Environ. Sci. Technol. 1993, 27, 883-888
Evaluation of the Sorption of Volatile Organic Compounds by Unsaturated Calcareous Soil from Southern Nevada Using Inverse Gas Chromatography Spencer M. Steinberg'
Department of Chemistry, University of Nevada-Las
Vegas, Las Vegas, Nevada 89 154-4003
Davld K. Kreamer
Department of Geoscience and Water Resource Management Program, University of Nevada-Las Las Vegas, Nevada 89 154-4003 The vapor phase sorption of several volatile organic compounds (VOCs) by a calcareous soil from Southern Nevada has been measured using inverse gas chromatography (IGC). For dry soil, sorption isotherms are apparently nonlinear, and finite desorption kinetics contribute to the chromatographic peak shapes. Competitive sorption between VOCs and water was examined by adding water to the carrier gas and by directly wetting the soil. Even a small addition of water drastically decreased the sorption of non-hydrogen-bonding VOCs by the soil and eliminated the effect of finite sorption kinetics on the chromatographic peak shapes. However,hydrogen-bonding compounds still exhibited nonlinear sorption isotherms and finite sorption and desorption kinetics in the presence of water.
Introduction The mobility of volatile organic compounds (VOCs) in the soil vadose zone is highly dependent upon the sorption and diffusion of these compounds within the soil matrix. Accurate predictions regarding the transport of VOCs in the vadose zone requires a detailed understanding of the rate and mechanism of sorption and desorption of substrates from the various binding sites in the soil. For both aqueous- and vapor-phase sorption on soils, there exists a spectrum of binding sites for VOCs, some of which can be characterized by rapid equilibration kinetics (instantaneous),while other sites are characterized by slow sorption and/or desorption kinetics (1, 2). Interaction with the rapid binding sites can be described by a sorption isotherm; however, a mathematical description of the slow sites requires the use of rate constants or more complicated diffusion expressions (3). We recognize that a complete model of volatile compounds in the vadose zone must account for both fast and slow sorption and desorption; however, we restrict our attention in this report primarily to binding sites that represent kinetically fast interactions. Other studies have treated the kinetically slow processes, which have been evoked to explain sorption hysteresis in soils, and persistent contamination of soil by various organic compounds in both the vadose and saturated zones (1-6). Results of the application of IGC to the characterization of sorption by soil samples from the Nevada test site (NTS) are reported in what follows. The effect of water vapor at various relative humidities (RH) on retention of trichloroethylene (TCE! on NTS soil was investigated. We also examined the sorption of tetrachloroethylene (PCE) l,l,l-trichloroethane (TCA), 1,1,2,2-tetrachlorethane (TeCA), benzene, toluene, ethylbenzene, pentane, diethyl ether, acetone, and acetonitrile at various soil water contents and at several different temperatures,
* To whom all correspondence should be addressed. 0013-938X/93/0927-0883$04.00/0
@ 1993 American Chemical Society
Vegas,
Various static and dynamic experimental methods have been used to derive equilibrium sorption data for volatile compounds by soils. Static methods involve the use of a sealed vessel containing the soil and an aqueous and or vapor phase (6-8). A known mass of sorbent is added, and the concentration of the sorbate is measured in the gas or aqueous phase following an equilibration period. Several promising dynamic methods have been published (6,9,10). Dynamic methods involve equilibration of the soil with a flowing stream containing the volatile compound. After the equilibration of the sample, the total concentration of material sorbed is measured. Among the dynamic methods, IGC provides a rapid reliable approach for quantifying sorption of various compounds by soils and sediments (9-1 I). In addition to providing sorption data, the gas chromatographic approach, as applied herein, also may provide insight into the kinetics of sorption and desorption. Inverse chromatography involves injecting a known quantity of organic compound into a gas chromatography (GC) or liquid chromatography column packed with a stationary phase. Observed chromatographicpeak morphology (peak shapes) and retention times can be utilized to quantify the sorption isotherm. The inverse nature of IGC is the characterization of an unknown stationary phase (in this application, soil) using well-characterized sorbate probes.
