Mechanistic Investigation of Non-Ideal Sorption Behavior in Natural

Article pubs.acs.org/est

Mechanistic Investigation of Non-Ideal Sorption Behavior in Natural Organic Matter. 1. Vapor Phase Equilibrium Katherine Young Bell† and Eugene J. LeBoeuf*,‡ †

CDM Smith, 210 25th Avenue North, Suite 1102 Nashville, Tennessee 37203, United States Department of Civil and Environmental Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States

‡

S Supporting Information *

ABSTRACT: Results from an experimental and modeling investigation of the influence of thermodynamic properties of highly purified natural organic matter (NOM) on observed equilibrium sorption/desorption behaviors of vapor phase trichloroethylene (TCE) is presented. Identification of glass transition (Tg) behavior in Leonardite humic acid and Organosolv lignin enabled evaluation of equilibrium and nonequilibrium sorption behavior in glassy and rubbery NOM. Specific differences in vapor phase equilibrium behavior in NOM above and below their Tg were identified. In the glassy state (below Tg), sorption of TCE is well-described by micropore models, with enthalpies of sorption characteristic of microporous, glassy macromolecules. Above Tg, sorptive behavior was well-described by Flory−Huggins theory, indicating that the mobility and structural configuration of rubbery NOM materials may be analogous to the characteristic sorption behavior observed in more mobile, rubbery macromolecules, including strong entropic changes during sorption. Results from this work provide further support that, at least for the samples employed in this study, NOM possesses macromolecular characteristics which display sorption behavior similar to synthetic macromolecules−an important assumption in conceptual sorption equilibrium models used in the analysis of the fate and transport of VOCs in the environment.

■

INTRODUCTION One of the most common threats of subsurface pollution in industrialized countries is the release of volatile organic compounds (VOCs) (e.g., refs 1−3). Remediation of VOCcontaminated sites is often complicated because the contaminants can readily move between aqueous and vapor phases as well as associate with soils and sediments (geosorbents) comprised of often complex agglomerations of minerals and natural organic matter (NOM). Predicting the fate and transport of VOCs in these heterogeneous matrices can thus be difficult due to many interacting processes such as sorption to mineral surfaces or organic matter, and diffusion through organic or inorganic porous materials. A substantial amount of research has focused on improving our understanding of mechanisms governing VOC transport in the subsurface environment (e.g., refs 4−12). For a given VOC, primary contributing factors influencing sorption and desorption behavior include (i) mineral and NOM content and structure, including surface area and microporosity; (ii) mineral/NOM aggregate structure; and (iii) relative humidity. Although sorption of various organic chemicals on saturated soil has been studied extensively, reports on the sorption of VOCs on unsaturated or dry soils are relatively few. And, of these studies, those that compare VOC vapor sorption on dry versus wet soil show that vapor sorption in dry soil is typically (i) greater than that of wet soils; (ii) nonlinear; and (iii) suppressed by the presence of water in a nonlinear manner.4−9 © 2012 American Chemical Society

While mineral matrices possess significant influences, especially under conditions of decreasing relative humidities, our emphasis here is on improving our understanding of the role of NOM matrices in influencing sorptive behavior of VOCs,10,11 focusing on NOM characteristics and environmental conditions in which NOM may play an especially important role, such as low relative humidities (e.g., 12) and low VOC relative vapor pressures (i.e., P/P0 less than 10−3). An improved understanding of the role of NOM in vapor sorption can thus provide improved means for focusing remedial actions on specific components of soils and sediments most responsible for controlling VOC uptake and release and/or changing environmental conditions to alter retention/release mechanisms, such as increasing soil temperatures to create thermal transitions in NOM matrices. NOM structure may be viewed as partially microcrystalline, but predominantly amorphous macromolecular solids that may undergo characteristic phase transitions between the glassy and rubbery states at their glass transition temperatures (Tgs) (e.g., refs 13−15). These states are distinguished by differences in macromolecular mobility, which correlate to significantly varying sorption behaviors in these materials. For example, Received: Revised: Accepted: Published: 6689

December 31, 2011 May 3, 2012 May 29, 2012 May 30, 2012 dx.doi.org/10.1021/es204726p | Environ. Sci. Technol. 2012, 46, 6689−6697

Environmental Science & Technology

Article

Table 1. Sorbent Characterization 13

elemental analysis (mass %)

glass transition (Tg) (°C)

C NMR spectra aromatic and phenolic C (%)

carboxyl and carbonyl C (%)

sorbent

C

H

N

O

Al

ash (%)

atomic O/C

atomic H/C

parrafinic C (%)

substituted aliphatic C (%)

