Effects of Mineral Compositions on Matrix Diffusion and Sorption of ⁷⁵Se(IV) in Granite

Exploring the migration behaviors of selenium in granite is critical for the safe disposal of radioactive waste. The matrix diffusion and sorption of ⁷⁵Se(IV) (analogue for ⁷⁹Se) in granite were systematically studied to set reliable parameters in this work. Through-diffusion and batch sorption experiments were conduct with four types of Beishan granite. The magnitudes of the obtained apparent diffusion coefficient (D_a) values are of the following order: monzogranite > granodiorite-2 > granodiorite-1, which is opposite to the sequence of the K_d values obtained from both the diffusion model and batch sorption experiments. The EPMA results of the granitic flakes showed that there was no obvious enrichment of Se(IV) on quartz, microcline and albite. Only biotite showed a weak affinity for Se(IV). Macroscopic sorption behaviors of Se(IV) on the four types of granite were identical with the sequence of the granitic biotite contents. Quantitative fitting results were also provided. XPS and XANES spectroscopy data revealed that bidentate inner-sphere complexes were formed between Se(IV) and Fe(III). Our results indicate that biotite can be representative of the Se(IV) sorption in complex mineral assemblages such as granite, and the biotite contents are critically important to evaluate Se(IV) transport in granite.