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Oxidation of Sulphites by Electrodes Made of Novel Materials for Use in Microbial Fuel Cells

Catalysts based on novel carbon forms (Fullerenes С60/С70, Higher Fullerenes and Double Wall Carbon Nanotubes) were applied to facilitate the process of oxidation of sulfites to sulfates. The electrodes with incorporated new catalysts are able to desulphurize and remove toxic pollutants. The electrochemical characterization included steady state polarization curve analysis. The electrodes that incorporate fullerene structures were found to yield the highest current densities. A short overview considers the existing sulfur oxidizing microbes that should facilitate the creation of a workable microbial fuel cell.


Introduction
The removal of pollutants while simultaneously generating energy will help safeguard the environment and enable the development of innovative fuel cells. In this regard a principle objective is to build new electrodes and catalysts able to remove toxic pollutants where sulfide is oxidized to sulfite through sulfite reduction by Dsr.
The sulfite generated by rDsr is then oxidized to sulfate by other enzymes [2]; (ii) oxidation of sulfite to sulfate by a mononuclear molybdenum enzyme known as sulfite oxidoreductase [3]. In bac-terial sulfate reduction, bacteria respire sulfate and yield sulfide.
The process consists of four paths [4,5]. It has been shown that organotrophic bacteria are capable of the oxidation of thiosulfate to tetrathionate (these bacteria are referred to as T-HSOB) and are found in the redox layer of the Black Sea [6].
Microbial Fuel Cells (MFCs) with graphite anodes and graphite cathodes were built in an aerobic seawater environment and in anoxic marine sediment [7]. The anode electrode was embedded in anoxic marine sediments while connected through electronic circuits to a similar electrode in the overlying aerobic seawater (the cathode), thus building a MFC. The MFC had a power yield of 0.01 W/m 2 and can supply electronic instrumentation [8]. The enrichment with microorganisms from the family Geobacteraceae on graphite anodes allowed these microorganisms to conserve energy, supporting their growth by oxidation of organic compounds with an electrode that served as the electron acceptor [9]. Advanced catalyst supports including carbon nanotubes, aerogels and graphene have been tested in the past [10]. Applied also in catalyst synthesis is a method to fabricate the electro-catalyst for the electrodes using a lyophilization process [11]. Key sources when describing the electrochemical oxidation of sulphites in alkaline solutions are [12,13].
These researchers showed that the anodic oxidation of sulphite irreversibly yields sulphate and dithionate. The oxidation of SO 3 2at the anode was found to initiate at 1.2 V vs SHE in alkaline solutions. An OHradical is added to the sulphite ion at relatively high potentials. The anodic oxidation of sulphite ions under alkaline conditions has been studied comprehensively by J Lu et al. [14].
The adsorbed species suffer deprotonation at pH < 7 and are then subject to oxidation. In solution, sulphite exists in the form HSO 3 and SO 3 2with the following equilibria between these species [15]: . Novel electro-catalytic materials such as higher fullerenes and carbon nanotubes are studied in our current research. Higher fullerenes are fabricated by applying the carbon arc method in a quartz reactor followed by sublimation.
These include the fairly stable species C 74 , C 78 , C 80 , C 82 , C 84 , C 86 , C 88 , C 90 , C 92 , C 94 , C 96 , C 98 , C 100 . Characteristic of higher fullerenes is that the bonding sites between the pentagon atom groups are usually found to be the most reactive.

Materials and Methods
In this study, we use "higher fullerenes", made by the method of Deener and Alford, also called "narrow gap fullerenes" [16]. Sodium sulfite (Na 2 SO 3 ), sodium chloride NaCl and higher order fullerenes were purchased together with manganese acetate and polypyrrole from Sigma Aldrich. Fullerenes C 60 /C 70 and DWCNTs were pur-chased from CEC Research, Houston, Texas. The Vulkan XC-72 particles with a particle size of 50 nm were purchased from the Cabot Corporation and prepared in accordance with [17]. The catalysts studied were lyophilized fullerenes C 60 /C 70 , higher fullerenes and DWCNTs (2-11 mg) dispersed in 6 ml of distilled water in an ultrasonic bath for 15 minutes. Subsequently, 40 mg of manganese acetate are slowly added to the aqueous suspension together with 60 mg of polypyrrole. These ternary mixtures were then baked at 180 °C for 12 hours in a Teflon autoclave. Thus, manganese oxides cover the fullerene and nanotube structures with polypyrrole binding.
The electrodes under investigation have a geometric area of 1and 10 cm 2 . The electrodes were prepared from a catalyst mixture and Teflonized carbon black (60 mg/cm 2 Vulcan XC-72 (35% Teflon)) as a binder [18]. The mixture is compressed on both sides of a stainless-steel collector at 150 °C and pressed at 300 kg/cm 2 ( Table 1).

XRD Analysis
Shown in Figure 1 is an XRD of the catalyst incorporating higher fullerenes and manganese oxides.  . Electrolyte 1М Na 2 SO 3 -132 g/l + 18 g/l NaCl.