Insights into the mechanism of fluoride adsorption over different crystal phase alumina surfaces

Abstract

Activated alumina is the most common adsorbent for purifying fluoride in water, however, little is known so far about the adsorption mechanisms and comparison of adsorption behaviors for F on different crystal phase alumina surfaces, which seriously obstacles the development of high-performance sorbents. Herein, employing the density functional theory approach, we have studied F adsorbed on α-Al2O3(0001), γ-Al2O3(110), and θ-Al2O3(010) surfaces. Results accentuate that the θ-Al2O3 (010) is the most reactive than ɑ-Al2O3 (0001) and γ-Al2O3 (110) for F adsorption and the high reactivity is mainly attributed to the high unsaturation level of Al atoms. Detailly, the most stable adsorption sites are top of Al1 site, bridge of Al6 and adjacent Al atom, and bridge of AlⅢ atoms for α, γ, θ-alumina, respectively. The bonding picture shows that the bonding between F and alumina surface is attributed to the hybridization between F-p orbitals and Al-s,p orbitals. In addition, the alumina surfaces are hydroxylated with water molecules when exposing to the atmosphere, exhibiting a great impact on the performance of purifying F element. Results suggest that the hydroxylated θ-Al2O3 (010) adsorbs F with the smallest adsorption energy than other hydroxylated alumina surfaces, exhibiting the lowest performance of purifying F element.