Environ. Sci. Technol. 1982, 16, 594-602
Reversible and Resistant Components of PCB Adsorption-Desorption: Isotherms Domlnlc M. Di Toro” and Lewls M. Horzempat
Environmental Engineering and Science Program, Manhattan College, Bronx, New York 1047 1 IResults
from laboratory equilibration studies indicate that sediment-adsorbed 2,4,5,2’,4‘,5’-hexachlorobiphenyl (HCBP) fractions may be comprised of both reversibly and strongly bound or resistant components. This evidence suggests that for many environmental modeling efforts it may be inappropriate to treat this and other PCB isomer adsorption reactions as either completely reversible or completely irreversible. The initial refinement of such models requires a means of estimating the relative magnitudes of the respective sorbed PCB fractions. To this end, a computational method has been derived to allow prediction of the magnitude of the reversible and more strongly adsorbed HCBP fractions from conventional isotherm data. The derivation, which is based upon experimentally observed linear HCBP isotherms, has also been applied to other organic molecules of similar isotherm character. This methodology provides an initial quantitative approximation of the strongly bound, resistant, sediment fractions while utilizing relatively simple experimental adsorption-desorption data. Results suggest that for montmorillonite clay, kaolinite clay, and natural lake sediment (Saginaw Bay, MI) linear isotherms effectively describe the relationships between both adsorbed HCBP fractions and the aqueous HCBP concentration. The implications of these and other model predictions for the cycling of PCB’s in natural waters are discussed.
Introduction The reversibility of the adsorption reaction between dissolved organic chemicals and naturally occurring soils, sediments, and suspended particles is of fundamental importance in the understanding of the fate of these chemicals in the environment. The strong particle-binding tendency of persistent organic chemicals such as DDT and PCB is well known, and its importance has been stressed many times (1-3). The issue of reversibility becomes critical if the adsorption-desorption behavior of a chemical is to be expressed quantitatively within the framework of mass balance equations. These are used in mathematical models of the transport of chemicals in soil column leaching laboratory and field experiments ( 4 ) and in models of the fate of chemicals in natural water systems (5-7). To date, with a notable exception to be discussed below, the formulations used to express the adsorption and desorption reactions assume reversible behavior, that is, at equilibrium the same isotherm applies for adsorption and desorption. The difficulty with this assumption is that for many organic chemicals and many naturally occurring adsorbents, laboratory adsorption and subsequent desorption experiments appear to demonstrate only partially reversible behavior. That is, the measured desorption does not conform to the adsorption isotherm. While these nonsingular or hysteric isotherms have been ascribed to a number of possible experimental artifacts, sufficient evidence exists to suggest that they do in fact reflect a fun+Present address: Envirosphere Corp., 2 World Trade Center, New York 10048 594
Environ. Sci. Technoi., Voi. 16, No. 9, 1982
damental property of the sorption process (8). Hysteretic isotherms were observed during laboratory studies utilizing an isomer of PCB, 2,4,5,2’,4‘,5’-hexachlorobiphenyl (HCBP), and various natural and clay adsorbents at low aqueous (1-100 ng/L) and adsorbent (10-100 mg/L) concentrations (9). The results of consecutive desorption studies suggested that this behavior was due to the existence of both readily reversible and more strongly adsorbed HCBP fractions. Although the longevity of the hysteresis effect requires further characterization, kinetic studies (to be discussed) have shown it to persist for environmentally significant time periods. Therefore, from a modeling perspective, there exists a need to account for this adsorption behavior in a quantitative and consistent way. It is the purpose of this paper to present a framework within which nonsingular behavior can be analyzed. Emphasis has been placed upon developing a mathematical treatment that can be easily incorporated into mass balance equations. While more mathematically rigorous .descriptions of the experimental HCBP data are possible, this treatment possesses the distinct advantage of being applicable to readily available experimental data. This presentation is restricted to linear isotherms since those have been found to be applicable to HCBP adsorptiondesorption. However, the basic framework is not confined to only this class of isotherms.
Theory A typical desorption experiment follows an adsorption experiment in which, at equilibrium, the adsorbed concentration r, = q / m (mass of adsorbate/mass of adsorbent), and the dissolved aqueous concentration, e,, are measured. The solids are separated, and the supernatant is discarded and replaced by adsorbate-free aqueous phase. At desorption equilibrium the adsorbed, ?-d, and aqueous, c d , concentrations are measured. If this procedure is repeated at constant temperature for different initial adsorbate concentrations, an isotherm results. The adsorption isotherm relates r, to c,, and for the linear case a partition coefficient for adsorption, 7ra, can be defined as the slope of the isotherm: r, = 7rac,. Similarly, if the desorption points fall on a straight line, a desorption “isotherm” can be defined, rd = ?T&, with “partition coefficient”, “d. The result is illustrated in Figure l a for HCBP adsorption-desorption with Saginaw Bay sediment (9).
