Sorption of ²³¹Pa on silica and the effect of humic acid

Sorption of protactinium on silica colloids was studied in the pH range of 1 to 12 in NaClO₄ medium using radiotracer technique. Silica was characterized using X-ray diffraction, light scattering and surface area measurements. The point of zero charge for silica colloids was about pH 2. The sorption of protactinium was about 98% in the pH range of 3 to 9 and was lower (70-80 %) below pH 3 and above pH 9. The quantitative sorption in the pH range 3 to 9 could be explained by surface complexation model. The reduction in sorption was attributed to electrostatic repulsion as the fraction of protactinium exists as cationic species at pH ≤ 2 and anionic species above pH 9. There was reduction in the sorption of protactinium in the presence of humic acid below pH 2 and above pH 10. Sorption of protactinium on silica in presence of 0.05M HF was about 99% between pH 3 to 8 and below 30% in the low and high pH region. Isotherm study revealed an exponential decrease in protactinium activity in solution with increase in silica. (author)