard. Mater. 262 (2013) 1–8. [26] M.V. Alipazaga, R.G.M. Moreno, N. Coichev, Synergistic effect of Ni(II) and Co(II) ions on the sulfite induced autoxidation of Cu(II)/tetraglycine complex, Dalton Trans. 13 (2004) 2036–2040. [27] Z. An, H. Zhang, Q. Wen, Z. Chen, M. Du, Desalination combined with copper(II) removal in a novel microbial desalination cell, Desalination 346 (2014) 115– 121. [28] L. Huang, Y. Shi, N. Wang, Y. Dong, Anaerobic/aerobic conditions and biostimulation for enhanced chlorophenols degradation in biocathode microbial fuel cells, Biodegradation 25 (2014) 615–632.
Contents lists available at ScienceDirect
Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Comparison of Co(II) reduction on three different cathodes of microbial electrolysis cells driven by Cu(II)-reduced microbial fuel cells under various cathode volume conditions Dan Wu a, Yuzhen Pan b, Liping Huang a,⇑, Xie Quan a, Jinhui Yang b a Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China b Experiment Center of Chemistry, Dalian University of Technology, Dalian 116024, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Co(II)-reduced MECs were
successfully driven by Cu(II)-reduced MFCs. Cathode materials in Co(II)-reduced MECs substantially affected system performance. Titanium sheet cathode was more favorable for Co(II) and Cu(II) reduction. Stainless steel mesh was more costeffective for Co(II) and Cu(II) reduction. Cathode volume affected performance depending on material of Co(II)-reduced MECs.
a r t i c l e
i n f o
Article history: Received 2 October 2014 Received in revised form 15 December 2014 Accepted 20 December 2014 Available online 27 December 2014 Keywords: Microbial electrolysis cell Microbial fuel cell Cathode Metal reduction
a b s t r a c t Reduction of aqueous Cu(II) and Co(II) is one critical step for simultaneous recovery of copper and cobalt, and recycle of spent lithium ion batteries, but suffers from consumption of large amount of energy and chemicals. Here we report Co(II)-reduced microbial electrolysis cells (MECCo) can be driven by Cu(II)reduced microbial fuel cells (MFCCu) for simultaneous Cu(II) and Co(II) recovery with no external energy consumption, and system performance was heavily dependent on cathode material of MECCo and cathode volumes in both MECCo and MFCCu. Either titanium sheet (TS) or stainless steel mesh (SSM) cathode achieved efficient Co(II) reduction whereas carbon rod (CR) cannot proceed this occurrence. While smaller cathode volumes in MFCCu led to appreciable Co(II) reduction (41.4 ± 3.8%) on the CR cathode, the highest Co(II) reduction using TS (45.0 ± 0.3%) or SSM (39.7 ± 3.6%) was obtained under smaller cathode volumes in both MFCCu and MECCo. Moreover, when a mixed Cu(II) and Co(II) catholyte was deliberately used as the influent of MFCCu and the effluent of MFCCu was subsequently imported into the connected MECCo for tentatively simultaneous Cu(II) and Co(II) recovery from simulated mixed wastes, this so-called sequential MFCCu and MECCo operation achieved Cu(II) reduction of 100% and Co(II) reduction of 65.3–72.0% using either TS or SSM cathodes. These results illustrate cathode material of MECCo and cathode volumes in both MECCo and MFCCu were critical for efficient Co(II) reduction in MECCo driven by MFCCu with achievements of simultaneous copper and cobalt recovery as well as no external energy consumption. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel./fax: +86 411 84708546. E-mail address: [email protected] (L. Huang). http://dx.doi.org/10.1016/j.cej.2014.12.078 1385-8947/Ó 2014 Elsevier B.V. All rights reserved.
122
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Nomenclature CR TS SSM SHE OCCs SC MFC MEC MFCCu MECCo YMFC,Cu
carbon rod titanium sheet stainless steel mesh standard hydrogen electrode open circuit conditions solution conductivity microbial fuel cell microbial electrolysis cell Cu(II)-reduced MFC Co(II)-reduced MEC yield of copper (mol/mol COD)
1. Introduction Copper and cobalt are elements with excellent ferromagnetic properties, relatively stable against corrosion and easily to handle, and thus useful in microelectronic fields such as lithium ion batteries. Recovery of copper and cobalt from spent lithium ion batteries is in consequence one of the primary objectives in the recycling of these wastes due to the shortage of natural ores and environmental/ecological considerations [1]. Conventional pyrometallurgical or hydrometallurgical processes, bioleaching, and electrochemical processes can recover Cu(II) and Co(II) after the spent battery dismantling occurs [1,2]. The great energy consumption, high cost and serious second pollution urge a turn to environmental friendly and cost-effective strategies. The newly developed Cu(II)-reduced microbial fuel cells (MFCCu) [3–6] and Co(II)-reduced microbial electrolysis cells (MECCo) [7,8] provide new approaches for individual copper and cobalt recovery, where Cu(II) and Co(II) can accept the cathodic electrons and be reductively formed as Cu and Co on the cathodes of MFCCu and MECCo, respectively based on their halfcell reactions in the cathodes and acetate oxidation in the anodes. The remaining challenges include simultaneously recovering Cu(II) and Co(II) from mixed wastes, in situ utilization of power from MFCCu, and minimizing or requiring no external power supply for MECCo. Despite exploration of MFCs integrated with either conventional processes or MECs for wastewater treatment with simultaneous production of hydrogen, alkali, cobalt, zinc, methane and formic acid [9–14], to date, no essential attention has been paid to the MECCo driven by MFCCu system as a promising technology for simultaneously Cu(II) and Co(II) recovery from wastes with no external energy consumption. Cathode potential is one important parameter in MECs where different chemicals are produced [15–17]. It is well known various material cathodes of MECs exhibit different overpotentials and considerably affect hydrogen production [15–20]. In the case of MECCo driven by MFCCu system, Co(II) reduction may occur concomitantly with hydrogen evolution due to their similar half-reaction potentials [7,8], particularly at much low metal concentrations with slow mass transfer (eg. 200 mg/L) [4,5]. We thus guess that various material cathodes of MECs may indirectly affect Co(II) reduction via hydrogen evolution in this MFCCu–MECCo system. Considering the usage of carbon rod (CR), titanium sheet (TS) and stainless steel mesh (SSM) as the cathodes of individual MECs/ MFCs for hydrogen evolution [15–19], these materials were chosen here as the cathodes of MECCo driven by MFCCu for suitably efficient Co(II) and Cu(II) reduction. A change in electrode distance in either MFCCu or MECCo is well known to change the internal resistances [21,22]. In the case of MECCo driven by MFCCu system, a close electrode distance through decrease in cathode volume in either MFCCu or both MFCCu and MECCo may improve system performance [13]. Together with
YMEC,Co yield of cobalt (mol/mol COD) Y MEC;H2 yield of hydrogen (mol/mol COD) CE coulombic efficiency (%) CEMFC,an anodic CE in MFCCu (%) CEMEC,an anodic CE in MECCo (%) CEMFC,ca cathodic CE in MFCCu (%) CEMEC,ca,Co cathodic CE in MECCo based on reduced Co(II) (%) CEMEC;ca;H2 cathodic CE in MECCo based on evolved hydrogen (%) gsys overall system efficiency (%) COD chemical oxygen demand (g/L)
considering various material cathodes of MECCo, smaller cathode volumes in MFCCu only or both MFCCu and MECCo are expected to further enhance reduction of Cu(II) in MFCCu and Co(II) in MECCo. In this study, a truly environmental friendly MECCo driven by MFCCu system was introduced, which not only extracted energy from wastewaters through the anodes of MFCCu and MECCo, and in situ utilized the energy of MFCCu to power MECCo, and thus avoided the need for external energy input in MECCo, but also achieved simultaneous copper and cobalt recovery. CR, SSM and TS were alternatively tested as the cathode of MECCo for suitably efficient system performance. Moreover, smaller cathode volumes in MFCCu only or both MFCCu and MECCo were explored for enhanced Cu(II) and Co(II) reduction. Following this investigation, a so-called sequential MFCCu and MECCo operation was studied, in which a mixed Cu(II) and Co(II) catholyte was deliberately used as the influent of MFCCu and the effluent of MFCCu was subsequently imported into the connected MECCo for tentatively simultaneous Cu(II) and Co(II) recovery from simulated mixed wastes. Multiple parameters including Cu(II) and Co(II) reduction, yields of copper, cobalt and hydrogen, anodic coulombic efficiency (CE), cathodic CE, and overall system efficiency were extensively used to evaluate system performance. Deeper insight into these aspects will enhance the system performance for simultaneous copper and cobalt recovery, and thus recycle of spent lithium ion batteries with no external energy consumption as discussed subsequently.
