Industry Environment
II: zinc, mercury and cadmium Mercury has a long history as an environmental pollutant. It first came to attention in the 1950’s with the identification of clusters of neurological cases diagnosed around Minamata in Japan, which became known as Minamata disease, although the toxic nature of mercury had long been appreciated. Minamata disease was found to be caused by the consumption of organic mercury compounds in seafood.
It was ultimately determined that the source of the pollution was the Chisso chemical factory, which released untreated wastewater directly into the Shiranui Sea. Methyl mercury was produced by Chisso as a by-reaction from the production of acetaldehyde using mercury sulphate as a catalyst. Organic mercury compounds are considerably more toxic than inorganic forms and rapidly accumulated in shellfish, which became biomagnified further up the food chain as organic mercury compounds are retained in tissue. At least 1,700 people died as a result of this incident.
The main anthropogenic inputs of mercury into the environment are from coal-burning power stations and the use of mercury in artisan gold mining. Elemental mercury is mixed with crushed ore, often by hand as shown in Figure 4. Mercury dissolves any gold present in the ore to form an amalgam, which can be recovered by evaporating off the mercury, usually with poor recovery rates. UNIDO estimates that a thousand of tonnes of mercury enter the environment each year from artisan gold mining worldwide[4].
Cadmium is a toxic metal that can be present in high concentrations in soil contaminated by mining, smelting and metal finishing. As a minor component of zinc ores, cadmium can substitute for zinc in the mineral sphalerite (ZnS) and was often discarded in spoil heaps around mines. The village of Shipham in Somerset, England, has elevated levels of cadmium in soil as a result of historical lead and zinc mining dating back to medieval times.
Although the average intake of cadmium is relatively high, epidemiological studies have found no evidence of health risks to the population, probably because locally grown produce represents a small percentage of resident’s diets [5].
Itai-itai disease is caused by cadmium poisoning, often from the consumption of crops grown in soil contaminated with cadmium, which causes severe joint pain and softening of bones. This was first diagnosed in Toyama prefecture, Japan, and linked to cadmium discharges into the Jinzu river from mining upstream. Irrigation of paddy fields with cadmium-rich river
Figure
2.Geochemical map of central England and Northern Ireland showing cadmium levels in soil. (British Geological Survey)
water exacerbated cadmium intake in diets composed mainly of rice and fish. Toxicologists believe that cadmium interferes with proteins that contain zinc due to the similarity in size between the zinc and cadmium ions. Cadmium remains in the body for a long time because there is little metabolism and it is only slowly excreted.
Zinc is essential as a trace element for biological functions. Nevertheless, the US EPA has set a secondary maximum contaminant level of 50 ppm in drinking water. Inhalation of zinc oxide fumes by metal workers can cause a temporary condition known as ‘zinc chills’ which produces uncontrollable shaking in an individual; a favourite trick played on apprentice plumbers. Zinc is not thought to have any long-term health effects; indeed zinc may limit the uptake of cadmium from soil and vegetables and provide some protection. Disturbingly, both the US and UK
Figure
3.Acid mine drainage at Rio Tinto, Spain,showing characteristic ‘yellow boy’ deposits of iron hydroxides. (Photo - Carol Stoker,NASA)
January/February 2012
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