influence of humic substance adsorptive fractionation on pyrene partitioning to dissolved and...

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Influence of Humic Substance Adsorptive Fractionation on Pyrene Partitioning to Dissolved and Mineral-Associated Humic Substances JIN HUR AND MARK A. SCHLAUTMAN* Departments of Environmental Engineering & Science and Geological Sciences, School of the Environment, Clemson University, Clemson, South Carolina 29634-0919 and Institute of Environmental Toxicology, Clemson University, Pendleton, South Carolina 29670 Changes in pyrene binding by dissolved and mineral- associated humic substances (HS) due to HS adsorptive fractionation processes were examined in model envi- ronmental systems using purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). For PAHA, carbon- normalized pyrene binding coefficients for nonadsorbed, residual fractions (K oc (res)) were different from the original dissolved PAHA K oc value (K oc (orig)) prior to contact with the mineral suspensions. A strong positive correlation between pyrene log K oc (res) and log weight-average molecular weight (MW w ) for residual PAHA fractions was observed, which was relatively independent of the specific mineral adsorbent used and hypothesized fractionation processes. A strong positive correlation between log K oc (ads) and log MW w was also found for PAHA fractions adsorbed to kaolinite at low mass fraction organic carbon levels, although the relationship was statistically different from the one found with residual PAHA fractions. The same trends and correlations found for PAHA were not observed with SRFA, suggesting that the impacts of HS adsorptive fractionation on changes in hydrophobic organic contaminants binding are also influenced by the source and other biogeochemical characteristics of HS. Introduction Binding/partitioning of hydrophobic organic contaminants (HOCs) to dissolved and/or mineral-associated humic sub- stances (HS) often governs their transport, reactivity, bio- availability, and ultimate fate in natural and engineered environmental systems (1-3). In many of these systems, organic carbon-normalized binding/partition coefficients (Koc) are critical factors in determining HOC distributions. Wide variations in Koc values for a given HOC have been observed for different dissolved bulk HS materials, and numerous studies have been conducted in the hopes of accounting for these variations based on bulk HS physico- chemical descriptors (4-9). However, it is questionable whether a single Koc value based on the original dissolved bulk HS is adequate to provide accurate representations of systems where HS adsorption by minerals leads to increased system complexities. For example, observed differences between HOC partitioning to dissolved versus adsorbed HS have been hypothesized as resulting from HS conformational changes that occur upon their adsorption to mineral surfaces (10-14) and/or because of HS adsorptive fractionation brought about by the preferential sorption of particular HS components (10, 13, 14). Molecular weight (MW) fractionation of HS due to selective adsorption by minerals can be monitored with size exclusion chromatography (SEC). For example, previous studies using SEC have shown deviations in the weight- average MW (MWw) values of residual (i.e., nonadsorbed) HS fractions remaining in solution versus the original HS MWw, demonstrating the preferential adsorption of certain size fractions within a bulk material (15-17). Several reports have suggested HS adsorptive fractionation as a possible explanation for the differences in Koc values measured for various HS fractions, such as the Koc value observed with mineral-associated HS, the original Koc value based on the dissolved bulk HS, and the Koc value for residual HS remaining in solution after HS adsorption. For example, Garbarini and Lion (10) reported that Koc values for trichloroethylene and toluene partitioning to a dissolved commercial humic acid were more than 2.5 times higher than their respective values determined for Al2O3 coated with the same source material and suggested selective HS adsorption as a possible expla- nation for their results. Jones and Tiller (14) found that supernatant solutions of nonadsorbed fractions of a soil humic acid yielded lower phenanthrene Koc values than did the original dissolved bulk material and also speculated that preferential adsorption of higher MW HS fractions were responsible for their observations. However, neither study cited above actually examined whether HS adsorptive fractionation was operative in their experimental systems. Using a small subset of terrestrial (IHSS soil humic acid and purified Aldrich humic acid) and aquatic HS (IHSS Suwannee River humic and fulvic acids), Hur and Schlautman (18) recently demonstrated that completely adsorbing these materials onto mineral surfaces significantly changed their pyrene Koc values relative to their original respective dissolved Koc values. In other words, in the absence of HS adsorptive fractionation effects, the process of adsorbing the different HS materials onto mineral surfaces changed their ability to bind pyrene. Parallel experiments with ultrafiltration fractions obtained from the purified commercial humic acid gave similar results (18). For both the ultrafiltration fractions and different HS materials, Hur and Schlautman (18) found strong positive (but different) correlations between pyrene log Koc values and log MWw for the originally dissolved and sub- sequently adsorbed HS materials. Therefore, even though Hur and Schlautman’s overall experimental approach mini- mized HS adsorptive fractionation effects with respect to pyrene partitioning, their results suggest that HOC binding by adsorbed HS will be affected by sorption processes that are selective with respect to the size of HS components (18). Because of the paucity of studies examining possible effects of HS adsorptive fractionation on HOC partitioning, additional knowledge is required to better understand these heterogeneous environmental systems. The overall objective of the present study was to investigate possible relationships between pyrene Koc values for adsorption-fractionated dis- solved and mineral-associated HS and their corresponding MWw values. Although solution chemistry is likely to influence both HS adsorptive fractionation and HOC partitioning (e.g., refs 9, 12, and 14), we chose to utilize only one fixed solution condition (0.1 M NaCl, pH 7) in the present study to focus This paper is part of the Walter J. Weber Jr. tribute issue. * Corresponding author phone: (864)656-4059; fax: (864)656-0672; e-mail: [email protected]. Environ. Sci. Technol. 2004, 38, 5871-5877 10.1021/es049790t CCC: $27.50 2004 American Chemical Society VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5871 Published on Web 10/14/2004

