Chemical Factors Influencing Metal Alkylation in Water - ACS

17 Chemical Factors Influencing Metal Alkylation in Water K. L. JEWETT and F. E. B R I N C K M A N National Bureau of Standards, Inorganic Chemistry Section, Washington, D.C. 20234 Downloaded by UNIV OF GUELPH LIBRARY on July 11, 2012 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch017

J. M. B E L L A M A Chemistry Department, University of Maryland, College Park, Md. 20742

Because of their toxic properties, the role that heavy metals play i n the environment has recently been the subject of increased study. It i s observed that heavy metals i n their inorganic forms exhibit relatively low toxicity toward biota when compared to these metals with certain carbofunctional groups attached. For example, alkylmercurials are from 10-100 times more toxic than inorganic mercury compounds (1,2), and similar effects are noted for organotins ( 2 ) . Many heavy metals, however, are released into the aquatic environment in relatively nontoxic form. They subsequently acquire this enhanced toxicity as organometals through environmental interactions involving both biological and non-biological processes ( 3 ) . It is therefore fundamental to environmental concerns to investigate the chemical factors which bring about this transformation from inorganic to organometal compounds i n aqueous media. Two important factors responsible for the conversion of heavy metals into organometals are ligand interactions and photochemical processes. In this paper an initial effort i s made to evaluate the potential importance of these factors i n bringing about metal transformations. Examples have now appeared i n the literature which show the occurrence of metal transformations i n aqueous solution (3), including a wide range of transmethylation reactions between heavy metal ions ( 4 ) . On the one hand, the known cellular metabolite methylcobalamin has been shown to transfer with ease i t s methyl group to mercuric ion in aqueous solution to form the highly neurotoxic methylmercury ion (5). In another study (6) , J e r n e l ô v et al.j have implicated anthropogenic alkyHead discharges i n formation of high methylmercury concentrations i n St. C l a i r River sediments. Hence we have an example of methylation also occurring environmentally i n apparently a straightforward metathetical reaction. Work has also been advanced i n the area of photochemistry. For example, a me thy1chromiurn bond has been formed i n the photolysis of tertiary-hutoxy radicals and chromium(II) i n aqueous 304

In Marine Chemistry in the Coastal Environment; Church, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

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305

s o l u t i o n (7). I t was found t h a t i n the presence of a c e t a t e i o n or a c e t i c a c i d i n aqueous s o l u t i o n , H g was photolyzed i n s u n l i g h t to form a methylmercury bond (8). We have examined t h i s l a t t e r r e a c t i o n and s i m i l a r ones. These l a s t r e a c t i o n s are c o n s i s t e n t with e a r l y f i n d i n g s (9) t h a t (RCC>2)2 9 compounds decompose i n prot i c o r g a n i c s o l v e n t s under u l t r a v i o l e t i r r a d i a t i o n to y i e l d a l k y l m e r c u r i a l s , RHg(OCOR). 2 +

H

Downloaded by UNIV OF GUELPH LIBRARY on July 11, 2012 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0018.ch017

Experimental Metal C o o r d i n a t i o n VS. Rate o f Reaction. Two o f the o r g a n o t i n compounds, Me3SnNC>3 and Me SnC10i (Me = CH3) were prepared (10) by Reacting Me3SnBr with an e q u i v a l e n t amount of the a p p r o p r i a t e s i l v e r s a l t i n methanol. The s o l u t i o n s were then f i l t e r e d and the s o l v e n t was removed from the o r g a n o t i n product by vacuum s e p a r a t i o n techniques. Both d e s i r e d t i n compounds were then sublimed twice in vacuo and found t o be s u i t a b l e f o r use by s p e c t r o m e t r i c t e c h niques. Other compounds used were found not t o r e q u i r e f u r t h e r purification. A l l s o l u t i o n s were prepared u s i n g d i s t i l l e d water; the presence o f d i s s o l v e d a i r had no d e t e c t a b l e e f f e c t on r e a c t i o n r a t e s observed. In order t o i n v e s t i g a t e the a b i o t i c m e t h y l a t i o n o f H g in water, the e f f e c t o f l i g a n d s on r e a c t i o n r a t e was s t u d i e d . I t had been p r e v i o u s l y determined (4) t h a t Me3Sn undergoes f a c i l e l o s s o f one methyl group t o q u a n t i t a t i v e l y form MeHg from aquo-Hg as shown by the f o l l o w i n g equation: 3

