Environ. Sci. Technol. 1994, 28, 2184-2190
Sorption of Perylene on a Nonporous Inorganic Silica Surface: Effects of Aqueous Chemistry on Sorption Rates Mark A. Schlautman’ and James J. Morgan
Environmental Engineering Science, California Institute of Technology, Pasadena, California 9 1125 The influence of solution chemistry on the initial rate of perylene adsorption to a nonporous inorganic silica surface was investigated. Fluorescence was utilized to monitor the loss of perylene from aqueous solution. At constant ionic strength, the initial rate of adsorption decreased with increasing pH for all background electrolyte compositions. The observed adsorption rates were correlated with the aqueous activity coefficient (riw) of perylene and the surface speciation of silica. At low pH, the rate of perylene adsorption appeared to depend solely on its yiw. At neutral to high pH, binding of cations at the silica surface became increasingly important in determining adsorption rates. Binding of Na+ at the silica surface decreased the rate of perylene adsorption, whereas binding of Ca2+at the surface increased the adsorption rate. The reasons for this difference are not presently known, but may relate to the structure of water at the solution-silica surface when different cations are Dresent in the interfacial region.
Introduction The distribution (i.e., partitioning) of nonionic hydrophobic organic compounds (HOCs) between water and surface soils or sediments has been shown to depend primarily on the hydrophobicity of the compound and the fraction of organic carbon (foe) in the sorbent (1-4). For sorbents with adequate amounts of organic carbon, normalizing observed partition coefficients by foc results in organic carbon partition coefficients (K,) which generally vary only by factors of 3-5 for a given solute. However, when little organic carbon is present in a sediment or aquifer (e.g.,fo,< 0.001),values for KO,have been observed to be much higher than would be predicted on the basis of sorption by organic material alone (5-8). In these loworganic systems, mineral surfaces are thought to contribute significantly to, and may even control, the sorption of HOCs. Because very little research has been done in this area, the interactions between nonionic HOCs and inorganic mineral surfaces are not well understood. Only recently, for example, has a critical review of the topic been completed (9). For sorbents with low amounts of organic carbon, enhanced sorption of nonionic HOCs is favored when the sorbent is a “swelling”or expandable clay material and/or the solutes contain polar functional groups (10, 11). Karickhoff (10) showed that for HOCs with polar moieties (e.g., simazine and biquinoline), more sorption than would be predicted by simple organic matter binding correlations occurs when the organic carbon to expanding clay matter ratio is less than approximately 0.033. The enhanced sorption of polar HOCs appears to result because the polar functional groups can participate in nonhydrophobic
* Author to whom correspondence should be addressed. Present address: Environmental, Ocean and Water Resources Division, Department of Civil Engineering, Texas A&M University, College Station, TX 77843-3136. 2184
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bonding reactions such as hydrogen bonding, complexation, and even covalent bonding with inorganic surfaces (11).
The above authors (9-11),however, have also noted that compounds with polar functional groups are not the only HOCs that can adsorb to mineral surfaces and that inorganic sorbents need not be expandable clay minerals. For example, Schwarzenbach and Westall (5) observed that aluminum oxide, kaolinite (a nonexpanding clay), and porous silica all adsorbed halogenated and aromatic solutes in the absence of organic carbon and that the specific surface area and mineral surface type had large impacts on the amount of sorption. With the exception of the silica, they found a highly significant positive linear correlation between the log of the binding coefficient (K,) and log of the octanol-water partitioning coefficient (Kow) for the sorbents. The micropores in the surface of the porous silica were suspected to be the major cause for the poor correlation for that material, presumably because of steric effects. Stauffer and MacIntyre (12)examined the sorption of four low-polarity HOCs (trichloroethene, 1,Zdichlorobenzene, naphthalene, 1-methylnaphthalene) on three minerals (aluminum oxide, gibbsite, goethite) and on a loworganic carbon aquifer material. In general, they observed that sorption decreased with increasing pH and increased with increasing ionic strength. There were direct relationships found between the observed sorption coefficients and sorbate molar volumes and negative logarithms of sorbate water solubilities. For naphthalene and 1,2dichlorobenzene, sorption coefficients were positively related to the surface areas of the different sorbents. Keoleian and Curl (13) examined the adsorption of tetrachlorobiphenyl on two natural kaolinite clays, one well crystallized and the other poorly crystallized. Equilibrium was achieved in less than 2 h, and the adsorption