Adsorption of Natural Organic Matter onto Carbonaceous Surfaces

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Environ. Sci. Technol. 2007, 41, 1238-1244

Adsorption of Natural Organic Matter onto Carbonaceous Surfaces: Atomic Force Microscopy Study JUSTIN M. GORHAM, JOSHUA D. WNUK, M. SHIN, AND HOWARD FAIRBROTHER* Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218

The microscopic structure of carbonaceous surfaces exposed to natural organic matter (NOM) under aqueous conditions has been explored using atomic force microscopy (AFM). Dismal Swamp Water was used as the NOM source, while highly ordered pyrolytic graphite (HOPG) served as a surrogate for the graphene sheets that characterize the surface of many carbonaceous materials in aquatic environments. Under acidic conditions, the HOPG surface was covered with a densely packed monolayer of NOM molecules. In some cases, aggregates of welldefined, individual NOM molecules were observed that exhibited a degree of registry with respect to the HOPG substrate. This suggests that adsorbate-substrate interactions play a role in moderating the structure of the adsorbate layer. As the pH increased, the concentration of adsorbed NOM decreased systematically because of increasingly repulsive interactions between adsorbates. Increasing the ionic strength produced a modest increase in the concentration of adsorbed NOM. Ca2+ ions exerted a more pronounced influence on both the surface coverage of adsorbed NOM molecules and the size of individual adsorbates because of the effects of intermolecular complexation. In contrast to the spherical structures observed by AFM under aqueous conditions, adsorbed NOM formed a mixture of “ringlike” assemblies and larger aggregates upon drying.

Introduction Natural organic matter (NOM), a ubiquitous component of aquatic environments, consists of a complex, polydisperse, and heterogeneous mixture of organic compounds principally formed from the biodegradation of plant and animal matter (1). Although NOM’s composition is strongly dependent upon the local environment, typical components include fulvic and humic acids, polysaccharides, lipids, proteins, and amino acids (1). Because of the presence of carboxylic acid and phenolic groups, NOM typically behaves as a polyanion in aqueous conditions (2). NOM plays an important role in many geochemical processes, including the fate and transport of trace metals and hydrophobic organic pollutants (3). The interfacial properties of carbon-based materials (e.g., activated carbons (ACs), fuel-based soot, and biomass char) are also altered by adsorbed NOM (4-6). The surface of ACs, for example, * Corresponding author phone: (410)516-4328; fax: (410)516-8420; e-mail: [email protected]. 1238

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is typically covered with NOM which blocks surface sites and micropores thus reducing their adsorptive capacity and effective lifetime (7). The heterogeneous and polydispersed nature of NOM, coupled with the complexity of typical carbonaceous materials in the environment, has meant that the adlayers formed by NOM on these surfaces are poorly understood (7-9). Consequently, the NOM adsorption onto carbon-based surfaces is currently described by isotherms and mathematical models that are inherently limited by their empirical nature (10). The development of more accurate adsorption isotherms and interaction models would benefit significantly from detailed microscopic information on the structure of carbonaceous surfaces under aqueous conditions in the presence of NOM. Atomic force microscopy (AFM) has been used effectively to study environmentally relevant liquid/solid interfaces because of its ability to image adsorbed structures under aqueous conditions with a spatial resolution on the nanometer length scale (11-13). One significant advantage of AFM over other analytical techniques, (e.g., transmission electron microscopy and vacuum-based surface spectroscopies) is the ability to extract information directly from the liquid/ solid interface (1). Changes in the structure and concentration of adsorbed species that occur as a result of drying can therefore be circumvented in AFM studies (14-16). Several recent AFM studies have focused on identifying the adlayers formed by NOM on atomically smooth mineral surfaces under aqueous conditions (14, 17-21). For example, the conformations and structures of adsorbed NOM formed on both mica and hematite have been examined as a function of NOM concentration, Ca2+ concentration, pH, and ionic strength (19). Results from this investigation revealed that the adsorbate layer is comprised of different structures whose detailed composition is sensitive to the pH and NOM concentration. For instance, on mica, aggregates of adsorbed NOM molecules were formed under mildly acidic conditions (pH 6) while highly ordered linear structures were observed under basic (pH 11) conditions. In contrast to mica, NOM adsorbed on hematite under acidic conditions (pH 4) formed both flattened disks and larger spherical structures (19). These results highlight the complexity of the adsorbate structures formed by NOM as well as the response of the adsorbate layer to the aqueous conditions and chemical composition of the substrate. In the present investigation, we employed highly ordered pyrolytic graphite (HOPG), a well-defined carbonaceous substrate. HOPG consists of extended graphene sheets that represent the dominant surface features of ACs and other forms of black carbon; HOPG also lacks any appreciable concentration of surface oxides. Furthermore, adsorbate layers can be resolved with nanometer scale resolution by AFM because HOPG is composed of large, atomically flat terraces. In contrast, typical BC materials such as ACs are extremely heterogeneous and poorly defined, and atomically smooth regions of the surface (necessary for detailed AFM analysis) cannot be accessed. Indeed, in several recent studies, these considerations have motivated the use of HOPG as a surrogate for the surface of carbonaceous materials (2225). In this study, AFM was used to probe the adlayers formed by NOM on carbonaceous surfaces under aqueous conditions using a reasonably well-characterized source of NOM (from the Great Dismal Swamp). Emphasis was placed on identifying the influence of aqueous conditions on the adsorbate 10.1021/es061793d CCC: $37.00

