The effect of initial pH on surface properties of ferric ion precipitates formed during microbial oxidation of ferrous ion by Leptospirillum ferriphilum in a CSTR

Abstract

While bioleaching is a proven technology for the efficient recovery of base metals from sulphide minerals, its sustenance is dependent on the continuous availability of ferric ion, Fe3+, in soluble form, in the bioleach liquor. However, the solubility of ferric ion is low at higher pH that it tends to precipitate, resulting in the formation of ferric ion precipitates. The formation of ferric ion precipitates in bio-hydrometallurgy decreases the leaching efficiency by trapping the leached metals in solution through an adsorption mechanism which is not well understood. Although the surface properties of the precipitate could be linked to its metal adsorption properties, there has not been a detailed study that gives any indication or explanation of the adsorption mechanism. Therefore, the aim of this study was to investigate the effect of initial pH on the surface properties of ferric ion precipitate and relate this to the adsorption characteristics of the precipitate for desired metals. Biooxidation experiments catalysed by Leptospirillum ferriphilum were conducted in a CSTR with a working volume of 1L. The biooxidation experiments were conducted at pH values of 1.3, 1.5, 1.7, 1.9 and 2.2 at a constant temperature of 35 0C for 14 days. The recovered precipitates were characterized by X-ray diffraction, elemental analyses, SEM, particle size distribution (PSD) and zeta potential. Zeta potential measurements were conducted to investigate what role initial pH plays in modifying the precipitate surface charge and what role the surface charge of each precipitate plays in the nature of adsorption of copper ions onto the precipitate surface. The amount of copper adsorbed onto the precipitate was quantified by the magnitude of the change in surface charge after adsorption experiments. Quantification results showed that the amount of ferric ions precipitates formed increased from 4.31g to 13.26g with an increase in initial pH (from 1.3 to 2.2). The results also showed that significant precipitation of ferric ion occurred during the exponential phase while insignificant precipitation was observed during the stationary phase.