Application of ionic liquids and low cost materials for bioenergy production and wastewater treatment in microbial fuel cells

Resumen

Global warming and depletion of fossil fuels have encouraged the development of novel technologies for producing renewable energies. In recent years, microbial fuel cells (MFCs) have become one of the most promising alternatives for bioenergy production. MFCs use microbial metabolism for wastewater treatment and simultaneously electricity production. Despite the many advantages of this technology, MFCs still show several limitations for their large scale application such as the high cost of materials (commercial membranes, catalyst, etc), their design, which sometimes hinder its operation in continuous mode, and their relative low performance. So far, many advances have been made to overcome these drawbacks and broaden the application range of MFCs, however further work is required for them to be economically feasible on a large scale. In this thesis, some of the limitations previously commented have been addressed. Regarding the high price of the commercial membranes (e.g. Nafion 117) and the low performance, novel polymer inclusion membranes (PIMs) based on ionic liquids (ILs) have been synthetized to replace expensive commercial membranes and increase its conductivity, and therefore the performance of the MFCs. To electrochemically characterize the PIMs and predict their behavior in MFCs, a method based on impedance spectroscopy was developed. Methyltrioctylammonium chloride -[MTOA+][Cl−]-, 1-methyl-3-octylimidazolium hexafluorophosphate -[OMIM+][PF6-]-, tributylmethylphosphonium methylsulphate -[P4,4,4,1+][MeSO4-]- and triisobutyl(methyl)phosphonium tosylate -[PI4,I4,I4,1+][TOS−]- were selected for preparing the membranes. The Internal resistance of the IL-based PIMs was measured by electrochemical impedance spectroscopy (EIS). This parameter was related to the power output of MFCs, since the resistance is inversely proportional to the conductivity of the membranes affecting the amount of energy produced. Results from EIS showed that, among the IL tested, [PI4,I4,I4,1+][TOS−]-based membrane exhibits the lowest internal resistance. Moreover, the use of this membrane as separator makes possible to produce higher power densities when compared with commercial membranes, and similar values in terms of COD removal. As regard MFC configuration, a novel air-cathode single chamber up-flow MFC was designed to operate in continuous mode. As separator, it includes an ionic liquid-based membrane-cathode assembly developed by our research group to reduce the internal resistance and increase the conductivity of the separator. The effects of the type of ionic liquid and the feed flow on the MFC performance were investigated. The results demonstrated that low feed flows increase the power output and the wastewater treatment capacity of MFCs regardless the type of ionic liquid used. Among the two ionic liquids tested, triisobutyl(methyl)phosphonium tosylate -[PI4,I4,I4,1+][TOS−]- and methyltrioctylammonium chloride -[MTOA+][Cl−]-, the best results were obtained using the -[PI4,I4,I4,1+][TOS−]-based membrane-cathode assembly at a feed flow of 0.25 mL min-1, the lowest flow rate. The maximum power output achieved was 12.3 W m-3anode, with a value of 60 % of COD removal. The configuration designed was capable of continuously producing electricity during two cycles of 168 h. Finally, in an attempt to minimize material costs, microalgae were evaluated as low cost substrate in terracotta-based MFCs. Microalgae were selected due to their abundance in nature and the problems associated with their uncontrolled growth. The results showed that microalgal biomass cannot be used by domestic wastewater microorganisms as carbon source to produce electricity in terracotta-based MFCs. However, this type of low cost MFC self-produces a catholyte solution with unique properties that, under optimal conditions, is suitable to degrade microalgae to be used as substrate in terracotta-based MFCs. The results confirmed that 5 days during real cycles of light/darkness (16:8) are sufficient to lysis microalgal biomass with catholyte solution. The energy produced after the addition of microalgae digested with catholyte under light/dark conditions is 1,800 times higher than in the case of MFCs being fed with fresh microalgae. Although further work is needed, this thesis successfully addresses some of the most important bottlenecks of MFCs such as their high cost material, relatively low performance and optimal design to operate in continuous mode. Future research efforts will focus on nutrient transport through the membrane to optimize the growth of microalgae in the cathodic chamber. This biomass algal could be used as substrate for the anode or for biodiesel production. However, the main purpose will be to fix the CO2 generated in the cathode by microalgae