Thursday, November 25, 2010

Presence of Arsenic in different geological environment.


In modern parlance, arsenic is viewed as being synonymous with “toxic”.
by
Dr. Nitish Priyadarshi


As old as recorded history, arsenic has existed through the centuries as a curative, a pigment, a cosmetic, on mirrors, part of alchemical lore, and most notoriously, the deadliest of poisons. In its various forms- as tasteless and odorless arsenic trioxide, the gold-bearing yellow arsenic sulfide, the deadly gas arsine, the practical alloy copper-arsenic, common pesticide white arsenic, and the electronics staple gallium arsenide-arsenic has served with fascinating ease human impulses both noble and wicked.

The long history of arsenic in science, medicine and technology has been over shadowed by its notoriety as a preferred poison in homicides. In modern parlance, arsenic is often viewed as being synonymous with “toxic”.

Widespread arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in India and Bangladesh and neighbouring countries. Presently 42 major incidents around the world have been reported on groundwater arsenic contamination. It is estimated that approximately 57 million people are drinking groundwater with arsenic concentrations elevated above the World Health Organization's standard of 10 parts per billion.

The notoriety and lingering concern about the potential effects of arsenic on various fauna and flora has inevitably engendered a lot of research on the many facets of this element in the environment. This paper represents an attempt to bring together the key research results from the different geological work.

Arsenic is considered to be a rare but ubiquitous element of the upper lithosphere. Arsenic is occasionally observed in the native state in nature. However it is more frequently found combined with sulfur, selenium, tellurium and also as sulfo salts and arsenides of various heavy metals such as copper, iron, nickel and cobalt. It also forms a number of pentavalent arsenate minerals that bear a close geochemical relationship with phosphates and vandates with which it can form some isomorphic compounds.

Arsenopyrite, which is the most abundant and widespread mineral of arsenic, is found in pegmatites but more frequently in high temperature gold-quartz veins, high temperature tin-veins, and in contact with metamorphic sulfide deposits.

The numerous oxidic minerals of arsenic observed in nature are a result of the oxidation of sulfide and arsenide deposits in contact with the free oxygen of the atmosphere. The arsenate mineral are the most preponderant of the oxide minerals. Some arsenate minerals have been observed in metamorphic rocks deep in the earth.

It is not surprising to find that there has been increased interest in coals, together with work on rocks, soils, plants and waste materials, probably because of possible adverse health effects of high concentrations. Arsenic with some other environmentally sensitive elements, coal tends to be seen as a major source of arsenic, whereas, where as it only contributes 1.8 % of the total emissions to the atmosphere, which is about the same as wood fuel. There is a wide range of values from less than 1 ppm to several hundred ppm, sometimes enrichment being related to nearly arsenic rich ores. Arsenic has similar chemical properties to phosphorus (P), the element immediately below arsenic on the periodic chart. During coal combustion, arsenic oxidizes and forms gaseous As2 O3 and enters the atmosphere. This volatilization causes concerns for governments of many countries because of environmental pollution due to extensive use of coal.

Although it has been stated categorically that arsenic is present in coal as arsenopyrite and that ‘little exists in any other form’, the only good evidence for the nature of the association of arsenic with pyrite has come from a detailed study of an eastern U.S. coal. It was found that the arsenic was most likely to be present in solid solution in the pyrite, and it was noted that the arsenic was predominantely in fractures in the coal and in microfractures in the pyrite. Arsenic was also detected at isolated points in some pyrite grains, perhaps because small arsenic and selenium bearing minerals inclusions formed and were incorporated into the pyrite at the time of crystallization. However, not all high pyrite coals contain high arsenic, an observations that has also been made in respect of many Australian bituminous coals. This probably means that some coals contain arsenic in other forms, for example organically associated, associated with clays, perhaps as arsenate ions or with phosphate minerals, where (As04)3- could replace some (PO4)3-. Studies need to be carried out on a variety of coals, in order to clarify the nature of the mineral association of arsenic in coals. The extent of organically associated of arsenic is not clear, although organic bonding has been suggested for many Bulgarian coals, for some low rank Canadian coals and for some low sulfur Siberian coals. For most coals, arsenic seems to be mainly associated with the mineral matter, with varying smaller amounts being associated with organic matter.

Many researchers recognize arsenic as a sulphophile element. The occurrence of arsenic in coal is chiefly associated with sulfide minerals, pyrite in particular , and subordinately with organic matter. Therefore, there is commonly a positive correlative relationship between arsenic and total sulfur content in coal.

The distribution of arsenic in a coal - bearing basin appears to have been profoundly modified by the combined effects of many factors , such as the diagenesis of arsenic matter derived from vegetal matter , or the surrounding rocks , the geochemistry of peat formation including pH and Eh conditions of the basin and many other variables. Syn- depositional tectonic activities also exert an impact on peat accumulation and on the quality of coal as well.

A high coal bearing index indicates a relative balance between the rate of basin subsidence and the compensative peat accumulation over a long period of deposition. The weaker the tectonic disturbance, the higher the coal – bearing index and the lower the arsenic concentration in the coal. High concentration of arsenic in coals reflects the relatively vigorous tectonic activities during deposition.

