Environ. Sci. Technol. 1997, 31, 2307-2312
Diffusion of Volatile Organic Compounds in Pressed Humic Acid Disks MEEI-LING CHANG,* SHIAN-CHEE WU, AND CHIH-YU CHEN Institute of Environmental Engineering, National Taiwan University, No. 71, Chou Shan Road, Taipei 106, Taiwan
The slow sorption and desorption of volatile organic compounds (VOCs) in soils are assumed to be partly controlled by intraparticle diffusion and diffusion in soil organic matter (SOM) and influence soil remediation. In this study, humic acid, a component of SOM, is used to simulate SOM as a partition medium for VOCs. The sorption kinetics of VOCs in dry, pressed humic acid disks was investigated by tracking the weight change of the sorbent with a microbalance. A diffusion model successfully describes the kinetics of sorption and desorption. The apparent diffusivities of toluene, n-hexane, and acetone in humic acid disks range from 10-8 to 10-9 cm2/s; those of desorption, which increase with temperature, are about 10-9 cm2/s. No significant amount of permanently bound sorbate residue could be observed. The enthalpy change for the sorption of toluene is -14 kcal/mol, and the activation energies for its diffusion during sorption and desorption are 10.1 and 15.7 kcal/mol, respectively. According to the results of this study, it is suggested that the sorption of VOCs in humic acid is a physical process and that the slow process of sorption in natural soils as reported in the literature is not entirely a consequence of diffusion in the humic acid.
Introduction The efficiency of extracting volatile organic compounds (VOCs) from contaminated soil during soil remediation processes depends upon the rate of VOC desorption from the soil matrix. It is often found that a fraction of sorbed VOCs is slowly released from soil particles and that some portion of sorbed VOCs will be permanently retained in the particles (1). Slow adsorption and desorption are proposed to be controlled by slow processes, such as slow diffusion in the micropores of soil particles (2-4), intraparticle diffusion of VOCs in soil particles (5), and slow migration in soil organic matter (SOM) (6-8). SOM, as a natural polymer, adheres to mineral surfaces and behaves like a partition medium for VOCs (9-12). Under natural conditions of high relative humidity, the predominant sorbent in soil is SOM rather than mineral surfaces (13, 14). Despite its high sorption capacity, SOM does not have an unusually large surface area (10). It is similar to a piece of plastic or rubber with a homogeneous interior composed of polymers instead of aggregates, allowing sorbate molecules to travel inside freely. The concept of sorbate molecules penetrating synthetic polymer mass by diffusion can be used to describe the sorption of VOC by SOM (15-19). A schematic model of VOC sorption by SOM is shown in Figure 1. The * Author to whom correspondence should be addressed (telephone/fax: 886-2-3629435; e-mail:
[email protected]).
S0013-936X(96)00903-0 CCC: $14.00
1997 American Chemical Society
FIGURE 1. Conceptual diagram of VOC sorption and diffusion into solid SOM. The proposed mechanisms are (1) surface adsorption, (2) multilayer formation and surface modification, and (3) penetration and slight swelling. transport mechanisms involved in the sorption process are believed to be attachment, surface modification, and diffusion. A direct measurement of VOC diffusivity in SOM, without interference from other mass transfer processes in soil aggregates or other soil constituents, is necessary for determining whether diffusion in SOM is slow and contributes to the sluggishness of the sorption and desorption processes. However, it is very difficult to isolate SOM from soils without changing the properties or the composition of the SOM. Humic acid has a molecular weight ranging from several hundred to perhaps over 300 000 and a polymer-like structure and contributes to a significant portion of SOM (20). Therefore, it might be a surrogate for natural SOM in the study of VOC sorption on SOM. Therefore, this paper seeks (1) to illustrate the adsorption and desorption kinetics of VOCs in a model SOM film made of humic acid powder, (2) to determine whether temperature and type of VOCs are affecting factors and whether a strong chemical association exists between VOCs and humic acid, and (3) to decide whether sorption on humic acid contributes to the sluggishness and irreversibility of slow VOC sorption on soils. Commercially available humic acid will be squeezed into a thin layer and used to simulate the natural SOM films, which are homogeneous and have high sorption capacity and small specific surface area, found on mineral surfaces. VOC vapor will penetrate into the model SOM layer only by diffusion. If edge effects (i.e. assuming that the length scale of film width is much greater than that of thickness) are ignored, variation of VOC concentration in the SOM film with a thickness of 2l can be described by a one-directional mass conservation equation:
∂q ∂2q )D 2 ∂t ∂x
(1)
∫ q(x,t) dx
M(t) ) 2S
l
(2)
0
In eqs 1 and 2 q (mg/cm3) is the VOC concentration in the SOM film at location x and time t, D is the apparent diffusivity of VOCs inside the SOM film, x is the axial dimension, M is the total sorbed mass, S is the surface area of one side of the film, and l is the half-thickness of the film. By measuring the instantaneous total weight of the SOM film with a microbalance and subtracting the initial weight of the clean film, we are able to track the sorbed mass of VOC, M(t), and subsequently estimate the best-fitting value of D.
