Heavy Metal Sorption at the Muscovite (001)–Fulvic Acid Interface

ARTICLE pubs.acs.org/est

Heavy Metal Sorption at the Muscovite (001)
Fulvic Acid Interface Sang Soo Lee,*,† Kathryn L. Nagy,† Changyong Park,‡,§ and Paul Fenter‡ †

Department of Earth and Environmental Sciences, 845 West Taylor Street MC-186, University of Illinois at Chicago, Chicago, Illinois 60607, United States ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States

bS Supporting Information ABSTRACT: The role of fulvic acid (FA) in modifying the adsorption mode and sorption capacity of divalent metal cations on the muscovite (001) surface was evaluated by measuring the uptake of Cu2+, Zn2+, and Pb2+ from 0.01 m solutions at pH 3.7 with FA using in situ resonant anomalous X-ray reflectivity. The molecular-scale distributions of these cations combined with those previously observed for Hg2+, Sr2+, and Ba2+ indicate metal uptake patterns controlled by cation
FA binding strength and cation hydration enthalpy. For weakly hydrated cations the presence of FA increased metal uptake by approximately 60
140%. Greater uptake corresponded with increasing cation
FA affinity (Ba2+ ≈ Sr2+ < Pb2+ < Hg2+). This trend is associated with differences in the sorption mechanism: Ba2+ and Sr2+ sorbed in the outer portion of the FA film whereas Pb2+ and Hg2+ complexed with FA effectively throughout the film. The more strongly hydrated Cu2+ and Zn2+ adsorbed as two distinct outer-sphere complexes on the muscovite surface, with minimal change from their distribution without FA, indicating that their strong hydration impedes additional binding to the FA film despite their relatively strong affinity for FA.

’ INTRODUCTION Natural organic matter (NOM) in both dissolved and solid forms plays a significant role in controlling the disposition of toxic heavy metal elements in the environment. Dissolved organic matter (DOM) binds metal ions in solution, changing their speciation and mobility.1
3 Various thermodynamic models have been developed to predict metal binding by DOM,4
6 although they are mostly case-specific and still need to be refined for general applications.7 In many soils and sediments, NOM, including DOM, binds to mineral surfaces and can significantly alter the uptake of metals to both the organic matter and minerals.8
12 Unlike the case for DOM-metal binding in solution, there are no models that describe the systematics of metal binding at the DOM
mineral interface. The development of such models requires observations at the molecular scale using surface-sensitive approaches to distinguish the various modes of ion
mineral
DOM interactions. Muscovite mica has often been chosen as a mineral substrate in experimental systems because its basal plane, the (001) surface, is similar to the dominant surfaces of many clay minerals, which, along with many micas, are the main constituents of argillaceous soils and sediments. The surface cleaves easily to provide a large atomically flat surface with a permanent negative charge (∼1e
per unit cell area, AUC). The morphology of adsorbed DOM on the basal surface of muscovite has been characterized by atomic force microscopy (AFM).13
16 The earlier AFM images were interpreted as showing the formation of aggregates of DOM on the r 2011 American Chemical Society

surface after reaction in 10
200 mg DOM/L solutions.13
15 The DOM aggregates were sparsely distributed leaving large areas of the surface exposed to solution. The coverage, size, and sorption stability of the aggregates increased with increasing cation concentration and decreasing pH, indicating that aggregate formation is related to the degree of cation
organic complexation and the hydrophobicity of the DOM. A more recent study showed that at acidic to near neutral pH DOM aggregates were located on top of a 5
36 Å thick organic film that covered the surface.16 The DOM film covered a larger area of the surface than the aggregates and therefore would be expected to affect the overall sorptive capacity of the underlying substrate. X-ray reflectivity (XR) is an in situ, nondestructive method suitable for probing the distribution of DOM and simultaneous uptake of metal cations at the mineral
water interface. Particularly, the approach measures surface signals averaged over a relatively large area (∼1 mm2), enhancing the sensitivity to an organic film with wide coverage and diminishing the sensitivity to sparsely distributed aggregates. Continuous organic films about 6
12 Å thick were modeled from XR data obtained on the muscovite (001) surface after reaction in 100 mg/kg H2O Elliott Soil Fulvic Acid II (ESFA) solutions at pH 2
6.17 At pH 3.7, Received: April 18, 2011 Accepted: October 5, 2011 Revised: September 15, 2011 Published: October 05, 2011 9574

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Table 1. Best-Fit Model Parameters of the RAXR Dataa sampleb

χ2 (R-factor)

inner-sphere peak c cIS

zIS

uIS

outer-sphere peak c cOS uOS

zOS

zbroad

broad peak c cbroad

ubroad

metal only muSr53.5

1.35 (0.006)

1.38(f)

0.014(5)

0.29(f)

4.52(6)

0.19(1)

0.66(7)

8.43(31)

0.06(2)

1.38(43)

muHg12.0 24

1.15 (0.005)

0.62(5)

0.06(1)

0.13(f)

3.58(4)

0.14(1)

0.68(6)

9.57(36)

0.06(2)

2.26(55)

muPb103.7 32

1.54 (0.012)

1.90(3)

0.29(4)

0.27(11)

4.39(42)

0.19(5)

1.60(43)

9.58(61)

0.05(1)

2.00(f)

muCu103.7 32 muZn103.7 32

1.32 (0.007) 1.23 (0.007)


0.14(19) 1.28(72)

0.03(1) 0.01(1)

0.29(f) 0.29(f)

3.97(4) 3.90(4)

0.28(4) 0.20(4)

0.92(11) 0.40(10)

7.69(106) 5.37(75)

0.19(7) 0.14(5)

3.45(88) 1.77(38)

muSr5ESFA3.5 21

1.18 (0.008)

1.38(f)

0.03(1)

0.25(f)

muHg1ESFA2.024

1.22 (0.004)

metal + FA 4.40(2)

0.20(1)

0.17(5)

6.66(9)

0.20(3)

1.28(11)

2.28(7)

0.15(2) e

1.28(11)

6.28(27)

0.47(5)

4.71(25)

inner-sphere peak c

outer-sphere peak c

diffuse profiled

zIS

cIS

uIS

zOS

cOS

uOS

zdiffuse

cdiffuse

k
1diffuse 5.4(15)

muPb10SRFA3.7

1.28 (0.008)

1.91(2)

0.27(3)

0.20(9)

2.91(12)

0.42(4) e

1.46(7)

7.24(33)

0.21(5)

muCu10SRFA3.7

1.08 (0.009)


0.14(f)

0.05(1)

0.29(f)

3.71(10)

0.22(3)

0.67(12)

4.61(54)

0.20(8)

2.9(11)

muZn10SRFA3.7

1.17 (0.013)

1.28(f)

0.00(1)

0.29(f)

4.07(5)

0.15(1)

0.58(7)

4.97(39)

0.28(11)

4.9(20)

a The numbers in parentheses indicate 1σ uncertainties of the last digit(s) of the fitting parameters. f indicates parameter fixed during fitting. b mu: muscovite, SRFA: Suwannee River Fulvic Acid, ESFA: Elliott Soil Fulvic Acid II. The subscripted number after each metal name indicates the metal concentration in units of 10
3 m. All FA solutions contained 100 mg/kg of a dissolved FA. The number at the end of each sample name indicates the solution pH. c zj, cj, and uj: height (Å), occupancy (atom per unit cell area, AUC), and distribution width (Å) of a Gaussian peak j. d zdiffuse, cdiffuse, k
1diffuse: height of the first peak (Å), total occupancy (atom/AUC), and debye length (Å) of the broad diffuse profile (see Supporting Information). e Modeled as a part of metal
FA complexes.

