MICROBIAL RECHARGEABLE BATTERY EMPLOYING WASTE WATER

TREATMENT AND CO ₂ SEQUESTRATION

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims priority to India provisional patent application number 202241036474 filed on June 24, 2022, titled “Microbial rechargeable battery employing waste water treatment and CO ₂ sequestration and methods of fabrication thereof’, the entire contents of which is herein incorporated by reference.

FIELD

The present disclosure relates to integration of microbial fuel cell (MFC) and microbial electrochemical cell (MECC) for waste water treatment, generation of electricity, and production of value-added products (VAP).

BACKGROUND

Use of non-renewable fossil fuels for energy generation is on the rise, wherein the such non-renewable fuels are rapidly declining in deposits. In addition, manufacturing processes have an impact on the environment, lead to climate change, and waste generation, hazardous by-products and end-products that are difficult to degrade or recycle are further challenges that need to be addressed. The need for alternative environmentally sustainable production technologies based on non-fossil fuel feedstocks is therefore greater than ever. Therein, microbial electrochemical technology (MET) is up-and-coming by using microorganisms, a potential energy source with rapidly renewable capacity, inexpensive cost, being friendly to environment and multi-use as a catalyst to speed up chemical reactions or even participants in processes. Moreover, this technology can produce biocatalytic chemicals, materials or fuels from renewable feedstocks (i.e., biomass) or from recyclable waste streams (i.e., organic waste and inorganic gaseous waste), thus supporting the development of a circular bioeconomy which is considered as a promising game changer in industrial and environmental issues.

Microbial electrochemical technology (MET) is a system which converts chemical energy stored in biodegradable organic substance into electric current and other chemicals by using microorganisms. In detail, microorganisms oxidize organic matter to release electrons which are captured by the anode of the system. Then, the electrons flow to the cathode by coupled wire and usage depends on human demands. Despite the advent nearly a century ago, MET has attracted special attention in the recent few years due to unique merits namely resources recovery, production bioproducts which is harmless to the environment and sustainability.

The process of wiring of microbial metabolism and electrodes in METs is called microbial extracellular electron transfer (EET) which is a role key of all METs. According to several reported research works, EET is affected by the potential difference between the final electron carrier and the anode. In anaerobic environment, synergic consortia of fermentative and bio electrogenic microorganisms are set up by aqueous solutions. The fermentative microorganism enables breaking down complex organic compounds into simpler structure such as acetate, ethanol, hydrogen gas, polymers and other long chain fatty acids. Those products will easily be oxidized by bioelectrogenic microorganisms. Then, the electron transfer process from electron donor to terminal electron acceptor happens outside the cell with an insoluble form and creates electric energy which is collected by electroactive microorganisms. Besides, the colonies of microorganisms in favourable conditions lead to the formation of biofilm whose ability is a conducting electron on solid-state electrodes. Therefore, EET is divided into two mechanisms i.e., Direct extracellular electron transfer, and Mediate extracellular electron transfer. It is also known from several research works that MET is used by the hybrid system between nature and related engineering viz. microbiology, electrochemistry, environmental and material sciences. Besides, the flexible valorization as a basis strength of MET helps to generate various products (electricity biofuel, biogas and platform chemicals) from any form of waste (solid, liquid and gaseous) in a sustainable way. Since MET started developing, it significantly contributes in diverse domain such as bioenergy, waste remediation, CO2 sequestration, bioelectronics, resource recovery, desalination and other areas.

As known in several prior arts, the classification of MET is based on its applications. The classification includes microbial fuel cell (MFC) to create bioelectricity, bioelectrochemical treatment (BET) to treat wastewater with complex properties while microbial electrolysis cells (MEC) are used for producing hydrogen and methane; microbial electrochemical snorkel (MES) generates platform chemicals, microbial desalination cell (MDC) can separate ions and electro-fermentation (EF) boosts the synthesis process of bio-based products which are also involved. These applications all have advantages in terms of sustainability, economy, environment and technical perspectives.

As disclosed in Inamuddin, M.F., Ahmer, A.M., Asiri 2019, Microbial Fuel Cells: Materials and applications, LLC, Chapter 1, 3, MFC is the most popular application corresponding to primary MET. It was found in 1911 by MC. Potter by his experiment of the electrical current generation from bacteria. Thenceforth, it has been developing and widely applied in many fields such as wastewater treatment, electricity generation and energy recovery. The working principle of MFC is the catalytic activity of certain bacteria through redox reaction to produce electrons from organic matters.

As disclosed in Ravi, P., Dipankar, D., Rajeeb, D., Valentina, E.B. 2020, Adaptive and Intelligent Contron of Microbial Fuel Cells, 1(2), 3—5, the three main components of MFC are the anode, the cathode and the membrane. In a MFC, the proton exchange membrane is placed in the middle of two compartments, one is the anode chamber with the anode and bio-catalyst while the cathode chamber includes the cathode and another bio-catalyst. The task of this membrane is to transfer protons between those compartments along with an electrical pathway connecting the two electrodes. The oxidation of organic compounds is catalyzed by the anode bio-catalyst, parallelly, the reduction of inorganic substances is catalyzed by the remaining catalyst. The reduced organic substances form a precipitate which removes the inorganic substance from the solution. Microorganisms in an anode chamber produce protons which are moved to the anode in order to create electrons.

As disclosed in Pant, D., Van Bogaert, G., Diels, L., Vanbroekhoven, K. 2010, A review of the substrates used in microbial fuel cells (MFCs) for sustainable energy production. Bioresour. Technol, 101, 1533-1543, by utilizing appropriate substrates, those electrons are transferred through the outer circuit.

As disclosed in Lovley, D. 2008, The microbe electric: conversion of organic matter to electricity. Curr. Opin. Biotechnol, 19, 564-571, electron transfer can occur in two mechanisms which are direct and indirect. Direct exchange allows electrons flow via a mediator and biofilm at the anode. Contrariwise, an external mediator assures electrons transfer from microbes to the surface of the anode in the circumstantial procedure. At the cathode, the combination of protons and electrons create electricity as well as the treated product.

