Among the heavy metals mercury (Hg) is one of the most toxic element which have effects on environment and also on the health of human.
In the modern era of innovation and civilization, pollutants and contaminates increase in the environment as a by-product of development. In these pollutants heavy metals have severe toxic effects on natural environment and also on the health of human.
A brief history of mercury is that it is present in the environment from 1500 BC in the tombs of Egypt, Hindus and ancient Chinese. The people gave name after the god of Roman “Mercury” that means mobility and speed. The physical characters of mercury are silvery white and present in liquid form at temperature and pressure.
It is found in the environment in three forms such as elemental mercury, organic (Methyl mercury) and inorganic compounds.
In the field of medical, mercury is used in thermometers, manometers, barometers and also use in dental amalgam. The Hg is also used in the field of science and as well as in the tube lights.
Naturally Hg is present in the Earth crust about 7×10-2 mg kg-1, in the oceans is 3×10-5 mg L-1 and in the air is 2-10 ng m-3. The mean residential time of Hg is 0.8-2 years in the atmosphere and 20-30 years in ocean water. Methyl mercury is present in the oceans 80 t year-1 that is transported from all the other sources, 240 t year-1 is removed from the ocean by photo chemical reaction and 40 t year-1 is taken up marine food web. The anthropogenic activity increases the Hg level in the environment by industrial process, phosphate, steel, metal smelting, gold extraction processes and the big source of Hg is the coal fired power plant (In 1999 40% of Hg emit in USA).
The Hg is also generating while the production of electronic and weather devices, incineration of municipal solid waste. In Japan, one the bad industrial disaster in the history was dumping Hg compounds in Minamata Bay. So, it is estimated that annual global Hg is emitted by the both natural and anthropogenic activity ranging from 5,000-8,000 metric tanns years-1. Increasing temperature can boost the Hg level in the fishes up to seven times the present rates, found by Swedish researchers.
Any scientist cannot know the exact mechanism of Hg enters into the food chain and in ecosystems it may be vary. At low concentration Hg is very toxic because of it high bio availability and toxicity. Microbes like bacteria take Hg in inorganic form and they convert it into methyl mercury by metabolism.
The bacteria that contain methyl mercury consumed by the next level of food chain. In plants, Hg accumulated in the forms of Hg2+, Hg and organic Hg. Aquatic plants also accumulate more organic form oh Hg (methyl mercury) than land plants. It is transported to many regions from its source globally because of it mobility and also a trans boundary pollutant.
Due to this Hg contaminate the areas of having no sources of Hg or zero Hg level. By inhalation inorganic Hg daily intake is 0.2 µg and if the level of Hg in drinking water is 0.5 µg L-1, it is considered that average daily intake is 1 µg and the maximum contamination level is 2 µg L-1. In the food of human Hg ranges from 2-20 µg. By ingestion the high amount of Hg can cause symptoms of cardiovascular collapse, dysphagia, abdominal pain, vomiting, shock, severe gastrointestinal damage, bloody diarrhea, loosening of the teeth, nephritis and acute renal failure. Primary results of acute poisoning are hemorrhagic colitis and gastritis and ultimately damage the kidneys.
In Minamata Bay, Japan, 3000 people suffered different severe Hg poisoning symptoms and death from a disease known as Minamata disease.
I am working in the Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad