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The performance of Cu2+ as dissolved cathodic electron-shuttle mediator for Cr6+ reduction in the microbial fuel cell

Abstract

The study investigates the performance of Cu2+ as dissolved cathodic electron-shuttle mediator (dcESM) for simultaneous Cr6+ reduction and electricity generation in a microbial fuel cell (MFC) at pH 2 and 4 conditions. The dcESM behavior of Cu2+ on carbon cloth (CC) catalyzes the reduction of Cr6+ into Cr3+ at pH 2 by undergoing redox reactions. However, at pH 4, a simultaneous reduction of Cu2+ and Cr6+ was observed. Cyclic voltammetry studies were performed at pH 2 and 4 to probe the dcESM behavior of Cu2+ for Cr6+ reduction on CC electrode. Also, at pH 2, increasing the concentration of Cu2+ from 50 to 500 mg L− 1 favors the Cr6+ reduction by reducing the reaction time from 108 to 48 h and improving the current production from 3.9 to 6.2 mA m− 2, respectively. Nevertheless, at pH 4, the efficacy of Cr6+ reduction and electricity generation from MFC is decreased from 63 to 18% and 4.4 to 1.1 mA m− 2, respectively, by increasing the Cu2+ concentration from 50 to 500 mg L− 1. Furthermore, the performance of dcESM behavior of Cu2+ was explored on carbon felt (CF) and platinum (Pt) electrodes, and compare the results with CC. In MFC, at pH 2, with an initial concentration of 100 mg L− 1, the reduction of Cr6+ in 60 h is 9.6 mg L− 1 for CC, 0.2 mg L− 1 for CF, and 51.3 mg L− 1 for Pt cathodes. The reduction of Cr6+ (initial concentration of 100 mg L− 1) at pH 4 in 120 h is 44.7 mg L− 1 for CC, 32.1 mg L− 1 for CF, and 70.9 mg L− 1 for Pt cathodes. Maximum power densities of 1659, 1509, and 1284 mW m− 2 were achieved when CF, CC, and Pt, respectively were employed as cathodes in the MFC.

Introduction

In recent years, hexavalent chromium (Cr6+) is exceedingly prevalent in various industrial effluents, and is often discharged from metallurgy, electroplating, leather tanning, and textile industries [1]. Cr6+ is a well-known mutagen, teratogen, and carcinogen [2]. The discharge of Cr6+ to the environment is of critical concern because: (i) of its non-biodegradable nature; (ii) it undergoes various transformations and forms toxic, carcinogenic compounds; and (iii) it is bioaccumulative [3]. The existing traditional treatment techniques for the removal of Cr6+ are ion exchange, adsorption/biosorption, coagulation-flocculation, chemical precipitation, electrochemical method, biological reduction, and membrane filtration [4]. Although these techniques are highly promising, long-term applications are often hindered due to high operational/maintenance costs, additional energy requirement, and the formation of large secondary toxic sludge.

In the past decade, microbial fuel cells (MFCs) have received enormous attention as a promising technology for wastewater treatment coupled with electricity generation [5,6,7]. MFCs are devices that use exoelectrogenic bacteria to oxidize the organic matter in the anode chamber, thereby producing protons and electrons. The protons drift internally through a proton exchange membrane (PEM), while the electrons migrate externally to the cathode chamber, where they are reduced to form water by an appropriate catholyte [8, 9]. The anode chamber of the MFC is highly versatile to treat simple organic compounds like acetate [10] and glucose [11] to complex wastewater such as brewery [12], distillery [13], and starch [14]. Besides, the cathode chamber of the MFC is successfully employed for treating metal-laden wastewater containing single metal ions such as cobalt [15], copper [16], silver [17], chromium [3, 18], and selenite [19]. Although MFCs offer promising solutions for the removal of Cr6+ [3, 18], the slow reaction kinetics and long operating time due to high cathodic overpotential hinder it from large-scale applications [20, 21]. Recently, Krishnani et al. [22] utilized various conductive polymers such as polyaniline, polypyrrole, polyaniline nanowires and palladium-decorated polyaniline for Cr6+ reduction due to their electrical properties (like that of a semiconductor) and mechanical strength. Pang et al. [20] have reported an MFC employed with graphite felt coated conductive polypyrrole that reduces the cathodic overpotential and improves the electron shuttling at the cathode-catholyte interface for Cr6+ reduction. Nevertheless, their long-term applications are still limited due to complicated synthesis methods, poor dispersibility, weak stability, and low conductivity [23].