2. Materials and methods 2.1. Reactor configuration Identical two-chamber MFCCu and MECCo were used in all experiments, with the chambers separated by a cation exchange membrane (CMI-7000 Membranes International, Glen Rock, NJ). Graphite felt (Sanye Co., Beijing, China) was served as the anodes of both MFCCu and MECCo. The anodic working volume in either MFCCu or MECCo was 20 mL. Carbon rod (CR, Chijiu Duratight Carbon Co., China) was always used as the cathode of MFCCu. CR, TA1 titanium sheet (TS) and SS 304 woven mesh (SSM) (99.9%, Qingyuan Co., China) were alternatively served as the cathode of MECCo with a same projected surface area of 12 cm2. These materials were cleaned before tests using 0.5 M H2SO4. 2.2. Inoculation and operation The anodes of both MFCCu and MECCo were inoculated from the anodes of operating MFCs running on acetate. The anolyte was composed of (g/L) sodium acetate, 1.0; KH2PO4, 4.4; K2HPO4, 3.4; NH4Cl, 1.3; KCl, 0.78; MgCl2, 0.2; CaCl2, 0.0146; NaCl, 0.5; trace vitamins and minerals [7].
123
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Prior to add the solutions into MFCCu and MECCo, the anolyte was sparged with ultrapure N2 gas for 15 min. Both MFCCu and MECCo were initially acclimated using deionized water as catholyte with an external resistor of 1000 X. After six-cycle refreshments, the catholyte in MFCCu and MECCo was replaced by aqueous CuCl2 and CoCl2, respectively in addition to an applied voltage of 0.2 V to MECCo (DC Power Supply PS-1502DD, Yihua, Guangzhou, China). After another two or three-refreshments with repeatable and stable electrode potential observation in both MFCCu and MECCo, the acclimation of MECCo and MFCCu was finished. The external power source in MECCo was replaced by the well-developed MFCCu and a low resistance of 10 X was used in the circuit in order to measure the circuit current, although this including a resistor resulted in an additional voltage loss in the system and thus the actual applied voltage over the anode and cathode was smaller than the power source of MFCCu. During the following entire experiments, MECCo was always powered by the MFCCu, thus exhibiting no consumption of external energy. For the initial investigations, aqueous CuCl2 (50 mg/L) was always added in the cathode of MFCCu with simultaneous supplementation of CoCl2 (50 mg/L) in the cathode of MECCo. Different cathode volumes (MFCCu25 mL–MECCo25 mL, MFCCu13 mL– MECCo25 mL, MFCCu13 mL–MECCo13 mL) were explored for efficient reduction of Cu(II) in MFCCu and Co(II) in MECCo under various material cathodes of CR, TS or SS in MECCo. In order to abate electron consumption for hydrogen evolution and benefit more for Co(II) reduction, slight acidic catholyte of pH 5.8 with solution conductivity (SC) 5.5 mS/cm was firstly set in MECCo [7,8] whereas generally reported catholyte of pH 2.0 with solution conductivity (SC) 8.7 mS/cm [3–6] was employed in MFCCu. Identical catholyte of pH 2.0 with SC 8.7 mS/cm in MFCCu and MECCo was then used to eliminate the medium differences except ions of Cu(II) in MFCCu and Co(II) in MECCo. Following this exploration, a synthetic mixed Cu(II) and Co(II) catholyte at a same concentration of 50 mg/L was deliberately used as the influent of MFCCu and the effluent of MFCCu was then imported to the MECCo, which was connected and powered by the MFCCu. This so-called sequential MFCCu and MECCo operation was tentatively explored to recover Cu(II) and Co(II) from simulated mixed wastes with no consumption of external energy. Two controls were also operated: one was run in open circuit conditions (OCCs) to examine changes of Cu(II) and Co(II) in the absence of current generation. In the second control, new bare cathodes lacking either Cu(II) or Co(II) were accordingly analyzed for redox behavior to exclude the roles of Cu(II) or Co(II) on electrode reactions. Unless otherwise stated, new cathodes were used for each operation cycle. All reactors were operated in fed-batch mode and all experiments were run in duplicate and maintained at room temperature (20 ± 3 °C). All of the inoculation and solution replacements were performed in an anaerobic glove box (YQX-II, Xinmiao, Shanghai).
