The Role of Natural Laccase Redox Mediators in Simultaneous Dye Decolorization and Power Production in Microbial Fuel Cells

Abstract

Redox mediators could be used to improve the efficiency of microbial fuel cells (MFCs) by enhancing electron transfer rates and decreasing charge transfer resistance at electrodes. However, many artificial redox mediators are expensive and/or toxic. In this study, laccase enzyme was employed as a biocathode of MFCs in the presence of two natural redox mediators (syringaldehyde (Syr) and acetosyringone (As)), and for comparison, a commonly-used artificial mediator 2,2 -azinobis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) was used to investigate their influence on azo dye decolorization and power production. The redox properties of the mediator-laccase systems were studied by cyclic voltammetry. The presence of ABTS and As increased power density from 54.7 ± 3.5 mW m −2 (control) to 77.2 ± 4.2 mW m −2 and 62.5 ± 3.7 mW m −2 respectively. The power decreased to 23.2 ± 2.1 mW m −2 for laccase with Syr. The cathodic decolorization of Acid orange 7 (AO7) by laccase indicated a 12-16% increase in decolorization efficiency with addition of mediators; and the Laccase-Acetosyringone system was the fastest, with 94% of original dye (100 mgL −1 ) decolorized within 24 h. Electrochemical analysis to determine the redox properties of the mediators revealed that syringaldehyde did not produce any redox peaks, inferring that it was oxidized by laccase to other products, making it unavailable as a mediator, while acetosyringone and ABTS revealed two redox couples demonstrating the redox mediator properties of these compounds. Thus, acetosyringone served as an efficient natural redox mediator for laccase, aiding in increasing the rate of dye decolorization and power production in MFCs. Taken together, the results suggest that natural laccase redox mediators could have the potential to improve dye decolorization and power density in microbial fuel cells. Energies 2018, 11, 3455 2 of 12The enzyme is widely utilised in the oxidation of phenolic and non-phenolic substrates such as dyes, pesticides, antibiotics etc.The redox potential of the substrate should be lower than that of laccase for oxidation to be thermodynamically feasible. The redox potential range for fungal laccase is between 0.4-0.8 V vs. standard hydrogen electrode (SHE), which is suitable for oxidation of phenolic substrates; for non-phenolic substrates that have a redox potential of >1.3 V vs. SHE, and cannot be oxidized directly by laccase, a redox mediator is required [5]. A redox mediator is a small molecular weight compound that is oxidized by the enzyme and reduced by the substrate continuously. They act as electron shuttles for large substrates that cannot access the active site of the enzyme, e.g., due to steric hindrance [6]. In laccase mediator systems (LMS), the enzyme oxidizes the mediators to form stable radicals with high redox potential that diffuse away from the enzyme active site and oxidize the substrates and get reduced in the process. In this way, laccase indirectly oxidises substrates that have high redox potentials or large molecular sizes [7] (...

Degradation of Azo Dye (Acid Orange 7) in a Microbial Fuel Cell: Comparison Between Anodic Microbial-Mediated Reduction and Cathodic Laccase-Mediated Oxidation

“…There are various studies that have investigated azo dye decolourization in the anode and others in the cathode (Huang et al, 2017;Lai et al, 2017;Mani et al, 2018). It is not clear which approach is the best as different studies used different organisms, operating conditions, cathode catalyst etc., making a direct comparison of decolourization rates for each system difficult ( Table 1).…”

Environmentally Friendly and Recyclable Natural-Mediator-Modified Magnetic Nanoparticles for Laccase-Catalyzed Decolorization

“…Furthermore, Lac stability and activity was decreased when incubated with arti cial these redox mediators in the long run. us, natural mediators have been used due to their environmental friendliness, low-cost, and good biocompatibility to laccase [38].…”

Exploring community evolutionary characteristics of microbial populations with supplementation of Camellia green tea extracts in microbial fuel cells

“…Therefore, natural resources or products were recently considered to replace such artificially synthesized compounds for sustainable applications. Several studies clearly revealed that appropriate supplementation of natural products abundant in electrochemically active substances could significantly promote simultaneous bioelectricity generation and wastewater treatment in MFCs [17] , [18] , [19] , [20] . Recently, Xu et al [20] extracted natural herbal substances abundant in anthocyanins to effectively increase nearly threefold efficiency of bioelectricity generation in MFCs.…”

“…Recently, Xu et al [20] extracted natural herbal substances abundant in anthocyanins to effectively increase nearly threefold efficiency of bioelectricity generation in MFCs. From the perspective of chemical structure, in fact these bioelectricity-stimulating substances were mostly polyphenols, which are abundant in natural plants [17] , [18] , [19] , [20] .…”

“…Considering stimulation of effective bioenergy extraction, the protagonist dealing with bioelectricity production in MFCs is electroactive microbes or mixed consortia, that effectively release electrons from oxidation of organic matter [20] . Of course, the bioactivities of the electricity-generating microorganisms (e.g., biofilm forming, electron-transporting characteristics) would directly influence the power-generating efficiency of MFCs [ 17 , 19 ]. Due to some inhibitory/antagonistic interactions to be taken place, it was not possible to guarantee that all of natural substances are appropriate sources to be natural RMs.…”