Adsorption of hexavalent chromium [Cr(VI)] onto a loamy sand soil was studied using batch and steady flow tests with contaminant solutions at pH 2, 5 and 7. In all the cases the adsorption of Cr(VI) decreased with increasing pH. The hexavalent chromium speciation and its presence as different oxyanions, according to the solution pH, were the main variables affecting the adsorption process. The influence of the ratio soil/solution concentration was also studied in flow systems at pH 2. Chromium retention increased with the increasing of its concentration in the influent solution. A two-site adsorption model was fitted to the breakthrough curves of hexavalent chromium solutions in order to estimate the Freundlich (k(F)) and Langmuir (S(max)) adsorption parameters, using CXTFIT code. These values were compared to those determined by batch tests and it was concluded that batch tests tended to underestimate these parameters. Nevertheless, they followed the same trend as the parameters determined in opened system, even when the pH of the initial solution was modified.
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 (R 2 = 0.99), and of lead (R 2 = 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 < R 2 < 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.
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