Abstract:

Fluoride contamination of water is a problem worldwide and has caused great concern. Our study focused on the removal of fluorides from aqueous solutions using newly prepared and regenerated activated alumina. To obtain a suitable adsorbent, industrial boehmite was calcined from 573 K to 1473 K and the sample was characterized. The X-ray diffraction patterns showed that the sample was transformed to γ-alumina (activated alumina) at temperatures from 773 K to 1173 K, and the BET dates showed that the specific surface area of the sample decreased gradually from the temperature of 773 K to 1173 K. In our study, the activated alumina calcined from 773 K to 973 K was selected as the defluoridation adsorbent, and dynamic adsorption was employed for the removal of fluorides from aqueous solutions. The breakthrough curves demonstrated that the adsorption capacity of the adsorbent decreased with increasing calcination temperature. To investigate the effect of initial fluoride concentration on the adsorption capacity, 15 mg·L^-1, 20 mg·L^-1, and 25 mg·L^-1 fluoride solutions were selected as the initial aqueous fluoride solution. As a result, the capacity of the adsorbent increased gradually with the increase in the initial fluoride concentration. In order to improve the capacity, we also studied the regeneration process in our experiment. When the activated alumina was saturated by the fluorides, it was regenerated with a NaOH solution (pH = 13.0, 13.3, 13.5) and activated with a HCl solution (0.1 mol·L^-1). By a comparison of the five desorption and Al³⁺ dissolution rates during the regeneration process, it was determined that the NaOH solution with pH 13.0 was the most suitable desorption agent. An analysis of the nitrogen adsorption-desorption isotherm showed that its sharpness was almost unchanged after regeneration, which indicated that the pore structure of the adsorbent was not destroyed during this process. The change in the specific surface area and isoelectric point for the five-times regenerated adsorbent were important impact factors for fluoride adsorption. The specific surface area of the regenerated adsorbent increased, and the study of the zeta potential demonstrated that the isoelectric point also increased after regeneration. To observe the adsorption effect of regenerated activated alumina, we performed an adsorption experiment after each regeneration. The breakthrough curves demonstrated that the regenerated activated alumina exhibited faster saturation and increased adsorption capacity compared to fresh activated alumina. To elucidate the adsorption mechanism, IR spectroscopy was employed to characterize the O―H band of the adsorbent. The change in the Al―O―H content of the activated alumina during regeneration was the main factor impacting the fluoride adsorption capacity of the activated alumina.

Key words: Defluoridation, Regeneration, Activated alumina, Breakthrough curve, Zeta potential