In addition to the solubilization of iron and nickel, concentrations of both soluble manganese and cobalt also increased as bioleaching of the laterite ore progressed (Figure 3
a). The source of these two transition metals in the ore is the oxyhydroxide mineral asbolane ((Ni,Co)x
O). The oxidation state of manganese in asbolane is +4 and, as its empirical formula indicates, it commonly contains both nickel and cobalt within its lattice structure. Concentrations of soluble manganese were highly correlated with those of cobalt during dissolution of the nickel laterite ore (R2
= 0.93; Figure 3
b). Since Mn(IV) is highly insoluble in pH 1.8 liquors, the implication is that asbolane was also subjected to reductive dissolution. In order to elucidate the mechanism of asbolane dissolution, the laterite ore was incubated in sterile acidic growth medium under aerobic conditions, in the absence or presence of Fe2+
. After seven days incubation, Mn concentration in the absence of added ferrous iron was 7.4 ± 1.1 mg/L and 35.6 ± 0.3 mg/L when ferrous iron was added. Likewise, the concentration of soluble Co was higher in ferrous iron-supplemented medium (6.3 ± 0.4 mg/L) than in iron-free liquor (2.0 ± 0.3 mg/L). These results imply that At. ferrooxidans
caused the reductive dissolution of asbolane indirectly, via the generation of ferrous iron formed during the reductive dissolution of goethite. As with iron and nickel, bioprocessing of the lateritic ore under aerobic conditions resulted in far smaller amounts of both manganese and cobalt being solubilised than when anaerobic conditions were imposed (40 vs.
140 mg, and 8 vs.
24 mg, respectively, over 20 days). Over 95% of the chromium in the limonite ore was also solubilised within the time frame of the experiments (data not shown). The source of this metal was the mineral chromite (FeCr2
) in which chromium has an oxidation state of +3 and iron of +2 (i.e
., both metals are in reduced states). Dissolution of chromite was therefore considered to have occurred by direct acid dissolution. The reductive conditions of the bioprocess would preclude oxidation of the chromium released to its highly toxic +6 oxidation state, such as would occur in alternative processing options, such as high pressure acid leaching.
) Changes in concentrations of soluble manganese (
) and cobalt (
) in bioreactor cultures of At. ferrooxidans
containing Ni laterite ore and elemental sulphur, incubated either under anoxic (solid symbols) or aerobic (open symbols) conditions); (b
) correlation between the amounts of manganese and cobalt solubilised under anoxic conditions.