Competitive Sorption of Pb(II) and Zn(II) on Polyacrylic Acid-CoatedHydrated Aluminum-Oxide SurfacesYingge 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, UnitedStates#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 formscoatings on minerals. Such coatings are expected to affectmetal−ion sorption due to abundant sorption sites in NOMand potential modifications to mineral surfaces, but sucheffects are poorly understood in complex multicomponentsystems. Using poly(acrylic acid) (PAA), a simplified analog ofNOM containing only carboxylic groups, Pb(II) and Zn(II)partitioning between PAA coatings and α-Al2O3 (1−102) and(0001) surfaces was investigated using long-period X-raystanding wave-florescence yield spectroscopy. In the single-metal−ion systems, PAA was the dominant sink for Pb(II) andZn(II) for α-Al2O3(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-Al2O3(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-Al2O3(1−102) sorption sites. In contrast, >99% of both metal ions sorbed to PAA when equi-molar Pb(II) and Zn(II) were addedsimultaneously to PAA/α-Al2O3(0001). PAA outcompeted both α-Al2O3 surfaces for metal sorption but did not alter theirintrinsic order of reactivity. This study suggests that single-metal−ion sorption results cannot be used to predict multimetal−ionsorption at NOM/metal−oxide interfaces when NOM is dominated by carboxylic groups.

■ INTRODUCTIONMinerals and humic substances (often referred to as naturalorganic matter (NOM)) are ubiquitous in soils and aquaticsystems and are often spatially associated due to the formationof NOM coatings on mineral surfaces.1−4 Such coatingspotentially induce significant modifications to mineral surfaceelectrostatic properties, such as reversing surface charge frompositive to negative, and provide abundant additional sorptionsites for metal ions.3,5−8 As a result, NOM coatings aregenerally assumed to play an important role in thebiogeochemical cycling of heavy metals in natural waters,soils, and sediments.3,5−8

Humic substances are natural biomacromolecules producedfrom 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, includingcarboxylic, amino, phenolic, and aromatic groups.5,9 As a resultof this complexity, many studies have used chemically andstructurally simple molecules as analogs of NOM. Polycarbox-ylic acids such as poly(acrylic acid) (PAA), a polymercontaining carboxylic functional groups in linear CH2−CH2chains, are often selected as simple surrogates for humicsubstances because of the general similarity of theirpolyelectrolyte properties and functional groups to those ofhumic substances.10−13 For example, PAA has been used as amodel compound for humic substances to study the environ-

Received: March 27, 2013Revised: July 30, 2013Accepted: September 11, 2013

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mental behavior of lanthanide and actinide ions in naturalmedia and to evaluate the long-term performance and safety ofnuclear waste repositories.11 PAA is also widely used in industryas a scale inhibitor, as a dispersant in papermaking, and as astabilizer and flocculant.12,13 Due to its very low environmentalimpact and high sorption capacity for metal ions, PAA is alsoconsidered as a promising sorbent in toxic heavy-metal removalfrom industrial effluents.12,14 For example, PAA is often used asa chelating agent in a technique known as polymer-assistedultrafiltration (PAUF) to help remove trace metals fromwastewater effluents. The strong effective binding of PAA totrace metal ions results in high removal efficiency and thedesired quality of treated water.14

There have been extensive studies of the interaction of humicsubstances with metal ions and mineral surfaces,5,10,15−17 and anumber of thermodynamic models have been developed topredict metal−ion binding by NOM although the generalapplicability of such models requires further improve-ments.5,15,16 Depending on the metal−ion affinities of NOMand mineral surfaces, and experimental conditions such as pHand the types and concentrations of metal ions, NOM caneither enhance metal−ion uptake by increasing the negativecharge on mineral surfaces and/or forming strong complexeswith metal ions,7,8,18,19 or decrease metal−ion sorption byphysically blocking the sorption sites on mineral surfaces.20,21

Moreover, metal−ion interactions at more complex NOM/mineral/water interfaces cannot be described simply as the sumof the behavior observed in individual binary systems.10,17

Therefore, metal−ion partitioning in multicomponent systemsapproaching the complexity of real systems is still poorlyunderstood. In addition, natural soil and aquatic systemscontain a variety of metals at concentrations at and above tracelevels.22,23 It is reasonable to expect that different types of metalions may compete for available sorption sites and potentiallyinterfere with each other in terms of uptake. Such competitivesorption effects could, in turn, impact the bioaccumulation andtoxicity of metal ions in the environment. Therefore,understanding these processes and determining if the findingsfrom single-metal−ion sorption studies are valid in multimetal−ion systems are essential for predicting the fate and transport ofmetal ions in natural environments.22−26 Only a few studies ofcompetitive sorption among multiple metal ions at NOM/mineral interfaces have been carried out to date.22−26