Recently, environmentally benign and cost-effective dissolved cathodic electron-shuttle mediators (dcESMs) have drawn wide attention as they mediate and expedite the reduction of Cr6+ in MFC. The dcESM can encourage electron transfer between the microbes or from cathode to the microbes, and/or from microbes to the electron-accepting compounds [24]. They exhibit reversible redox reactions and thereby improve the kinetics of Cr6+ reduction by diminishing the electrical repulsion between the negatively charged cathode and the electron acceptors (Cr2O72− or CrO42−) [21, 25]. Also, due to their high solubility, they can quickly equilibrate the charges with cathode and electron-accepting substances. Liu et al. [26] reported improved reduction of Cr6+ using H2O2 as a dcESM. However, its long-term operation was hindered due to poor oxygen reduction kinetics. Wang et al. [21] noticed Fe3+ as a dcESM, decreases diffusion resistance and cathodic overpotential, and hence enhances the Cr6+ reduction. Although Fe3+ improves the Cr6+ reduction, the power production was decreased by 36% due to the loss of Fe3+ via reduction.

As an alternative, copper (Cu2+) can be used as a dcESM due to its excellent catalytic behavior. Cu2+ improves the biocathode performance in MFC by promoting the electron transfer between cathode and microbes [27,28,29]. Recently, Li and Zhou [25] have explored Cu2+ as a sole dcESM for Cr6+ reduction in the abiotic cathode of the MFC. When Cu2+ is employed as a dcESM for Cr6+ reduction, the high reduction potentials of Cr6+ and Cu2+ (at 25 °C; Eqs. (1) and (2), respectively) lead them electrochemically reduce to chromium oxide (Cr2O3) and Cu0, respectively under closed-circuit conditions of the MFC [2, 25, 30].

$$ {\mathrm{Cr}}_2{\mathrm{O}}_7{2}^{\hbox{-} }+8{\mathrm{H}}^{+}+6{\mathrm{e}}^{\hbox{-}}\leftrightarrow {\mathrm{Cr}}_2{\mathrm{O}}_3+4{\mathrm{H}}_2\mathrm{O}\kern0.75em \left(\mathrm{E}{}^{\circ}=1.33\;\mathrm{V}\right) $$
(1)
$$ {\mathrm{Cu}}^{2+}+2{\mathrm{e}}^{\hbox{-}}\leftrightarrow {\mathrm{Cu}}^0\ \left(\mathrm{E}{}^{\circ}=0.337\;\mathrm{V}\right) $$
(2)

However, the dcESM behavior of Cu2+ is precisely influenced by cathode potential (Eh) and pH of the catholyte [16]. The Eh and pH of Cu2+ mediated Cr6+ reduction is calculated (calculations are shown in the Supplemental Materials) and is shown in Fig. 1a. When pH drops below a critical value (pH ≤ 3.2; Fig. 1a), Cu2+ behaves as a dcESM by undergoing a redox process which expedites the reduction of Cr6+ as shown in Eqs. (3) and (4) (at 25 °C). For instance, Cu2+ that are reduced to Cu1+ acts as the electron donor for the reduction of Cr6+ to Cr3+, while at the same time, the Cu1+ is oxidized back to Cu2+ (Eqs. (3) and (4)). This is because, when pH drops below a critical value, the oxidation state of Cu is + 2 (Fig. 1a); hence, it undergoes the redox process and exhibits dcESM phenomenon. While the valence state of Cr is + 6 and it thermodynamically favors the reduction of Cr6+ to Cr3+ in the presence of an ideal electron donor, Cu2+.

