چكيده به لاتين
Wastewaters containing sulfur compounds require treatment due to their toxicity and detrimental environmental impacts. Biological removal through microbial oxidation is considered a viable alternative to conventional physico-chemical methods. Among sulfur-oxidizing microorganisms, green and purple sulfur bacteria (anaerobic photosynthetic bacteria) capable of utilizing sunlight have recently attracted significant research interest. In this study, the thiosulfate oxidation capacity of bacteria enriched from three indigenous consortia (effluent from a leather factory in Tabriz, and two sulfur-rich lakes in Qorveh and Sarein) was investigated. In the first section (the chemolithotrophic sulfur bacterium enriched from the leather factory effluent), the impact of S/N ratios (1, 1/5, 2, and 2/5) on product distribution was assessed. The results indicated that higher S/N ratios (2 and 2/5) were more effective for elemental sulfur production, with the maximum sulfur yield (26% of removed thiosulfate) observed on the second day of the experiment at an S/N ratio of 2/5. Additionally, the factors influencing growth and dominance of green and purple sulfur bacteria from the two sulfur lakes were evaluated after enrichment and acclimatization under various conditions. Biological oxidation of thiosulfate (in the range of 1000–3000 ppm) showed that the enriched green sulfur bacterium could completely remove thiosulfate up to 2000 ppm. However, oxidation rate and removal efficiency declined at 3000 ppm, identifying this concentration as inhibitory. Furthermore, mixotrophic growth at 500 ppm acetate enhanced both bacterial growth and oxidation rate. For the purple sulfur bacterium, complete thiosulfate removal was observed at concentrations between (300–800 ppm), whereas higher concentrations had inhibitory effects, slowing and eventually halting growth. A two-level factorial experimental design was employed to optimize and evaluate the interaction of key factors (thiosulfate, bicarbonate, and light at different levels for both bacterial species) on two responses: thiosulfate removal percentage and sample coloration, the latter serving as an indicator of growth and dominance of the target species. The results showed that efficient oxidation of sulfur compounds requires a balanced ratio of electron donor (thiosulfate) to electron acceptor (bicarbonate). Lower thiosulfate concentrations, as expected and consistent with preliminary tests, led to higher removal efficiencies. Bicarbonate and light intensity significantly affected both response variables in green and purple sulfur bacteria, though their positive effects were limited to specific concentration ranges, beyond which inhibitory effects emerged. Interaction among the factors was found to play a critical role in achieving optimal responses. Optimization results for the green sulfur bacterium demonstrated that under conditions of 1583 ppm thiosulfate, 3700 ppm bicarbonate, and high light intensity (15 klux), it was possible to achieve 75% thiosulfate removal and 25% sample coloration with a desirability of 0/87. Based on the findings of this study, green and purple sulfur bacteria—with the ability to oxidize thiosulfate up to 2000 ppm and lower energy requirements compared to other species—appear to be promising candidates for industrial applications.