Competitive metal−ion sorption effects at mineral surfaceshave been found to range from none or weak to fairly strong,depending on the experimental conditions including the typesof metal ions and mineral surfaces, the initial metalconcentrations used, and reaction kinetics.23−25 Althoughsurface complexation models (SCM) have been used to predictmetal−ion uptake in single-metal−ion systems, SCM modelsbased on simple systems cannot correctly predict multiplemetal−ion uptake in natural soil mixtures.26 Therefore,competitive sorption effects are largely unknown in multi-metal−ion, multisubstrate systems under realistic conditions.This is due in part to the general lack of appropriate analyticaltools capable of determining both the chemical speciation andspatial distributions of elements in these complex systems.3,6−8

Here we studied the competition between aqueous Pb(II)and Zn(II) ions for sorption sites on poly(acrylic acid) (PAA)-coated, hydrated single-crystal α-Al2O3 (0001), and (1−102)surfaces. These mineral substrates were chosen as modelsbecause the aluminol sites present on these surfaces arerepresentative of those of Al-(oxyhydr)oxide phases such as

gibbsite and boehmite in soils and aquatic systems. In addition,α-Al2O3 can be obtained in oriented single-crystal formsrequired for the grazing-incidence X-ray standing wave studiesreported below, whereas the other more common Al-(oxyhydr)oxide phases are not available as single crystals.Finally, the structures of the hydrated α-Al2O3 (0001) and (1−102) surfaces have been determined by crystal truncation roddiffraction studies27,28 and their sorption properties have beenwell-studied.3,6,29,30

We used the long period X-ray standing wave fluorescentyield (LP-XSW-FY) method to measure Pb(II) and Zn(II)partitioning on PAA-coated single-crystal α-Al2O3 (0001) and(1−102) surfaces. LP-XSW-FY is a grazing-incidence spectro-scopic technique for characterizing the spatial and chemicaldistribution of elements in single- or multilayered sam-ples.3,6,31−35 It is element-specific, nondestructive, and hashigh sensitivity to elements at low concentrations (≥10−8 M)within a particular layer of material as well as at the buriedinterfaces between layers.3,6,31−35 As a result, this technique hasbecome an effective tool for probing element distributions atvarious types of interfaces, including electrochemical inter-faces,31,32 biological membranes,31 mineral/water interfaces,32

and mineral surfaces coated with microbial biofilms or thinorganic films.3,6,31,32,34,35

■ EXPERIMENTAL METHODSPreparation of Metal−Oxide Surfaces and PAA Thin

Films. The α-Al2O3 substrates used in this study arecommercially available, highly polished α-Al2O3 (0001) and(1−102) single-crystal 5 cm diameter wafers (Saint-GobainCrystals & Detectors Co.). These surfaces were prepared usinga chemical cleaning procedure described in our previousstudies.3,6,27,28 In brief, all substrates were cleaned with acetone,then washed in 10−3.5 M sodium hydroxide for 20 min, andsubsequently washed in 10−2 M nitric acid for an hour. Eachchemical washing step was followed by multiple rinses withMilli-Q water. The washed crystals were then baked at 350 °Cfor 4 h to minimize excess carbon on the surfaces. The cleaningprocedure was repeated as necessary until the concentrations ofmetal surface impurities was undetectable (<0.1%) and theorganic (adventitious) carbon content at the crystal surfaceswas less than ∼10% as determined by X-ray photoelectronspectroscopy (XPS) (Surface Science S-Probe, monochromaticAl Kα radiation).Polyacrylic acid sodium salt (25% by weight, MW = 240 000

Da, Aldrich Chemico Co. Ltd.) was used to prepare 2% PAAstock solutions with N2-sparged 0.01 M NaNO3 aqueoussolution as the background electrolyte. The solution pH wasadjusted to pH 6.0 ± 0.05 by using 1N NaOH. The solutionwas equilibrated overnight and sparged with nitrogen gas. ThepH of the solution was adjusted to 6.0 by adding small volumesof 0.01 N NaOH or 0.01 N HNO3 solution where necessary.The PAA solution was then filtered using a 0.2 μm Nalgenefilter. Reagent grade Pb(NO3)2 and Zn(NO3)2 (J.T. Baker)were used to prepare metal−ion stock solutions. For LP-XSW-FY sample preparation, aliquots of PAA stock solution andPb(NO3)2 and Zn(NO3)2 stock solutions were mixed toprepare the desired final solutions with specific [PAA],[Pb(II)], and/or [Zn(II)].Clean single-crystal α-Al2O3 (0001) and (1−102) substrates

were placed in individual Petri dishes, and 200 μL of thespecific metal−ion containing PAA solution was pipetted ontothe crystal surface. A damp Kimwipe was kept inside the closed

