Metal-induced chromium(VI) sorption by two calcareous soils

Abstract

Competing anions, notably SO 2- 4 and H₂PO- 4 , reduce Cr(VI) sorption; however, the role of cooperative cations in Cr(VI) retention has merited less attention. This research studied the effect of Cd, Cu, Ni, and Zn on sorption of Cr(VI) in 2 calcareous soils: Calcixerollic Xerochrept (CX) and Paralithic Xerorthent (PX). Sorption kinetics and isotherms were combined with sequential extractions and fractional factorial designs. Most cation sorption took place during the first hour, whereas sorption of Cr(VI) was slower. Without the cooperative presence of these cations, no sorption of Cr(VI) was detected in the range 0–0·35 mM. Cd, Cu, Ni, and Zn augmented the sorption of Cr(VI) by the calcareous soils; however, in both soils the amount of Cr(VI) retained was much lower than cation sorption. In the presence of cation, sorption of Cr(VI) was first observed at an equilibrium concentration of 0·07 mM for the CX soil and 0·17 mM for the PX soil. For higher concentrations, sorption of Cr(VI) by both soils was described by a constant distribution isotherm. More than 80% of Cr(VI) sorbed by soils was extracted as a non-exchangeable fraction using the Tessier et al . sequential procedure. Fractional factorial designs indicated that although the 4 cations favoured sorption of Cr(VI), Cu had the greatest positive influence.