Significant role of interlayer in strontium adsorption on weathered biotite

Abstract

Weathering of micaceous minerals play a significant role in the adsorption and migration of radioactive strontium. Weathering changes the structure of minerals and the capacity of adsorption sites on their surfaces. The regulatory mechanism of environmental factors on radioactive strontium on weathered micaceous minerals remains to be sorted out and clarified. In this work, we studied the adsorption behavior of Sr2+ on weathered micaceous minerals with different weathering degrees via batch experiments and spectroscopic approaches. The batch experiment results indicated that the adsorption capacity of Sr2+ increased from 5.36 mg/g (Na-Bio) to 19.89 mg/g (SCa-Bio) as the weathering degree of biotite increased. The outer-sphere complexation and electrostatic attraction/ion exchange mainly governed the adsorption of Sr2+ on weathered biotite. The type of cation co-existing in solution and the weathering degree of biotite regulate Sr2+ adsorption into the interlayers. Owing to their similar ionic properties (e.g., hydration energy and hydrated radius), Sr2+ competed more fiercely with Ca2+ for adsorption sites. Hydrated Na+ and Sr2+ could enter and adsorb on the interlayer, but the difference is that the hydrated Na+ collapsed the interlayer of weathered biotite (from 15.0 Å to 11.8 Å), while the hydrated Sr2+ expanded the interlayers (from 12.0 Å to 14.8 Å). Nevertheless, the hydrated Sr2+ could occupy most interlayers when Na+ and Sr2+ co-existed in the solution. It was noted that dehydrated K+ present in the interlayer of weathered biotite could collapse the expanded interlayer of biotite. The d-spacing of weathered biotite (e.g., SCa-Bio) would decrease from 15.0 Å to 10.4 Å, significantly limiting the interlaminar adsorption of Sr2+. Furthermore, this inhibitory effect was based on the high weathering degree of micaceous minerals and the noticeable interlayer expansion. XPS results demonstrated that the co-existing cations hardly influence the adsorption form of Sr2+ on weathered micaceous minerals. The above findings provide intrinsic insights into the understanding of radioactive strontium transport and fate in the environment.