data were fit by using a one-parameter linear isotherm model. Upon normalizing by the specific surface area of each clay, the adsorption constants for the two kaolinite sorbents were equivalent. They proposed that the specific surface area measured by BET Ndg) can be used to normalize adsorption constants for nonporous, nonexpanding minerals because it gives the total external surface area of particles. For expanding clays and microporous minerals, however, they speculated that normalization by BET Nz(g) surface area would not be useful. Keoleian and Curl (13) postulated that the small adsorption constants and the linearity of the isotherms suggested a nonspecific physical adsorption mechanism and that both the hydration of the aluminum oxide surface and the hydrogen bonding of water to the silanol groups of kaolinite were energetically more favorable interactions than the van der Waals energy of attraction between tetrachlorobiphenyl and the surface. Murphy et ai. (14) investigated the adsorption of three HOCs to hematite and kaolinite. Because the mineral surfaces were pretreated to remove organic carbon, sorption of the HOCs was low. Single0013-936X/94/0928-21S4$04.50/0
0 1994 American Chemical SOClety
concentration (i.e., one-point) adsorption constants, normalized by BET NZ(g) surface areas, were higher for kaolinite than for hematite. They speculated that the higher sorptivity on kaolinite resulted because the siloxane and gibbsite basal planes on its surfaces contain few ionizable hydroxyl sites and thus exhibit more hydrophobic characteristics than does a hematite surface. In contrast to Schwarzenbach and Westall (5) and Curtis et al. (6), Murphy et al. (14) observed that adsorption constants for the different HOCs on a particular mineral did not increase uniformly with KO,. However, because only one of the HOCs used by Murphy et al. (14) was a nonpolar compound, other surface reactions in addition to hydrophobic bonding were probably occurring. The most extensive investigation of (nonpolar) HOC adsorption by bare mineral surfaces to date has been by Backhus (15). She examined the sorption of a series of HOCs [polycyclic aromatic hydrocarbons (PAHs) and chlorinated benzenes] to three inorganic surfaces (kaolin, silica, alumina). Her studies showed that adsorption was complete within approximately 10 h and that isotherms were linear up to aqueous phase daturation. In agreement with Schwarzenbach and Westall (5)and Curtis et al. (61, Backhus (15) observed that the association of an organic solute with an inorganic surface increased with increasing HOC aqueous activity coefficient (Ti”). The adsorption of a particular HOC to the different inorganic surfaces resulted in similar association constants when normalized by the available surface area. Backhus (15) concluded that the process by which nonpolar HOCs associate with inorganic surfaces is driven primarily by changes in the interactions and organization of the solvent water and not by a specific interaction of the HOCs with inorganic surfaces. This same driving force, the so-called “hydrophobic effect” (16-18), is primarily responsible for partitioning of HOCs between organic carbon and water. The observation that adsorption constants for different HOCs with a particular mineral surface increase uniformly with solute hydrophobicity (5,6,12,15)is consistent with the hydrophobic effect. For example, it has been shown in solubility and adsorption studies that the removal of each methylene group from aqueous solution contributes -RT to the total energy of reaction for aliphatic organic compounds (16, 17, 19,201. Previously, we reported on the effects of aqueous chemistry on the binding of a nonionic, nonpolar HOC (perylene) by dissolved and adsorbed aquatic humic substances (21, 22). Because perylene is a very hydrophobic PAH (log KO,> 6), even in the presence of humic materials it adsorbs appreciably to inorganic surfaces, as verified by its uptake on the fluorescence cells utilized in these investigations. During these previous studies, we observed trends developing for perylene sorption to the cells with varying solution chemistry conditions. Because of the potential importance of mineral sorption of HOCs in low-organic carbon environments, the adsorption of perylene by this nonporous silica surface was investigated further. The focus of this paper is primarily on the initial rate of the adsorption reaction between perylene and the inorganic cell surface. A simple hypothesis guiding the present study was that the rate of adsorption of an HOC by a bare mineral surface would depend on the aqueous activity coefficient, yiw,of the hydrophobic solute and on the relative hydrophobicity (i.e., polarity) of the inorganic surface. Because both yiw and the surface polarity are
affected by the aqueous chemistry of natural waters, a systematic study of the effects of pH, ionic strength, and the presence of bivalent cations was undertaken. The objectives of this investigation, therefore, were to (1) determine the initial rate of adsorption of perylene to an inorganic surface and (2) investigate the effects of varying solution chemistry on the rate of adsorption.
