Chemical Modeling in Aqueous Systems - ACS Publications

27

Effect of the P h y s i c o c h e m i c a l Their

Form

of T r a c e Metals o n

A c c u m u l a t i o n by B i v a l v e M o l l u s c s

FLORENCE L. HARRISON

Downloaded by PURDUE UNIV on December 8, 2016 | http://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch027

Lawrence Livermore Laboratory, University of California, Environmental Sciences Division, P.O. Box 5507, Livermore, CA 94550

Bivalve molluscs effectively concentrate many trace elements (1-13). They are f i l t e r feeders and as such maintain a flow of water through their g i l l s for feeding, respiration, and the removal of metabolic wastes. Trace metals occur in many physicochemical forms in water, and thus can enter animals by their ingestion of living and nonliving particulate material suspended in the water and from the sorption of substances dissolved in the water. We know neither the rates of accumulation nor the effects of physicochemical form on accumulation of many of the c r i t i c a l elements in most animals. Many coastal ecosystems have elevated levels of metals and radionuclides (14). Anthropogenic sources of stable isotopes of metals include sewage disposal plants, electroplating plants, and mining and dredging operations; sources of radioactive isotopes include effluents from nuclear power plants and submarines, medical establishments, and uranium ore mining. The pollution from most of these operations results from routine or accidental discharges and are either continuous or episodic. Models have been developed to predict concentration changes in bivalve molluscs after increased amounts of these pollutants are αis charged into their environment. These models can be used to determine the conditions needed to maintain healthly populations of tbese animals and to minimize adverse effects on man from their consumption. The latter is important because certain metals are implicated in acute health problems in man, and a continued concern exists about the dose to man from radionuclides released into aquatic environments. In the presentation that follows>we w i l l consider the mathematical models that have been developed and the model parameters required to predict concentration changes in the animals. In addition, experimental data w i l l be provided that indicate the sensitivity of model parameters to differences i n physicochemical form of the elements in the water and to differences in metabolic responses among species. These kinds of 0-8412-0479-9/79/47-093-611$06.00/0 © 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

612

CHEMICAL

MODELING IN AQUEOUS

SYSTEMS

i n f o r m a t i o n w i l l p r o v i d e an i n d i c a t i o n o f t h e r e l i a b i l i t y o f m o d e l p r e d i c t i o n s and o f the a r e a s i n w h i c h a d d i t i o n a l d a t a a r e needed.


M a t h e m a t i c a l M o d e l s and M o d e l

Parameters

Mathematical Models. The a c c u m u l a t i o n o f an e l e m e n t by any pathway c a n i n v o l v e a number o f d i f f e r e n t p r o c e s s e s . I f the r a t e - d e t e r m i n i n g p r o c e s s can be d e s c r i b e d m a t h e m a t i c a l l y , a model c a n b e d e v e l o p e d to p r e d i c t c h a n g e s i n c o n c e n t r a t i o n w i t h t i m e and l o c a t i o n . A c o n s i d e r a b l e e f f o r t h a s b e e n made to d e v e l o p m o d e l s to p r e d i c t the d i s t r i b u t i o n o f r a d i o n u c l i d e s r e l e a s e d i n t o the e n v i r o n m e n t (_15). The t y p e s o f m o d e l s d e v e l o p e d to p r e d i c t c o n c e n t r a t i o n s o f r a d i o n u c l i d e s i n a q u a t i c organisms i n c l u d e e q u i l i b r i u m (lj>, 17_, 18) and dynamic m o d e l s ( 1 £ > 2 0 ) . The t y p e o f m o d e l to b e u s e d i n a g i v e n s i t u a t i o n d e p e n d s on the n a t u r e o f the r e l e a s e and on the p r o p e r t i e s o f t h e ecosystem. When r e l e a s e s o f m e t a l s or r a d i o n u c l i d e s are c o n t i n u o u s and s t e a d y - s t a t e c o n d i t i o n s a r e p r e s e n t , an e q u i l i b r i u m m o d e l s u c h as a c o n c e n t r a t i o n f a c t o r m o d e l c a n b e used. I n t h i s c a s e , t h e i m p o r t a n t p a r a m e t e r needed f o r the m o d e l i s the c o n c e n t r a t i o n f a c t o r , t h e r a t i o o f the c o n c e n t r a t i o n i n t h e a n i m a l to t h a t i n t h e w a t e r . The c o n c e n t r a t i o n i n t h e a n i m a l i s d e t e r m i n e d t h e n by m u l t i p l y i n g the c o n c e n t r a t i o n i n the w a t e r by t h e c o n c e n t r a t i o n f a c t o r . I n the c a s e o f a c c i d e n t a l e p i s o d i c r e l e a s e s , a dynamic s i t u a t i o n e x i s t s i n which organisms a c c u m u l a t e the m a t e r i a l f o r a r e l a t i v e l y s h o r t p e r i o d and t h e n lose i t with a c h a r a c t e r i s t i c time c o n s t a n t . In this case,the i m p o r t a n t p a r a m e t e r s needed f o r the m o d e l a r e b i o l o g i c a l t u r n o v e r r a t e c o n s t a n t s and c o n c e n t r a t i o n factors. Model Parameters. C o n c e n t r a t i o n f a c t o r s were d e t e r m i n e d f o r l a r g e numbers o f b i v a l v e m o l l u s c s . V a l u e s were o b t a i n e d by d e t e r m i n i n g the c o n c e n t r a t i o n o f s t a b l e a n d / o r r a d i o a c t i v e n u c l i d e s i n a n i m a l s and w a t e r t h a t were c o l l e c t e d d i r e c t l y f r o m t h e e n v i r o n m e n t and t h a t were s a m p l e d i n l a b o r a t o r y e x p e r i m e n t s . T h e s e d a t a were c o m p i l e d f o r u s e i n m o d e l s to p r e d i c t r a d i o n u c l i d e c o n c e n t r a t i o n s i n whole organisms or t h e i r t i s s u e s . In g e n e r a l , f o r b i v a l v e m o l l u s c s a s i n g l e v a l u e i s g i v e n f o r each element. The t u r n o v e r o f t r a c e m e t a l s i n o r g a n i s m s d e p e n d s on d y n a m i c p r o c e s s e s o f exchange w i t h elements i n the e n v i r o n m e n t . Compartments o f e l e m e n t s a r e i d e n t i f i e d f r o m a m a t h e m a t i c a l a n a l y s i s o f the changes i n c o n c e n t r a t i o n d u r i n g a c c u m u l a t i o n or loss. The r e s o l u t i o n o f c o m p a r t m e n t s i s l i m i t e d by e x p e r i m e n t a l e r r o r , and the c o m p a r t m e n t s t h a t c a n be i d e n t i f i e d a r e t h o s e whose c o n c e n t r a t i o n s d i f f e r s i g n i f i c a n t l y i n t h e i r e x p o n e n t i a l change. T h e s e c o m p a r t m e n t s may be p h y s i o l o g i c a l , s t r u c t u r a l , o r c h e m i c a l e n t i t i e s , and t h e i r m e t a b o l i c r o l e s may n o t b e known. The t u r n o v e r t i m e o f an e l e m e n t i n an o r g a n i s m depends upon t h e o r g a n i s m and e l e m e n t . In small organisms w i t h a l a r g e

Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


27.

Physiochemical Form of Trace Metals

HARRISON

613

surface-to-volume r a t i o , the turnover time o f monovalent elements such as Na or Cs may be minutes, whereas i n large organisms and m u l t i v a l e n t elements, months may be r e q u i r e d . The turnover i s a l s o a f u n c t i o n o f the metabolism o f the element by the organism. The q u a n t i t i e s accumulated by organisms when the concentrations are increased i n the water d i f f e r g r e a t l y f o r those elements that are and are not under homeostatic c o n t r o l . Turnover r a t e s were determined by f o l l o w i n g the accumulation or l o s s o f r a d i o n u c l i d e s from animals i n the f i e l d or i n the l a b o r a t o r y . They were determined also by f o l l o w i n g the increased q u a n t i t i e s o f trace metals i n animals that were exposed to increased q u a n t i t i e s o f the element i n the water. The change i n concentration o f a trace element i n an organism at any time may be described by: dC/dt = k£W(t) - k C ( t ) ,

(1)

0

where W(t) C k£ = k = Q

= the concentration i n the water at time, t , = the concentration i n the organism, the b i o l o g i c a l accumulation rate constant, the b i o l o g i c a l loss rate constant.

At steady-state c o n d i t i o n s , dC/dt = 0, and k W = k C(s), £

(2)

Q

where C(s) = the concentration conditions.

i n the organism a t steady state

Assuming f i r s t order k i n e t i c s and a constant concentration i n the water, equation 1 upon i n t e g r a t i o n becomes: k.W -k t C(t) = [1 - e ° ] . (3) ο S u b s t i t u t i n g C(s) = k|W/k i n t o equation 3 , we have: Q

C(t) = C(s) [1 - e " * ^ ] .

(4)

In those s i t u a t i o n s where the c o n c e n t r a t i o n i n the water i s known, concentration f a c t o r s (CF) can be s u b s t i t u t e d f o r concentrations i n the animal to g i v e : -k t CF(t) = CF(s) [1 - e ° ] (5) (


614

CHEMICAL

MODELING

IN

AQUEOUS

SYSTEMS

where CF(t) = the c o n c e n t r a t i o n f a c t o r i n the organism a t time, t , CF(s) = the c o n c e n t r a t i o n f a c t o r i n the organism a t steady-state c o n d i t i o n s . The l o s s of stable or r a d i o a c t i v e n u c l i d e s may be described by: C(t) = C ( i ) [ e "

k o t

],

(6)


where C(i)

= the i n i t i a l c o n c e n t r a t i o n i n the organism.

From k , the b i o l o g i c a l ha I f - l i f e (T%) o f a trace element i n an organism may be determined from the r e l a t i o n s h i p : 0

k ο

V a r i a t i o n s i n Model Parameters R e s u l t i n g from Species D i f f e r e n c e s Whole Body Radionuclide Concentration Factors and Turnover Rates. A s e r i e s o f experiments were performed to determine the turnover r a t e s and c o n c e n t r a t i o n f a c t o r s o f Co, Cs, Mn, and Zn i n the marine clam Mya a r e n a r i a and the oyster Crassostrea gigas. The r e s u l t s published p r e v i o u s l y (21, 2_2) are included f o r comparison. Radionuclide accumulation was followed i n l a b o r a t o r y systems i n which the concentrations of s t a b l e elements were kept constant (220. Changes i n concentrations were followed i n c r i t i c a l t i s s u e s as w e l l as the e n t i r e body. The loss of r a d i o n u c l i d e s was followed i n animals that had accumulated the r a d i o n u c l i d e s for 48 d i n the l a b o r a t o r y . A f t e r exposure to the s t a b l e and r a d i o a c t i v e n u c l i d e s , they were t r a n s f e r r e d to n o n r a d i o a c t i v e , u n f i l t e r e d , c i r c u l a t i n g seawater(at the Marine Laboratory o f C a l i f o r n i a State U n i v e r s i t y , Humbolt) i n which b i o l o g i c a l loss of the r a d i o n u c l i d e s was followed. In the oyster C. g i g a s , l a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n f a c t o r s and turnover rates o f Co, Cs, Mn, and Zn were found i n the s o f t t i s s u e s (Table I ) . The highest c o n c e n t r a t i o n f a c t o r measured was f o r Zn and the lowest f o r Cs. In the clam M. a r e n a r i a , large d i f f e r e n c e s i n c o n c e n t r a t i o n f a c t o r s and turnover rates of these elements were a l s o found (Table I ) . Cobalt had the highest c o n c e n t r a t i o n f a c t o r and Cs had the lowest. In both animals, two compartments were i d e n t i f i e d f o r most elements. D i f f e r e n c e s i n the percentages of the element i n each compartment were found for those elements which had two i d e n t i f i a b l e compartments. Comparison of the c o n c e n t r a t i o n f a c t o r s f o r Mn and Co shows more than an order of magnitude


27.

