Influence of metal loading on the mode of metal retention in a natural clay

Clay, which is a natural fine-grained soil, was subjected to various levels of contamination by the heavy metal Zn, in a series of batch sorption experiments conducted at pHs of 3-7. Zinc speciation in the soil was subsequently analyzed using a sequential extraction technique, which fractionated the retained Zn into exchangeable, carbonate, reducible, organic, and residual fractions. The results showed that when Zn loading increased, the increase in Zn sorption by the clay was rather limited in acidic conditions. The Zn distribution between the dissolved phase and each fraction of the sorbed phases could be described with either the Freundlich or the Langmuir adsorption equations. If favorable pH existed, at high Zn loadings, precipitation was the most effective process of Zn removal from the dissolved phase. At low Zn loading levels, a relatively higher proportion of Zn was retained in the more persistent fractions. These findings indicated that the predominant mode of heavy metal retention in a soil can be significantly influenced by the level of soil contamination by the metal, in addition to geochemical factors and behaviors of the metal itself.