+

2 +

+

+

Me Sn

+

+ Hg

3

2 +

H

2°

> Me Sn

2 +

+ MeHg

2

2+

+

(1)

Reactions were monitored by proton magnetic resonance (pmr) employing procedures d e s c r i b e d elsewhere (.4 #11) · F i g u r e 1 shows a c h a r a c t e r i s t i c pmr spectrum. Areas under these curves were measured by p l a n i m e t r y and normalized t o a d j u s t f o r i n t e n s i t i e s due t o the v a r y i n g amounts o f protons. At any time, extent o f r e a c t i o n (X) i s t h e r e f o r e equal to 3/2·(Area M e S n ) / ( A r e a Me3Sn )· i m S n ] . or 3· (Area MeHg )/(Area Me Sn ) · [Me Sn ].. These data were then subjected t o r e g r e s s i o n a n a l y s i s and the r e s u l t s are summarized i n Table I . M e t h y l a t i o n o f mercury was found t o obey the second order r a t e law, 2+

+

2

3

+

+

U

i

a

V

-

+

3

À

+

3

TO

where A and Β represent the i n i t i a l c o n c e n t r a t i o n s of r e a c t a n t s and χ i s the extent o f r e a c t i o n at any g i v e n time ( t ) . F i g u r e 2 shows a p l o t o f data t y p i c a l l y obtained. Within experimental e r r o r , e.g., ± oa. one standard d e v i a t i o n , both the formation of methylmercury and d i m e t h y l t i n ions have the same slope as p r e d i c ted i n equation 1. The l i n e s shown through these data p o i n t s i n



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Figure I. Characteristic nuclear magnetic spectrum of all protonated metal species indi cated by Equation 1

I

0

1 5

1

10 SECONDS Χ 1 0

1

ι

15

20

ι 25

3

Figure 2. When production of either MeHg* or Me Sn * is incorporated in the 2nd order rate expression shown on the ordinate, slopes (k ) for either species are in reasonable agreement. For all Cl~ dependent reactions discussed here, d [ M ] /dt = d[Me Sn ^ /dt. 2

2

2

2

2


17.

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307

Table I R e l a t i o n s h i p Between T o t a l C h l o r i d e and Mercuric Ion Concentrations o f Transmethylation


T o t a l CI T o t a l Hg

3. For each o f these r e a c t i o n s , a p p r o x i mately 0.05 M s o l u t i o n s o f H g C l were prepared by d i s s o l v i n g t h a t compound i n t o s a l i n e s o l u t i o n o f a p p r o p r i a t e c o n c e n t r a t i o n . The other r e a c t a n t (CH3)3SnCl was prepared i n d i s t i l l e d water. There was no evidence t h a t use o f N a as a counter i o n a f f e c t e d k i n e t i c 2

+


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results. A combination e l e c t r o d e was used t o make pH measurements o f r e a c t a n t s o l u t i o n s . For the s p e c i f i c case Cl/Hg = 3, the pH o f a thermostated r e a c t i o n mixture was measured as a f u n c t i o n of time, and found t o f o l l o w the simple r e l a t i o n s h i p , k t = [ H ] .