HARRISON

615


d i f f e r e n c e between the values for oysters and clams. Greater s i m i l a r i t y was found i n turnover r a t e s than c o n c e n t r a t i o n f a c t o r s ; Co, Mn, and Zn had turnover times o f 2 to 3 mo i n both TABLE I C o n c e n t r a t i o n F a c t o r s and H a l f - L i v e s i n S o f t Tissues

Concentration Factor


Element Zinc Cobalt (_22) Manganese Cesium (22)

1200 50 35 10

Cobalt (21) Manganese Zinc Cesium (21)

790 590 320 5

Half-life,d Compartment Compartment 2 1 A. Oysters 98 ( i o o ) 130 (62) 6 (38) 98 (76) 4 (24) 6 (60) 2 (37) Β. Clams 120 (100) 70 (70) 8 (30) 110 (67) 30 (33) 60 (25) 4 (75) a

Value i n p a r e n t h e s i s i s the percent of the t o t a l body element i n t h e compartment.

s p e c i e s , whereas f o r Cs i t was < 1 wk. These data on c o n c e n t r a t i o n f a c t o r s , turnover r a t e s , and compartment s i z e s suggest d i f f e r e n t metabolic pathways o f these elements i n both an ima 1 s. Tissue Turnover o f R a d i o n u c l i d e s . Oysters and clams that had accumulated e i t h e r 54Mn and 65Zn or 60Co and 137 for about lh mo i n the l a b o r a t o r y were t r a n s f e r r e d to u n f i l t e r e d oceanic water, and changes i n concentrations o f the r a d i o n u c l i d e s i n the t i s s u e s were followed for about 5 mo. Large d i f f e r e n c e s were found i n the r a t e s o f r a d i o n u c l i d e s loss from the t i s s u e s o f both the o y s t e r s (Figure l)and the clams (Figure 2 ) . Three p a t t e r n s o f loss during the experimental p e r i o d were observed. From many t i s s u e s , the l o s s was monophasic, from others i t was b i p h a s i c , but i n some there was an increase or a period o f l i t t l e change before the loss occurred. The l a s t p a t t e r n was seen for 65Zn i n some t i s s u e s o f oysters and clams and i n d i c a t e s that z i n c may be m o b i l i z e d from some t i s s u e s f o r accumulation i n others. The accumulation o f M n , C o , Z n , and C s , was followed i n M. a r e n a r i a under c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s . The changes i n c o n c e n t r a t i o n f a c t o r s o f the r a d i o n u c l i d e s during accumulation d i f f e r e d i n each t i s s u e , and, i n a given t i s s u e , w i t h each r a d i o n u c l i d e (Figure 3). Only f o r 1 3 7 were steady-state c o n d i t i o n s approached i n a l l the t i s s u e s , even though the accumulation was followed for more than 5 mo. The s c a t t e r i n the data during the sampling p e r i o d was g r e a t e s t i n Cs

54

6 0

6 5

1 3 7

C 8


CHEMICAL MODELING IN AQUEOUS SYSTEMS

Crassostrea gigas Gills


Muscle MO [\ I I I

I

k

i Λ

10 1 -

ioo U

Ε Ε & i

l

Mantle —ι—Γ

"

J

J

Ί

Viscera 1—I Γ

J

I L

J

I L

A—

Manganese I I I L

0.1

£

l

J

I

I—L

J

I I L

Digestive gland in—ι—r IL

J

I L

ι -I

ιο

0

ί Zinc J I I L

1

« 100 K]

1

1

J Ί

1

I L 1 1 Γ

!

A

0.1 100

Cesium _J L J L Ί—I—I

Γ

J

I I L

ζΓΊ—I—I—Γ

J

I—I Γ

^ : J

I

Ί—I

I

L

1—Γ

L

Ί—I—I—Γ

J

I L

ζ~ι—ι—r

J i

1

1

L 1

1

10 1 0.1

Cobalt _J I I L 50

150

250

J

50

I L

150 250

J

50

I I L

150 250

J 50

I L 150 250

50

150 250

Time, days Figure 1. Percent of initial radioactivity in tissues of Crassastrea gigas during loss period


27.

617


HARRISON

Mya arenaria Gills


Muscle

100

prn—ι—

10

Ί

\

-

_

1 0.1 1000

τ—r

-\

Manganese

ι Ί

Αχ

I

Zinc

Viscera

Mantle

Γ

J

L

J

L

J

L

J

L

X

Digestive gland

ί

ι

1—

-

V

-

Γ

100 10 1

J

100 ξ

L 1

r-

Cesium 10 10.1

J

L

100 Cobalt

10 1 0.1 0

J

L

50 100 150 0

50 100 150 0

J

L

50 100 150 0

J 50

L 100 150 0

50 100 150

Time, days

Figure 2. Percent of initial radioactivity in tissues of Mya arenaria during loss period


CHEMICAL MODELING IN AQUEOUS

SYSTEMS


618

Figure 3.

Concentration factors in tissues of My a arenaria during accumulation period



27.