Ba2+ adsorbed to the muscovite surface from a premixed BaCl2
ESFA solution mostly as an apparent inner-sphere (IS) complex.18 The electron density of the fulvic acid (FA) film was higher with Ba2+ than without Ba2+, implying that some Ba2+ was located within the film. However, the amount and distribution of Ba2+ in the film could not be quantified because XR is not element-specific. Resonant anomalous X-ray reflectivity (RAXR)19 was applied to probe the distribution of Sr2+, a cation having affinity for organic matter similar to that of Ba2+,20 at the muscovite
ESFA interface.21 The results showed that about 10
40% of the adsorbed cation accumulated in the outer part of a FA film at pH 3.5
5.5 while the remaining Sr2+ adsorbed on the muscovite surface as an outer-sphere (OS) complex.21 Compared to these cations, Hg2+, a cation with a greater affinity for organic matter,20,22,23 did not form any discrete IS or OS complex on the muscovite surface reacted in a premixed Hg(NO3)2 and ESFA solution at pH 2. Instead, a large fraction of Hg was incorporated within the FA film, resulting in increases both in metal uptake (by ∼140% compared to that without FA) and in film thickness (by ∼20% compared to that without Hg).24 The way in which dissolved metals interact with the muscovite (001)
solution interface in the presence of DOM must depend on specific properties of the cations, but those trends are unclear at present. Here we present new XR and RAXR data for Pb, Zn, and Cu uptake onto the muscovite (001) surface from premixed metal
FA solutions at acidic pH, a condition typically observed in carbonate-depleted clayey organic-rich soil layers, e.g., in some humults ultisols or spodozols,25,26 or in many metal-contaminated environments, e.g., acid mine drainages27,28 and associated pit lakes.29 The results for total coverage and adsorbed cation distribution are combined with those observed previously for Ba, Sr, and Hg and characterized systematically as a function of the

relative cation affinity for organic matter and the cation hydration strength.

’ EXPERIMENTAL SECTION Sample Preparation. Experimental solutions were prepared by dissolving a high-purity (g99.99%) nitrate of Cu, Zn, Sr, or Pb (Aldrich Chemical Co., Inc.), Suwannee River Fulvic Acid (SRFA) from the International Humic Substances Society (IHSS), or both together in deionized water (DIW; ∼18.2 MΩ). Solutions containing SRFA were prepared with a relatively high FA content (100 mg/kg H2O; i.e., mg of dry FA in 1 kg of DIW) to ensure the formation of a FA film on the muscovite surface over the time of the reaction (>2 h).16,17 High concentrations of metal cation (5
10  10
3 m; molality) were used to minimize competitive effects of hydronium and other cations sourced from the muscovite and FA. The high concentrations also controlled the ionic strength of the solutions without addition of other electrolytes, which could increase the complexity of the system. Therefore, the observed structural changes are expected to derive purely from changing cation
FA interactions. The pH of the SRFA solution without any adjustment was 3.7, close to the log K1 value (3.81) of the FA.30 The pH of all other solutions was also adjusted to 3.7 using high-purity 0.1 M HNO3 except that for muscovite reacted in a 5  10
3 m Sr(NO3)2 solution without FA (muSr53.5, Table 1) whose pH was 3.5 for comparison to results from the previous experiment conducted in a premixed 5  10
3 m Sr(NO3)2 and 100 mg/kg ESFA solution at the same pH (muSr5ESFA3.5, Table 1).21 Prepared solutions were stored in brown polypropylene bottles in a refrigerator until used. For each experiment, a gem-quality single crystal muscovite (Asheville Schoonmaker Mica Company) was cleaved to expose a fresh (001) surface and immersed vertically in a 50-mL centrifuge 9575

