PB91-196493
Interaction of Vapour phase Organic Compounds with Indoor Sinks
Acurex Corp., Research Triangle Park, NC
Prepared for:
Environmental Protection Agency, Research Triangle Park, NC
1991
-------
AEERL-P-661
I. REPORT NO.
EPA/600/J-qynfiQ
TECHNICAL REPORT DATA
(Please rsad fnsimc lions on the reverse before com pie ft
4. TITLE AND SUBTITLE
The Interaction of Vapor Phase Organic Compounds
with Indoor Sinks
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7.AUTHon(s)B.Tichenor (EPA);
(Univ. of AK), L. Sparks (E
(Acurex)
,_ Z. Guo (A cur ex), J. Dunn
EPA), and M. Mason
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Acurex Corporation
P. O. Box 13109
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO,
68-C2-4701 (Acurex)
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air and Energy Engineering Research Laboratory
Research Triangle Park, NC 27711
13. TYPE OF REPORT MND PERIOD COVERED
Journal article; 6/89-2/90
14. SPONSORING AGENCY CODE
EPA/600/13
is. SUPPLEMENTARY NOTES AEERL project officer is Bruce A.
2991. Indoor Air, 1, 23-35 fl99l).
Tichenor, MD-54. 919/541-
te. ABSTHACTThe paper reports on the development of sink models based on fundamental
mass transfer theory. The interaction of indoor air pollutants with interior surfaces
(i. e., sinks) is a well known, but poorly understood, phenomenon. Studies have
shown that re-emissions of adsorbed organic vapors can contribute to elevated con-
centrations of organics in indoor environments. Research is being conducted in
small environmental test chambers to develop data for predicting sink behavior.
The results of experiments conducted to determine the magnitude and rate of adsorp-
tion and desorption of vapor phase organic compounds for several materials are
presented. Five materials were evaluated: carpet, painted wallboard, ceiling tile,
window glass, and upholstery. Two organic compounds were tested with each mater-
ial: tetrachloroethylene (a common cleaning solvent) and ethylbenzene (a common
constituent of petroleum based solvents widely used in consumer products). Results
of the experimental work show the relevant sink effect parameters for each material
tested and compare the sorptive behavior of the two organic compounds evaluated.
An indoor air quality model was modified to incorporate adsorption and desorption
sink rates. The model was used to predict the temporal history of the concentration
of total vapor phase organics in a test house after applying a wood finishing product.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.iDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution
Mathematical Models
Mass Trant'er
Organic Compounds
Vapors
Test Chambers
Pollution Control
Stationary Sources
Indoor Air
Sinks
13B
12A
14G
07C
07D
14B
P. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report}"
Unclassified
2O. SECURITY CLASS (This page)
Unclassified
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EPA/600/J-91/069
PB91-196493
Indoor Air. I. 23-35 (1991)
1991 Danish Technical Presi DK-Copenhogen
imic
The Interaction of Vap
Compounds with Indoor Sinks
Bruce A. Tiehenor1, Zhishi Guo\ James E. Dunn3, Leslie E. Sparks'
and Mark A. Mason2
Abstract
*The interaction of indoor air pollutants with ulterior
surfaces (i.e., sinks) is a well known, but poorly under-
stood, phenomenon. Studies have shown that re-emis-
sions of adsorbed organic vapours can contribute to ele-
vated concentrations of organic* in indoor environ-
ments. Research is being conducted in small environ-
mental test chambers to develop data for predicting
sinfe behaviour. Tliis paper reports on the development
of sink models based on fundamental mass transfer
theory. The results of experiments conducted to deter-
mine the magnitude and rate of adsorption and desarp-
tion of vapour phase organic compounds for several
materials are presented. Five materials were evaluated:
carpet, painted waliboard, ceiling tile, window glass,
and upholstery. Two organic compounds were tested
with each material: tetrachloroethylene (a common
cleaning solwni) >'nd ethyibenzene (a common consti-
tuent ofpetrakum-oased solvents widely used in con-
sumer products). The results of the experimental work
are presented showing the relevant sink effect para-
meter for each material tested and comparing the sorp-
tax behaomiT of the mo organic compounds evaluat-
ed. An indoor air quality (IAQ) model was modified
to incorporate adsorption and desorption sink rates.
The model WK used to predkt the temporal history of
Manuscript received: 10 April 1990
Accepted for publication: 24 September 1990
1 U.S. Environmental Protection Agency
Air and Energy Engineering Research Laboratory
Indoor Air Branch (MD-54)
Research Triangle Park, NC 27711, USA
' Acurex Corporation
Environmental Systems Division
Research Triangle Park, NC 27709, USA
' University of Arkansas
Department of Mathematical Sciences
Fayeiteville, AR 72701, USA
the concentration of total vapour phase organia in a
test house after application of a wood finishing pro-
duct. The predicted results are presented and compared
to measured values. Suggestions for further research on
indoor sinks are presented.
