B. C. Raymahashay
Professor of Geology
Department of Civil Engineering
Indian Institute of Technology
Kanpur - 208 016


The original source of heavy metals in coal are basement rocks surrounding the basin of deposition. Weathering of these rocks generates a flux of metals into plant debris and associated sediments. There is progressive increase in metal concentration during coal formation and combustion into fly ash. When fly ash derived metals are finally introduced into river or ground water systems, they are fractionated once again into the organic and inorganic constituents of soils and sediments. Thus a biogeochemical cycle of metals is completed.


According to the definition suggested by Nriagu (1977), Environmental Biogeochemistry deals with basic and applied inquiry about chemical changes in the atmosphere, biosphere, hydrosphere and lithosphere brought about by the activities of man. Biogeochemistry obviously has strong linkages with geosciences. Therefore, it is not unusual to find a detailed discussion of elemental cycles e.g. those of carbon, nitrogen, phosphorus and sulphur in standard geochemical literature.

The carbon cycle in particular is tied up with the early history of the earth and the evolution of the atmosphere and oceans. Starting with the off-quoted calculations by Rubey (1951), several geochemists have confirmed that the carbon in ancient sedimentary rocks was acquired by quantitative removal from the earth s early atmosphere which was rich in CO2 gas. Every time we burn fossil fuels, we take part in what Holland (1972) has described as man s greatest geochemical experiment . The consequences of putting the CO2 back to the atmosphere are now part of the global warming controversy.

In the present discussion we would look at combustion of coal from a narrower anthropocentric view point . In doing so we would follow heavy metals in coal from origin through utilization to re-deposition in recent sediments. The components of this specific cycles are (i) derivation by rock weathering, (ii) concentration during coal formation, (iii) retention in ash and (iv) assimilation in soils and sediments.


In a typical coal basin, plant debris are believed to have been deposited in shallow waters along with products of weathering of basement rocks under a humid climate. For example, in the Gondwana basins, weathering of igneous and meta-sedimentary rocks occurring in the shield areas must have contributed a variety of heavy metals to the coal deposits (Sahu, 1987). Many rock-forming minerals incorporate trace elements by substitution in their crystal structures. Some others, after temporary mobilisation, are retained in the clay fraction of the weathering profile. On the other hand, metals occurring as sulphides are highly mobile during weathering in the presence of oxygenated water. This may lead to local high concentrations of dissolved Cu, Zn, Mo, Ag, Hg, Pb, and associated Se, As and Cd in circulating waters (Drever, 1988).

The sedimentary rocks associated with coal are dominated by fine grained clastics (Hemingway, 1968). A typical distribution of selected elements (Table 1) shows that their concentration is relatively high in shale. This enrichment is primarily due to adsorption on the surface of fine grained particles. Other processes which influence the occurrence of heavy metals in sedimentary rocks are precipitation following oxidation/reduction and reaction with organic matter (Krauskopf, 1979).

Table 1. Concentration of selected elements (ęg/gm) in rocks. From Drever, 1988.

                Granite         Shale           Sandstone       Limestone
Cu               20              45                 2               4
Zn               50              95                16              20
Pb               17              20                 7               9
Mo                1               2.6               0.2             0.4
As                2              13                 1               1
Ag                0.04            0.07              -               -
Hg                0.03            0.4               0.03            0.04
Se                0.05            0.6               0.05            0.9
Cd                0.13            0.3               -               0.03


The conversion of plant matter to coal is essentially a biochemical process. At the first stage of peat formation, lignin is selectively preserved and converted to humic acids under the action of micro-organisms and their enzyme systems (Flaig, 1968; Given et al., 1973). Development of bituminous coal from peat involves decrease in cellulose and increasing modification of lignin, intermediate products and humic acid. While humification is the major process at peat stage, time, temperature and pressure are important factors for development of coals of higher rank.

It is believed that coals contain evidence for preservation of original biogenic accumulation of trace elements (Nicholls, 1968). The trace element content is usually determined by analyzing the ashes of various lithotypes. For example, from a study of Indian coals, Ghosh et al. (1987) concluded that Pb and Co had been supplied by the woody portion of the proto- coal material whereas Ga, Nb, Ni, Cr and In came from the non-woody portion. Similarly, V, Mn, Sr, La and Ba were derived from an extraneous inorganic source while Cu, Mo and a part of the available Cr were attributed to both organic and inorganic source. The elements associated with the organic fraction may represent primary concentration in the plant tissues and further enrichment during selective decay. Other possible mechanisms are adsorption from ground water and formation of organo-metallic compounds. Polyvalent metals interact with humic acid to form complexes of varying stability. Three possibilities have been considered by Stevenson (1977) and Krauskopf (1979). These are (a) substitution of H of a carboxyl group forming a salt-like linkage, (b) direct linking with the carbon atom of an organic and (c) formation of a chelate compound with the metal occurring at the centre of a ring structure.

Trace elements including heavy metals can be associated with the inorganic fraction by several mechanisms. Among these Nicholls (1968) has listed (i) occurrence in the crystal lattice of detrital minerals delivered to the coal basin, (ii) sorption from circulating water during the initial peat formation stage and during later diagenesis, (iii) precipitation from circulating water under the prevailing Eh and pH conditions during peat formation and subsequent diagenesis, (iv) introduction of mineral matter after coal formation.