VOL. 31, NO. 8, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2307
TABLE 1. Parameters of Humic Acid Disks expt A
B
C
D1a
D2a
E
F
weight (mg) 82.5 65.8 101.2 38.3 38.3 48.8 64.8 thickness (mm) 0.52 0.37 0.75 0.25 0.25 0.33 0.48 density (g/cm3) 1.30 1.44 1.28 1.23 1.23 1.21 1.11 a 1 and 2 represent duplicate experiments of the same disk. All disks are of the same diameter, 12.45 mm.
FIGURE 3. Apparatus for kinetic study of sorption and desorption.
FIGURE 2. SEM photographs of a humic acid disk: (a) surface and (b) cross-sectional view of a broken edge. The pictures show that the pressed disk is homogeneous and contains few pores.
Materials and Methods Preparation of Humic Acid Pellets. Humic acid powder (Aldrich Chemical Co.) was pressed into a thin disk under a pressure of 12.7 N/m2 for 1 min. The humic acid disk was then carefully retrieved from the presser and examined with a microscope. Six disks, each 12.45 mm in diameter, of various thicknesses ranging from 0.25 to 0.75 mm were made. These disks weighed from 38.3 to 101.2 mg, and their average density was 1.26 g/cm3 (Table 1), which was close to that of dry peat and muck reported by other researchers (21). The disks were oven-dried (103 °C) overnight and stored in a dessicator before use. Homogeneous disks can be made following the above procedure. Figure 2 shows the SEM photographs of the pellets. The surface is so smooth (Figure 2a) that only a few cracks can be found. The exposed inner surface of a broken edge displays the homogeneity and lack of pores inside the disk (Figure 2b). VOCs. Three VOCs, toluene, n-hexane, and acetone (99.8%+, Merck Co.), were used. Among common aromatics, toluene has relatively high diffusivity. n-Hexane, a six-carbon
2308
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 31, NO. 8, 1997
n-paraffin, may have a representative diffusivity for n-paraffins of higher carbon numbers because it was found that the diffusivity for n-paraffin in polyisobutylene at 35 °C leveled off to a constant value after five-carbon molecules (17). Acetone is the most polar among the three. Sorption Experiment. The experimental apparatus is illustrated in Figure 3. The disk was hung on the platinum stirrup on the sample side of an electric microbalance (Cahn 200) with sensitivity of 1 µg. The stirrup and the disk were enclosed in a glass gas chamber. To remove sorbed impurities on the disk, the disk was oven-dried and purged with 50 mL/ min N2 gas (HC-free, purity 99.999%) in the gas chamber for more than 20 h until the decrease in weight became unnoticeable. The microbalance was placed in a small room with the temperature maintained at 24 ( 1 °C, and the temperature of sorption chamber was controlled by a water bath (at 25 ( 0.1, 35 ( 0.1, or 45 ( 0.1 °C). Heating tape was taped on the inlet gas tube so that the inlet gas could be maintained at high sorption temperature. A steady flow of gas was pumped into the bottle containing either toluene, n-hexane, or acetone liquid, and the effluent was mixed with a flow of pure N2 gas. The concentration in the gas phase was constant during sorption. The maximun VOC removal rate in our experiments was 100 µg/min could keep the concentration of VOC in the chamber from varying. When the rate of change of the weight could not be differentiated from the base noise of the microbalance, which was about 2 µg/5 h, the sorption experiment was terminated. The concentration of inlet VOC gas, sampled from the side port located behind the mixing chamber, was determined by GC/PID (China Chromatograph, with PID, HNU PI52-02A) or GC/FID (Hewlett-Packard 5890 II). Estimating Diffusivity. If a sorption experiment is carried out with a clean humic acid disk suspended in gas phase at a constant gaseous VOC concentration, then the initial condition inside the disk is
q(x,0) ) 0
(3)
Assuming the sorbates in the sorbed-on-surface phase and those in the gaseous phase are in equilibrium after a short period of surface adsorption and surface modification process, then the boundary condition at the disk surface (x ) 0) is a constant VOC concentration (qe) in equilibrium with the VOC concentration in the gas phase:
q(0,t) ) qe
(4)
Using the experimental results, qe can be estimated from the ultimate sorbed mass Me using
qe ) Me/2Sl
(5)
