Surface complexation of fluoride at the activated nano-gibbsite water interface

The drawbacks of existing fluoride removal processes are due to low efficiencies and less mechanistic understanding of the process. The acid–base protolysis and surface complexation constants for fluoride and thermally activated nanogibbsite were investigated in this study for the first time. Fluoride adsorption on heat activated gibbsite nanoparticles (HGNP) was determined as a function of pH, concentration of background electrolyte and adsorbate loading. XRD evidenced the HGNP is in a transient state of the formation of χ-alumina as a poorly crystalline solid. An immense increase in surface area was obtained from 50 to 399 m2/g by the heat activation. Macroscopic data obtained from the batch experiments suggested an enhanced fluoride removal by HGNP with an adsorption maximum around pH 6. The adsorption quantified by the diffuse double layer surface complexation model indicated an inner sphere complexation mechanism at 0.526 mM of F̄ with monodentate mononuclear complexes. An increase of fluoride concentration to 1.351 mM demonstrated a multilayer complex formation which was also confirmed by the isotherm data modeling with two distinct sorption maxima. XPS evidenced similar results showing two peaks at 684.7 and 688.5 eV for weak and strong bonded fluoride on HNGP surface. Fluoride adsorption maximum was reported as 4 × 10−6 mol/m2 based on Langmuir data fitting.