Performance And Mechanism Of Arsenic And Fluoride Removal From Water By Alumina-based Adsorbents

Adsorption is one of the most suitable techniques for polishing arsenic and fluoride concentration in effluent to meet the regulation criteria, and for suppling pollutants-free water to small community and scattered household. Most of the published work on adsorption have been focusing on adsorbent preparations, structure characterization, performance investigation. The correlation between the adsorption performances and the bulk and surface properties of reported adsorbents were not fully investigated yet. Features or properties contributed to high adsorption performance were not fully understood, while these clues are important for preparing new efficient adsorbents for field applications. In addition, mass transfer of pollutants in the outersuface and within the particles are highly correlated with the kinetics of pollutants removal. In this study, alumina-based adsorbents were prepared, the property-performance correlation and mass transfer study of ASO43- and F" adsorption on these adsorbents were investigated.Three mesoporous aluminas (MAs) with different surface areas and pore volumes were successfully synthesized. The three adsorbents were extensively characterized to determine their key structural properties that account for their good adsorption capacities and fast kinetics for sequestering As(V) from water. MAI showed the largest As(V) adsorption capacity of 175.7 mg/g among the three. No direct relationship was observed between the mesoporous pore properties and the adsorption capacities of these mesoporous aluminas. The extraordinarily large adsorption capacity of MAI was correlated to its strongly disordered state confirmed by a pair distribution function (PDF) analysis, its large amorphous content fraction measured by selective chemical extraction (SCE), its high Al-OH surface site density determined by a surface titration method, and its Al-0 coordination environment (AlO4 and AlO5) identified by 27Al NMR. Besides its excellent adsorption capacity, MA1 also had the fastest adsorption kinetics. All As(V) uptake kinetics data were modeled using the homogeneous surface diffusion model (HSDM). The intraparticle surface diffusion coefficients (Ds) were numerically determined. The fast As(V) uptake kinetics of MAI were explained by it having the smallest particle size from the HSDM calculations. The MAI was calcined at different temperatures, the structural performance of Al2O3 were affirmed by XRD, PDF, SAED for studying the correlation between temperature and the structural of adsorbent and adsorption capacity. The amorphous content and the adsorption capacity were decreased with increasing calcination temperature. MA effectively decreased As(V) concentrations in spiked well water to well below the WHO’S maximum level of As(V) in drinking water, indicating its great potential as a practical adsorbent candidate.Magnetic alumina aerogel (MAA) was prepared by amending aluminum with magnetite. The largest adsorption of MAA was 43.9 mg/g, the adsorption fluoride of magnetic alumina aerogel was decreased with the increasing the ration of Fe/Al. The aturation magnetization of 40.12 emu/g was observed, which makes the separation of the adsorbent realizable after batch adsorption. Batch adsorption results revealed that fluoride adsorption on the adsorbent followed the pseudo-second-order kinetics model. The intraparticle mass diffusion was well fitted using a homogeneous surface diffusion model (HSDM) and the intraparticle surface diffusion coefficients (Ds) were numerically determined. General co-existing anions did not show any inhibition effects on F- uptake by MAA except for slight inhibition by HCO3- and PO43-. Spike experiments demonstrated that MAA was effective in F" removal. Al 2p and Fe 2p XPS spectra of MAA before and after F" removal firmly evidenced that both the alumina and iron oxide phases contributed to F- adsorption on the surface. Considering its adsorption performance and easy separating properties, the adsorbent is considered a promising candidate for fluoride removal from water with a high application potential in future.