Experimental Section Materials. Perylene (Aldrich, 99+ 3’ 2 pure) was the fluorescent hydrophobic organic probe used in this study. Relevant chemical and physical properties of this nonpolar, nonionogenic PAH compound were tabulated previously (21). A conservative fluorescent probe, rhodamine 110 (Lasergrade),was obtained from Kodak. Both probes were used without further purification (23). Solution preparations and additional information were reported previously (21, 22). The model for mineral surfaces was the interior walls of fused silica fluorescence cells (Spectrosil, Starna Cells Inc.) having a path length of 10 mm. Fused silica cells contribute very little background fluorescence relative to quartz cells and are generally recommended for fluorescence work. The choice of this inorganic surface provided several benefits: (1) the ability, using fluorescence, to measure the rates of adsorption on short time scales (i.e., minutes), which has not been previously reported; (2) the capability to follow the adsorption process without having to separate the solid and aqueous phases; and (3) the applicability of fused silica as a model nonporous mineral surface having well-known surface characteristics. The fluorescence cells were cleaned between experiments by soaking and rinsing with copious amounts of methanol and then deionized, distilled water (DDW;Corning MegaPure System) to assure perylene removal. Cell cleanliness was monitored with fluorescence and absorbance measurements, and in no case was the sorption of humic materials to the cell walls detected. To assure repeatability between experiments, the cells were filled with DDW when not in use so that the surfaces would remain fully hydrated. Procedure. The initial rate of perylene adsorption to the inorganic silica surface of fluorescence cells for various aqueous chemistry conditions was studied in the presence and absence of humic materials by following a procedure adapted from Backhus and Gschwend (24). The procedure enables the kinetics of perylene partitioning between the humic material and water to be separated from the kinetics of perylene adsorption to the cell walls. Detailed experimental procedures have been reported previously (21,22). Briefly, a typical experiment consisted of pipeting 3 mL of a sample of known pH, background electrolyte@),and humic material concentration (typically 0-10 mg/L) into a fluorescence cell and measuring the appropriate background absorbance and fluorescence intensities. The sample was then spiked with 10pL of a combined perylenerhodamine 110stock solution, and the cell was immediately placed in a (dark) stirring module of the spectrophotometer. Initial perylene concentrations were nominally 1.6 nM. After a 3-min period, which allowed the perylenehumic reaction to be completed, fluorescence and absorbance measurements for perylene were made as a function of time for a total period of 40 min. Spectroscopic measurements were also made for rhodamine 110 to account for the slight variation in spiking volume among the different samples. Environ. Sci. Technol., Vol. 28, No. 12, 1994 2185
Different relative rates of mixing were used to investigate the importance of stirring in the cells. Because stirring rates were not quantified, however, the different rates of mixing are described only qualitatively (e.g., no mixing, moderate mixing, and rapid mixing). The different mixing conditions were obtained by using the same (approximate) flow control valve settings on the air-driven stirring module. Stirring in the cells was achieved using a Tefloncovered micro stir bar. Experiments showed that sorption of perylene by the stir bar was negligible over the 40-min period examined, but became substantial over much longer time periods (23). The insignificance of short-term perylene sorption by the stir bar was verified by two separate techniques. In the first, comparisons of fluorescence loss with time among samples with and without a stir bar (no mixing for each) revealed no significant differences. In the second, separate extractions of the cells and the stir bars after 40-min showed perylene uptake by the stir bars to be small.