$$ {{\mathrm{H}\mathrm{CuO}}_2}^{\hbox{-} 1}+3{\mathrm{H}}^{+}+{\mathrm{e}}^{\hbox{-}}\leftrightarrow {\mathrm{Cu}}^{+}+2{\mathrm{H}}_2\mathrm{O}\kern0.48em \left({\mathrm{E}}_0=1.73\ \mathrm{V}\right) $$
(3)
$$ 3{\mathrm{Cu}}^{+}+{{\mathrm{H}\mathrm{CrO}}_4}^{\hbox{-} 1}+7{\mathrm{H}}^{+}\leftrightarrow {\mathrm{Cr}}^{3+}+3{\mathrm{Cu}}^{2+}+4{\mathrm{H}}_2\mathrm{O}\kern0.6em \left({\mathrm{E}}_0=1.178\ \mathrm{V}\right) $$
(4)
Fig. 1
figure1

a The equilibrium Eh–pH diagram of Cr-Cu-H2O system; b The experimental set-up of MFC

On the other hand, at pH above a critical value (pH ≥ 3.2), Cu2+ undergo reduction to stable cuprous oxide (Cu2O) (Eq. (5) and Fig. 1a), hindering the dcESM performance of Cu2+ on Cr6+ reduction.

$$ 2{\mathrm{Cu}}^{2+}+{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{\hbox{-}}\leftrightarrow {\mathrm{Cu}}_2\mathrm{O}+2{\mathrm{H}}^{+}\kern0.72em \left({\mathrm{E}}_0=0.203\ \mathrm{V}\right) $$
(5)

Although the mechanism of Cu2+ in mediating Cr6+ reduction has been demonstrated, the dcESM behavior of Cu2+, particularly in acidic conditions, has not yet been reported in any literature. Hence, the present study intends to: (i) demonstrate the dcESM behavior of Cu2+ on Cr6+ reduction at pH 2 and 4 (the two pH conditions were selected based on the above theoretical evidence) using carbon cloth (CC) as the electrode in MFC and elucidate the mechanism by cyclic voltammetry (CV) analysis; (ii) determine the effect of Cu2+ concentration for Cr6+ reduction and electricity production in the MFC; and (iii) compare the performance of Cu2+ as dcESM on CC with carbon felt (CF) and platinum (Pt) cathodes in the MFC for Cr6+ reduction and electricity generation.

Materials and methods

MFC construction

The experimental setup of an MFC is shown in Fig. 1b. A two-chambered reactor (each chamber having dimensions 10.5 × 10 × 12 cm; 500 mL capacity; 300 mL working volume) was made with a plexiglass acrylic tube. The chambers were arranged directly adjacent to each other by a PEM (Nafion 117; Sigma-Aldrich; projected surface area of 50.24 cm2), and a square-ring with rubber gaskets were held in between the PEM to maintain air-tight condition. PEM was subsequently pretreated with 30% H2O2, deionized water, 0.5 M H2SO4, and deionized water again, for 1 h each [31].

The anode was made up of a low molecular heterocyclic aminopyrazine (Apy)-reduced graphene oxide (r-GO) hybrid coated CC (r-GO-Apy-CC; 25 cm2). The anode material was selected based on one of our previous studies [23], where the r-GO-Apy-CC electrode was found to exhibit excellent bioelectrocatalytic activity for both bacterial adhesion and current generation. Electrodes such as CC (25 cm2; Synergic India Pvt. Ltd. India), CF (25 cm2; Synergic India Pvt. Ltd. India) and Pt (25 cm2; Kevin Scientific, Chennai) were used as cathodes, and are connected externally to the anode by a copper wire. The electrodes were previously soaked in deionized water for 24 h, subsequently dried in the oven at 100 °C for 15 min and were placed at 5 cm apart on either side of the PEM. The anode and cathode chambers were continuously purged with nitrogen to maintain anaerobic conditions. The anodic and cathodic pH was continuously monitored by employing an online pH sensor provided with a multi-channeled data acquisition system (Aqua controller Model 980 AP, Adsensor; India). Fluctuations in the pH of the two chambers were monitored by using online indicators (Fig. 1b).