KEYWORDS:
Indoor sinks, Vapour phase organics, Adsorption,
Desorption, Mass transfer, IAQ models
Introduction
The interaction of indoor air pollutants with
interior surfaces (i.e., sinks) is a well known
phenomenon. While it is possible that the
mass transfer mechanism could be absorp-
tion (i.e., for a water soluble compound to a
moist surface), this paper assumes that the
mechanism is adsorption. Researchers have
developed models and data to deal with the
adsorption of particles and NOj to a variety
of indoor surfaces (Nazaroff and Cass, 1989;
Yanagisawa et al., 19S7) and, for NO2, the
generation of NO from the surface has also
been reported (Billick and Nagda, 1987; Spi-
cer et aL3 1989). Berglund et al. (1988) pres-
ented an extensive review of the literature on
adsorption and desorption of vapour phase
organic compounds in indoor environments,
including data on a ventilation system heat
exchanger. The same authors reported that
organic compounds adsorbed on building
materials in a 7-year-old preschool were re-
emitted (i.e., desorbed) when placed in a
small environmental test chamber ventilated
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24 8. Tichenor el a!.; VOC Interactions with Indoor Sinks
with clean air (Berglund et al., 1989). Zsol-
nay el al. (1987) reported on the transport of
Hndane vapours between treated wood and
cotton cloth. Gebefugi and Korte (1989) ex-
tended this research to include other fungi-
cides and additional textile materials. The
Danish Town Hall Study provided empirical
evidence that re-emission of indoor pollu-
taots, including vapour phase organies, from
fleecy (i.e., large surface area) materials may
contribute to the Sick Building Syndrome
(Nielsen, 1987; Nielser., 1988; Valbjorn and
Skov, 1987). Seifen and Schmahl (1987) re-
ported on research to determine the adsorp-
tion and desorption behaviour of several or-
ganic compounds and several surface mater-
ials, but concluded "... no rule can be de-
disced to predict the sorption behaviour of
different VOC with different materials".
Dunn and Tichenor (1988) presented data
showing the effect of sinks in small environ-
mental test chambers used to determine
emission rates of organic compounds for la-
tex caulk and moth repellant, and they illu-
strated the use of adsorption and desorption
rate constants in mass balance models- Ti-
chenor et al. (1988) reported on studies in an
IAQ test house where para-dkhlorobenzene
was measured in significant concentrations
several days after the source (moth repellant)
was removed. Measurements in the same test
house, taken 12 months after additional
moth repellant emission tests, showed con-
centrations of para-dichlorobenzene well
above the outdoor concentration, indicating
the re-emission of para-dkhlorobenzene
previously adsorbed on interior surfaces.
In view of the significant impact of sinks
on indoor concentrations of vapour phase or-
ganic compounds, research is being conduc-
ted to develop data for predicting sink beha-
viour. This paper reports on the develop-
ment of sink models and the results of ex-
periments conducted to determine the mag-
nitude and rate of adsorption and desorption
of vapour phase organic compounds for sev-
eral materials. In addition, an IAQ model
was modified to incorporate adsorption and
desorption sink rates, and the model was
used to predict the temporal history of the
conce .ration of total vapour phase organics
in a test house after application of a wood
finishing product. The concentrations pre-
dicted by the model are presented and com-
pared to measured values. Finally, sugges-
tions for further research on indoor sinks are
presented.
Theory
Adsorption Isotherms
The study of adsorptive/desorptive beha-
viour of gas molecules with solid surfaces has
resulted in several well known adsorption
isotherm equations (Daniels and Alberty,
1961).
The Langmuir isotherm assumes a mono-
layer of molecules on a homogeneous surface
with all adsorption sites mutually indepen-
dent and identical (i.e., all sites require the
same energy to adsorb molecules). At equi-
librium, the Langmuir isotherm can be rep-
resented by:
k,Q(l-8) = k/6
(1)
where ka is the adsorption rate constant; Ce is
the equilibrium concentration of die adsor-
bate in the gas phase; k/ is a desorption con-
stant; and 8 is the proportion of available ad-
sorption sites occupied. For the low vapour
concentrations of single compounds used in
the experiments discussed below, it can be
assumed that the occupied sites are a very
small fraction of the available sites (ie., 8
< < 1). Thus, equation (1) can be simplified
to a linear form:
= k/0
(2)
Since the equilibrium mass per unit area on
the sink, M*, is directly proportional to 8, it
follows that:
k,Cc = k,]Ms (3)
where kd is the desorption rate constant. For
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B. Tichenor el ol.: VOC Interactions wilh Indoor Sinks 25
Moss {*}
to chamber
Chamber
k'J
Sink
(x-y-w)
f.
