| Hydrogen sulphide is a colourless toxic sulphuric compound. Emitted into the air from various industrial and chemical settings, it would be hardly harmful to people and cause water and air pollution. The biooxidation of ferrous iron to ferric by Acidithiobacillus ferrooxidans is a significant step in removal of hydrogen sulphide. Iron oxidizes H2S to elemental sulphur, and then A. ferrooxidans could turn the Fe2+ into Fe3+. In this paper a new bio-chemical double stage reactor technology is developed. In the former stage H2S is absorbed by the ferric solution in the chemical reactor. Then in the second one ferrous iron in the solution is oxidized by A. ferrooxidans in the biological reactor. This method is currently attracting the interest of researchers as a new technique of purifying air because this bacteria is able to treat not only low but also high concentrations of H2S gas, requires little equipment, is inexpensive and produces only elemental sulphur. In this paper, a perfect strain of Acidithiobacillus ferrooxidans was isolated and was immobilized efficiently, a new bio-chemical double stage reactor technology was built for removal of hydrogen sulphide, the reactors and new medium were designed. Through these studies, the main results and conclusions obtained are as follows.1st. A strain of A. ferrooxidans, which could bear low pH and high initial ferrous iron concentration, was isolated by domestication and screening using a reactor designed by our lab. The bacteria had a better adaptability for pH. The oxidation rate of ferrous iron was improved 50% by the bacteria than before at pH 1.8.2nd. A. ferrooxidans cells were immobilized using the complex of PVA solution and sodium alginate solution crosslinked with 1-5%Ca(NO3)2 is described. The beads formed by the method are not adhesive, and have high mechanical strength, less biological toxicity, and high stability.This technique not only can avoid the agglomeration of PVA gel beads and the toxicity of saturated boric acid simultaneously, but also could efficiently improve and control PVA network structure. The results gained in this research demonstrate that the cell immobilization technique with complex of PVA and sodium alginate is a promising method for the bacteria. The excellent performance can be attributed to the structure of PVA beads. Firstly, Calcium alginate gels formed in the acclimation phase can both facilitate the formation of hydrogen bonds between PVA molecules and promote the formation of the interpenetrating gel network. Then in batch and continue culture processes, the network structure was further optimized with the diffusion of calcium alginate and the reproduction of immobilized cells, which may benefit the transport of substrate and product. However, in addition to the stable structure of PVA beads with affluent pores, the high activity of immobilized cells may also be attributed to the protection of sodium alginate, the nontoxicity of coupling agent to cells and so on. 3rd. A packed bed reactor was built and was run constantly. The results indicated that the optimal pH and temperature of the immobilized cells hadn’t much change, but the biological activity was improved largely. There were some jarosite deposits produced in the process. This phenomenon may be good for cells immobilization and the fluidity of the immobilized beads. However, it would be a disadvantage if the deposit was produced overly.The formation of precipitation is not directly achieved by microbe. The increase of Fe2+ oxidation rate, the production of precipitation is rather obvious. In other word, the formation of precipitation is distinctly related to Fe2+ oxidation rate. The increase of Fe2+ concentration and the consumption of H+ would facilitate the production of the precipitation. The X-ray diffractometer (XRD) data indicated that Sediments forming during oxidation of ferrous sulfate by free cells and immobilized cells in batch culture was identified as H3OFe3(SO4)2(OH)6 and NH4Fe3(SO4)2(OH)6 respectively. This difference indicates that the medium of A. ferrooxidans must be changed after being immobilization, especialy in the process of H2S removal.4th. In the bio-chemical double stage system, biological reaction and chemical absorbability were divided and incorporated through the medium of iron at the same time. So the activity and stability were better with no straight impact by H2S. As a result, the technology has a perfect efficiency and hasn’t the second time pollution.5th.9K medium is regarded as an optimal medium for Acidithiobacillus ferrooxidans culture, especially for suspended Acidithiobacillus ferrooxidans. However, the iron concentration is insufficient and some precipitation would be formed when the medium is used in chemical-biological process for H2S removal. From the results of analysis, the iron concentration73.539% and pH 64.296% are the most influential factors, the pH 64.296% and (NH4)2SO4 34.765% were the most influential factors for the precipitation. Based on the research, a novel optimized liquid mixture for H2S removal were established:(NH4)2SO4 1 g/L, K2HPO40.2 g/L, KCl 0.1 g/L、Ca(NO3)20.01 g/L, MgSO4·7H2O 0.5 g, iron 17 g/L with initial pH 1.8.6th. A better adaptable and flexible reactor system was developed and designed in order to incorporate the biological part and chemical part. Pilotscale experiment indicated that H2S removal efficiency may reach 99% at the optimal operation condition such as 31℃, pH of 1.5-1.8, residence time of 2.5 h, maximum concentration of inlet 10000 ppm of H2S.7th. Based on the study in this paper, bio-chemical double stage reactor technology can be used for H2S removal of biogas, natural Gas and industrial tail gas. |
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