+

2

P h o t o l y s i s of Heavy Metals Compounds. Approximately 0.05 M d i s t i l l e d water s o l u t i o n s o f mercuric a c e t a t e and methylmercurie acetate were prepared i n the dark. Some of each s o l u t i o n was placed i n t o t h i n - w a l l fused s i l i c a nmr tubes, which were maintained i n d i r e c t contact with a "pencil-probe" u l t r a v i o l e t l i g h t (253.7 nm) source. While c o n t r o l tubes were maintained i n the dark, the r e a c t i o n tubes were i r r a d i a t e d i n ten-minute increments up t o a t o t a l o f 100 minutes. A f t e r each exposure t o r a d i a t i o n , both c o n t r o l and r e a c t i o n tubes were examined by pmr spectrometry. Extent o f r e a c t i o n f o r the l a t t e r was determined r e l a t i v e t o the c o n c e n t r a t i o n measured i n the c o n t r o l tubes, u t i l i z i n g methylmetal peak areas as b e f o r e , as w e l l as the methyl peak area i n a c e t a t e i o n . A s o l u t i o n o f 0.05 M mercuric a c e t a t e i n deuterium oxide (D2O) was a l s o i r r a d i a t e d and v o l a t i l e decomposition products were examined. D i s t i l l e d water s o l u t i o n s (0.05 M) o f sodium a c e t a t e and o f t h a l l i u m ( I ) acetate were a l s o i r r a d i a t e d as a f u n c t i o n o f time. Experimental c o n d i t i o n s were the same as those used i n p h o t o l y s i s of mercury compounds. Gas e v o l u t i o n and metal p r e c i p i t a t i o n were c h a r a c t e r i s t i c f o r the i r r a d i a t e d a c e t a t e s o l u t i o n s c o n t a i n i n g Hg(II) o r T l ( I ) but not sodium acetate alone. V o l a t i l e decomposition products were analyzed by gas chromatography and mass spectrometry. For element a l mercury and t r a c e organomercury a n a l y s e s , a combination gas chromatograph-mercury s p e c i f i c atomic a b s o r p t i o n apparatus was used (13). Results and D i s c u s s i o n 2 +

The a b i o t i c t r a n s m e t h y l a t i o n o f H g i n water was examined by measuring the e f f e c t on the r a t e constant o f v a r y i n g the r a t i o o f t o t a l c h l o r i d e to t o t a l mercury (Cl/Hg) i n equation 3. The r e s u l t s are shown i n F i g u r e 3. In the a l 1 - p e r c h l o r a t e system, i.e. Cl/ Hg = 0, no r e a c t i o n was found t o occur. With i n c r e a s i n g Cl/Hg r a t i o s the r a t e o f r e a c t i o n reaches a maximum a t Cl/Hg = 4 and decreases t h e r e a f t e r . When n i t r a t e i o n was s u b s t i t u t e d f o r p e r c h l o r a t e i n the s p e c i f i c case o f Cl/Hg = 2, r a t e constants f o r t h i s and the p e r c h l o r a t e r e a c t i o n were not s i g n i f i c a n t l y d i f f e r e n t . In F i g u r e 3 i t i s apparent t h a t a s i g n i f i c a n t change i n the r a t e o f r e a c t i o n occurs when the t o t a l c h l o r i d e i o n c o n c e n t r a t i o n introduced i n t o equation 2 i s a l t e r e d w i t h r e s p e c t t o the concent r a t i o n o f mercury present i n t h i s system. Since c h l o r i d e i s a very s t r o n g c o o r d i n a t i n g l i g a n d and mercury i s known t o form a number o f complex s p e c i e s i n aqueous s o l u t i o n (14), we sought t o 3



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Figure 3. Results of chloride ion dependence study on reaction rate. Regression analyses of these data yielded relative standard errors in the k values (slopes) from 3^6%. 2


