Environ. Sci. Technol. 1990, 2 4 , 135-142
Sampling Bias Caused by Materials Used To Monitor Halocarbons in Groundwater Glenn W. Reynolds,+John T. Hoff,' and Robert W. Glllham
Waterloo Centre for Groundwater Research and Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, Canada, N2L 3G1
w Laboratory experiments were conducted to evaluate materials used in the construction of groundwater monitors for their potential to cause sampling bias. Ten materials were exposed to low concentrations of five halogenated hydrocarbons in water for periods up to 5 weeks. Borosilicate glass was the only material that did not diminish the halocarbon concentrations. Three metals, including stainless steel, apparently transformed the compounds. Six synthetic polymers, including poly(tetrafluoroethy1ene) and rigid poly(viny1 chloride), absorbed the compounds. The sorption rates were dependent on flexibility of the polymer, water solubility of the compound, solution volume to polymer surface area ratio, and temperature. A diffusion model explained the concentration histories of solutions exposed to polymers, and the diffusion mechanism was confirmed by direct measurement of halocarbon distributions in several of the polymers. The experimentally determined diffusivities and polymer-water partition coefficients for polyethylene were consistent with literature data.
Introduction The importance of obtaining representative samples of groundwater for the determination of organic contaminants has motivated many studies of sampling techniques. Systematic errors arising from volatilization (1-4, sorption-desorption (3-10), and leaching (11, 12) have been documented. Work in our laboratory has focused on evaluating the potential of synthetic polymers and other materials used in groundwater monitors to absorb and release common organic contaminants (13-16). Synthetic polymers are widely used in the construction of groundwater monitoring equipment. The fact that synthetic polymers are also employed to extract and preconcentrate water samples for the determination of soluble organics suggests that sorption might be a common source of error in sampling groundwater for organic contaminants. A question of current importance is whether to use poly(tetrafluoroethylene) or less expensive rigid poly(viny1 chloride) pipe to encase monitoring wells. Another question concerns the use of polymer tubing to convey water samples to the surface. A quantitative understanding of the processes by which organic compounds are absorbed by synthetic polymers is requisite to the design and effective use of sampling equipment constructed from synthetic polymers. This paper reports the results of laboratory experiments to measure the sorption of halogenated organic compounds by synthetic polymers and other materials used in groundwater monitors. Two techniques were employed: (1) Conventional sorption measurements were carried out to determine the rates under conditions resembling those in a well; diffusivities and polymer-water partition coefficients were estimated from these data. (2) A scanning Present address: Gartner Lee Limited, 140 Renfrew Drive, Markham, Ontario, Canada, L3R 8B6. 0013-936X/89/0924-0135$02.50/0
electron microscope with an energy-dispersive X-ray analyzer was used to confirm the mechanism of sorption and to measure the diffusivities independently. This paper interprets the results of the experiments in light of other studies of the sorption of organic compounds by synthetic polymers and suggests a strategy for controlling sampling bias.
