Competitive sorption/desorption of Zn, Cd, Pb, Ni, Cu, and Cr by clay-bearing mining wastes

The competitive sorption–desorption of trace metal ions by kaolinite-bearing coal mining wastes in Upper Silesia and by smectite-bearing waste from a basalt quarry in Lower Silesia, both in Poland, were studied. Generally, at higher clay contents, the sorption on both kaolinite wastes followed the same sequence of efficiency: Cr ~ Zn > Pb > Cu > Ni ≥ Cd, while the sequence for the smectite waste was Zn > Pb > Cd > Cu ≥ Ni > Cr. The retention of the metal ions followed the same sequence for all clay-bearing wastes: Cr > Pb > Cu > Zn > Ni ≥ Cd. The sorption of the clay-bearing coal mining wastes was considerable, though the kaolinite and illite were the main components. There were no significant differences between both kaolinite-bearing wastes in the sorption/desorption capacity. In spite of this, the highest pH (approx. 8) of the KaW-1 kaolinite-bearing waste and the presence of iron (hydr)oxides may be crucial in explaining the observed differences. The clay waste samples showed similar retention for all metal ions except Pb. The highest retention was observed for Cr. Lead showed a lower retention, and the difference among clay wastes was noticeable. The amounts of metal ions sorbed and desorbed by the clay wastes depended on the type of clay waste, its composition and grain size, trace metal type and concentration, solid/solution (s/l) ratio, pH and the presence of other trace metal ions. It is concluded that in addition to adsorption, precipitation controlled the removal of trace metal ions from multimetal solutions.