Environ. Sci. Technol. 1999, 33, 3918-3923
Determination of Uptake Kinetics (Sampling Rates) by Lipid-Containing Semipermeable Membrane Devices (SPMDs) for Polycyclic Aromatic Hydrocarbons (PAHs) in Water J A M E S N . H U C K I N S , * ,† JIMMIE D. PETTY,† CARL E. ORAZIO,† JON A. LEBO,† RANDAL C. CLARK,† VIRGINIA L. GIBSON,† WILLIAM R. GALA,‡ AND KATHY R. ECHOLS† Columbia Environmental Research Center (CERC), U.S. Geological Survey, 4200 New Haven Road, Columbia, Missouri 65201, and Chevron Research and Technology Company, 100 Chevron Way, Richmond, California 94802
The use of lipid-containing semipermeable membrane devices (SPMDs) is becoming commonplace, but very little sampling rate data are available for the estimation of ambient contaminant concentrations from analyte levels in exposed SPMDs. We determined the aqueous sampling rates (Rss; expressed as effective volumes of water extracted daily) of the standard (commercially available design) 1-g triolein SPMD for 15 of the priority pollutant (PP) polycyclic aromatic hydrocarbons (PAHs) at multiple temperatures and concentrations. Under the experimental conditions of this study, recovery-corrected Rs values for PP PAHs ranged from ≈1.0 to 8.0 L/d. These values would be expected to be influenced by significant changes (relative to this study) in water temperature, degree of biofouling, and current velocity-turbulence. Included in this paper is a discussion of the effects of temperature and octanol-water partition coefficient (Kow); the impacts of biofouling and hydrodynamics are reported separately. Overall, SPMDs responded proportionally to aqueous PAH concentrations; i.e., SPMD Rs values and SPMD-water concentration factors were independent of aqueous concentrations. Temperature effects (10, 18, and 26 °C) on Rs values appeared to be complex but were relatively small.
required for estimation of analyte water concentrations from concentrations in SPMD lipid were described earlier (2). Although SPMDs have been successfully used in a variety of field studies (3-19), laboratory studies are needed that will elucidate SPMD contaminant sampling rates, exchange coefficients, and partition coefficients, thereby enabling estimations of analyte concentrations in environmental waters. Also, the potential effects of environmental variables such as temperature, water velocity-turbulence, biofouling, and analyte concentrations must be better defined to more accurately estimate ambient chemical concentrations from SPMD data. We conducted flow-through exposures of SPMDs to three concentrations of priority pollutant (PP) polycyclic aromatic hydrocarbons (PAHs) at three temperatures. Static exposures were also conducted to measure the partition coefficients of the PAHs between SPMD components and water and to explore the possible effects of salinity. Results from our research into the effects of water velocityturbulence, particulate and dissolved organic carbon, and biofouling on Rss are reported separately (20-22). Modeling. The original modeling work focused on using concentrations of contaminants measured in the lipid compartments of exposed SPMDs to estimate ambient water concentrations (2, 23). Because of the convenience of dialyzing intact SPMDs (analytes are recovered in an enriched state) and because of the additional quantities of analytes recoverable from exposed membranes, it is common practice for investigators to analyze whole SPMDs. Thus, models are needed that will permit estimation of ambient aqueous chemical concentrations from chemical concentrations in whole SPMDs (i.e., both lipid and membrane). It was shown in earlier studies (1, 24) that both membrane and lipid concentrations of moderate-to-very high-Kow compounds rise throughout 28 days of flow-through exposure and that steady-state ratios of concentrations in membrane and lipid compartments are approached but are generally not achieved. Thus, a multicompartment (lipid, membrane, and water) differential equation model such as that reported by Gale (25) should provide improved pictures of SPMD component exchange kinetics and mediating variables but not necessarily more accurate estimates of analyte concentrations in ambient water. In this work we use a simple modeling approach to minimize complexity (i.e., Occam’s razor) and because of the limited knowledge of chemical exchange coefficients among the lipid, membrane, and water phases. To allow inclusion of the residues recovered from SPMD membranes in our water concentration estimation algorithm, we express the membrane as a lipid-equivalent volume. Then, the equilibrium SPMD-water partition coefficient (KSPMD) can be written as