0
I
I
I
Results and Discussion
Methods Verification. (A) Rate of Perylene Adsorption to Cell Walls. The adsorption of PAHs to inorganic surfaces (in the presence and absence of humic substances) was modeled on short time scales by Backhus and Gschwend (24)as a first-order, reversible rate process: PAHd F= PAH,,,
(1)
where the subscripts d and ads refer to PAH molecules that are dissolved and associated with an inorganic surface, respectively. Because fluorescence intensity is proportional to [PAHd], the rate of PAH adsorption to an inorganic surface (Le., loss of PAHd from solution) can be monitored by fluorescence (however, see the two caveats below). For the initialcondition [PAHdl= [PAHTI,where [PAHT] is the total PAH concentration (i.e., [PAHTI = [PAHdl+ [PAHad,]),Backhus and Gschwend (24)derived the following equation to describe the observed fluorescence versus time:
where FOis the fluorescence at time zero and k , and k , are the first-order forward and reverse rate constants for adsorption to the surface. Equation 2 is of the form
F =B
+ Me-kobt
(3)
where kobs is a fluorescence decay constant, B is the predicted PAH fluorescence at equilibrium, and M = FO - B, a measure of the adsorbed PAH predicted at equilibrium. These parameters were obtained for each experiment by fitting eq 3 to measured fluorescence intensities of the spiked samples with a nonlinear leastsquares curve-fitting program. The reaction in eq 1 can be described equally well by assuming a mass transfer process:
where cy is a mass transfer coefficient and the subscript eq denotes the equilibrium measurement. The solution is of the same form as eq 3 with kobs equivalent to the mass 2188
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transfer coefficient cy. Because of the difficulty in uncoupling mass transfer from adsorption reactions, kobs is used here only as a measure of the initial rate of adsorption (e.g., an observed rate constant). The initial rate of perylene adsorption to fluorescence cell walls was studied over short time periods (40 min). In the presence and absence of humic materials, measurements of perylene fluorescence decreased exponentially with time in accord with eq 3 (e.g., Figure 1). The distinct and parallel curves obtained for various humic material concentrations indicate that (1)the binding of perylene by humic substances is very fast relative to the adsorption of perylene by the silica surface and (2) the two reactions are relatively independent of each other on short time scales (24). In other words, the presence of the inorganic surface does not interfere with the humic-perylene binding reaction and the presence of humic substances does not affect the initial rate of perylene adsorption to silica. This was verified by the similarity of kobs values in the presence and absence of humic substances at fixed solution chemistry and mixing conditions. Humic materials in the system only change the initial perylene solution concentration (i.e., the initial amount of perylene available to be adsorbed at the silica surface). In effect, samples with the same amount of total perylene and varying humic material concentrations are analogous to data points in an isotherm experiment with regard to the amount of perylene adsorbed at a silica surface versus the concentration of dissolved perylene (24). Two important caveats should be noted concerning the use of fluorescence loss as an indication of PAH adsorption to mineral surfaces. First, our experiments revealed that adsorbed perylene could be completely recovered from fluorescence cell walls in 5-10 min with one methanol extraction (data not shown), The quantitative recovery of perylene is a direct measure of its adsorption to the inorganic surface. Without this information, the decay in fluorescence intensity with time (e.g., Figure 1) is only a measure of perylene being lost from solution. Because other processes (e.g., photodegradation) can potentially deplete PAH concentrations in water, adsorption of perylene to the cell walls can only be inferred without mass balance data. Second, in the study of Backhus and
Table 1. Effect of Mixing on the Initial Rate of Perylene Adsorption (kot,s)8~b 0.001 M
4.65 f 0.37 (11) 4.48 f 0.52 (12) 3.48 f 0.62 (10)
4.44 f 0.36 (11) 4.19 f 0.43 (12) 2.98 f 0.57 (12)
rapid mixing moderate mixing no mixing
0.1 M
1mM Ca2+
4.80 f 0.37 (13) 4.78 f 0.48 (12) 3.74 rt: 0.63 (14)
4.85 f 0.39 (13) 4.80 f 0.47 (12)
0.01 M
ndd
a pH 4. * Values for kobs (h-1) are reported f one standard deviation for the number of samples in parentheses. Total ionic strength of 0.1 M. d nd, not determined.
Table 2. Aqueous Chemistry Effects on the Initial Rate of Perylene Adsorption Coefficient ( y i ~ ) a , b 0.001 M 0.01 M 0.1 M 1mM Ca2+
(kobs)
and on the Perylene Activity
PH 4
PH 7
pH 10
4.19 f 0.43 (12) 4.48 f 0.52 (12) 4.78 f 0.48 (12) 4.80 f 0.47 (12)
4.08 rt: 0.41 (13) 4.02 f 0.32 (11) 3.98 f 0.40 (13) 4.33 f 0.44 (12)
3.40 & 0.41 (11) 3.19 A 0.20 (11) 3.01 & 0.35 (12) 3.70 f 0.32 (10)
