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

• Organomodified diatomaceous earth was utilized as an adsorbent.
• It was used for the removal of Cr(III), Cr(VI) and Pb(II) from aqueous solutions.
• The adsorption equilibrium was well described by Langmuir, Freundlich, Temkin and D–R isotherms.
• Adsorption kinetics of these heavy metals was found to conform to pseudo-second order model.
• The adsorption of Cr(III), Cr(VI) and Pb(II) on diatomaceous earth is exothermic and spontaneous.

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 (10–200 mg/L), adsorbent dosage (0.01–0.30 g/50 mL), pH (1.0–8.0), contact time (10–260 min), ionic strength (0.01–0.20 M) and temperature (25, 35 and 45 °C) were investigated. The adsorption process was relatively fast and equilibrium was established within 180 min for DAT and 120 min for DAT-HDTMABR. Maximum adsorption of Pb(II), Cr(III) and Cr(VI) was observed at pH 5, 4.5 and 2, respectively. Thus, pH 4.5 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.