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identify which chlorohydroxymercury complexes were present under the varying [Cl~]/[Hg ] ratios used i n the above rate studies, and to ascertain which species are actually necessary for the transmethylation reaction. I t i s postulated that the most reactive intermediate mercury complex w i l l show a variation i n r e l a tive abundance which w i l l p a r a l l e l the changes i n the reaction rate shown i n Figure 3. It i s also useful to note here that previous work i l l u s t r a t e d i n Figure 4 predicts that a large pH range has l i t t l e effect on the nature of [ H g C l ] " species, whereas small changes i n chloride ion concentration produce significant changes i n the relative abundances of chloromercury complexes. Thus, the shaded ellipse represents pH and pel values found by us i n the estuarine environment (3), whereas the conditions approximately covered i n our present laboratory investigation are shown by the dotted ellipse. During the course of each reaction i n the current study, the pH of each solution characteristically changed by approximately one pH unit. Attempts were therefore made to quantitatively relate the change i n [H ] with the second order rate equation. These tests include S- [H ] vs. time, and S/[H ] vs. time, and 5- [H ] V Vs. time where S represents the second order rate expression shown i n equation 2. Least squares regression analysis for a l l of the above cases incorporated data for any one run i n three ways: total data points vs. time, f i r s t half of the data points vs. time and second half of data points vs. time. Since our rate data typically included 13-18 nmr spectra taken over an eight hour period, the latter two analyses were deemed necessary i n the event that there was a significant change i n kinetics during such extended periods. In any of the calculations incorporating [H ], [H+] V , or 1/[H ], we found significantly poorer f i t s than for those runs not involving these hydrogen ion concentration terms. We therefore conclude that within the precision of our methods (of. Table I ) , the transfer of methyl from tin(IV) to mercury(II) i s not directly pH dependent i n saline media. A computer program, modified from one originally developed by Swedish workers (15) to calculate the most abundant mercury species at given pH and pCl values, indicated that several of fourteen possible [ H g ( 0 H ) C l ] " ~ complexes exist under conditions of environmental interest. The three complexes shown i n Figure 5 predominate under such conditions, and account for over 99% of a l l mercury(II) present i n the t o t a l system. While HgCl2 decreases with increasing chloride ion concentration, HgCli^ shows the reverse trend. The only chloromercury species to reach a maximum concentration over this environmental s a l i n i t y range i s HgCl3~. It was previously determined that the rate of appearance of either of the products formed i n equation 1 i s equal to the f o l lowing expression: 2+

2

n


n

+

+

+

+

2

+

2

+

2

m

m

n

n




JEWETT ET AL.

Figure 4. Stability fields for Hg(OH) Cl species determined from equilibria for mercury complexes as a function of pH and pCl m

n


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d

M

^ ^

g

^ = k ( 2

o b s d )

(4)

[Total Mercury] [Organotin]

I t i s assumed t h a t o r g a n o t i n s p e c i e s are i n v a r i a n t with changing Cl/Hg r a t i o s i n these aqueous s o l u t i o n s , and t h i s view i s c o n s i s t e n t with the low s t a b i l i t y constants reported f o r [ ( C H 3 ) 3 S n C l ] ~ complexes i n aqueous s o l u t i o n (16). I f the r a t e o f product format i o n i s dependent o n l y (or c h i e f l y dependent) on the presence o f a s p e c i f i c chloromercury s p e c i e s , [ H g C l ] ~ , a new r a t e constant k ' , can be d e f i n e d : 1

n

n

2

n

n


2

d

t

M

f f

+

1

2

n

= k '[(Hgcl ) " ][Organotin] 2

n

(5)

