Identification of dynamic leaching kinetics of stabilized, water-soluble wastes.

A one-dimensional diffusion model based on Fick's second law with a non-zero surface concentration at the solid-solution interface was developed to calculate effective calcium and sulfate diffusion coefficients of composites placed in saltwater. A regression method was used to identify the leaching kinetics. The regression method decomposes the stabilized PG leaching processes into diffusion, surface wash-off, and immediate and long-term precipitation. The immediate surface precipitation of both calcium and sulfate ions occurred only in three of the PG composite combinations. The effective diffusion coefficients of calcium (2.58-4.68 x 10(-13) m2 s(-1)) and sulfate (2.77-5.02 x 10(-13) m2 s(-1)) obtained from the regression method are similar to those obtained from methods of cumulative flux and daily flux associated with the simple diffusion model, provided that the leaching processes do not deviate significantly from that of the diffusion. The ratio (1.13) of effective sulfate to calcium diffusion coefficients obtained using the regression analysis is statistically consistent with the theoretical value (1.31), which further justifies the regression method. The research also implies that the leaching processes of calcium and sulfate ions stop after a certain period of time (300-900 d for calcium and 80-170 d for sulfate) and that the precipitations of calcium and sulfate affect the leaching processes. The regression method can be used to identify the leaching mechanisms and to predict the long-term stability of the stabilized wastes.

Stabilization of phosphogypsum using class C fly ash and lime: assessment of the potential for marine applications.

Phosphogypsum (PG, CaSO(4).H(2)O), a solid byproduct of phosphoric acid manufacturing, contains low levels of radium ((266)Ra), resulting in stackpiling as the only currently allowable disposal/storage method. PG can be stabilized with class C fly ash and lime for potential use in marine environments. An augmented simplex centroid design with pseudo-components was used to select 10 PG:class C fly ash:lime compositions. The 43cm(3) blocks were fabricated and subjected to a field submergence test and 28 days saltwater dynamic leaching study. The dynamic leaching study yielded effective calcium diffusion coefficients (D(e)) ranging from 1.15 x 10(-13) to 3.14 x 10(-13)m(2)s(-1) and effective diffusion depths (X(c)) ranging from 14.7 to 4.3mm for 30 years life. The control composites exhibited diametrical expansions ranging from 2.3 to 17.1%, providing evidence of the extent of the rupture development due to ettringite formation. Scanning electron microscopy (SEM), microprobe analysis showed that the formation of a CaCO(3) on the composite surface could not protect the composites from saltwater intrusion because the ruptures developed throughout the composites were too great. When the PG:class C fly ash:lime composites were submerged, saltwater was able to intrude throughout the entire composite and dissolve the PG. The dissolution of the PG increased the concentration of sulfate ions that could react with calcium aluminum oxides in class C fly ash forming additional ettringite that accelerated rupture development. Effective diffusion coefficients and effective diffusion depths alone are not necessarily good indicators of the long-term survivability of PG:class C fly ash:lime composites. Development of the ruptures in the composites must be considered when the composites are used for aquatic applications.