HARRISON

Thysiochemical Form of Trace Metals

619

the d i g e s t i v e gland and stomach and the v i s c e r a . Also i n these t i s s u e s higher f r a c t i o n a l standard d e v i a t i o n s i n the concentrations were found i n the e i g h t animals s a c r i f i c e d a t each sampling time. The turnover r a t e s o f r a d i o n u c l i d e s were measured i n b i v a l v e molluscs both i n the f i e l d and i n the laboratory (21-30). Seymour (27) obtained a b i o l o g i c a l h a l f - l i f e o f 300 d for ^Zn i n C. gigas and Wolfe (7) an e c o l o g i c a l h a I f - l i f e o f 347 d i n C. v i r g i n i c a . F r a z i e r (_10) c a l c u l a t e d a h a l f - l i f e on the order o f 40 to 50 d f o r Zn i n v i r g i n i c a , a value that i s c l o s e r to the 100 d reported here for gigas. Some r e s u l t s i n d i c a t e that the rates obtained i n the f i e l d vary s e a s o n a l l y ; such v a r i a t i o n i s suggested by the turnover r a t e s o f ^Zn i n C. gigas (26) and o f Zn and Cu i n virginica ( 11). F r a z i e r (K)) found that the uptake of metals by oysters t r a n s f e r r e d to contaminated environments depends upon the season. Uptake by Cj_ v i r g i n i c a was r a p i d i n the summer and f a l l , but was low i n the e a r l y s p r i n g . Rates o f accumulation and l o s s may d i f f e r s i g n i f i c a n t l y . George and Coombs (31) followed the accumulation and loss o f Cd i n M y t i l u s e d u l i s . The rate o f Cd uptake was 18 times f a s t e r than that o f e x c r e t i o n . They concluded that the slower e l i m i n a t i o n i s a consequence o f a need to d e t o x i f y and store Cd by an i m m o b i l i z a t i o n mechanism. Whole Body Stable Element Concentration F a c t o r s . Concentrations o f some s t a b l e elements i n populations o f oyster C. gigas and the clam Saxidomus n u t t a l l i were monitored to determine the normal seasonal changes i n concentrations. For each sample, the soft t i s s u e s were separated from the s h e l l , r i n s e d i n seawater, and pooled to give a composite sample. Ten to twelve clams and 50 to 100 oysters were d i s s e c t e d a t each sampling. The wet t i s s u e s were weighed and d r i e d i n an oven a t 103°C f o r at l e a s t 48 h. Samples were weighed a f t e r ashing to constant weight a t 450 °C i n a muffle furnace. Elemental analyses were performed by neutron a c t i v a t i o n (32). The concentration i n the s o f t t i s s u e s o f oysters on an ash weight b a s i s was Zn - Fe > Mn > Co (Table I I ) . The c o n c e n t r a t i o n f a c t o r s o f the s t a b l e elements, Co, Mn, and Zn were higher than those determined by r a d i o n u c l i d e studies i n the laboratory ( c f . Tables I and I I ) . The d i f f e r e n c e s between the r a d i o a c t i v e and s t a b l e n u c l i d e c o n c e n t r a t i o n f a c t o r s may be due to the presence i n the organism of slowly exchangeable or nonexchangeable compartments. Steady-state c o n d i t i o n s o f stable and r a d i o a c t i v e isotopes would not be reached i n such compartments during the period o f most laboratory experiments, and the c o n t r i b u t i o n of these compartments to the concentration f a c t o r would be overlooked. Because most stable element analyses do not d i s t i n g u i s h compartments but give the t o t a l amount i n a l l compartments, the concentration f a c t o r o f a stable isotope would be l a r g e r than


CHEMICAL

620

MODELING IN AQUEOUS

TABLE I I T r a c e M e t a l C o n c e n t r a t i o n s and C o n c e n t r a t i o n i n the Oyster Crassostrea gigas


Concentration Element

( y g / g wet w e i g h t ) Range Mean

Zinc

100

Manganese Cobalt Iron

10 0.06 100

73-140 3.1-21. 0.008-0.21 38.

-380

Seawater (yg/i) 6 1.2 0.1 3

SYSTEMS

Factors

Concentration Factor 17,000. 8,300. 600. 33,000.

t h a t o f the r a d i o i s o t o p e . O p h e l (_33) i n d i c a t e d t h a t i n some b i o t a s u c h a s i t u a t i o n e x i s t s f o r S r and p e r h a p s f o r C o . D i f f e r e n c e s i n c o n c e n t r a t i o n f a c t o r s b e t w e e n s t a b l e and r a d i o a c t i v e i s o t o p e s may r e s u l t a l s o f r o m o t h e r f a c t o r s . I f more t h a n one c h e m i c a l f o r m o f t h e e l e m e n t a r e p r e s e n t i n t h e e n v i r o n m e n t , t h e y may b e s u b j e c t to d i f f e r e n t t u r n o v e r and concentration processes. Seasonal effects could r e s u l t i f d i f f e r e n t p h y s i c o c h e m i c a l forms dominate d u r i n g p a r t s o f the y e a r . L a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n s o f C o , F e , M n , and Z n d u r i n g t h e s a m p l i n g p e r i o d were f o u n d i n o y s t e r s c o l l e c t e d a t n e a r - m o n t h l y i n t e r v a l s o v e r a number o f s e a s o n s ( F i g . 4 ) . C o n c e n t r a t i o n s o f Z n r a n g e d f r o m 3300 t o 9 0 0 0 , o f Fe f r o m 2000 t o 9 1 0 0 , o f Mn f r o m 270 to 7 8 0 , and o f Co f r o m 2 . 5 to 5 . 5 y g / g a s h w e i g h t d u r i n g t h e 17 mo p e r i o d . Seasonal f l u c t u a t i o n s i n c o n c e n t r a t i o n s o f these metals i s c l e a r l y e v i d e n t . The p a t t e r n o f changes i n F e , Z n , and Co were s i m i l a r ; c o n c e n t r a t i o n s were h i g h i n t h e s p r i n g and l a t e f a l l , and low i n t h e summer and p a r t of winter. The f a l l p e a k was s e e n i n b o t h 1972 and 1973. The p a t t e r n o f c h a n g e o f Mn was o p p o s i t e f r o m t h e o t h e r t h r e e , i . e . , i t s c o n c e n t r a t i o n was low when t h e o t h e r s were h i g h . L a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n b e t w e e n some o f t h e s a m p l e s were m e a s u r e d . The c o n c e n t r a t i o n o f Z n on O c t o b e r

1,1973,

was 4100 and on November 10, 1973, i t was 9000 y g / g a s h weight. T h i s approximate d o u b l i n g o f the c o n c e n t r a t i o n i n about 6 wks i n d i c a t e s a v e r y r a p i d r a t e o f c h a n g e . The d i f f e r e n c e s i n r a t e s o f change i n c o n c e n t r a t i o n s u g g e s t t h a t t h e d y n a m i c s o f a c c u m u l a t i o n and l o s s may d i f f e r d u r i n g t h e y e a r . L a r g e d i f f e r e n c e s i n c o n c e n t r a t i o n s o f C o , F e , and Zn w i t h t i m e were o b s e r v e d i n c l a m s a l s o ( F i g u r e 5 ) . C o n c e n t r a t i o n s o f Fe r a n g e d f r o m 600 to 1 5 , 0 0 0 , o f Z n f r o m 250 t o 500, and o f Co f r o m 6 . 5 to 15.5 u g / g a s h w e i g h t . S e a s o n a l t r e n d s were n o t as o b v i o u s i n c l a m s as i n o y s t e r s , b u t c y c l i c c h a n g e s d u r i n g t h e 18-mo i n t e r v a l were a p p a r e n t . ?