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Influence of Humic SubstanceAdsorptive Fractionation on PyrenePartitioning to Dissolved andMineral-Associated HumicSubstances†

J I N H U R A N D M A R K A . S C H L A U T M A N *

Departments of Environmental Engineering & Science andGeological Sciences, School of the Environment,Clemson University, Clemson, South Carolina 29634-0919 andInstitute of Environmental Toxicology, Clemson University,Pendleton, South Carolina 29670

Changes in pyrene binding by dissolved and mineral-associated humic substances (HS) due to HS adsorptivefractionation processes were examined in model envi-ronmental systems using purified Aldrich humic acid (PAHA)and Suwannee River fulvic acid (SRFA). For PAHA, carbon-normalized pyrene binding coefficients for nonadsorbed,residual fractions (Koc(res)) were different from the originaldissolved PAHA Koc value (Koc(orig)) prior to contactwith the mineral suspensions. A strong positive correlationbetween pyrene log Koc(res) and log weight-averagemolecular weight (MWw) for residual PAHA fractions wasobserved, which was relatively independent of thespecific mineral adsorbent used and hypothesizedfractionation processes. A strong positive correlation betweenlog Koc(ads) and log MWw was also found for PAHAfractions adsorbed to kaolinite at low mass fraction organiccarbon levels, although the relationship was statisticallydifferent from the one found with residual PAHA fractions.The same trends and correlations found for PAHA werenot observed with SRFA, suggesting that the impacts of HSadsorptive fractionation on changes in hydrophobicorganic contaminants binding are also influenced by thesource and other biogeochemical characteristics of HS.

IntroductionBinding/partitioning of hydrophobic organic contaminants(HOCs) to dissolved and/or mineral-associated humic sub-stances (HS) often governs their transport, reactivity, bio-availability, and ultimate fate in natural and engineeredenvironmental systems (1-3). In many of these systems,organic carbon-normalized binding/partition coefficients(Koc) are critical factors in determining HOC distributions.Wide variations in Koc values for a given HOC have beenobserved for different dissolved bulk HS materials, andnumerous studies have been conducted in the hopes ofaccounting for these variations based on bulk HS physico-chemical descriptors (4-9). However, it is questionablewhether a single Koc value based on the original dissolvedbulk HS is adequate to provide accurate representations ofsystems where HS adsorption by minerals leads to increased

system complexities. For example, observed differencesbetween HOC partitioning to dissolved versus adsorbed HShave been hypothesized as resulting from HS conformationalchanges that occur upon their adsorption to mineral surfaces(10-14) and/or because of HS adsorptive fractionationbrought about by the preferential sorption of particular HScomponents (10, 13, 14).

