Formation of polyelectrolyte complexes with the major elements Fe and Al and the trace elements U and Cu during heterotrophic microbial leaching of rocks

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Abstract
A heterotrophic bacterial microflora, selected by treatment of a brown forest soil with thymol or lipids (0.1%), enhanced uranium and copper solubilization from an aplite and a granitic rock during the first stages. Uranium or copper were solubilized simultaneously with the major elements Fe, AI, Mg, and Ca in the form of low molecular weight (2000 to 2900) organo‐metallic compounds, which were easily dissociated after a treatment on a cation exchange resin (H+ form) but not with an anionic resin (HCOO∼ form). These organo‐metallic complexes formed under aerated, (Eh, +300 to +500 mv), acidic (pH 2.5 to 5) conditions by an association of metallic cations (UO2 2+, Cu2+, Fe2+, Fe3+ , Al3+) with simple organic acids (i.e., oxalic, isocitric, citric, succinic, hydroxy‐benzoic, and coumaric acids) via their carboxylic and phenolic groups. As shown by potentiometric titration, carboxylic groups seemed to be preferentially involved in the complexing processes. The results suggest that simple organic molecules or complexes which exist in natural environments, particularly in soils, are only a transitory state before their association to form polyanions and polycations or polynuclear like complexes. These processes may be involved in the mechanisms of trace and major element mobilization in aerated acidic organic environments (e.g., spodosols). The experimental conditions can also be used to select microorganisms, in particular bacteria, able to form complexing agents and to grow in acid conditions.