bk-1983-0225.ch006

6 Partition Models for Equilibrium Distribution of Chemicals in Environmental Compartments P. J. McCALL, R. L . SWANN, and D. A. LASKOWSKI

Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: August 30, 1983 | doi: 10.1021/bk-1983-0225.ch006

Dow Chemical, Midland, MI 48640

D i s t r i b u t i o n of organic chemicals among environmental compartments can be d e f i n e d in terms of simple e q u i l i b r i u m expressions. P a r t i t i o n c o e f f i c i e n t s between water and air, water and soil, and water and b i o t a can be combined to c o n s t r u c t model environments which can provide a framework f o r p r e l i m i n a r y evaluat i o n of expected environmental behavior. T h i s a pproach is p a r t i c u l a r l y u s e f u l when little data is a v a i l a b l e s i n c e p a r t i t i o n c o e f f i c i e n t s can be estimated with reasonable accuracy from c o r r e l a t i o n s between prope r t i e s . I n a d d i t i o n to i d e n t i f y i n g those environmental compartments i n which a chemical is likely to r e s i d e , which can a i d in d i r e c t i n g f u t u r e research, these types of models can provide a base f o r more elaborate k i n e t i c models. Increased production and use of chemicals have created a need to b e t t e r understand the f a t e and e f f e c t s of chemicals i n the environment. Recognition of environmental concerns by regul a t o r y agencies has l e d to new l e g i s l a t i o n aimed a t f i n d i n g answers to important questions regarding the d i s t r i b u t i o n and behavior of chemicals i n the environment. H i s t o r i c a l l y l a b o r a t o r y t e s t s have i n v e s t i g a t e d i n d i v i d u a l process a s s o c i a t e d with movement ( s o i l l e a c h i n g , v o l a t i l i t y , a d s o r p t i o n , etc.) and t r a n s f o r mation ( s o i l degradation, h y d r o l y s i s , p h o t o l y s i s , etc.) of chemicals. The current t h r u s t i n environmental chemistry i s to i n t e g r a t e environmentally meaningful l a b o r a t o r y data i n t o a r e a l i s t i c d e s c r i p t i o n of the " r e a l world" behavior of a chemical. The underlying goal of t h i s research i s to reach i n the most e f f i c i e n t manner and i n the s h o r t e s t time p o s s i b l e , a r e l i a b l e assessment of f a t e . Several approaches have evolved which, i n general, can be described as models of the environment or p a r t s of the environment. Models g e n e r a l l y f a l l i n t o two c a t e g o r i e s ; p h y s i c a l or mathematical. P h y s i c a l models o f t e n termed microcosoms or model ecosystems attempt to i s o l a t e a r e p r e s e n t a t i v e segment of the 0097-6156/83/0225-0105$06.00/0 © 1983 American Chemical Society

Swann and Eschenroeder; Fate of Chemicals in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

106

F A T E OF C H E M I C A L S I N T H E E N V I R O N M E N T

environment w i t h i n which the f a t e of a chemical i s observed i n order to d e s c r i b e i t s f a t e . Mathematical models attempt to d e f i n e a l l the important processes which a c t on a chemical i n the e n v i ronment and i n c o r p o r a t e them i n t o a d e s c r i p t i o n of chemical behavi o r as a f u n c t i o n of the v a r i a b l e s a c t i n g on the system. The type of model approach taken depends on the degree of accuracy and prec i s i o n expected and the questions asked of the model. Two c l a s s e s of mathematical models have been developed: those which a r e s p e c i f i c and attempt to d e s c r i b e the t r a n s p o r t and degr a d a t i o n of a chemical i n a p a r t i c u l a r s i t u a t i o n ; and those which are general or " e v a l u a t i v e and attempt to g e n e r a l l y c l a s s i f y the behavior of chemicals i n a h y p o t h e t i c a l environment. The type of modeling discussed here, e q u i l i b r i u m p a r t i t i o n i n g models, f a l l i n t o the l a t t e r category. Such models attempt, with a minimum of information, to p r e d i c t expected environmental d i s t r i b u t i o n patterns of a compound and thereby i d e n t i f y which environmental compartments w i l l be of primary concern.