At the middle of the disk there is no concentration gradient:
∂q (l,t) ) 0 ∂x
(6)
The analytical solutions for the fraction of equilibration reached (i.e. the ratio of the sorbed mass Mt and the ultimate mass at infinite time Me) are available in Crank (eqs 36 and 37 in Chapter 1 of ref 17) and can be expressed as
Mt/Me ) f(t)
(7)
where f(t) is a dimensionless solution with a value of zero at t ) 0 and a value of unity when t approaches infinity. Desorption was carried out by purging 50 mL/min N2 after the sorption process was completed, and the weight was measured with a microbalance. The criterion for interrupting the desorption process was the same as that for sorption experiments. Once the humic acid disk, after having equilibrated with VOCs for an infinite length of time, is placed into a chamber with clean air, the appropriate initial condition for the desorption process is
q(x,0) ) qe
(8)
and the boundary conditions are
q(0,t) ) 0
(9)
and eq 6. The analytical solution for the variation in sorbed amount with respect to time during desorption is
Mt/Me ) 1 - f(t)
(10)
where f(t) has the same meaning as in eq 7. By using this diffusion model, diffusivity can be determined from the uptake or loss of the film during sorption or desorption. When Mt/Me ) 0.5 (t ) t1/2), the relationship between diffusivity and t/4l2can be derived as follows (halftime method) (17):
D ) 0.04919/(t1/2/4l2)
(11)
This method has been used to determine the diffusivity of toluene into butyl rubber by other researchers (18).
Results and Discussion Seven sets of experimental sorption results (A-F), including one duplicate (D1 and D2), were obtained for three VOCs (Figures 4-6; Table 2). The gas-phase VOC concentrations ranged from 1.92 mg/L (P/P0 ) 0.0088) for toluene to 25.8 mg/L (P/P0 ) 0.038) for acetone. The maximum ratio of equilibrium sorbed mass to the disk weight was 1%, which did not significantly affect the thickness of the disk during sorption. Two-Stage Sorption Process. At least two mechanisms were involved in each sorption run except for the runs using a higher acetone concentration (expt C) and toluene at higher temperature (expt F). A first sorption stage with relatively small sorption capacity during the first 2.5 h is followed by a second sorption stage with much higher capacity lasting for several days. Similar two-stage results have been reported for the sorption of organic compounds in polymers, such as allyl chloride in poly(vinyl acetate) and toluene (liquid) in poly(aryl ether ether ketone) (19, 22). Models of diffusion in polymers may be applied to explain this phenomenon. Activation energy is required to loosen the polymer molecular chain to form a suitable space or free volume for penetrant molecules (23), and the energy involves
intermolecular and intramolecular effects (Chapter 2 of ref 17); thus, time is required for VOCs to “solvate” the target surface. Following the locations, from the outer gas phase to the interior of the sorbent mass, which the penetrating VOC molecules will encounter, a series of mechanisms are supposedly involved: (1) VOC vapor adsorption to solid surface, (2) vapor multilayer formation and solid surface modificationssufficient attachment to allow multilayer adsorption, and (3) penetration and slight swelling of the whole solid as the vapor penetrates. As the swelling front progresses through the solid, the molecules behind it penetrate more easily, thereby increasing diffusivity. Since the ratio of equilibrium sorption mass to the weight of humic disk used in this study (Tables 1 and 2) is 20 kcal/mole for a chemisorption process (24) and can be calculated with the van’t Hoff equation
d ln Kp ∆H ) dT RT2
(14)
where R is the ideal gas constant (1.987 kcal/mol‚K), T is the absolute temperature (K), and Kp is the equilibrium SOMair partition coefficient. The enthalpy change of toluene sorption is -14 kcal/mol. Since ln Kp is linear with 1/T (Figure 7b), the enthalpy change
TABLE 2. Experimental Conditions and Results of Sorption and Desorption exptl run
VOC van der Waal vola (L/mol) VOC concn (mg/L) P/P0 temp (°C) approx 1st-stage sorption mass (µg) lag time for diffusion process (h) equilibrium sorption mass (µg) obsd Kp × 10 [(mg/g)/(mg/L)] Da × 109 b (cm2/s) Da × 109 b (cm2/s) a
Cited from ref 26.
