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

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2014-11, Vol.462, p.124-130
• Main Authors: Vithanage, Meththika, Rajapaksha, Anushka Upamali, Bootharaju, M.S., Pradeep, T.
• Format: Article
• Language: English
• Subjects: Activated; Adsorption; Complexation; Diffuse double layer; Fluorides; Gibbsite nanoparticles; Nanostructure; Surface chemistry; Surface complexation modeling; X-ray photoelectron spectroscopy
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2014.09.003

•Heat activation converted crystalline nano-gibbsite to a poorly crystalline solid.•Surface area increased enormously by heat activation from 50 to 399m2/g.•F̄ formed a monodentate mononuclear complex at activated GNP in low concentrations.•Multilayer complex formation was confirmed by the isotherm modeling. 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 399m2/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 0.526mM of F̄ with monodentate mononuclear complexes. An increase of fluoride concentration to 1.351mM 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 688.5eV for weak and strong bonded fluoride on HNGP surface. Fluoride adsorption maximum was reported as 4×10−6mol/m2 based on Langmuir data fitting.