Fluoride Action Network

The role of fluoroaluminate complexes on the adsorption of fluoride onto hydrous alumina in aqueous solutions

Source: Journal of Colloid and Interface Science - in press | November 6th, 2019 | Authors: Lin J, Chen Y, Hong X, Huang C, Huang CP.
Location: International
Industry type: Water Treatment

Abstract

Activated alumina (AA) has been extensively applied in the defluorination of industrial wastewaters and groundwater. Although the dissolution of AA due to formation of fluoroaluminate complexes (AlFx3-x), especially in acidic condition, has been observed, its role on fluoride uptake by alumina has not been discussed in any previous literature, most of which consider F as the sole adsorbed species. The present study described the effect of fluoroaluminate complexes on fluoride adsorption onto alumina. Results indicated that fluoroaluminate complexes, major fluoride species at pH < 6, were responsible for total fluoride adsorbed. Free fluoride ions were adsorbed mainly in the alkaline pH region, e.g., pH > 6. The dissolution of AA during defluorination was measured and analyzed by the thermodynamic solubility model. The surface concentration of F and AlFx3-x were calculated considering electrostatic interactions. Characterization of fluoride-laden AA by XPS revealed that the fraction of surface Al-F species decreased with pH, which suggested the transition of the surface fluorinated species to that of free fluoride ions. The stability constants of four surface complexes, namely, AlOH-FAl2+, AlOH-F2Al+, AlOH2+-F and AlOH-F, were 106.88, 105.36, 102.72 and 102.36, respectively. Obviously fluoroaluminate complexes exhibited stronger chemical bonds with the surface hydroxy species than free fluoride.