Molecular weight (MW) fractionation of HS due toselective adsorption by minerals can be monitored with sizeexclusion chromatography (SEC). For example, previousstudies using SEC have shown deviations in the weight-average MW (MWw) values of residual (i.e., nonadsorbed)HS fractions remaining in solution versus the original HSMWw, demonstrating the preferential adsorption of certainsize fractions within a bulk material (15-17). Several reportshave suggested HS adsorptive fractionation as a possibleexplanation for the differences in Koc values measured forvarious HS fractions, such as the Koc value observed withmineral-associated HS, the original Koc value based on thedissolved bulk HS, and the Koc value for residual HS remainingin solution after HS adsorption. For example, Garbarini andLion (10) reported that Koc values for trichloroethylene andtoluene partitioning to a dissolved commercial humic acidwere more than 2.5 times higher than their respective valuesdetermined for Al2O3 coated with the same source materialand suggested selective HS adsorption as a possible expla-nation for their results. Jones and Tiller (14) found thatsupernatant solutions of nonadsorbed fractions of a soilhumic acid yielded lower phenanthrene Koc values than didthe original dissolved bulk material and also speculated thatpreferential adsorption of higher MW HS fractions wereresponsible for their observations. However, neither studycited above actually examined whether HS adsorptivefractionation was operative in their experimental systems.

Using a small subset of terrestrial (IHSS soil humic acidand purified Aldrich humic acid) and aquatic HS (IHSSSuwannee River humic and fulvic acids), Hur and Schlautman(18) recently demonstrated that completely adsorbing thesematerials onto mineral surfaces significantly changed theirpyrene Koc values relative to their original respective dissolvedKoc values. In other words, in the absence of HS adsorptivefractionation effects, the process of adsorbing the differentHS materials onto mineral surfaces changed their ability tobind pyrene. Parallel experiments with ultrafiltration fractionsobtained from the purified commercial humic acid gavesimilar results (18). For both the ultrafiltration fractions anddifferent HS materials, Hur and Schlautman (18) found strongpositive (but different) correlations between pyrene log Koc

values and log MWw for the originally dissolved and sub-sequently adsorbed HS materials. Therefore, even thoughHur and Schlautman’s overall experimental approach mini-mized HS adsorptive fractionation effects with respect topyrene partitioning, their results suggest that HOC bindingby adsorbed HS will be affected by sorption processes thatare selective with respect to the size of HS components (18).

Because of the paucity of studies examining possibleeffects of HS adsorptive fractionation on HOC partitioning,additional knowledge is required to better understand theseheterogeneous environmental systems. The overall objectiveof the present study was to investigate possible relationshipsbetween pyrene Koc values for adsorption-fractionated dis-solved and mineral-associated HS and their correspondingMWw values. Although solution chemistry is likely to influenceboth HS adsorptive fractionation and HOC partitioning (e.g.,refs 9, 12, and 14), we chose to utilize only one fixed solutioncondition (0.1 M NaCl, pH 7) in the present study to focus

† This paper is part of the Walter J. Weber Jr. tribute issue.* Corresponding author phone: (864)656-4059; fax: (864)656-0672;

e-mail: [email protected].

Environ. Sci. Technol. 2004, 38, 5871-5877

10.1021/es049790t CCC: $27.50 2004 American Chemical Society VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5871Published on Web 10/14/2004

on how HS adsorptive fractionation and/or HS conforma-tional changes upon their adsorption to mineral surfaceswould affect subsequent pyrene partitioning. Special em-phasis was given to systems having low mass fraction organiccarbon ( foc) levels of adsorbed HS, because adsorptivefractionation effects are maximized under these conditions(17). In addition, the results of Murphy et al. (11) suggest thatdifferences in HOC partitioning to dissolved versus mineral-associated HS will be most important at low foc levels. Carewas taken in the present study to ensure that phase separationdid not lead to disruption of the equilibrium conditionsestablished during the initial sorption equilibration period(s)(10, 12, 19). For example, Laor et al. (13) observed that whenresidual HS were removed from their experimental systems,measurable concentrations of the previously mineral-boundHS (2-6 mg of C/L) were released back into solution duringtheir subsequent HOC sorption equilibration step. Therefore,to avoid problems of this type, pyrene partitioning to thevarious HS compartments was evaluated without removingresidual HS from the HOC-HS-mineral systems to minimizepotential disruption of system equilibria.

