Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 178

8.1 INTRODUCTION

The commonly used binder system for stabilization and solidification (S/S) of wastes
is described in Chapters 4 through 6. As discussed in Chapter 7, many ingredients
in waste may interfere with the setting and hardening in different ways. In order to
improve the efficacy of S/S of concerned contaminants in the waste, many additives
have been tried out in S/S processes, and a number of them are used commercially,
especially at treatment, storage, and disposal facilities (TSDFs). Additives can be
grouped into three basic categories: (1) metal stabilizers, (2) organic stabilizers, and
(3) processing and anti-inhibition aids.
Reaction products that form can be dependent on competing reactions and on
the order of addition of reactants. The rate of addition, degree of mixing, and
temperature may also affect the nature of the reaction products. Thus, the nature,
dosage, and timing for addition of an additive may have a significant effect on the
performance of the S/S waste forms.

8.2 METAL STABILIZATION

In the management of hazardous waste, more is known regarding the treatment of
metals than about the fixation, destruction, or immobilization of any other hazardous
constituent group. Metals cannot be destroyed and so must be converted to their
least soluble or reactive form to prevent reentry into the environment. The mecha-
nisms for metal stabilization consist of pH control and buffering, speciation/precip-
itation/re-speciation, oxidation/reduction, and sorption/ion exchange. Usually a com-
bination of mechanisms is active and the metals are often not present as simple
compounds.

8.2.1 P H C ONTROL AND B UFFERING

Amphoteric metals form compounds that are soluble in both acidic and basic solu-
tions. 1 Most metals are amphoteric and exhibit minimal solubility somewhere in the
range of pH 8 to 11, 2 as shown in Figure 7.3. It should be understood, however, that
these curves were determined for the pure metal hydroxides in water. The actual
solubility minima for individual metals will vary somewhat in complex systems. For
precipitation to occur with optimum effect, the binding reagent should be added in
such proportions as to ensure that optimum pH conditions are maintained for the
main contaminants present. 3 If there are several metal contaminants, the final desired
pH must be a compromise because no two metals have minima at the same pH. If
this is not adequate, speciation or other treatment alternatives may be needed.
In order to adjust the pH of the system within a range where heavy metals have
the lowest solubility, additives can be added to control and buffer pH, including
acids, alkalis, lime, caustic soda, soda ash, and ferrous sulfate. Acids can be used
to solubilize metals so that treatment methods can be effective. If required, the most
common acids used in S/S are sulfuric and hydrochloric.
In addition to pH adjustment, lime supplies additional calcium for reaction and
can react with certain interfering organics (see Section 8.4). Flyash and kiln dust