Green Energy Production and Integrated Treatment of Pharmaceutical Wastewater Using MnCo₂O₄ Electrode Performance in Microbial Fuel Cell

Abstract

The wastewater produced by the pharmaceutical industry is highly organic and toxic. Dual-chambered microbial fuel cells (DMFCs) may represent a sustainable solution to process wastewater while simultaneously recovering its energy content. DMFCs are bio-electrochemical devices that employ microorganisms to transform the chemical energy of organic compounds into electrical energy. This study aims to demonstrate the feasibility of a DMFC with a manganese cobalt oxide-coated activated carbon fiber felt (MnCo₂O₄-ACFF) electrode to treat pharmaceutical industry wastewater (PW) and exploit its energy content. The proposed technology is experimentally investigated considering the effect of the organic load (OL) on the system performance in terms of organic content removal and electricity production. As per the experimental campaign results, the optimum OL for achieving maximum removal efficiencies for total chemical oxygen demand, soluble oxygen demand, and total suspended solids was found to be 2 g COD/L. At this value of OL, the highest current and power densities of 420 mA/m² and 348 mW/m² were obtained. Therefore, based on the outcomes of the experimental campaign, the (MnCo₂O₄-ACFF) electrode DMFC technique was found to be a sustainable and effective process for the treatment and energy recovery from PW.

1. Introduction

Pharmaceutical compounds of various kinds, including antibiotics and analgesics, are produced and used on a massive scale around the world. Over the last few years, the pharmaceutical industry has significantly developed [1]. Indeed, these pharmaceutical compounds are useful to prevent, treat, and cure various infections and diseases [2] and therefore are largely employed in the sectors of animal and human health [3]. The sources of pharmaceutical pollutants include various industries; hospital effluents; agricultural runoffs; and sewers. The term “pharmaceutical wastewater” (PW) predominantly refers to the wastewater with pollutants produced during the production of pharmaceuticals [4]. Various pollutants such as aspirin, diclofenac, ibuprofen, naproxen and paracetamol are present in this pharmaceutical wastewater. The European Union and the US Environmental Protection Agency define pharmaceutical pollutants as pollutants of growing concern [5]. These pharmaceutical pollutants may affect humans in many ways such as heart arrhythmia, immunological system trouble, hepatic dysfunction, and bone marrow suppression [6]. In particular, antibiotic pollution, especially in case of long-term exposure, negatively affects human health, particularly for those suffering from chronic illnesses including obesity, diabetes, and asthma [7]. During the COVID-19 epidemic, globally, there was also a significant increase in antidepressant contamination [8]. Moreover, the usage of antibiotics affects the surface and groundwater quality, whereas the release of diclofenac and ketoprofen in surface water bodies may cause circulatory abnormalities and cardiac defects in fishes such as Danio rerio and Clarias gariepinus [9].

To overcome these problems, the proper treatment of PW is essential. Various methods such as chlorination, ultrafiltration, ozonation, electrocoagulation, adsorption and sono-electrochemical catalytic oxidation may be implemented to remove the pollutants from PW [10]. Although these methods are effective in treating PW, they demand a significant amount of space, energy, and expensive chemicals, and also hazardous by-products are produced from them. Hence, effective and eco-friendly methods, such as bio-electrochemical systems, can be used to treat the PW. Dual-chambered microbial fuel cells (DMFCs) can effectively remove the hazardous contaminants from the wastewater producing at the same time electricity sustainably by catalytic oxidation and chemical reduction [11]. Pollutant removal efficiency and energy generation are the parameters used to evaluate the performance of DMFCs [12]. As an example, Khalili et al. (2017) [13] utilized DMFCs for treating septic wastewater, finding a maximum COD removal efficiency of 50% and power output of 6.4 W/m³.

DMFCs convert the pollutants present in wastewater into electricity, employing microorganisms as a biocatalyst [14]. A proton exchange membrane was positioned between the two chambers (anode and cathode) in a conventional DMFC. An electrical load in the circuit is the external resistance that normally connects both chambers. Protons and electrons are produced by the inoculum microorganisms, which biologically break down the wastewater in the anode chamber [15]. After passing through the proton exchange membrane, the protons form a water molecule that reacts with oxygen, and the electrons are then transferred in a closed-circuit mode. Therefore, as a result, power is produced and pollutants are removed from the wastewater [16,17]. DMFCs have several advantages such as less sludge production, easy scalability and flexibility, enhanced removal of organic pollutants, etc.

