PB87-231759
Design of Laboratory Systems Cor
Controlling the Activity of
Moderately Volatile Organic Compounds
Corvallis Environmental Research Lab., OR
Aug 87
V
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TECHNICAL REPORT DATA
(fttttt rred /munttrioni en l/it rtvene btfert comphrtnt)
I.MI'OKTNO. 2.
EPA/600/3-87/030
J. RECIPIENT'S ACCESSION NO.
*. TITlI ANO SUBTITLE
The design of laboratory systems for controlling the
i.'-tivity of moder 'ely volatile organic compounds
5. REPORT DAT!
August 1987
B. PERFORMING ORGANIZATION COOt
,'jofin°l3esta11, Bruce Brownawell, Ronald Hilburn and
Gerald Schuytema
1. PERFORMING OROANIZATION REPORT NO.
». PERPORMINO ORGANIZATION NAMI ANO ADDRESS
Oregon State University Chemistry Department
and US EPA, Corvallis, OR
10. PROORAM ELEMENT NO.
11. CONTRACT/BRANT NO.
CR 811894
12_SPQN»0RING AOCNCT NAMt AND ADDRESS
tnvironmenfai Research Laboratory
Office of Research and Development
US Environmental Protection Agency
Corvallis, OR 97333
13. TYJPE OF REPORT ANO PERIOD COVEREO
Design Manual
14. SPONSORING AOENCV COOt
600/02
16. IU»Ll"tNT*BY NOTES
16. ASSTRACT
The gas-phase transfer of hexachlorobenzene (HCB) from the solid to the chambers
used for sediment-water bioaccumulatirn tests was Investigated.
The physical properties of HCB found in the literature are summarized. Significant
discrepancies weru found among different methods. The chief sources in uncertainty in
these data were due to variations In temperature, and the possible incomplete equili-
bration of the solid with the gas. These items are discussed in detail.
The theory for transfer from gas to a liquid is developed. The effects of
association of HCB with humate and other competing processes (head space losses,
adsorption to food, organisms, sedimentjare included.
Experimental d'-.ta from the use of the apparatus in actual bioaccumulation experi-
ments are discussed.
1». REV WORDS AND DOCUMENT ANALYSIS
l. DESCRIPTORS
b.lOENTI# ICRS/OPEN INOCO TUMI
C. COSATI Fkld/Cioup
Release to Public
IB. StCURIT V CLASS /ThO Ktpcrl)
Unclassified
31. NO. 0' PAGES
66
IJ. PRICE
CPA Form 1120*1
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EPA/600/3-87/030
August 1987
t> b87- ^ J 17 by
The Design of Laboratory Systems
for Controlling tha Activity of Moderately Volatile
Organic Compounds
Report on work performed under
Cooperative Agreement No. CR 811894-01-1
Corvallis Environmental Research Laboratory
United States Environmental Protection Agency
Corvallis, OR 97333
by
John Uestall, Bruce Brovnavell, Ronald Hiiburn
Department of Chemistry
Oregon State University
Corvallis, OR 97331
and
Gerald Schuytema
Corvallis Environmental Research Laboratory
United States Environmental Protection Agency
Corvallis, OR 97333
ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CORVALLIS, OR 97333
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NOTICE
This document has been reviewed in accordance with
U.S. Environments! Protection Agency policy and
approved for publication. Mention of trade names
or commercial products does not constitute endorse-
ment or recommendation for use.
ii
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Table of Contents
Acknowledgments
Abstract
1. Overview
1-1
2. Physical Properties of Hexachlorobenzene
2-1
3. Theory
3-1
Henry's Law.
Transfer at air-water interfaces: the film model.
Transfer across air-water Interfaces in laboratory and field
environments.
Transfer between gas bubbles and solution.
Change in concentration in solution.
Association with humic matter.
Competing processes.
Description of experiments.
Analysis for HCB by gns chromatography.
Results and discussion.
Vapor pressure and gas phase concentration.
Solubility knu Henry's constant
Kinetics
Association with humate
Equilibrium assumption.
4. Experiments
4-1
5. Applications, Conclusions, Recommendations
5-1
iii
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Acknowledgments
The authors gratefully acknowledge the coooperation and assistance of
several scientists at the Corvallls Environmental Research Laboratory,
U.S.E.^.A., in particular Gary Chapman, Pill Griffis, Dan Krawczyk,
A1 Nebeker, and Spencer Peterson.
iv
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Abstract
The gas-phase transfer of hexachlorobenzene (HCB) from the solid to
the chambers used for sediment-water bioaccumulation tests was
investigated.
The physical properties of HCB found in the literature are
summarized. Significant discrepencies were found among different
methods. The values determined in this study were (with standard
deviation in parentheses): vapor pressure of HCB at 295 K: 1.37 (0.02)
mPa; saturation concentration of HCB in air at 295 K: 0.554 (0.007)
nmol/L; the delivery rate of HCB with apparatus: 0.26 pmol/s;
saturation concentration of HCB in water at 295 K: 18 (1) nmol/L;
saturation concentration of HCB in water with 1 mg/L humate at 295 K:
21 (1) nmol/L; saturation concentration of HCB in water with 50 mg/L
humate at 295 K: 62 (2) nmol/L; transfer rate constant for HCB between gas
and water in apparatus: 5.8 x 10"^ (0.7 x 10"^) s"*. The chief
sources in uncertainty in these data were due to variations In
temperature, and the possible incomplete equilibration of the solid with
the gas. These items are discussed in detail.
The theory for transfer from gas to a liquid is developed. The
effects of association of HCB with humate and other competing processes
(head space losses, adsorption to food, organisms, sediment) are included.
Experimental data from the use of the apparatus in actual
bioaccumulation experiments are discussed.
v
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i-1
1. OVERVIEW
The accumulation of organic pollutants by aquatic organisms is of
concern when considering regulatory criteria and disposal options.' Not
only must.the source of chemical to the organism be considered (e.g.,
sediments, pore water, overlying water), but also the environmental impact
(e.g., lethal and sublethal effects within an ecosystem, and the "route
back to man" and effect on human health).
Carefully controlled laboratory exposure experiments are necessary
to study biological effects and accumulation of organic chemicals as a
function of dose, concentration, and mode of exposure. A fundamental
problem in designing these experiments Involves dosing with target
compound in such a way that the chemical activity of r.he compound can be
controlled in each of the important compartments of the experiment. There
are many ways of introducing organic chemicals to biological exposure
experiments, whether they be batch or flow-through in design.
Typically, the target compound is introduced to the aqueous phase in
a pulse-mode or with sequential aliquots over the course of the
experiment, via contaminated sediments if they are present, or through
contaminated food added at discreet time intervals. Contaminant exposure
levels are difficult to control, because chemical equilibration between
food, sediment, water, and other possible important phases can be slow in
many cases. The target compound of interest may also volatilize, be
degraded, or be bloaccumulated to such an extent that concentration of
chemical may be depleted with time in other compartments of the
experiment. Finally, measured aqueous phase concentrations of hydrophobic
organic compounds may not reflect tho dissolved phase activity due to
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1-2
interactions with colloidal or hunlc-type organic natter (Means and
Wijayaratn«, 1982; McCarthy et al., 19R5; Brownawell and Farrington,
1966).
One way to achieve a known and constant dissolved phase activity of
a neutral organic compound is to bubble continuously a solution with a gas
strean of controlled concentration. This method of dosing is also
attractive due to the fact that oxygen aeration is performed without loss
<^f volatilization of target compound from the system. In this report, the
I
theory of gas-liquid transport of moderately volatile organic compounds is
presented and discussed in terms of the practical experimental variables
that must be considered when designing experiments. Gas-water transfer of
one model hydrophobic organic compound, hexachlorobenzene (HCB), is
examined in detail and results from fundmental experiments are interpreted
in terms of the theoretical models developed.
HCB has several properties which make it an excellent model compound
for this study: (1) the hydrophobicity of HCB indicated by its high
octanol-water partition coefficient and relatlvley low water solubility is
representative of a large range of pesticides and other xenobiotic organic
compounds, (11) HCB is highly resistant to biological or chemical
aegradation, simplifying experimental procedures for long term experiments
and interpretation of bioaccumulatlon studies, (ili) the high analytical
sensitivity afforded by capillary gas chromatography with electron capture
detection for KCB allows low level aqueous phase measurements despite the
low solubility of HCB, (iv) the moderately high Henry's Law constant of
HCB is somawhet higher than most chlorinated hydrocarbons and pesticides,
which facilitates the air-water exchange of HCB in gas bubbling
experiments, and (v) HCB is a stable, ubiquitous, environmental
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1-3
contaminant which has been widely used as a pusticide, and is also a
stable by-product in the synthesis of many chlorinated solvents and
pesticides (Versar Inc., 1986). HCB is a suspected carclnogan and an EPA
priority pollutant. Biological effect and chemical transport data ar-s
needed to understand better the transport and fate of this compound in
aquatic environments.
