Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 98

6.4.5 Conclusion........................................................................................ 135
References.............................................................................................................. 135

6.5 Hydroceramic Concretes.............................................................................. 137
6.5.1 Introduction ...................................................................................... 137
6.5.2 Hydroceramics versus Geopolymers ............................................... 138
6.5.3 Manufacture of Hydroceramic Concrete ......................................... 138
6.5.3.1 Materials ........................................................................... 138
6.5.3.2 Mixing............................................................................... 141
6.5.3.3 Curing ............................................................................... 141
6.5.3.4 Waste Pretreatment ........................................................... 142
6.5.4 Leach Behavior ................................................................................ 142
6.5.4.1 Intrinsic Solubility ............................................................ 142
6.5.4.2 Standard Leach Protocols ................................................. 143
6.5.5 Application ....................................................................................... 148
References.............................................................................................................. 148

6.1 PHOSPHATE STABILIZATION AND

IMMOBILIZATION OF HEAVY METAL AND
RADIONUCLIDE CONTAMINANTS

Robert C. Moore

Phosphate treatment has been evaluated for many applications including immobili-
zation of contaminants in contaminated soils, stabilization of incineration and mine
wastes, and as a waste form for heavy metal and mixed waste. As phosphates, certain
contaminants have been shown to be thermodynamically stable and insoluble over
most conditions encountered in the environment. 1-4 Contaminants amenable to phos-
phate treatment include lead, cadmium, zinc, copper, uranium, neptunium, pluto-
nium, and europium. The solubility of these contaminants as phosphates is substan-
tially less than in other forms. For example, the solubility of lead phosphate is more
than 40 orders of magnitude lower than lead as a carbonate, oxide, or sulfate. Table
6.1.1 gives a list of metal phosphate solids along with their aqueous solubility.
In addition to decreased solubility, phosphate treatment decreases the bioavail-
ability of contaminants. 7-11 Phosphate has been demonstrated to significantly reduce
the bioavailability of cadmium, lead, copper, zinc, and arsenic. 12,13 Insoluble forms
of lead, including phosphates, in lead mine waste may have resulted in unexpected
low lead levels in the blood of children living in proximity. 9
Dissolved, liquid forms of phosphate are used when fast kinetics are desired, while
solid forms slowly release and provide a constant source of phosphate. 12,14 Dissolved
forms include phosphoric acid and aqueous solutions of soluble phosphate salts; solid
forms include natural phosphate rock and calcium apatite. The solubilization of the
contaminant is often the rate-controlling step in phosphate treatment. Low pH favors
faster kinetics because of the increased solubility of metal compounds.
Figure 6.1.1 illustrates different methods of implementing in situ treatment.
Applying a solid or soluble phosphate compound to the surface or mixing the