The influence of layer charge on Zn2+ and Pb2+ sorption by smectites

Two smectites with different layer charge localization were used to characterize the chemical and structural aspects of minerals treated with 1.9 x 10(-2) M and 1 M heavy metal solutions (Zn2+ and Pb2+). Natural and treated smectites were analysed by: (1) X-ray powder diffraction first at 25 degrees C and relative humidity of 60% and then at increasing temperature up to 400 degrees C; (2) thermogravimetric and differential thermal analysis; (3) X-ray fluorescence; (4) atomic absorption and UV visible spectrophotometries. The sorption of both metals depends on 2:1 layer features and is enhanced when the total layer charge is mostly due to substitution in the octahedral sheet. The variability between Zn2+- and Pb2+-treated smectites in the interlayer water content depends both upon cation electrostatic features and upon smectite layer charge localization. The greatest water content was observed in [Zn2+]1 M-treated smectite showing charge imbalance in the octahedral sheet. In smectites treated with [Zn2+]1M solutions, an additional thermal reaction occurs at a temperature of similar to 380 degrees C, and the d(001) spacing, which is 10 Angstrom at a temperature of similar to 200 degrees C, gradually decreases in the temperature range 200-400 degrees C. In smectites treated with 1 M [Pb2+] solutions this value is reached at a temperature of similar to 150 degrees C and remains quite constant up to 400 degrees C.