 2007 American Chemical Society Published on Web 01/17/2007

FIGURE 1. (a) Topographic and (b) phase images of HOPG under aqueous conditions in the absence of NOM; (c) topographic and (d) phase images of NOM from the Great Dismal Swamp adsorbed onto HOPG at pH 6.3. A line scan (white line) across two individual steps on the HOPG substrate is shown as an insert in a. In c, adsorbed NOM is evidenced by regions of lighter color, indicating elevated (adsorbed) features. A comparison of c and d demonstrates the ability of phase imaging to differentiate adsorbed NOM from the HOPG substrate on the basis of the different physicochemical properties of these two materials. layer including pH, ionic strength, Ca2+ concentration, and the effect of drying.

Experimental Section Source of Natural Organic Matter. NOM (pH 4.4) was obtained from the Great Dismal Swamp (GDS) in southeastern Virginia. For AFM, experiments, to ensure that the residual NOM consisted of dissolved organic carbon (DOC, ∼98-100 mg carbon per liter) the GDS water was filtered using a glass fiber filter (Whatman GF/C, nominal pore size ≈ 1.2 µm). Size exclusion chromatography reveals that the NOM is composed principally of ≈3 kDa sized macromolecules with a smaller fraction centered at approximately 30 kDa. A summary of the chemical composition and macromolecular size distribution of the NOM used in this study is included in Figure 1 of the Supporting Information (hereafter referred to as SI). This information can also be found in ref 26. Substrate. HOPG (Grade 2; 10 × 10 × 1 mm; SPI Supplies) was adhered to the AFM sampling plate. Prior to each AFM

experiment, the HOPG surface was peeled using adhesive tape to remove surface layers exposed to the effects of air oxidation and contaminant adsorption. This creates an atomically smooth HOPG surface suitable for AFM imaging (see Figure 1a). Furthermore, X-ray photoelectron spectroscopy (XPS) analysis reveals that surfaces generated in this way are chemically homogeneous and are dominated (>98%) by carbon (SI, Figure 2). AFM Experiments. To image the adsorbate layer formed by NOM, the Dismal Swamp Water (DSW) was added dropwise onto the HOPG substrate until it was completely covered. The adsorbate layer was then allowed to equilibrate for 1 h prior to analysis. Images recorded after shorter equilibration times often exhibited “streaking” because of the displacement of weakly bound NOM molecules by the AFM tip. Unless noted, images were recorded under aqueous conditions. The adsorbate layers formed by NOM on HOPG were imaged using tapping mode AFM (Pico SPM LE; Agilent Technologies). In this mode, torsional forces between the VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) Example of a well-ordered adlayer formed by NOM under acidic conditions. (b) “Enlarged” AFM image of the area highlighted by the white box in a. Examples of regions on the HOPG substrate that divide NOM aggregates are indicated by white arrows in a. Examples of “voids” within individual NOM aggregates are indicated by white arrows in b. (c) Image analysis of adsorbed species observed in b; the diameter of individual adsorbates are plotted vs their measured heights. tip and the sample are minimized, reducing the likelihood that imaging will displace soft, easily deformed adsorbates. Images were obtained using silicon nitride tips coated with a magnetic Co-Cr layer. The probes used in this investigation have a maximum tip radius of ∼90 nm, a nominal force constant of 3.5 N/m, and oscillate at a frequency of ∼75 kHz in air and ∼37 kHz in water. In any AFM experiment, the finite size of the AFM tip dilates the 2-D footprint of adsorbed features. In contrast, the height of adsorbed species can be resolved at the angstrom scale (19). In AFM experiments, the tip amplitude, scan speed, and servo gain were optimized to achieve the clearest images of the adsorbate layer. Particular care was taken to find an amplitude setting that produced detailed images without displacing any adsorbed species. The reproducibility of the adsorbate structures was verified by taking 8 × 8 µm2 images of the adsorbate layer at several locations on the HOPG substrate. For each image, the topography, phase, and amplitude were recorded. Between each series of AFM experiments, any adsorbed NOM was removed from the tip by immersing it first in Milli-Q water and then in acetone before drying in air. Influence of pH. The pH of the GDS water was adjusted by dropwise addition of 0.1 M NaOH or HCl. To ensure that 1240

