INERTIZATION OF CADMIUM ENRICHED HAZARDOUS RESIDUE: AN INERT APPROACH - Management of Hazardous Residues Containing Cd - page 143

waste fixation are Portland cement, Pozzolans, Bentonite and Zeolite as
reported by Page et al. (2003). They explained that stabilization and
solidification with cement or pulverized fuel ash is a viable solution and
concluded that the disadvantage of this option is the highly embodied energy
from cement, transportation of the additives and increased bulk material for
disposal.
3.6.1. CementBased Technique (Retention of Cadmium in Cements)
The retention of cadmium in cements is based on immobilization
mechanism. The main immobilization mechanism of inorganic pollutants is:
(1) hydroxide precipitation due to high pH, (2) silicates precipitation, (3)
incorporation into calcium containing hydrated phases, and (4) sorption onto
high surface area C-S-H (Macpee and Glasser 1993).
Many reported studies on the immobilization of cadmium in Portland
cement. Toxicity control leaching protocol (TCLP) tests indicate that the
retention is effective (at contents less than 0.1 mg/L, i.e. 9 X 10 -6 mol/L) when
cadmium is introduced as insoluble cadmium hydroxide or soluble cadmium
nitrate (Cartledge et al., 1990; Herrera et al., 1992). Cartledge et al., 1990 have
found that cadmium (Cd/cement mass ratio = 0.1) introduced as hydroxide is a
source of nucleation sites for the precipitation of C-S-H and portlandite
[Ca(OH) 2 ]. When cadmium is introduced as a soluble salt (nitrate of chloride),
precipitation of Cd(OH) 2 occurs immediately as the pH increases due to
clinker dissolution. This suggested that the main immobilization mechanism is
just the physical entrapment.
3.6.2. Glass CeramicsBased Technique
Several matrices have been studied for immobilization of toxic element,
such as glasses, ceramics and cements. Glass ceramics are also envisaged due
to their high chemical flexibility or lower diffusion coefficient. Moreover,
their high density of grain boundary makes them very resistive to brutal crack
(Shamy and Douglas, 1972; Höland and Beall, 2002; Watchman 1996). In
glass-ceramics, when toxic elements are incorporated in specific sites inside
crystals (embedded by a glassy matrix), this process corresponds to a double
barrier protection from any release of the hazardous elements into the
environment.
Calcium-rich glass-ceramics appear as good candidates to immobilize
cadmium and lead because calcium has a large ionic radius and then calcium
crystal sites are expected to host voluminous cations like those of Cd and Pb.
Note that such Ca sites (in silicate perovskite) were shown to host large