and the crystal products. The potentials of cathode and anode were collected by a data logger using an automatic data acquisition system (PISO-813, Hongge Co., Taiwan). The cathode redox behavior in MECCo was studied using cyclic voltammetry (CV, CHI 650, Chenhua, Shanghai) at a scan rate of 1.0 mV/s. Linear sweep voltammetry (LSV) was performed using a potentiostat (CHI 650, Chenhua, Shanghai) at a scan rate of 0.1 mV/s and power density was calculated based on per cathode working volume (W/m3). The three electrode system for both LSV and CV consisted of a working electrode (cathode electrode), counter electrode (platinum plate), and Ag/AgCl reference electrode (195 mV versus a standard hydrogen electrode, SHE). All potentials shown were corrected to a SHE. 2.4. Calculations Reduction of Cu(II) in MFCCu (%) and Co(II) in MECCo (%), yields of copper in MFCCu (YMFC,Cu, mol/mol COD), cobalt in MECCo (YMEC,Co, mol/mol COD) and hydrogen in MECCo (Y MEC;H2 , mol/mol COD), anodic coulombic efficiencies (CEs) in MFCCu (CEMFC,an, %) and MECCo (CEMEC,an, %), cathodic CEs in MFCCu (CEMFC,ca, %) and MECCo (CEMEC,ca,Co, % and CEMEC;ca;H2 , %), and overall system efficiency (gsys, %) were modified and calculated as Eqs. (1)–(11):
CuðIIÞreduction ð%Þ ¼
Cui Cut 100% Cui
ð1Þ
CoðIIÞreduction ð%Þ ¼
Coi Cot 100% Coi
ð2Þ
Y MFC;Cu ¼
Y MEC;Co ¼
Y MEC;H2 ¼
Cui Cut 64
V MFC;ca;l
ð3Þ
MFC V MFC;an DCOD 32
Coi Cot 59
V MEC;ca;l
ð4Þ
MEC V MEC;an DCOD 32
nH2 ;t V MEC;ca;g
ð5Þ
MEC V MEC;an DCOD 32
R CEMFC;an ¼
96; 485
R CEMEC;an ¼
CEMFC;ca ¼
96; 485
Idt
4DCODMFC V MFC;an 32
Idt
4DCODMEC V MEC;an 32
100%
ð6Þ
100%
ð7Þ
a1 V MFC;ca;l ðCui Cut Þ 96; 485 R 100% Idt
CEMEC;ca;Co ¼
a2 V MEC;ca;l ðCoi Cot Þ 96; 485 R 100% Idt
CEMEC;ca;H2 ¼
a3 V MEC;ca;g nH2 ;t 96; 485 R 100% Idt
ð8Þ
ð9Þ
2.3. Measurements and analyses Total chemical oxygen demand (COD) in the anolyte was measured using standard methods. The concentrations of Cu(II) and Co(II) in catholyte were measured by an atomic absorption spectrophotometer (AAnalyst 700, PerkinElmer). Hydrogen produced in the headspace of MECCo cathode was analyzed using a gas chromatograph (GC7900, Tianmei, Shanghai) and a molecular sieve column (TDX-01, 60–80, 4 mm 2 m) with argon as the carrier gas. Scanning electronic microscopy (SEM) (QUANTA450, FEI company, USA) equipped with an energy dispersive X-ray spectrometer (EDS) (X-MAX 20 mm2/50 mm2, Oxford Instruments, UK), and X-ray diffraction (XRD-6000, Shimadzu LabX, Japan) were used to examine the morphologies of the electrode after copper and cobalt recovery,
gsys ¼
ð10Þ
ðCui Cut Þ V MFC;ca;l a1 þ ðCoi Cot Þ V MEC;ca;l a2 þ nH2 ;t V MEC;ca;g a3 4 ðV MFC;an DCODMFC þ V MEC;an DCODMEC Þ 32 100% ð11Þ
Cui and Coi are the initial Cu(II) and Co(II) concentrations in the catholytes whereas Cut and Cot are the Cu(II) and Co(II) concentrations at an operation time of t (mol/L); nH2 ;t is hydrogen concentration in headspace of the cathodes at t (mol/L); VMFC,an and VMEC,an are anolyte volumes in MFCCu and MECCo, respectively (L); VMFC,ca,l
124
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
and VMEC,ca,l are catholyte volumes in MFCCu and MECCo, respectively (L); VMEC,ca,g is volume of headspace in the cathodes of MECCo (L); a1 (2 mol/mol), a2 (2 mol/mol) and a3 (2 mol/mol) are the number of electrons required for Cu(II) and Co(II) reduction as well as hydrogen evolution, respectively. DCODMFC and DCODMEC are the cumulative COD consumptions over a set period of t in the anodes of MFCCu and MECCo, respectively (g/L); I is circuit current (A); 64, 59, and 32 are the atom/molecule weights of Cu, Co and O2, respectively (g/mol); 4 is the molar number of electrons required for oxygen reduction (mol/mol); 96,485 is Faraday constant (C/mol e).
3. Results and discussion 3.1. Reactor setup Following inoculation, anode potentials in both MFCCu (Fig. 1A) and MFCCo (Fig. 1C) experienced gradual decrease. The initial five or six-cycle refreshments using deionized water as catholyte resulted in the decrease of anode potentials in both MFCCu and MECCo to 0.23 to 0.25 V, values frequently observed using acetate as an anodic fuel. These results demonstrate the gradual development of anodic biofilms and the finishing of biofilms acclimation at five or six-cycle refreshments. A shift from deionized water to 50 mg/L CuCl2 solution in the catholyte of MFCCu substantially decreased the cathode potentials (Fig. 1A), mainly ascribed to the lower half cell potential of Cu(II) reduction than O2 reduction [3]. In the case of MECCo acclimation and after the replacement of deionized water by 50 mg/L CoCl2 in the cathode, an applied voltage of 0.2 V resulted in higher anode than cathode potentials (Fig. 1C), always observed in MECs. This trend appeared stably and repeatedly in the subsequent three-cycle refreshments (Fig. 1C), implying the suitability of this MECCo for the following experiments. In addition, we cannot exclude the occurrence of cat-
alytic water hydrolysis here due to the applied voltage of 0.2 V slightly lower than the 0.3 V typically used even with Pt catalysts [23]. There was a general increase trend in open circuit voltage (OCV) and power production from MFCCu with the prolonged acclimation period, achieving similar values of 0.64 V and 4.3 W/m3 at an acclimation period of 9 d, and 0.65 V and 4.6 W/m3 at 12 d (Fig. 1B). This power of approximate 0.11 mW was either nearly 2-fold as the reported at a similar Cu(II) concentration and with a large working volume of 1.0 L [4], or lower than the 0.62 mW at a much higher Cu(II) concentration of 800 mg/L and with a small working volume of 14 mL [6]. Power overshoot, which the cell voltage and current dropped very quickly and resulted in a doubling back of the power density curve, was observed at an acclimation time of 12 d (Fig. 1B). Shorter acclimation periods of 6 d and 9 d led to the disappearance of power overshoot, reflecting electron transfer limitation at acclimation time of 12 d in MFCCu (Fig. 1B). The limitation was most likely related to a slow response from the acclimated anodic biofilms to adjust to the external resistance. Cu(II) removal in MFCCu was attributable to both cathodic reduction and chemical adsorption, in which cathodic reduction occupied 30.6 ± 3.3% and chemical adsorption possessed 7.1 ± 1.0% (Fig. 1D). Different from Cu(II) removal, no Co(II) reduction was observed on the CR cathode and a Co(II) removal of 16.0 ± 0.7% was completely ascribed to Co(II) adsorption (Fig. 1D). These results clarify the contribution of cathodic reduction and chemical adsorption to removals of Cu(II) in MFCCu and Co(II) in MECCo. 3.2. Comparison of CR, TS and SSM performances After a 12-d acclimation, both MFCCu and MECCo were well developed (Fig. 1) and used for Co(II) and Cu(II) recovery. As shown in Fig. 2A, no Co(II) reduction was observed on the CR cathode during an operation period of 6 h whereas appreciable Cu(II) reduction
Fig. 1. (A) MFCCu acclimation, (B) polarization curve of MFCCu periodically during acclimation, (C) MECCo acclimation, and (D) removals of Cu(II) in MFCCu and Co(II) in MECCo (cathode volume in both MFCCu and MECCo: 25 mL; CR as the cathode of MECCo; MECCo at an applied voltage of 0.2 V; arrows indicate when anodes were fed with fresh medium).
