Fulvic Acid Sorption on Muscovite Mica as a Function of pH andTime Using In Situ X-ray Reflectivity

Sang Soo Lee,*,† Paul Fenter,‡ Changyong Park,‡ and Kathryn L. Nagy†

Department of Earth and EnVironmental Sciences, 845 West Taylor Street MC-186, UniVersity of Illinoisat Chicago, Chicago, Illinois 60607, and Chemical Sciences and Engineering DiVision, Argonne National

Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439

ReceiVed NoVember 5, 2007. ReVised Manuscript ReceiVed April 21, 2008

Interfacial structures of the basal surface of muscovite mica in 100 mg kg-1 Elliott Soil Fulvic Acid II solutionswere investigated using in situ X-ray reflectivity. Molecular-scale variations in the thickness and internal structureof the fulvic acid (FA) film were observed and quantified as a function of pH (2–12) and reaction time (3–500 h atpH 3.7). At pH e6, the electron-density profile of the FA layer sorbed on the muscovite surface was composed ofone near-surface peak followed by a broad peak that diminished in electron density with distance from the surface.The presence of the near-surface peak is attributed to condensation of FA molecules during sorption. The apparentthickness of the FA layer decreased from 12.3 to 7.2 to 6.4 Å as pH increased from 2 to 3.7 to 6, respectively. AtpHg8.5, a distinct interfacial structure was observed, consisting of sharper peaks similar to those previously observedfor muscovite in the absence of FA. These peaks are most likely composed of smaller aqueous species, such as H2Omolecules, metal ion impurities from FA, and Na+ from NaOH. The FA sorbed on the muscovite surface at pH 3.7maintained a relatively constant thickness after 3 hours. However, the electron density of the near-surface FA peakincreased by about 24% from 3 to 12 hours, and remained relatively constant from 12 to 500 hours. The electron densityof the more distant part of the sorbed FA layer increased slightly after 12–50 hours of reaction but then decreased,and the broad peak flattened by 500 hours. Internal structural changes are possibly due to the slow sorption rate ofFA molecules, or a fractionation effect, i.e., continuous subsitution of smaller FA molecules by larger FA molecules.

Introduction

Soil is one of the most important global carbon reservoirs innature. The amount of carbon stored in soils as both organic(1550 Gt) and inorganic (950 Gt) forms is larger than the sizesof the atmospheric pool (760 Gt) and the biotic pool (560 Gt).1

Among these, the largest fraction is humic substances (HS),2

which are formed by microbial degradation of biomoleculesfollowed by secondary biochemical synthesis reactions.3 At theearth’s surface, HS interact with subsurface materials includingminerals. The sorption and desorption of HS on mineral surfacesis important because these processes can control the diffusiveflux of dissolved organic carbon (DOC), which potentiallytransforms to greenhouse gases.4–6 HS-coated soil particles caneasily aggregate, and the aggregates improve soil quality andlong-term agricultural sustainability.6,7 Furthermore, these pro-cesses can influence the environmental distribution, speciation,and bioavailability of metals and organic pollutants,8–15 change

the reactive properties of mineral surfaces,16–23 and alter mineraldissolution and precipitation reactions.24–28

A major fraction of fine-sized soil particles is composed ofphyllosilicate clay minerals that provide a large quantity of reactivesurface area, part of which is basal surface area with a highnegative charge that depends on the clay mineralogy. It has beensuggested that the adsorption of HS on the basal surface of clayminerals mainly takes place via cation bridging and hydrophobic

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bonding.29,30 Cation bridging can occur when divalent cationsadsorbed on negatively charged clay surfaces electrostaticallyattract negatively charged functional groups of HS to thesurfaces.31 However, hydrophobic bonding can be effective whenthe solution ionic strength is sufficient to screen negative chargesin HS or when protons at acidic pH neutralize the net-negativecharge on the organic molecules.32 Nonetheless, the directdemonstration of sorption mechanisms and processes is chal-lenging because HS have supramolecular structures containinga variety of macromolecules with different individual structures,sizes, and numbers of reactive groups.33–37 In fact, the frac-tionation of organic molecules by sorption on mineral surfacesoccurs because of the heterogeneity and polydispersity of HS.Presumably fast-adsorbing low-molecular-weight compounds aresuccessively displaced from the surface by slowly adsorbingcompounds of higher molecular weight.20,29,31,38–42

Atomic force microscopy (AFM) techniques have been usedto observe the morphology of HS sorbed on mineral surfaces.Muscovite mica is often the selected sorbent because of its large,atomically flat basal surface that has a composition and permanentcharge similar to those of many sheet-silicate clay minerals.Using tapping-mode AFM in air, Balnois et al. 43 observed sorbedhumic acid (HA) with small globular shapes that were 0.5-2 nmin height on dried surfaces of muscovite after reaction in solutionscontaining 10 mg/L HA and 5-500 mM NaCl or 5 mM CaCl2

at pH >5. However, drying samples can alter the configurationof sorbed HS molecules, although it is usually assumed that asmall amount of water is present within the macromolecules tosustain their structures.44 Using tapping-mode AFM in solution,Maurice and Namjesnik-Dejanovic45 observed that aquatic HSformed ring-shaped aggregates with a mean height of 3.5 nm in0.01 M CaCl2 solution containinge100 mg C/L peat fulvic acid(FA) at pH ∼5. In a 0.1 M LiCl solution at pH 3 containing 25mg C/L natural organic matter (NOM) from freshwater wetlands,they observed that HS sorbed on the muscovite surface asaggregates with flattened spherical shapes with thicknesses of0.4-0.9 nm.46 However, the results of Gibson et al.47 indicatedthat muscovite that reacted in 10 mg/L of HS solution was coveredby a coherent surface film with thickness ranging from <0.5 nm(at pH 4.9) to 3.6 nm (at pH 2) and that small discrete particles,

which are described in most AFM studies as typical HS sorbedon a muscovite surface, are on top of this film rather than on theoriginal muscovite surface. To measure film thickness, theyscratched a reacted surface with the AFM tip in contact modeto remove weakly sorbed HS and then measured the heightdifference between scratched and unscratched surfaces in tappingmode. However, this method required the manipulation of thesample in a dry state and could have altered the surfacecharacteristics. Moreover, because HS are soft and can be easilyremoved by the AFM tip, the information from AFM imagesmay be distorted by tip-sample interaction.46

X-ray reflectivity is a method for probing atomic- or molecular-scale structures near surfaces and at interfaces; it is nondestructiveand can be applied in solutions. Therefore, it is appropriate forthe in situ investigation of soft organic films sorbed on mineralsurfaces.48–51 The method has been used to determine the in situinterfacial structure of water and cation sorbates on the muscovitesurface in solution with atomic-scale resolution.52–54 Recently,Lee et al.55 observed an approximately 1-nm-thick film structurefor three FAs sorbed on the muscovite surface at pH 3.7 usingthis technique. In this article, we report in situ X-ray reflectivitydata for the muscovite (001) surface in solutions of soil FA overa range of pH covering most natural conditions. Interfacial electrondensity profiles derived from the data provide, with angstrom-scale resolution, information on the thickness and internal structureof the sorbed FA film, both of which vary systematically as afunction of pH and reaction time.

Materials and MethodsSample Preparation. ASTM-V1 grade natural muscovite

(Asheville Schoonmaker Mica Company) was used in the experi-ments. Each crystal had a basal surface area of 25 mm × 25 mmand was ∼0.1 mm thick. The unit cell area of the basal surface plane,AUC, is 46.72 Å2, and the length of a unit cell along the surface-normal direction, d, is 19.9564 Å.54 Each muscovite crystal wascleaved to expose an atomically smooth, fresh surface and im-mediately immersed vertically in a 50 mL centrifuge tube containingan FA solution. FA solutions were prepared using Elliott Soil FulvicAcid II (ESFA II) from the International Humic Substances Society(IHSS). ESFA II consists of C (50.12 wt %), H (4.28 wt %), O(42.61 wt %), N (3.75 wt %), S (0.89 wt %), and P (0.12 wt %) withwater (11.2 wt%) and ash content (1.00 wt%). The major functionalgroups are carboxylic (13.24 mol/kg C) and phenolic (2.27 mol/kgC) moieties.56 Each solution (30 g) contained 3 mg of ESFA IIresulting in 100 mg kg-1 of FA. The initial pH of the solution wasapproximately 3.7, which is close to the pK1 value of the FA (3.67;cf, pK2 ) 9.53 56). Other FA solutions were prepared at pH valuesadjusted to 2, 6, 8.5, 8.9, and 12 using from 1 to 0.01 M high-purityHNO3 and NaOH solutions. Two solutions at similar pH (8.5 and8.9) were prepared to test the reproducibility of the system. Theionic strength was not fixed at a constant value in these experimentsfor the primary reason that additional chemical components would

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7818 Langmuir, Vol. 24, No. 15, 2008 Lee et al.

complicate the interpretation of the X-ray reflectivity data that aresensitive only to total electron density. In addition, backgroundelectrolytes can interact with both FA and the muscovite surface,causing changes that occur to different extents in their respectiveelectrical double layers as well as effective charge.53–55 Theexperimental solution compositions mimic various natural waters inthat both the ionic strength and pH vary. For example, near-neutralpH freshwater solutions often have low ionic strengths, and solutionsat extreme pH values often have relatively higher ionic strengths,reflecting the increased solubility of many common minerals underthese conditions. In the two experiments in which the pH was adjustedsubstantially (i.e., pH 2 using HNO3 and pH 12 using NaOH), theobserved interfacial structure could have been affected by theadsorption of H+ and Na+ on the muscovite surface and/or into theFA film. Such effects were considered in interpreting the structureof the sorbed FA. At pH 3.7-8.9, the ionic strength of the FAsolutions in which no or only small amounts (e6 × 10-5 m) ofNaOH were added should be controlled mostly by the chargedfunctional groups of the FA (e.g., the concentration of the carboxylicgroups is 7 × 10-4 m in a 100 mg kg-1 ESFA II solution 56). Allprepared solutions were stored in dark-brown Teflon bottles to preventphotodegradation2,57 and were refrigerated at 4 °C until used in theexperiments.

