The primary goal of the Tank Side Cesium Removal (TSCR) system, under development by Washington River Protection Solutions, LLC (WRPS), is to remove entrained solids and Cs-137 (137Cs) from the Hanford tank waste supernate to expedite production of low-activity waste (LAW). Ion exchange (IX) testing of 10.9 L of waste from Hanford tank 241-AP-105 (AP-105), performed by Pacific Northwest National Laboratory, utilized a lead-lag-polish column format, with a bed volume of 10 mL per column, to decontaminate tank waste supernate using crystalline silicotitanate as the ion exchange media.
The AP-105 Cs ion exchange processing test, discussed elsewhere, resulted in a shorter transition zone (i.e., steeper load curve) than those defined by wastes from tanks 241-AP-107 and 241-AW-1021. The shorter transition zone was indicative of a matrix effect retarding Cs capacity. Therefore, aliquots of spent CST from the lead, lag, and polish columns were subjected to a digestion protocol to quantify analytes retained by the CST and extrapolate the impact on Cs capacity. The spent CST was digested using a combination of 5 M HNO3 and H2O2 with vigorous heating and stirring. Due to the radiation dose accompanying the 137Cs on the CST columns, a secondary Cs separation by ammonium molybdophosphate embedded in polyacrylonitrile (AMP-PAN) was performed to separate the 137Cs from the CST so the samples could be contact-handled for analysis outside of a shielded facility.
A summary of the inductively coupled plasma optical emission spectroscopy (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) results is shown in Table S.1, reporting selected analytes retained by the CST. Resource Conservation and Recovery Act (RCRA) hazardous metals, including Ba and Pb, were retained by the spent CST. Additionally, it was observed that the transition metals that likely occur in the tank waste as divalent ions, including Cd, Ca, Pb, Sr, and potentially Fe, are retained by the CST by IX and/or other mechanisms (e.g., precipitation/ filtration by the CST bed). The loading of these analytes decreased from the lead to lag to polish column, with the highest concentration found on the lead column. The same phenomenon was not observed for K, which was the second-most prevalent analyte retained by CST, behind Ca. The K loaded uniformly across the three columns.
Small amounts of Al were quantified in the CST digestate solutions. The mechanism of Al retention is likely due to formation of the zeolitic aluminosilicates on the surface of CST. The spent CST also retained a portion of the uranium from the IX feed.
Compositional integrity of the CST was assessed by calculating the recovery of the main CST components (Ti, Nb, and Zr) quantified in spent CST compared to the initial concentrations reported for pre-treated CST. No discernable leaching of CST components was observed in AP-105 effluent, which was validated by the quantitative recovery of the CST components in the digested CST from AP-105 processing.