Sorption-desorption of lead (II) and mercury (II) by model associations of soil colloids

Sorption by soil colloids largely determines the bioavailability of heavy metals and their movement in soil and aquatic environments. Due to soil constituents' interactions, the sorption behavior of natural soil colloids may not correspond to the simple sum of their individual constituents. In this work, sorption of Pb(II) and Hg(II) ions by binary and ternary model particles containing Wyoming montmorillonite (SW), poorly crystallized ferrihydrite (Ferrih), and soil humic acid (HA) was investigated and the results obtained were compared with the sorption behavior of the individual constituents. For single sorbents, Pb(II) sorption was high on HA, moderate on SW and zero on Ferrih, whereas Hg(II) sorption decreased in the order: HA >> Ferrih > SW. Ferrihydrite coatings on SW had little effect on Pb(II) and Hg(II) sorption by the clay. Humic acid coatings on SW significantly enhanced sorption of both heavy metals, whereas, unexpectedly, HA coatings on Ferrih did not enhance heavy metal sorption. This last result was attributed to blockage of the functional groups of HA responsible for heavy metal sorption (such as carboxylic groups) as a result of their interaction with the Ferrih surface. A similar behavior was observed for Pb(II) in ternary particles containing HA. Sorption by the model associations was highly reversible. The results of this study confirmed that the sorptive behavior of colloidal particles for heavy metals is not the simple sum of the contributions of the single constituents, indicating the usefulness of considering polyphasic model sorbents to achieve a more realistic interpretation of the sorption process in soil.