Leaching Processes and Evaluation Tests for Inorganic Constituent Release from Cement-Based Matrices - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 239

(EDTA), nitrilotriacetic acid (NTA), citrate, and oxalate are common chelating agents
found in industrial waste streams, applied to remediate metals-contaminated soils or
sometimes detected in leachates of hazardous waste landfills. 78-81 In addition, dissolved
organic matter incorporated into the S/S materials containing certain waste types (e.g.,
sludges, soils) or transferred to the matrix from the surrounding fill may act as chelating
agents. 82
Large organic molecules surround and covalently bond with metal ions, forming
a stable complex and effectively withdrawing certain metal ions from solution. 83 The
result of chelation is that the apparent liquid solubility limit relaxes, total elemental
solubility increases (sometimes by several orders of magnitude), and the solid-liquid
partitioning is driven toward dissolution of a solid mineral phase. Although most
metal species are affected, lead, cadmium, and copper are particularly susceptible
to chelation. 84
A study on chelation of cement-solidified spent reactor decontamination resins
concluded that mobilization of cationic metal/radionuclides is significantly enhanced
at the highly alkaline pH of cement waste forms. 85 In this study, concentrations of
> 10 -4 M EDTA were shown to mobilize divalent transition metal Ni and Co, while
higher concentrations (> 10 -3 M) of picolinate mobilized Sm 3+ , Th 4+ , NpO 2 + , UO 2 + ,
and oxides of Pu. S/S treatment using highly binding chelants with cationic met-
als/radionuclides was discouraged.
10.3.2.3.3 Anions
Anions in solution may influence the chemistry of the constituents in the pore
solution through competitive adsorption, complexation, or common ion effects. For
example, the presence of sulfate and phosphate ions may result in precipitation of
metal ions such as lead, barium, and calcium, lowering the activity of these constit-
uents in the pore solution. 11,33 Conversely, the introduction of acetate, ammonium,
and chloride ions may increase solubility of some contaminants (e.g., copper, lead)
through formation of coordination complexes. 83 The increase in metal solubility at
high pH is attributed to the formation of complex metal hydroxide anions.
10.3.2.3.4 Redox Potential
Environmental reducing conditions and gradients in redox potential can alter the
chemistry of waste systems over long periods of time, leading to precipitation or
dissolution of some contaminants. 86 For example, Cr, Tc, and Mn retention is
increased under low E h conditions, 7 while retention of arsenic decreases as As(V)
is reduced to As(III). 33 Redox gradients and reducing conditions may result from
waste material characteristics, biological activity, or external sources.
The redox potential of pure portland cement by nature is mildly oxidizing, with
E h measurements between +100 and +200 mV. The few electro-active components
inherent to cement paste include dissolved oxygen and trace amounts of sulfite SO 3 2-
in pore water and small quantities of insoluble Fe 2+ and Mn 2+ . Blended cements,
especially those with significant replacement of cement with blast furnace slag, may
exhibit overall reductive capabilities. 54
A significant fraction of saturated disposal scenarios are likely to have charac-
teristically low E h 7 in the range of -200 to -500 mV. 87 Decomposition of organic