0 Moderate mixing, * Values for kobs (h-l) are reported strength of 0.1 M.
f
yiw
(x10-7)
8.72 8.79 9.46 9.46
one standard deviation for the number of samples in parentheses. Total ionic
c. n- I
I
d.V
5.0
I
A
I
Moderate Mixing
I
/
DH 7
I
3.54
25 lo-’
10-9
lo-’
ionic Strength (M)
ionic Strength (M)
Figure 2. Effects of mixing on the initial rate of perylene adsorption to silica. All data are for samples at pH 4. Open symbols connected by lines are for samples in NaCl solutions. Single (solid) points at 0.1 M ionic strength are for samples with 1 mM Ca2+.
adsorption to silica. All data are for moderate mixing conditions. Open symbols connected by lines are for samples In NaCl solutions. Single (solid) points at 0.1 M ionic strength are for samples with 1 mM Ca2+.
Gschwend (24) and in this study, the observed decreases in PAH fluorescenceupon adsorption to inorganic surfaces resulted because of the removal of the fluorescing species from aqueous solution and (more importantly) from the path of fluorescence detection (23). Contrary to two reported studies (25,26),physical adsorption of PAHs to bare mineral surfaces does not result in the static quenching of PAH fluorescence. For example, the use of inorganic materials as substrates for studying the photochemistry of physically adsorbed organic molecules and the use of fluorescent organic molecules to probe the solidliquid interfacial region of inorganic colloids have been extensively documented (27). (B) Rate of Mixing. The effect of different stirring rates on the short-term adsorption of perylene to the fluorescence cell walls was examined for three conditions: no mixing, moderate mixing, and rapid mixing. Mixing effects were investigated at pH 4 for various background electrolyte compositions. A notable effect on the initial rate of perylene adsorption was observed for the different stirring rates, as shown in Figure 2. Although more vigorous mixing increased the value O f kobs for all samples, the increase was relatively modest between the moderately and rapidly mixed samples (Table 1). These results are consistent with the observations of Backhus (15). She
examined the adsorption of perylene to glass walls and observed that equilibration was faster for samples shaken on a wrist-action shaker than for samples that had no mixing. Because kobs increases when the mixing rate is increased, a mass transfer step must contribute to the initial rate of perylene adsorption when no mixing is maintained. Similarly, mass transfer limitations are also observed for moderately mixed conditions, but their influence on the initial rate of perylene adsorption is expected to be much less important. Aqueous Chemistry Effects on the Rate of Perylene Adsorption. For all aqueous chemistry experiments, the rate of mixing was qualitatively characterized as moderate. Average values of hobs for the samples having similar solution chemistry conditions are listed in Table 2, and the effects of aqueous chemistry on the initial rate of perylene adsorption are shown in Figure 3. At constant ionic strength, values of kobs decreased with an increase in pH for all samples, both with and without the addition of 1mM CaC12. For the samples containing only NaCl as the background electrolyte, the pH effect increased with increasing ionic strength. In NaCl solutions, increases in the electrolyte concentration resulted in an increase in kobs for samples at pH 4 whereas the opposite trend was observed at pH 7 and pH 10. The presence of 1mM Ca2+
Flgure 3. Effects of aqueous chemistry on the initial rate of perylene
Environ. Sci. Technol., Vol. 28, No. 12, 1994 2187
Table 3. Reactions and Intrinsic Equilibrium Constants for Surface Ionization and Electrolyte Adsorption on Fused SilicasSb lo@> 80
SiOHZ++ SiOH + H+ pKal = -3.2 SiOH + SiO- + H+ pKa2 = 7.2 Na+ + SiOH SiO-Na + H+ p*KNat = 6.7 Ca2++ SiOH + SiO-Ca+ + H+ p*Kca2+= 3.8 Ca2++ HzO + SiOH + SiO-CaOH + 2H+ P*KCaOH+ = 3.8 C1 = 1.295 F/m2, Cz = 0.2 F/m2,Ns = 5 sites/nm2
Increasing [NaCI]
c 0
0
Constantsfrom Rea and Parks (28)are for use with a triple-layer surface complexationmodel where C1 and C2 are the inner and outer Helmholtz capacities,respectively,and& is the site densityof surface hydroxyl groups. The formulas of surface complexes indicate the assumed location of constituents of the complex. The entire surface complex is located directly on the surface unless a dash is present in the formula. If a dash is present, everythingto the left of the dash is located on the surface and everything to the right of the dash is located at the outer Helmholtz plane.
