Complexatíon of Trivalent Actinide Ions (Am³⁺, Cm³⁺) with Humic Acid: The Effect of Ionic Strength

Abstract

The effect of ionic strength on the complexatíon of Am 1+ and Cm 3+ by humic acid is investigated at pH 6 with three different experimental methods: ultrafiltration, UV-Visible spectroscopy, and time resolved laser fluorescence spectroscopy (TRLFS). The ionic strength is adjusted by NaCl0 4 and varied from 0.001 M to 5.0 M. The metal ion concentration is changed from 9.95 X10" 9 mol/L Cm 3+ to 1.04X 10" 5 mol/L Am 3+ , while the humic acid concentration is varied from 5.5 X10" 8 mol/L to 1.87 XI 0~5 mol/L. This assures a wide range of metal ion to humic acid concentration ratios to be examined. At pH 6, the loading capacity (LC) of Gohy-573 humic acid for trivalent actinide ions is found to change with ionic strength : LC = -(0.126+0.012) J~1 + (0.683±0.014).A relatively small effect of ionic strength on the complexatíon constant can be ascertained. The constant at 7=0 evaluated by the SIT model is : log ß(0) = 6.52 ± 0.05 and an average value of the constants determined over the whole range of ionic strength under investigation is: log ßUJ = 6.24±0.14.

On the study of Th(IV)-humic acid interactions by competition sorption studies with silica and determination of global interaction constants

“…% of carbon, pH = 7.9 for GoHy-573; 0.080 g C/L, 56.6wt. % of carbon, pH = 7.8 for Gohy-2227), and W HA are available in [93,104] (4.91 × 10 −3 eq/g for Gohy-2227; 5.38 × 10 −3 eq/g for GoHy-573). The global interaction constant used is log GL HA β(Th 4+ ) = 21.3 from our present study.…”

Radioactivity, Geochemistry, and Health

“…Humic substances have experimentally been shown to strongly complex the trivalent actinides (Artinger et al, 1998;Czerwinski et al, 1996;Morgenstern et al, 2000); Th(IV) (Nash and Choppin, 1980); U(VI) and probably U(IV) (Czerwinski et al, 1994;Zeh et al, 1997); and Np(V) (Kim and Sekine, 1991;Marquardt and Kim, 1998;Marquardt et al, 1996;Rao and Choppin, 1995) at mildly acidic to neutral pH. Under basic conditions, actinide-humic substance complexation is strongly a function of the carbonate concentration, because carbonate competes effectively with the humic acid as a ligand (Unsworth et al, 2002;Zeh et al, 1997).…”

“…Little data are available for tetravalent actinides, but Tipping (1993) suggests, based on thermodynamic modeling, that these should be even more strongly complexed by humic substances than other oxidation states. Several studies have shown that actinide-humic acid complexes are thermodynamically stable in high ionic strength solutions (Czerwinski et al, 1996;Labonne-Wall et al, 1999;Marquardt et al, 1996). However, destabilization of humic colloids at high ionic strength (Buckau et al, 2000) and competition for humic acid sites by divalent metal cations (Marquardt et al, 1996;Tipping, 1993) may limit the importance of colloidal transport of actinides in brines.…”

Environmental geochemistry of radioactive contamination.

“…Humic colloids are stable and occur in concentrations up to 0.4 g L -1 in dilute, shallow groundwaters overlying the Gorleben test site in Germany (Buckau et al, 2000). Humic substances have experimentally been shown to strongly complex the trivalent actinides (Czerwinski et al, 1996;Artinger et al, 1998;Morgenstern et al, 2000); Th(IV) (Nash and Choppin, 1980); U(VI) and probably U(IV) (Czerwinski et al, 1994;Zeh et al, 1997); and Np(V) (Kim and Sekine, 1991;Rao and Choppin, 1995;Marquardt et al, 1996;Marquardt and Kim, 1998) at mildly acidic to neutral pH. Under basic conditions, actinidehumic substance complexation is strongly a function of the carbonate concentration, because carbonate competes effectively with the humic acid as a ligand (Zeh et al, 1997;Unsworth et al, 2002).…”

“…Several studies have shown that actinide-humic acid complexes are thermodynamically stable in high ionic strength solutions (Czerwinski et al, 1996;Marquardt et al, 1996;Labonne-Wall et al, 1999). However, destabilization of humic colloids at high ionic strength (Buckau et al, 2000) and competition for humic acid sites by divalent metal cations (Tipping, 1993;Marquardt et al, 1996) may limit the importance of colloidal transport of actinides in brines.…”