Study On The Modification Of Fly Ash And Removal Of Fluoride In Aqueous Solution Using Modified Fly Ash

Fly ash, a waste product of coal combustion in thermal power plants, consists mainly of silica, alumina, and iron oxide. Physical or chemical reactions occur between these oxides and many compounds, which prove that fly ash might appear some adsorption properties. However, it is concluded by other studies that fly ash won't show strong adsorption capacity unless it is treated or activated for its complex consistent and network structure.In this paper, high efficient adsorbent was prepared by some methods of modification using fly ash as raw materials. Methods of modification of fly ash were discussed to select an appropriated method. Conditions of modification of fly ash and operating methods of adsorption, such as reaction time, temperature, pH and dosage, were investigated, based on the influences of conditions on the adsorption capacity. The thermodynamics and kinetics in the process of adsorption of fluoride on modified fly ash have been analyzed. The main results were as follows:(1) The modification methods of fly ash were investigated to prepare adsorbent with good ability to remove fluoride from aqueous solution. Based on the adsorption capacity, fly ash treated with sonochemical and sulfuric acid, had greatest adsorption capacity.(2) Based on the process of modification, proper conditions of modification of fly ash were obtained. The characteristics and modification mechanism of modified fly ash were discussed and analyzed based on data from X-ray Diffraction (XRD), Infrared Spectra (IR), Scanning Electron Microscopy (SEM), and Transmission Electron Microscope (TEM). The results showed that the effect of the concentration of sulfuric acid solution and the drying temperature on the adsorption capacity of fly ash modified is greater. Si-Al network structure of the raw fly ash is damaged after the action of the sulfuric acid under the ultrasonic, so that the soluble Si, Al, Fe within the fly ash particles were released and the crystalline phases of Ca and Fe appeared, which made Si-O-Fe, Al-O-Fe structure was formed. At the same time, under the ultrasonic environment, sulfuric acid corrosion for fly ash has been strengthened which made the surface of the fly ash be uneven and the specific surface area increase.(3) Optimum adsorption conditions of fluoride in aqueous solution on sonochemical and sulfuric modified fly ash were obtained. The results showed that the conditions such as the concentration of F- , temperature, and pH were significant factors affecting the adsorption capacity of F- on. modified fly ash.(4)The results of kinetics and thermodynamics suggested that the adsorption of the F- on the modified fly ash was H-class isotherm adsorption and was fitted well with the Langmuir isotherm adsorption model. The adsorption kinetics was agreement with the pseudo-second-order model and intrparticle diffusion model, which showed that the adsorption process was controlled by surface adsorption.