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the concentration of NOM in the GDS remained constant, the added volume of acid/base was less than 0.1 mL. Influence of Ionic Strength and Ca2+. The effect of ionic strength and Ca2+ ion concentration on the adsorbate layer formed by NOM on HOPG was investigated by adding appropriate amounts of 0.1 M NaCl and CaCl2 to the DSW, respectively. In each case, the pH of DSW was adjusted to 6.3. Imaging Adsorbed Structures under Ambient Conditions. The effect of drying on the structure of NOM was also investigated. In these studies, a NOM-containing solution was allowed to equilibrate on an HOPG substrate as described previously. The substrate was then rinsed thoroughly with milli-Q water at the same pH as the NOM-containing solution, dried under nitrogen, and then imaged under ambient conditions.

Results and Discussion Figure 1 shows representative topographic and phase images of HOPG under aqueous conditions in the absence (Figure 1a and 1b) and presence of NOM (Figure 1c and 1d). In the absence of NOM, the surface is characterized by large, atomically smooth terraces with steps characteristic of the HOPG basal plane. These structural features are highlighted

FIGURE 3. Influence of pH on the adlayers formed by NOM on HOPG; (a) pH 4.0, (c) pH 6.3, and (d) pH 9.1. In b, an “adsorbate-free” region was created in the adsorbate layer formed at pH 4.0 by decreasing the oscillating amplitude set point of the AFM tip; a line scan analysis through the adsorbate-free region shows that the height of the adsorbate layer is ≈1.5 nm. The step edges can be identified as the thin irregular lines in c and d. by the line scan shown as an insert in Figure 1a. The chemical homogeneity of the HOPG surface is reflected in the featureless phase image (Figure 1b). In both topographic and phase images, steps on the HOPG surface are evidenced as thin, irregular lines that propagate for at least several hundred nanometers. A comparison of Figure 1a and 1c illustrates that NOM adsorbs to the HOPG surface in patches. The irregular 2-D footprint of these patches suggests that they are composed of aggregates of individual NOM molecules. Although the measured height of the adsorbed structures is fairly uniform (≈1 nm), a smaller concentration of larger sized structures is also observed; an example is shown in the bottom-middle of Figure 1c. Occasionally, extended linear adsorbates, typical of polysaccharides (1), were also observed, but these appear to be only minor components of DSW. Because of the difference in their physiochemical properties, phase imaging provides an independent means to differentiate soft materials, such as NOM, adsorbed onto comparatively “hard” substrates, such as HOPG. This approach has been employed in previous studies to distinguish NOM adsorbed onto mica and hematite (19). An example of the ability of phase imaging to verify the presence of adsorbed NOM on HOPG and to identify the structure of the adlayer is shown if we compare Figure 1b and d. In accord with

