Influence of Humic Substance Adsorptive Fractionation on Pyrene

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

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 mineralassociated humic substances (HS) due to HS adsorptive fractionation processes were examined in model environmental systems using purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA). For PAHA, carbonnormalized pyrene binding coefficients for nonadsorbed, residual fractions (Koc(res)) were different from the original dissolved PAHA Koc value (Koc(orig)) prior to contact with the mineral suspensions. A strong positive correlation between pyrene log Koc(res) and log weight-average molecular weight (MWw) 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 Koc(ads) and log MWw 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 substances (HS) often governs their transport, reactivity, bioavailability, 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 physicochemical 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 †

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]. 10.1021/es049790t CCC: $27.50 Published on Web 10/14/2004

 2004 American Chemical Society

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 weightaverage 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 explanation 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 subsequently adsorbed HS materials. Therefore, even though Hur and Schlautman’s overall experimental approach minimized 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 dissolved 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 VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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on how HS adsorptive fractionation and/or HS conformational changes upon their adsorption to mineral surfaces would affect subsequent pyrene partitioning. Special emphasis was given to systems having low mass fraction organic carbon ( foc) levels of adsorbed HS, because adsorptive fractionation effects are maximized under these conditions (17). In addition, the results of Murphy et al. (11) suggest that differences in HOC partitioning to dissolved versus mineralassociated HS will be most important at low foc levels. Care was taken in the present study to ensure that phase separation did not lead to disruption of the equilibrium conditions established during the initial sorption equilibration period(s) (10, 12, 19). For example, Laor et al. (13) observed that when residual HS were removed from their experimental systems, measurable concentrations of the previously mineral-bound HS (2-6 mg of C/L) were released back into solution during their subsequent HOC sorption equilibration step. Therefore, to avoid problems of this type, pyrene partitioning to the various HS compartments was evaluated without removing residual HS from the HOC-HS-mineral systems to minimize potential disruption of system equilibria.

Experimental Section Materials. Purified Aldrich humic acid (PAHA) and Suwannee River fulvic acid (SRFA) were selected for use as a terrestrial and aquatic HS, respectively. Pyrene (Fluka, 99.5+% purity) was selected as the model HOC. Kaolinite and hematite were obtained from Sigma and Alfa, respectively, and used without further treatment. Physicochemical characteristics of all materials used have been previously reported (9, 17). Analytical Methods. A total organic carbon (TOC) analyzer (Shimadzu model 5050) was used to quantify HS concentrations in the aqueous phase. A spectrophotometer (Beckman model DU640) was used to measure absorbances for calculating HS specific ultraviolet absorbance (SUVA) values and pyrene fluorescence inner-filter corrections (9, 19). Pyrene concentrations in aqueous solutions or hexane extracts were quantified by a luminescence spectrophotometer (Perkin-Elmer, LS-5B) using external standards. The excitation/emission wavelengths (nm/nm) for pyrene were 336/373, and slits were set for bandwidths of 3 nm for excitation and 5 nm (in aqueous solutions) or 20 nm (in hexane extracts) for emission. Relative precisions of 1 and 3% were routinely obtained for absorbance/fluorescence and TOC measurements, respectively. Size exclusion chromatography (SEC) with ultraviolet detection at 254 nm was used to determine apparent MWw values of dissolved/residual HS samples, generally following the methodology recommended by Zhou et al. (20). Additional details for the SEC measurements have been reported previously (9, 17). Because MWw values for PAHA samples determined with our SEC system are dependent on their concentrations, all PAHA SEC chromatograms were extrapolated to a concentration of 0 mg/L to obtain consistent, infinite dilution MWw values (17). Pyrene Partitioning to Residual HS. A conceptual model of pyrene partitioning and a flowchart of the experimental procedures followed in this study are presented in Figure 1. Detailed experimental procedures for HS adsorption by kaolinite and hematite have been reported previously (17, 18). Pyrene Koc values with the residual HS, designated here as Koc(res), were determined using the supernatant HS solutions resulting from the HS adsorption experiments. A fixed concentration (80 µg/L) of pyrene was spiked into each residual HS sample. Due to the limited concentrations and volumes of residual HS samples, a modified fluorescence quenching 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 that fluorescence intensity is proportional to the concentration of free pyrene (i.e., pyrene not associated with HS). Pre5872

