Critical Review
pubs.acs.org/est
Critical Review of Low-Density Polyethylene’s Partitioning and
Diffusion Coefficients for Trace Organic Contaminants and
Implications for Its Use As a Passive Sampler
Rainer Lohmann*,†,‡
†
Graduate School of Oceanography, University of Rhode Island, 215 South Ferry Road, Narragansett, Rhode Island 02882, United
States
‡
Geowissenschaften, Universität Tübingen, Hölderlinstrasse 12, 72074 Tübingen, Germany
S Supporting Information
*
ABSTRACT: Polyethylene (PE)-water equilibrium partitioning constants,
KPEw, were reviewed for trace hydrophobic organic contaminants (HOCs).
Relative standard deviations were <30% for phenanthrene, anthracene,
fluoranthene, and pyrene implying excellent reproducibility of KPEw across
laboratories and PE sources. Averaged KPEw values of various HOCs were
best correlated with aqueous solubility, logC w sat (L): logK PEw =
−0.99(±0.029)logCwsat(L) + 2.39(±0.096) (r2 = 0.92, SE = 0.35, n =
100). For 80% of analytes, this equation predicted logKPEw within a factor
of 2. A first-order estimation of KPEw can be obtained assuming constant
solubility of the compounds in the PE, such that the variation in Cwsat(L)
determines the differences in KPEw. For PE samplers, KPEw values do not
change with the thickness of the PE sampler. The influence of temperature
on KPEw seems dominated by solubility-changes of the compound in water,
not in PE. The effect of salt is rather well understood, using a Schetschenow-style approach. The air-PE partitioning constant,
KPEa, can be approximated as the ratio of KPEw/Kaw (the air−water partitioning constant). A critical review of diffusivities in PE,
DPE, suggests that best results are obtained when using the film-stacking method. A good correlation is then found between DPE
and molar volume, Vm (Ǻ 3/mol): logDPE (m2/s) = 0.0145(±0.001)Vm + 10.1(±0.20) (r2 = 0.76, SE = 0.24, n = 74).
■
INTRODUCTION
Passive sampling of trace organic compounds has become a
widely accepted means of measuring concentrations of truly
dissolved compounds in water and atmosphere. Initially,
semipermeable membrane devices (SPMDs), developed by
Huckins and co-workers, were used to mimic the uptake of
hydrophobic organic contaminants (HOCs) by fish.1,2 These
consisted of a low-density polyethylene (LDPE) polymer bag
filled with a lipid-like material, triolein. A large body of
literature has been published on the various aspects of
establishing dissolved concentrations based on the use of
SPMDs, mostly as kinetic uptake samplers.1−8 SPMDs remain
widely used, although their analysis can be difficult due to the
presence of triolein.
Over the past decade, other, generally single-phase, polymers
have been increasingly used, such as silicone (used in solidphase matrix extractions, SPMEs), polyoxymethylene (POM),
and simple PE samplers. Booij et al. (1998)6 noted the
possibility of just using the PE matrix without triolein filling,
followed by initial field trials by Müller et al. (2001).9 Adams et
al. (2007) published a comprehensive study into the effects of
temperature, salinity, and time on the uptake of HOCs by PE
sheets.10
© 2011 American Chemical Society
This review will focus on PE sheets as simple samplers of
HOCs and other trace organic compounds. LDPE sheets have
been used as passive samplers in a wide range of matrices, such
as in sediments’ porewater,11−14 the water column,10,14−16 and
atmosphere17,18 as well as to assess the activity gradients across
sediment−water14,19 and water−air15,20 interfaces.
At equilibrium, the partitioning constant between PE and
water, KPEw, is defined as
KPEw =
CPE
Cw
(1)
where CPE and Cw are the concentrations of the HOC in PE
(mol/kg PE) and freely dissolved in water (mol/L H2O),
respectively.
As compared to other matrices for passive sampling, PE is
the simplest (in its chemical makeup) and cheapest polymer
available. PE is produced in 3 types − low density (LDPE),
linear low density (LLDPE), and high density (HDPE). LDPE
is made up ca. 20−40 long and short chain branches per 1000
Received:
Revised:
Accepted:
Published:
606
August 4, 2011
November 26, 2011
November 29, 2011
November 29, 2011
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partitioning constants between octanol and water (Kow),
octanol and air (Koa), air and water (Kaw), the compounds’
subcooled liquid aqueous solubility (Cwsat(L), in mol/m3) and
solubility S (kg/L).
For PAHs, these were the final adjusted values (FAV)
published in the literature;38 values for PAHs not reported were
based on the (FAV) properties’ correlation with molecular
weight (MW). For PCBs, Kow values were taken both from
Hawker and Connell (1988)39 and Schenker et al. (2005).40
FAV data were only available for selected congeners;40 their
FAV values were regressed against Hawker and Connel
(1988)’s data39 to extrapolate to all congeners, for both Kow
and Cwsat(L) values. FAV data were also used for PCDDs41 and
for various OCPs.40 In addition, hexadecane-water partitioning
constants, Khdw, and molar volume (Vm) values were obtained
from the SPARC online calculator.42 SPARC uses solvation
models that describe the intermolecular interaction upon
placing an organic solute molecule in a solvent system.43
These are detailed in Tables S1−4.
carbon atoms off the main PE chain. Less branching (15−30
short chain branches per 1000 carbon atoms) results in LLDPE.
By further reducing branching off the PE chain, the crystallinity
increases to >70%, and HDPE is obtained.21 LDPE is made via
free radical polymerization using an initiator molecule that
radicalizes and attacks ethylene units to form chains.
