Mechanism of bioleaching of coal fly ash by Thiobacillus thiooxidans

Bioleaching of aluminum and iron from coal fly ash (CFA) by Thiobacillus th iooxidans (T thiooxidans) bacteria is considered. The interactions between bacteria, metabolic products, CFA particles, and leaching products were stu died. It is demonstrated that bacterial growth and the amount of metals lea ched from the CFA are coupled through biological and chemical interactions, which involve the various components in this system. Bioleaching experiments were performed batch wise by suspending up to 10% ( w/v) CFA in E thiooxidans growth medium containing cell inoculum for a typi cal 3 week period of time. Samples were taken periodically from leached sus pensions and relevant parameters including metals' concentrations, cell cou nts, pH and extracellular polymeric substances (EPS) were determined. The results show that under the same conditions, similar leaching levels ar e obtained by sulfuric acid and bioleaching sf CFA, and the contribution of other metabolites is insignificant. CFA inhibits the growth rate through t wo major effects. The first is due: to the alkaline components released by the CFA that cause a rise in the pH, and a corresponding delay in growth. T he second is attributed to the random attachment of the bacteria to both th e sulfur particles (the energy source) and the barren CFA particles, result ing in a so-called "dilution effect" of the sulfur particles, and an inhibi tion of the initial growth rate. However, after an adaptation period of the bacteria the subsequent growth rate, the maximal cell concentration and mi nimal pH were similar to those obtained in the control experiment, irrespec tive of CFA content. Enhanced excretion of EPS was observed in the presence of CFA as well as in calcium and barium enriched growth media. It is presu med that the mechanism of EPS production is related to the presence of the particulate solid phase. (C) 2001 Elsevier Science B.V. All rights reserved .