b
A
B
hexane 0.1753 6.46 0.0096 25 10 3.72 418 8.62 13.8 3.7
acetone 0.1124 4.47 0.0065 25 5 9.50 27 0.914 4.4
C acetone 25.8 0.038 25 not obvious (no) 216 0.824 6.5 4.7
D1
D2
E
F
toluene 0.1499 2.07 0.016 25 11 2.06 367 46.3 6.8 1.3
toluene
toluene
toluene
2.44 0.019 25 12 2.37 341 36.3 6.4 1.1
19.2 0.0088 35 17 2.59 196 20.9 6.8 1.8
7.06 0.020 45 less obvious 1.59 412 8.97 20.9 6.9
Calculated by half-time method.
FIGURE 7. (a, top) Plot of ln Da and ln Dd versus 1/T (K) for toluene sorption, yielding activation energies of 10.1 and 15.7 kcal/mol for sorption and desorption, respectively; and (b, bottom) plot of ln Kp versus 1/T (K) for toluene sorption, yielding enthalpy change of -14 kcal/mol. Temperature ranges from 25 to 45 °C. is not affected by temperatures from 25 to 45 °C. The relatively low enthalpy change shows that the sorption process is a physical exothermic process. No chemical bond is formed or broken during sorption. Desorption. The desorption processes require about 40120 h to complete. It is for some reason difficult to determine the endpoint of equilibrium. Sorbed VOCs can be desorbed from humic acid, but a minor fraction of mass could not be recovered during these desorption experiments as a result of irreversibility or experimental error. Ignoring this minor deviation and using the original weight of the humic acid disk as the desorption endpoint does not affect the estimation for desorption diffusivity. Desorption is initially fast followed by a slow period. Simulation of desorption diffusivity Dd calculated by the half-time method fits the desorption data well (Table 2; Figures 4-6).
The desorption rate is slower than the sorption rate by a factor of 4-5. The activation energy of toluene desorption (15.7 kcal/mol) is higher than that of sorption by a factor of 1.6, resulting in a lower desorption rate. There may be some matrix deformation in humic acid due to swelling after VOCs penetrate, which creates a more favorable environment and causes VOCs to remain. This indicates the existence of concentration-dependent sorption and/or changes in sorbent properties with sorption. Both are consistent with the sorption model proposed previously in this paper. VOC Effect. Among the three VOCs tested, n-hexane has the highest sorption diffusivity and toluene has the highest Kp. There seems to be no apparent relationship between diffusivity and partition tendency among different VOCs. Acetone, with the smallest van der Waal volume and lowest Kp, however, has similar (or even lower than hexane) diffusivity inside humic acid. Physical hindrance may not be the only factor retarding the migration of sorbate molecules in the humic acid matrix. Weak molecular interactions (e.g., hydrogen bonding between acetone and hydroxyl group of humic acid) may play a role in controlling the mobility of sorbate molecules. Sorbent and Humidity Effects. There was no attempt to humidify the system in this study. The SOM phase was not water-saturated, as it normally is in the natural subsurface. In all but the most arid climates or in soil remediation practices that purge the soil using hot dry air, soil moisture (even in the unsaturated zone) is likely to be sufficient to provide almost 100% humidity in the gas phase, such that micropores (and intrapolymeric regions of polyelectrolytes such as SOM) are likely to be water-filled. Under these conditions, the SOM matrix becomes more hydrophilic and the Kp for nonionic compounds changes; VOC diffusion coefficients also differ from those measured in such humidified conditions. Therefore, a polymer diffusion model that fails to consider the effects of moisture is inappropriate for describing the slow rate of desorption observed in natural soil. Further research is necessary to investigate the change of diffusivity, the activation energy, the enthalpy change of sorption, and the partition coefficient under humidified conditions to establish a better model to describe the sorption of VOCs in SOM under moisturized conditions. It is reasonable to believe that the thickness of natural SOM film on soil mineral surfaces or discrete particles is only 30-1000 nm as given in ref 25 and by observation with electric microscope in this laboratory. If humic acid is the only constituent of natural SOM, the time scale of VOC penetration in humic acid film estimated by the observed diffusivity is approximately several seconds for both sorption and desorption. It does not seem likely that penetration through humic acid is a limiting slow process. Mechanisms other than diffusion in humic acid control the slow sorption of VOCs in natural soils. Also, sorption on humic acid does not
VOL. 31, NO. 8, 1997 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2311
contribute significantly to irreversibility, since there was
