A physiologically based kinetic model for bacterial sulfide oxidation.

In the biotechnological process for hydrogen sulfide removal from gas streams, a variety of oxidation products can be formed. Under natron-alkaline conditions, sulfide is oxidized by haloalkaliphilic sulfide oxidizing bacteria via flavocytochrome c oxidoreductase. From previous studies, it was concluded that the oxidation-reduction state of cytochrome c is a direct measure for the bacterial end-product formation. Given this physiological feature, incorporation of the oxidation state of cytochrome c in a mathematical model for the bacterial oxidation kinetics will yield a physiologically based model structure. This paper presents a physiologically based model, describing the dynamic formation of the various end-products in the biodesulfurization process. It consists of three elements: 1) Michaelis-Menten kinetics combined with 2) a cytochrome c driven mechanism describing 3) the rate determining enzymes of the respiratory system of haloalkaliphilic sulfide oxidizing bacteria. The proposed model is successfully validated against independent data obtained from biological respiration tests and bench scale gas-lift reactor experiments. The results demonstrate that the model is a powerful tool to describe product formation for haloalkaliphilic biomass under dynamic conditions. The model predicts a maximum S° formation of about 98 mol%. A future challenge is the optimization of this bioprocess by improving the dissolved oxygen control strategy and reactor design.

Pathways of sulfide oxidation by haloalkaliphilic bacteria in limited-oxygen gas lift bioreactors.

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.

Inhibition of microbiological sulfide oxidation by methanethiol and dimethyl polysulfides at natron-alkaline conditions.

To avoid problems related to the discharge of sulfidic spent caustics, a biotechnological process is developed for the treatment of gases containing both hydrogen sulfide and methanethiol. The process operates at natron-alkaline conditions (>1 mol L(-1) of sodium- and potassium carbonates and a pH of 8.5-10) to enable the treatment of gases with a high partial CO(2) pressure. In the process, methanethiol reacts with biologically produced sulfur particles to form a complex mixture predominantly consisting of inorganic polysulfides, dimethyl disulfide (DMDS), and dimethyl trisulfide (DMTS). The effect of these organic sulfur compounds on the biological oxidation of sulfide to elemental sulfur was studied with natron-alkaliphilic bacteria belonging to the genus Thioalkalivibrio. Biological oxidation rates were reduced by 50% at 0.05 mM methanethiol, while for DMDS and DMTS, this was estimated to occur at 1.5 and 1.0 mM, respectively. The inhibiting effect of methanethiol on biological sulfide oxidation diminished due to its reaction with biologically produced sulfur particles. This reaction increases the feasibility of biotechnological treatment of gases containing both hydrogen sulfide and methanethiol at natron-alkaline conditions.

Effects of methanethiol on the biological oxidation of sulfide at natron-alkaline conditions.

The effects of methanethiol (MT) on biological sulfide oxidation were studied in a continuously operated bioreactor, in which chemolithoautotrophic bacteria belonging to the genus Thioalkalivibrio convert hydrogen sulfide (H2S) at natron-alkaline conditions. Previous bioreactor experiments have shown that always a fraction of the H2S is oxidized to sulfate and thiosulfate. This is unwanted, as it leads to caustic requirements for pH control and the formation of a bleed stream to discharge these compounds from the process. The current research shows that due to the addition of MT, sulfate formation is prevented. As a result, all supplied H2S is completely converted into elemental sulfur. Treatment of a continuous supply of 51.0 mM day(-1) H2S and 79 microM day(-1) MT was feasible for a prolonged period, with 99 mol% selectivity for sulfur formation. A part of the MT reacts with the freshly produced sulfur particles to form dimethyl disulfide (DMDS) and dimethyl trisulfide (DMTS). Results indicate that MT, DMDS, and DMTS partly adsorb onto the biosulfur particles. At concentrations above 10 microM, these volatile organic sulfur compounds induce biomass decay.

The effect of pH on thiosulfate formation in a biotechnological process for the removal of hydrogen sulfide from gas streams.

In a biotechnological process for hydrogen sulfide (H2S) removal from gas streams, operating at natronophilic conditions, formation of thiosulfate (S2O3(2-)) is unfavorable, as it leads to a reduced sulfur production. Thiosulfate formation was studied in gas-lift bioreactors, using natronophilic biomass at [Na+] + [K+] = 2 mol L(-1). The results show that at sulfur producing conditions, selectivity for S2O3(2-) formation mainly depends on the equilibrium between free sulfide (HS(-)) and polysulfide (Sx(2-)), which can be controlled via the pH. At pH 8.6, 21% of the total dissolved sulfide is present as Sx(2-) and selectivity for S2O3(2-) formation is 3.9-5.5%. At pH 10, 87% of the total dissolved sulfide is present as Sx(2-) and 20-22% of the supplied H2S is converted to S2O3(2-), independent of the H2S loading rate. Based on results of bioreactor experiments and biomass activity tests, a mechanistic model is proposed to describe the relation between S2O3(2-) formation and pH.

Sulfide oxidation at halo-alkaline conditions in a fed-batch bioreactor.

A biotechnological process is described to remove hydrogen sulfide (H(2)S) from high-pressure natural gas and sour gases produced in the petrochemical industry. The process operates at halo-alkaline conditions and combines an aerobic sulfide-oxidizing reactor with an anaerobic sulfate (SO(4) (2-)) and thiosulfate (S(2)O(3) (2-)) reducing reactor. The feasibility of biological H(2)S oxidation at pH around 10 and total sodium concentration of 2 mol L(-1) was studied in gas-lift bioreactors, using halo-alkaliphilic sulfur-oxidizing bacteria (HA-SOB). Reactor operation at different oxygen to sulfide (O(2):H(2)S) supply ratios resulted in a stable low redox potential that was directly related with the polysulfide (S(x) (2-)) and total sulfide concentration in the bioreactor. Selectivity for SO(4) (2-) formation decreased with increasing S(x) (2-) and total sulfide concentrations. At total sulfide concentrations above 0.25 mmol L(-1), selectivity for SO(4) (2-) formation approached zero and the end products of H(2)S oxidation were elemental sulfur (S(0)) and S(2)O(3) (2-). Maximum selectivity for S(0) formation (83.3+/-0.7%) during stable reactor operation was obtained at a molar O(2):H(2)S supply ratio of 0.65. Under these conditions, intermediary S(x) (2-) plays a major role in the process. Instead of dissolved sulfide (HS(-)), S(x) (2-) seemed to be the most important electron donor for HA-SOB under S(0) producing conditions. In addition, abiotic oxidation of S(x) (2-) was the main cause of undesirable formation of S(2)O(3) (2-). The observed biomass growth yield under SO(4) (2-) producing conditions was 0.86 g N mol(-1) H(2)S. When selectivity for SO(4) (2-) formation was below 5%, almost no biomass growth was observed.