For the analysis of modes of occurrence
For the analysis of modes of occurrence in coal samples, a six-step SCEP  was employed in the present study. This leaching procedure, with a major advantage that whether specific elemental forms occur or are absent in different SCEP products depending on their solubility in the various reagents , has been successfully used in many studies on modes of occurrence of trace elements in coal , , , ,  and . Six types of elemental occurrence in coal were identified using this leaching method (Fig. 2), including water-soluble, ion-exchangeable, carbonate, organic-bonded, silicate, and sulfide . The water-soluble trace elements were extracted from the mixture of ∼4-g coal sample and 60-ml distilled water; the ion-exchangeable elemental forms were obtained from the water-soluble-extracted residue by the addition of 60-ml NH4AC. After that, the organic Cy3 NHS ester and mineral fraction were separated by 1.47-g/cm3 CHCl3. The organic- and carbonate-associated elements were, respectively, dissolved by HNO3 + CHClO4 and HCl. Finally, the silicate- and sulfide-associated fractions were obtained from the residue of carbonate-extracted mineral fraction using 2.89-g/cm3 CHBr3, and then were digested by the mixture of HNO3 (65%, v/v) and HF (40%, v/v). Note that ultra-pure water (18.2 MΩ cm), prepared by a Milli-Q™ A10 system (Millipore) and guaranteed reagent (HNO3), was further purified by a DuoPUR acid purification system (Milestone) for the SCEP and ICP-MS analyses. However, like any analytical techniques, SCEP also has some limitations. For example, 100% extraction of a component in any one stage of the SCEP is difficult to achieve  and . It is possible that fine-grained minerals (e.g., clay) dispersed in the organic matter would be difficult to separate from organic matter. In an attempt to overcome this problem, statistical analysis has been performed on the results from leaching steps . The approach is similar to that adopted in the analysis of geochemical data generated from the analysis of a suite of coal samples .