Sorption of PB and B on Soils and their Separated Clay Fractions

Abstract

Pollution is one of the main threats possessing a potential risk for man and animals, especially the potentially toxic elements (PTE) which are considered non-biodegradable and can persist in soils for years. Clay minerals are thought to be among the main factors immobilizing PTE in soils and; therefore, these minerals control their bioaccessibility. Thus, the current research aims at determining the mechanisms by which the potentially toxic element Pb beside of the metalloid B are retained in a clayey non-calcareous soil and a clayey calcareous one under different pH values i.e. 5, 7 and 9. The clayey non-calcareous soil is characterized by a clay content of 58.25% and CaCO3 content of 24.8 g kg-1 while the clayey calcareous one is characterized mainly by a clay content of 40.75% and CaCO3 content of 289.9 g kg-1. The clay fraction of the investigated soils was separated and identified of the types of the clay minerals dominating in these soils using X-ray diffraction analyses. Montmorillonite was the dominant clay mineral in the non-calcareous clayey soil, whereas kaolinite and palyorskite were the dominant ones in the calcareous clayey one. Adsorption of Pb and B on soils and clay minerals increased gradually with increasing the initial concentration of these ions in the equilibrium systems, especially with increasing the pH of theses systems from 5 to 9. In this concern, adsorption of the investigated ions by the clay minerals exhibited much higher values than those occurred on the investigated soils. Freundlich isotherm was the most efficient model fitting Pb sorption on both soils and clay minerals, whereas, Langmuir isotherm model seemed to be more appropriate for fitting the sorption data of B on the investigated soils and separated clay minerals. In conclusion, adsorption of Pb and B on clay minerals seemed to be the dominant mechanism controlling their mobility in soils even at pH 9. The actual sorption capacities and affinities in soils remained much lower than that occurred by the separated clay minerals. To what extent can clay minerals retain PTE versus a clayey and a calcareous clayey soils was a matter of concern in this study.