Formation and colloidal behaviour of elemental sulphur produced from the biological oxidation of hydrogensulphide

The formation and aggregation of elemental sulphur from the microbiological oxidation of hydrogensulphide (H 2 S) by a mixed population of aerobic Thiobacillus -like bacteria has been investigated. Sulphide is formed during the anaerobic treatment of wastewaters which contain oxidized sulphur compounds such as thiosulphate, sulphite and sulphate. This sulphide has to be removed from the effluent solution of anaerobic reactors because of its detrimental characteristics e.g. toxicity, corrosiveness, oxygen demand and bad odour. Also the biogas produced in the anaerobic treatment plants generally will contain substantial amounts (up to 3% v/v) of hydrogensulphide. For removing the sulphide, conventional physico-chemical sulphide-removing processes can be applied. The processes are based on the oxidation of sulphide with peroxide, hypochlorite or permanganate or the precipitation of sulphide with iron(III)chloride. Major drawbacks of these methods are the high costs for chemicals and the production of excessive amounts of chemical sludge. An alternative method for the chemical sulphide removal comprises the oxidation of sulphide with bacteria. At the Department of Environmental Technology (WAU) a process was developed in the mid eighties in which sulphide is oxidized into elemental sulphur. Since sulphur is an insoluble compound it can be removed from the water-phase which leads to a reduction of the total S-content. The formed sulphur can be re-used in, for instance, bioleaching processes or it can be used as a raw material for sulphuric acid production after undergoing a purification step. The objective of this PhD-research was to optimize the biological sulphide removing process which concerned the development of 1) an oxygen control strategy for maximizing the sulphur production and 2) a sulphur removal method. In order to achieve the objectives, it was necessary to understand the colloidal properties of the biologically produced sulphur particles.Chapter 1 presents a general introduction on physico-chemical and biological methods for sulphide removal and a general overview on the sulphur chemistry. In Chapters 2 and 3 experimental results concerning the oxidation of sulphide into sulphur and sulphate are described. The oxidation of sulphide to sulphate yields more energy than the formation of elementary sulphur and consequently the micro-organisms tend to form sulphate rather than sulphur. In environmental technology however, the formation of the non-soluble sulphur is preferred. It was shown that at sulphide loading rates of up to 175 mg S 2-.L -1.h -1complete conversion of sulphide into sulphur only proceeds if a stoichiometrical amount of oxygen is supplied, that is 0. 5 mol of oxygen per mol of sulphide. At higher oxygen to sulphide consumption ratios increasing amounts of sulphate are formed, even when the oxygen concentration remains below 0. 1 mg .L -1. This value is in the range of the lower detection-limit of the currently available oxygen sensors which means that these probes are not suited to the accurate control of the oxygen dosage. An appropriate alternative for the oxygen measurements is the application of the redox-state of the solution. Although the redox-potential is a so called 'mixed-parameter', which means that its value is determined by several dissolved compounds, e.g. sulphide, thiosulphate, oxygen and maybe also certain 'unknown' compounds, it has been shown that a linear relationship exists between the sulphide concentration and the redoxpotential. According to Eckert sulphide-ions have a much higher current exchange density than oxygen-ions which means that the electron exchange with the platinum electrode surface is much higher for sulphide than for oxygen. The measured redox-potential is therefore kinetically determined rather than thermodynamically. The optimal redox range for sulphur formation is -147 ± 5 mV/H 2 at a temperature of 30°C and pH 8.Dynamic experiments conducted in a fed-batch reactor revealed that the organisms are capable of switching between sulphur and sulphate formation within 0.5 h. This is far below the maximum doubling time of e.g. T hiobacillus O and T hiobacillus denitrificans, indicating that one metabolic type of organism can perform both reactions. Sulphide auto-oxidation primarily leads to the formation of thiosulphate. Its presence was recognized immediately after an increase of the sulphide loading rate during experiments conducted in a continuous flow reactor. In such a situation the biological oxidation capacity obviously becomes a limiting factor.In order to develop an appropriate sulphur removal step, the physico-chemical properties of biologically produced sulphur particles had to be known. Steudel et al. encountered the presence of long-chain polythionates ( -S0 3 -S n -S03-) in a sulphur dispersion formed by acidophilic Thiobacillus ferrooxidans species. They formulated a 'vesicle-model' to describe the appearance of these sulphur particles. In such a vesicle the orthorhombic sulphur crystals are included within a network