Chemistry of methylmercurials in aqueous solution

Chemosphere No. 4, PP 147 - 152, 1973.

Pergamon Press.

Printed in Great Britaia.

CHEMISTRY OF METHYLMERCURIALS IN AQUEOUS SOLUTION N. L. Wolfe, R. G. Zepp, J. A. Gordon, and G. L. Baughman U. S. Environmental Protection Agency, National Environmental Research Center-Corvallis, Southeast Environmental Research Laboratory, College Station Road, Athens, Georgia 30601

(Received im USA Ii Juae 1973; received in UK f o r publication 25 Juae 1973)

Abstract The kinetics of acid cleavage of dimethylmercury At 25°C and pH 5, D M M h a s

(DMM) was studied in aqueous solution.

an acidolysis half-life of about 33 years.

Cleavage of dimethyl-

mercury by mercuric species is pH-dependent and is slow at the concentrations of mercuric species in solution in the aquatic environment. sunlight photodegradation.

Dimethylmercury and methylmercuric ion do not undergo

The evaporative loss of dimethylmercury from water can be compared

with oxygen diffusion based on the ratio of transfer constants. Introduction Recently the ecological effects of mercury and mercury compounds in the environment have become a matter of increased concern. I' 2

Both organomercurials and inorganic mercury compounds

as well as elemental mercury have been reported to result in formation of methyl and dimethylmercury (DMM) in the aquatic environment, s

A model 4 similar to Figure I has been proposed for

these transformations, but lacks detail pertaining to mechanisms and kinetics.

(C~%)2Hg (CH3)=Hg Hg°

•

Hg ++ =

CHsHg +

04-0c~ c~-Hg*/~ Figure i.

Model for the transformation of mercury in the environment. 4

Furthermore, previously reported studies of organomercurial reactions were limited to organic solvents, s

Extrapolation of these data to aqueous systems is especially precarious considering

the hydrolysis of mercurials in water (eqs i-5). s

147

148

No. 4

He0 ~ CI~Hg + + X-

C~HgX

(I)

H~O CHsHg +

•

CHsHgOH + H +

(2)

H~O ~gX~

=

H g + + + 2x"

Hg++ + HeO

=

HgOll+ + He0

(3)

HgOH + + H~

~

(4)

Hg(0H)e + H+

(5)

We report some chemical kinetic data required to evaluate the possible chemical role of some pathways shown in Figure i. Experimental All organomercury compounds were obtained co~nercially and purified before use. acidolysis and photolysis experiments, disappearance of D M M w a s concentration determined relative to an internal standard. analyzed by the dithizone technique, v

In

followed with GLC and the

Methylmercury compounds were

In desymmetrization reactions, disappearance of Hg II

was followed by reducing aliquots with stannous chloride and then analyzing for elemental mercury by flameless atomic absorption spectroscopy.

Rate constants were obtained from a

computer analysis of data employing a least squares program. Results and Discussion Acidolysis of dimethylmercury.

Acid cleavage of DMM to give methylmercury ion and

methane (eq 6) has been proposed as a possible chemical route for degradation of DMM in acidic natural waters. ~,4,s

~0 C~g~

+H*

~

C%Hg + +~

(6)

To help evaluate this reaction as a pathway for chemical transformation, rate constants were determined at different temperatures in water. activation parameters.

The rate is dependent on both D~fl~and acid concentrations and obeys the

kinetic expression in eq 7. to different temperatures;

d[DMM] dt

They are presented in Table I along with their

=

With activation parameters, the rate constant may be extrapolated for HCI acidolysis data at 2 ~ C, k = 7.33 X I0-s g/mole sec.

k i D S ] [H+ ]

(7)

No. 4

149

Table i.

Acid

Kinetic Data for Acidolysis

Temperature °C

k X l0s g/mole sec

65

7.31 + 0.i a

HCI

85

HClO~

48.8

65

50.2

+ 2b

ion concentrations

(t½) can be calculated by eq 8. approximately

33 years.

