Sorption Modeling of Strontium, Plutonium, Uranium, and Neptunium Adsorption on Monosodium Titanate

Abstract

We examined the ability of various equilibrium isotherms to replicate the available data for the adsorption of strontium (Sr), plutonium (Pu), uranium (U) and neptunium (Np) on monosodium titanate (MST) during the treatment of simulated and actual Savannah River Site high-level waste. The data comes from numerous experimental studies conducted between 1999 and 2002. The analysis considered 29 isotherm models from the literature. As part of this study, we developed a general method for selecting the best isotherm models. The selection criteria for rating the isotherms considered the relative error in predicting the experimental data, the complexity of the mathematical expressions, the thermodynamic validity of the expressions, and statistical significance for the expressions.The Fowler Guggenheim-Jovanovic Freundlich (FG-JF), the Fowler GuggenheimLangmuir Freundlich (FG-LF) and the Dubinin-Astashov (DA) models each reliably predicted the actinide and strontium adsorption on MST. The first two models describe the adsorption process by single layer formation and lateral interactions between adsorbed sorbates while the Dubinin-Astashov model assumes volume filling of micropores (by osmotic pressure difference). These two mechanisms include mutually exclusive assumptions. However, we can not determine which model best represents the various adsorption mechanisms on MST. Based on our analysis, the DA model predicted the data well. The DA model assumes that an initial sorption layer forms after which networking begins in the pore spaces, filling the volume by a second mechanism. If this mechanism occurs in MST, as the experimental data suggests, then we expect all the empty and closed spaces of MST to contain actinides and strontium when saturated. Prior microstructure analyses determined that the MST surface is best described as heterogeneous (i.e., a semi-crystalline outer layer on an amorphous core) or composite material for adsorption. Therefore, we expect the empty spaces (of nanometer size) between the crystalline units in the fibrous material to provide sorption area for the actinides and strontium. Additional conclusions from this study follow.Since each of the three models work reliably, we recommend use of the computationally simplest model as the primary tool until future work can differentiate between the two mechanisms. The Dubinin-Astashov model possesses a simpler mathematical form with fewer parameters and operations. WSRC-TR-2003-00180Page 5 of 66The experimental data for actual and simulated wastes generally showed consistent agreement. However, the data sets do include considerable variance from a number of causes including the following.• The plutonium sorption data appears most consistent (e.g., between actual and simulated waste) and most easily predicted. Since plutonium removal efficiency proves most important for the process design efforts, this consistency of the data proves especially beneficial.• Extremely high mass loadings of uranium on MST result in multilayer sorption behavior an...

Results of Supplemental MST Studies

“…Historical data suggests reasonable ability to predict actual waste performance based on these studies. 4 • The omission of sludge solids otherwise normally present in actual wastes (during the initial SWPF MST strike and MST reuse) will not cause the simulant waste to deviate substantially from the behavior of actual waste. Past experiments do indicate that sludge solids add complexity to the radionuclide behavior -and especially that of strontium -but with minor relative impact.…”

Review of Experimental Studies Investigating the Rate of Strontium and Actinide Adsorption by Monosodium Titanate

“…al evaluated published isotherm models for the adsorption of Sr, Pu, Np, and U onto MST using experimental data collected from numerous tests at SRNL. 6 Of the 29 isotherm models investigated, the Dubinin-Astashov (DA) model provided good predictions for the adsorption of Sr and actinides with the fewest number of parameters and operations. Subsequently, Fondeur et.…”

Sorption Behavior of Monosodium Titanate and Amorphous Peroxotitanate Materials Under Weakly Acidic Conditions

“…[19][20][21][22][23] Sorption of heavy metals and other nonradioactive metals found in industrial waste streams receives less attention, however, a number of examples can be found. Sorption of barium, chromium, mercury and zinc onto MST, [24][25][26] sorption of cadmium onto lithium, sodium, and potassium-titanates, [27,28] and sorption of copper, zinc, and nickel onto lithium-titanate are reported.…”