of long- chain polythionates. Synthetically formed 'LaMer' sulphur, which is formed by the acidification of a sodium thiosulphate solution, belongs also to this vesicle model. In more recent papers, Steudel applies the vesicle-model to all types of biologically produced sulphur, formed by e.g. neutrophilic thiobacilli and phototrophic chromatiaceae species. However, electrophoretic mobility measurements and flocculation experiments, as described in Chapter 4, show a clear difference between sulphur originating from our reactor system and 'LaMer' sulphur. Since the sulphur particles in our system are formed under slightly alkaline conditions, i.e. pH 8, they don't belong to the 'vesicle' model. Polythionates were reported to be stable only under acidic conditions. Steudel attributes the hydrophilic character of the biologically produced sulphur to the presence of negatively charged sulphonic-groups, whereas we have evidence that (bio)polymers are attached to the sulphur core. From dynamic light-scattering measurements it can be seen that the particle diameter reduces at increasing salt concentrations which is indicative of an inward migration of an adsorbed layer. These (bio)polymers very likely contain charged groups, such as carboxylic, phosphate and ammonium groups which give the sulphur its overall hydrophilic carboxylic, phosphate and ammonium groups which give the sulphur its overall hydrophilic character. X-ray measurements of freshly formed sulphur particles indicate the presence of orthorhombic sulphur crystals (S 8 ) which are known to be hydrophobic. These crystals are therefore present in the inner-part of the sulphur particles. The negative surface charge of the particles can be measured by potentiometric titrations. Results of such titration experiments are described in Chapter 5. The point of zero charge ( pzc ) was determined at pH 5.8. At higher pHvalues the surface becomes more negatively charged whilst at pH-values below 5.8 a positive charge was measured. The pzc does not however correspond with the iso-electrical point ( iep ), i.e. the pH at which the electrophoretic mobility is zero. The iep is located at a pH below 3. A possible explanation is that within the adsorbed polymer layer charge distribution occurs. Although at pH 5.8 the overall surface charge is zero, the charge of the outside of the polymer layer may be slightly negative, as follows from the electrophoretic mobility measurements, while the charge of the inner polymer side is more positive. This positive charge is attracted to the S 8 nucleus because of its high electron density.In this study it was shown that biologically produced sulphur particles have the ability to aggregate into larger clusters, particularly at high sulphide loading rates. However, increasing salt concentrations lead to a deterioration of the aggregation process, indicating that not only DLVO-interactions are involved but also factors such as entrapment of sulphur particles within the biomass/sulphur film and crystallisation of the elemental sulphur particles attribute to the sulphur aggregation. The effect of certain well defined polymers was investigated in order to improve the understanding of the effect of certain complex dissolved polymers on the sulphur-aggregation such as tannins, humic acids or additives used in the paper industry (Chapter 5). It was found that longchain polymers especially affect the sulphur-aggregation detrimentally. These compounds are dissipated from the water-phase and adsorb onto the sulphur particles. In the case of approaching sulphur particles covered with these long-chain polymers approach, an increased entropy results, leading to lateral repulsion. This then hampers the aggregation of the sulphur particles. Similar observations have been made for cationic polymers but not for anionic polymers. Besides chemical factors, physical factors also play an important role in the formation of a well-settleable sulphur sludge. Fluid shear forces disintegrate the sludge. For this reason we developed a new reactor-type for sulphide oxidation, i.e. an expanded sludge bed reactor. In this reactor, shear-forces due to aeration of the reactor suspension are avoided (Chapter 6). In this new reactor type, sludge particles are formed which have an average size of about 3 mm and a mean sedimentation velocity exceeding 25 m .h -1. The sulphur content of the sludge amounted to 92% whilst the rest fraction presumably consists of active biomass, as follows from aerobic activity measurements. Because biomass is immobilized within the sludge high loading rates are achieved, viz. 14 g HS .L -1.d -1whilst only 6 g HS .L -1.d -1could be obtained for a free cell suspension. The maximal applicable sulphide loading could indeed be higher but in the experimental assembly the recirculating flow, necessary for oxygen suppletion, reached an excessive level resulting in extreme liquid upward velocities. As a consequence, wash-out of biomass occured. Under the condition that fatty acids are present in the influent, such as acetate and propionate, anaerobic conditions within the sludge prevail, leading to the reduction of sulphur into sulphide.