During reaction periods, the centrifuge tubes were covered withaluminum foil to reduce exposure to light. For the pH-seriesexperiments, muscovite crystals were maintained in the solutionsfor 2 to 4 h. For the time-series experiments, muscovite crystalswere submerged in pH 3.7 FA solutions for 3, 12, 50, and 500 h.After reaction, each wet muscovite crystal was transferred to a thin-film sample cell,52 drenched with the reaction solution, and coveredwith Kapton film. Excess solution in the sample cell was removedby gravitational drainage until a homogeneous solution film a fewmicrometers thick was maintained above the muscovite surface onthe basis of visual inspection, at which time the sample cell wassealed. The muscovite remained in contact with the solution film foran additional 1 to 2 h prior to the reflectivity measurements whilethe sample was aligned and evaluated.

X-ray Reflectivity Measurements. Specular X-ray reflectivitywas measured in situ at beamline 11-ID-D of the Basic EnergySciences Synchrotron Radiation Center (BESSRC), Advanced PhotonSource (APS), Argonne National Laboratory. The incident beamsize defined by slits was 0.1-0.4 mm (vertical) × 0.5-1.0 mm(horizontal) with an incident flux of ∼1011 photons/s. The X-rayflux reflected from the muscovite surface was measured using aCCD detector,58 except for the experiment at pH 3.7 for 3 h whichwas measured by rocking scans using a scintillation detector andreported by Lee et al.55 The uncertainties of the measured reflectivitieswere determined by counting statistics.59 Collecting one full dataset usually required about 30-60 min with a CCD detector andabout 4 h with a scintillation detector.55 The experimental stationwas dark during all reflectivity measurements. Reflectivity (R) isdefined as the ratio of the reflected to incident X-ray flux and ismeasured as a function of momentum transfer, q ) (2π/d) L, whered and L are the length of a unit cell along the surface-normal directionand a Bragg index, respectively. During measurements, the stabilityof the system including sample alignment, interfacial structure, andsolution layer thickness was monitored periodically at two referencepoints: the surface fiducial at q ) 0.85 Å-1 (corresponding to L )2.7) and the bulk fiducial at q) 1.83 Å-1 (corresponding L) 5.8).55

The data showed less than a 4% average fluctuation in reflectivityat the reference points (Figure S1 in Supporting Information). Allmeasurements at the reference points are included in the model fits.

Modeling the Interface Structure. Reflectivity data were fit bya parametrized electron-density model that consists of three sublayers:(1) the ideal muscovite substrate lattice, (2) the interfacial speciesincluding relaxed muscovite surface layers and any layers of surface-adsorbed species, and (3) bulk water above the surface. The atomsin the sublayers are specifically assigned by their occupancies (c),distances from the muscovite surface (z), and vibrational amplitudes(u). However, FA is a mixture of macromolecules that cannot besimply described as a specific arrangement of multiple atoms.Therefore, sorbed FA was modeled as a composite layer (referredto as a generic FA model) that is described in the next paragraph.The reflectivity calculated from a given model is expressed as

R(q)) (4πre ⁄ qAUC)2|FUCFCTR +Fint +FW|2 (1)

where re ) 2.818 × 10-5 Å is the classical electron radius and AUC

is the area of the unit cell in the ab plane. Each F is a structure factordefined as F ) Σj cjfj(q) exp(iqzj) exp[-(quj)2/2], where fj(q) is theatomic scattering factor with the expression summed over all atomswithin the substructure of interest. FUC, FCTR, Fint, and FW are thestructure factors for a unit cell of muscovite, crystal truncation rod(CTR) scattering (FCTR ) 1/[1 - exp(-iqd/2)] 59), the interfacialregion including the relaxed muscovite surface layers and any surface-adsorbed species, and bulk water, respectively. FUC was calculatedon the basis of the crystallographic parameters obtained from theX-ray diffraction (XRD) analysis.54 The atoms within the top twounit-cells from the surface were allowed to relax from their idealpositions in the bulk crystal to yield best-fit results.54,55 Surface-adsorbed species represent solution species positioned close to themuscovite surface, resulting in an electron density that is distinctfrom that of bulk water. Initial fits were obtained by fitting thesolution profiles above the surface with a “water-equivalent” profile,in which all components above the surface (including water, adsorbedions, and FA) are treated as water molecules (i.e., two hydrogenatoms and one oxygen atom).55

Adsorbed ions (e.g., Na+) and water molecules near the muscovitesurface could be expressed by a Gaussian distribution. FA is a mixtureof various macromolecules and so cannot be easily described withthe same model. Because we initially expected that sorbed FA wouldform a homogeneous film with finite thickness, we attempted tomodel the layer as a discrete electron-density block using two errorfunctions (Gaussian smeared step functions60). However, thisapproach was unsuccessful because the block structure was generallyunstable during the fitting procedure (e.g., resulting in a thicknessclose to zero when the positions of its two boundary functionsapproached too close to each other) generating a poor quality of fit(i.e., a large �2 value, which is defined below). A better quality offit could be obtained using models composed of multiple Gaussianpeaks (e.g., a two-peak model fit for the pH 6 data shown in FigureS2a of the Supporting Information). When multiple Gaussian-peakmodels were used, the quality of fit normally improved as the numberof peaks increased (e.g., for data at pH 3.7, �2, which is definedbelow, improved from 3.43 to 2.79 to 1.62 using one, two, and threeGaussian peaks, respectively) because of the increased number offitting parameters, which in turn can simulate more detailed interfacialstructures. By increasing the number of peaks, the resultant electron-density profile converged into a pattern composed of a near-surfacepeak followed by a broad pattern with an electron density thatdiminished away from the muscovite surface. On the basis of thepattern, a generic FA model was developed that is composed of asingle Gaussian peak and a broad profile modeled by the summationof multiple overlapping peaks with interdependent parameters.55

By using interdependent parameters, the number of parameters usedto describe the multiple peaks can be reduced (e.g., 30 parametersare required for 10 individual peaks compared to only 5 parametersfor the same number of peaks using the generic FA model), witha comparably good quality of fit (Table 1). These peaks are distributed

(57) Wu, F. C.; Mills, R. B.; Cai, Y. R.; Evans, R. D.; Dillon, P. J. Can. J.Fish. Aquat. Sci. 2005, 62, 1019–1027.

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FulVic Acid Sorption on MuscoVite Mica Langmuir, Vol. 24, No. 15, 2008 7819

with an equal spacing, dFA, and occupancies of the peaks vary asa function of the distance from the muscovite surface. The occupancyof the nth peak of the sorbed FA, cFA,n is defined as

cFA,n ) cFA,1e-λFA(n-1) (2)

where cFA,1 is the occupancy of the first fulvic peak and λFA is thedecay constant of the FA occupancy with distance from the muscovitesurface. Because the peaks are distributed with an equal spacingdFA, the structure factor for the total FA, FFA, can be expressed as

FFA )FFA,0 +FFA,1 × 1- e�(q)n

1- e�(q)(3)

where FFA,0 and FFA,1 are the structure factors of the independentpeak and the first peak of the interdependent peaks, respectively,and �(q) is defined as �(q) ) iqdFA - λFA.

The distribution of water molecules above the surface-adsorbedspecies was modeled by a layered-water profile.52–55 This modelconsists of a series of equally spaced Gaussian peaks with an averageelectron density equal to that of bulk water. The peaks are broadenedexponentially as the peak positions recede from the muscovite surface,resulting in a flat profile far from the interface that represents therandomly oriented molecules of bulk water.

The structural parameters and statistical uncertainties of the modelwere estimated by fitting the measured data using the least-squaresfitting procedure. The goodness of fit was measured statisticallyusing scaled �2 () [∑k(Ik - Icalc)2/σk

2]/(N - Np)) values, where Ik

and Icalc are the measured and calculated reflected intensity,respectively, σk is the uncertainty of the kth data point, and N andNp are the numbers of data and fitting parameters, respectively. Thebest-fit model was determined on the basis of the smallest �2 value,which should be close to 1 for an ideal case. The �2 value for a givendata set is sensitive to the quality of raw data, and a smaller �2 valuemay actually result from lower-quality data with higher uncertainties(i.e., larger σk values). Consequently, it is useful to compare therelative quality of fit among data sets using another measure, the R

factor () ∑k|(I - Icalc)/Ik|/N). The R factor determines the fractionaldeviation between measured and calculated data irrespective of themeasured uncertainties. The value is expected to approach theminimum error in the measured data as the �2 value ideally approachesa value of 1.

