Microbial fuel cells for chromium (VI) bioremediation using copper-metal organic framework containing anode

Abstract

The present disclosure provides a copper-metal organic framework anchored Cu foil (Cu-MOF / Cu-Foil) anode for a Microbial Fuel Cell (MFC), wherein the MFC uses a Cu foil cathode wherein the MFC helps in hexavalent Chromium (Cr (IV)) bioremediation, and power generation The Cu-MOF gets transformed to Cu2O nanoflowers in-situ under the operation conditions of MFC, which extends the electrocatalytic activity and stability. The MFC achieves bioremediation of 26 mg / l of Cr (VI) within a day, and the complete bioremediation within 5 days. The open circuit voltage (OCV), current density, and power density of the MFC are 0.983 V, 800 to 1027 mA / m2, and 980 to 1049 mW / m2 respectively. The coulombic efficiency attained by the MFC is 39.2% whereas the energy efficiency of the MFC is 35.42%. A method of preparation of the Cu-MOF / Cu-Foil anode is provided.

Description

[0001]TITLE MICROBIAL FUEL CELLS FOR CHROMIUM (VI) BIOREMEDIATION USING COPPER-METAL ORGANIC FRAMEWORK CONTAINING ANODE CROSS REFERENCE TO RELATED APPLICATIONS The present application is based upon and claims priority to India complete patent application number 202341056659 filed on August 24, 2023, the entire contents of which is herein incorporated by reference. FIELD The present disclosure relates to Microbial Fuel Cells (MFC), more particularly relates to a dual chambered MFC including metal organic frameworks (MOF) containing anode for Cr (VI) bioremediation, and simultaneous power generation. DEFINITION As used in the present disclosure, the following terms are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used indicates otherwise. As used herein, the terms “Cu-MOF deposited Cu foil anode” or “Cu-MOF / Cu foil” or just “Cu-MOF anode” may have been used interchangeably, which means the anode of the Microbial Fuel cell (MFC) of the present disclosure, which is fabricated by depositing Cu-MOF on a Cu-Foil substrate. As used herein, the terms “Cr2O3layer formed on Cu foil cathode” or “Cr2O3 / Cu foil” may have been used interchangeably, which means the deposition of Cr2O3on the Cu-foil cathode of the Microbial Fuel cell (MFC) of the present disclosure, after partial or complete Cr (VI) bioremediation. As used herein, the terms “H3BTC” or simply “BTC” may have been used interchangeably to mean 1,3,5-benzenetricarboxylic acid. BACKGROUND Owing to the increasing trend of industrialisation, increased metal-bearing solid wastes resulting in contamination of aquatic systems, worrying depletion of the natural resources, and growing scarcity of metals have been becoming increasingly a concern. Hexavalent form of chromium (Cr (VI)), owing to its highly soluble nature in water, is the most mobile form of Cr in the aquatic environment, which is why the threshold concentration of chromium in drinking water is set at 50 µg / L by the drinking water guidelines. In the United States and Canada, average Cr (VI) levels in drinking water range from 0.2 to 2 µg-Cr (VI) / L. The US Environmental Protection Agency (US EPA) acknowledges Cr (VI) as a harmful element, in living species, Cr (VI) is carcinogenic, genotoxic, teratogenic and mutagenic. Physico-chemical techniques currently used for Cr (VI) removal are not environmentally friendly and use a large number of chemicals. Such toxic Cr (VI) is released to environment by industrial processes such as leather tanning, electroplating, metal production, mining, textile industry, aircraft industry, wood preservation, mud drilling, pigment / dye manufacturing, and cement production. In addition, the domestic sewage, sludge from anaerobic digesters for production of biogas and biofertilisers, and primary clarifiers in wastewater plants also significantly contribute to the Cr (VI) contamination. Thus, bio-remediation of Cr (VI) becomes inevitable. There are various techniques currently available such as sedimentation, filtration, chemical precipitation, ion exchange, electrochemical remediation and adsorption of Cr (VI) and bioremediation. Among these available techniques, bio-electrochemical remediation offers advantages including self-sustainability, minimal sludge generation, low maintenance and low cost, whereas the other methods require reducing agents, sophisticated membrane systems, tedious regeneration procedures, high maintenance and cost. The beneficial remediation of metal bearing waste streams by microbial fuel cells (MFC) governs synchronous metal contaminants refinement, water purification and simultaneous energy generation. Recently, in wastewater treatment processes, there are promising concepts like ‘wealth from waste’ and ‘zero liquid discharge’, which help to treat the water as well as recovers valuable metals / materials from waste. In specific, the bioremediation of Cr (VI) results in formation of adequate amount of Cr2O3 nanomaterial using MFC based treatment. In an acidic environment, due to abundance of protons, facile Cr (VI) to Cr (III) reduction occurs. However, in MFC, owing to the neutral pH condition, the reaction kinetics of bioremediation of Cr(VI) becomes slower. Furthermore, in dual-chambered MFC, the oxidation of organic carbon by microbial community at anode generates electrons and protons, while the electrons are transferred to the cathode through an external circuit, the protons move to the cathode via membrane / salt bridge. As the availability of electrons at cathode to reduce Cr (VI) to Cr (III) is more, MFC performance increases. Thus, the advancement in MFC performance could be accomplished with the establishment of efficient electron generating anode. In general, the conventional electrodes including carbon cloth, graphite rod, carbon paper and Nickel foams were extensively used in MFC. These electrodes however require sequential pre-treatment, external electrocatalyst modification strategies, including their high cost are countable hindrances in the way of ubiquitous commercialization of MFCs. It is believed that the direct implantation of electroactive sites on anode material is highly anticipated to provide the efficacious MFC performance and durability. This would enhance the microbe-Cr (VI) and microbe-anode interaction and lead to extensive bioremediation process and energy production. Amid the various reported porous material such as aerogel, solgel, zeolite and carbonaceous material, Metal Organic Frameworks (MOF) are considered to be alluring electrode (anode), as the porous coordinated MOF offers high specific surface area, well-aligned crystal structure, low densities and good thermal stability, which can act as a robust nano-electrocatalyst with large surface area, having functional exterior and pores. Hence, the present inventors envisage a rational design and development of Cu foil-based anode materials in order to address the challenges associated with the bioremediation of hexavalent chromium and simultaneous power generation with dual-chambered MFC. The present inventors aim to synthesise a suitable MOF to be used in the anode of MFC, where the MOF offers expedient electron generation, having unique morphological and structural features, where the pores and voids present in the MOF govern predominant adsorption and diffusion of the electrolyte, while obviating the demand of complex processes and high-end technology instruments, and preventing formation of toxic chemicals and gases, yet, at the same time the MOF requires a convenient route of synthesis. The present inventors also envisage suitable sources of microorganisms for the MFC that synergistically deliver the benefits of zero cost, that are abundant in nature, and are environmentally benign. SUMMARY The present disclosure provides a copper-metal organic framework anchored Cu foil (Cu-MOF / Cu-Foil) anode for a Microbial Fuel Cell (MFC), wherein the MFC uses a Cu foil cathode wherein the MFC helps in hexavalent Chromium (Cr (VI)) bioremediation, and power generation. The Cu-MOF gets transformed to Cu2O nanoflowers in-situ under the operation conditions of MFC, which extends the electrocatalytic activity and stability. The anode and cathode arrangement facilitates the electron transfer process between active Cu-MOF (anode) centres and Cu foil (cathode) in the MFC. The MFC achieves bioremediation of 26 mg / l of Cr (VI) within a day, and the complete bioremediation within 5 days. The open circuit voltage (OCV) of the MFC is 0.983 V, current density of MFC is 800 to 1027 mA / m2, and power density of MFC is 980 to 1049 mW / m2. The coulombic efficiency attained by the MFC is 39.2% whereas the energy efficiency of the MFC is 35.42%. In an aspect of the invention, a method of preparation of Cu-MOF / Cu foil anode for the MFC is provided. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS Fig. 1 is a pictorial representation of antimicrobial activity (AMA) and antimicrobial resistivity (AMR) of Cu-MOF in phosphate-based and non- phosphate-based solutions. Fig.2 illustrates pictorial representation of Cu-MOF dehydration process Fig. 3A (a, b, c, d) shows the field emission scanning electron microscopy (FE- SEM) images of (a) Cu foil and (b-d) Cu-MOF deposited Cu foil anode (Cu- MOF / Cu foil). Fig. 3B illustrates a schematic representation of formation mechanism of Cu- MOF on Cu foil surface. Fig. 4A (a, b, c, d, e, f) illustrates (a to d) Field emission scanning electron microscopy (FESEM) images of Cu-MOF deposited Cu foil anode (Cu2O / Cu foil); (e and f) FESEM images of Cr2O3layer formed on Cu foil cathode (Cr2O3 / Cu foil); Fig.4B illustrates Energy-dispersive X-ray spectra (EDS) of Cu2O / Cu foil. Fig. 4C illustrates Energy-dispersive X-ray spectra (EDS) of Cr2O3 layer formed on Cu foil cathode (Cr2O3 / Cu foil). Fig. 5A illustrates the reaction mechanism of formation of Cu2O from Cu-MOF deposited Cu foil (Cu-MOF / Cu foil). Fig. 5B illustrates the schematic representation of formation of Cu2O from Cu- MOF deposited Cu foil (Cu-MOF / Cu foil). Fig.6 (a, b, c, d) illustrates XRD patterns of (a) bare Cu foil, (b) Cu-MOF / Cu foil, (c) Cu2O / Cu foil, and (d) Cr2O3 layer formed on Cu foil cathode (Cr2O3 / Cu foil). Fig. 7 illustrates FTIR patterns of Cu-MOF / Cu foil anode (before bioremediation through MFC) and the transformed Cu2O / Cu foil anode (after bioremediation through MFC). FIG. 8A illustrates Ultraviolet-Visible spectrum of effluent at different time intervals. FIG. 8B illustrates Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV- DRS) studies of solid state Cr2O3 (deposited on Cu foil cathode). Fig. 8C illustrates electronic configuration of Cr3+ground state in octahedral symmetry consists of 4A2g (ground state) and 4T1g (excited state). Fig. 9A illustrates Cyclic voltammetry (CV) studies of Cu2O / Cu foil anode based MFC from day 1 to day 13. Fig. 9B illustrates CV studies of Cu2O / Cu foil anode based MFC from day 15 to day 40. Fig. 9C illustrates comparison of CV studies for Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC from day 1 to day 40. Fig. 9D illustrates: Gibbs free energy of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC from day 1 to day 40. Fig. 10A illustrates Electrochemical Impedance Studies (EIS) of Cu2O / Cu foil anode based MFC from day 1 to day 13. Fig.10B illustrates EIS of Cu2O / Cu foil anode based MFC from day 15 to day 40. Fig.10C illustrates EIS of bare Cu foil anode based MFC from day 1 to day 40. Fig. 10D illustrates equivalent circuit model of Cu / Cu2O anode based MFC from day 1 to day 40. Fig. 11A illustrates Solution Resistance (Rs) plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC from 1 to 40 days. Fig. 11B illustrates Charge transfer resistance (R2) plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC from 1 to 40 days. Fig. 11C illustrates Diffusion resistance (R3) plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC from 1 to 40 days. Fig.12A illustrates Electrical conductivity plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC. Fig.12B illustrates Ionic conductivity plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC. Fig. 12C illustrates bar diagram showing influential increment in electrical conductivity and ionic conductivity of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC. Fig. 13A illustrates Chronoamperometric studies of Cu2O / Cu foil anode based MFC from day 1 to day 13 having the (insert) t-1 / 2vs. current plot. Fig. 13B illustrates Chronoamperometric studies of Cu2O / Cu foil anode based MFC from day 15 to day 40 having the (insert) t-1 / 2vs. current plot. Fig. 13C illustrates Chronoamperometric studies of bare Cu foil anode based MFC from day 1 to day 13 having the (insert) t-1 / 2vs. current plot. Fig. 13D illustrates Chronoamperometric studies of bare Cu foil anode based MFC from day 15 to day 40 having the (insert) t-1 / 2vs. current plot. Fig. 13E illustrates stability test for Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 10 hours at 0.5 V. Fig.14A illustrates Diffusion coefficient (D) profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC up to 4.2*10-2cm2 / s. Fig.14B illustrates Diffusion coefficient (D) profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC up to 10*10-4cm2 / s. Fig. 14C illustrates bar diagram showing Diffusion coefficient calculated by Randles‐Sevcik equation (Drs) and Diffusion coefficient calculated by Cottrell equation (Dct) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. Fig. 15A illustrates current density and power density profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 40 days. Fig. 15B illustrates bar diagram showing average current density (CD) and power density (PD) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. Fig. 15C illustrates voltage efficiency (VE) and Coulombic efficiency (CE) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. Fig. 15D illustrates energy efficiency profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 40 days. Fig. 15E illustrates bar diagram showing average voltage efficiency (VE), coulombic efficiency (CE) and energy efficiency (EE) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. 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 copper-metal organic framework anchored Cu foil (Cu-MOF / Cu-Foil) anode for a Microbial Fuel Cell (MFC), wherein the MFC uses a Cu foil cathode wherein the MFC helps in hexavalent Chromium (Cr (VI)) bioremediation, and power generation. The Cu-MOF gets transformed to Cu2O nanoflowers in-situ under the operation conditions of MFC, which extends the electrocatalytic activity and stability. The anode and cathode arrangement facilitates the electron transfer process between active Cu-MOF (anode) centres and Cu foil (cathode) in the MFC. Cu-MOF of the present disclosure owing to its unique morphological and structural features, such as the divalent Cu2+centre coordinated to four oxygen atoms of 1,3,5-benzene tricarboxylic acid (H3BTC) ligand and water molecules, it offers an expedient route of synthesis. Fig. 1 is a pictorial representation of antimicrobial activity (AMA) and antimicrobial resistivity (AMR) of Cu-MOF in phosphate-based solutions (PBS) and non-phosphate-based solutions. As shown in Fig. 1, the anti-microbial activity (AMA) of Cu was noticeably reduced in inorganic phosphate based medium (PBS). The release of free Cu+ion occurs in the case of Cu2O in any non-phosphate solutions leading to the toxic effect to bacterial community and death. In the case of PBS, there is a formation of water insoluble copper phosphate (Cu2(PO4)2) which prohibits the leaching of Cu+ion, and generates Anti-Microbial Resistivity (AMR) trait and subsequently prevents cell damaging in biofilm. Fig.2 illustrates pictorial representation of Cu-MOF dehydration process. As illustrated in Fig. 2, the coordination of Cu2+and carboxylate groups of H3BTC create the paddle wheel organization, where the easily removable axial aqua moiety contributes Cu-MOF, which is a Lewis acid property. Materials All the experiments were performed using procured chemicals without any further purification. − Agar-agar [99%] from, RANKEM, − Ammonium chloride [NH4Cl, 99.5 %] from SRL, − 1,3,5-benzene tricarboxylic acid [H3BTC, 98 %] from Alfa Aeasar, − Dipotassium hydro orthophosphate [K2HPO4, 99%] from RANKEM, − Hydrochloric acid [HCl, 38%] from Alfa Aeasar, − Nitric acid [HNO3, 72%] from RANKEM, − Potassium dihydrogen orthophosphate [KH2PO4, 99%] from RANKEM, − Potassium chloride [KCl, 99.5 %] from SRL, − Potassium dichromate [K2Cr2O7, 99.9%] from SRL. − Plastic boxes, PVC tube [diameter 20 mm], and straws were purchased from local shops. − Fruit waste (Citrus limetta) were collected from a juice shop at Guduvancherry, Chennai, India. − Cow dung were collected from regional cow farm at Guduvancherry, Chennai, India, and utilised as fresh (< 10 min after excretion). Preparation of Cu-MOF / Cu foil anode The Cu-MOF / Cu foil anode for the MFC was prepared by growing Cu-MOF films directly on Cu foil by one pot acid etching protocol using the following steps. Step 1: − Cleaning the surface of a Cu foil with ethanol: water (1:2) mixture; − Subjecting the surface cleaned Cu foil to sonication with 1M HCl solution and air drying to form a deoxygenated Cu foil. Step 2: − Soaking the deoxygenated Cu foil in 0.1M HNO3solution for a sufficient time in order to effect activation of the Cu surface, which is for about 30 mins; − Preparing a solution of 1,3,5-benzenetricarboxylic acid (H3BTC) and methanol in a ratio of 1:1; − Adding the H3BTC-methanol solution in a dropwise manner to the Cu-foil and / with stirring at 80oC for sufficient time (for about 6-8 hours) in order to complete formation of precipitate of cyan blue coloured Cu-MOF on Cu foil; − Washing the cyan blue coloured Cu-MOF containing Cu foil with ethanol and drying at 90oC for a sufficient time (about 10-15 hours). In an exemplary embodiment (Example 1), the preparation of Cu-MOF / Cu foil anode (for the MFC) is provided. Example 1: Preparation of Cu-MOF / Cu foil anode (for the MFC) A Cu foil of dimension 3 cm x 1 cm was surface cleaned with ethanol: water (1:2) mixture, followed by sonication with 1M HCl solution for 3 mins and air dried for 30 mins to form the deoxygenated Cu foil. The deoxygenated Cu foil was soaked in 0.1M HNO3solution for 30 mins, followed by the dropwise addition of 22 mM of 1,3,5-benzenetricarboxylic acid (H3BTC) dissolved in 3 ml methanol, and stirred at 80oC for 6 hrs. The resultant precipitate of cyan blue coloured Cu-MOF deposited on Cu foil was obtained. The as-prepared Cu-MOF / Cu foil was then washed with ethanol and dried at 90oC overnight to form the Cu-MOF / Cu foil anode for the MFC. Fabrication of Microbial Fuel Cell (MFC) A dual chambered MFC was fabricated having an anode chamber, and a cathode chamber where: − Anode chamber includes Cu-MOF / Cu foil anode of the present disclosure, 0.05 M Phosphate Buffer Solution (PBS) (pH-7), a mixed culture gut bacteria containing material such as cow dung, and organic waste material such as food waste or fruit waste. The PBS, the mixed culture gut bacteria containing material, and the organic waste are added in appropriate amounts in order to effect energy source consumption by bacteria to grow on the anode and form the biofilm. − Cathode chamber includes a Cu foil cathode, 0.05 M Phosphate Buffer Solution (PBS) (pH-7), and waste water containing Cr(VI). The PBS and the waste water containing Cr(VI) are added in appropriate amounts in order to effect maintenance of the pH at 7. − The anode and the cathode chambers are connected by a salt bridge (KCl : Agar in a ratio of 1:2). − The cathode and the anode (electrodes) are connected through electrical leads to an external source of electric power with the polarity being selected to direct the electrolyte flow of anions to the anode and the electrolyte flow of cations to the cathode. − The MFC encompasses the cathode chamber and the anode chamber, where the anode chamber is sealed to be anaerobic. The inventors tested several waste water samples for their chromium concentration. Among several waste water samples tested, they found 26 ppm (26 mg / l) K2Cr2O7 as the maximum concentration of K2Cr2O7 in the waste water samples, and 5 ppm (5 mg / l) K2Cr2O7as the minimum concentration of K2Cr2O7in the waste water samples. Owing to this tested inference, the inventors used 26 ppm (26 mg / l) K2Cr2O7 as a representation of the chromium waste water sample containing the maximum concentration of K2Cr2O7. The role of mixed culture gut bacteria containing material, and the organic waste in the present disclosure are as below: − mixed culture gut bacteria containing material to act as a natural microbe source. − organic waste material that includes natural reducing sugars. While for the purpose of the present disclosure, the inventors have used cow dung as mixed culture gut bacteria containing material, and fruit waste as organic waste material, a person skilled in the art, however can use materials that serve the similar purpose of the mixed culture gut bacteria containing material, and the organic waste in order to achieve the intended performance of the MFC of the present disclosure. In an aspect of the invention, with economic and environmental considerations, the anode chamber and the cathode chamber are made of low-cost, recyclable and reusable containers. In an aspect of the present disclosure, the low-cost containers for the anode chamber and the cathode chamber are plastic boxes. The MFC was operated under aerobic condition for bioremediation and simultaneous power generation. In an exemplary embodiment (Example 2), the fabrication of the MFC using the Cu-MOF / Cu foil anode, and Cu foil cathode is provided. Example 2: Fabrication of Microbial Fuel Cell (MFC) A dual chambered MFC was fabricated having an anode chamber, and a cathode chamber where: − 250 ml size plastic boxes were taken for each of anode chamber and cathode chamber. − Anode chamber includes the Cu-MOF / Cu foil anode (3 cm x 1 cm) as prepared in the present disclosure (in example 1), 50 ml of 0.05 M Phosphate Buffer Solution (PBS) (pH-7), and 30 gm of semi-solid cow dung as procured in its natural form, and 300 gm of semi-solid fruit waste as procured in its natural form. − Cathode chamber includes the Cu foil cathode, 50 ml of 0.05 M Phosphate Buffer Solution (PBS) (pH-7), and 400ml of 26 ppm K2Cr2O7. − The Anode and the cathode chambers are connected by a salt bridge (KCl : Agar in a ratio of 1:2). − The cathode and the anode (electrodes) are connected through electrical leads to an external source of electric power with the polarity being selected to direct the electrolyte flow of anions to the anode and the electrolyte flow of cations to the cathode. − The MFC encompasses the cathode chamber and the anode chamber, where the anode chamber is sealed to be anaerobic. Morphological studies of Cu-MOF / Cu foil anode Metal centres coordinated with organic ligands possess 3-D arrangement to form an octahedron morphology with square-shaped pores, which leads to the formation of porous spongy architecture that facilitates the diffusion and adsorption of electrolyte and ions. Fig. 3A (a, b, c, d) shows the field emission scanning electron microscopy (FE- SEM) images of (a) Cu foil and (b-d) Cu-MOF deposited Cu foil anode (Cu- MOF / Cu foil). As shown in Fig. 3A (a, b, c, d), Cu foil was uniformly decorated with octahedral shaped Cu-MOF with average size of 2-3 μm. The well-aligned and intergrown Cu-MOF crystals governs surface roughness compared to bare Cu foil and helps towards the adhesion and growth for biofilm on Cu-MOF. Fig. 3B illustrates a schematic representation of formation mechanism of Cu- MOF on Cu foil surface. As shown in Fig. 3B, Cu2+generated from Cu foil by acid etching reacts with BTC ligand resulting in the homogenous formation of Cu-MOF. The Cu foil in the anode acts both as a conducting support as well as a source of Cu2+ions. Fig. 4A (a, b, c, d, e, f) illustrates (a to d) Field emission scanning electron microscopy (FESEM) images of Cu-MOF deposited Cu foil anode (Cu2O / Cu foil); (e and f) FESEM images of Cr2O3layer formed on Cu foil cathode (Cr2O3 / Cu foil). As shown in Fig. 4A (a, b, c, d), the morphological investigation outlines that under electrochemical condition, Cu2O microflowers with densely packed nanosize petals with interspaces and voids act as electron and ion transfer passages, lower the charge transfer resistance and avail more accessible active sites, which also acts as a microrough substrate to trap the microbes and leads to the formation of stable biofilm. The sockets provided by hierarchical voids and inter sheet spaces of Cu2O / Cu foil promotes bacteria adhesion and rapid electron transfer, which guaranteed the promotion of electrocatalytic process. The integrated biofilm formations with effectual bacterial accumulation on Cu2O increase the interfacial properties of electrode material by extended bacterial cell and increased Cu2O contact. The FESEM observation of in-situ formed Cu2O / Cu foil authenticates the decoration of Cu2O microflowers on Cu foil and the approximate diameter of the flower is found to be 7.49 μm consisting of 81.97 nm thickened nanosheet petals. Under operating conditions of MFC, the in-situ formation of Cu2O was originated from the generation of Cu2+from water soluble Cu-MOF, where the MOF acts as a soft template by avoiding the utilisation of any surfactants and surface directing agents. Here, the released Cu2+is reduced to Cu2O by the natural reducing sugars (sucrose / glucose) present in the fruit waste. As shown in Fig. 4A (e, f), the Cr2O3 nanoparticle decorations over Cu foil cathode were with the average particle size of 241 nm. Fig.4B illustrates Energy-dispersive X-ray spectra (EDS) of Cu2O / Cu foil. Fig. 4C illustrates Energy-dispersive X-ray spectra (EDS) of Cr2O3layer formed on Cu foil cathode (Cr2O3 / Cu foil). As shown in Fig. 4B, the EDS studies attribute to the presence of C, N, O, Cu, P and K, where the potassium and phosphor originates from the buffer solution and nitrogen from the biofilm. Figs. 4 e and f shows that Cr2O3nanoparticle decorations over Cu foil cathode were with the average particle size of 241 nm. As shown in Fig. 4C, the elemental composition of obtained Cr2O3 over Cu foil cathode was analysed employing EDS, which indicates the presence of Cu, O, Cr, K, P, where the potassium and the phosphorous originates from the buffer solution and nitrogen from the biofilm. Fig. 5A illustrates the reaction mechanism of formation of Cu2O from Cu-MOF deposited Cu foil (Cu-MOF / Cu foil). Fig. 5B illustrates the schematic representation of formation of Cu2O from Cu- MOF deposited Cu foil (Cu-MOF / Cu foil). As shown in the reaction mechanism in Fig. 5A [equations (1), (2), (3), and (4)], and the schematic representation in Fig.5B: − the Cu2+ion react with the aldehyde group of reducing sugar and converted to Cu2O sheets. − Cu2+to Cu+reduction is also done through the hydrolysis reaction under an aqueous electrochemical condition. The schematic representation of formation of flowers from octahedral MOF consists of following mechanism: In aqueous PBS medium, Cu-MOF solubilize to release Cu2+. The released Cu2+ions react with the reducing sugars and results in Cu2O nanoparticles. With increased time, nanoparticles aggregate and give rise to nanosheets formation, orders hierarchically and leads to the formation of Cu2O microflowers. Structural analysis of bare Cu foil, Cu-MOF / Cu foil anode (before bioremediation), Cu2O / Cu foil anode (after bioremediation, i.e., after in-situ nanoflower formation during the operation of the MFC i.e., transformation of Cu- MOF to Cu2O on the anode Cu foil surface), and Cr2O3 / Cu foil (after bioremediation i.e., after Cr2O3 layer formed on the Cu surface) Fig. 6 (a, b, c, d) illustrates XRD patterns of (a) bare Cu foil, (b) Cu-MOF / Cu foil, (c) Cu2O / Cu foil, and (d) Cr2O3 layer formed on Cu foil cathode (Cr2O3 / Cu foil). As shown in Fig. 6(a), bare Cu foil exhibit XRD peaks at 43.73o, 50.85o, 74.50o, and 90.30ocorresponding to the Cu crystal planes of (111), (200), (220), and (311) respectively. As shown in Fig.6(b), the obtained XRD peaks of Cu-MOF / Cu foil correspond to respective planes of FCC lattice structured Cu-MOF crystals. The Cu-MOF / Cu foil anode exhibits XRD peaks between 10oto 40o, corresponding to 222, 400, 422, 333, 440, 600, 551, 555, 860, 773. As shown in Fig. 6(c), along with the XRD planes of Cu foil, Cu2O / Cu foil exhibit additional XRD peaks at 29.70o, 37.00o, 42.40o, 53.5oand 61.50°, indicating the crystal planes of (110), (111), (200), (211), and (220) respectively for the cubic structured Cu2O with d-spacing values of 2.44 Å. The additional peaks at 27.17o, 33.79o, 53,42oand 56.90orefers to the Cu3(PO4)2.3H2O. As shown in Fig. 6(d), Cr2O3 / Cu foil (Cr2O3deposited on the Cu-foil cathode) with rhombohedral structured Cr2O3 nanoparticles exhibited peaks at 29.40o, 36.40o, 42.10o, and 61.10orepresenting (012), (110), (113), and (214) planes, respectively. Upon comparison of XRD patterns of Cu-MOF, Cu2O / Cu foil and Cr2O3 / Cu foil, the diffraction peaks of Cu foil shifted to higher 2Ѳ values, indicating the hindrance of Cu foil surface. Vibrational studies of Cu-MOF / Cu foil anode (before bioremediation), Cu2O / Cu foil anode (after bioremediation, i.e., after in-situ nanoflower formation during the operation of the MFC i.e., transformation of Cu-MOF to Cu2O on the anode Cu foil surface). Fig. 7 illustrates FTIR patterns of Cu-MOF / Cu foil anode (before bioremediation through MFC) and the transformed Cu2O / Cu foil anode (after bioremediation through MFC). As shown in Fig. 7 (Before MFC), the FTIR spectra of Cu-MOF / Cu foil exhibit characteristic bands at 1583, 1414, and 1626 cm-1attributed to the C=C and C=O symmetric stretching vibration modes, enunciating from the carbon backbone of BTC ligand. The stretching vibrations of O-H and C-C bonds appeared at 3640 and 1640 cm-1. The band at 719 cm-1corresponds to the Cu-O stretching vibration mode. The vibrational stretching modes of N-H present in amide groups of protein is observed at 3246 cm-1, whereas the peaks at 2989, 1714 and 1098 cm-1attributes to the C-H stretching frequency in methylene group of fatty acids, amide type I in protein stretching frequency and (PO4)2-group P-O vibrations. As shown in Fig. 7 (after bioremediation through MFC) the FTIR spectra is more specific with broad peak between 3500 to 3000 cm-1attributed to the asymmetric stretching of hydroxyl groups that are hydrogen bonded to each other, the shoulder peaks near 3000 cm-1correspond to the overtone of adsorbed water molecules, peaks exhibiting characteristic bands at 1583, 1414, and 1626 cm-1are attributed to the C=C and C=O symmetric stretching vibration modes of BTC ligand. The high intensity peak around 1100 cm-1corresponds to the C-H stretching frequency. The Cu-O stretching frequency is noticed at 644 cm-1. All the peaks are well defined in comparison to Fig 7 (before bioremediation through MFC). The obtained details on functional group analysis affirm the presence of biofilm enrichment on the surface of Cu2O. The strong peak at 644 cm-1highlights Cu-O bond stretching in Cu2O with Cu+centres and Cu exists in cuprous state. Electronic spectroscopy analysis FIG. 8A illustrates Ultraviolet–Visible spectrum of effluent at different time intervals. FIG. 8B illustrates Ultraviolet–Visible Diffuse Reflectance Spectroscopy (UV- DRS) studies of solid state Cr2O3 (deposited on Cu foil cathode). Fig. 8C illustrates electronic configuration of Cr3+ground state in octahedral symmetry consists of 4A2g (ground state) and 4T1g (excited state). As shown in Fig. 8A, during bioremediation of Cr (VI), the concentration of Cr (VI) decreased drastically from day 1 to day 15 with the precipitation of Cr2O3. Fig. 8A also shows that, On day 1, the peaks at 359, 434 and 261 nm, depicts the presence of Cr (III) and Cr (VI) respectively. As shown in Fig.8B, the UV-DRS spectrum of d3system based Cr2O3, where the peaks at 444 and 643 nm are contributed to the 4A2g / 4T1g, 4A2g / 4T2g allowed transitions of Cr(III) ions. Fig. 8C shows the electronic configuration of