To date, wastewater from different industries such as rice mills, dairy sector, surgical cotton and distilleries has been treated using DMFC technology [18]. However, the treatment of PW using manganese cobalt oxide-coated activated carbon fiber felt (MnCo₂O₄-ACFF) electrode DMFCs in pollutant removal and power production has not yet been explored. So, the novelty of this work consists of demonstrating the feasibility of using (MnCo₂O₄-ACFF) electrode DMFCs for green electricity production and PW treatment. More in detail, the work aims to (i) determine the impact of OL on (MnCo₂O₄-ACFF) electrode DMFC treatment for PW; (ii) assess the impact of using phosphate buffer as a catalyst to improve electricity production; and (iii) assess the performance of the proposed solution for pollutant removal and electrical energy production.

2. Materials and Methods

2.1. Collection of Sample and Preliminary Characterization

PW was collected from Kausikh Therapeutics (P) Ltd. in Gerugambakkam, Chennai, Tamil Nadu, India. In order to analyze its physicochemical characteristics, including pH, total solids (TSs), total suspended solids (TSSs), total dissolved solids (TDSs), biological (BOD), and chemical oxygen demand (COD), the collected sample was kept at 4 °C. The overall physicochemical characteristics of PW, shown in

Table 1. The analyzed physico-chemical parameters of PW sample.

Parameter	Value
pH	8.3
Total Solids (TSs)	5800 (mg/L)
Total Dissolved Solids (TDSs)	1800 (mg/L)
Total Suspended Solids (TSSs)	4000 (mg/L)
Total Chemical Oxygen Demand (TCOD)	4500 (mg/L)
Biochemical Oxygen Demand (BOD)	1300 (mg/L)

, were determined through the APHA (2017) [19] methods. The total chemical oxygen demand (TCOD) comprises both SCOD and PCOD:

-

Soluble chemical oxygen demand (SCOD)—Soluble form of organics;

-

Particulate chemical oxygen demand (PCOD)—Insoluble form of organics.

The bacteria in the inoculum can access more easily the soluble form of biomass rather than the solid form. Hence, in this study, the amount of contaminants oxidized in PW during its treatment was determined using TCOD and SCOD parameters. This analysis of COD includes sulfuric acid (H₂SO₄), a silver catalyst, and a strong oxidant—typically potassium dichromate (K₂Cr₂O₇)—with a known volume of PW. In a reflux system, the resulting mixture is heated for a predetermined period. The unreacted dichromate is then titrated with a standard ferrous ammonium sulfate (FAS) solution to determine the amount of COD. Equations (1) and (2) were utilized to determine the amount of COD (in mg/L) and COD removal efficiency (as %), respectively.

$$COD=\left(F_B-F_S\right)·M_F·8000$$

(1)

$$\mathrm{C}\mathrm{O}\mathrm{D} \mathrm{r}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l} \mathrm{e}\mathrm{f}\mathrm{f}\mathrm{i}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{n}\mathrm{c}\mathrm{y}=\frac{{BT}_{COD}-{AT}_{COD}}{{BT}_{COD}}$$

(2)

where ${\mathit{F}}_{\mathit{B}}$ is the ferrous ammonium sulfate (FAS) consumed in the blank, ${\mathit{F}}_{\mathit{S}}$ is the FAS consumed in the sample, ${\mathit{M}}_{\mathit{F}}$ is the FAS molarity, 8000 is the oxygen milliequivalent weight × 1000 mL/L, ${\mathit{B}\mathit{T}}_{\mathit{C}\mathit{O}\mathit{D}}$ is the COD present in the sample before treatment, and ${\mathit{A}\mathit{T}}_{\mathit{C}\mathit{O}\mathit{D}}$ is the COD present in the sample after treatment.

