Arsenic and the Water of Death
Pabitra Mukhopadhyay has written an excellent article focusing attention on a serious environmental problem, that of arsenic in drinking water. Some areas of the U.S. have high arsenic levels in groundwater, but they are in less populated regions that have been able to find other sources for drinking water. However, many of the tube well drilled in Bangladesh and West Bengal, the adjoining Indian state, bring water to the surface that is highly contaminated with arsenic. The problem is serious as it was not discovered for a long time, and many people have been sickened or died from arsenic poisoning. Many more have been exposed to levels that will cause further health problems. Exposure at even low levels may cause cancer later as arsenic is a potent carcinogen. While Pabitra’s article dealt with the arsenic situation in general, this is an account of the physical and chemical processes related to arsenic contamination in ground water.
Arsenic is a reactive metalloid that is rarely found uncombined in nature. There are at least 50 minerals that have arsenic as a component. Most of these occur in areas that have had volcanic activity in the past. Weathering breaks down the minerals and streams pick up the arsenic compounds and carry them along. Evaporation, particularly in desert areas, will concentrate the arsenic compounds and the arsenic concentration may become very high in pools and shallow lakes. In many areas, the minerals eventually becomes covered with sediments so that they do not affect surface waters. However, water in aquifers still may come into contact with the minerals and leach out the arsenic compounds. Wells drilled into those aquifers will bring arsenic to the surface and the arsenic levels in the water may be too high for safe drinking water.
Human activities such as mining brings arsenic minerals to the surface, and leaching from mine tailings contaminates surface waters and shallow aquifers. Mobilization of arsenic in the sedimentary aquifers has been attributed to changes in the geochemical environment due to agricultural irrigation. In deeper wells, elevated arsenic concentrations are associated with compaction caused by groundwater withdrawals. A more recent concern is that our increasing use of coal may lead to increased levels of arsenic in groundwater. Coal contains an average amount of about 14.5 parts per million (ppm). That is a trace amount but since we burn about 4.5 billion tons of coal each year, coal mining brings about 67,000 tons of arsenic to the surface each year. When coal is burned, the arsenic ends up mostly in the coal ash, which is then disposed of in landfills, cements, and even by agricultural use. Once in the environment, some of the arsenic ultimately finds its way into the groundwater.
Arsenic in water is in the form of either As(V) compounds, called arsenates, or As(III) compounds, called arsenites. As(III) species are more toxic than As(V) species. However, one form may be converted into the other by chemical reactions depending on the environment. Oxidation converts As(III) to As(V) and reduction reactions convert As(V) to As(III). Water tests do not discriminate between the forms of arsenic and water standards usually have a standard for the total arsenic level of no more than 10 parts per billion, though some areas have even stricter standards. For comparison, the standard for cyanide in drinking water is around 200 ppb, so arsenic is about 20 times more toxic than cyanide. Also, arsenic is bioaccumulated and a carcinogen and the chances of poisoning or cancer goes up as the total exposure over time.
Toxicity: Arsenates are chemically similar to phosphates, and their toxic effect is expressed by interfering with reactions involving phosphates. Arsenate toxicity occurs when it replaces phosphate in the oxidative phosphorylation processes. This leads to mitochondrial impairment and inhibition of glycolytic energy metabolism, which causes cell damage and muscular weakness. Many biological systems carry out reducing reactions that convert As(V) to the more toxic As(III). As(III) is more reactive and forms strong bonds with functional groups such as the thiolates of cysteine and the imidazolium nitrogens of histidines. Arsenic toxicity depends on its chemical form and some plants and animals accumulate arsenic as relatively nontoxic organoarsenic molecules. Plants grown using water high in arsenic may not themselves be toxic.
Arsenate is predominant in water containing high levels of dissolved oxygen, while As(III) species occur under more reducing conditions such as found in deeper wells and anaerobic environments. The distribution of arsenite-oxidizing bacteria in upper layers and arsenate-reducing bacteria in lower depths of the sediments impact the type of arsenic released into nearby tubewell groundwater.
Removal: Metal arsenites are much more soluble than the corresponding metal arsenates. Arsenates are more likely to be removed from water by being adsorbed by solid phases, such as sediments and soils. As(V) compounds exist in water in ionized forms that may be removed by precipitation with many metal ion, principally iron ions. In the pH ranges found in drinking water, As(III) exist as H3AsO3 which does not ionize enough to combine with metal ions, so As(III) is hard to remove by normal precipitation methods.
To overcome this problem, a very innovative and cost effective arsenic removal technology has been devised at Jadavpur University by Dr. Bhaskar Sengupta, and his colleagues. The method, subterranean arsenic removal (SAR), uses aerated groundwater that is recharged back into the aquifer to create an oxidation zone. The oxidation zone created by the aerated water boosts the activity of the arsenic-oxidizing microorganisms and oxidizes As(III) to As(V), which is then precipitated by the iron ions present in the water. No chemicals are used and the method has a very long operational life. This method shows great promise in making the water well contaminated with arsenic safer to drink.
(C) 2011 J.C. Moore
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