Utilization of indigenous bacteria should be considered to establish a successful
biohydrometallurgical process. In this study, mixrotrophic iron-oxidizing bacterial
consortia consisting of Comamonas testosteroni, Alicyclobacillus ferrooxydans and
Pantoea septic which were isolated from Indonesian mineral ores were examined to
determine their abilities to recover nickel from limonite and saprolite ores in the
bioleaching experiments using stirred tank reactors. The nickel bioleaching
experiments inoculated with the bacterial consortia were carried out using coarse
limonite ores and weathered saprolite ores with pulp density of 10% w/v. Abiotic
controls were also carried out replacing the inocula by the sterile medium. The
bioleaching processes were monitored by measuring Ni and Fe contents and pH of the
leaching solution as well as the total bacterial enzymatic activity measured as FDA
hydrolytic activity. The effect of leaching on the mineralogy of laterite ores was
investigated by the scanning electron microscope equipped with energy-dispersive
spectroscopy (SEM-EDS) and X-ray powder diffraction (XRD). After 28 days of
incubation, the FDA hydrolytic activity was observed in both bioleaching experiments
containing limonite (17.2 ????g fluorescein/mL) and saprolite ores (16.9 ????g
fluorescein/mL). The leached Ni and Fe in the bioleaching experiments containing
limonite ores (30% Ni and 5.6% Fe) was greater than that in abiotic controls (1% Ni and
0.1% Fe) with the pH range of 2.5 to 3.5. However, the bacterial consortia were less
capable of bioleaching of Ni (2.5%) with the similar leached Fe (6%) from the saprolite
ores. In abiotic controls, the medium pH remained relatively constant (pH 6). It was
concluded that these bacterial isolated as the consortium were capable of nickel
bioleaching (precious metal) more effectively than iron (gangue metal),
applicable to the commercial processing of the difficult-to-process low-grade nickel
laterite ores in Indonesia.