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 Al-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 Al-PO unit within the surface structure is released, decreasing the zeta potential as the pH decreases. In contrast, in alkaline environments, the Al/Si-OH hydroxyl group loses protons, causing a decrease in the zeta potential with increasing pH. Furthermore, PGPs can effectively immobilise SeO, SeO, I, and IO 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.

中文翻译：

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磷酸活化偏高岭土基地质聚合物的表面化学和放射性核素阴离子固定潜力


磷酸活化的偏高岭土基地质聚合物（PGP）已被研究作为放射性核素处置的有希望的选择。然而，缺乏对PGP表面化学性质的了解阻碍了对其及其应用的进一步研究。本研究通过 zeta 电位测量、结构表征和浸出实验探索了 PGP 的结构相关静电特性和阴离子固定电位。这些发现表明，酸活化触发偏高岭土的地质聚合，产生新的 Al-PO 单元（x = IV，VI 代表 VI-和 V-配位），其比例控制 PGP 的表面电荷。 PGP 在大约 2-5 的平衡 pH 范围内具有正电荷，并在大约 pH 4 时表现出最大正 zeta 电位。在酸性条件下，表面结构内的 Al-PO 单元被释放，随着 pH 值的升高，zeta 电位降低减少。相反，在碱性环境中，Al/Si-OH 羟基失去质子，导致 zeta 电位随着 pH 值的增加而降低。此外，PGP 可以通过稳定/固化 (S/S) 有效固定 SeO、SeO、I 和 IO 阴离子。这种固定主要是通过阴离子和 PGP 带正电表面之间的静电吸引来促进的。重要的是，即使在固化或随后的浸出之后，固定过程也不会导致 PGP 的基质结构发生显着改变。