MOBILITY OF CADMIUM IN COAL FLY ASH - Management of Hazardous Residues Containing Cd - page 179

C ONCLUSION
The various methods of sequential extraction of Cd from coal fly ash
(Psenner, Tessier et all., BCR, Sposito, Kersten i Förstner, Rudd, Gatehouse)
were studied in order to characterize chemical fraction and was it found the
yield of extraction was varied from 36.3% (BCR) to 92.1% (Tessier) of
cadmium content in coal fly ash (19.0 mg · kg -1 ).
The cadmium content in particular fractions using sequential extraction
methods was in order exchangeable fraction < carbonate < oxide < organic
(sulfide) by Tessier, Gatehouse, Kersten and Förstner methods, while other
studied procedures (Rudd, Psenner, BCR, Sposito) did not give any regular
trends in Cd amount for individual chemical fraction.
The best method for determination of fractions soluble in water and
soluble in acids (carbonate and adsorbed) was obtained by Rudd method. This
seems to be important in the context of important issue of transfer of cadmium
from coal fly ash to coal fly ash solutions in natural conditions.
Optimum conditions of Cd chemical fraction extraction were established.
In order to extract exchangeable fraction of cadmium (pH = 7) 0.5 mol l -1
MgCl 2 solution (0.5 h in 25°C - Tessier) should be applied. Next, in order to
elute adsorbed and carbonate fraction the extraction by means of 0,5 mol l -1
KF (pH = 6,5, 16 h in 25°C - Rudd) is recommended to apply. In order to
extract oxide fraction of cadmium the use of 0.1 mol l -1 NH 2 OH · HCl in 0.01
mol l -1 HNO 3 (pH = 4.5, 24 h in 25°C), next 0.1 mol l -1 oxalate buffer (pH = 3,
24 h at 25°C) - Kersten and Förstner is recommended, which should be
followed by extraction with 4 mol l -1 HNO 3 (16 h in 80°C - Sposito) for
sulfide fraction and 30% H 2 O 2 in HNO 3 (pH = 2, 5 h in 80°C) for organic
fraction and finally followed by extraction with 3.2 mol l -1 CH 3 COONH 4 in
20% HNO 3 (30 min in 25°C - Tessier). The extraction of Cd from coal fly ash
residue should be performed by mineralization in HNO 3 /HClO 4 (2 h in 90 °C)
by Tessier's method.
The characterization of mobility fraction of Cd soluble in water and in
acids (carbonate and adsorb fractions) was performed. It were found that Rudd
and Sposito methods yielded maximum Cd amounts under these conditions.
Mobility fractions of Cd contain of 22% of its total concentration in the fly ash
in environmental conditions. Their method for cadmium determination is
proposed for highest delectability for fractions of interest.