During extrusion of LDPE into its desired shape, lubricants
(such as fatty acids or presumably technical nonylphenol16) and
additives (e.g., antioxidants, flame retardants, colorings, photochemical stabilizers, etc.) are sometimes added, depending on
the intended application.22 Potential filling materials for LDPE
include calcium carbonate, talcum, soot, silica gel, and
alumina.22
■
LDPE PROPERTIES
LDPE’s density is generally between 0.91−0.925 g/cm3.23 Its
crystallinity is on the order of 40%23 but varies within a range of
approximately 35−55% (also depending on how crystallinity is
measured24−26). The simplest way to estimate PE’s crystallinity
is from its density (ρ), assuming a linear combination of ideal
amorphous (ρ = 0.85 g/cm3) and crystalline regions (ρ = 1.0 g/
cm3).21 The crystalline regions (crystallites) are on the order of
10−15 nm.24 It is generally agreed that diffusion and
partitioning are reserved to the amorphous phase of LDPE.25,26
At room temperature, LDPE is above its glass transition
temperature (Tglass) of ∼−125 °C and below its melting
temperature of ∼100−110 °C.21 At ambient temperature the
amorphous region has molecular motion, and crystallization
continues until equilibrium is reached.23
Water is almost insoluble in PE. Sorption values of 2 × 10−5
kg/kg PE were reported, but even these could have − at least
partially − been due to interaction with impurities in the PE.27
■
RESULTS AND DISCUSSION
1). Measured PE-Water Partitioning Constants (KPEw).
The most appropriate predictors for the measured KPEw values
would be equilibrium partitioning constants between another
long aliphatic chain and water, such as Khdw. Few measured
Khdw values exist in the literature, though. Previous studies
suggested that Khdw as predicted from commercial software
(COSMOtherm) was providing better estimates of Khdw than
Khdw(SPARC).16,32 The usefulness of COSMOtherm will largely
depend on testing predicted values on a much larger set of data
and making this available for researchers using PE as a sampling
tool for the compounds of interest.
Poly parameter linear free energy relationships (pp-LFER)
might also become a convenient tool in prediction log Khdw, but
they suffer from a limited set of measured (Abraham’s)
parameters for HOCs of interest. Recent work also highlighted
that even these measured parameters need to be critically
assessed, and possibly corrected.44
So, different predictors of KPEw values are compared and
discussed here, notably Kow, Cwsat(L), S, MW, Khdw (SPARC),
or Khdw by Schwarzenbach et al. (2003)45 (log Khdw = 1.21 log
Kow −0.43) for neutral or weakly polar compounds. For each
compound class (PAHs, PCBs, PBDEs, other contaminants)
those viewed as most reliable are given in the main text (all
regressions are available in Tables S9−S12).
a). PAHs. For PAHs, we note a very good reproducibility of
KPEw for a wide range of PAHs determined by different
laboratories. For phenanthrene, anthracene, fluoranthene,
pyrene, chrysene, and benz(a)anthracene at least 5 measurements were reported in the literature (Table S5). Relative
standard deviations (RSD) were <30% for phenanthrene,
anthracene, fluoranthene, and pyrene (and <40% for chrysene
and benz(a)anthracene) implying excellent reproducibility of
KPEw across laboratories and LDPE sources (calculated for the
original, nonlog values). There was a distinct decrease in
reproducibility of KPEw for higher MW PAHs (RSD 50−90%)
and for some of the lower MW PAHs. The higher variability of
the lower MW PAHs can probably be explained by evaporative
losses during extraction and volume reduction. Some of the
variability reflects interlaboratory and possibly LDPE-manufacturing differences. For silicone rubber Smedes et al. (2009)
observed minor variability with different batches by the same
silicone, but major differences by different manufacturers (0.2−
■
MATERIALS AND METHODS
Published KPEw partitioning constants were taken from the
literature. For PAHs, six studies reported values,10,11,28−32 five
for PCBs,10,11,28,30,31 and several studies published values for
various organo-chlorine pesticides (OCPs),30−33 polychlorinated dibenzo-p-dioxins (PCDDs),10 polybrominated diphenylethers (PBDEs),34,35 and contaminants of emerging concern.16
In general data were generated using gas chromatography
coupled to mass spectrometric detection (GC-MS), with the
exception of a couple of studies using electron capture
detection (ECD) for PCBs.28,31 Contrary to studies validating
the partitioning of HOCs by SPME fibers, where nondepletion
of the aqueous phase can be obtained,36 for PE-water
partitioning studies, HOCs were extracted and quantified in
both water and PE. Only the water phase was measured when
determining uptake kinetics of phenanthrene, pyrene, and PCB
#52.10 Most KPEw values were already reported on a volumemass basis (L H20/kg PE); the others28,32 were converted to a
mass concentration for LDPE assuming an average density of
0.91. Studies were generally conducted at 20−25 °C, and a
further temperature correction was not performed here. The
average of the KPEw values at 13 and 30 °C values reported by
Booij et al. (2003) was used here.28 Several studies determined
KPEw in saltwater.28,29,31 These were corrected to freshwater
assuming a Setschenow constant of 0.35 (see below) for all
investigated HOCs.37
As far as possible, internally consistent (i.e., adjusted for
thermodynamic consistency) physicochemical properties were
chosen for the various compound classes investigated here: the
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0.5 log units).30 Yet these differences are contained within the
30−40% RSD of KPEw for key PAHs (0.1−0.2 log units),
suggesting that they are overall of minor importance for LDPE.
For most PAHs measured KPEw were at or below their
respective Kow (Figure 1a). Yet for higher MW compounds,
high r2 value and a low standard error (SE)
log KPEw = 1.22( ± 0.046)log K ow
− 1.22( ± 0.24)
(r 2 = 0.92, SE = 0.27, n = 65)
(2)
Excluding the measurement for PAHs with RSD > 40% (the
PAHs with MW < phenanthrene, or MW > chrysene), resulted
in the following best fit
log KPEw = 0.99( ± 0.066)log K ow
− 0.07( ± 0.33)
(r 2 = 0.84, SE = 0.20, n = 43)
(3)
In this correlation, the SE dropped to 0.2. Another benefit of
the correlation based on selected PAHs is that the slope is not
significantly different from 1, implying that the free energy
terms are similar for the compounds in both PE-water and
octanol-water partitioning. As the PAHs mostly undergo van
der Waals interaction in both PE and octanol, a slope of ∼1 can
be expected. Equations 2 and 3 differ significantly from
previous correlations of logKPEw − logKow for PAHs with a
slope of 1.5 and an intercept of −2.5 (albeit using different Kow
values).30
The Khdw correlation proposed by Schwarzenbach et al.
