Environ. Sci. Technol. 2007, 41, 547-554
Hydration-Assisted Sorption of a Probe Organic Compound at Different Peat Hydration Levels: The Link Solvation Model E. R. GRABER,* L. TSECHANSKY, AND M. BORISOVER Institute of Soil, Water and Environmental Sciences, The Volcani Center, Agricultural Research Organization, Bet Dagan, 50250, Israel
Sorption isotherms of phenol on Pahokee Peat as model natural organic matter (NOM) have been measured at different partial NOM hydrations (water activities). Sorption at a given phenol solution concentration is substantially smaller in the lower water activity systems than in higher water activity systems, reaching a sorption maximum at an intermediate water activity. Such cooperative phenol uptake at interim water activities as a result of NOM hydration (hydration-assisted sorption) is predicted by the link solvation model (LSM), whereby water enhances the disruption of the noncovalently cross-linked NOM structure, creating new sorption sites. The LSM is herein extended to account for the observed direct relationship between isotherm linearity and water activity. The extended LSM provides an excellent description of phenol sorption isotherm data at nine different NOM hydration levels with a single set of three unique parameters. The successful fit of the LSM supports the conceptual model of creation of new sorption sites for sorbate molecules in the hydrated organic matter sorbent, accompanied by competition for those new sites by water molecules at high water activities.
Introduction The role of soil moisture in diminishing the sorption of organic compounds at mineral surfaces has been long recognized and extensively studied (1-12). Works examining the effect of hydration status of natural organic matter (NOM) on sorption isotherms of organic chemicals are fewer (6, 8, 13-16), although recently, this topic has been attracting more attention. This is because the structure of the NOM phase, which controls the sorption of many organic chemicals in soils, undergoes substantial changes upon hydration (swelling, increased flexibility, conformation changes, and shifts in ionization status of polar functional groups, refs 17-21). Thus, it can be anticipated that hydration-driven changes in NOM may strongly affect sorption interactions of organic compounds. Yet, the general understanding of the influence of NOM-associated water on NOM interactions with organic compounds is incomplete. This lack of knowledge also contributes to a limited understanding of the mechanisms that control sorption of organic compounds by NOM from water, and by NOM at varying water activity. To elucidate the effect of NOM hydration on sorption of organic compounds, we embarked on a series of studies of * Corresponding author phone: 972-3-968-3307; fax: 972-3-9604017; e-mail:
[email protected]. 10.1021/es061274a CCC: $37.00 Published on Web 12/10/2006
2007 American Chemical Society
sorption of probe organic compounds on a model NOM (Pahokee Peat; International Humic Substances Society; IHSS) under completely hydrated and completely dry conditions (13-15, 22). For compounds that lack the ability to undergo strong specific (H-bonding) interactions with NOM (15), such as trichloroethylene, nitrobenzene, carbon tetrachloride, and acetophenone, activity-based sorption was found to be the same or slightly lower in hydrated peat than in dry peat (8, 13, 15). In contrast, activity-based sorption of strongly interacting compounds (e.g., phenol, m-nitrophenol, pyridine, benzyl alcohol) was much greater in hydrated peat than in dehydrated peat (13, 15), an effect termed “hydrationassisted sorption.” These observations were explained by a new conceptual model invoking the formation of sorption sites upon NOM hydration (13-15, 22). The site formation mechanism involves cooperative water-assisted disruption of noncovalently linked NOM contacts formed by functional groups, with sorption of organic sorbates at moieties of the disrupted contact. These moieties are usually less available for compound sorption in dry NOM due to strong inter- and intraNOM interactions (such as H-bonding, proton-transfer phenomena, bridging via metal cations). Strong specifically interacting compounds successfully compete with the solvent for moieties at the disrupted contact, while weakly interacting compounds do not. It was shown that the greater a compound’s ability to undergo specific interactions with NOM, the greater the hydration-assisted sorption effect, thus demonstrating the importance of such noncovalent