Thallium adsorption onto phyllosilicate minerals

Area of Science:

• Environmental Science
• Geochemistry
• Soil Science

Background:

• Phyllosilicate minerals are key in retaining thallium (Tl) in soils and sediments.
• Understanding Tl(I) adsorption mechanisms is vital for predicting its environmental fate and bioavailability.
• Micaceous minerals are known to strongly bind monovalent cations like cesium (Cs).

Purpose of the Study:

• To investigate the adsorption mechanisms of thallium (Tl(I)) onto various phyllosilicate minerals.
• To compare Tl(I) adsorption behavior with that of cesium (Cs) and rubidium (Rb).
• To elucidate the role of mineral surfaces and interlayer sites in Tl(I) retention.

Main Methods:

• Macroscopic adsorption isotherms were measured for Tl(I) onto illite, smectite, muscovite, vermiculite, and a soil clay fraction.
• X-ray absorption spectroscopy (XAS) was used to characterize the adsorbed Tl species.
• Adsorption experiments were conducted using Na- and K-saturated mineral samples.

Main Results:

• Three distinct Tl(I) uptake pathways were identified: high-affinity inner-sphere adsorption at frayed edges/collapsed zones, intermediate-affinity inner-sphere adsorption on planar surfaces, and low-affinity adsorption in interlayers.
• Tl(I) adsorption at frayed edges and vermiculite interlayers can create new sites for further adsorption.
• A shift in Tl(I) uptake from edge to planar sites was observed with increasing Tl(I) loading on the soil clay fraction.

Conclusions:

• The adsorption of Tl(I) onto phyllosilicate minerals follows similar trends to Cs and Rb.
• Concepts developed for (radio)cesium retention are applicable to understanding Tl(I) behavior in soils, sediments, and rocks.
• Phyllosilicate mineralogy significantly influences the dominant Tl(I) adsorption pathways and overall retention capacity.