Outline of the report. The objectives of this report are to define
clearly physical transport models for air-water transfer of any volatile
organic compound, to discuss the physical meaning of the kinetic models in
terms of experimental design, and to discuss critically some aspects of
the results obtained in gas-bubbling experiments performed with HCB as a
model compound.
The report is divided into the following sections; a review of the
physical properties of HCB; the theory of exchange of solutes at air-water
interfaces; an interpretation of experiments in which HCB was transferred
from the solid phase to the liquid phase via the gas phase; and a summary
of results and recommendations for future study.
The basic physical properties which control the behavior of HCB in
biological exposure experiments are reviewed critically. This section
provides the background for the physical chemical parameters which are
required to model air-water exchange and biological uptake, provides a
basis for comparing the physical-chemical property estimates derived from
this work, and also provides insight into the quality of critical chemical
property data that exist for low-solubllty, low-vapor pressure organic
compounds of environmental significance.
Physical transport models for volatile chemicals are developed in a
general way by considering a microscopic view of mass transfer across an
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1-4
air-water interface. The two-film diffusive transfer model Is discussed
to provide a vhyslcally Intuitive understanding of the factors that affect
transfer. A mathematical model to describe the time i!ep«;ndent water
concentration of HOB, or other volatile species, is developed in two
steps: (1) the transfer of HCB between water and individual rising
bubbles; and (11) the increase in the concentration of HCB In the liquid.
Also considered in the theoretical section of this report is the effect
that an association of HCB with a solute such as sodium humate will have
on both air-water equilibrium and transfer kinetics. Finally, the effect
of competing chemic&l or biological processes on air-water exchange is
discussed, with particular emphasis on the effect of headspace losses.
The experimental section is organized to answer several questions
which are sug6ested from the theoretical sections as important in
modelling air-water exchange of HCB and properly designing biological
exposure experiments for this compound:
i. the vapor pressure and the concentration of HCB in air in
equilibrium with the solid, and the rate of delivery of HCB
through a particular experimental set-up;
11. the solubility of HCB in wate? and its rate of uptake in water
from ar. HCB-saturated gas;
ill. the effect of hunlc material suspended in the aqueous phase on
the total acount of HCB in the aqueous phase.;
lv. the effect of humlc material suspended In the aqueous phase on
the exchange rrte of HCB between the aqueous and gaseous
phases.
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1-5
Theoretical and experimental results are discussed in the last
r.ection of the report. Th
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2-1
2. HEXACHLOaPBENZENE ¦ PHYSICAL PROPERTIES
Reliable information on physical properties are required for accurate
estimates of the equilibrium distributions and transport rates of organic
compounds, such as HCB, in aquatic systems. For expjnple, sorption of
neutral hydrophobic organic compounds to natural sediments and soils can be
w?ll predicted from the water solubility or octanol-water partition
coefficient, KQW, of the compound (Karickhoff et al., 1979; Schwarzenbach
and Westell, 1981). Predictive models for bioconcentration of neutral
organic compounds by aquatic organisms have also been based on solubility
and Kov correlations (Chiou et al., 1977; Mackay, 1982). The rate and
degree of air-water exchange for volatile species depends cn the Henry's Law
consr.ant. In this part of the report is presented a review of reported
Kow's, aqueous soubilities, vapor pressures, and Henry's Law constants for
HCB (Table 2.1), and a discussion of the quality of these data.
Octanol-water partition coefficients
Kqw determinations are generally performed by analysis of the
aqueous phase after equilibration with octanol, usually with the
equilibration and separation being made in a centrifuge tube. Table 2.1
shows reasonable agreement emong the four KQW determinations performed by
this method, with an average log Kow value of 5.36 ± 0.13. The
generator column method used by Miller et al. (1984) gave a log Kow value
of 5.47, consistent with the batch equilllbration methods.
Careful examination of the literature reveals large standard errors
associated with log KQW measurements greater than 5.0, and particularly
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2-2
Table 2.1. Physical properties of hexachlorobenzene.
Log Xow (-C If given) method
batch equilibration
Senerator column
reverbi- phaso HPLC
5.44
5.50
5.31 (2?.)
5.20 (20-25)
5.47
6.52
5.75
6.71
6.06
6.53 calculated from fragments
Water solubility (*C If given)
5.0 Mg/L (25) shake flask
5.0, 4.9 (20)
5.2-14 (20)
47.0
5.4 (20)
Vapor pressure
(*C) nun Hg
mPa
generator column
micro-column method
(15)
0.40 x
10"3
0.53
(20)
1.09 x
10'5
1.43
(25)
1.91 x
10"5
2.51
(35)
6.40 x
10-5
8.42
(25)
2.25 r.
10"5
2.96
(25)
2.92 x
10'5
3.84
Henry'
s Law constant -
(23)
0.054
(23)
0.029
(20)
0.034
(25)
0.059
gas saturation
gc estimate on Apolane-87
gc estimate on BP-1
H/RT (cm^ water/cm"' air)
gas stripping
gas saturation
reference
Brlggs (1981)
Chlou et al. (1982)
Uatarai fit al. (1982)
Platford et al. (1982)
Miller et al. (1984)
McDuffie (1981)
Garst and Wilson (1984)
Rapaport and Eisenrlch
(1984)
Burkhard (it al. (1985)
Yalkowsky et al. (1979)
Yalkowsky et al. (1979)
Ch'.ou et al. (1982)
Hashimoto et al. (1982)
Miller et al. (1984)
Hashimoto et al. (1982)
Farmer et al. (1980)
Verschueren (1983)
Farmer et al. (1980)
Farmer et al. (1980)
Bidleman (1984)
Bldleman (1984)
Atlas et al. (1982)
Atlas et al. (1983)
calculation assumes vapor pressure of
1.09 x 10* xm Hg and water solubility of 5.0 ug/L
calculation assumes vapor pressure of
1.91 x 10 mm Hg and water solubility of 5.0 ug/L
Molar Mass - 284.8 g/mole
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2-3
for compounds having log Kow's greater Chan 6.0. This scatter id due to
the difficulty of measuring very low concentrations of chemicals in the
aqueous phase, and to the presence of micro-emulsions of octano) Which can
be present In the aqueous phase. It Is not uncommon for the range of
reported octanol-water partition coefficients to span more than an order of
mi^nltude for the more highly chlorinated biphenyls (Bro'mawell, 1986), for
instance.
There has been considerable effort in recent years to develop
reverse-phase KPLC methods for estimating the Kow of organic compounds
(e.g., Weber ec al., 1986; Chlu et al., 1986). These methods have the
advantage that phase separations are complete, detectable concentrations are
readily achieved, analysis time is rapid, and multlcomponent determinations
are easily performed. In this approach, log Kow is correlated with
retention time, retention volume, or capacity factor of a set of
"calibration" standards. An inherent problem arises in choosing the correct
value of log Kow for compounds having large enough Kow's to cooprlse a
good calibration range. RP-HPLC approaches also assume that the mechanism
of association of large hydrophobic solutes with chemically-bonded
reverse-phases is similar to solute-solvent interactions in octanol. The
four log Kow estimates listed in Table 2.1 for the RP-HPLC method have a
large standard error (± 0.49 log units) and hnve an average KQV which
lS greater than one log unit above that determined by the equilibration
methods.
Estimation methods based on the additivity of molecular fragments to
the notal log KQW are often used when experimental values are absent or
uncertain. Yalkowsky et al. (1979) have reported a log Kow *52 for
HCB calculated by the method of Nys and Rekker (1974). This deviation from
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2-U
experimentally determined values nay be due Co non-addltivity of chlorine
fragment contributions In this approach.
The variation of Kow with temperature for HCB cannot be determined
accurately from these literature values, but is not expected to be to
greater than the errors associated with tho experimentally determined values
given 1p Table 2.1. Leo et al. (1971) report an average temperature
dependence of approximately 2%/degree in Kow for a wide range of organic
compounds.
Aoueour. solubility
Solubility determinations of low solubility compounds (< 100 ftg/L)
are fraught with many of the same experimental difficulties that are
discussed above for octanol-water experiments. In the traditional
shake-flask method, solubility determinations of solids may be Influenced by
slow dissolution kinetics, or the presence of small crystals or
micro-aggregates suspended lri the aqueous phase. Filtration of the aqueour
phase is often performed to minimize contamination from undissolved
compound, but unfortunately it is rarely specified whether adsorption of
dissolved compound to the filter or filtration apparatus was taWn into
account. The Impact of adsorption can be minimized by saturating all
filtration apparatus surfaces before sampling. Hashimoto (1982) reported
that membrane filters adsorbed considerably more refiltered HCB than did
glass fibur filters. An Increased number of workers are measuring
solubilities of low solubility compounds with generator column methods
adapted from May et al. (1978). In these methods, the passing aqueous phase
equilibrates with a large surface area of coated sand or glass beads, and
all post column surfaces are saturated before sampling begins.
