Bioleaching of silica sand using bioreducing bacteria (<i>Shewanella</i> strains)

Abstract

Silica sand is generally not pure when mined; often associated with iron hydroxide and oxy-hydroxide impurities which lowers its industrial value and requires purification before use. To join the global community towards environmental consciousness, protection and effective energy utilization, this paper considers microbial purification in a closed non-growth system as an alternative for iron removal. The anaerobic removal of Fe(III)-bearing impurities from the mineral was investigated using small scale microcosm experiments conducted with Fe(III) in silica sand as the sole electron acceptor and lactate as the electron donor in the presence of pure cultures of Shewanella strains (S. putrefaciens CIP8040, S. putrefaciens CN32, S. oneidensis MR-1, S. algae BrY and S. loihica) and anthraquinone 2,6 disulphonate (AQDS) serving as electron shuttling mediator. The reduction of Fe(III) from silica sand was measured as the production of Fe(II) in HCl extracts using modified p-Phenanthroline techniques. Absorbance was measured using UV vis-spectrophotometry at 510nm. Up to 17.6% of the iron bearing impurities (~117mg of bioreducible Fe2O3 per 100g of silica sand) was successfully removed after 15 days. Addition of AQDS as electron transport mediator enhanced the rate and extent of bioreduction by facilitating the exchange of electrons between the iron reducing bacteria and the iron-bearing phase in the mineral. This study revealed that all the selected commercially available Shewanella strains tested were able to reduce and leach iron (III) from Silica sand by coupling oxidation of lactate to anaerobic respiration except Shewanella loihica whereas Shewanella algae BrY was the most efficient.

Keywords: Silica sand, iron impurities, iron reducing bacteria, Shewanella strains bioreduction.