(w)
Fig. 1 T\ist chamber schernolsc
with sink: mass balance equa-
tions
Exit (y)
x = Mass to chamber over time t {dx/dt = R(t)]
y = Mass from chamber over time t [dy/dt =kz (x-y-w)}
w = Moss in the sink [dw/dt = k3(x—y—w)—k4w]
a sink with an area of A, the mass balance
equation is:
(4)
Also, an equilibrium constant, k,., can be de-
fined as:
= MJCC
(5)
The Freundlich isotherm, originally deve-
loped as an empirical fit to non-Langmuir
isotherms, was later derived by assuming an
exponential distribution of adsorption site
energies. The Freundlich isotherm can be
represented by:
m = kC
(6)
where m is the mass adsorbed per unit of ad-
sorbent and k and a are empirical constants.
The BET (Brunauer-Emmett-Teller) iso-
therm provides for multilayer adsorption.
For the low vapour concentrations encoun-
tered in indoor environments, it is assumed
that the available adsorption sites will not be
covered, so the BET isotherm is probably
not appropriate.
Application to Chamber Testing
Dunn and Tichenor (1988) proposed a set of
ordinary differential equations to describe
the mass balance m a test chamber using the
schematic shown in Figure 1:
dx/dt = R(t)
dy/dl = k2(x-y-w)
dw/dt = k3(x-y-w) -
(7)
(8)
(9)
where x is the mass that enters the chamber
over time t; y is the mass that exits the
chamber over time t; w is the mass in the
sinks R(0 is the *nPut rate to *e chamber; k2
is the air exchange rate, N; k3 is the rate con-
stant to the sink; and kj is the rate constant
from the sink.
Expressing the sink term in a form consis-
tent with the linear form of Langmuir's
equation gives:
dw/dt = k,CV-k4MA (10)
where the concentration in the chamber, C
= (x-y-w)/V; V is the chamber volume; M is
the mass per unit area in the sink; and A is
the sink area, (Note that w = MA.)
At equilibrium, dw/dt = 0, Thus,
k3CV = k
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26 B. Tichenor et a!.: VOC Interactions with Indoor Sinks
Substitution of equations (7), (8), and (10)
and replacing ka and k4 yield:
VdC/dt = RdHsfCV-k.CA + kjMA (13)
or
dC/dt =
R(t)/V-NC-k4CA/V + k.,M A/V (14)
Substituting L = A/V in equation (14) gives:
dC/dt = P.(t)/V - NC - kaCL + kdML (15)
Appropriate units for equation (15) are:
C = concentration, mg/mj
t = time, h
R(t) = emission rate of source, mg/h
V = chamber volume, m3
N = air exchange rate, h"1
ka = adsorption rate, m/h
L = ratio of sink area to chamber volume,
m"1
kd = desorption rate, h'
M = mass per unit area in sink, mg/m2
Substituting for k3 and ki in equation (Ib;
yields:
AdM/dt = Ak,C-AkdM
or simply
dM/dt = kaC
(16)
(17)
Thus, if Langmuir adsorption processes are
assumed, equations (15) aiid (17) describe
the rate of change of concentration and mass
in the sink, respectively, for small chambers.
Experimental Methods
Experiments to determine adsorption and
desorption rates for vapour phase organic
compounds were conducted in small (53
litre) environmental test chambers (Tichenor
et al.31988; Tichenor, 1989).
Test Materials and Compounds
Five sink materials were evaluated: (a) carpet
- nylon fibre pile bound with a styrenebma-
diene latex to a jute backing; (b) wailboard
(gypsum drywaU) - finished with an interior
latex paint; (c) ceiling tile - 1.5 cm thick
composite material (mineral fibre, starch,
clay, gypsum perlite paper fibre, silicate); (d)
window glass; and (e) upholstery - throw pil-
low with 50% polyester/50% cotton fabric
covering a polyester fibre fill. Each material
was tested with two organic compounds: (a)
tetrachloroethylene (perchloroethylene) - a
common cleaning solvent and (b) ethylben-
zene - a common constituent of petroleum-
based solvents widely used in consumer pro-
ducts.
Test Procedures
A sample of the sink material was placed in a
test chamber supplied with clean, humidified
air (45% RH, 23 °C) at a nominal rate of 1 air
change per hour (ACH). At the start of each
test, a portion of the inlet flow was replaced
by flow from a constant temperature oven
that held a permeation tube containing the
test compound. The test compound was ad-
ded to the chamber inlet flow at a constant
rate for 48 hours, then the flow from the per-
meation oven was shut off and replaced by
flow from the clean air system.