Inoculation

The anode chamber of the MFC was inoculated with anaerobic sludge collected from anaerobic digester of the sewage treatment plant, Nesapakkam, Chennai, India. The sludge was washed with 0.85% NaCl (w/v) solution and subjected to heat shock pretreatment (100 °C, 2 h) to suppress the activity of methanogens [32]. Sodium acetate (pH value 7.0) was used as the carbon source in the anode chamber of the MFC. For inoculation, 50 mL of dewatered anaerobic sludge was added to 250 mL of synthetic wastewater containing macronutrients as NH4Cl, 125 mg L− 1; NaHCO3, 125 mg L− 1; MgSO4·7H20, 51 mg L− 1; CaCl2·2H2O, 300 mg L− 1; FeSO4·7H2O, 6.25 mg L− 1 and 1.25 mL L− 1 of trace metal solution as reported in Lovley and Phillips [33]. Nitrogen gas was purged continuously to maintain anaerobic conditions in the anode and cathode chambers. Synthetic electroplating wastewater was used as a catholyte and was prepared by mixing an appropriate quantity of potassium dichromate (K2Cr2O7; 99%; Sigma-Aldrich) and copper sulfate (CuSO4; 99%; Sigma-Aldrich) with deionized water. The initial concentrations of Cu2+ and Cr6+ were maintained at 100 mg L− 1. To study the effect of Cu2+ concentration on Cr6+ reduction, the initial concentration of Cr6+ was maintained at 100 mg L− 1, and the different concentrations of Cu2+ of 50, 100, 300, 500 mg L− 1 were added. The conductivity of the catholyte was improved by adding NaCl (11.7 g L− 1). The pH of the influent solution was adjusted with H2SO4 (0.1 M) and NaOH (0.1 M). All the experiments were carried out at ambient temperature.

Measurements and analyses

Hexavalent chromium was analyzed by UV-Vis spectrophotometer (UV-1800 PC, Shimadzu) at 540 nm. Total copper was measured using Atomic Absorption Spectrophotometry (AAnalyst 700, Perkin Elmer) after sampling at regular intervals. The removal efficiency was calculated using Eq. (6).

$$ \mathrm{Removal}\ \mathrm{efficiency}\ \left(\%\right)=\frac{\left(A-B\right)}{A}\times 100 $$
(6)

where A and B are the initial and observed concentrations in mg L− 1.

A precise 4-channel potentiostat (VSP 300 Biologic; India) was employed to investigate the electrochemical characteristics of the system. Current (I) was calculated according to Ohm’s law, I = V/R, where R is the external resistance. Power (P) is the product of voltage V and I (P = IV). Power and current densities were calculated by dividing the respective terms by the cathode surface area (m2). The polarization study was performed by employing the I-V characterization technique using a three-electrode system, where the working electrode was a cathode, the counter electrode was an anode, and reference electrode was saturated Ag/AgCl (+ 0.197 V vs. SHE), that was placed close to the cathode.

The electrochemical activity of the electrode was examined by CV analysis in a separate single-cell system. The experiment was performed in a high purity quartz glass beaker (500 mL) with an airtight polytetrafluoroethylene cap, mounted over silicon encapsulated polytetrafluoroethylene ring. The cell system consists of a counter electrode of standard Pt, reference electrode of Ag/AgCl, and the working electrode of CC. CV analysis was conducted in the cell system deployed with Cu2+ mediated Cr6+ solution (1:1), and the performance was compared with a controlled solution of Cr6+ and Cu2+ solution. During CV analysis, the working electrode potential in controlled Cu2+ and Cu2+ mediated Cr6+ solutions were linearly scanned from + 1 V to − 1 V at a scan rate of 10 mV s− 1. In controlled Cr6+ solution, the potential was linearly scanned from + 0.5 to + 1.5 V for pH 2, and + 0.5 to 1.0 V for pH 4 at a scan rate of 10 mV s− 1. High–resolution scanning electron microscopy (FEI Quanta 200 FEG) equipped with energy–dispersive x-ray spectroscopy was employed to confirm Cr3+ and Cu2O monolayer formation on cathode surfaces for a specific period.