When a data set was fit with the generic FA model, an increasein the number of layers (n) in the interdependent set of peaks usuallyresulted in smaller �2 values. Above approximately 10 layers, thechange in fit quality was relatively small (<1% improvement of �2

values by including one more layer in the data fitting). The genericFA model reported in this study used n ) 10. The parameters fromthe best-fit models are used to generate the electron-density profilesof the interfaces as a function of distance from the muscovite surface.The electron-density profiles in this article are broadened by theexperimental resolution of the data (π/qmax ) 0.5-0.6 Å, where qmax

is the maximum q value ranging from 5.31 to 5.95 Å-1 for thereflectivity data sets) as described previously.61

Determination of Thickness and Occupancy of the FA Layer.The thickness of the FA film sorbed on the muscovite surface wasquantified from the electron-density profile derived from the best-fitmodel. The electron-density profile was also used to estimate theamount of FA in the film over a specific area of the muscovitesurface. However, for this calculation it must be kept in mind thatthe electron-density profile is averaged laterally over a large areaof the muscovite surface (typically, several tenths of a mm2), makingit impossible to distinguish FA molecules individually or as aggregates(presumably for a size of 10-1000 nm2), which under certainexperimental conditions (e.g., AFM) may be observed as sparselydistributed discrete particles rather than a continuous film.

As described in the previous section, the generic FA model wasdeveloped to have two components (a single peak and 10 interrelatedpeaks) on the basis of the electron-density pattern of the sorbed FAlayer at pH e6, which consists of an electron-dense part near themuscovite surface and a broadly humped or tailing pattern fartherfrom the muscovite surface. However, the two model components

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Table 1. Parameters for Aqueous Species in the Muscovite-FA Solution Interfacial Region from the Best-Fit Models of the X-rayReflectivity Dataa

sample �2(R factor)

generic FA modelc

single peakd broad profilee layered waterf

zFA,0 (Å) cFA,0 (Weq) uFA,0 (Å) zFA,1 (Å) cFA,1 (Weq) dFA (Å) λFA uFA (Å) zW (Å) dW (Å) uW (Å) ubar

pH SeriespH 2 3.15 (0.089) 2.18 (21) 4.33 (155) 0.77 (13) 3.45 (57) 2.63 (90) 1.12 (36) 0.51 (17) 0.90 (f) 9.40 (18) 6.26 (12) 2.65 (13) 0.39 (8)pH 3.7b 1.62 (0.064) 2.67 (8) 5.44 (47) 0.88 (5) 4.60 (34) 1.19 (11) 0.59 (2) 0.17 (3) 0.90 (f) 12.62 (31) 8.45 (41) 3.53 (26) 1.31 (18)pH 6 2.77 (0.092) 1.96 (4) 2.54 (28) 0.50 (5) 3.01 (12) 1.86 (15) 0.73 (5) 0.25 (3) 0.90 (f) 11.36 (21) 9.03 (30) 3.91 (15) 1.54 (40)

first peakg second peakh

z1 (Å) c1 (Weq) u1 (Å) z2 (Å) c2 (Weq) u2 (Å)

pH 8.5 1.85 (0.075) 1.75 (1) 2.06 (12) 0.13 (7) 3.18 (2) 4.24 (23) 0.64 (6) 5.72 (6) 2.52 (23) 0.92 (6) 0.92 (11)pH 8.9 1.36 (0.084) 1.76 (3) 1.99 (25) 0.28 (6) 3.11 (4) 4.13 (34) 0.67 (7) 5.78 (8) 3.08 (13) 1.14 (6) 0.74 (10)pH 12 1.93 (0.064) 1.71 (2) 1.64 (18) 0.23 (8) 3.01 (3) 3.90 (18) 0.57 (4) 5.68 (9) 3.00 (14) 1.03 (4) 0.87 (10)

generic FA modelc

single peakd broad profilee

zFA,0 (Å) cFA,0 (Weq) uFA,0 (Å) zFA,1 (Å) cFA,1 (Weq) dFA (Å) λFA uFA (Å)

Time Series3 hb 1.62 (0.064) 2.67 (8) 5.44 (47) 0.88 (5) 4.60 (34) 1.19 (11) 0.59 (2) 0.17 (3) 0.90 (f) 12.62 (31) 8.45 (41) 3.53 (26) 1.31 (18)12 h 2.45 (0.056) 2.62 (5) 7.16 (61) 1.00 (5) 5.10 (17) 1.79 (20) 0.85 (1) 0.21 (2) 1.04 (18) 13.66 (19) 8.16 (15) 3.89 (27) 0.58 (16)50 h 1.67 (0.062) 2.74 (4) 7.28 (37) 0.99 (3) 5.13 (9) 1.58 (11) 0.66 (3) 0.22 (2) 0.79 (9) 12.88 (36) 8.08 (38) 3.33 (19) 1.09 (12)500 h 1.28 (0.069) 2.55 (2) 7.26 (25) 1.00 (2) 4.98 (7) 1.48 (8) 0.97 (3) 0.22 (4) 0.90 (f) 13.80 (113) 8.57 (26) 4.85 (31) 1.00 (f)

a Occupancy is expressed in the dimensionless unit of the water equivalent, Weq, and the electron density is normalized to the number of electrons in oneH2O molecule per AUC. The numbers in parentheses indicate standard deviations of the last digits of the fitting parameters. f: parameters fixed during datafitting. b Refit from Lee et al.55 c In the generic FA model, a single peak is described as a Gaussian distribution, and a broad profile is modeled by the summationof multiple overlapping peaks with interdependent parameters as described in Materials and Methods. d zFA,0, cFA,0, and uFA,0: distance from the muscovitesurface, occupancy, and distribution width of the independent FA peak, respectively. e zFA,1 and cFA,1: distance from the muscovite surface and occupancyof the first peak of the multiple peaks composing the broad FA profile, respectively. dFA, λFA, and uFA: distance between two peaks, decay constant of FAoccupancy, and distribution width of the multiple peaks composing the broad FA profile, respectively. f zW, dW, and uW: distance from the muscovite surface,distance between two water layers, and distribution width of the first Gaussian peak of the layered water model, respectively. ubar indicates how quickly thepeak width increases for successive water layers away from the muscovite surface. g z1, c1, and u1: distance from the muscovite surface, occupancy, anddistribution width of the first peak, respectively. h z2, c2, and u2: distance from the muscovite surface, occupancy, and distribution width of the second peak,respectively.

7820 Langmuir, Vol. 24, No. 15, 2008 Lee et al.

do not necessarily represent two individual components of the overallFA layer, which can change shape as a function of solutioncomposition. For example, the near-surface FA peak at pH 2 isasymmetric because of partial overlap with the broad profile fittingcomponent and therefore could not be described by a single Gaussianpeak. (See Results for details.) Moreover, the FA pattern usuallyoverlapped with layered water components of the model. Conse-quently, it is not straightforward to determine the thickness andoccupancy of any individual part of the modeled FA layer on thebasis of the best-fit parameter values.

The thickness and occupancy of the sorbed FA layer were estimatedon the basis of the total electron-density profile derived from thebest-fit model. Figure 1 illustrates how these values are extractedfrom a total electron-density profile. Local minima in the totalelectron-density profile are determined over the distance range of0 to 20 Å from the muscovite surface. The first minimum, whichis closest to the muscovite surface, corresponds to the intersectionpoint between the muscovite surface and the sorbed FA and is referredto as the lower boundary of the FA layer. The second minimum,which is positioned within the FA layer, corresponds to the internalboundary between the surface peak and the tailing pattern of the FAlayer. The third minimum, which is the point where the FA tailing

pattern intersects with bulk water, is referred as the upper boundaryof the FA layer.

The thickness of the overall FA layer is defined as the distancebetween the lower and upper boundaries. However, it should benoted that the upper boundary is difficult to determine preciselybecause of the potentially diffuse FA-water interface and the poorelectron-density contrast of FA to bulk water. The thickness reportedin this study is, therefore, an apparent thickness; that is, the rangeof distance over which the electron density of the modeled FA layeris distinct from that of muscovite and bulk water. The occupanciesof the surface peak and the total FA layer are calculated by integratingthe electron density between the lower and the internal boundaryand between the lower and the upper boundary, respectively. Thisleads to an upper estimate of the FA occupancy because it ignoresany incorporation of water molecules within the FA film. It is difficultto distinguish the uncertainties of the thickness and occupancy fromthe parameters of the best-fit model because these parameters areoften strongly correlated. For instance, the distribution width (uFA)of the peaks within the FA model often showed high correlationwith the other parameters and was fixed when the covariance waslarge (i.e., r2 > 0.9, where r is the Pearson’s correlation coefficient).As a result, the fitting parameters of the best-fit model for the pH3.7 data are slightly different from the values previously reportedby Lee et al.,55 although they showed the same quality of fit (i.e.,�2 value and R factor) and produced the identical electron-densityprofile. Therefore, instead of standard deviations, we provide rangesfor FA thickness and occupancy that are estimated by determiningthe boundaries for two derived electron-density profiles from onedata set. These profiles are generated using parameters from thebest-fit model and bracketing those related to occupancy (i.e, cFA,0,cFA,1, and λFA, which are relatively less correlated to one another;r2 < 0.4) by adding or subtracting their σ values.