functional group links in NOM organization (13, 14). The conceptual model is shown schematically in Figure 1. Supporting this conception, pyridine sorption in systems of varying extents of NOM solvation (by acetonitrile) exhibited a cooperative effect of the solvent on pyridine sorption at intermediate solvation extents (acetonitrile activity 0.3-0.6), suggesting that a number of points along the NOM-contact were solvated simultaneously to achieve extensive solvationassisted sorption. Pyridine sorption exhibited a maximum at acetonitrile activity between 0.7 and 0.8 and declined at higher acetonitrile activities. This suggests that at higher levels of solvation, solvent molecules successfully competed with sorbate molecules for moieties of the disrupted contact (23). The net result of the tradeoff between solvent-assisted penetration of organic compound molecules into functional group linked-contacts, versus competition between sorbate and solvent molecules for new sites at those disrupted contacts will depend on the ability of the compound to interact with the newly created sites. To formalize the conceptual model, an analytical sorption isotherm model (Link Solvation Model; LSM) describing penetration and interactions of solute and/or solvent molecules in noncovalent inter- and intramolecular contacts in the NOM structure was developed (22, 24). The isotherm model successfully described the observed cooperative solvent effect (i.e., simultaneous participation of several solvent molecules) on sorption of pyridine by NOM, and reproduced the experimentally observed maximum in pyridine uptake at an intermediate active solvent activity (23). The LSM can describe different situations such as sorbate interactions with NOM links disruptable by water molecules alone (typically considered part of NOM swelling upon hydration) and simultaneous penetration of solute and solvent molecules into NOM contacts that are less available for only sorbate or solvent molecules. The conceptual LSM model can also explain oft-observed desorption hysteresis, as cooperatively disrupted NOM links can hardly be exVOL. 41, NO. 2, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic of hydration-assisted sorption conceptual model. In this representation, the m parameter of the LSM ) 9. Organic matter fragment is adopted from part of the 2-dimensional molecular structure of humic acid given in (41). pected to return to their initial equilibrium condition upon desorption. Support for this model has been recently published (16) in a study of gas phase sorption of 188 compounds on leonardite humic acid at up to three different hydration levels. It was found that polar aromatic compounds and aliphatic alcohols sorbed more strongly in hydrated leonardite humic acid than in dry humic acid, while nonpolar compounds preferred dry humic acid. These results were attributed to the link solvation mechanism. This data set relies almost exclusively on single point sorption data (16), and as such, can only afford real comparisons if sorption isotherms for all the compounds are indeed linear. Until now, a test of the LSM conception and model by measuring sorption isotherms of a probe compound under a full range of partial organic matter hydration levels has been lacking. The express purpose of the current study was to fill this gap. By varying hydration levels of the model organic matter, we can probe the effect of different extents of waterinduced changes in the sorbent (link disruption, sorbatesolvent competition, etc.) on sorption. By comparing the extent of sorption at a given hydration level to the extent of sorption in the dry system (defined as the hydration effect), we can distinguish between situations where (i) the level of hydration is too low for contact disruption; (ii) the extent of hydration is sufficient for disruption but water activity in the sorbent phase is too low to compete with strongly interacting compounds for newly formed sorption sites at disrupted contacts (hydration-assisted sorption); and (iii) the extent of hydration is ample for disruption, but water activity in the sorbent is sufficient to compete for newly created sorption sites (decrease in hydration-assisted sorption). We also test for cooperativity in hydration-assisted sorption as a function of both water activity and sorbate activity. Cooperativity can be important for understanding related phenomena such as desorption hysteresis. Finally, this data set enables us to refine and generalize the LSM sorption model for hydrated systems.
LSM Theoretical Background The following is an abbreviated description of the LSM development, found in its entirety in a previous publication (24) and summarized in ref 22. The model is predicated on the assumption that sorbate-sorbent interactions occur at sites that are distributed throughout the NOM sorbent. The sites are considered to consist of a noncovalently linked contact formed by chemical groups or fragments, depicted as {-P‚‚‚P′-}. The entire {-P‚‚‚ P′-} link may be a sorption site, as in the traditional Langmuir model. In our departure from the traditional models, we conceive of a {-P‚‚‚ P′-} 548
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link that may also be disrupted by a sorbate molecule (denoted by A), or by a sorbed solvent molecule (denoted by S), thus creating new sorption sites. The disruption may be slow, resulting in slow sorption kinetics. The link disruption mechanism is formalized in terms of chemical equilibria:
{-P‚‚‚P′-} + A h {-PA‚‚‚P′-}
KA )
θPA‚‚‚P′ θPP′aA
(1)
{-P‚‚‚P′-} + A h {-P‚‚‚AP′-}
K ′A )
θP‚‚‚AP′ θPP′aA
(1′)
{-P‚‚‚P′-} + 2A h {-PA‚‚‚AP′-} KA2 )
{-P‚‚‚P′-} + S h {-PS‚‚‚P′-} KS )
θPA‚‚‚AP′ θPP′a2A
θPS‚‚‚P′ θPP′aS
{-P‚‚‚P′-} + S h {-P‚‚‚SP′-} K ′S ) θP‚‚‚SP′
(2) (3) (3′)
θPP′aS
{-P‚‚‚P′-} + 2S h {-PS‚‚‚SP′-} KS2 )
θPS‚‚‚SP′ θPP′a2S
{-P‚‚‚P′-} + A + S h {-PA‚‚‚SP′-} KAS )
{-P‚‚‚P′-} + A + S h {-PS‚‚‚AP′-} K ′AS )
(4)
θPA‚‚‚SP′ θPP′aAaS (5) θPS‚‚‚AP′ θPP′aAaS (5′)
These equilibrium equations exhaust all possibilities for A and S molecules to disrupt an asymmetric {-P‚‚‚P′-} link. For all the above reactions, we introduced the equilibrium constants KA, K′A, KA2, KS, K′S, KS2, KAS, and K′AS (eqs 1-5′, respectively) where θ denotes the fraction of differently solvated links, and aA and aS the equilibrium activities of A and S molecules. The pure liquid state is the reference state for compound activity. Considering equilibrium constants for the chemical equilibrium equations and a mass balance for “concentration” of links (Cop), a local sorption isotherm was obtained, which, for the case of sorption from dilute solutions, reduces to a Langmuir-like expression for sorbate uptake SA (eq 6):
SA )
CoPkAeffaAB 1 + kAeffaAB
(6)
where kAeff is the sum of equilibrium constants of reactions 1 and 1′. The affinity portion of this Langmuir-like expression includes the B term that is dependent on solvent activity (aS) and is defined, for sorption at asymmetric NOM links, according to eq 7:
B)
SA )
Rπ × sin (nπ)
( ) ( ) ( ) ( ) ( ) ( )
1+
KA K A2 KA
aA +
aA +
[
KS 2
KS KS 2 KS
1+
× anA ×
aS
KA 2 KA
aA +
KS2 KS
aS
1 + kSeffaS + KS2a2S
[
( ) KS 2
1+
( ) KSm
am-1 S
aS
SA KS ) SA(Y) 1 + KSaS + KS2a2S
]
]
n
(8)
n
(9)
This solvent effect (eq 9) is directly related to the B term (eq 7) of the local sorption isotherm. From eq 9 it is clear that when (KS2)/(KS) is greater than KS, there may be an increase in sorbate uptake upon addition of the solvent S. At sufficiently high solvent activities (aS), a maximum in the solvent effect-solvent activity dependence will result. The general isotherm model was expanded to account for the cooperative nature of the hydration/solvation effect on sorption (24). When an NOM contact is disrupted, we considered there are m sites for sorption (as distinct from two sites assigned for disrupted contact {-P‚‚‚P′-} in the original scheme (eqs 1-5′). Cooperativity was accounted for by replacing eqs 4 and 5 with eqs 10 and 11.
{-P‚‚‚P′-} + mS h {-PS‚‚‚Sm-1P′-} KSm )
]
n
(12)
Equation 12 was shown to adequately describe the increase in sorbate uptake of a compound with increasing solvent activity (24). The ratio (KSm)/(KS) represents solvation of the link fragments not involved directly in the interaction between a sorbate molecule and one site of the disrupted contact. This solvation is the driving force for solvation (hydration)assisted sorption. At sufficiently high solvent activities (aS), a maximum in the solvent effect-solvent activity dependence will result, as caused by an increase in the KSmam S term, describing sorbate-solvent competition. In this model, independent adjustable parameters include KS, KSm, and m.