Introduction Semipermeable membrane devices (SPMDs) are receiving increasing attention for their utility as passive, in situ samplers of organic contaminants in water and in air. The devices, when configured for monitoring applications, generally consist of a neutral lipid (i.e., triolein) enclosed in layflat nonporous (having transient as opposed to fixed cavities) polymeric tubing (1) such as low-density polyethylene (LDPE). Most of the basic theory and mathematical models * Corresponding author phone: (573)876-1879; fax: (573)876-1896; e-mail: James•
[email protected]. † Columbia Environmental Research Center (CERC). ‡ Chevron Research and Technology Company. 3918
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 21, 1999
KSPMD ) KLw(VL+KmLVm)/VSPMD
(1)
CSPMD-e ) CwKLw (VL+KmLVm)/VSPMD
(2)
and
where VL, Vm, and VSPMD are the volumes of the lipid, membrane, and the lipid plus membrane, respectively, KML is the membrane-lipid partition coefficient, CSPMD-e is the analyte concentration (ng/L) in the whole SPMD at equilibrium, and Cw is concentration (ng/L) in water. Since we use volume terms instead of mass, it is convenient that both triolein and LDPE have about the same specific gravity, i.e., 0.91. Using this lipid-equivalent approach and assuming 10.1021/es990440u CCC: $18.00
1999 American Chemical Society Published on Web 09/29/1999
TABLE 1. Selected Physicochemical Properties of Priority Pollutant PAHs minimal box dimensionsc (Å) compds
MWa
no. of ringsb
length (L)
breadth (B)
depth (D)
moleculard volume (Å3)
water solubility (g/m3)
log Kowe
naphthalene acenaphthylene acenaphthene fluorene phenanthrene anthracene fluoranthene pyrene benz[a]anthracene chrysene benzo[b]fluoranthene benzo[k]fluoranthene benzo[a]pyrene dibenz[a,h]anthracene indeno[1,2,3-cd]pyrene benzo[g,h,i]perylene
128.2 152.2 154.2 166.2 178.2 178.2 202.3 202.3 228.3 228.3 252.3 252.3 252.3 278.4 267.0 276.3
2A 2A1C 2A1C 2A1C 3A 3A 3A1C 4A 4A 4A 4A1C 4A1C 5A 5A 5A1C 5A1C
8.9 8.8 8.8 11.1 11.5 11.7 10.7 11.4 13.7 13.6 13.6 13.3 13.6 15.6 13.2 11.5
7.2 8.4 8.1 7.2 7.7 7.2 9.0 9.5 9.4 7.7 9.3f 9.1 8.9 9.3 10.0 10.2
3.1 3.1 3.2 3.1 3.1 3.1 3.1 3.1 3.1 4.4 4.5 3.1 3.1 3.1 3.1 3.1
126.9
30.2 3.93 3.93 1.90 1.18 0.076 0.260 0.135 0.011 0.0019 0.0140 0.0080 0.0038 0.0005 0.0005 0.0003
3.45 4.08 4.22 4.38 4.46 4.54 5.20 5.30 5.91 5.61 5.78 6.20 6.35 6.75 6.51 6.90
148.8 160.4 169.5 170.3 187.7 186.0 212.9 212.2 230.3 231.1 228.6 255.4 244.3
Molecular weight. b A ) aromatic, C ) nonaromatic. c Calculated for minimal molecular energy configuration, using Alchemy III. d Molecular volume data from Mackey et al. (28). e Preferred or selected values from Mackay et al. (28). f Exact value uncertain because of problems associated with shape and deviation from planarity. a
aqueous diffusion layer control of uptake rate for the whole device, the analyte concentration in a whole SPMD is given by
CSPMD ) CwKSPMD(1 - exp[-kwAt/KLw(VL+KmLVm)]) (3) where kw is the boundary layer mass transfer coefficient, A is area, and t is time (days [d]). In the case of membrane control of uptake rates
CSPMD ) CwKSPMD(1 - exp[-kmKmwAt/KLw(VL+KmLVm)]) (4) where km is the mass transfer coefficient in the membrane and Kmw is the membrane-water partition coefficient. Simplifying eqs 3 and 4 and solving for Cw
Cw ) CSPMD/KSPMD(1 - exp[-ket])
(5)
where ke is the group kwA/KLw(VL+KmLVm) or kmKmwA/ KLw(VL+KmLVm). These two versions of ke represent the elimination rate constant or exchange coefficient (t-1) for overall uptake and elimination, unless biofilm control of uptake rates (20, 21) is operative. When ket is small (