Based on the need of applications, the material of these components can be selectively chosen. Also, they must meet the commercial requirements which are being cost-affordable, sustainable, readily available and highly effective. Typically, carbon nanotubes are used as material for the anode because of good electrical conductivity and chemical stability. Apart from carbon nanotubes, graphite plates as another suitable option which are the simplest material with an inexpensive cost and perform well on almost surfaces. Besides, the cathode of MFC has a higher demand on what it made of since it is an important determinant to the whole system. Ordinarily, the three layers of the cathode in MFC are the diffusion layer, the catalyst and a conducting support material which requires high mechanical strength, catalytic capacity to improve the redox reaction and good electronic/ionic transfer properties, respectively. The most common candidate for the material of the cathode is ferricyanide as an electron acceptor, however, its drawback is that the diffusion can affect long-term productivity. Furthermore, hydrogen, phosphate, oxygen, hydrogen peroxide, manganese oxide or copper chloride are used as the oxidizer.

As disclosed in Sonia, M., Tiquia-Arashiro, Deepak Pant. 2019, Microbial Electrochemical Technologies, CRC Press. Taylor & Francis Group, I, 3-4; III, 114-117, the MEC is operated as either direct or mediate electron transfer. In the direct mechanism, redox proteins on the membrane are used as the linking species, the protrusions from cells such as pili and nanowire carry out electron conveyance which intend to increase in anaerobic environment. On the other hand, the mediate electron transfer has no physical interaction between bacteria and electrodes, it employs dissolved redox species as the linking species. The mediator to move electrons can be endogenous or exogenous. It also enables to reverse the oxidation reaction at the cathode. Since, it is an upgrade of MFC, the application domain of MEC is diverse and can be classified into four major groups: product synthesis, wastewater treatment, resource recovery and biosensing.

MET is also facing many challenges despite of its potential to be mature. The first challenge is the high operational cost and low power output. Although, microbes or fungi are very cheap source of labors, the materials for electrodes are expensive, especially the catalyst. In order to ensure the electrochemical interaction between electrodes and microorganism as well as improve the voltage output, electroactive biofilm tends to be thicker and denser. Thus, it becomes the deciding factor in efficiency of the process and commercialization of this technology. So far, the 3D electrode is being positively investigated to make an ideal biofilm. According to literature, the 3D electrode offers a favourable habitat for bacteria growth and controls the anodic potential to support energy for bacteria absorb ability which helps to accelerate the electron transfer within the cells/biofilms and from cells to electrode. However, when more energy is utilized by bacteria to increase the productivity, the more energy input is required. another obstacle of MET, (as disclosed in Zhang, T. & Tremblay, P. 2015, Current challenges and future perspectives on emerging bioelectrochemical technologies, Frontiers in Micro., 9), comes from its wide range of microbes which is always considered as a strength. There is a huge gap between the number of microorganisms which has capacity to exchange electron with solid surfaces or meditator are discovered and the number of in-depth studies about them. Only few of these species are deeply researched wherefore this shortage might affect the chances in finding out more bacteria for the development of MET. Besides, the electron transfer mechanism from the cathode towards microbes has not been clearly studied while this is important to invent the applications.

Table 1 depicts the drawbacks of application of different materials in the given methods.

Table 1

The inventors of the present disclosure felt that municipal or community sewage water contains a huge amount of organic substances that can be used as energy source in MFC. The measure of bioelectricity generated by MFC in the wastewater treatment minimizes the electricity requirement of conventional treatment of wastewater. In addition, MFC produce 50-90% less solid disposal. In general, MFC can completely oxidize acetate, propionate and butyrate substrates to carbon dioxide and water. Hybrid substrate as a combination of anodophiles and electrophiles are more efficient for treating wastewater. Specific microbes possess ability to remove sulfides in wastewater. By combining the MFC waste water treatment, MECC - CO2 sequestration, forming an integrated MFC-MECC for waste water treatment and CO2 sequestration can produce a microbial rechargeable battery (MRB). SUMMARY

The present disclosure provides a Microbial Rechargeable Battery (MRB) formed by the integration of a microbial fuel cell (MFC) for generation of electricity, which acts as the discharging unit; and a microbial electrochemical cell (MECC) for storage of electricity, which acts as the charging unit. The anode chamber of the MFC is in contact with the wastewater, wherein a mixed culture inoculum is added to the wastewater. The microbes in the wastewater sludge act as a biofilm on the MFC anode, which treat the waste water into clean water, in the process they generate electrons which are taken up by a redox mediator at the cathode chamber of MFC. The clean water is collected, whereas the reduced redox mediator is circulated to be anolyte of the MECC unit. The microbes in the anode chamber of MFC also produce CO2 and H2; the CO2 and H2 are fed to the cathode chamber of the MECC. The cathode of the MECC is in contact with a mixed culture inoculum in the MECC cathode chamber, wherein the microbes act as a biofilm on the MECC cathode, wherein the biofilm on the MECC cathode converts the CO2 and H2 into value added products.

Reactions involved in MFC:

Overall reaction:

C12H22O11+ O2 ^MFC ► 13H ₂ O + 12CO ₂

Half-cell reaction at anode:

C12H22O11 + 13H ₂ O - ► 12CO ₂ + 48H ⁺ + 48e-

Half-cell reaction at cathode:

O2 + 4H ⁺ + 4e - ► 2H ₂ O

Reactions involved in MECC:

Overall reaction:

MECC

H2CO3 + 4CO ₂ + 6H2 - ► CH3CH2OH + CH3CHO + CH3OH + 4O ₂

Half-cell reaction at anode:

(Fe(CN) ₆ ) ⁴ - (Fe(CN) ₆ ) ³ - + e- Half-cell reaction at cathode:

CO2 + H2O - ► H2CO3

H2CO3 + 4CO ₂ + 6H2 - ► CH3CH2OH + CH3CHO + CH3OH + 4O ₂

In the MRB of the present disclosure, during the charging phase (MECC), electrical energy will be consumed to form acetate and other salts from CO2, while during the discharging phase (MFC), electrical energy is generated by the consumption of wastewater and/or acetate or other salts.