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Figure 2. Total electron-density profile derived from the best-fit model of XR for muscovite (001) in contact with a 100 mg/kg SRFA solution at pH 3.7 (muSRFA3.7). Those for muscovite (001) in a 100 mg/kg ESFA solution at pH 3.7 (muESFA3.7)18 and a 100 mg/kg PPFA solution at pH 3.6 (muPPFA3.6)18 are plotted for comparison. The electron density was normalized to that of bulk water, and plotted with a band indicating the 1σ uncertainty as a function of height from the surface. The profile below 0 Å (corresponding to the muscovite substrate beneath the top oxygen plane) is not shown. Figure 1. Normalized RAXR signal using the resonant amplitude normalization [(|Ftot(q,E)|2
|FNR(q)|2)/(2|FNR(q)|), where Ftot and FNR are total and nonresonant structure factors, respectively] for muscovite in solutions containing heavy metals (Pb, Cu, and Zn in comparison to Hg and Sr)21,24 in the absence (cyan circles) and presence (pink squares) of FA. Spectra are offset by 2 units, and open and filled symbols are used alternately for easier visual comparison. Each spectrum is labeled with the q value (Å
1) where the spectrum was measured. The curves through data points are calculated intensities derived from the best-fit models. Solid (for solid symbol data) and dashed (for open symbol data) gray horizontal lines guide theoretical reflectivity when there is no resonant atom at the interface. The deviation of the RAXR signal from the reference lines is proportional to the ion coverage at small q.24

tube containing one of the solutions for at least 2 h,17 after which the wet muscovite was transferred to a thin-film sample cell for XR measurements as described previously.17,18,21,24 Specular X-ray Reflectivity. Measurements were made in situ at beamlines 6-ID-B (MU-CAT) and 11-ID-D (BESSRC), Advanced Photon Source, Argonne National Laboratory (Figure S1 in Supporting Information (SI)). X-ray experiments on samples containing FA were conducted in the dark. The stability of the experimental samples over the measurement time of approximately 1 h was confirmed by periodically measuring reflectivity at two reference points defined by momentum transfer values q = 0.85 and 1.83 Å
1. Only experiment muPb10SRFA3.7 (muscovite in 10  10
3 m Pb and SRFA at pH 3.7; Table 1) showed a small but significant (∼6%) variation in reference point reflectivities. Measurements for muCu10SRFA3.7 and muZn10SRFA3.7 were duplicated with separately prepared samples. The XR data were fit with parameterized models (SI text) to obtain optimized structures for the total electron density profiles at the interfaces. Resonant Anomalous X-ray Reflectivity. Measurements were obtained by scanning the photon energy (E) near the X-ray absorption edge of the target metal at fixed q values (from 0.25
0.35 to 3.3
4.3 Å
1) (Figure 1). One set of data typically included 10
20 spectra measured over 3
6 h. The stability of the experimental system was monitored by periodically repeating measurements at low q values (0.25
0.57 Å
1). The RAXR signals at the reference points varied less than 5% in amplitude, except for muPb10SRFA3.7, which showed a slow but continuous decrease

in signal amplitude (e.g., ∼20% decline after 3 h). This implies that adsorbed Pb was mobile during X-ray exposure. A similar result was observed after 2 h of X-ray measurements of adsorbed Hg on a pre-FA-coated muscovite (001) surface at pH 2.0.24 The RAXR data were fit by a model with two cation positions (represented as inner-sphere (IS) and outer-sphere (OS) positions in Table 1) near the muscovite surface, followed by a broad ion profile. Initial cation distributions were guided by a semiquantitative cation profile derived from model-independent analysis31 (Figures S2 and S3). For data collected in a pure metal-salt solution, the broad profile was modeled using a single Gaussian peak.24,32 With FA, a slightly better quality of fit could be obtained using a broad asymmetric peak simulated by overlapping a series of equally spaced Gaussian peaks whose occupancies decrease exponentially as a function of distance from the surface (SI text).