Experimental Section
aS
where R and n are parameters of exponential heterogeneity of sorption sites, and kSeff is the sum of the equilibrium constants of reactions 3 and 3′. Considering low sorbate activities, the solvent effect term defined as the ratio of sorbate uptake from a given solvent (SA) to sorbate uptake from an inert solvent (e.g., nhexadecane; SA(Y)) at a given sorbate activity, may be derived from eq 8:
1+
[
(7)
1 + KSaS + KS2a2S
KA2
Then, it is possible to define the “solvent effect” for the cooperative extension in parallel to the definition in eq 9:
SA KS ) SA(Y) 1 + KSaS + KSmam S
KS2 1+ a KS S
where the ratio (KS2)/(KS) describes link solvation. When (KS2)/ (KS) > KS, adding a solvent can increase the B term; when aS increases, KS2a2S becomes important and the B term decreases. To generalize this homogeneous site isotherm to a distribution of varied site energies, we exploit the classical conception of site distribution as an exponential function of energy. The general sorption isotherm is given by eq 8:
1+2
{-P‚‚‚P′-} + A + (m-1)S h {-PA‚‚‚Sm-1P′-} KASm-1 ) θPA‚‚‚Sm-1P′ (11) θPP′aAam-1 S
θPS‚‚‚Sm-1P′ θPP′am S (10)
Materials. Pahokee Peat obtained from the IHSS (49% organic carbon (OC), 3.3% N, 4.3% H, 0.5-1.2% S; ref 14) was freezedried (moisture content 3.2% w/w) prior to sorption experiments. Phenol (C6H5OH; 99%, BDH Chemicals Ltd, Poole England) was used without additional purification. It was selected as the probe sorbate because its hydration-assisted sorption was previously observed in a fully hydrated system as compared with a completely dry system (15), was used without additional purification. Sorption Experiments. Phenol sorption was examined at nine different NOM moisture contents (3.2-40.3% w/w) at eight initial phenol solution concentrations (range 202000 mg/L) at 23 ( 2 °C. The maximal initial phenol concentration was well below the concentration at which meaningful hydrogen-bonding related phenol association in n-hexadecane may be anticipated (25, 26). Sorption experiments were carried out in 2 mL glass vials equipped with Teflon-lined screw-caps. Experimental moisture contents were obtained by adding predetermined amounts of water (microliter amounts) to the dry peat (0.025-0.05 g). Moistened peat was aged in the experimental vials for 10 days to allow time for initial redistribution of moisture in the peat sample (21, 27). Then, phenol solutions in n-hexadecane (1-1.5 mL) were added, and the sorbent/solution system was mixed in the dark by horizontal shaker. The amount of water in the sorbent phase was sufficiently large that any possible water redistribution between the sorbent, air phase and n-hexadecane phase could not change the amount of water associated with the NOM phase and its moisture content (maximal amount of water which could dissolve into n-hexadecane 1) can be thought of as “hydration-assisted sorption.” A number of important features of the hydration effect can be observed in Figure 3. At relatively low water activities (to about 0.4), hydration-assisted sorption of phenol at all three equilibrium solution concentration levels is about the same, and relatively low. However, between 0.4 and 0.7 water activity, a strong increase in hydration-assisted sorption is observed at all three solution concentration levels, with a direct relationship between the extent of hydration-assisted sorption and Ce (the higher the Ce, the greater the hydration effect at a given water activity). The hydration effect peaks at a water activity of between 0.68 and 0.80, and at higher water activities, a decrease in hydration effect is observed. The extent of the decrease is not significantly different for the three cases. The inflection and sharp rise in hydration effect over a water activity range of 0.4-0.7 are indicative of the involvement of several water molecules in sorption of a sorbate molecule. Thus, this trend demonstrates the cooperative nature of the hydration effect at interim water activities. Conceptually, the results shown in Figures 2 and 3 are predicted by the link solvation model (23, 24), which requires a certain level of water activity to obtain cooperative opening of the noncovalently cross-linked sorbent structure and creation of new sorption sites for sorbate molecules (Figure 1). At some higher level of water activity, competition between water molecules and solute molecules for the newly opened sites is predicted by the model to result in a relative reduction in hydration-assisted sorption, as indeed observed for water activities exceeding 0.8. Both creation of new sorption sites and competition for sorption sites can result in an increase in sorption isotherm linearity, as observed (Table S2 and Figure S2; Supporting Information). These results for phenol are qualitatively very similar to our previous results demonstrating solvent (acetonitrile)-
