Silica nanopowders/alginate composite for adsorption of lead (II) ions in aqueous solutions

• Synthesis of silica nanopowders/alginate composite.
• Adsorption of lead (II) by silica/alginate nanocomposite.
• Study of the effect of operational parameters on the adsorption process.
• Kinetic, isotherm, thermodynamic, and equilibrium of the adsorption process.

The aim of the present work was to study the Pb(II) ions adsorption by entrapped silica nanopowders within calcium alginate to determine the isotherm, kinetic and thermodynamic of the adsorption process. According to the results, an optimal initial pH of 5.0 was found for the Pb(II) adsorption. The adsorbed Pb(II) ions reached to 36.51 mg/g as the contact time increased to 180 min at an initial Pb(II) concentration of 50 mg/L. However, a contact time of 90 min was selected as equilibrium time because of no significant increase in Pb(II) adsorption after this time. The results of isotherm and kinetic studies showed the Langmuir isotherm and pseudo-second order kinetic model were the best fitted models (R2 > 0.999). The maximum adsorption capacity for Pb(II) adsorption onto entrapped silica nanopowders was estimated to be 83.33 mg/g. According to the Dubinin–Radushkevich (D–R) isotherm model, Pb(II) adsorption onto the composite follows a chemical mechanism (E = 10 kJ/mol). Negative ΔG° and ΔH° values indicated spontaneous and exothermic nature of the Pb(II) adsorption onto entrapped silica nanopowders, respectively. In addition, the results of continuous flow mode study exhibited that increasing empty-bed contact time (EBCT) from 5 to 20 min resulted in increasing the specific throughput (St) from 8.42 to 11.22 mL/g, respectively.