The MRB of the present disclosure involves stable sequential operation of a bioanode (of MFC unit), and a biocathode (of MECC unit), and their counter electrodes.

In an aspect of the present disclosure, the bioanode (of MFC unit), and the biocathode (of MECC unit) are microbial films on graphite or graphite-conducting polymer composite electrodes, wherein the counter electrodes are of graphite.

In an aspect of the present disclosure, the redox mediators may be selected from but not limited to hexaferricyanates, dichromates, permanganates, or organic quinones.

The redox mediator is internally replenished between MECC and MFC units whereas CO2 and wastewater are both internally and externally replenished.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Fig. 1 is a representation of integrated microbial fuel cell (MFC) as discharging unit and Microbial electrochemical cell (MECC) as charging unit of the Microbial Rechargeable Battery (MRB).

Fig. 2 illustrates a Flow chart of Fabrication of Microbial Fuel Cell (MFC) for waste water treatment. Fig. 3 illustrates a Flow chart of Fabrication of Microbial Electrochemical Cell (MECC) for Carbon dioxide sequestration.

Fig. 4 illustrates Potential vs time triangular signal in a Cyclic voltammetry (CV) (Prior Art).

Fig. 5 illustrates a basic CV for a reversible redox process (Prior Art).

Fig. 6 illustrates CV of MFC from fabrication to death.

Fig. 7 is a representation of the processes occurring in the MFC.

Fig. 8 illustrates Nyquist plot of MFC from fabrication to death.

Fig. 9 illustrates Electrical equivalent circuit (EEC) fit of the Nyquist Plot of MFC.

Fig. 10A illustrates the trend in Solution resistance (Rl) of the MFC with time.

Fig. 10B illustrates the trend in Anodic polarisation resistance (APR) (R2) of the MFC with time.

Fig. 10C illustrates the trend in Cathodic polarisation resistance (CPR) (R3) of the MFC with time.

Fig. 11 illustrates Anodic polarisation resistance (APR) (R2) and Anodic Capacitance (AC) (C2) of the MFC.

Fig. 12 illustrates Cathodic polarisation resistance (CPR) (R3) and Cathodic Capacitance (CC) (C3) of the MFC.

Fig. 13A illustrates Optical microscopy images of Combined culture biofilm in the MFC during days 1-9

Fig. 13B illustrates Optical microscopy images of Combined culture biofilm growth in the MFC seen after 9 days.

Fig. 14A, and 14B illustrates Scanning electron microscopy (SEM) images of anodic bacterial culture in the MFC.

Fig. 15 illustrates power density and Anodic capacitance of the MFC with time.

Fig. 16 illustrates Columbic Efficiency (CE) and Energy Efficiency (EE) of the MFC with time.

Fig. 17 illustrates Agarose gel electrophoresis image of 16S rDNA gene cloned plasmid (pDM20 vector) of the bacteria developed using sample collected from anolyte of MFC. Fig. 18 illustrates Agarose gel electrophoresis of the 16S rRNA (of the bacteria developed using sample collected from anolyte of MFC) amplified PCR product using forward primer and a reverse primer by cloned plasmid as DNA, TAE buffer IX, stained with ethidium bromide.

Fig. 19 illustrates the percentage of microbes depicted in TA clone plasmid from MFC anolyte

Fig. 20A illustrates the electrochemical processes of Microbial electrochemical cell (MECC) from 24 to 864 hours.

Fig. 20B illustrates the electrochemical processes of Microbial electrochemical cell (MECC) from 1104 to 1320 hours.

Fig. 21 A illustrates equivalent circuit (EC) diagram to fit Electrochemical Impedance Spectroscopy (EIS) data of MECC(l).

Fig. 2 IB illustrates equivalent circuit (EC) diagrams to fit Electrochemical Impedance Spectroscopy (EIS) data of MECC(2), and MECC(3).

Fig 22A illustrates Nyquist plot of CO2 sequestration in cathode chamber of MECC(l) with 150ml/min as CO2 flow rate.

Fig 22B illustrates Nyquist plot of CO2 sequestration in cathode chamber of MECC(2) with 250ml/min as CO2 flow rate.

Fig 22C illustrates Nyquist plot of CO2 sequestration in cathode chamber of MECC(3) with 500ml/min as CO2 flow rate.

Fig. 23 A illustrates Resistance (R) vs time (days) for MECC(l)

Fig. 23B illustrates Resistance (R) vs time (days) for MECC(2)

Fig. 23C illustrates Resistance (R) vs time (days) for MECC(3)

Fig. 24 illustrates Agarose gel electrophoresis image of 16S rDNA gene cloned plasmid (pDM20 vector) of the bacteria developed using sample collected from catholyte of MECC.

Fig. 25 illustrates Agarose gel electrophoresis of the 16S rRNA (of the bacteria developed using sample collected from catholyte of MECC) amplified PCR product using forward primer and a reverse primer by cloned plasmid as DNA, TAE buffer IX, stained with ethidium bromide. Fig. 26 illustrates the percentage of microbes depicted in TA clone plasmid from MECC’s catholyte.

Fig. 27 illustrates Gas chromatography-mass spectrometry (GC-MS) peaks of ethanol and acetate identified through the CO2 reduction by MECC.

Fig. 28 illustrates FTIR spectra of the liquid sample collected from catholyte of MECC after CO2 reduction by the Microbial rechargeable battery (MRB) of the present disclosure.

Fig. 29 illustrates the Galvanostatic Charge/Discharge (GCD) profile of the MRB for 50 cycles.

Fig. 30 illustrates the specific capacitance of the MRB for 50 cycles.

Fig. 31 illustrates the specific capacitance of the MRB for 2000 cycles.