Strictly speaking, it is incorrect to use the term isotherm for the desorption curve since in the presence of nonreversibility the desorption points obtained depend on the previous adsorption and the details of the desorption step rather than being strictly governed by the causal relationship rd = ?TdCd. However, with this restriction kept in mind, the procedure is well defined, and for the sake of a simple nomenclature, this curve is called a single desorption isotherm. Many examples of these isotherm pairs exist in the literature for various adsorbate-absorbent systems: dieldrin-montmorillonite (10);p,p’-DDT-humic acid (11); aldicarb, terbufos-soil (12); PCB (aroclor 1254)-estuarine sediment (13). The nonsingular nature
0013-936X/82/0916-0594$01.25/0
0 1982 Amerlcan Chemical Society
concentration to the column were again to increase. What is the proper isotherm relationship in this case? Such questions and the desire for a more concise description of the adsorption-desorption data leads to the analysis described below.
( a ) Single Desorption
(b) Consecutlve Desorption Aqueous HCBP Concentratlon,
c, (ng H C B P / t )
Figure 1. Hexachloroblphenyladsorption-desorption isotherms, Saginaw Bay sediment, station no. 50: (a) adsorption and single desorption data (m = 220 rng/L); (b) adsorption and consecutive desorption data (m = 1100 mglL). Data are averages of three replicates.
of the isotherms is a consequence of the desorption partition coefficient, Pd, being greater than the adsorption partition coefficient, a,. A more refined and time-consuming experimental procedure is, after adsorption, to extend each initial desorption step by performing subsequent multiple desorptions. The points generated in this fashion can also be described by an isotherm, which may be termed the consecutive desorption isotherm as shown in Figure l b (9). Examples of these consecutive desorption isotherms with significant nonreversibility have also been reported for various adsorbate-adsorbent systems: atrazine-soil (14,15); Picloram-loam (4); Fluometuron-fine sandy loam (16);2,4,5T-clay (17)and loam (18);Parathion-soil (19),montmorillonite (20), and sandy loam (21); Diuron-soil (15) and lake sediment (22). It was first shown for Atrazine (14) that each consecutive desorption isotherm, corresponding to each adsorption point, can be represented as a Freundlich isotherm. However, the isotherm parameters were found to depend upon the individual adsorption concentrations. Thus although the description of the desorption data is adequate, it is not concise. This framework has been successfully applied to a mass balance analysis of picloram-soil column leaching experiments (4). At the start of the experiment the initial adsorption at each computational node within the column follows the adsorption isotherm as concentrations increase. Then the influent concentration is decreased, and as the computed concentrations within the column begin decreasing, the isotherm branches off along the appropriate desorption isotherm with the appropriate interpolated parameters. An interesting question arises if the influent
Material and Methods The sorption of PCB’s to sediments was investigated in a series of experiments conducted using tritiated 2,4,5,2’,4’,5’-hexachlorobiphenyl (HCBP) and a series of natural lake sediment (Saginaw Bay, MI) and clay mineral (montmorillonite and kaolinite) samples. Details of sediment preparation and analysis as well as procedures for radiochemical stock solution preparation are presented elsewhere (9). Sorption experiments were conducted by equilibrating sediment suspensions in 25-mL Corex brand (Corning) centrifuge tubes for 3-h adsorption and 2-h desorption time periods. Preliminary experiments suggested that these time periods were sufficient to achieve equilibrium. Samples were analyzed following equilibration for both total sediment and water and following centrifugation (7000 rpm for 15 min) for aqueous HCBP concentrations. Total and aqueous HCBP concentrations were determined by analyzing the tritium contents of both total and aqueous samples with a Beckman LS 150 liquid scintillation counter. Desorption isotherms were determined by replacing the equilibrium aqueous fraction resulting from adsorption with equivalent volumes of distilled water. All sample data points represent the average of triplicate replicates. Results The results of experimental consecutive desorption studies indicate that there exists a significant component of the adsorbed HCBP that is extremely difficult to desorb. The problems involved in experimentally achieving zero aqueous concentration make it uncertain as to whether or not all of this fraction is ultimately desorbable under the varying physical-chemical conditions of natural waters. However, it is clear that for HCBP (see Figure l b and other data presented in ref 9) and for other adsorbatesadsorbent systems cited previously, the particulate HCBP concentrations for the initial few desorptions are significantly above the adsorption isotherm. That is, a portion of the adsorbate does not significantly desorb even at low aqueous concentrations. To idealize the situation, consider Figure 2. The adsorption isotherm is assumed to be linear as is the initial stages of the consecutive desorption isotherm, which is presumed to describe the behavior of the readily reversible fraction. This idealization is important since it limits the applicability of the analysis to those aqueous concentrations for which the consecutive desorption data conform to the linear assumption. Define the resistant (subscripted by 0) component concentration, ro, as the extrapolated intersection of the consecutive desorption isotherm and the ordinate. Define the reversible components (subscripted by x) at adsorption, r, and at the first desorption, r,d, as the difference between the observed adsorption and desorption sedimentadsorbed concentration and the extrapolated resistant concentration: (1) r,, = ra - ro (2) rxd = r d - r,, This component is reversible since it responds to the change in aqueous concentration from c, at adsorption equilibrium to c d at desorption equilibrium. Environ. Sci. Technol., Vol. 16, No. 9, 1982
595
Table I. Isotherm Parameters for HCBP Adsorption-Desorption adsorbent desorption concn (m), adsorption adsorbenta mg/L time, h time, h Saginaw Bay no. 50 55 2 3 Saginaw Bay no. 5 0 55 48 48 220 Saginaw Bay no. 50 3 3 montmorillonite 220 3 3 kaolinit e 1000 3 3 a
partition coefficients, (L/kg) =a
=d
n0
nX
Figure
17000 18100 12200 6600 1060
25100 60900 25800 15600 3290
5270 12700 9170 4250 730
11700 5400 3070 2350 3 27
6 6 3 4 5
The aqueous phase is distilled water for all experiments; temperature = 23 “C.
