Uranium(VI) complexation by humic acid under neutral pH conditions studied by laser-induced fluorescence spectroscopy

Abstract

The complexation of U(VI) with Aldrich humic acid (HA) was studied at pH 7 under exclusion of CO2. Using two independent laser-induced spectroscopic methods, time-resolved laser-induced fluorescence spectroscopy (TRLFS) and TRLFS with ultrafast pulses (fs-TRLFS), the formation of the ternary U(VI) mono hydroxo humate complex UO2(OH)HA(I) by reaction of UO2OH+with HA was studied. Assuming that all proton exchanging functional groups of the HA are able to contribute to the complex formation, a mean stability constant of 6.58 ± 0.24 was derived for UO2(OH)HA(I). Alternatively, the analytical data were evaluated based on the metal ion charge neutralization model resulting in a complexation constant of 6.95 ± 0.10 and a loading capacity of 0.76 ± 0.28. An overall complexation constant of logβ0.1M= 14.89 ± 0.54 was calculated for the total reaction of U(VI) with HA starting from the non-hydrolyzed UO22+ion. This value agrees very well with literature data. Taking into account the UO2(OH)HA(I) complex, the speciation of U(VI) in presence of HA was recalculated. It was found, that the formation of UO2(OH)HA(I) can significantly influence the U(VI) distribution in the environmentally relevant pH region. As a consequence, the mobility of U(VI) in natural aquifer systems could be enhanced.