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Petri dish to increase the relative humidity and minimizeevaporation of the PAA solution. After 3 h of equilibration,each sample was spin-coated at 3500 rpm for 60 s to form a dryPAA film using a Headway Research, Inc. EC101DT spincoater. Atomic force microscopy (AFM) (Veeco multimodescanning probe microscope) and X-ray reflectivity were used tocharacterize the coated samples as described below.LP-XSW-FY Measurement and Data Analysis. Detailed

information about the principles and applications of LP-XSW-FY technique can be found in two review papers.31,32

Synchrotron X-ray reflectivity and LP-XSW-FY experimentswere performed using a general-purpose Kappa diffractometerat the GeoSoilEnviro Consortium for Advanced RadiationSources (GSECARS) Sector on the beamline 13-ID-C at theAdvanced Photon Source (APS). The monochromatized andfocused X-ray beam was collimated to 1.0 mm vertical and 20μm horizontal. Samples were mounted vertically in a Teflonsample cell covered with Kapton film (a polyimide film), andthe space over the sample was purged by dry He gas duringexperiments. X-ray reflectivity measurements were made beforeand after LP-XSW-FY measurements by scanning X-rayincidence angle between 0.0 and 0.5° and monitoring theintensities of the incident (I0) and reflected (I1) X-rays usingN2-filled gas ionization chambers. Pb Lα and Zn Kαfluorescence yield data were collected at 14 keV incident X-ray energy (wavelength λ = 0.8856 Å) using a four-elementsilicon drift detector (SII NanoTechnology, Vortex-ME4)coupled to digital X-ray processor electronics (X-rayInstrumentation Associates, xMAP). LP-XSW-FY fitting wasdone using the procedure described in previous studies.3,6,32−35

Details of the experimental setup and model fitting for LP-XSW-FY are provided in the Supporting Information (SI).

■ RESULTS AND DISCUSSION

Pb(II) and Zn(II) Single-Metal−Ion Distributions atPAA-Coated α-Al2O3 (1−102) Single-Crystal Surfaces.Figure 1 shows a typical X-ray reflectivity profile collected for a

PAA-coated α-Al2O3 (1−102) surface (PAA = 2%, 0.01 MNaNO3, pH 6.0). Consistent with our previous X-rayreflectivity studies of biofilm- and PAA-coated alumina surfaces,the critical angle of the α-Al2O3 (1−102) surface was found tobe 0.165 ± 0.010° at an incident X-ray energy of 14 keV.3,6,35

Unlike the biofilm-coated metal−oxide surfaces,3,35 a significantoscillatory feature was observed in X-ray reflectivity profiles ofthe PAA-coated alumina surfaces indicating that the PAA filmsare relatively smooth with well-defined thicknesses. The filmthickness and roughness obtained from fitting the X-rayreflectivity data are in good agreement with those obtainedfrom direct AFM measurements as shown in Figure 1. Thenumber and magnitude of oscillations in X-ray reflectivity andLP-XSW-FY profiles increase with increasing film thickness anddecreasing interfacial roughness (see simulations presented inFigure S1 of the SI). LP-XSW-FY is very sensitive to themetal−ion loading as shown in Figure S2 of the SI. The shapeof the LP-XSW-FY profiles changes as the metal−iondistribution between the film and surface changes. As themetal−ion loading on the surface increases, the FY signal inhigh-angle region increases, which suggests that more metalions partition onto the α-Al2O3 (1−102) surfaces. As a result,quantitative LP-XSW-FY fitting is required to obtain metal−iondistributions between the PAA coatings and the aluminasubstrates.Figure 2A−D show X-ray reflectivity and LP-XSW-FY

