Pathways of Sulfide Oxidation by Haloalkaliphilic Bacteria in Limited-Oxygen Gas Lift Bioreactors

Abstract

Physicochemical processes, such as the Lo-cat and Amine-Claus process, are commonly used to remove hydrogen sulfide from hydrocarbon gas streams such as landfill gas, natural gas, and synthesis gas. Biodesulfurization offers environmental advantages, but still requires optimization and more insight in the reaction pathways and kinetics. We carried out experiments with gas lift bioreactors inoculated with haloalkaliphilic sulfide-oxidizing bacteria. At oxygen-limiting levels, that is, below an O(2)/H(2)S mole ratio of 1, sulfide was oxidized to elemental sulfur and sulfate. We propose that the bacteria reduce NAD(+) without direct transfer of electrons to oxygen and that this is most likely the main route for oxidizing sulfide to elemental sulfur which is subsequently oxidized to sulfate in oxygen-limited bioreactors. We call this pathway the limited oxygen route (LOR). Biomass growth under these conditions is significantly lower than at higher oxygen levels. These findings emphasize the importance of accurate process control. This work also identifies a need for studies exploring similar pathways in other sulfide oxidizers such as Thiobacillus bacteria.

Influence of methanethiol on biological sulphide oxidation in gas treatment system

“…Sulphur mainly formed below −370 mV, whereas sulphate was the main end product at ORP values above −320 mV. These findings are in good agreement with those of Klok et al [11] and Van den Bosch et al [10] In the transition zone between −370 and −320 mV (grey area in Figure 4), the selectivity for sulphur and sulphate formation was lower (Figure 4(a)-(b)) than outside this area; consequently, the rate of abiotic thiosulphate formation was higher in this zone (Figure 4(c)). Methanethiol had a clear effect on the selectivity for sulphate formation in the transition zone (Figure 4(b)), for example, at −350 mV; as methanethiol inhibited biological sulphate formation (Figure 4(b)), thiosulphate formation increased as a result of abiotic sulphide oxidation (Figure 4(c)).…”

“…Growth was seriously hampered at −450 mV. According to the work by Klok et al, [11] SOB gain most energy for their growth from sulphate production (Equation (2)), and much less from sulphur formation (Equation (1)). As there was insufficient oxygen for sulphate production (Equation (2)), very little growth was possible at −450 mV.…”

“…[10] Furthermore, it should be noted that at an O 2 /H 2 S supply ratio of 0.5 mol mol −1 no oxygen is available for sulphate production, while at a ratio of 2 mol mol −1 sulphate is the sole end product. [11] Sour gas streams also often contain organic sulphur compounds or thiols, which inhibit the SOB, [12] leading to lower sulphide oxidation rates. Consequently toxic sulphide will accumulate which, in turn, will lead to the inhibition of reaction (1), thereby preventing the formation of hydroxyl ions that are needed to absorb H 2 S from the sour gas.…”

“…[19] Hence, in this study experiments were carried out at constant O 2 /H 2 S supply ratios within each experiment. [11] Moreover, to explain our experimental results, we studied the inhibition of cytochrome c oxidase activity, the enzyme responsible for sulphate formation, by methanethiol in cell-free extracts obtained from different experiments. Finally, we also studied the inhibition mode for sulphide oxidation by estimating the unknown kinetic parameters in rate equations.…”

Biotechnological removal of H2S and thiols from sour gas streams under haloalkaline conditions

“…In the literature these pathways can also be found as limited oxygen route (LOR) and full oxygen route (FOR) [111]. Oxidation of sulfide via LOR occurs under limited oxygen conditions (low redox conditions) and leads to the sulfur formation, whereas oxidation via FOR leads to the sulfate formation.…”