Fluoride toxicity and mitigation strategies in acidophilic bioleaching microorganisms

Bioleaching is an established process for sulfidic ores and is increasingly applied to the recycling of industrial residues. However, unlike ores, many residues like sludge contain inhibitory elements, among which fluoride poses a major challenge due to its toxicity toward acidophilic microorganisms even at low concentrations. This study systematically investigated fluoride tolerance in pure and mixed cultures of various acidophilic sulfur- and iron-oxidizing bacteria commonly used for bioleaching, including Acidithiobacillus spp., Leptospirillum spp., and Sulfobacillus thermosulfidooxidans. Fluoride toxicity was found to be substrate-dependent. During sulfur oxidation, A. thiooxidans displayed the highest fluoride tolerance (0.5 mM F⁻), whereas S. thermosulfidooxidans showed complete inhibition. In contrast, iron-oxidizing bacteria demonstrated increased fluoride tolerance, with S. thermosulfidooxidans remaining active at 1.5 mM F⁻ when grown on ferrous iron. Mixed cultures showed enhanced fluoride tolerance during sulfur oxidation but reduced tolerance during iron oxidation. pH was identified as a critical factor influencing fluoride toxicity due to increased formation of undissociated HF at low pH. To mitigate fluoride inhibition, fluoride complexation with ferric iron or aluminum was evaluated. For A. ferrooxidans, iron oxidation resumed at Fe³⁺:F⁻ ratios of 7.5:1, while other cultures required ratios of at least 10:1. Aluminum complexation required Al:F⁻ ratios between 1:1 and 2:1, depending on the culture and growth conditions. Overall, fluoride inhibition during bioleaching is influenced by multiple factors, including pH, ferric iron concentration, and the fluoride dissolution rate. Early addition of aluminum is recommended to prevent microbial inhibition and ensure stable bioleaching performance. KEY POINTS: • Higher fluoride tolerance was observed during iron oxidation. • S. thermosulfidooxidans remained active up to 1.5 mM F⁻. • Fluoride toxicity is strongly pH dependent due to increased HF formation at low pH. • Effective fluoride complexation requires higher Fe³⁺:F⁻ ratios (> 7.5:1) than Al3⁺:F⁻ ratios (> 1:1).