Interaction of niobium with iron-oxide colloids and the role of humic acid

In this work, we report the sorption of Nb on iron oxides and the effect of humic acid. Iron oxides viz. goethite, hematite and magnetite were chemically synthesised and characterised by X-ray diffraction, particle size, surface area and zeta potential measurement. The sorption of Nb on all the three iron oxides was low (∼40%) at pH 1, increased to ∼ 90% at pH 8 and decreased marginally above pH 8. The effect of humic acid on the sorption was very small. Thermodynamic parameters viz. activation energy, enthalpy, entropy, free energy and sticking probability were calculated to understand the mechanism of the sorption process. Although the enthalpy was positive, the free energy change was negative i.e. the sorption was entropy driven process. The sorption followed pseudo-second-order kinetics and sticking probability model indicated that the process was chemisorption. This study is important to understand the probable migration of 94Nb (half life 20300 y) during underground storage of radioactive waste.