(2003)45 (logKhdw = 1.21logKow − 0.43) resulted in Khdw values
exceeding their respective Kow (Figure S1, Table S9) such that
measured KPEw were further away from predicted partitioning
into a long-chain hydrocarbon than into octanol. This questions
whether the predicted logKhdw45 values are accurate for higher
MW apolar compounds. Correlation coefficient and standard
error were of course the same as for Kow (Table S9). SPARCpredicted Khdw resulted in slightly better r2 and SE, implying
that SPARC predict this partitioning value adequately for
PAHs. In contrast, Hale et al. (2010)32 and Sacks and Lohmann
(2011)16 reported that SPARC does not perform well in
predicting KPEws for a range of OCPs and semipolar
contaminants.
As an alternative, Cwsat(L) was used as a correlating property,
assuming that for all compounds, their preference to dissolve in
PE will be dictated entirely by their (in)compatibility with
water (as all compounds can only interact with PE via van der
Waals forces). This resulted in the best correlation with
measured KPEw for PAHs (Figure 1b), slightly better than what
was previously observed for correlations with Kow
log KPEw = − 0.85( ± 0.023)log C w sat(L)
Figure 1. Log KPEw for PAHs versus (a) log Kow or (b) log Cwsat(L).
The predicted line is based on a slope of −1 and an intercept of 2.4
(eq 8).
+ 2.85( ± 0.071)
(r 2 = 0.94, SE = 0.27, n = 65)
some of the measured values exceeded Kow, probably reflecting
measurement artifacts in the respective experiments. Based on
possible interactions between solvent and solute, octanol can
accommodate solutes via van der Waals, polar and hydrogenbonds interactions, while polyethylene can only offer van der
Waals interactions. In addition, polyethylene is not as flexible as
a liquid solvent. For both reasons, KPEw is expected to be lower
than Kow (although the energetic cost of cavity formation is
higher in octanol than in apolar solvents).
When plotting measured KPEw versus proxy parameters, such
as Kow, good correlations were obtained for PAHs, both with a
(4)
Lastly, previous work suggested the use of MW as a good proxy
for partitioning constants.30,38 For the log KPEw compiled here,
MW was not as good a predictor as log Cwsat(L), log Khdw
(SPARC), or log Kow (Table S9).
b). PCBs. For PCBs, the observed reproducibility of KPEw
values determined by different laboratories was not as good as
for PAHs (Table S6). Excellent agreement was apparent for
PCB 52, where five studies report a mean log KPEw = 5.5 (range
5.4−5.6) with a RSD of 20% (based on the nonlog values).
RSDs < 30% were obtained for PCBs 18, 44, 101, and 138 but
not for PCBs 28, 66, 105, 110, 118, 128, 153, and 180 (all
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congeners with n ≥ 3). For other congeners, reproducibility
varied up to 100% RSD between reported values. As opposed
to results for PAHs, there was no obvious relationship between
MW and degree of agreement of KPEw reported by different
laboratories. For most PCBs measured KPEw were at or below
their respective Kow (Figure 2a), but values from one study
were notably present above the 1−1 line for numerous
congeners.31 If values from that study were excluded, the
average RSD for all reported KPEw dropped to 29% (from 45%),
suggesting reasonably good agreement of KPEws for PCBs, too.
When plotting measured KPEw versus Kow, good correlations
were obtained for PCBs (which are identical to results by
Smedes et al., 2009)30
log KPEw = 1.14( ± 0.041)log K ow
− 1.14( ± 0.26)
(r 2 = 0.91, SE = 0.24, n = 79)
(5)
39
Kow values by Hakwer and Connell (1988) gave similar
results (Figure S2). Data by Smedes et al. (2009)30 questioned
the goodness of Hawker and Connell’s Kow values, where a
strong ortho-substition effect was predicted.
Correlating all measured KPEw versus Cwsat(L) resulted in an
equally good fit as observed with Kow for all PCBs
log KPEw = − 0.99( ± 0.035)log C w sat(L)
+ 2.37( ± 0.14)
(r 2 = 0.91, SE = 0.23, n = 79)
(6)
The regression value for the slope of −1 implies that
partitioning from water into PE is indeed entirely driven by the
congeners’ hydrophobicity: A congener that is ten times less
soluble in water enriches in PE ten times more (Figure 2b). It
also suggests that the solubility of PCBs in PE is a constant, at
∼102.4 mol/L; though with a different intercept than for PAHs
(eq 4).
Excluding the measurement for PCBs from the one study
which frequently scattered above the 1−1 line31 slightly
improved r2 and decreased SE for all correlations (Table
S10). These correlations were not statistically different from the
previous correlations, suggesting that the excluded study,
overall, reported the same trends, albeit less precise.
In the case of PCBs, various correlations were equally strong
in their predictive power of log KPEw values, such as unified log
Kow, log Cwsat(L), and log Khdw (Schwarzenbach et al., 2003,45
Figure S2), which were superior to the use of MW, log Kow
from Hawker and Connell, or log Khdw (SPARC).
c). Various Other Contaminants. For numerous other
OCPs, PCDDs, and PBDEs, log KPEw values were also
determined (Tables S7, S8), but values were rarely determined
by more than one laboratory. Internally consistent physicochemical properties were only available for approximately half
of them (Table S3, S4). It should be noted that even these
adjusted properties especially for OCPs are not as constrained
as values for PAHs and PCBs, as no within compounds group
correlations can be established for a diverse set of structures.
For PBDEs, high values of r2 and low SE were determined for
a wide range of properties, especially log Cwsat(L) and MW
(Table S11). For the remaining other compounds, including
various OCPs, alkylphenols, and PCDDs, correlations were
poorer (higher SE) than for PAHs, PCBs, or PBDEs (Table
Figure 2. Log KPEw for PCBs versus (a) log Kow or (b) log Cwsat(L).
The predicted line is based on a slope of −1 and an intercept of 2.4
(eq 8).
S9−11). Without the data for methoxychlor, the correlation
with Cwsat(L) was very strong
log KPEw = − 1.07( ± 0.056)log C w sat(L)
+ 2.16( ± 0.12)
(r 2 = 0.91, SE = 0.34, n = 35)
(7)
The correlation parameters were not significantly different from
those obtained for PCBs (eq 6). This also suggests that the
available solubility data46 and/or the measured KPEw for
methoxychlor are not accurate. In the absence of using log
Cwsat(L) values, the next best predictor was using Khdw
predicted from SPARC, while MW performed very poorly
(Table S11).
