Synthesis of hydrous CeO2 polypyrrole nanocomposite as a rapid and efficient adsorbent for defluoridation of drinking water

Abstract

Contamination of water by fluoride ions is one of the major threats to human life and thus must be removed from drinking water. A novel CeO₂ polypyrrole nanocomposite (HCeO₂@PPy) was fabricated by co-precipitation of CeO₂ nanoparticles followed by in situ chemical oxidative polymerization. The as-synthesised adsorbent was characterized using a variety of physico-chemical techniques which included Fourier Transform Infrared Spectroscopy (FTIR), X-Ray diffraction (XRD), Braunauer-Emmett-Teller (BET), Field Emission Scanning Electron Microscopy/Energy Dispersive (FE-SEM/EDS), High Resolution-Transmission Electron Microscopy (HR-TEM), X-ray Photoelectron Spectroscopy (XPS) and point-of-zero charge (pzc) determination. The HCeO₂@PPy nanocomposite adsorbent was applied for the removal of F⁻ ions from aqueous solution. Application of the adsorption modelling, revealed that the Langmuir adsorption isotherm model best described the process with a maximum adsorption capacity of 70.92 mg g⁻¹, at an optimum pH of 6.0 ± 0.2, 0.03 g adsorbent dose and 25 °C. Rapid kinetic studies revealed that the pseudo-second-order model gives a better description of the adsorption process. Thermodynamic data showed that the adsorption process was physical, spontaneous and feasible. The behaviour of the amino functional groups within HCeO₂@PPy moiety and the nanometal oxide surface hydroxides at different pHs along with the pHₚzc, FTIR and XPS spectra analyses were used to explain the mechanism of adsorption. The mechanism was conceived to be anionic exchange of hydroxyl group with the F⁻ ions and electrostatic attractions of protonated hydroxyl on the surface of the adsorbent as well as the nitrogen atoms of amino groups from the polypyrrole (PPy) moiety with F⁻ ions. The determined thermodynamic parameters, enthalpy change (ΔH° = −26.33 kJmol⁻¹) and Gibbs free energy change (ΔG° = −19.074 to −10.028 kJ mol⁻¹) indicated the exothermic and spontaneous nature of the sorption process. Further evaluation on the as-prepared adsorbent exposed a moderately selective material which exhibited excellent removal ability of F⁻ ions from ground water at its natural pH to below WHO stipulated levels with an adsorption-desorption efficiency of up to three cycles. Therefore, hydrous CeO₂ polypyrrole nanocomposite has revealed great potential for water defluoridation.