Interaction between montmorillonite and pollutants from industrial waste-waters: exchange of Zn2+ and Pb2+ from aqueous solutions

The interactions between montmorillonite (sample SAz-1 from the Source Clay Repository of the Clay Minerals Society) and Zn2+ and Pb2+ solutions of different ionic strengths (from about 10−5 to 1 M) are studied in order to observe changes in the clay-solution system and mineral crystal chemistry. In both types of solution the stationary state of exchange, attained within 20 min, depends on the solutionʹs ionic strength. The uptake can be interpreted by means of a Langmuir-type equation with Zn2+ somewhat more easily exchanged than Pb2+. The shifting of the d(001) spacing from 15.3 Å (natural sample) to 12.8 Å (Zn2+-exchanged montmorillonite) and 12.4 Å (Pb2+-exchanged montmorillonite) suggests that the kind of interlayer cation affects the c dimension of the layer. Upon heating, the exchanged montmorillonite layer collapses (d(001),Zn = 9.5 Å; d(001),Pb = 10.0 Å) and the closest packing is attained at T = 200°C for Pb2+ - and at 380°C for Zn2+-exchanged samples. In addition to reactions observed for natural and Pb2+-exchanged samples in the temperature ranges 20–250°C and 500–700°C, the thermal analyses of Zn2+-exchanged montmorillonite show a reaction at about 380°C which can be related either to strongly bonded water molecules or to “brucitic-like” interlayers or to Zn2+-exchanged octahedra.