profiles collected for Pb(II) partitioning at PAA-coated α-Al2O3 (1−102) surfaces as a function of Pb(II) concentration(0.5 to 50 μM), with a 3-h equilibration time at pH 6.0. At allPb(II) concentrations tested, the LP-XSW-FY profiles show amaximum in the lower angle region at ∼0.11°, suggesting thatPb(II) partitions dominantly into the PAA films. As Pb(II)concentration decreases, the FY intensity at ∼0.165° increases,suggesting that increasing amounts of Pb(II) partitioned ontothe α-Al2O3 (1−102) surfaces. Fitting results for Pb(II)distributions at the PAA-coated α-Al2O3 (1−102) surfaces arelisted in Table 1. Fitting parameters of the X-ray reflectivitydata are provided in Table S1 (SI). Note that the fits of the LP-XSW-FY profiles in the high-angle region show discrepancieswith the experimental data due to the presence of thebackground scattering; however, such discrepancies do notaffect the final fitting results of metal−ion partitioning (see theSI in section on X-ray reflectivity and LP-XSW-FY datacollection and fitting). Pb(II) sorbs dominantly (∼63%) in thePAA coatings at the lowest Pb concentration studied (0.5 μM),and the amount of Pb(II) sorbed onto the α-Al2O3 (1−102)surfaces increased from less than 1% to ∼37% as Pb(II)concentration decreased from 50 μM to 0.5 μM (Table 1). X-ray reflectivity and LP-XSW-FY profiles for Zn(II) are similarto those of Pb(II) (see Figure S3A−C of the SI). LP-XSW-FYfitting results listed in Table 1 show that Zn(II) partitioningonto the α-Al2O3 (1−102) surfaces increased from ∼1% to∼31% as Zn(II) concentrations decreased from 50 to 0.5 μM.Thus, the PAA coatings were found to be the dominant sinksfor both Pb(II) and Zn(II) at all concentrations tested insingle-metal−ion systems.Figure 3 compares the partitioning of Pb(II) and Zn(II),

individually, between PAA coatings and the underlying α-Al2O3(1−102) surfaces. The partitioning of both Pb(II) and Zn(II)into PAA increased on α-Al2O3 (1−102) surfaces as metal−ionconcentration decreased. The concentrations of Pb(II) andZn(II) adsorbed on the α-Al2O3 (1−102) surfaces were foundto be comparable at all metal−ion concentrations considered

Figure 1. X-ray reflectivity data and fit for a typical PAA-coated α-Al2O3 (1−102) surface and comparison with the film thickness androughness values obtained from atomic force microscopy (AFM)analysis: PAA = 2%, 0.01 M NaNO3, pH 6.0. The AFM image is a 3Dplot of the PAA-coated α-Al2O3(1−102) surface. The trench shown inthe image is the region where the soft PAA film was carefully removedfrom the hard α-Al2O3 (1−102) surface using a sharp razor blade. Thefilm thickness was determined by measuring the distance from the topof the trench to the bottom of the substrate.

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(0.5−50 μM). At the highest metal−ion concentration (50μM), more than 99% of both Pb(II) and Zn(II) partitionedinto the PAA coatings.The interaction of Pb(II) and Zn(II) with mineral surfaces,

bacterial surfaces, and humic substances has been studiedextensively in the past.5,23,24,29,30,36−39,42,43 Metal sorption ontomineral surfaces, bacterial surfaces, and NOM can be describedby a generalized equation as follows:3

+ → −+ +KL Me L Me2 2 (1)

where Me represents divalent metal ions, L represents theligand or the reactive sites on mineral or bacterial surfaces, andK is the apparent stability constant. Table 2 lists some reportedvalues for site densities of functional groups of various types oforganics as well as mineral surfaces and binding constants ofPb(II) and Zn(II) to these functional groups.3,12,36−42

The stability constant of Pb(II) on PAA (6.75/7.0)12,41 ishigher than that of the strong binding sites (6.0)3 on α-Al2O3(1−102) surfaces. Thus, PAA coatings on these surfaces areexpected to dominate Pb(II) sorption. In addition, thecarboxylic groups of PAA have a much higher site densityand stability constant for Pb(II) as compared to the carboxylicgroups of other types of organics or bacterial surfaces. Forexample, Pb(II)−PAA complexes are much stronger thanPb(II) complexes on bacterial surfaces, thus Pb(II) is expectedto partition more strongly into the PAA coatings compared tothe biofilm coatings on alumina surfaces we have studiedpreviously.3,35

Although log K values for Pb(II) sorption on PAA (6.75/7.0)12,41 are higher than those of the strong binding sites (6.0)on α-Al2O3 (1−102) surfaces, the difference may not besignificant because of reported differences in log K values in the

Figure 2. X-ray reflectivity and LP-XSW-FY profiles for Pb(II) partitioning on PAA-coated α-Al2O3 (1−102) surfaces (PAA = 2%, 0.01 M NaNO3,pH 6.0, equilibration time = 3 h): (A) [Pb(II)] = 50 μM; (B) [Pb(II)] = 5 μM; (C) [Pb(II)] = 1 μM; (D) [Pb(II)] = 0.5 μM. Gray open circlesrepresent X-ray reflectivity data, black solid lines represent X-ray reflectivity fit, black open diamonds represent FY data, red solid lines representPb(II) FY fit.