I
1000
.-t
Desorption, (+)
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ar
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Adsorption Desorption
n
+0
(a)
0
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Adsorptlon,
bm
(0)
~
0
b ”
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n
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1
0
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/
0
0
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ca
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n m
Y
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v)
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Aqueous HCBP Concentration, c, (ne H C B P I I )
Flgure 2. Schematic illustration of the definitions of the resistant, r o , and the reversible components of adsorption, rxar and desorption, rxd, with the assumption of a ilnear consecutive desorption isotherm.
P
e UI P
a
Reversible Component
I
C
From the geometry of the linear isotherms or from the algebraic derivation given in Appendix I, the reversible and resistant component concentrations can be computed from each pair of adsorption (r,, c,) and single desorption (rd, cd) values. The equation for the resistant component concentration is (3) ro = (rd - Pra)/(l - P) where = cd/c, The reversible component concentrations follow by difference (eq 1 and 2). If this idealization is a useful characterization, then the reversible component should follow an isotherm that does not distinguish between either the adsorption or the desorption points but is related only to the aqueous concentration. That is the reversible component isotherm should not display any hysteresis. Figure 3c, which contains the data in Figure l a (and replotted in Figure 3a) analyzed by using eq 1-3, shows that indeed all the reversible adsorption and desorption points are described by a single, in this case linear, isotherm: rx = T,C (4) independent of which adsorption-desorption data pairs are considered or whether all the adsorption or all the desorption data are examined, i.e., whether rx = rm or rxd and c = c, or cd, respectively. Figure 3b illustrates that for these data the resistant component is a linear function of the equilibrium adsorption aqueous concentration, c,, leading to an isotherm for the resistant component: ro = roc, (5) 596
Environ. Scl. Technol., Vol. 16, No. 9, 1982
t
s (C)
v)
Desorption, (+)
I
l lL
I
I
1 1
IIIi ca 7’d
I
I
1
l 1 1 l L
100
A q w o u s HCBP Concentration, c, (ng HCBP/!)
Flgure 3. Hexachlorobiphenyladsorption-desorption isotherms, Saginaw Bay sediment, station no. 50 (rn = 220 mg/L): (a) adsorption and single desorption data and linear isotherms; (b) resistant component estimates from eq 3 and linear isotherm; (c) reversible component linear isotherm, adsorptlon (eq 1) and desorption (eq 2) estimates of the reversible component.
Since the adsorption isotherm is also linear, the resistant component can also be expressed in terms of the adsorbed concentration, r,, that is: ro = rga/ra.The experimental conditions and isotherm parameters for these data are listed in Table I.
Relationship to Single Desorption Isotherm The formulation presented above is based upon the consecutive desorption isotherm (Figure lb). It is of interest to inquire what is the relationship of this analysis to the single desorption isotherm (Figure la). It can be shown (Appendix 11) that, if the single desorption and adsorption isotherms are linear and essentially all the adsorbate mass is associated with the particles at the start of the desorption, then the reversible and resistant com-
Table 11. Adsorption and Consecutive Desorption Isotherm Parameters adsorbate
adsorbent
Ka, (ng/kg)/ (ng/L )Na
Na
NaINd
atrazine atrazine picloram 2,4,5-T
mohave soil Walla Walla soil norge loam Glendale clay loam
0.21 2.61 0.18 0.616
1.0 0.85 0.94 0.792
2.3 2.3 2.22-2.98 2.30
a
no? L/kg 0.12 0.87-1.08 0.095-0.125 0.20-0.24
n,,O L/kg 0.091 0.77-0.96 0.060-0.073 0.12-0.15
ref 14 14 14 17
n o and n, were calculated by using eq 17 and 13 and the experimental range in e,.
ponents also follow linear isotherms, eq 4 and 5, with partition coefficients
maa(ad - a,) = 1 + m(ad - ?fa) (7) These relationships provide the connection between the conventional adsorption, (a,) and single desorption (ad) partition coefficients and the partition coefficients of the reversible (ax)and resistant (ao)components. In addition, they establish the fact that linear adsorption and single desorption isotherms imply linear reversible and resistant component isotherms. Since the reversible isotherm applies at both the adsorption and single desorption aqueous concentration and ro is constant for an adsorption-desorption experiment, each consecutive desorption isotherm can be expressed as r = ro + r x c (8) which is linear. Thus these results lead to the conclusion that an analysis using linear adsorption and consecutive desorption isotherms (Figure 2) is consistent with linear adsorption and single desorption isotherms (Figure la), and both types of isotherms are reasonable representations of linear adsorption-desorption data. The reversible and resistant component partition coefficients can be obtained either from an analysis of the individual data, eq 1-3, or from the partition coefficients a, and a d , and eq 6 and 7. This symmetry is especially useful in analyzing previously published adsorption-desorption data for which only the adsorption and single desorption isotherms are presented. A comparison of both these methods is presented in Figure 4, which presents HCBP adsorption and single desorption data for montmorillonite and kaolinite, respectively. The resistant (Figure 4b) and reversible component estimates (Figure 4c)are computed by using eq 1-3. The linear isotherms are eq 4 and 5 using average a, and a d computed from a fit of the adsorption and single desorption data (Figure 4a and eq 6 and 7) for the partition coefficients. The agreement is basically due to the observed linearity of the adsorption and single desorption isotherms.
