The Adsorption of Organomercury Compounds from Seawater onto

22 The Adsorption of Organomercury Compounds from Seawater onto Sedimentary Phases Cristie Dalland, Eva Schumacher, and Mary L. Sohn

Downloaded by CORNELL UNIV on October 6, 2016 | http://pubs.acs.org Publication Date: April 21, 1986 | doi: 10.1021/bk-1986-0305.ch022

Department of Chemistry, Florida Institute of Technology, Melbourne, F L 32901

Adsorption isotherms and conditional adsorption con stants (Kads) were determined for the adsorption of diphenyl mercury and phenylmercuric ion (introduced as phenylmercuric acetate) from a seawater matrix onto several solid phases. Diphenyl mercury was found to adsorb onto humic acid, but no adsorption was detected onto bentonite, amorphous Fe(0H) or hydrous Mn0 . The value of Kads was found to decrease with increasing ionic strength for the di phenyl mercury - humic acid system in seawater. As the concentraction of suspended humic acid increased, the nonlinearity of the diphenyl mercury - humic acid system became more pronounced at an increasingly lower diphenyl mercury concentration. Phenylmercuric ion adsorbed most strongly onto humic acid although adsorption onto Fe(0H) andMn0 was detected and Kads values were determined. Kads values for PMA with humic acid, Mn0 and Fe(0H) were found to decrease with decreasing salinity. 3

2

3

2

2

3

The s p e c i a t i o n , c o n c e n t r a t i o n s and r e s i d e n c e t i m e s o f d i s s o l v e d substances i n n a t u r a l w a t e r s a r e dependent on many f a c t o r s and p r o c e s s e s . Important f a c t o r s i n c l u d e t e m p e r a t u r e , pH, redox p o t e n t i a l , i o n i c s t r e n g t h and t h e c o n c e n t r a t i o n s o f o t h e r d i s s o l v e d s p e c i e s such as o r g a n i c and i n o r g a n i c l i g a n d s as w e l l as t h e presence o f suspended p a r t i c u l a t e and c o l l o i d a l m a t t e r . Important p r o c e s s e s i n a d d i t i o n t o r a t e o f i n p u t , and b i o c h e m i c a l c y c l i n g i n c l u d e p r e c i p i t a t i o n , c o m p l e x a t i o n , c o a g u l a t i o n and a d s o r p t i o n onto sus pended p a r t i c u l a t e m a t t e r . The f o c u s o f t h i s s t u d y i s t h e p r o c e s s o f a d s o r p t i o n . The a d s o r p t i o n o f s u b s t a n c e s onto s o l i d suspended phases i s a p r i m a r y mechanism f o r t h e removal o f d i s s o l v e d m a t t e r from t h e water column and t h e consequent a c c u m u l a t i o n o f many substances i n sediments. Of major concern i s t h e v a l u e o f t h e a d s o r p t i o n c o n s t a n t (Kads) which i s a measure o f t h e s t r e n g t h o f t h e a d s o r p t i o n i n t e r a c t i o n . Know l e d g e o f t h e v a r i a t i o n i n Kads w i t h changes i n e x p e r i m e n t a l c o n d i 0097-6156/86/0305-0369S06.00/0 © 1986 American Chemical Society

Sohn; Organic Marine Geochemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1986.