HARRISON



27.


621

CHEMICAL

622

MODELING IN AQUEOUS

SYSTEMS


Saxidomus nuttalli

J

F

M

A

M J J A Time months

S

O

N

D

f

Figure 5. Concentration of metals in soft tissues of the clam Saxidomus nuttalli



27.

HARRISON

Thysiochemical Form of Trace Metals

623

Seasonal f l u c t u a t i o n s i n metal c o n c e n t r a t i o n s were described for the oyster C. v i r g i n i c a (JL, ICO ; elevated l e v e l s i n the summer are followed by decreased l e v e l s i n the w i n t e r . F r a z i e r (10) found that Mn concentrations i n C. v i r g i n i c a were p o s i t i v e l y c o r r e l a t e d only w i t h those of Fe; Fe concentrations c o r r e l a t e d with Cd and Zn; and Cd, Cu, Fe, and Zn concentrations were h i g h l y c o r r e l a t e d w i t h each other. We found Co, Fe, and Zn to behave s i m i l a r l y i n C. gigas and Mn to vary i n v e r s e l y . F r a z i e r (JJO), i n reviewing the hypotheses suggested f o r the c o n t r o l o f metal uptake, categorized these hypotheses i n t o four groups r e l a t e d t o : 1) s h e l l growth and metabolism, 2) feeding, 3) spawning, and 4) p h y s i c a l chemistry o f the metal e i t h e r i n the water or a t the water-membrane i n t e r f a c e . He i d e n t i f i e d two fundamentally d i f f e r e n t patterns o f behavior i n C^ v i r g i n i c a e x e m p l i f i e d by Mn and Zn. The concentrations o f Mn i n s o f t t i s s u e are c o r r e l a t e d s i g n i f i c a n t l y w i t h s h e l l growth, whereas Zn concentrations d r a m a t i c a l l y decrease before the period o f maximum s h e l l growth. The changes i n Zn seem to be c l o s e l y r e l a t e d to gonadal development and spawning. D i f f e r e n c e s i n the p a t t e r n s o f changes i n Mn and Zn were a l s o evident i n the r e s u l t s reported here on C. gigas. However, no data are a v a i l a b l e that r e l a t e the changes d i r e c t l y to s h e l l growth or to reproductive changes. Seasonal f l u c t u a t i o n s i n metal c o n c e n t r a t i o n s were reported for the mussel M y t i l u s e d u l i s (34). Mussels from the Elbe/Cuxhaven s i t e i n Germany have highest concentrations normally i n l a t e winter and s p r i n g , and the lowest i n l a t e summer and autumn. Bryan (_35) reported annual f l u c t u a t i o n s i n the content o f Co, Cu, Fe, Mn, N i , Pb, and Zn i n s c a l l o p s . In g e n e r a l , highest c o n c e n t r a t i o n s were found i n the winter and lowest i n the summer and autumn. The data a v a i l a b l e demonstrate seasonal changes i n element c o n c e n t r a t i o n i n a number o f b i v a l v e molluscs. The maximum and minimum concentrations do not occur a t the same time o f the year i n a l l ecosystems. This i s expected because seasonal changes are not concurrent i n a l l ecosystems. I d e n t i f i c a t i o n has not been made o f the r e l a t i v e c o n t r i b u t i o n to these changes o f v a r i a t i o n s i n the amounts and physicochemical forms o f the elements i n the water and i n the metabolic processes i n the organisms. However, seasonal changes can be s i g n i f i c a n t and should be taken i n t o c o n s i d e r a t i o n i n model development. E f f e c t o f P h y s i c a l and Chemical F a c t o r s Presence and Absence o f P a r t i c l e s . Turnover o f Co and Cs i n Mya a r e n a r i a were measured by f o l l o w i n g both the accumulation and loss o f r a d i o n u c l i d e s . However, the c o n d i t i o n s under which the accumulation were measured d i f f e r e d from those o f l o s s . During the accumulation p e r i o d , the animals were maintained i n f i l t e r e d water i n a closed system, whereas during the loss p e r i o d , they


624

CHEMICAL

M O D E L I N G IN

AQUEOUS

SYSTEMS

were maintained i n once-through, u n f i l t e r e d seawater that contained the food organisms and p a r t i c u l a t e m a t e r i a l to which they are exposed normally. Fluxes of Cs and Co i n the clam during the accumulation and loss periods were c a l c u l a t e d (Table I I I ) . The f l u x of an element i n the t i s s u e s i s the product of


TABLE I I I Fluxes i n Mya a r e n a r i a Determined During and Loss Experiments (21)

Body p a r t

Cesium , yg/d/g (1x10*) Accumulation Loss

Mantle G i l l s and l a b i a l palps Muscle D i g e s t i v e gland and stomach Viscer a

Accumulation

Cobalt , yg/d/g (1x10*) Accumulation Loss

8.1

8.8

5.5

1.2

14. 3.0

14. 5.1

3.5 3.7

14. 0.4

16. 3.4

9.3 7.1

47. 4.4

100. 21.