assisted sorption for pyridine on peat (23). The phenol data represented in Figure 3, however, is the first detailed data set (to the best of our knowledge) to demonstrate water-assisted sorption in NOM over a complete range of water activities with cooperative involvement of water molecules in the organic compound sorption process. LSM Extension and Fitting of Phenol Data. In the version of the LSM published earlier (24) based on the pyridine data set (23), solvation-related assistance in sorption was taken to be independent of solute activity, and isotherm nonlinearity was expressed in terms of a Freundlich model exponent that was independent of solvent activity. In contrast to these two activity-independent terms based on earlier data, the current phenol data set is distinguished by (i) the extent of hydration-related assistance in sorption (hydration effect) of phenol is dependent on solute activity (Figure 3); and (ii) the Freundlich exponent (n) is linearly dependent on water activity (Figure S2). Therefore, we herein extend the LSM to account for these differences, and then fit the phenol sorption data using the extended model. This is accomplished in the following way: (i) In the original LSM development (24), the ratio (KSm)/ (KS) (shown here in eq 12) was introduced to replace the ratio (KASm-1)/(KA) based on the simplifying assumption that interactions with link fragments are additive. KASm-1 is the equilibrium constant for the simultaneous penetration of one sorbate (A) molecule and (m-1) solvent (S) molecules into an NOM link, and KA is the equilibrium constant for penetration of a single A molecule into the link. For the current extension, this simplifying assumption is eliminated, and eq 12 is transformed into a more general form:
SA )
[
1+
( ) KASm-1
]
n
am-1 S
KA Rπ × anA × sin (nπ) 1 + KSaS + KSmam S
(13)
Considering the hydration effect for a full range of sorbate activity, eq 13 can be rewritten as follows:
SA )
( )
aA n Rπ n × × × aA,max a sin (nπ) A,max n KASm-1 1+ am-1 S KA aA n ) SA(Y),max × × m a 1 + KSaS + KSmaS A,max KASm-1 m-1 n 1+ aS KA
[
( )
]
( )
[
( )
1 + KSaS + KSmam S
]
(14)
where SA(Y),max is the concentration of compound A in the sorbent phase in the absence of active solvent S (aS ) 0) at a maximal reference activity aA,max of compound A. (ii) The linear dependence of the Freundlich exponent n on water activity was accounted for empirically according to the observed relation: n ) 0.21aS + 0.55 by incorporation into the n term of eq 14. Then, to obtain the solvent effect (SA)/(SA(Y)), SA (eq 14) for a given solvent activity (aS) was divided by SA(Y) (eq 14) for a solvent activity of zero (aS ) 0):
Solvent Effect )
( )
SA aA ) SA(Y) aA,max
n-n(Y)
[
1+
×
( ) KASm-1 KA
am-1 S
1 + KSaS + KSmam S
]
n
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(15)
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where n is the exponent of the Freundlich model fit for the sorption isotherm at a given solvent (water) activity (aS), and n(Y) is the Freundlich exponent for the dry NOM system (aS ) 0). Since maximal sorbate activity aA,max is unity when referred to the pure liquid state, the ultimate equation to be used for data correlation is as follows:
Solvent Effect )
[
1+
( ) KASm-1
]
n
am-1 S
KA SA ) (aA)n-n(Y) × SA(Y) 1 + KSaS + KSmam S (16)
Considering that n is known for all systems including n(Y), eq 16 has four independent fitting parameters: m, denoting the number of sites available for solvent interaction upon link disruption, and the equilibrium parameters: (KASm-1)/ (KA), KS, and KSm. The ratio (KASm-1)/(KA) denotes the assistance given by solvent molecules to penetration of the sorbate molecule into the disrupted NOM link. The KS parameter signifies NOM link disruption by a single solvent molecule only, while the KSm parameter indicates link disruption via m solvent molecules where m > 1. According to the model, a single set of parameters from eq 16 should be able to describe the solvent effect at different solute and water activities.
