Retention of Cr(VI) and Pb(II) on a loamy sand soil: kinetics, equilibria and breakthrough

Abstract

Illegal discharges are of great concern among industry activities, since they occur under uncontrolled conditions. In most cases, effluents are acidic and the concentrations of heavy metals are very high. With this in mind, the main goal of this study was to evaluate the sorption on a soil of two of the most toxic heavy metals, Cr(VI) and Pb(II), in those conditions.

A loamy sand soil was collected in Oporto, Portugal. Batch equilibrium and sorption kinetics were evaluated using both metals solutions, with concentrations ranging from 50 mg L−1 to 200 mg L−1, at pH 2 and 5, between 2 h and 288 h. To evaluate the sorption equilibrium, eight isotherm models were fitted to experimental data. The best adjustments were observed for the Redlich–Peterson and Khan models for the adsorption of chromium (R2 = 0.99), and of lead (R2 = 0.99), respectively. The sorption kinetics was evaluated using three models – Elovich, pseudo first order and an empirical power function. The retention of lead was almost instantaneous and the empirical power function described better the sorption kinetics of chromium (0.89 < R2 < 0.99).

In addition, flow experiments were performed with effluents of both metals (50 mg L−1) at pH 2 and 5, for about 90 h. Results revealed a high retention of chromium and a weak retention of lead, for low pH values. FTIR analyses to the column samples revealed that clay minerals have an important role in the retention of both metals.