Adsorptive studies of toxic metal ions of Cr(VI) and Pb(II) from synthetic wastewater by pristine and calcined coral limestones

Abstract

This research work reports on novel materials obtained when coral limestones (PCLS) was calcined at 800 and 900 °C to produce CLS-800 and CLS-900. The materials were characterized by SEM, BET, XRD, FTIR and UV-Vis. The calcined materials exhibited BET surface areas of 64.10 and 63.28 m2/g for CLS-800 and CLS-900 respectively which are significantly higher than that of the starting material (PCLS) 20.45 m2/g. XRD analysis revealed that PCLS was dominated by CaCO3 aragonite while CLS-800 and CLS-900 was a mixture of calcite and CaO. FTIR results revealed that all adsorbents exhibited peaks for (-CO32−) and (CaO). UV-Vis results revealed that CLS-800 and CLS-900 were both blue and red when compared to the starting material. PCLS, CLS-800 and CLS-900 were used as adsorbents for Cr(VI) and Pb(II) ion solutions. The maximum adsorption capacities for Cr(VI) and Pb(II) were 99.12 and 98.42 mg/g onto CLS-800 at pH 2 and 8 respectively. The adsorption kinetic data suited PFO kinetic model better than PSO. While isotherm data best fitted Freundlich model. The removal of Cr(VI) onto all adsorbents was favoured at low temperature whilst Pb(II) favoured high temperature. Hence, negative (ΔHo) values for Cr(IV) ions indicated exothermic process while positive (ΔHo) values for Pb(II) ions indicated an endothermic process. The adsorption mechanism was through synergistic processes; hydrogen bonding, complexation, electrostatic, surface deposition and intraparticle diffusion. The reusability of the adsorbents was tested for four cycles.

Keywords

• Batch adsorption
• Calcium oxide
• Calcium carbonate
• Calcined coral limestones
• Cr(VI)
• Pb(II)