Arsenic is used as an additive in various metal alloys and wood preservation. Its toxic properties are exploited in the formulation of arsenical herbicides and insecticides. Geogenic Arsenic is largely responsible for most human poisoning. Due to this environmental health impact, As toxicity has been researched and documented more extensively than any other metalloids.
TOXICITY TO PLANTS AND ANIMALS
Arsenic contamination of soil and water poses a serious threat to plants and animals. Plants and microorganisms are known to accumulate As in their tissues and exhibit a certain degree of tolerance. However, at high concentrations, As is toxic to nearly all forms of life.
Biomass production and yields of a variety of crops have been shown to reduce significantly at high concentrations of As in soils.
Table 2; Showing the risks of arsenic contamination in terrestrial environments (Courtesy Ongley, L. K., Armienta, A., and Mango, H. (2003).
|Soil||36050–10070–10||Yield reduction in barley; plants showed symptoms of As toxicity and P deficiency Reduction in growth of vegetative and root system in tomatoes|
|Soil||0, 15, 20, 30, 50, and 100 as power station fly ash or disodium hydrogen arsenate||50% yield reduction in wheat, barley, and oats.Sensitivity to As was in the order: oats >wheat > barley|
|Soil||100||Decreased the height of the apple tree: 100%growth inhibition at above 100 mg kg_1|
|Seedling beds||1000 and 2000||Substantial growth reduction in white spruce seedlings|
|Soil||0–280 kg As ha-1(fine sandy loam soil)0–560 kg As ha-1 (clay soil).||Significant growth reduction in cotton andSoya bean|
|Soil||NaAsO2 applied at ratesup to 720 kg As ha-1||As toxicity persisted for four growing seasonsin potatoes and peas|
|Water &nutrientsolutions||0.01, 0.1, or 1.0 mM PbCl2 or Na2HAsO4 in 1% agar þ modified Arnon and Hoagland solution.||Growth inhibition of pea seedlings at all concentrations. As resulted in more growth inhibition than Pb|
|Soilless culture||1.0–5.5||No phytotoxic effect on radish|
|Soilless culture||1.0–5.0||Organic arsenicals (MAA > DMA) morephytotoxic than inorganic As to turnip,accumulating above the threshold for
As in food crops (1.0 mg kg-1)
|Nutrient solution||0–10||Significant yield reduction in tomato (notissue chlorosis or necrosis was observed)|
|Growth medium||0.5–50 _M As||Growth inhibition of mung beanabove 2.2 _g g_1 of As in the dry mass|
|Water culture||0, 0.04, 0.4, 4.0, and 20||Increasing As decreased plant dry weight incabbage. Most As remained in theroots with only 10–25% transported to
the tops, _2% entered the inner leaves
|Green algae inculture medium||78.7 _g liter_1 As(III)159.3 _g liter_1 As(V)12.4 (MMA)
|Raising phosphate concentration in themedium increased As(V) toxicity tofreshwater green alga Scenedesmus obliguus|
|Earthworms||>400 Up to 8000PDA (phenyldichloroarsine),As(III) and As(V) at varied concentrations||Caused total fatality to earthwormsTolerated by Lumbricus rubellus and Dendrodrilus rubidus toleratedToxicity follows: PDA > As(III) >
As(V) and 24 h LD50 values 189.5, 191.0,
and 519.4 _mol kg_1, respectively
RISKS TO ANIMALS AND HUMANS
Drinking water is the most important source of dietary intake of As by animals and humans. However, food also forms a source of As exposure. The occurrence of inorganic As in drinking water has been identified as a source of risk for human health even at relatively low concentrations. As a consequence, more stringent safer limits for As in drinking water have been proposed.
The acute toxicity of As compounds in humans is a function of their rate of removal from the body. Arsine is considered to be the most toxic form, followed by As(III), As(V) and organic As compounds (MMA and DMA). Lethal doses in humans range from 1.5 mg kg-1(diarsenic trioxide) to 500 mg kg -1of body weight (DMA). Acute As intoxication associated with the ingestion of contaminated well water has been reported in many countries.