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2-5
There is reasonable agreement among Che solubility measurements given
in Table 2.1 with the notable exception of the value of 67.0 /ig/L reported
by Miller et al. (1984), who used the generator column method. Hc&rever,
Hashimoto (1982) measured a solubility for HCB by a micro-column method of
5-4 Mg/l* which is in good agreement with those solubilities determined by
the shake-flask method.
It is difficult to estimate what effect temperature has on the
solubility of HCB. Several large polycyclic aromatic hydrocarbons have a
positive solubility dependence on temperature, with solubilities increasing
4-8%/degree (Wauchope and Getzen, 1972; May et al., 1978). However,
p-dichlorobenzene'8 solubility decreases with increasing temperature over a
range of environmentally relevant temperatures (Wauchope and Getzen, 1972).
It is not easy to predict temperature effects on solubility, without
experimental data, because the hcacs of solution are not constant with
temperature. The solubility data in Table 2.1 suggest that the temperatuie
dependence of HCB solubility Is nor. great between 20 and 25*C.
The few direct measurements of the vapor pressure of HCB which have
been made near room temperature have employed the gas saturation method.
This technique is similar to 'he generator column approaches used for
solubility measurements. The vapor pressure of HCB is seen to have a
greater dependence on temperature than does solubility, increasing by about
15-30%/degree.
Bidleman (1984) has estimated the vapor pressures of many chlorinated
hydrocarbons by a capillary gas chromatography method in which Che retention
times at several temperatures are correlated to retention times of
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2-6
hydrocarbons with known vapor pressures on noupolar columns. Because the GC
method estimates the vapor pressure o£ the supercooled liquid, the data
listed in Table 2.1 have been converted to vapor pressures of solid HCB by
correcting for the heat of fusion by the following equation:
log (P^) - - ASm(Tm - T)/2.303 RT (2.1)
where is the entropy of melting, given by Miller et al. (1984) as
UU.8 J mol'^ K"^ for HCB, Tm and T are the melting (230*C) and
ambient temperatures, in Kelvin, Pfi and P^ are the solid and supercooled
liquid vapor pressures at temperature T, and R is the ideal gas constant
(8.314 J mol*^ K"*). The GC determinations agree with experimentally
determined values within the uncertainties of the estimation method.
Henrv'3 Law constant
The Henry's Law constant relates the concentration of a solute in the
liquid phase to its concentration or vapor pressure in the gas phase. (The
constant is defined in Equations 3.2 and 3.5 in the next part of the
report). Table 2.1 lists two experimental determinations of H/RT, and two
values calculated from the vapor pressure and solubility data in Table 2.1.
It might be fortuitous that the four values agree within a factoi of two,
given the uncertainties in experimental data and the unknown dependence of
solubility with temperature. The extent to which H depends on temperature
cannot be ascertained at this point.
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3-1
3. fHEQRY
Introduction
Continuous contact of an aqueous solution with bubbles of a *gas
stream containing a known concentration of an organic compound, such as
HCB, can be an effective way to maintain a known and constant activity of
that compound dissolved in solution. This method is especially attractive
for ecotoxicological studies where aeration is also necessary tc supply
oxygen to the system. Solving the equations for the transfer of HCB (or
I
other neutral organic compounds) from incoming gas to aqueous solution
allows for the understanding and optimization of experimental design, such
that near saturation conditions in the aqueous phase can be obtained, anJ
possible effects of competing transfer rates can be evaluated.
'.."his part of the report contains (1) a brief discussion of general
theory describing the transport of volatile gases or organic compounds
across air-water interfaces; (ii) the analytical solution for the transfer
of chemical species between a bubble and water, from which the change of
water concentration as a function of time is derived; and (ill) a
discussion of competing rate processes which may affect the concentration
of HCB in solution, with particular emphasis on predicting the r.ffect of a
loss of HCB from solution into the headspace of experimental systems.
Further information on the air-water transfer of volatile compounds in
environmental systems can be found in the review by Thomas(1982).
Henry's Law: Equilibrium of a solute between liould and yas phases.
A moderately volatile solute such as hexachlorobenzene will
distribute Itself between closed liquid and gas phases according to the
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i-i
reaction
Cj. - Cg H' (3.1)
where represents the concentration of the solute in the liquid phase
(mol/L), Cg represents the concentration of the solute in the gas phase
(rool/L), and H' is the dimensionless Henry's Law constant. The mass
action equation corresponding to Reaction 3.1 is
CS
H' - (3.2)
C1
Occasionally the activity of the solute in the vapor phase is
expressed in terms of partial pressure instead of concentration. Although
the dimensionless Henry's constant given above will be used throughout
this report, the relationship In terms of pressure Is given below for
reference. The conversion between units of concentration and partial
pressure can be made by way of the ideal gas law,
p V - N R T (3.3)
where p is the partial pressure (kPa), V is the volume (dm ), N is the
number of moles, R is the gas constant (8.314 J mol'1 K**), and T is
the absolute temperature (K). The concentration is then related to the
partial pressure by
N p
C • <3.«)
8 V RT
and the Henry's Law constant (mol dm""* kPa"1) in terns of partial
pressure is
P
H - -- - H' RT (3.5)
C1
Other forms of the Henry's constant are found, but they will not be used
in this report.
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3-3
Rate of transfer at eas-lloutd Interfaces: the film model.
One of the goals of the transport model Is to describe the
concentration of the solute In one phase or the other as a function of
time. The differential equation that leads to that function Is developed
below.
The rate of transfer of a solute from the liquid phase to the gas
phase, or from the gas phase to the liqiuld phase, depends in a complex
way on many factors such as mixing of the gas phase, nixing of the liquid
phase, the affinity of the solute for the gas phase and the liquid phase,
and the dlffuslvity of the solute In both phases. Often very simple
conceptual models are used with operational parameters to relate observed
rates of transfer to observed experimental conditions such as rate of
mixing, molecular diffusivity and Henry's Law constant of compound being
transferred.
Two such models (Morel, 1983) are (i) the surface renewal model and
(11) the film model. In this study, we have little basis for selecting
one model or the other, since very few of the factors listed above were
varied. In particular, the transfer of only one compound was considered.
Thus almost any model in which flux is directly proportional to
concentration difference between the two phase? would be acceDtable. In
this study the commonly used film model has been used as the basis for the
transfer equations. However, it is importanc to bear in mind that other
conceptual models could lead to the same final equations.
According to the film model (Figure 3.1), the completely mixed bulk
gas phase and completely mixed bulk liquid phase are separated from each
other by a stagnant gas film and a stagnant liquid film. For both films
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3-4
FILM MODEL
Liquid Gos
Film Film
Bulk
Liquid
Bulk
Gas
C, (bulk)
0
Figure 3.1. The film model. The gas phase concentrations are plotted In
terms of the equivalent liquid phase concentration to allow easy
comparison betwen scales. In the figure, Che solute is moving from the
bulk of the gas phase to the bulk of the liquid phase. If the
concentration in the liquid phase does not change, the concentration in
the gas phase will approach C^(eq), as in a bubble leaving solution in
equilibrium with the liquid. If the concentration in the ga* phase does
not change, the concentration in the liquid phase will approach Cj(eq),
as in a liquid eventually equilibrating with an inflowing gas stream.
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3-5
the flux through the film is equal to a mass transfer coefficient times
the concentration difference across the film:
1 dN
Ji ki (ci * ci > (3 6)
A fit
1 dN n
J ------- k (C - C ) (3.7)
* A dt 8 8 8
2 • 1
where is the flux (aol dm s ) across film i, A is the
o
interfacial area (dm), N is the number of moles, t is time (s), is
the heterogeneous mass transfer coefficient for film i (da s~^), is
the bulk concentration in phase i, and C^z~0 is the concentration in
phase i at the interface. Flux out of the liquid into the gas is defined
as positive, and flux into the liquid from the gas is defined as
negative. The concentration of the solute in the liquid phase and the gas
phase at the Interface (z-0) are assumed to be at equilibrium, related by
Henry's Lav (Equation 3.2).
At steady state the flux through the gas film is equal to the flux
through the liquid film; this net flux is denoted by J:
J - J^(steady state) - Jg(steady state) (3.8)
A combination of Equations 3.2, 3.6-3.8 yields this overall steady state
flux in terms of the bulk concentrations,
J - k (Cx - C^eq)),
(3.9)
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3-6
where k (dm s"*) is che effective mass transfer coefficient
1 1 1
- - - + (3.10)
k k, k H'
1 6
and C^(eq) is defined as Che liquid phase concentration that would be In
equilibrium with the bulk gas phase concentration,
C
C,(eq) - -& (3.11)
H'
Note that the effective mass transfer coefficient, k, depends on the
Henry's law Constant, as well as k^ and kg. It is not the objective
of this study to examine transport in terms of these parameters as they
appear in Equation I*.10. Nonetheless, the equation is still important in
allowing us to consider the effects of mixing in the gas phase and the
liquid phase and to address questions such as, will stirring of the liquid
phase affect transfer rate? In the next section the application of the
film model to environmental and laboratory systems Is discussed.