2.2. Fabrication of MnCo₂O₄-ACFF Electrode

In 80 mL of distilled water, 24.89 mg of manganese chloride (Mncl₂), 70.1 mg of cobalt chloride (CoCl₂), and 19.1 mg of urea (CH₄N₂O) were initially dissolved. MnCl₂, CoCl₂ and CH₄N₂O were bought from Merck Darmstadt, Germany. After that, the activated carbon fiber felt was immersed in this solution for 16 h at 180 °C in an autoclave covered with Teflon. The coated membranes were allowed to cool to room temperature and then washed with distilled water. Thereafter, they were vacuum-dried for 24 h at 60 °C. The coated electrodes were characterized by cyclic voltammetry and electrochemical impedance spectroscopy [20].

2.3. Experimental Setup of DMFC

The experimental setup of the DMFC is shown in Figure 1. The DMFC was realized by using plexiglass. Here, the anode and cathode chambers of the DMFC were maintained in a batch mode with 500 mL operational capacity. The chambers were fabricated with the dimensions of 11.2 × 7 × 6 cm³. An electrode made of MnCo₂O₄-ACFF measuring 10 × 10 = 100 cm² and 3mm thick was placed in each chamber. The electrode is a primary component in determining the performance of the DMFC. When compared to other commonly used electrode types, MnCo₂O₄-ACFF in the DMFC offers several benefits, including strong mechanical properties, excellent chemical stability, conductivity, surface area, increased adsorption ability, better results in power density, current production, and charge transfer [21]. Thus, in this investigation, a MnCo₂O₄-ACFF electrode was used. First, 300 mL of PW was filled in the anode chamber up to the brim, in which 200 mL of anaerobic digested sludge taken from a sewage treatment plant was kept at the bottom. The sludge was utilized as an inoculum [22]. The anode chamber consists of a sampling port which is employed to add and remove the effluent. The DMFC’s performance was examined at ambient temperature, and for 110 days, the anode chamber was kept in anaerobic conditions. Due to its high porosity, MnCo₂O₄-ACFF promotes the growth of biofilms. A small quantity of Bromo Ethane Sulfonate (BES-50 mM) was included in the anode chamber before starting the experiment in order to prevent methane production, because methane generation would have an adverse effect on power production [23]. The reactor was set up in the acclimatization phase for 30 days. The microbes in the inoculum normally require 24 to 72 h to acclimatize and to start biological decomposition. The effluent from the pharmaceutical industry is rich in pollutants such as diclofenac, which is very difficult to decompose. The PW shows a complex system made up of numerous additional wastes from different processes used in the production and processing of antibiotics. The inoculum may therefore require more time to adapt to the PW. During the acclimatization phase, the reactor functioned in an open circuit mode without any external resistance.

A phosphate buffer solution (KCl—0.13 g/L, NH₄Cl—0.31 g/L, Na₂HPO₄—4.58 g/L, NaH₂PO₄.H₂O—2.45 g/L) was added to the cathode chamber during the acclimatization phase in order to attract more protons from the anode [24]. At the same time, the anode chamber was filled with PW. One aeration port and one wire point for circuit connection were present in the cathode chamber. The purpose of this aeration opening was to allow oxygen to enter the cathode chamber and to receive more protons from the anode end. Before the PW and phosphate buffer were added to the anode and cathode chambers, their pH levels were adjusted to 7. A proton exchange membrane (PEM) sustained by a gasket (Nafion 117, perfluorinated membrane: 10 × 10 cm², Sigma Aldrich, thickness (0.1833 mm), density (2.0 g/cm²), conductivity (0.083 sec/cm)) was used to connect the anode and cathode chambers. After reaching an open circuit voltage of 180 mV, the reactor was switched to closed circuit mode.

2.4. Operational Parameters

The DMFC was tested at different values of OL (0.5–4.5 g COD/L). At these various OLs, the removal efficiencies of solids and COD as well as the total power produced were examined. The pH of the PW was regularly monitored inside the chambers of the DMFC.

2.5. Experimental Study

The voltage across the external circuit which includes the resistor was measured at particular time intervals using a digital multimeter. Samples were analyzed every two days to assess the characteristics of the PW in triplicate. According to Jayashree et al. [25], Ohm law was used to determine power, $\mathit{P}$, as reported in Equation (3).

$$\mathit{P}=\mathit{V}·\mathit{I}$$

(3)

where

-

P—power;

-

$\mathit{V}$—voltage;

-

$\mathit{I}$—current intensity.