11

P a r t i t i o n i n g Models In i t s simplest form a p a r t i t i o n i n g model evaluates the d i s t r i b u t i o n of a chemical between environmental compartments based on the thermodynamics of the system. The chemical w i l l i n t e r a c t with i t s environment and tend to reach an e q u i l i b r i u m s t a t e among compartments. HamakerCl) f i r s t used such an approach i n attempting to c a l c u l a t e the percent of a chemical i n the s o i l a i r i n an a i r , water, s o l i d s s o i l system. The r e l a t i o n s h i p s between compartments were chemical e q u i l i b r i u m constants between the water and s o i l ( s o i l p a r t i t i o n c o e f f i c i e n t ) and between the water and a i r (Henry's Law c o n s t a n t ) . T h i s model, as i s t r u e with a l l models of t h i s type, assumes that a l l compartments a r e w e l l mixed, at e q u i l i b r i u m , and a r e homogeneous. At t h i s l e v e l the r a t e s of movement between compartments and degradation r a t e s w i t h i n compartments a r e not considered. Mackay (_2,3) b u i l d i n g upon the e a r l i e r work of Baughman and L a s s i t e r ( 4 ) advanced the development of p a r t i t i o n i n g modeling u s i n g the concept of f u g a c i t y . Here chemical e q u i l i b r i u m or part i t i o n c o e f f i c i e n t s between two phases a r e expressed as an "escaping tendency" the chemical exerts from any given phase. Thus, when a system i s a t e q u i l i b r i u m the f u g a c i t y i n each compartment matches that i n any other compartment. Fugacity has the u n i t s of pressure and p a r t i t i o n c o e f f i c i e n t s between two phases a r e the r a t i o of the f u g a c i t y c a p a c i t i e s i n each phase. In Mackay s development of an e q u i l i b r i u m model a s l i c e of the earth i s s e l e c t e d as a u n i t world or model ecosystem. Fugaci t i e s a r e c a l c u l a t e d f o r each compartment of the ecosystem and the o v e r a l l d i s t r i b u t i o n p a t t e r n s of a given chemical a r e p r e d i c t e d . In a s i m i l a r approach M c C a l l et a l . (_5) have d e f i n e d a model ecosystem which represents a u n i t world, however, t h i s development i n c o r p o r a t e s standard chemical e q u i l i b r i u m expressions i n t o a 1



6.

MCCALL ET A L .

Equilibrium Distribution Partition Models

107

s e r i e s of simultaneous equations to p r e d i c t d i s t r i b u t i o n . Mathe m a t i c a l l y both approaches a r e e s s e n t i a l l y the same with the exception that d i f f e r e n t u n i t s are used. The end r e s u l t i s the same. A model i s obtained which p r e d i c t s d i s t r i b u t i o n p a t t e r n s of chemicals i n a simulated environment r e p r e s e n t a t i v e of a segment of the world. The g o a l i s not to p r e d i c t a c t u a l expected environmental c o n c e n t r a t i o n s , but to p r e d i c t expected behavior. To which phase i s the substance l i k e l y to migrate; w i l l a p e s t i c i d e a p p l i e d to s o i l l e a c h or be v o l a t i l e ; w i l l a chemical accumulate i n the b i o t i c compartment; and so on. The method of u s i n g f u g a c i t y c a l c u l a t i o n s w i l l be d i s c u s s e d l a t e r i n t h i s symposium, t h e r e f o r e a d e t a i l e d d e s c r i p t i o n w i l l not be given i n t h i s paper. The d e s c r i p t i o n of e q u i l i b r i u m models using chemical e q u i l i b r i u m expressions w i l l be discussed with the r e c o g n i t i o n that the two approaches are very much the same. Environmental P a r t i t i o n