DETAILED DESCRIPTION

The preferred embodiments of the present disclosure will be described in detail with the following disclosure and examples. The foregoing general description and the following detailed description are provided to illustrate only some embodiments of the present disclosure and not to limit the scope of the present disclosure. The disclosure is capable of other embodiments and can be carried out or practiced in various other ways.

Unless otherwise specified, all the technical and scientific terms used herein have the same meaning as is generally understood by a person skilled in the art pertaining to the present disclosure.

Headings are used solely for organizational purposes, and are not intended to limit the disclosure in any way.

The use of the singular includes the plural unless specifically stated otherwise. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well. The use of “or” means “and/or” unless stated otherwise. As used herein, the terms “comprises” and/or “comprising” specify the presence of the stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. Furthermore, to the extent that the terms “includes,” “having,” “has,” “with,” “composed,” “comprised” or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”

As used herein, ranges and amounts can be expressed as “about” a particular value or range. “About” is intended to also include the exact amount. Hence “about 5 percent” means “about 5 percent” and also “5 percent.” “About” means within typical experimental error for the application or purpose intended.

It is to be understood that wherein a numerical range is recited, it includes all values within that range, and all narrower ranges within that range, whether specifically recited or not.

Reference throughout this specification to “one embodiment,” “an embodiment,” “one example,” or “an example” means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “one example,” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, or characteristics may be combined in any suitable combinations and/or sub -combinations in one or more embodiments or examples. In addition, it should be appreciated that if any figures are provided herewith, they are for explanation purposes to persons ordinarily skilled in the art and that the drawings of them are not necessarily drawn to scale. In this specification, certain aspects of one embodiment include process steps and/or operations and/or instructions described herein for illustrative purposes in a particular order and/or grouping. However, the particular order and/or grouping shown and discussed herein are illustrative only and not limiting. Those of skill in the art will recognise that other orders and/or grouping of the process steps and/or operations and/or instructions are possible and, in some embodiments, one or more of the process steps and/or operations and/or instructions discussed above can be combined and/or deleted. In addition, portions of one or more of the process steps and/or operations and/or instructions can be re-grouped as portions of one or more other of the process steps and/or operations and/or instructions discussed herein. Consequently, the particular order and/or grouping of the process steps and/or operations and/or instructions discussed herein do not limit the scope of the disclosure.

The present disclosure provides a Microbial Rechargeable Battery (MRB) formed by the integration of a microbial fuel cell (MFC) for generation of electricity by converting wastewater into clean water, which acts as the discharging unit; and a microbial electrochemical cell (MECC) that converts CO2 into acetates, carbonates, bicarbonates and stores the generated electricity, which acts as the charging unit.

Fig. 1 illustrates the working principle of the MRB of the present disclosure.

The anode chamber of the MFC is in contact with the wastewater, wherein a mixed culture inoculum is added to the wastewater.

The microbes in the wastewater sludge act as a biofilm on the MFC anode forming a bioanode, which treat the waste water into clean water, in the process they generate electrons which are taken up by a redox mediator at the cathode chamber of MFC. The clean water is collected, whereas the reduced redox mediator is circulated to the anode chamber of the MECC unit to act as anolyte in the MECC unit. The microbes in the anode chamber of MFC also produce CO2 and H2; the CO2 and H2 are fed to the cathode chamber of the MECC. The cathode of the MECC is in contact with a mixed culture inoculum in the MECC cathode chamber, wherein the microbes act as a biofilm on the MECC cathode, wherein the biofilm on the MECC cathode converts the CO2 and H2 into value added products such as acetates, carbonates, bicarbonates, alcohols, aldehydes, and ketones.

In an aspect of the disclosure, the mixed culture inoculum in the MFC anode chamber, and the mixed culture inoculum in the MECC cathode chamber are the same inoculum.

Reactions involved in MFC:

Overall reaction:

C12H22O11+ O2 ^MFC ► 13H ₂ O + 12CO ₂

Half-cell reaction at anode:

C12H22O11 + 13H ₂ O - ► 12CO ₂ + 48H ⁺ + 48e-

Half-cell reaction at cathode:

O ₂ + 4H ⁺ + 4e - ► 2H ₂ O

Reactions involved in MECC:

Overall reaction:

MECC

H2CO3 + 4CO ₂ + 6H2 - ► CH3CH2OH + CH3CHO + CH3OH + 4O ₂

Half-cell reaction at anode:

(Fe(CN) ₆ ) ⁴ - (Fe(CN) ₆ ) ³ - + e-

Half-cell reaction at cathode:

CO2 + H2O - ► H2CO3 In the MRB of the present disclosure, during the charging phase (MECC), electrical energy will be consumed to form acetate and other salts from CO2, while during the discharging phase (MFC), electrical energy is generated by the consumption of wastewater and/or acetate or other salts.

The MRB of the present disclosure involves stable sequential operation of a bioanode (of MFC unit), and a biocathode (of MECC unit), and their counter electrodes.

In an aspect of the present disclosure, the bioanode (of MFC unit), and the biocathode (of MECC unit) are microbial films on graphite or graphite-conducting polymer composite electrodes, wherein the counter electrodes are of graphite.

In an aspect of the present disclosure, the redox mediators may be selected from but not limited to hexaferricyanates, dichromates, permanganates, or organic quinones.

Materials and Methods

- Waste water is collected from the drain outlet of the domestic residence in Chennai, India.

- The microbial culture inoculum contributing to the biofilm (at the MFC anode, and the MECC cathode) are formed by mixing different anaerobic bacteria culture in a petri dish experiment.

- The catholyte in MFC and the anolyte in MECC is a mixture of equal proportion of IM Potassium ferricyanide and IM potassium ferrocyanide (IM Potassium ferri/ferro cyanide) procured from E-Merck and used without any further purification.

- CO2 source in MECC is magnesium carbonate from SRL chemicals, India,

- Cone. HC1 is laboratory grade and procured from SRL chemicals India and used without further purification. Fabrication of MFC for waste water treatment

Fig. 2 illustrates a flow chart of fabrication of Microbial Fuel Cell (MFC) for waste water treatment.

