Selective uptake of Re(VII) by amine microcapsules

The selective uptake of Re(VII) from nitric acid solutions was investigated by using amine microcapsules (MCs). In this study, amine extractants (primene JMT, tri-n-octylamine TOA, tridodecylamine, n-methyldioctylamine), which are effective for the extraction of oxonium anions by an anion-exchange reaction, were encapsulated in the calcium alginate gel polymer (CaALG) by using its highly immobilizing ability. The characterization of MCs was examined by SEM/EPMA, and the uptake properties and the distribution of Re(VII) ions were estimated by batch method. The particle size of MCs was about 700 μm, and fine droplets of amines of amines were uniformly encapsulated in the matrices. TOA microcapsule (TOA-CaALG) exhibited relatively large distribution coefficient (Kd) above 104 cm3/g for Re(VII) in the presence of 10-2 M NHO3. The uptake rate of Re(VII) on TOA-CaALG in the presence of 1 M HNO3 was attained equilibrium within 10 h and high uptake percentage above 70% was obtained. The Kd value of Re(VII) for TOA-CaALG tended to increase with increasing HNO3 concentration at 10-3-10-2 M NHO3, and had a maximum around 0.1 M HNO3. In order to examine the selectivity of Re(VII) for TOA-CaALG, the uptake behaviors of other metal ions were estimated. The order of Kd value at 0.1 M HNO3 was Re(VII)>W(VI)>Cr(VI)-MO(VI). The uptake isotherm of Re(VII) followed a Lanmuir-type adsorption equation and the maximum adsorption capacity was estimated to be 1.38 mmol/g. The Re(VII) ions in 0.1 M HNO3 were also effectively extracted with TOA-CaALG in the column operation. The loaded Re(VII) ions were successfully eluted with an eluent of highly concentrated HNO3 solution. These findings suggest that the MCs enclosing amine are stable in highly concentrated HNO3 solution and seem to be effective for the selective uptake of Re (VII). (author)