OCCURRENCES AT MINERAL BACTERIA INTERFACE DURING OXIDATION OF ARSENOPYRITE BY THIOBACILLUS-FERROOXIDANS

The combination of an improved bacterial desorption method, scanning e lectron microscopy (SEM), diffuse reflectance and transmission infrare d Fourier transform spectroscopy, and a desorption-leaching device lik e high-pressure liquid chromatography (HPLC) was used to analyze bacte rial populations (adhering and free bacteria) and surface-oxidized pha ses (ferric arsenates and elemental sulfur) during the arsenopyrite bi ooxidation by Thiobacillus ferrooxidans. The bacterial distribution, t he physicochemical composition of the leachate, the evolution of corro sion patterns, and the nature and amount of the surface-oxidized chemi cal species characterized different behavior for each step of arsenopy rite bioleaching. The first step is characterized by a slow but strong adhesion of bacteria to mineral surfaces, the appearance of a surface phase of elemental sulfur, the weak solubilization of Fe(ll), As(III) , and As(V), and the presence of the first corrosion patterns, which f ollow the fragility zones and the crystallographic orientation of mine ral grains. After this short step, growth of the unattached bacteria b egins, while ferrous ions in solution are oxidized by them. Ferric ion s produced by the bacteria can oxidize the sulfide directly and are re generated by Fe(ll) bacterial oxidation. At this time, a bioleaching c ycle takes place and a coarse surface phase of ferric arsenate (FeAsO( 4)xH(2)O where x approximate to 2) and deep ovoid pores appear. At the end of the bioleaching cycle, the high concentration of fe(lll) and A s(V) in solution promotes the precipitation of a second phase of amorp hous ferric arsenate (FeAsO4.xH(2)O where x approximate to 4) in the l eachate. Then the biooxidation process ceases: The bacteria adhering t o the mineral surfaces are coated by the ferric arsenates and the conc entration of Fe(lll) on the leachate is found to have decreased greatl y. Both oxidation mechanisms (direct and indirect oxidation) have been stopped. (C) 1995 John Wiley and Sons, Inc.