Experimental Methods The sorption experiments were designed to examine the sorption rates associated with materials used in groundwater monitoring wells. Ten materials were evaluated: borosilicate glass tubing, 316 stainless steel tubing, aluminum tubing, galvanized steel sheet, rigid poly(viny1 chloride) rod (RPVC), poly(tetrafluoroethy1ene) tubing (Teflon, PTFE), Nylon 6,6 plate (NYL), polypropylene tubing (PP),low-density polyethylene tubing (LDPE), and latex rubber tubing (LAT). Most of these are or have been commonly used as well casing, screening, or transfer-line tubing materials; glass and latex rubber were included to represent the extremes of minimal and maximal sorption. The five organic compounds used in the experiments were l,l,l-trichloroethane (TRI), 1,1,2,2-tetrachloroethane (TET), hexachloroethane (HEX), bromoform (BRO), and tetrachloroethylene (TEY). Among these compounds are some of the most common organic contaminants in groundwater. Each material was washed, cut into pieces, and distributed equally among 30 160-mL glass hypovials. An additional 15 hypovials, which did not contain materials to be tested, served as controls. The hypovials were filled without headspace from a glass carboy containing an aqueous solution of the compounds at concentrations ranging from 20 to 45 pg L-l. These concentrations are similar to the drinking water standards for low molecular weight halocarbons. The hypovials were immediately sealed with Teflon-faced silicone rubber septa and stored in the dark a t 22 "C until sampled. The hypovials were not shaken continuously but were occasionally tilted back and forth to simulate near-stagnant conditions in a monitoring well. The shapes of the materials varied, but the ratio of the volume of solution to the surface area of material was nearly constant, being -0.35 cm. This ratio would be experienced in a monitoring well cased with material having an internal diameter of 1.4 cm. Two hypovials containing the material to be tested and one control hypovial were sacrificed at the following times: 5, 15, and 30 min; 1, 3, 6, and 12 h; 1, 2, and 4 days; and 1, 2, 3, 4, and 5 weeks. The concentrations of the five compounds in the solutions were determined in duplicate by GLC. A modified pentane extraction was used to preconcentrate the samples (17). The extracts were injected into a Hewlett-Packard 5710A gas chromatograph equipped with a 63Ni electron-capture detector. The column was packed with 10% UCON polar 50 HB5100 on 80-100-mesh Chromosorb. The detection limits (pg L-l) were approximately 3.0 for TRI, 1.0 for BRO, 0.5 for TET and TEY, and 0.05 for HEX.
0 1989 American Chemical Society
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Environ. Sci. Technol., Vol. 24, No. 1, 1990
135
Additional experiments were conducted to determine the effects of temperature, concentration, and solution volume to polymer surface area ratio on sorption of the compounds by LDPE. To evaluate the effect of temperature, a set of hypovials containing LDPE was carried through the above procedure except that the hypovials were stored at 10 "C. To evaluate the effect of the volume to area ratio, a set of hypovials containing LDPE was carried through the procedure at a ratio of 0.60 cm. To determine whether the initial concentration would affect the rate of sorption, LDPE pieces were inserted into 16 hypovials as previously described. The vials were filled with buffered organic-free water and capped. Duplicate vials were spiked to known concentrations by injecting small aliquots of concentrated methanol stock solution containing the five compounds. The initial concentrations varied from approximately 5 to 100 pg L-I for all of the compounds except HEX, which varied from approximately 2.5 to 50 pg L-I. The hypovials were stored at 22 O C and sampled after 157 min. Concentration distributions within the polymers were measured directly with a scanning electron microscope (SEM; JEOL Model JSM-840, operated at 25 keV) equipped with an energy-dispersive X-ray analyzer (EDAX; Kevex Analyst 8000). The compounds were those used in the sorption experiments, and the polymers were RPVC, PTFE, PP, and LDPE. A 6-cm segment of the polymer tubing was suspended in a stoppered Erlenmeyer flask containing a saturated solution of the compound. The solution was stirred continuously by a magnetic stirrer. After 30-360 min of exposure, the piece was removed from the solution and a 2-mm transverse section was cut from the middle. The section was immediately mounted on an aluminum stub with graphite cement and placed into the vacuum chamber of a gold-coating device. After a 30-nm layer of gold was deposited, the stub was quickly transferred to the sample chamber of the SEM. Typically, 7 min elapsed between removing the piece from the solution and measuring the concentration distribution. The electron beam caused the halogen atoms to fluoresce, and it was assumed that the rate of X-ray production was proportional to the concentration of the compound. No attempt was made to determine the absolute concentration. For a qualitative determination of a concentration profile, the electron beam was slowly scanned across the cut surface of the section and the intensity of the halogen X-rays produced was photographed; X-ray dot maps were also used to visualize concentration distributions. For a more quantitative determination, a series of discrete line analyses (30-s counting times) was made a t 100-pm intervals across the cut surface of the specimen. Similar techniques have been used to study the interdiffusion of poly(viny1 chloride in poly(capro1actone) (18). Results and Discussion Sorption Experiments. The data for each experiment (material-compound combination) were plotted as a concentration history showing relative concentration versus time. The relative concentration was computed by dividing the concentration remaining in solution, C, by the initial concentration, Co. To facilitate comparisons, the experimental data were grouped according to compound or material and plotted on common axes. Illustrative plots are shown in Figure 1. The polymer data were presented in Reynolds and Gillham (14) and the complete data set is contained in Reynolds (13). Each plotted point is the mean of four measurements (duplicate measurements from duplicate hypovials). The standard deviation of the relative concentration was usually less than 0.03; error bars 136
Environ. Sci. Technol., Vol. 24, No. 1, 1990
1 min
1 hour
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1.2
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i
f
10
08
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V
B
' S T A I N L E ~ SSTEEL
I
z 0
-I-
08-
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ALUMINUM
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0.21
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02-
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lo2 lo3 1o4 lo5 T I M E (min) Figure 1. Relative concentration of bromoform, CICo,versus time in (A) control hypovials, (B) hypovials containing glass and metals, and (C) hypovials containing polymers. The smooth curves in (C) represent least-squares fits to the data for the diffusive transport model (eq 5).