2). Predicting logKPEw for Organic Contaminants.
Combining measured averaged KPEw for all compounds resulted
in the following best-fit, in which Cwsat(L) explained 92% of the
variance of measured values
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different from −1, implying that for apolar and semipolar
compounds, their solubility in PE can be approximated as
constant. The magnitude of the KPEw is then almost entirely
dominated by the solubility in water, which spans almost 6
order of magnitude for the HOCs considered here. With this
best-fit, KPEw values of OCPs are mostly overpredicted by
around a factor of 2, which is not surprising in view of the
semipolar nature of several OCPs included here (leading to a
reduced solubility in PE). In contrast, KPEw for most PAHs are
underpredicted (by around a factor of 2). This is possibly
linked to the smaller molecular size of planar PAHs relative to
chlorinated HOCs of similar log Cwsat(L) values (see the SI),
resulting in enhanced solubility in the PE sheets. In a recent
critical review of lipid−water partitioning, PAHs also displayed
higher values than PCBs or PCDDs relative to their Kow
values.47
Razzaque and Grathwohl (2002) demonstrated the usefulness of solubility (S, in kg/L) in predicting organic carbon−
water partitioning coefficients for HOCs and suggested a
maximum solubility of HOCs in organic carbon of around
0.054 kg/kg.48 The correlation of average logKPEw with logS
was almost as good as for log Cwsat(L) with a slope of −1
log KPEw = − 0.99( ± 0.029)log C w sat(L)
+ 2.39( ± 0.096)
(r 2 = 0.92, SE = 0.35, n = 100)
(8)
PCBs contribute just under half of all data (n = 45), PAHs (n =
19), PBDEs (n = 14), and other compounds (n = 22) roughly a
fifth each (Figure 3a). The correlation is basically identical to
log KPEw = − 1.06( ± 0.033)log S
− 1.60( ± 0.22)
(r 2 = 0.91, SE = 0.37, n = 100)
(9)
The intercept of eq 9 represents a maximum solubility of
HOCs in PE of 0.027 kg/kg, not significantly different from the
value derived for organic carbon. By correcting for the
crystallinity (unaccessible organic carbon in PE), the values
become identical.
The correlation of logKPEw versus logKow does not result in as
good a correlation (Figure S4). The results are biased, as
expected, and most OCPs fall below the best-fit trendline,
outside of a factor of ±2, while most PAHs are above the
trendline, also outside a factor of ±2. For semipolar compounds
(here defined as compounds with a hydroxyl or epoxy group),
Kow overpredicts partioning into apolar PE. Only 60% of data
are within a factor of ±2 of the best-fit trendline.
The use of logKhdw, as predicted from SPARC, was not as
good as using logCwsat(L), but it was still a better predictor than
the use of Kow (Table S12). About two-thirds of measured data
fall within a factor of ±2 of the best-fit trendline (Figure 3b),
with most OCPs being overpredicted
log KPEw = 0.74( ± 0.030)log Khdw(SPARC)
+ 0.42( ± 0.21)
(r 2 = 0.86, SE = 0.46, n = 100)
Figure 3. Average log KPEw for PAHs, PCBs, and other organic
contaminants versus (a) log Cwsat(L) and (b) log Khdw (predicted
from SPARC). Error bars are 1 standard deviation for PAHs and PCBs
but indicate total range for OCPs (only duplicate measurements were
made). The dashed lines indicate ± factor of 2 around the trendline.
(10)
To predict the KPEw of an unknown compound, eq 8 will
yield the best estimate. If Cwsat(L) is neither measured nor can
it be estimated, a second-best option is to use Khdw (SPARC),
eq 10. The use of Kow should be avoided as much as possible,
unless the compound displays no functional groups. Best fit
values of the KPEw for the various compounds are included in
the SI (Tables S5-PAHs; S6-PCBs; S7-PBDEs; S8-other
compounds).
3). Effect of LDPE Thickness. The effect of LDPE
thickness on partitioning was directly compared in a few
studies focusing on OCPs,32 PBDEs,35 and PAHs.10 No
that for PCBs alone (eq 6) or considering all individual
measurements (n = 194, Figure S3, Table S12). 80% of data are
within a factor of ±2 of the best-fit trendline. This is the most
reliable prediction of KPEw for various organic compounds
partitioning from water into PE, just requiring knowledge of
Cwsat(L). In this correlation, the slope is not significantly
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significant differences were observed in either study. Similarly, a
look at the published KPEw values for various PAHs shows no
apparent change in partitioning with thickness (Figure S5).
Regressions of log KPEw values versus thickness, ranging from
25−100 μm, were not significant at p = 0.05, and slopes of the
regressions were not significantly different from zero (Table
S13). These results strongly imply that uptake of HOCs by PE
is an absorptive process and that surface adsorption does not
play a significant role for the uptake of neutral molecules by PE.
This is in contrast to results for POM, where partitioning
coefficients varied with thickness.29 In contrast to POM-based
samplers, LDPEs are generally purchased from commercial
companies, such that in-house shaving/cutting is not necessary.
It could also reflect on the chemically simple nature of LDPE
sheets, yielding similar results regardless of actual production/
extrusion process.
4). PE-Air Partitioning Constants (KPEa). As opposed to
measurements of the partitioning of HOCs between PE and
water, little data exist that quantified the use of PE as samplers
of gas-phase organic compounds. This is the more surprising, as
SPMDs haven been frequently used as passive air samplers.49−52
Bartkow et al. (2004) presented field-derived KPEa for 3
PAHs (fluorene, phenanthrene, and anthracene) based on the
comparison between their accumulation in PE sheets and gasphase concentrations from active high-volume sampling.17 In a
follow-up study by Kennedy et al. (2007), a more rigorous
correlation of KPEa versus Koa was suggested for PAHs53
Figure 4. KPEa for selected PAHs as a function of Koa.
contaminants in water and in polyethylene. These factors most
likely oppose each other − decreasing temperatures will cause
decreased solubility of (solid) organic contaminants in water,
while at the same time also affecting (probably lowering) the
compound’s solubility in LDPE. KPEw at a temperature (T)
different than 298 K can be calculated using a modified version
of the van’t Hoff equation
log KPEa = 0.78log K oa + 1.6
(r 2 = 0.84, n = 6)
ΔH
Of key interest is whether such correlations can be derived and
used beyond PAHs. As long as we can safely assume that water
uptake of PE is negligible, then the partitioning process of
HOCs into PE will be the same whether the PE is deployed in
water or air. Water uptake was reported to be less than 0.2%
after immersion of LDPE in water at 20 °C for 1 year.54 In this
case, KPEa can be predicted from KPEw/Kaw.
