Friday, April 10, 2009

Paleoenvironmental implications of the Boron content of coals.

Paleoenvironmental implications of the Boron content of coals with special reference to Jharkhand coals of India.
by
Dr. Nitish Priyadarshi
Abstract:
71 channeled samples of coals of Permian age from eight coalmines in the Jharkhand State of India were collected. The boron content ranges from BDL to 35ppm. and is well within the range of most world coals. Average ash% (30.12) is high. Boron was analyzed in coal ash using Spectrophotometer. The close similarity of boron in the coals under study and other lower Gondwana basins of India are broadly attributed to the uniform sources. Coals under study have low boron content, and were deposited under fresh water influence during the early stages of coalification.

Introduction:
The geochemistry of coal is an integral part of any modern study dealing with coal characterization, owing to the possible presence in the coal of toxic and industrially undesirable elements that exceed the legal limits for emission of such substances. A coal seam formed from material deposited in a brackish water environment may contain undesirable elements; for example, sulphur and elements that form sulphides and sulphates. Boron is an element that is sensitive to the environment of deposition and, therefore, can be used to delineate the area(s) of a coalfield influenced by brackish water conditions during deposition.

Interest in the elemental composition of coal has been on the increase worldwide mainly as a result of growing environmental problems.

Our interest in the boron content of coals is of several reasons. It has been suggested that boron presumably derived from the coke may affect the mechanical properties of certain steels. Coal has been considered as a source of graphite for use as a moderator in nuclear reactors, but the content of boron in the graphite must not exceed 2 ppm. The levels of soluble boron coming from washery wastes and fly ash disposal areas should be checked to ensure that undesirable amounts of boron are not being added to nearby rivers or lakes. Some fly ashes could be useful as soil supplements, but plants should be monitored to ascertain whether the boron from fly ash is enhancing or retarding their growth. Similar effects should also be considered during reclamation projects after coal –mining. From geochemical point of view, the content of boron is interesting because it indicates whether the environment involved fresh, mildly brackish, or brackish water conditions during the early stages of coalification. The investigation has been done on Australian coals (Swaine, 1971) and Canadian coals (Goodarzi, 1988; Banerjee and Goodarzi, 1990; Gentzis et.al.1990).

The Permian coal deposits constitute 98% of the total coal reserves of the country. The
geology of the ancient Indian shield consists of extensively mineralized rock in vicinity
of Gondwana coal basins, it is but normal that the coal ash would be rich in many heavy
metals derived from the respective terrains.

Objectives:
The purpose of this paper is to determine the boron content in different coal seams of different coalfields of Jharkhand State, to discuss the mode of occurrence of Boron in the coals and to evaluate the relation between the boron content in Permian coals of Jharkhand and environment of deposition.

Methodology:

71 channel samples of Gondwana coals of Jharia, East Bokaro, Ramgarh, South Karanpura, North Karanpura and Hutar coalfield (fig.1) were collected for study. Samples were ashed in platinum crucibles in muffle furnace at 5000c. for 6 hours. Boron was determined in the ash through Spectrophotometer (Beckman DB-G, Grating Spectrophotometer). Analysis as described by Pollock (1975) was followed.
DISCUSSION AND CONCLUSION:
Boron content in coals:

Results for boron in coal are given in table-1. The range for world coals being 0.5-2456ppm boron, but most would probably be between 5 and 400ppm boron (Swaine, 1990). Average concentration (19.70 ppm) of boron in coal samples under study is well within the range of most world coals. Boron content show little variation in all the coal samples collected from Karharbari and Barakar Formations. Average ash% is high (30.12). At best this is blendable variety.

Mukherjee et.al (1982) studied spectrochemically the coals of Karharbari and Barakar Formation of the coal field under study and observed that Boron content is in range of 10-30 ppm in Dobari Quarry of Jharia coal field, 5 ppm in Kusunda open cast mine of Jharia coal field, 10-60 ppm in East Bokaro coal field, 10-20 ppm in Ramgarh coal field, 10-30 ppm in Argada coal field, 5-15 ppm of Dakra seam of North Karanpura coal field and 5-10 ppm in Hutar coal field.

Boron content in the other Indian coals is in range of 25-28ppm boron in Rajmahal Purnea belt, 5-27ppm in Mahanadi coalfields, 5-31ppm in Son valley, 10-30ppm in Satpura valley, 18-21ppm in Wardha coalfield, and 12-38ppm in Godavari valley (Mukherjee et.al.1982). 10-30ppm boron has been reported from Tertiary coals of NW India (Chandra and Singh, 1994). The close similarity of boron in the coals under study and other coals of Gondwana basins may be broadly attributed to the more uniform nature of the major contributory sources of the Gondwana coals.

