Mechanism of fluoride inhibition on bioleaching bacteria and competitive complexation of ferric ions

A bottleneck occurs in the application of bioleaching technology to fluoride-containing ore. The reason for this is that fluorine has a strong inhibitory effect on leaching bacteria with the dissolution of fluorine-containing gangue minerals. In this study, we use the chemical properties of fluorine to convert F ions by adding substances that can form stable complexes with F^-, which enables the leaching bacteria to tolerate high fluoride environments. In this research, we studied the inhibition mechanism of fluorine on bacteria, and identified its true toxic form (HF). We found that fluoride exhibited a transmembrane inhibitory effect on bacteria. Under fluoride stress conditions, the concentration of intracellular fluoride was significantly higher than that of a non-fluorinated control group, which was about 18% dry cell. We selected common Fe³⁺ ions in the bioleaching system, and studied the competitive complex detoxification of Fe³⁺ to F^-. Our thermodynamic analysis results show that Fe³⁺ can compete with HF in first-order competitive complexation reactions whereby the HF complex structure is converted to FeF_n^3-n. In the presence of ferric ions, we found that the bacteria could tolerate F^-concentrations up to 1.0 g·L^-1. Our analysis of the Fe and F complex species indicates that bacteria could grow normally when the concentration of Fe³⁺ ions was five times greater than that of F^- ions. Correspondingly, the proportion of FeF²⁺ components in the solution was ≥ 45%, and the concentration of free fluoride was 2.87×10^-5 mol·L^-1. The complexation mechanism shows that as the ratio of F^--Fe³⁺ decreases, the concentration of the ligand is relatively lower. Based on the coordination chemistry, the complex of fluoride and iron moves in a lower coordination direction, and the Fe and F complex species can be controlled by adjusting the concentration ratio of F^- and Fe³⁺ in the medium, therefore making it possible for bacteria to grow in a high-fluorine environment.