Adsorption and Desorption of Thallium(I) By Soils.

Thallium is highly toxic but its fate in soil has not been well understood. In Taiwan, elevated levels of Tl in soils have been detected due to its application in the electronic industries. To understand the interaction mechanism of Tl with soils, this study conducted Tl(I) adsorption and desorption experiments for six soil samples, which included permanent-charge and pH-dependent-charge soils. The Tl(I) adsorption capacities of permanent-charge soils were higher than those of the pH-dependent charge soils. The ratios of Tl(I) adsorption maximum to CEC were determined to be 21 – 39% for the variable-charge soils and approximately 60% for the permanent-charge soils. The adsorption/desorption isotherms of each soil were both fitted to Freundlich equation to obtain n values, which were subsequently used to calculate hysteresis coefficient. The values of the hysteresis coefficients of all the soils increased with the initial Tl(I) concentration. The permanent-charge soils exhibited a higher hysteresis coefficient than the pH-dependent charge soils. These results indicated that a fraction of Tl(I) may be retained by the clay minerals in the soils. However, because the desorption behaviors of Tl(I) were generally reversible, Tl(I) is expected to be leached readily in soil, indicating a high environmental risk of Tl(I) pollution in soil.