Experimental assessment of occurrences and stability of lead-bearing minerals in bacterial biofilms

Bio-induced precipitation of lead-bearing minerals is investigated in bacterial biofilms grown by Shewanella oneidensis MR-1 under aerobic conditions. Under the different conditions investigated, thermodynamic calculations establish that the stable mineral phases expected to precipitate are either wulfenite PbMoO4 or cerussite PbCO3. However, observations by electron microscopy show that the first solids precipitated within hours at the experimental solution/biofilm interface are crystals of about 20 nm in diameter of pyromorphite Pb5(PO4)3(OH,Cl). In Mo-bearing systems, the precipitation of the thermodynamically-predicted wulfenite phase is delayed compared to the abiotic experiment and is observed only after seven days of lead exposure. The initial lead phosphate crystals observed on the extracellular polymeric substances are assumed to result from concurrent local abundances of adsorbed Pb2+ ions and phosphate groups released by metabolically active cells. Scanning electron microscopy observations of samples milled by focused ion beam reveal effective diffusion-limited precipitation of pyromorphite within the well-preserved biofilm porosity.