Arsenic is the twentieth most abundant component in the earths crust and 12th most abundant component in the human organic structure. It has been used as a homicide since the in-between Ages. However, As is besides known as a curative agent every bit early as 400 BC. For illustration, Fowler solution ( arsenite solution incorporating 7.6 * 106 ?g As/l ) has been used since the nineteenth century for the intervention of leukaemia, psoriasis, chronic bronchial asthma and besides as a quinine water. The day-to-day dose was frequently every bit high as 3000 ?g. As has besides extensively been used as pesticides, weedkillers, wood preservatives, industry of dyestuffs, chemical warfare gases, glass industry, electronics, and growing advancing agent ( 3 ) . The find of inauspicious wellness effects due to As exposure led to reduced use of As. For illustration, the usage of As salts in the agribusiness went down drastically since 1970 ‘s with 70 % of the world-wide production As trioxide applied in the agribusiness in 1970 to merely 45 % in 1980 ( 4 ) . The use of As in the glass fabrication industry is besides reduced ( 5 ) . The current utilizations of As compounds are as clarifier in the glass industry, as wood preservative, in semiconducting materials, as a drying agent and defoliant in agribusiness ( 6, 7 ) . One of the present uses of As as curative agent is in Cancer intervention. Recently in 2000 randomised clinical tests in the US led the Food and Drug Administration ( FDA ) to O.K. As trioxide for intervention against leukaemia ( 8 ) .
Arsenic Chemistry of relevancy
As in the environment can happen in several oxidization provinces ( -3, 0, +3, +5 ) and as both organic and inorganic As. However, in natural Waterss inorganic As is prevailing and is found as oxyanions of trivalent arsenite ( H3As ( III ) O3 ) or pentavalent arsenate ( e.g. ( H2As ( V ) O4 – ) . In surface Waterss organic As compounds may be produced by biological activity, but are seldom of import quantitatively. Figure 1 shows constructions of some of the As compounds referred in the text. Redox possible and pH are the most of import commanding factors in As speciation. As ( V ) is more prevailing under oxidising conditions and As ( III ) is more prevailing under cut downing conditions. The rate of oxidization of As ( III ) to As ( V ) or decrease of As ( V ) to As ( III ) depends on many factors like pH, Eh, the presence of microorganisms, the presence of other oxidising agents like manganese, Fe, etc. , exposure to visible radiation and temperature ( 9,10,11,12,13,14 ) . By and large the rate of oxidization of As ( III ) to As ( V ) is really low ( 15 ) in natural Waterss, but can continue measurably in several yearss in strongly alkalic or acerb solutions. The speciation of As is of import in commanding the mobility, bioavailability and toxicity. As ( V ) with pK1 and pK2 values of 2.2 and 7.1 is present in the anionic signifier, whereas As ( III ) with pK1 value of 9.2 is present in impersonal signifier in natural Waterss. The mobility of As is chiefly controlled by the presence of metal ( hydr ) oxides and since at the pH of natural H2O As ( III ) is present as a impersonal species and As ( V ) is present in anionic signifier the sorption of As ( V ) is strongly favored. Therefore As ( III ) is more nomadic compared to As ( V ) in impersonal to somewhat acidic pH scope. The presence of other viing ions besides have an consequence on the mobility of As ( 16, 17 ) . For illustration high concentrations of P can desorb both As ( V ) and As ( III ) . Similarly in the absence of other ions and copiousness of available surface sites sorption of As ( III ) can besides take topographic point ( 18, 19 ) . Under highly cut downing conditions and acidic pH with high concentration of decreased S precipitation of orpiment 4 ( As2S3 ) , realgar ( AsS ) , FeAsS or other sulphide minerals is favorite taking to take down As concentrations ( 14, 20 ) .
Arsenic is readily absorbed when ingested in dissolved signifier ( 80-90 % ) or inhaled ( 30-85 % ) . As ( V ) , whether organic or inorganic, is better absorbed from the GI piece of land compared to As ( III ) . In contrast to inorganic arsenic, neither mono- methyl arsenic nor di-methyl arsenic binds strongly to biological molecules in worlds ( 22 ) .