the loss rate constant and the c o n c e n t r a t i o n i n the animal. R e l a t i v e l y good agreement for l ^ C s between the c a l c u l a t e d values of f l u x e s from l a b o r a t o r y accumulation data and those from f i e l d loss data were obtained; the r a t e constants determined by the two methods were s i m i l a r . These r e s u l t s suggest that the chemical form of the r a d i o n u c l i d e was s i m i l a r and that the major source o f the l ^ C s £ ^ i £ f m the water; no s i g n i f i c a n t d i f f e r e n c e s were detected between the two s i t u a t i o n s . Greater d i f f e r e n c e s were found i n the f l u x of Co during the two periods than i n that of Cs. Much l a r g e r f l u x e s of Co were measured during loss than during up t a k e , i n the g i l l s , the d i g e s t i v e gland and stomach, and the v i s c e r a . These d i f f e r e n c e s may r e f l e c t the a v a i l a b i l i t y of food or may i n d i c a t e that c o b a l t f o l l o w s a d i f f e r e n t metabolic pathway during accumulation and l o s s . Thus, c o b a l t may be deposited more r a p i d l y i n a compartment than i t can be m o b i l i z e d from i t . We i n v e s t i g a t e d the e f f e c t s of p a r t i c u l a t e m a t e r i a l i n another s e r i e s of experiments (_25). Oysters were introduced i n t o a discharge canal o f a nuclear p l a n t before scheduled r e l e a s e s of r a d i o a c t i v i t y . The q u a n t i t i e s of the r a d i o n u c l i d e s accumulated during the r e l e a s e were measured during a p e r i o d of normally high ( J u l y ) and a p e r i o d o f normally low b i o l o g i c a l p r o d u c t i v i t y (December). During each p e r i o d , the animals were d i v i d e d i n t o two groups: one received untreated seawater, and the other f i l t e r e d seawater. During the J u l y experiment, the oysters were placed i n the canal at 1700 h the evening before the r e l e a s e . Some oysters o r

t

e

c

a m s

s

r 0



27.

625


HARRISON

were removed from the water a t 0850 h ( p r e r e l e a s e ) ; the r e l e a s e occurred at 0900 h and terminated a t 1100 h. Only ^Zn was detected i n the p r e r e l e a s e animals maintained i n n o n f i l t e r e d seawater (Table I V ) . The r e l a t i v e l y higher concentrations i n the p r e r e l e a s e oysters maintained i n f i l t e r e d water compared to those i n n o n f i l t e r e d water may be because o f increased a v a i l a b i l i t y r e s u l t i n g from the leaching o f p a r t i c l e s that had been deposited on the f i l t e r during the 8-h i n t e r v a l during which the p a r t i c l e s were c o l l e c t e d . Desorption o f r a d i o n u c l i d e s from the p a r t i c l e s on the f i l t e r would occur any time the water flowing through the f i l t e r had a lower s p e c i f i c a c t i v i t y than that i n which the p a r t i c l e s were o r i g i n a l l y suspended. In the animals sampled at 1100 h ( p o s t r e l e a s e ) , concentrations o f a l l r a d i o n u c l i d e s were elevated. Concentrations i n p o s t r e l e a s e oysters i n n o n f i l t e r e d seawater were higher than those i n f i l t e r e d seawater f o r a l l r a d i o n u c l i d e s . The amounts accumulated per hour i n animals i n f i l t e r e d water compared to those i n n o n f i l t e r e d water were lower by 95% f o r C o , by 78% f o r Mn, by 52% f o r Z n , and by 40% f o r > C s . On December 4, 1973, the p r e r e l e a s e oysters were removed a t 0900 h, and the p o s t r e l e a s e were removed a t 1230 h; animals were placed i n the discharge canal water a t 1530 h on December 3. Compared w i t h the expected concentrations i n the water, the amounts o f most r a d i o n u c l i d e s accumulated per hour during the release were lower i n December than i n J u l y (Table I V ) . The d i f f e r e n c e s between the q u a n t i t i e s accumulated i n oysters held i n f i l t e r e d vs. n o n f i l t e r e d water suggest that p a r t i c l e s p l a y an important r o l e i n the accumulation^ o f elements. P a r t i c l e s i n the water may be l i v i n g microorganisms, organic d e t r i t u s , i n o r g a n i c m a t e r i a l , or any combination o f the three and may vary i n q u a n t i t y , both w i t h time and l o c a t i o n . The r o l e o f p a r t i c l e s i n r a d i o n u c l i d e accumulation d i f f e r e d w i t h each r a d i o n u c l i d e . I n oysters, ^ C o p p to be accumulated p r i m a r i l y from the suspended p a r t i c u l a t e f r a c t i o n , whereas 134,137ç appears to be accumulated p r i m a r i l y from the soluble f r a c t i o n . The d i f f e r e n c e s between the amounts o f r a d i o n u c l i d e s accumulated i n J u l y and December probably r e s u l t e d p r i m a r i l y from d i f f e r e n c e s i n the composition o f the suspended p a r t i c l e s ; p a r t i c l e loads were 25 and 15 mg/1 dry weight, r e s p e c t i v e l y (25). Lack o f c o r r e l a t i o n was reported i n the l e v e l s o f Cu and Zn i n f i l t e r e d water and those i n oysters (_36). This poor degree o f c o r r e l a t i o n may be r e l a t e d to the i n g e s t i o n o f suspended p a r t i c u l a t e m a t e r i a l i n the seawater (2_, _37_). Because the p a r t i c l e s may vary i n q u a n t i t y and composition both w i t h time and l o c a t i o n i n an environment, the q u a n t i t i e s o f those elements w i t h high a f f i n i t i e s f o r p a r t i c l e s that are accumulated by organisms should vary correspondingly. E f f e c t o f Physicochemical Form. The physicochemical form o f the element may a f f e c t the q u a n t i t i e s accumulated. To assess the 6 0

1 3 4

54

6 5

1 3 7

a

e a r s

s



detected.

6.28

80 ± 7 157 ± 11 26

70 ± 10 60 ± 12 0

9.31

ND ND 0

30 ± 8 40 ± 8 3

ND 498 ± 20 166

ND 310 ± 12 103 349.

60 ± 25 200 ± 30 14 2.97

467.

ND 500 ± 22 162

ND 324 ± 15 108

130 ± 30 200 ± 40 23

423.

258.