370

O R G A N I C M A R I N E GEOCHEMISTRY

t i o n s c a n p r o v i d e v a l u a b l e i n s i g h t i n t o t h e mechanism o f a d s o r p t i o n processes. The m a j o r i t y o f a d s o r p t i o n s t u d i e s d e a l w i t h t h e i n t e r a c t i o n s between m e t a l i o n s and e i t h e r r e a l o r model sediment phases. Amor phous Fe(OH)^, hydrous MnO^, c l a y s ( b e n t o n i t e , i l l i t e , k a o l i n i t e ) , SiO^, A l ^ O ^ and o r g a n i c m a t t e r ( o f t e n humic s u b s t a n c e s ) a r e t h e s o l i d phases most f r e q u e n t l y s t u d i e d . E q u i l i b r a t i o n o f hydrous o x i d e s o l i d phase a d s o r b e n t s w i t h a m e t a l i o n a d s o r b a t e i s n o r m a l l y a c c o m p l i s h e d by m e c h a n i c a l s h a k i n g of t h e sample over a t i m e p e r i o d e x t e n d i n g from s e v e r a l hours t o s e v e r a l days when w o r k i n g w i t h model s o l i d phases w h i c h were p r e pared i n t h e l a b o r a t o r y . An i n i t i a l e q u i l i b r a t i o n i s u s u a l l y reached w i t h i n one o r two hours and i s f o l l o w e d by a p e r i o d o f much l e s s e x t e n s i v e a d s o r p t i o n w h i c h may e x t e n d over a much l o n g e r time p e r i o d (1-3) However, sediments and suspended m a t t e r i s o l a t e d from n a t u r a l systems seem t o r e q u i r e l o n g e r t i m e p e r i o d s of seven t o twenty days b e f o r e an a c c u r a t e d e t e r m i n a t i o n o f Kads can be made ( 4 , 5 ) . I n a d d i t i o n , when w o r k i n g w i t h n a t u r a l s e d i ments, t h e major r o l e p l a y e d by adsorbed o r g a n i c c o a t i n g s on s o l i d phases must be c o n s i d e r e d ( 2 , 6 ) . The e f f e c t o f pH on m e t a l i o n a d s o r p t i o n i s u s u a l l y q u i t e d r a m a t i c w i t h t h e p e r c e n t o f m e t a l adsorbed onto a s u r f a c e i n c r e a s i n g from near z e r o t o a maximum, o v e r a range o f s e v e r a l pH u n i t s (1-4,7) as pH i n c r e a s e s . I n c r e a s e d i o n i c s t r e n g t h o f t h e s u p p o r t i n g medium t y p i c a l l y r e s u l t s i n a d e c r e a s e i n Kads due t o increased competition f o r a v a i l a b l e surface a c t i v e s i t e s (7,8). However, t h e v a l u e o f Kads f o r some m e t a l s w i t h n a t u r a l suspended sediments has been found t o i n c r e a s e w i t h i n c r e a s i n g s a l i n i t y ( c o n d i t i o n s s i m i l a r t o e s t u a r i n e m i x i n g ) , s u g g e s t i n g removal o f m e t a l i o n s from s o l u t i o n by c o a g u l a t i o n ( 9 ) . S o l i d phase concen t r a t i o n i s a l s o a c r i t i c a l parameter a f f e c t i n g Kads v a l u e s . The i n v e r s e r e l a t i o n s h i p between t h e c o n c e n t r a t i o n o f suspended adsorb ing s o l i d s and Kads i s most r e a d i l y e x p l a i n e d by a s o l i d - s o l i d i n t e r a c t i o n which decreases t h e a v a i l a b i l i t y o f the strongest b i n d i n g s i t e s ( 9 ) . The i n t r o d u c t i o n o f l i g a n d s i n t o a m e t a l i o n - s o l i d adsorbent e q u i l i b r i u m c o n s i d e r a t i o n can l e a d t o e i t h e r an i n c r e a s e o r d e c r e a s e i n Kads. L i g a n d s w h i c h a r e n o t adsorbed w i l l d e c r e a s e m e t a l a d s o r p t i o n by competing w i t h t h e adsorbent f o r t h e m e t a l , w h i l e enhancement o f m e t a l a d s o r p t i o n can o c c u r i f t h e l i g a n d i s adsorbed and a d d i t i o n a l complexing f u n c t i o n a l groups a r e s t i l l a v a i l a b l e on t h e adsorbed l i g a n d f o r m e t a l i o n i n t e r a c t i o n (2). The r e l a t i v e importance o f t h e v a r i o u s s o l i d phases t o t r a c e m e t a l d i s t r i b u t i o n s i n n a t u r a l a q u a t i c systems i s dependent on t h e m e t a l i o n o f i n t e r e s t . How e v e r , t h e f r a c t i o n o f m e t a l adsorbed t y p i c a l l y i n c r e a s e s as t h e p e r c e n t a g e o f o r g a n i c m a t t e r i n t h e sediment i n c r e a s e s (9). The s o r p t i o n b e h a v i o r o f m e t a l i o n s onto s o l i d s u r f a c e s i s o f t e n d e s c r i b e d by a Langmuir i s o t h e r m . The v a l u e o f Kads can be determined from t h e s l o p e o f t h e i s o t h e r m w h i c h i s u s u a l l y l i n e a r i n t h e range o f low m e t a l i o n c o n c e n t r a t i o n s ( 3 ) . S t u d i e s i n v o l v i n g t h e a d s o r p t i o n o f o r g a n i c s u b s t a n c e s onto s o l i d phases have l a r g e l y c e n t e r e d around o r g a n i c p e s t i c i d e s because o f t h e e n v i r o n m e n t a l s i g n i f i c a n c e o f t h e s e t o x i c sub s t a n c e s . The e x t e n t o f a d s o r p t i o n o f B r o m a c i l onto f r e s h w a t e r



22.

D A L L A N D ET AL.

