Characteristics of CI~- adsorption by constant charge soils and variable charge soils

The adsorption of chloride ions by three variable charge soils and four constant charge soils of China were measured under different pH and various concentrations of KCI. The results showed that although the amount of CI- adsorbed by soils increased with increasing CI- concentration, the linear relationship was only found in constant charge soils. For variable charge soils, within the range of 0.5 to 5 mmol/L of CI- added, the adsorption can be described by the Langmuir equation satisfactorily. And the adsorption energy parameter (K) calculated from the equation was similar in magnitude. When equal CI- added, the amount of CI- adsorption by different soils decreased in the sequence Latersol red earth red latersol yellow brown earth brown earth, dark brown earth and black soil, which was agreed with the order of their positive charges. The adsorption of chloride by constant charge soils, which caused by co-adsorption with K+ mainly, was a little and a negative adsorption was usually observed at low concentrations. The adsorption of CI- decreased with increasing with pH. However, the decreasing speed was higher in variable charge soils than that in constant charge soils.