9.05

12.8

4.85

Amount accumulated per hour during the r e l e a s e

C

8.0 ± 2.7 210 ± 6.3 101

1.0 ± 1.0 99 ± 4.0 49

Cs

200 ± 20 490 ± 20 145

1 3 7

1.0 ± 1.0 14 ± 3.8 7

Cs

46 ± 5.1 72 ± 3.6 13

1 3 4

ND 340 ± 6.7 170

Zn

ND 160 ± 4.8 80

6 5

150 ± 25 750 ± 30 300

Co

ND 260 ± 7.7 130

a

6 0

ND 120 ± 4.7 60

Mn

i n Oysters (25)

'Radionuclide c o n c e n t r a t i o n was c a l c u l a t e d from the known d i l u t i o n of the source m a t e r i a l .

)

ND, not

December 4, 1973 N o n f i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h) F i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h) Seawater c o n c e n t r a t i o n (pCi/1)

D

J u l y 31, 1973 N o n f i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h ) F i l t e r e d seawater P r e r e l e a s e (pCi/kg) P o s t r e l e a s e (pCi/kg) Rate (pCi/kg h) Seawater concentration ( p C i / l )

54

TABLE IV R a d i o n u c l i d e Concentrations


27.

HARRISON

627


e f f e c t s o f the physicochemical form o f Cu and Zn on accumulation, groups o f twelve oysters were held i n a flow-through system i n seawater c o n t a i n i n g ^ Cu, ^^Zn, and e i t h e r g l y c i n e (1 χ 10" M\ ethylenediaminetetraacetate (EDTA, 1 χ 10~ M), "yellow s t u f f " (2 mg/1), or c l a y (5 mg k a o l i n / 1 ) . Animals were removed from the test s o l u t i o n s , d i s s e c t e d , weighed, and the r a d i o n u c l i d e s q u a n t i f i e d . A f t e r a 24 h exposure, the c o n c e n t r a t i o n f a c t o r s i n the oysters d i f f e r e d both w i t h the t e s t m a t e r i a l and w i t h the element (Table V ) . In the t i s s u e s , c o n c e n t r a t i o n f a c t o r s o f both elements g e n e r a l l y were high i n the g i l l s and the d i g e s t i v e gland and stomach and low i n the muscle and the blood. The data on physicochemical forms suggest that t h i s i s an important f a c t o r i n the accumulation o f elements. Concentration f a c t o r s both increased and decreased. George and Coombs (31) reported that complexation to EDTA, humic or a l g i n i c a c i d s , or p e c t i n doubled the r a t e s o f accumulation and f i n a l t i s s u e concentrations i n e d u l i s and e l i m i n a t e d a lag p e r i o d . Whether the changes i n the q u a n t i t i e s accumulated are due to d i f f e r e n c e s i n rates of transport of the d i f f e r e n t forms o f metals across b i o l o g i c a l membranes, to changes i n feeding r a t e s stimulated by the presence o f organic m a t e r i a l i n the water, or to i n t e r a c t i o n s of metals and l i g a n d s i n the water that a l t e r the q u a n t i t i e s o f the b i o l o g i c a l l y a v a i l a b l e forms o f the metal i s not known. However, d i f f e r e n c e s i n turnover r a t e s from laboratory and f i e l d studies may be explained i n p a r t by t h i s f ac tor. E f f e c t o f Concentration. To p r e d i c t the amounts o f r a d i o a c t i v e or stable n u c l i d e s o f an element that may be accumulated when increased q u a n t i t i e s are present i n the water, we must know i f the c o n c e n t r a t i o n o f the stable n u c l i d e i s under metabolic c o n t r o l . To t e s t for a homeostatic mechanism for Zn and Mn i n oysters and f o r Co and Cs i n clams, groups o f oysters and clams were exposed to increased concentrations o f these elements i n an attempt to saturate any r e g u l a t o r y mechanisms. The r e s u l t s reported e a r l i e r for Co and Cs on clams (21) are included for comparison to those for Mn and Zn i n o y s t e r s . The accumulation o f ^ Mn and 65Zn f m the water by oysters was followed i n groups o f animals held i n d i f f e r e n t concentrations of Mn and Zn i n the water (2, 5, or 10 Pg/1) and the same c o n c e n t r a t i o n o f 65Zn j 5 4 ^ Concentration f a c t o r s of both ^Kn j 6 5 ^ were lower i n animals maintained i n water c o n t a i n i n g 5 or 10 g/1 (Figure 6 ) . This r e d u c t i o n was not l i n e a r , however, as a f i v e - f o l d increase i n c o n c e n t r a t i o n d i d not r e s u l t i n a f i v e - f o l d decrease i n concentration factor. The accumulation o f ^Co and ^ C s from the water by clams was followed at concentrations o f 0.5, 2.5, and 12.5 ug/1 o f s t a b l e Co and Cs (21). Clams do not seem to regulate Cs i n the range o f concentrations t e s t e d , but do seem to 4


6

6

4

r 0

a n c

a n c

η#

n

3 7


CHEMICAL

628

MODELING IN AQUEOUS

SYSTEMS

TABLE V


C o p p e r - 6 4 and Z i n c - 6 5 C o n c e n t r a t i o n F a c t o r s i n O y s t e r S o f t T i s s u e s A f t e r 2 4 - h E x p o s u r e to T e s t M a t e r i a l s Body Part

None (control)

Mantle G i l l s and l a b i a l palps S h e l l muscles D i g e s t i v e gland and stomach Viscera Body f l u i d Remains Whole body Mantle G i l l s and l a b i a l palps S h e l l muscles D i g e s t i v e gland and s t o m a c h Viscera Body f l u i d Remains Whole

body

Glycine

Clay

Yellow Stuff

EDTA

100

A. 160

Copper 120

120

30

210 60

340 60

280 30

170 20

90 7

160 100 10 30 80

230 220 10 30

180 80 5 30 80

120 90 4 14 60

50 30 2 6 20

82

60

1.1

1

1

0

65 Zinc

16

B. 23

31 4

37 4

170 23

153 37

3.0 0.1

10 9 0.8 2

13 12 0.8 2

82 64

1.0 0.9

4 24

86 61 4 11

10

12

57

49

0.1

D 3


0.1 0.1

27.