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liminary studies suggested that Koc(res) values for PAHA might show a dependency on its concentration with the present method, so all samples with a high concentration of PAHA were diluted to 3 mg of C/L prior to adding pyrene, thus ensuring that the measured Koc(res) values would be comparable across a wide PAHA concentration range. Koc values determined with the present technique matched those results obtained from Stern-Volmer analyses that used 3 mg of C/L as the maximum HS concentration (data not shown). All samples were mixed on a reciprocating shaker at low speed for 20 min to attain equilibrium (12, 19, 21) before removing aliquots for fluorescence and absorbance measurements. Residual HS-associated pyrene, freely dissolved pyrene and residual HS concentrations were all quantified separately to determine Koc(res) values with the following equation (19):

Koc )

[pyr-HS] [pyr]free[HS]

(1)

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 dissolved pyrene in HS samples were determined by fluorescence using external standards, and pyrene concentrations associated with the residual dissolved HS were calculated by mass balance. Pyrene Partitioning to Mineral-Bound HS over a Low foc Range. These pyrene partitioning experiments mimicked the HS adsorption experiments, except for the spiking of pyrene (100 µg/L) into centrifuge tubes that contained appropriate concentrations of HS and minerals. First, the same HS concentrations as were used in the HS adsorption experiments were added to centrifuge tube that contained a 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 shaker for 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 freely dissolved pyrene + pyrene associated with residual dissolved HS) was quantified by hexane extraction and subsequent fluorescence measurement. The pyrene concentration adsorbed to the mineral-associated HS was then determined by mass balance. Although Johnson and Amy (22) reported successful quantification of pyrene in HS samples using hexane extraction and fluorescence measurement, we observed a slight increase in the fluorescence of HS-equilibrated hexane control solutions. Therefore, a modified hexane extraction method was developed and utilized to quantify total dissolved pyrene concentrations. Briefly, the background fluorescence of hexane solutions after equilibration with residual HS in the absence of pyrene was measured and used to make fluorescence corrections. Preliminary studies showed pyrene recovery percentages of 100.5 (( 1.3) and 99.1 (( 2.8) in the presence of 10 and 50 mg of C/L, respectively, HS solutions.

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

FIGURE 1. (a) Conceptual model for pyrene partitioning in this study. (b) Flowchart of experimental procedures used. important surface sites for HS adsorption by ligand exchange processes (11, 12, 17, 18, 23-27). For example, HS components likely adsorb relatively uniformly on the hematite surface but may be restricted primarily to the edge sites on kaolinite (11, 17, 18). However, the higher adsorption of PAHA versus SRFA on kaolinite and hematite suggests the additional involvement of hydrophobic interactions for its enhanced adsorption (17, 18). Pyrene Partitioning to Residual HS. Residual HS MWw values are plotted as a function of equilibrium HS concentration in Figure 2a. Deviations of these values from their corresponding original MWw values demonstrate the MW fractionation of PAHA and SRFA upon their adsorption to kaolinite and hematite. A detailed discussion and explanation for the adsorptive fractionation results shown in Figure 2a have been provided in a previous paper (17). Koc values for the residual HS, Koc(res), can be compared to the Koc values measured for the dissolved original bulk materials, designated here as Koc(orig) (Figure 2b). Despite the scattering observed in a few of the data points, some overall trends can be observed, particularly for PAHA. For example, a visual comparison between panels a and b of Figure 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), who reported a positive relationship between log Koc for pyrene and log MWw for several dissolved bulk aquatic HS. In

addition, Hur and Schlautman (9) demonstrated that positive correlations existed between pyrene log Koc values and log MWw of both dissolved ultrafiltered PAHA size fractions and a small subset of different dissolved bulk HS from diverse origins. The Koc(res) values for PAHA shown in Figure 2b also reflect such a general trend. Because of the high degree of fractionation, this trend was most evident for the residual PAHA equilibrated with kaolinite. Jones and Tiller (14) collected supernatant HS solutions from sorption experiments of a soil humic acid onto kaolinite and determined phenanthrene Koc(res) values using the Stern-Volmer technique. In general, they found an overall average Koc(res) value that was lower than Koc(orig) for the dissolved bulk soil humic acid. Here, however, we do not simply show the dissimilarity between Koc(orig) and Koc(res) values but demonstrate that the relative Koc(res) values likely depend on the extent of HS molecular weight fractionation, which in turn appears to be influenced by the particular mineral present in the system and the type of HS and its biogeochemical characteristics. For example, in contrast to PAHA, SRFA did not exhibit the same degree of MWw fractionation and variation in its Koc(res) values (Figure 2). However, it is apparent that the majority of its Koc(res) values were lower than its Koc(orig), which is in general agreement with the lower residual SRFA MWw values obtained versus its original MWw. A log Koc-log MWw graph was constructed for the residual PAHA samples obtained after adsorption equilibration with VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) MWw and (b) pyrene Koc(res) values for residual HS remaining in solution after adsorption by minerals as a function of equilibrium HS concentration. Solid and dashed lines represent the 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 data are from ref 17.

FIGURE 3. Correlation between pyrene log Koc(res) and log MWw for residual PAHA fractions. Dashed line corresponds to the linear regression equation shown. (O) PAHA and kaolinite, (0) PAHA and hematite. Log Koc data for PAHA ultrafiltration fractions ([) from ref 9 are included for comparison. MWw values for the ultrafiltration fractions were recalculated for an infinite dilution condition (see text). kaolinite and hematite (Figure 3). As expected, positive relationships were found between these two parameters for both mineral systems, although the trend was more apparent with kaolinite because of its wider range of MWw values. 5874

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Interestingly, Figure 3 shows that a single correlation generally characterizes the correct trend for both mineral systems, suggesting that the log Koc-log MWw relationship for residual HS may be relatively insensitive to the specific mineral adsorbent used. This is noteworthy because different MW components of PAHA are preferentially adsorbed to kaolinite versus hematite, and thus the respective structural characteristics for the residual PAHA solutions are not likely to be the same for the two mineral systems (17). To further investigate the PAHA log Koc-log MWw relationship, data points obtained from ultrafiltered PAHA size fractions (9) were also compared with the trend established for the sorption-fractionated PAHA components. It is expected that physical processes are predominant during size fractionation via ultrafiltration, whereas both chemical and physical processes are likely involved in mineralpromoted adsorptive fractionation. Nevertheless, it is apparent that the data points for the ultrafiltered size fractions do not deviate appreciably from the trend obtained with the adsorption-fractionated PAHA, suggesting that the positive relationship between log Koc(res)-log MWw may be largely unaffected by any particular HS fractionation process. This finding may be analogous to an observation recently reported by Kitis et al. (28), who found that the disinfection byproduct formation potentials of several dissolved natural organic matter samples fractionated by alternative approaches (e.g., adsorption by granular activated carbon or hematite, ultrafiltration) were strongly correlated with their SUVA values, despite the different fractionation procedures used. In contrast with PAHA log MWw values, no positive trends were observed between log Koc(res) values and SUVA or MWwnomalized SUVA values for PAHA and SRFA (Figure 2 of the Supporting Information). In fact, negative relationships between these parameters were observed with PAHA, consistent with previous findings by Hur and Schlautman (9). The relatively weaker extent of fractionation observed with SRFA versus PAHA also resulted in no strong positive correlation between its log Koc(res) and log MWw values (Figure 3a of the Supporting Information). Pyrene Partitioning to Mineral-Bound HS over a Low foc Range. Apparent MWw values of the adsorbed HS fractions in low ( Koc(orig). Although our reasoning above is speculative, additional studies to elucidate between these two potentially competitive effects would be helpful in reconciling the apparent discrepancy noted above between our result and those of previous investigators. Koc(ads) values for hematite-associated PAHA were typically larger than those for kaolinite-bound PAHA at comparable foc, except when foc became greater than 0.0014 (Figure 4b). This apparent mineral surface effect on PAHA binding of pyrene is consistent with the findings of Murphy et al. (11), who reported higher anthracene Koc(ads) values for hematite versus kaolinite coated with a peat humic acid, and Hur and Schlautman (18), who obtained higher pyrene Koc(ads) values with PAHA and PAHA ultrafiltrated fractions adsorbed on hematite versus kaolinite at pH 4 in the absence of adsorptive fractionation. From Figure 4, comparison of Koc(ads) and adsorbed MWw values for PAHA reveals that the relative decrease in Koc(ads) after PAHA adsorption was greater for kaolinite than hematite, suggesting that the impact of conformational changes upon adsorption is dependent on the particular mineral (11, 18). To further investigate conformational effects versus adsorptive fractionation effects, we examined the trend between pyrene log Koc(ads) and log MWw values for kaoliniteadsorbed PAHA (Figure 5). For comparison, the residual PAHA data points and trendline previously shown in Figure 3 are also presented. Despite the very narrow range in adsorbed MWw values obtained in our experiments, it can be seen that the adsorbed PAHA MWw does influence pyrene partitioning as quantified by the significant positive correlation (r ) 0.79) between log Koc(ads) and log MWw. It is also clear that the slope of the regression line for adsorbed PAHA is approximately six times higher than that obtained for residual PAHA, a difference in slopes that is statistically significant at both the 95 and 99% confidence levels. This statistical difference in slopes also was observed when considering only the residual PAHA MWw values that were equivalent to those obtained with adsorbed PAHA (see Supporting Information). As discussed above, such a comparison suggests that although the relative trend of pyrene Koc(ads) may be influenced by PAHA adsorptive fractionation effects, the absolute values are likely determined by conformational changes of PAHA components upon adsorption. For example, at low surface coverage, adsorbed PAHA components may adopt more open structures that are unfavorable for pyrene binding, whereas higher surface coverage may increase pyrene accessibility to adsorbed PAHA components. This speculation may be consistent with a recent observation of aggregated HS VOL. 38, NO. 22, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Supporting Information Available Description of the assumptions and procedures used to calculate Koc(ads), regression results in support of Figure 5, and figures showing HS adsorption results and correlations between pyrene log Koc(res) values and various residual HS properties. This material is available free of charge via the Internet at http://pubs.acs.org.