DSC

TMA

organosolv lignin Leonardite humic acid

65.8

5.37

0.08

28.7

N/A

0.0

0.44

0.08

20

33

43

4

69 ± 2

69 ± 3

63.2

3.64

1.17

31.0

N/A

2.47

0.49

0.06

21

9

45

25

74 ± 3

74 ± 3

characteristics using differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA) as described by LeBoeuf and co-workers.13,15 Briefly, thermal characterization involved placing NOM samples into a DSC or TMA instrument, drying under a nitrogen purge gas at 105 °C, then scanning a temperature range from subzero (typically, starting at −40 °C) and ramping up through observed Tgs (defined here in terms of inflection point of the heat capacity curve). Summaries of sorbent properties and surface areas are provided in Tables 1 and S1, respectively. Batch isotherms were conducted using a glass ampule system for evaluating the long-term sorption behavior of VOCs.29 Ampules were filled with an appropriate mass of NOM and baked at 105 °C for 24 h under vacuum to remove physi-sorbed water. Following free-space analysis (using a Micromeritics ASAP 2010, Norcross, GA), trace 14C-labeled TCE liquid or vapor was injected into each ampule which was immediately flame-sealed. Ampules were equilibrated at 15, 25, or 35 °C for 90 days. Following the sorption period, vapor-phase TCE concentration was evaluated through injection into the liquid scintillation fluid, and then counted on a Packard (Meriden, CT) TRI-CARB 3170TR/SL liquid scintillation analyzer. Above the Tg of NOM materials employed in this study, the batch isotherm method could not be utilized due to physical limitations of the higher experimental temperatures. (Note: both experimental methods were evaluated at 35 °C to confirm reproducibility of results (see Figure S1).) Sequential dosing isotherms were thus performed using standard protocols on a Micromeritics ASAP 2010 at 75, 95 and 115 °C.30 Prior to performing isotherms, liquid TCE was purified according to the distillation procedure described in Micromeritics Application Note 95.31 Samples were degassed at 105 °C, except for the 115 °C isotherms in which case the degas temperature was 115 °C, and analyzed using an automated sequential dosing procedure. Briefly, this sequential dosing method first determines the dead space within the sample container, then adds an initial dose of the sorbing gas to the evacuated sample container equal to the known dead space, plus an additional increment (or dosage) of sorbate. The amount of sorbate sorbed by the sample is determined by difference in pressure from that originally dosed and that at equilibrium following each incremental dose. Incremental doses are then added as required until the total amount of sorbate sorbed is brought to a predetermined relative pressure for a given temperature.30 Sorption Data Analysis. Numerous models have been used to describe vapor-phase equilibrium sorption behavior in whole soils, sediments, organic matter, and model soil systems. These models include the Freundlich equation,32 the Langmuir equation,33 and the Brunauer, Emmett, and Teller (BET) model,34,35 all with varying degrees of success.

the rubbery state, with its inherent free-ranging and relatively rapid molecular mobility may be viewed as a fluid-like media with which sorbates may partition and diffuse through Fickiandominated processes.16,17 In the glassy state, however, free segmental molecular motions are restricted which may result in a much more rigid structure, dominated by the presence of semipermanent, internal micro- and nanoscale voids.18 Here, nonlinear adsorption processes often dominate,19 and diffusion becomes inherently non-Fickian in nature, where the diffusing molecule may be limited by the relaxation rate of the macromolecular matrix. This macromolecular view of NOM has been used to describe sorption into internal “holes” (pores or voids) of nanometer dimensions in glassy NOM20−22 which may be filled following a sum of Langmuir terms.23 The extended Flory− Huggins theory has also been used to describe sorption behavior in NOM.24,25 Although researchers have described NOM as a macromolecular sorption medium, few have evaluated this macromolecular sorption behavior in NOM materials above and below their glass transition temperatures.14,26−28 Additionally, there has been no work to date directly comparing the equilibrium sorption behavior of VOCs in dry NOM materials in both the glassy and rubbery states. This study improves our understanding of vapor-phase equilibrium sorption and desorption behavior in both the glassy and rubbery states to evaluate how the macromolecular state and mobility of NOM directly affect the sorptive behavior of these systems. Further, it provides for the first time reports of sorption enthalpies of vapor-phase VOC sorption above and below known glass transition temperatures within dry NOM.

■

EXPERIMENTAL SECTION Materials and Methods. Trichloroethylene (TCE) was used as a probe sorbate. Radiolabeled TCE was prepared by mixing a 500 μCi quantity of 14C-labeled TCE (Sigma, St. Louis, MO) with 5 mL of unlabeled HPLC-grade (99.5%) TCE (Mallinckrodt, Paris, KY). The trace mixture was stored in 8mL glass (Pyrex) ampules at −15 °C until 4 h prior to use. Certified ACS methanol (≥99.8%) and ScintiSafe Econo 1 Cocktail were purchased from Fisher Scientific (Springfield, NJ). A terrestrial humic acid and hardwood lignin were selected for use as sorbents because of their availability in a highly purified form as well as prior use in investigations of sorptive interactions with environmental contaminants.14,27 Leonardite (standard grade) humic acid (LHA) was obtained from the International Humic Substances Society (IHSS) and Organosolv lignin (OL) was obtained from Aldrich Chemical Co. (St. Louis, MO). Characterization of NOMs included elemental, 13C NMR, surface area, and microporosity analyses using nitrogen and carbon dioxide. Sorbents were also analyzed for their thermal 6690

dx.doi.org/10.1021/es204726p | Environ. Sci. Technol. 2012, 46, 6689−6697


Article

Figure 1. Low-temperature equilibrium sorption isotherms for (a) OL and (b) LHA.

Table 2. Sorption Model Parameters for TCE on OL and LHA: Low Temperatures 15 °C

model

25 °C

35 °C

composite

Organosolv lignin Dubinin−Raduskevich W0 [cm3/g, liquid] βE [kJ/mol] Dubinin−Astakhov W0 [cm3/g, liquid] βE [kJ/mol] n [-]

Dubinin−Raduskevich W0 [cm3/g, liquid] βE [kJ/mol] Dubinin−Astakhov W0 [cm3/g, liquid] βE [kJ/mol] n [-] a

0.047 < 0.050 < 0.053b 6.19a < 6.60 < 7.01 R2 = 0.992

0.048< 0.052 < 0.055 6.63 < 7.05 < 7.46 R2 = 0.993

0.050 < 0.053 < 0.057 7.44 < 7.81 < 8.17 R2 = 0.995

0.048 < 0.050 < 0.052 6.99 < 7.30 < 7.61 R2 = 0.983

0.057a < 0.062 < 0.066b 5.01a < 5.46 < 5.87b 1.21a < 1.34 < 1.47b R2 = 0.999

0.057 < 0.069 < 0.082 4.65 < 5.55 < 6.49 1.08 < 1.32 < 1.56 R2 = 0.998 Leonardite humic acid

