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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)

The influence of addition order and contact time in the system hematite (α-Fe 2 O 3 ) -humic acid (HA) -thorium (IV) was studied in batch experiments. Thorium (IV) is considered here as a chemical analogue of other actinides (IV). The sorption isotherms were acquired varying pH in the range 2-10, and HA concentration in the range 1-100 mg/L. As already observed by numerous authors, thorium (IV) retention was hindered when HA and hematite were equilibrated beforehand during 24 hours. As it has been observed in a previous study, this 2 effect was drastic when the ratio between humic and surface (iron oxide) sites exceeds a critical value. However, when HA were added after a 24 hours equilibration of the hematitethorium (IV) system, thorium (IV) was barely desorbed from the iron oxide surface.Furthermore, no drastic effect of the ratio between humic and surface sites could be evidenced, as the increase of HA concentration only results in a slight monotonic decrease in Th(IV) retention. Increasing contact time between components of the systems only indicated slight Th(IV) retention variation. This was interpreted as a consequence of slow kinetic controls of both the Th(IV)-HA complexation and HA-hematite sorption.

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