Sorption of niobium on colloidal silica and the effect of humic acid

Sorption of niobium on silica colloids was studied using radiotracer technique in the pH range of 2–11 in NaClO4 medium. 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 niobium was >99 % in the pH range of 3–9 and was slightly lower (~95 %) below pH 3 and above pH 9. The quantitative sorption in the pH range 3–9 could be explained by surface complexation model. The small reduction in sorption was attributed to the existence of small fraction of niobium as cationic species at pH ≤ 2 and anionic species above pH 9. Presence of humic acid was found to have negligible effects on the sorption characteristics.