It has been suggested that correlation with ash content and occurrence with respect to the marginal part of the coal seams are parameters useful in sorting out the various processes by which heavy metals are incorporated in coal. On the other hand, local conditions can considerably influence the variation from one coal seam to another.


During combustion of pulverized coal at temperatures of the order of 900 to 1500oC in thermal power plants, the solid component of the residue enters the flue gas stream in the form of fine grained particulate matter. This is termed Fly Ash because of its quick dispersion into the environment. The most important aspect of fly ash generation is its enormous quantity. It is estimated that several million tonnes of ash is produced annually from the major power plants in India which use coal with 30 to 40 percent ash. It is interesting to review the fate of heavy metals contained in coal during this combustion process.

Watt and Thorne (1965) concluded that fly ash predominantly contained glassy spheres, opaque spheres and spongy particles. The other components were crystalline phases (quartz, mullite, hematite and magnetite) and partially burnt coal particles. By burning clay minerals typically occurring with coal under laboratory conditions, these authors demonstrated that spongy particles (cenosphere) were produced at relatively low temperatures whereas clear glass particles formed at higher temperature. A part of the quartz could be residual from original coal while the remaining quartz and mullite represent products of thermal break down of clays. Iron sulphides and carbonates in coal are converted to oxides.

While the main constituents of coal undergo combustion as described above, the trace elements are partitioned according to their volatilization temperature and the particles (Davison et al., 1974). Elements like Pb, Zn, Cd, Hg and As are volatile at the furnace temperature but they condense on the ash matrix at the cooler downstream part of the ash duct (Sahu, 1991). The non-volatile metals remain in the oxide and silicate phases of ash. As it is difficult to accommodate these metals in the crystal lattice of quartz and associated silicates, they are likely to form a surface labile layer on the fine grained particles (Sahu, 1987). This leads to a situation which is of great significance for environmental impact namely (a) the heavy metals including toxic elements are enriched several times in fly ash compared with the original coal (Table 2) and (b) they are easily leachable in contact with water (Ferraiolo et al., 1990).

Table 2. Enrichment of heavy metals in fly ash. Adopted from Sahu, 1987.

               Coal           Fly Ash        Enrichment
               (ęg/gm)        (ęg/gm)        (Fly Ash/Coal)
As              5.9             61              10.3
Cd              0.19             1.45            7.6
Cr             20.2            131               6.5
Co              6               38               6.3
Cu             18              120               6.7
Ni             15.1             98               6.5
Pb             30               70               2.3
Zn             37              210               5.7
U               1.4             11.6             8.3


Fly ash is discharged into surrounding land and water bodies through wet and dry deposition. It is well known that constituents of soils and sediments are effective sinks for heavy metals (Singh and Subramanian, 1984). Their interaction with fly ash derived metals is no different. Even when coal dust from mining operations is added to the suspended load of nearby rivers, the heavy metal content shows a significant increase. For example, Raveendranath and Raymahashay (1988) observed that the Zn content in sediments went through a maximum (900 ppm) at the confluence of the effluent channel of a coal mine with the local stream compared with the upstream and downstream segments. It is expected, therefore, that metal enriched fly ash will show the same effect.

Patel and Pandey (1987) compared the analysis of soils collected from locations contaminated and uncontaminated by fly ash fall out around the Korba power plant. Their data listed in Table 3 show that with reference to the metal content of top 30 cm of the uncontaminated soils as background, the values in contaminated soils have higher values up to 90 cm below ground surface. It can be concluded that the metals deposited at the surface had permeated into deeper levels and were retained by soil constituent.

Table 3. Concentration of metals (ęg/gm) in Korba fly ash and soils. Adopted from Patel and Pandey, 1987.

          Fly Ash     Uncontaminated                 Contaminated Soil
                      Soil (0-30 cm)            (0-30cm) (30-60cm) (60-90cm)
Cu         53.5           5.5                     22.0      15.0      10.5
Co         31.5           3.5                     13.3       8.8       5.5
Ni         83.5           7.5                     27.5      20.3      11.3
Pb         46.5           4.5                     18.0      11.8       6.5
Zn        115.0          12.0                     40.8      31.8      21.0
Bi          6.5           Nil                      1.0      Nil       Nil
Sb         63.75          Nil                      7.5       2.3      Nil

In a detailed study of the binding sites of fly ash derived copper in river sediments, Ravichander et al., (1994) found that adsorption by the inorganic constitutes (illite, chlorite, Kaolinite and quartz) was much less compared with the organic fraction. Further characterization of the organic matter revealed that humic acid was much more effective for Cu adsorption compared with fulvic acid and humin. This is apparently due to the relatively higher value of their stability constant of Cu - humic acid complexes.

These data tend to show that the major factors controlling the fractionation of fly ash derived heavy metals into recent sediments are adsorption on surface active inorganic constituents and formation of organo-metallic complexes. These, therefore, are similar to the factors responsible for incorporation of these metals in coal to start with.


The pathway of heavy metals in coal starts with weathering of basement rocks surrounding the basin of deposition. The plant debris and mineral matter accumulate these metals by adsorption and complexation. They are further enriched during selective decay and coalification. During utilization of coal deposits, dust from mining operations and fly ash from power plants introduce the metals once again into the sedimentary environment. This effectively closes the specific biogeochemical cycle that we have considered.


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