DISTRIBUTION OF ARSENIC IN WORLD COALS WITH SPECIAL REFERENCE TO NORTH KARANPURA COALFIELD, JHARKHAND, INDIA.
Dr. Nitish Priyadarshi
The origin of coal
Coal is a result of the accumulation and slow decay of plant remains in sedimentary strata. It undergoes in situ compaction under water with time, accompanied by biochemical processes such as decomposition due to bacterial action, dehydration, loss of volatile compounds (e.g. methane, higher hydrocarbons, carbon dioxide and nitrogen) and densification to form various ranks of coal depending on environmental conditions. In absence of atmospheric oxygen, the plant matter is further degraded by the action of anaerobic bacteria, which extract and utilize oxygen from organic molecules containing oxygen like lignin.
Where does the arsenic come from ?
Arsenic, one of the potentially hazardous trace elements, is usually concentrated in the sulphidic minerals of coal.
Many authors interpret the arsenic accumulation in terms of arsenic concentration during decay of plant matter in the humic layers. Arsenic is absorbed by the plants from the soil or crust.
Arsenic contributed by the surface as well as underground circulating waters during the primary stages of coal formation.
Arsenic deposited through the hydrothermal solutions during the igneous activity in and around coal basins.
The enrichment of arsenic and other trace elements in coal is governed by the following factors (Swaine, 1962).
· Duration of supply of arsenic during the initial stages of coal formation.
· pH and Eh conditions in the depositional basin.
· Variety and concentration of the supplied constituents.
· Microstructural frame work of the coal seams.
· Porosity of the overlying and underlying rocks.
· Rate of sedimentation and tectonics of the coal basin.
The arsenic content of coal samples worldwide is highly variable, with an average value around 5 mg kg-1 and extreme high values of up to 35,000 mg kg-1 in coals from endemic arsenosis areas in China.
Arsenic in coals:
It is not surprising to find that there has been increased interest in Arsenic in coals, together with work on rocks, soils, plants and waste materials, probably because of possible adverse health effects of high concentrations. The arsenic content in coal has been reported to be as high as hundreds or even more than one thousand ppm, suggesting that under certain geological and geochemical conditions arsenic can be considerably enriched. In comparison, the average arsenic concentration in earth’s crust is low ( a Clarke value of 2.2 ppm). The enrichment coefficient for arsenic is close to the maximum enrichment coefficients of the trace elements in coaly materials such as Be, Ge, and Ga (Zhou and Ren,1992).
Arsenic is present in coal as arsenopyrite and that little exists in any other form. Minkin et.al. (1984) found that arsenic was most likely to present in solid solution in pyrite, and Finkelman et.al. (1979) noted that arsenic was predominantly in fractures in the coal and in microfactures in the pyrite. For most coals, arsenic seems to be mainly associated with the mineral matter, with varying smaller amounts being associated with organic matter.
Arsenic has similar chemical properties to phosphorus (P), the element immediately below arsenic on the periodic chart. It is well known that coal fly ash contains arsenic that can leach into receiving- water reservoirs. During coal combustion, arsenic oxidizes and forms gaseous As2 O3 and enters the atmosphere. This causes concerns for governments of many countries because of environmental pollution due to extensive use of coal.
Organic-inorganic affinity of Arsenic:
The relationship between the concentration of an elements and ash content has been used as a first order approximation of the elements organic/inorganic affinity. Factors governing elements partitioning between organic and inorganic phases have been discussed by a member of investigators. If the concentration of arsenic and other elements increases with increasing ash content, i.e. a positive correlation, the arsenic may be characterized as having an inorganic affinity.
Alternatively, if the arsenic concentration decreases with increasing ash content, a negative correlation, the arsenic may be characterized as having an organic affinity.
Distribution of arsenic in world coals:
The concentration of arsenic in coal is commonly below 10 ppm (Zhou and Ren, 1992). Arsenic in most world coals is 0.5-80 ppm (Swaine, 1990). Difference in arsenic levels between Gondwana coals and coals from the Northern Hemisphere are reflected in the mean values (as ppm As), namely, 1.5 (Australia), 4 (South Africa), 15 (United Kingdom) from Swaine (1990), and 24.6 in 7351 samples (United States) from Bragg et.al. (1998). Concentrations of arsenic in Chinese coals are between 0.21 and 32000 ppm (Ren et al. 1999). In general, the arsenic content of most Canadian coals is low as compared with the range for most world coals. The mean values for arsenic in the Bulgarian coal deposits range from 2 to 58 ppm (Eskenazy,1995). The Miocene Cayirhan coals from the Beypazari basin of Turkey have 32-148 ppm arsenic (querol et al., 1997). The concentration of arsenic in the Gokler coal samples in Gokler coalfield of Turkey range from 170 to 3854 ppm (av. 833 ppm), with a geometric mean of 670 ppm. The means indicate that these coals contain more arsenic than most world coals (Karayigit et al., 2000). Arsenic concentration in coals of Hat Creek Deposit of British Columbia, Canada varies from 4.03 ppm to 52.7 ppm. (Goodarzi, 1987). The concentration of arsenic varies from 2.6 ppm to 138.1 ppm in coals of Teruel Mining district in northeast Spain (Querol et al. 1992).
Arsenic in the Permian coals of North Karanpura Coalfield of Jharkhand State of India:
The North Karanpura coalfield, a western most member in the east-west chain of the Damodar Valley Basin, forms a large expanse of coal bearing sediments spread over Hazaribag, Ranchi and Palamau districts of Jharkhand State. It covers a total area of around 1230sq. Km. For the arsenic study, samples from coal from Badam, Kerendari, KDH, Rohini, Dakra and Karkatta were analysed by the author. Molybdenum-blue Colorimetry was used as the chemical technique for arsenic determination as recommended by the International Standard Organisation. Concentration of arsenic in coal samples range from from less than 0.01 to 0.49ppm with an arithmetic mean of 0.15ppm. (Priyadarshi, 2004). Concentration of arsenic is low compared to most world coals. Average ash% is very high (up to 32.51%). The low arsenic concentrations of the coal studied could be related to the geological characteristics of the source area in the basin and to a resulting low degree of arsenic mineralization (realgar or orpiment) of the synsedimentary solutions, which resulted in a paucity of arsenic in the system.
Impact of arsenic on health:
Arsenic is an environmental hazard and the reduction of drinking water arsenic levels is under consideration. People are exposed to arsenic not only through drinking water but also through arsenic contaminated air and food. Arsenic is emitted to the air by coal combustion as some coals are unusually high in arsenic because of geologic factors. Some of the common examples of arsenic poisoning are Skin lesions including keratosis of the hands and feet, pigmentation on the trunk, skin ulceration and skin cancers. Toxicities to internal organs, including lung dysfunction, neuropathy and nephrotoxicity have also been identified in some parts of China where the coal containing high arsenic burned inside the home in open pits for daily cooking and crop drying, producing a high concentration of arsenic in indoor air. Arsenic in the air coats and permeates food being dried producing high concentrations in food.
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Dr. Nitish Priyadarshi