The MFC cathode chamber contains 50mM Potassium dihydrogen orthophosphate buffer and IM Potassium ferricyanide as redox mediator.

Fabrication of MECC for CO2 sequestration

Fig. 3 illustrates a flow chart of fabrication of Microbial Electrochemical Cell (MECC) for CO2 sequestration.

The MECC anode chamber contains 50mM Potassium dihydrogen orthophosphate buffer and IM Potassium ferricyanide as redox mediator.

Electrochemical characterization

The electrochemical characterization of the MFC for waste water treatment and MECC for CO2 sequestration as well as the integrated MFC-MECC are carried out independently employing Zahner-Zennium E4 electrochemical workstation using techniques such as cyclic voltammetry (CV), electrochemical impedance studies (EIS) and galvanostatic charge-discharge profile (GCD).

Electrochemical characterization of the MFC for waste water treatment

The electrochemical characterization via cyclic voltammetry (CV), linear sweep voltammetry (LSV), Electrochemical impedance spectroscopy (EIS) and Galvanostatic charge/discharge (GCD) were carried out.

Cyclic Voltammetry studies of the MFC for waste water treatment

CV is widely utilized method for the study of redox species For the CV studies, the current at the working electrode is recorded and monitored by applying triangular potential with time from initial to final voltage shown in Fig. 4 (Prior Art) and Fig. 5 (Prior Art). The potential is scanned positively in forward direction and then in negative reverse direction towards original potential to complete one cycle. The complete forward and reverse process constitutes single or multiple cycles within the same cell. The resulting graph is a plot of response current at the working electrode to the applied voltage.

For the MFC of the present disclosure, CV is carried out to indicate the bacterial development on anode as well as the mechanism of charge transfer, where the working electrode is the anode, The CV was performed between -1 to +1 V at varying scan rates from 1-100 mV/s, and recorded in either a two-electrode or a three-electrode mode.

Fig. 6 illustrates CV of MFC from fabrication to death at the load of ImV/s. The flow of waste water was 300ml/min.

From Fig. 6, it can be inferred that the MFC are not electroactive (between 24 to 72 hrs) from the time of fabrication indicating negligible microbial development at anode. After 96hrs and up to 168hrs, wide oxidation hump over 0.5 to 0.8 V, reduction hump over -0.14 to -0.36 V reveal existence of microbial activity. This suggests growth of additional electrogenic microorganisms at the anode of the MFC causing increase in the charge transfer rate. The redox peak intensity lowered from 192 to 288 hrs verifying negligible activity by bacteria. The total reduction of carbohydrates, acetates and other energy sources occurred within 240 hrs. At 312 to 576hrs, no redox peak is noticed showing decreased bacterial activities thus verifying the complete reduction of energy sources within 240 hrs. The specific capacitance of waste water treatment is determined with the equation 1,

C _S p= f 7fedV7(vmAV) - equation 1 where, Idis indicate discharge current (Amps), ldis dV represent the integral area under CV, m indicate the mass active material in gms, v represent scan rate (Vs ^-1 ),

AV is potential gradient (in V).

The average specific capacitance of the MFC is estimated as 0.14 mFg ¹ .

Fig. 7 is a representation of the processes occurring in the MFC. From Fig. 7, it can be inferred that sugar source oxidized by bacteria generates H ⁺ ions and electrons in anodic chamber. H ⁺ generated at biofilm/anolyte interface pass through salt bridge into catholyte. Once H ⁺ passes through salt bridge and reaches catholyte, it protonates the buffer medium to maintain neutral pH. At the same time, electrons generated passes through external circuit from anode to cathode. These processes develop some resistance in MFC called as lumped resistance. The accumulation of charges at the anolyte/biofilm interface developed capacitance.

Electrochemical Impedance studies (EIS) of MFC

As resistance plays a major role in the MFC performance, EIS is employed to measure internal resistance (Rin) and capacitance. In general, polarization studies and Current interrupt method was employed to obtain Ri _n . Comparing to conventional technique, EIS is quite effective to assess charge transfer, mass transfer, ohmic, double layer capacitance and biofilm monitoring for MFC.

Fig. 8 illustrates Nyquist plot of MFC from fabrication to death.

From Fig. 8 it is understood that the reduction of Anodic Polarization Resistance (APR) during initial 1-9 days and higher Anodic Capacitance (AC) influencing the electricity production. The microbial growth on the anodic surface is firmly influenced by the APR and confirmed by EIS studies as discussed here. Studies on resistances in fabricated MFC

Fig. 9 illustrates Electrical equivalent circuit (EEC) fit of the Nyquist Plot.

Fig. 10A illustrates the trend in Solution resistance (Rl) of the MFC with time.

Fig. 10B illustrates the trend in Anodic polarisation resistance (APR) (R2) of the MFC with time.

Fig. 10C illustrates the trend in Cathodic polarisation resistance (CPR) (R3) of the MFC with time.

As shown in Fig. 10A, Ri is contributed by membrane and electrolyte. The electrochemical kinetics and biokinetics lead to resistance caused by polarization. Ri is 61.52 to 79.58 Q (1-9 days) and reduces to 10.45 Q, indicating no seepage of anolyte and catholyte via salt bridge. Ri is invariant for 15-24 days. APR (R2(Q)) is 140 times less than literature from 1-9 days.

As shown in Fig. 10B, R2 increases from 43 to 913.8 Q at the end of 24 days. As shown in Figure 10, R2 on 15 ^th day is still 6 times less than 11 ^th day R2 of the literature. The rise in R2 is caused by the change in accumulation at EDL leading to reduced biochemical kinetics and biocapacitance.

As shown in Fig. 10C, CPR (RTQ)) is initially of the order of 200 Q and increases eventually to 7.5 kQ at the end of 17 days. Further decrease in R3 to 1.58 kQ at 24 days lead to ORR.

It can be inferred that, in MFC, R2 and R3 dominate over Ri.

