Fixation of Zn2+ and Pb2+ by a Ca-montmorillonite in brines and dilute solutions: Preliminary results

This paper presents preliminary results on the fixation of Zn2+ and Pb2+ by a purified Wyoming Ca-montmorillonite, in different experimental conditions: variable (3 · 10−5 < I < 3 · 10−2M or constant (I = 3.10−2 M) low ionic strength solution, and high (I = 3 M) ionic strength solutions at 20°C and 50°C. In low ionic strength solutions, the fixation of Zn2+ and Pb2+ appears to be controlled by ionic exchange mechanism; Zn2+ and Pb2+ adsorption are strongly favoured over Ca2+ adsorption, Pb2+ being more strongly fixed than Zn2+. In high ionic strength solutions, for both metals, amounts of exchanged heavy metals are very small. The fixation is higher in 1 M CaCl2 than in 3 M NaCl solutions. Temperature has different effects on zinc and lead fixation. At 50°C, all the adsorption isotherms are shifted to higher values, except for lead in 1 M Cacl2 solutions. XRD characterizations of samples exchanged in dilute solutions show a decrease of hydration states with increasing Zn2+ and Pb2+ contents in the interlamellar space. Systematic determinations of CEC were performed on these samples, using Cu(EDA)2Cl2. The CEC decreases with increasing Pb2+ in clay, indicating that a part of exchanged Pb2+ ecomes unexchangeable with (Cu(EDA)2)2+. On Zn-exchanged samples, the CEC remains constant.