The concentration of the test compound
was monitored at the outlet of the chamber
by gas chromatography. Compounds were
identified by retention time and quantified
by electronic integration of detector re-
sponse. Tetrachloroethylene was quantified
using an electron capture detector (ECD);
ethylbenzene was quantified using a flame
ionization detector (FID). Samples were col-
lected using gaslight syringes, automated gas
sampling valves equipped with fixed volume
sample loops, or solid sorbent sampling
tubes containing graphitized carbon adsor-
bents. Syringe and sampling loop samples
were injected directly to the packed column
of a gas chromatograph equipped with both
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B. Tichcnor el ol: VOC Interactions with Indoor Sinks 27
*E
j[
c
b
u
u
0
O
9-
E -
7-
6-
5-
4 •
3 -
2-
1 -
0 •
Adsorption
icol "No Sink" Curve
sorpllan
v,
1 _*_ ' , — i r-f* ran 1i
Fig. 2 Chamber test (etbyiben-
zeno/eorpcl): adsorption and
desorplion represented by
crosshatciied areas
20
40 15 60
Time (hrs)
FID and ECD- Sorbent samples were therm-
ally desorbed first to a cryogenic trap and
then to a gas chromatograph equipped with a
fused silica column and an FID-
Samples were collected as frequently as
possible for the first few hours in order to
define the rising portion of the concentration
vs. time curve {see Figure 2). Once the con-
centration reached an apparent equilibrium
value, the sampling frequency was decreased.
Forty-eight hours after the start of the test,
the injection of the organic compound was
terminated. At this point, the sampling fre-
quency was again increased. Sampling conti-
nued until the concentration reached the
quantification limit of the method.
Test Conditions
A wide variety of test conditions was used.
Initial testing was conducted on carpet with
tetrachloroethylene to investigate the effect
of concentration and temperature on the
sorption behaviour. Subsequent tests with
the other sink materials and with ethylben-
zene %vere conducted under fewer conditions.
Tests to determine the sink behaviour of
empty chambers were also carried out Table 1
summarizes the conditions for all the tests.
AH tests were conducted at a nominal 1 ACH
and a water vapour content equivalent to
45% relative humidity at 23 °C [Note that te-
trachloroethylene is designated as PCE (per-
chloroethylene) to avoid confusion with
TCE (trichloroethylene).]
Data Analysis
Data obtained from the small chamber tests
were analysed assuming a Langmuir type ad-
sorption. These analyses yielded values for:
Mc, the mass per unit area on the sink at
equilibrium; ka, the adsorption rate constant;
kd, the desorption rate constant; and k,., the
equilibrium constant.
Determination of
Given the experimental procedures described
above, the mass in the sink is assumed to be
at equilibrium at the time (Q the flow of the
test compound is stopped (see Figure 2). The
total mass of the test compound exiting the
chamber from time i^ to the ?nd of the test is
the sum of (a) the mass of -ie compound in
the chamber air at time t> and (b) the mass of
the compound emitted from the sink. The
total mass exiting the chamber from t, to the
end of the test is obtained from the area
under the curve (Ac) described by the data
points multiplied by the air exchange rate
(N) and the chamber volume (V). In this
case, the AC was calculated using the trape-
zoid rule. The mass of the compound in the
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28 B, Tichenor ct at.: VOC Inter actions with Indoor Sinks
Table 1 Tesl conditions for sink experiments
Material
Carpet
Wallboard
Ceiling
tile
Window
glass
Upholster}'
(pillow)
Empty
chamber
Tetrachloroeihylene
chamber air is the
don at equilibrium
lumck f\f\ tt^tlc*
Compound
PCE1
PCE
Ethylbenzene
PCE
Elhylbenzene
PCE
Elhylbenzene
PCE
Elhylbenzene
PCE
Ethylbenzene
PCE
PCE
Elhylbenzene
= Perchloroethylene (PCE)
Concentration
range (rog/m")
6-50
14-15
8-9
19-26
il-13
23-28
10-11
19
10-11
28-29
8-11
5-38
12-13
10-11
Temperature
<°C)
23
35
2?
23
23
23
23
23
23
23
23
23
35
23
No. of
tests
10
2
2
2
2
2
2
1
2
2
2
4
2
2
product of the concentra- where
(Ce) and the chamber vo
fN
— V" ~*~
icl +kVHiN + k_I +
^a1-* *SJ/ 11* «Vjij
. V.N. va
*SJ.T'»*"NJJ /">| \
= (A^NV -
(18)
where M,. is the equilibrium mass per unit
area (mg/m2) in the sink at time t^.
Determination of k,, and kd
Equations (15) and (17) can be solved analy-
tically for certain simple cases. If the portion
of the concentration vs. time curve from I, to
the end of the curve is considered., the initial
conditions are: t = 0, QO) = C« and M(0)
= M. = C^ka/kd) [from equation (3)J. For
these conditions the solutions for equations
(15) and (17) are:
C(t) =
M(t) =
(20)
Values for k, and k,t were obtained by fitting
equation (19), where Q is a known experi-
mental value, to the chamber concentration
vs. time data from t, to the end of the test
using a nonlinear regression curve fit rou-
tine; in this case, NUN by SAS (1985). Fig-
ures 3 and 4 show the data and the fitted
Langtnuir adsorption curve for two of the
experimental runs (ethylbenzene/wallboard
and tetrachloroethylene/carpet); the theoreti-
cal "no-sink" curve, defined by C(t)=CceMl,
is also shown.
