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Trace Metal Ion Partitioning at Polymer Film-Metal Oxide Interfaces: Long-Period X-ray Standing Wave Study Tae Hyun Yoon,*,† Thomas P. Trainor,‡ Peter J. Eng,§ John R. Bargar,| and Gordon E. Brown, Jr.†,| Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, Department of Chemistry and Biochemistry, University of Alaska Fairbanks, Fairbanks, Alaska 99775, GSECARS, University of Chicago, Chicago, Illinois 60637, and Stanford Synchrotron Radiation Laboratory, SLAC, 2575 Sand Hill Road, MS 69, Menlo Park, California 94025 Received November 5, 2004. In Final Form: February 20, 2005 The distributions of Pb(II) and As(V)O43- ions in the interfacial region between thin poly(acrylic acid) (PAA) coatings and R-Al2O3(0001), R-Al2O3(1-102), and R-Fe2O3(0001) single-crystal substrates were studied using long-period X-ray standing wave fluorescent yield (XSW-FY) and X-ray reflectivity techniques. The PAA film serves as a simplified analogue of natural organic matter (NOM) coatings on mineral surfaces. Such coatings are often assumed to play an important role in the partitioning and speciation of trace heavy metals in soils and aquatic systems. On the R-Al2O3(1-102) surface, Pb(II) ions were found to preferentially bind to the PAA coating, even at sub-micromolar Pb(II) concentrations, and to partition increasingly onto the metal oxide surface as the Pb(II) concentration was increased ([Pb(II)] ) 5 × 10-8 to 2 × 10-5 M, pH ) 4.5; 0.01 M NaCl background electrolyte). This observation suggests that the binding sites in the PAA coating outcompete those on the R-Al2O3(1-102) surface for Pb(II) under these conditions. The As(V)O43oxoanion partitions preferentially to the R-Al2O3(1-102) surface for the As(V)O43- concentrations examined (1 × 10-7 to 5 × 10-7 M, pH ) 4.5; 0.01 M NaCl background electrolyte). Partitioning of Pb(II) (at 1 × 10-7 M and pH 4.5) was also examined at PAA/R-Al2O3(0001), and PAA/R-Fe2O3(0001) interfaces using XSW-FY measurements. Our results show that the PAA coating was the dominant sink for Pb(II) in all three samples; however, the relative order of reactivity of these metal oxide surfaces with respect to Pb(II) sorption is R-Fe2O3(0001) > R-Al2O3(1-102) > R-Al2O3(0001). This order is consistent with that found in previous studies of the PAA-free surfaces. These XSW results strongly suggest that the characteristics of the organic film (i.e., binding affinity, type, and density of binding sites) as well as metal oxide substrate reactivity are key factors determining the distribution and speciation of Pb(II) and As(V)O43- at organic film/metal oxide interfaces.
1. Introduction Solid-water interfaces are of great significance in many areas of science and engineering, including aqueous geochemistry, contaminant transport in groundwater, wastewater treatment, heterogeneous catalysis, atmospheric aerosol chemistry, corrosion science, and colloid science.1,2 Moreover, in many natural settings, natural organic matter (NOM) is ubiquitous and can form coatings on mineral surfaces that may cause significant changes in interfacial properties.3-12 NOM coatings on mineral surfaces can act as (1) a competing sorbent for pollutant * Author for correspondence: e-mail, taeyoon@ pangea.stanford.edu; phone, 650-725-0580; fax, 650-725-2199. † Stanford University. ‡ University of Alaska Fairbanks. § University of Chicago. | Stanford Synchrotron Radiation Laboratory. (1) Brown, G. E., Jr.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. Rev. 1999, 99, 77-174. (2) Brown, G. E., Jr.; Parks, G. A. Int. Geol. Rev. 2001, 43, 963-1073. (3) Niehoff, R. A.; Loeb, G. I. J. Marine Res. 1974, 32, 5-12. (4) Hunter, K. A.; Liss, P. S. Nature 1979, 282, 823-825. (5) Baliestrieri, L.; Brewer, P. G.; Murray, J. W. Deep Sea Res. 1981, 28A, 101-121. (6) Davis, J. A. Geochim. Cosmochim. Acta 1982, 46, 2381-2393. (7) Tipping, E.; Cooke, D. Geochim. Cosmochim. Acta 1982, 49, 7580. (8) Murphy, E. M.; Zachara, J. M. Geoderma 1995, 67, 103-124. (9) Kaiser, K.; Guggenberger, G. Org. Geochem. 2000, 31, 711-725. (10) Kaiser, K.; Guggenberger, G. Eur. J. Soil Sci. 2003, 54, 219236.