’ RESULTS AND DISCUSSION Fulvic Acid Sorption on the Muscovite (001) Surface. The total electron-density profile for muSRFA3.7 (Suwannee River FA adsorbed on muscovite at pH 3.7) has a broad peak near 2.5 Å followed by a broader profile with the maximum electron density located near 4.6 Å (Figure 2). This pattern is similar to those determined previously for FA sorbed on muscovite from a 100 mg/kg Elliott Soil FA II (ESFA) solution at pH 3.7 (muESFA3.7) and a 100 mg/kg Pahokee Peat FA (PPFA) solution at pH 3.6 (muPPFA3.6).18 The broad peak near the surface has a lower electron density in muSRFA3.7 than in muESFA3.7 and muPPFA3.6. This suggests that the fraction of SRFA adsorbed directly on the surface either has a lower electron density or covers less of the surface than the similar fraction of ESFA or PPFA. The higher electron density for the PPFA sample may be explained by its higher ash content (4.61 wt %) compared to that for SRFA (0.46 wt %) or ESFA (1.00 wt %).18,30 The next peak in the electron-density profile of muSRFA3.7 extends to about 7 Å from the surface and is narrower and closer to the surface compared to that of muESFA3.7. The muSRFA3.7 profile also has a third electron-dense region above ∼7 Å, indicating that a fraction of sorbed SRFA molecules extends farther from the surface. A similar pattern is observed in muPPFA3.6, but is relatively less prominent in muESFA3.7. 9576

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Figure 3. Fractional change in heavy metal uptake at the muscovite (001)
solution interface in the presence of 100 mg/kg FA in relation to the calculated molar ratio between metal
FA complexes and free metal cation in solution and the metal cation hydration enthalpy. The molar ratio [Me2+
FA]/[Me2+] was calculated based on the solution composition (Table S2). Fractional changes in cation coverage (cMeFA
cMe)/cMe (%)) (see text) were calculated based on the RAXR results of five metals (marked in a pink dashed pentagon; except Ba). The color map with contours, generated by the best-fit empirical expression (eq 1), illustrates the trends in how the hydration enthalpy of the cation and affinity of the cation for FA under the experimental conditions control metal uptake at the interface. Total electron-density profiles derived from the best-fit models of the interfacial structure with adsorbed metal in the absence (blue dashed line) and presence (red solid line) of FA are also shown. Refer to Table 1 for sample codes, and Figure 2 for descriptions of the axes for profiles associated with individual elements. The electron-density profiles measured in FA solutions without metals (green dot-dashed line) are plotted for comparison. The element-specific profiles of adsorbed metals in the absence and presence of dissolved FA are shown in sky-blue and pink shaded areas, respectively. Note that the distribution of Ba is estimated based on the difference between total electron-density profiles (e.g., muBa5ESFA3.7
muESFA3.7) and was not determined by RAXR.

Table 2. Characteristics of Fulvic Acid Films Adsorbed on the Muscovite (001) Surface in the Absence and Presence of Heavy Metals average layer density (FWeq) samplea muSRFA3.7 muPb10SRFA3.7

layer thickness (Å) 6.0(5.9
6.3) 25.8(24.7
26.5)

with metals

without metals 1.02(0.97
1.07)

1.29(1.25
1.32) 1.11(1.07
1.14)

muCu10SRFA3.7

8.0(7.9
8.1)

1.09(1.04
1.14) 0.99(0.94
1.04)

muZn10SRFA3.7

8.1(8.0
8.2)

1.12(1.11
1.16) 1.02(0.96
1.07)

muESFA2.017

12.1(11.9
12.1)

1.12(1.07
1.16)

muHg1ESFA2.024

14.9(14.7
14.9)

1.34(1.29
1.39) 1.13(1.08
1.18)

muESFA3.717,18 muSr5ESFA3.521

7.2(7.1
7.2) 10.9(10.9
11.0)

1.02(0.96
1.07) 1.08(1.05
1.12) 0.99(0.96
1.03)

a

Refer to Table 1 for sample codes. The numbers in parentheses are the ranges of the values calculated from the lower (
σ) and upper (+σ) limits of electron-density profiles derived from the best-fit models.