Finally the flux in Equation 3.9 can be transformed Into an equation
in change in concentration in the liquid phase, by multiplying the flux
2
(Equation 3.9) by lnterfacial area A (dm ) and dividing by the liquid
volume (dm^):
dC. k A
---- - - --- (C1 - C^eq)) (3.12)
d t Vj
Equation 3.12 is the fundamental equation for first order change In
concentration. As mentioned In the introduction to this section, this
equation can be derived by considerations other than those of the film
model; the form of the equation remains the same, but the Interpretation
of the constant k is different.
-------
3-7
Tfflngpgrt .cross air-water Interfaces In laboratory and field environments
The exchange of gases or neutral oiganic compounds between air and
wxter can be represented an a first order process with the net flctx across
che interface described by Equation 3.9. The mass transfer coefficient k
is dependent on the diffusivity and Henry's Law constant of the compound,
and the mixing conditions of the experiment.
Several methods have been used to estimate the mass transfer
qoefficients of individual chemicals under different environmental and
laboratory conditions. Most commonly, a two-fiim model has been used
(Llss and Slater, 1974). It is assumed that the bulk phases are well
mixed and the main resistance to transport is due to molecular diffusion
processes in both gas and liquid interfacial layers Uhen tha overall
mass transfer coefficient is written on a liquid-phase basis, as in
Equation 3.9, the relationship between k and the individual liquid and gas
phase mass transfer coefficients (k^ and kg, repecLively) is given by
Equation 3.10.
The values of k^ are much lower than kg due to slower rates of
molecular diffusion In liquids. Measured ratios of are Che
range of 0.003 to C.02 in the environment (Llss and Slater, 1974; M&ckay
et al., 1979). Estimates of k^ and kg have been reported by Smith et
al. (1981) for several polycycllc hydrocarbons (PAH), under a range of
mixing conditions in laboratory experiments. From those values it is
possible to estimate approximate vqlues of mass transfer coefficients
which can be expected for HCB. This estimate is made by setting the ratio
of transfer coefficients of two compounds equal Co Che reciprocal of che
ratio of the square roots of their molar mauses (Llss and Slater, 1974).
-------
3-8
Table 3.1 shows Che estimates of Dass transfer coefficients of HCB
for laboratory conditions (from the data of Smith et al., 198]), and for
typical open ocean conditions (Llss and Slater, 1974). The mass transfer
coefficients Increase greatly when the solution is nixed, with the value
for the open ocean falling in the range of the stirred experimental
vessels.
The ratio kj/kg is seen to be in the range 0.03 to 0.003 for
environmental and field systems. According to Equation 3.10, the mass
transfer will be controlled by the liquid film when H' » kj/kg ""d uy
the gas film when H' « kj/kg. Thus for compounds with H > C.l,
liquid film control is anticipated, and for compounds with H' < 0.001, gas
film control.
The value of H' reported for HCB is 0.029 (Atlas et al., 1984); it
has been estimated from solubility vapor pressure measurements to be 0.031
in the experimental section of this report. Thus transfer of HCB is
likely to be controlled primarily, but not exclusively, by the liquid
film. It can be seen from Table 3.1 that most of the resistance to HCB
transfer is in the liquid phase, with a variables contribution from the gas
phase depending of mixing in each phase. This Indicates that increasing
mixing in th-? liquid phase will generally promote transfer of HCB.
However, it is important to note that the effect of gas phase resistance
cannot be ignored when trying to estimate k for HCB, as it can for
chemicals with greater values of H'.
In the following discussions of laboratory experiments, only one
compound, hence only one v-ilue of H', is being considered. Therefore
differentiation between gas film and liquid film control is not so
important, since Equation 3.12 is generally valid. However, part of the
-------
3-9
Table 3.1 Extrapolated values of kg, , and k for HCB under different
mixing conditions based on laboratory volatilization experiments of
Smith et ul. (1981). k(HCB) - k(PAH)(MWpAH/MWHCB)1/2. k is
calculated by Equation 3.10. Mass transfer coefficients predicted for
open ocean conditions are based on kg and k^ values of H2O and
CO2, respectively, in a similar manner (I.lss and Slater, 1974).
Experimental k k1 k k1/k
Conditions ® ®
(pm / s)
stagnant (no mixing) 190 1.0 0.86 0.0052
stirred at 100 rpm 620 -\5 6.A 0.015
stirred at 200 rpm 890 26 13 0.029
stirred at 200 rpm 6300 28 25 0.0045
+ 3-4 m/s wind
open ocean 2100 22 16 0.010
(typical conditions)
-------
3-10
study involves the associjtion of HCB with humlc material; the association
with humlc material Is operationally equivalent to altering H' of HCB, If
all chemical reactions are fast compared to transport. In this case,
differentiation of film control becomes important again. Furthermore, it
should be clear that changing the mixing conditions (e.g., changing the
shape of the vessel or gas flow rate) will affect k.
-------
3-11
Transfer Rate From Gas Bubbles to Solution
Equation 3.12 Is a valid approximation for many gas-llquld
interfaces. To solve the transport equation for transfer of HCB from
bubbles to solution, in terms of experimental variables, it is useful to
write the equation for transfer between the water column and a single
bubble as it rises through the water column. Reformulation of Equation
3.12 in terms of the g,as phase concentration in a bubble gives:
dCb(t) \ s
dt V. H*
D
(Cb(t) - Cb(eq)) (3.13)
where C^(t) (mol/L) is the time-varying concentration in the bubble,
kb (dm/s) is the effective mass transfer coefficient across tlx*
bubble-water interface (as given in Equation 3.10), (dm ) is the
area of the bubble-water Interface, (L) is the volume of the bubble,
and C^(eq) (mol/L) is the gas phase concentration that would be in
equilibrium with the bulk water concentration:
C^eq) - Cx H' (3.14)
This equilibrium concentration is assumed to Le constant over the lifetime
of one bubble, that is, the change in concentration in the water is slow
compared to that in the bubble.
Equation 3.13 can be solved for the concentration in the bubble as
it leaves the solution by separating variables and integrating over the
lifetime of a bubble, t^ (s):
V A f
(cb(tb) - Cb(eq)) - (Cb(t-0) - Cb(eq)) exp( ) (3.15)
Vb H'
where (^(t-O) is the concentration of the solute in the inflowing gas
stream. It can be seen that for residence time of the bubble t^
-------
3-12
sufficiently long compared to bubble leaves the
solution in equilibrium with the water.
Equation 3.15 can be rearranged to express the degree of approach to
equilibrium more quantitatively in terns of the parameter f, defined as
the actual change in the concentration in the bubble from entry to exit,
divided by the change in concentration if the bubble were to leave the
solution at equilibrium:
f
Cbu-0) - Cb(eq)
1 - exp( -
VHjV
)
\ »'
<3.16)
-------
3-13
Change In liquid-phase concentration.
The quantity that is measured in these experiments is the change in
concentration of the solute in the liquid. To calculate this change, two
time scales are Involved: the timescale of a single bubble, discussed
above, and the timescale for the equilibration of the liquid with the
inflowing gas stream, co be discussed here.
The discussion that follows is valid for a completely mixed liquid
phase. At any time t during the course of the equilibration of the liquid
with the inflowing gas stream, the rate of change in the concentration of
HCB in the liquid phase is simply the change in concentration in the gas
stream from entry to exit, multiplied by the volumetric flow rate of the
gas, Qg (L/s), divided by the volume of liquid (L):
dCl(t) Qr
------ - (C.(t-O) - C (t)) (3.17)
dt v1 b b ^
The bubble phase concentrations in Equation 3.17 can be transformed to
liquid phase concentatlons through Equations 3.11, 3.14, and 3.15 t- ,„?id
the differential equation in terms of C^(t) alone:
dC (t)
k' (C (t) - C (eq)) (3.18)
dt
where the effective rate constant, k' (s"*)t
Q H'
k' - ( 1 - exp( ) )
V1 Vb »'
g f (3.19)
is composed of two factors: g - QgH'/V^, which is related to the rate
of supply of the solute through the gas stream to the liquid, and f,
related to the degree of equilibration of a single bubble with the
solution (Equation 3.16).
-------
3-14
Finally Equation 3.18 can be integrated to yield the concentration
in the liquid as a function of time:
(C1(c) - C1) - (C^t-0) - C^eq)) exp(-k' t) (3.20)
where C^(eq) Is the concentration in the liquid that would be in
equilibrium with the bulk concentration in the inflowing gas, as described
in Equation 3.11.
It can be seen from Equation 3.20 that the aqueous concentration
increases with time and assymptotically approaches C^(eq), with a time
constant given by Equation 3.19. The rate at which C^(t) approaches
C^(eq) can be Increased by increasing gas flow rate, increasing t^ or
the effective height of the water column, increasing k^ (possibly by
increasing mixing), increasing A^/V^ by decreasing the bubble size, or
by decreasing the volume of water, V^. The rate of equilibration will
also increase with H' by an amount dependent on the other variables as
given by Equations 3.19 and 3.10.