A comparison of field-derived KPEa for PAHs mentioned
above and those derived from KPEw/Kaw (Figure 4) showed very
good agreement up to log Koa = 8.5. As the field derived
correlation (eq 11) relied on a narrow range of PAHs to define
the trendline, the use of KPEw/Kaw is probably more accurate
and reliable in predicting KPEa values (Table S14). Ma et al.
(2010) Koa and Kaw values results in the following prediction38
log KPEa(PAHs) = 0.97log K oa − 0.42
{
/ R)
1 −1
}
298 T
KPEw(T ) = KPEw(298)e( PEw
(13)
where KPEw (T) and KPEw (298) are PEw partitioning constants
at temperature T (K) and 298 K, respectively, ΔHPEw is the
enthalpy of PEw partitioning (kJ/mol), and R is the universal
gas constant (8.3143 J/mol/K).
Several studies have tried to determine the net effect of
temperature changes on PE-water partitioning, with mixed
results. Booij et al. (2003) exposed LDPE strips to water
enriched with various PAHs and PCBs from two generator
columns.28 These experiments were conducted in a water bath,
set to 2, 13, and 30 °C during different runs. In most cases,
increasing temperature resulted in decreasing KPEw, with the
exception of chrysene, benzo(e)pyrene, and benzo(a)pyrene
(Table 1). Values of ΔHPEw ranged from −20 kJ/mol to −50
kJ/mol for PAHs and were around −10 kJ/mol for PCBs.
Adams et al. (2007) deduced the temperature-effect on KPEw
for phenanthrene, pyrene, and PCB 52 by measuring changes in
their dissolved concentrations at different temperatures (Table
1).10 By comparing results to a control experiment, other
factors (adsorption to surfaces) could be ruled out, thereby
establishing a connection between Cw and KPEw. Their results
showed decreasing dissolved concentration for all HOCs
studies (and increasing KPEw) with decreasing temperature.
Values of ΔHPEw ranged from −20 (phenanthrene) to −30 kJ/
mol (pyrene) and were around −10 kJ/mol for PCB 52. Other
studies reported inconclusive results (e.g., refs 16 and 29).
A comparison with published values of either the
E
) or the
compound’s excess enthalpy of solvation (ΔHsol
compound’s internal energy of octanol−water partitioning,
Uow,55 shows some agreement between results (Table 1). For
PAHs up to pyrene, measured ΔHPEw values are within a factor
(11)
(12)
Two further aspects of the KPE‑a − Koa correlation should be
mentioned. Similar to the results discussed for the partitioning
of HOCs between PE and water, KPEa values are below their
respective Koa ones. The slope of the correlation is not
significantly different from one, suggesting that PAHs undergo
similar interactions in octanol and PE. Included in Figure 4 are
hexadecane-air partitioning constants (Khda), based on Khdw,
divided by their Kaw.45 The excellent agreement between Koa
and Khda suggests that these values are accurate. This suggests
that uptake of gas-phase HOCs by PE is not as efficient as their
uptake by octanol or hexadecane, probably due to the rigid
matrix of the polymer compared to liquid solvents.
5). Environmental Factors Affecting PE’s Partitioning
Constants. a). Temperature. Temperature will affect KPEw
in two possible ways − by changing the solubilities of the
611
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Environmental Science & Technology
Critical Review
E
Table 1. Derived Enthalpies of PE-Water Partitioning, ΔHPE‑w, Compared to Excess Enthalpy of Solvation, ΔHsol
, and Internal
Energies for Octanol-Water Partitioning, Uow, (All in kJ/mol) for Selected HOCs
Booij et al., 2003
PCB 28
PCB 52
PCB 118
acenaphthene
phenanthrene
fluorene
pyrene
benz(a)anthracene
chrysene
benzo(e)pyrene
benzo(a)pyrene
a
−9
−11
−26
−22
−43
−46
−34
-
28
(2−30 °C)
Booij et al., 2003
28
(13−30 °C)
Adams et al., 2007
10
(4−23 °C)
E a
ΔHsol
−39 /−33
b
+12
−28
−28
−37
−36
−5
−15
+28
+32
−12
−18
−29
−27
−34
−40
−35
−45
−37
−24
−50
c
ΔUowd
ΔUowf
−28
−38
−23
−29e
−21
−24
−24
−23
−23
−25
−24
e
−19
−19
−19
−23
−23
−25
Summarized by Shiu and Ma (2000).81 bFor PCB 29. cFor PCB 30. dFAV by Beyer et al. (2000).55 eDefault value. fMeasured by Lei et al. (2000).82
E
of 2 of ΔHsol
and Uow. For PCBs, measured ΔHPEw are all
E
around −10 kJ/mol, while both ΔHsol
and Uow imply enthalpies
of at least 20 kJ/mol.
E
In this context, ΔHsol
reflects the T-influence on the
compound’s solubility of the pure condensed phase and
water, while Uow describes the energy associated with the Teffect of a subcooled liquid solute partitioning between two
dissolved phases. The latter is arguably more representative of
the PE-water partitioning. In addition, Uow values derived for
internally consistent properties are constrained by further
measurements, hence are probably more accurate.
Overall, these results suggest that temperature effects on
KPEw are governed by changes in the compound’s aqueous
solubility and that solubility changes in PE are of minor
importance. As the experimental data are limited, it is suggested
to use a default value of ΔHPEw of −25 kJ/mol for HOCs
(though compound-specific Uow values are available for selected
PAHs, PCBs, PCDD/Fs, and OCPs). This value is close to
most Uow values from the literature.40,55
No data were found on the temperature effect on KPEa.
