Adsorption of Pb(II), Cr(III) and Cr(VI) from aqueous solution by surfactant-modified diatomaceous earth: Equilibrium, kinetic and thermodynamic modeling studies

Abstract

The adsorption of Cr(III), Cr(VI) and Pb(II), ions onto natural (DAT) and organomodified (DAT-HDTMABR) diatomaceous earth from aqueous solutions was studied in a batch adsorption system. Factors influencing Pb(II), Cr(III) and Cr(VI) adsorption such as initial metal ion concentration (10–200mg/L), adsorbent dosage (0.01–0.30g/50mL), pH (1.0–8.0), contact time (10–260min), ionic strength (0.01–0.20M) and temperature (25, 35 and 45°C) were investigated. The adsorption process was relatively fast and equilibrium was established within 180min for DAT and 120min for DAT-HDTMABR. Maximum adsorption of Pb(II), Cr(III) and Cr(VI) was observed at pH5, 4.5 and 2, respectively. Thus, pH4.5 value was used throughout the whole experiments. The adsorption characteristics of the two adsorbents were generally very similar. The adsorption data obtained was well described by Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (D–R) isotherm models. The maximum metal adsorption capacities assumed by a Langmuir model were on the order of Pb2+>Cr3+>Cr6+. Adsorption energy (E) for adsorption of Cr(III), Cr(VI) and Pb(II) species by DAT and DAT-HDTMABR has been computed from the D–R equation. Adsorption energy evaluated for the adsorption of those three metal cations showed that an ion-exchange mechanism (physisorption) was operative. A comparison of the kinetic models on the overall adsorption rate showed that the adsorption system can be best described by the pseudo-second-order kinetics. Different thermodynamic parameters viz., Gibbs free energy ΔG, enthalpy, ΔH, entropy, ΔS, activation energy, Ea, and sticking probability, S* have also been evaluated, and the adsorption was feasible, spontaneous, and exothermic in nature.