Elucidation of Gram-Positive Bacterial Iron(III) Reduction for Kaolinite Clay Refinement

Improving the electrochemical characteristics and performance of a neutral all-iron flow battery by using the iron reduction bacteria

At present, the all-iron redox flow batteries (RFBs) have greater application potential due to high accessibility of electrolytes compared to traditional RFBs. Meanwhile, although electroactive bacteria can accelerate the electrons transfer, their potential to improve the performance of RFBs has been overlooked. Previously, we had confirmed that ferrous-oxidizing bacteria (FeOB) could enhance the performance of an all-iron RFB, therefore we conducted several batch experiments and chronopotentiometry experiments by using the ferric-reducing bacteria (FeRB) or mixed culture (FeOB and FeRB) to demonstrate whether they have the same or stronger effects on Fe3+-DTPA/Na4[Fe(CN)6] RFB. The results showed that the experimental reactors could achieve higher charging current density and initial cathodic potential during constant voltage charging process. The electrochemical impedance spectroscopy data and cyclic voltammetry curves demonstrated that the polarization impedance increased slower and reduction peak potential of experimental groups also emerged a positive shift compared to CK. According to chronopotentiometry experiments results, the microbes could function at maximum 0.3 M, 12 mA/cm2, and also improved the charging specific capacity. Combined the SEM pictures and microbial composition analysis, the main functional electroactive FeRB were Alcaligenes, Corynebacterium and Bacillus, which indicated to have important potential in improving the performance of RFBs.

Impact of Bacillus species on Fe reduction of kaolin in bioleaching: surface, structural, and chemical studies

Conventional techniques to remove Fe impurities in kaolin typically involve high environmental impact and cost. Alternative methods have been focused on the use of bioleaching where Fe in kaolin is reduced with microorganisms. Early results established a noticeable effect of the bacteria on the redox state of Fe, but knowledge gaps persist such as details on the bacterial-kaolin interactions during attachment of bacteria onto kaolin surface, the metabolites produced by bacteria, and changes in Fe(II)/Fe(III) ion equilibria in solution. To bridge these gaps, this study was conducted to determine the detailed physicochemical changes in bacteria and kaolin during bioleaching through surface, structural, and chemical analysis. Bioleaching experiments were conducted for 10 days where each of the three Bacillus sp. was put in contact (at 9 × 10⁸ CFU) with 20 g of kaolin powder using 200 mL of 10 g/L glucose solution. All samples treated with bacteria showed increasing trends in Fe(III) reduction up until day 6 or 8 followed by a slight decrease towards the end of the ten-day period. Examination of scanning electron microscope (SEM) images suggests that bacterial activity damaged the edges of kaolin particles during bioleaching. Ion chromatography (IC) results showed that during bioleaching, Bacillus sp. produced organic acids such as lactic acid, formic acid, malic acid, acetic acid, and succinic acid. EDS analysis of kaolin before and after bioleaching showed Fe removal efficiencies of up to 65.3%. Analyses of color properties of kaolin before and after bioleaching showed an improvement in whiteness index of up to 13.6%. KEY POINTS: • Dissolution of iron oxides by Bacillus species proven with phenanthroline analysis. • Organic acid type and concentration unique to species detected during bioleaching. • Whiteness index of kaolin is improved after bioleaching.

Elucidation of Iron(III) Bioleaching Properties of Gram-Positive Bacteria

Microbial-based iron reduction is an emerging technology used as an alternative to conventional chemical-based iron reduction. The iron reduction in kaolin refinement is vital for enhancing its commercial value. Extensive studies on microbial-based iron reduction mainly focus on Gram-negative bacteria, whereas little is understood about Gram-positive bacteria's mechanism and potential application. This study aims to investigate the iron-reducing mechanism of two Gram-positive bacterial isolates, Bacillus cereus (B. cereus) and Staphylococcus aureus (S. aureus). By varying the growth environment of bacteria and monitoring the biochemical changes during the process of iron reduction, the results show that Gram-positive bacterial iron reduction performance depends on the medium composition, differing from Gram-negative bacteria-based reduction processes. Nitrogen-rich medium facilitates the microbial basification of the medium, where the alkaline conditions impact the microbial iron reduction process by altering the gene expression involved in intracellular pH homeostasis and microbial growth. This discovery will contribute to the mineral refining processes and promote the development of microbial-based bioprocesses for ore purification, while also laying the foundation for investigating other Gram-positive bacterial iron-reducing ability.