371

Adsorption of Organomercury Compounds

sediments was found t o c o r r e l a t e s t r o n g l y w i t h t h e p e r c e n t o f o r g a n i c m a t t e r i n t h e sediments w h i l e weaker a d s o r p t i o n onto c l a y s and amorphous Fe(OH)^ s u r f a c e s was i n d i c a t e d (10,11,12). I n many c a s e s , t h e e x t e n t o f a d s o r p t i o n o f n e u t r a l o r g a n i c compounds i s i n v e r s e l y r e l a t e d t o s o l u b i l i t y (13). With respect t o t h e e f f e c t o f t h e c o n c e n t r a t i o n o f s o l i d phase on Kads, r e s u l t s seem t o be s i m i l a r t o those o b t a i n e d i n m e t a l i o n s o l i d i n t e r a c t i o n s . Kads v a l u e s f o r t h e p a r t i t i o n i n g o f DDT, Kepone, L i n d a n e , and H e p t a c h l o r d e c r e a s e w i t h i n c r e a s i n g sediment c o n c e n t r a t i o n . The e f f e c t o f pH on t h e a d s o r p t i o n o f o r g a n i c substances i s dependent on t h e pK o f t h e substance c o n s i d e r e d (14,15). The s o r p t i v e b e h a v i o r o f o r g a n i c substances i s u s u a l l y b e s t d e s c r i b e d by a F r e u n d l i c h i s o t h e r m (10,13,14,16). The v a l u e o f Kads can be determined from t h e y - i n t e r c e p t o f a l o g - l o g p l o t o f the modified F r e u n d l i c h equation: S . = Κ - C ads ads

l / n

(1)

S r e p r e s e n t s t h e c o n c e n t r a t i o n o f t h e a d s o r b a t e i n t h e s o l i d phase, while C represents the e q u i l i b r i u m concentration i n the bulk s o l u t i o n . The a d s o r p t i o n i s o t h e r m o f d i p h e n y l mercury and humic a c i d i s i l l u s t r a t e d i n F i g u r e 1. Nature o f T h i s Study. The f o l l o w i n g r e p r e s e n t s p r e l i m i n a r y r e s u l t s of an i n i t i a l s t u d y on t h e a d s o r p t i v e b e h a v i o r o f two organometall i c s . Numerous o r g a n o m e t a l l i c s comprise t h e a c t i v e i n g r e d i e n t o f v a r i o u s p e s t i c i d e s . Examples i n c l u d e p h e n y l m e r c u r i e s a l t s , d i p h e n y l mercury, t r i p h e n y l , t r i b u t y l , and t r i c y c l o h e x y l t i n s a l t s , as w e l l as o r g a n o a r s e n i c compounds ( T a b l e 1 ) . Because o f t h e t o x i c i t y of t h e s e substances t h e i r b e h a v i o r and a s s o c i a t i o n s i n n a t u r a l water systems i s a m a t t e r o f concern. T a b l e I . Commercial and A g r i c u l t u r a l Uses o f Some Compound D i p h e n y l mercury Phenylmercury a c e t a t e Phenylmercury b o r a t e Phenylmercury c h l o r i d e Phenylmercury h y d r o x i d e Phenylmercury i o d i d e Ethylmercury c h l o r i d e Methylmercury c h l o r i d e Methylmercury i o d i d e Fentin acetate Fentin hydroxide

Organomentallics

Use Fungicide Eradicant fungicide Paint mildewstat Seed treatment f u n g i c i d e Lumber t r e a t m e n t f u n g i c i d e Fungicide Seed treatment f u n g i c i d e Seed t r e a t m e n t f u n g i c i d e Mercury a n a l y s i s reagent Fungicide Fungicide

M a t e r i a l s and Methods. L a b o r a t o r y g l a s s w a r e was soaked i n 50% n i t r i c a c i d o v e r n i g h t and then r i n s e d r e p e a t e d l y w i t h d i s t i l l e d water. A l l l i q u i d s o l u t i o n s were s t o r e d i n p o l y e t h y l e n e c o n t a i n e r s . Stock s o l u t i o n s o f d i p h e n y l mercury and p h e n y l m e r c u r i e a c e t a t e were p r e p a r e d by d i s s o l v i n g weighed p o r t i o n s o f t h e pure s o l i d s ( o b t a i n e d from t h e U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency, P e s t i c i d e s and I n d u s t r i a l Chemicals R e p o s i t o r y ) i n d i s t i l l e d water and were



372


log

C

F i g u r e 1. F r e u n d l i c h i s o t h e r m f o r t h e a d s o r p t i o n o f DPM onto 150 ppm humic a c i d i n 50% seawater.



22.

DALLANDETAL.