The power density (PD) and current density (CD) were calculated in correspondence with the surface area of the anode.

2.6. Coulombic Efficiency (CE)

Equation (4) was used to calculate the Coulombic efficiency, $\mathit{C}\mathit{E}$, as proposed by Tamilarasan et al. (2017) [23].

$$CE=\left(8·I\right)/\left(F·q·∆COD\right)$$

(4)

where

-

8—numerical constant;

-

$F$—Faraday constant (96,500 C/mol);

-

$q$—Flow of wastewater (m³ /s);

-

$∆COD$—Removal rate of COD (g/L).

-

$CE$ measured in this study suggested that methanogenic bacteria may be present in the inoculum. In the steady state of the DMFC, the polarization curve corresponding to each OL was measured by changing the external resistance (16,000–100 Ω).

3. Results and Discussion

3.1. DMFC’s Acclimatization Phase

The initial phase, known as the acclimatization phase, lasted for 30 days in which the microbial population adjusted to its new surroundings. The DMFC was tested at 0.68 g COD/L fixed OL in an open-circuit mode. At this acclimatization phase, 38% COD removal efficiency was obtained. Until the voltage and current density were stabilized, the DMFC was operated. Once the reactor was stabilized, the CE, electricity production and the removal efficiencies of organics and solids were assessed by varying the OL regularly.

3.2. Effect of OL in DMFC on TCOD and SCOD Removal

The effect of TCOD in the DMFC at various OLs is shown in Figure 2. To determine the effect of the DMFC on TCOD and SCOD, a range of the OL from 0.5 to 4.5 g COD/L was tested.

As illustrated in Figure 2, the highest TCOD removal efficiency of 89% was obtained at 2 g COD/L OL. At this 2 g COD/L OL, the concentration of TCOD was reduced from 2000 to 220 mg/L. As the OL assumed the values of 2.5 and 3 g COD/L, the TCOD removal efficiency reduced to 79% and 68%, respectively. The reason behind this decrease was that the methanogenic microorganisms were not active beyond 2 g COD/L OL. Such a result was comparable to the one obtained in the work of Poureshghi Oskouei et al. [26], in which the inactivity of methanogens reduced the TCOD removal efficiency.

A pattern similar to that of TCOD removal efficiency was observed for the SCOD removal efficiency, as shown in Figure 3. It is worth noticing that the highest SCOD removal efficiency of 91% was detected in correspondence with an OL equal to 2 g COD/L. At this OL of 2 g COD/L, the concentration of SCOD was reduced from 1300 to 117 mg/L. As the OL increased to 2.5 COD/L and 3 g COD/L, the SCOD removal efficiencies diminished to 82% and 72%, respectively.

The bacterial population in the dense biofilm is a significant factor in wastewater treatment [27]. Electrogenic and non-electrogenic microbes, such as methanogenic microorganisms in the anode chamber, significantly contribute to the achievement of maximum TCOD and SCOD removal efficiencies. Microbes that are electrically charged, or electrogenic, can help to produce electricity by directly transferring electrons to the electrode. Complex organic molecules are broken down into simpler components by non-electrogenic bacteria. Thus, these microbes altogether improved the COD removal efficiency [28].

The performance of the DMFC depends extensively on various operating conditions, types of electrodes and substrates, and reactor design.

Table 2. COD removal efficiencies in various types of wastewater and MFC reactors.