Cr3+ground state in octahedral symmetry consists of 4A2g (ground state) and 4T1g (excited state), which indicates that all the Cr(VI) are reduced to Cr(III) by MFC. Electrochemical studies: The electrochemical performances of the MFC were carried out by two electrode mode at a scan rate of 1 mV / s at neutral pH using Cu2O / Cu as anode, and its comparison with bare Cu foil as anode employing admiral electrochemical workstation at ambient temperature. The electro-analytics was performed with techniques including Cyclic Voltammetry (CV), Electrochemical Impedance Studies (EIS) and Chronoamperometry (CA) for a period of 40 days. Cyclic voltammetry (CV) studies Fig. 9A illustrates Cyclic voltammetry (CV) studies of Cu2O / Cu foil anode based MFC from day 1 to day 13. Fig. 9B illustrates CV studies of Cu2O / Cu foil anode based MFC from day 15 to day 40. Fig. 9C illustrates comparison of CV studies for Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC from day 1 to day 40. As shown in Fig. 9C, Bare Cu foil displays minimal background current with significant redox peaks, respective to the existence of conductive network and electroactive sites. Although Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC show similar voltammetric behaviour, Cu2O / Cu foil exhibits higher current, owing to enhanced surface area and electrical conductivity. As shown in Fig. 9A and Fig. 9B, under the operating conditions of MFC, Cu2O / Cu foil nanocomposite results in shifting of the redox peak owing to the prolonged exposure of anodes to the electrolyte and adsorption of complexes on its surface. On increasing the contact time of bare Cu foil in the electrolyte provides minimal electron transport behaviour, whereas Cu2O / Cu foil shows noticeable response, indicating prolonged electron conducting trait. The p-type semiconducting property of Cu2O develops the change in atomic charge and behaves as a backbone for transport of electron that has been generated by the biofilm formed on the surface of Cu2O. Furthermore, the Cu2O flowers on Cu foil increases the surface defects and disorders, which promotes the electrical conductivity and ensures MFC activity. Amidst bare Cu foil and Cu2O / Cu foil studied here, Cu2O / Cu foil generates maximum response as demonstrated by the augmented anodic current density of 1150 mA / cm2. The flower architecture of Cu2O facilitates closely stacked multi-sheet structure with labile oxygen functionalities, which ease the adsorption of the electrolyte. The interlayer cavities and voids of flowers promote the beneficial accommodation and diffusion of the electrolyte, which in turn enhances the accessibility of electrons towards the active sites of Cu2O / Cu foil. Furthermore, the oxygen vacancies of Cu2O leads to the generation of defects, dislocations and disorders in Cu2O / Cu foil matrix that increases the micro-strains and active sites, which improves the surface adsorption and conduction phenomenon. The enhanced hole-electron transfer kinetics at a heterojunction of electron generating biofilm and p-type semiconductor Cu2O reduces the Fermi level, resulting in the generation of rapid electron transport channels for anodic performances. Higher anodic performance promotes the Cr(VI) remediation to greater extent with high proton diffusion and redox nature. Further, the hierarchical decoration of Cu2O flowers on conducting structure of Cu foil reduces the utility of binder and surfactant as well as inhibits the inhomogeneity arising due to coating. It also increases the electrochemical stability of the MFCs under harsh electrochemical perturbations. The combinatorial advantages evince the effectual development in structural, morphological and electrical properties, which facilitates the efficient electron transfer and result in improved performance of MFC. Furthermore, upon increasing the duration of contact of Cu2O / Cu foil anode in the electrolyte, steady performances of MFC are noticed, which ensures the antifouling trait of Cu2O / Cu foil under harsh electrochemical treatment, where the bare Cu foil exhibits loses on prolonged exposure to the electrolyte. Electrochemical-thermodynamic interpretation The respiration (glycolysis) and fermentation process of heterotrophic microbe breaks up organic matter to small units such as pyruvate, acetate, lactate, glucose and carbon dioxide, which is evident from the Gibbs free energy (ΔG) of their oxidation process. Under anaerobic condition, the exogenous oxidants (electron acceptor) receive the electron transferred from the respiratory chain via NADH (kreb’s cycle). In general, compared to aerobic respiration, the energy gain in anaerobic digestion is lower due to the less positive redox potentials of electron acceptors. Besides, microbes from MFC extract large amount of ΔG to build biofilm and to head the redox reactions. In theory basics, the ΔG required for the functionality of MFC based on glucose, Cu(I / II)and Cr(VI / III)redox cascades, can be derived from electrode reactions with respect to Standard Hydrogen Electrode (SHE) as provided in the following equations. Anodic reaction C6H12O6+ 6H2O 6CO2+ 24H++24e- (EOanode)= - 0.43V) (5) Cathodic reaction 14H++ Cr2O72-+ 6 e- Cr3++ 7H2O (EOcathode)= 1.32V) (6) Net reaction C6H12O6 + Cr2O72-6CO2 + 2Cr3++ 10H++ 18e- + H2O (EOcell)= 1.75V) (7) ΔGƟnet= nF(EƟbio- EƟCr (VI / III)) (8) ΔGƟbio= nF(EƟdonor- EƟacceptor) (9) Here in equation.9, EƟdonor = EƟglucose (EO= - 0.43V) (I / II)are: - ΔGƟnet= -3595.03 kJ / mole. In practical applications, there is no complete oxidation of substrate, the microorganism oxides glucose to pyruvate (ΔG= -225 kJ / mole) or acetate (-206 kJ / mole), where for one electron, the amount of ΔGelecis -89.7 kJ / mole with electron acceptor (MET). Fig.9D illustrates: Gibbs free energy of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC from day 1 to day 40. As shown in Fig. 9D, the ΔG values for constructed MFC assembly with Cu2O / Cu and Cu foil anode for duration of 40 days, calculated using modified Butler-volmer equation, (10) io = Current density (A / m2), n= number of electrons transferred, F= faraday’s constant (C / mole), Kb= boltzmann constant (m2kg / S2K), T= temperature (K), C= concentration (M), A= area of the electrode (m2), h= plank’s constant (m2kg / S), ΔG= gibbs free energy (kJ / mole), R= gas constant (J / mole.K). Cu2O / Cu foil anode and bare Cu foil anode exhibited average ΔGcalcof -100.2 and -98.9 kJ / mole respectively on operation of MFC, which is lower as the spontaneity of the redox reaction expressed by the negative sign of the free energy calculated. The role of Cu+as electron acceptor on conductive copper substrate stimulates the extended redox shuttle with advantages including greater physical contact and electrochemical nernstian behaviour. This facilitates the regeneration of acceptor species for prolonged duration. In addition, the standard redox potential of Cu(I / II)species (0.34 V) is close to organic matter potential (0.43V) and more negative than the Cr(VI / III)(1.32 V), govern low overpotential at electrode surface. Aforementioned factors with spontaneity in the process attributing the effectual conversion of Cr(VI) to Cr(III), where Cu(I / II) shuttle satisfies the electron transfer process from biofilm to circuit and increases the MFC performances. Electrochemical Impedance Studies The backbone of internal resistance including ohmic, mass transfer and kinetics through resistive, capacitive and inductive impedances were studied by electrochemical impedance (EI) technique, which provides quantified data and promotes better interpretation of the system. To comprehend the electrical performances of Cu2O / Cu anode and bare Cu foil anode, the Electrochemical Impedance Studies (EIS) were performed in the frequency range of 0.01 mHz to 0.1 kHz at 50 mv / s AC perturbation amplitude. Fig. 10A illustrates Electrochemical Impedance Studies (EIS) of Cu2O / Cu foil anode based MFC from day 1 to day 13. Fig.10B illustrates EIS of Cu2O / Cu foil anode based MFC from day 15 to day 40. Fig.10C