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
125
Fig. 2. (A) Co(II) and (B) Cu(II) reduction, (C) applied voltage and (D) cathode potential in MECCo, (E) circuit current, and (F) cyclic voltammetry tests with CR, TS or SSM (cathode volume in MFCCu or MECCo: 25 mL; catholyte in MFCCu: pH 2.0 with SC 8.7 mS/cm; catholyte in MECCo: pH 5.8 with SC 5.5 mS/cm).
of 77.3 ± 0.9% was achieved in MFCCu at the same 6 h (Fig. 2B). The no occurrence of Co(II) reduction using CR was mainly ascribed to lower applied voltages of 0.03–0.18 V (Fig. 2C) and higher cathode potentials of 0.34 to 0.17 V (Fig. 2D), both of which were insufficient for proceeding Co(II) reduction. Under the present experimental condition, the potential required for Co(II) reduction was more negative than 0.37 V. A high circuit current of 13.7 ± 1.5 A/m3 with CR (Fig. 2E) may have contributed to the high Cu(II) reduction (Fig. 2B) and the appreciable CEMFC,ca (Table 1). Different from CR, both TS and SSM successfully proceeded Co(II) reduction, reaching 28.3 ± 1.1% (Fig. 2A) with YMEC,Co of 0.30 ± 0.02 mol/mol COD (Table 1) (TS), and 23.6 ± 0.5% (Fig. 2A) with 0.22 ± 0.00 mol/mol COD (Table 1) (SSM) at 6 h. In addition, a higher CEMEC;ca;H2 of 14.2 ± 0.2% with TS than 3.9 ± 0.5% using SSM (Table 1), was in agreement with other observations that TS was one favorable electrode for hydrogen evolution in individual MECs [19]. It cannot be excluded that the more cobalt produced on the cathode may have been more beneficial for hydrogen
evolution, shown as the positive correlation of hydrogen production and Co(II) reduction with TS or SSM (Fig. 2A and Table 1). It was also understandable the higher applied voltages of 0.26– 0.35 V (Fig. 2C) and more negative cathode potentials of 0.52 to 0.59 V (Fig. 2D) led to higher Co(II) reduction and hydrogen production. Concomitantly, similar Cu(II) reduction of 37.6% was observed with TS and SSM (Fig. 2B). These results stress the importance of cathode of MECCo on efficient Cu(II) and Co(II) reduction in this system. Compared to the no occurrence of Co(II) reduction on the CR cathode (Fig. 2A), Co(II) reduction with either TS or SSM (Fig. 2A) somewhat consumed circuit electrons, in consistent with lower circuit currents of 0.17–0.18 mA (Fig. 2E). In the open circuit controls (OCCs), only 15.8 ± 0.7% of Cu(II) was removed (Fig. 2B), reflecting the importance of cathodic electrons on Cu(II) reduction. Cyclic voltammetry analysis on the CR electrode showed the presence of one reduction peak of 0.31 V with the presence of Co(II) (Fig. 2F), which was more negative than the 0.23 V with SSM and 0.12 V with TS. These results demonstrate the more benefi-
126
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Table 1 Comparison of yields of copper, cobalt and hydrogen, and operational efficiencies in the MECCo driven by MFCCu system under various material cathodes of MECCo and operational conditions.
gsys (%)
Operational conditions
Cathode of MECCo
Anodic CEs CEMFC,an (%)
CEMEC,an (%)
CEMFC,ca (%)
Cathodic CEs CEMEC,ca,Co (%)
CEMEC;ca;H2 ð%Þ
MFCCu25– MECCo25
CR TS SSM
42.2 ± 7.2 35.8 ± 0.0 45.0 ± 1.5
46.3 ± 7.9 53.1 ± 0.0 47.7 ± 1.6
35.2 ± 6.4 36.6 ± 0.7 35.8 ± 3.9
0.7 ± 2.5 28.4 ± 1.6 22.7 ± 0.2
0.0 ± 0.0 14.2 ± 0.2 3.9 ± 0.5
MFCCu13– MECCo25
CR TS SSM
43.3 ± 1.2 32.8 ± 7.6 45.0 ± 1.1
60.5 ± 1.6 63.0 ± 7.2 86.4 ± 2.1
13.6 ± 1.0 23.6 ± 7.9 18.7 ± 1.7
18.1 ± 1.9 34.4 ± 0.3 16.3 ± 0.1
MFCCu13– MECCo13
CR TS SSM
55.8 ± 3.5 32.8 ± 1.1 54.2 ± 3.2
77.9 ± 4.9 63.0 ± 2.1 104 ± 6.2
12.4 ± 1.5 27.8 ± 3.3 19.6 ± 2.1
pH: 2.0, SC: 8.7 mS/cm
CR TS SSM
55.8 ± 3.5 34.3 ± 1.1 75.5 ± 1.1
77.9 ± 4.9 65.9 ± 2.1 145 ± 2.0
Sequential MFCCu and MECCo
CR TS SSM
58.7 ± 5.3 19.8 ± 2.7 44.1 ± 0.0
82.0 ± 7.4 27.6 ± 3.7 61.6 ± 0.0
Yield YMFC,Cu (mol/ mol COD)
YMEC,Co (mol/ mol COD)
Y MEC;H2 (mol/ mol COD)
8.7 ± 0.6 18.6 ± 0.6 16.6 ± 0.9
0.29 ± 0.00 0.26 ± 0.00 0.32 ± 0.02
0.00 ± 0.02 0.30 ± 0.02 0.22 ± 0.00
0.00 0.15 ± 0.01 0.04 ± 0.01
0.4 ± 0.2 21.9 ± 5.4 6.4 ± 0.1
7.6 ± 0.5 16.9 ± 2.6 12.2 ± 0.6
0.12 ± 0.01 0.15 ± 0.02 0.17 ± 0.01
0.20 ± 0.01 0.43 ± 0.10 0.28 ± 0.01
0.00 ± 0.00 0.27 ± 0.04 0.11 ± 0.03
0.0 ± 0.0 22.5 ± 0.8 12.1 ± 1.9
0.0 ± 0.0 22.1 ± 0.2 7.2 ± 1.2
4.0 ± 0.2 15.6 ± 0.9 13.8 ± 0.8
0.14 ± 0.00 0.18 ± 0.02 0.21 ± 0.01
0.00 ± 0.00 0.28 ± 0.00 0.25 ± 0.03
0.00 ± 0.00 0.28 ± 0.01 0.15 ± 0.02
13.9 ± 0.0 32.8 ± 1.6 17.9 ± 0.9
0.0 ± 0.0 14.6 ± 1.1 9.0 ± 0.2
0.0 ± 0.0 23.7 ± 0.4 0.0 ± 0.0
4.5 ± 1.8 16.1 ± 0.5 13.4 ± 0.7
0.15 ± 0.01 0.23 ± 0.02 0.27 ± 0.01
0.00 ± 0.00 0.19 ± 0.01 0.25 ± 0.00
0.00 ± 0.00 0.31 ± 0.02 0.00 ± 0.00
16.9 ± 1.6 45.9 ± 6.2 20.4 ± 0.0
0.0 ± 0.0 27.2 ± 2.2 13.6 ± 0.0
0.0 ± 0.0 17.4 ± 2.8 1.1 ± 0.1
5.8 ± 0.0 10.3 ± 0.9 9.0 ± 0.5
0.18 ± 0.00 0.18 ± 0.00 0.18 ± 0.00
0.00 ± 0.00 0.15 ± 0.01 0.17 ± 0.00
0.00 ± 0.00 0.09 ± 0.00 0.01 ± 0.00
