The role of hydrogen sulphide in environmental transport of mercury

Abstract

HYDROGEN SULPHIDE may be evolved in anoxic sulphur-containing river or lake sediments1. The effect of the presence of sulphide on the availability of inorganic mercury for biological methylation has been considered previously. Essentially mercury in the form of Hg(0) or Hg(II) may be converted into mercuric sulphide in anoxic sediments2. It might seem that this very insoluble material would not, in practice, be available for methylation and that sulphide formation would act as a method of removing mercury from availability to the general environment (that is, it would act as a mercury sink). However in a complex natural system (aquarium sediment) it has been shown that HgS may, in fact, be methylated in the sediments although to produce quantities of methyl mercury substantially less than those produced using a more soluble mercury substrate2. Therefore, although H2S in a sediment would be expected to reduce the rate of mercury methylation considerably by the formation of HgS, it would not be expected to prevent formation of methyl mercury entirely. The effect of H2S on methyl mercury itself has also been investigated using a purely inorganic model3. An aqueous solution of methyl mercuric chloride (1 mg dm−3) loses methyl mercury steadily over 3 days after initial exposure to H2S. A volatile product was evolved into the space above the solution. This product was trapped in a sodium carbonate–disodium phosphate solution, extracted with toluene and shown to co-migrate with methyl mercury on thin layer plates of silica gel. It was concluded that the evolved product of the reaction system was a volatile organo-sulphur derivative of mercury that reverted to methyl mercury on extraction. These observations suggest that reaction between methyl mercury and H2S generated in sediments reduces the amount of methyl mercury available for accumulation by fish and shellfish and represents a powerful mechanism adding to the general mobility of mercury compounds in the environment. It is therefore important to identify the unknown mercury compound evolved from the system described above. We have now investigated the interaction of H2S with River Mersey (England) inter-tidal estuary sediment samples in order to determine its effect on methyl mercury levels in real systems. We have also investigated similar reactions in a model system, leading to the identification of the previously unknown volatile mercury species responsible for the transport of mercury in the work of Rowland et al.3