Change in Resistances (the Rs) and the effect on Capacitances (the Cs) over time The change in Rs and Cs of MFC with time is provided in Fig. 10 - 12. The parameters fitted in the equivalent circuit and its values are shown in Table 2.

Fig. 11 illustrates Anodic polarisation resistance (APR) (R2) and Anodic Capacitance (AC) (C2) of the MFC. Fig. 12 illustrates Cathodic polarisation resistance (CPR) (R3) and Cathodic Capacitance (CC) (C3) of the MFC.

Table 2 Values of the Parameters in EC; Rs in Q and Cs in pF

Effect of combined culture is higher on the performance of MFC than individual culture. The combined culture bacteria in the in the MFC of the present disclosure produced maximum power density of 1.104 W/m ² at the end of life of MFC, and the R2, C2 values being 0.45 KQ and 0.0015 F respectively. R3 and C3 are 3.12 kQ and 0.135 F. C2 of MFC employing geobacter is 0.0028 F whereas R2 being 5.084 Q on 1 ^st day. But the combined culture based MFC showed 0.0036 F and 0.035 kQ on 1 ^st day. 11 ^th day lead to 913 Q which is far less than the C2 of MFC in literature (Table 2). Thus it is clear that mixed culture inoculum employed in the present studies showed higher performance than the single culture bacterium reported so far in the literature.

Performance of MFC employing various bacterial cultures is given in Table 3.

Table 3

Optical microscopic analysis of biofilm

The growth of biofilm is visualized by optical microscope to understand (i) the topography of the bacterial growth on the substrate, (ii) morphology of the microbes (shape and geometry) and (iii) the fittest microbes among the mixed culture that predominates the biofilm.

Fig. 13A illustrates Optical microscopy images of Combined culture biofilm in the MFC during days 1-9

Fig. 13B illustrates Optical microscopy images of Combined culture biofilm growth in the MFC seen after 9 days.

Fig. 14A, and 14B illustrates Scanning electron microscopy (SEM) images of anodic bacterial culture in the MFC. As shown in Fig. 13A, and Fig. 13B, the citrus substance (acts as substrate for bacteria growth) present in the waste water leads to combined culture bacteria growth with subsequent fungus growth (Fig. 14A and Fig. 14B) with time. MFC Performance

Power density trend of MFC with time is provided in Table 4, which indicates that MFC fabricated with citrus energy source produce maximum power on day 8 and decreases on day 9 due to fungus growth. 803 mW/m ² is obtained as maximum power density indicating stable bacterial film from 1-9 days.

Table 4

Columbic Efficiency (CE) and Energy Efficiency (EE) of MFC

For comparison with the literature, very low CE of 11% for glucose based MFC is reported so far, and 24% CE for dual chambered POME based MFC is known so far. The highest CE of a MFC reported so far is 24.4% for 1 g/1 of glucose based dual chamber MFC. As shown in Fig. 16, the highest CE of 30.8% is obtained from the MFC of present disclosure. This CE value is 1.26 times larger than POME based MFC and 2.8 times more than glucose based MFC.

The EE obtained for the MFC of the present disclosure is 2.09% and is in agreement with the literature.

Water parameters before and after treatment by MFC

Samples are collected from the municipal sewage water, Guduvancherry, Chennai, India. The location was chosen because of the large number of residential complexes located in that area. Black colour samples were collected around 100m from the residential outlet. pH, EC, TSS, TDS, TS, BOD in five days and COD were obtained for MFC.

The waste water parameters and dissolved metal ion concentration in the waste water is determined by A AS and provided in Tables 5 and 6. It is inferred that the concentration of the metal ions are higher than the standards provided by Indian Pollution Control Board (PCB) and Indian Standards Institute (ISI).

Table 5

Parameters evaluated for the waste water before MFC treatment

Table 6

Dissolved metal ion concentration in waste water

From Table 6 it is inferred that it is not only Cr, but other heavy metals and hazardous metals like Pb and Zn are also present in very high concentrations in the regions where the waste water was collected.

After MFC treatment, the outlet water is tested for the above parameters and the results are demonstrated in Table 7.

Table 7

The clean water outlet tested for drinkable/utility water parameters

From Table 7, it is inferred that MFC had acted on the sewage waste water by using the sludge, organic and other minerals constituents in it as energy source for the microbes in the inoculum grown as biofilm on the anodic graphite electrode. The waste water to potable water conversion efficiency of the MFC is 30.8% as shown in Fig. 16.

16S rRNA sequencing analysis developed using sample collected from anolyte of MFC

Colonies of bacteria were developed using sample collected from anolyte of MFC. The DNA isolation was done using isolation kit and 98 colonies were randomly selected for restreaking. The isolated colonies were allowed to grow for 24 hrs in 5 ml Luria Bertani Broth with 50 pg ml ^-1 ampicillin at 37°C, at pH of 6.8 and 250 rpm. Plasmids were isolated using EZ-10 Spin Column Plasmid DNA Minipreps Kit (Bio Basic) as per the manufacturer's instructions. Plasmids were sequenced at the Eurofins Genomics India Pvt Ltd. Sequencing was done in one direction only using the sequencing primer M13F (5'-TGTAAAACGACGGCCAGT -3') and when the amplicon in the vector was found to be in a reverse orientation, M13R (5'-CAGGAAACAGCTATGAC-3') sequences were trimmed to remove vector sequences using the VecScreen tool (http:// www. ncbi. nlm. nih. gov /tools/vecscreen/) (Fig. 17 and Fig. 18). Chimeric sequences were detected by using the CHECK_CHIMERA utility at the Ribosomal Database Project and removed from the analyses. The bacterial species responsible for waste water treatment is identified by 16srRNA sequencing shown in Fig. 19. It is identified that the biofilm possesses Geobacter metallireducens as the major microbial community that treat waste water into potable water efficiently. Electrochemical characterization of MECC for CO2 sequestration

The major advantages of the microbial electrochemical systems are (i) bacteria are ubiquitous in the environment and feed on all carbon sources such as organic, inorganic, metallic and plastics. This versatile feeding habit of microbes in bio electrochemical cells produce a wide range of products from electricity to hydrogen or methanol to other value-added products depending on the source fed on by the bacteria. In the present disclosure, the carbon source for the bacteria is from CO2 generated in the MECC while treating waste water and also CO2 from atmosphere being fed into MECC (Fig. 1).

