Lability and sorption of heavy metals as related to chemical, physical, and mineralogical characteristics of highly weathered soils

Abstract

Purpose

Heavy metal lability, probably, is the most important isolated factor to cause toxicity in plants and organisms in soils. Sorption of heavy metals, in turn, affects directly the amount of their labile forms in soils. Therefore, to assess sorption and quantify labile forms of Cd, Cu, Ni, Pb, and Zn, adsorption and incubation studies were carried out.

Materials and methods

The adsorption experiment consisted of a ${\text{12}} \times {\text{5}} \times {\text{1}}0$ factorial design with 12 samples, five metals, and ten doses. An incubation experiment and metal extraction by cationic exchange resin in membrane form, with capacity to exchange 2.80 mol_c kg⁻¹ dry resin, were conducted in samples from A and B horizons of six highly weathered soils from Minas Gerais, Brazil.

Results and discussion

The data from the adsorption experiment was fitted to nonlinear Langmuir and Freündlich models to give the b and k constants, which were used in a correlation study with soil properties. Cationic resin was used to extract the labile metal forms of heavy metals, and low concentrations of the labile forms were determined for all metals and soils in all incubation times.

Conclusions

Factors that influenced Langmuir maximum adsorption and Freündlich buffer capacity were pH and CEC, as chemical characteristics, and goethite or hematite contents, as mineralogical attributes. Negative charge density and hydrolyzed species formation may have been main factors related to maximum adsorption and buffer capacity for most heavy metals. Low contents of labile metal forms obtained by cationic resin extraction were attributed to high intensity of metal adsorption reaction onto soil colloids. Greatest lability was found in soils with mineralogy dominated by gibbsite and kaolinite, while lowest metal lability was determined in soils with higher hematite and goethite contents. Due to their specificities, tropical soils should have more research toward the understanding of the relationship soil vulnerability to heavy metals pollution and availability/lability of these important toxic substances in the environment.