Cluster formation and gelation are studied in a colloidal model system with competing short-range attractions and long-range repulsions. In contrast to predictions by equilibrium theory, the size of clusters spontaneously formed at low colloidal volume fractions decreases with increasing strength of the short-range attraction. Moreover, the microstructure and shape of the clusters sensitively depend on the strength of the short-range attraction: from compact and crystalline clusters at relatively weak attractions to disordered and quasi-linear clusters at strong attractions. By systematically varying attraction strength and colloidal volume fraction, we observe gelation at relatively high volume fraction. The structure of the gel depends on attraction strength: in systems with the lowest attraction strength, crowding of crystalline clusters leads to microcrystalline gels. In contrast, in systems with relatively strong attraction strength, percolation of quasi-linear clusters leads to low-density gels. In analyzing the results we show that nucleation and rearrangement processes play a key role in determining the properties of clusters and the mechanism of gelation. This study implies that by tuning the strength of short-range attractions, the growth mechanism as well as the structure of clusters can be controlled, and thereby the route to a gel state.

In the biotechnological desulfurization process under haloalkaline conditions, dihydrogen sulfide (H2S) is removed from sour gas and oxidized to elemental sulfur (S8) by sulfide-oxidizing bacteria. Besides S8, the byproducts sulfate (SO42–) and thiosulfate (S2O32–) are formed, which consume caustic and form a waste stream. The aim of this study was to increase selectivity toward S8 by a new process line-up for biological gas desulfurization, applying two bioreactors with different substrate conditions (i.e., sulfidic and microaerophilic), instead of one (i.e., microaerophilic). A 111-day continuous test, mimicking full scale operation, demonstrated that S8 formation was 96.6% on a molar H2S supply basis; selectivity for SO42– and S2O32– were 1.4 and 2.0% respectively. The selectivity for S8 formation in a control experiment with the conventional 1-bioreactor line-up was 75.6 mol %. At start-up, the new process line-up immediately achieved lower SO42– and S2O32– formations compared to the 1-bioreactor line-up. When the microbial community adapted over time, it was observed that SO42– formation further decreased. In addition, chemical formation of S2O32– was reduced due to biologically mediated removal of sulfide from the process solution in the anaerobic bioreactor. The increased selectivity for S8 formation will result in 90% reduction in caustic consumption and waste stream formation compared to the 1-bioreactor line-up.

Biological desulfurization under haloalkaliphilic conditions is a widely applied process, in which haloalkalophilic sulfide-oxidizing bacteria (SOB) oxidize dissolved sulfide with oxygen as the final electron acceptor. We show that these SOB can shuttle electrons from sulfide to an electrode, producing electricity. Reactor solutions from two different biodesulfurization installations were used, containing different SOB communities; 0.2 mM sulfide was added to the reactor solutions with SOB in absence of oxygen, and sulfide was removed from the solution. Subsequently, the reactor solutions with SOB, and the centrifuged reactor solutions without SOB, were transferred to an electrochemical cell, where they were contacted with an anode. Charge recovery was studied at different anode potentials. At an anode potential of +0.1 V versus Ag/AgCl, average current densities of 0.48 and 0.24 A/m2 were measured for the two reactor solutions with SOB. Current was negligible for reactor solutions without SOB. We postulate that these differences in current are related to differences in microbial community composition. Potential mechanisms for charge storage in SOB are proposed. The ability of SOB to shuttle electrons from sulfide to an electrode offers new opportunities for developing a more sustainable desulfurization process.

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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.

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In the biodesulfurization (BD) process under halo-alkaline conditions, toxic hydrogen sulfide is oxidized to elemental sulfur by a mixed culture of sulfide oxidizing bacteria to clean biogas. The resulting sulfur is recovered by gravitational settling and can be used as raw material in various industries. However, if the sulfur particles do not settle, it will lead to operational difficulties. In this study, we investigated the properties of sulfur formed in five industrial BD facilities. Sulfur particles from all samples showed large differences in terms of shape, size, and settleability. Both single crystals (often bipyramidal) and aggregates thereof were observed with light and scanning electron microscopy. The small, non-settled particles account for at least 13.6% of the total number of particles and consists of small individual particles with a median of 0.3 µm. This is undesirable, because those particles cannot be removed from the BD facility by gravitational settling and lead to operational interruption. The particles with good settling properties are aggregates (5–20 µm) or large single crystals (20 µm). We provide hypotheses as to how the differences in sulfur particle properties might have occurred. These findings provide a basis for understanding the relation between sulfur particle properties and formation mechanisms.

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