The CV and EIS studies of MECC for CO2 sequestration is discussed here. The operation time of MECC (1) from 24 to 1320 hrs with CO2 flow rate of 150ml/min, MECC (2) from 24 to 316 hrs with CO2 flow rate of 250ml/min, MECC (3) from 24 to 240 hrs with CO2 flow rate of 500ml/min.

Complete reduction of CO2 to alcohol and aldehyde is observed to be within 600 hrs for MECC (1), 192 hrs for MECC (2) and 216 hrs for MECC (3) as investigated by material and electrochemical characterization.

EIS of MECC for CO2 sequestration

The Electrochemical processes and internal resistances (Rint) of fabricated MECCs are obtained using EIS and fitting the equivalent circuit (EC) via Zman software (Fig. 20A - Fig. 22C).

Fig. 20A illustrates the electrochemical processes of Microbial electrochemical cell (MECC) from 24 to 864 hours.

Fig. 20B illustrates the electrochemical processes of Microbial electrochemical cell (MECC) from 1104 to 1320 hours. Fig. 21A illustrates equivalent circuit (EC) diagram to fit Electrochemical Impedance Spectroscopy (EIS) data of MECC(l).

Fig. 21B illustrates equivalent circuit (EC) diagrams to fit Electrochemical Impedance Spectroscopy (EIS) data of MECC(2), and MECC(3).

Fig 22A illustrates Nyquist plot of CO2 sequestration in cathode chamber of MECC(l) with 150ml/min as CO2 flow rate.

Fig 22B illustrates Nyquist plot of CO2 sequestration in cathode chamber of MECC(2) with 250ml/min as CO2 flow rate.

Fig 22C illustrates Nyquist plot of CO2 sequestration in cathode chamber of MECC(3) with 500ml/min as CO2 flow rate.

Solution resistance (Rl) and CPE (Q), Anodic and cathodic resistance (R2 & R3) are listed in Tables 8-10.

EIS analysis of MECC(l) is shown in Table 8 and Fig. 23A. At 24hrs, Ri= 85.89 . This Ri is 37 times less than reported value (3.2 ± 0.5 k ). As the days progress, Ri increases to I I 0.3Q after 192 hrs because of hindrance in movement of protons.

R2 is 41.45 Q at 24hrs and is 243 times less than the reported value (10.1 ± 0.5 k ) and it reaches 47.9 Q after 144 hrs. R2 eventually increase gradually to 61.46 at 384hrs and remain constant till 864hrs confirming the effective bacterial growth on anode upto 864 hrs and higher charge transfer rate. MECCs containing microbial biofilm have less anodic charge transfer resistance and contribute to Rmt of the MECCs. The increasing of R2 to 153.2 Q at 1104 hrs and 135.7 Q at 1320 hrs was due to less effective biofilm and slow charge transfer rate.

CPE (Qi) is 0.0009 to 0.0030 F and ai is 0.7 to 0.49 from 24 to 1320 hrs indicating diffusion process. The value of cathode resistance R3 is 6828 to 1.163 k from 24 to 1320 hrs and attributes to the cathodic reduction process. The CPE (Q2) is 4 to 29 mF from 24 to 1320 hrs and a2 increased from 0.7 to 0.9. The increase of ‘a’ factor attributes to ideal capacitor behaviour.

EIS analysis of MECC(2) is shown in Table 9 and Fig. 23B. Ri is 400.05 Q at 24hrs and increases to 568Q at 192 hrs. This behaviour is caused by the hindrance in transfer of protons. R2 is 3.76 k at 24 hrs which agrees well with reported data (3.2 ± 0.5 kQ) and it reduced to 2.82 kQ at 192 hrs confirming the effective development of biofilm on graphite anode and higher charge transfer rate. Qi value is 0.001 to 0.2 F from 24 to 72 hrs and ai is > 0.5 indicating ideal capacitor behaviour. Qi decreases and ai is near zero after 72 to 192 hrs indicating distorted resistance. The Warburg resistance W, has small variation from 24 to 192 hrs and accounts for the formation of H3O ⁺ and subsequently H2 gas in the catholyte. The formation of hydronium ions can be initiated by the infinite dilution of the catholyte leading to evolution of hydrogen gas and eventual to reduction of CO2 to value added products in MECC.

EIS analysis of MECC(3) is shown in Table 10 and Fig. 23C. The Ri is 400.05 Q at 24 hrs and reduces to 3.865 Q after 216 hrs indicating the higher proton transfer process. The R2 is 9 kQ at 24 hrs and is 3 times greater than the reported value (3.2 ± 0.5 k ) and reduces to 690.91 Q after 216 hrs confirming the effective development of biofilm on graphite anode and higher rate of charge transfer. Qi is 1.103m to 3.48pF from 24 to 216 hrs and ai is > 0.5 upto 168 hrs indicating ideal capacitor and decreases to zero after day 8 indicating distorted resistance. The Warburg resistance W has small variation from day 1 to day 9 and accounts for the formation of H3CP due to CO2 concentration of catholyte and leading to evolution of hydrogen gas and eventual conversion into value added products in the MECCs. Table 8

EC parametric values of MECC(l) Table 9

EC parametric value of MECC(2) Table 10

EC parametric value of MECC(3)

16S rRNA sequencing analysis of bacteria developed using sample from catholyte of MECCs

Fig. 24 illustrates Agarose gel electrophoresis image of 16S rDNA gene cloned plasmid (pDM20 vector) of the bacteria developed using sample collected from catholyte of MECC.