Type of Reactor	Anode Material	Type of Wastewater	Maximum COD Removal Efficiency (%)	Reference
Dual-chambered MFC	Carbon cloth	Septic tank wastewater	50	Khalili et al. (2017) [13]
Air-cathode MFC	Carbon cloth	Tannery wastewater	88	Sawasdee et al. (2016) [30]
Dual-chambered MFC	Preheated carbon felts	Molasses wastewater	50	Lee et al. (2016) [31]
Dual-chambered MFC	Plain graphite plate	Sugar industrywastewater	79.8 ± 1.5	Naina et al. (2020) [32]
Algal cathode MFC	Graphite plate	Kitchen wastewater	73.5	Naina et al. (2020) [33]
Air-cathode MFC (fed-batch mode)	Graphite rod	Slaughterhouse wastewater	72	Mateo-Ramírez et al. (2017) [34]
Moving bed biofilm reactor-MFC	Carbon fiber paper	Pulp/paper wastewater	65.6	Chen et al. (2020) [35]
Air-cathode	Carbon felt	Synthetic wastewater	78	Ghadge et al. (2015) [36]
Dual-chambered MFC	Activated carbon fiber felt	Seafood processing wastewater	83	Jayashree et al. (2016) [25]
Dual-chambered MFC	(MnCo2O4—Activated carbon fiber felt)	Pharmaceutical wastewater	89	This study

shows the COD removal efficiencies in various types of wastewater and MFC reactors. The SCOD removal efficiency (91%) obtained in this study was found to be higher than the one observed in the work of Lee and Lin [29] in which 44% of the SCOD removal efficiency was obtained while treating organic wastewater in MFCs.

3.3. Impact of OL on TSS Removal Efficiency

Figure 4 shows the impact of OL on TSS removal efficiency and concentration.

In correspondence with an OL of 2 g COD/L, a TSS removal efficiency of 85.5% was observed. At this value of the OL, the concentration of TSS was reduced from 2000 to 290 mg/L. The frequent generation of volatile fatty acids in the anaerobic environment of the anodic chamber was the reason for this high TSS removal efficiency. These acids have the potential to partially inhibit bacterial activity and decrease the pH of the anodic chamber. At 2.5 g COD/L of OL, the lowest TSS removal efficiency of 78% was achieved. The reason behind this decline in TSS removal efficiency is that in the anode portion of the H-shaped DMFC, intermediate compounds are produced when fermentable bacteria in the PW undergo fermentation at higher OLs. In turn, these compounds may reduce the TSS removal efficiency by suppressing microbial activity. This phenomenon occurs when certain metabolic pathways and enzymes become activated, as some organic substrates are available. Once the microbial population adapts to the wastewater environment, it can improve its metabolic capacities, which is necessary for them to degrade the complex organic compounds present [37].

3.4. OL’s Impact on Electricity Production

During the acclimatization phase, on the 16th day, 100 mV (open circuit voltage) was attained. Since the microbes in the inoculum require more time to stabilize, the same open circuit mode was kept for an extra 14 days. Then, the PW was sent to the DMFC at various OLs in a closed circuit mode. The amount of power produced is seriously affected by changing the OL in wastewater treatment [38,39]. The polarization curve that was produced in the DMFC while treating PW at various OLs is shown in Figure 5a,b.

Based on the investigations of Sleutels et al. [40] and Naveenkumar et al. [41], the DMFC’s ability to produce electricity is affected by wastewater loading. Figure 5a clearly shows that at 2 g COD/L OL, the highest values of current and power densities of 420 mA/m² and 348 mW/m², respectively, were obtained. High nutrient availability to the microbes and the efficiency of inoculum microbes in utilizing the PW may be the cause for this high power density and current density. Testing an OL of 2.5 g COD/L, the values of 250 mW/m² and 362 mA/m² were measured for power density and current density, respectively. Ohmic loss may cause this reduction in power and current densities. Ohmic losses, also called resistive losses, occur in DMFCs as a result of the electrical resistance of the electrons [42]. Electrons experience electrical resistance when they go through the various cell components of the DMFC. Thus, the efficiency and performance of the DMFC may be affected by these losses. Ohmic losses could be a significant obstacle to the transfer of electrons in the reactor’s open circuit mode [15].

Figure 5b illustrates that the maximum voltage (956 mV) and current density (420 mA/m²) were detected when the OL was fixed at 2 g COD/L. This may be explained considering that the microbial population is more responsive to the substrate, consuming more substrate to keep its metabolic level at this OL [43]. Hence, there was an increase in the oxidation of organic compounds, which led to the production of more protons and electrons. This resulted in higher voltage, power density, and current density.

When an OL of 1 g COD/L was tested, the output voltage and current density were found to be 700 mV and 256 mA/m², respectively. The reason behind this reduction is that at this OL value, the microbes are not able to sustain higher voltages.

