DISSOLUTION KINETICS OF THO2 IN ACID AND CARBONATE MEDIA

The dissolution kinetics of microcrystalline ThO2 in acidic and carbon ate media has been investigated using a thin-film continuous flow reac tor. The dissolution rate of microcrystalline ThO2 is proportional to logH+ in the pH range 2.4-3.5, and proportional to logCO32- in the logCO32- range -1.7 to -3.7. The dissolution rates were too low to be measured in the region where HCO3- is the dominant carbonate specie s in solution. The dissolution rates are surface reaction controlled. The following empirical rate laws were fitted to the experimental data : r(diss) = k(H+)H+(a) where a = 0.93+/-0.42 and k(H+) = 10(-4.9+/-0 .39) 1 m(-2) h(-1). GRAPHICS where b = 0.88+/-0.51 and GRAPHICS = 10(-5.4+/-1.2) 1 m(-2) h(-1). Using a general isotherm for metal oxide surface protonation, the rate law for the proton promoted dissolution kinetics can be rewritten in terms of the concentration of protonated surface sites {=ThOH2+} as r(diss) = k(s){=ThOH2+}(as) where a(s) = 4 .43+/-1.95 and k(s) = 10(14.8+/-0.39) h(-1). This is consistent with a mechanism where the rate determining step is the detachment of a meta l ion from the oxide after a fast initial surface protonation reaction . For the carbonate promoted dissolution, the lack of data for the sur face speciation of ThO2 in the presence of carbonates prevents further modelling of the reaction mechanism.