Biologically produced sulfur particles and polysulfide ions

This thesis deals with the effects of particles of biologically produced sulfur (or 'biosulfur') on a biotechnological process for the removal of hydrogen sulfide from gas streams. Particular emphasis is given to the role of polysulfide ions in such a process. These polysulfide ions are formed from reaction of sulfide with biologically produced sulfur. The basic concepts of this H 2 S removal process were developed at the department of Environmental Technology of Wageningen University and the process is currently commercialized byPaquesB.V. and Shell Global Solutions Int. B.V. as the Shell-PaquesorThiopaqÔprocess. It was originally designed for the removal of H 2 S from 'biogas', which is a product from anaerobic wastewater treatment plants. Currently it is also used for the treatment of natural gas.The process basically consists of a gas absorber and a bioreactor. In the gas absorber, H 2 S is absorbed from an untreated gas stream by an alkaline liquid. The dissolved hydrogen sulfide (HS-) is then led into the bioreactor where it is biologically oxidized to elemental sulfur by sulfide-oxidizing bacteria, mainly of the genusThiobacillus. The solid sulfur formed is separated from the liquid by sedimentation and a liquid recycle flows from the bioreactor back to the gas absorber. To prevent oxidation of sulfide to sulfate, oxygen-limiting conditions are applied in the bioreactor. Production of elemental sulfur is preferred over the production of sulfate as elemental sulfur is less harmful than sulfate. Furthermore hydroxyl ions, consumed in the absorption of H 2 S in the alkaline liquid, are regenerated upon oxidation of sulfide to elemental sulfur. This saves costs of dosing caustic (NaOH) to the process. Solid elemental sulfur is also easily separated from the solution by sedimentation and the resulting sulfur product can be reused.Elemental sulfur has a very low solubility in water and the solid material is formed in the shape of particles, or 'globules'. In chapter 2 an overview is given of reported studies on the properties of biologically produced sulfur. This concerns sulfur as it is produced in the biotechnological process by Thiobacilli, but also sulfur produced by other sulfide-oxidizing bacteria. Sulfide-oxidizing bacteria produce elemental sulfur as an intermediate in the oxidation of hydrogen sulfide to sulfate. Sulfur produced by these microorganisms can be stored in sulfur globules, located either inside or outside the cell. Excreted sulfur globules are colloidal particles that are stabilized against aggregation by electrostatic repulsion orstericstabilization. The formed elemental sulfur has some distinctly different properties as compared to "normal" inorganic sulfur. The density of the particles is for instance lower than the density of orthorhombic sulfur and the biologically produced sulfur particles have hydrophilic properties whereas orthorhombic sulfur is known to be hydrophobic. The nature of the sulfur and the surface properties of the globules are different for sulfur produced by different bacteria. For example, the globules produced by phototrophic bacteria appear to consist of long sulfur chains terminated with organic groups, whereaschemotrophicbacteria produce globules consisting of sulfur rings (S 8 ). Extracellularly stored sulfur globules produced byAcidithiobacillusferrooxidans have hydrophilic properties that can probably be explained by a vesicle structure consisting mainly of polythionates (-O 3 S-S n-SO 3-). Adsorbed organic polymers, such as proteins, provide hydrophilic properties to sulfur produced by a mixed culture of normal'>Thiobacilliin a biotechnological H 2 S removal process. The small particle size and the hydrophilic properties of these particles provide advantages over other commercially available forms of sulfur in bioleaching and fertilizer applications.A central question in this thesis is how biologically produced sulfur particles can affect the absorption of H 2 S in the gas absorber. A possibly occurring effect is an enhancement of the absorption rate of gaseous H 2 S by a heterogeneous reaction of the dissolved sulfide with biologically produced sulfur particles. To assess the role of this reaction in the enhancement of the H 2 S absorption rate, the kinetics of this reaction has been studied, as has been described in chapter 3 . The kinetics of the heterogeneous reaction between dissolved sodium sulfide and biologically produced sulfur particles has been studied by measuring the formation of polysulfide ions, S x2-, in time. Experiments were performed at pH 8.0 and at temperatures ranging from 30 to 50°C. The obtained data were fitted with a reaction rate model. To describe the heterogeneous reaction kinetics, a detailed reaction mechanism for the polysulfide formation reaction has been proposed. Furthermore, a decreasing particle size and anonuniformparticle size distribution were incorporated in this reaction rate model. Results for the reaction of sulfide withbiosulfurwere compared with an earlier published study on the reaction of sulfide with "inorganic" granular sulfur. Whereas the reaction rate of sulfide with granular sulfur was limited by diffusion, the reaction rate of sulfide with biologically produced sulfur is limited by chemical reaction. This is probably caused by the smaller particle size or the specific hydrophilic properties of the biologically produced sulfur. This makes the surface of thebiosulfurparticles more easily available for reaction than the surface of granular sulfur.