Enhancing the activity of iron-oxidising bacteria: A case study with process liquors from heap bioleaching of a complex sulphide ore

- Fe2+ concentrations up to 16 g/L did not inhibit the studied iron-oxidisers.
- Al3+ concentrations of 6 g/L and below enhanced iron oxidation rates.
- High cadmium, sulphate, and vanadium concentrations decreased iron oxidation rates.
- Ammonium supplementation to bioheaps would enhance the heap bioleaching process.
- High inhibitory ion content increases microbial sensitivity to nitrogen deficiency.

In the present study, six process liquors (PL1 - PL6) originating from heap bioleaching of a complex sulphide ore were examined to reveal factors limiting microbial activity in the bioheaps. PL4 had the lowest iron oxidation activity even though its indigenous iron-oxidising community was diverse (Acidithiobacillus, Leptospirillum, Acidiferrobacter, and Sulfobacillus species). Shake flask experiments at 27 °C revealed that ferrous iron (Fe2+) and aluminium (Al3+) concentrations up to 16 and 12 g/L, respectively, were not inhibitory for the iron-oxidising microorganisms in PL4. In addition, Al3+ concentrations of ≤ 6 g/L were shown to enhance iron oxidation rates. High correlation between increased concentrations of cadmium (Cd), sulphate (SO42−), and vanadium (V) and decreased iron oxidation rates was detected when comparing process liquors 1-6. Moreover, possible nutrient limitation in PL4 was delineated by selectively supplementing it with macro- and micronutrients. Supplementation of 320 mg/L of nitrogen (as NH4+) to PL4 significantly increased iron oxidation rates from 20 mg/L/h (no nutrient supplementation) to 160 mg/L/h and would likely also enhance the heap bioleaching process. Additionally, microorganisms growing in high inhibitory ion concentrations (e.g. Cd2+) were shown to be more sensitive to nitrogen deficiency than microorganisms growing in more dilute liquors.