- - .’
I
,
- n u - - - - - -
5
4
6
’ .
/
I
c
I
I
I
I
7
8
9
10
11
PH
(0.1 M total ionic strength) had little effect on kobs at pH 4 relative to monovalent electrolyte solutions. At pH 7
and pH 10, however, the presence of ea2+increased the rate of perylene adsorption markedly compared with NaCl solutions of similar total ionic strength. From these results, a rudimentary understanding of the role of aqueous chemistry in the process(es) by which nonpolar, nonionic HOCs associate with inorganic surfaces can be obtained. As stated earlier, we expected the rate of perylene adsorption to depend on its yiw and on the relative hydrophobicity/polarity of the silica surface. Any observed changes in the adsorption rate, therefore, would result because of a change in either one or both of these parameters. To gain insight into the molecular-level adsorption mechanism(s), the effects of aqueous chemistry on HOC association with mineral surfaces must be understood. Changes in aqueous chemistry can affect both HOC and mineral surface properties, as well as their interactions with water and eachother. Examples of these changes are the “salting out” of HOCs with increasing salinity and the protonation-deprotonation of surface hydroxyl groups on metal oxides. Because yiw and the surface hydrophobicity of silica are both affected by aqueous chemistry conditions, interpretation of the effects of pH, ionic strength, and the presence of Ca2+on perylene adsorption to silica can be complicated. By understanding how changes in aqueous chemistry affect the properties of the silica surface and dissolved perylene separately, however, an idea of the important processes driving the adsorption reaction can be obtained. To characterize the effects of solution chemistry on the inorganic silica surface, surface chemical reactions of the cell walls were modeled using SURFEQL and equilibrium constants from Rea and Parks (28). SURFEQL is a computer code adapted from MINEQL (29) to solve equilibrium surface speciation calculations (30). The model-dependent equilibrium constants from Rea and Parks (28) are applicable for use with the triple-layer surface complexation model. The intrinsic equilibrium constants were originally determined for pyrogenic silica and were utilized (with slight modifications) by Rea and Parks (28) to simulate inorganic ion adsorption on quartz. Pertinent reactions and corresponding equilibrium constants are shown in Table 3. The surface speciation of silica for the solution chemistry conditions of this study is shown in Figure 4. It can be seen that neutral surface 2188
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1
+
40-
K
I
__ >SiOH >s10-
/
_ .
20-
-
/-
>StO-No >SIO-Co’
/
/
/
/-
/ /
-
/
0--
c
/
______-----
/
I
1
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I
PH Figure 4. Speciation of an inorganic sillca surface using a triple-layer surface complexation model and the surface parameter values from Table 3. (a) Simulations shown are for NaCl concentratlonsof 0.001, 0.01, and 0.1 M, respectively. (b) Simulation shown is for 1 mM CaClz at a total ionic strength of 0.1 M.
hydroxyl groups dominate the distribution over the pH range of most natural waters and that adsorption of metal cations becomes important only at neutral to high pH values. In NaCl solutions, the net charge on the silica surface is negative above approximately pH 2. With the addition of 1 mM CaC12 (0.1 M total ionic strength), the net charge at the surface becomes positive above pH 4 because of the surface exchange reactions in which some bivalent cations replace protons at surface hydroxyl sites (Table 3, Figure 4b). The effect of solution chemistry on HOCs can be quantified with an aqueous activity coefficient (23, 31). Briefly, the solubility of perylene in various salt solutions was estimated by use of the Setschenow relationship and perylene solubility data in distilled water (32),and values of y i w were then calculated after correcting for the energy “cost” of melting the solid solute. These values are listed in Table 2. In Figure 3, it is observed that the initial rate of perylene adsorption was affected by pH, ionic strength, and the presence of Ca2+. Because the driving force for perylene removal from aqueous solution depends on the fugacity of the hydrophobic solute, an increase in yiwshould result in a faster rate of adsorption to an inorganic surface if the adsorption reaction is not too complicated (e.g., ratelimiting step is an elementary process). Similarly, if a