0.053 < 0.073 < 0.093 4.91 < 6.23 < 7.56 1.10 < 1.44 < 1.77 R2 = 0.997

0.050 < 0.056 < 0.062 6.13 < 6.74 < 7.34 1.38 < 1.64 < 1.90 R2 = 0.985

0.021a < 0.022 < 0.024b 7.69a < 8.19 < 8.70b R2 = 0.994

0.021 < 0.023 < 0.024 8.01 < 8.49 < 8.98 R2 = 0.992

0.022 < 0.024 < 0.025 8.98 < 9.34 < 9.71 R2 = 0.996

0.021 < 0.022 < 0.023 8.49 < 8.82 < 9.14 R2 = 0.987

0.024a < 0.027 < 0.030b 6.30a < 7.03 < 7.76b 1.18a < 1.40 < 1.62b R2 = 0.998

0.024 < 0.031 < 0.039 5.35 < 6.71 < 8.07 1.05 < 1.36 < 1.67 R2 = 0.995

0.024 < 0.031 < 0.038 6.51 < 7.78 < 9.05 1.22 < 1.51 < 1.80 R2 = 0.998

0.023 < 0.025 < 0.028 7.43 < 8.09 < 8.76 1.40 < 1.63 < 1.86 R2 = 0.989

Lower 95% confidence level for model coefficients. bUpper 95% confidence level for model coefficients.

Sorption into internal “holes” in glassy NOM has been described using a sum of Langmuir terms,23 however, a more appropriate approach may be to utilize pore-filling models which have been widely used in describing equilibrium sorption in activated carbons,36 coals,37 and soils and sediments hypothesized to contain trace quantities of high-surface-area carbonaceous material.38,39 Most pore-filling models are based on the Polanyi potential theory, which assumes that adsorption takes place when the strength of the potential energy field is great enough to compress the sorbate to a partial pressure greater than its vapor pressure. Because the energy field is independent of temperature, homologous temperature data should fall on the same curve. The most widely used equations to describe the shape of the characteristic curve are the Dubinin−Raduskevich (D−R) and the Dubinin−Asktakhov (D−A) models. These equations take the form ⎡ ⎛ RT ln(P /P) ⎞n⎤ 0 W = W0exp⎢ −⎜ ⎟⎥ ⎢⎣ ⎝ βE 0 ⎠ ⎥⎦

where W is quantity of sorbate adsorbed at P/P0, W0 is the total micropore volume, R is the universal gas constant, T is the experimental temperature, E0 is the characteristic energy related to the sorbate, β is the sorbate specific affinity coefficient, and n is 2 for the D−R model and adjustable in the D−A model. In rubbery NOM, Flory−Huggins theory describes a threedimensional dilation in which a macromolecular network takes up solvent and swells to a point of equilibrium where the free energy decrease from mixing is balanced by the free energy increase due to stretching of macromolecule chains. In a binary system such as in vapor sorption, the following form applies:40 ln γ = ln φ + (1 − φ) + χ (1 − φ)2

(2)

where γ is the sorbate activity or relative pressure of the penetrant, φ is the volume fraction of sorbate, and χ is the Flory−Huggins interaction parameter which comprises both enthalpic and entropic components41 χ = χH + χS

(1) 6691

(3)



Article

Figure 2. High-temperature equilibrium sorption isotherms for (a) OL and (b) LHA.

Table 3. Sorption Model Parameters for TCE on OL and LHA: High Temperatures 75 °C

95 °C

115 °C

3.92 × 10−3a < 4.45 × 10−3 < 5.06 × 10−3b 1.01 < 1.03 < 1.06b R2 = 0.999

4.16 × 10−3 < 4.64 × 10−3 < 5.18 × 10−3 1.07 < 1.08 < 1.10 R2 = 0.999

7.89 × 10−4 < 1.03 × 10−3 < 1.34 × 10−3 1.24 < 1.29 < 1.33 R2 = 0.999

−7.68 × 102a < −5.36 × 102 < −3.04 × 102b 6.14 × 102a < 1.07 × 103 < 1.53 × 103b −7.67 × 102a < −5.38 × 102 < −3.10 × 102b R2 = 0.999

2.71 × 103 < 7.46 × 103 < 1.22 × 104 −2.43 × 104 < −1.48 × 104 < −5.35 × 103 2.64 × 103 < 7.37 × 103 < 1.21 × 104 R2 = 0.999 Leonardite humic acid

3.11 × 105 < 4.46 × 105 < 5.81 × 105 −1.16 × 106 < −8.91 × 105 < −6.21 × 105 3.10 × 105 < 4.45 × 105 < 5.80 × 105 R2 = 0.989

6.29 × 10−4a < 7.66 × 10−4 < 9.32 × 10−4b 0.627a < 0.664 < 0.702b R2 = 0.996

1.02 × 10−3 < 1.24 × 10−3 < 1.52 × 10−3 0.817 < 0.852 < 0.887 R2 = 0.998

4.27 × 10−3 < 5.68 × 10−3 < 7.57 × 10−3 1.17 < 1.22 < 1.26 R2 = 0.998

−1.79 × 10−4a < −1.54 × 104 < −1.29 × 104a 2.56 × 104a < 3.06 × 104 < 3.55 × 104b −1.76 × 104a < −1.51 × 104 < −1.27 × 104b R2 = 0.999

−6.20 × 104 < −5.20 × 104 < −4.20 × 104 8.37 × 104 < 1.04 × 105 < 1.24 × 105 −6.16 × 104 < −5.16 × 104 < −4.17 × 104 R2 = 0.999

1.41 × 105 < 2.65 × 105< 3.90 × 105 −7.78 × 105 < −5.30 × 105 < −2.81 × 105 1.40 × 105 < 2.64 × 105< 3.89 × 105 R2 = 0.995

model

Organosolv lignin Freundlich, Q = f{P/P0} K [mol/g] n [-] Flory−Huggins χ0 [-] χ1 [-] χ2 [-]

Freundlich, Q = f{P/P0} K [mol/g] n [-] Flory−Huggins χ0 [-] χ1 [-] χ2 [-] a

Lower 95% confidence level for model coefficients. bUpper 95% confidence level for model coefficients.

synthetic macromolecules.42,43 The implications of this finding are that the entropic contribution to mixing may actually become negative. Thus, the system is likely moving to a more ordered state with a concurrent reduction in free volume which may limit macromolecular mobility and affect the characteristics of sorption behavior. All sorption models were fit by nonlinear regression methods in Origin (OriginLab Corporation, Northampton, MA).

The enthalpic contribution is a combinatorial term related to solubility parameters of the sorbate and sorbent

χH =

⎛ V1 ⎞ 2 ⎜ ⎟ (δ − δ ) 2 ⎝ RT ⎠ 1

(4)

where χH is the enthalpic component of Flory−Huggins interaction parameter, V1 is the molar volume of sorbate, R is the universal gas constant, and δ1 and δ2 are the solubility parameters of the sorbate and sorbent, respectively, at temperature T. This allows only for positive values of χH, where specific sorbate-macromolecule interactions that yield negative χHs are not addressed. The entropic contribution, χS, is due to configurational changes upon dissolution of macromolecules in the sorbate and in practice taken to be a constant between 0.3 and 0.4 (usually 0.34). Because the Flory−Huggins interaction parameter is a function of the square of the difference of the solubility parameters, the interaction parameter is typically positive with values ranging from 0 to 1. However, negative values have been measured for χ in

■

RESULTS AND DISCUSSION Thermodynamic characterization of LHA and OL indicates that these materials are in the glassy state at 15, 25, and 35 °C (below the Tg ∼ 70 °C), and therefore likely possess limited macromolecular mobility. The rigidity resulting from limited mobility at these temperatures should be manifested in sorption behavior that is reflected by micropore filling and condensation. Isotherms are shown in Figures 1a, S2, and S4 for OL and 1b, S3, and S5 for LHA, with results of model fits for the Dubinin models summarized in Table 2, and linear, Freundlich, and 6692



Article

Figure 3. Flory−Huggins interaction parameters for (a) OL and (b) LHA.

models often provide excellent fits to experimental data, here we also employ a Flory−Huggins isotherm model in an effort to capture more specific macromolecular characteristics of the sorption process. High-temperature isotherms for OL and LHA are shown in Figure 2a and b, and Freundlich and Flory− Huggins model fits are shown in Table 3. The Flory−Huggins interaction parameter, χ, may display a temperature and/or concentration dependence. High-temperature experiments demonstrate that both concentration and temperature are important in describing the behavior of these materials. Interaction parameters are shown in Figure 3a and b for OL and LHA, respectively. The concentration dependence of the interaction parameter, χ, may be described as an empirical power series40,48,49

BET models provided in Table S2. Although other researchers were able to fit vapor-phase sorption data for various geosorbents using the Langmuir and BET models,4,45 success of these fits may be influenced by their exploration of relative pressures in the classical BET range (e.g., P/P0 = 0.05 to 0.3). The lower relative pressures (P/P0 = 5.0 × 10−04 to 2.0 × 10−02) used in this study generally fall in ranges that best represent micropore filling phenomena, and are not necessarily fit well by the Langmuir or BET models. As noted above, the D−R and D−A models differ mathematically only by the model exponent, n. Examination of 95% confidence intervals for n for OL and LHA show a statistical overlap of the exponents for the homologous temperature isotherm developed from the series of sorption data in the glassy state. The n value for both OL and LHA for the homologous temperature isotherm, however, is significantly different from 2, suggesting better fit by the D−A model. Additional evidence of the microporous nature of these NOM materials is the similarity in adsorption potentials (the quantity E = βE0) for two different sorbates, TCE and carbon dioxide. Polanyi44 observed that for a fixed adsorption volume, adsorption potentials for different sorbates on the same material could be related by their affinity coefficient, β. Adsorption potentials experimentally derived for TCE and CO2 are shown in Tables 2 and S2, respectively. If the NOM materials display microporous sorption behavior, then the product E should be equal for the two systems as long as the sorbent material retains its glassy character. Adsorption potentials for the two sorbates indicate that the values are similar, especially considering that these quantities were derived using two different experimental methods. Dubinin−Raduskevich adsorption potentials for TCE in OL and LHA are 7.30 and 8.82 kJ/mol, respectively; for CO2 in the same materials, E = 8.52 and 9.36 kJ/mol. Comparisons with nitrogen sorption data, however, do not provide this correlation, similar to results of Carmo et al.45 This likely reflects a limitation of collecting isotherm data at liquid nitrogen temperatures due to the potential of retarded diffusion and limited accessibility of the probe molecule into internal pores.23 At temperatures of 75, 95, and 115 °C, the NOM materials are expected to be in the rubbery state as indicated by their glass transition temperatures. The sorption of sorbates into soft or rubbery NOM has typically been modeled using a linear or Freundlich model with the assumption that partitioning is marked by a Freundlich exponent near 1.46,47 Although these

