Impact of inoculum activity on bioleaching: avoiding the lag phases

• Starting up of bioleaching depended on the iron oxidising activity of the inoculum. • Lag phase on bioleaching can be avoid by controlling inoculum activity. • Bacterial iron oxidising activity was measured by offline OUR. Reliable and standardised procedures are needed to inoculate bioleaching bioreactors effectively. These procedures help avoid unproductive initial stages and support the establishment of bioleaching as a robust and industrially viable metal extraction process. The lag phase typically occurs at the start of bioleaching and has traditionally been linked to bacterial adaptation to new conditions. In this work we propose that the reproducibility and kinetic of the bioleaching starting up of metallic sulphides depend mainly on the bacterial activity of the inoculum. It is postulated that the lag phase in bioleaching can be avoided or significantly reduced by using an inoculum with sufficient activity to prevent Fe(II) accumulation. This is critical for reaching the Oxidation-Reduction Potential (ORP) threshold required to leach target minerals in the ore. To test this hypothesis, we conducted bioleaching experiments with three sulphidic materials of varying composition and pretreatment. The main variable was the microbial activity of the inoculum, quantified using offline Oxygen Uptake Rate (OUR) measurements under non-limiting conditions regarding O 2 or Fe(II). The results demonstrate that, when optimal aeration and fluid dynamic conditions are guaranteed, the microbial activity during the starting process represents a critical variable. As a result, the use of an inoculum with 7.5 times higher activity resulted in a 46.7 % reduction in the lag phase of pyrite bioleaching.