Theory Detailed development of IGC theory has been presented and reviewed by others (8,11,12).An experimental variant of IGC which has been called the elution characteristic point method (ECP) was utilized in this study. This approach is briefly reviewed here in order to clarify the assumptions that are implicit in this work and the limitations of these measurements. The ECP method can be implemented using conventional GC equipment. The ECP method assumes that the chromatographic process can be described by the ideal chromatography equation (11,12): (dc/dt)(l+ (rn/VcE)(dq/dc))= -u(dc/dx)
(1)
The various terms in eq 1are defined as follows: c = gasphase concentration (mol/mL);Q = soil concentration (mol/ g); rn = mass of soil (g); V , = volume of column (mL); E = column porosity; t = time (min); x = position along length of column (cm); u = carrier velocity (cm/time). This simple equation is derived from mass balance considerations and ignores molecular diffusion, dispersion, or finite sorption and desorption kinetics. Furthermore, it is assumed that the carrier gas is noncompressible and that the analyte makes a negligible contribution to the total gas (carrier + analyte). The effects of carrier gas compressibility and finite concentrations have been conEnviron. Sci. Technol., Vol. 27, No. 5, 1993 883
sidered by others (12). The most important effect of carrier compressibility is a change in the flow rate due to the pressure drop along the column. As explained in the Experimental Section, we have adjusted our results to the average carrier flow rate in the column. Equation 1can be rearranged to yield the following (11, 12):
I
t , = to (1+ ( m i ~ , c ) ( a q i w )
(2) where (dq/dc) is the first derivative of the sorption isotherm; and t, and toare the retention times for a VOC probe at a particular concentration and for an unretained probe (methane), respectively. Since t , is a function of the first derivative of the sorption isotherm, a nonlinear isotherm will result in an asymmetric chromatographic peak. When the sorption isotherm is linear, the interpretation of the inverse chromatogram is simplified, since t , becomes independent of concentration. IGC analysis should then result in asymmetric chromatographic peak, with retention times independent of the mass of sorbate injected. Rearranging eq 2 results in the following:
I
1
@-A
-63-
K = ( t ,- to)(FJm) = ( q / c ) (3) where K is the partition coefficient (mL/g), and F, is the volumetric flow rate (mL/min). Equation 3 can be used to calculate the partition coefficient K from the chromatographic retention time.
Flgure 1. Experimental apparatus: (A) carrier gas flow regulators; (B) water-filled impinger;(C) mixingtee; (D) pressure gauge; (E) gas injection valve; (F) soil column; (G) FID detector; (H) chart recorder; (I) oven.
Experimental Section The chromatographic conditions utilized in this study are as follows. All soil columns were constructed of 4 mm (i.d.) X 10 cni glass. These relatively short columns minimized the effects of carrier compressibility. The average pressure drops observed were 0.19 and 0.07 atm for the clay and silt and the medium-to-fine sand columns, respectively. The glass columns were deactivated with dimethyldichlorosilane (DMCS) before being packed with soil. Soil was held in place with plugs of DMCS silanized glass wool. The calculated porosity for these columns was about 0.6; however, data analysis did not require the evaluation of porosity. All chromatography was performed on an HP5710 gas chromatograph equipped with two flame-ionization detectors (FID). The only modifications to this instrument were the installation of a Valco gas injection valve (0.25-mLloop) and a precision manometer in the carrier gas line just before the injection valve. A distilled-water impinger was also installed in the oven for the addition of water to the carrier gas. A diagram of the instrument used in this study is shown in Figure 1. For most of the data reported here, injections were performed with the gas injection valve. The valve loop was filled with air saturated with the vapor of one of the various probe compounds. The mass injected was varied by diluting the saturated vapor with ambient laboratory air in an all-glass syringe (i.e., loo%, 50%, 10%). The concentrations of the 11 VOCs in their saturated vapor were calculated using published equations that relate the partial pressure of the pure liquids to temperature (13). This injection procedure was found to be reproducible within 5 % , Some experiments were also conducted with injections of 1-4 pL of neat liquid. Experiments were performed at column temperatures that ranged from 25 to 120 "C; however, in the presence of water vapor, the maximum temperature was 50 "C.