In correspondence with an OL of 2.5 g COD/L, the voltage and current density were found to be 800 mV and 340 mA/m², respectively. The output voltage and current density decreased at higher OLs. This may occur due to the following reasons: at higher OLs, instead of electrogenic bacteria, the microbial communities in the anode chamber might have utilized the available substrate and oxidized it more successfully. This would have led to a reduction in the output voltage and current density [44]. Inadequate transfer of protons to the cathode chamber and biofilm thickening also results in low voltage and current density [45].

To effectively treat wastewater using DMFCs, the internal resistance must be reduced, so that higher voltage, power density, and current density can be obtained. Many factors, including the membrane, electrolyte, anode electrode, material of the electrode, rate at which wastewater is fed into the reactor, layout of the reactor, and distance between the electrodes might influence the DMFC’s internal resistance [46]. Indeed, microbial adhesion and electron transportation are affected by the surface area of the electrode and the electrode’s material. To enhance the electron transmission and reduce the internal resistance, high surface area conductive materials can be employed.

Buffer concentration can also enhance the efficiency of DMFCs. In the current experiment, a 50 mol/m³ concentration of phosphate buffer was added to the cathode chamber. The rapid oxidation of organic materials has made the buffer facilitate the transportation of protons and electrons, which increased the voltage. The buffer solution simultaneously increased the electrical output by reducing the ohmic losses (electrolyte resistance).

3.5. Coulombic Efficiency

Figure 6 illustrates the effect of the OL on the CE. It is clear that the CE trend gradually decreased at higher OL. This was comparable to the findings of Kim et al. [47] in which a similar trend of CE was observed. An increase in CE is one of the main requirements for the field implementation of DMFCs.

By determining the CE, the reactor’s stability, which is also correlated with the removal efficiency of COD, was ensured. In this study, a progressive improvement in COD removal efficiency was noted throughout the initial phase of the reaction despite the reactor’s low stability and the decrease in CE. The reduction in CE indicates that in spite of high substrate availability, a limited amount of biodegraded substrates can only be used as donors of electrons in the power production. A small quantity of substrate utilization during anaerobic processes like fermentation and methane production instead of electrogenic bacterial metabolism would decrease the CE.

The addition of BES prevents methane production, but some methanogenic bacteria may be left behind, decreasing the CE. At 2 g COD/L OL, the maximum CE, equal to 46%, was obtained. At lower OLs, less substrate is available to methanogenic bacteria. Hence, this phenomenon enhanced the CE. In addition to that, a higher COD removal efficiency would result in a higher CE. In this study the highest COD removal efficiency was obtained at 2 g COD/L OL, which increased the CE. The CE decreased to 40% in correspondence with an OL of 2.5 g COD/L. It occurs due to the electrogenic bacteria’s competition with other bacteria during intense saturation conditions in the surface of the anode.

4. Importance and Future Prospects of the Study

The current study demonstrated at the lab scale that DMFC technology may be an effective method to remove pollutants and produce sustainable energy from PW. To attain the maximum efficiency on a large scale, however, several technical, socioeconomic and scientific problems need to be addressed. The design of DMFCs should be made in such a way that it should be eco-friendly and economically feasible at the industrial scale. Research should be focused to make economically feasible electrodes, and it also should improve the process efficiency. In future, energy balance analysis and economic evaluation can be performed to assess the feasibility of the proposed solution on a large scale.

5. Conclusions

A novel and effective method of removing pollutants and generating electricity from PW using (MnCo₂O₄-ACFF) electrode DMFCs was implemented in this study. To evaluate the feasibility of this DMFC treatment process, various values of OL were tested. It was found that 2 g COD/L of OL maximized the process performance. The maximum TCOD, SCOD, and TSS removal efficiencies of 89%, 91%, and 85.5% were obtained at the optimum value of OL (2 g COD/L). At this condition, also the highest values of voltage, current density, power density, and CE were observed, which were equal to 956 mV, 420 mA/m², 348 mW/m², and 46%, respectively. The DMFC produced electricity continuously for 160 days while treating the PW. Thus, the research findings indicated that the suggested technological solution is potentially effective in treating PW and also for power production.