Occurrence of Dimethylmercury ((CH₃)₂Hg) in Organic Solvents

Liang, L., Wilhelm, S. M., Pang, P., "Occurrence of Dimethylmercury ((CH₃)₂Hg) in Organic Solvents", Materials and Geoenvironment, Vol. 51 (2), 1968-1971 (2004).

ABSTRACT

A highly volatile and toxic mercury species, (CH₃)₂Hg, was found in a newly opened 4 L methanol reagent bottle. The compound was identified using three technically independent methods, independent standards, and quantified to be 75 µg/L (as Hg in (CH₃)₂Hg). Some commonly used organic solvents such as toluene, methylene dichloride, isopropanol, and many crude oil samples have been found to contain Hg fractions with behavior similar to the volatilization of (CH₃)₂Hg. This finding raises the questions: "Is the occurrence of (CH₃)₂Hg rare or common?" and "In addition to man-made, is (CH₃)₂Hg naturally occurring in natural gases and/or crude oil?"

Note from author S. Mark Wilhelm:

The identification of dimethylmercury (DMHg) as a contaminant in methanol is certainly an interesting observation. It is very unlikely that all methanol contains DMHg. In fact, checks of other methanol sources at Cebam did not reveal measurable quantities. Presently we are pursuing the manufacturer to reveal the source of the particular bottle of methanol that was contaminated but without success so far. Finding 75+ ppb of DMHg in methanol does beg the question as to how it came to be there. My guess is that it came from a contaminated feed to a reformer, most likely mercury in naphtha or LPG, but possibly methane as well.

Methanol is made a variety of ways but the vast majority of commercial methanol is made from synthesis gas. Syngas is produced by steam reforming of methane, LPG or naphtha to produce a mixture H₂, CO, CO₂, and water. In steam reforming of natural gas, methane and steam are combined in a reactor with a catalyst (Ni) at a temperature between 700 and 1100 C and at 10 to 50 bar pressure. Methanol is made from purified syngas in tubular reactors packed with catalyst (many commercial types but Cu/ZnO on alumina typically). The overall reaction is CO + 2H₂ ↔ CH₃OH. Methanol synthesis reactors operate at temperatures between 250 and 350 C and at 30-100 bar.

If the mercury in methanol came from the the syngas, then it existed in the feed to the reformer as either elemental mercury or as DMHg. The catalytic dissociation of hydrocarbon to make syngas would dissociate any DMHg that might be present in the feed. Whatever mercury that exits the syngas reactor would certainly be in elemental form given the reducing atmosphere (hydrogen) and very elevated temperature. It seems much more likely that the methanol synthesis reaction methylates elemental mercury rather than for DMHg to survive the process of methane reformation to syngas and syngas reaction to methanol. All of this assumes, of course, that the methanol in question came from a conventional petrochemical process and was not contaminated in purification steps.

Naphtha and LPG containing trace amounts of mercury are fairly common. Investigations of mercury in petrochemical manufacture have identified mercury in feeds to crackers in several instances. In Asia, mercury in chemical grade naphtha has a specification limit of 1 ppb because of its deleterious effect on hydrogenation catalysts and cryogenic equipment. Mercury in crude oil distills preferentially to LPG and naphtha cuts. While it is believed that most mercury in LPG and naphtha is elemental, it is by no means certain that this is the case. In fact, in the rare circumstance that mercury speciation of naphtha has been attempted (Tao, H.; Murakami, T.; Tominaga, M.; Miyazaki, A., J. Anal. At. Spectrom., 1998, 13, 1085H) dialkylmercury is the dominant species (no Hg⁰ found!) detected in two "condensate naphtha" samples tested. The origins of the samples in Tao's study are not revealed, unfortunately.



Order the entire paper