cial TS than SSM, and the most unfavorable CR for Co(II) reduction in this system. Current densities at potentials more negative than 0.78 V exhibited a similar more negative trend on the three various material electrodes (Fig. 2F), mainly ascribed to the acute occurrence of hydrogen evolution [16,18]. In the controls with the absence of Co(II), very weak peaks for both Co(II) reduction and hydrogen evolution were observed (Fig. 2F), reflecting the critical roles of Co(II) in these reactions on the electrode. 3.3. System performance under smaller cathode volumes in either MFCCu only or both MFCCu and MECCo For CR, a smaller cathode volume of 13 mL in MFCCu achieved Co(II) reduction of 41.4 ± 3.8% (Fig. 3A) compared to no occurrence with a 25 mL cathode volume (Fig. 2A). This was mainly ascribed to higher applied voltages (Fig. 3D), more negative cathode potentials in MECCo (Fig. 3F) and higher circuit currents (Fig. 3E), resulted from higher voltage output and power generation (Fig. 3B) from MFCCu with this smaller cathode volume [21,22]. Compared to the 25 mL cathode volume (Fig. 2A), more electrons consumed for Co(II) reduction with this 13 mL cathode volume reasonably led to lower Cu(II) reduction (67.5 ± 1.4%) (Fig. 3B). However, the smaller cathode volumes in both MFCCu and MECCo completely impeded Co(II) reduction on the CR cathode (Fig. 3A), explained by lower applied voltages (Fig. 3D) and higher cathode potentials (Fig. 3F). This no occurrence of Co(II) reduction may have saved more electrons and thus led to higher Cu(II) reduction (79 ± 0.3%) than those of other materials (Fig. 3B). For TS and SSM, smaller cathode volumes in either MFCCu only or both MFCCu and MECCo generally improved reduction of Co(II) in MECCo and Cu(II) in MFCCu. TS had Co(II) reduction of 36.5 ± 9.2% (Fig. 3A) and Cu(II) reduction of 46.9 ± 6.1% (Fig. 3B) in MFCCu13– MECCo25, and 45.0 ± 0.3% (Fig. 3A) and 54.0 ± 1.8% (Fig. 3B) in MFCCu13–MECCo13, higher than 28.3 ± 1.1% (Fig. 2A) and 37.5 ± 0.6% (Fig. 2B) in MFCCu25–MECCo25. While gsys was heavily dependent on cathode materials and appeared lower with a 13 mL cathode volume in MFCCu (Table 1), this smaller cathode volume substantially improved operational efficiencies in MECCo with TS including CEMEC,an, CEMEC,ca,Co, CEMEC;ca;H2 , YMEC,Co and Y MEC;H2 (Table 1). Comparatively, SSM exhibited Co(II) reduction of 23.4 ± 0.7% (Fig. 3A) and Cu(II) reduction of 39.7 ± 3.6% (Fig. 3B) in MFCCu13–MECCo25, and 39.7 ± 3.6% (Fig. 3A) and 66.0 ± 5.2% (Fig. 3B) in MFCCu13–MECCo13, compared to 23.6 ± 0.5% (Fig. 2A) and 37.5 ± 0.6% (Fig. 2B) in MFCCu25–MECCo25. Smaller cathode
volumes in MFCCu reasonably increased voltage output and power production (Fig. 3C) [21,22], and thus higher applied voltages (Fig. 3D) and more negative cathode potentials in MECCo (Fig. 3F), both of which favored Co(II) reduction with either TS or SSM. The fact that some CEMEC,an exceeded 100% (Table 1) was in good agreement with previously reported [7,9,13,18]. The smaller cathode volumes in either MFCCu only or both MFCCu and MECCo improved circuit current with CR or SSM (Fig. 3E). Circuit current with TS, however, was invariable to the change of cathode volume (Figs. 3E and 2E). This result was mainly ascribed to the higher resistance of TS than SSM and CR [19,24], resulting in the ignorance of resistance change in response to cathode volume change in either MFCCu only or both MFCCu and MECCo. Considering the more benefit of a 13 mL cathode volume in either MFCCu using CR, or both MFCCu and MECCo with TS and SSM (Fig. 3A), CR was thus performed in MFCCu13–MECCo25 whereas TS and SSM were operated in MFCCu13–MECCo13 in the subsequent experiments. 3.4. System performance under identical catholytes in MFCCu and MECCo Practical application of the self-driven MFCCu and MECCo system requires an identical catholyte in both MFCCu and MECCo due to the coexistence of Cu(II) and Co(II) in the same wastes. Identical catholyte of pH 2.0 with SC 8.7 mS/cm in MECCo and MFCCu was thus explored, compared to the catholyte of pH 5.8 with SC 5.5 mS/cm in MECCo and pH 2.0 with SC 8.7 mS/cm in MFCCu mentioned above. For all the three materials, catholyte of pH 2.0 with SC 8.7 mS/cm possessed a low resistance and reasonably resulted in higher circuit currents (Fig. 4A), lower applied voltages (Fig. 4B) and higher cathode potentials of MECCo (Fig. 4C) than that of pH 5.8 with SC 5.5 mS/cm. Cyclic voltammetry showed an identical reductive peak at 0.42 to 0.40 V on both the CR and SSM electrodes (Fig. 4D). Comparatively, TS exhibited a slightly positive shift to 0.34 V with a high peak current of 2.80 A/m2 (Fig. 4D), implying the more efficient Co(II) reduction on the TS electrode than the others. All the reductive peaks of 0.40 V (CR), 0.34 V (TS) and 0.42 V (SSM) (Fig. 4D) were more negative than those of 0.31 V (CR), 0.12 V (TS) and 0.23 (SSM) in pH 5.8 with SC 5.5 mS/cm (Fig. 2F), implying more favorable of pH 5.8 with SC 5.5 mS/cm for Co(II) reduction. Either no occurrence of Co(II) reduction with CR or decreased Co(II) reduction (29.8 ± 1.1%) and YMEC,Co (0.19 ± 0.01 mol/mol COD) using TS were observed at pH 2.0 with SC 8.7 mS/cm
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
127
Fig. 3. (A) Co(II) and (B) Cu(II) reduction, (C) applied voltage and (D) cathode potential in MECCo, (E) polarization curve of MFCCu, and (F) circuit current with small cathode volumes in either MFCCu only or both MFCCu and MECCo (catholyte in MFCCu: pH 2.0 with SC 8.7 mS/cm; catholyte in MECCo: pH 5.8 with SC 5.5 mS/cm).
