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1 Overview As a new type of energy source, biogas is being used more and more widely. Chinese environmental regulations stipulate strictly that when using biogas as an energy source, the H2S content in the biogas gas must not exceed 20 mg·m-3. Whether in industrial or domestic gases, it is necessary to remove H2S as much as possible. When biogas is produced by anaerobic fermentation equipment, especially during medium- or high-temperature fermentation, it contains a large amount of H2S. Since there is a lot of water vapor present in biogas, water, together with H2S in the biogas, accelerates the corrosion and blockage of metal pipes, valves, and flow meters. Additionally, SO2 generated upon the combustion of H2S combines with water vapor present in the combustion products to form sulfurous acid, which causes corrosion on the metal surfaces of equipment. It also leads to pollution of the atmospheric environment and can affect human health. Therefore, it is necessary to remove H2S from biogas before using it. In the past, chemical methods were used for biogas desulfurization, which can be divided into several approaches such as alkaline absorption, chemical adsorption, chemical oxidation, and high-temperature thermal oxidation. The operating costs of chemical desulfurization are very high; the main reasons are the large amount of chemical reagents required during operation and the high energy consumption, plus the formation of new sulfur-containing compounds, which can cause new environmental pollution if not properly managed. Biological desulfurization is a new technique that replaces chemical desulfurization, and it can overcome the shortcomings of chemical desulfurization in many aspects. In the biological desulfurization process, two main types of microorganisms are involved, namely photoautotrophic microorganisms and chemoautotrophic microorganisms. This article reviews the basic principles behind the removal of H2S from biogas by these two major categories of microorganisms. 2 Principles of desulfurization by photoautotrophic microorganisms Green sulfur bacteria are strictly anaerobic photoautotrophic microorganisms. In the presence of light and inorganic nutrients, green sulfur bacteria can use CO2 to synthesize new cellular materials, while simultaneously converting S2- into elemental sulfur which is then released outside the cells. These characteristics make green sulfur bacteria highly suitable for biological desulfurization processes. The desulfurization reaction proceeds as follows: It is worth noting that under certain conditions, green sulfur bacteria will further oxidize elemental sulfur to SO42-. Therefore, there is a relationship between the intensity of light and the desulfurization process – insufficient light impairs desulfurization, while excessive light leads to the formation of SO42-. Only under appropriate lighting conditions can sulfides be completely converted into elemental sulfur without the formation of SO42-. Thus, it is essential to strictly control the reaction conditions when using green sulfur bacteria for desulfurization. 3 Principles of desulfurization by chemoautotrophic microorganisms Chemoautotrophic microorganisms use CO2 as a carbon source and obtain energy in the process of oxidizing S2-. In the presence of organic carbon sources, some strains of autotrophic microorganisms can utilize organic carbon sources for heterotrophic metabolism. Chemoautotrophic microorganisms convert S2- into elemental sulfur, and this can occur both under aerobic conditions and under anaerobic conditions. In aerobic conditions, oxygen serves as the electron acceptor, while in anaerobic conditions, nitrate can be used as the electron acceptor. Many chemolithoautotrophic microorganisms can use elemental sulfur, H2S, thiosulfates, and organic sulfides as electron donors; representative microorganisms among them are Thiobacillus ferrooxidans, Thiobacillus denitratans, Thiobacillus sulfidaris, and Thiobacillus sulfuroxidans. The process by which chemoautotrophic microorganisms convert S2- into elemental sulfur is as follows: when oxygen is the limiting factor in the biochemical reaction, elemental sulfur is the main product; when S2- is the limiting factor, the main product is SO42- rather than elemental sulfur. 