loo
10'
were not indicated for the sake of clarity. The control concentrations remained essentially constant, indicating that interactions with the hypovials were too small to be measured. Glass was the only material of the 10 tested that did not cause a reduction in solution concentration for at least one compound over the 34-day period of observation. Of the metals, stainless steel was the least reactive, causing reductions of only two compounds, BRO and HEX; the reductions were substantial however, amounting to 70% after 5 weeks. Aluminum caused reductions greater than or equal to 90% for four of the five compounds, the exception being TEY. Galvanized steel caused reductions greater than or equal to 99% for all compounds. The time for a 50% reduction in solution concentration can be used as a measure of the rate of compound disappearance. Table I shows that the compound disappearance rates decreased in the order galvanized steel > aluminum > stainless steel. The order in which the compounds disappeared from solution was BRO > HEX > TRI > T E T > TEY. It is unlikely that sorption would substantially deplete halocarbon concentrations in hypovials containing impenetrable materials a t the volume to surface area ratios and solution concentrations used in these experiments. Gillham and O'Hannesin (16)measured sorption of monoaromatic hydrocarbons by polymers and stainless steel using very similar methods, and no losses were observed for stainless steel. Sorption of organic contaminants by glass has been observed (7,19), but for compounds having much lower water solubilities. The reduction of halocarbon concentrations in vials containing the metals is believed to have been caused by
Table I. Times for 50% Reduction of Solution Concentration in the Hypovials Containing the Metals 50% reduction timea/min
metal
BRO
HEX
TRI
TET
TEY
stainless steel aluminum galvanized steel
45000
45000 3000 45
>50000 8000
>50000
1200 45
>50000 >50000 1000
90
23000 170
Compounds are identified in the first paragraph of Experimental Methods.
reactions involving the metal surfaces or metal ions released from the surfaces. Vogel et al. (20) summarized the current understanding of abiotic transformations of halogenated aliphatic compounds in natural water. Polyhalogenated aliphatic compounds can undergo reductive hydrogenolysis in the presence of transition metals and transition-metal complexes. In this reaction, a hydrogen atom replaces a halogen substituent and the transition metal is oxidized. Such reactions are relatively slow in nature, but the rates would be higher with an abundant supply of reduced metal present. Halocarbon concentrations declined rapidly after an initial delay period in vials containing the metals, whereas concentrations declined more gradually in vials containing the polymers. (See Figure 1.) The delay may be related to the time required to deplete dissolved oxygen or for significant concentrations of transition-metal ions to accumulate. It was also observed that the order in which the compounds disappeared was different when metals were present than when polymers were present. (See below.) The more halogenated compounds were generally removed before the less halogenated compounds in solutions exposed to the metals, and this is consistent with the postulated mechanism. Interpretation of the order of compound disappearance is complicated by the fact that all of the compounds were initially present. The less halogenated compounds could thus be augmented by reductive hydrogenolysis of the more halogenated compounds. Unidentified peaks were seen in the chromatograms from hypovials containing the metals; the peaks were probably reaction products. The fact that stainless steel showed the lowest activity of the three metals in the sorption