χ = χ0 + χ1 ϕ2 + χ2 ϕ2 2

(5)

where χ0, χ1, χ2 are second-order polynomial fitting parameters and ϕ2 is the macromolecule volume fraction. Model parameters for χ are presented with the Flory−Huggins model fits for high-temperature sorption data in Table 3. The plot of high-temperature OL data (Figure 2a) shows that OL isotherms are convex to the relative pressure axis. This isotherm shape is commonly observed for sorption of organic vapors in rubbery macromolecules.48,49 Data (Figure 3a) are shown with a Flory−Huggins model fit with the interaction parameter for each temperature being described by eq 2. In general, the interaction parameter decreases with a decreasing macromolecule volume fraction, implying that as the volumetric uptake of solvent in the NOM increases, there is a reduction in the disorder (negative entropy) of the OL-TCE system. This suggests that the environment into which the sorbate molecules are dissolving is becoming more like that of the pure penetrant. The concentration of TCE dissolved in the OL increases as χ decreases, consistent with the notion that the mixture is more compatible with TCE than the macromolecule alone. Similar observations have been reported for the vapor sorption of acetone in rubbery poly(dimethylsiloxane).48 LHA-TCE isotherms, however, display a convex shape only at 115 °C, a near linear shape at 95 °C, and a concave shape at 75 °C. One possible explanation is that LHA has not fully transitioned into the rubbery state since its Tg is reported to be 74 ± 3 °C.50 At 75 °C there may still be regions of the macromolecule that remain inflexible, contributing to a partially rigid matrix. This distribution of mobility is further evidenced by the work of 6693



Article

structure, and uptake of solvent could induce behavior which would account for the negative entropic contribution to the interaction parameter. Enthalpy of Sorption. The isosteric heat of sorption, or enthalpy of sorption, ΔHsorption, provides additional insight into the energies associated with sorption processes.54 In the glassy state, this property may be determined from parameters derived from micropore isotherm models. Chiang et al.55 derived an expression that directly relates the characteristic adsorption energy to the enthalpy of sorption

LeBoeuf and co-workers who have found multiple glass transitions in NOM materials.13,50,51 Sorbate concentration affects the interaction parameter, as does temperature. Both OL and LHA systems show an increasing interaction parameter with increasing temperature. The enthalpic contributions to the interaction parameter were estimated for the TCE-OL and TCE-LHA pairs from literature values for the solubility parameters of TCE, OL, and LHA using eq 4 and are provided in Table 3. Note that the expression for the enthalpic contribution to the interaction parameter (eq 4) only allows χH to vary with temperature, but not concentration, but by definition, the enthalpic contribution will never be less than zero. From the experimental evaluation of the Flory−Huggins parameters and estimates of χH (Table 4), the entropic contribution to the interaction parameter must

βE0 π (1 + αT ) (6) 2 −1 where α [T ] is the thermal coefficient of expansion for the sorbate,56 and other parameters have been previously defined. This relationship not only provides information about the energy of interaction between the sorbate and sorbent, but also allows for the calculation of the entropic contribution to adsorption since the Gibbs free energy of reaction (ΔG) can easily be estimated.55 Heats of sorption for TCE in OL and LHA were determined using the characteristic energy parameters derived from micropore model fits (eq 6) as presented in Table 5. The ΔH =

Table 4. Enthalpic Contributions to the Interaction Parameter χH (cal/cm3)0.5 temperature (°C)

organosolv lignin

Leonardite humic acid

75 95 115

0.33−0.57 0.31−0.54 0.30−0.51

0.47−0.69 0.44−0.65 0.42−0.62

Table 5. Enthalpy of Sorption for TCE in NOM

be negative, except in the 115 °C data. This is consistent with other work that often tends to show that the major contribution to the interaction parameter derives from the χS component.41 Examination of both the OL and LHA data reveals that the interaction parameter for TCE generally increases with increasing temperature for each system. Although most synthetic macromolecule systems tend to show an opposite trend (i.e., a decrease in χ with temperature),43,52,53 it has been reported that TCE can exhibit an increasing interaction parameter with increasing temperature in synthetic macromolecules such as poly(ethyl acrylate), poly(caprolactone), and poly(n-butyl methyacrylate).43 This argument, however, should be tempered by the high pKa of TCE, suggesting only weak hydrogen bond formation between TCE and the sorbent relative to the likely much stronger hydrogen bonds formed between intramacromolecular acidic and basic oxygen functional groups. Consideration should also be given to the physical significance of the entropy of a macromolecular system. The entropic contribution of the interaction parameter is generally thought to be related to the configurational arrangement of the macromolecular matrix. An increase in the system entropy can be related to an increase in the disorder while a decrease (negative entropy) can be attributed to a rearrangement to a more ordered, lower energy state. Both NOM materials possess glass transitions near the lowest experimental temperature. As the experimental temperature is increased from 75 to 95 to 115 °C, there may be a collapse of the pore structure, followed by a reordering to a lower-energy, more thermodynamically stable state. One possible mechanistic interpretation then, is that the NOM at 115 °C should be the most relaxed system and introduction of chemical potential may not cause much additional relaxation or molecular reordering. In this highly ordered state, introduction of additional sorbate molecules may cause the matrix to rearrange to a more disordered state (positive entropy) to accommodate the additional chemical potential. At temperatures nearer Tg, there is potential for relaxation of the macromolecular

