Principles of Microbial Metal Leaching
4.1 Leaching Mechanisms
Mineralytic effects of bacteria and fungi on minerals are based mainly on three principles, namely acidolysis, complexolysis, and redoxolysis. Microorganisms are able to mobilize metals by (1) the formation of organic or inorganic acids (protons); (2) oxidation and reduction reactions; and (3) the excretion of complexing agents. Sulfuric acid is the main inorganic acid found in leaching environments. It is formed by sulfur-oxidizing microorganisms such as thiobacilli.A series of organic acids are formed by bacterial (as well as fungal) metabolism resulting in organic acidolysis, complex and chelate formation (BERTHELIN, 1983). A kinetic model of the coordination chemistry of mineral solubilization has been developed which describes the dissolution of oxides by the protonation of the mineral surface as well as the surface concentration of suitable complexforming ligands such as oxalate, malonate, citrate, and succinate (FURRER and STUMM, 1986). Proton-induced and ligand-induced mineral solubilization occurs simultaneously in the presence of ligands under acidic conditions.
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