Competitive Sorption of Pb(II) and Zn(II) on Polyacrylic Acid-Coated Hydrated Aluminum-Oxide Surfaces Yingge Wang, † F. Marc Michel, †,‡,▽ Clement Levard, †,○ Yong Choi, § Peter J. Eng, ∥ and Gordon E. Brown, Jr. †,‡,⊥,#, * † Surface & Aqueous Geochemistry Group, Department of Geological & Environmental Sciences, Stanford University, Stanford, California 94305-2115, United States ‡ Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, MS 69, 2575 Sand Hill Road, Menlo Park, California 94025, United States § Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States ∥ Consortium for Advanced Radiation Sources, University of Chicago, Chicago, Illinois 60637, United States ⊥ Department of Photon Science, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States # Department of Chemical Engineering, Stauffer III, Stanford University, 381 North-South Mall, Stanford, California 94305-5025, United States * S Supporting Information ABSTRACT: Natural organic matter (NOM) often forms coatings on minerals. Such coatings are expected to affect metal−ion sorption due to abundant sorption sites in NOM and potential modifications to mineral surfaces, but such effects are poorly understood in complex multicomponent systems. Using poly(acrylic acid) (PAA), a simplified analog of NOM containing only carboxylic groups, Pb(II) and Zn(II) partitioning between PAA coatings and α-Al 2 O 3 (1−102) and (0001) surfaces was investigated using long-period X-ray standing wave-florescence yield spectroscopy. In the single- metal−ion systems, PAA was the dominant sink for Pb(II) and Zn(II) for α-Al 2 O 3 (1−102) (63% and 69%, respectively, at 0.5 μM metal ions and pH 6.0). In equi-molar mixed-Pb(II)−Zn(II) systems, partitioning of both ions onto α-Al 2 O 3 (1−102) decreased compared with the single-metal−ion systems; however, Zn(II) decreased Pb(II) sorption to a greater extent than vice versa, suggesting that Zn(II) outcompeted Pb(II) for α-Al 2 O 3 (1− 102) sorption sites. In contrast, >99% of both metal ions sorbed to PAA when equi-molar Pb(II) and Zn(II) were added simultaneously to PAA/α-Al 2 O 3 (0001). PAA outcompeted both α-Al 2 O 3 surfaces for metal sorption but did not alter their intrinsic order of reactivity. This study suggests that single-metal−ion sorption results cannot be used to predict multimetal−ion sorption at NOM/metal−oxide interfaces when NOM is dominated by carboxylic groups. ■ INTRODUCTION Minerals and humic substances (often referred to as natural organic matter (NOM)) are ubiquitous in soils and aquatic systems and are often spatially associated due to the formation of NOM coatings on mineral surfaces. 1−4 Such coatings potentially induce significant modifications to mineral surface electrostatic properties, such as reversing surface charge from positive to negative, and provide abundant additional sorption sites for metal ions. 3,5−8 As a result, NOM coatings are generally assumed to play an important role in the biogeochemical cycling of heavy metals in natural waters, soils, and sediments. 3,5−8 Humic substances are natural biomacromolecules produced from the breakdown of plants, animals, fungi, and bacteria. 5,9 These natural organic macromolecules are weak polyelectro- lytes and have various compositions, sizes, and conformations and a number of different types of functional groups, including carboxylic, amino, phenolic, and aromatic groups. 5,9 As a result of this complexity, many studies have used chemically and structurally simple molecules as analogs of NOM. Polycarbox- ylic acids such as poly(acrylic acid) (PAA), a polymer containing carboxylic functional groups in linear CH 2 −CH 2 chains, are often selected as simple surrogates for humic substances because of the general similarity of their polyelectrolyte properties and functional groups to those of humic substances. 10−13 For example, PAA has been used as a model compound for humic substances to study the environ- Received: March 27, 2013 Revised: July 30, 2013 Accepted: September 11, 2013 Article pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es401353y | Environ. Sci. Technol. XXXX, XXX, XXX−XXX