All three FA samples demonstrate the common feature of an approximately 10 Å thick film with a structure composed of a

directly adsorbed fraction of FA having a higher electron density and a remnant tail that has a lower electron density. Effect of Fulvic Acid on Pb Uptake on Muscovite. The amount of adsorbed Pb is enhanced in the presence of FA as shown by the RAXR data for muPb10SRFA3.7 vs muPb103.732 (Table 1). The total coverage of Pb [0.90(7) atom per unit cell area, AUC = 46.72 Å2]33 in the presence of FA is about twice as high as that (∼0.5 atom/AUC) needed to compensate the muscovite surface charge (∼1e
/AUC), implying that some Pb is bonded to sorbed FA molecules. Comparing the electrondensity profiles of muPb103.7 and muPb10SRFA3.7 shows that the distribution of the additional Pb matches that of the FA film, indicating a direct association of Pb with the sorbed FA (Figure 3). The total electron density of the FA film in muPb10SRFA3.7 is also higher than that in muSRFA3.7 (Table 2) because of the presence of electron dense Pb in the layer. A large fraction of Pb is adsorbed as an apparent IS complex to the muscovite surface, while it is not possible to distinguish an OS complex because the modeled electron density of this species would be superimposed on that of Pb
FA complexes (Figure 3). The muPb10SRFA3.7 profile shows a small increase in the Pb distribution located immediately 9577

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Environmental Science & Technology adjacent to the surface (i.e., 10
6 M Pb in solution.34 The TER-XSW approach is better suited for investigating the distribution of an ion within a large-scale matrix (such as biofilms), but it has intrinsically a lower spatial resolution than RAXR (e.g., ∼25 Å vs ∼1 Å, respectively). Therefore, it is unknown if the fraction of Pb reported near the surface34 represents a true surface-adsorbed species. The fractional enhancement of cation uptake by FA (compared to that without FA) is larger for Pb than Sr (Table 1 and Figure 3). The dissimilar atomic-scale distributions of the cations in muPb10SRFA3.7 and muSr5ESFA3.5 indicate that the uptake is controlled by different sorption mechanisms. Whereas these two cations have similar hydration enthalpies, Pb has a stronger affinity for FA and forms organic complexes in the solution. Strontium does not complex significantly with FA in the solution. The enhanced sorption of Sr is, unlike Pb, localized in the outer part of the FA film, suggesting that, although Sr and FA were mixed in solution prior to adsorption, the Sr adsorbed independently after the FA.21 A similar result using only XR data had been observed for Ba, which is slightly less strongly hydrated than Sr but has a similar affinity for FA.18 The total electron-density profile above the muscovite surface for muBa5ESFA3.7 is higher than that of muESFA3.7, and can be attributed partly to an accumulation of Ba in the layer (Figure 3). The distribution of Pb throughout the FA film indicates that its enhanced uptake results mostly from sorption of metal
organic complexes, similar to results for Hg at pH 2.0 (muHg1ESFA2.0) (Figure 3).24 The total coverage of Hg [0.62(5) Hg/AUC], however, was smaller than that of Pb, in part because of greater competition by hydronium for sorption sites at pH 2.0 compared to pH 3.7 (Table 1). Also, little Hg occurred as distinct IS and OS species. The Hg
FA complexes would have been protonated at pH 2.0, resulting in decreased electrostatic repulsion at the surface and increased hydrophobicity of the complexes.2,17,35,36 Effect of Fulvic Acid on Copper and Zinc Uptake on Muscovite. The element-specific electron-density profiles of muCu10SRFA3.7 and muZn10SRFA3.7 show broad peaks at about 4 Å from the surface (Figure 3). The heights and occupancies of these peaks match those of the adsorbed OS (OSads) complex of Cu or Zn from solutions without FA (i.e., muCu103.7 and muZn103.7, respectively),32 indicating that the OSads species is not altered by the presence of FA. For each metal there is a second broader peak extending to ∼10 Å from the surface. This broader distribution is similar to that of an extended OS (OSext) complex observed in muCu103.7 and muZn103.7 (i.e., without FA).32 The position of the OSext species in the absence of FA is stabilized by multiple layers of water molecules, including those in higher-order hydration shells of the cation and the hydration layer at the muscovite surface.32 The lack of a significant increase in metal uptake in the presence of FA was unexpected especially for Cu2+ which has been reported to bind strongly to organic matter.4,37 Calculations based on the nonideal competitive adsorption-Donnan model show that the amount of Cu2+ bonded to SRFA in solution at

