Biocarbon from Sewage Sludge As Anode Catalyst for the Production of Bioelectricity in an MFC

Abstract

Over the last few years, the use of novel anode carbon catalysts from biomass as raw materials in Microbial Fuel Cells (MFC) has been increasingly investigated due to their catalytic properties and low cost compared to noble metals such as Pt. Nevertheless, it is acknowledged that the use of a catalyst in an MFC is not sufficient to generate bioenergy. A biofilm of electroactive bacteria must be grown on the catalyst surface, forming a bioanode. The combination of catalyst and biofilm oxidizes organic matter contained in substrate, promotes the electron transfer kinetics and thus enhances the generation of bioelectricity. In this work, a biosolid from waste water sewage sludge from the food industry has been pyrolyzed and activated with KOH in N2 atmosphere in order to obtain a biocarbon. The biocarbon has been further functionalized with methanol in different concentrations (0.15, 0.5 and 1.0 M) aiming to enhance its biocompatibility with electroactive Bacillus subtilis (B. subtilis), used as microorganism. With these components, bioanodes have been fabricated using gas diffusion carbon cloths on which a catalytic layer containing the biocarbon have been painted using the brush method, followed by the growth of a biofilm of B. subtilis for 6 days in broth media. The biocarbon was physicochemically characterized by XRD, SEM-EDS and FESEM. XRD patterns indicate that the material is amorphous in nature with a broad (002) peak attributed to graphitic structures. Some reflections attributed to crystalline phases are also observed. EDS analysis shows the presence of C, O, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti and Fe in the biocarbon. On this matter, the biosolid shows a composition of 72.0, 18.96, 2.96 and 0.22 (wt. %) of C, O, Si and K, respectively. The biocarbon has a reduction in C content (29.23 wt. %), with an increase in O (23.54 wt. %) and K (17.29 wt. %), attributed to the activation with KOH. Moreover, the biocarbon shows biocompatibility with B. subtilis as indicated by morphology analysis by FESEM. The biocarbon + B. subtilis bioanodes have been characterized in a half-cell with pharmaceutical waste water (PWW, pH= 6.1) as the electrolyte. The cyclic voltammograms (CVs) show their biocatalytic activity, generating a high current density from the PWW compared with the electrodes without biofilm. The results indicate that biocarbons from waste water sewage sludge are promising catalysts for MFC anode applications, due to their biocompatibility with bacteria and because their chemical composition may enhance the microbial metabolism mechanism, enhancing the performance of the bioelectrochemical device.