3.1 Principles of biological desulfurization technology The basic principle behind this desulfurization technology is to bring biogas containing H2S into contact with a sodium carbonate solution containing chemoautotrophic microorganisms; once H2S is absorbed by the alkaline solvent, it is converted into elemental sulfur or sulfates through microbial catalysis. The main reactions that take place in the bioreactor are as follows: Currently, this technology is one of the most mature desulfurization methods available worldwide, and it has the following advantages: (1) Safety: All biological desulfurization systems operate in a closed system, and the H2S in biogas is completely absorbed; there is no free H2S downstream of the absorber, so there is no risk of poisoning or injuries, nor any environmental pollution. (2) Cost savings: This skill requires less investment, with a small number of essential equipment and instruments. The operating cost is low, and fewer operators are required for production, thereby reducing labor costs; no chemical catalysts are needed, and biological catalysts do not become inactive – they regenerate on their own and do not require replacement. Few chemicals are required during operation, which helps to cut production costs; the operational costs and maintenance expenses associated with this technology are very low. (3) High efficiency: This technique ensures that the H2S content in the desulfurized natural gas is less than 4 ppmv ； Moreover, it offers great operational flexibility; it can handle H2S concentrations in the range of 50 ppmv to 100 vol.%, and pressures ranging from 1 to 100 barg, providing high flexibility to cope with peak H2S loads. The skill’s operation process is simple, with few control and monitoring systems; there are no complicated control circuits, making operation and protection straightforward and easy. It is suitable for small gas fields with high H2S concentrations, offering better economic efficiency; moreover, the equipment using this technology features stable performance, reliable operation, and good economic returns. 3.2 Principles of Iron Salt Absorption Biological Desulfurization The basic principle of iron salt absorption biological desulfurization is that during the absorption process, H2S is oxidized to elemental sulfur by Fe3+. Under acidic conditions (pH=1.2–1.8), ferrooxidans facilitates the conversion of Fe2+ to Fe3+, which can then be reused in the absorption process. The relevant reactions are as follows: These substances possess a relatively high redox potential, enabling them to convert H2S into elemental sulfur without further oxidizing elemental sulfur to sulfates. The resulting elemental sulfur is recovered after separation, and then Fe2+ is converted to Fe3+ through the metabolism of Thiobacillus ferrooxidans, allowing for recycling. Therefore, most researchers believe that this method has low energy consumption, minimal investment requirements, and reduced waste emissions, making it more suitable for the process of biogas desulfurization. 4 Discussion (1) In terms of the ability of photoautotrophic microorganisms to remove H2S from biogas, green sulfur bacteria are a relatively ideal choice, as they can utilize inorganic carbon sources. Moreover, these bacteria have a high desulfurization efficiency, and the metabolic product, elemental sulfur, is released outside the cells, making it easier to separate. However, light and bacteria require a large amount of radiation energy during the conversion process; since the formation of sulfur nanoparticles in the reaction system leads to a **decrease in light transmittance, which in turn affects the desulfurization efficiency, it is difficult to make this approach feasible from an economic standpoint. If breakthroughs can be achieved in reducing the energy consumption of light sources and increasing their power, this approach holds broad market prospects for application. (2) Under strict control of oxygen supply, using chemoautotrophic microorganisms to remove H2S from biogas holds great potential for commercial application, especially since there are already precedents for the use of two-stage desulfurization technologies in practice (the Shell-Parker technology). This skill features that it does not affect the recovery and utilization of biogas, nor does it cause new environmental pollution. Other methods, such as using iron salts to absorb and remove H2S and then regenerating the iron salt absorption solution through biological oxidation to reuse those iron salts, have become new areas of active research in recent years. (3) Biological desulfurization techniques must meet several conditions: first, they must have reliable efficiency; second, they require few nutrients; third, elemental sulfur in the biomass must be easily separable. 5 Conclusion Compared with traditional physical and chemical desulfurization methods, which require high energy consumption and incur significant treatment costs, biological desulfurization encourages the search for low-energy, highly efficient, economical, and advanced treatment solutions. Biological desulfurization technology has opened up new directions for research and application in this field. Although biological desulfurization technology holds promising industrial application prospects, it is still largely in the research and development phase, and it faces many challenges that pose obstacles to its rapid advancement. On the one hand, researchers need to seek out bacterial strains with desulfurization capabilities and study their desulfurization effects; on the other hand, they must apply knowledge from biotechnology and genetic engineering to enhance their activity, stability, and selectivity, in order to achieve excellent desulfurization results and make new breakthroughs in H2S removal technologies for biogas.