STUDIES ON ELECTROCHEMICAL PROPERTIES OF SOILS Ⅵ. ADSORPTION OF IONS BY RED SOILS IN RELATION TO THE ELECTRIC CHARGE OF THE SOIL
Zhang Xiao-nian, Jiang Neng-hui, Shao Zong-chen, Pan Shu-zheng and Zhang Wan-gen (Nanjing Institute of Soil Science, Academia Sinica)
In order to study the adsorption of cations and anions by red soils in relation to the electric charge of the soil, electrolyte solutions of KCl, NH_4Cl and K_2SO_4 in concentrations of 0.001, 0.01 and 0.1N were equilibrated with soil samples at different pH. Two soil samples were used for the study, one was red earth with a permanent negative charge of 7.4 m.e. per 100 gms. and a positive charge of 2.8 m.e. per 100 gms. at pH 2 and the other was a lateritic soil with a permanent negative charge of 3.3 m.e. per 100 gms. and a positive charge of 5.8 m.e. per 100 gms. (iso-electrie point at about pH 4.7) The pH at which a soil showed neither positive adsorption nor negative adsorption for Cl~- ions was called the point of zero adsorption of the soil under the specified conditions. This point was thought to be closely related to the relative magnitude of the effective positive charge density and the effective negative charge density of the soil. It was shown that the point of zero adsorption for the lateritic soil was higher than that for the red earth. The point decreased with the decrease of ion concentration for the red earth, whereas ion concentration had no remarkable effect on the lateritie soil. The pH at which a soil adsorbed equal amounts of cations and anions was called the iso-ionic point of the soil. Besides the electric charge of the adsorbent, this point was also strongly affected by the valence type and the concentration of electrolytes. The iso-ionic point for 1:2 type electrolyte was higher than that for 1:1 type electrolyte. In the former case the point decreased with the inerease of concentration, while in the latter case the point practically unchanged irrespective of the change in concentration for the lateritic soil although for the red earth the point decreased with the increase of concentration too. The removal of free iron oxides from the soil resulted in the decreases of both the point of zero adsorption and the iso-ionic point as a consequence of the decrease of positive charge. It was found that although both the electric charge and the adsorption of cations or anions were strongly affected by the pH of the solution, the effected extent were different. When expressed as the ratio of adsorbed cations to the amount of negative charge or the ratio of adsorbed anions to the amount of positive charge of adsorbent, the former increased with the increase of pH and the latter increased with the decrease of pH. It thus seems to imply that besides the change of electric charge of the soil, pH also plays an important direct effect on ion adsorption.