Under the geological and geochemical conditions described above, the secondary arsenic enrichment occurs in part of coal beds. Because the secondary arsenic enrichment results from deuterogenic tectonic (mainly faults ) and hydrothermal activities, the enrichment is local in extent. Thus within the same mining area, the arsenic distribution in coal can be divided into two major distribution types : synsedimentary and secondary.

Deuterogenic arsenic concentration in coal is controlled to some extent by the original sulfur content of the coal. This is manifested by the apparent strong positive correlation of arsenic and total sulfur in the coal.

According to different geo-scientists basalts and diabases contain an average of 2.0ppm arsenic and gabbros 1.4ppm. At present a value of 1.5ppm arsenic may be assigned to basaltic rocks. Fifty-six individual samples of granitic rocks analyzed by different workers gave an average of 1.6ppm arsenic. The average for granitic rocks may be taken as 1.5ppm. Thus the granitic average does not differ enough from the basaltic and gabbroic averages (1.5 and 1.4ppm, respectively). Rhyolitic rocks and silicic glasses are higher in arsenic than other common rock types. The average content of arsenic in igneous rocks may be taken as 1.5ppm arsenic based on the average for granitic rocks, basalts and gabbros.

In nonmarine carbonaceous shales and near shore marine shales and claystones , the arsenic content is not related to the organic carbon content of the samples. However, in offshore marine samples arsenic is concentrated in high carbon – samples. Arsenic occurs in some samples in syngenetic pyrite but also is present in relatively large amount in samples that contain little pyrite. In general, arsenic is present in iron sulfides, clay minerals ( possibly in adsorbed form), organic matter etc in shales. Because of the wide variation of arsenic among the shales, it is not easy to obtain a precise average. At present the average for shales may be taken as 13ppm arsenic. A tentative average for sandstones may be taken as 1ppm arsenic. Analysis of many sandstone samples from the various parts of the world are desirable. Cherts usually contain about 1ppm arsenic. An average of 1ppm arsenic may be given for limestones and dolomites.

Data on the arsenic content of metamorphic rocks are not abundant, and therefore, behavior of arsenic is metamorphic reactions is not well known. Arsenic is likely to be lost in the transformation of slates and graywackes into schists and gneiss in regional metamorphism. In metamorphosed sedimentary iron ores containing coarsely crystalline hematite the arsenic seemed to have been removed by some leaching process.

Many arsenic compounds are water soluble and hence arsenic contamination of water can occur readily. Water is the major means of transport of arsenic in the environmental compartments. Sedimentation of arsenic in association with iron and aluminium may sometimes be considerable. Rivers and lakes generally contain less than 0.01 mg/1.arsenic. The concentration in ground water depends on the arsenic content of the bedrock. Where ever arsenic is present in natural waters, it is most often found as the anion, either as arsenate or arsenite.

The solubility of arsenic oxide in water is 2.05 g. As2 O3 per 100 g. of water at 25 0 C. The solubility of arsenic sulfide As2 S3 in water in extremely low.

Arsenic as a tendency to become precipitated in the hydralyzates. It is more concentrated near the surface than deep in the sediments. It is enriched in oxidate sediments, chiefly by absorption on ferric hydroxide. The manganese – rich oxidates are lower in arsenic than the iron rich type.

Arsenic is present in air mainly in the particulate form as inorganic arsenic. Though both tri and pentavalent forms occurs in air, the pentavalent form is more predominant than the trivalent form. Methylarsenic is also present in small amounts in air of suburban , urban and industrial areas.

Data on the arsenic content of soils were summarized by different scientists. Their results showed that about 30% of the soils contained less than 5ppm arsenic, about 50% contained 5 to 10ppm and about 20% contain more than 10ppm (parts per million). Most of the recent data on arsenic in various soils give less than 10ppm arsenic. The average value of arsenic in soils probably lies in the range of 5 to 10ppm arsenic. Thus soils are enriched in arsenic compared to igneous rocks.

Arsenic, in small quantities, is a universal contaminant of plants and animals and may sometimes be notably concentrated in organisms, e.g. in land plants growing in soil rich in arsenic and in marine and fresh – water organisms , such as fishes , mollusks , crustaceans , plankton , and some brown algae.

Though organic arsenic compounds are beneficial as a growth stimulant for animals and arsenic compounds have been used as medicines , there is no firm evidence that arsenic in any form is essential to man. In fact arsenic compounds are proved to be toxic. The toxicity of arsenic compounds depends on the chemical and physical form of the compound, the route by which it enters the body and dose and duration of exposure. In man, subacute and chronic arsenic poising may be insidious and pernicious. The symptoms of mild chronic poisoning are fatigue and lot of energy. In more severe intoxication the following symptoms may be observed: kidney degeneration, tendency to edema , liver cirrhosis , bone marrow injury , gastrointestinal catarrh , polyneuritis , exfoliate dermatitis and altered skin pigmentation.

No true tolerance of arsenic has ever been demonstrated. During chronic exposure, trivalent arsenic accumulates mainly in bone , muscle , and skin and to a lesser degree in the liver and kidneys.

A W.H.O task group, applying the linear non-threshold model estimated that a life time exposure to arsenic in drinking water at a concentration of 0.2 mg/1 gave a 5% risk of getting cancer of the skin.
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