Heavy Metal Cation-Induced Cl-Adsorption by Variable Charge Soils

Three variable charge soils were chosen to investigate the effects of three heavy metal cations on Cl- adsorption by the soils with batch method. The results showed that the presence of heavy metal cations induced the increase of Cl- adsorption by the variable charge soils compared with that in Na+ system. The ability of heavy metal cations to enhance Cl- adsorption followed the order: Pb(Ⅱ) Cu(Ⅱ) Cd(Ⅱ), consistent with the quantity of these cations adsorbed by the soils. It was found that the enhancement of Cl- adsorption by heavy metal cations in these soils followed the order: latosol yellow soil red soil, which was also related with the amount of heavy metal cations adsorbed by the three soils. Cl- was mainly adsorbed on the positive sites of the soils through electrostatic attraction between soil surface and cations. The enhancement of Cl- adsorption by heavy metal cations involved in three possible mechanisms: (1) specific adsorption of heavy metal cations increased soil positive surface charge and thus increased Cl- adsorption by the soils; (2) the presence of heavy metal cations decreased system pH and resulted in the increase of soil positive surface charge, and thus led to an increase of Cl- adsorption; (3) the complex such as MCl+ was formed between heavy metal cations and Cl-, when MCl+ was adsorbed by the soils, the amount of Cl- adsorbed by the soils increased. While among the three mechanisms, the first one and the second one were the main mechanisms.