Nutrient effect on the biological leaching of a black-schist ore.

The purpose of the study was to examine the influence of inorganic N (NH4+, NO3-) and phosphate on the biological oxidation of a sulfidic black-schist ore which contained pyrrhotite as the main iron sulfide. Iron was initially solubilized as Fe2+ from the ore and subsequently oxidized to Fe3+ in shake flask experiments. Under these experimental conditions, iron dissolution from pyrrhotite was mainly a chemical reaction, with some enhancement by bacteria, whereas the subsequent Fe2+ oxidation was bacterially mediated, with negligible contribution from chemical oxidation. Phosphate amendment did not enhance Fe2+ oxidation. Chemical analysis of leach solutions with no exogenous phosphate revealed that phosphate was solubilized from the black-schist ore. Ammonium amendment (6 mM) enhanced Fe2+ oxidation, whereas the addition of nitrate (6 and 12 mM) had a negative effect. An increase in the temperature from 30 to 35°C slightly enhanced Fe2+ oxidation, but the effect was statistically not significant. The precipitation of potassium jarosite was indicative of Fe2+ oxidation and was absent in nitrate-inhibited cultures because of the lack of Fe2+ oxidation. The black-schist ore also contained phlogopite, which was altered to vermiculite in iron-oxidizing cultures.