Table 1 Sequential chemical extraction procedure

Water-leachable

A 3-g sample of coal was transferred into a container. Add 20 mL of Milli-Q water at room temperature. Let it stand for 24 h. The suspension was then centrifuged at 3500 rpm for 20 min.

Ion-exchangeable

20 mL of ammonium acetate (1 N) was added to the dried residual solids at room temperature for 24 h, suspension was then centrifuged at 3500 rpm for 20 min.

Organic matter-bound

The dried residual solid was treated with CHCl₃ (1.47 g/cm³). After 24 h of stirring at room temperature, a float-sink separation was performed by centrifugation (3500 rpm for 20 min). The float fraction (1.47 g/cm3) of the sample was dried at 40°C, then treated with 12 mL of an oxidizing mixture (HNO₃: HCl = 3:1) and 3 mL HF in a Telfon beaker which was put in a microwave oven (at 600 W for 30 min; at 1200 W for 25 min; and at 1200 W for 30 min).

Carbonate-bound

The sink fraction (1.47 g/cm³) was dried at 40°C, and then treated with 20 ml HCl (0.5%). The suspension was then centrifuged at 3500 rpm for 20 min.

Silicate-bound

The dried residual solid was treated with CHBr₃ (2.89 g/cm³). After 24 h stirring at room temperature, a float-sink separation was performed by high-speed centrifugation (5000 rpm for 20 min). The float fraction (2.89 g/cm³) of the sample was dried at 40°C, and then treated with 12 mL of an oxidizing mixture (HNO₃: HCl = 3:1) and 3 mL HF in a Telfon beaker which ws put in a microwave oven (at 600 W for 30 min; at 1200 W for 25 min; and at 1200 W for 30 min).

Sulfide-bound

The sink fraction (2.89 g/cm³) was dried at 40°C, and then treated with an oxidizing mixture (HNO³: HCl = 3:1) and HF in a Telfon which was put in a microwave oven (at 600 W for 30 min; at 1200 W for 25 min; and at 1200 W for 30 min).