Complexation behaviour of neptunium with humic acid

Abstract

The complexation of Np(V) with humic acid (HA) was studied under environmental conditions (pH ≥ 6, concentration of HA 1-150 mg/L) over a wide range of metal ion concentrations (10 −14 to 10 −4 mol/L). The experimental methods used for the determination of the complexation constants were continuous electrophoretic ion focusing, anion exchange, and ultrafiltration in combination with radiometric measurements, and, as a direct speciation method, UV/Visspectroscopy. The interaction between Np(V) and humic acid has been described with the charge neutralization model including a correction for different loading capacities (LC) which makes possible the comparison of results obtained at various experimental conditions. Furthermore, the degree of deprotonation has also been used to calculate the complexation constants. The complexation constants calculated for Np(V) with the LC-concept vary with the metal ion concentration in the range between 10 −14 and 10 −4 mol/L. At metal concentrations below 10 −11 mol/L, a complexation constant log β LC = 5.0 ± 0.3 was obtained. In the concentration range 10 −7 to 5 × 10 −4 mol/L, the log β LC values decrease continuously to 3.7 ± 0.4 (10 −5 -5 × 10 −4 mol/L). The latter is in agreement with direct measurements (UV/Vis-spectroscopy) at this concentration and data from the literature.

Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH, ionic strength, carbonate and humic acid

“…For systems with HA, the speciation code was modified by integrating the metal ion charge neutralization model (Kim and Czerwinski, 1996), the Np(V) humate complexation constant log ß = 3.6 (Marquardt and Kim, 1998;Seibert et al, 2001; and the pH function of the loading capacity (LC) with LC = À0.589 + 0.101 pH (Sachs et al, 2004).…”

Humic acid model substances with pronounced redox functionality for the study of environmentally relevant interaction processes of metal ions in the presence of humic acid

“…These were successfully applied to investigate environmentally relevant geochemical interaction processes of HA with metal ions (Kumke et al, 2005;Křepelová et al, 2006;Mibus et al, 2007;Plaschke et al, 2002;Pompe et al, 1996Pompe et al, , 1998Pompe et al, , 2000Sachs et al, 2002Schmeide et al, 2003Schmeide et al, , 2005Schmeide et al, , 2006Bernhard, 2009, 2010;Seibert et al, 2001).…”

Competitive effect of iron(III) on metal complexation by humic substances: Characterisation of ageing processes

“…This requirement arises from the fact that interaction of metals with humic substances has been found to be dependent on the concentration ratio (of what to what?) (Bidoglio et al, 1991;Seibert et al, 2001). In the present study, we have examined the influence of Fe(III) on the humic complexation of Eu(III) as an analogue for trivalent actinides (Am, Cm), employing 59 Fe and 152 Eu as radioactive tracers to enable precise measurements in very dilute systems to be made.…”