373

s t o r e d under r e f r i g e r a t i o n f o r a maximum o f 100 days. Both d i p h e n y l mercury (DPM) and p h e n y l m e r c u r i c a c e t a t e (PMA) a r e f u n g i c i d e s used i n l i m i t e d n o n a g r i c u l t u r a l areas o n l y (17,18). Hydrous manganese o x i d e s and amorphous i r o n o x i d e s were p r e p a r e d i n t h e l a b o r a t o r y a c c o r d i n g t o t h e methods d e s c r i b e d by Oakley e t a l (_3 ). The a d d i t i o n o f manganese s u l f a t e s o l u t i o n t o a s l i g h t l y b a s i c p o t a s s i u m permanganate s o l u t i o n produces a s u s p e n s i o n o f hydrous MnO^. A s u s p e n s i o n o f Fe(OH)^ i s produced by s i m p l y a d j u s t i n g a f e r r i c n i t r a t e s o l u t i o n t o a pH o f 8.0 w i t h a d i l u t e sodium h y d r o x i d e s o l u t i o n . Both s u s p e n s i o n s were washed r e p e a t e d l y w i t h seawater and s t o r e d i n seawater f o r s e v e r a l days. B e n t o n i t e was o b t a i n e d from F i s c h e r S c i e n t i f i c . Humic a c i d was o b t a i n e d from A l d r i c h . An a d d i t i o n a l humic a c i d sample (BV) which was e x t r a c t e d from Chesapeake Bay sediments, was used i n one s e t o f a d s o r p t i o n e x p e r i m e n t s . The e x t r a c t i o n and p u r i f i c a t i o n o f t h i s sample has been d e s c r i b e d i n d e t a i l elsewhere ( 1 9 ) . F u r t h e r c h a r a c t e r i z a t i o n o f BV humic a c i d i s a l s o p r e s e n t e d e l s e w h e r e ( 2 0 ) . The a d s o r p t i o n o f d i p h e n y l m e r c u r y (DPM) and p h e n y l m e r c u r i e i o n (PM) was s t u d i e d on t h e s o l i d phases d e s c r i b e d above (hydrous manganese o x i d e s , amorphous i r o n o x i d e s , humic a c i d and b e n t o n i t e c l a y ) . The s o l i d phase (5-15 mg) was added t o 25-50 ml o f f i l t e r e d seawater y i e l d i n g s o l i d phase c o n c e n t r a t i o n s of a p p r o x i m a t e l y 100 t o 400 ppm suspended m a t t e r . The c o n c e n t r a t i o n o f seawater was a l s o v a r i e d i n order to study the v a r i a t i o n of a d s o r p t i v e behavior w i t h changes i n i o n i c s t r e n g t h . The s u s p e n s i o n was t h e n s p i k e d w i t h e i t h e r DPM o r PMA t o y i e l d c o n c e n t r a t i o n s o f o r g a n o m e t a l l i c which v a r i e d from 0.10 t o 3.5 ppm. The range i n o r g a n o m e t a l l i c c o n c e n t r a t i o n used f o r t h i s s t u d y was determined by t h e s e n s i t i v i t y of t h e d e t e c t i o n method and t h e s o l u b i l i t y o f DPM and PMA i n seawater. The s u s p e n s i o n s were then a g i t a t e d on a shaker t a b l e f o r 48 h o u r s . A d s o r p t i o n c o n s t a n t s which were measured a f t e r 10 days were i d e n t i c a l t o c o n s t a n t s measured a f t e r a 48 hour e q u i l i b r a t i o n p e r i o d . Thus a l l a d s o r p t i o n measurements were made a f t e r a 48 hour equilibration period. Samples were then removed from t h e shaker t a b l e and c e n t r i fuged. I n i t i a l l y , f i l t r a t i o n was employed t o s e p a r a t e l i q u i d and s o l i d phases, but t h e o r g a n o m e t a l l i c s were s t r o n g l y adsorbed onto t h e f i l t e r paper, so t h i s p r o c e d u r e was abandoned and c e n t r i f u g a t i o n was adopted. The c o n c e n t r a t i o n o f DPM o r PMA r e m a i n i n g i n s o l u t i o n was measured on a P e r k i n - E l m e r Model 460 atomic a b s o r p t i o n s p e c t r o p h o t o m e t e r employing t h e M e r c u r y - H y d r i d e System (MHS-10). The r e d u c t i o n o f p h e n y l m e r c u r i e c a t i o n s t o e l e m e n t a l mercury has p r e v i o u s l y been r e p o r t e d ( 2 1 , 2 2 ) . The average r e l a t i v e p r e c i s i o n of t h e method was 1.8%. L i n e a r c a l i b r a t i o n c u r v e s f o r DPM and PMA were o b t a i n e d f o r almost t h e e n t i r e s o l u b i l i t y ranges o f t h e s e compounds i n seawater ( F i g u r e 2 ) , The c a l i b r a t i o n c u r v e f o r DPM became n o n l i n e a r near t h e s o l u b i l i t y l i m i t (3.5 ppm f o r DPM and >5.0 ppm f o r PMA). The d e t e c t i o n l i m i t s f o r DPM and PMA were 0.10 ppm and 0.04 ppm r e s p e c t i v e l y , i n seawater. Because o f t h e r e l a t i v e l y h i g h d e t e c t i o n l i m i t s f o r DPM and PMA, i t was n o t p o s s i b l e t o work i n t h e ppb o r lower c o n c e n t r a t i o n range which would be e n v i r o n m e n t a l l y more r e l e v a n t . The amount o f o r g a n o m e t a l l i c adsorbed by t h e s o l i d phase was