(Fig. 4E and Table 1), compared to 41.4 ± 3.8% and 0.20 ± 0.01 mol/ mol COD (CR) and 45.0 ± 0.3% and 0.28 ± 0.00 mol/mol COD (TS) at pH 5.8 with SC 5.5 mS/cm (Fig. 4E and Table 1). In addition, hydrogen evolution termed as CEMEC;ca;H2 and Y MEC;H2 in pH 2.0 with SC 8.7 mS/cm was completely impeded on the CR and SSM cathodes (Table 1). Lower resistances with CR and SSM than the other materials [19,20,24] presumably resulted in the lower applied voltages (Fig. 4B) and higher cathode potentials (Fig. 4C), both of which were unsatisfactory for hydrogen reduction [15–17]. On the other hand and for SSM, the negative effects of lower applied voltages (Fig. 4B) and higher cathode potentials (Fig. 4C) due to catholyte of pH 2.0 with SC 8.7 mS/cm may have been compensated by the higher circuit currents (Fig. 4A), explaining the similar Co(II) reduction in these two catholytes (Fig. 4E). For all the three materials, Cu(II) reduction (Fig. 4F) and YMFC,Cu (Table 1) in pH 2.0 with SC 8.7 mS/cm were reasonably higher, mainly attributable to the higher circuit currents (Fig. 4A). Despite the more beneficial pH 5.8 with SC 5.5 mS/cm for Co(II) reduction on the CR and TS cathodes of MECCo (Fig. 4E), Cu(II)
would be undesirably precipitated as Cu(OH)2 if the same catholyte was also used in MFCCu [4,5]. Based on this consideration, catholyte used for subsequent sequential MFCCu and MECCo operation, and morphology and product analysis preparation was uniformly maintained at pH 2.0 with SC 8.7 mS/cm. 3.5. Performance of sequential MFCCu and MECCo Compared to single Cu(II), a mixed Co(II) and Cu(II) influent to MFCCu decreased open circuit potential and power production (Fig. 5A), and thus abated applied voltages (Fig. 5B) and increased cathode potentials in MECCo (Fig. 5C) with all the three materials. There was no appreciable difference in circuit current between single Cu(II)/Co(II), and a mixed Co(II) and Cu(II) influent (Fig. 5D). TS exhibited the highest applied voltage (Fig. 5B), and the lowest cathode potential (Fig. 5C) and circuit current (Fig. 5D), followed by SSM. Cu(II) and Co(II) were considerably reduced in this sequential MFCCu and MECCo, achieving 100% and 65.3–72.0%, respectively using either TS or SSM, among which amounting to 70.0–85.5%
128
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Fig. 4. (A) applied voltage and (B) cathode potential in MECCo, (C) circuit current, and reduction of (E) Co(II) and (F) Cu(II) in an identical catholyte of pH 2.0 with SC 8.7 mS/cm in both MECCo and MFCCu, compared to those of pH 5.8 with SC 5.5 mS/cm in MECCo and pH 2.0 with SC 8.7 mS/cm in MFCCu. (D) Cyclic voltammetry tests in pH 2.0 with SC 8.7 mS/cm (CR: MFCCu13–MECCo25; TS and SSM: MFCCu13–MECCo13).
of total Cu(II) was reduced in MFCCu (Fig. 5E) and 61.3–68.7% of total Co(II) was reduced in MECCo (Fig. 5F). These results were higher than those in single Cu(II) and Co(II) influent at identical catholyte (Fig. 4E and F), reflecting the catalysis roles of either Co(II) ion in Cu(II) reduction or Cu(II) ion in Co(II) reduction, consistent with previously reported [2,25,26]. TS had similar YMFCs,Cu (0.18 ± 0.00 mol/mol COD) and YMECs,Co (0.16 ± 0.01 mol/mol COD) to SSM (Table 1). While the Cu(II) and Co(II) reduction rates could not be compared with conventional electrochemical processes due to greatly different operational conditions and reactor architectures [1,2], the Cu(II) reduction rate here was 4.8 times of individual MFCs at a higher Cu(II) concentration of 200 mg/L and a higher pH of 4.7 [3–5], and 1.4 times of that at a Cu(II) of 100 mg/L and a more acidic pH of 3.0 [27] whereas the Co(II) reduction rate was
equivalent to previous individual MECs with an energy consumption of 1.6–4.0 kWh/kg Co [7,8]. Despite a high circuit current using CR (Fig. 5D), the lowest applied voltage (Fig. 5B) and the highest cathode potential in MECCo (Fig. 5C) led to the no occurrence of Co(II) reduction (Fig. 5F), similar to the single Cu(II)/Co(II) influent (Fig. 4A–C and E). For all the three materials, an identical small amount of Co(II) (10.5%) was removed in MFCCu (Fig. 5E), mainly ascribed to chemical adsorption on the cathode. Comparatively, much more Co(II) (61.3–68.7%) was removed on the TS or SSM cathode of MECCo (Fig. 5F), reflecting the selectivity of MFCCu and MECCo for cobalt recovery. While Cu(II) was mainly reduced in MFCCu (Fig. 5E), the residual Cu(II) in the effluent of MFCCu (10.5–30.0%) was completely removed in the subsequent MECCo with all the three mate-
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
129
Fig. 5. (A) voltage output and power density in MFCCu, (B) applied voltage and (C) cathode potential in MECCo, (D) circuit current, changes of Cu(II) and Co(II) concentrations in sequential (E) MFCCu and (F) MECCo operation at an identical catholyte of pH 2.0 with SC 8.7 mS/cm in both MFCCu and MECCo (CR: MFCCu13–MECCo25; TS and SSM: MFCCu13–MECCo13).
rials (Fig. 5F), illustrating the more efficiency of sequential MFCCu and MECCo operation for Cu(II) removal. In view of purity of final products on the cathodes of sequential MFCCu and MECCo, however, mixed copper and cobalt were somewhat unavoidably formed, mainly ascribed to the more or less chemical adsorption of Co(II) in MFCCu (Fig. 5E) and the Cu(II) reduced in the subsequent MECCo (Fig. 5F). In view of material cost, SSM (73 $/m2) was much lower than TS (450 $/m2). Taken together with the appreciable Co(II) reduction on the SSM cathode with simultaneous Cu(II) reduction in MFCCu (Fig. 5E and F), the substantially low cost of SSM as well as its easily produced and efficiently performed significantly enhanced the economic feasibility of this self-driven MFCCu–MECCo system for simultaneous Cu(II) and Co(II) recovery.