ΔHsorption (kJ/mol) temperature ( °C)

organosolv lignin

Leonardite humic acid

15 25 35

−7.79 −8.39 −9.37

−9.67 −10.1 −11.2

ΔHsorption values for OL were calculated to be −7.79, −8.39, and −9.37 kJ/mol for 15, 25, and 35 °C respectively; ΔHsorption values for LHA were calculated to be −9.67, −10.1, and −11.2 kJ/mol for 15, 25, and 35 °C, respectively. To date, there are no reported values for vapor-phase TCE sorption into NOM materials using micropore models; therefore, a direct comparison with published studies is not possible. However, studies have evaluated sorption of organic solvents into activated carbons and carbon molecular sieves, reporting values of ΔH in the range of −18 to −45 kJ/mol for sorption of benzene and methylethylketone onto activated carbon,55 and −32 to −76 kJ/mol for sorption of carbon tetrachloride, chloroform, benzene, and methylene chloride onto four different activated carbons at multiple temperatures.57 Such values reflect a similar physical sorption process, albeit likely enhanced from expected increased interactions between the activated carbon and the sorbates used in those studies relative to interactions of TCE with OL and LHA. It is insightful to also examine the entropic energy contribution to the sorption process in the glassy state. Because sorption into a microporous sorbent may be viewed as a predominantly physical process, there should be a relatively small entropic contribution unless there is significant change in the macromolecular structure of the sorbent due to swelling and/or structural rearrangement associated with sorbate/ sorbent interaction. Dubinin micropore models are based on the assumption that the free energy of adsorption is provided by the total energy required to condense the sorbate from the gas state to the liquid state and is equal to the Gibb’s free energy. As such, the entropic contribution can be determined from the Gibb’s free energy of reaction: 6694


Environmental Science & Technology ΔS =

−ΔG + ΔH T

Article

conceptual sorption equilibrium models used in the analysis of the fate and transport of VOCs in the environment. Examination of nonequilibrium vapor-phase sorption processes of VOCs in NOM is provided in the companion study to this effort. Together, these studies provide additional identified needs to explore impacts of this research on practical applications of remediation technologies such as influence of temperature or steam on VOC retention and extraction.

(7)

The entropy of TCE sorption in NOM was determined to be −0.10 to −0.04 kJ/mol/K for OL and −0.10 to −0.05 kJ/mol/ K for LHA. These entropic contributions are on the order of the enthalpic contributions noted above for OL and LHA. Further, these data are consistent with other studies evaluating sorption into microporous materials.55 In rubbery macromolecular systems, use of Flory−Huggins equilibrium isotherms does not provide the appropriate parameters for use in calculating isosteric heats of sorption. Although ΔH cannot be determined from the high-temperature isotherm parameters, an effort was made to estimate this value from the Clausius−Clapeyron equation, where absolute pressure (P) is plotted against the inverse temperature (1/T) for a constant uptake:4 ΔHsorption

⎡ ∂(ln P) ⎤ = R⎢ ⎥ ⎣ ∂(1/T ) ⎦mass

■

ASSOCIATED CONTENT

* Supporting Information S

Additional sorbent characterization and model fit information, including comparison of batch and sequential dosing isotherm methods. This information is available free of charge via the Internet at http://pubs.acs.org/.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; tel.: 615-343-7070; fax: 615-322-3365.

(8)

If data derived for OL and LHA in the rubbery state at 75, 95, and 115 °C possess regions of overlapping mass uptake, then the Clausius−Clapeyron equation can be applied. Analysis of experimental data showed that ΔH for OL could not be estimated at any uptake due to poor model fit. LHA, however, yielded values of +76 to +86 kJ/mol heat of sorption for uptakes of 5 × 10−6 to 1 × 10−5 mol TCE/g LHA. These large positive heats of sorption, while unreasonable for favorable sorption, are consistent with Flory−Huggins theory in predicting large negative entropic contributions associated with the introduction of additional sorbate molecules that may cause the fluid-like, relatively ordered macromolecular matrix to rearrange to a more disordered state to accommodate the extra chemical potential. Observations of NOM Tgs, combined with evaluation of vapor phase sorption behavior above and below the Tg, provides additional insights into specific contributions of macromolecular mobility on sorption mechanisms. First, the methodology developed to expand evaluation of isotherms into the low-pressure pore-filling regions has allowed for development of further evidence of the microporous character of NOMs in the glassy state. Second, vapor-phase experiments, conducted in pure NOMs above the glass transitions of these materials, show differences in the mobility and structural configuration of these macromolecules, as evidenced by strong entropic changes during sorption. This increasingly mobile, rubbery medium is more consistent with dissolution into a rubbery macromolecule than sorption into a microporous solid. It should be noted, however, that such observed differences do not fully discount the fact that NOM is very heterogeneous in nature and, while Tgs have been detected in both sorbents, it is highly likely that domains or regions possessing rubbery or glassy characteristics will exist at both below and above the observed Tgonly when there is sufficient mobility or immobility of the majority of the matrix would one expect to observe these differences (e.g., it is likely that specific sorption mechanisms may not be easily differentiated for experimental temperatures at the Tg (e.g., 70 °C for either sorbent)). Finally, this work provides further substantiation that, at least for the samples employed in this study, NOM possesses macromolecular characteristics that display sorption behavior similar to synthetic macromoleculesan important assumption in

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the insightful and very helpful comments provided by the three reviewers of this manuscript. This material is based upon work supported by the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.