One Important limiting case exists whe;» f - 1, and the bubbles leave
the solution in equilibrium. When this is the case, k' - H'Qg/V^, and
the Henry's constant can Le determined experimentally from the rate.
-------
3-15
Association with Humlc Matter
In the presence of humlc matter the solubility of HCB Is observed to
exceed that in the absence of humlc matter. Similar observations lhave
been reported by Chlou et ai. (1986), and Callaway et al. (1984). This
apparent enhancement of the solubility can be Interpreted through two
different mechanisms: (1) the association of HCB with humlc material In
solution, or (11) the modification of the properties of the solvent. The
fact that Lhe effect Is observed ar. relatively low concentrations of
humate tends to favor the first mechanism, but a contribution from the
second cannot be ruled out. To a first approximation both mechanisms can
be represented by the same linear model, and the distinction between
mechanisms is not necessary. The simple linear model will be developed
here in terms of the association mechanism.
In assessing the interaction of HCB with humlo. material, it is
necessary first to distiguish between the concentration of free HCB, C^,
the concentration of HCB associated with humlc material, Cfl, and the
total HCB concentration in solution, Cj,
CT - Cx + Ca (3.21)
According to the association mechanism, the operational mass action
equacion for the binding of HCB to humate is
C1 Ch *h " Ca <3"22>
where is the operational stability constant (L mg'^) and is
the humate concentration In mg L~^. The mathematical form of the
association mechanism presented above Is identical to that of the
partition mechanism, which has been used to describe the distribution of
nonpolar hydrophobic compounds between sediments and water. Here the term
-------
3-16
association is preferred to partition, since partition implies the
distribution between two three-dimensional phases.
If the solution concentration of free HCB is established at Ithe
saturation value by contact with the saturated vapor, and If the total
humate concentration Is in great excess over the total HCB concentration,
then the operational stability constant can be determined from the total
HCB concentration and the free HCB concentration (determined in the
I'isence of humate) through a comD.'nation of Equations 3.21 and 3.22:
Thus the apparent association constant can be determined from the
concentration of HCB in solutions, with anJ without humic matter, by
maintaining the solutions in equilibrium with a constant gas-phase
activity of HCB. In addition to this equilibrium effect of hunate, humate
will also affect the rate of transfer of HCB as expressed in
Equation 3.19, by altering the apparent Henry's constant. This effect is
described below.
The simplest model of kinetics for transfer of HCB to the humate
solution involves three assumptions, in addition to those of the film
model: (i) association-dissociation kinetics of the humate-HCB complex are
Infinitely fast compared to transport kinetics; (ii) the liquid film
transport coefficient of the humate-HCB complex is identical to that of
the frse HCB; and (ill) the humate Is in great excess over the HCB and
uniformly distributed, not accumulating significantly *t the air-water
interface. (One might expect that humic materials would accumulate at the
interface; the impact of an enrichment of humic materials at the interface
is discussed with the results.) Under the three assumptions listed
-------
3-17
above, the two-film model with humate Is of the same mathematical form as
the model without humate, with tfeo replacements: the free HCB
concentration is replaced by the total. HCB concentration CT> arid
the conventional Henry's constant H' is replaced by an effective Henry's
constant H'', defined
C
H" - -5- (3.24)
CT
This effective Henry's constant is related to the conventional Henry's
constant through the association constant and humate concentration by
combining Equations 3.2, 3.21, 3.22, and 3.24:
H'
H" - (3.25)
(1 + *h V
Thus, the effective Henry's constant varies with humate concentration and
binding intensity. Furthermore, the equilibrium and kinetics of HCB
transfer in the presence of humate is analogous to that in the absence of
humate, with the effective Henry's constant replacing the true constant.
The question then becomes, how do the kinetic constants depend on the
Henry's constant?
In Equation 3.19 it was shown •¦hat the constant k' is composed of
two factors: g, which is the rate constant if each bubble left the
solution at equilibrium, and f, the fraction of equilibrium approached by
each bubble. For very small values of H', the factor g obviously becomes
very small. The value of k^ in Equation 3.10 approaches k^H', and the
value of f approaches l-exp^^^t^/V^) in Equation 3.16. Thus the
apparent rate constant for equilibration with the incoming gas is very
small, but proportional to H'. The physical interpretation of this
-------
3-18
situation is that the distribution of HCB to the liquid phase is favored.
The equilibration of the liquid with the incoming gas is relatively r.low,
being ultimately limited by the rate of supply of tha solute through the
gas stream. This condition is tru* regardless of whether the bubbles
leave the solution stripped of the solute or the bubbles leave partially
full due to short residence time compared to the gas-film limited
transport.
For large values of H', the factor g is large, but f becomes very
small. In the limit of very large H', the two effects cancel each other,
and the observed rate constant becomes independent of H'. The physical
interpretation of this situation is that distribution of HCB to the gas
phase is favored. Transfer is liquid film controlled, and the
concentration of HCB in a bubble is virtually unchanged during the course
of the bubble's rise through the solution. The tendency of a larger
Henry's Law constant no decrease the amount of solute required to saturate
the solution, and to speed equilibration, is offset by the tendency of the
increased Henry's coefficient to decrease the driving force across the
liquid film, and to retard equilibration.
Thus we expect the effect of complexation of HCB by huraata to have
no effect or to decrease the overall rate constant, i.e., the rate at
which ths total concentration of HCB in solution approaches its
equilibrium value.
-------
3-19
Competing processes.
The equations above describe the transfer kinetics of HCB from gas
to water in the absence of other sources or sinks for HCB. Additional
reactions which might be important in controlling C^(t) Include
volatilization of HCB from water to the headspace of the experimental
apparatus, exchange with container walls, sediments, dissolved organic
matter, organisms, or food, and any other reactions which might afreet HCB
concentration in solution. Once equilibrium conditions exist in the
experimental vessel, introduction of clean food, organisms, or sediments
that are not in equilibrium with dissolved HCB will decrease the dissolved
concentration, if the amount of HCB exchanged to that phase is significant
compared to the total amount in solution. If an equilibrium partition
coefficient, Kp (L/kg) is used to describe partitioning into a solid
phase, then the effect of HCB uptake on C^ can be neglected only when
m Kp « V^, where m (kg) is the dry weight of the solid phase. To
maintain a constant, saturated concentration of HCB in solution, the mass
of such solids per unit volume of solution should be kept below 1/Kp.
Experiments in which C^(r.) does not approach equilibrium with
Incoming vapor, C^(eq), in the absence of non-equilibrium phases in the
vessel suggest a loss of HCB out of the system across the air-water
interface, into the headspace of the apparatus. This loss can be
significant if the headspace gas is mixed with outside air of C lower
O
than that exiting the water column. Alternatively, HCB could be Injected
into the headspace by eerosol formation as bubbles burst at the air-water
interface. If we ignore this latter possibility, the effect of a
headspace loss on the overall air-water exchange kinetics can be included
easily in the model developed previously.
-------
3-20
The flux of HCB out of the liquid at the free headspace surface can
be described by an equation slmlar to Equation 3.9
1 dN
- k (C.(t) - C.(eq.s)) (3.26)
A dt 8 1 i
s
where k£ Is the mass transfer coefficient at the water-headspace
interface of area Ag, and C^(eq,s) is the concentration in the liquid
that would be in equilibrium with the concentration in the gas of the
headspace, assumed to be constant.
From Equation 3.26 and a similar equation for the flux at the
bubble-liquid interface, the differential equation for the time dependent
concentration of HCB in water can be developed:
dC.(t) k A
(C.(t) - C.(eq,s)) - k' (C (t) - C (eq,b)) (3.27)
dt V1
The steady state concentration in the liquid phase (C^(ss)) is found by
setting the derivative equal to zero and rearranging terms:
k A C (eq,s) + k' V C (eq,b)
C (ss) - i i..i (3.28)
k A + k' V.
s s 1
It is seen from Equation 3.28 that the effects of headspace loss are
minimized when the Ag/V^ is minimized, when k'/ks is maximized
(increasing Q, Ab/Vb, and tfa), or when C1(eq,s) approaches
Cj^eq.b).
Representative calculations with Equation 3.28 illustrate the
possible importance of volatilization losses across the headspace. It is
assumed that k' is 6 x 10"^ s"^ (data from these experiments with
. i
round bottom flasks), k_ is 1.4 x 10 cm/s (5.0 cm/hr) (extrapolation
s
9
of data from Southworth, 1979), equals one liter, A,. is 100 cm ,
-------
3-21
ind C(eq,s) is zero, 1.6., we assume that the headspace is so rapidly
nixed with clean air that the concentration at the surface is effectively
tero. The resulting C^(ss) is equal to 0.041 C(eq,b); headspace losses
ire seen to be very important.