3.6. Morphology of the cathode and product confirmation In order to confirm products on the cathodes, totally 6 repeated batch cycles were particularly performed under the optimal conditions of CR cathode in MFCCu13–MECCo25, and TS and SSM cathodes in MFCCu13–MECCo13. Together with considering products on the CR cathode of the connected MFCCu, totally 6 samples were accordingly obtained for SEM, EDS and XRD analysis. Since morphology and product on CR cathode of MFCCu under conditions of CR, TS and SSM cathode of MECCo were very similar and for simplicity, only one sample results about CR cathode of MFCCu were presented here. As shown in Fig. 6A, rose-like crystals were clearly observed on the CR cathode of MFCCu. Cu signals at 0.98, 8.06 and 8.96 keV s were detected using EDS analysis (Fig. 6B), demonstrat-
130
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
Fig. 6. SEM observation on (A) the CR cathode of MFCCu, and (D) CR, (G) TS, and (J) SSM cathode of MECCo after reduction of (A) Cu(II) and (D, G and J) Co(II). EDS (B, E, H and K) and XRD (C, F, I and L) analyses on products of (A) Cu(II) and (D, G and J) Co(II) reduction. (CR: MFCCu13–MECCo25; TS and SSM: MFCCu13–MECCo13.)
ing the formation of copper product. The ratio of Cu and O was approximate 11, much higher than the 2 as a final product of Cu2O, implying most Cu(II) was reduced to pure copper [4–6]. XRD spectra showed copper were clearly present in the powder collected from the cathode of MFCCu with a stronger Cu(1 1 1) relative peak intensity at 43.2° than others of Cu(2 0 0) at 50.3° and Cu(2 2 0) at 77.1° (Fig. 6C), all of which closely matched with the reported pure copper [4–6]. These results confirm the occurrence of reduction reaction from Cu(II) to Cu(0). For the CR cathode of MECCo, particles were distributed sparsely over the surface (Fig. 6D). The binding energy peaks at 0.79, 6.95 and 7.69 keV s by EDS analysis represented that cobalt was a main
cathode product, which was also reflected by a high ratio of Co and O of 7 (Fig. 6E). The XRD patterns closely matched that of metal Co (Co0) with standard peaks at 1 1 1, 2 0 0, and 2 2 0 degree in 2h (Fig. 6F). Compared to CR, the surface of TS was clearly covered by uniform particles (Fig. 6G) while similar binding energy peaks by EDS analysis (Fig. 6H) and characteristic peaks of Co(0) by XRD analysis (Co(1 1 1), Co(2 0 0) and Co(2 2 0)) (Fig. 6I) were both observed. Comparatively, larger particles were formed on the SSM cathode (Fig. 6J) in addition to weaker binding energy peaks in EDS (Fig. 6K) and the absence of Co(2 0 0) at 53.1° in XRD analysis (Fig. 6L). These results demonstrate the somewhat dependence of crystal forms of final products on the cathode materials of
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
131
the complexity of the treatment process, hence creating more challenges for process optimization and maintenance. While copper and cobalt recovered from the sequential MFCCu and MECCo operation need to be further separated and purified, Co(II) remaining in the effluent of this system still need to be recovered in order for the requirement of national wastewaters release standard criterion. In addition, considering the more complexity of actual wastewaters than acetate used in the present anodes as well as the lower solution conductivity of 1.0 mS/cm in actual wastewaters than the 5.5 mS/cm in the present anodes, results obtained here will be different from those using actual wastewaters and most likely higher than the latter. Enhanced Cu(II) and Co(II) reduction with development of continuous flow systems together with considering actual wastewaters are thus needed to advance this new technology. Further investigations in this direction are certainly warranted.
4. Conclusions
Fig. 7. Theoretical cathode potentials for half-reactions of (A) Cu(II) to Cu(I), Cu(II) to Cu and Cu(I) to Cu, and (B) Co(III) to Co(II), Co(III) to Co3O4 and Co(II) to Co, at a same Cu(II) or Co(II) concentration of 50 mg/L and different pH values.
MECCo. However, a similar ratio of Co and O of 7 under conditions of CR, TS or SSM cathode implies the invariable products on these three material cathodes. The products formed on the cathodes mainly depend on substrate concentration and pH value [4–6]. Under the present experimental condition, cathode theoretical potentials at different pHs were calculated based on the Nernst equations (Fig. 7). Considering the slight change of 0.1–0.2 pH units in the effluent of both MFCCu and MECCo at an identical initial pH of 2.0, Cu(II) and Co(II) were reductively changed to Cu(0) (Fig. 7A) and Co(0) (Fig. 7B), respectively, further confirming the final products in MFCCu and MECCo. Cu(0) and Co(0) crystals can be also attributable to the cathodic strict anaerobic environment. Under an aerobic/facultative environment, dissolved oxygen can compete with species of Cu(II) and phenols for cathodic electrons, resulting in the formation of partial reduction products [3–7,28]. The primary functions of this system are not only its environmental benefit to remove Cu(II) and Co(II) from the spent lithium ion batteries but also economic benefit to recover Cu(II) and Co(II) with the present market values of $3.3 and $16.8 per pound, respectively and no consumption of external energy. The in-site utilization of electricity from MFCs may also provide more environmental benefits compared to direct harvest electricity from MFCs [20]. Therefore, this self-driven MFCCu–MECCo system is an environmental friendly method to treat wastewaters from spent lithium ion batteries with the potential economic benefit. While this MECCo driven by MFCCu system represents a true sustainable process and is new to simultaneous recovery of Cu(II) and Co(II) from wastes, there are still many challenges to enable practical applications. The MFCCu coupled with MECCo might add up to
The MECCo driven by MFCCu system exhibited concomitant Cu(II) and Co(II) reduction with no consumption of external energy. System performance was heavily dependent on the cathode material of MECCo and cathode volumes in both MECCo and MFCCu. Co(II) was efficiently reduced using either TS or SSM, and CR cannot proceed this occurrence. While smaller cathode volumes in MFCCu achieved appreciable Co(II) reduction of 41.4 ± 3.8% on the CR cathode, the highest Co(II) reduction using TS (45.0 ± 0.3%) and SSM (39.7 ± 3.6%) was obtained under smaller cathode volumes in both MFCCu and MECCo. Sequential MFCCu and MECCo operation efficiently reduced Cu(II) and Co(II), achieving 100% and 65.3–72.0%, respectively with either TS or SSM. These results demonstrate cathode material of MECCo and cathode volumes in both MECCo and MFCCu were critical for efficient Co(II) reduction in MECCo driven by MFCCu with achievements of copper and cobalt recovery as well as no external energy consumption. Acknowledgments The authors gratefully acknowledge financial support from the Natural Science Foundation of China (Nos. 51178077 and 21377019), the National Basic Research Program of China (No. 2011CB936002), Specialized Research Fund for the Doctoral Program of Higher Education ‘‘SRFDP’’ (No. 20120041110026), and Program for Changjiang Scholars and Innovative Research Team in University (IRT_13R05). References [1] J. Xu, H.R. Thomas, R.W. Francis, K.R. Lum, J. Wang, B. Liang, A review of processes and technologies for the recycling of lithium-ion secondary batteries, J. Power Sources 177 (2008) 512–527. [2] M.B.J.G. Freitas, V.G. Celante, M.K. Pietre, Electrochemical recovery of cobalt and copper from spent Li-ion batteries as multilayer deposits, J. Power Sources 195 (2010) 3309–3315. [3] A. Ter Heijne, F. Liu, R. van der Weijden, J. Weijma, C.J.N. Buisman, H.V.M. Hamelers, Copper recovery combined with electricity production in a microbial fuel cell, Environ. Sci. Technol. 44 (2010) 4376–4381. [4] H. Tao, M. Liang, W. Li, L. Zhang, J. Ni, W. Wu, Removal of copper from aqueous solution by electrodeposition in cathode chamber of microbial fuel cell, J. Hazard. Mater. 189 (2011) 186–192. [5] H. Tao, L. Zhang, Z. Gao, W. Wu, Copper reduction in a pilot-scale membranefree bioelectrochemical reactor, Bioresour. Technol. 102 (2011) 10334–10339. [6] S. Cheng, B. Wang, Y. Wang, Increasing efficiencies of microbial fuel cells for collaborative treatment of copper and organic wastewater by designing reactor and selecting operating parameters, Bioresour. Technol. 147 (2013) 332–337. [7] L. Jiang, L. Huang, Y. Sun, Recovery of flakey cobalt from aqueous Co(II) with simultaneous hydrogen production in microbial electrolysis cells, Int. J. Hydrogen Energy 39 (2014) 654–663.
