Modelling the adsorption of mercury onto natural and aluminium pillared clays

INTRODUCTION

The removal of heavy metals by natural adsorbent has become one of the most attractive solutions for environmental remediation. Natural clay collected from the Late Cretaceous Aleg formation, Tunisia was used as a natural adsorbent for the removal of Hg(II) in aqueous system.

METHODS

Physicochemical characterization of the adsorbent was carried out with the aid of various techniques, including chemical analysis, X-ray diffraction, Fourier transform infrared and scanning electron micrograph. Batch sorption technique was selected as an appropriate technique in the current study. Method parameters, including pH, temperature, initial metal concentration and contact time, were varied in order to quantitatively evaluate their effects on Hg(II) adsorption onto the original and pillared clay samples. Adsorption kinetic was studied by fitting the experimental results to the pseudo-first-order and pseudo-second-order kinetic models. The adsorption data were also simulated with Langmuir, Freundlich and Temkin isotherms.

RESULTS

Results showed that the natural clay samples are mainly composed of silica, alumina, iron, calcium and magnesium oxides. The sorbents are mainly mesoporous materials with specific surface area of <250 m(2) g(-1). From the adsorption of Hg(II) studies, experimental data demonstrated a high degree of fitness to the pseudo-second-order kinetics with an equilibration time of 240 min. The equilibrium data showed the best model fit to Langmuir model with the maximum adsorption capacities of 9.70 and 49.75 mg g(-1) for the original and aluminium pillared clays, respectively. The maximum adsorption of Hg(II) on the aluminium pillared clay was observed to occur at pH 3.2. The calculated thermodynamic parameters (∆G°, ∆H° and ∆S°) showed an exothermic adsorption process. The entropy values varied between 60.77 and 117.59 J mol(-1) K(-1), and those of enthalpy ranged from 16.31 to 30.77 kJ mol(-1). The equilibrium parameter (R (L)) indicated that the adsorption of Hg(II) on Tunisian smectitic clays was favourable under the experimental conditions of this study.

CONCLUSION

The clay of the Aleg formation, Tunisia was found to be an efficient adsorbent for Hg(II) removal in aqueous systems.

Effects of impurities on the removal of heavy metals by natural limestones in aqueous systems

Effects of impurities on the removal of heavy metals by natural limestones in aqueous solutions were studied by evaluating various factors including limestone concentration, pH, contact time and temperature. Solutions of Pb(II), Cd(II), Cu(II) and Zn(II), prepared from chloride reagents at a concentration of 10 mg/L, were studied in a batch method. Four natural limestone samples, collected from the Campanian-Maastrichtian limestone beds in Tunisia, were used as adsorbents. Sorption experiments indicated that high removal efficiencies could be achieved. Limestone samples containing impurities, such as silica, iron/aluminum oxides and different kinds of clay minerals, demonstrated enhanced sorption capacity, nearing 100% removal in some cases. Kinetic experiments showed that the sorption of metal ions occurred rapidly at a low coverage stage, and that solutions were nearly at equilibrium after 60 min. Data trends generally fit pseudo-second order kinetic, and intra-particle diffusion, models. The following conditions were found to promote optimum, or near-optimum, sorption of heavy metals: 1) contact time of more than 60 min, 2) pH = 5, 3) >3 g/L limestone concentration and 4) T = 35 °C. The results of this study suggest that the limestones from northern Tunisia, that contain higher amounts of silica and iron/aluminum oxides, are promising adsorbents for the effective removal of toxic heavy metals from wastewaters.