Sorption of alkaline earth metal ions on hydrous thorium oxide

The surface charge characteristics (protonation constant, pK(a1)*, deproton ation constant, pK(a2)* and point of zero charge, PZC) of hydrous thorium o xide (HThO) prepared by direct precipitation with NaOH have been studied by batch pH-titration using mixtures of (LiCl + HCl or LiOH) solutions at a t otal ionic strength of 0.1 mol dm(-3) at 298, 323 and 343 K. While the prot onation capacities and pK(a1)* are independent of temperature, the capaciti es and pK(a2)* for the deprotonation reaction decrease with increase in tem perature. The protonation reaction shows a larger entropy change and the de protonation reaction contributes more to the overall enthalpy change. Sorpt ion 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 inc rease below the PZC and the sorption edge in general, shifts to higher pH w ith increase in temperature. Analysis of the data indicates that near neutr al hydrolytic species of Mg2+ and mono-hydroxo species of Ca2+, Sr2+ and Ba 2+ are involved in the sorption process, which could be explained on the ba sis 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 show s the sequence: Ba2+ > Ca2+ > Mg (2+) similar to Sr2+ which indicates that the surface hydroxyl groups on HThO have weak field strength.