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Iron Fe2+ oxidation, Monod constants, microbial consortium, biomass determination, Acidithiobacillus ferrooxidans

Abstract

Bioleaching is a process of biological extraction of metals from minerals conducted by microorganisms that utilize various metabolic mechanisms to convert insoluble into soluble forms of metals. While bioleaching presents an advantageous method for extracting valuable metals from natural deposits, it also entails the inadvertent release of these metals into the environment by naturally existing microorganisms within heaps and ores. Such a phenomenon contributes to the mobility of toxic metals, which is crucial for the environment. Currently, significant attention is directed toward contemporary industrial zones using circular economy for metal recovery, while investigating and monitoring historical and exploited mining areas remains relatively limited. This issue is particularly severe in post- mining areas. Mining and processing of ores generate a significant amount of waste, which is typically stored in open-air conditions. As a result of the natural bio-oxidation of sulfide minerals, acidic effluents are formed, which can pose an environmental threat due to the presence of sulfates(VI) and toxic elements such as arsenic, cadmium, chromium, copper, lead, mercury, nickel or zinc (Newsome and Falagán, 2021).

The biodiversity of microorganisms capable of oxidizing metal compounds includes archaea, bacteria, fungi, and yeast (Sajjad et al., 2019); these are resistant to high levels of metal concentration and high acidity. Naturally occurring and the most extensively characterized is the bacterium  Acidithiobacillus ferrooxidans (Bonnefoy and Holmes 2012). Several studies of bioleaching have been carried out using A. ferrooxidans monoculture (Zhang et al. 2018; Santaolalla et al. 2021; Kang and Wang 2022) and also mixed cultures (Wang et al. 2014; Heydarian et al. 2018). Many scientific  studies investigated the growth kinetics of pure cultures of A. ferrooxidans using 9K medium and FeSO4 as substrate (Silverman and Lundgren 1959). However, the kinetic constants varied considerably and required consideration of additional parameters such as product inhibition, initial Fe 3+ ion  concentration, and inoculum size. A model for the growth kinetics of A. ferrooxidans, including all assumed parameters, was presented for the monoculture (Molchanov et al. 2007).

So far, no growth kinetics model has been presented for the consortium isolated from arsenic- containing waste. Our research showed that such a consortium consists predominantly of A. ferrooxidans, and Acidiphillum cryptum. We determined a standard curve for the relation of cell biomass [mg]  against absorbance λ = 500 nm and Monod equation constants. We also investigated surface characteristics, such as surface charge, which are important when bacteria are in contact with the mineral surface (e.g. in bioleaching). It was shown that in the case of A. ferrooxidans, zeta potential depends  on their growth history (Blake et al. 1994). To test whether changing culture conditions with different iron(II) content affects the surface charge of bacterial cells, the zeta potential was measured using Zetasizer 2000 (Zetasizer, Malvern, United Kingdom) at a constant ionic strength of 10 -3 M KCl, pH  2.0. The oxidation rate of iron was measured using the titrimetric method. Our research aims to better understand the acidophilic bacteria consortium's kinetic growth isolated from arsenic-bearing waste. The expected results will contribute significantly to the fields of metal recovery and environmental  remediation.

Acknowledgement

This work was supported by the National Science Centre, Poland, under grant no. 2021/43/D/ST10/02784.

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