Results and discussion

The dcESM behavior of Cu2+ for Cr6+ reduction

The dcESM behavior of Cu2+ on Cr6+ reduction (initial concentrations of 100 mg L− 1; 1:1 ratio) was characterized on CC cathode in MFC at pH 2 and 4. The temporal behavior of Cu2+ and Cr6+, and corresponding variations in pH were monitored at pH 2 and 4. The experimental result at pH 2 displays a large reduction in Cr6+ to Cr3+ with an efficiency of 99.9% at 84 h of operating time (Fig. 2a). In contrast, the reduction of Cu2+ was not significant at pH 2. A slight decrease (25%) in the Cu2+ concentration was noticed within the initial few hours, probably, due to the reduction of Cu2+ to Cu1+ (Eq. (3)). However, at pH 4, a considerable reduction of Cu2+ along with Cr6+ was observed.

Fig. 2
figure2

a Temporal behavior of Cr6+ and Cu2+ and corresponding change in the pH value at pH 2; b Cyclic voltammograms of CC electrode in Cu2+, Cr6+, and Cr6+ & Cu2+ mediated solution at pH 2

The results were interpreted that the reduction of Cr6+ in the presence of Cu2+ involves heterogeneous reactions. At pH 2, Cr6+ and Cu2+ are highly protonated by surrounding the H+ ions and predominantly exist in the form of HCrO4− 1 and HCuO2− 1, respectively as shown in the reactions Eqs. (7) and (8). During the initial 10 h, a decrease in the pH was noticed from the initial pH (pH 2) due to the hydrolysis of Cu2+ which involves an increase in the H+ ions in the solution as shown in Eq. (8).

$$ {\mathrm{Cr}}_2{{\mathrm{O}}_7}^{2\hbox{-} }+{\mathrm{H}}_2\mathrm{O}\leftrightarrow 2{{\mathrm{H}\mathrm{CrO}}_4}^{\hbox{-} } $$
(7)
$$ {\mathrm{Cu}}^{2+}+2{\mathrm{H}}_2\mathrm{O}\leftrightarrow {{\mathrm{H}\mathrm{CuO}}_2}^{\hbox{-} 1}+3{\mathrm{H}}^{+} $$
(8)

Besides, a slight reduction in the Cu2+ (25%) was observed at this stage, probably due to the cathodic reduction of HCuO2− 1 to Cu+ in the presence of H+ ions as explained in Eq. (3) [34]. However, a steady-state reduction in Cr6+ concentration was observed throughout the experiment. This could be correlated with Eqs. (3) and (4) and explained as: the Cu2+ that is reduced to Cu1+ acts as an electron donor for the reduction of Cr6+ to Cr3+, while at the same time, the Cu1+ is oxidized back to Cu2+. Correspondingly, the pH of catholyte was increased owing to the consumption of H+ ions as indicated in Eqs. (3) and (4). The color of the wastewater was changed from orange-yellow to greenish-yellow after 48 h, indicating, the complete reduction of Cr6+ to Cr3+.

