Treatment of fluoride in well-water in Khanhhoa, Vietnam by aluminum hydroxide coated rice husk ash

In this study, aluminum hydroxide coated rice husk ash [RHA/Al(OH)3] was synthesized and used for adsorption of fluoride from aqueous solution. The amorphous material containing 20% (w/w) of Al2O3 was obtained by stirring the mixture of RHA burned at 700°C for 60 min and 0.1 m AlCl3 solution for 30 min. This material possessed porous ordered structure with uniform particle size in the range of 50–100 nm and Brunauer-Emmett-Teller (BET)-specific surface area of 81 m2 g−1. The isotherm data were fitted well to Langmuir model, and the maximum monolayer adsorption capacity was found to be 8.2 mg g−1 at 25°C. The kinetic results revealed that the pseudo-second-order rate model fitted the experiments data better than the pseudo-first-order one and demonstrated the chemical nature of the adsorption of fluoride. Thermodynamic parameters in the temperature range 30–70°C showed that the adsorption was a spontaneous and an endothermic process. RHA/Al(OH)3 can be used for well treatment of fluoride in contaminated well-water in the Ninhhoa district (Khanhhoa province). With the initial fluoride concentration (10.1 mg l−1) and the 2 h of treatment process with the dose of 4.0–7.0 g l−1, the content of fluoride decreased to 0.5–1.5 mg l−1, which met the acceptable limit of WHO.