We f u r t h e r expect k2* t o be g r e a t e r than K2 s i n c e the react i v e mercury s p e c i e s i s present a t lower c o n c e n t r a t i o n s under these conditions. Moreover, the energy o f a c t i v a t i o n f o r the r e a c t i o n shown i n equation 1 has been p r e v i o u s l y determined (4,11)- A r e l a t i v e l y low value o f 14.2 kcal/mole i s c o n s i s t e n t with a r e a c t i o n i n v o l v i n g Me3Sn and HgCl3~, i n t h a t l a r g e coulombic r e p u l s i o n s are not expected i n formation o f the a c t i v a t e d complex (17). Although i o n i c i n t e r f e r e n c e s have hampered attempts t o d i r e c t l y determine p e l values i n l a b o r a t o r y experiments, r e f i n e d p C l approximations have l e d t o s e v e r a l c o n c l u s i o n s . When u s i n g the m o d i f i e d computer program along with the estimated p C l v a l u e s f o r the l a b o r a t o r y r e a c t i o n c o n d i t i o n s , i t was again found that only HgCl3~ reaches a maximum c o n c e n t r a t i o n i n the Cl/Hg range f o r which k , reaches a maximum. Inasmuch as the r e a c t i o n shown i n equations 1 and 3 i s f i r s t order i n mercury, and the r a t e constant reaches a maximum when the Cl/Hg r a t i o i s v a r i e d , we would expect a chloromercury complex t o show such s i m i l a r behavior. Since there i s only one mercury s p e c i e s e x h i b i t i n g the expected behavior under l a b o r a t o r y c o n d i t i o n s , we conclude t h a t HgCl3~ i s the o n l y or most important rate-determining mercury complex. One case f o r u s e f u l l y extending the present f i n d i n g s becomes apparent. In our survey (4) , we have p r e v i o u s l y examined the following reaction: H0 C d C l + (CH ) S n C l No Reaction (6) +

2 (

2

2

3

3

Chlorohydroxycadmium i o n s analogous t o the present Hg cases are mainly d i s s o c i a t e d under these c o n d i t i o n s (18). Consequently, r e a c t i o n may be mainly hindered by the a n t i c i p a t e d l a r g e energy of a c t i v a t i o n r e q u i r e d f o r the two p o s i t i v e l y charged aquated species, e.g. Cd and Me3Sn , t o undergo transmethylation. In a d d i t i o n , p r e l i m i n a r y r e s u l t s suggest that some h a l i d e i o n ( C l ~ or Br") must a l s o be p r e s e n t along w i t h oxo-anions (NO3"" or ClOi^") to i n s u r e t h a t methyl t r a n s f e r occurs. That i s , the presence of other 2 +

+

y



17.


JEWETT ET AL.

