Organomercury soils

NMR has been applied to the determination of organomercury compounds in non-saline sediments and humic and fulvic acids in soil and saline sediments. 

Kimura and Miller [28] have also studied the decomposition of organic fungicides in soil to mercury vapour and to methyl- or ethylmercury compounds and devised methods for the determination of these compounds in the vapours liberated from the soil sample. The mixed vapours of mercury and organomercury compounds is passed successively through bubblers containing a carbonate-phosphate solution to absorb organic... 

Kimura and Miller [29] have described a procedure for the determination of organomercury (methylmercury, ethylmercury and phenylmercury compounds) and inorganic mercury in soil. In this method the sample is digested in a steam bath with sulphuric acid (0.9M) containing hydroxy ammonium sulphate, sodium chloride and, if high concentrations of organic matter are present, potassium dichromate solution. Then, 50% hydrogen... 

Langmyhr et al. [45] have applied cold vapour atomic absorption spectrometry to the determination of organomercury compounds in soils and sediments. 

Many human tragedies have been caused by organomercury compounds in food. In the twentieth century, mercury compounds, usually the most toxic types, were applied as fungicides to seeds. Large-scale poisoning occurred in Iraq in 1971-1972 as a result of an appalling error, when seeds contaminated with Hg compounds were sold for consumption. After that event, the application of Hg compounds in farming practice declined, but a fair amount of the element remains in the soil. 

The isolation of analytes by leaching, as is often done in soil analysis, is often dictated by the difficulties encountered when dissolving samples completely. However, also for speciation work, where the analytes are labile such procedures are often the only way to obtain the analyte solutions (see e.g. Ref. [687] for the isolation of organomercury compounds from fish samples). 

This element is a chalcophile, and in unweathered rocks is most commonly found as the mineral cinnabar (HgS). In soil environments, the cationic form, is most common, as the reduced oxidation state (+1) has a limited stability range. Reduction to the metallic elemental form, H, is easily achieved in soils by both biological and chemical reactions. Elemental mercury is somewhat volatile, and the vapor is extremely toxic to organisms. Under anaerobic conditions at least, soil microbes methylate mercury, forming volatile organomercury compounds that are bioavail-able and present a health hazard. At the same time, however, anaerobic conditions can convert Hg into the exceedingly insoluble sulhde, HgS. Some of the more important transformations possible for mercury in soil are summarized in Figure 9.9. 

The condition for uptake of the pesticide by the plant is given by its persistence or system characteristics. The most persistent pesticides are particularly organometallic (most of them organomercury) compounds or derivatives of chlorinated hydrocarbons. Depending on the concentration in the soil, they can be identified especially in root crops, carrot, radish, beet, potatoes and plants giving oil. The rate of uptake of these substances by plants are different even in the case of substances with a similar persistence [29, 30]. The uptake can also be different in the case of identical plants [31]. Solid pesticides penetrate more easily into agricultural products from sandy and clay soils [32]. 

Metbylmercury, in turn, is decomposed abi-otically and by bacteria containing mercuric reductase and organomercurial lyase these demethylating strains of bacteria are common in the environment and have been isolated from water, sediments, soils, and the G1 tract of humans and other mammals. Other mechanisms can reduce inorganic Hg + to Hg in freshwater, with Hg production higher under anoxic conditions, in a chloride-free environment, and at pH 4.5. Metbylmercury concentrations in tissues of marine fishes can now be detected at levels >10.0p,g/kg tissue using graphite furnace sample preparation techniques and atomic absorption spectrometry. 

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