concentration

(ppm)

F i g u r e 2. C a l i b r a t i o n c u r v e s f o r PMA and DPM i n seawater.


22.



375

c a l c u l a t e d as t h e d i f f e r e n c e between t h e s p i k e and t h e amount r e m a i n i n g i n s o l u t i o n a f t e r t h e e q u i l i b r a t i o n p e r i o d o f 48 h o u r s . V a l u e s o f a d s o r p t i o n c o n s t a n t s were determined g r a p h i c a l l y , as d e s c r i b e d below.


R e s u l t s and D i s c u s s i o n D i p h e n y l Mercury A d s o r p t i o n . A d s o r p t i o n o f DPM from seawater onto amorphous i r o n h y d r o x i d e , manganese o x i d e and b e n t o n i t e c l a y was not d e t e c t e d i n t h i s s t u d y . A comparison o f s t a n d a r d d i p h e n y l mercury s o l u t i o n s i n seawater w i t h i d e n t i c a l s o l u t i o n s t o w h i c h sediment phase had been added and shaken f o r 48 hours was r o u t i n e l y performed as p a r t of t h e i s o t h e r m d e t e r m i n a t i o n . There was no s i g n i f i c a n t d i f f e r e n c e i n t h e c o n c e n t r a t i o n of d i s s o l v e d d i p h e n y l mercury f o r s t a n d a r d .versus s t a n d a r d p l u s s o l i d phase f o r any of t h e s u s p e n s i o n s of amorphous, Fe(OH)^, MnO^, o r b e n t o n i t e i n seawater, i m p l y i n g no s i g n i f i c a n t a d s o r p t i o n of DPM from seawater onto t h e s e phases under t h e c o n c e n t r a t i o n s s t u d i e d . I f lower c o n c e n t r a t i o n s o f DPM c o u l d have been used (ppb o r l o w e r ) i t i s p o s s i b l e t h a t a d s o r p t i o n might have been d e t e c t e d . A d s o r p t i o n of DPM from seawater by humic a c i d was r e c o r d e d and t h e v a l u e of Kads was determined by a s i m p l e l e a s t squares a n a l y s i s of t h e l i n e a r p o r t i o n of t h e i s o t h e r m t o be 1.11 f o r a 150 ppm s u s p e n s i o n o f humic a c i d i n seawater ( T a b l e I I ) . T h i s low v a l u e o f Kads i s not unexpected, s i n c e one would expect t h e i n t e r a c t i o n between t h e n e u t r a l n o n p o l a r DPM m o l e c u l e and t h e suspended humic a c i d t o be due t o a r a t h e r weak m o l e c u l a r a t t r a c t i o n . Table I I . V a l u e s of A d s o r p t i o n C o n s t a n t s f o r DPM onto Humic A c i d (150 ppm) as a F u n c t i o n o f Seawater C o n c e n t r a t i o n Kads

(L/G)

1.11 1.25 3.10 1.28

(1.07,

% Seawater BV)

100 50 25 0

The e f f e c t o f changing i o n i c s t r e n g t h on t h e v a l u e o f Kads f o r t h e DPM - humic a c i d system was i n v e s t i g a t e d by d i l u t i n g seawater w i t h d i s t i l l e d water. The pH of t h e s e systems v a r i e d from 7.0 t o 7.6 and depended o n l y on t h e r e l a t i v e amounts of seawater and f r e s h w a t e r . T h i s s m a l l v a r i a t i o n i n pH s h o u l d not s i g n i f i c a n t l y a f f e c t t h e a d s o r p t i v e b e h a v i o r o f DPM. Schwartz (14) found no pH dependence f o r t h e s o r p t i v e b e h a v i o r of o r g a n i c s when t h e s e sub s t a n c e s a r e p r e d o m i n a n t l y i n t h e i r m o l e c u l a r forms. The s l i g h t decrease i n pH w i t h f r e s h w a t e r a d d i t i o n m i r r o r s t h e changes w h i c h would occur i n n a t u r a l b r a c k i s h systems where s a l t and f r e s h water mix. The e f f e c t o f d e c r e a s i n g i o n i c s t r e n g t h on t h e v a l u e o f Kads i s shown i n T a b l e I I . The p r o g r e s s i o n from seawater t o a m i x t u r e of 25% seawater ( 7 5 % d i s t i l l e d w a t e r ) l e d t o an i n c r e a s e i n Kads f o r t h e DPM-humic a c i d system. However, a t 100% d i s t i l l e d w a t e r , the v a l u e o f Kads decreased. The t r e n d o f d e c r e a s i n g Kads w i t h i n c r e a s i n g i o n i c s t r e n g t h i s