Reported Uoa are on the order of −80 kJ/mol, which represents
a good proxy value for calculating changes in KPEa as a function
of temperature.40,55
KPEw increases by a factor of 2 for a deployment of 5 °C as
compared to one at 25 °C using ΔHPEw = −25 kJ/mol. Relative
to room temperature, KPEa increases 10-fold at the 5 °C
deployment (based on ΔHPEa = −80 kJ/mol).
b). Ionic Strength. Dissolved salts in aqueous solutions are
known to reduce the solubility of HOCs in a process referred to
as ‘salting out’. An empirical approach based on early work by
Setschenow (1889)56 is commonly used to describe the change
in aqueous solubility in the presence of salts (Cwsalt)
al. (2003)45 suggests increasing Ks with increasing size of the
HOCs (e.g., Ks (benzene) = 0.2; naphthalene = 0.28;
benzo(a)pyrene = 0.34). In contrast, experimental work by
Jonker and Muijs (2010) using SPME fibers did not find such
an effect for a wide range of PAHs and suggested the use of Ks
= 0.35 M−1.57 This values covers, within uncertainties, most
values summarized by Schwarzenbach et al.45 for PAHs and
PCBs and is suggested as a reasonable Ks for estimating the
effect of salt on KPEw of typical HOCs (PAHs, PCBs, PCDDs,
PBDEs).
Increasing polarity of the solutes decreases the effect of salt,
as suggested by Schwarzenbach et al. (2003)45 and also
observed by Sacks and Lohmann (2011)16 for triclosan and
alkylphenols (Ks = 0.15−0.31).
For typical deployments in coastal and marine environments,
the salt concentration will reach 0.5 M (30 practical salinity
units, psu). Assuming a Kis of 0.35 for an apolar HOC, this
increases KPEw by 50% for an open ocean deployment. No salt
effect on KPEa is expected.
c). Pressure. For deployments in the deeper waters of lakes
and oceans, the potential effect of pressure on partitioning
constants needs to be assessed. To date, the effect of pressure
on KPEw has not been addressed in the literature. For every 10
m depth in the water column, an additional atmosphere (100
kPa) is added. A PE sampler deployed at 1000 m suddenly faces
101 atm. The most obvious effect of pressure will be a
compression of the polymer matrix, thereby decreasing its free
volume available for diffusion, retarding diffusion, and limiting
uptake.
Parks and Richards (1949) predicted little change in
crystallinity with increasing pressure (around 1% for 100
atm).58 This was experimentally confirmed, with minor changes
in crystallinity (1−2%, within experimental error) for high
molecular weight PE up to 1800 atm.59 According to Wang et
al. (2007) the effect of increasing pressure has a rather limited
effect on the self-diffusion coefficient of toluene in polystyrene
at elevated temperatures.60 Increasing the pressure from 0.1 to
50 MPa (equivalent to 5,000 m depth) reduced the diffusion
coefficient by a mere 0.3 log units. This can be explained by a
partial loss of free volume in the polymer at higher pressures.
This suggests that there is a minor kinetic effect of pressure,
causing slower equilibration, and possibly a decrease in KPEw
itself (although crystallinity itself is not affected by pressure).
d). pH. The effect of pH was investigated by Sacks and
Lohmann (2011).16 For the semipolar molecules (triclosan and
alkylphenols) investigated, the KPEw of the neutral molecules
C wsalt = C w10−Ks[salt]
(14)
−1
where Ks is the salting out constant (M ), and [salt] is the
total molar ionic strength (M) of the aqueous solution.
Ionic strength is not expected to affect a pure hydrophobic
polymer like PE. KPEws should therefore increase inversely
proportional to the compounds’ decreased aqueous solubility.
This was indeed verified by Adams et al. (2007),10 who could
show that the presence of salt increased KPEw of phenanthrene
and pyrene as predicted based on eq 14. For a 0.1 molar NaCl
salt content, KPEw increased by 7%.
Xie et al. (1997) suggested a generic Ks value of 0.27 for
HOCs, representing an average of reported literature values.37
The data compiled by Xie et al. (1997)37 and Schwarzenbach et
612
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LDPE film, as the importance (resistance) of diffusion in the
sheet becomes more important. Indeed, for all HOCs
considered, faster DPE values were deduced for the 51 μm
thick LDPE sheet (Figure S6).
There are two exceptions to the better values obtained from
the film stacking method. Rusina et al. (2007)64 derived DPE
values for four PAHs. While the values for naphthalene and
fluoranthene agree with their 201063 measurements, those for
benzo(a)pyrene and dibenz(a,h)anthracene were much lower.
This was presumably caused by an insufficient initial
equilibration of the ‘source’ PE sheets in their earlier work.
The other set of outliers is based on the work of Al-Malaika et
al. (1991)67 with 2-hydroxybenzophenones. This is probably
due to the fact that these are the only compounds including
multiple polar groups and a wide range of molecular weights
included in Figure S6.
To extrapolate DPE values beyond those few measured,
initially correlations with MW were used initially; but as MW
contains no direct information on molecular volume, the
obtained correlations are not ideal. Rusina et al. (2010)63 used
total surface area (TSA, in Å2) instead, a molecular property
that was established and advanced in the 1980s. Pearlman and
co-workers in particular proposed TSA to describe the available
surface area for solvation and published results for selected
compound groups (PAHs and PCBs).72,73 The code used to
perform the calculations is presumably part of a commercial
software developed by the authors.74
This suggests that an alternative needs to be used, as TSAs
are not (freely) available for many compounds. There are
several approaches that calculate the molar volume of
compounds (Vm). In this review, SPARC-generated Vm (Ǻ 3/
mol)42 were used, as they are based on the same computation
algorithm and freely available. Using Vm(SPARC) already
improved r2 (r2 = 0.84 versus 0.81 using TSA) and decreased
SE (from 0.18 to 0.17) for the original Rusina et al. (2010)
data.63 A compilation of DPE versus Vm (SPARC) is shown in
Figure 5, based on the studies using the PE film stacking
method except for the values reported by Hale et al. (2010)32
for 51 μm PE film. For several PAHs, DPE values were reported
was approximately 100 times larger than that of the
deprotonated species. This suggests that PE can be considered
a neutral molecule only sampler, as would be expected
considering its apolar makeup.
6). Diffusivity of HOCs in PE. Once the HOC has left the
gas- or dissolved phase, it diffuses within the PE matrix.
