Comparative study of strontium adsorption on muscovite, biotite and phlogopite

Micaceous minerals are the natural materials that can block radioactive strontium (Sr) released in the environment, and their adsorption capacity and mechanism are highly divergent owing to the different properties of micas. In this work, we comparatively studied the adsorption of Sr(II) on three typical micas, muscovite, biotite and phlogopite. The effects of pH, contact time, ionic strength, and background electrolyte were evaluated. It was found that phlogopite and muscovite had the largest solid-liquid distribution coefficient (K_d) for a reaction time of 48 h under acidic and alkaline conditions, respectively. Under alkaline conditions, as the reaction time increased to 44 days, phlogopite and muscovite showed the highest and lowest K_d, respectively. The K_d for Sr(II) adsorption on biotite and phlogopite increased with increasing pH but decreased with increasing pH for muscovite. X-ray diffraction analysis revealed that the interlayer weathering of phlogopite (a new diffraction peak appeared at 2-theta of ~6.1°) occurred along with the adsorption of Sr(II) below pH 9.0 under 0.01 mol/L NaCl. Furthermore, the adsorption of Sr(II) was significantly inhibited in the presence of 10^-5 and 10^-2 mol/L Cs⁺, resulting in similar adsorption capacity for phlogopite and muscovite at pH ~4.1. Consequently, the difference in Sr(II) adsorption on muscovite, biotite and phlogopite mainly came from the synergistic process of adsorption and weathering, which induced the differences in availability of interlayer sites among micas over a certain time.