Experimental SectionMaterials. Purified Aldrich humic acid (PAHA) and SuwanneeRiver fulvic acid (SRFA) were selected for use as a terrestrialand aquatic HS, respectively. Pyrene (Fluka, 99.5+% purity)was selected as the model HOC. Kaolinite and hematite wereobtained from Sigma and Alfa, respectively, and used withoutfurther treatment. Physicochemical characteristics of allmaterials used have been previously reported (9, 17).

Analytical Methods. A total organic carbon (TOC) analyzer(Shimadzu model 5050) was used to quantify HS concentra-tions in the aqueous phase. A spectrophotometer (Beckmanmodel DU640) was used to measure absorbances forcalculating HS specific ultraviolet absorbance (SUVA) valuesand pyrene fluorescence inner-filter corrections (9, 19).Pyrene concentrations in aqueous solutions or hexaneextracts were quantified by a luminescence spectropho-tometer (Perkin-Elmer, LS-5B) using external standards. Theexcitation/emission wavelengths (nm/nm) for pyrene were336/373, and slits were set for bandwidths of 3 nm forexcitation and 5 nm (in aqueous solutions) or 20 nm (inhexane extracts) for emission. Relative precisions of 1 and3% were routinely obtained for absorbance/fluorescence andTOC measurements, respectively. Size exclusion chroma-tography (SEC) with ultraviolet detection at 254 nm was usedto determine apparent MWw values of dissolved/residual HSsamples, generally following the methodology recommendedby Zhou et al. (20). Additional details for the SEC measure-ments have been reported previously (9, 17). Because MWw

values for PAHA samples determined with our SEC systemare dependent on their concentrations, all PAHA SECchromatograms were extrapolated to a concentration of 0mg/L to obtain consistent, infinite dilution MWw values (17).

Pyrene Partitioning to Residual HS. A conceptual modelof pyrene partitioning and a flowchart of the experimentalprocedures followed in this study are presented in Figure 1.Detailed experimental procedures for HS adsorption bykaolinite and hematite have been reported previously (17,18). Pyrene Koc values with the residual HS, designated hereas Koc(res), were determined using the supernatant HSsolutions resulting from the HS adsorption experiments. Afixed concentration (80 µg/L) of pyrene was spiked into eachresidual HS sample. Due to the limited concentrations andvolumes of residual HS samples, a modified fluorescencequenching technique was utilized to determine Koc(res)instead of the more typical Stern-Volmer analysis (4, 9, 19),although both approaches are based on the assumption thatfluorescence intensity is proportional to the concentrationof free pyrene (i.e., pyrene not associated with HS). Pre-

liminary studies suggested that Koc(res) values for PAHA mightshow a dependency on its concentration with the presentmethod, so all samples with a high concentration of PAHAwere diluted to 3 mg of C/L prior to adding pyrene, thusensuring that the measured Koc(res) values would be com-parable across a wide PAHA concentration range. Koc valuesdetermined with the present technique matched those resultsobtained from Stern-Volmer analyses that used 3 mg of C/Las the maximum HS concentration (data not shown). Allsamples were mixed on a reciprocating shaker at low speedfor 20 min to attain equilibrium (12, 19, 21) before removingaliquots for fluorescence and absorbance measurements.

Residual HS-associated pyrene, freely dissolved pyreneand residual HS concentrations were all quantified separatelyto determine Koc(res) values with the following equation (19):

where [pyr-HS] is the HS-associated pyrene concentration(µg/L), [pyr]free is the freely dissolved pyrene concentration(µg/L), and [HS] is the dissolved residual HS concentration(mg of C/L). For this study, concentrations of freely dissolvedpyrene in HS samples were determined by fluorescence usingexternal standards, and pyrene concentrations associatedwith the residual dissolved HS were calculated by massbalance.

Pyrene Partitioning to Mineral-Bound HS over a Lowfoc Range. These pyrene partitioning experiments mimicked