Depending on the analyte, the current and the thickness of the
PE matrix, the overall kinetics of sorption by the passive
sampler are limited by diffusion across the aqueous boundary
layer, the diffusion within the PE or a combination of both.
Under typical deployments of thin LDPE membranes (<100
μm) in Lakes and Oceans, the uptake will be boundary-layer
controlled (see below). Yet, as LDPEs become more widely
used, applications might arise where the samplers are
deliberately used in a membrane-controlled uptake mode.
Examples are the potential towing behind a fast moving object
(ship) or the prolonged deployment of very thick membranes
in remote locations (eg., buoys). In these situations it is
important to better understand the diffusion within the LDPE
membrane, as detailed below.
Two basic approaches are used to determine the diffusivity of
analytes in PE. In the ‘spinning’ approach, the PE sampler is
placed in a well stirred aqueous solution, to overcome control
by boundary layer diffusion. At increasing time intervals, either
the analytes decrease in water, or their increase in the PE are
measured. Alternatively, the PE sheet can also be spiked with
target analytes and the desorption measured over time.
In the ‘film stacking’ approach, the analytes are initially
spiked in a thin PE film. This source PE film is pressed against
numerous others, clean PE films with known thickness.61 After
given diffusion times, the individual PE sheets are taken apart
and analyzed separately to measure the progress of diffusion.
While the former approach is limited by ensuring that the rate
controlling step is the diffusion within the PE membrane, in the
latter approach the source material should be homogeneous
and the applied pressure not cause any change in diffusivity in
the PE membranes. For volatile analytes, such as solvents, the
desorption of the molecules from a saturated PE sheet can
determined gravimetrically over time.62
Published diffusion coefficients of various organic analytes in
PE film (DPE) have been determined using the various methods
mentioned above. Within each study, a clear trend is visible
showing decreasing diffusion coefficients with increasing size/
molecular weight of the analytes. Yet when comparing DPE
values from different studies, a wide range is reported. DPE
values for phenanthrene and pyrene vary by at least a factor of
10 (Figure S6). In view of the good agreement for KPEw
sorption coefficients, this highlights the challenging aspect of
isolating the diffusion within PE from boundary layer effects.
A closer look at the reported data shows a separation of
reported diffusivities according to the method by which they
were determined (Figure S6). Higher DPE values were generally
obtained from the film-stacking method for a wide range of
PAHs, PCBs, and other organic compounds,63−67 whereas
methods that relied on spinning,10,32,68 evaporation,62 pervaporation,69,70 and gravimetry71 reported lower DPE values.
This suggests that most methods were limited by boundary
layer control of the diffusion, resulting in lower DPE values than
if diffusion was only within the PE membrane. Such an effect
can indeed be observed in the data by Hale et al. (2010), in
which the authors determined DPE values for 25 and 51 μm
thick LDPE sheets in the same spinning experiments.32
Aqueous boundary-layer control is less important for thicker
Figure 5. Measured log DPE for selected organic compounds versus
their molar volume, Vm (predicted from SPARC). (All studies used the
PE film stacking method except for Hale et al. (2010).32 Only values
for naphthalene and fluoranthene from Rusina et al. (2007)64 were
included in the best fit).
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Critical Review
from different studies, with good agreement: DPE were reported
as −12.5, −12.7 and −13.2 for fluoranthene, −13.3 and −13.4
for pyrene, and −13.6 and −13.7 for benzo(a)pyrene (Table
S15).
The following correlation was obtained between DPE and
Vm(SPARC)
Another approach is to use LDPE samplers of different
thicknesses to confirm that equilibrium in the field has been
reached.17 This approach, of course, only works for environmental settings where nearly stable concentrations can be
expected, such as the remote atmosphere/ocean.
Including PRCs equilibrated within the LDPE sheet opens
up variable deployment times in the field.4,76 This has been
shown for PE sheets inserted into bed sediments,11,13 the water
column,10,77 and air.18
PRCs are compounds similar in properties to the target
analytes (such as isotopically labeled compounds) which are
added to the PE sheet prior to deployment. PRCs are not
present in the environment, ensuring their continuous flux out
of the PE sampler. Knowing the PRC concentrations prior to
(CPRC,t=0) and post deployment (CPRC,t), the overall in situ
exchange rate constant, ke (1/day) can be determined as
log DPE = 0.0145( ± 0.001)Vm(SPARC)
+ 10.1( ± 0.20)
(r 2 = 0.76, SE = 0.24, n = 74)
(15)
Only values for naphthalene and fluoranthene from Rusina et
al. (2007)64 were included in the best fit. This correlation is
basically identical to the best-fit obtained using the Rusina et al.
(2010)63 data, except that additional studies and compounds
(total of 38 PCBs, 20 PAHs, 14 OCPs, butylated hydroxyanisol
and butylated hydroxytoluene) were included (Tables S15−
17). Knowing these DPE, the data reported by Adams et al.
(2007)10 can be reanalyzed and the thickness of the aqueous
boundary layer (δw) deduced
δw =
Dw × δPE
1
×
KPEw
DPE,app − DPE
ke = ln
CPRC, t = 0
CPRC, t
/t
(17)
where t is the overall deployment length (days). This approach
is only valid under aqueous boundary layer control or in the
linear uptake phase of the sampler (once diffusion is limited by
internal diffusion, ke is no longer linear any longer). Similar to
the results for SPMDs, aqueous boundary layer control is
expected for all HOCs with log KPEw > 3.6. 75 In this case, ke is
inversely proportional to KPEw, and the use of several PRCs
enables to derive ke values for all analytes of interest.75
Booij and Smedes (2010) have suggested a nonlinear leastsquares method to better characterize sampling rates based on
dissipation rates of PRCs, including those that are either
insignificantly or completely lost.78
The actual uptake of the analytes by the PE sampler is then
calculated as
(16)
where δPE is the half-thickness of the PE sheet, and DPE,app is the
apparent diffusion coefficient reported for PE.
Results from eq 16 indicate that in the experimental setup
used by Adams et al. (2007)10 the aqueous boundary layer
thickness ranged from 30−140 μm, in-line with expectations for
a stirred solution that was still in a laminar flow-regime at the
passive sampler interface.75
The diffusivity of compounds in LDPE is a strong function of
temperature, with increasing diffusivity at higher temperatures,
but this has not been addressed in greater detail. Aminabhavi
and Naik (1998) measured the changes in DPE for alkanes with
increasing temperatures: From 298 to 323 K, DPE increased 2fold for hexane but 70-fold for pentadecane.71 Al-Malaika et al.