A separate CV analysis was conducted to confirm the dcESM behavior of Cu2+ on Cr6+ reduction at pH 2 using a single cell system with CC as a working electrode (Fig. 2b). At pH 2, CV analysis of controlled Cu2+ solution exhibits well-defined reduction and oxidation peaks at − 0.36 and + 0.36 V, respectively, which could be attributed to the dcESM behavior of Cu2+ on CC electrode (Fig. 2b) [35]. In a mixture of Cu2+ and Cr6+ solution, CV analysis was repeated for 70 cycles at a scan rate of 10 mV s− 1. During the forward scan, a large cathodic current was drawn after 0.23 V was due to the combined reduction of Cu2+ and Cr6+, and on the reverse scan, the peak observed at − 0.34 V was correlated with the oxidation of Cr2+ to Cr3+ (Cycle 5; Fig. 2b) [36]. The additional peak observed at − 0.01 V (Cycle 30; Fig. 2b) can be attributed to the oxidation of Cu1+ to Cu2+. While continuing the scan up to 70 cycles, the peak at − 0.34 V was observed to be diminished due to the stable and insoluble Cr3+ formation. Concurrently, the peak observed at − 0.01 V was found to increase due to the increases in Cu1+ concentration. The CV study confirms the dcESM behavior of Cu2+ on Cr6+ reduction at pH 2 and complements with the reported experimental results.

The proposed mechanism for the reduction of Cr6+ and Cu2+ at pH 2 and 4 are shown in Fig. 3. At pH 4, simultaneous reduction of Cu2+ and Cr6+ was observed with removal efficiencies of 71 and 56%, respectively (Fig. 4a). At this pH condition, dcESM behavior of Cu2+ was not observed; instead, simultaneous reduction of Cr6+ and Cu2+ was noticed. This could be due to the stable form of Cu2O which hinders the dcESM behavior of Cu2+ for Cr6+reduction. However, simultaneous reductions of Cu2+ and Cr6+ were occurred owing to the high cathodic potential as presented in Eqs. (1) and (5). The pH was observed to increase from 4 to 5.54 and the color of the effluent was changed from orange-yellow to pale yellow. In CV analysis, the peak observed at − 0.34 and − 0.01 V at pH 2 conditions were absent at pH 4 and can be attributed to the irreversibility of the Cu2+ and Cr6+ as stable Cu2O and Cr2O3, respectively (Fig. 4b).

Fig. 3
figure3

The proposed mechanism for the reduction of Cr6+ and Cu2+ at pH 2 and 4

Fig. 4
figure4

a Temporal behavior of Cr6+ and Cu2+ and corresponding change in the pH value at pH 4; b Cyclic voltammograms of CC electrode in Cu2+, Cr6+, and Cr6+ & Cu2+ mediated solution at pH 4

Effect of Cu2+ concentration on Cr6+ reduction and electricity production

Studies were performed by varying the Cu2+ concentration at pH 2 and 4 to understand the effect of Cu2+ and its dcESM behavior on Cr6+ reduction and bioelectricity generation. The results elucidate that, at pH 2, the presence of Cu2+ is highly favorable for the reduction of Cr6+. Increasing the Cu2+ concentration from 50 to 500 mg L− 1 reduces reaction time from 108 to 48 h for the complete reduction of Cr6+ (Fig. 5a). Similarly, the current production was improved from 3.94 to 6.24 mA m− 2 by increasing the Cu2+ concentration from 50 to 500 mg L− 1 (Fig. 5b). However, at pH 4, the presence of Cu2+ decreases the reduction of Cr6+. By increasing the Cu2+ concentration from 50 to 500 mg L− 1, the reduction efficiency of Cr6+ was observed to decrease from 63 to 18% (Fig. 5c). Correspondingly, the response in the current density was decreased from 4.4 mA m− 2 (616 mV) to 1.1 mA m− 2 (155 mV), respectively (Fig. 5d).