Results
The experimental results, including values
for Q, Ma ka kd, and k« are presented in Ta-
bles 2-5. The relative standard deviations of
the estimates for ka and kd ranged from 7%
to 20%, with an average of 14%. Table 2 pro-
vides the results for carpet tested with tetra-
-------
B. Tkhenor cl ol.: VOC Interactions with Indoor Sinks
Data
ongmur Adsorption
No Sink" Curv«
Fig, 3 Chamber lesf (ethjrlben-
icnc/wollboordj: Langrnuir and
"no sink" predictions
B !0 12
Time
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30 B. Tichenor ci ol.: VOC Inleioctions wilh Indoor Sinks
Table 3 Experimental results - letrochloroelhytcne ond woltboard, ceiling file, and pillow
Sink
material
Wallboard
Ceiling lile
Pillow
ACH
0.982
0.994
1.03
1.03
0.923
0,925
Q
(nig/m1)
19.2
25.8
22.6
28.4
28.4
28.6
Mt
(rng/ni1)
2.67
3.79
3.68
4.61
632
8.17
k,
(m/h)
0.22
0.20
0.09
0,11
0,02
0.03
kd
1.6
1,4
0.54
0.69
0.09
0.1 1
k,
(m)
0.14
0-15
0.16
O.i6
0.22
0.29
1) The sample area (A) for the wallboard and ceiling tile was 0.14 m>; for the pillow, 0.266 m-'.
2) The effective chamber volume (V) for the wallboard and ceiling die tests was 0.051 m1; for the pillow,
0.046 m1.
Table 4 Experimental results - elhylbenzene and carpet, wollboard, ceiling tile, ond pillow
Sink
material
Carpet
Wallboard
M
Ceiling tile
Pillow
ACH
0.945
0.982
0.975
0.949
0.971
0.988
0,956
0.991
c,
(mg/m5)
8.2
8.6
11.4
127
9.8
11.2
7.6
10.7
M.
(mgtar)
7.79
8.29
3.40
3.83
4.30
4.26
1.73
3.41
k,
(w/h)
0.07
0.08
0.58
032
0.25
0.23
0.004
0-005
K,
(h-)
0.08
0.08
1.9
1.1
0.58
0.60
0.018
0.015
k.
(m)
0.95
0.96
0.30
0.30
0.44
0.38
0.23
0.32
f)° The sample area (A) for the carpet, wallboard, and ceiling tile was 014 m'; for the pillow^,0.266 m!-
2) The effective chamber volume (V) for (he carpet, wallboard, and ceiling ate iests was 0.051 m'; for the
Notes:
Th'
Thi
pillow, 0.046m'.
Table 5 Average adsorption and desorp»bn rate constants and equilibrium constants - Langmuir iso« ->rrn model
PCE1
Elhylbenzene
Rate
constant
k,(m/h)
kd (h"1)
Mm)
k,(m/h)
kj (h •')
Mm)
Carpet
0.13
0.13
0.97
0.08
0.08
0.95
Carpet
(35 °C)
0.30
0.59
0.52
—
—
Wallboard
0.21
1.5
0.14
0.45
1.5
0.30
Ceiling
tile
0.10
0.61
0.16
0.24
0.59
0,41
Pillow
0.03
0.10
0,25
0.004
0.016
0.27
•Teirachloroeihykne = Pcrchloroethylene (PCE)
Note: All lests were conducted at 23 °C, except where noted.
-------
B. Tithenor ct ol,: VOC Interactions with Indoor Sinks 31
60
„ 40
n
I
£ 30
20
10
Fig. 5 Cor relation between M.
ond C«: teiracWotocthylcnc/
carpel
10
20
30
50
60
ehloroeihylene; Table 3 presents the results
for the remainder of the tetrachloroethylene
experiments; Table 4 provides information
on the experiment conducted with ethyl-
benzene; Table 5 presents a summary of ihe
Langmuir rate constanis for all the tests.
Note that the tests conducted on the empty
chamber and on window glass showed no
sink effect, so no results are presented for
these experiments.
Discussion
Applicability of Langmuir Isotherm
Figure 5 presents a least squares plot of M«.
vs. Cc for tetrachloroethylene and carpet at
23 °C, The plot shows a linear relationship
with a slope of 0.97 and an r2 correlation of
0.97. This linearity confirms the applicabil-
ity of the Langmuir equilibrium at low con-
centrations represented by equations (3) and
(5) [i.e., M^kAijQ^ktCJ, with the slope
of the line equal to the average k,. value for
the 10 tests (see Table 5).
