Concomitant Zn–Cd and Pb retention in a carbonated fluvio-glacial deposit under both static and dynamic conditions

Abstract

The chemical and physical processes involved in the retention of 10 −2 M Zn, Pb and Cd in a calcareous medium were studied under saturated dynamic (column) and static (batch) conditions. Retention in columns decreased in order: Pb ≫ Cd ≈ Zn. In the batch experiments, the same order was observed for a contact time of less than 40 h and over, Pb ≫ Cd > Zn. Stronger Pb retention is in accordance with the lower solubility of Pb carbonates. However, the equality of retained Zn and Cd does not fit the solubility constants of carbonated solids. SEM analysis revealed that heavy metals and calcareous particles are associated. Pb precipitated as individualized Zn–Cd–Ca– free carbonated crystallites. All the heavy metals were also found to be associated with calcareous particles, without any change in their porosity, pointing to a surface/lattice diffusion-controlled substitution process. Zn and Cd were always found in concomitancy, though Pb fixed separately at the particle circumferences. The Phreeqc 2.12 interactive code was used to model experimental data on the following basis: flow fractionation in the columns, precipitation of Pb as cerrusite linked to kinetically controlled calcite dissolution, and heavy metal sorption onto proton exchanging sites (presumably surface complexation onto a calcite surface). This model simulates exchanges of metals with surface protons, pH buffering and the prevention of early Zn and Cd precipitation. Both modeling and SEM analysis show a probable significant decrease of calcite dissolution along with its contamination with metals.