-3.1 + 0.6

21.3 + 0.8 -

-5.3 + 1.4 =Calculated

reaction conditions

much greater than DMM concentrations,

For example,

Thus, acidolysis

t½

22.1 + 0.I

b Single determination.

conditions where pseudo-first-order

or hydrogen

AS% = eu

+ 0.I a

a Average value of two determinations, by the computer program ACTE.

Under environmental

AH# c kcal/mole

8.06 _+ 0.3b

85

to buffering

of Dimethylmercury. a

at pH 5, the half-life

can become significant

can prevail due the half-life

for this reaction would be

at 25°C only at very low pH's.

0.693

(8)

[Yl k Desymmetrization

of dimethylmercury.

chemical pathway for degradation

CH3HgCHo

[HgI I ]

Another reaction of DMM suggested as a possible

is cleavage by inorganic mercuric

species. 4

+ Hg II P~O 2CHsHg +

(9)

= [Hg++ ] + [HgOH'+] + [Hg(OH,a)]

(10)

The kinetics of this reaction employing Hg(CI04) z in water were determined; are given in Table 2. concentration Also,

the rate constants

At constant pH, the reaction rate was dependent on both DMM and total

of inorganic mercuric

the rate constants

species

[Hg II] and obeyed the expression

increased as the pH was decreased.

that Hg(CIO~) e hydrolyzes species being pH-dependent

given in eq ii.

Hietanen and Sill~n s have shown

to give Hg(OH)e , HgOB* and Hg ++ , the relative concentrations (eqs 3-5).

The increase in rate constant at lower pH is therefore

due to an increase in Hg ++ and/or HgOH + ion concentrations. 9 d[Dl~4] dt

~bs

of each

[Dm~] [~gH]

(ll)

No. 4

150

A half-life

for reaction 9 can be calculated

much larger than that of DMM.

if the Hg II concentration

Under these pseudo-first-order

conditions,

is assumed to be

the following expres-

sion is obtained: 0.693 t½

=

[Hgll ] k~bs

Using a concentration

(121

of 0.03 ppb for Hg II, the average value reported for mercury in

natural waters, e and k = i X I0S 6/mole sec at pH = 5.8, t½ is about 50 days. desymmetrization

of DMM becomes environmentally

more significant

Thus,

the

as [Hg II] increases and as pH

decreases.

Table 2.

pH-Dependence

pH

of the Desymmetrization

of Dimethylmercury

in Water at 27° C. a

k 6/mole sec

pH

5 X 104

5.8

I × i0s

2 X 104

9.0

2 X 10- I

1.2 3.1

k 6/mole sec

a Atomic absorption method.

Stability

of dimethylmercury

to oxygen and base.

Secondary and tertiary dialkylmercury

compounds have been reported to undergo slow oxidation by molecular oxygen in organic solvents l° whereas

primary dialkylmercury

In our studies, solution at 8 ~ C .

Dl~was

compounds

are reported to be less susceptible

to oxidation. I°

found to be stable for 24 hours in an oxygen-saturated

Dimethylmercury

aqueous

was also found to be stable to one molar KOH for 20 hours at

8~c. Photochemistry

of methylmercurials.

Takehara at el. 11 reported

chloride

(i0-s M) in water was 76% decomposed

mercuric

iodide was reported to undergo photochemical

lengths greater than 290 nm. le of methylmercurials

after exposure

To help evaluate

in the aquatic environment,

exposure

no photochemical

to 17.1 hours of mid-day

extensively

degraded

~

the significance the ultraviolet

Methyl-

to light of wave-

of photochemical

degradation

(uv) spectra of CHsHgl ,

solution were recorded.

As predicted

reaction was observed for CHsHgCI , CHsHgOH , and CHsHg + after

sunlight.

9~/o) after 3.7 hours.

However,

CHsHgBr was 10% degraded and CHsHgl was

In agreement with vapor phase photolytic

reported by other investigators Is DI~ was not decomposed The decomposition

for two hours to sunlight.

reaction on exposure

CHsHgBr , CHsHgCI , CHsHgOH , and CHsHg + (10-4 M) in aqueous by the uv spectra,

that methylmercuric

in water on direct photolysis

results (> 290 nm).

of neither DMM nor CI~Hg + was sensitized by acetone or singlet oxygen.

