Characterization of eluted metal ions by sequential extraction from Krepoljin coal basin, Serbia: Mechanisms of metal interaction

Eight samples of powdered brown coal (Krepoljin coal basin, Miocene age) were successively extracted by two extractants at room temperature: bidistilled water, Me1, and 2 M MgCl2 solution, Me2. Large amounts of Ni and Zn are leached from the coal deposit by water. On the contrary, no significant amounts of Pb and Cu are washed under normal conditions. Also, the mobility of Zn and Ni especially depends, to a high extent, on ionic strength. It can be assumed that the humic acids co-extracted during water leaching, Me1, and being a highly competitive ligand for metal ions, will act as a disturbing agent in metal speciation procedure in coals. This assumption is supported by the obtained significant correlation between the Me1:Me2 ratio and the humic acid complex stability constants