LSM Fit to Phenol Data and Model Implications It was determined that the KS-dependent term could be omitted while retaining the ability of the model to describe the data. Setting KS to 0 means that NOM link disruption is not possible if only one solvent molecule penetrates into the contact, supporting the concept of solvent effect cooperativity. Thus, the number of fitting parameters for eq 16 was reduced to 3. The fit of the three fitting parameter version of eq 16 for all sets of phenol uptake data (all hydration levels) is shown in Figure 3; fitting parameters and statistics are given in the figure caption. The single set of three parameters provides a very successful description of the combined data from the nine isotherms. Here, the m parameter (the number of sites available for solvent interaction upon link disruption ( standard error) is computed to be equal to 9.8 ( 0.6. The ratio (KASm-1)/(KA), returned as 19 000 ( 7100, is responsible for the phenomenon of hydration-assisted sorption, while the much smaller KSm parameter (253 ( 96) represents sorbate-solvent competition for newly created sites. The KSm parameter becomes important only at high solvent activities. In the pyridine-acetonitrile system studied earlier (23), the m parameter for acetonitrile as the active solvent was found to be 5.8 ( 1.4, which, considering the differences between the two sets of experiments (different sorbate, different active solvent, different batch of peat) and the assumptions inside the model, is comparable to the value for m determined in the current experiment (9.8 ( 0.6). The disparity between m values may stem from variability in the sorbent, as it was not from the same batch in the two experiments. Alternatively, it may be conjectured that the difference in m parameters reflects a role for the size of the solvent molecules in cooperative penetration (acetonitrile being larger than water), or differences in the ability of the solvent (acetonitrile vs water) to interact with NOM moieties. The successful fit portrayed in Figure 3 demonstrates the power of the extended LSM model, and supports the conceptual model of creation of new sorption sites for sorbate molecules in the hydrated organic matter sorbent (Figure 1), accompanied by competition for those new sites by water molecules at high water activities. Considering the flexibility of the LSM, it should be possible to use it to consider sorption mechanisms of sorbate molecules of varying structures from 552
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different solvents or solvent mixtures on a variety of sorbents. Furthermore, the LSM provides a mechanistic explanation for sorption hysteresis (see Figure 1), which can be explicitly tested in future studies. Future directions could also include the effect of relative size of sorbate vs solvent molecule, i.e., testing whether in the presence of larger sorbate molecules, fewer smaller solvent molecules are required for link disruption. Currently, the LSM model considers that a single solvent molecule is replaced by a single sorbate molecule (represented by the m - 1 term; seen schematically in Figure 1). This is not necessarily the case and should be tested, so that the LSM can be generalized if needed. The model does not account for nonspecific partitioning of solute molecules into the sorbent or sorption at sites that are available without recourse to disruption by a solvent, but can be extended to include such sorption domains, if required. Examination of the physical limitations inherent in the model (e.g., use of an exponential function to describe site heterogeneity) may be another direction for future model development. Organic Matter Organization and its Role in Sorption of Organic Compounds: In Context. The observed monotonic increase in phenol isotherm linearity as a function of hydration extent and maximum in sorption at an interim hydration level are both predicted by the LSM, as a direct result of the increase in number of sorption sites upon hydration, and competition for those sites at high hydration levels. It is worthwhile examining whether the extended dual mode model (EDMM) conception suggested for interpreting sorption phenomena in polymers (32) and applied for natural organic matter sorbent (33) can also explain this data set. The EDMM (32) envisions the glassy phase to consist of two sorption domains - dissolution and holes, while the rubbery phase consists of a single dissolution domain:
qe ) [KD exp(σS*)]Ce +
[S0(1 - S*/Sg)]bCe 1 + bCe
(17)
