ADSORPTION OF NP(IV) ON QUARTZ IN CARBONATE SOLUTIONS

Adsorption of neptunium(IV) on quartz in the presence of carbonate was studied by a batch method. The effect of reduction as well as of carbonate concentration was examined on surface-based distribution coefficient (K(a)). The observed K(a) was interpreted taking into account the sorption of Np(OH)2(CO3)22-, Np(OH)4(CO3)24- and Np(OH)5-. The hydroxycarbonate species Np(OH)2(CO3)22- and Np(OH)4(CO3)24- were regarded as the main species affecting the adsorption below pH 11, and their K(a) was determined to be 10(1.70 +/- 0.06) ml/m2 for Np(OH)2(CO3)22- and 10(0.67 +/- 0.23) ml/m2 for Np(OH)4(CO3)24-. From the literature value of Np(IV) solubility, the formation of Np(OH)5- with the stability constant of < 10(46) was suggested, and the sharp increase of K(a) at higher pH was well explained by Np(OH)5-. The K(a) of Np(OH)5- was predicted to be > 10(5.9) ml/m2.