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 Zn 2 + and Pb 2 + 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 solution' s ionic strength. The uptake can be interpreted by means of a Langmuir-type equation with Zn 2 ÷ somewhat more easily exchanged than Pb 2 +. The shifting of the d(ool ) spacing from 15.3 (natural sample) to 12.8 ,~ (Zn2+-exchanged montmorillonite) and 12.4/~ (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~ooJ ~.zn = 9.5 ~,; d~oo~).ph = 10.0 ,~) and the closest packing is attained at T= 200°C for Pb 2 +- and at 380°C for Zn2+-exchanged samples. In addition to reactions observed for natural and Pb 2 ÷ -exchanged samples in the temperature ranges 20- 250°C and 500-700°C, the thermal analyses of Zn 2 ÷-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 Zn 2 +-exchanged octahedra.