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Competitive sorption of Pb(II) and Zn(II) on Shewanella oneidensis MR-1 biofilm-coated single-crystal α-Al 2 O 3 (1 −1 0 2) and α-Fe 2 O 3 (0 0 0 1) surfaces was investigated using long-period X-ray standing wave-florescence yield (LP-XSW-FY) spectroscopy. In situ partitioning of aqueous Pb(II) and Zn(II) between the biofilms and underlying metal-oxide substrates was probed following exposure of these complex interfaces to equi-molar Pb and Zn solutions (0.01 M NaNO 3 as background electrolyte, pH = 6.0, and 3-h equilibration time). At higher Pb and Zn concentrations (⩾10 −5 M), more than 99% of these ions partitioned into the biofilms at S. oneidensis /α-Al 2 O 3 (1 −1 0 2)/water interfaces, which is consistent with the partitioning behavior of both Pb(II) or Zn(II) in single-metal-ion experiments. Thus, no apparent competitive effects were found in this system at these relatively high metal-ion concentrations. However, at lower equi-molar concentrations (⩽10 −6 M), Pb(II) and Zn(II) partitioning in the same system changed significantly compared to the single-metal-ion systems. The presence of Zn(II) decreased Pb(II) partitioning onto α-Al 2 O 3 (1 −1 0 2) substantially (∼52% to ∼13% at 10 −7 M, and ∼23% to ∼5% at 10 −6 M), whereas the presence of Pb(II) caused more Zn(II) to partition onto α-Al 2 O 3 (1 −1 0 2) surfaces (∼15% to ∼28% at 10 −7 M, and ∼1% to ∼7% at 10 −6 M). The higher observed partitioning of Zn(II) (∼28%) at the α-Al 2 O 3 (1 −1 0 2) surfaces compared to Pb(II) (∼13%) in the mixed-metal-ion systems at the lowest concentration (10 −7 M) suggests that Zn(II) is slightly favored over Pb(II) for sorption sites on α-Al 2 O 3 (1 −1 0 2) surfaces under our experimental conditions. Competitive sorption of Pb(II) and Zn(II) at S. oneidensis /α-Fe 2 O 3 (0 0 0 1)/water interfaces at equi-molar metal-ion concentrations of ⩽10 −6 M showed that the presence of Pb(II) ions decreased Zn(II) partitioning onto α-Fe 2 O 3 (0 0 0 1) significantly (∼45% to −7 M, and ∼41% to 3% at 10 −6 M), whereas adding Zn(II) caused only small changes in Pb(II) partitioning (∼59% to ∼47% at 10 −7 M, and ∼26% to ∼23% at 10 −6 M), suggesting that Pb(II) strongly outcompetes Zn(II) for sorption sites on S. oneidensis -coated α-Fe 2 O 3 (0 0 0 1) surfaces. Our study implies that caution should be taken when applying results obtained from partitioning studies of single-metal-ion systems to mixed-metal-ion systems at complex biofilm/mineral interfaces.

Microbial biofilms are often present as coatings on metal-oxide surfaces in natural and industrial environments and may induce significant changes in the partitioning behavior and speciation of aqueous metal ions, which in turn can impact their transport and fate. In this study, long-period X-ray standing wave-fluorescence yield (LP-XSW-FY) spectroscopy was used to measure under in situ conditions the partitioning of aqueous Pb(II) and Zn(II) between multilayer Shewanella oneidensis MR-1 biofilms and highly polished, oriented single-crystal surfaces of α-Al2O3 and α-Fe2O3 as a function of metal-ion concentration and time at pH 6.0. We show that after 3-h exposure time, Pb(II) binds preferentially to the α-Al2O3 (1–102) and α-Fe2O3 (0 0 0 1) surfaces at low Pb concentration ([Pb] = 10−7 M) and then increasingly partitions into the biofilm coatings at higher concentrations (10−6 to 10−4 M). In contrast, Zn(II) partitions preferentially into the biofilm coating for both surfaces at all Zn concentrations studied (10−7 to 10−4 M). In comparison, the α-Al2O3 (0 0 0 1) surface has a low affinity for both Pb(II) and Zn(II), and the biofilm coatings are the dominant sink for both ions. These findings suggest that in the presence of S. oneidensis biofilm coatings, α-Al2O3 (0 0 0 1) is the least reactive surface for Pb(II) and Zn(II) compared to α-Al2O3 (1–102) and α-Fe2O3 (0 0 0 1). They also show that Zn(II) has a lower affinity than Pb(II) for reactive sites on α-Al2O3 (1–102) and α-Fe2O3 (0 0 0 1) at [Me(II)] of 10−7 M; at 10−5 M, the bulk of the metal ions partition into the biofilm coatings. At longer exposure times (20–24 h), both Pb(II) and Zn(II) increasingly partition to the metal-oxide surfaces at [Me(II)] = 10−5 M and pH 6.0, indicating possible reaction/diffusion-controlled sorption processes. Pb LIII-edge and Zn K-edge grazing-incidence extended X-ray absorption fine structure (GI-EXAFS) measurements suggest that both Pb(II) and Zn(II) ions may be complexed by carboxyl groups in S. oneidensis biofilms after 3-h exposure at pH 6.0 and [Me(II)] = 10−5 M. In contrast with Burkholderia cepacia, which was used in our previous studies of monolayer biofilm-coated metal-oxide surfaces (Templeton et al., 2001), S. oneidensis MR-1 forms relatively thick biofilm coatings (6–20 μm) that are rich in reactive functional groups and are expected to dominate metal-ion adsorption. Our results show that even thick and highly reactive biofilms like S. oneidensis do not cause much change in the intrinsic chemical reactivities of the underlying metal-oxide surfaces with respect to aqueous Pb(II) and Zn(II) and don’t block reactive sites on the metal-oxide surfaces; instead they reduce the rate of Pb(II) and Zn(II) sorption onto these surfaces.

The effect of cell wall-associated extracellular polymeric substances (EPS) of the Gram-negative bacterium Shewanella oneidensis strain MR-1 on proton, Zn(II), and Pb(II) adsorption was investigated using a combination of titration/batch uptake studies, surface complexation modeling, attenuated total reflectance – Fourier transform infrared (ATR-FTIR) spectroscopy, and Zn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. Both unmodified (wild-type (WT) strain) and genetically modified cells with inhibited production of EPS (ΔEPS strain) were used. Three major types of functional groups (carboxyl, phosphoryl, and amide groups) were identified in both strains using ATR-FITR spectroscopy. Potentiometric titration data were fit using a constant capacitance model (FITEQL) that included these three functional groups. The fit results indicate less interaction of Zn(II) and Pb(II) with carboxyl and amide groups and a greater interaction with phosphoryl groups in the ΔEPS strain than in the WT strain. Results from Zn(II) and Pb(II) batch adsorption studies and surface complexation modeling, assuming carboxyl and phosphoryl functional groups, also indicate significantly lower Zn(II) and Pb(II) uptake and binding affinities for the ΔEPS strain. Results from Zn K-edge EXAFS spectroscopy show that Zn(II) bonds to phosphoryl and carboxyl ligands in both strains. Based on batch uptake and modeling results and EXAFS spectral analysis, we conclude that the greater amount of EPS in the WT strain enhances Zn(II) and Pb(II) uptake and hinders diffusion of Zn(II) to the cell walls relative to the ΔEPS strain.