the HS adsorption experiments, except for the spiking ofpyrene (100 µg/L) into centrifuge tubes that containedappropriate concentrations of HS and minerals. First, thesame HS concentrations as were used in the HS adsorptionexperiments were added to centrifuge tube that containeda fixed mineral suspension and solution composition (i.e.,0.1 M NaCl, pH 7, 50 or 10 g/L of kaolinite or hematite,respectively). All samples were equilibrated on the shakerfor 72 h, based on preliminary rate studies. After centrifuging(3000 rpm, 30 min) each sample to separate the solid phase,the total concentration of dissolved pyrene (i.e., equals freelydissolved pyrene + pyrene associated with residual dissolvedHS) was quantified by hexane extraction and subsequentfluorescence measurement. The pyrene concentration ad-sorbed to the mineral-associated HS was then determinedby mass balance. Although Johnson and Amy (22) reportedsuccessful quantification of pyrene in HS samples usinghexane extraction and fluorescence measurement, we ob-served a slight increase in the fluorescence of HS-equilibratedhexane control solutions. Therefore, a modified hexaneextraction method was developed and utilized to quantifytotal dissolved pyrene concentrations. Briefly, the backgroundfluorescence of hexane solutions after equilibration withresidual HS in the absence of pyrene was measured and usedto make fluorescence corrections. Preliminary studies showedpyrene recovery percentages of 100.5 (( 1.3) and 99.1 (( 2.8)in the presence of 10 and 50 mg of C/L, respectively, HSsolutions.

Results and DiscussionHS Adsorption by Minerals. PAHA and SRFA adsorptionresults are shown in Figure 1 of the Supporting Information.In general, their isotherms on kaolinite and hematiteexhibited Langmuir-type behavior (17), consistent withprevious reports of HS adsorption behavior (11, 15, 16, 18,23-25). Despite the nearly 10-fold higher surface area basedconcentration of kaolinite (715 m2/L) versus hematite (74.1m2/L) in each centrifuge tube, greater adsorption of each HSwas observed with hematite. This result likely relates to thedifferent distributions, concentrations, and acidities ofmineral surface hydroxyl groups, which are expected to be

Koc )[pyr-HS]

[pyr]free[HS](1)

5872 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 22, 2004

important surface sites for HS adsorption by ligand exchangeprocesses (11, 12, 17, 18, 23-27). For example, HS compo-nents likely adsorb relatively uniformly on the hematitesurface but may be restricted primarily to the edge sites onkaolinite (11, 17, 18). However, the higher adsorption of PAHAversus SRFA on kaolinite and hematite suggests the additionalinvolvement of hydrophobic interactions for its enhancedadsorption (17, 18).

Pyrene Partitioning to Residual HS. Residual HS MWw

values are plotted as a function of equilibrium HS concen-tration in Figure 2a. Deviations of these values from theircorresponding original MWw values demonstrate the MWfractionation of PAHA and SRFA upon their adsorption tokaolinite and hematite. A detailed discussion and explanationfor the adsorptive fractionation results shown in Figure 2ahave been provided in a previous paper (17).

Koc values for the residual HS, Koc(res), can be comparedto the Koc values measured for the dissolved original bulkmaterials, designated here as Koc(orig) (Figure 2b). Despitethe scattering observed in a few of the data points, someoverall trends can be observed, particularly for PAHA. Forexample, a visual comparison between panels a and b ofFigure 2 reveals that the relative magnitudes for PAHA Koc-(res) values generally follow the residual PAHA MWw values.This is consistent with the findings of Chin et al. (5), whoreported a positive relationship between log Koc for pyreneand log MWw for several dissolved bulk aquatic HS. In

addition, Hur and Schlautman (9) demonstrated that positivecorrelations existed between pyrene log Koc values and logMWw of both dissolved ultrafiltered PAHA size fractions anda small subset of different dissolved bulk HS from diverseorigins. The Koc(res) values for PAHA shown in Figure 2b alsoreflect such a general trend. Because of the high degree offractionation, this trend was most evident for the residualPAHA equilibrated with kaolinite. Jones and Tiller (14)collected supernatant HS solutions from sorption experi-ments of a soil humic acid onto kaolinite and determinedphenanthrene Koc(res) values using the Stern-Volmer tech-nique. In general, they found an overall average Koc(res) valuethat was lower than Koc(orig) for the dissolved bulk soil humicacid. Here, however, we do not simply show the dissimilaritybetween Koc(orig) and Koc(res) values but demonstrate thatthe relative Koc(res) values likely depend on the extent of HSmolecular weight fractionation, which in turn appears to beinfluenced by the particular mineral present in the systemand the type of HS and its biogeochemical characteristics.For example, in contrast to PAHA, SRFA did not exhibit thesame degree of MWw fractionation and variation in its Koc-(res) values (Figure 2). However, it is apparent that themajority of its Koc(res) values were lower than its Koc(orig),which is in general agreement with the lower residual SRFAMWw values obtained versus its original MWw.