If k' and are maintained constant, C^(ss) is a linear
function of C(eq,s). If C(eq,s) - C(eq,b)/2, that is, the head space gas
rere a naif-half mixture of clean air and gas stream, the value of C(ss)
2
rould r«e equal to 0.52 Cj(eq,b). If As were decreased to l.C cm
ind the assumptions in the above two examples were maintained, Cj(ss)
rould be equal to 0.81 and 0.90 C^(eq,b), respectively.
It is obvious that it is necessary to decrease the mixing ol' clean
ir into Che headspace to minimize effectively this potential loss
echanlsm.
-------
3-22
gwnmfliry
The rate laws which describe air-water transfer of neutral organic
compounds, such as HCB, in gas bubbling experiments have been developed.
The time•dependent aqueous phase concentration, C|(t), of a volatile
compound is given by:
lCjL(t) - C1(eq)) - tC^Ct-O) - C^eq)) exp(-k't)
approaches C^(eq), the equilibrium concentration with respect to
the incoming vapor concentration, with a time constant k'. The constant
1c' c^n be describeu in ter~s of experimental parameters and provides
insight into the controlling factors which should be conridered when
optimizing gas bubbling experiments
H'
k' - -8-- ( 1 - exp( ) )
V1 Vb »'
The overall rate of tranter can be increased with increasing the flow rate
of bubbles (Q_). by decreasing the volume of water (V,), or by using a
B 1
compound with a higher Henry's Law constant
-------
3-23
exponent in this equation becomes sufficiently negative and bubbles leave
in equilibrium with the water, then k' cannot be increased by further
control of these latter variables.
Equations to describe the aqut-us concentration of HCB in gas
bubbling experiments can be complicated by additional sources or sinks of
HCB. Exchange of HCB with container walls, sediments, organisms. And food
nay affect C^ when the total capacity of these phases approaches th&t of
the total volume of water. The mass of nor-equllibrium phases per volume
of water introduced to experiments should be minimised.
Volatilization across the air-vater interface into the headspace can
provide an important loss mechanism for HCB when the vapor in the
headspace is allowed to mix with even a small amount of clean air.
Equations are developed which show that in a steady-state situation
headspace losses are .Minimized when the interfacial area at the headspace
is small, when the headspace concentration of HCB approaches equilibrium
with the water (low mixing with clean air), and generally when k' is
optimized, as described above. Mixing of clean air into the headspace can
be minimized by keeping the interfacial area of the effluent gas flow low,
or by bubbling the outflow through a water trap to prevent back diffusion
of clean air.
-------
4-1
U. EXPERIMENTS
A series of experiments wera performed to characterize the transfer
of hexachlorobenzcne from the solid phase, through the gas phase to the
liquid aqueous phase. The experiments involved both the addition of HCB
to the liquid and stripping of HCB from the liquid.
These experiments are outlined in Table 4.1. All of the experiments
were variations on the same basic design. Gas was bubbled through a
liquid; during the contact between the gas and the liquid, solutes are
exchanged between the two phases. The liquids used were ethyl jetate,
which extracts HCB from the vapor phase with high efficiency, and water,
with and without suspended humic material. The objectives of the
experiments were to determine:
i. the vApor pressure and the concentration of HCB in air in
equilibrium with the solid, and the rate of delivery of HCB
through a particular experimental set-up;
ii. the solubility of HCB in water and its rate of uptake in water
from an HCB-saturated gas;
Hi. the effect of humic material suspended in the aqueous phase on
the total amount of HCB in the aqueous phase;
iv. the effect of humic material suspended in the aqueous phase on
the exchange rate of HCB between the aqueous and gaseous
phases.
The source of the hurtle materials used in these experiments was
humic acid from Aldrich. Malcolm and MacCarthy (1986) have discussed
limitations on the use of this material in research or. soils and water.
Humic materials were added to samples through a spike of a well mixed
solution containing 1 g/L of humic acid. No attempt was made to separate
soluble and Insoluble material in the concentrated solution.
-------
4-2
Table 4.1. Description of Experiments
\
Experiment £&£ Liquid SglvtP
1. Uptake Air/HCB Ethyl Acetate HCB
2. Uptake Aii/HCB 0.010 M NaCl HCB
3a.Uptake Air/HCB 0.D10 M NaCl +1.0 rag/L Na Humate HCB
3b.Uptake Air/HCB 0.010 M NaCl + 50. mg/L Na Humate HCB
-------
4-3
Description of Experiments.
The experimental apparatus for the transfer of HCB from the solid to
the liquid is shown in Figure 4.1. Compressed air pressure was s^t at
40 kPa (5 psi) over atmospheric pressure. The air was passed through a
drying column and a molecular sieve to remove trace impurities, and flow was
regulated by a needle valve.
The needle valve was placed downstream of the molecular sieve to
reduce re-equilibration time after an adjustment of the valve, and upstream
of the HCB column to avoid the potential of pressure build-up, and
explosion, in the HCB column. The flow rate was measured with a bubble flow
meter attached to the exit port of the system. After an adjustment of the
needle valve, time was allowed for the system pressure to re-equilibrate.
Flrv was measured at several times during the course of an experiment and
found to be relatively constant.
From the needle valve, the air was equilibrated with water and passed
through a 25 x 2.5 cm column filled with 0.5 cm diameter glass beads coated
with solid HCB. (In the experiments with ethylacetate, the water
equilibration step was bypassed.) The beads were coated by preparing a
slurry of 2 g HCB in enough diethyl ether to cover the beads; when the
slurry became thick, the beads were transferred to a tray, no more than 1
layer of beads deep for final drying.
The gas stream was then bubbled into the sample liquid through a glass
frit of porosity 40-60 pm. The sample liquid was contained in a
three-liter round bottom flask. The gas leaving the sample vessel was
passed through a water trap to limit back diffusion of HCB-free air into the
vessel. The gas passing out of the water trap was passed through Tenax
resin to trap HCB.
-------
4-4
EXPERIMENTAL APPARATUS
B
Figure 4.1. Apparatus for transferring HCB from the solid phase to the
liquid phase via the gas phase. A. compressed air; B. regulator set at 35
i
-------
4-5
The entire apparatus was operated in a fume hood. The three-liter
flask was maintained at 295 K in a water bath. The column with HCB was
not placed in the water bath, due to its limited resistance to corrosion
and leaks. However, for further studies, a thermostatted column for the
HCB is highly recommended due to the strong dependence of HCB vapor
pressure on temperature (Table 2.1).
The liquid in the round bottom flask was sampled with a j;1ass pipet
through a ground-glass stoppered sampling port. The volume of liquid
withdrawn for analysis was replaced by an equal volume of HCB-free
solvent. A correction for this dilution was made in the data analysis
(vide infra).
Three factors appeared to be Important in minimizing losses of HCB
to the walls of the vessel and the headspace: (i) minimizing hoadspace in
the sample flask by filling the flask close to capacity; (J.i) minimizing
loss of HCB across the solution-headspace interface by use of a water trap
on the exit poin of the sample flask; and (iii) minimizing the glass
surface to solution volume ratio through use of a round-bottom flask. No
experiments were conducted to quantify the effects of these three
measures, but performance of the system seemed to Improve as the measures
were introduced.
-------
4-6
Analysis for HCB bv Gas Chromatography.
The standards for the calibrations were made up in three different
media: (1) toluene, (tl) 0.001 tt NaHCOj, and (ill) 0.001 M NaHCO^,
20 mg/L sodium humate.
Laboratory standard solutions of pentachlorobenzene (PeCB), which is
the internal standard, and HCB were prepared from the pure solids
(Supelco, 99%) by adding a known mass of the compounds to a 2b mL
volumetric flask and filling to the mark with toluene. The concentration
of the laboratory scandard HCB was 3.10 ug/mL and PeCB 16.20 ug/mL.
The nonaqueous calibration standards were prepared by adding a known
volume of the laboratory standards to a 40 mL glass bottle with a Teflon
lined screw cap containing 3 mL of toluene. Aluminum foil was placed over
the mouth of the bottle to prevent the sample from contacting the Teflon
cap liner. The volume of HCB was varied and the volume of PeCB was held
constant. The solutions were placed on a vortex mixer for 90 seconds,
then added by pipette to crimp-top autosampler vials.
The aqueous calibration standards were prepared by adding a known
volume of the laboratory standards to a 40 mL glass bottle with a Teflon
lined screw cap containing 20 mL of the aquoou.3 solution. Aluminum foil
was placed over the mouth of the bottle to prevent the sample from
contacting ths teflon lining. The solutions were placed on a vortex mixer
for 90 seconds, 3 mL of toluene were added, the solutions mixed for
another 90 seconds, then the phases allowed to separate. Three (in some
cases only two) aliquots of the toluene phase were removed by pipette and
placed in separate crimp-top autosampler vials; the separate subsamples
allowed for replication of the gas chromatography step of the analysis.
There were no replicates of the extraction procedure in these
-------
U-l
calibrations. Samples were extracted and analyzed by the sane procedure
described above for the standard's.
The samples were run on a Perkln-Elmer Model Sigma 2000 gas
chromatograph with an electron capture detector, programmable temperature
vaporizing injector, Model LCI 100 integrator, and Model AS-2000
autosampler. A Perkin-Elmer bonded methylsllicone column (10 m x 0.25 nu?
was used isothermally at 180 "C, with split injection at 275 °C. Each
calibration involved solutions with standards of five different
concentrations; two or three replicate injections of each standard were
made. Peak height and peak area for both HCB and PeCB were recorded.
Peak height was used for quantitation. Relative standard deviation was 4%
for replicate injections. Some of the samples early in the program were
analyzed by a similar procedure on a different gas chromatograph.
-------
4-8
The raw data obtained from all of these experiments are tine series
of concentrations in the sample vessel. The raw data for Experinfents
1 - 3 are presented in Tables 4.2 - 4.4.
The data for time and concentration were interpreted in terms of two
adjustable parameters in th. jdel developed in Section 4 of this report:
the saturation concentration, C(eq), and the rate constant, k'. The
equations that were actually used in the determination of these constants,
corrected for sampling volume, are presented in Table 4.5.
The values of C(eq) and k' determined by this method are presented
in Table 4.6. The fundamental constants that have been derived from these
results are summarized in Table 4.7. It can be Feen that the fundamental
constants determined in this study agree well with Chose of Atl?.s et al.
(1984), discussed in Part 2 of this report.
In the following section, the results presented in Table 4.7 are
discussed in detail, including the determination of: (i) the vapor
pressure and vapor phase saturation concentration of HCB through uptake
experiments with an organic liquid; (ii) the saturation concentration of
HCB in water and Henry's constant through vapor phase uptake experiments;
(ill) the degree of equilibration of a bubble as it passes through
solution; (iv) the association of HCB with dissolved organic matter
(equilibrium and kinetics) through saturation concentration and gas-liquid
exchange kinetics. The values of apparent Henry's constants, degree of
equilibration, and association constants are summarized in Table 4.8.
-------
4-9
Table 4.2. Data from Experiment 1. Transfer of HCB from air to
ethylacetate.
Time (hr) C(HCB)(^g/L) Standard
Deviation®
16.67 1.24 .05
17.83 1.45 .05
18.83 1.91 .06
19.75 1.67 .23
23.08 2.14 .27
24.00 2.19 .28
24.83 2.33 .29
40.25 3.74 .43
44.50 3.86 .15
48.35 4.45 .17
a Calculated by propagation of error from slope and intercept of
calibration curve and replicate injections of sample (cf. Peters, Hayes,
Hieftje, 1976)
-------
4-10
Table 4.3. Data from Experiment 2. Transfer of HCB from air to
0.01 M NaCl.
Time (hr)
C(HCB) (jjg/L)
Standard
Deviation'
18.1
1.53
0.24
23.7
1.85
0.29
25.5
2.72
0.46
26.5
2.27
0.38
41.6
2.78
0.47
43.5
2.62
0.44
45.7
2.59
0.44
47.4
2.70
0.43
49.3
2.49
0.39
65.4
3.48
0.59
67.4
3.75
0.60
71.0
3.69
0.59
73.3
3.66
0.59
90.7
4.26
0.69
95.0
4.29
0.69
99.2
4.14
0.67
185.4
5.08
0.83
189.7
5.24
0.85
192.6
4.86
0.79
209.6
4.95
0.80
212.7
4.65
0. 75
214.4
4.36
0.71
233.7
4.74
0.77
239.0
4.53
0.76
240.4
4.07
0.68
256.0
3.99
0.66
259.4
4.16
0.69
262.4
4.07
0.68
280.5
4.69
0.78
280.5
4.83
0.80
280.5
4.56
1.09
287.3
4.70
0.78
303.2
3.72
0.62
309.0
3.53
0.56
327.0
3.70
0.59
353.4
3.84
0.62
449.6
3.77
0.60
449.6
4.12
0.66
495.6
4.28
0.69
495.6
4.32
0.70
a Calculated by propagation of error from slope and intercept of
calibration r.urve and replicate injections of sample (cf. Peters, Hayes,
Hieftje, 1976)
-------
4-11
Table 4.4. Data from Experiment 3.a
Transfer of HCB from air to 0.01 P. NaCl with 1.0 mg/L humic matte
Time (hr) Concentration (^g/D
0.0
0.00
27.0
3.00
93.7
6.20
142.2
6.64
195.0
5.64
267.7
5.39
307.0
5.34
359.5
5.44
431. 5
5.40
475.5
6.26
526.5
5.99
629.0
5.07
Transfer of HCB from air to 0.01 M NaCl with 50.0 mg/L huinic matter
Time (hr) Concentration (jig/L)
0.0
0.00
20.5
4.61
45.7
5.04
113.7
11.00
138.6
11.16
163.1
13.46
187.6
13.55
204.8
14.35
276.8
14.69
329.7
15.73
348.4
16.72
469.3
16.47
a Standard deviation estimated at 0.7 pg/L for all determinations
-------
A-12
Table 4.5. Equations' used Co defino models. The object function, of which the
weighted sum of squares was minimized, was Y(i) - C(i) -CexP(i). The
function C(i) Is given balow; the values of CexP(i) are given in Tjables
4. 2-it . 5.
a. Experiment
Determination of C by uptake of HCB in etliylacetate.
O
ct(i)
c„ Q.
8 8
i
E
J-l
c,(j-D
where:
<^(1), c(i)
Cg - 0.554 nmol/L
Q - 0.463 mL/s,
O
Vx - 3.00 L,
Va - 0.100 L,
time series of concentrations in the liquid phase;
concentration in gas phase (Table 4.6)
gas flow rate
volume of liquid In sample vessel
volume of sample withdrawn for analysis (and
replaced with HCB-free ethyl acetate)
B. Experiments 2. and 3. Determination of C^(eq) and k' (kJ.ner.ic model) for
uptake of HCB in aqueous solutions.
Cj ( i) - C^eq) + [C1(l-l)[l-Va/V1] - C^eq)] exp( - k' [ t ( i) -1< i -1) ] |
where:
C^(i), t(l) time series of concentrations in the liquid phase;
Q - 0.463 mL/s, gas flow rate
O
- 2.90 L, volume of liquid in sample vessel
V. - 0.100 L, volume of sample withdrawn for analysis (and
a
replaced with HCB-free aqueous solution)
-------
4-13
Table 4.6. Optimal values of parameters C(eq) and k' found for model.
Experiment It' (SD)b C(eq) (SD)b
[s"1) (nmol/L]
1. Uptake 0.554c (0.007)c
HCB
Ethyl Acetate
2. Uptake 5.8E-6 (0.7E-6) 18 (1)
HCB
Water
3a. Uptake B.7E-6 (2.8E-6) 21 (1)
HCB
Water
Huroate
1 mg/L
3b. Uptake 2.5E-6 (0.2E-6) 62 (2)
HCB
Water
Huraate
SO mg/L
a Values In table determined by weighted nonlinear least squares analysis of
experimental concentration vs. time data, corrected for sampling volume,
according to equations in Table 4.5.
b Standard Deviation determined from propagation of error through weighted
nonlinear least squares
c In this case the concentration of HCB in air.
-------
4 -14
Table 4.7. Summary of Results8*'3
Vapor pressure of HCB at 295 K
1.37 (0.02) mPa
Saturation concentration of HCB in air at 295 K
0.554 (0.007) nmol/L
Delivery race of HCB with apparatus
0.26 pmol/s
Saturation concentration of HCB in water at 295 K
18 (1) nmol/L
Saturation concentration of HCB in water with 1 mg/L humate at 295 K
21 (1) nmol/L
Saturation concentration of HCB in vater with 50 mg/L humate at 295 K
62 (2) nmol/L
Transfer rate constant for HCB between gas and water in apparatus
5.8 x 10"6 (0.7 x 10'6) s'1
a The numbers in parentheses are the standard deviation determined from
propagation of error through weighted nonlinear least squares.
^ In this table, the results that are discussed in detail in the text
summarized for convenience. Several importanc qualifications on the accuracy
and precision of these values are discussed in Che text; these values should
not be cited or used without a full appreciation of these qualifications.
-------
4-15
Table 4.8. Results of Uptake Experiments®
Experiment H'' SD fb SD Kh k SD
h *
(L / rag!
2. Uptake 0.031 0.03 1.2 0.2
HCB
Water
3a. Uptake 0.026 0.01 2.0 0.8 0.17 0.17
HCB
Water
Humate
(1 mg/L)
3b. Uptake 0.0089 0.0005 1.7 0.3 0.05 0.01
HCB
Water
Humate
(50 mg/L)
a Values in table calculated from values in Table 4.6 and equations
discussed in text. The values are:
H'' the (effective) Henry's constant (Equation 3.24);
f the fractional approach to equilibrium of a bubble (Equation 4.4);
the effective association constant of HCB with humate (Equation 3.23).
k As discussed in the text, values of f significantly greater than one
are physically unreasonable, indicating error in the data or insufficiency of
the model.