mineral surface is “hospitable” to a hydrophobic molecule, one might expect the rate of adsorption to depend on the degree of this characteristic. Because yiwincreases with increasing ionic strength (Table 2), a concurrent increase in kobs would be expected for systems in which the energy driving adsorption results from solute fugacity. This may explain the trend observed at pH 4 in Figure 3. Because very little charge is developed on the silica surface at pH 4, the surface charge density is relatively low (Figure 4). Regardless of the composition of the background electrolyte, essentially 100% of the sites are neutral hydroxyl groups. A similar increase in kobs with increasing ionic strength is not observed in NaCl solutions at pH 7. Because changes in pH have little effect on yiwfor perylene, a driving force similar to that exerted at pH 4 should be operative at pH 7; therefore, the surface properties appear to be impeding the rate of adsorption at this pH. In Figure 4a, it is observed that increases in the NaCl concentration at pH 7 result in increasing amounts of Na+ bound at the silica surface. At pH 10, the fraction of sites binding Na+ becomes quite significant with increasing ionic strength. Consistent with the results at pH 7, a considerable decrease in kobs with increasing NaCl concentration at pH 10 is observed. Trends between silica surface speciation and rates of perylene adsorption can also be seen in the presence of Ca2+. At pH 4, very little Ca2+ is associated with the inorganic surface (Figure 4b) and the effect of Ca2+on the rate of perylene adsorption is minimal relative to a NaCl solution (Figure 3). At pH 7 and 10, however, the binding of Ca2+is appreciable at the silica surface; large differences in adsorption rates compared with those for NaCl solutions are also observed for samples with Ca2+at these pH values. Interestingly, while the binding of Na+to silica suppresses the initial rate of perylene adsorption, the presence of surface-bound Ca2+appears to favor the rate of adsorption. The reasons for these observations are not clear. I t may be possible that the presence of Ca2+ near the surface somehow alters the structure of water (i.e.,the organization of water molecules) in the interfacial region in a manner that makes the surface more receptive to hydrophobic molecules. Although the presence of Na+ at the silica surface also influences the perylene adsorption rate, its effect is opposite to that observed for Ca2+. It is obvious that more investigations are needed to fully understand the effects of these, and other, adsorbed cations on HOCmineral surface interactions. Significance for Environmental Systems. Prior studies of nonpolar HOC adsorption on inorganic surfaces have conflicted on the role of the mineral surface in the adsorption reaction. For example, some researchers have observed that the type of surface was important in HOC adsorption (5, 141, whereas others have found that differences in HOC adsorption for their minerals could be explained by differences in surface area (13,15). Several researchers have shown that a linear free energy relationship exists between the extent of nonpolar HOC adsorption (normalized by surface area) on bare minerals and HOC yiwS (e.g., refs 9 , 1 5 , 2 3 , and 31). From these compilations, it appears that both arguments on the role inorganic surfaces play in adsorption may be correct, depending on the range of yiwused to evaluate the data. In the studies of Schwarzenbach and Westall ( 5 )and Murphy et al. (14), discrepancies among values were found for different sorbents, but adsorption was not examined over a wide
range of HOC yiws. If all of the above referenced compilations are examined over a narrow range of solute hydrophobicity, differences in the extent of adsorption are indeed observed for various inorganic surfaces. When the data are examined over a wide range of solute hydrophobicity, however, a clear correlation exists when adsorption constants are normalized by the surface area of the minerals. This suggests that the primary driving force for HOC adsorption to inorganic surfaces is solute hydrophobicity, whereas the type of inorganic surface is a less important parameter. Changes in aqueous chemistry can affect the characteristics of both an HOC and a mineral surface. For nonpolar, nonionogenic HOCs, the primary effect will be changes in solute fugacity from ionic strength differences that can be incorporated into yiwthrough a correction with the Setschenow relationship. From the rate results of this preliminary study, we propose that the influence of aqueous chemistry, as it affects the surface speciation of minerals, may be another ancillary parameter for nonpolar HOC adsorption to inorganic minerals. These solution chemistry effects may be similar to the differences in mineral type, because each can affect the relative surface hydrophobicities/polarities. Future studies will investigate whether the aqueous chemistry effects observed here for adsorption rates can have a similar impact on equilibrium adsorption results.
