Effect of biofilm coatings at metal-oxide/water interfaces II: Competitive sorption between Pb(II) and Zn(II) at Shewanella oneidensis/metal-oxide/water interfaces

Competitive sorption of Pb(II) and Zn(II) on Shewanella oneidensis MR-1 biofilm-coated single-crystal alpha-Al2O3 (1-102) and alpha-Fe2O3 (0001) 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 NaNO3 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/alpha-Al2O3 (1-102)/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 alpha-Al2O3 (1-10 2) substantially (similar to 52% to similar to 13% at 10(-7) M, and similar to 23% to similar to 5% at 10(-6) M), whereas the presence of Pb(II) caused more Zn(II) to partition onto alpha-Al2O3 (1-102) surfaces (similar to 15% to similar to 28% at 10(-7) M, and similar to 1% to similar to 7% at 10(-6) M). The higher observed partitioning of Zn(II) (similar to 28%) at the alpha-Al2O3 (1-102) surfaces compared to Pb(II) (similar to 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 alpha-Al2O3 (1-102) surfaces under our experimental conditions. Competitive sorption of Pb(II) and Zn(II) at S. oneidensis/alpha-Fe2O3 (0001)/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 alpha-Fe2O3 (0001) significantly (similar to 45% to <1% at 10(-7) M, and similar to 41% to 3% at 10(-6) M), whereas adding Zn(II) caused only small changes in Pb(II) partitioning (similar to 59% to similar to 47% at 10(-7) M, and similar to 26% to similar to 23% at 10(-6) M), suggesting that Pb(II) strongly outcompetes Zn(II) for sorption sites on S. oneidensis-coated alpha-Fe2O3 (0001) 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. (C) 2016 Elsevier Ltd. All rights reserved.