Chemical Modeling in Aqueous Systems - ACS Publications

13 Oceanic Elemental Scavenging PETER G. BREWER and WEI MIN

HAO

Downloaded by STONY BROOK UNIV SUNY on June 4, 2018 | https://pubs.acs.org Publication Date: March 19, 1979 | doi: 10.1021/bk-1979-0093.ch013

Woods Hole Oceanographic Institution, Woods Hole, MA 02543

The phenomenon of scavenging, or adsorption onto s o l i d s u r faces, i s f r e q u e n t l y invoked by marine chemists as an important c o n t r o l on the d i s t r i b u t i o n of the chemical elements i n seawater. Kranskopf (1) i n an e a r l y paper commented on the importance of adsorption as a c o n t r o l on the abundance of minor elements i n seawater, and other papers, too numerous to mention, have since a s c r i b e d geochemical importance to t h i s process. Unfortunately, attempts to use surface chemical theory, such as that given i n Stumm and Morgan (2^) or i n the elegant review by Parks (3) , have had mixed success. One approach has been to study the surface chemistry of a w e l l - d e f i n e d s o l i d phase, such as δ-Μηθ2 (4) or i l l i t e and beachsand 05), i n the l a b o r a t o r y and to apply these data to a set of f i e l d observations. However, the p a r t i c l e s present i n seawater and assumed to be r e s p o n s i b l e f o r scavenging are not w e l l c h a r a c t e r i z e d and are of complex composition (6^, 7_, 8), and the v a l i d i t y of applying l a b o r a t o r y r e s u l t s from pure phases i s questionable. Here we attempt a d i f f e r e n t approach; f i r s t l y i n c o l l a t i n g the v a r i o u s scavenging r a t e constants which have been d e r i v e d , p u t t i n g these on a common b a s i s and examining t h e i r chemical c o r r e l a t i o n s , and secondly i n asking what surface chemical p r o p e r t i e s must be a t t r i b u t e d to deep ocean p a r t i c u l a t e matter i n order to e x p l a i n the observed e f f e c t s . A knowledge of these p r o p e r t i e s would not only have considerable academic merit, but would be of great p r a c t i c a l use i n p r e d i c t i n g the f a t e of other chemical species of r a d i o n u c l i d e s i n the deep ocean. Most of the observed oceanic elemental removal r a t e s ob tained from f i e l d s t u d i e s r e s u l t from a complex mix of b i o l o g i c a l , p h y s i c a l and chemical processes and e x t r a c t i n g the component due to any one i d e a l i z e d process, such as adsorption, i s d i f f i c u l t . A p o s s i b l e exception to t h i s g e n e r a l i t y l i e s i n the v a r i o u s pap ers which describe scavenging as an i n s i t u process o p e r a t i n g i n the deep ocean by means of a one-dimensional a d v e c t i o n - d i f f u s i o n scavenging model (9_> 10). We now r e a l i z e that the data set (11) used by C r a i g (10) was p a r t i a l l y d e f i c i e n t ; however, h i s papers were exemplary and the concepts have s i n c e found wide a p p l i c a t i o n

0-8412-0479-9/79/47-093-261$05.00/0 © 1979 American Chemical Society Jenne; Chemical Modeling in Aqueous Systems ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

262

CHEMICAL MODELING IN AQUEOUS SYSTEMS

(12, 13). In such a model, the d i s t r i b u t i o n of a chemical element i s c o n t r o l l e d by mixing between an upper and lower boundary and some production or removal process. The assignment of a mechan ism to e x p l a i n t h i s process i s to some extent i n t u i t i v e ( f o r i n stance, a d e f i c i t over atmospheric e q u i l i b r i u m i s a t t r i b u t e d to r e s p i r a t i o n ) ; however, f o r many metal i o n s , the e x p l a n a t i o n of the observed removal r a t e by an a d s o r p t i v e mechanism i n the deep a p h o t i c ocean appears to be most l i k e l y .


Methods and Data Thorium. The short residence time of thorium i n seawater and i t s a f f i n i t y f o r s o l i d phases have long been recognized. Broecker e_t_ a l . (14) have examined the residence time of thorium i n surface seawater and i t s i m p l i c a t i o n s regarding the f a t e of r e a c t i v e p o l l u t a n t s . Krishnaswami e_t a l . (12) have examined the i s o t o p i c composition of deep P a c i f i c p a r t i c u l a t e matter samples obtained by l a r g e volume i n s i t u pumping. From t h e i r observations of the uT h and U - T h p a i r s , the a c t i v i t y of the Th on f i l t e r s and the o b s e r v a t i o n that on ^20% of the T h i n sea water e x i s t s i n a p a r t i c u l a t e s t a t e , they d e r i v e a r a t e constant f o r a d s o r p t i o n onto a s o l i d phase of 8 χ 10" s e c or 10° * yr"" . 2 3 8

2 3 i +

2 3 4

2 a u

2 3 0

2 3 i f

8

- 1

4

1

Lead. The c o l l e c t i o n and a n a l y s i s of seawater samples f o r s t a b l e l e a d presents severe problems (see the r e p o r t of C. Patterson's group ( 1 5 ) ) , and a l l r e l i a b l e estimates of the r a t e constant f o r the removal of l e a d d e r i v e from observations of the 2 1 0 _ 2 2 6 d i s e q u i l i b r i u m (9) i n the deep sea. C r a i g et a l . (9_) d e r i v e d a scavenging r a t e constant f o r P b of VL.8 χ 10~ y r " , based upon the h a l f - l i f e of P b (22 y r s ) and i t s observed ap proximately 50% d e p l e t i o n from i t s parent R a . This d e p l e t i o n has been amply confirmed (16, 17, 18); however, Bacon et_ a l . (13) have shown that the P b a c t i v i t y of marine p a r t i c u l a t e matter i s inadequate to account f o r the observed d e f i c i e n c y and postu l a t e d a d s o r p t i o n at the sediment-water i n t e r f a c e as a dominant mechanism. Their removal r a t e constant f o r a d s o r p t i o n by s i n k i n g p a r t i c l e s was estimated to be i n the range I O " " to 1 0 ~ " yr" . We w i l l take 1 0 ~ " yr"" as an average r a t e constant. p b

R a

2 1 0

2

1

2 1 0

2 2 6

23lf

2

2

7

3

3

1 5

1

1

Copper. C r a i g (10) and Brewer (19) have p o s t u l a t e d that copper i s scavenged i n deep ocean water. More recent data given by Boyle et_ a l . (20) r e v e a l s i g n i f i c a n t l y lower concentrations and c o n v i n c i n g evidence f o r the scavenging process. Boyle et a l . (20) have used an a d v e c t i o n - d i f f u s i o n - s c a v e n g i n g model and c a l c u l a t e a removal r a t e constant of 10~ y r " . As pointed out by C r a i g (10) and Brewer and Murray (21), such models y i e l d net r a t e s g i v i n g the d i f f e r e n c e between production from decomposing p l a n k t o n i c d e b r i s and consumption due to a d s o r p t i o n onto more s t a b l e p a r t i c l e s . The r a t e constant of 10" must then be a lower l i m i t ; the upper l i m i t i s unknown but may be estimated 3

1

3

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

13.

BREWER AND HAO

Oceanic Elemental Scavenging

263

by p o s t u l a t i n g t h a t , were no a d s o r p t i o n to occur, copper might be a l i n e a r f u n c t i o n of phosphate and summing the p o s t u l a t e d produc t i o n and observed consumption r a t e s . The removal r a t e constant f o r copper obtained i n t h i s way i s 20% higher than the value given by Boyle e_t a_l. (20) and i n the range 1 0 ~ * y r " .


3

0 5

1

N i c k e l . The marine geochemistry of n i c k e l has been d i s cussed by S c l a t e r et a l . (22) and i n Boyle (23). The general trend of the v e r t i c a l p r o f i l e s i s to show an i n c r e a s e w i t h depth and a p a r t i a l co-variance w i t h the n u t r i e n t s phosphate and s i l i con. Scavenging of n i c k e l was not discussed i n these papers; how ever, the chemical s i m i l a r i t i e s of Pb, Cu and N i would suggest that some a d s o r p t i o n must occur, the question being to what de gree. We have used the data given by Boyle (23) f o r GEOSECS S t a t i o n 219 i n the Bering Sea and t r e a t e d t h i s i n the same way as f o r copper, examining the d e v i a t i o n s from the most s t r o n g l y b i o p h i l i c element, phosphate. In t h i s way, we estimate a maximum removal r a t e f o r n i c k e l of 0.65 χ 10" nmol kg yr and a max imum removal r a t e constant of 1 0 · y r " . 3

_ Ι +

2

- 1

- 1

1

Cadmium. The data a v a i l a b l e f o r cadmium show marked i n creases w i t h depth, and M a r t i n et_ a^. (24) have observed a strong c o r r e l a t i o n of cadmium w i t h phosphate i n C a l i f o r n i a n c o a s t a l waters. Boyle (23) gives cadmium data f o r GEOSECS S t a t i o n 219. The observed c o r r e l a t i o n w i t h phosphate i n d i c a t e s that scavenging of C d must be very s m a l l and our estimate of the r a t e constant i s an upper l i m i t of % 1 0 ~ yr" . 2 +

5 , 2 5

1

Discussion The r a t e constants f o r removal, presumably by a d s o r p t i o n , given i n the preceding s e c t i o n were a l l obtained from deep Pac i f i c data and were d e r i v e d from the a p p l i c a t i o n of a one-dimen s i o n a l a d v e c t i o n - d i f f u s i o n - s c a v e n g i n g model. There are geograph i c a l d i f f e r e n c e s ; however, the d i f f e r e n c e s i n p r o p e r t i e s such as pH and the amount of suspended matter (P. G. Brewer, unpublished data, 1978) w i l l not be l a r g e . I f the d e r i v e d r a t e constants have any v a l i d i t y , they should e x h i b i t some c o r r e l a t i o n w i t h important chemical p r o p e r t i e s r e l a t i n g to a d s o r p t i o n . We know l i t t l e of the s p e c i f i c d e t a i l s of the s o l i d - s o l u t i o n i n t e r f a c e i n n a t u r a l waters. Neihof and Loeb (25, 26) have examined the surface charge of p a r t i c u l a t e matter i n sea water and the r o l e of ad sorbed organic matter i n determining t h i s charge. In t h e i r ex periments, a l l s o l i d surfaces assumed a moderately e l e c t r o n e g a t i v e charge i n n a t u r a l sea water; i n U.V. i r r a d i a t e d , o r g a n i c f r e e water, the s o l i d s e x h i b i t e d t h e i r own c h a r a c t e r i s t i c charges. Loeb and Niehof (27) have f u r t h e r obtained o p t i c a l data on t h i s adsorbed f i l m which are c o n s i s t e n t w i t h a polymeric macromolecul a r c h a r a c t e r . I t i s g e n e r a l l y accepted that pH i s the master v a r i a b l e governing the extent of a d s o r p t i o n of metal ions at


264


oxide-water i n t e r f a c e s ( 2 ) , and that a d s o r p t i o n i n c r e a s e s w i t h i n c r e a s i n g pH. W i t h i n these broad c o n s t r a i n t s there i s c o n s i d e r a b l e l a t i tude i n choosing a model which might r e a l i s t i c a l l y d e s c r i b e the i n s i t u a d s o r p t i v e process. James and Healy (28) have developed an important model i n which the f r e e energy of a d s o r p t i o n (AG ads) r e s u l t s from the d i f f e r e n c e i n e l e c t r o s t a t i c c o n t r i b u t i o n s between coulombic a t t r a c t i o n (AG coul) and r e p u l s i o n due to change i n s o l v a t i o n energy (AG s o l v ) . The complete process i s governed by


AG ads = AG coul + AG s o l v + AG chem

(1)

where AG chem i s an a d j u s t a b l e parameter which appears as a small d i f f e r e n c e between the two l a r g e terms. A p o i n t of i n t e r e s t i s that i n t h i s model M qZ+ ions show minimal tendency to adsorption due to unfavorable AG s o l v terms. O'Connor and Kester (5) d i s cussed the James and Healy model, but opted f o r a model i n which M qZ+ was exchanged f o r surface bound hydrogen i o n : a

a

Z +

M q a

(z-i)+ + HX = MX

+

H

+ .