The average layer density, expressed as FWeq ) [Weq/(LFA × AUC)]× 30, is the 3D water-equivalent density of the film (Weq/(LFA ×AUC)) normalized to the number density of bulk water (1 H2O moleculeper 30 Å3), where LFA is the thickness of the FA layer. Hydrogenatoms contribute relatively little to either the electron density ormass density of FA and water (i.e., the fraction of H in ESFA II isonly 4.28 wt % compared to the fraction of C equal to 50.12 wt %).Because most of the FA consists of light elements such as C, O, andN and water’s mass is mostly O atoms, the electron-density ratiobetween the FA layer and bulk water is similar to the mass-densityratio of the same materials. This implies that the average layer density,which is a value normalized to the electron density of bulk water,is comparable to the specific density of sorbed FA (i.e., the massdensity of FA normalized to that of bulk water).

Results

Reflectivity Data. In situ reflectivity data were measured overranges of L values from about 0.7 to 16.9-18.9 correspondingto q values from about 0.22 to 5.31-5.95 Å-1. The reflectivityof muscovite in 100 mg kg-1 ESFA II solutions varies greatlyas a function of q, ranging from 10-5 to 10-10 with sharp peakscorresponding to the tails of the substrate Bragg peaks (Figures2 and 3). The larger variations related to the tails of the bulkBragg reflections often mask the smaller variations related to theinterfacial structure. To enhance the visibility of the changes inabsolute reflectivity, the measured reflectivity was normalizedto the calculated reflectivity from the best-fit model of the pH3.7 data after 3 h of reaction (shown in Figure S3 of the SupportingInformation). The largest differences between the curves can beobserved near the reflectivity minima, which correspond to datapoints that have a smaller contribution from bulk reflections.

To represent the interfacial structure changes as a function of pHand reaction time qualitatively, the reflectivity was plotted for eachof four scattering conditions (q ) 0.79, 1.48, 2.68, and 3.62 Å-1)near reflectivity minima (Figure 4). Within the pH series, the valuesof reflectivity for the pH 8.5 and 8.9 data are similar to each other

Figure 1. (a) Schematic showing how to determine the boundaries andthickness of a sorbed FA layer from the total electron-density profile(thick black solid line). The intrinsic model components are also shownfor the muscovite surface (thick gray solid line), a generic FA modelcomposed of one Gaussian peak (gray short-dashed line) followed bya broad profile (gray long-dashed line), and layered water (gray dotted-dashed line). Three boundaries (lower, internal, and upper) are determinedat the points where the electron densities are minimized. The range ofa surface FA peak is defined between the lower and internal boundaries,followed by a tailing pattern between the internal and upper boundaries.The thickness of the total FA layer is calculated on the basis of thedistance between the lower and upper boundaries. (b) Details of 10overlapping peaks (thin solid lines with colors gradually fading fromblack to gray) composing the broad profile of the generic FA model.The example is the electron-density profile of the best-fit model formuscovite reacted in a 100 mg kg-1 ESFA II solution at pH 3.7 for 3 has reported by Lee et al.55

FulVic Acid Sorption on MuscoVite Mica Langmuir, Vol. 24, No. 15, 2008 7821

under all four scattering conditions, indicating that the interfacedid not change substantially over this small pH range and alsothat it was reproducible. Over the broad pH range of 2 to 12, thevalues of reflectivity vary with pH at all selected q values,indicating that the interfacial structures have changed at bothlarge (e.g., the presence and absence of an organic film) andsmall (e.g., change of fine structure within the film) scale (FigureS4a in Supporting Information). There is also a systematic trendunder each scattering condition of decreasing reflectivity frompH 2 to 8.5-8.9, which suggests systematic changes in interfacialstructure. The time-series data at pH 3.7 show obvious changesin reflectivity at low q (0.79 and 1.48 Å-1), suggesting that thecharacteristics of the overall interfacial structure, such as thethickness or total amount of sorbed FA, may have changed duringthe experiment. Compared to the low-q data, the high-q data(2.68 and 3.62 Å-1) show little change, indicating that the portionof the sorbed FA that was changing with time is apparentlyassociated with the relatively broadly distributed part of theinterfacial profile (which will contribute to the reflectivity signalsignificantly only at low q), whereas the more narrowly distributedcomponents that contribute at high q (e.g., as a result of specificallyadsorbed species) were less affected by these temporal changes(Figure S4b in Supporting Information).

Effect of pH on Muscovite-Fulvic Acid InterfacialStructure. The quality of fit (i.e., the �2 values and R factors)of the best-fit models shows that the aqueous species near themuscovite surface at pH 2-6 were adequately expressed usingthe generic FA model (Table 1). The electron-density profilesshow distinct distributions composed of one peak near the sur-

Figure 2. Specular X-ray reflectivity of muscovite in 100 mg kg-1 ESFAII solutions at pH from 2 to 12. Each graph is offset for visual comparisonexcept for the pH 8.5 and 8.9 data, which are superimposed to test thereproducibility of the system at those pH values. Lines through eachdata set are the calculated reflectivity from the best-fit model under thegiven pH condition. The calculated reflectivity from the best-fit modelfor the pH 2 data (red line) is superimposed on the pH 12 data to showthe differences between the two data sets. Pink arrows indicate the pH12 data whose reflectivities are significantly different from the calculatedvalues from the best-fit model for the pH 2 data. The largest changesoccur near the reflectivity minima, and the variations of the absolutereflectivity near the minima (indicated by orange vertical lines) areshown in Figure 4a in detail.

Figure 3. Specular X-ray reflectivity of muscovite reacted in 100 mgkg-1 ESFA II solutions at pH 3.7 for 3 to 500 h. Each graph is offsetfor visual comparison. Each line indicates the calculated reflectivityfrom the best fit model under the given pH condition. The variationsof the absolute reflectivity near the reflectivity minima (indicated byorange vertical lines) are shown in Figure 4b in detail.

Figure 4. Reflectivity of muscovite in 100 mg kg-1 ESFA II solutionsat q ) 0.79 (9), 1.48 (O), 2.68 (2), and 3.62 (0) Å-1 as a function of(a) pH and (b) reaction time. The solid and dashed lines connect reflectivityvalues calculated from the best-fit models for each data set at those qvalues.

7822 Langmuir, Vol. 24, No. 15, 2008 Lee et al.

face followed by a broad humped pattern. At pH 2, the derivedelectron-density profile shows a broad asymmetric FA peakextending up to about 6.3 Å from the muscovite surface (Figure5a). The occupancy of the peak was 10.5 (9.0-12.2)Weq (waterequivalents, i.e., the electron density normalized to the numberof electrons in one water molecule) at pH 2 but decreased to 6.3(5.7-6.8)Weq at pH 3.7 and to 4.2 (3.8-4.7)Weq at pH 6 (Table2). The thickness and occupancy of the total FA film alsodecreased with increasing pH (Figure 6). At pH 2, the thicknessof the total FA layer was 12.3 (12.0-12.6) Å, and the occupancywas 21.7 (19.3-24.5)Weq. This results in an average density ofthe FA film at pH 2 of 1.13FWeq. At pH 3.7, the thickness andoccupancy were reduced to 7.2 (6.7-8.9) Å and 11.5(10.3-14.8)Weq, respectively. The calculated average density ofthe FA film at pH 3.7 was 1.02FWeq, which is also smaller thanthat at pH 2 (1.13FWeq). At pH 6, the FA film thickness (6.4(6.0-6.6) Å) became slightly less than that at pH 3.7. However,the occupancy of the total FA film (11.0 (9.8-12.2)Weq) remainedsimilar to that at pH 3.7, resulting in a more electron-dense FAlayer (i.e., 1.10FWeq) than that at pH 3.7.

Contrary to the data at pH 2-6, the reflectivity data at pH8.5-12 could not be successfully fit using the generic FA model.Attempts to fit the data with the generic FA model generatedelectron-density profiles with sharp peaks at approximately 2and 3 Å from the muscovite surface, followed by multiple peaksdue to the interdependency of the model FA layers, and all resultshad a lower quality of fit (Figure S2b of the SupportingInformation). This indicates that the solution near the muscovitesurface under these pH conditions has a discrete peak structurerather than a broad distribution pattern and requires a model

consisting of two sharp Gaussian peaks followed by layeredwater for successful fits of the data (Table 1 and Figure 5b). Thepositions of the first peak closest to the muscovite are similarto one another (1.71(2)-1.76(3) Å from the surface) over thepH range from 8.5 to 12, whereas the occupancies of the peaksdecrease as the pH increases (2.06(12) f 1.99(25)Weq f1.64(18)Weq for the data at pH 8.5, 8.9, and 12, respectively).However, the changes are slight when the derived uncertaintiesare included. The second peaks are also positioned at a similardistance from the muscovite surface (3.01(3)-3.18(2) Å) andhave similar occupancies ranging from 3.90(18)Weq to4.24(23)Weq. Both surface peaks are followed by a water structurethat clearly shows an oscillation pattern with an approximately3 Å periodicity that is dampened with distance from the muscovitesurface. In contrast, the electron-density profiles at pH 2-6 showthat water above the FA film is almost featureless.

