Sorption of alkaline earth metal ions on hydrous thorium oxide

The surface charge characteristics (protonation constant, pK(a1)*, deprotonation constant, pK(a2)* and point of zero charge, PZC) of hydrous thorium oxide (HThO) prepared by direct precipitation with NaOH have been studied by batch pH-titration using mixtures of (LiCl + HCl or LiOH) solutions at a total ionic strength of 0.1 mol dm(-3) at 298, 323 and 343 K. While the protonation capacities and pK(a1)* are independent of temperature, the capacities and pK(a2)* for the deprotonation reaction decrease with increase in temperature. The protonation reaction shows a larger entropy change and the deprotonation reaction contributes more to the overall enthalpy change. Sorption of Mg2+, Ca2+, Sr2+ and Ba2+ at 298, 323 and 343 K from 10(-3) mol dm(-3) of respective chloride solutions as a function of pH shows an abrupt increase below the PZC and the sorption edge in general, shifts to higher pH with increase in temperature. Analysis of the data indicates that near neutral hydrolytic species of Mg2+ and mono-hydroxo species of Ca2+, Sr2+ and Ba2+ are involved in the sorption process, which could be explained on the basis of the surface hydrolysis model. Sorption of alkaline earth metal ions as a function of concentration in the range 10(-3) to 10(-2) mol dm(-3), at initial pH of 7, 8 and 9 and analysed by the D'Arcy and Watt equation shows the sequence: Ba2+ > Ca2+ > Mg (2+) similar to Sr2+ which indicates that the surface hydroxyl groups on HThO have weak field strength.