Fig. 25 illustrates Agarose gel electrophoresis of the 16S rRNA (of the bacteria developed using sample collected from catholyte of MECC) amplified PCR product using forward primer and a reverse primer by cloned plasmid as DNA, TAE buffer IX, stained with ethidium bromide.

Fig. 26 illustrates the percentage of microbes depicted in TA clone plasmid from MECC’s catholyte.

Colonies of bacteria were developed using sample collected from catholyte of MECCs. The DNA isolation was done using isolation kit and 98colonies were randomly selected for restreaking. The isolated colonies were allowed to grow for 24 hrs in 5 ml Luria Bertani Broth with 50 pg ml ^-1 ampicillin at 37°C at pH of 6.8 and 250 rpm. Plasmids were isolated using EZ-10 Spin Column Plasmid DNA Minipreps Kit (Bio Basic) as per the manufacturer's instructions. Plasmids were sequenced at the Eurofins Genomics India Pvt Ltd. Sequencing was done in one direction only using the sequencing primer M13F (5'- TGTAAAACGACGGCCAGT-3') and when the amplicon in the vector was found to be in a reverse orientation, M13R (5'-CAGGAAACAGCTATGAC-3') sequences were trimmed to remove vector sequences using the VecScreen tool (http:// www. ncbi. nlm. nih. gov /tools/vecscreen/) (Fig. 24 and Fig. 25). Chimeric sequences were detected by using the CHECK_CHIMERA utility at the Ribosomal Database Project and removed from the analyses. The bacterial species responsible for CO2 reduction is identified by 16srRNA sequencing shown in Fig. 26.

Confirmation of CO2 reduction by MRBs to Ethanol and acetaldehyde

The confirmation of the formation of products of reduction of CO2 into value added products such as ethanol, acetates, acetaldehyde, methanol and methane is carried out by two different techniques such as GC-MS and FTIR.

Fig. 27 illustrates Gas chromatography-mass spectrometry (GC-MS) peaks of ethanol and acetate identified through the CO2 reduction by MECC.

Table 11 demonstrates the products formed and the % yield of the same from the above-mentioned techniques.

Table 11

The percentage yield of different products by bio-electrochemical reduction of CO2 by MECC

Fig. 28 illustrates FTIR spectra of the liquid sample collected from catholyte of MECC after CO2 reduction by the Microbial rechargeable battery (MRB) of the present disclosure. FTIR analysis of the liquid obtained in the catholyte of the MECC part of MRB also indicates formation of multiple products possessing functional groups such as primary alcohol, secondary alcohol, aldehyde, ketonic, aliphatic hydrocarbons so on and so forth (Fig 28 and Table 12).

Table 12

The functional group identification of the products in the FTIR Examples:

The present disclosure will now be explained in further detail by the following examples. These examples are illustrative of certain embodiments of the disclosure without limiting the scope of the present disclosure.

Example:

A microbial rechargeable battery (MRB) prepared according to the teachings of the present disclosure using: the MFC as fabricated and characterised in the present disclosure, and the MECC(3) as fabricated and characterised in the present disclosure. MECC(3) was chosen for the MRB over MECC(l) and MECC(2) due to its higher conversion and yield of value added products from CO2.

Galvanostatic Charge/Discharge profile of MRB

GCD profile is determined at lA/g and operated from -0.6 to 0.6 V with respect to standard Ag/AgCl. The Voltage-time profile is linear GCD profile for 50 cycles.

Fig. 29 illustrates the Galvanostatic Charge/Discharge (GCD) profile of the MRB for 50 cycles.

The cycle stability of MRB in which MFC acts as the charging unit and MECC as discharging unit is 2000 cycles.

Fig. 30 and Fig. 31 illustrate the specific capacitance profile of the MRB for 50 cycles and 2000 cycles respectively.

Initially the specific capacitance of MRB is found to be 558 mF/g. In the second cycle 9% decrease in the capacitance is noticed. The reason for the reduction from the 1 ^st cycle to 2 ^nd cycle can be due to first level reduction of CO2 into ethanol, methanol, methane, acetaldehyde and acetic acid, thus making the solution acidic. After 2000 cycles, the capacitance reduces by 91%. The experimentally observed capacitance for MRBs is larger than the theoretical capacitance up to 50 ^th cycles.

After reaching 100 cycles, the capacitance reduces to 125 mF/g. The reason for this observation of higher experimental value could be attributed to higher efficiency of conversion of CO2 by discharging unit (MECC) of the MRBs and involves faradic process.

The MRBs assisted in reduction of CO2 into value added products revealed initial specific capacitance value of 0.558 F/g (50 cycles) reducing to 0.250 F/g and 0.051 F/g until 2000 cycles. Here MFC produced Vmax as 0.79 V and Pmax of 388.5 mW/m ² .

Advantages:

The MRB of the present disclosure has the following non-limiting advantages.

- Integration of wastewater treatment with CO2 sequestration.

- Charging and discharging unit combined making microbial rechargeable battery.

- A complete closed loop energy production and storage device operated by harmless microbes.

- Production of clean water and electricity generation, conversion of CO2 into salts leading to storage of electricity.

- Closed system with CO2 from atmosphere is fed as source for sequestration and hence a clean, green technology.

- Replenishing of source is external as well as internal (wastewater and CO2) whereas the replenishing of chemicals (such as redox mediator) is completely internal.

- Design and integration of the wastewater treatment unit with CO2 sequestration

- As the microbial culture and biofilm depends on the source of wastewater, the process is novel, unique and not attempted elsewhere. - End products are clean water, CO2 converted into value added products such as acetates, bicarbonates, or carbonates, electricity generation and storage.

- Upon need, the charging unit shall be connected to the load and charge the load without affecting the other processes occurring in the system.

Applications:

The MRB of the present disclosure has the following non-limiting industrial applications.

- Waste recycling - Waste to energy processes

- Low power to moderate power applications such as toys, drones, e-cycle, e- rickshaw

- Low power electronics and communication devices Although the present disclosure is described in terms of certain preferred embodiments, it is to be understood that they have been presented by way of example, and not limitation. Thus, the present disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.