illustrates EIS of bare Cu foil anode based MFC from day 1 to day 40. Fig. 10D illustrates equivalent circuit model of Cu / Cu2O anode based MFC from day 1 to day 40. The equivalent circuit model was generated by EI fitting the nyquist plot in EC-lab. The obtained parameters of circuit model are plotted in Fig.11A – 12C where the resistance components of Cu2O / Cu foil were compared to Cu foil anode, which gives an information on role of anode material on MFC function and Cr (VI) remediation. Fig. 11A illustrates Solution Resistance (Rs) plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC from 1 to 40 days. Owing to the copper excess defects and electron rich environment of Cu2+1O, there is influential electron transfer which results in low solution resistance (Rs) of Cu2O / Cu foil anode (191.7 Ω) than the bare Cu foil anode (205.6 Ω). The non- redox active centres of bare Cu foil need more activation energy for the proceeding reaction, whereas the electroactive Cu2O foster the electron transfer process with adverse electron transport chain from biofilm. Upon daily operation of MFC there is a steady increment in the Rs of Cu2O / Cu foil due to the impediment in proton transfer, reaches the maximum value of 661.6 Ω. Cu foil has shown dynamic property with maximum Rs of 1171 Ω, due to its non- catalytic and inferior electrode kinetics behaviour. Fig. 11B illustrates Charge transfer resistance (R2) plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC from 1 to 40 days. A function of charge transfer resistance vs. time postulates the details on electron transport, biofilm role, and electrical conductivity throughout the durability of fuel cell operation. The charge transfer resistance (R2) is directly related to the electrochemical reaction kinetics of cathode in MFC. The low R2pledges an augmentation in electric conductivity, capacitance and charge transfer of electrodes by subduing the activation energy barrier. As shown in Fig. 11B, the observed decrement in R2 was observed for Cu2O / Cu foil, can meritoriously increase the fuel cell performance, which triggers the cathodic reaction kinetics and yields high Cr(VI) remediation related to bare Cu foil anode. The reduction in R2 is efficient with the redox mediators in a MFC unit via the charge transfer overpotential decrement. The impeccable electron transfer was rendered by the Cu2O / Cu foil, Cu+ion acts as an electron acceptor. The electrochemical reaction rate is inversely proportional to the R2, which represents by the diameter of the semicircle. It is observed that on 5thday, the R2 for Cu2O / Cu foil becomes 8 folds lower than the day 1, which signifies the stable biofilm formation and electron transportation pathway. The electrogenic bacteria with Cu+electron shuttles coins coherent increase in charge transfer by overwhelming the activation losses and ameliorates the Cr(VI) reduction kinetics at cathode sequentially. In the case of bare Cu foil, the high R2values were observed due to its bare and smooth surface, which lacks biofilm adhesion and growth. The constant phase element Q2 lies within a range of 0.00037-0.0387 with a2>0.5, represents the capacitive behaviour of the anode material. Fig. 11C illustrates Diffusion resistance (R3) plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC from 1 to 40 days. The diffusion resistance (R3) is proportionate to the concentration of dissolved redox species in an electrolyte which is inversely related to the thickness of diffusion layer. Owing to the inefficient ion transfer between microbes and electrode on day 1, the higher R3was observed for Cu2O / Cu foil (2.4 KΩ) and Cu foil (958.3 Ω) MFCs. On day 5, the R3of Cu2O / Cu foil and Cu foil were reached 309.4 Ω and 5.5 KΩ, respectively, suggesting that the stable biofilm formation on Cu2O / Cu foil electrode enhances the ion transport trait and increases the fuel cell operation. The synergistic morphological and structural merit of Cu2O enhances the bacterial and acceptor (Cu+) interaction and minimizes the internal resistances of the cell. The neutral PBS based electrolyte medium retains ionic conductivity of the system, facilitates the ion transport and equates the electro neutrality. In addition, the dissolved ions which contributes the charge carrier also reduces the Rs by stable and defined utilisation of organic matter and attributes steady power and current generation. Fig.12A illustrates Electrical conductivity plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC. As shown in Fig. 12A, the electrical conductivity for Cu2O / Cu foil is twofold greater than the bare Cu foil, owing to the morphological and structural advantages of Cu2O flowers formed by packing of nano-petals. Fig.12B illustrates Ionic conductivity plot of Cu2O / Cu foil anode based MFC and Cu foil anode based MFC. As shown in Fig. 12B, the average ionic conductivity of Cu2O / Cu and bare Cu foil MFC units were 18.69 and 2.46 mS / cm, supporting the D values obtained via Randles-Sevcik equation-based calculation under “Diffusion coefficient calculation” below. Fig. 12C illustrates bar diagram showing influential increment in electrical conductivity and ionic conductivity of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. Bar diagram of Fig. 12C shows the influential increment in electric and ionic conductivity of Cu2O / Cu foil system, where the high conductivity indicates the high proton and charge transfer processes and coins the advantage towards Cr(VI) reduction process. Diffusion coefficient calculation The ChronoAmperometric (CA) studies of Cu2O / Cu and Cu foil for 40 days at 0.5 V for 3000s, the t-1 / 2values were calculated using plot between i vs t-1 / 2. Fig. 13A illustrates Chronoamperometric studies of Cu2O / Cu foil anode based MFC from day 1 to day 13 having the (insert) t-1 / 2vs. current plot. Fig. 13B illustrates Chronoamperometric studies of Cu2O / Cu foil anode based MFC from day 15 to day 40 having the (insert) t-1 / 2vs. current plot. Fig. 13C illustrates Chronoamperometric studies of bare Cu foil anode based MFC from day 1 to day 13 having the (insert) t-1 / 2vs. current plot. Fig. 13D illustrates Chronoamperometric studies of bare Cu foil anode based MFC from day 15 to day 40 having the (insert) t-1 / 2vs. current plot. Fig. 13E illustrates stability test for Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 10 hours at 0.5 V. As illustrated in Fig. 13E, the stable current production was observed for Cu2O / Cu for 10 hours whereas bare Cu foil faces current lose after 2 hours. The diffusion coefficient (D) values of the MFC are calculated using Cottrell equation as well as Randles‐Sevciks equation. The redox shuttle gets complete as the proton migrates from anode to cathode and balances the net charge, the influential diffusion of the proton transfer governs substantial increment in redox behaviour. The diffusion coefficient of MFC modules were calculated using the Randles‐Sevciks equation (quasi reversible process; ΔEp ≠ 57 mV) as follows from CV data, current (A), D = diffusion coefficient (cm2 / sec), A= area of the electrode (cm2), C= concentration (mole / cm3), and ν= scanrate (V / sec). Cottrell equation I = zFAC (D / pit)1 / 2 where z is the number of charge involved in the reaction, F is Faraday’s constant, A is the area of the electrode, C is the initial concentration of the electrolyte, D is the diffusion coefficient, t is the time of the reaction. Fig.14A illustrates Diffusion coefficient (D) profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC up to 4.2*10-2cm2 / s. Fig.14B illustrates Diffusion coefficient (D) profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC up to 10*10-4cm2 / s. As shown in Fig. 14A, the average D value was obtained as (4.0±0.3) x 10-2and (3.1±0.3) x 10-2cm2 / s for Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC respectively for 40 days, which is 2-fold higher than the reported values. As shown in Fig. 14B, the average D value of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 40 days were 2.33 x 10-4and 1.91 x 10-4cm2 / s, respectively. Fig. 14C illustrates bar diagram showing Diffusion coefficient calculated by Randles‐Sevcik equation (Drs) and Diffusion coefficient calculated by Cottrell equation (Dct) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. The diffusion coefficient data indicates that the Cu2O / Cu foil anode facilitates faster transportation and migration of ions from the electrolyte to electrolyte / electrode interface and leads to better Cr(VI) reduction in MFC. The obtained average D values for respective processes are 2.92 x 10-3and 1.43 x 10-3cm2 / s. The significance difference in the D values from CV and CA studies suggests the dependency of diffusion coefficient on the time domain. MFC studies and Cr bioremediation Fig. 15A illustrates current density (CD) and power density (PD) profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 40 days. As shown in Fig.15A, the Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC exhibit open circuit voltages (OCV) of 0.983 V and 0.580 V, respectively and falls to 0.4 V and 0.17 V respectively after 40 days (i.e., after completion of bioremediation by MFC) (experimentally calculated, but not shown in the figure). The calculation of OCV after 40 days (after bioremediation by MFC) When the OCV is 1 V, the respective CD is 800 mA / m2Therefore, when the OCV is 0.4V, the corresponding CD becomes: 800 mA / m2* 0.4 V / 1V = 320 mA / m2From Fig. 15A, it can be observed that the voltage lower limit after bioremediation for Cu2O / Cu foil anode based MFC to be 0.4 V. A similar calculation of voltage lower limit after bioremediation is done for bare Cu foil anode based MFC, which is 0.17 V. The dotted lines in Fig.15A depict the variation of OCV based on current density and power density before bioremediation (by MFC), and the solid lines depict the variation of OCV based on current density and power density after bioremediation (by MFC). As shown in Fig. 15A, Cu2O / Cu and Cu foil exhibit, power densities of 1076.7 and 580 mW / m2, respectively in which Cu2O / Cu foil produces high power, attributed to the advantages of Cu2O flowers on Cu matrix. The direct growth of Cu2O flowers on Cu foil effectively reduces the activation barrier towards better electron transport kinetics and delivers an enhanced current density of 1163 mA / m2, as shown in Fig.15A, Fig. 15B illustrates bar diagram showing average current density (CD) and power density (PD) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. As shown in Fig. 15B, both CD and PD attain very high values in the case of Cu2O / Cu foil anode based MFC in comparison to the bare Cu foil anode based MFC. Cu2O / Cu foil holds high electrochemical stability due to the combinatorial properties of individual components and their compatibility and strong interaction between the biofilm (formed over the Cu2O surface) and Cu2O. The water- insoluble contents of stable biofilm including protein, cellulose, lignin, pectin and hemicellulose are considered as the fundamental hydrophobic skeleton leading to the better hydrolytic stability under an aqueous medium. The effectual growth of Cu2O flowers on the Cu foil via direct etching from Cu foil limits the agglomeration and leach-out of Cu2O, which simultaneously facilitates the efficacious diffusion of glucose and transport of electrons / ions, leading to the consistent MFC performance over a period of time. Fig. 15C illustrates voltage efficiency (VE) and Coulombic efficiency (CE) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. Fig. 15D illustrates energy efficiency profiles of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC for 40 days. Fig. 15E illustrates bar diagram showing average voltage efficiency (VE), coulombic efficiency (CE) and energy efficiency (EE) of Cu2O / Cu foil anode based MFC and bare Cu foil anode based MFC. The dotted lines in Fig. 15C depict the VE and CE before bioremediation (by MFC), and the solid lines depict the VE and CE after bioremediation (by MFC). As illustrated in Fig. 15C, the voltage efficiency (VE) of the Cu2O / Cu foil anode based MFC is 45%, and for bare Cu foil anode based MFC the VE is 35%. As shown in Fig. 15C, the maximum CE obtained was 39.2% for Cu2O / Cu foil anode based MFC and 23.64% for bare Cu foil anode based MFC. As shown in Fig. 15D, the maximum EE obtained was 35.42% for Cu2O / Cu foil anode based MFC, and 13.6% for bare Cu foil anode based MFC. As shown in the bar diagram of Fig. 15E, the maximum efficiency profile is due to the greater electron transfer from biofilm to Cu2O / Cu foil anode. The natural microbe source (cow dung) constitutes of crude fibre, protein, cellulose and carbohydrate along with microorganisms, where these biomolecules act as a bridging platform for the adsorption of microbes on Cu2O. A physical barrier developed by polysaccharide and polypeptide matrixes of cow dung embeds an antimicrobial resistance to the bacterial community to Cu+ions. In addition, on long term exposure of bacteria and Cu+, the mobile genetic elements (MGE) develop Cu resistance determinants, which increase the tolerance of bacteria to Cu+and inhibit the antimicrobial properties of Cu+. Besides, the phosphate buffer solution (PBS) in MFC condition prohibits the free Cu+ion / radical generation by the effect of solvation, which also impedes the antimicrobial activity of Cu+. The resulted biofilm over the surface of Cu2O / Cu foil is developed uniformly, where the bacterial community were trapped between the petal voids of the flower and interspaces of flowers. Hence, the better electron transport was governed via the biofilm to Cu2O / Cu foil anode, where the biofilm generates electron through the decomposition of organic matter. The MFC achieves bioremediation of 26 mg / l of Cr (VI) within a day, and the complete bioremediation within 5 days. Advantages: The present disclosure has the following non-limiting advantages. − The pores and voids present in Cu-MOF govern predominant adsorption and diffusion of the electrolyte. − During the working of MFC, the Cu-MOF transformed to Cu2O nanoflowers in-situ; where the in-situ nanoflowers generation increases the electrocatalytic activity, conductivity and stability of the Cu-MOF / Cu foil anode under harsh electrochemical conditions. − Specifically, the designed synthetic strategy generates Cu-MOF with no external precursor on Cu foil by direct growth method and the MFC unit directs Cu-MOF to Cu2O in-situ formation by obviating the demand of complex processes and high-end technology instruments, and preventing formation of toxic chemicals and gases. − Formation of Cu2O flowers on Cu foil as anode, the electron acceptor and electron transfer property of the anode material is improved, resulting in decrement in internal resistance owing to the developed wettability, porosity, and surface biocompatibility, which in turn increased the power and bioremediation efficiency of the MFC. − The utilisation of natural resources such as cow dung and Citrus limetta waste, in the MFC of the present disclosure as a source of microorganism, synergistically delivers advantages of zero cost, environmentally benign nature and abundance. − After the lifespan of the MFC of the present disclosure, the digested sludge can be utilised as natural fertilizers, while providing a solution to solid waste management and disposal. − The cost and maintenance of MFC were simplified by replacing generally used membranes with a salt bridge, and generally used acrylic / glass reactors by low-cost materials such as plastic boxes, which are also reusable, and recyclable too. − Co-generation process in which MFC is utilized to generate bioelectricity as well as Cr(VI) bioremediation − utlization of MOF based anodes increases the current and power density outputs that can be used to charge low power electronics Applications: The present disclosure has the following non-limiting industrial applications. − Bioremediation of hexavalent Chromium (Cr(VI)) containing environmental pollutants. − Power generation. 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.

