Thorium(IV) and neptunium(V) uptake from carbonate containing aqueous solutions by HDTMA-modified natural zeolites

The ²³²Th-uptake ([Th(IV)]° = 9.7 × 10^?5 M) from carbonate solutions ([CO ₃ ² ]_tot = 0.25 M, 9.0 < pH_c < 10.8) by raw and HDTMA-modified HEU-type zeolitic-, chabazitic- and phillipsitic-tuffs was investigated. The strong uptake by the HDTMA-tuffs at pH_c≈9 was assigned to the Th(CO₃) ₅ ^6? and ThOH(CO₃) ₄ ^5? predominance. The sorption coefficients (R _d) decreased with increasing pH_c indicating carbonate competition. Enhanced R _d values for pH_c > 10.5 are likely due to ThO₂(am)-precipitation. The ²³⁷Np-uptake ([Np(V)]° = 2.6 × 10^?5 M) from carbonate solutions ([CO ₃ ² ]_tot = 0.25 and 3.0 × 10^?4 M) by raw and HDTMA-modified HEU-type zeolitic tuff and pulverized pure heulandite crystals was studied under Ar-atmosphere at 6 < pH_c < 11. The R _d values for both elements indicated the modified tuffs potential to remove tetravalent- and pentavalent actinides from environmental matrices.