Stabilization/Solidification Additives - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 186

effectiveness was premised to be a function of the pore dimensions, chemical compo-
sition, and cation concentration of each zeolite. In addition, the uranium remained sorbed
under redissolving conditions. Clinoptilolite proved effective in sorbing 60 Co from
solution. 40 Surfactant modification made clinoptilolite effective in sorbing anions (chro-
mate). 37,38 The sorbed chromate was resistant to water extraction.
Clinoptilolite or other zeolites have proven effective in sorbing metals (Cd, Cr, Cu,
Fe, Mn, Ni, Pb, Zn) when mixed with contaminated compost, soil, or waste. 42-45 Even
though sorption makes the metals less leachable by acids, it has been demonstrated that
metals (Pb and Zn) sorbed on clinoptilolite are more bioavailable and biotoxic than the
same metals in untreated soil. 42 Clinoptilolite decreased cesium leaching from a cement-
flyash grout, but increased anion leaching. 28
The results of Colella 40 and Serne et al. 28 are in agreement on clinoptilolite
increasing cesium leach resistance. 28,40 The strontium leach resistance did not
increase upon adding clinoptilolite; but it is not clear this is a confirmation, since
the cement-flyash grout gave such a high baseline leach resistance for strontium. In
summary, clinoptilolite is expected to increase the leach resistance of cesium, lead,
and cobalt, but not strontium, cadmium, copper, and zinc cations, or any anions
(chromate, pertechnetate, selenate, and nitrate) without surfactant modification of
the surface. Zeolites do increase the leach resistance of uranium, but apparently their
effectiveness depends on the zeolite pore size.

8.2.4.4 Vermiculite and Other Sorbents

Vermiculite strongly sorbed cesium and strontium. 48 More strontium sorbed as the
pH increased, but the sorbed strontium readily extracted into 0.1 N EDTA. 48 The
same source reported cesium sorption being unaffected by pH and the EDTA solu-
tion, 48 whereas another source reported that the cesium distribution ratio increased
from about 10 to 400 as the pH increased from about 2 to 12. 49 The reason for the
discrepancy is not clear, but there are differences between the two studies; e.g.,
source of vermiculite. Both sources indicated high affinity of vermiculite for cesium
at high pH, which may be important agreement for the purpose of cement stabili-
zation. The sodium form vermiculite was superior to both the potassium form and
the calcium form for cesium sorption. 49 Incomplete conversion of vermiculite into
organophilic clay allowed the simultaneous sorption of both anionic radioiodine and
cationic radio-cesium and strontium, but the vermiculite distribution ratio signifi-
cantly decreased for both cations. 50
Cesium was displaced from vermiculite saturated with cesium by the following
cations, presented in the order from greatest to least displacement: 51

K > Ce ≈ Y ≈ La > Sr ≈ Ca ≈ Ba > Li
K > Na > Cs (isotopic exchange) ≈ Li

Potassium displaced about 40% of the cesium loaded on vermiculite, greatest
of all the ions studied. Only about 10% was displaced during isotopic exchange,
with more being displaced if the vermiculite was not saturated with cesium. Ver-
miculite columns removed 99.5% of 137/134 Cs and 60 Co from aqueous solution, but