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

Abstract

http://www.radiochimacta.deInternational audienceThe influence of humic acids (HA) on the retention of thorium (IV) onto the surface of silica colloids is investigated. Thorium is considered as an analogue of tetravalent actinides (U, Np, Pu), except for the fact that it has no f electrons. Silica (SiO2) is chosen as a model surface because it is a component of many minerals and because of its weak HA sorption properties. Retention experiments are performed by batch procedure at constant ionic strength (I = 0.1M NaClO4), at various pH (2 to 9) and HA concentrations (1–100 mg/L). The sorption of Th(IV) onto colloidal amorphous silica is characterised using a surface complexation model. The ternary system (i.e. HA-Th-Silica) clearly shows the influence of HA on Th(IV) retention. This can lead to a strong reduction of the amount of Th(IV) sorbed onto silica in the presence of HA compared to silica colloids without HA, due to a predominant part of thorium present in solution as humic complexes. In a pH range where no organic coating onto silica occurs, there is a competitive reaction between Th(IV) sorption onto the silica surface and onto HA reactive functional groups. In this case, Schubert's method is applied to obtain a global interaction constant for the Th(IV)-HA system. The large interaction constant values indicate a strong affinity of HA for Th(IV)

Influence of Addition Order and Contact Time on Thorium(IV) Retention by Hematite in the Presence of Humic Acids

“…The known interaction of thorium (IV) with natural organic matter observed in aquatic, marine and groundwater systems seems to contradict the latter hypothesis (22)(23)(24). Moreover, strong interaction constants were determined for Th(IV) and Np(IV) with humic acid in pH ranges relevant to natural waters (28,58,59). Thus, the strength of the interactions between HA and metal ions does not seem to be the main factor implied.…”

“…Under the same conditions, inorganic carbon concentration has been estimated to be less than 5 × 10 -5 M (53, 54), which induces only a weak influence on Th(IV) speciation (28,55).…”

Thorium speciation in seawater

“…In general, they indicate that Th forms strong complexes with humic acids (Nash and Choppin, 1980;Choppin and Nash, 1981;Artinger et al, 2003;Reiller et al, 2003) and acid polysaccharides (Guo et al, 2002b;Quigley et al, 2002;Passow et al, 2006-this issue). Murphy et al (1999) used laboratory experiments to demonstrate that in the presence of humic acids (Perdue et al, 1980) and acid polysaccharide-containing colloidal organic matter (COM) , Th(IV) sorption to hematite was greatly enhanced.…”

Complexation of Radionuclides with Humic Substances

“…The retention of Th decreases strongly with an increase in HA concentration indicating the formation of organic species that govern Th behaviour [46]. Global interaction constants can be determined from experimental data obtained at various pH in the range from 6 to 8 ( Figure 5) [47]. These interaction constants are very high, indicating very strong organic complexes with Th(IV).…”

“…These interaction constants are very high, indicating very strong organic complexes with Th(IV). The formation of hydroxo-humates of Th(IV) is assumed in [46][47]. Figure 6 illustrates the effect of formation of such organic complexes on Th speciation as a function of HA concentration.…”