Table 1. Pb(II) and Zn(II) Partitioning Results for PAA-Coated α-Al2O3 (0001) and (1-102) Surfaces As a Function of Metal−ion Concentration (PAA = 2%, 0.01 M NaNO3, pH 6.0, Reaction Time = 3 h)

Pb(II) partitioning Zn(II) partitioning

substrate added [Pb(II)] added [Zn(II)] % at surface % in PAA STDEV % at surface % in PAA STDEV

α-Al2O3(1−102) 50 μM <1 >99α-Al2O3(1−102) 5 μM 6.0 94.0 5.3α-Al2O3(1−102) 1 μM 28.4 71.6 8.9α-Al2O3(1−102) 0.5 μM 36.9 63.1 4.0α-Al2O3(1−102) 50 μM 1.4 98.6 <1.0α-Al2O3(1−102) 5 μM 12.1 87.9 5.5α-Al2O3(1−102) 1 μM 28.6 71.4 3.8α-Al2O3(1−102) 0.5 μM 30.8 69.2 5.2α-Al2O3(1−102) 50 μM 50 μM 0.9 99.1 <1.0 1.1 98.9 <1.0α-Al2O3(1−102) 5 μM 5 μM 2.2 97.8 0.5 8.7 91.3 5.3α-Al2O3(1−102) 1 μM 1 μM 10.1 89.9 3.8 24.5 75.5 8.3α-Al2O3(1−102) 0.5 μM 0.5 μM 12.1 87.9 4.1 24.2 75.8 2.0α-Al2O3(0001) 50 μM 50 μM <1 >99 <1 >99α-Al2O3(0001) 5 μM 5 μM <1 >99 <1 >99α-Al2O3(0001) 0.5 μM 0.5 μM <1 >99 <1 >99

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literature. There are potentially some very strong binding siteson α-Al2O3 (1−102) surfaces where metal ions prefer tosorb,23,43 particularly at very low metal concentrations whenthese strongest binding sites are available. The partitioning ofboth Pb(II) and Zn(II) ions onto α-Al2O3 (1−102) at lowmetal concentrations found in this study suggests that a numberof these very strong sorption sites are present on α-Al2O3 (1−102) surfaces and can effectively compete with strongly bindingcarboxylic groups in PAA. A log K of 6.03 for the strong bindingsites on α-Al2O3 (1−102) surfaces represents an averagestability constant for strong sites. In addition, this value was nota direct measurement but was obtained from Langmuir fittingof Pb(II) sorption on B. cepacia biofilm-coated α-Al2O3 (1−102) surfaces, where several assumptions were made.3

As shown in Table 2, the log K of the AlO−Pb2+monodentate complexes on γ-Al2O3 is comparable to the logK of Pb(II) sorption on α-Al2O3 (1−102) single-crystal surfaces(6.0) reported by Templeton et al. (2001). However, the log Kof AlO−PbOH+ monohydroxyl complexes (10.2) on γ-Al2O3 is

about 3−4 log units higher.42 In addition, grazing-incidence X-ray absorption fine structure (GI-XAFS) spectroscopy studiesfor Zn(II) and Pb(II) ion sorption at α-Al2O3 (1−102) surfacesfound that both metal ions form predominantly mononuclearedge-sharing bidentate complexes.29,30 It is expected that thelog K value of bidentate complexes should be higher than thatof the monodentate complexes. Therefore, the presence of verystrong sorption sites on α-Al2O3 (1−102) surfaces is likely andthese sites can effectively compete with PAA coatings formetal−ion sorption.Thus, caution needs to be taken when using the log K and

site density approach to predict the partitioning of metal ions incomplex systems. As mentioned above, reported stabilityconstants for metal−ion binding on mineral surfaces andorganics could also vary substantially as a function of organicmolecule molecular weight, metal-to-ligand ratio, pH, and ionicstrength in solution.12 Among various divalent metal ions,Cu(II) and Ni(II) interactions with PAA have been studied themost, and reported values of the stability constant vary nearly 3orders of magnitude for Cu(II) and Ni(II).12,40 Some studiesreport a slightly higher stability constant for Cu(II) thanPb(II),40 whereas others report the opposite.12,44 Morlay et al.(1999) suggested that metal stability constants for reactivefunctional groups should only be used for order-of-magnitudeestimates because of this lack of agreement.12,42 In addition,direct comparison of stability/binding constants is notstraightforward without knowledge of the type of complexesformed in PAA coatings or on α-Al2O3 (1−102) surfaces.To the best of our knowledge, there are no direct log K data

available for Zn(II) binding to PAA films under conditionssimilar to those for Pb(II) binding to PAA films.12,40,45 Tomidaet al. reported the stability constant of Pb(II)−PAA complexesis 2.2 log units higher than that of Zn(II)−PAA complexes.12

Based on the stability constants of Pb(II)−PAA complexesdetermined in two independent studies,12,40 a stability constantranging from 4.8 to 5.8 is expected for Zn(II)−PAA complexes(Table 2), suggesting a lower affinity of Zn(II) for carboxylicgroups in PAA films compared to Pb(II). The Pb(II) > Zn(II)sorption order has also been observed in many sorptionexperiments for organics including bacterial surfaces.36−38 For

Figure 3. Single-metal−ion partitioning of Pb(II) and Zn(II) betweenPAA films and α-Al2O3 (1−102) surfaces from LP-XSW-FYmeasurements (PAA = 2%, 0.01 M NaNO3, pH 6.0, equilibrationtime = 3 h).

