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 CO 2 . Using two independent laser-induced spectroscopic methods, timeresolved laser-induced fluorescence spectroscopy (TRLFS) and TRLFS with ultrafast pulses (fs-TRLFS), the formation of the ternary U(VI) mono hydroxo humate complex UO 2 (OH)HA(I) by reaction of UO 2 OH + 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 UO 2 (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 log β 0.1 M = 14.89 ± 0.54 was calculated for the total reaction of U(VI) with HA starting from the non-hydrolyzed UO 2 2+ ion. This value agrees very well with literature data. Taking into account the UO 2 (OH)HA(I) complex, the speciation of U(VI) in presence of HA was recalculated. It was found, that the formation of UO 2 (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.

Water Quality of Medium Size Watercourse Under Baseflow Conditions: The Case Study of River Sutla in Croatia

“…In addition, at the pH around 7, such as measured in our study, Sachs et al (2007) reported significant formation of uranium complexes with humic substances, which are abundant organic ligands of natural origin, and made up approximately 60% of the carbon dissolved in aquatic systems (Peña-Méndez et al 2005). As a consequence of the increased use of fertilizers in the modern agriculture, additional input of humic substances into the river water can occur due to their use as additives to fertilizers (Peña-Méndez et al 2005), which could resulted in uranium complexation and further concentration increase in the river water at agriculturally impacted downstream sites.…”

Structural characterization of U(VI) surface complexes on kaolinite in the presence of humic acid using EXAFS spectroscopy

“…TRLFS measurements [11] showed that, even if HA is present, U(VI) prefers to sorb directly onto kaolinite rather than via HA. Aqueous U(VI)-humate complexes themselves do not show any fluorescence at all under the applied measuring conditions [12]. So, if U(VI) would be bound to kaolinite by HA, no fluorescence of U(VI) in the U(VI)-HA-kaolinite samples should be observed.…”

Complexation parameters for the actinides(IV)-humic acid system: a search for consistency and application to laboratory and field observations

“…Using the log * β • n in [47] for hydrolysis and log * β 1.4.I = −3.8 for U(OH) 4 HA(I), and other constants in Table 3 for UO 2 + , using analogy with NpO 2 + [56], and UO 2 2+ [11,78], speciation calculations can be done under the conditions given in [31, Table 1] for SCWHA and SCWHA/C.…”