Figure A.2. Kinetic data from series of iron ore reductive dissolution experiments using filter-sterilized primary effluent wastewater as carbon and energy source.
(vi) A. putrefaciens 200 was capable of growth on a medium consisting of primary effluent wastewater supplemented with yeast extract (0.25 g/L);
the microorganism was unable to grow or reduce iron on yeast extract (pH 7) alone. The doubling time for batch cultures grown on wastewater medium was approximately 90 minutes ( 40 minutes on lactate medium).
(vii) A. putrefaciens 200 was capable of catalyzing the reductive dissolution of iron from low-grade iron ore No. 1589 using primary effluent wastewater as its sole substrate for microbial growth and reducing power.
Initial reductive dissolution rates (1.0 x 10-5 M· hr- 1) were approximately 33% of the rate measured when batch cultures were grown on defined lactate media.
(viii) Oxygen utilization rates (3 x 10-4 M · hr- 1) of batch cultures grown on wastewater media were also 33% of the rate measured when batch cultures were grown on defined lactate media.
IV. SUMMARY
It appears that Alteromonas putrefaciens strain 200 is a suitable
microorganism for use in a microbially-catalyzed process for the bio-extraction (reductive dissolution) of iron from low-grade iron ore. Scale-up of such a process for commercial application is made even more attractive by the ability of the
microorganism to utilize a relatively inexpensive organic substrate (primary effluent wastewater) as the carbon and energy source. Yeast extract was added to the
wastewater prior to the growth period as a nutrient supplement. Yeast extract is generally included in complex media to provide such essential growth factors as vitamins and trace metals for microbial cultivation.
Results from the batch experiments described above indicate that the rate of reductive dissolution of iron from low-grade ore is affected by several factors,
including the concentration of iron ore (as Fe(III)) provided to the batch culture, the presence of complexing agents NTA and EDTA, and the identity and
concentration of substrate provided for microbial growth and reducing power. At saturating solid Fe(III) concentration (10 mM iron ore as Fe(III)], the addition of equimolar EDTA and NTA resulted in a measurable increase in the initial and overall reductive dissolution rates. Whether the complexing agents increase total soluble Fe(III) or complex such inhibitory metals as Zn, Cu, Hg, and Pb contained within the iron ore is not known. A decrease in iron ore particle size enhances reductive dissolution rates, probably by increasing the surface area available for microbial attachment. Microorganism/Fe(III) particle contact is required for iron reduction by A. putrefaciens 200 (Arnold et al., 1988). Particle size effects, however, are not as pronounced as previously thought most likely because of the highly aggregated nature of Fe(III)-oxide particles at neutral pH (Stumm and Morgan, 1981).
Substitution of primary effluent wastewater for lactate as substrate during growth and reductive dissolution experiments resulted in a 67% reduction in reductive dissolution rates. In addition, oxygen utilization rates were nearly 30%
of the rate measured when batch cultures were provided with lactate as reducing power. This is most likely a result of the electron flow limitations imposed on microbial metabolic processes by the less desirable substrate form. Log-phase growth rates for cultures grown on wastewater media were nearly two-fold slower than the values measured for batch cultures grown on lactate medium. A late
log-phase cell density of approximately 2.0 x 109 cells/ml (A600 = 1.0) was obtained after 24 hours of growth on wastewater medium as opposed to 12 hours in the case of lactate medium. Nevertheless, the initial rates of iron ore reductive dissolution
(1.0 x 10-5 M·hr- 1) observed in the wastewater substrate/iron ore batch experiments are of sufficient magnitude to allow scale-up to a commercially viable batch process using the experimental details described here as the fundamental protocol. Since reductive dissolution rates after one hour are relatively slow, an additional step should include recycle of the ore remaining in the reactor ( after the initial one hour treatment) into a series of batch reactors for similar bioleaching treatment with Alteromonas putrefaciens 200.
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