-------
4-16
Vapor pressure and gas phase concentration at saturation. The first
quantity to be determined is the vapor pressure and gas phase saturation
concentration of HCB. For this determination, Experiment 1 was conducted:
air was passed over solid HCB and then through ethylacetate. If (i) the
vapor is in equibrium with the solid, and (ii) all of the HCB is extracted
from the bubbles during their passage through ethylacetate, then the
saturation concentration can be calculated from the rate of change of
concentration of HCB in the ethylacetate:
dC, Q C
--1- (4.1)
dt V
and the vapor pressure from the ideal gas law:
p - C& R T (4.2)
The validity of the assumptions (i) and (ii) above have been supported
qualitatively by other experiments in which the gas flow rate was varied,
and the linearity of the increase in concentration of HCB in ethylacetate
with time. The vapor pressure found from Equations 4.2 was 1.37 mPa,
which agrees well with other determinations (Table 2.1),
These data indicate that HCB can be delivered at unit activity at
the rate of
dN .
C Q - 0.2o pmol s (4.3)
dt 88
A major sorce of uncertainty in the number reported for this concentration
is in the control of the temperature of the column with the solid HCB,
since it was not thermostatted.
-------
4-17
Solubility and Henry's Constant. The next step is to calculate the
water solubility and the dimensionless Henry's Law constant. The water
solubility found from Experiment 2 (Table A.6), v;as combined with ^the gas
solubility from Experiment 1 to yield the Henry's Law constant. The value
of H' found from these c'ata and Equation 3.2 was 0.031 (0.003). This
value is within the range of those reported in the literature. The
relative standard deviation is calculated to be approximately 10%.
Kinetics. The uptake of HCB by aqueous solutions is shown as a
function of time in Figure 4.2. The experimental data and the model,
which is defined in Tables 4.5 and 4.6, are shown. The effect of sampling
on the concentration is quite evident.
The kinetic constant k' is composed of two factors, one of which is
related to the degree to which a bubble approaches equilibrium with the
liquid during its passage through the water column. This approach to
equilibrium can be quantified in terms of the quantity f, defined in
Equation 3.16 as the actual change in the concentration of the bubble from
entry to exit, divided by the change if the bubble left at equilibrium:
cb(t-°) - cb(tb) , Wb , V „ „
f - - 1 - exp( ) - (4.4)
C^t-O) - Cb(eq) H' Vb H'Qg
For Experiment 2 the value of f calculated from Vj_, k', H', and Qg is
1.3 (0.3). A value of f greater than one is physically unreasonable,
indicating that the extraction of HCB from a bubble exceeds that predicted
if the bubble leaves the solution at equilibrium. Therefore we interpret
this value of f to mean that the bubbles leave the solution very close to
equilibrium with the solution. An alternative interpretation, involving
the a priori assumption of equilibrium between bubbles and solution, is
examined at the end of this section.
-------
4-18
60
m
40
20
2000
1000
0
t (ks)
Figure U.2. Uptake of HCfi by 0.01 M NaCl { . ) ; by 0.01 M NaCl vith
1 mg/L huroate ( • ); by 0.01 H NaCl with 50 mg/L humate ( a ); and the
kinetic model (solid lines). Data points with error bars (standard
deviation of mean) are shown. The solid lines are the model
calculations. The experimental conditions and equations of the model are
given in Table 6.5; the adjustable parameter values used In the model are
given in Table 6.6. The discontinuities in the calculated concentrations
are due to the volume removed for analysis.
-------
4-19
Association with Humate. Experiment 3 allows the association of HCB
with humate to be assessed. The experimental data and the model for the
uptake of HCB in solutions containing humate are shown In Figure 4.2.
From these data both the equilibrium association constant of HCB with
humate and the uptake rate can be found.
From the values of C(eq) from in Table 4.6 and Equation '1.23, the
operational contents of - 0.17 and 0.05 L mg"^ are determined for
the 1 mg/L and 50 mg/L solutions, respectively. Only 14% of the HCB is
bound in the first solution and 70% in the second. Considering the low
degree of binding the lack ol agreement between these two parameters is
not too disturbing; the data show that binding of HCB by this humic
material is significant only at relatively high concentrations of hjmate.
According to the arguments presented in Part 3, the effect of
humates on transfer kinetics can be assessed by using an effective Henry's
constant (Equation 3.25) in the kinetics equations, Equations 3.20 and
3.19. From this approach it is expected that complexatlon by humates will
have no effect or decrease the rate constant. A decrease appears to be
apparent in the data presented in Table 4.6.
However, when the value of f is calculated (Table 4.8), It is again
significantly greater than one, ;he maximum physically reasonable value.
This value of f suggests that the extraction of HCB from a bubble exceeds
the equilibrium concentration in the water. Two alternatives to this
physically unreasonable situation seem most plausible: (1) the assumption
in the model are incorrect, or (11) on account of experimental error, the
values of f are not significantly different from one, And the bubbles are
actually very close to equilibrium with the solution.
-------
4-20
One of the assumptions In the model was that HCB was homogeneously
distributed through the solution. However it is known tht humlc material
is enilched at the air-water interface. If the humic material in (these
tests was selectively accumulated at the bubble-water interface, the
association of HCB would occur to a higher degree than 4n the bulk of
solution, and the extraction from the bubble would occur at. a. rate faster
than predicted on the basis cf bull: concentrations. A quantitative model
of this process can be worked out, but the limited amount of data
available and the number of unknown parameters makes this pursuit of
little benefit at this time.
Equilibrium Assumption. The values of f in Table 4.8 are all
greater than one, the maximum physically reasonable value under the
assumptions of the model. Another way of interpreting these values of f
is that the Initial uptake of HCB by the solution is too rapid to be
consistent with the relatively low plateau concentration attained with
time. Among the many possible explanations of this apparent
inconsistency, two appear to be most plausible: (i) the vapor phase
concentration varies with time due to temperature fluctuations, and (ii)
the bubbles really do leave The vessel at equilibrium with the liquid, and
the anomalously high values of f are simply a result of covarlar.ce among
the two adjustable paraneters, Cfuq) and k'. If the first explanation is
correct, the interpretation of the dat.-i p,iv«:
-------
4-21
The equations for this equilibrium model are presented in Table A.9,
the values of the adjustable parameters in Table A.10, and the function
superimposed on the data in Figure 4.3. It is seen that for the {feiueous
solution (no humates) the equilibrium model fics the data about as
satisfactorily as the kinetic model, and the factor f has a plausible
value. For both of the. humate solutions, the value of C(eq) is
unreasonably high, in an attempt to match the high initial uptake rate.
Thus our tentative conclusions are: (i) the equilibrium model is
satifactory for the solution without humate; (ii) the kinetic model is
necessary to describe uptake in the presence of humic material. Callaway
et si. (1984) reached similar conclusions in a study of more volatile
compounds. The uptake rate in th<3 presence of humlc material Is faster
than predicted on the basis of the equilibrium model, wic.h adjusted Henry's
constant, suggesting the enhancement of uptake due to an enrichment of
humic material at the air-water interface. The validity of these
conclusions is subject to uncertainty about the effects of small
variations in te.-:pertature.
-------
4-22
Table 4.9. Equation used to define equilibrium model. The object function, of
which the weighted sum of squares was minimized, was Y(i) C(l) -CexP(i).
The function C(l) Is given below; the values of CexP(i) are given in Tables
4.3-4.4.
Experiments 2. and 3. Determination of C1(eq) (equilibrium model) for uptake
of HCB in aqueous solutions.
Q C (eq)
C (i) - C (eq) + [C (l-l)[l-V /V.] - C.(eq)] exp{- [t(l)-t(i-l)])
i I I a I J.
i 1 E
wliere:
C^i), t(i)
Q - 0.463 niL/s,
O
C - 0.554 nrnol/L
6
Vx - 2.90 L,
Va - 0.100 L,
time series of concentrations in the liquid phase;
gas flow rate
concentration of HCB In gas phase
volume of liquid in sample vessel
volume of sample withdraw! for analysis (and
replaced with HCB-free aqueous solution)
-------
4-23
Table 4.10. Optimal v-.lues of parameters C^(eq) found for equilibrium
model.
Experiment
C(eq) (SD)1
[nraol/L]
2.
Uptake
HCB
W.i te r
3a.
Uptake
HCB
Water
Humate
1 mg/L
3b.
Uptake
HCB
Water
Humate
50 mg/L
20 1
22 1
91 U
a Values in table determined by weighted nonlinear least squares analysis of
experimental concentration vs. time data, corrected for sampling volume,
according to equations in Table 4.9.
k Standard Deviation determined from propagation of error through weighted
nonlinear least squares
------- |