132
D. Wu et al. / Chemical Engineering Journal 266 (2015) 121–132
[8] L. Huang, L. Jiang, Q. Wang, X. Quan, J. Yang, L. Chen, Cobalt recovery with simultaneous methane and acetate production in biocathode microbial electrolysis cells, Chem. Eng. J. 253 (2014) 281–290. [9] M. Sun, G. Sheng, L. Zhang, C. Xia, Z. Mu, X. Liu, H. Wang, H. Yu, R. Qi, T. Yu, M. Yang, An MEC–MR-coupled system for biohydrogen production from acetate, Environ. Sci. Technol. 42 (2008) 8095–8100. [10] Y. Wang, X. Liu, W. Li, F. Li, Y. Wang, G. Sheng, R. Zeng, H. Yu, A microbial fuel cell-membrane bioreactor integrated system for cost-effective wastewater treatment, Appl. Energy 98 (2012) 230–235. [11] Y. He, F. Yan, H. Yu, S. Yuan, Z. Tong, G. Sheng, Hydrogen production in a lightdriven photoelectrochemical cell, Appl. Energy 113 (2014) 164–168. [12] K.R. Fradler, I. Michie, R.M. Dinsdale, A.J. Guwy, G.C. Premier, Augmenting microbial fuel cell power by coupling with supported liquid membrane permeation for zinc recovery, Water Res. 55 (2014) 115–125. [13] L. Huang, B. Yao, D. Wu, X. Quan, Complete cobalt recovery from lithium cobalt oxide in self-driven microbial fuel cell – microbial electrolysis cell systems, J. Power Sources 259 (2014) 54–64. [14] D. Cui, Y. Guo, H. Lee, H. Cheng, B. Liang, F. Kong, Y. Wang, L. Huang, M. Xu, A. Wang, Efficient azo dye removal in bioelectrochemical system and postaerobic bioreactor: optimization and characterization, Chem. Eng. J. 243 (2014) 355–363. [15] A. Kundu, J.N. Sahu, G. Redzwan, M.A. Hashim, An overview of cathode material and catalysts suitable for generating hydrogen in microbial electrolysis cell, Int. J. Hydrogen Energy 38 (2013) 1745–1757. [16] E. Ribot-Llobet, J.Y. Nam, J.C. Tokash, A. Guisasola, B.E. Logan, Assessment of four different cathode materials at different initial pHs using unbuffered catholytes in microbial electrolysis cells, Int. J. Hydrogen Energy 38 (2013) 2951–2956. [17] Q. Wang, L. Huang, H. Yu, X. Quan, G. Chen, Recent developments of graphene electrodes in bioelectrochemical systems, Acta Phys. Chim. Sin. 29 (2013) 889– 896.
[18] Y.M. Zhang, M.D. Merrill, B.E. Logan, The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells, Int. J. Hydrogen Energy 35 (2010) 12020–12028. [19] Q. Chen, J. Liu, Y. Liu, Y. Wang, Hydrogen production on TiO2 nanorod arrays cathode coupling with bio-anode with additional electricity generation, J. Power Sources 238 (2013) 345–349. [20] W. Li, H. Yu, Z. He, Towards sustainable wastewater treatment by using microbial fuel cells-centered technologies, Energy Environ. Sci. 7 (2014) 911– 924. [21] Y. Fan, S. Han, H. Liu, Improved performance of CEA microbial fuel cells with increased reactor size, Energy Environ. Sci. 5 (2012) 8273–8280. [22] L. Zhang, J. Li, X. Zhu, D. Ye, Q. Liao, Anodic current distribution in a liter-scale microbial fuel cell with electrode arrays, Chem. Eng. J. 223 (2013) 623–631. [23] B.E. Logan, K. Rabaey, Conversion of wastes into bioelectricity and chemicals by using microbial electrochemical technologies, Science 337 (2012) 686–690. [24] A.M. Couper, D. Pletcher, F.C. Walsh, Electrode materials for electrosynthesis, Chem. Rev. 90 (1990) 837–865. [25] Y. Liu, J. Shen, L. Huang, D. Wu, Copper catalysis for enhancement of cobalt leaching and acid utilization efficiency in microbial fuel cells, J. Hazard. Mater. 262 (2013) 1–8. [26] M.V. Alipazaga, R.G.M. Moreno, N. Coichev, Synergistic effect of Ni(II) and Co(II) ions on the sulfite induced autoxidation of Cu(II)/tetraglycine complex, Dalton Trans. 13 (2004) 2036–2040. [27] Z. An, H. Zhang, Q. Wen, Z. Chen, M. Du, Desalination combined with copper(II) removal in a novel microbial desalination cell, Desalination 346 (2014) 115– 121. [28] L. Huang, Y. Shi, N. Wang, Y. Dong, Anaerobic/aerobic conditions and biostimulation for enhanced chlorophenols degradation in biocathode microbial fuel cells, Biodegradation 25 (2014) 615–632.