Summary and Conclusions The effects of solution chemistry on the initial rate of perylene adsorption to a nonporous inorganic silica surface were important. At constant ionic strength, the rate of adsorption decreased with increasing pH for all samples, in both the presence and the absence of Ca2+. In NaCl solutions, increases in the electrolyte concentration resulted in an increase, a small decrease, and a larger decrease in adsorption rates for samples at pH 4, 7, and 10, respectively. The presence of Ca2+had little effect on the rate of perylene adsorption at pH 4 relative to NaCl solutions; however, at pH 7 and 10, the presence of Ca2+ greatly increased the adsorption rate compared with NaCl solutions of similar total ionic strength. The aqueous activity coefficient of perylene and the surface speciation of silica were determined for the solution chemistry conditions examined in an attempt to correlate observed adsorption rates with solute and surface properties. Changes in the observed rate of adsorption at pH 4 appear to depend solelyon the fugacity of perylene, because little change in the surface speciation of silica occurs with varying electrolyte composition at this pH. At pH 7 and 10,however,the binding of cations at the inorganic surface becomes more important. In NaCl solutions at pH 7, little variation in the rate of adsorption is observed with increasing ionic strength because the increased binding of Na+ at the silica surface apparently offsets the increasing perylene fugacity. At pH 10,the decreasing surface affinity for perylene suppresses the solute’s increasing fugacity and the rate of adsorption decreases with increasing NaCl concentration. Binding of Ca2+at the silica surface had an effect on the rate of perylene adsorption opposite to that of Na+; at pH 7 and 10, the adsorption rate increased in the presence of Ca2+compared with NaCl solutions. A possible explanation for the contrasting results observed with these two cations is that the manner in which Na+ Environ. Sci. Technoi., Vol. 28, NO. 12, 1994 2189
and Ca2+structure water molecules near the silica surface differs. The role of a mineral surface in the adsorption of nonpolar HOCs has been debated by previous researchers. As discussed by Karickhoff (IO),initial model development for describing adsorption reactions in complex systems depends on elucidating the essential phenomenological behavior while neglecting “second-order” effects. In the context of this work, we propose that the adsorption of a particular HOC to inorganic surfaces will depend primarily on the surface area of the minerals. Variations in adsorption for different types of inorganic surfaces and/ or for the changing surface chemical characteristics of a particular mineral species brought about by varying aqueous chemistry conditions will then be observed as second-order effects (Le., of secondary importance in select applications). Acknowledgments
We gratefully acknowledge Deb Backhus, Steve Eisenreich, and Phil Gschwend for their helpful discussions, Elizabeth Carraway for the nonlinear least-squares curvefitting program used to analyze fluorescence data, and the three reviewers for their constructive comments. This work was supported by grants from the Andrew W. Mellon Foundation, William and Flora Hewlett Foundation, Smith and Louise Lee Memorial Endowment, San Francisco Foundation (Switzer Foundation Environmental Fellowship), and the American Water Works Association (Larson Aquatic Research Support Ph.D. Scholarship). Elements of this paper were presented at the Symposium on Physical-Chemical Processes Controlling Contaminant Mobility in Aquatic Environments at the 207th American Chemical Society Meeting in San Diego, CA, March 1318, 1994. Literature Cited Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979, 13, 241-248. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science 1979,206, 831-832. Means, J. C.; Wood, S. G.; Hassett, J. J.; Banwart, W. L. Environ. Sci. Technol. 1980, 14, 1524-1531. Chiou, C. T.; Peters, L. J.; Freed, V. H. Science 1981,213, 683-684. Schwarzenbach, R. P.; Westall, J. Environ. Sci. Technol. 1981, 15, 1360-1367. Curtis, G. P.; Roberts, P. V.; Reinhard, M. Water Resour. Res. 1986,22, 2059-2067. Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,25, 1223-1237. Ball, W. P.; Roberts, P. V. Environ. Sci. Technol. 1991,25, 1237-1249. Backhus, D. A.; Gschwend, P. M.; Eisenreich, S. J. Environ. Sci. Technol. submitted for publication. Karickhoff, S. W. J . Hydraul. Eng. 1984, 110, 707-735. Curtis, G. P.; Reinhard; M.; Roberts, P. V. In Geochemical Processes at Mineral Surfaces; Davis, J.A., Hayes, K. F.,
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Received for review March 14, 1994. Revised manuscript received August 4, 1994. Accepted August 5, 1994.” 0
Abstract published in Advance ACS Abstracts, September 15,
1994.
