The effect of Fe-oxidizing bacteria on Fe-silicate mineral dissolution

Acidithiobacillus ferrooxidans are commonly present in acid mine drainage ( AMD). A. ferrooxidans derive metabolic energy from oxidation of Fe2+ presen t in natural acid solutions and also may be able to utilize Fe2+ released b y dissolution of silicate minerals during acid neutralization reactions. Na tural and synthetic fayalites were reacted in solutions with initial pH val ues of 2.0, 3.0 and 4.0 in the presence of A. ferrooxidans and in abiotic s olutions in order to determine whether these chemolithotrophic bacteria can be sustained by acid-promoted fayalite dissolution and to measure the impa ct of their metabolism on acid neutralization rates. The production of almo st the maximum Fe3+ from the available Fe in solution in microbial experime nts (compared to no production of Fe3+ in abiotic controls) confirms A. fer rooxidans metabolism. Furthermore, cell division was detected and the total cell numbers increased over the duration of experiments. Thus, over the pH range 2-4, fayalite dissolution can sustain growth of A. ferrooxidans. How ever, ferric iron released by A. ferrooxidans metabolism dramatically inhib ited dissolution rates by 50-98% compared to the abiotic controls. Two sets of abiotic experiments were conducted to determine why microbial iron oxid ation suppressed fayalite dissolution. Firstly, fayalite was dissolved at p H 2 in fully oxygenated and anoxic solutions. No significant difference was observed between rates in these experiments, as expected, due to extremely slow inorganic ferrous iron oxidation rates at pH 2. Experiments were also carried out to determine, the effects of the concentrations of Fe (2+), Mg 2+ and Fe3+ on fayalite dissolution. Neither Fe2+ nor Mg2+ had an effect on the dissolution reaction. However, Fe3+, in the solution, inhibited both s ilica and iron release in the control, very similar to the biologically med iated fayalite dissolution reaction. Because ferric iron produced in microb ial experiments was partitioned into nanocrystalline goethite (with very lo w Si) that was loosely associated with fayalite surfaces or coated the A. f errooxidans cells, the decreased rates of accumulation of Fe and Si in solu tion cannot be attributed to diffusion inhibition by goethite or to precipi tation of Fe-Si-rich minerals. The magnitude of the effect of Fe3+ addition (or enzymatic iron oxidation) on fayalite dissolution rates, especially at low extents of fayalite reaction, is most consistent with suppression of d issolution by interaction between Fe3+ and surface sites. These results sug gest that microorganisms can significantly reduce the rate at which silicat e hydrolysis reactions can neutralize acidic solutions in the environment. (C) 2001 Elsevier Science B.V. All rights reserved.