376


the same t r e n d t y p i c a l l y seen when s t u d y i n g m e t a l i o n a d s o r p t i o n (7, 8 ) . I t i s somewhat s u p r i s i n g t h a t DPM, a n e u t r a l m o l e c u l e , e x h i b i t s t h i s same t r e n d . I t had been e x p e c t e d t h a t i n c r e a s i n g i o n i c s t r e n g t h would have had a s a l t i n g out e f f e c t on the DPM, i n c r e a s i n g t h e tendency o f t h e DPM t o a s s o c i a t e w i t h the o r g a n i c humic phase. However, t h e t r e n d from 100% t o 25% seawater was j u s t the o p p o s i t e , and suggests t h a t DPM and t h e i o n s of seawater may be competing f o r the same t y p e of a d s o r p t i o n s i t e s . The r e v e r s a l of t h i s t r e n d , o r t h e d e c r e a s e i n Kads when one p r o g r e s s e s from 25% seawater t o d i s t i l l e d water can be e x p l a i n e d i f one c o n s i d e r s t h e s o l u b i l i t y b e h a v i o r o f the adsorbent, humic a c i d . Humic a c i d was found t o be f a i r l y i n s o l u b l e i n s o l u t i o n s v a r y i n g from 25% t o 100% seawater. These s o l u t i o n s a r e c h a r a c t e r i z e d by a f a i n t c o l o r a t i o n even a f t e r t h e humic a c i d has been c e n t r i f u g e d out. As mentioned e a r l i e r , f i l t r a t i o n of these samples r e s u l t e d i n r e t e n t i o n of DPM by the f i l t e r paper, p r o b a b l y due t o a d s o r p t i o n . The c o l o r a t i o n o f t h e c e n t r i f u g a t e was c o n s i d e r a b l y d a r k e r f o r t h e d i s t i l l e d water experiments, s u g g e s t i n g t h a t the humic a c i d was more s o l u b l e i n t h i s medium. Thus t h e i n c r e a s e d c o n c e n t r a t i o n o f d i s s o l v e d humic a c i d i n d i s t i l l e d water l e d t o g r e a t e r s o l u b i l i z a t i o n of DPM and i t s s o r p t i v e b e h a v i o r was thus m o d i f i e d by t h e g r e a t e r c o n c e n t r a t i o n of d i s s o l v e d humic a c i d . T a b l e I I l i s t s two v a l u e s f o r Kads a t 100% SW. The b r a c k e t e d v a l u e i s t h a t e v a l u a t e d u s i n g a humic a c i d e x t r a c t e d from e s t u a r i n e sediments. The s e d i m e n t a r y sample was o b t a i n e d from t h e Chesapeake Bay near t h e town of B i v a l v e (BV). Because the v a l u e s of Kads were so s i m i l a r f o r t h e e s t u a r i n e humic a c i d and t h e c o m m e r c i a l l y p r e pared s o i l humic a c i d , use of the e s t u a r i n e sample was d i s c o n t i n u e d and the s o i l humic a c i d was used c o n s i s t e n t l y thoughout t h e s t u d y . The e f f e c t of c o n c e n t r a t i o n of suspended a d s o r b e n t on s o r p t i v e b e h a v i o r i n a seawater m a t r i x was s t u d i e d by d e t e r m i n i n g adsorp t i o n i s o t h e r m s f o r DPM and 94, 150, 200,400, and 1000 ppm humic a c i d . The c o r r e s p o n d i n g v a l u e s o f Kads ( T a b l e I I I ) show no d e f i n i t e t r e n d w i t h r e s p e c t t o i n c r e a s e d adsorbent c o n c e n t r a t i o n . T h i s i s not t o t a l l y unexpected i n view o f O'Connor and C o n n o l l y * s o b s e r v a t i o n t h a t systems w i t h low Kads v a l u e s do not show d r a m a t i c changes of Kads w i t h changes i n suspended m a t t e r c o n c e n t r a t i o n s (9). Only systems w i t h h i g h Kads v a l u e s t y p i c a l l y show d e f i n i t e d e c r e a s e s i n Kads w i t h i n c r e a s e s i n a d s o r b e n t c o n c e n t r a t i o n . T a b l e I I I . V a l u e s o f A d s o r p t i o n C o n s t a n t s f o r DPM onto Humic A c i d as a F u n c t i o n of Suspended Humic A c i d C o n c e n t r a t i o n Kads (L/g)