No. 4

151

Evaporative loss of dimethylmercury.

Since DMM has a high vapor pressure (50 mm Hg at

2 0 . ~ C ) 14 and does not dissociate in solution, evaporative loss from the aquatic environment must be considered.

Using the method of Tsivoglou Is with the relationship between transfer

constants and molecular dianeters

(KI/Ke = de/dl) , the ratio of transfer coefficients for

oxygen and DMM ~6 is calculated to be K~/Kz = 2.4.

Using Tsivoglou's experimental reaeration

data 15 for a moderately turbulent section of the Jackson River (dump 14), D M M w o u l d calculated half-life of 12 hours in this reach of the river.

have a

These calculations also predict

that elemental mercury would be lost from solution at a rate 2.3 times faster than DMM. Conclusions The above studies show that DI~ and methylmercuric ion react (C-Hg bond cleavage) at a negligible rate with hydrogen ion, molecular oxygen, or hydroxide ion under environmental conditions. degradation.

They also show that DMM and methylmercuric ion do not undergo photochemical The reaction of D M M w i t h

Hg II species, however, could be important.

Conversely, the evaporative process may be of major significance in the loss of D~4 from aquatic ecosystems and will be enhanced by higher levels of turbulence. Disclaimer Mention of commercial products does not necessarily constitute endorsement by the Environmental Protection Agency. References i L. Friberg et al., Nord. Hy~. Tidskr., 52, Suppl. 4, Chapter 4 (1971). 2

F. M. D'Itri, Technical Report No. 12 to the Michigan (USA) House of Representatives, Chapter III (1971).

3 (a) S. Jensen and A. Jernel~v, Nature, 223, 753 (1969); (b) J. M. Wood, C. G. Rosen, and S. F. Kennedy, Nature, 220, 173 (1968);

(c) T. Fagerstr~m and A. JernelBv, Water Research,

5, 121 (1971).

A. JernelBv, "Chemical Fallout," M. Miller and G. Berg, Eds., Charles C. Thomas, Publisher, Springfield, Illinois, 1969, Chapter 4.

s F. R. Jensen and B. Rickborn, "Electrophilic Substitution of Organomercurials," McGraw-Hill, New York, N. Y., 1968.

No. 4

152

6 (a) S. Hietanen and L. D. Sill~n, Acta Chem. Scand., 6, 747 (1952); Walton, 7

(b) T. D. Waugh, H. F.

and J. A. Laswick, J. Phys. Chem., 59, 395 (1955).

G. Gran, Svensk Paperstidn., 53, 234 (1950).

s (a) S. I. Shibko and N. Nelson, Enviromnental Research, 4, 23 (1971);

(b) P. A. Krenkel,

R. S. Reimers, E. B. Shin, and W. D. Burrows, "Mechanisms

of Mercury Transformations

Bottom Sediments," Technical Report No. 27, Environmental

and Water Resources Engineering,

Vanderbilt University,

Nashville,

Tennessee,

in

1971.

9 A more detailed report on this reaction is being prepared for publication.

Io T. G. Brilkina and V. A. Shushunov, "Reactions of Organometallic Peroxides," Chemical Rubber Co., Cleveland, ii

H. Takehara, M. Kotakemori,

12 K. C° Bass, Or~anometal.

Compounds with Oxygen and

Ohio, 1969.

and T. Kajimura, Nippon No~ei Ka~aku Kaishi, 39, 448 (1965).

Chem. Rev., i, 391 (1966).

~3 (a) R. Gomer and W. A. Noyes, Jr., J. Amer. Chem. Soe., 71, 3390 (1949); and H. W. ThompSon,

Trans. Faraday Soc., 33, 501 (1937);

(b) J. W. Linnett

(c) A. Terenin, J. Chem. Phys., 2,

441 (1934).

14 H. W. Thompson et al., Trans. Faraday Soc., 32, 681-5 (1936).

~s E. C. Tsivoglou,

"Tracer Measurement of Stream Reaeration," USDI, Federal Water Pollution

Control Administration,

Washington,

D. C., 1967.

16 The molecular diameter of DI~ was calculated from the Vanderwaals radii.