Partition of Cd, Cu, Pb and Zn among mineral particles during their sorption in soils

Abstract

Heterogeneity of soil mineral particles may lead to the misinterpretation of bulk sorption data on their role in metal sorption, which may be resolved through the direct observation of metal distribution among the soil particles after the sorption. Analytical transmission electron microscopy was used to study metal (Cd, Cu, Pb, Zn) sorption characteristics of clay minerals, Fe-oxyhydroxides and their natural assemblages in soils with contrasting pH under single element and competitive situations. Iron-oxyhydroxides sorbed higher metal amounts than clay mineral particles mostly, and the sorption capacity of the latter phases were more affected by competition. Iron-oxyhydroxide particles acted as effective metal sorbents in soils not only as individual particles but also as coatings on other mineral particles, and the structural Fe in clay minerals may also resulted in higher metal sorption in some cases. Besides the direct observation of metal sorption onto soil mineral particles, metal precipitates could be identified directly, and the role of organic surfaces in metal sorption could be inferred indirectly in certain cases. Comparison of metal affinity sequences and their partition characteristics between the bulk soil and different mineral particles were found to be an effective tool to specify the most active mineral components in metal sorption at given soil and sorption condition.