Claims

CLAIMS 1. A method of preparation of a copper-metal organic framework anchored Cu foil (Cu-MOF / Cu-Foil) anode for a Microbial Fuel Cell (MFC) comprising the steps of: − cleaning the surface of a Cu foil with a mixture of ethanol and water in a ratio of 1:2; − subjecting the surface cleaned Cu foil to sonication with 1M HCl solution and air drying to form a deoxygenated Cu foil; − soaking the deoxygenated Cu foil in 0.1M HNO3solution for 30mins; − preparing a solution of 1,3,5-benzenetricarboxylic acid (H3BTC) and methanol in a ratio of 1:1; − adding the H3BTC-methanol solution in a dropwise manner to the Cu- foil with stirring at 80oC for 6-8 hours to complete formation of precipitate of cyan blue coloured Cu-MOF on Cu foil; and − washing the cyan blue coloured Cu-MOF containing Cu foil with ethanol and drying at 90oC for 10-15 hours.

2. The anode prepared by the method as claimed in claim 1, wherein the Cu- MOF / Cu foil anode exhibits XRD peaks between 10oto 40o, corresponding to 222, 400, 422, 333, 440, 600, 551, 555, 860, 773.

3. The anode prepared by the method as claimed in claim 1, wherein the Cu- MOF in the Cu-MOF / Cu foil anode during the electrochemical process conditions of the MFC gets in-situ transformed to Cu2O nanoflowers on the Cu foil surface of the anode, wherein the in-situ transformed Cu2O / Cu foil anode exhibits XRD peaks between 30oto 60o, which corresponds to 110, 111, 200, 211, 220.

4. The anode prepared by the method as claimed in claim 1, wherein the anode exhibits a highly porous structure, which facilitates the facile formation of biofilm.

5. The anode prepared by the method as claimed in claim 1, wherein the anode exhibits a high surface area.

6. A Microbial Fuel Cell (MFC) comprising, an anode chamber, and a cathode chamber, wherein − The anode chamber includes a copper-metal organic framework anchored Cu foil (Cu-MOF / Cu-Foil) anode, 0.05 M Phosphate Buffer Solution (PBS) (pH-7), a mixed culture gut bacteria containing material, and organic waste material; − Cathode chamber includes a Cu foil cathode, 0.05 M Phosphate Buffer Solution (PBS) (pH-7), and waste water containing Cr(VI); and − a salt bridge that connects the anode and the cathode chambers, wherein, the cathode and the anode are connected through electrical leads to an external source of electric power with the polarity being selected to direct the electrolyte flow of anions to the anode and the electrolyte flow of cations to the cathode.

7. The MFC as claimed in claim 6, wherein the waste water containing Cr(VI) has a concentration of 26 ppm K2Cr2O7.

8. The MFC as claimed in claim 6, wherein the Cu-MOF on the surface of the Cu foil gets transformed to Cu2O nanoflowers in-situ under the electrochemical operation conditions of MFC, which extends the electrocatalytic activity and stability.

9. The MFC as claimed in claim 6, wherein the Cu-MOF in the Cu-MOF / Cu foil anode during the electrochemical process conditions of the MFC gets in-situ transformed to Cu2O nanoflowers on the Cu foil surface of the anode, wherein the in-situ transformed Cu2O / Cu foil anode exhibits XRD peaks between 30oto 60o, which corresponds to 110, 111, 200, 211, 220 planes.

10. The MFC as claimed in claim 6, wherein the mixed culture gut bacteria containing material is cow dung.

11. The MFC as claimed in claim 6, wherein the organic waste material is fruit waste.

12. The MFC as claimed in claim 6, wherein the MFC exhibits an open circuit voltage (OCV) of 0.983 V.

13. The MFC as claimed in claim 6, wherein the MFC exhibits an open circuit voltage (OCV) of 0.4 V after completion of bioremediation by MFC.

14. The MFC as claimed in claim 6, wherein the MFC exhibits current density of 800 to 1027 mA / m2.

15. The MFC as claimed in claim 6, wherein the MFC exhibits power density of 980 to 1049 mW / m2.

16. The MFC as claimed in claim 6, wherein the MFC exhibits coulombic efficiency of 39.2%.

17. The MFC as claimed in claim 6, wherein the MFC exhibits energy efficiency of 35.42%.

18. The MFC as claimed in claim 6, wherein the MFC exhibits highly porous voids with high surface area that facilitates facile formation of biofilm.

Images