Fig. 5
figure5

Effect of Cu2+ concentration at pH 2: a temporal reduction of Cr6+; b response in the current density. Effect of Cu2+ concentration at pH 4: a temporal reduction of Cr6+; b response in the current density

At pH 2, increasing the Cu2+ concentration improves the reduction of Cr6+ and electricity generation from MFC. This could be due to the dcESM behaviour of Cu2+ improves the kinetics of Cr6+ reduction by diminishing the electrical repulsion between the negatively charged cathode and the Cr2O72−anions in the catholyte [21, 25]. As the rate of electron transfer improves, the cathodic over potential decreases resulting in an improvement in the generation of electricity. On the other hand, at pH 4, increasing the concentration of Cu2+ decreases the reduction kinetics of Cr6+ as well as the responses in the current density. At pH 4, Cu2+ was electrochemically reduced to Cu2O (Eq. (5) and Fig. 1a) and deposited over the electrode surface (Fig. 7b). The deposition of Cu2O increases with increase in the Cu2+ concentration. This results in higher cathodic overpotential at the cathode-catholyte interface that hinders the kinetics of Cr6+ reduction and current production at pH 4. In both the pH conditions, the temporal response of the current density was found to be decreased as the experiment progresses. The trend can be correlated with the decrease in the catholyte concentration due to the reduction of Cr6 and can be theoretically explained by the Nernst equation as in Eqs. (9) and (10).

$$ E\mathrm{cathode}-E0=-\frac{2.303\ RT}{nF}\mathit{\log}\frac{\left[ Cr3+\right]}{\left[ Cr6+\right]} $$
(9)
$$ \upeta \mathrm{c}=-\frac{2.303\ RT}{nF}\mathit{\log}\frac{\left[ Cr3+\right]}{\Big[ Cr6+} $$
(10)

According to the Eq. (10), overpotential (ηc) becomes more negative as the concentration of Cr3+ increases, causing a decrease in the cell potential [37, 38]. Eventually, the response in the current density decreases as the experiment progresses. When the rate of reduction of Cr6+ reaches an asymptotic state, the response of the current density exhibits an analogous behavior to the concentration of Cr6+.

The dcESM behavior of Cu2+ on CF and Pt electrodes

The dcESM behavior of Cu2+ on Cr6+ reduction (initial concentrations of 100 mg L− 1; 1:1 ratio) was characterized on Pt and CF in MFC at pH 2 (Fig. 6a) and pH 4 (Fig. 6b), and the results were compared with CC electrode. It was observed that, at pH 2, the reduction of Cr6+ to Cr3+ in 60 h is 99.8% for CF cathode, 48% for Pt cathode, and 90% for CC cathode. As expected, not much reduction of Cu2+occured in all the electrodes at pH 2 (Fig. 6a). The color of the wastewater was changed from orange-yellow to greenish-yellow, and subsequently, a stable blue color solution when CC and CF were employed as the cathode material. However, in the case of Pt as a cathode, the color of the wastewater was changed moderately from orange-yellow to pale yellow. At pH 4, the simultaneous reduction of Cr6+ and Cu2+ was observed for Pt and CF electrodes. The reduction of Cr6+ at pH 4 in 120 h is 68% for CF cathode, 29% for Pt cathode, and 56% for CC cathode (Fig. 6b). Similarly, Cu2+ reduction in 120 h is 75% for CF cathode, 50% for Pt cathode, and 71% for CC cathode. The results indicate that the dcESM behavior of Cu2+ on Cr6+ reduction is exhibited not only on CC but also in CF and Pt cathodes in MFC.

Fig. 6
figure6

dcESM behavior Cu2+ on CC, CF, and Pt: a at pH 2; b at pH 4

The surface morphological characteristics of the Cr3+ and Cu2O on the surface of Pt, CC, and CF were analyzed by performing the SEM analysis (Fig. 7a, b, and c, respectively), and the image clearly shows the nucleation of Cr3+ or Cr2O3 and/or Cu2O on the cathode surface. Furthermore, polarization studies were performed with CC, CF and Pt to understand the efficiency of electrode material for power production (Fig. 7d). The maximum power densities of 1659 mW m− 2 (4.9 mA m− 2), 1509 mW m− 2 (5.5 mA m− 2), and 1284 mW m− 2 (5.1 mA m− 2) were achieved when CF, CC, and Pt were employed as cathode materials. The study confirms that carbon-based electrode materials are ideal for bioelectricity generation from MFC.

