The influence of calcium, sodium and bicarbonate on the uptake of uranium onto nanoscale zero-valent iron particles

This work investigates the influence of calcium (Ca), sodium (Na) and bicarbonate (HCO<inf>3</inf>^-) on the uptake of uranium (U) onto nanoscale zero-valent iron particles (nZVI). Solutions tested contained U at 1mg/L, NaHCO<inf>3</inf> ranging from 0 to 100mg/L and CaCl<inf>2</inf> or NaCl ranging from 625 to 719mg/L to normalise the ionic strength. Mine water containing a similar concentration of U (1.03mg/L) and HCO<inf>3</inf>^- at 845mg/L was also tested as a natural analogue. All solutions were stored in sealed glass jars in the open laboratory with headspace comprising ambient air. Results demonstrate Ca, Na and HCO<inf>3</inf>^- as having no significant inhibitive influence on the efficacy of nZVI for initial U removal from solution, with ≥95.7% uptake recorded for all systems studied after 0.5h reaction. Similar U retention (>97.1%) was recorded throughout the entire 672h experiment for all solutions with Ca absent. In contrast, partial U desorption in the latter stages of the experiment was recorded for all solutions with Ca present, with 87.3%, 85.2% and 84.7% removal recorded after 672h for solutions containing 0, 10 and 100mg/L HCO<inf>3</inf>^-, respectively, and 10.9% removal recorded for the mine water. Maximum U removal onto nZVI was recorded as directly proportional to HCO<inf>3</inf>^- concentration for solutions with Ca absent, however, no trend was identified for the Ca-bearing solutions. Overall results demonstrate Ca as having a significant inhibitive influence on the long-term retention (e.g. >48h) of U on nZVI, which is independent of HCO<inf>3</inf>^- concentration when also present at <100mg/L.