Bacterial effects on the mobilization of cations from a weathered Pb-contaminated andesite

Mobilization of contaminant Pb and other cations from a weathered andesite was examined by conducting water-rock leaching experiments to measure relea se rates of the major and minor elements found in the bulk rock. Experiment s were conducted for over 600 h, at pH 3 and 6.5, as a function of grain si ze, and in the presence and absence of a common subsurface bacterial specie s, Bacillus subtilis. The acid leaching experiments exhibited enhanced Ph m obilities. The contact area between mineral surfaces and water controlled t he amount of most elements leached into solution. Typically, bacteria accel erated the leaching of trace elements. However, major element levels were i nversely correlated with bacterial content, and generally lower major eleme nt concentrations were observed in the presence of bacteria, relative to th e bacteria-free systems. This effect was likely caused by bacterial enhance ment of secondary mineral precipitation rates. Extremely low concentrations of Pb were leached into solution during the course of the experiments unde r conditions most closely approximating natural conditions (i.e., near neut ral pH). Moreover, the release rate of Pb relative to other elements was lo w. Pb was associated with iron oxides precipitated in the rock pore spaces, and the experiments suggest that the iron oxides dissolved to a lesser ext ent than did other minerals in the rock matrix. The mobility of the Pb asso ciated with these iron oxides was less than that of other heavy metals that were likely associated with the surrounding matrix. (C) 1999 Elsevier Scie nce B.V. All rights reserved.