previous studies on mineral surfaces (17), Figure 1 emphasizes the importance of aggregation effects in determining the 2-D structure of the adsorbate layer formed by NOM. The most clearly resolved and well-ordered adlayers formed by NOM on HOPG were observed occasionally under acidic conditions. An example of such an adlayer is shown in Figure 2a. A portion of this image (white box in 2a) is also shown in higher resolution in Figure 2b. Discrete aggregates of well-defined, clearly resolved, spherical structures are observed in both Figure 2a and b. Within each aggregate, voids are observed where the underlying substrate is exposed at the liquid/solid interface. One interesting feature of Figure 2a and 2b is that the aggregates display a degree of registry with respect to the underlying substrate; specifically, they are aligned parallel to the HOPG step edges. This implies that π-π interactions between HOPG’s graphene sheets and adsorbed NOM can influence the 2-D structure of the adlayer. The majority of the adsorbates observed in Figure 2a and b are imaged as similarly sized spherical structures. We believe that these adsorbates are individual NOM macromolecules. Image analysis (Figure 2c) reveals that the majority of these individual NOM molecules exhibit heights ranging between 0.5 and 1.5 nm and lateral diameters from 30 to 45 nm. These results are consistent with size exclusion chromatography (SI, Figure 1) that shows NOM in the GDS is dominated by VOL. 41, NO. 4, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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similarly sized macromolecules with an average molecular weight of approximately 3 kDa (26). The dimensions of the NOM molecules observed in Figure 2 are also similar to those measured in previous studies by AFM for similarly sized NOM (2-2.3 kDa) adsorbed on mica under acidic conditions (19). Figure 2a and b shows that a smaller fraction of larger-sized NOM molecules are also present. These larger adsorbates appear as brighter, “lighthouse” like features. Image analysis (Figure 2c) reveals that these species exhibit heights in excess of 2.0 nm and comprise less than 10% of the adsorbed species. This is consistent with the small fraction of higher molecular weight (>30 kDa) macromolecules detected by size exclusion chromatography (SI, Figure 1) (26). Influence of pH. Figure 3a, c, and d reflects the adsorbate layer’s response to changes in pH. At pH 4.0, the surface is blanketed by a densely packed adlayer of macromolecules (Figure 3a) obscuring the native steps and terraces of the HOPG substrate. The thickness of this adlayer was measured by rastering the AFM tip over a 1.2 × 1.2 µm2 area at a decreased oscillation amplitude. This increases the force applied by the tip enough to displace adsorbed NOM molecules. The adsorbate layer thickness could then be determined by measuring the cross-sectional height of this “adsorbate-free” region (Figure 3b) under regular imaging conditions. The height of the adsorbate layer was estimated to be ≈1.5 nm (insert to Figure 3b). On the basis of the measured height of typical NOM molecules (1-2 nm, see Figure 2c), we conclude that the thickness of the adlayer formed on HOPG under acidic conditions consists of only one or two adsorbed NOM molecules. As the pH increases, the concentration of adsorbed NOM decreases systematically. For example, at pH 6.3 the surface concentration of adsorbed NOM decreased compared to pH 4.0 (compare Figure 3c and 3a). At pH 9.1, the concentration of adsorbed NOM decreases further (Figure 3d) and the resultant adlayer is “patchy” with relatively few adsorbed species. The microscopic changes in the adsorbate layer observed in Figure 3 are consistent with batch studies that have shown NOM sorption onto ACs decreases at higher pH (7-9). Under acidic conditions, NOM macromolecules are predominantly neutral, and adsorption is favored by attractive intermolecular forces between adsorbates that include hydrogen bonding, van der Waals, and π-π interactions. At higher pH, however, deprotonation of carboxylic and phenolic functional groups leads to a higher charge density on the adsorbed NOM (SI, Table 1). Consequently, electrostatic repulsion between adsorbed macromolecules increases and the adsorbed concentration decreases. At a macroscopic level, this phenomena explains why carbon fouling of ACs is favored under acidic conditions (27). Influence of Ionic Strength and [Ca2+]. As revealed by phase imaging, Figure 4a and b illustrates the effect that ionic strength and [Ca2+] exert on the structure of the adsorbate layer (the corresponding topographic images are shown in Figure 4 of the SI). Figure 4a demonstrates that increasing ionic strength leads to a higher concentration of adsorbed species, presumably because electrostatic repulsions between negatively charged NOM adsorbates are more effectively screened. In comparison to the effect of ionic strength, increasing the [Ca2+] produces a more dramatic affect on both the size and surface coverage of adsorbed species. For example, in the native DSW where the [Ca2+] is only ≈0.15 mM (see SI, Table 1), the surface coverage of adsorbed species is relatively small and the step edges and terraces of the HOPG substrate can clearly be resolved (Figure 1c). As the [Ca2+] increases, however, there is a marked increase in the surface coverage. Indeed, for [Ca2+] > 1.5 mM, the adlayer almost obscures the HOPG substrate. In addition to its effect on surface coverage, 1242

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FIGURE 4. AFM phase images showing the influence of (a) ionic strength and (b) [Ca2+] concentration on the adlayers formed by NOM on HOPG. In each experiment, the pH was maintained at 6.3.