Effect of Time on Muscovite-Fulvic Acid InterfacialStructure. The X-ray reflectivity data for muscovite reacted inFA solutions at pH 3.7 for 3 to 500 h (Figure 3) were fit usingthe generic FA model (Table 1). The electron-density profilesderived from the best-fit models suggest that the overall structureof the FA film was relatively constant during the reaction timeperiod. All electron-density profiles show the pattern of oneelectron-dense peak close to the muscovite surface followed bya broadly humped tailing structure (Figure 7). The thickness ofthe FA layer calculated from the total electron density profilesalso suggests that the FA layer maintained a nearly constantthickness (7.2-8.4 Å) within the uncertainties throughout thereaction (Figure 8). However, the internal structure of the FAfilm changed over the reaction time period. First, the occupancyof the near-surface peak calculated from the total electron-densityprofiles increased over the initially short time interval (Table 2).From 3 to 12 h, the occupancy of the surface peak increased from6.3 (5.7-6.8)Weq to 7.8 (7.1-8.5)Weq without any significantchanges in the peak positions (2.67(8) and 2.62(5) Å after 3 and12 h reactions, respectively). However, from 12 to 500 h, theoccupancy of the surface peak remained relatively constant.Second, the electron-density pattern within the tailing structureafter the surface peak changed with time. The electron-densityprofile of the solution after 3 h of reaction shows a weak, humpedpattern ∼5.7(3) Å away from the muscovite surface, but theoverall shape of the tailing structure is relatively flat (Figure 7).After 12 h, the electron-density profile shows that the broadlyhumped pattern was enhanced in electron density at ∼6.3(2) Å,which is slightly farther from the muscovite surface than theposition after 3 h of reaction. After 50 h, the humped patternbecame sharper, and the position moved a little closer to themuscovite surface (6.3(2)f 5.8(1) Å). Finally, after 500 h, thehumped pattern maintained a similar position (6.1(2) Å) but wasless electron-dense and broader than that after 50 h, resulting inthe relatively flat overall structure similar to that after 3 h ofreaction time.

The changes in the internal structure of the FA layer result invariations of the total electron density of the FA sorbed on themuscovite surface. The calculated occupancy of the entire FAlayer was 11.5 (10.3-14.8)Weq after 3 h but increased to 14.9(13.3-16.5)Weq after 12 h. As the result, the average layer densityincreased from 1.02FWeq to 1.14FWeq when increasing the reactiontime from 3 to 12 h. After 12 h, the occupancy of the FA layerremained almost constant within uncertainty (Table 2). Theaverage layer density after 12 h also did not change significantly(1.14FWeq f 1.15FWeq f 1.10FWeq after 12, 50, and 500 h,respectively) up to 500 h.

Figure 5. Derived electron-density profiles for the best-fit models of theinterfacial structure of muscovite in 100 mg kg-1 ESFA II solutions atpH (a) from 2 to 6 and (b) from 8.5 to 12. The electron-density profileof the muscovite-water interface at pH 5.552 is also shown forcomparison. The electron density was normalized to that of bulk water.The electron-density profile of the muscovite below 0 Å is not shown.

FulVic Acid Sorption on MuscoVite Mica Langmuir, Vol. 24, No. 15, 2008 7823

Discussion

Batch sorption experiments are generally conducted at aconstant ionic strength using a fixed dissolved electrolyteconcentration. The electrolyte compresses the Debye length ofthe electric double layer near a charged mineral surface andreduces the contribution from long-range electrostatic interactionbetween solution species and the mineral surface.62 However,

the main purpose of this study was to determine the verticalprofile of DOM sorbed on a pristine mineral surface. It wasnecessary to minimize the number of solution components tosimplify the interpretation of the data because the XR measure-ment is not element-specific, and electrolyte ions adsorb tomuscovite surfaces53,54 and can change the muscovite (001)interfacial structure in DOM solutions.55 Investigation of thedistribution of adsorbed electrolyte ions, using resonant anomalousX-ray reflectivity (RAXR),53,63 from solutions containing DOMis ongoing, and the results presented in this article provide, inpart, a baseline for the interpretation of those data. In this section,we interpret the angstrom-scale structure of the FA layer sorbedon the muscovite (001) surface and discuss why the film structurechanges as a function of pH and reaction time in relation toNOM sorption phenomena observed by others using approachesincluding batch experiments, size-exclusion chromatography(SEC) combined with ultraviolet (UV) spectroscopy, attenuatedtotal reflectance Fourier-transform infrared (ATR-FTIR) spec-troscopy, and AFM.

FA Sorption as a Function of pH. At pH e6, the electron-density profiles of solutions near the muscovite surface showedthat FA was distributed as a film that consists of a broad peaknear the muscovite surface and a broadly humped tailing pattern.The absence of any finer structure within the FA film reflects the

(62) Langmuir, D. Aqueous EnVironmental Geochemistry; Prentice-Hall: UpperSaddle River, NJ, 1997; p 600.

(63) Park, C.; Fenter, P. A.; Sturchio, N. C.; Regalbuto, J. R. Phys. ReV. Lett.2005, 94, 076104-1-4.

Table 2. Characteristics of FA Films Adsorbed per Unit Cell Area of the Muscovite Surface Calculated from the Total Electron-DensityProfiles a

total FA layer near-surface peak

sample thickness (Å) occupancy (Weq)c average layer density (FWeq)d occupancy (Weq)c

pH SeriespH 2 12.3 (12.0-12.6) 21.7 (19.3-24.5) 1.13 (1.03-1.25) 10.5 (9.0-12.2)pH 3.7b 7.2 (6.7-8.9) 11.5 (10.3-14.8) 1.02 (0.99-1.07) 6.3 (5.7-6.8)pH 6 6.4 (6.0-6.6) 11.0 (9.8-12.2) 1.10 (1.05-1.18) 4.2 (3.8-4.7)

Time Series3 hb 7.2 (6.7-8.9) 11.5 (10.3-14.8) 1.02 (0.99-1.07) 6.3 (5.7-6.8)12 h 8.4 (8.0-8.7) 14.9 (13.3-16.5) 1.14 (1.07-1.22) 7.8 (7.1-7.5)50 h 7.9 (7.6-8.2) 14.2 (13.4-15.2) 1.15 (1.13-1.19) 7.7 (7.3-8.1)500 h 8.2 (7.6-8.9) 14.1 (11.8-16.4) 1.10 (1.00-1.18) 7.8 (6.8-9.2)

a The numbers in parentheses are the ranges of the values calculated from the electron-density profiles generated using lower (-σ) and upper (+σ) limitsof occupancy parameters from the best-fit models. b Refit from Lee et al. 55 c Weq: water equivalent, with the electron density normalized to the number ofelectrons in one H2O molecule per AUC. d FWeq: average number of water equivalents normalized to that of bulk water in a 1-Å-thick layer over the unit-cellarea, AUC.

Figure 6. Occupancy, thickness, and average layer density of the FAlayer at pH 2, 3.7, and 6 calculated from the total electron-density profilesderived from the best-fit models. Lines connecting the points are guidesto the eye.

Figure 7. Derived electron-density profiles for the best-fit models of theinterfacial structure of muscovite in 100 mg kg-1 ESFA II solutions atpH 3.7 after 3, 12, 50, and 500 h. The electron density was normalizedto that of bulk water. The electron-density profile of the muscovitebelow 0 Å is not shown.

Figure 8. Occupancy, thickness, and average layer density of the totalFA layer as a function of reaction time calculated from the total electron-density profiles derived from the best-fit models. Lines connecting thepoints are to emphasize the trends with reaction time.

7824 Langmuir, Vol. 24, No. 15, 2008 Lee et al.

macromolecular characteristics of the FA film (i.e., the muchlarger molecular size than that of water). The characteristicelectron-density pattern that decreases away from the muscovitesurface might be explained by previous observations that thedistribution of the mass and size of FAs, which are mixtures ofvarious organic molecules extracted from natural waters andsoils, can be quantified by mathematical functions (e.g., log-normal distribution of the molecular weights of aquatic FAreported by Cabaniss et al.64). However, an internal structureshowing two distinct parts (broad peak and tail) cannot be solelydue to this effect (compare the electron-density profiles in Figure5a to the profile in Figure S5 of the Supporting Information) butmay be related to the fact that FA molecules contain a varietyof functional groups that can interact with muscovite in differentways. The broad peak near the muscovite surface can beinterpreted as FA fractions that adsorb directly on the muscovitesurface. The remainders of the directly sorbed FA moleculesmay then be contained in the broadly humped pattern that extendsto 6-12 Å, depending on the solution pH, from the muscovitesurface. Using X-ray reflectivity, Cheng et al. 52 observed thatwater molecules adsorbed directly on the muscovite surface arepositioned at 1.3(2) and 2.5(2) Å in deionized water. In FAsolutions, the distinct water peaks are absent, indicating that theportion of FA adsorbed directly on the muscovite surface islikely hydrophobic and repels water molecules from thesurface65,66 and/or the sorbed FA disrupts the ordered structureof sorbed water molecules on the muscovite surface.55 Theformation of the electron-dense peak near the surface could bealso related to the flattening of the FA molecules via sorptionon the muscovite. Flattening may occur as a result of confor-mational changes of sorbed molecules that enhance the stabilityof the molecules by increasing the number of functional groupsbonded on mineral surfaces67 and has often been observed invarious ex situ43,68,69 and in situ AFM studies.45,46,70

