biodesulfurization

Biodesulfurization is a biotechnological approach to removing sulfur from fossil fuels using microorganisms. It aims to reduce sulfur content in fuels such as diesel and gasoline to lower sulfur dioxide emissions during combustion, while preserving much of the fuel’s hydrocarbon structure.

In many well-studied systems, desulfurization proceeds via the 4S pathway, a sulfur-specific enzymatic route found in certain bacteria such as Rhodococcus and Gordonia species. The pathway targets sulfur-containing heterocycles like dibenzothiophene (DBT). Enzymes DszC, DszA, and DszB catalyze a stepwise oxidation and cleavage that converts DBT first to DBTO and then to a sulfur-free product, typically 2-hydroxybiphenyl, with inorganic sulfite released in the process. The carbon skeleton of the hydrocarbon fragment is largely retained, which helps maintain fuel quality.

Substrates include various organosulfur compounds present in crude oil and its fractions, notably DBT, benzothiophene, and other sulfur-containing aromatics. Biodesulfurization is particularly researched for diesel fractions, where regulatory limits on sulfur are stringent. The process can be implemented in ex-situ bioreactors (treating refinery streams outside the process train) or, less commonly, in-situ within refinery units. Many implementations use biphasic or two-liquid phase partitioning systems to improve mass transfer between the oil phase and the aqueous microbial phase and may employ surfactants or special reactor designs.

Advantages include mild operating conditions and selective sulfur removal without extensive hydrocarbon degradation. Limitations involve slow reaction rates, mass transfer challenges, toxicity of sulfur compounds to the cells, limited substrate range, and economic competitiveness. Research continues into optimized strains, process configurations, and integrated refinery strategies to enhance practicality.