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pH 3.7 should be larger than that of Sr2+ at pH 3.5 (Table S3). Zn2+ has a slightly smaller affinity for organic matter compared to Cu2+,38 but still has a larger affinity than Ba2+ and Sr2+, especially for binding to phenolic or thiolic groups (Table S2). The cations Cu2+ and Zn2+ may occur mainly in nonsorbing metal
organic complexes that remain in solution during the experiments.39,40 Extended X-ray absorption fine structure (EXAFS) spectroscopy of Zn in organic-rich soils with a relatively high metal content (0.5
10 mg Zn/g of soil) at near neutral pH (5.6
7.3) showed that most Zn is bonded to organic matter with first-shell coordination ligands that are a mixture of oxygen (or nitrogen) and sulfur.41 Copper(II) complexed with multiple ligands (e.g., malate or malonate groups) and formed five- and six-membered chelate ring structures in solutions containing DOM (∼300
500 g C/kg) and Cu (100
6500 mg/kg) at pH 4.5 and 5.5 as determined by EXAFS spectroscopy.42 If the positive charge of each metal cation is shielded by ligands, then sorption to the muscovite would be controlled by the net charge and hydrophobicity of the cation
organic complex. This phenomenon also helps to explain the RAXR results for Hg, in which the enhanced metal coverage observed at pH 2.0 declined at higher pH24 in part because of deprotonation of functional groups that did not bind Hg and in part because of increased hydrolysis of Hg to form neutral inorganic species. It is also possible that some Zn2+ and Cu2+ formed complexes within sorbed aggregates13
15 which occurred at low surface coverage below the detection limits of XR and RAXR. A large fraction of Zn and Cu in the solutions should have been present in simple (hydrated) ionic form according to thermodynamic calculations (Table S2). Therefore, some aqueous Zn2+ and Cu2+ might have interacted with the sorbed FA film, similarly to Ba2+ and Sr2+.18,21 Part of the sorbed Zn and Cu identified as OSext species might be bonded within the outer region of FA; however, the locations of the peaks assigned to OSext species do not match well the location of the outer region of the film, supporting the interpretation that the majority of the cations are independently adsorbed species. It is not possible to determine why Zn2+ and Cu2+ sorbed to the FA film to a lesser extent than Sr2+ and Ba2+ based on the limited data in the current study. The reason may be related in part to the difference in hydration strength of the cations. The results show that the total amount of metals, especially within the FA layer, increases as the magnitude of the cation hydration enthalpy decreases (i.e., the hydration strengths become weaker) (Figure 3). This relationship indicates that strongly hydrated cations are less sorptive to the SRFA film than to the muscovite (001) surface at pH 3.7. Changes in the Internal Structure of Adsorbed FA by Adsorbed Metal Cations. The presence of metal cations leads to changes in the thickness, layer density, and detailed structure of the FA film on the muscovite (001) surface. In muCu10SRFA3.7 and muZn10SRFA3.7, the total electron density at 6
8 Å above the surface is greater than in muCu103.7 and muZn103.7 (Figure 3). This height range does not match the heights of any adsorbed Cu or Zn species, indicating that it is a region dominated by adsorbed FA. The position is 1
2 Å higher above the surface than the outer region of the film in muSRFA3.7, suggesting that the presence of the OS Cu and Zn complexes effectively increases the thickness of the organic film (Table 2). The overall average layer density increases mainly because of the added electron density from the cations. The layer densities corrected for the occupancy of the cations are 0.99
1.02 FWeq 9578

