Trace element concentrations, ashing time

Another study of the variation of trace element concentrations with ashing time at 500°C in covered and uncovered platinum crucibles indicated that ashing time had no effect (Table V). As previously noted, the loss of boron from the uncovered crucibles stabilized at a relatively constant concentration in less than 5 hrs. Molybdenum and vanadium, which show losses with increasing temperatures, show no apparent ashing time dependence. 

Table V. Ashing Time vs. Trace Element Concentrations for Coal C—17601 (ppm)°...

Cumulative leaching plots (cumulative mass of element leached per mass dry weight of ash vs. cumulative mass of leachate collected vs. mass dry weight of ash) for Pb, Cu, and Zn are shown in Fig. 12. The plots are constructed using measured element leachate concentrations and volumes of leachate collected. In many cases, the detection limit of the element was used as the concentration, so that the traces are conservative. Over time, the pH of leachates has dropped from about 12 to about 9. Data for Cd are not shown because of the high number of detection limit values that were observed in both the unamended and amended lysimeters. 

Most of the readily soluble surface-associated elements may have been depleted in the Drax and Meaford weathered ashes, compared with the fresh ash from Drax, because of their prolonged contact time with water. The Drax ash samples have been deposited 17 years and the Meaford samples some 40 years. Loss of the readily soluble elements from the weathered fly ashes would therefore be anticipated. The results, shown in Table 3, clearly demonstrate that this is the case. The concentrations of Ca, Na, K, SO4-, B, Cr, Cu, Li, Ni, Hg, and Mo are all much lower, with some of the trace elements below the detection limits. The only exception is N03, and the field studies, discussed later, show that fertilizers are probably responsible. 

In the photographic procedure, the lack of a suitable internal standard for exposure correction, the attempt to record and determine all elements on one generalized exposure, and the very high concentration of the trace elements in the ash (for some samples as much as 33 times the amount reported in the coal) caused a poor relative standard deviation. However, of the 13 elements determined, only Co, Ni, Cr, and V were less precise than 20%, a level which we feel is suitable for a photographic method. 

Combining the fusion technique with ICPES measurements gives a rapid and accurate method for the ash elemental analysis. The total analysis time needed is 20-25 minutes per sample. However, although the fusion procedure is excellent for the determination of all major elements, it is not suitable for the determination of trace elements, because the final solution (1 L) is too dilute for detection of trace elements. If the solution volume is kept small, extremely high concentrations of lithium and boron in the solution give an undesirable high background spectrum for trace element measurements. Hence, it is necessary to resort to a separate procedure where both trace and major elements can be simultaneously determined. 

The principal limitations of ESCA include the inability to detect elements present at trace concentrations within the analytical volume, and insufficient lateral resolution to characterize single micrometer-sized particles. The inability to characterize trace species is illustrated in Figure 10 for a sample of coal fly ash particles (11). The fly ash results from the noncombustible mineral components of the coal and consists largely of fused iron oxides and aluminosilicates (42). In addition, most elements are present in at least trace concentrations (22, 42), and many of these elements are highly enriched in the surface region of the particles (evidence for this will be discussed in the next section). However, the ESCA spectrum acquired over several hours of counting time indicates only the presence of detectable surface S and Ca in addition to the fly ash matrix constituents. 

Indirect FAA methods, in which the hydrocarbon matrix is eliminated and the analyte is concentrated, could have been applied to all elements studied by the Project if a large enough sample were used. However, since indirect techniques still may encounter chemical (interelement) interferences, a minimum of 5 ml of solution must be available so that standard additions techniques can be applied. Some trace metals have been determined in petroleum by indirect FAA after a 100-g sample had been ashed (17), However, ashing such large samples, particularly crudes or residual fractions, is difficult and time consuming. Sulfated ash procedures were used in the Project to prepare various petroleum matrices for determining Cd, Co, Mo, Ni, and V by FAA. However, procedures were developed only for the first three elements, and cross-check data were collected only for cadmium. Since alternate techniques had greater sensitivity and allowed smaller samples to be ashed, flame atomic absorption was not widely used. 

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