313

p o s s i b l e n e g a t i v e l y charged HgL3" (i.e.* L = N03~, Ac", etc.) complexes may not be s u f f i c i e n t f o r t r a n s m e t h y l a t i o n . We are c u r r e n t l y reexamining t h i s p r o s p e c t f o r r e a c t i o n a t a s u i t a b l e pH where c o o r d i n a t i o n of cadmium i s s h i f t e d by excess C l ~ t o an abundance o f [CdCl3]~. Good aqueous m e t h y l a t i n g agents such as t r i m e t h y l t i n i o n or t r i m e t h y l e a d i o n (£#11) may then become e f f e c t i v e i n methyl t r a n s f e r to cadmium, although the f a t e o f any intermediate methylcadmium species i n s a l i n e waters i s p r e s e n t l y unknown. Through these s t u d i e s one c l e a r l y sees t h a t the p o t e n t i a l e f f e c t s o f c o o r d i n a t i o n and i o n i c e q u i l i b r i a on organometal t r a n s formations may indeed be v e r y s i g n i f i c a n t . P h o t o l y s i s , however, may have a s i m i l a r o r an even more widespread impact on e n v i r o n mental t r a n s f o r m a t i o n s o f metals. We have t h e r e f o r e i n i t i a t e d s t u d i e s seeking t o assess the scope and extent o f t h i s i n f l u e n c e on metal t r a n s f o r m a t i o n s . When an aqueous s o l u t i o n of mercuric a c e t a t e i s i r r a d i a t e d , the r e s u l t s as a f u n c t i o n o f time are shown i n F i g u r e 6. Note t h a t a c e t a t e i o n (Ac") decreases i n an almost e x p o n e n t i a l manner. Methylmercury i o n i s r a p i d l y formed and i n c r e a s e s t o a maximum c o n c e n t r a t i o n , but t h i s s p e c i e s f i n a l l y d i m i n i s h e s i n concentrat i o n upon f u r t h e r i r r a d i a t i o n . Dimethylmercury o n l y forms a f t e r methylmercury has reached i t s maximum c o n c e n t r a t i o n . I t appears t h a t dimethylmercury reaches some p o i n t where i t s r a t e o f format i o n approximates the r a t e of decomposition, but f i n a l l y t h i s s p e c i e s a l s o decreases i n c o n c e n t r a t i o n . During the course of r e a c t i o n , gas e v o l u t i o n and metal p r e c i p i t a t i o n are observed. These products have a l s o been c h a r a c t e r i z e d . By u s i n g gas chromatography and mass spectrometry, ethane and carbon d i o x i d e were i d e n t i f i e d . Mercury metal was determined u s i n g a gas chromatograph i n tandem with m e r c u r y - s p e c i f i c atomic absorpt i o n spectrophotometer (GC-AA) which i s d e s c r i b e d elsewhere (13). When mercuric a c e t a t e (0.05 M) i n D 2 O was i r r a d i a t e d and v o l a t i l e products subsequently examined by mass spectrometry, no i n c o r p o r a t i o n o f deuterium was observed. Only ethane and carbon d i o x i d e were again d e t e c t e d . Decomposition o f r e a c t i v e methyls p e c i e s must t h e r e f o r e i n v o l v e some concerted b i m o l e c u l a r process i n which s o l v e n t (H 0 or D 0) p r o t o l y s i s i s not a c t i v e l y i n v o l v e d . At the p o i n t where methylmercury reaches maximum concentrat i o n both methylmercury and a c e t a t e i o n s are observed t o be o f comparable c o n c e n t r a t i o n s . Consequently, s i m i l a r behavior f o r the i r r a d i a t i o n o f methylmercurie a c e t a t e i s a n t i c i p a t e d . A s i m i l a r p a t t e r n i s seen f o r product formation a f t e r methylmercuric i o n reaches i t s maximum c o n c e n t r a t i o n (Figure 6) when compared with the r e s u l t s found on i r r a d i a t i n g methylmercuric a c e t a t e as shown i n F i g u r e 7. The s l o p e s o f each o f the s p e c i e s represented i n F i g u r e 7 show much the same behavior as t h e i r analogues i n F i g u r e 6. The f i n a l products f o r t h i s r e a c t i o n are i d e n t i c a l t o those observed f o r the p h o t o l y s i s o f mercuric a c e t a t e . P h o t o l y s i s o f systems i n v o l v i n g mercury(II) i n the presence 2

2


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îoo-l

Ι.β

1.4

1.2

I. Ο

.·

β

.4

.2

pCl Figure 5. At the chloride ion concentrations (pCl) shown, the above mercury species are expected to occur

SBC. * 10

2

Figure 6. Relative concentrations for the various protonated reactants and products were determined during pho tolysis relative to a control sample which was maintained in the dark



JEWETT ET

AL.

Metal Alkyfotion in Water

Figure 7. The photolysis of methylmercuric acetate is seen to follow a reaction sequence similar to that observed for mercuric acetate after halfreaction