MONTMORILLONITE
1000
E
P
“0
Relationship to Freundlich Isotherms Previous investigators have analyzed consecutive desorption isotherms within the framework of Freundlich isotherms (4,14,17). The adsorption isotherm is represented by
r, = K,caNa (9) and each consecutive desorption isotherm is represented by: rd = KdCdNd (10) where the Freundlich parameters Kdand Nd are different for each consecutive desorption isotherm. This is una-
KAOLINITE
Adsorption,
(0)
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Resistent C o m p o n e n t Adlorptlon Isotherm
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Reverslble C o m p o n e n t Isotherm
Reversible c o m p o n e n t Isotherm
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1
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0 ’ 1
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100
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A ~ Y B O Y SHCBP C o n c e n t r a t i o n .
E,
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Flgure 4. Hexachlorobiphenyl adsorption-desorption isotherms montmorillonite, (m = 220 mg/L) and kaolinite (m = 1000 mg/L): (a) adsorption and single desorption data and linear isotherms; (b) resistant component estimate (eq 3) and linear isotherm; (c) reversible component linear isotherm, adsorption (eq 1) and desorption (eq 2) estimates of the reversible component.
voidable since each consecutive desorption isotherm intersects the adsorption isotherm at the adsorption concentration, rd = ra at cd = c,, so that Kd = K,c,(Na-Nd) (11) which indicates that the consecutive desorption Freundlich parameter, Kd, is a nonlinear function of the initial adsorption concentration, c,. However a peculiar result has been noted: the ratio N,/Nd is approximately constant (N,/Nd 2.3). Table I1 presents some examples. It is of interest to investigate the implication of this result with respect to an analysis based upon reversible and resistant components. As shown in Table 11, the adsorption isotherms are almost linear, N , e l, and the partition coefficient is the slope of the isotherm a, =
&,/de,
= K,N,c,~~-~
(12)
The consecutive desorption isotherm, when approximated as a straight line with a nonzero intercept (eq B), has a slope a,. If this slope is equated to the slope of the Freundlich consecutive desorption isotherm at the point of intersection of the adsorption isotherm, then
Environ. Sci. Technol., Vol. 16, No. 9, 1982
597
and by using the equality of the isotherms at the intersection (eq 11) T, = KaNdcaNal-l
(14)
and using eq 1 2 for K, yields ax =
(Nd/Na)ra
(15) Since Nd/N, is approximately constant and a, is constant for linear adsorption isotherms and almost constant for N, 1,the reversible partition coefficient is the same for each consecutive desorption isotherm. Hence to the extent that a linear approximation of the consecutive desorption isotherm is a reasonable representation, the fact that Nd/N, is the same for each consecutive desorption isotherm is a reflection of the fact that all the consecutive desorption isotherms can be represented by a single reversible component with partition coefficient nx. The fact that the desorption Freundlich constant, Kd, is a function of the adsorption concentration, c,, eq 11,is reflected in the fact that the resistant component given by eq 8 ro = r, - r,c, (16) can be expressed in terms of the Freundlich parameters (eq 9 and 15), yielding
-
ro = (1- Nd/N,)K,CaNn (17) Since Nd,JN,is constant for every consecutive desorption isotherm, ro is a function only of the adsorption concentration, c,. For a linear adsorption isotherm, N, = 1, and the resistant partition coefficient is TO = (1- Nd/Na)Ka (18) which is also the same for each consecutive desorption isotherm. Hence, eq 15 and 18 represent all the consecutive desorption isotherms consistently in terms of two parameters, ro and rx,as compared to the individual Freundlich isotherms for each consecutive desorption isotherm. The observational fact that Nd/N, is constant for each isotherm is the key to this simplification. The resistant and reversible component partition coefficients corresponding to the Freundlich parameters are listed in Table 11. The indicated ranges are the effect of the slight nonlinear adsorption isotherm. The linear approximation of the adsorption isotherm results in only slight changes in a. and a,. The critical approximation is that the consecutive desorption isotherm is linear. This is reasonable until fairly low aqueous concentrations are achieved after multiple desorptions, as discussed previously. Discussion The proposed linear approximation describing the binding of organics to sediments is clearly an oversimplification of the actual process. In the case of HCBP, experimental evidence (9) suggests that under certain chemical conditions the binding to sediments during consecutive desorption may be described by a curvilinear isotherm that may or may not ultimately demonstrate complete desorbability. It is also quite possible that the actual adsorption process may involve binding to sites of a gradation of energies rather than the two arbitrarily defined fractions (reversible and resistant). Nevertheless, there exist distinct advantages for data analysis that result from treating HCBP and other organic adsorption study results in terms of this approximation. For a sediment-adsorbed organic molecule such as HCBP, it may not be experimentally feasible to generate consecutive desorption isotherms under all of the chemical 598
Environ. Sci. Technol., Vol. 16, No. 9, 1982