ion species, (2) a physical barrier inhibiting the transport of ions to mineral surface binding sites, (3) a passivating layer blocking high affinity surface sites, and/or (4) a charged medium that modifies the electrical double layer properties at the mineral-water interface. As a consequence, NOM coatings on mineral surfaces may cause dramatic changes in many of the physicochemical properties of mineral particles, including their sorption capacity and reactivity to various pollutant ions and the kinetics of sorption/desorption reactions. However, due mainly to experimental difficulties in probing trace element distributions at organic film-mineral interfaces under in situ conditions, only a few such studies have been conducted,13-19 and thus our knowledge of these distributions is limited. The long-period X-ray standing wave method, combined with X-ray reflectivity measurements, is well suited for (11) Ransom, B.; Bennett, R.; Baerwald, R.; Hulbert, V.; Burkett, P. Am. Mineral. 1999, 84, 183-192. (12) Ransom, B.; Bennett, R.; Baerwald, R.; Shea, K. Marine Geol. 1997, 138, 1-9. (13) Templeton, A.; Trainor, T.; Spormann, A.; Brown, G. E., Jr. Geochim. Cosmochim. Acta 2003, 67, 3547-3557. (14) Templeton, A. S.; Trainor, T. P.; Traina, S. J.; Spormann, A. M.; Brown, G. E., Jr. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 11897-11902. (15) Wang, J.; Caffrey, M.; Bedzyk, M.; Penner, T. J. Phys. Chem. 1994, 98, 10957-10968. (16) Wang, J.; Bedzyk, M.; Caffrey, M. Science 1992, 258, 775-778. (17) Wang, J.; Caffery, M. J. Am. Chem. Soc. 1995, 117, 3304-3305. (18) Trainor, T. P.; Templeton, A. S.; Brown, G. E., Jr.; Parks, G. A. Langmuir 2002, 18, 5782-5791. (19) Libera, J. A.; Gurney, R. W.; Nguyen, S. T.; Hupp, J. T.; Liu, C.; Conley, R.; Bedzyk, M. J. Langmuir 2004, 20, 8022-8029.
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probing trace element distributions within thin organic film layers (>10 nm) on polished metal oxide surfaces under in situ conditions.13-19 X-ray standing wave (XSW) techniques are based on the generation of well-defined intensity fields from the superposition of two coherently related X-ray beams (i.e., the incident and reflected beams). The interference of these two beams provides a spatially localized periodic probe with a length scale equivalent to the XSW period, D, which is given by D ) λ/(2 sin θ), where 2θ is the angle between the incident and reflected beams and λ is the X-ray wavelength. To generate long-period XSWs, the angle of incidence is chosen in the regime of total external reflection of the substrate, and therefore the resulting XSW period, D, is much longer than that of typical Bragg diffraction XSW (D ) 0.1-0.4 nm). These long-period standing waves are well suited for studying biological and environmental samples with submicrometer-thick coatings of organic matter or microbial biofilms. In addition to good distance resolution in the z-dimension (∼1 nm),20 the XSW-FY method is also sensitive to trace concentrations of elements distributed at the solid/organic layer interface and in the organic layer. For fluorescing atoms with a distribution function N(z) in the organic layer-solid interfacial region, the net fluorescence yield, Y(θ), is given by
Y(θ) ∝
∫ N(z) I(z,θ) dz
(1)
where I(z,θ) is the XSW field intensity. I(z,θ) may be readily calculated based on the optical properties of the composite substrate, allowing trace element distributions to be determined using model distribution functions for N(z).13 Enhancement of the XSW intensity can be produced in samples with smooth (roughness R-Al2O3(1-102) > R-Al2O3(0001).44-46 In addition, Templeton et al.14 showed that the order of reactivity of these substrates with respect to aqueous Pb(II) ions is not altered by a continuous B. cepacia biofilm coating. However, as presented in Tables 2 and 4, the stability constant for the PAA-Pb(II) complex (i.e., ML2 species) is typically larger than those of higher affinity mineral surface binding sites (i.e., M1). Moreover, the stability constant for the higher affinity Pb(II)-humate complex (i.e., log KPb-S2) is 2.1 to 3.4 units larger than those of higher affinity mineral surface binding sites. Therefore, in the case of a mineral