Effects of Amorphous Al(OH)3on the Desorption of Ca2+, Mg2+, and Na+from Soils and Minerals As Related to Diffuse Layer Overlapping

Abstract

The overlapping of the diffuse layers of the electric double layers between phyllosilicates and Fe(III) and Al(III) oxides results in compensating of some negative surface charge on phyllosilicates by the positive surface charge on Fe(III) and Al(III) oxides and thus should affect the distribution of cations in the diffuse layers on phyllosilicates. In this article, the batch experimental method was used to investigate the desorption of Ca 2+ , Mg 2+ , and Na + from an Alfisol, an Ultisol, a bentonite, and a kaolinite induced by amorphous Al(OH) 3 added and to evaluate the effect of the overlapping of the diffuse layers on the release of cations from the minerals and the soils. The presence of amorphous Al(OH) 3 was found to enhance the desorption of Na + , Ca 2+ , and Mg 2+ from the soils and the minerals; the effect of amorphous Al(OH) 3 on the desorption of Ca 2+ , Mg 2+ , and Na + increased with its amount added. The increase of ionic strength depressed the diffuse layer and decreased the overlapping of diffuse layers and thus decreased the contribution of diffuse layer overlapping to the release of Ca 2+ and Mg 2+ from soils and minerals. This study suggests that the overlapping of the diffuse layers of the electric double layers of the negatively charged phyllosilicates and the positively charged Al(OH) 3 accelerated the release of Ca 2+ , Mg 2+ , and Na + from the soils and minerals as a result of the decrease in their effective charge density.