conditions of concern from an environmental modeling standpoint. For instance, it has been observed that the magnitude of the adsorbed and readily desorbed HCBP sediment fractions is a function of a variety of parameters including aqueous composition and sediment concentration (9). This suggests that a large number of consecutive desorption isotherms would be required to adequately define the environmental behavior of HCBP. However, for certain chemical conditions such as high (>lo00 mg/L) sediment concentrations it is extremely difficult to obtain consecutive desorption isotherms at the aqueous HCBP concentrations of concern in natural water systems. The extremely small aqueous HCBP concentrations that result from sediment HCBP release during successive desorption cycles possess large experimental uncertainties, require a prohibitive number of experimental data points, and lead to ill-defined isotherms. From the standpoint of environmental modeling the linear approximation offers a means of refining currently used assumptions such as complete reversibility for organic adsorption to sediments. From the environmental chemical perspective the approximation can be utilized to separate reaction variables and provide an aid in interpreting organic binding mechanisms. These applications may be demonstrated by a consideration of the results for both multiple cycle adsorption-desorption and kinetic studies. In order to incorporate environmental cycling within mass balance calculations there exists a need to provide a method of analyzing the behavior of adsorption and desorption under changing aqueous PCB concentrations. (An example would be sediments initially exposed to high PCB concentrations subsequently migrating into environments characterized by lower but varying aqueous PCB levels.) The isotherm treatment of the present study has been utilized to predict how such changes would affect the sediment-adsorbedPCB fractions. Model predictions have been compared with the results of laboratory experiments designed to simulate the cycling process described above. As discussed in detail in the methods section, these experiments involved an adsorption step followed by a single desorption (the standard isotherm experiment) and following this a series of consecutive adsorptions. For small incremental increases in the aqueous concentration during the adsorption steps, small such that they do not exceed the initial adsorption concentration, c,, the resistant component should be constant and only the reversible component should vary. That is, this portion of the data should follow the consecutive desorption isotherm r = ro a,c cd < c C c, (19) where ro = roc,. When the aqueous concentration exceeds c,, more resistant sites should be available and the data should follow the adsorption isotherm
+
r = (no+ a,)~ c > c, (20) Figure 5 presents the results for HCBP binding to Saginaw Bay sediment and montmorillonite. The initial adsorption, single desorption, and the subsequent consecutive adsorption data are differentiated in the legend. The range of the quadruplicated data is as indicated. For Saginaw Bay sediment, the theoretical prediction is followed almost exactly. The agreement for montmorillonite is less satisfactory although it also clearly shows a change in behavior as the aqueous concentration exceeds c,. These results provide at least a preliminary answer to the question posed at the outset: namely, how are these isotherms to be interpreted if the aqueous concentration, c ( t ) , is changing in time during a mass balance calculation? With local equilibria, the resistant component reacts in response to
ADSORPTION TIME
1601
+
+
+
f
Adsorption Desorption Consecutive Adsorption
-2
1 o4
- 4 8 HR
ADSORPTION TIME
HR
I
l
o
4
n
1
c
.-U
lZ0I
m
I
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.
Ad8orptlon Isotherm
R e s i s t a n t Component Adeorptlon Isotherm
,
0
0
V
n
Montmorillonite
rn
(5) ALIWOYS
HCBP c o n c e n t r a t ~ o n ,
C.
~ n gHCBP/I)
Figure 0. Hexachlorobiphenyladsorption-desorption isotherms, Saginaw Bay sediment, station no. 50 (m = 55 mg/L), effect of adsorption time (see Figure 3 captlon). Dotted isotherms are the 2-h results repiotted for visual reference.
-
Aqueous HCBP Concentratlon, c, (ng HCBP/I)
Figure 5. Hexachlorobiphenyladsorption-desorption and consecutive adsorption, Saginaw Bay sediment, station no. 50 (m = 1100 ppm) and montmorillonite (m = 1100 ppm and 2 mequlv/L of NaHCO, buffer).
the maximum aqueous concentration achieved up to that time: ro = ?roc,&) (21) c,,(t) = max c ( t ) (22) t whereas the reversible component follows the linear reversible isotherm
rx = Txc(t) (23) in all circumstances. Since this interpretation is based upon rather limited experimental data it should be regarded with caution. However it does suggest the advantages of applying the proposed model to scenarios of this type as opposed to assuming a completely reversible formulation for HCBP adsorption-desorption. Kinetics The assignment of reversible and resistant sediment HCBP components in accordance with the method described above also offers potential advantages in the utilization of kinetic data to evaluate binding mechanisms. Of particular interest is the effect of time on the relationship between the reversibly and resistant HCBP components. The results of experiments designed to assess these effects can be seen from the comparison (presented in Figure 6) of the adsorption, single desorption, reversible, and resistant isotherms found for 2- and 48-h adsorption times. (It has been found that increasing the desorption time has no effect on the desorption isotherms.) The adsorption partition coefficient is slightly increased (T, = 1700 1800 L/kg), but the desorption partition coefficient
-
is markedly increased ( ~ =d 25000 61 000 L/kg). As a result the resistant component partition coefficient increased by approximately a factor of 2 (r0= 5100 12800 L/kg), and the reversible component partition coefficient decreases (T, = 11 800 5400 L/kg) by approximately the same factor. Since the partition coefficient can be thought of as the product of a binding-strength constant and a capacity factor, the question arises as to which factor is responsible for the increase in a0 with time. That is, are HCBP molecules being bound more strongly at a fixed number of sites or is the number of strongly bound sites increasing with time. Although the reversible component data are quite scattered there is some suggestion that the increase in the resistant fraction is accompanied by an equal and opposite decrease in the magnitude of the reversibly bound fraction. This would seem to imply that a change in the capacity factor of the resistant component is occurring rather than a change in the binding strength. Such an effect could conceivably result from a transformation of reversible binding sites to strong binding sites (metastable complex stable configuration). If the magnitude of the reversible partition coefficient had remained constant during the experiment it would have implied that separate sites are responsible for reversible and resistant binding of HCBP. Clearly, more experimental data are warranted before a choice of change in capacity factor vs. bindingstrength constant can be made with certainty. However, the results again illustrate the utility of considering the data in terms of reversible and resistant components.