■

REFERENCES

(1) Lerner, D. N.; Burston, M. W.; Bishop, P. K. Hydrogeology of the Coventry Region (Uk) - an Urbanized, Multilayer, Dual-Porosity Aquifer System. J. Hydrol. 1993, 149, 111−135. (2) Squillace, P. J.; Moran, M. J. Factors associated with sources, transport, and fate of volatile organic compounds and their mixtures in aquifers of the United States. Environ. Sci. Technol. 2007, 41, 2123− 2130, DOI: 10.1021/es061079w. (3) Kobus, H.; Barczewski, B.; Koschitzky, H.-P. Groundwater and Subsurface Remediation: Research Strategies for in-Situ Technologies; Springer, 1996. (4) Chiou, C. T. Partition and Adsorption of Organic Contaminants in Environmental Systems; Wiley-Interscience, 2002. (5) Chiou, C. T.; Shoup, T. D. Soil Sorption of Organic Vapors and Effects of Humidity on Sorptive Mechanism and Capacity. Environ. Sci. Technol. 1985, 19, 1196−1200. (6) Chiou, C. T.; Shoup, T. D.; Porter, P. E. Mechanistic Roles of Soil Humus and Minerals in the Sorption of Nonionic OrganicCompounds from Aqueous and Organic Solutions. Org. Geochem. 1985, 8, 9−14. (7) Ong, S. K.; Lion, L. W. Mechanisms for Trichloroethylene Vapor Sorption onto Soil Minerals. J. Environ. Qual. 1991, 20, 180−188. (8) Ong, S. K.; Lion, L. W. Effects of Soil Properties and Moisture on the Sorption of Trichloroethylene Vapor. Water Res. 1991, 25, 29−36. (9) Rhue, R. D.; Rao, P. S. C.; Smith, R. E. Vapor-Phase Adsorption of Alkylbenzenes and Water on Soils and Clays. Chemosphere 1988, 17, 727−741. (10) Smith, J. A.; Chiou, C. T.; Kammer, J. A.; Kile, D. E. Effect of Soil-Moisture on the Sorption of Trichloroethene Vapor to VadoseZone Soil at Picatinny Arsenal, New Jersey. Environ. Sci. Technol. 1990, 24, 676−683. 6695



Article

(11) Steinberg, S. M.; Pignatello, J. J.; Sawhney, B. L. Persistence of 1,2-Dibromoethane in Soils - Entrapment in Intraparticle Micropores. Environ. Sci. Technol. 1987, 21, 1201−1208. (12) Cabbar, H. C. Effects of humidity and soil organic matter on the sorption of chlorinated methanes in synthetic humic-clay complexes. J. Hazard. Mater. 1999, 68, 217−226. (13) DeLapp, R. C.; LeBoeuf, E. J.; Bell, K. D. Thermodynamic properties of several soil- and sediment-derived natural organic materials. Chemosphere 2004, 54, 527−539, DOI: 10.1016/S00456535(03)00710-0. (14) LeBoeuf, E. J.; Weber, W. J. A distributed reactivity model for sorption by soils and sediments. 0.8. Sorbent organic domains: Discovery of a humic acid glass transition and an argument for a polymer-based model. Environ. Sci. Technol. 1997, 31, 1697−1702. (15) Young, K. D.; Leboeuf, E. J. Glass transition behavior in a peat humic acid and an aquatic fulvic acid. Environ. Sci. Technol. 2000, 34, 4549−4553, DOI: 10.1021/es00889j. (16) Berens, A. R. Effects of Sample History, Time, and Temperature on Sorption of Monomer Vapor by PVC. J. Macromol. Sci. Phys. 1977, B14, 483−498. (17) Berens, A. R. Transport of Organic Vapors and Liquids in Polyvinyl-Chloride. Makromol. Chem., Macromol. Symp. 1989, 29, 95− 108. (18) Weber, W. J.; Leboeuf, E. J.; Young, T. M.; Huang, W. L. Contaminant interactions with geosorbent organic matter: Insights drawn from polymer sciences. Water Res. 2001, 35, 853−868. (19) Vieth, W. R.; Howell, J. M.; Hsieh, J. H. Dual Sorption Theory. J. Membr. Sci. 1976, 1, 177−220. (20) Huang, W. L.; Weber, W. J. A distributed reactivity model for sorption by soils and sediments. 11. Slow concentration dependent sorption rates. Environ. Sci. Technol. 1998, 32, 3549−3555. (21) White, J. C.; Pignatello, J. J. Influence of bisolute competition on the desorption kinetics of polycyclic aromatic hydrocarbons in soil. Environ. Sci. Technol. 1999, 33, 4292−4298. (22) Xing, B. S.; Pignatello, J. J. Time-dependent isotherm shape of organic compounds in soil organic matter: Implications for sorption mechanism. Environ. Toxicol. Chem. 1996, 15, 1282−1288. (23) Pignatello, J. J. Soil organic matter as a nanoporous sorbent of organic pollutants. Adv. Colloid Interfaces 1998, 76, 445−467. (24) Georgi, A.; Kopinke, F. D. Validation of a modified FloryHuggins concept for description of hydrophobic organic compound sorption on dissolved humic substances. Environ. Toxicol. Chem. 2002, 21, 1766−1774. (25) LeBoeuf, E. J.; Weber, W. J. Reevaluation of general partitioning model for sorption of hydrophobic organic contaminants by soil and sediment organic matter. Environ. Toxicol. Chem. 1999, 18, 1617− 1626. (26) LeBoeuf, E. J.; Weber, W. J. Macromolecular characteristics of natural organic matter. 2. Sorption and desorption behavior. Environ. Sci. Technol. 2000, 34, 3632−3640, DOI: 10.1021/es991104g. (27) Gunasekara, A. S.; Simpson, M. J.; Xing, B. S. Identification and characterization of sorption domains in soil organic matter using structurally modified humic acids. Environ. Sci. Technol. 2003, 37, 852− 858, DOI: 10.1021/es026151e. (28) Gunasekara, A. S.; Xing, B. S. Sorption and desorption of naphthalene by soil organic matter: Importance of aromatic and aliphatic components. J. Environ. Qual. 2003, 32, 240−246. (29) Young, K. D.; LeBoeuf, E. J. Development of a glass ampoule system for evaluation of long-term sorption/desorption behavior of vapor phase volatile organic compounds in geosorbents. Analyst 2001, 126, 1816−1819. (30) Micromeritics. ASAP 2010 Accelerated Surface Area and Porosimetry System Operator’s Manual; Micromertics, 19951997. (31) Micromeritics. Preparation of Hydrocarbons or Other Liquids as Vapor Sources for Gas Adsorption Using the ASAP 2010, 1995 ed.; Micromertics, 1995. (32) Lin, T. F.; VanLoy, M. D.; Nazaroff, W. W. Gas-phase transport and sorption of benzene in soil. Environ. Sci. Technol. 1996, 30, 2178− 2186.