Mode of occurrence:

The boron in coals is thought to be mostly organically bound. An inverse relationship between boron and the ash content of the coals has been mentioned by many authors as being indicative of the organic affinity of boron (Butler,1953; Goodarzi,1988; Swaine, 1990; Beaton et al.,1991). Inorganically bound boron is usually associated with the clay minerals, mainly illite (Bouska and Pesek, 1976; Kler et.al., 1987). But in the research area we cannot make any assumptions regarding organic or inorganic affinity of boron as the correlation value between ash% and boron is low (r = -13.6). It may be organically bound.

Relation between the boron content in coals of Jharkhand and environment of deposition:
Boron as an indicator of the paleosalinity of the sedimentation environment has been a subject of many investigations. Goldschmidt and Peters (1932) pointed out for the first time the relation between the high boron content in sea water and the boron content in sea water and the boron content in marine sediments. Later Goldschmidt (1958) stated:
"The supply of boron from the ocean among the various types of sediments, really dominates the geochemistry of this element".

Further work on Swedish sediments indicated that the boron concentrations in marine and non-marine sediments differ significantly (Landergren,1945). The boron content of seawater is 4440 ppb, while that of river water is 10 ppb (Li, 1982).

This is the basis for the use of boron as a salinity indicator. The basis for using the amount of boron in clays, coals, or other materials as an indicator is that seawater contains 4.6ppm boron compared with less than about 0.1ppm boron in most rivers and other terrestrial waters (Goodarzi and Swaine, 1994). The question is, do clays and coals assimilate boron from the waters in which they are deposited and retain it during the diagenesis and later processes? There is an experimental evidence that clays remove some boron from aqueous solutions. For example Kaolinite, Montmorillonite, and Illite extract boron from solutions, but not to the same extent (Hingston, 1964).

Another experimental study showed that the removal of boron from natural waters depended on both the salinity and the boron content of the solution. These experiments confirmed that adsorption is the mechanism for the initial intake of boron. In general, Illites fix more boron than Kaolinites or Montmorillonites, depending mainly on the boron concentration in solution, but also on pH, ionic strength, and temperature (Goodarzi and Swaine, 1994).

In view of the usefulness of the boron content of clays for indicating, at least approximately, the degree of salinity in terms of marine, brackish water, and fresh water, several attempts have been made to use the boron content of sediments associated with coals. Shales from the part of Appalachian coal basin in Pennsylvania, U.S.A. was investigated and was found the following mean values for boron content: 44ppm boron (fresh water), 92ppm boron (brackish water), and 115ppm boron (marine) (Degens, et.al. 1957;Keith and Degens, 1959). On the basis of work on some sediments in Ruhr region of Germany it was suggested values of 15 to 45ppm boron for fresh water conditions and 90 to 190ppm for marine conditions (Ernst et.al. 1958). In the Hat Creek coals in the south-central British Columbia there are two of the thickest sub-bituminous coal deposits in the world, and formed in a fresh water, lacustrine environment (Goodarzi and Van der Flier-Keller,1988). They contain 5 to 32 ppm boron, which indicates freshwater conditions consistent with other evidence (Goodarzi and Gentzis,1987).
The above studies support the use of boron along with other parameters for determination of the depositional environments of sediments.

On the basis of different results and further work, it was suggested that the following scale of value is applicable (Swaine,1971).

Up to 40ppm boron: fresh water-influenced coals.
40 to 120ppm boron: brackish-water influenced coals.
> 120ppm boron : marine, seawater influenced coals.

Following a reappraisal of earlier work and taking into account of recent work on Canadian coals and Australian coals (Goodarzi and Swaine,1994) , it is proposed that the terms identifying the degrees of salinity and their associated boron concentrations should be changed to fresh water (F), mildly brackish water (MB) and brackish water (B). The new range and categories are.

Up to 50ppm boron: fresh water-influenced coals (F).
50 to 110ppm boron: mildly brackish-water influenced coals (MB).
> 110ppm boron: brackish-water influenced coals (B).

According to above classifications it may be concluded that the Jharkhand coals, which contain low boron, content, were fresh- water influenced during the early stages of coalification.

Acknowledgement:

Author is grateful to Mr. F. Goodarzi, Geological Survey of Canada, for his useful suggestions to improve the paper.

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