Fitting the kinetic data (i.e., C vs. t) using
equations (19) and (21) to obtain k, and kd
values based on a Langmuir sorption process
shows a good fit for ceiling tile and wall-
board (see Figure 3). On the other hand, the
fit of the data for the pillow and carpet daia
indicates that ihe desorption process for
these materials deviates from the Langmuir
assumptions (see Figure 4). Thus, it appears
that Langmuir kinetics apply to relatively
flat, smooth surfaces (e.g.( ceiling tile, wall-
hoard), but may be inappropriate for more
complex surfaces (e.g., carpet and upholst-
ery). In spite of the relatively poor fit of the
kinetic data for carpel and pillow, it is felt
that the values of ka, kj, and kc for these ma-
terials can be used to compare their sorption
behaviour with other materials and to com-
pare the sink effects of different compounds.
Sink Strength
The capability of a sink material to adsorb
indoor air pollutants is represented by k,.; the
higher the kc value, the greater the mass ad-
sorbed on the sink at a given concentration
(i.e., the stronger the sink). An analysis oi
variance (AOV) of the k,. values in Tables 2 -
4 shows a significant (p = 0.06) COM-
POUND X MATERIAL interaction (i.e.,
the differences in the kc values among the
materials depend on the compound). Carpet
was a significantly stronger sink (p < 0.01)
than the other three materials for both com-
pounds, while no significant differences in k«.
were detectable (p = 0.01) among the other
three materials for either compound. As sta-
ted previously, glass was not a sink for either
compound.
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32 B. Tichenor et ol. • VOC Interactions with Indoor Sinks
The sink strengths of the two compounds
tested arc also different. Ceiling tile adsorbed
significantly more (p < 0.01) cihylbeii/cne
than ictrachloroethyicnc. A similar trend (p
= 0.05) was evident for wallboard. For car-
pet and pillow, no differences (p = 0.79 and
p - 0.80, respectively) in sink strength were
observed between the two compounds.
Adsorption and Desorption Rates
Values ."or ka and kd were analysed to deter-
mine differences in adsorption and dcsorp-
tion rates for the materials and compounds
tested. An AOV indicated a significant MA-
TERIAL X COMPOUND interaction for k,
(p < 0.01), but not for kd (p = 0.99). Other
findings include:
- wallboard and ceiling tile adsorbed ethyl-
benzene at significantly higher rales (p <
0.01) than carpel and pillow. Elhylbenzcne
adsorption by wallboard was faster than by
ceiling tile; the adsorption rates for carpel
and pillow were not significandy different;
- wallboard adsorbed tetrachloroethylene at
significantly higher rates (p < 0.01) than
the pillow, with carpet and ceiling tile rep-
resenting intermediate, but indistinguish-
able, values;
- ethylbenzene adsorbed at a significantly
higher rate than tetrachloroethylene on
the wallboard (p < 0.01) and ceiling die
(p < 0.01). There were no significant dif-
ferences in the adsorption rates for the two
compounds for carpet (p = 0.22) and pil-
low (p = 0.68);
- wallboard desorbed at a significandy high-
er rate (p < 0.01) than the other three ma-
terials. Ceiling tile desorbed at a signific-
antly higher rate (p < 0.01) than carpet
and pillow, which are indistinguishable;
- there was no significant difference (p =
0.56) in the desorpdon rates for ethylben-
zene and tetrachloroethylene for any of the
sink materials.
Effect of Temperature
Only limited data were collected to investi-
gate the effect of temperature. Based on ther-
modynamics, both the adsorption and de-
sorption rates should be higher at elevated
temperatures. The values for k, and kj for the
tetrachloroethylene/carpet tests conducted at
35 °C indeed show significantly higher values
(p < 0.001) than those observed at 23 °C.
The significantly lower value of kc at 35 °C
occurred because the desorption rate was ele-
vated more than the adsorption rate.
Practical Implications
The data presented herein, while limited, do
confirm that common indoor materials ad-
sorb and subsequently re-emit vapour phase
organic compounds. The type of material
and compound affects the rates of adsorption
and desorption as well as the amount of ma-
terial adsorbed.
Recent experiments conducted at an in-
door air quality (1AQ) test house (Jackson et
al, 198?) provide further confirmation of the
importance of indoor sinks. A wood stain
was applied to a bare oak floor, and the con-
centration of total vapour phase organics
(based on sampling with Tenax/charcoal and
analysis with gas chromatograph/fiame ioni-
zadon detector) was followed over time as
shown in Figure 6.
An IAQ model (Sparks, 1988) was used to
evaluate the data. The emission rate of total
organics from the stain was calculated from:
ER,01 =
(22)
where, ERIO, is the total emission rate (mg/
m'-h); EI^ is the emission rate during the
application of the product (mg/rn2-h); ER» is
the initial emission rate of the drying phase
(mg/rn2 h); k is the decay constant for the
emission rate during drying (h"1); and t is the
time (h). The following values were used:
ER1W = 15,000 mg/mMt for the first 0.4
hours; ERo = 17,000 mg/m2-h; k = 0.4 tv1.