experiments is probably related to the fact that it is the most inert with respect to corrosion. All of the polymers reduced the solution concentrations of at least three of the five compounds tested. The total extent of reduction ranged from not detectable to 99%, and the time for a 50% reduction ranged from 10 to over 50000 min. A nonparametric analysis of variance, the Quade test (21), was used to determine the statistically significant trends in compound disappearance rates. Rankings were developed by comparing concentration history plots for the experiments having a solution volume to polymer surface area ratio of approximately 0.35 cm and a temperature of 22 "C. The results indicate that both factors, polymer and compound, are significant at a level of 1% . The rates vary more among polymers than among compounds. The compound disappearance rates decrease in the order LAT > LDPE > PP > NYL > PTFE > RPVC, and the corresponding trend for the compounds is TEY > HEX > TRI > BRO > TET. Multiple comparison tests indicate that not all of the inequalities are significant at a level of 5%. For example, the rates for RPVC and PTFE cannot be distinguished, nor can those of NYL and PP. A nonparametric ANOVA of Gillham and OHannesin's (16) sorption experiment data for polymers also reveals significant trends in compound disappearance rate. The order for the polymers is flexible PVC > LDPE > PTFE
> poly(viny1idene fluoride) > rigid PVC, and the order for the compounds is p-xylene > m-xylene > ethylbenzene > o-xylene > toluene > benzene. As before, not all of the inequalities are significant. Considering both data sets, it is evident that a qualitative relationship exists between the type of polymer and the rate at which the compounds were removed from solution. The sorption rates were highest for the most flexible polymers (LAT, flexible PVC, LDPE) and lowest for the most rigid polymers (RPVC, epoxy-fiberglass, PTFE). This is consistent with the results of Barcelona et al. (9),who conducted similar experiments with five polymer tubing materials. It is also apparent that the less soluble compounds were removed from solution more rapidly than the more soluble compounds. (See section on diffusive transport model.) SEM Experiments. The concentration distributions of halogenated hydrocarbons in LDPE and PP clearly showed penetration and diffusion of the compounds in the polymers. Figure 2A shows a lineman photo of the concentration of TEY in LDPE tubing. The concentration profiles of halocarbons in LDPE and PP resembled those expected for diffusion from a solution of constant concentration into a plane sheet. When a semiinfinite solid is exposed to a solution of constant concentration, and the diffusivity of the solute is constant, the concentration of the solute in the solid has the form (22) c ( x , t ) / ( C K )= erfc [ ~ / ( 4 D t ) l / ~ ] (1) where c ( x , t ) is the concentration in the solid (g ~ m - ~C) , is the concentration in the solution (g ~ m - ~K) ,is the equilibrium partition coefficient (dimensionless), D is the diffusivity of the solute in the solid (cm2 s-l), x is the penetration distance in the solid (cm), and t is the exposure time (s). Profiles approximating this form were observed for short exposure times in LDPE and PP. The rounding of the outer edges of the profiles in Figure 2A is probably due to relaxation that occurred after the polymer was removed from the solution. The concentration profiles of TRI, BRO, and TEY were measured as accurately as possible in LDPE with the technique described earlier. An illustrative plot of X-ray intensity versus penetration distance is shown in Figure 2B. The diffusivities, calculated by fitting the profiles to eq 1 at c/CK = 0.5, ranged from 3 X to 8 X cm2 s-l. The SEM-measured