(2)

The r e a c t i o n was determined by an e q u i l i b r i u m constant as: κ.

•

W

(

2

-

1

H

(K.) given A

1

(3)

z

S c h i n d l e r (29, 30) has proposed a s i m i l a r model i n that M + ions are adsorbed, yet adsorption i s understood i n terms o f s u r face complex formation w i t h deprotonated surface OH-groups as l i g a n d s . His schematic example using S i as a t y p i c a l oxide s u r face i s :

> >

HO HO HO

Si - 0

HO^

H0^"

•Si

Z+ + M'

.-0 Z+ _ + M*

HO HO HO

>

Si - 0 - M

(z-i)+

(4)

(Z-2)+ (5)

\ Q

The f r e e OH i o n i s a powerful l i g a n d . Dugger e_t a l . (31) have pointed out that the l i g a n d p r o p e r t i e s o f the surface OH groups are not g r e a t l y modified by the attached s i l i c o n . I f the S c h i n d l e r model i s c o r r e c t , then the derived r a t e constants should bear a s i g n i f i c a n t r e l a t i o n s h i p to the s t r e n g t h of the i n t e r a c t i o n of the metal i o n w i t h 0H~. I n Table"I and Figures 1 and 2, we show the r a t e constants derived from f i e l d



13.

Oceanic

BREWER AND HAO

Elemental

Scavenging

11

IO" 10

-io

io-

Ni

Cd

9

10 -8 Pb

κ;

'D

Cu

10-

10" 10 -5

1 0 _4

/

/

4"

10 -3 10

1

1

10-1

SCAVENGING

io-2 1 0 - 3 i o - 4 io-5 1 0 - 7 RATE

CONSTANT

(YR ~ ) 1

Figure 1. Plot of the scavenging rate constants derived from advection-diffusionscavenging models against the stability constant for simple hydroxo complexes, * K i



266


Cu Ni

Cd

Pb

10

1

1

1

IO"

SCAVENGING

2

3

4

5

6

10" 10" 1 0 " 10" 10" '

RATE

CONSTANT

(YR' ) 1

Figure 2. Plot of the scavenging rate constants derived from advection-diffusionscavenging models against the stability constant *β for reaction with two hydroxo groups 2


BREWER AND HAo

13.

Oceanic

Elemental

267

Scavenging

measurements compared to the s t a b i l i t y constants f o r hydroxo-complexes "K;L and *Ε>2· c o r r e l a t i o n lends c o n s i d e r a b l e support to the argument that the i n t e r a c t i o n w i t h surface OH-groups i s a dominant c o n t r o l on the i n s i t u a d s o r p t i v e process. The r a t e con s t a n t s derived here may be c r i t i c i z e d i n that they c o n t a i n s e v e r a l assumptions; however, the geochemist i s a f f o r d e d some r e l i e f i n an e q u a l l y wide choice of constants. The constants used by S c h i n d l e r (29) are a l s o given i n Table I . The value f o r ' $2 1h Figure 2 f a l l s s i g n i f i c a n t l y away from the other ions r e f l e c t i n g the com p a r i s o n of t e t r a v a l e n t and d i v a l e n t ions and the i n t e n s i t y of i n t e r a c t i o n w i t h a second OH-group. T n e


c

i n

TABLE I Comparison of the adsorption r a t e constants derived from f i e l d s t u d i e s w i t h values f o r * K j and *$2 s e l e c t e d (a) i n t h i s paper and (b) by S c h i n d l e r (29).

Element

Rate Constant (yr" )

Log %

4

-3.7

Th

10°*

Pb

10" ·

Cu

ΙΟ"" ·

Ni

Chemical Modeling in Aqueous Systems - ACS Publications

Recommend Documents