Coefficients

S o i l S o r p t i o n Constant - Soil/Water ( K ) . The d i s t r i b u t i o n of a chemical between s o i l and water can be described with an e q u i l i b r i u m expression that r e l a t e s the amount of chemical sorbed to s o i l or sediment to the amount i n the water a t e q u i l i b r i u m . o c

K

d

=

PS chemical/g s o i l yg chemical/g water

where Kd = s o r p t i o n c o e f f i c i e n t yg chemical/g s o i l = c o n c e n t r a t i o n of adsorbed chemical yg chemical/g water = c o n c e n t r a t i o n of chemical i n solution The primary a c t i v e s u r f a c e that i n t e r a c t s with the chemical i n the s o r p t i o n process has been shown to be the organic f r a c t i o n of the s o i l ( 6 - 1 0 ) . Therefore, the s o r p t i o n c h a r a c t e r i s t i c s of a chemical can be normalized to o b t a i n s o r p t i o n constant based on organic carbon ( K ) which i s e s s e n t i a l l y independent of any s o i l . Q C

K oc

- yg chemical/g organic carbon yg chemical/g water

^\

T h i s value, l i k e other p a r t i t i o n c o e f f i c i e n t s i s a measure of the hydrophobicity of a chemical. The more h i g h l y sorbed, the more hydrophobic a substance i s . Henry's Law Constant - Water/Air ( K ) . The d i s t r i b u t i o n of a chemical between water and a i r i s an expression of Henry's Law which can be w r i t t e n as f o l l o w s ( 1 1 ) . w

C =

w

water C . air

=

T(WS) 16.04 PM

=

1

m

H

w


F A T E OF CHEMICALS I N T H E E N V I R O N M E N T

108 where K w C air

r e c i p r i c o l of Henry's Law Constant (H)

C water


P M T WS Such a r e l a t i o n s h i p d e s c r i b e s how a chemical w i l l p a r t i t i o n between water and the atmosphere under e q u i l i b r i u m c o n d i t i o n s and i s a p p r o p r i a t e only f o r d i l u t e s o l u t i o n s which a r e t y p i c a l l y observed i n the environment. C e r t a i n hydrocarbons d e s p i t e possessing r e l a t i v e l y low vapor pressures, may tend to p a r t i t i o n s i g n i f i c a n t l y toward the a i r . This i s l a r g e l y a r e s u l t of t h e i r c o r respondingly low water s o l u b i l i t i e s which r e s u l t i n low values f o r K . Therefore, chemicals which have low values f o r K have a greater tendency to p a r t i t i o n towards the a i r and v o l a t i l i z e from solution. w

w

B i o c o n c e n t r a t i o n Factor - Fish/Water (BCF). The p a r t i t i o n ing of a chemical between water and f i s h i s yet another express i o n of the hydrophobic nature of the chemical. The r a t i o of chemical i n the f i s h to that i n the water at e q u i l i b r i u m i s def i n e d as the b i o c o n c e n t r a t i o n f a c t o r . BCF =

yg chemical/g yg chemical/g

fish water

(4)

where BCF = B i o c o n c e n t r a t i o n f a c t o r yg chemical/g f i s h = c o n c e n t r a t i o n of chemical i n f i s h yg chemical/g water = c o n c e n t r a t i o n of chemical i n water The b i o c o n c e n t r a t i o n f a c t o r , although u s u a l l y r e l a t e d to f i s h i s a c t u a l l y an estimate of the bioaccumulation p o t e n t i a l f o r b i o t a i n general. D i f f e r e n t organisms may bioconcentrate a given chemi c a l to a l e s s e r or greater degree, however with d i f f e r e n t chemic a l s , the r e l a t i v e ranking with respect to b i o c o n c e n t r a t i o n w i l l be e s s e n t i a l l y the same f o r a l l s p e c i e s . C o r r e l a t i o n s Between P a r t i t i o n C o e f f i c i e n t s . As has been p r e v i o u s l y discussed, environmental p a r t i t i o n c o e f f i c i e n t s ar*e to a l a r g e extent a measure of a chemical's tendency to p a r t i t i o n between aqueous and organic media. C o r r e l a t i o n s between v a r i o u s combinations of p a r t i t i o n coe f f i c i e n t s have appeared i n the l i t e r a t u r e . Water s o l u b i l i t y has


6.