As pH increased from 2 f 3.7 f 6, the occupancy of thesurface FA peak decreased from 10.5(16)Weq f 6.3(5)Weq f4.2(4)Weq (Table 2). The enhanced electron density near themuscovite surface at lower pH indicates that more FA sorbeddirectly on the surface. The increased sorption of FA on themuscovite surface can be related to enhanced hydrophobicattraction at lower pH. At lower pH, highly concentratedhydronium ions (H3O+) would adsorb on the negatively chargedmuscovite surface and partially neutralize the surface to enhanceits hydrophobicity. Major functional groups, such as carboxylicand phenolic moieties, are neutralized by protonation, and FAmolecules become more hydrophobic as well.29,30,42,43,71 Also,at low pH protonated functional groups of FA, such as aminegroups, can be electrostatically attracted by the negatively chargedmuscovite surface, although the attraction should be weakerbecause the surface will be partially neutralized by the adsorbedhydronium ions. From XRD and dry combustion elementalanalyses on size-fractionated natural agricultural soil (from near

Waseca, MN), it was observed that HS sorbed on the fine clayfractions composed mostly of smectite and its interstratified phaseshad higher N contents than HS sorbed on coarse fractionsdominated by quartz.6 In addition, FA contains impurities suchas salts and mineral particles. Adsorption or precipitation ofelectron-dense cations or solid particles would also contributeto the electron density of the surface peak. Lee et al.55 observedthat Pahokee peat FA (IHSS) with a higher impurity content(4.61 wt %) formed a more electron-dense surface peak on themuscovite surface than ESFA II with a lower impurity content(1.00 wt %).

At pH g8.5, the electron-density profiles of the solutionsshowed no apparent broad distribution pattern near the muscovitesurface, which implies that there was no distinct continuous FAfilm structure on the muscovite surface under these pH conditions(Figure 5b). However, this result does not exclude the possibilityof FA molecules or aggregates sorbed individually on themuscovite surface. X-ray reflectivity is sensitive to the averageelectron-density distribution along the surface-normal direction.Therefore, individual FA particles that sorb sparsely on themuscovite surface do not contribute significantly to the totalelectron density and may not be distinguished by this method.Also, it must be kept in mind that the basal surfaces of manyphyllosilicates, which have permanent negative charge, may reactdifferently with DOM from edge surfaces that have pH-dependentcharge, so the lack of a continuous layer at high pH on themuscovite (001) surface may not represent the sorption structureon edge surfaces.

The first peak closest to the muscovite surface (1.71-1.76 Å)at pHg8.5 may correspond to the position of dissolved ions andwater molecules in ditrigonal cavities. This position is comparableto the positions of various cations sorbed directly in the ditrigonalcavities of the muscovite surface (i.e., as inner-sphere complexes),such as 1.67(6)-1.77(7) Å and 2.15(9)-2.16(2) Å for K+ andCs+, respectively,54 1.26(22) Å for Sr2+,53 and 1.98(2) Å forBa2+ .55 HS usually contain various metals, such as Al40 or Cuand Zn,72 as impurities. Assuming that the ash content (1.00 wt%) in ESFA II is mostly composed of soluble salts, the maximumconcentration of metal ions from the FA may be on the orderof 10-5 to 10-7 m in a 100 mg/kg ESFA II solution. Thisconcentration of metals would be too small to affect the interfacialstructure at pH 12 where highly concentrated Na+ (10-2 m fromNaOH used for pH adjustment) was present but would competewith Na+ (at a concentration of (4-6) × 10-5 m) for adsorptionat negatively charged sites on either the muscovite or FA at pH8.5-8.9. If other metal ions were to adsorb on the surface, thenthey would not be distinguished in these measurements fromwater molecules and/or Na+ within the limit of data resolution(0.5-0.6 Å). The decrease in the occupancy of the peak from2.06(12)Weq at pH 8.5 to 1.64(18)Weq at pH 12 may result fromthe substitution of more electron-dense metal ions from the FAby less electron-dense Na+ ions, used to adjust the pH to thehigher value, in ditrigonal cavities of the muscovite surface.

The second peak at 3.01(3)-3.18(2) Å from the muscovitesurface, may include sorbed FA. In that case, the position of thepeak implies that FA may be sorbed via bridging cationspositioned in ditrigonal cavities. At high pH, most FA will havea net negative charge, and the direct sorption of FA on thenegatively charged muscovite surface must be hindered byelectrostatic repulsion. Therefore, cation bridging may be themajor sorption mechanism, especially in the high-pH solutions

(64) Cabaniss, S. E.; Zhou, Q.; Maurice, P. A.; Chin, Y.-P.; Aiken, G. R.EnViron. Sci. Technol. 2000, 34, 1103–1109.

(65) Feng, X.; Simpson, A. J.; Simpson, M. J. Org. Geochem. 2005, 36, 1553–1566.

(66) Simpson, A. J.; Simpson, M. J.; Kingery, W. L.; Lefebvre, B. A.; Moser,A.; Williams, A. J.; Kvasha, M.; Kelleher, B. P. Langmuir 2006, 22, 4498–4503.

(67) Filius, J. D.; Lumsdon, D. G.; Meeussen, J. C. L.; Hiemstra, T.; vanRiemsdijk, W. H. Geochim. Cosmochim. Acta 2000, 64, 51–60.

(68) Namjesnik-Dejanovic, K.; Maurice, P. A. Colloids Surf., A 1997, 120,77–86.

(69) Wilkinson, K. J.; Balnois, E.; Leppard, G. G.; Buffle, J. Colloids Surf.1999, 155, 287–310.

(70) Plaschke, M.; Romer, J.; Klenze, R.; Kim, J. I. Colloids Surf., A 1999,160, 269–279.

(71) Buffle, J.; Mota, A. M.; Simoes Goncalves, M. L. S. J. Electroanal.Chem. 1987, 223, 235–262.

(72) Raspor, B.; Nurnberg, H. W.; Valenta, P.; Branica, M. Mar. Chem. 1984,15, 217–230.

FulVic Acid Sorption on MuscoVite Mica Langmuir, Vol. 24, No. 15, 2008 7825

that contains significant amounts of Na+.73 However, thedistribution width of this second peak is ∼0.6(1) Å, correspondingto a fwhm of 1.5(2) Å, which may be too narrow for themacromolecules of FA. If this peak corresponds to FA, then themolecules must be small or have a flattened morphology (e.g.,ring shape with much larger lateral diameter than thickness).Although FA molecules can be flattened, it is unlikely that theyform such a sharp electron-density peak because this would requirethat all atoms of the various functional groups in polydisperseFA molecules be positioned in a narrow range of distance fromthe muscovite surface.

The flattened structure that is inferred for the FA moleculesat pH 2-6 is different from this configuration observed at higherpH. The electron-density profiles of the sorbed FA at lower pHsclearly show the separation of two distinct partssa directly sorbedpart and a remainder partsand the total FA layer is broadlydistributed over a relatively wide range of distance (6.4-12.3Å) from the surface. It is possible that the second peak observedin the higher pH data might be broader than the peak derivedfrom the best-fit model, but that it appears asymmetric and thuswould not have been fully describable by the symmetric Gaussianpeak used in the model. To test this, we attempted to fit the databy simulating the second peak using 10 inter-related Gaussianpeaks (i.e., the broad profile of the generic FA model) insteadof a single Gaussian peak. The fit gave a large decay constantfor the FA occupancy (λFA) resulting in the formation of a singleGaussian-like shape (e.g., when λFA ) 4, the occupancy of thesecond peak of the interdependent FA peaks [i.e., cFA,2 ) cFA,1

exp(–λFA)] is less than 2% of that of the first peak). This testresult implies that the second peak is more likely composed ofsmaller aqueous species, such as water molecules, rather thanlarger FA molecules.

The calculated thickness of the FA layer at pH 2 is 12.3 Åwhereas the thickness ranges from 6.2 to 7.2 Å at pH 3.7-6. Thethickness of the FA film at pH 2 is about twice that at pH 3.7and 6, which implies the formation of a multilayer. Multilayersmay form at lower pH because of an increased attractive forcebetween FA molecules (i.e., hydrophobic effect).28,41 In solution,de Wit et al.74 estimated that the diameters of hydrated FAmolecules, assuming a spherical model, ranged from 12 to 17Å. On the basis of the viscosity measurement by Avena et al.75

the calculated diameter of hydrated Laurentian FA ranged from26 to 42 Å. The thickness of the FA layer determined from thisstudy is comparable to but a little thinner than the estimated sizesof the hydrated FAs in solution. The difference is likely dueagain to the flattening of FA molecules upon sorption to themuscovite surface. Using tapping-mode AFM in 0.01 M CaCl2

and ae100 mg C/L aquatic HS solution at pH ∼5, Maurice andNamjesnik-Dejanovic45 observed that HS molecules and ag-gregates sorbed on the muscovite surface had a flattened ringshape with a diameter of 491(97) Å and a height of 35(17) Å.Using the same technique, they also observed that a single NOMmolecule sorbed on the muscovite surface in a 0.1 M LiCl and25 mg C/L freshwater wetland NOM solution at pH 3 had aflattened spherical shape with a mean lateral diameter of 34 Åand a mean thickness of 6 Å.46 The thickness of a single FAmolecule measured from their AFM studies is in good agreementwith the thicknesses of the FA layers determined in this study.

