ABSTRACT

69 Modification of pumice particle surfaces by aluminum oxide coating was found effective in creating hard surface sites for fluoride adsorption, in accordance with the hard and soft acids and bases (HSAB) concept. Aluminum oxide coated pumice (AOCP) reduced fluoride concentration in model water from 5.0 ±0.2 mg/L to 1.5 mg/L in approximately 1 hr, in batch adsorption experiments using an adsorbent dose of 10 mg/L. Contrary to expectations, thermal treatment of A OCP aimed of further improving its performance, instead reduced the fluoride removal efficiency. The equilibrium adsorption of fluoride by AOCP conformed reasonably to five isotherm models in the order: Generalized model > Langmuir type 2 > BET >Temkin >Dubinin-Radushkevich; with a Langmuir maximum adsorption capacite of 7.87 mg/g. AOCP exhibited good fluoride adsorption within the pH range, 6-9, which makes it possible to avoid pH adjustment with the associated cost and operational difficulties, especially if it is to be used in remote areas of developing countries. Based on results from kinetic adsorption experiments, it was observed that at a neutral pH of 7.0 ±0.1 which is a more suitable condition for groundwater treatment, fluoride adsorption by AOCP was fairly faster in the initial period of contact than a grade of activated alumina (AA) that was tested, the commonly used adsorbentfor water defluoridation. AOCP is thus promising and could also possibly be a usefulfluoride adsorbent.