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of a number of other known b a c t e r i a l metabolic products i n aqueous s o l u t i o n has a l s o been attempted. Thus, s t o i c h i o m e t r i c amounts o f acetone, ethanol or p r o p i o n i c a c i d , when i r r a d i a t e d i n the presence o f aqueous mercuric c h l o r i d e , have a l s o been found t o produce the same organomercury s p e c i e s , a l b e i t a t d i f f e r e n t r a t e s . P h o t o l y s i s o f mercury compounds has a l s o been found t o occur i n very d i l u t e s o l u t i o n s even under normal l a b o r a t o r y f l u o r e s c e n t l i g h t i n g . A s o l u t i o n o f 20 ppm (yg/g) mercury (as HgCl2) and 40 ppm a c e t a t e (as NaAc), p l a c e d i n a quartz cuvette, was found t o produce t r a c e amounts o f methylmercury i o n , dimethylmercury, and mercury metal. The amounts o f these same s p e c i e s were found to be i n s i g n i f i c a n t i n the c o n t r o l s o l u t i o n which was maintained i n the dark. Again, the presence o f each o f the products was v e r i f i e d by GC-AA a n a l y s i s . T h i s study i m p l i e s that some mercury compounds normally used i n the l a b o r a t o r y may be undergoing t r a n s f o r m a t i o n s by j u s t s i t t i n g on the s h e l f . A survey o f the p h o t o l y s i s o f a c e t a t e i o n i n the presence o f other metal i o n s has a l s o been i n i t i a t e d . Among other metals, t h a l l i u m was s e l e c t e d f o r i n i t i a l study because o f the r e c e n t demonstration o f i t s b a c t e r i a l u t i l i z a t i o n and t r a n s p o r t (19). Thallium(I) shows evidence o f involvement i n a p h o t o l y t i c process s i m i l a r t o t h a t seen f o r H g ( I I ) . Thus, when t h a l l i u m ( I ) a c e t a t e i s i r r a d i a t e d a decrease i n acetate c o n c e n t r a t i o n occurs? however, i f a s o l u t i o n of sodium a c e t a t e i s s i m i l a r l y i r r a d i a t e d , no diminu t i o n o f acetate c o n c e n t r a t i o n i s observed over the same p e r i o d of time. A l s o , as with the case f o r Hg, continued i r r a d i a t i o n of T l ( I ) acetate f i n a l l y produces f r e e metal, along w i t h gaseous products. Two important f a c t o r s emerge from these r e s u l t s : (1) p r e s ence o f a metal c a t i o n capable o f a p p r o p r i a t e ( p o s s i b l y bidentate) c o o r d i n a t i o n o f the acetate moiety may be necessary f o r e f f e c t i v e p h o t o a l k y l a t i o n t o occur; and (2) p o s s i b l e formation o f a t r a n s i e n t methylthallium(I) s p e c i e s , s i m i l a r t o those r e a c t i o n intermediates reported f o r Pd(II) and P t ( I I ) (4), forms but r a p i d l y decomposes t o observed products. That i s , as soon as the organothal1ium s p e c i e s i s produced i t undergoes f u r t h e r r e a c t i o n , presumably d i s p r o p o r t i o n a t i o n t o T l ( I I I ) and the metal (20). T h i s prospect i s c u r r e n t l y under i n v e s t i g a t i o n , and e f f o r t s are being made to i d e n t i f y e l u s i v e intermediate s p e c i e s , and any p o s s i b l e s t a b l e m e t h y l t h a l l i u m ( I I I ) by-products. These s t u d i e s c l e a r l y i n d i c a t e t h a t s a l i n i t y and p h o t o l y s i s may have important r o l e s i n b r i n g i n g about organometal transformat i o n s i n the environment, and should be considered i n f u r t h e r s t u d i e s o f the r o l e o f metals i n the environment. S a l i n i t y has been demonstrated t o r e g u l a t e the r a t e o f one k i n d of transmethyla t i o n r e a c t i o n t o an important extent, and may be a key f a c t o r i n other cases. In a d d i t i o n , p h o t o l y s i s has been demonstrated t o be a process d e s i r a b l e f o r f u r t h e r study. Even i n the ppm range methy1a t i o n of mercury has been demonstrated as a f a c i l e process occurr i n g under t y p i c a l l a b o r a t o r y l i g h t i n g c o n d i t i o n s . T h i s l a s t p o i n t


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w i l l bear f u t u r e c o n s i d e r a t i o n s on common a n a l y t i c a l methods employed f o r both environmental or l a b o r a t o r y samples. The aforementioned r e s u l t s represent model s t u d i e s i n t o the e f f e c t s that l i g a n d i n t e r a c t i o n s and p h o t o l y s i s may p l a y i n the environment. I t i s through our understandings a t t a i n e d i n these s t u d i e s t h a t we may more capably i n v e s t i g a t e appropriate reac t i o n s occuring under environmental c o n d i t i o n s .