Fig. 7
figure7

SEM image of Cr and Cu adsorbed on the surface of cathode: a Pt; b CC; c CF; d Polarization curve

Practical implication to scale up MFC technology

In summary, MFCs apply a simple redox principle in which the sole influencing factor for the entire electrochemical reductions is the degradation of organic matter (The details are provided in Supplementary Materials) by exoelectrogenic microorganisms at the anode chamber. At the cathode, Cr6+ electrochemically reduces to Cr3+ by accepting the electrons from the anode chamber. The use of dissolved electron–shuttle mediators reduces the activation energy at the cathode-electrolyte interface and improves the cathode performance for Cr6+ reduction and bioelectricity generation. In fact, Cu2+ is a potential contaminant, and extreme consumption of copper leads to severe toxicological concerns, such as nausea, contractions, convulsions, or even death [16, 39]. Hence, incorporating Cu2+ as a dissolved mediator in MFC is a sustainable approach because Cu2+ not only enhances the reduction of Cr6+ but also reduces to its most stable, Cu2O form, simultaneously. The findings demonstrated in the present study are highly significant to the electroplating industry where a combination of Cr6+ and Cu2+ are discharged at high acidic conditions from washing, rinsing, batch dumps, and processing and/or operational units. Presently, chemical coagulation/precipitation is the most widely adopted treatment technique for electroplating wastewater. However, this technique involves high operational costs due to the consumption of large amounts of chemicals and the generation of a huge quantity of sludge. Instead, the use of Cu2+ as a dcESM for Cr6+ reduction is a cost-effective method as it does not require any addition of chemicals, and both often co-exist in the effluents discharged from electroplating or mining industries. However, further studies are required to evaluate the long-term operation conditions and process economy of MFC over influent. Also, pilot/full-scale studies are needed for the practical implementation of this technology in industries.

Conclusions

In the present work, dcESM phenomenon of Cu2+ on Cr6+ reduction in MFC is reported using CC, CF, and Pt electrodes. The dcESM behavior of Cu2+ for Cr6+ reduction is highly influenced by Eh and pH of the catholyte in MFC. In acidic conditions, when the pH is below a critical point of 3.2, Cu2+ improves the reduction of Cr6+ to 99.9% at 84 h of operating time. On the other hand, above the critical pH point, simultaneous reduction of Cu2+ and Cr6+ was observed with removal efficiencies of 71 and 56%, respectively. Hence, the Cu2+ and its role as dcESM in MFC is highly advantageous as it not only enhances the Cr6+ reduction but also undergoes electrochemical reduction into non-toxic Cu2O. Since Cu2+ on Cr6+ often co-exist in the wastewater from electroplating or mining industries, and due to their synergy, the study provides an entirely new concept of ‘using a pollutant to treat another one’ with simultaneous generation of energy.

Availability of data and materials

The data used to support the findings of this study are available from the corresponding author upon request.

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Acknowledgements

The authors gratefully acknowledge the financial support of DST, India [No. SR/WOS-A/ET-1017/2014] in carrying out this research work.

Funding

This work was supported by DST, India [No. SR/WOS-A/ET-1017/2014].

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Praveena Gangadharan planned and conducted the experiments. Indumathi M Nambi supervised the project. The author(s) read and approved the final manuscript.

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Correspondence to Praveena Gangadharan.

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Gangadharan, P., Nambi, I.M. The performance of Cu2+ as dissolved cathodic electron-shuttle mediator for Cr6+ reduction in the microbial fuel cell. Sustain Environ Res 30, 19 (2020). https://doi.org/10.1186/s42834-020-00059-3

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Keywords

  • Microbial fuel cell (MFC)
  • Heavy metal removal
  • Hexavalent chromium
  • Copper
  • Wastewater treatment
  • Bioelectricity generation