Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 181

Rice hull ash is an amorphous biogenetic silica. 12 Because of its sorptive and
alkali-reactive nature, rice hull ash has some unusual properties. Under alkaline
conditions, the amorphous silica reacts slowly to produce soluble silicates, which
can then react with toxic metals ions to form low-solubility metal silicates. At the
same time, the soluble silicate can react with available calcium from cement, or
other polyvalent metal ions, to set and harden the system in a controlled manner.
The advantage of this method over that of most soluble silicate processes is that the
slow, continuous generation of soluble silicate provides a reserve capacity analogous
to the action of buffers in a pH-control system. As metal hydroxides and other species
slowly dissolve in the alkaline environment of the waste form, they can then become
re-speciated as the “silicate.” The process is patented in the U.S. 13 and elsewhere.

8.2.2.3 Carbonates

Precipitation by the addition of carbonates has not been widely used for S/S. Car-
bonates decompose at low pH to release carbon dioxide and only a few metal
carbonates (e.g., cadmium, barium, and lead) are less soluble than their correspond-
ing hydroxides.

8.2.2.4 Phosphates and Trisodium Phosphate

Although many metals can form low-solubility phosphates with PO 4 3- , the results
solubility-wise are quite variable due partly to the complexity of phosphate chem-
istry. In recent years, considerable work has been done on the use of phosphates for
metal fixation. Phosphates can consist of monomeric PO 4 3- , and various polymeric
species having the potential to sequester the metals as water-soluble species. Phos-
phates alone (without any S/S reagents) can be effective in the stabilization of lead
in some wastes, and this is the most common application for phosphate. 14 The
resulting products are different from those in most stabilization processes in that
they exhibit very low solubility over a wide pH range. Cementitious materials can
be added to improve the physical characteristics of the treated waste if desired, in
which case the process is usually considered to be a cement- or pozzolan-based
process with phosphates as additives.

8.2.2.5 FeSO 4 Co-Precipitation

Co-precipitation of metals with hydrous oxides formed from iron salts has been used
in S/S. 15 The co-precipitation reduces metal solubility to levels below those obtained
with hydroxide precipitation. Optimum results are obtained when the Fe(II)/Fe(III)
ratio is between 1:1 and 1:2.

8.2.2.6 Other Inorganic Complexing Agents

Some metals or metalloids can be present as anions and require a different treatment
approach. For example, arsenic can be present as the arsenate ion, AsO 4 3- . Addition
of either calcium or ferric ion results in the formation of highly insoluble inorganic
arsenates: calcium arsenate, Ca 3 (AsO 4 ) 2 and ferric arsenate, FeAsO 4 . In the unusual