However, it should be noted that the thicknesses determinedhere may not correspond to the thickness of single molecules.Because X-ray reflectivity yields an average electron-densitydistribution at an interface, it is difficult to distinguish individualFA molecules (or aggregates) using this technique if they aresparsely distributed on the muscovite surface. Plaschke et al.70

reported that the coverage of individual HA particles sorbed onthe muscovite surface at pH 3-8 ranged from 5 to ∼10% in 0.1M NaCl and 100 mg/L HA solutions using tapping-mode AFM.With X-ray reflectivity, individual particles with such lowcoverage may be indistinguishable from bulk water when theyare averaged with similar electron-dense water molecules fillingthe space between the particles. Therefore, we have defined thethickness in this study to represent sorbed FA as a continuousfilm rather than the average height of individual particles. TheAFM study by Gibson et al.47 showed that discrete particles,which are described in most AFM studies as HS sorbed on themuscovite surface, were located on the top of a coherent film andnot on the original muscovite surface at pH 2 and 4.9. They alsoobserved that the thickness of the film changed as a function ofpH. After 30 min of reaction in Suwannee River HA solutionat pH 2, the muscovite surface was covered by ∼36 Å of the HAfilm. This is thicker than the FA film determined from the electron-density profile at pH 2 (12.3 Å). The difference may be due tothe types of HS used. The HA studied by Gibson et al.47 is morehydrophobic and easily coagulated and/or precipitated at low pHthan the FA used in this study.76–78 Moreover, the AFMmeasurements were conducted in air, and drying may alter theHA structure on the surface. However, both studies show thatthicker organic films can form at lower pH, probably as a resultof the enhanced hydrophobicity and electrostatic attraction. Athigher pH, the thickness of the film determined by Gibson etal.47 decreased as pH increased, and the film structure disappearedat pHg9. This trend is similar to the change in the film thicknessdetermined in this study. The absence of film structure at higherpH implies that the electrostatic repulsion between the negativelycharged muscovite surface and negatively charged functionalgroups in HS reduces the amount of HS sorbed on the muscovitesurface. Furthermore, at higher pH, the increased repulsive forcebetween negatively charged HS molecules will cause thedispersion of HS into small particles79 and, as a result, hamperthe formation of film structure, which presumably requires theaggregation and networking of HS molecules, on the muscovitesurface. Accompanying the decrease in FA film thickness, thedecrease in the amounts of sorbed FA with increasing pH maybe the result of an increased repulsive force.20,42,77

On the basis of the occupancy of the FA layer, the total massdensity of the sorbed species within the layer was calculated.The calculated value can be comparable to the number of sorbedFA molecules when the layer is composed of mostly FA andmost sorbed FA molecules are confined within the layer. In otherwords, the calculated amounts of FA could include some numberof water molecules incorporated within the layer because thedata were measured in solution. Also, as mentioned before, thecalculated values may not fully include the electron densitiesfrom sparsely distributed large FA molecules or aggregates, partsof which extend farther away from the muscovite surface thanthe upper boundary defined on the basis of the total electron-

(73) Sposito, G. The Chemistry of Soils; Oxford University Press: New York,1989; p 277.

(74) de Wit, J. C. M.; van Riemsdijk, W. H.; Koopal, L. K. EnViron. Sci.Technol. 1993, 27, 2005–2014.

(75) Avena, M. J.; Vermeer, A. W. P.; Koopal, L. K. Colloids Surf. 1999, 151,213–224.

(76) Hayes, M. H. B.; MacCarthy, P.; Malcolm, R. L.; Swift, R. S. HumicSubstances II: In Search of Structure; John Wiley & Sons: Chichester, U.K.,1989; pp 3-31.

(77) Schroth, B. K.; Sposito, G. EnViron. Sci. Technol. 1998, 32, 1404–1408.(78) Shin, H. S.; Rhee, S. W.; Lee, B. H.; Moon, C. H. Org. Geochem. 1996,

24, 523–529.(79) Myneni, S. C. B.; Brown, J. T.; Martinez, G. A.; Meyer-Ilse, W. Science

2000, 286, 1335–1337.

7826 Langmuir, Vol. 24, No. 15, 2008 Lee et al.

density profile. The occupancy of the FA layers calculated fromthe total electron-density profiles ranges from 11.0Weq to 21.7Weq

(Table 2) corresponding to 0.70-1.39 mg/m2. This range ofvalues is in agreement with the amount of NOM sorbed ontovarious mineral powders as calculated from ex situ batch sorptionexperiments (Table 3). However, direct comparison of thequantities is difficult because the powdered minerals have multiplecrystallographic sites, various compositions (e.g., oxides vssilicates) and different surface properties, (e.g., specific surfaceareas or pH values of point of zero charge, pHpzc) and werereacted with different kinds of NOMs (e.g, HAs or FAs) fromdifferent sources (e.g., surface waters, soil, or peat).

FA Sorption as a Function of Reaction Time. The electron-density profiles derived from the best-fit models show that theoverall structure of the FA film remains relatively constant after

3 h (Figure 7). This suggests that FA forms a stable film on themuscovite surface in at most 3 h. However, the electron-densitypattern within the FA film shows that the internal film structurechanges over time. The change in internal structure may occurbecause of the slow adsorption rate of FA on the muscovitesurface. On the basis of the pK1 value (3.67) of the FA in thesolutions at pH 3.7, about 50% of the carboxylic functional groupsmay be deprotonated and possess negative charge. Becausemuscovite has a permanent negative charge on the cleaved basalsurface, adsorption may be retarded by the electrostatic barriergenerated from repulsive forces between FA and the muscovitesurface.82 Namjesnik-Dejanovic and Maurice46 observed that atleast 2 h of equilibration time was required to obtain stable AFMimages of HS sorbed on the muscovite surface at pH 3-11. Theincrease in the occupancy of the surface FA peak from 3 to 12 hindicates that the interface may not be fully equilibrated with the

(80) Hur, J.; Schlautman, M. A. J. Colloid Interface Sci. 2003, 264, 313–321.(81) Stumm, W.; Morgan, J. J. Aquatic Chemistry, 3rd ed.; John Wiley and

Sons: New York, 1996. (82) Avena, M. J.; Koopal, L. K. EnViron. Sci. Technol. 1999, 33, 2739–2744.

Table 3. Reported Values of the Amounts of NOM Sorbed onto Various Mineral Powders from Batch Sorption Experiments

NOMa minerals (pHPZC)b electrolytes pH reaction time (h) amount sorbed (mg/m2)

HS from surface Esthwaite water20 goethite (8.4) 2 mM NaCl 5.0 16 3.8d

5.5 3.4d

6.0 3.0d

6.5 2.4d

7.0 1.9d

7.5 1.5d

8.0 0.5d

8.5 0.6d

goethite (8.2) 2 mM NaCl 7.0 2.0d

goethite (7.0) 2 mM NaCl 7.1 2.4d

hematite (6.3) 2 mM NaCl 6.9 0.2d

peat HS from Aldrich Chemie, Germany41 γ-alumina (7.4) 0.1 M NaNO3 3.3 20 4.95e

9.7 2.00e

SRHA31 γ-alumina (8.4) 1 mM NaCl 4.0 24 1.15d

7.0 0.36d

10.0 0.11d

10 mM NaCl 4.0 1.10d

7.0 0.48d

10.0 0.18d

0.1 M NaCl 4.0 1.25d

7.0 0.70d

10.0 0.31d

7.0 1.07d

10.0 0.99d

1 mM NaCl 4.0 0.77d

10 mM NaCl 4.0 0.77d

0.1 M NaCl 4.0 0.84d

HA from Aldrich, U.K.30 vermiculite (∼2.5)c 2.0 48 2.9d,f

4.0 2.5d,f

6.0 2.5d,f

8.0 1.8d,f

10.0 1.6d,f

12.0 3.7d,f

SR-NOM39 iron oxide (7.5) 10 mM NaCl 4.0 not given 0.88d,g

SRFA42 goethite (7.0-7.5) 10 mM NaCl 5.8 24 1.61d,g

SR-NOM40 goethite (∼7.5)c 10 mM NaCl 4.0 24 0.60d,g

kaolinite (5.3) 0.20d,g

swamp FA from VA40 goethite (∼7.5)c 0.50d,g

kaolinite (5.3) 0.16d,g

peat HS80 kaolinite (4.2) 0.1 M NaCl 7.0 72 0.23d,g

hematite (8.2) 7.0 0.84d,g

SRFA80 kaolinite (4.2) 7.0 0.09d,g

hematite (8.2) 7.0 0.64d,g

SRFA28 boemite (9.4) 10 mM NaCl 3.0 48 0.36f

5.0 0.36f

7.0 0.31f

9.0 0.21f

11.0 0.12f

a NOM (natural organic matter), HS (humic substances), SRHA (Suwannee River humic acid), HA (humic acid), SR-NOM (Suwannee River natural organicmatter), SRFA (Suwannee River fulvic acid), and FA (fulvic acid). b pHpzc: pH of point of zero charge. c values from Stumm and Morgan.81 The pHpzc ofvermiculite was estimated to be similar to that of montmorillonite. d The Langmuir isotherm was used for the calculation. e The Frumkin-Fowler-Guggenheim(FFG) isotherms were used for the calculation. f Recalculated from the graphical data. The sorption data from Zhou et al.30 did not reach the full surface coverage.g Recalculated with an assumption that the weight fraction of C is 50% of the total mass of NOM.