2.2 1.1 3.6 1.5 2.3

Although

Concentration

of Humic A c i d (ppm) 94 150 200 400 1000

l e a s t squares a n a l y s i s o f t h e l i n e a r p o r t i o n s o f t h e


22.

DALLANDETAL.


377


a d s o r p t i o n isotherms do not i n d i c a t e a s y s t e m a t i c change i n Kads w i t h changes i n suspended humic a c i d c o n c e n t r a t i o n , the c o r r e s p o n d i n g isotherms ( F i g u r e 3) show a d e f i n i t e and m e a n i n g f u l p a t t e r n . As the c o n c e n t r a t i o n of suspended humic a c i d i n c r e a s e s , the non l i n e a r b e h a v i o r of the i s o t h e r m i s i n i t i a t e d a t a lower and lower DPM c o n c e n t r a t i o n . T h i s suggests t h a t as the c o n c e n t r a t i o n of adsorbent i n c r e a s e s , n o n i d e a l ( n o n l i n e a r ) b e h a v i o r i s a t t a i n e d a t a c o r r e s p o n d i n g l y lower c o n c e n t r a t i o n of a d s o r b a t e (DPM). T h i s i n t u r n i m p l i e s fewer e n e r g e t i c a l l y f a v o r a b l e a d s o r p t i o n s i t e s a v a i l able at higher adsorbate c o n c e n t r a t i o n , implying s o l i d - s o l i d i n t e r a c t i o n r e s u l t i n g i n the p a r t i a l e l i m i n a t i o n of the a v a i l a b i l i t y of c e r t a i n a d s o r p t i o n s i t e s (9). Thus, i n s p e c t i o n of the adsorp t i o n i s o t h e r m s ( F i g u r e 3) demonstrates a t r e n d c o n s i s t e n t w i t h t h a t seen i n o t h e r s t u d i e s (_9) a l t h o u g h Kads v a l u e s do not m i r r o r t h i s . Phenylmercurie Adsorption. P h e n y l m e r c u r i e i o n was chosen f o r t h i s study p r i m a r i l y because of i t s s t r u c t u r a l s i m i l a r i t y t o DPM, the major d i f f e r e n c e , o t h e r than t h e l o s s of one benzene r i n g , b e i n g the charge. The presence of the (+1) charge g r e a t l y m o d i f i e s the a d s o r p t i o n b e h a v i o r of t h e s p e c i e s w i t h r e s p e c t t o DPM. P h e n y l m e r c u r i e i o n was i n t r o d u c e d t o the v a r i o u s s o l i d phaseseawater suspensions as an aqueous s o l u t i o n of p h e n y l m e r c u r i e a c e t a t e (PMA). A d s o r p t i o n of p h e n y l m e r c u r i e i o n onto a l l s o l i d phases s t u d i e d , except b e n t o n i t e c l a y , was noted a t s o l i d concent r a t i o n s of 150 ppm (suspended). A d s o r p t i o n isotherms f o r humic a c i d , hydrous MnO^ and amorphous Fe(OH)^ a r e shown i n F i g u r e 4. V a l u e s of Kads f o r p h e n y l m e r c u r i c i o n and these s o l i d phases a r e l i s t e d i n T a b l e IV. T a b l e IV. V a l u e s of A d s o r p t i o n C o n s t a n t s f o r PMA Kads

(L/g)