A log Koc-log MWw graph was constructed for the residualPAHA samples obtained after adsorption equilibration with

FIGURE 1. (a) Conceptual model for pyrene partitioning in this study. (b) Flowchart of experimental procedures used.

VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5873

kaolinite and hematite (Figure 3). As expected, positiverelationships were found between these two parameters forboth mineral systems, although the trend was more apparentwith kaolinite because of its wider range of MWw values.

Interestingly, Figure 3 shows that a single correlation generallycharacterizes the correct trend for both mineral systems,suggesting that the log Koc-log MWw relationship for residualHS may be relatively insensitive to the specific mineraladsorbent used. This is noteworthy because different MWcomponents of PAHA are preferentially adsorbed to kaoliniteversus hematite, and thus the respective structural charac-teristics for the residual PAHA solutions are not likely to bethe same for the two mineral systems (17).

To further investigate the PAHA log Koc-log MWw

relationship, data points obtained from ultrafiltered PAHAsize fractions (9) were also compared with the trendestablished for the sorption-fractionated PAHA components.It is expected that physical processes are predominant duringsize fractionation via ultrafiltration, whereas both chemicaland physical processes are likely involved in mineral-promoted adsorptive fractionation. Nevertheless, it is ap-parent that the data points for the ultrafiltered size fractionsdo not deviate appreciably from the trend obtained with theadsorption-fractionated PAHA, suggesting that the positiverelationship between log Koc(res)-log MWw may be largelyunaffected by any particular HS fractionation process. Thisfinding may be analogous to an observation recently reportedby Kitis et al. (28), who found that the disinfection byproductformation potentials of several dissolved natural organicmatter samples fractionated by alternative approaches (e.g.,adsorption by granular activated carbon or hematite, ultra-filtration) were strongly correlated with their SUVA values,despite the different fractionation procedures used.

In contrast with PAHA log MWw values, no positive trendswere observed between log Koc(res) values and SUVA or MWw-nomalized SUVA values for PAHA and SRFA (Figure 2 of theSupporting Information). In fact, negative relationshipsbetween these parameters were observed with PAHA, con-sistent with previous findings by Hur and Schlautman (9).The relatively weaker extent of fractionation observed withSRFA versus PAHA also resulted in no strong positivecorrelation between its log Koc(res) and log MWw values(Figure 3a of the Supporting Information).

Pyrene Partitioning to Mineral-Bound HS over a Lowfoc Range. Apparent MWw values of the adsorbed HS fractions

in low (<0.002) foc systems were calculated following theideal mixture approach described in Hur and Schlautman(9). The approach is based on the concepts of mass balanceand adherence to the Beer-Lambert law for UV absorbanceand calculates adsorbed MWw values from their correspond-ing residual MWw values, corresponding relative fractions ofresidual UV absorbing components, and the original bulkHS MWw value. In general, MWw values for the adsorbed HSfractions increased with increasing foc (Figure 4a). Theseresults may appear to be counterintuitive for some systems(e.g., PAHA + kaolinite) on the basis of the residual HS sampletrends previously shown in Figure 2a. However, it is importantto remember that the increasing equilibrium HS concentra-tions shown in Figure 2, and likewise the increasing foc valuesshown in Figure 4, result from the increasing initial HSconcentrations used in each centrifuge tube. In other words,as the initial HS concentration increases relative to theconstant mineral suspension concentration, the overallpercentage of HS components adsorbed decreases (Figure1 of the Supporting Information) due to the limited sorptionsites. Consequently, the HS components that are preferen-tially adsorbed by the minerals increasingly occupy thelimited sorption sites with increasing initial HS concentration.As a result, the adsorbed HS MWw deviates further from theoriginal bulk MWw as foc increases. For the PAHA-hematitesystem, complete adsorption of all PAHA componentsoccurred under our experimental conditions and thus alladsorbed MWw values calculated to the original PAHA MWw

value.

FIGURE 2. (a) MWw and (b) pyrene Koc(res) values for residual HSremaining in solution after adsorption by minerals as a function ofequilibrium HS concentration. Solid and dashed lines represent thecorresponding original values for dissolved PAHA and SRFA,respectively. (O) PAHA and kaolinite, (0) PAHA and hematite, (b)SRFA and kaolinite, (9) SRFA and hematite. MWw data are from ref17.