MCCALL ET A L .

Equilibrium Distribution Partition Models

109

been r e l a t e d to n-octanol/water r a t i o s ( 1 2 13, 14), bioconcentrat i o n factors(15^, 16i) and s o i l s o r p t i o n constants(17-20) . N-octanol/water r a t i o s have been c o r r e l a t e d with b i o c o n c e n t r a t i o n f a c t o r s (21, 22, 23) and s o i l s o r p t i o n constants have been c o r r e l a t e d with n-octanol/water ratios(18^, 1_9, 24) . More r e c e n t l y Kenega and Goring have given c o r r e l a t i o n equations f o r a l l combinations of these parameters(25). The f o l l o w i n g c o r r e l a t i o n equations were used i n the estimat i o n of p a r t i t i o n c o e f f i c i e n t s used i n t h i s paper. >

lnWS(ppm) = -1.7288 InK - 0.01(MP-25) + 15.1621 (20) In K = In K -0.7301°° (24) In BCF = 0.935 In K -3.443 (_25) o c

o w


o w

The advantages of developing such c o r r e l a t i o n s i s that once any of the parameters i s known i t i s then a simple process to estimate the others. T h i s i s p a r t i c u l a r l y u s e f u l i n e a r l y evaluat i o n of chemical p a r t i t i o n i n g i n the environment. From a l i m i t e d amount of information on a chemical, f o r example, i t s vapor pressure, water s o l u b i l i t y and melting p o i n t , other p a r t i t i o n i n g parameters can be estimated and used i n simple ecosystem models to evaluate the chemical's expected environmental d i s t r i b u t i o n . P a r t i t i o n i n g In Model Ecosystems An ecosystem can be thought of as a r e p r e s e n t a t i v e segment or model of the environment i n which one i s i n t e r e s t e d . Three such model ecosystems w i l l be d i s c u s s e d (Figures 1 and 2 ) . A t e r r e s t r i a l model, a model pond, and a model ecosystem, which combines the f i r s t two models, are described i n terms of e q u i l i b r i u m schemes and compartmental parameters. The s e l e c t i o n of a p a r t i c u l a r model w i l l depend on the questions asked regarding the chemical. For example, i f one i s i n t e r e s t e d i n the p a r t i t i o n i n g behavior of a s o i l - a p p l i e d p e s t i c i d e the t e r r e s t r i a l model would be employed. The model pond would be s e l e c t e d f o r aquatic p a r t i t i o n ing questions and the model ecosystem would be employed i f o v e r a l l environmental d i s t r i b u t i o n i s considered. P a r t i t i o n c o e f f i c i e n t s can then be combined to d e s c r i b e the ecosystem, assuming a l l the compartments are w e l l mixed such that e q u i l i b r i u m i s achieved between them. This assumption i s genera l l y not true of an environmental system s i n c e t r a n s f e r r a t e s between compartments may be slower than transformation r a t e s w i t h i n compartments. Therefore, e q u i l i b r i u m i s never t r u l y approached, except f o r perhaps with very s t a b l e compounds. However, such s i m p l i f i c a t i o n s can g i v e an i n d i c a t i o n i n t o which compartments a chemical w i l l tend to migrate and can provide a mechanism f o r ranking and comparing chemicals. Consider the model ecosystem i n F i g u r e 2, chosen to represent a s l i c e of the environment. The dimensions have been s e l e c t e d to represent a 1000 m x 1000 m square s u r f a c e which c o n t a i n s a 10 km


110

FATE OF CHEMICALS I N T H E E N V I R O N M E N T

MODEL ENVIRONMENTS Terrestrial: Downloaded by CORNELL UNIV on May 18, 2017 | http://pubs.acs.org Publication Date: August 30, 1983 | doi: 10.1021/bk-1983-0225.ch006

K

KQC

W

Soil Air

bk-1983-0225.ch006

Recommend Documents