In-Situ Monitoring and Solubility Modeling of Sodium Phosphate Using Online PAT Tools
Author(s)
Cardenas Ocampo, Viviana
Advisor(s)
Editor(s)
Collections
Supplementary to:
Permanent Link
Abstract
The Hanford site contains 56 million gallons of radioactive waste that was produced starting in the Manhattan project and ending in the cold war. The Hanford tank waste consists of supernatant liquid, salt cake, and sludge. The alkaline liquid phase is rich
in salts, including sodium phosphate. The blockages have previously caused delays in operations and an approximate cost of 40 million dollars from the replacement of clogged
underground piping. The crystallization of material that has previously clogged pipelines at Hanford may potentially crystallize during processing at the Hanford Waste Treatment Plant. However, optical spectroscopy techniques, such as infrared and Raman spectroscopy, have demonstrated potential for online monitoring of solutions and slurries and may be able to detect inadvertent crystallization quickly or before crystallization has occurred. In this work, in-situ and ex-situ measurements are used to identify the hydrate of sodium phosphate that crystallizes at high pH and to build a solubility model based on the Pitzer framework.
In this study, Process Analytical Technology (PAT) tools were employed to monitor different forms of sodium phosphate that crystallize from cooling crystallization at different concentrations, temperatures, and cooling rates. A Mettler Toledo EasyViewer probe visually inspected the crystals, facilitating differentiation of morphologies. Raman spectroscopy
was used to analyze the solid and solution phases; Raman spectroscopy can distinguish phosphate and hydrogen phosphate anions in the solution phase while also detecting
solid crystals. Attenuated Total Reflectance – Fourier Transform Infrared (ATR-FTIR) spectroscopy was expected to measure the solution phase only; ATR-FTIR absorbance
varies directly with solute concentration, offering insights into dissolved phosphate ions.
The sodium phosphate crystalline phases formed were characterized using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Raman Spectroscopy, and X-ray Diffraction (XRD). DSC and TGA results indicated the formation of hydrated trisodium phosphate species, and Raman spectroscopy data, both in situ and ex situ, were consistent with these thermal analyses. However, XRD analysis suggests that the crystallized
phases may include trisodium phosphate hemihydrate and disodium hydrogen phosphate. Raman spectroscopy, collected in air, showed PO3−4 peaks at ∼942 and 1006 cm−1
and no detectable HPO2−4 features. Because the crystals progressively lose water under ambient conditions, they trend toward lower hydrate forms over time. Therefore, in situ Raman and ATR-FTIR measurements are essential for capturing the true crystalline and solution species at the moment of formation before washing and air-drying alters them.
The sodium phosphate crystalline phases formed were characterized using Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Raman Spectroscopy, and X-ray Diffraction (XRD). DSC and TGA results indicated the formation of hydrated trisodium phosphate species, and Raman spectroscopy data, both in situ and ex situ, were consistent with these thermal analyses. However, XRD analysis suggests that the crystallized
phases may include trisodium phosphate hemihydrate and disodium hydrogen phosphate. Raman spectroscopy, collected in air, showed PO3−4 peaks at ∼942 and 1006 cm−1
and no detectable HPO2−4 features. Because the crystals progressively lose water under ambient conditions, they trend toward lower hydrate forms over time. Therefore, in situ Raman and ATR-FTIR measurements are essential for capturing the true crystalline and solution species at the moment of formation before washing and air-drying alters them.
To interpret and generalize the experimental solubility behavior of sodium phosphate in alkaline media, a thermodynamic model was developed to relate solubility to temperature and ionic interactions. Experimental solubility data across a range of temperatures (25–55◦C) and fixed sodium hydroxide concentration (3 molal) were used to parameterize a thermodynamic solubility model based on the Pitzer framework. The resulting model captured the complex interactions within the Na3PO4-NaOH-H2O system, demonstrating good accuracy within the studied temperature range.
This research demonstrates that integrating in-situ ATR-FTIR and Raman spectroscopy with EasyViewer provides a comprehensive, real-time picture of phosphate crystallization
under strongly alkaline conditions. The EasyViewer offered continuous visualization of crystal size and shape evolution. ATR-FTIR tracked the solution phase PO3−4 band (∼1003
cm−1) with baseline-corrected absorbance that is linearly proportional to concentration, allowing us to quantify dissolved phosphate throughout each heating-cooling cycle. Simultaneously, Raman spectroscopy captured both solid and liquid signatures: the PO3−4 v1 stretch shifted from 936 cm−1 in clear solution to 942 cm−1 once crystals appeared, and a 1006 cm−1 v3 band appeared only in the presence of a slurry, evidence that a trisodium phosphate hydrate was the phase crystallized. The absence of HPO2−4 Raman bands further confirmed that the solid crystallized consisted only PO3−4 species.
Sponsor
Date
2025-09-24
Extent
Resource Type
Text
Resource Subtype
Thesis (Masters Degree)