The Hanford Site currently stores approximately 56 million gallons of radioactive tank waste that varies in chemical composition and contains highly radioactive fission products like Cs-137. The Tank-Side Cesium Removal (TSCR) system implements a series of ion exchange columns to process the material, removing 99.9% of the radioactivity from the waste. Specifically, the TSCR system filters solid materials and utilizes the sorbent crystalline silicotitanate (CST) to remove Cs-137 from the supernate before immobilization and vitrification at the Waste Treatment Plant (WTP) Low-Activity Waste (LAW) facility. Concentrations of group 1 alkali cations like sodium and potassium have demonstrated an impact on the capacity of Cs uptake in CST, but the effects of various anion compositions is lesser-known. To better understand how anions like hydroxide and nitrite affect CST performance, batch contacts were performed with and without potassium at varying anion concentrations with a sodium concentration of 5.6 M to mimic TSCR operating conditions. Results showed that optimizing anion composition can double the Cs uptake of CST. Distribution coefficients decreased with increased concentrations of nitrate whereas hydroxide, nitrite, and carbonate exhibited increased distribution coefficients at higher concentrations. Additionally, a method adapted from Marcos-Arroyo was developed to interpolate Cs activity coefficients between anion compositions. The observed impacts of anion composition and the developed interpolation method provide valuable insights for ongoing cleanup efforts at the Hanford site and offer a deeper understanding to optimize future TSCR operations.
