Photochemical formation of methylmercuric compounds from mercuric acetate

Chemosphere No. 3, PP 131 - 133, 1973.

Pergamon Press.

Printed in Great Britain.

PHOTOCHEMICAL FORMATION OF METHYLMERCURIC COMPOUNDS FROM MERCURIC ACETATE

Hirokatsu Akagi and Eigo Takabatake The Institute of Public Health, Minato-ku, Tokyo, Japan (Received in Japan 3 May 1973; ~eceived in UK for publication 14 May 1973)

In the course of studies on photochemical alkylation of inorganic mercuric compounds in aquatic environment, the authorsI have observed that irradiation with sunlight converted mercuric chloride into methylmercuric chloride in the presence of a small amount of acetic acid, methanol or ethanol, and into both methyl- and ethylmercuric chlorides in the presence of propionic acid. Among these alcohols and fatty acids, acetic acid which occurs widely in nature as a degradation product of various organic materials, proved to be the most effective methyl donor for the formation of methylmercuric compounds. This communication describes a study of the photochemical methylation using mercuric acetate, a hypothetical intermediate of the reaction, under aqueous conditions. A 20 W blacklight lamp having the same spectral distribution as sunlight was used to maintain a stable light intensity through the experiments. A solution of 1 mmole of mercuric acetate in 100 mlof water in a quartz container was irradiated by the lamp placed 15 cm distant while being stirred slowly by a magnetic stirrer at 20° . After various intervals, samples were acidified with hydrochloric acid and methylmercuric chloride formed was extracted with benzene from the reaction mixture and determined by ECD gas chromatography as described previously.I No methylmercuric compound was produced in a non-lrradiated control. As shown in Fig. 1, the amount of methylmercuric compound in the reaction mixture increased rapidly reaching 124.47 i~moles after 96 hours. This is compared with the value of 0.06 pmoles obtained by irradiation of a solution of mercuric chloride and acetic acid in water.

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120

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168

Irradiation time (hour) Fig. 1. Photochemical Formation of Methylmercuric Compound from Mercuric Acetate in Water

The aqueous solution of mercuric acetate decomposed on standing to produce a yellow precipitate which redissolved on the addition of a small amount of acetic acid. However, after the addition of 3 or 5 tamales of acetic acid to the reaction mixture, the yield of methylmercuric compound was reduced to about one-thousandth. To investigate the influence of the yellow precipitate on the yield of methylmercuric compound, several experiments using different molar ratios of mercuric acetate or mercuric oxide (yellow powder) and acetic acid were carried out under the same conditions. The amounts of methylmercuric compound formed after 48 hours irradiation are listed in Table I. Of these reaction mixtures, (A) and (B) were heterogeneous, containing yellow precipitate, and the others were clear solutions. The irradiation of 1 mmole of mercuric oxide in the presence of 2 mmoles of acetic acid produced the same amount of methylmercuric compound as obtained by the irradiation of mercuric acetate without added acetic acid. The yields of methylmercuric compound in the heterogeneous systems were about 1000 times greater than those in the clear solutions. The results show that mercurlc acetate can be replaced by mercuric oxide, and that the presence of yellow precipitate in the reaction mixture enhances the yield of methylmercuric

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compound. Further, the data suggest that mercuric acetate is hydrolyzed to mercuric oxide and acetic acid in water and that the mercuric oxide produced stimulates the light-induced methylation of inorganic mercury. Table I. Effect of Acetic Acid on the Formation of Methylmercuric Compound from Inorganic Mercury

Reaction mixture

Inorganic mercury mmole Hg(OAc) 2

(A)

(C)

1 1

(D) (E) (F)

HgO

1

(B)

Acetic acid mmole

1 I I

Methylmercuric compound found I~m°le

0

50.96

2

52.73

3

0.09

5

0.06

5

0.07

7

0.06

Reaction mixture, 100 ml, was irradiated by 20 W blacklight lamp for 48 hours. When hydrochloric acid or sodium chloride was added to the irradiated reaction mixture, a white precipitate was produced in proportion to the amount of methylmercurlc chloride isolated. This precipitate turned black when added to dilute ammonium hydroxide. These results indicate that mercuric compound is reduced to mercurous compound along with the formation of methylmercuric compound. Janzen et al .2 demonstrated the production of both methyl and acetoxy radicals in the photolysis of mercuric acetate in benzene solution. The similar free radical methanism might also proceed under the aqueous conditions mentioned above, but further works are required to elucidate the precise mechanismof this reaction. It is likely that sunlight irradiation of mercuric acetate under aqueous conditions results in the formation of methylmercuric compounds in the same way as in these experiments, which would explain the production in nature of methylated product from mercuric compounds under conditions where mercuric acetate is released.

References 1) H. Akagi and Y. Sakagami, J. Hyg. Chem. (Japan), 18(6), 358 (1972) 2)

E . G . Janzen and B. J. Blackburn, J. Amer. Chem. Soc., 91,4481 (1969)