i n Seawater

S o l i d Phase

4.5 5.3 50

P h e n y l m e r c u r i c i o n showed a marked p r e f e r e n c e f o r humic a c i d (Kads = 5 0 ) a l t h o u g h a d s o r p t i o n onto two of the t h r e e i n o r g a n i c phases was pronounced w i t h r e s p e c t t o t h e b e h a v i o r of DPM. The charge o f t h e p h e n y l m e r c u r i c i o n a l l o w s i n t e r a c t i o n s t o occur w i t h a d s o r p t i o n s i t e s on Mn0 and F e ( 0 H ) ~ , w h i c h were not r e a c t i v e towards DPM. The e f f e c t of changing i o n i c s t r e n g t h on the v a l u e of Kads f o r PMA a d s o r p t i o n was a l s o i n v e s t i g a t e d . D e c r e a s i n g the s a l i n i t y of the humic a c i d s u s p e n s i o n , l e d t o a v e r y pronounced d e c r e a s e i n Kads. I n 100% seawater, Kads was e v a l u a t e d as 50 L/g ( T a b l e IV) w h i l e a t 25% seawater Kads dropped t o a v a l u e of 1.2. Decreasing s a l i n i t y of t h e MnO^ and Fe(OH)^ systems y i e l d e d s i m i l a r r e s u l t s . At 25% and 50% seawater, no a d s o r p t i o n of PMA onto e i t h e r phase was detected. These r e s u l t s a r e s i m i l a r t o the r e s u l t s of L i e t a l . f o r m e r c u r i c i o n a d s o r p t i o n ( 2 3 ) . I n t h a t s t u d y , a d e c r e a s e i n the a d s o r p t i o n c o n s t a n t f o r m e r c u r i c i o n onto sedimentary phases w i t h ?

0



378


F i g u r e 3. A d s o r p t i o n i s o t h e r m s f o r DPM and v a r i o u s con c e n t r a t i o n s o f suspended humic a c i d i n seawater.





F i g u r e 4. A d s o r p t i o n i s o t h e r m s o f PMA seawater.

and s o l i d phases


380



d e c r e a s i n g s a l i n i t y was d e s c r i b e d . Thus t h e a d s o r p t i v e b e h a v i o r o f PMA ( a charged i o n i c s p e c i e s ) w i t h r e s p e c t t o s a l i n i t y changes, was found t o be somewhat s i m i l a r t o t h a t o f m e r c u r i c i o n . Conclusions. D i p h e n y l mercury, a n e u t r a l o r g a n o m e t a l l i c compound, which does n o t c o n t a i n any markedly a c i d i c o r b a s i c f u n c t i o n a l groups was found t o adsorb o n l y onto humic a c i d . No s o r p t i v e b e h a v i o r c o u l d be d e t e c t e d w i t h r e s p e c t t o b e n t o n i t e , MnO^ o r Fe(0H)„. The r a t h e r low v a l u e o f Kads i n d i c a t e s a s i m p l e m o l e c u l a r a t t r a c t i o n . However, t h e d e c r e a s e i n Kads w i t h an i n c r e a s e i n i o n i c s t r e n g t h i n d i c a t e s t h a t DPM i s competing w i t h t h e m e t a l i o n s i n seawater f o r a d s o r p t i o n s i t e s . The i n c r e a s i n g n o n l i n e a r i t y o f a d s o r p t i o n i s o t h e r m s w i t h i n c r e a s i n g suspended humic a c i d concent r a t i o n i s a l s o s i m i l a r t o r e s u l t s o b t a i n e d f o r m e t a l i o n adsorp tion. P h e n y l m e r c u r i c i o n was found t o a s s o c i a t e most s t r o n g l y w i t h humic a c i d , a l t h o u g h a d s o r p t i o n onto MnO^ and Fe(OH)^ was a p p r e c i c i a b l e . A d s o r p t i o n onto b e n t o n i t e was n o t d e t e c t a b l e f o r e i t h e r organometallic. A l t h o u g h f u r t h e r s t u d i e s a r e needed on t h e b e h a v i o r o f organo m e t a l l i c p e s t i c i d e s , t h e r e s u l t s o f t h i s study i n d i c a t e t h a t n e u t r a l o r g a n o m e t a l l i c s can be e x p e c t e d t o a s s o c i a t e w i t h o r g a n i c phases such as humics. Charged o r g a n o m e t a l l i c s , such as p h e n y l m e r c u r i c i o n a s s o c i a t e most s t r o n g l y w i t h o r g a n i c phases, a l t h o u g h a d s o r p t i o n onto some i n o r g a n i c phases does o c c u r . Obviously, a d d i t i o n a l r e s e a r c h i s needed t o g e n e r a l i z e about t h e a d s o r p t i v e b e h a v i o r o f o r g a n o m e t a l l i c s o t h e r than o r g a n o m e r c u r i a l s . Adsorp t i o n s t u d i e s on o r g a n o t i n compounds a r e c u r r e n t l y i n p r o g r e s s . Acknowledgments Acknowledgement i s made t o t h e Donors o f t h e P e t r o l e u m Research Fund, a d m i n i s t e r e d by t h e American C h e m i c a l S o c i e t y , f o r t h e support o f t h i s r e s e a r c h . The a u t h o r s a l s o w i s h t o thank R o l f Sohn for preparation of f i g u r e s . Literature

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The Adsorption of Organomercury Compounds from Seawater onto

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