| The geochemical cycling of trace elements in soil and aquatic systems is strongly influenced by sorption and precipitation reactions that occur at metal (hydr)oxide surfaces. However, the electrical and chemical properties of the mineral-water interface can be dramatically modified by the formation of highly hydrated, highly reactive microbial biofilms. We have developed synchrotron-based experimental approaches to quantitatively probe the distribution and speciation of Pb and Se sequestered within complex Burkholderia cepacia biofilms coating Al- and Fe-oxides. X-ray absorption spectroscopy was used to determine the speciation of the metal(loid) ions (e.g., mode of metal binding, changes in redox state) associated with the biofilms vs. the metal-oxide surfaces. XSW measurements were used to profile the vertical distribution of metal(loid) ions within microbial biofilms formed on single-crystal surfaces, and X-ray microscopy was used to map the lateral heterogeneities in metal(loid) distribution within the biofilms. The spectroscopic data were coupled to images of the biofilms obtained using epifluorescent, transmission electron (TEM) and scanning electron (SEM) microscopy.; X-ray standing wave data show that Pb and Se ions rapidly penetrate the Burkholderia cepacia biofilms and preferentially bind to highly reactive sites on the Al- and Fe-oxide surfaces. Once these reactive sites are saturated, sorption to surface functional groups present within the biofilms becomes quantitatively important. Therefore, B. cepacia biofilms do not passivate the intrinsic reactivity of the metal-oxide surfaces. Moreover, structural fitting of Pb LIII-EXAFS spectra collected for Pb(II) sorbed to B. cepacia/goethite composites, coupled to macroscopic uptake data, show that the presence of B. cepacia enhances Pb(II) uptake at goethite surfaces, primarily through the stabilization of bidentate, binuclear Pb(II) complexes.; The metabolic activity of B. cepacia has a dramatic effect on the speciation and bioavailability of Pb and Se. At micromolar Pb 2+ concentrations, B. cepacia induces the biomineralization of pyromorphite, a sparingly soluble Pb-phosphate phase, adjacent to the outer-membrane. For Se, B. cepacia catalyzes the rapid reduction of Se(IV) and partial reduction of Se(VI) to elemental Se(0). The Se species differentially segregate at the biofilm/metal-oxide interface, where Se(VI) and Se(0) are associated with the biofilms whereas Se(IV) is preferentially bound to the underlying mineral surface. |
