Promoting autotrophic sulfate reduction and elemental sulfur recovery in bioelectrochemical systems

Resumo

Industrial activities such as paper, pharmaceutical, mining, food processing, etc. generate wastewater with high sulfate content. Sulfate as such is not very harmful to health, but if it is poured into rivers or sewage systems, the microorganisms known as sulfate reducing bacteria (SRB) can transform it into hydrogen sulfide. Hydrogen sulfide is a compound with bad odour, is corrosive and has been shown toxic at low concentrations. For these reasons, the treatment of sulfate-rich effluents is essential. In addition, the recovery of elemental sulfur from these effluents in order to be reused as fertilizer or raw material in the industry is an opportunity to recover resources in the framework of the circular economy. Bioelectrochemical systems (BES) are a novel technology based on the ability of some bacteria to exchange electrons with a solid electrode. Lastly, the study of the BES has focused on the treatment of wastewater and the recovery of products thanks to the activity of the microorganisms that colonize the electrodes. In this thesis, the use of BES for the treatment and recovery of sulfur compounds was studied, specifically, the treatment of these wastewaters with sulfate in a biocathode. The system allows the reduction of sulfate at a biocatode while at the anode electrolysis of water occurs to generate the necessary electron flow. The microorganisms that colonize the surface of the cathode use the hydrogen produced from the electrons to transform the sulfate into hydrogen sulfide. However, the results obtained showed that thanks to the water electrolysis that takes place at the anode an oxygen flow to the cathode is generated, allowing the growth of microorganisms capable of producing sulfur from hydrogen sulfide, called sulfide oxidizing bacteria (SOB). The influence of pH of the cathode compartment and the cathode potential was studied in order to improve sulfate removal and sulfur production. It was observed that neutral pH (pH = 7) was more beneficial since an acidic pH (pH = 5.5) could inhibit the activity of the SRB and a basic pH (pH = 8.5) required more energy to achieve similar results due to the limitation in the production of hydrogen at a high pH. Regarding the potential of the cathode, it could be observed that lower potentials led to greater sulfate removal rate, but from a potential of -1.0 V vs. SHE, the system could not increase the removal rate. In addition, the impact of real wastewater coming from a flue gas desulphurization system in the system was also studied. It was observed that with real water the sulfate removal decreased, however, the production of elemental sulfur increased. Finally, since the oxygen flow from the anode to the cathode could not be controlled with the previous systems, two new configurations were designed to improve the production of elemental sulfur. The first one consisted in the addition of an electrochemical cell to oxidize the hydrogen sulfide at the anode, allowing the control of the potential and thus controlling the production. The second configuration consisted in the addition of a fuel cell with a cathode exposed to the air taking advantage of the capacity of the hydrogen sulfide to be oxidized at an anode spontaneously and thus produce energy instead of requiring it in the oxidation process.