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B.lkhenor et ol.: VOC Interactions with indoor Sinks 33
1000
100
f
"5 A
fig. 6IAQ test house experiment
[wood slain): model predictions
too
200 300
Time (hrs)
The value for ER^p was developed from on-
going experiments; the values for ER,, and k
were obtained from chamber tests (Tichenor
and Quo, 1988).
Figure 6 shows the results of the model
runs for three cases; (a) assuming no sink ef-
fect; (b) assuming Laiipmuir type sinks; and
(c) assuming re-emitting sinks are non-
Langmuir (noted as "Best Fit"). Note that
the prediction assuming "no sink" fails to ac-
count for the increased concentration due to
the re-emitting sinks after the first few hours
of the test.
The curve labelled "Langmuir sink" was
predicted by the model with the sink beha-
viour described by equation (17) and using
the chamber derived values for k, and kd
based on the ethylbenzene tests. The test
house carpet values (ka = 0.08 m/h; kd =
0.08 h'1) were taken directly from the cham-
ber data. Single values for all wall and ceiling
surfaces (k, = 0.35 m/h; kd = 1.0 h'1) were
based on averages of the wallboard and ceil-
ing die chamber data. The "Langmuir sink"
curve does indicate re-emissions from the
sinks, but ii does not adequately predict the
time history of these re-emissions. The fail-
ure of the Langmuir isotherm data to accura-
tely predict the sink behaviour could be
caused by several factors:
- the ethylbenzene data do not properly ac-
count for the sink behaviour of the com-
plex petroleum-based solvent emitted
from the stain;
- the test house sink surfaces are different
and more complex than the surfaces evalu-
ated in the chambers; and
- the desorption behaviour is not adequately
defined by a Langmuir process. (Note the
previous discussion of the poor fit of the
chamber kinetic data for carpet and pil-
low.)
The curve labelled "Best Fit" was predicted
by the model assuming all sink surfaces be-
haved according to a Langmuir adsorption
process and a non-Langmuir desorption pro-
cess. The rate of change of mass in the sink
was described by:
(23)
where, k, and A have been previously de-
fined; C, is the concentration at time t, (mg/
m3);kd" is the desorption rate constant; M, is
the mass in the sink at time t (mg/m2); and n
is an empirical constant. Note that the de-
sorpdon portion of this equation is analo-
gous to a Freundlich isotherm described
previously [see equation (6)]. The following
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34 B. Tichenor ei ol.: VOC Inferddions with Indoor Sinks
values, obtained from analyses of the cham-
ber data for ethylbenzene/carpetj were used
in the IAQ model: ka = 0.1 m/h; k/ =
0.004; and n = 1.5. The fit of the model (see
the "Best Fit" line) is quite good for the sink
behaviour described by equation (23). The
theoretical basis for this equation is weak,
and farther research is required to refine this
approach.
Conclusions
Experiments conducted to examine the beha-
viour of indoor sinks vis-a-vis vapour phase
organic compounds have clearly shown ad-
sorption to and re-emission from common
indoor materials for two typical indoor air
pollutants. Of the five materials tested, only
window glass had no measurable sink effect.
Both the type of material and the type of or-
ganic compound affect the rates of adsorp-
tion and desorpuon, as well as the equilibri-
um mass in the sink. Higher temperatures
increase both the adsorption and desorption
rate constants. Fundamental Langmuir ad-
sorption theory appears adequate to describe
the behaviour of smooth materials, such as
wallboard and ceiling die. The desorption
kinetics of rough, complex materials, such as
carpet and upholstery, appear to be governed
by non-Langmuir processes. Experiments
conducted in an IAQ test house confirm the
importance of sinks in controlling the levels
of vapour phase organics over extended per-
iods. If appropriate adjustments are made to
account for non-Langmuir desorption pro-
cesses, IAQ models can adequately predict
the impact of sinks on the concentration/
time history in full scale (i.e., test house) en-
vironments.
Research Needs
Further research is needed to increase under-
standing of the behaviour of indoor sinks
with respect to vapour phase organic com-
pounds:
- examination of the equilibrium and kine-
tic behaviour of additional sink materials
and additional organic compounds, in-
cluding mixtures of vapour phase orga-
nics;
- evaluation of the behaviour of sinks under
a wider range of conditions (i.e., tempera-
tt -, humidity, concentration);
- examination of the behaviour of sinks ex-
posed to dynamic, non-steady source
emissions;
- development of sink models that account
for non-Langmuir desorption processes.
Such models should be capable of predict-
ing re-emissions from sinks over long per-
iods (e.g., for weeks or months);
- incorporation of sink models into IAQ
models and validation in full-scale indoor
environments. Methods for simplifying
complex, multiple sinks and for dealing
with vapour phase organic mixtures are
needed.
A preliminary version of this paper was presented
at ttie Fifth International Conference on Indoor
Air Qtiality and Comfort, Toronto, Canada, July
1990.