-
-
-
Summary It is useful to summarize the experimental basis for the component hypothesis. The definition of the resistant component follows from two observations: (1) the single desorption isotherm is not coincident with the adsorption isotherm, that is, this pair of isotherms display hysteresis; (2) the consecutive desorption isotherm can be approximated by a straight line, at least for the first few consecEnviron. Sci. Technol., Vol. 16, No. 9, 1982
599
utive desorptions. These are distinct observations, and each can be examined in the light of experimental evidence. The presence of hysteresis in the adsorption and single desorption isotherms can be ascribed to a number of phenomena and possible experimental artifacts. A recent review (8) concluded that hysteresis persists in most cases in spite of experimental modifications designed to eliminate the phenomena. Our experimental investigations for HCBP (9)have evaluated the influence of increasing the desorption times (no reduction in hysteresis was observed) and the influence of using the dilution method to produce the desorption isotherm (hysteresis was still present). Thus HCBP hysteresis is not being caused by insufficient desorption time or by the centrifugation used to separate the solids prior to the desorption experiment. The second observation, that the consecutive desorption isotherm is linear, is experimentally observed for the first few consecutive desorptions. After that the data are inconclusive in one case (Figure lb), and at a lower sediment concentration, they suggest that in fact the consecutive desorption isotherm decreases toward the origin. It is for this reason that the component responsible for this behavior is termed resistant. That is, the resistant component does not desorb at all for the first few consecutive desorptions and may or may not subsequently desorb. Its existence accounts for the observed hysteresis in the single desorption experiments, and it has been argued above that experimental artifacts are not the cause for hysteresis. The question of the linearity of the consecutive desorption isotherm can be interpreted in terms of the resistant component consecutive desorption isotherm: is it a horizontal straight line indefinitely (as shown in Figure 2) or is it initially a horizontal straight line that eventually curves to the origin as aqueous concentrations approach zero? In either case it is clear that the resistant component consecutive desorption isotherm is a straight line initially and that it is linear for the consecutive adsorption data presented in Figure 5. These observations justify the assumption employed in this analysis. Further detailed experimental investigations are required to establish the shape of the resistant component consecutive desorption isotherm. The issue is significant since it relates to the ultimate desorbability of PCB from previously contaminated sediments. But this issue should not be confused with the existence of hysteretic isotherms. The hypothesis employed in this analysis sharpens the question of ultimate desorbability since it is clear that the issue is the behavior of the resistant component at aqueous concentrations approaching zero rather than the overall utility of separating the data into the two components by using the method proposed in this paper.
Conclusions From an empirical point of view, the analysis of HCBP isotherm data in terms of reversible and resistant components is quite successful in providing a concise and unified description. The full adsorption and desorption isotherm data set is represented by two partition coefficients, r0 and rX.The relationship between consecutive desorption isotherms and a single desorption isotherm has been clarified for the linear isotherm case at least, and they have been shown to be equivalent in the sense that either can be used to obtain the relevant partition coefficients for the components. The consecutive adsorption experiments provide preliminary confirmation that the reversible and resistant components behave in accordance with their expected properties. The reversible component exhibits 800 Envlron. Sci. Technol., Vol. 16, No. 9, 1982
no hysteresis in response to changes in aqueous concentrations, and the resistant component reacts only to increases in aqueous concentrations that exceed the initial adsorption concentration. The use of the method described in this paper to estimate the reversible and resistant components of adsorption-desorption provides additional insight into the influence of kinetics, sediment type, and aqueous-phase modifications (e.g., altering the pH) since it is possible to observe the effects on each of the components individually. Perhaps such investigations will help to clarify the mechanisms responsible for hysteretic isotherms and clarify the issue of ultimate desorbability.