diffusivities, which were determined at saturation concentrations, are -1 order of magnitude higher than those obtained from the sorption experiments at trace concentrations. (See below.) Diffusivity often depends on sorbed penetrant concentration for condensable organic vapors in polymers (23). In a study by Rogers et al. (24), the diffusivities of several organic vapors in LDPE increased by factors of 7-18 as activity increased from zero to 1. It was not possible to measure the diffusivities in PTFE, because the halocarbon concentrations were below detection. Only the concentration profile of BRO could be measured in RPVC, due to interference from the large amount of chlorine present in the polymer. This profile was unlike those seen in LDPE and PP in that it was sharp Environ. Sci. Technol., Vol. 24, No. 1, 1990 137
Table 11. Values for log ( K 2 D ) / c m 2s-I Obtained by Fitting the Sorption Experiment Data to the Diffusive Transport Model (es 5 )
log (PD)"/cm2 s-l TRI
polymeP
AC/cm
TEY
HEX
RPVC PTFE NYL PP LDPE LDPE LDPE* LAT
0.337 0.358 0.321 0.365 0.372 0.602 0.372 0.355
-7.8 -5.7 -6.4 -4.0
-8.0 -7.9 -5.7 -4.4 -4.0
e
e
-4.8
-4.3 -3.9
-4.4 -3.8
e -4.1
-4.4
BRO
TET
d
-8.5
-8.0 -7.5 -6.5
d
d -9.2
-5.0
-6.4 -6.6 -5.2 -5.2
-6.6 -7.3
-6.1
-6.7
-4.1
-4.4
-5.9 -6.2
"The polymers and compounds are identified in the first paragraph of Experimental Methods. bThe experiment was done a t 10 "C;all other experiments were done at 22 "C. 'A = (solution volume)/(polymer surface area). dAmount of sorption was too small to estimate P D . eExperiment was not done.
actions between the penetrant molecules and the polymer (23, 25). Diffusive Transport Model. In the following sections, the sorption experiment data are analyzed by a diffusive transport model and the values of the sorption parameters obtained are compared with relevant data from studies of the sorption of organic vapors by synthetic polymers. The experimental system is represented by an infinite sheet of polymer material suspended in a well-stirred solution of limited volume. One-dimensional geometry was used for convenience and generality; this simplification should not introduce large errors into the calculation of the sorption parameters. The thickness of the sheet is 2L (cm) and the thickness of the solution in contact with both sides of the sheet is 2 A (cm). The equations that relate the concentration in the sheet, c ( x , t ) , to that in the solution, C ( t ) , are (ref 22, pp 56-60) d c / d t = D(d2c/dx2)
(2)
( A / K ) ( d C / d t ) = rD(dc/dx) a t x = h5 and t
C = Co; c
= 0 for -L
< x < L at t
>0
=O
(3) (4)
where K is the polymer-water partition coefficient (dimensionless) and D is the diffusivity (cm2s-*). Both K and D are assumed to be constant. The exact solution is an infinite series; an approximate solution, appropriate for small t , is (26)
C / C o = exp(T/a2) erfc [ ( T / c Y ~ ) ' / ~ ]
0
02
04
06
08
10
12
DISTANCE (mm)
Flgure 2. (A) (Top) X-ray linsscan photo of a transverse section cut from the middle of a 6-cm length of LDPE tubing that was exposed to a saturated solution of TRI for 80 min. Superimposed on the secondary electron image of the section are two traces of CI K a X-rays generated by scanning the electron beam horizontally across the section: upward deflection indicates increasing intensity. The curved inside and outside edges of the section are seen at the extreme left and right margins of the photo. (B) (Bottom) chlorine K a X-ray intensity (counts, 30-s counting time) versus distance in LDPE plate (- 1.2-mm thickness) after exposure to a saturated solution of TET for 165 min. Each plotted point is the mean of four measurements;the 95% confidence intervals are indicated. The curves drawn in the figure represent eq 1 with D equal to 7 X lo-* cm2 s-'.
