Comparing Fluoride Adsorption Capacity onto Different Types of Bone Char

摘要

Fluoride adsorption onto three types of bone char (BC), i.e. pig bone char (PBC), chicken bone char (CKBC) and cow bone char (CBC), was investigated. Hydroxyapatite (HAP, Ca_5(PO_4)_3OH), which can replace its hydroxyl groups with fluoride ions, is the major component of BC. The adsorption data of PBC, CKBC and CBC fitted well with the pseudo-second order model and the Langmuir isotherm, indicating that fluoride ions were removed by ion-exchange mechanism. X-ray powder diffraction (XRD) analysis revealed that CKBC contains a higher percentage of HAP than other bone chars, but the specific surface areas of CKBC (62.80 m~2/g-BC) was less than PBC (83.79 m~2/g-BC) and CBC (103.11 m~2/g-BC). However, the maximum fluoride adsorption capacity on CBC, 0.678 mg/g-BC, was higher than those of other bone chars. These result indicated that fluoride adsorption capacity depended more on the specific surface area of BC than the HAP contents. The fluoride adsorption capacity on BC reached the highest at ca. pH 6 which was lower than the points of zero charge. Although the adsorption capacity was not the highest, PBC was found to be the most suitable adsorbent for practical application of defluoridation because of less color released in treated water than CBC.