Following (32, 33), qe is sorbed concentration at equilibrium, Ce solution concentration at equilibrium, KD distribution coefficient for the dissolution domain (according to the model, identical in both glassy and rubbery phases), σ a function of the Flory-Huggins interaction parameter, Sg the isothermal glass transition concentration, and S* the effective plasticizing concentration, whereas S* ) SD + f SH, SD, SH are the dissolution and hole domain concentrations, respectively, and f is the ratio of plasticizing ability of a hole to a dissolution species (0 e f e 1). When S* g Sg, the second term in the equation drops out, leaving a linear or exponential term (depending on σS*) that intersects the origin. Given that the second term drops out after plasticization, then by definition, sorption in the glassy phase exceeds that of the rubbery phase at the same solution concentration, as long as sorption in the two dissolution domains is indeed comparable as in the model (eq 17). When the EDMM was applied to sorption by NOM, indeed sorption in the two dissolution domains was reported to be linear and very similar (within a factor of 2), and substantially less than in the glassy hole domain (less by a factor of 10) (33). Greater sorption at a given solution concentration in the proposed glassy domain as compared with the proposed rubbery domain of NOM was also reported in other studies (34-36). By the same token, sorption in the glassy phase is expected to be nonlinear due to the hole-filling domain, and linear in the rubbery phase. Given this, hydration-caused plasticization would be anticipated to exhibit an increase in isotherm linearity accompanied by lower sorption at a given solution concentration. However, the current data set exhibits a monotonic increase in phenol isotherm linearity as a function of hydration extent (Figure S2; Supporting Information), and
at the same time, a maximum in sorption at an interim hydration level (Figure 3). Therefore, the current data set cannot be interpreted on the basis of this EDMM concept. Recently it was reported that the glass transition temperature (Tg) of peat varies as a function of peat hydration level, with Tg reaching a maximum at an intermediate moisture content (37). It was hypothesized that at an interim moisture content, water acts as an “antiplasticizer” by forming hydrogen-bond linkages between organic polymer side chains, resulting in greater glassiness. According to the hypothesis, at higher moisture contents, glassiness is reduced because water-based cross-links are unable to prevent multiple molecular motions in highly hydrated and swollen NOM, while at lower moisture contents, the structure is less glassy because there is insufficient water to create numerous water-based cross-links. For such a postulated maximally glassy condition at an interim moisture content, phenol sorption could be expected to be maximal, indeed as observed here (Figures 2 and 3). However, sorption isotherms should also exhibit a maximum in nonlinearity at an interim moisture content. This is not the case for the data reported herein, whereby isotherm linearity increases monotonically as a function of moisture content (Figure S2; Supporting Information). Hence, the current data set also cannot be interpreted on the basis of the “water as antiplasticizing agent” concept. Nonetheless, the LSM conception may have certain intersections with the idea of NOM pore deformation induced by sorbate molecules or by a conditioning agent that causes irreversible pore deformation (38-40) as controlling sorption of organic compounds. This is because molecular-size distances between organic fragments of the NOM matrix or possible molecular-size pores should be associated with significant interactions between the “walls” of voids. These significant interactions between NOM elements forming molecular-size voids may make “between void wall interactions” hardly distinguishable from noncovalent NOM linking in the LSM model. Additionally, the same multi-linked nature of NOM contacts seen in the framework of the LSM as responsible for cooperative hydration/solvation effect on sorption of organic compounds, is expected to result in irreversibility of both NOM link disruption and pore deformation, thus causing sorption/desorption hysteresis of organic compounds (Figure 1).
Acknowledgments We greatly appreciated the efforts of the reviewers. E.R.G. acknowledges support from the Israel Science Foundation (grant no. 162/05).
Supporting Information Available A tabulation of salient experimental details and conditions, Freundlich model fit parameters and fit statistics for the isotherms, figures showing the kinetics of phenol sorption, n versus water activity, and S/Ce versus S for isotherm experiments. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review May 28, 2006. Revised manuscript received October 6, 2006. Accepted October 25, 2006. ES061274A