Table 2. Site densities of various types of bacteria, metal−oxide surfaces, and a humic acid and their binding affinities log K forPb(II) and Zn(II)

type of surface site type site density Pb(II)logK Znl(II)logK

S. oneidensis MR-136 carboxyl 1.08 × 10−3 mol/gdrya 4.6 4.1

Enterobacteriaceae37 carboxyl 5.0 (±0.7) x10−4 mol/gdry 3.9 (±0.8) 3.3 (±0.1)A. macleodii subsp. f ijiensi 38 EPS 5.2 4.4humic acid39 carboxyl 4.9 × 10−3 mol/gdry log KS1 = 3.4b

log KS2 = 8.75(c)

PAA12 carboxyl 1.4 × 10−2 mol/gdry 6.75(d)/7.00 4.8(e)-5.8(e)

acetic acid12 carboxyl 3.5α-Al2O3 (0001)

3 strong 5.6 × 10−8 mol/m2(f) 5.35weak 1.1 × 10−6 mol/m2(f) 2.7

α-Al2O3 (1−102)3 strong 2.5 × 10−6 mol/m2(f) 6.0weak 5.1 × 10−6 mol/m2(f) 3.55

γ-Al2O342 monodentate(g) AlO− + Me2+ = AlO−-Me2+ 6.4(h)

monohydroxyl complex(g) AlO− + MeOH+ = AlO−-MeOH+ 8.7−10.2(i) 10.1(h)

aThe value was converted from wet to dry weight using dry/wet ratio 1:8. blog KS1 is the log K value for weak binding sites. (c)log KS1 is the log Kvalue for strong binding sites. (d)The value 6.75 is from ref 41 (e)These values are estimated from stability constants for Pb-PAA from refs 12 and 40assuming the dissociation constant of PAA used is the same. (f)Site densities modified from Templeton et al., 2001,3 see ref 35 (g)Me represents asmetal ions such as Pb and Zn. (h)At metal concentration 5 × 10−7 M. (i)The values vary as a function of Pb(II) concentration ranging from 2.9 ×10−4 to 5 × 10−7 M.

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example, Ha et al. (2010)36 studied Pb(II) and Zn(II) sorptiononto S. oneidensis MR-1 bulk cells and found that the stabilityconstants of Pb(II) ions were higher than those of Zn(II) ions.However, no data are available for Zn(II) binding onto α-Al2O3(1−102) surfaces that would allow further comparisons withPb(II) partitioning at such complex interfaces.Competitive Sorption between Pb(II) and Zn(II) at

PAA-Coated α-Al2O3 (1−102) Single-Crystal Surfaces.LP-XSW-FY fitting results for Pb(II) and Zn(II) distributionsbetween the PAA coatings and α-Al2O3 (1−102) surfaces atequi-molar concentrations of both Pb(II) and Zn(II) arepresented in Table 1 (data and fitting are presented in FigureS4A−C of the SI). The PAA films are the dominant sinks forboth Pb(II) and Zn(II) ions for all metal−ion concentrationstested even at the lowest metal−ion concentration (0.5 μM).However, both Pb(II) and Zn(II) ions were found toincreasingly partition onto α-Al2O3 (1−102) surfaces asmetal−ion concentrations decreased. Pb(II) and Zn(II)partitioning onto α-Al2O3 (1−102) surfaces increased from∼1% to ∼12% and from ∼1% to ∼24%, respectively, as Pb(II)and Zn(II) concentrations decreased from 50 to 0.5 μM (Table1). This trend is consistent with single-metal−ion Pb(II) andZn(II) partitioning as a function of metal−ion concentration.The dominance of PAA films in Pb(II) sorption is expectedbecause the log K values for Pb(II)−PAA complexes are higherthan those for Pb(II) complexes formed at the strong bindingsites on α-Al2O3 (1−102) surfaces (Table 2). However, theincreased partitioning of both Pb(II) and Zn(II) ions onto α-Al2O3 (1−102) at lower metal−ion concentrations suggests thepresence of strong sorption sites on α-Al2O3 (1−102) surfacesthat have higher or comparable stability constants relative tothe PAA coatings.Figure 4 shows the effect of adding competing ions on the

