Adsorption Characteristics of Calcined Oyster Shell for the Removal of Fluoride

Abstract

Objectives Groundwater is contaminated with fluoride by industrial activities. In Korea, oyster shell is considered as a waste and cannot be stored for more than 6 months, and it was made as a fertilizer and supplied to farms. But it cannot be used in farms because of odors and salinity. In this study, an experiment was performed to remove fluoride using calcined oyster shell and the characterization of fluoride adsorption on calcined oyster shell was investigated. Methods The oyster shell was calcined to enhance its adsorption capacity for fluoride and the temperature of thermal treatment on oyster shell ranges from 100℃ to 900℃. Kinetic adsorption test, equilibrium adsorption test, thermodynamic adsorption test, adsorption experiment according to pH of fluoride solution, adsorption experiment in the presence of competitive ions in fluoride solution, and adsorption experiment under different dose of adsorbents were performed using oyster shell thermally treated at 700℃ (OS700). Results and Discussion Oyster shell thermally treated at 700℃ was found to have the highest fluoride removal. Equilibrium for fluoride adsorption onto OS700 was achieved within 1 h at a fluoride concentration of 300 mg/L. Equilibrium experiments showed that fluoride adsorption onto OS700 was better fitted by Langmuir model than the Freundlich model, inferring that fluoride adsorbs onto OS700 as a mono-layer. Thermodynamic experiments showed that fluoride adsorption onto OS700 is endothermic and non-spontaneous. The increase of pH from 3 to 11 decreased the amount of fluoride adsorption onto OS700 from 86.4 mg/g to 68.5 mg/g. The presence of both CO32- and PO43- decreased the fluoride adsorption onto OS700, but the influence of Cl- concentration on fluoride adsorption was insignificant. Although the fluoride removal percentage increased with the increase of dose amount of OS700, the amount of fluoride adsorption on unit mass of OS700 decreased. Conclusions In this study, the fluoride adsorption characteristics of the OS700 were analyzed. OS700 is expected to be used as an effective and environmentally friendly adsorbent to remove fluoride in water. Key words: Adsorption, Fluoride, Oyster Shell, pH, Thermal Treatment