Helium (He) and nitrogen (Nz) were used as carrier gasses in these studies. Sorption was unaffected by changing flow rates from 10 to 40 mLimin, although flow rates of about 10 mL/min were generally used. Methane was used as an unretained probe in all of these experiments. The retention volume of methane was unaffected by temperature, moisture, or mass injected. Chromatography experiments were conducted from 0% to 52 % relative humidity. Different relative humidities were established by mixing dry carrier gas with carrier gas that was passed through a water-filled impinger which was maintained at the same temperature as the soil column. The relative flow rates of the water-saturated and dry carriers were adjusted using flow regulators. The relative humidity of the carrier was determined by trapping water vapor exiting the soil column over a 24-h period using a column containing a combination of 50 % CaS04and 50 % Mg(C104)Z(approximately 25 g each). Trapped water was quantified gravimetrically. Assuming ideal gas behavior, the vapor pressure of water was then calculated from the total amount of water collected and the total volume of carrier that had passed through the column during the 24-h period. The relative humidity was calculated by comparing this calculated vapor pressure with the tabulated vapor pressure of water at the column temperature (13). For several experiments, higher water contents (2.85% and 11%)were obtained in the soil columns as follows. The soil columns were completely saturated with deionized water and then eluted with carrier for 24-48 h. The water loss from the soil columns was monitored gravimetrically. When the desired water contents were obtained, the columns were then eluted with the carrier that had passed through the impinger, and IGC measurements were conducted. Column weights were monitored before and after the IGC measurements and did not change.
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Environ. Sci. Technol., Vol. 27, No. 5, 1993
250
Table I. NTS Average Soil Characteristics Debris Flow Sample fraction coarse sand ( x > no. 5) medium-to-fine (no. 20 > x > 200) clay and silt (no. 200 > x )
% soil (wiw)~
m2igb
%OC ( W / W ) ~
35-70
10.7 f 1.6 10.1 f 1.0
1.2 0.50 f 0.12
0.4-5
21.5 f 2.0
1.2
25-60
Percent contribution to total soil. Surface area by BET method. Percent organic carbon of oven-dried soil.
200
4.1 mg
A
/
150
E E "
Throughout these experiments the GC detector temperature was maintained at 250 "C. The gas injection valve body and the packed column injector were heated to 180 "C using an auxiliary heater. Carrier flow rates which were generally about 10 mL/min were measured using a soap bubble flowmeter. The flow rates through the column were corrected to the soil column temperature and to the average column pressure using the following equation:
,
I
3.4 mg
0 C
0
: 100
a
50
2.1 mg
(4) ... ........
0
where F,and F, are the column flow rate and the measured flow rate; T, and T , are the column and flowmeter temperatures; P, and P, are ambient laboratory air pressure and the vapor pressure of water; and j is a pressure gradient correction factor (12).j is given by:
0
2
4
6
a
10
Time (minutes)
Flgure 2. IGC of three masses of TCE on the clay and silt fraction. The soil column contained 2.0 g of soli. The conditions are He carrier at 12.2 mL/min, 0% RH, and 100 "C.
Teller method (15), and organic carbon contents were measured using an automated dry combustion procedure (16).
where Pi and Po are the inlet and outlet pressures, respectively. Chromatographic data were recorded using a strip chart recorder, and peak areas were integrated using a planimeter. To determine if any of the VOCs were irreversibly sorbed by the soil column, IGC peak areas were compared to peak areas obtained when the soil column was replaced with a column of 10% SE-30 on acid-washed Chromasorb-W. This analysis indicated no loss of VOC in the soil columns. All injections were performed in triplicate, and at least three different masses of each VOC were injected for evaluation of the soil-vapor partition coefficients (Table 11). FID response factors were determined using external standards. Less than 5% variation was observed in retention times for replicate injections. Column-to-column variations in partition coefficients derived from this analysis were less than 10% Soil used in this study was obtained from the Nevada Test Site in the vicinity of Frenchman Flat. This material is comprised chiefly of alluvial deposits with very poorly developed to unapparent soil horizons. X-ray diffraction analysis of the soil showed that the soil consisted chiefly of calcite, quartz, and anorthite. The various soil size fractions were separated using a dry sieving procedure (14). Sieving was performed for 10 min on air-dried soils. Soil surface area and organic carbon contents were measured and are reported in Table I. Surface areas were measured by N2 adsorption using the Brunaur-Emmet-
.
Results and Discussion A t 0%-15% RH, the chromatograms obtained with N2 as the carrier gave split or bimodal peaks. We hypothesized that this was a nonequilibrium effect possibly due to slower (diffusion-controlled) transfer of the probe into soil micropores in the presence of the heavier carrier gas, and we believe that this slower transfer resulted in a portion of the injected compound being quickly eluted from the column. Bimodal peaks were not observed with He. Therefore, all low humidity measurements (