FIGURE 3. Correlation between pyrene log Koc(res) and log MWw

for residual PAHA fractions. Dashed line corresponds to the linearregression equation shown. (O) PAHA and kaolinite, (0) PAHA andhematite. Log Koc data for PAHA ultrafiltration fractions ([) fromref 9 are included for comparison. MWw values for the ultrafiltrationfractions were recalculated for an infinite dilution condition (seetext).

5874 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 22, 2004

Attributing HOC sorption to the organic carbon in low foc

sorbents is confounded by the potential contribution ofmineral surfaces to the overall sorption process. The as-sumptions and procedures we used to calculate pyrenepartition coefficients for mineral-associated HS (Koc(ads))are described in the Supporting Information. Pyrene bindingby kaolinite-associated PAHA was consistently lower thanits Koc(orig) value (Figure 4b), except for the highest foc value(∼0.0014) examined. Note that this reduction in pyrenebinding occurred despite the fact that the adsorbed PAHAfractions had higher overall MWw values as compared to theoriginal value (Figure 4a). This observation suggests thatPAHA components undergo conformational changes uponadsorption to kaolinite, such that the resulting conformationsprovide lower pyrene binding affinities at low foc values. Ouroverall lower Koc(ads) results with kaolinite-associated PAHAversus Koc(orig) appear to be in general agreement with twoprevious reports. Murphy et al. (11) reported experimentalKoc values for anthracene on HS-coated kaolinite that weregenerally lower than those predicted by its octanol-waterpartition coefficient. Jones and Tiller (14) reported generallylower extents of phenanthrene binding by a soil humic acidadsorbed on kaolinite than that prior to adsorption.

The positive trend we observe between Koc(ads) and foc

for kaolinite-associated PAHA (Figure 4b) is at odds, however,with the trends reported in the two previous studies (11, 14).This apparent discrepancy may have resulted, in part, fromthe various attempts made here and by the previous

researchers to subtract the bare kaolinite surface contributionto total HOC sorption in the presence of adsorbed HS. Inother words, without access to molecular-level informationof the location(s) and conformation(s) of adsorbed HScomponents (29), it is impossible to truly understand howthey are covering a mineral surface and how this mayultimately affect such a bare mineral surface correction tototal HOC sorption. The fact that our solute was morehydrophobic than the ones used in the previous studies mayhave exacerbated this problem. However, we believe that amore plausible explanation for the different trend weobserved relates to the interplay that occurred betweenadsorbed HS conformational effects and HS adsorptivefractionation effects. In other words, we believe that suchconformational effects are behind the overall lower Koc(ads)values for kaolinite-associated PAHA versus its Koc(orig) value,but that the trend of increasing Koc(ads) with foc (Figure 4b)would be expected because the overall MWw for the adsorbedPAHA components also increases with foc due to adsorptivefractionation. For the highest foc condition tested (∼0.0014),it appears that adsorptive fractionation effects ultimatelyovercome the conformational effects so that Koc(ads) > Koc-(orig). Although our reasoning above is speculative, additionalstudies to elucidate between these two potentially competi-tive effects would be helpful in reconciling the apparentdiscrepancy noted above between our result and those ofprevious investigators.

Koc(ads) values for hematite-associated PAHA were typi-cally larger than those for kaolinite-bound PAHA at com-parable foc, except when foc became greater than 0.0014(Figure 4b). This apparent mineral surface effect on PAHAbinding of pyrene is consistent with the findings of Murphyet al. (11), who reported higher anthracene Koc(ads) valuesfor hematite versus kaolinite coated with a peat humic acid,and Hur and Schlautman (18), who obtained higher pyreneKoc(ads) values with PAHA and PAHA ultrafiltrated fractionsadsorbed on hematite versus kaolinite at pH 4 in the absenceof adsorptive fractionation. From Figure 4, comparison ofKoc(ads) and adsorbed MWw values for PAHA reveals that therelative decrease in Koc(ads) after PAHA adsorption wasgreater for kaolinite than hematite, suggesting that the impactof conformational changes upon adsorption is dependenton the particular mineral (11, 18).

