Electron shuttling in haloalkaliphilic sulfide oxidizing bacteria

Hydrogen sulfide (H2S) is a toxic, odorous and corrosive gas that can be present in various gas streams, such as natural gas and biogas. To prevent emission of sulfur compounds, such as sulfur dioxide, these gas streams require treatment. As a cost-effective and environmentally friendly alternative to physico-chemical processes, a biological gas desulfurization process has been developed. This process uses a mildly alkaline sodium (bi)carbonate solution to remove H2S from gas streams in an absorber column. In an aerated bioreactor, a mixed population of sulfide oxidizing bacteria (SOB) converts dissolved sulfide into predominantly elemental sulfur (S0) using oxygen. Biological formation of sulfate (SO42-) and chemical formation of thiosulfate (S2O32-) are enhanced at higher oxygen levels. These by-products impair the sustainability of the process as they consume caustic (NaOH) and require removal via a bleed stream. Although the bioreactor is operated under oxygen-limited conditions, formation of (thio)sulfate is inevitable in the current process.

To increase the selectivity for sulfur formation of the biotechnological desulfurization process, an anaerobic bioreactor with sulfidic conditions was placed in between the absorber column and the aerated bioreactor: the ‘dual-reactor line-up’. With this line-up, a selectivity for elemental sulfur formation of 97% was achieved, compared to 76% in the traditional 1-bioreactor line-up, which resulted in a reduction of 90% in NaOH consumption and bleed stream formation. The sulfidic conditions in the anaerobic bioreactor suppress the metabolic pathway for sulfate formation and resulted in a shift of the microbial population. The bacteria that became dominant were suggested to be limited to elemental sulfur formation and could not form sulfate. Furthermore it was found that bacteria removed part of the sulfide in the anaerobic bioreactor, thereby shuttling sulfide and/or electrons from the anaerobic to the aerated bioreactor. A direct effect of the sulfide uptake in the anaerobic bioreactor is a lower sulfide concentration entering the aerated bioreactor, decreasing chemical formation of thiosulfate.

To apply the dual-reactor line-up in industry, more insight is required in the effect of the sulfide concentration and HRT in the anaerobic bioreactor on the process performance. Furthermore, the effect of pH on process performance was investigated. To achieve a high selectivity for elemental sulfur formation (>95%), either the HRT in the anaerobic bioreactor should be at least 20 minutes, or the sulfide concentration should be at least 0.5 g L-1. A higher pH resulted in lower selectivity for elemental sulfur formation (the selectivity for S0 formation was 88% at pH 9.1 and 96% at pH 8.5). Furthermore, the biological sulfide uptake increased at higher sulfide concentrations and at higher pH. The concentration of polysulfides, which are formed due to an equilibrium reaction between HS- and elemental sulfur, increases at higher sulfide concentrations and higher pH. Hence, the results suggest that biological sulfide uptake in the anaerobic bioreactor is related to polysulfide.

The role of the bacteria on the absorption of H2S from the gas stream in the absorber column was also investigated. Previously, the absorption of H2S into the process solution was considered to be purely physico-chemical. First, it was shown that activity of SOB increases with temperature between 25-45 °C. Subsequently, the H2S absorption efficiency at different temperatures was determined. Whereas chemical absorption improves at lower temperatures, in our experiment the H2S absorption efficiency increased at higher temperatures. Also when the biomass concentration in the stream to the absorber increased, the H2S absorption efficiency improved. These results indicate that SOB enhance the absorption of H2S and thus play a role in the removal of H2S from gas streams.

In the biological desulfurization process, bacteria use oxygen as electron acceptor for sulfide oxidation. It was shown that bacteria taken from the biodesulfurization process can also use the anode of an electrochemical cell as electron acceptor. In batch tests it was demonstrated that bacteria remove sulfide from solution under anaerobic conditions (without presence of an electron acceptor). When these ‘charged’ SOB were subsequently transferred to an electrochemical cell, electric current was produced in the absence of dissolved sulfide. The ability of the bacteria to take up sulfide and release electrons at the anode is called ‘electron shuttling’.

The principle of electron shuttling was also applied in a continuous system, consisting of an anolyte uptake chamber and an electrochemical cell. Sulfide was continuously supplied to the anaerobic anolyte uptake chamber and removed from solution by bacteria. The charged bacteria were circulated over the anode side of the electrochemical cell, where bacteria continuously produced electric current in the absence of dissolved sulfide. After several days of operation, the main end product became sulfate and not elemental sulfur. The experimental results indicated that planktonic bacteria oxidized sulfide to sulfur and that the biofilm oxidized sulfur to sulfate.

In the aerated bioreactor, the air flow to the aerated bioreactor is controlled via online measurement of the oxidation/reduction potential (ORP). The ORP was considered to be mainly dependent on the concentration of sulfide and oxygen in the bioreactor. However, both sulfide and oxygen are rapidly consumed and their concentrations in the aerated bioreactor are below the detection limit. In batch tests, it was shown that the ORP is not only dependent on sulfide or oxygen, but is influenced by bacteria. Furthermore, the aerated bioreactor of the biodesulfurization process was operated at various redox setpoints. Bacteria from the aerated bioreactor were transferred to an electrochemical cell and current production was measured, demonstrating that bacteria release more electrons (i.e. contain more charge) when the ORP was lower. These measurements were subsequently used to calibrate a model to describe the relation between redox and charge storage, which showed that selectivity for sulfate formation increases when bacteria contain less charge.

The new process line-up decreases chemical consumption and waste stream formation of biological gas desulfurization considerably, which is especially important when treating gas streams with a high sulfur load. The ability of bacteria to release electrons to an electrode in the absence of dissolved sulfide provides new opportunities to recover energy from sulfide, which potentially could further increase the sustainability of biological gas desulfurization.