HARRISON



Crassostrea gigas

629

Mya arenaria

Time, days Figure 6. Concentration of metals in soft tissues of Crassostrea gigas and Mya arenaria exposed to different levels of metals in the water. Manganese and zinc: (A) 2; (M) «5; and (%) 10 ng/L. Cobalt and cesium:(%) 0.5; (f) 2.5; and i | j 12.5



630

CHEMICAL

MODELING IN AQUEOUS SYSTEMS

regulate Co a t the higher c o n c e n t r a t i o n s ; the Co c o n c e n t r a t i o n f a c t o r s were lower (Figure 6 ) . Shuster and P r i n g l e (_5) exposed C^_ v i r g i n i c a to increased l e v e l s o f metals i n the water under c o n t r o l l e d l a b o r a t o r y experiments. They reported an approximate doubling o f metal concentrations i n the t i s s u e s upon doubling the metal i o n c o n c e n t r a t i o n i n the water. The uptake o f Cd by e d u l i s at low concentrations was d i r e c t l y p r o p o r t i o n a l to the concentrations i n the seawater w i t h a maximum concentration f a c t o r o f 167 a t 0.7 mg Cd/1 ( 3 1 ) . At higher c o n c e n t r a t i o n s , the c o n c e n t r a t i o n f a c t o r decreased, which was considered to i n d i c a t e a s a t u r a t i o n o f the a v a i l a b l e b i n d i n g s i te s. Boyden (38) reports that Cd c o n c e n t r a t i o n f a c t o r s o f P a t e l l a were 10 times g r e a t e r i n a Cd-contaminated environment than could be expected from a p r o p o r t i o n a l increase r e l a t e d to environmental c o n c e n t r a t i o n s . On the other hand, Cu c o n c e n t r a t i o n f a c t o r s o f Ostrea e d u l i s , C. gigas, and e d u l i s were g r e a t l y decreased i n a g r o s s l y Cu-contaminated environment. Although r e g u l a t i o n o f turnover o f some elements at elevated concentrations i s suggested from the data, the mechanisms are not c l e a r l y e s t a b l i s h e d , and the response i s species dependent. The changes i n c o n c e n t r a t i o n f a c t o r s may not be a c e l l u l a r but instead may be an organismic response to the t o x i c e f f e c t s o f the increased l e v e l s o f metals. Oysters decreased the time the s h e l l was opened at increased concentrations o f Cu (_13). A c l o s u r e response was a l s o found i n mussels ( F . L . H a r r i s o n and D.W. R i c e , unpublished data, 1978). The t o x i c responses may i n c l u d e s h e l l c l o s u r e , changes i n pumping and feeding r a t e s , and changes i n respiration. Conclusion The turnover o f s t a b l e and r a d i o a c t i v e n u c l i d e s o f trace metals d i f f e r s g r e a t l y w i t h animal s p e c i e s , element, time, and physicochemical form o f the metal i n the water. Concentration f a c t o r s and turnover rates o f a given element can range s e v e r a l orders o f magnitude i n value i n d i f f e r e n t b i v a l v e molluscs. The r e l i a b i l i t y o f models developed to p r e d i c t c o n c e n t r a t i o n change i s dependent on the s e l e c t i o n o f input parameters appropriate f o r the s i t u a t i o n under c o n s i d e r a t i o n . E q u i l i b r i u m models are used to assess the environmental impact o f power p l a n t s i t i n g . The use o f a s i n g l e , maximum c o n c e n t r a t i o n f a c t o r for b i v a l v e molluscs as input i n t o the model i n t h i s s i t u a t i o n i s appropriate f o r screening purposes, i . e . , to determine whether the maximum c r e d i b l e value would impact the environment. However, when more r e a l i s t i c estimates are r e q u i r e d , s e l e c t i o n o f c o n c e n t r a t i o n f a c t o r s a p p l i c a b l e to the s i t e , species, and s i t u a t i o n i s necessary. Dynamic models are used i n the case o f a c c i d e n t a l r e l e a s e s where time i s an important c o n s i d e r a t i o n . The use o f appropriate


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rate constants and concentration f a c t o r s even i n s i m p l i f i e d dynamic models can r e s u l t i n b e t t e r approximations of the c o n c e n t r a t i o n maxima and c o n c e n t r a t i o n changes that can be expected than the use o f e q u i l i b i u m models. Acknowledgements


The author thanks John W. Dawson and David W. R i c e , J r . f o r their technical assistance. Work performed under the auspcies o f the U.S. Department of Energy by the Lawrence Livermore Laboratory under c o n t r a c t number W-7405-ENG-48. Abstract Bivalve molluscs effectively concentrate numberous elements. To maintain healthy populations of molluscs and to minimize adverse effects on man from their consumption, we must be able to predict changes in concentration after stable and radioactive nuclides are released into their environment. Both equilibrium and dynamic predictive models have been developed. Model parameters needed include concentration factors and turnover rate constants. Concentration factors and rate constants determined experimentally in the oyster Crassostrea gigas and the clam Mya arenaria differ widely with species and element. The physical form of the element in the water affects turnover also; accumulation of radionuclides of Co, Cs, Mn, and Zn is greater in water containing suspended particles. The chemical form of the element in the water affects its accumulation. When glycine, ethylenediaminetetraacetate (EDTA), "yellow stuff", or clay are added to seawater, the accumulation of Cu and Zn by the oyster differs with each test material; glycine increases and EDTA decreases the accumulation of both elements. The concentrations of stable Co, Fe, Mn, and Zn in oysters and of Co, Fe, and Zn in clams fluctuate. In oysters, seasonal changes were evident. Concentrations of Co, Fe, and Zn seem to vary together, whereas that of Mn varied inversely. Before accurate predictions can be made of the accumulation of stable and radioactive nuclides of elements by bivalve molluscs, we need concentration factors, rate constants, and information on the effects of metabolic and environmental factors on these parameters for each animal species of interest. Literature Cited 1. 2.

Galtsoff, P.S. The American oyster, Crassostrea virginica gemlin. U.S. Fish. Wildl. Serv. Fish Bull. 64, 480 p. (1964). Brooks, R.R. and Rumsby, M.G. The biogeochemistry of trace element uptake by some New Zealand bivalves. Limnol. Oceanogr.


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