partitioning of Pb(II) or Zn(II) between PAA coatings and α-Al2O3 (1−102) surfaces. Adding equi-molar concentrations ofZn(II) to single-Pb(II) system increased Pb(II) partitioning inPAA and decreased Pb(II) partitioning onto α-Al2O3 (1−102)surfaces (Figure 4A) at lower concentrations (≤1 μM). On theother hand, adding equi-molar concentrations of Pb(II) tosingle-Zn(II) system resulted in an insignificant increase inZn(II) partitioning in PAA and a decrease in Zn(II)partitioning onto α-Al2O3 (1−102) surfaces (Figure 4B). Forexample, Pb(II) and Zn(II) sorption onto the α-Al2O3 (1−102)surface in the mixed-metal−ion systems decreased from ∼37%to ∼12% and ∼31% to ∼24%, respectively, compared to thesingle-metal−ion systems (Table 1).Addition of competing ions to each single-metal−ion system

decreased metal−ion partitioning onto α-Al2O3 (1−102)surfaces and increased metal partitioning into PAA; however,this effect is more pronounced for Pb(II) than for Zn(II). At allmetal−ion concentrations tested, Pb(II) partitioning into PAAfilms was found to be higher than that of Zn(II) (Table 1). Incontrast, Pb(II) partitioning onto α-Al2O3 (1−102) surfaceswas found to be less than that of Zn(II) in mixed-metal−ionsystems. These observations indicate that Zn(II) outcompetedPb(II) for sorption sites on α-Al2O3 (1−102) surfaces, whereasPb(II) outcompeted Zn(II) for carboxylic sites in PAA underour experimental conditions.Unfortunately, there are no direct log K thermodynamic data

available for Zn(II) interaction with PAA and α-Al2O3 (1−102)surfaces, and thus a thorough analysis of competitive Pb(II) andZn(II) sorption in such complex systems is not currentlypossible. However, some available stability constants in similar

systems can be used to rationalize the observed trend. Asshown in Table 2, the stability constant for Zn(II)−PAAcomplexes is expected to be around 1−2 log units lower thanPb(II)−PAA complexes. Thus, Pb(II) is expected to out-compete Zn(II) for sorption sites in PAA films. On the otherhand, the stability constant of Pb(II) on γ-Al2O3 was found tobe lower than that of Zn(II) for monohydroxyl monodentatecomplexes. Coston et al.46 studied Pb(II) and Zn(II) sorptionon natural Al- and Fe-oxide surface coatings on an aquifer sandand found that removing the Al-oxide coatings significantlyreduced Zn(II) sorption relative to Pb(II) sorption, suggestingthat Zn(II) preferred to form complexes with reactive sites onthe Al-oxides more than Pb(II).42,46 The above analysissuggests that Zn(II) may have a lower affinity for PAA filmsand a higher affinity for the α-Al2O3 (1−102) surface relative toPb(II). Therefore, higher partitioning of Zn(II) onto the α-Al2O3 (1−102) surfaces is expected when Pb(II) is present inthe system.Although Pb(II) and Zn(II) showed comparable sorption

behavior in single-metal−ion systems, the partitioning of Pb(II)and Zn(II) in mixed-metal−ion systems was observed to bequite different possibly due to the competitive sorptionbetween Pb(II) and Zn(II) in PAA and on α-Al2O3 (1−102)surfaces. This finding indicates that multimetal−ion sorption atcomplex NOM/mineral interfaces cannot be predicted fromthe behavior observed in single-metal−ion systems. Similarresults have been found in our recent study of competitive

Figure 4. Results of LP-XSW-FY analysis of competitive sorptioneffects between Pb(II) and Zn(II) on PAA-coated α-Al2O3 (1−102)surfaces (PAA = 2%, 0.01 M NaNO3, pH 6.0, equilibration time = 3h): (A) the effect of adding Zn(II) ions on Pb(II) partitioning; (B) theeffect of adding Pb(II) ions on Zn(II) partitioning.

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Pb(II) and Zn(II) sorption at S. oneidensis/α-Al2O3(1−102)/water interfaces.47 In that study we found that Pb(II)outcompeted Zn(II) for the sorption sites in the biofilms,whereas Zn(II) outcompeted Pb(II) for the sorption sites on α-Al2O3 (1−102) surfaces.47 Furthermore, the single-metal−ionpartitioning study did not predict the trend observed in mixed-metal−ion systems at complex biofilm/mineral interfaces.47