substrate coated by PAA or HA, it is more probable that mineral surface binding site will be “blocked” or “altered” by organic coatings with higher affinity binding sites. To test this possibility, we performed XSW-FY experiments on Pb(II) partitioning between a continuous PAA coating and R-Fe2O3(0001), R-Al2O3(1-102), and R-Al2O3(0001) surfaces. As shown in Figure 6 and Table 3 (samples 6-a, 6-b, and 6-c), the majority of Pb(II) ions are partitioned into the PAA coating for all three metal oxide substrates, which can be explained by the higher binding affinity of ML2 binding sites within the PAA coating. However, we also observe an increase of adsorbed Pb(II) species as the intrinsic reactivities of the metal oxide surfaces increase, indicating that the surface binding sites have not been “blocked” and the relative differences in surface reactivities with respect to Pb(II) have not been seriously altered by the presence of the PAA coating. This observation is consistent with the relative reactivities of biofilm-mineral interfaces with respect to aqueous Pb(II)14 and is also similar to the results by Zachara et al.47 for a Co(II)mineral-humic acid system. (44) Bargar, J. R.; Towle, S. N.; Brown, G. E., Jr.; Parks, G. A. Geochim. Cosmochim. Acta 1996, 60, 3541-3547. (45) Bargar, J. R.; Towle, S. N.; Brown, G. E., Jr.; Parks, G. A. J. Colloid Interface Sci. 1997, 185, 473-492. (46) Bargar, J. R.; Trainor, T. P.; Fitts, J. P.; Chambers, S. C.; Brown, G. E., Jr. Langmuir 2004, 20, 1667-1673. (47) Zachara, J. M.; Resch, C. T.; Smith, S. C. Geochim. Cosmochim. Acta 1994, 58, 553-566.
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4. Conclusions Analyses of XSW-FY profiles and X-ray reflectivity curves for several PAA-coated R-Al2O3 and R-Fe2O3 substrates reacted with aqueous Pb(II) and As(V)O43provided quantitative information on the partitioning of these ions between the organic coating and the single crystal metal oxide substrates under various experimental conditions. Most of the Pb(II) ions preferentially bind to carboxyl functional groups in the PAA coating on the same surfaces even at sub-micromolar Pb(II) concentrations and begin to partition into the PAA coating as Pb(II) concentration was increased. In addition, most of the As(V)O43ions preferentially adsorb onto the R-Al2O3(1-102) surface. These observations suggest that the binding sites in the PAA coating outcompete those of R-Al2O3(1-102) surface for Pb(II), whereas the opposite is true for As(V)O43-. This result is not surprising given the negative charge of the carboxyl functional groups in the PAA coating and the positive charge of the R-Al2O3(1-102) surface at pH 4.5. Additionally, the effect of a PAA coating on intrinsic differences in metal oxide surface reactivities with respect to aqueous Pb(II) was also tested; Pb(II) partitioning was found to follow the same trends as that found for the bare surfaces (i.e., R-Fe2O3(0001) > R-Al2O3(1-102) > R-Al2O3(0001)). These results for PAA-coated metal oxides surfaces are not consistent with earlier predictions that organic coatings fundamentally alter the reactivities of mineral substrates and also lead to the suggestion that the characteristics of the organic coating on a mineral surface (i.e., binding affinity, type and density of binding sites) as well as intrinsic mineral surface reactivities are key factors determining the distribution and speciation of pollutant species at mineral-organic coating-water interfaces. Acknowledgment. We wish to acknowledge the support of NSF Grants CHE-0089215 (Stanford University CRAEMS on Chemical and Microbial Interactions at Environmental Interfaces) and CHE-0431425 (Stanford Environmental Molecular Science Institute). We also wish to thank to Dr. Alexis S. Templeton, Joe Rogers, and other APS and SSRL staff members for the help with XSWFY/X-ray reflectivity measurements. This work was performed at GeoSoilEnviroCARS (sector 13), Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation sEarth Sciences (EAR-0217473), Department of EnergysGeosciences (DE-FG02-94ER14466), and the State of Illinois. The APS is supported by the U.S. Department of Energy, Basic Energy Sciences under Contract No. W-31-109-Eng-38. LA047271Y