Literature Cited 1. Hartung, R. and Dinman, B. D., Ed., "Environmental Mercury Contamination", Ann Arbor Sci. Publ. (1972), pp. 341-345. 2. Barnes, J . M. and Magos, L., Organomet. Chem. Rev. (1968), 3, 137-150. 3. Brinckman, F. E. and Iverson, W. P., ACS Symposium on Marine Chemistry in the Coastal Environment (T. Church, Ed.) (1975), paper 22. 4. Jewett, K. L. and Brinckman, F. Ε . , Preprints of Papers, Div. Environ. Chem. Amer. Chem. Soc. (1974), 14, 218-225. 5. Agnes, G . , Bendle, S., H i l l , Η. Ο. Α . , Williams, F. R., and Williams, R. J . P., Chem. Comm. (1971), 850-851. 6. Jernelöv, Α., Lann, H . , Wennergren, G . , Fagerström, T . , Asell, P., and Andersson, R., "Analyses of Methylmercury Concentration in Sediment From the St. Clair System", unpublished report of the Swedish Water and Air Pollution Research Laboratory (1972). 7. Ardon, M., Woolmington, K. and Pernick, Α., Inorg. Chem. (1971) 10, 2812. 8. Agaki, H. and Takabatake, Ε . , Chemosphere (1973), 3, 131-133. 9. Razuvaev, G. A. and Ol'deleop, Υ. Α., Doklady Akad. Nauk S.S. S.R., (1955), 105, 738-740. 10. Clark, H. C., O'Brien, R. J., Inorg. Chem. (1963), 2(4) 740-4. 11. Jewett, K. L. and Brinckman, F. E. (1975), submitted for publication. 12. Johansson, L., Coord. Chem. Rev. (1974), 12, 241-261, and references cited therein. 13. Blair, W., Iverson, W. P. and Brinckman, F. E., Chemosphere (1974), 3, 167-174. 14. Gilmore, J . T . , Environ. Letters (1971), 2, 143-152. 15. Anfält, T . , Dyrssen, D., Ivanova, Ε . , and Jagner, D., Svensk Kemisk Tidskrift (1968), 80, 340-342. 16. Cassol, Α., Magon, L., and Barbieri, R., Inorg. Nucl. Chem. Letters (1967), 3, 25-29. 17. Abraham, M. H. and Spalding, T. R., J. Chem. Soc. (A) (1968), 2530-35. 18. Weber, Jr., W. J. and Possalt, H. S., in "Aqueous Environmen tal Chemistry of Metals", Ann Arbor Sci. Publ. (1974), pp. 255-264. 19. Schneiderman, G. S., Garland, T. R., Wildung, R. Ε . , and Drucher, Η., Abstract 74th Ann. Mtg. Amer. Soc. Microbiol., Chicago, Ill.(May, 1974), p. 2. 20. Kurosawa, Η., and Okawara, R., Inorg. Nucl. Chem. Letters (1967), 3, 21-23.


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318 Ac knowle dgemen t s


The authors are g r a t e f u l f o r f i n a n c i a l support o f these s t u d i e s by the NBS O f f i c e o f A i r and Water Measurements, E n v i r o n mental P r o t e c t i o n Agency and Department o f the Navy. We thank Dr. R o l f Johannesen f o r h i s v a l u a b l e a s s i s t a n c e i n nmr and computer c a l c u l a t i o n s . We a l s o thank Mr. Richard Thompson (pmr measurements) and American U n i v e r s i t y Research P a r t i c i p a t i o n Program students, Lee Silberman and Steven Wagner (data c o l l e c tion) f o r t h e i r a i d .


Chemical Factors Influencing Metal Alkylation in Water - ACS

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