Influence of microorganisms on biotite dissolution: An experimental approach

Microbially assisted dissolution of biotite was studied using four environmentally important species (Bacillus subtilis, Shewanella putrefaciens, Streptomyces acisdiscabies, and Schizophyllum commune) incubated for 35 days in batch reactors at slightly alkaline pH conditions (ca. pH 9.5). For comparison we performed a control experiment without biological component. Dissolution was monitored by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis of elements (Al, Fe, K, Mg, Mn, Si, and Ti) released into the leach solution. Structural and chemical alterations of biotite were analyzed by a directly evolved gas analysis system (DEGAS) and transmission electron microscopy (TEM) coupled with electron-energy loss spectroscopy (EELS) and energy-dispersive X-ray (EDX) analysis. Our results show that biotite dissolves incongruently with a preferential release of the interlayer cations followed by the octahedral and subsequently the tetrahedral site. Dissolution rates of biotite calculated on the basis of Si release into the solution are similar for abiotic and biotic experiments, on the order of 10−13 mol/(m2 s). However, calculated K release rates as well as Mg/Si and K/Si element ratios show a preferential dissolution of these elements from biotite in the presence of microorganisms. For example, in comparison to the control experiment we observe that B. subtilis enhances the Si-normalized Mg and K solute concentrations by about 50%. DEGAS and EDX analyses reveal that the release of K+ from the interlayer is associated with an uptake of Na+, H3O+, and NH4+. These substitutions are strongest for biotite exposed to microorganisms, e.g., in experiments with B. subtilis the uptake of NH4+ is 6 times higher than in the control experiment. Altogether, these results substantiate the considerable influence of microorganisms on the dissolution of biotite.