and steplike. It was also observed that RPVC was swelled and plasticized by the BRO solution. Non-Fickian behavior is often observed in glassy polymers a t high concentrations of organic penetrant. In such systems sorption is controlled by the slow, relative to diffusion, relaxation (swelling) of polymer structure induced by strong inter138
Environ. Sci. Technol., Vol. 24, No. 1, 1990
(5)
where C and Co are the solution concentrations a t t and t = 0, respectively, T = D t / L 2 , and CY = A / K L . This function was fitted to the concentration histories by an iterative least-squares procedure. Since the value of A was known (it is the ratio of solution volume to polymer surface area), the value of P O (cm2s-l) was calculated for each polymer. The concentration histories generally conformed to eq 5, except when t was large and CY was not small, indicating that the model provides an adequate representation of the experimental data. The agreement between the model and the concentration histories for BRO can be seen in Figure 1C. The logarithms of P O are given in Table 11. The 95% confidence limits for the log ( P O )values were usually within k0.15 of the estimate. The log ( P O )values for the two different solution volume to polymer surface area ratios agree within experimental error, providing additional evidence that the model is appropriate. Because the volume to surface area ratios were approximately constant (except for one experiment), the log ( P O ) values reflect the trends in compound disappearance rate previously discussed. The P O values can be converted to sorption
Table 111. Logarithms of Diffusivity, D (cm2 s-I), Polymer-Water Partion Coefficient, K (Dimensionless), and Water Solubility, S (mg L-l), for the Halogenated and Monoaromatic Hydrocarbons in LDPE at 22 OC
compd” C,/C, TET BEN BRO TRI TOL EtBEN p-XYL HEX TEY
0.24 0.18 0.17 0.10
0.06 0.02 0.02 0.01 0.01
log log D/cmZ exptlb est‘ exptld 1.4
1.5 1.5 1.8 2.1 2.5 2.5 2.8 2.8
1.5 1.6 1.7 2.2 2.2 2.6 2.6 2.9 2.7
-8.7 -8.1 -8.4 -8.4 -8.4 -9.1 -9.0 -9.7 -9.7
1% Se/mg L-I L-’ 3.5 3.2 3.5 3.2 2.7 2.1 2.1 1.7 2.2
-1
I
I
200
I
300
I
400
B O I L I N G TEMPERATURE ( K )
BEN,benzene; TOL, toluene; EtBEN, ethylbenzene; p-XYL, p-xylene; the other compounds are identified in the first paragraph of Experimental Methods. Calculated by using the following information: A = 0.372 cm; L = 0.053 cm. ‘The estimated K values and ref 30 were calculated by eq 6 using data from Figure 3 (Ks) and 31 (KH).dThe P D values were taken from ref 15 and this work. ‘The water solubilities are for 20 OC (14, 15).
half-times by the formula, tllz = 0.585A2/PD. The sorption half-times ranged from less than 20 min for HEX and TEY in LDPE and LAT to over 200 days for BRO and T E T in RPVC and PTFE. For the experiments in which sorption equilibrium was attained, it is possible to calculate K and D by using the following equations: a = C,/(C, - C,), K = A / a L , and D = ( P D ) / P ,where C, is the solution concentration a t equilibrium. The calculations were performed for the halogenated and monoaromatic hydrocarbons in LDPE; data for the latter compounds were taken from Gillham and O’Hannesin (16). The results (Table 111)show that the trends in log K and log D are opposite, in keeping with other studies (23),and that the trend in log (PO) is determined by the trend in log K. The logarithms of the water solubilities of the compounds are significantly correlated (0.1% level) with both log K and log ( P D ) . Polymer-Water Partition Coefficients. The purpose of this and the following section is to show that the experimental values of K and D for LDPE are consistent with literature data. In this section, the experimental values for K are compared with literature values derived from Henry’s law solubility coefficients and Henry’s law constants for air-water partitioning. The following assumptions are implicit: (1) the concentrations of organic compounds used in the experiments were low enough that Henry’s law is valid, and (2) K can be calculated by the formula
K = KsKH
I
(6)
where Ks is the Henry’s law solubility coefficient for the vapor in the polymer (mol m-3 kPa-’) and KH is the Henry’s law constant for air-water partitioning (m3kPa mol-’). The first assumption is necessary because solubility coefficients for organic vapors in polymers are generally dependent on concentration. It is probably valid because the activities of the organic solutes used in the experiments were of the order of Further evidence is provided by the nonequilibrium sorption experiments, which showed that sorption rate was independent of initial concentration over the range tested. The second assumption is expected to be valid, because the Henry’s law solubility coefficients should not be affected by the small amount of water absorbed by LDPE (
