Surface chemistry and radionuclide anion immobilisation potential of phosphoric acid-activated metakaolin-based geopolymers

Abstract

Phosphoric-acid-activated metakaolin-based geopolymers (PGPs) have been investigated as promising options for the disposal of radionuclides. However, a lack of understanding of the surface chemical properties of PGPs has hindered further research into them and their application. This study explored the structure-related electrostatic properties and anion immobilisation potentials of PGPs via zeta potential measurements, structural characterisation, and leaching experiments. These findings suggest that acid activation triggers the geopolymerisation of metakaolin, resulting in new Alx-PO units (x = IV, VI representing VI- and V-coordination), the ratio of which controls the surface charge of PGPs. PGPs possess a positive charge in the equilibrium pH range of approximately 2–5 and exhibit a maximum positive zeta potential at approximately pH 4. Under acidic conditions, the AlVI-PO unit within the surface structure is released, decreasing the zeta potential as the pH decreases. In contrast, in alkaline environments, the AlVI/Si-OH hydroxyl group loses protons, causing a decrease in the zeta potential with increasing pH. Furthermore, PGPs can effectively immobilise SeO32−, SeO42−, I−, and IO3− anions through stabilisation/solidification (S/S). This immobilisation is primarily facilitated by electrostatic attraction between the anions and the positively charged surface of the PGPs. Importantly, the immobilisation process does not cause significant alterations to the matrix structure of the PGPs, even after solidification or subsequent leaching.