Notation dissolved aqueous concentration of HCBP at adsorption equilibria (ng/L) dissolved aqueous concentration of HCBP at desorption equilibria (ng/L) Freundlich adsorption parameter [ (ng/ kg)/ (ng/ L)Nal Freundlich consecutive desorption parameter [(ng/kg)/ (ng/L)Ndl adsorbent concentration (kg/L) Freundlich adsorption exponent Freundlich consecutive desorption exponent sediment-bound concentration of HCBP at adsorption equilibria (ng/g) sediment-bound concentration of HCBP at desorption equilibria (ng/g) resistant component of adsorbed HCBP (ng/g) reversible component of adsorbed HCBP at adsorption equilibria (ng/g) reversible component of adsorbed HCBP at desorption equilibria (ng/g) partition coefficient for adsorption (L/ kg) partition coefficient for single desorption (L/kg) partition coefficient for the resistant component (L/kg) partition coefficient for the reversible component (L/kg)
Acknowledgments
We are pleased to acknowledge the assistance and cooperation of the following members of the EPA Large Lakes Research Station, Grosse Ile, MI: Nelson Thomas, William Richardson, Michael Mullin, and John Filkens. Our group at Manhattan College, Maureen Casey, John Jeris, Robert Thomann, Donald O’Connor, John Mancini, and Joanne Guerriero, are also thanked. Appendix I Derivation of Reversible and Resistant Component Concentrations. The equilibrium sediment-bound concentrations of HCBP that result from an adsorption experiment, r, at aqueous concentration c,, and a subsequent desorption, rd at aqueous concentration cd, are assumed to be the sum of a resistant component, ro, and reversible components, r,, and rxd, that is (AI) r, = ro + rxa rd = ro rxd
+
The resistant component is the same for both adsorption and desorption since it is assumed not to be affected by the desorption step, whereas the reversible component reacts to the lower aqueous concentration at desorption and decreases from rxato rx. at equilibrium.
It is further assumed for this derivation that the reversible component follows a linear isotherm with partition coefficient axso that the reversible component concentrations are related to the aqueous concentration by the equations rxa = "xca (-43) rxd = "xcd (A41 Using eq A2 to solve for ro and substituting eq A4 for rxd yield ro = rd - T,Cd (A5) Substituting eq A3 for a, and using eq A1 for rxayield ro = rd - (cd/C,)(r, - ro) (A6) The final result for the resistant component concentration is rd - ora ro = 1-P where p = cd/c,. The reversible component concentrations are obtained by subtraction using the defining eq A1 and A2: (AB) r,, = r, - ro (A9) rxd= rd - ro The assumption that the reversible component follows a linear isotherm is checked by examining a composite plot of r,, vs. c, and rxdvs. cd for linearity. It is significant to note that the derivation does not involve a mass balance argument, so the equations are applicable even if the experimental vessel is acting as a third phase with adsorption and desorption characteristics of its own. Also it is not necessary that the desorption be carried out by removing all the supernatant after centrifugation or that a correction be made for any residual aqueous phase. In fact the equations apply to whatever method is used to accomplish the desorption, for example, use of an imrniscihle organic solvent as a third phase to accomplish the desorption (15). It is only necessary that the linearity assumption for the reversible component is satisfied and that its partition coefficient is unaffected by the desorption method employed. If the linearity assumption is found to be inappropriate, it is still possible to separate the components. The derivation using nonlinear reversible isotherms (e.g., a Langmuir or Freundlich equation) is similar with the exception that an unknown parameter (the Langmuir binding constant and the Freundlich exponent, respectively) remain in the equations for ro and rr Thus an iterative fitting procedure is required to make the estimates. Appendix 11 Relationship between Linear Adsorption, Single Desorption, Nonexchangeable, and Consecutive Desorption Isotherms. It is the purpose of this derivation to show that if the adsorption and single desorption isotherm are linear, then the resistant component and the consecutive desorption isotherm are also linear. The linearity assumptions for adsorption and desorption are expressed by the equations r, = sac, (AW (All) rd = adcd The resistant component concentration is given by (eq A7)
which can be expressed in terms of aqueous concentrations only using eq A10 and A l l :
A relationship is available between cd and c, if the desorption is carried out by removing essentially all the supernatant and replacing it with adsorbate-free solvent (or equivalently that the mass of adsorbate associated with the residual supernatant is negligible) and that the vessel desorption is negligible. Then the mass of adsorbate prior to solvent addition, mr,, is equal to the total mass of adsorbate in the dissolved and particulate phases: mr, = cd mrd (A141
+
where m is the concentration of adsorbent present. Substituting eq A10 and A l l yields the desired relationship: maaca cd = 1 mad
+
This relationship in eq A13 yields
so that ro is linearly related to c, and by the definition of ao, eq 4:
Hence, if the linearity and mass balance equations are applicable, the resistant component partition coefficient can be computed from the adsorption and single desorption partition coefficients. Similarly the consecutive desorption isotherm can also be shown to be linear. This is equivalent to showing that the slope of the consecutive desorption isotherm, which is the partition coefficient of the reversible component, is the same at the adsorption and desorption aqueous concentrations. The reversible component concentration at adsorption equilibrium is given by eq A8: rxa = r, - ro or, by using eq A10 and A16:
(AW
which simplifies to
Similarly the reversible component concentration at desorption equilibrium is given by eq A9: rxd = rd - ro (A21) ma*("d - a,) 1 + mad (A221 - 1 m(rd - n,) m", cd where eq A l l and eq A16 have been used for rd and ro and eq A15 for c,. Combining terms and simplifying yield
+
Hence the reversible component isotherm has the same slope at c, (eq A20) and cd (eq A23), so the consecutive isotherm is linear, and further, the reversible component partition coefficient is given by Environ. Sci. Technol., Vol. 16, No. 9, 1982
601
Environ. Sci. Technol. 1982, 16, 602-606 Pa
Ta
= 1 + m(r, - a,)
(A241
It should be noted that these relationships can be easily corrected for the mass associated with the residual supernatannt if its volume is known by simply adding that mass to the left-hand side of eq A14.
