| Abstract: | Complexation of europium (III) and UO22+ by humic acid was found to be very fast (< 1 m). However, passage through ion exchange resin removed a fraction which decreased as the contact time increased over a 2–3 day period. The dissociation kinetics for UO22+-humate were described by an equation with five first order terms with rate constants ranging from 40.6 m−1 to 5.64×10−3 m−1. The percentage of UO22+ dissociating by the slower paths, comparable to that eluting with the humic acid through the ion exchange resin is assigned to internal site-bound cations (strong binding). The faster dissociating fraction corresponded roughly to the UO22+ removed by the cation exchange resin (weak binding). The time dependence of the weak to strong binding ratio was associated with change in the conformation of the macromolecular humic acid as the cations neutralized the charge repulsions of the anionic humate. |