To further investigate conformational effects versusadsorptive fractionation effects, we examined the trendbetween pyrene log Koc(ads) and log MWw values for kaolinite-adsorbed PAHA (Figure 5). For comparison, the residual PAHAdata points and trendline previously shown in Figure 3 arealso presented. Despite the very narrow range in adsorbedMWw values obtained in our experiments, it can be seen thatthe adsorbed PAHA MWw does influence pyrene partitioningas quantified by the significant positive correlation (r ) 0.79)between log Koc(ads) and log MWw. It is also clear that theslope of the regression line for adsorbed PAHA is ap-proximately six times higher than that obtained for residualPAHA, a difference in slopes that is statistically significantat both the 95 and 99% confidence levels. This statisticaldifference in slopes also was observed when considering onlythe residual PAHA MWw values that were equivalent to thoseobtained with adsorbed PAHA (see Supporting Information).As discussed above, such a comparison suggests that althoughthe relative trend of pyrene Koc(ads) may be influenced byPAHA adsorptive fractionation effects, the absolute valuesare likely determined by conformational changes of PAHAcomponents upon adsorption. For example, at low surfacecoverage, adsorbed PAHA components may adopt more openstructures that are unfavorable for pyrene binding, whereashigher surface coverage may increase pyrene accessibility toadsorbed PAHA components. This speculation may beconsistent with a recent observation of aggregated HS

FIGURE 4. (a) MWw and (b) pyrene Koc(ads) values for mineral-associated HS as a function of foc. Solid and dashed lines representthe corresponding original values for dissolved PAHA and SRFA,respectively. (O) PAHA and kaolinite, (0) PAHA and hematite, (b)SRFA and kaolinite, (9) SRFA and hematite. MWw values forhematite-bound PAHA are assumed equal to the original value dueto complete adsorption (see text).

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structures adsorbed to the basal plane surfaces of muscoviteand hematite using in-solution atomic force microscopy (29).It was reported therein that ring structures with nanoporosity,which may be important for HOC binding, were moreabundant at higher HS concentrations. This explanationwould also be consistent with the fact that Koc(ads) valuesincreased slightly with foc (r ) 0.66) for hematite-associatedPAHA, despite no change in overall adsorbed MWw (Figure4).

Adsorptive fractionation effects on pyrene binding provedto be much less important for SRFA, as demonstrated by theweaker trends (r ) 0.46, 0.26, and 0.45 for hematite, kaolinite,and the two combined, respectively) between log Koc(ads)and log MWw (Figure 3b of the Supporting Information). Thisappears to be reasonable, considering that the aquatic fulvicacid is less heterogeneous than the terrestrial humic acid (9,17, 18). In fact, since SRFA is an allochthonous HS (i.e., hasa terrestrial origin), one can argue that its precursor materialshave already undergone fractionation and that what weidentify as the aquatic fulvic acid is actually the fractionatedresidual organic material components that have remainedin solution. Therefore, our results with SRFA and PAHAconfirm the expectation that the importance of mineralsurface adsorptive fractionation on HOC partitioning is likelyto be dependent on the HS source and their other bio-geochemical characteristics.

AcknowledgmentsElements of this paper were presented at the symposium“Physicochemical Processes in Environmental Systems: ASymposium in Honor of Professor Walter J. Weber, Jr.”,Division of Environmental Chemistry, 226th AmericanChemical Society National Meeting, New York, NY, Sep-tember 2003. M.A.S. gratefully acknowledges the postdoctoralopportunity provided to him early in his career by Walt Weberat the University of Michigan. Comments from threeanonymous reviewers and the assistant editor, Lynn Katz,improved the focus of the paper and are greatly appreciated.Funding for the work reported herein was provided by theNational Science Foundation (Grant 9996441) and the U.S.Department of Agriculture (SC-1700133). The contents ofthis paper do not necessarily reflect the views and policiesof NSF or USDA, nor does the mention of trade names orcommercial products constitute endorsement or recom-mendation for use.

Supporting Information AvailableDescription of the assumptions and procedures used tocalculate Koc(ads), regression results in support of Figure 5,and figures showing HS adsorption results and correlationsbetween pyrene log Koc(res) values and various residual HSproperties. This material is available free of charge via theInternet at http://pubs.acs.org.

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FIGURE 5. Correlation between pyrene log Koc(ads) and log MWw

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Received for review February 10, 2004. Revised manuscriptreceived August 29, 2004. Accepted September 1, 2004.

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