Significance of Aqueous Cation Composition on Heavy Metal Mobility in a Natural Clay

Heavy metal mobility in soils is influenced by various soil variables and environmental factors, including the composition of the soil-aqueous phase. This study used the sequential extraction technique to study how major cations could affect heavy metal sorption and speciation in a natural clay. The sorption of Zn, Pb, and Cd by the soil in the presence of${\rm Na}^{+}$and${\rm Ca}^{2+}$and the resulting metal speciation in the sorbed phases were examined. Sorption experiments were performed at pH levels of 3 to 8. Sorptions occurring in both single and multiple heavy metal systems were investigated. The results show that${\rm Na}^{+}$and${\rm Ca}^{2+}$significantly inhibited Zn, Pb, and Cd sorption in the acidic pH range only. The reduction in the exchangeable sorption was responsible for a large part of the reduction in the total Zn, Pb, and Cd sorption in the presence of${\rm Na}^{+}$and${\rm Ca}^{2+}$. Specific adsorptions of heavy metals in other fractions of sorbed phases were only marginally affected by the composition of cations. The reduction in the exchangeable sorption increased with increasing concentration of${\rm Na}^{+}$and${\rm Ca}^{2+}$. In the presence of a major cation, the competitive adsorption among the heavy metals seemed to have only a minor effect on individual heavy metal sorption.