چكيده به لاتين
Sulfur compounds come into environment from a variety of sources and their removal is essential due to environmental hazards and toxicity to humans. In this study, we aimed to investigate the factors affecting bio-removal of sulfide/thiosulfate and its products distribution (elemental sulfur with various applications against sulfate with pollution and secondary problems) in order to achieve maximum pollutant removal and elemental sulfur production (desirable product) in the shake flask and bioreactor by a native consortium of Atashkouh soil. Bio-oxidation of thiosulfate (700–4000mg/l) showed that the initial consortium (OC) was able to completely remove low concentrations and produce elemental sulfate and sulfur while removal decreased with increasing concentration and reached complete inhibition at 4000mg/l. To enhance biological oxidation, OC was adapted to thiosulfate (AT) resulting in complete oxidation of 3000mg/l thiosulfate (compared to 45% by OC) and formation of 62% sulfate and 14% elemental sulfur (desirable product). Oxidation of sulfide (being more toxic than thiosulfate) was further investigated, and due to the simultaneous bio-chemical oxidation of sulfide, chemical oxidation was first examined and it was found that sulfide was rapidly converted to chemical products, such as thiosulfate (dominant product). To improve aerobic sulfide bio-oxidation, four set of experiments were performed. Firstly, AT-consortium was used to oxidize sulfide at low concentration (500mg/l) where complete sulfide removal and production of 84% sulfate (bio-specific product) were observed. However, rising sulfide concentration to 2000mg/l led to reduction of oxidation due to sulfide toxicity with production of only 26% sulfate and 64% thiosulfate (chemical product). Secondly, the removal of 2000mg/l of sulfide was investigated using the adatpted-OC consortium to sulfide (AS) and it was found that the toxicity of this pollutant reduced sulfate production to 15% compared to 35.5% with the OC-consortium. Medium pH adjustment was also carried out however bio-oxidation results were not satisfactory. Inspired by the conversion of sulfide (toxic) into thiosulfate (less toxic) during chemical oxidation, finally a hybrid process was proposed to reduce the toxicity and improve oxidation of high sulfide concentration (2000mg/l) which enhanced biological oxidation with 55% increase in sulfate formation over conventional bio-oxidation. Despite the success of this hybrid process, due to the remaining thiosulfate, optimization of thiosulfate oxidation using AT-consortium was performed to increase the removal and production of elemental sulfur as the final product. Screening was performed among 7 primary factors by a fractional factorial method (27-2) and 6 important factors werer used in response surface methodology (RSM)) to single and multiple optimize thiosulphate removal (Y1), elemental sulfur (Y2) and sulfate (Y3). The results of single optimization showed that maximum Y1 and Y3 occurred at high stirring speed, low Thiosulfate concentrtion, longer time and lower temperature while elemental sulfur production (Y2) decreased under these conditions. Multiple optimization using desirability function was therefore used for synchronization of maximum thiosulfate removal and elemental sulfur production as well as 50% sulfate production. The final optimum levels of agitation speed: 144rpm, pH: 6.07, temperature: 24.7°C, inoculum percentage: 19/5%, time: 2.62day and thiosulfate concentration: 2.87g/l were obtained. Additionally, two a magnetically stirred bioreactor (0.5l) and a impeller stirred bioreactor (1l) were exploited to investigate the effect of direct aeration on thiosulfate oxidation. In the small bioreactor at 1vvm, the initial rate of thiosulfate oxidation decreased from 0.83 to 0.72mmol-Sl-1hr-1 with increasing thiosulfate concentration from 4000 to 6000mg/l, while elemental sulfur production reached its maximum of 40%. Investigation of the effect of aeration rate (1-2vvm) on this bioreactor showed that aeration had a positive effect on thiosulfate oxidation rate at 4000mg/l and a negative effect on elemental sulfur production and oxidation time. In order to increase the removal efficiency at 1vvm aeration, thiosulfate oxidation was therefore examined under optimal conditions of experimental design at different stirring speed (30-9orpm) in stirred bioreactor where the highest elemental sulfur of 46% was achieved at 6orpm and 40h alongside the complete thiosulfate removal. As our consortium showed successful oxidation of thiosulfate into elemental sulfur, its dominant species was isolated and identified as Ochrobactrum sp. by 16 rRNA method. Our indigenous consortium was therefore proved to be a superior candidate for industrial applications due to its high capacity in sulfide and thiosulfate oxidation as well as enhanced production of elemental sulfur as high volume (bioreactor) product.