Quantification of Pb(II) and Zn(II) Binding to PAACarboxylic Groups and Alumina Surface Sites. In thisstudy, the use of PAA allows us to quantify our results due tothe fact that the number of carboxylic groups is known for agiven mass of PAA coating. As a result, we can estimate thenumber of Pb(II) and Zn(II) ions associated with the PAAcoatings and the underlying alumina substrates. The PAA filmsused contain much larger amounts of reactive COOH sites(calculated to be 1.4 × 10−3 mol/m2 based on a 1000-Å thickPAA film with PAA density of 0.92 g/cm3) compared to thetotal sites available on the alumina surfaces (7.6 × 10−6 mol/m2

for α-Al2O3 (1−102) surfaces, and 1.16 × 10−6 mol/m2 for α-Al2O3 (0001) surfaces as shown in Table 2). The metal−ionloadings used here are low compared to both PAA carboxylicfunctional groups and alumina surface sites. A rough estimate ofthe total number of metal ions in the systems shows that themetal ions used do not saturate the surface sites even at thehighest metal ion concentration of 50 μM. For example, forboth Pb(II) and Zn(II) present at 50 μM, the total amount ofPb(II) and Zn(II) ions is roughly estimated to be 3.4 × 10−7

mol/m2. So the competition we observed in this study forPb(II) and Zn(II) sorption on α-Al2O3 (1−102) surfaces is notcaused by filling up the surface sites. The highest metal−ionloading ([Pb(II)] = [Zn(II)] = 50 μM) would not even fill upthe strong sorption sites of the α-Al2O3 (1−102) surface (2.5 ×10−6 mol/m2 as shown in Table 2).Competitive Sorption between Pb(II) and Zn(II) at

PAA-Coated α-Al2O3 (0001) Single-Crystal Surfaces.Table 1 lists Pb(II) and Zn(II) distributions between PAAcoatings and α-Al2O3 (0001) surfaces from LP-XSW-FYanalysis (see Figure S5A−C of the SI). More than 99% ofPb(II) and Zn(II) was found to partition into the PAA coatingseven at the lowest metal−ion concentration (0.5 μM). In thiscase, PAA coatings dominated the sorption of both Pb(II) andZn(II) ions. This is expected because the log K values forPb(II)−PAA complexes are higher than those for Pb(II)complexes formed with abundant weak binding sites on α-Al2O3 (0001) surfaces (Table 2). The higher partitioning ofboth Pb(II) and Zn(II) onto α-Al2O3 (1−102) surfaces thanonto α-Al2O3 (0001) surfaces in the presence of PAA coatingsat low metal−ion concentrations (≤5 μM) suggests a higherreactivity of α-Al2O3 (1−102) surfaces toward metal ions incomparison to α-Al2O3 (0001) surfaces. This observation isconsistent with previous studies of Pb(II) and Zn(II)partitioning at Shewanella oneidensis MR-1 biofilm-coated α-Al2O3 (0001) and (1−102) surfaces,35 Pb(II) partitioning atPAA-coated α-Al2O3 (0001) and (1−102) surfaces at pH 4.5,6

and Pb(II) partitioning at Burkholderia cepacia biofilm-coatedα-Al2O3 (0001) and (1−102) surfaces,3 as well as previousstudies of Pb(II) sorption at uncoated metal oxide surfaces,29

where the intrinsic order of reactivity for α-Al2O3 (0001) and(1−102) surfaces has been found to be α-Al2O3 (1−102) ≫ α-Al2O3 (0001). These observed differences in reactivity of the α-Al2O3 (0001) and (1−102) surfaces have been explained bydifferences in the structures of the hydrated surfaces.27,28,48

Therefore, the presence of biofilm- or organic-coatings,

including highly reactive PAA films, does not change theintrinsic order of reactivity of α-Al2O3 (0001) and (1−102)surfaces.

■ ASSOCIATED CONTENT

*S Supporting InformationSupporting Information for this article includes the detailedinformation about X-ray reflectivity and LP-XSW-FY datacollection and fitting, one table for LP-XSW-FY fittingparameters, and five additional figures (Figures S1−S5) forLP-XSW-FY data and fitting. This material is available free ofcharge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*(G.E.B.) Phone: +1 650-723-9168; fax: +1 650-729-2199; e-mail: gordon.brown@stanford.edu.

Present Addresses▽ (F.M.M.) Department of Geosciences, Virginia PolytechnicInstitute and State University, Blacksburg, Virginia 24061,United States.○(C.L.) CEREGE, Europole Mediterraneen de l′Arbois, BP 80,13545 Aix en Provence, Cedex 04, France

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This study was supported by NSF Grant CHE-0431425(Stanford Environmental Molecular Science Institute). TheLP-XSW-FY data reported in this paper were collected atGSECARS at APS, Argonne National Laboratory. GSECARS issupported by the National Science FoundationEarthSciences (EAR-1128799) and the Department of EnergyGeosciences (DE-FG02-94ER14466). APS is supported by theU.S. Department of Energy, Office of Science, Office of BasicEnergy Sciences, under Contract No. DE-AC02-06CH11357.

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