Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 190

More stable complexes such as citrates, EDTA complexes, oxalates, gluconate,
and a wide variety of non-chelate organometallics can sometimes be sorbed onto
activated carbon, clays, or other similar sorbents. This approach has worked for
nickel and chromium and probably arsenic and cadmium chelates. 66 The sorbed
organometallic compounds may then be solidified using portland cement or another
solidification reagent.
If neither of these approaches work then the metal must be released from the
organic so that typical treatment processes for metals can be applied. This usually
involves oxidative destruction of the organic molecule using strong oxidants such
as potassium permanganate, potassium persulfate, or hypochlorite, and the possible
use of elevated temperature. A plausible reaction for the hypochlorite oxidation/destruc-
tion of the EDTA ligand, necessary for precipitation/removal of Ni 2+ (aq) → Ni(OH) 2 (s),
is depicted below:

NiEDTA 2- + 12 ClO - → H 2 NCH 2 CH 2 NH 2 + 8 CO 2 + Ni 2+ (aq) + 12 Cl - + 4 OH -
↓
Ni(OH) 2 (s)

With either oxidizing or reducing agents, species other than the target compound
may compete for the reagent. As a result, large additions of oxidizing agent may be
necessary, and this becomes very expensive and time consuming. Therefore, a non-
oxidative method for handling complexed metals, such as sorption, is preferred.
Other problems can complicate the oxidative destruction of organic complexes. For
example, if chromium is present, it will be oxidized to Cr +6 and must then be reduced
before precipitation.

8.6 PROCESSING AND ANTI-INHIBITION AIDS

The first additives used probably came from the concrete industry. More and more,
additives are being used in chemical systems for purposes other than stabilization.
Cement-based systems may contain many additives, such as given in Table 8.1. The
purposes of additives, in addition to those already discussed, are many and varied.
Among the more common purposes are control of setting, retarders, water-reducing
agents, increase or control viscosity, and physical property development.
Pozzolan/portland cement processes consist primarily of silicates from poz-
zolanic-based materials like flyash, kiln dust, pumice, or blast furnace slag and
cement-based materials like portland cement. 67 These materials chemically react
with water to form a solid cementitious matrix, which improves the handling and
physical characteristics of the waste. They also raise the pH of the water, which may
help precipitate and immobilize some heavy metal contaminants. Pozzolanic and
cement-based binding agents are typically appropriate for inorganic contaminants.
The effectiveness of this binding agent with organic contaminants varies.