Early reprecipitation of sulfate salts in coal biodesulfurization processes using acidophilic chemolithotrophic bacteria

This study evaluated the effect of three sulfate salt-based culture media on the reprecipitation of sulfur under the action of two types of bacterial inoculum, a pure strain of Acidithiobacillus ferrooxidans (ATCC 23270) and a consortium of this strain and Acidithiobacillus thiooxidans (ATCC 15494), in a biodesulfurization process for coal (particle size < 0.25 mm) from the 'La Guacamaya' mine (Puerto Libertador, Cordoba, Colombia). All of the experiments were periodically monitored, with measurements taken of pH, cell concentration, iron concentration, and pyrite oxidation. Additionally, mineralogical analyses were conducted on the initial and final coal samples, through scanning electron microscopy with an energy-dispersive X-ray spectrometer. The results showed that sulfate reprecipitation occurred primarily, and nearly entirely, during the first 3 days of the process. While all the treatments obtained high levels of mineral oxidation, the reprecipitation processes decreased in media with low concentrations of sulfate, leading to the higher final removal of inorganic sulfur. The bioassays revealed that after 15 days, the maximum pyrite oxidation (86%) and inorganic sulfur removal (53%) was obtained with the treatments using the Kos and McCready culture media. The bacteria evaluated were found to have a great ability to adapt to very simple culture media with minimal nutrient concentrations, and even with some nutrients absent (as in the case of magnesium).