REDOX COUPLED SURFACE COMPLEXATION MODELING OF PU(IV) SORPTION ON KAOLINITE

The complex oxidation states of plutonium(Pu) is a major factor that hinders the understanding of Pu transport in the subsurface due to Pu in different oxidation states possess different affinities for surface. Kaolinite, a common clay mineral, is considered an important sorbent for radionuclides. Here we studied the Pu(IV) sorption at water-kaolinite interface at both aerobic and anaerobic condition with pH 2-8. Experiments were initially spiked with Pu(IV), but it was partially oxidized to Pu (V/VI) under aerobic condition while partially reduced to Pu(III) under anaerobic condition. Pu(IV) was the predominant sorbed species on kaolinite under both aerobic and anaerobic condition. For the aerobic condition, a zone of decrease of sorption was seen at intermediate pH 4-6, which was due to the ingrowth of aqueous Pu(V) with increasing pH. For anaerobic, the presence of Pu(III) in the aqueous phase lead to a notable shift in the pH sorption edge to higher pH values due to the lower affinity of Pu(III) for kaolinite relative to Pu(IV). A surface complexation model, for the first time, coupled the plutonium oxidation states Pu(III), Pu(IV), Pu(V), and Pu(VI) , was used to model the data. The model suggested that under aerobic condition the cation exchange sites on kaolinite dominate Pu sorption at pH<4; aluminol and silanol kaolinite edge sites contribute to Pu sorption at pH 4-8. For modeling Pu sorption on kaolinite at anaerobic condition, stability constants for Am(III) sorption on alumina and silica gel was firstly modeled through fitting reference experimental data. The model indicated that for alumina only edge sites contribute to the sorption while for silica gel both edge sites and exchange sites contribute to the sorption. The modeled stability constants for edge sites were raised significantly to fit the Pu sorption on kaolinite at anaerobic condition, which indicated a possible surface mediated oxidation of Pu(III) to Pu(IV) occurred at the edge sites. The modeled stability constant for cation exchange was unchanged. The effect of ionic strength of solution on sorption was also examined, the obvious increase of sorption with decrease ionic strength at low pH 2-4 provided circumstantial proof of the cation exchange process dominating sorption at low pH values.