Diethylenetriaminepentaacetic acid (DTPA), a multi-dentate acidic chelating agent containing five carboxyl and three amine groups all capable of protonation, is a pentabasic acid. To separate minor actinide [MA(III)] and rare earth [RE(III)] elements effectively from a HNO₃ solution containing DTPA generated in the MAREC process, a functional polymeric composite impregnated with a novel macroporous silica-based diglycolamide compound, N,N,N′,N′-tetraoctyl-3-oxapentane-1,5-diamide (TODGA), was prepared. Thus, the impregnated functional polymeric composite (TODGA/SiO₂-P) was obtained by impregnating and immobilizing TODGA molecules in the pores of the SiO₂-polymer (SiO₂-P) particles (ca. 50 μm diameter) via a vacuum sucking technique.

The effects of H⁺ ions within the concentration range 0.01–3.0 M and of NO₃⁻ ions within the range 0.1–3.115 M on the adsorption of RE(III) ions onto TODGA/SiO₂-P were investigated at 298 K. It was found that in the presence of 0.05 M DTPA, the DTPA species strongly affected the adsorption of RE(III) ions. The adsorption capability of TODGA/SiO₂-P depended on the competitive reactions of RE(III) ions with two species, viz. H₄DTPA⁻ and H₂DTPA³⁻. The distribution coefficient (K_d) for RE(III) ions increased with increasing NO₃⁻ concentration, with one RE(III) ion reacting with two NO₃⁻ ions. The adsorption mechanism of RE(III) ions towards the TODGA/SiO₂-P polymeric composite in HNO₃ solution containing 0.05 M DTPA was suggested as:

RE 3 + + H 4 DTPA - + 2 NO 3 - + 3 TODGA / SiO 2 - P ⇋ RE ( H 4 DTPA ) ( NO 3 ) 2 ⋅ 3 TODGA / SiO 2 - P

The formation of H₄DTPA⁻ and RE(H₄DTPA)(NO₃)₂ which are both capable of adsorbing with TODGA/SiO₂-P was the main reason why RE(III) ions exhibited such significant adsorption in the presence of 0.05 M DTPA.