Aluminum (hydr)oxide coated pumice for fluoride removal from drinking water: Synthesis, equilibrium, kinetics and mechanism

Intake of excess fluoride (beyond 1.5 mg/L, WHO guideline) for long periods can result in the incidence of fluorosis. Adsorption is widely considered the most appropriate technology for water defluoridation, if a suitable adsorbent is available. Several studied adsorbents have shown certain degrees of adsorption capacity, however, applicability of most is limited either due to lack of socio-cultural acceptance, high cost or effectiveness only in extreme pH conditions. The search for alternative adsorbents thus remains of interest. 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 h, using an adsorbent dose of 10 mg/L. Contrary to expectations, thermal treatment of AOCP aimed at 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 maximum capacity 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 quite comparable or perhaps fairly faster in the initial period of contact than that of activated alumina (AA), the commonly used adsorbent for water defluoridation. AOCP is thus promising and could also possibly be a useful fluoride adsorbent.