&nbsp;The effect of the specific properties of biologically produced sulfur on the equilibrium constant of the reaction between biologically produced sulfur and sodium sulfide is described in chapter 4 . Results were compared with the equilibrium of the reaction of sulfide with "inorganic" elemental sulfur. The equilibrium can be described by an equilibrium constantp K x. For biologically produced sulfur,p K x = 9.10±0.08 (21°C) and 9.17±0.09 (35°C) with an average polysulfide chain length x = 4.91±0.32 (21°C) and 4.59±0.31 (35°C). Thep K xvalue for biologically produced sulfur is significantly higher than for reaction of dissolved sodium sulfide with "inorganic" sulfur (p K x= 8.78; 21°C). This difference is probably caused by specific binding of polysulfide ions in the organic polymer layer, adsorbed on the surface of the sulfur particles.As was described in chapters 3 and 4, polysulfide ions can be produced from reaction of dissolved hydrogen sulfide with biologically produced sulfur. In a hydrogen sulfide removal process, these polysulfide ions can be oxidized to formthiosulfate. The basic concept of the H 2 S removal process is the absorption of H 2 S from a gas stream, in which OH-is consumed, and the subsequent biological oxidation of HS-to elemental sulfur (OH-is produced). By oxidation of polysulfide ions tothiosulfate, the biological oxidation of sulfide to elemental sulfur is prevented, resulting in a net loss of OH-. This means that an OH-solution has to be dosed to the process, which can involve considerable costs. To controlthiosulfateformation in the H 2 S removal process, more knowledge of the chemical oxidation of polysulfide ions in aqueous solutions is required. Therefore, the kinetics of this reaction and the reaction products formed, have been studied, which is described in chapter 5 . Experiments were performed in phosphate buffered solutions at pH 7 to 12, at temperatures between 20 and 40°C, and an ionic strength between 0.05 and 0.50 M. Polysulfide solutions were mixed with a buffer solution of known dissolved oxygen concentration, after which the decrease in the oxygen concentration of the solution was measured in time. The rate of oxygen consumption can be described by the empirical relationd[O 2 ]/d t=-k[S x2-][O 2 ] 0.59 . The reaction rate constant k is moderately dependent on pH and goes through a maximum at pH 10. The rate of oxygen consumption for polysulfide solutions is approximately four times higher than for sulfide solutions. At pH values below 9, reaction products were formed according to S x2-+ 3/2 O 2®&nbsp;S 2 O 32-+( x-2) S 0 . At pH values higher than 9, morethiosulfateand additional sulfide were formed, which is attributed to the low chemical stability of the sulfur of oxidation state zero, formed upon polysulfide oxidation. The results strongly suggest that hydrolysis of this 'nascent' elemental sulfur to HS-and S 2 O 32-is faster than hydrolysis of crystalline inorganic sulfur or colloidal particles of biologically produced sulfur, and has a significant contribution already at 30°C and pH 10.With knowledge of the measured reaction kinetics from chapter 3 it is possible to calculate the theoretical contribution of the heterogeneous reaction between sulfide andbiosulfurto the enhancement of the H 2 S absorption rate. This is described in chapter 6 . Based on a pseudo-first order approximation of the kinetics, theoretical enhancement factors have been calculated. These have been compared with experimentally determined enhancement factors. Experiments on H 2 S absorption in a suspension containing small (<&nbsp;3mm)biosulfurparticles have been performed in a stirred cell reactor with a constant gas-liquid interfacial area. The observed enhancement of H 2 S absorption in these experiments can be explained by the heterogeneous reaction. Here, large hydrophobic particles could not be kept in suspension and therefore only smaller hydrophilic particles were present (dp < 3mm). However, the observed enhancement of H 2 S absorption in a gas absorber column, coupled with a bioreactor, cannot be explained by the heterogeneous reaction alone. In these experiments both small hydrophilic particles and larger, more hydrophobic particles were present (dp up to 20mm). In the gas absorber column, there was therefore a relatively low available specific surface area.Furthermore, a highkLand low [S x2- ] prevented enhancement due to the heterogeneous reaction.A more likely explanation for enhancement of the H 2 S absorption rate in this series of experiments is the more hydrophobic behavior of the larger particles. A local increase of the hydrophobic sulfur particle concentration near the gas/liquid interface and specific adsorption of H 2 S at the particle surface can result in an increase in the H 2 S absorption rate.In the aerated bioreactor of the biotechnological H 2 S removal process, sometimes stable foam is formed, which contains fairly large amounts of solid elemental sulfur. For a good operation of the process it is required to control foam formation and therefore knowledge of the nature of the foam and the mechanism of foam formation is needed. The effect of biologically produced sulfur particles on foam formation is particularly interesting. In chapter 7 a study on foam formation in aqueous suspensions of biologically produced sulfur particles is described, with the objective of describing trends and phenomena that govern foam formation in a biotechnological H 2 S removal process. Air is bubbled through a suspension and the development of the foam height in time is measured, showing essentially two types of foam: unstable foam of constant foam height and stable foam with a rapidly increasing foam height. The transition between these types of foam can occur when the local sulfur concentration near the surface of the liquid is higher than a critical concentration, so that a stable network structure can be formed. Sulfur particles are transported to the top of the liquid by flotation. Upon foam formation large aggregates of sulfur fall apart into smaller fractions. Especially the larger fraction of the sulfur particles is present in the foam, indicating that these particles have the righthydrophobicityto form a network structure. Furthermore, polysulfide anions were found to have antifoaming properties in biologically produced sulfur suspensions, either because of the changing of the surface properties of the biologically produced sulfur or because of the antifoaming properties of the hydrophobic elemental sulfur formed upon the chemical oxidation of polysulfide ions.