Coal is a natural product that is created through compressed organic matter containing virtually every element in the periodic table – mainly carbon, along with some heavy metals. These heavy metals refer to any metallic chemical element that has a high density and is toxic at low concentrations. The concentrations of these heavy metals vary in coal by its geographic region. Small amounts of heavy metals can be necessary for health, but too much may cause acute or chronic toxicity. Many of the heavy metals released in the mining and burning of coal are environmentally and biologically toxic elements, such as lead, mercury, nickel, tin, cadmium, antimony, and arsenic, as well as radio isotopes of thorium and strontium.
Heavy metal analysis in coal has typically been a laborious and time consuming process. Even with multielement emission techniques, the sample preparations involving destruction of the sample matrix has limited sample throughput thus not allowing any subsequent speciation. Under this background, the ICP-OES technique can be used as a viable technology for its wide elemental coverage, very low detection limit (upto ppb level), fast analysis time (all elements at once), wide analytical working range, simple spectra of the constituents elements and high matrix tolerance. At the central lab of MSK, Kolkata, an attempt have been made to analyze some of the trace constituent heavy metals such as Cr, Ti, Cu, Ni, Pb, Zn from the as received coal using the recently procured ICP-OES.
The digestion of the coal samples have been carried out in the microwave digestor using the typical inorganic acids. The pressure used during the digestion process is as high as 50 bar and the power is kept typically at 500 Watt. Total time spent for such individual sample digestion is around 40 minutes. Very clear solution of the coal samples have been obtained from such digestion process without any sedimentation at the bottom. Heavy metals such as Cr, Ti, Cu, Ni, Pb, Zn in the range from wt% to ppm level in coal samples have been determined using the axial mode of the CCD detector avoiding any possible matrix interferences from any other analyte. The constituent elements are being determined using the axial mode of the detector to yield highly accurate results. Fig. 1 shows the calibration curves made for all the analytes as stated above using different concentrations (50 and 100 mg/L).
Fig. 1: Calibration curves made for the quantitative measurement of the heavy metal analytes in coal samples
Table 1 shows quantitative measurement data of constituent heavy metals for some of the as received coal samples. During each time of measurements the standard sample SARM 20 was measured from its solution and accordingly standard correction was done. It can be seen that the values obtained for the standard sample are quite close to their declared values and hence justifies the accuracy of the above mentioned technique.
Table 1: Heavy metal constituents as determined for few of the coal samples (with Standard SARM 20) through ICP-OES technique
|Serial No.||Constituent heavy metals detected thorugh ICP-OES in ppm|
|Standard SARM 20 decleared||–||18||25||26||6300||17|
|SARM 20 obtained||–||16||27||25||6273||20|
Dr. (Mrs.) Saswati Ghosh & Mr. Arnab Banerjee