Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 179

can also be used, among other things, for pH adjustment. Buffers, such as calcium
carbonate, sodium carbonate, soda ash, and magnesium oxide, are used to keep the
pH within the desired range.
Many metal contaminants of concern form highly insoluble hydroxides in a
basic cement matrix. This is by far the most common method used to reduce
solubility of metals. The alkalinity of this matrix can be supplemented with sodium
hydroxide and other alkali hydroxides, if necessary.

8.2.2 S PECIATION /P RECIPITATION /R E -S PECIATION

For metals where pH adjustment and hydroxide precipitation is insufficient, some
other additives are used to preferentially speciate metals into less soluble forms.
Additives used to speciate, precipitate, and re-speciate include sulfides, sulfur and
organo-sulfur compounds; soluble silicates and rice hull ash; carbonates; phosphates
(especially important for lead) and trisodium phosphate; FeSO 4 co-precipitation;
other iron compounds (also used for oxidation/reduction); other inorganic complex-
ing agents; and organic complexing agents. Sulfides and silicates are used more than
carbonates, phosphates, and iron co-precipitation.

8.2.2.1 Sulfides, Sulfur, and Organo-Sulfur Compounds

Metal sulfides are not amphoteric and, as such, treatment of metal cations with sulfide
(usually Na 2 S or NaHS, occasionally CaS) or sulfide-producing reagents can reduce
the amount of leachable metal contaminants significantly below that possible with
simple hydroxide precipitation. Figure 8.1 shows typical solubility behavior of some
metal sulfide constituents of concern in environmental work. It should be understood,
however, that these curves were determined for the pure metal sulfides in water. The
actual solubility minima for individual metals will vary somewhat in complex systems.
However, metal sulfides can resolubilize in an oxidizing environment. 4 And the pH
must be kept above 8 to prevent possible evolution of hydrogen sulfide gas. The sulfide
should be added before any S/S reagents because the calcium, magnesium, and iron
present in these reagents will compete for the soluble sulfide ion.
Elemental sulfur, sulfides, and organic sulfides have been used to reduce the
solubility of mercury compounds through the formation of insoluble mercuric sul-
fide. The most common way to reduce mercury solubility is by the use of various
sulfur compounds, typically sodium or calcium sulfide. Both the mercury hydroxide
and oxide are too soluble (see Figure 8.2 generated from published data). 5 In an
alkaline system, sulfur and OH - react to form polysulfide ion, S x 2- , that in turn reacts
with polyvalent metal ions to form very low-solubility compounds. Mercury solu-
bility does increase with the concentration of excess dissolved sulfide anions, similar
to increasing metal solubility with excess hydroxide anions at high pH, so it is
important to not use a large excess of sulfide.
Organo-sulfur compounds are primarily used for wastes containing mercury.
Organo-sulfur compounds such as hexadecyl mercaptan also appear to work well. 6,7
Other reagents that have been used include thiourea 8 and polydithiocarbamates. 9