Figures: Structures of some of the As compounds referred in the text ( after ( 89 )
Health effects and hazard Appraisal
As toxicity depends on many factors like species, sum of exposure, continuance of exposure, nutritionary position, methylation capacity, familial conditions, bioavailability, Se consumption and presence of co-carcinogenic factors like exposure to sunlight, coffin nail smoke ( 24,23,25,26,27,28,29,49 ) . As methylation is considered to be a detoxification way, because the pentavalent organic As metabolites ( DMA and MMA ) were the most common signifiers found in urine samples of open worlds. These signifiers are readily excreted and less toxic than inorganic As. However, some recent surveies identified trivalent metabolites of organic As, DMA ( As ( III ) ) and MMA ( As ( III ) ) , in urine samples of homo exposed to As ( sited in ( 30 ) ) . Laboratory surveies have shown that these organic As ( III ) metabolites are more toxic than inorganic As ( III ) ( 31,32,33 ) . Some of the early clinical symptoms of acute effects include abdominal hurting, emesis, diarrhea, muscular hurting, and failing ( 34 ) . Organic As is considered to be less toxic than inorganic As and the toxicity decreases in the order of AsH3 & A ; gt ; As ( III ) & A ; gt ; As ( V ) & A ; gt ; organic arsenic compounds. The deadly dose of As scope from 1.5 mg/kg organic structure weight for As2O3 to 500 mg/kg of organic structure weight for DMA ( 1 ) . Chronic exposure to low concentrations of As can do cuticular alterations like tegument pigmentation, hyperkeratosis and ulceration. Other chronic non cuticular effects of As are diabetes ( 35 ) , effects on cardiovascular system ( 36 ) , high blood pressure ( 37 ) , respiratory effects ( 38 ) , inauspicious gestation results etc ( 22 ) . Chronic exposure to As is besides documented to be carcinogenic in worlds ( 1 ) at high exposures. However, treatment is still traveling on about the threshold bound for carcinogenic action of As, since both US and non-US-based surveies showed merely positive correlativity between exposure to As and happening of malignant neoplastic disease at As concentrations above 100 ?g/l ( 39,40,41,42,23 ) Exposure to As can happen through nutrient, H2O and air and tabular array 1 shows general exposure degree through these assorted paths ( 51 ) . Harmonizing to the values in table 1, the major path of exposure to entire As is through nutrient. However, the exposure through nutrient did non have much attending, since more than 90 % of As in nutrient is in the signifier of organic As. Water is the 2nd major path of exposure to entire As, and As is chiefly present every bit inorganic As in natural Waterss. However, in countries with elevated As concentrations exposure of As through H2O becomes the major path. The first survey related to exposure to As through imbibing H2O and prevalence of tegument malignant neoplastic disease was reported from Taiwan ( 52 ) . Based on this survey WHO lowered the maximal allowable degree from 50 ?g/l to 10 ?g/l ( 1 ) . Table 2 depicts the recognition of As in imbibing H2O as a cause for inauspicious wellness effects in the signifier of lowered guideline values through old ages ( 53 ) . In ulterior old ages more surveies besides reported prevalence of internal malignant neoplastic diseases like vesica, liver, kidney and lung due to As exposure through imbibing H2O ( 49,42,21,50,44,45 ) . The latency period for development of tegument and internal malignant neoplastic diseases are reported to be 6- & A ; gt ; 40 old ages and depends on many factors like exposure degree, continuance of exposure, smoking wonts, nutritionary position, gender, sunshine and familial conditions ( 46, 1, 47 ) . The estimated life clip extra hazard of acquiring skin malignant neoplastic disease in the US population at 50 ?g/l is 3/1000 ( 22 ) and for the amount of internal malignant neoplastic diseases it is 1/100 ( 48 ) . This hazard estimation would be even higher for general population in Bengal delta with a organic structure weight of 50 kilograms, devouring 4-6 liters of H2O and alimentary deficient nutrient. For more information see ( IV ) .