Literature Cited

FIGURE 5. Correlation between pyrene log Koc(ads) and log MWw for kaolinite-associated PAHA fractions ([). Solid line corresponds to the linear regression equation shown. The open symbols and dotted line are from Figure 3 and are included for comparison. Note that complete adsorption of PAHA by hematite resulted in a constant adsorbed MWw and therefore is not shown in the graph. structures adsorbed to the basal plane surfaces of muscovite and 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 more abundant at higher HS concentrations. This explanation would also be consistent with the fact that Koc(ads) values increased slightly with foc (r ) 0.66) for hematite-associated PAHA, despite no change in overall adsorbed MWw (Figure 4). Adsorptive fractionation effects on pyrene binding proved to be much less important for SRFA, as demonstrated by the weaker 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). This appears to be reasonable, considering that the aquatic fulvic acid is less heterogeneous than the terrestrial humic acid (9, 17, 18). In fact, since SRFA is an allochthonous HS (i.e., has a terrestrial origin), one can argue that its precursor materials have already undergone fractionation and that what we identify as the aquatic fulvic acid is actually the fractionated residual organic material components that have remained in solution. Therefore, our results with SRFA and PAHA confirm the expectation that the importance of mineral surface adsorptive fractionation on HOC partitioning is likely to be dependent on the HS source and their other biogeochemical characteristics.

Acknowledgments Elements of this paper were presented at the symposium “Physicochemical Processes in Environmental Systems: A Symposium in Honor of Professor Walter J. Weber, Jr.”, Division of Environmental Chemistry, 226th American Chemical Society National Meeting, New York, NY, September 2003. M.A.S. gratefully acknowledges the postdoctoral opportunity provided to him early in his career by Walt Weber at the University of Michigan. Comments from three anonymous 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 the National Science Foundation (Grant 9996441) and the U.S. Department of Agriculture (SC-1700133). The contents of this paper do not necessarily reflect the views and policies of NSF or USDA, nor does the mention of trade names or commercial products constitute endorsement or recommendation for use. 5876

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Received for review February 10, 2004. Revised manuscript received August 29, 2004. Accepted September 1, 2004. ES049790T

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