Literature Cited Mayer, F. L., Hamelink, J. L., Eds. “Aquatic Toxicology and Hazard Evaluation”; Proc. First Annual Symposium on Aquatic Toxicology, ASTM STP 634; ASTM: Philadelphia, PA, 1977. Marking, L. L., Kimberle, R. A., Eds. “Aquatic Toxicology Proc. Second Annual Symposium on Aquatic Toxicology”; ASTM STP 667; ASTM: Philadelphia, PA, 1979. Hague, R., Ed. “Dynamics, Exposure, and Hazard Assessment of Toxic Chemicals”; Ann Arbor Science: Ann Arbor, MI, 1980. van Genuchten, M. Th.; Davidson, J. M.; Wierenga, P. J. Soil Sei. SOC.Am. Proc. 1974, 38, 29-35. Baughman, G. L.; Lassiter, R. R. In “Estimating the Hazard of Chemical Substances to Aquatic Life“; Cairns, J., Dickson, K. L., Maki, A. W., Eds.; ASTM STP 657; ASTM: Philadelphis, PA, 1978; pp 35-70. Thomann, R. V., Di Toro, D. M., submitted for publication. O’Connor, D. J.; Schnoor, J. L. Manhattan College Summer Institute Proceedings, 1980. Rao, P. S. C.; Davidson, J. M. In “Environmental Impact of Nonpoint Source Pollution”; Overcash, M. R., Davidson, J. M., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; pp 23-67.
(9) Horzempa, L. M.; Di Toro, D. M. Water Res., in press. (10) Huang, J.-C.; Liao, C. J. Sanit. Eng. Diu., ASCE 1970,19, 1057-1078. (11) Pierce, R. H.9 Jr.; O h Y , C. E.; Felbeck, G. T.9 Jr. Geochim. Cosmochim. Acta 1980, 24, 20-26. (12) Felsot, A.; Dahm, P. A. J. Agric. Food Chem. 1979, 27, 557-563. (13) Wildish, D. J.; Metcalfe, C. D.; Akagi, H. M.; McLeese, D. W. Bull. Environ. Toxicol. 1980, 24, 20-26. (14) Swanson, R. A.; Dutt, G. R. Soil Sei. SOC.Am. Proc. 1973, 37, 872-876. (15) Rao, P. S. C.; Davidson, J. M.; D. P. Kilcrease.
“Examination of Nonsingularity of Pesticide AdsorptionDesorption Isotherms for Soil Pesticide Systems”; Agron. Abstr. 1982, 34. (16) Savage, K. E.; Wauchope, R. D. Weed. Sci. 1974, 22,
106-110. (17) van Genuchten, M. Th.; Wierenga, P. J.; O’Connor, G. A. Soil Sei. SOC.Am. J. 1977, 41, 278-285. (18) Koskinen, W. C.; O’Connor, G. A.; Cheng, H. H. Soil Sci. SOC.Am. J. 1979,43,871-874. (19) Kishk, F. M.; Abu-Sharar, T. M.; Bakry, N. M.; Abou-Donia, M. B. Arch. Environ. Contam. Toxicol. 1979,8,637-645. (20) Bowman, B. T.; Sans, W. W. Soil Sei. SOC.Am. J . 1977,41, 514-519. (21) Bowman, B. T. Can. J. Soil Sci. 1979,59, 435-427. (22) Peck, D. E.; Convin, D. L.; Farmer, W. J. J. Enuiron. Qual. 1980,9, 101-106.
Received for review November 3, 1980. Revised manuscript received October 9,1981. Accepted April 5,1982. The research described in this report was conducted under EPA research grants CR805229 and CR807853.
A 1O-pm Cutpoint Inlet for the Dichotomous Sampler James B. Wedding,* Michael A. Welgand, and Theodore C. Carney Aerosol Science Laboratory, Colorado State Unlversity, Affiliated with the Research Institute of Colorado, Fort Collins, Colorado 80523
The United States Environmental Protection Agency is considering an alternative standard requiring the collection of a specific size fraction of atmospheric aerosol termed the thoracic fraction. This new standard provides an impetus for the inlet development described herein. An ambient air inlet for the dichotomous sampler has been designed, fabricated, and performance tested. It is in compliance in all aspects with the proposed U.S. Environmental Protection Agency Federal Reference Method performance specifications. The device operates at 16.7 L/min and has an aerodynamic particle size of 10 pm associated with a 50% effectiveness value, which is commensurate with the anticipated changes in the air quality standards. The system is independent of all environmental conditions, most importantly windspeed. The inlet employs a unique, omnidirectional cyclone fractionator. Under dynamic conditions the inlet was tested with solid particles and exhibited no evidence of particle bounce. H
Introduction The U.S.Environmental Protection Agency (EPA) under the auspice of the Clean Air Act Amendment of 1977 is mandated to review the total suspended particulates (TSP) standard for its applicability to the protection of health and welfare. The present standards for particulate matter are 75 pg/m3 average annual limit (geometricmean) and 260 pg/(m3 24-h) limit (geometric mean) for particles