(1991) reported activation energies of DPE for UV stabilizers on
the order of 80−100 kJ/mol from 278−313 K.67
7. Equilibration Times for Organic Contaminants in
PE. Knowing the molecular diffusivities of the HOCs, the time
to reach equilibrium in the field can be calculated. This will be
estimated for 2 scenarios in which the polymer-internal
diffusion represents the time-limiting step. In the first, the
time to reach 95% equilibrium in a typical 50 μm thick LDPE
sheet under internal diffusion control is calculated. This could
be the case for such a sheet exposed to a very fast current in the
air or water, as deployed off a ship or aircraft. The calculation is
based on the work by Crank and Park (1968)61 and suggests
that deployment times of 24 h are sufficient to reach
equilibrium for a compound with DPE = 10−14 m2/s (or a Vim
∼ 270 cm3/mol; covering all PAHs, PCBs, PCDD/Fs, and
PBDEs up to 8 bromines).61
In the second approach, the thickness of an LDPE sampler is
calculated such that a 2 month deployment is entirely
controlled by internal diffusion (if deployed on a remote
buoy, for example). This could potentially avoid the use of
performance reference compounds (PRCs) in the field. A 5
mm thick LDPE sheet would satisfy this requirement.
Naphthalene would be at 98% equilibrium, while HCHs
would be 33% equilibrated, PCB 209 and OCDD at ∼14%.
CPE = C wKPEw(1 − e−ket )
(18)
The time to reach equilibrium can also be calculated knowing
the thickness of the aqueous boundary layer. Two scenarios are
estimated using a 51 μm thick PE sampler, in which the value of
δw is either 10 μm (representative of a very turbulent water
flow) or 500 μm (typical for a very quiescent water movement).
For the sake of simplicity, we ignore that δw actually depends
on Dw.79 This is detailed below for compounds that behave like
PAHs, for which all relevant properties (DPE, Dw in m2/s, and
KPEw) are estimated based on the PAHs’ MW (g/mol)
log DPE = − 0.0137MW − 10.01
(19)
(see ref 63)
log Dw = − 7.57 − 0.71logMW
(20)
(see ref 45)
log KPEw = 0.0272MW − 0.63
(21)
(see eq 3 and ref 38)
In this case, ke is calculated based on the two additive
resistances in δw and δPE
1
1
=
×
ke
KPEw × δPE
1
δw
Dw
+
δPE × KPEw
DPE
(22)
Under high-flow conditions, a typical PAH like pyrene (MW
202) equilibrates within 2 days, while benzo(a)pyrene (MW
614
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Environmental Science & Technology
Critical Review
Figure 6. Predicted % equilibrium as a function of molecular weight and time (days) for PAH-like molecules in a 51 μm thick PE sampler assuming
either a) a very turbulent or b) a very quiescent flow.
of internal diffusion relative to aqueous boundary layer control.
Unfortunately, the effects of temperature and pressure on DPE
are not well characterized. This would also help better
understand if there is a true size cutoff for large MW HOCs
into PE and how this changes with pressure and temperature.
Once DPE are better constrained, sufficiently thick PE samplers
could be deployed in the field without the need to use PRCs, as
equilibrium could be calculated based on membrane diffusion.
256) takes 4 weeks (Figure 6a). Even under these turbulent
conditions, the aqueous boundary layer is the rate limiting step
for all PAHs with a MW > 150 (e.g., acenaphthylene). This
equates to aqueous boundary layer control for PAHs with log
KPEw > 3.8, basically confirming the conclusions reached for
SPMDs (see above). Under low-flow conditions, the aqueous
boundary layer dominates ke for all PAHs. Pyrene takes 2
months to equilibrate, while benzo(a)pyrene reaches only
∼50% equilibrium even after 1 year of deployment (Figure 6b).
8. Implications for the Use of PE Samplers in the
Environment. There is very good agreement of KPEw values
for traditional HOCs (PAHs, PCBs, PBDEs) from different
laboratories and LDPEs used but lesser so for semipolar
compounds and OCPs. By and large, this suggests that PEs
could indeed be used across the world for global scale passive
sampling of air or water80 and that properties summarized here
should be applicable in all environments. For nonpolar
compounds KPEw can be estimated (±factor 2) assuming
constant solubility in LDPE, as the variation in Cwsat(L)
determines the differences in KPEw. Concerning semipolar
compounds, their KPEw are mostly lower by ∼ factor of 2
compared to predictions with Cwsat(L), but further work is
required to obtain reliable measurements and predictions.
There is a surprising lack of knowledge of KPEa for HOCs other
than PAHs. This should be investigated in laboratory and field
studies with the aim of establishing/verifying that KPEa can be
predicted as KPEw/Kaw. The effect of temperature on KPEw and
KPEa needs to be better constrained, while the salting-out is well
described. For potential deployments of LDPE in deep water or
at high altitude, the effect of pressure on K values needs to be
investigated.
Best results of DPE are obtained when using the film-stacking
method. If stirred systems are chosen for further studies of DPE,
thicker PE sheets should to be used, thereby increasing the role
■
ASSOCIATED CONTENT
S Supporting Information
*
Additional information on selected physicochemical properties
of all analytes and their measured respectively best-fitted KPEw,
KPEa, and DPE values. This material is available free of charge via
the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
■
ACKNOWLEDGMENTS
Corresponding Author
*Phone: (401) 874-6612. Fax: (401) 874-6811. E-mail:
[email protected].
R.L. acknowledges an Alexander-von-Humboldt fellowship
while at the University of Tübingen (Germany) and funding
from EPA’s Great Lakes Restoration Initiative Award GLAS #
00E00597-0, Great Lakes Air Deposition Program Award #
GLAD 2010-5 and CICEET Research Grant #000-1894
supporting passive sampler research at URI. Helpful comments
from Drs. Abhijit Deshpande and Susy Varughese (IIT Madras,
India), Peter Grathwohl (University of Tübingen), Kees Booij
(NIOZ), and 4 anonymous reviewers are kindly acknowledged.
615
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■
Critical Review
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