(33) Shimizu, Y.; Takei, N.; Terashima, Y. Roles of Solid Components on the Sorption of Trichloroethylene (TCE) onto Natural Solids from the Vapor-Phase. Water Sci. Technol. 1994, 30, 1− 11. (34) Chiu, C. H.; Lin, T. F.; Young, C. C. Short-term sorption of toluene vapor onto natural organic materials. Water, Air, Soil Pollut. 2003, 144, 223−241. (35) Dural, N. H.; Chen, C. H.; Puri, R. K. Adsorption equilibrium of carbon tetrachloride on dry soils. Chem. Eng. Commun. 1997, 162, 75− 92. (36) Ramirez, D.; Sullivan, P. D.; Rood, M. J.; Hay, K. J. Equilibrium adsorption of phenol-, tire-, and coal-derived activated carbons for organic vapors. J. Environ. Eng.-ASCE 2004, 130, 231−241, DOI: 10.1061/(Asce)0733-9372(2004)130:3(231). (37) Senel, I. G.; Guruz, A.; Yucel, H. Characterization of pore structure of Turkish coals. Energy Fuels 2001, 15, 331−338. (38) Xia, G. S.; Ball, W. P. Polanyi-based models for the competitive sorption of low-polarity organic contaminants on a natural sorbent. Environ. Sci. Technol. 2000, 34, 1246−1253. (39) Pignatello, J. J.; et al. Nonlinear and competitive sorption of apolar compounds in black carbon-free natural organic materials. J. Environ. Qual. 2006, 35, 1049−1059, DOI: 10.2134/Jeq2005.0362. (40) Serad, G. E.; Freeman, B. D.; Stewart, M. E.; Hill, A. J. Gas and vapor sorption and diffusion in poly(ethylene terephthalate). Polymer 2001, 42, 6929−6943. (41) Cowie, J. M. G.; Arrighi, V. Polymers: Chemistry and Physics of Modern Materials, 3rd ed.; CRC Press, 2008. (42) Huang, J. C.; Deanin, R. D. Concentration dependency of interaction parameter between PVC and plasticizers using inverse gas chromatography. J. Appl. Polym. Sci. 2004, 91, 146−156, DOI: 10.1002/App.13111. (43) Tian, M. M.; Munk, P. Characterization of Polymer-Solvent Interactions and Their Temperature-Dependence Using Inverse GasChromatography. J. Chem. Eng. Data 1994, 39, 742−755. (44) Polanyi, M. Section III.-Theories of the adsorption of gases. A general survey and some additional remarks. Introductory paper to section III. Trans. Faraday Soc. 1932, 28, 316−333. (45) Carmo, A. M.; Hundal, L. S.; Thompson, M. L. Sorption of hydrophobic organic compounds by soil materials: Application of unit equivalent Freundlich coefficients. Environ. Sci. Technol. 2000, 34, 4363−4369. (46) Sawhney, B. L., Brown, K. W. Reactions and Movement of Organic Chemicals in Soils; Soil Science Society of America and American Society of Agronomy, 1989. (47) Xing, B. S.; Pignatello, J. J. Dual-mode sorption of low-polarity compounds in glassy poly(vinyl chloride) and soil organic matter. Environ. Sci. Technol. 1997, 31, 792−799. (48) Singh, A.; Freeman, B. D.; Pinnau, I. Pure and mixed gas acetone/nitrogen permeation properties of polydimethylsiloxane [PDMS]. J. Polym. Sci. Polym. Phys. 1998, 36, 289−301. (49) Freeman, B. D. In Comprehensive Polymer Science; Aggarwal, S.L., Russo, S., Eds.; Pergamon Press, 1992; p 167. (50) DeLapp, R. C.; LeBoeuf, E. J.; Chen, J.; Gu, B. H. Advanced thermal characterization of fractionated natural organic matter. J. Environ. Qual. 2005, 34, 842−853, DOI: 10.2134/Jeq2004.0241. (51) Zhang, L.; LeBoeuf, E. J.; Xing, B. S. Thermal analytical investigation of biopolymers and humic- and carbonaceous-based soil and sediment organic matter. Environ. Sci. Technol. 2007, 41, 4888− 4894, DOI: 10.1021/Es063106o. (52) Schuld, N.; Wolf, B. A. Solvent quality as reflected in concentration- and temperature-dependent Flory-Huggins interaction parameters. J. Polym. Sci. Polym. Phys. 2001, 39, 651−662. (53) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley, 1989. (54) Young, T. M.; Weber, W. J. A Distributed Reactivity Model for Sorption by Sails and Sediments 0.3. Effects of Diagenetic Processes on Sorption Energetics. Environ. Sci. Technol. 1995, 29, 92−97. 6696



Article

(55) Chiang, H. L.; Huang, C. P.; Chiang, P. C. The adsorption of benzene and methylethylketone onto activated carbon: Thermodynamic aspects. Chemosphere 2002, 46, 143−152. (56) Chemical Rubber Company. CRC Handbook of Chemistry and Physics; Boca Raton, FL. (57) Chiang, Y. C.; Chaing, P. C.; Huang, C. P. Effects of pore structure and temperature on VOC adsorption on activated carbon. Carbon 2001, 39, 523−534.

6697


Mechanistic Investigation of Non-Ideal Sorption Behavior in Natural

Recommend Documents