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Title: Kinetics and Equilibrium Sorption Models: Fitting Plutonium, Strontium, Uranium and Neptunium Loading on Monosodium Titanate (MST)

Abstract

The Dubinin-Astashov (DA) isotherm parameters for U, Pu, Sr and Np have been updated to include additional data obtained since the original derivation. The DA isotherms were modified to include a kinetic function derived by Rahn to describe sorbate loading from the beginning of sorption up to steady state. The final functions describe both kinetic and thermodynamic sorption.

Authors:
Publication Date:
Research Org.:
SRS
Sponsoring Org.:
USDOE
OSTI Identifier:
890062
Report Number(s):
WSRC-MS-2006-00127
TRN: US0604612
DOE Contract Number:
DE-AC09-96SR1850
Resource Type:
Journal Article
Country of Publication:
United States
Language:
English
Subject:
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY; ADSORPTION ISOTHERMS; KINETICS; NEPTUNIUM; PLUTONIUM; SORPTION; STRONTIUM; THERMODYNAMICS; SODIUM COMPOUNDS; TITANATES; URANIUM; SORPTIVE PROPERTIES

Citation Formats

Fondeur, F. Kinetics and Equilibrium Sorption Models: Fitting Plutonium, Strontium, Uranium and Neptunium Loading on Monosodium Titanate (MST). United States: N. p., 2006. Web. doi:10.1080/01496390600745495.
Fondeur, F. Kinetics and Equilibrium Sorption Models: Fitting Plutonium, Strontium, Uranium and Neptunium Loading on Monosodium Titanate (MST). United States. doi:10.1080/01496390600745495.
Fondeur, F. Wed . "Kinetics and Equilibrium Sorption Models: Fitting Plutonium, Strontium, Uranium and Neptunium Loading on Monosodium Titanate (MST)". United States. doi:10.1080/01496390600745495. https://www.osti.gov/servlets/purl/890062.
@article{osti_890062,
title = {Kinetics and Equilibrium Sorption Models: Fitting Plutonium, Strontium, Uranium and Neptunium Loading on Monosodium Titanate (MST)},
author = {Fondeur, F},
abstractNote = {The Dubinin-Astashov (DA) isotherm parameters for U, Pu, Sr and Np have been updated to include additional data obtained since the original derivation. The DA isotherms were modified to include a kinetic function derived by Rahn to describe sorbate loading from the beginning of sorption up to steady state. The final functions describe both kinetic and thermodynamic sorption.},
doi = {10.1080/01496390600745495},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Mar 08 00:00:00 EST 2006},
month = {Wed Mar 08 00:00:00 EST 2006}
}
  • The DA isotherm parameters for U, Pu, Sr and Np have been updated to include additional data obtained since the original derivation. The DA isotherms were modified to include a kinetic function derived by Rahn to describe sorbate loading from the beginning of sorption up to equilibrium. The final functions describe both kinetic and thermodynamic sorption. We selected the Rahn function to describe radionuclide sorption because it originates from diffusion and absorption controlled sorption. An investigation of the thermal behavior of radionuclide sorption on MST as shown by this data revealed the sorption process is diffusion (or transport) controlled (inmore » solution). Transport in solution can in theory be accelerated by vigorous mixing but the range of available mixing speed in the facility design will probably not be sufficient to markedly increase radionuclide sorption rate on MST from diffusion-controlled sorption. The laboratory studies included mixing energies hydraulically-scaled to match those of the Actinide Removal Process and these likely approximate the range of energies available in the Salt Waste Processing Facility.« less
  • 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 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 themore » expressions. The Fowler Guggenheim-Jovanovic Freundlich (FG-JF), the Fowler Guggenheim-Langmuir 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 later al 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. 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.« less
  • A possible disposition pathway for the residue from the abandoned In-Tank Precipitation (ITP) sends the material from Tank 48H in increments to Saltstone via aggregation in Tank 50H. After entering Tank 50H, the amount of fissile material sorbed on MST may increase as a result of contacting waste solutions with dissolved uranium and plutonium. SRNL recommends that nuclear criticality safety evaluations use uranium and plutonium loadings onto MST of 14.0 {+-} 1.04 weight percent (wt %) for uranium and 2.79 {+-} 0.197 wt % for plutonium given the assumed streams defined in this report. These values derive from recently measuredmore » for conditions relevant to the Actinide Removal Process (ARP) and serve as conservative upper bounds for uranium and plutonium loadings during the proposed transfers of MST from Tank 48H into Tank 50H.« less
  • High-Level Waste (HLW) is a waste associated with the dissolution of spent nuclear fuel for the recovery of weapons-grade material. It is the priority problem for the U.S. Department of Energy's Environmental Management Program. Current HLW treatment processes at the Savannah River Site (Aiken, SC) include the use of monosodium titanate. The local structural speciation of sorbed U varied with loading but not for Sr. Sorbed Sr exhibited specific adsorption as partially-hydrated species whereas sorbed U exhibited specific adsorption as monomeric and dimeric U(VI)-carbonate complexes. Sorption proved site specific. These differences in site specificity and sorption mechanism may account formore » the difficulties associated with predicting Sr and U loading and removal kinetics using MST.« less
  • High-Level Radioactive Waste (HLW) is the priority problem for the U.S. Dept. of Energy's Environmental Management Program. Current HLW treatment processes at the Savannah River Site (Aiken, SC) include the use of monosodium titanate (MST, similar to NaTi{sub 2}O{sub 5}xH{sub 2}O) to concentrate radioactive strontium (Sr) and actinides. Mechanistic information about radionuclide uptake will provide us with insight about the reliability of MST treatments. We characterized the morphology of MST and the chemistry of sorbed Sr{sup 2+} and uranium [U(VI)] on MST with x-ray based spectroscopic and electron microscopic techniques. Sorbed Sr{sup 2+} exhibited specific adsorption as partially-hydrated species, whereasmore » sorbed U exhibited site-specific adsorption as monomeric and dimeric U(VI)-carbonate complexes. These differences in site specificity and mechanism may account for the difficulties associated with predicting MST loading and removal kinetics.« less