Spectroscopic study (TRLFS and EXAFS) of the kinetics of An(III)/Ln(III) humate interaction

Summary

Humic acid metal ion interaction shows a complex kinetic behavior. Dissociation kinetics reported in the literature reach from hours to basically irreversibility. Different suggestions for the responsible mechanisms have been made. One possibility is variations in the metal ion humate complex environment, subject to this study. The dissociation kinetics of Eu(III)/Tb(III)/Cm(III) humate complexes is investigated with purified humic acid solution (Gohy-573(HA), pH=6.0, I=0.1M (NaClO₄)) and a humic rich groundwater (Gohy-532, pH=7.5). The different kinetic modes are verified by both scavenging of dissociated metal ions by an cation exchanger added after different metal ion humic acid contact times, and isotope exchange under stationary conditions, i.e. without change in metal ion humic acid ratio. The metal ion humate complex environment is studied by time resolved laser-induced laser fluorescence spectroscopy (TRLFS) and EXAFS. TRLFS shows that slower dissociating Cm(III) humate has a peak maximum at 600.5nm contrary to the fast dissociating ones with a peak maximum around 602.8nm. EXAFS also shows differences between slow and fast dissociating complexes (Tb(III), with an average Tb(III) oxygen distance of 2.37Å for an equilibrium mixture, and 2.33Å for only the slow dissociating complex. TRLFS shows that complexes with fast and slow dissociation kinetics have returned to the original distribution three days after removal of the fast dissociating metal ions and thus the different kinetic modes are in exchange equilibrium with each other. Furthermore, the isotope exchange investigations verify that humic acid europium flocculate basically behaves like dissolved/dispersed humate complexes.