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B. Tichenor el at.: VOC Interactions with Indoor Sinks 35
References
Bcrgiund, B., Johansson, I., and LindvaH, T. (1988) "Ad-
sorption and desorption of organic compounds in in-
door materials", Proceedings of Heallhy Buildings '88,
Stockholm, Swedish Council for Building Research,
Vol. 3, pp. 299-309.
Bergtod, B., Johansson, I., and Lindvall, T. (1989) "Vo-
latile organic compounds from building materials in a
simulated chamber study", Environment International,
15,385-388.
Billick, I. and Nagda, N. (1987) "Reaction decay of ni-
trogen dioxide". In: Proceedings of Indoor Air '87, Ber-
lin, Institute for Water, Soil, and Air Hygiene, Vol. 1,
pp. 311-315.
Daniels, E and Alberty, R. (196!) Physical Chemistry, 2nd
ed., New York, John Wiley & Sons.
Dunn, J. and Tichenor, B. (1988) "Compensating for
sink effects in emissions test chambers by mathemati-
cal modeling", Atmospheric Environment, 22,885-894.
Gebefugi, I. and Korte, F. (1989) Indoor Accumulation of
Semivolatiles. Paper presented at 82nd Annual Meet-
ing of Air and Waste Management Association, Paper
No. 89-86.1, Anaheim, CA.
Jackson, M., Clayton, R., Stephenson, E., Guyton, W.,
and Bunch, J. (1987) "EPA's indoor air quality test
house, 1. Baseline studies". In: Proceedings of the 198?
EPAIAPCA Symposium on Measurement of Toxic and
Related Air Pollutants, EPA-600/9-87-010 (NTIS
PB88-113402), 104-108, Research Triangle Park, NC.
Nazaroff, W. and Cass, G. (1989) "Mass-transport aspects
of pollutant removal at indoor surfaces", Environment
Imermlional, 15,567-584.
Nielsen, E (1987) "Potential pollutants - their import-
ance to the Sick Building Syndrome - and their re-
lease mechanism". In: Proceedings of Indoor Air '87,
Berlin, Institute for Water, Soil, and Air Hygiene, Vol.
2, pp. 598-602.
Nielsen, E (1988) "The importance of building materials
and building construction to the Sick Building Syn-
drome", Proceedings of Healthy Buildings 'SS, Stock-
holm, Swedish Council for Building Research, Vol. 3,
pp. 391-399.
SAS Institute, Inc. (1985) SAS User's Guide: Statistics,
Version 5 Edition, Gary, NC.
Seifert, B. and Schmahl, H. (1987) "Quantification of
soiption effects for selected organic substances pres-
ent in indoor air". In: Proceedings of Indoor Air '87,
Berlin, Institute for Water, Soil, and Air Hygiene, Vol.
1, pp. 252-256.
Sparks. L. (1988) Indoor Air Quality Model - Version 1.0,
EPA-600/8-88-097a (NTIS PB89-133607), U.S. EPA,
Air and Energy Engineering Research Laboratory,
Research Triangle Park, NC.
Spicer, C, Coutant, R.» Ward, G., Joseph, D., Gaynor, A.,
and BilKck, I. (1989) "Rates and mechanisms of NO*
removal from indoor air by residential materials", En-
vironment International! 15,643-654.
Tichenor, B. (1989) Indoor Air Sources: Using Small En-
vironmental Test Chambers to Charaaaize Organic Emis-
sions from Indoor Miaeriah and Products, EPA-69Q/8-
89-074 (NTIS PB90-H0131), U.S. EPA, Air and En-
ergy Engineering Research Laboratory, Research Tri-
angle Park, NC.
Tichenor, B. and Guo, Z. (1988) "The effect of ventila-
tion on emission rates of wood finishing materials",
Proceedings of Heallhy Buildings 'SS Stockholm, Swed-
ish Council for Building Research, Vol. 3, pp. 423-
432.
Tichenor, B., Sparks, L., White, J., and Jackson, M.
(1988) Evaluating Sources of Indoor Air Pollution. Paper
presented at 81st Annual Meeting of the Air Pollution
Control Association, Dallas, TX, (Paper No. 88-
110.2).
Valbjom, O. and Skov, P. (1987) "Influence of indoor cli-
mate on the Sick Building Syndrome prevalence". In:
Proceedings of Indoor Air 87, Berlin, Institute for
Water, Soil, and Air Hygiene, Vol. 2, pp. 593-597.
Yanagisawa, Y, Fasano, A., Spengler, J., Ryan, E, and
Billick, I. (1987) "Removal of nitrogen dioxide by in-
terior materials". In: Proceedings of Indoor Air '87, Ber-
lin, Institute for Water, Soil, and Air Hygiene, Vol. 1,
pp. 376-380.
Zsolnay, A., Gebefiigi, I., and Korte, E (1987) "The use
of factorial design to estimate synergistic effects in the
environment". In: Proceedings of Indoor Air '87, Ber-
lin, Institute for Water, Soil, and Air Hygiene, Vol. 1,
pp. 265-269.
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