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(1 FWeq corresponds to the electron density of bulk water = 0.33 e
/Å3)18 similar to that for muSRFA3.7 (1.02 FWeq) (Table 2). The electron density g10 Å above the surface is slightly higher than that when metal cations are absent, suggesting that the presence of small amounts of OSext species can induce some additional sorption of FA at the interface. Interfacial Hg and Pb, considered to be largely in the form of metal
organic complexes, also increased the electron density (g1.3 FWeq) of the FA films (Table 2). Similarly to the experiments for Cu and Zn, the densities adjusted by subtracting the contributions from the metals are comparable to those for the pure FA layers, indicating this increase results from the presence of the metals alone (Table 2). However, the greater layer thicknesses (g15 Å) must result from additional FA on the surface. For Sr and Ba, the prominent enhancement in the electron density observed in the outer part of the FA film confirmed the incorporation of these cations in this specific location. The overall film thickness increased slightly (Table 2), indicating that the adsorbed metals attracted more FA to the surface. Empirical Model of Metal Uptake at the Mineral
NOM Interface. At the muscovite
FA solution interface three effective ligands, the waters of hydration, the FA, and the mineral surface, compete to bind metal cations. Effects of this competition are characterized here for the first time for a group of divalent cations in terms of the metal coverage and its molecularscale distribution at the interface. The fractional changes in the amount of cation uptake relative to the system without FA [= (cMeFA
cMe)/cMe, where cMeFA and cMe are the metal ion coverages (atom/AUC) with and without FA, respectively] for all metals (except Ba) were characterized by RAXR, and can be expressed empirically in terms of the dependence on cation
FA binding strength and cation hydration enthalpy: ðcMeFA
cMe Þ=cMe ¼ 3:2ðlog10

’ ASSOCIATED CONTENT

KFA
0:49Þ


1:011ðjΔHhyd j
2130Þ þ 0:62

DOM molecules, which could sorb differently to mineral surfaces than other fractions of DOM.22,23 For less strongly hydrated cations, the fractional coverage increases in the order Ba2+ ≈ Sr2+ < Pb2+ < Hg2+, in sequence of increasing cation affinity for FA. This trend is related, at the molecular-scale, to a transition from additional uptake of metal in the outer part of the FA film (presumably by electrostatic effects) to uptake effectively throughout the entire film via sorption of metal
FA complexes. At the same time, sorbed FA does not appear to alter the binding of the strongly hydrated Cu and Zn to the mineral surface, in these cases demonstrating the importance of metal binding to the bare mineral surface. Weakly hydrated cations with a smaller affinity for organic matter tend to bind electrostatically to both the negatively charged muscovite surface and the negatively charged functional groups of the FA film formed on the muscovite surface. In soils or sediments where the concentrations of these cations are relatively small these cations may be readily displaced by background cations, such as Ca2+ or Na+. A moderately hydrated cation with a larger affinity for organic matter can also sorb as organic complexes, and may be less exchangeable and therefore less mobile, at least at low pH. As pH increases these metal
organic complexes may be released to solution owing to increased electrostatic repulsion between the muscovite surface and the sorbed NOM as a consequence of deprotonation reactions of the NOM. These results confirm that the complex interactions among ions, NOM, and mineral surfaces can be monitored systematically. This and similar types of molecular-scale characterization will be essential in the development of more robust predictive models for assessing the transport of toxic metals in nature.

bS ð1Þ

where KFA = [Me2+
FA]/[Me2+] is the molar ratio of cation
FA complexes to the free cation species in solution calculated based on the reported metal
ligand binding constants (Table S2) and ΔHhyd is the cation hydration enthalpy (kJ/mol). We note that KFA and ΔHhyd are not necessarily independent variables. The apparent high precision of some model parameters (SI) is mostly a result of the limited number of data points. This simple empirical equation may not explain fully the complex nature of metal
FA
muscovite interactions. However, the equation effectively reproduced the observed data trends [with a good quality of fit (i.e., χ2 < 1 and R-factor