TABLE 1. Variation in the Height and Lateral Diameter of Adsorbates Observed in Figure 4 as a Function of Ca2+ Concentration [Ca2+]/mM

height (nm)

diameter (nm)

0.14 1.2 1.6 2.0

0.6 ( 0.2 2.6 ( 1.0 3.0 ( 0.9 4.1 ( 0.6

34 ( 5 43 ( 8 73 ( 12 99 ( 14

analysis of Figure 4b and the corresponding topographic image (SI, Figure 4) reveals that the size of individual adsorbates is also influenced by the [Ca2+]. Image analysis indicates that the average height and diameter of individual adsorbates increases systematically with increasing [Ca2+], as shown in Table 1. For example, upon increasing the [Ca2+] from 0.14 mM to 2.0 mM, the average height of individual adsorbates increases from 0.6 to 4.1 nm while their average lateral (x-y) diameter increases from 34 to 99 nm. A greater concentration of larger adsorbed structures has also been observed when Ca2+ was used as a background electrolyte in AFM studies of NOM adsorbed on mica (19). The synergistic interaction between Ca2+ and NOM is also supported by ex situ XPS studies. Results from these experiments (SI, Figure 3) reveal that a measurable Ca signal is only observed when HOPG is exposed to both NOM and

both affected by drying. Numerous “ringlike” structures are observed that exhibit heights that range from 1.5 ( 0.3 nm with outer diameters 150 ( 22 nm (Figure 5, bottom). Some of the ringlike structures are not completely closed and appear segmented. Similar structures have been reported in studies of humic acid adsorbed on mica (17, 18). We believe that the ringlike structures observed in the present study are NOM aggregates that collapsed to form a ring to maximize intramolecular π-π and van der Waals interactions in the absence of hydration forces. Figure 5 also reveals the presence of several irregularly shaped NOM aggregates with dimensions (average height 6.5 nm) significantly larger than anything observed under aqueous conditions. These aggregates probably result from the adsorption of NOM onto the HOPG surface during the drying process. A comparison of Figure 5 with the AFM images obtained under aqueous conditions clearly demonstrates that significant differences exist in the adsorbate layer created by NOM in the absence of water. Indeed, such drying effects have provided much of the motivation to use analytical techniques such as AFM to probe environmental interfaces in situ at the liquid/solid interface. The present investigation indicates that under environmentally relevant conditions, carbonaceous surfaces will include adsorbed NOM. Results from this study also suggest that NOM and Ca2+ ions are likely to influence the surface characteristics and physicochemical properties of carbonbased nanomaterials, whose surfaces are characterized by graphene-like structures, in aquatic environments.

Acknowledgments

FIGURE 5. (top) Effect of drying on the adsorbate layer formed by NOM on HOPG. (bottom) Line scan through one of the numerous ringlike assemblies observed under ambient conditions. Ca2+. In contrast, no Ca signal was detected when HOPG was [Ca2+]

exposed to the same in the absence of NOM. This result supports the idea that the presence of both Ca2+ and NOM in solution results in the formation of a Ca-containing complex that binds strongly to the HOPG surface. The HOPG surface consists of extended graphene sheets that lack any surface oxides or other polar functional groups (see SI, Figure 2). As a result, the influence of Ca2+ on the adsorbate layer cannot be due to any specific adsorbatesubstrate interactions. The influence of Ca2+ ions is also not simply a result of increasing ionic strength. This is shown explicitly in Figure 4, where the ionic strength of 3.6 mM and 6.0 mM NaCl is identical to 1.2 mM and 2.0 mM CaCl2, respectively. Consequently, changes in the adsorbate layer observed in Figure 4 must be due to the effect of Ca2+ ions on the structure of NOM. The specific ion effect of Ca2+ is a result of its ability to promote intermolecular complexation/ bridging between NOM molecules (28). This leads to the formation of aggregates that contain multiple macromolecules held together by electrostatic interactions between Ca2+ ions and carboxylate anions. As the size of individual adsorbates increases, Figure 4b shows that their surface coverage also increases. This effect can be ascribed to two factors: (1) the preferential adsorption of higher molecular weight macromolecules (29-31) and (2) the decrease in electrostatic repulsion between adsorbed macromolecules because of the addition of Ca2+. Drying Effects. Figure 5 shows the AFM image obtained under ambient conditions after HOPG was exposed to DSW at pH 3.1 for ≈14 h. A comparison of Figure 5 with Figures 1-4 illustrates that the structure of NOM and the adlayer are

The authors are grateful to Professor Charlie O’Melia and his research group for supplying us with the Dismal Swamp Water (DSW). This research also benefited from useful discussions with Dr. Haiou Huang regarding storage and filtration of the DSW prior to use.

Supporting Information Available Chemical characterization of the Dismal Swamp Water (DSW), macromolecular size distribution of NOM measured by size exclusion chromatography, chemical characterization of the HOPG (highly ordered pyrolytic graphite) surface, effect of NOM exposure on the surface composition of HOPG in the presence and absence of Ca2+, influence of ionic strength, and Ca2+ concentration on the microscopic structure of the adlayer formed by NOM on HOPG. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review July 27, 2006. Revised manuscript received November 25, 2006. Accepted November 28, 2006. ES061793D