The effect of changes in pH
and the presence of ligands on the uptake of zinc ions by three types of clay
mineral (kaolinite, illite and montmorillonite) has been investigated. In
alkaline media the clay suspension acts as a nucleation centre for polymeric
hydroxy species, and the major role of many ligands is to mask the
precipitation process. Uncharged and negatively charged species are not sorbed
to any measurable extent. In acidic media the adsorption capacity of the clays
for zinc increases with pH and possible mechanisms are considered. For
kaolinite and illite the controlling process appears to be the attachment of
hydroxy species to particular sites on the particle edges; with montmorillonite
ion exchange at negative lattice sites appears predominant. Of particular
interest is the apparent affinity between montmorillonite and species
containing nitrogen functional groups.
The role of niobium carbides in the localised corrosion initiation of 20Cr-25Ni-Nb advanced gas-cooled reactor fuel cladding