FulVic Acid Sorption on MuscoVite Mica Langmuir, Vol. 24, No. 15, 2008 7827

FA solution within 3 h. On the basis of the X-ray reflectivitymeasurements, Lee et al.55 observed that Ba ions could passthrough a FA coating and adsorb directly to the muscovite surfaceafter prereaction in a 100 mg kg-1 ESFA II solution for 2 h,which might indicate incomplete coverage of the surface by FAafter only 2 h of reaction.

If the muscovite surface is not fully covered with FA moleculesin a solution, then areas lacking FA will be covered with watermolecules. The electron density of the surface peak at a giventime will correspond to the total number of electrons from bothwater and FA molecules per unit cell area. In this case, the electrondensity of the surface peak at a given time, csurf (t), can beexpressed as

csurf(t)) cFA,surf(1- e-kt)+ cW,surfe-kt (4)

where cFA,surf and cW,surf are the occupancies (Weq) of FA andwater molecules at full surface coverage by each species,respectively, k is a rate constant (h-1), and t is a reaction time(h). Under the initial condition (t ) 0) before reaction with FA,only water molecules will be present on the muscovite surface.In this state, the total electron density from water molecules thatare directly sorbed on the muscovite surface is assumed to be4.6(6)Weq, which is the amount sorbed in both ditrigonal cavitiesand surface oxygen sites per AUC.52 The fractions of FA andwater molecules and the rate constant were obtained using aleast-squares fitting procedure, and the plot of calculated electrondensity of the surface peak is shown in Figure 9. It should benoted that only five data points (t ) 0, 3, 12, 50, and 500 h) wereused to estimate three parameters (cFA,surf, cW,surf, and k). Theresults yield cFA,surf and cW,surf values of 7.8(1)Weq and 4.6(2)Weq,respectively, and a rate constant, k, of 0.26(6) h-1. From thesevalues, it is estimated that about 50% of the muscovite surfaceis covered with FA after ∼3 h. In a 100 mg kg-1 FA solutionat pH 3.7, at least 9 h of reaction is required to obtain more than90% of the FA coverage on the muscovite surface.

The fractionation of FA molecules by sorption on the muscovitesurface can also enhance the electron density of the surface peak.This fractionation can occur because of the heterogeneous andpolydispersecharacteristicsofFA.Thesorptionof smallermoleculesmaybekinetically favoredbecausefasterdiffusionmayallowsmallermolecules to approach mineral surfaces more easily whereas thesorption of larger molecules may be thermodynamically favoredbecause they may have more functional groups that are availableto bind with the surfaces simultaneously.20,29,31,38–42 Cumulativeeffects of multiple-site binding to the surfaces can be large and

adsorption can be irreversible because of the improbability thatall bonds break simultaneously.83,84 The fractionation of organicmatter by sorption to minerals has often been observed in previousstudies. Results from batch laboratory experiments by Ochs etal.41 showed that the surface coverage of HS sorbed on γ-aluminadecreased from 20 to 110 h, indicating the redistribution of HSon the surface. High-pressure size-exclusion chromatography(HP-SEC) results showed that higher-molecular-weight, morehydrophobic, and more aromatic NOM components are pref-erentially sorbed to goethite and kaolinite at pH 4 after 24 h ofreaction.40 From adsorption rate experiments combined with anSEC system, Hur and Schlautman80 observed that an initial rapiduptake of smaller HS molecules by kaolinite and hematite wasfollowed by slow substitution of the smaller molecules withlarger HS molecules. They observed that sorption fractionationof a peat HA on kaolinite at pH 7 continued even after 120 hwhereas the total amount of sorbed HA reached an equilibriumstate within 24 h. Therefore, a slow sorption rate of FA on themuscovite surface and the substitution of smaller, less-electron-dense FA molecules with larger, more-electron-dense FAmolecules may result in a change in occupancy of the surfaceFA peak with time. The low-q data measured after 500 h ofreaction showed a difference in intensity from those measuredafter 50 h of reaction (e.g., data at q ) 0.79 Å-1 in Figure 4a),suggesting that the interfacial structure may not be equilibratedeven after the longest reaction time applied in this study. Thedynamic characteristics of FA sorption are expected to lead toa continuous change in the film structure, which is observed hereas a change in the electron-density pattern within the layer (Figure7).

Summary

Our results show that in situ X-ray reflectivity can providedirect and quantitative information on the structural characteristicsof NOM films sorbed on the basal surface of muscovite micawith angstrom-scale vertical resolution. This method is nonde-structive and therefore adequate for the study of soft NOM orother organic films on solid surfaces.

Ourexperimental systemrepresentsnaturalenvironments inwhichrelatively fresh mineral surfaces are exposed to natural waterscontaininghighlyconcentratedDOM.Muscovite isaprimarymineralthat weathers from igneous and metamorphic rocks and occurs asa detrital phase in sediments and soils. It is also analogous to manysecondary clay minerals, such as illite, vermiculite, and smectite,in terms of its composition, structure, and charged surface. Theexperimental systems at acidic to neutral pH values may representfreshwater wetland environments or fast-flowing pore waters inshallow soil where the background electrolyte concentrations arerelatively low. The systems at high pH could be similar to organic-rich estuarine environments or shallow groundwaters in contactwith mafic or ultramafic rocks.

At pH 2-6, we observed that FA sorbed on the muscovitesurface as relatively thin films (6 to 12 Å thick) with an angstrom-scale structure consisting of one electron-dense near-surface peakfollowed by a broad less-electron-dense region. The layerdescribed by this electron-density pattern decreased in boththickness and occupancy until it could no longer be observed atpH g8.5, where electrostatic repulsion between FA moleculesand the muscovite surface would hinder the formation of acontinuous film. Observations of the film structure at pH 3.7 asa function of time showed that FA maintained a relatively constant

(83) Collins, M. J.; Bishop, A. N.; Farrimond, P. Geochim. Cosmochim. Acta1995, 59, 2387–2391.

(84) Henrichs, S. M. Mar. Chem. 1995, 49, 127–136.

Figure 9. Change in the electron density of the FA surface peak as afunction of reaction time. The data at t ) 0 h52 is not shown. The dashedline shows the calculated electron density of the FA surface peak fromthe best-fit result using eq 4.

7828 Langmuir, Vol. 24, No. 15, 2008 Lee et al.

thickness of about 7 to 9 Å after 3 to 500 h of reaction. Althoughthe total thickness of the layer reached an apparent equilibriumstate after 3 h, the coverage of the FA surface peak appeared tostabilize only after ∼10 h, and the distribution of electron densitywithin the entire film continued to change for at least 500 h.These internal changes may be related to the condensation andfractionation of FA molecules, which occur relatively slowly.The results demonstrate the utility of in situ X-ray reflectivityin characterizing adsorbed complex natural organic films includingtheir thickness and internal structures, which change at theangstrom-scale level while equilibrating with both the surfaceand solution. Structures of sorbed organic films on mineralsurfaces are important to understand because they may affect theextent and rate of other mineral-water reactions, including ionadsorption, dissolution, and precipitation.

Acknowledgment. This research was supported by theGeosciences Research Program, Office of Basic Energy Sciences,United States Department of Energy under grant DE-FG02-06ER15364 to the University of Illinois at Chicago and contractDE-AC02-06CH11357 to Argonne National Laboratory. Partial

support was provided by the National Science Foundation undergrant EAR-0455938 to the University of Illinois at Chicago. TheX-ray reflectivity data were measured at beamline 11-ID-D ofthe Basic Energy Sciences Synchrotron Radiation Center(BESSRC), Advanced Photon Source (APS). Use of the AdvancedPhoton Source was supported by the U.S. Department ofEnergy, Office of Science, Office of Basic Energy Sciences,under Contract DE-AC02-06CH11357 to UChicago Argonne,LLC, as operator of Argonne National Laboratory. Thoughtfulcomments from reviewers and the senior editor were used inrevising the manuscript.

Supporting Information Available: X-ray reflectivity at thebulk and surface fiducials as a function of measurement time, comparisonof the electron-density profiles from the best-fit models using two differentmodel approaches, fractional change in X-ray reflectivity as a functionof pH and reaction time, comparisons of the structure factors calculatedfrom the best-fit models, and calculated electron-density profile of thesorbed FA molecules with a log-normal mass distribution. This materialis available free of charge via the Internet at http://pubs.acs.org.

LA703456T

FulVic Acid Sorption on MuscoVite Mica Langmuir, Vol. 24, No. 15, 2008 7829