Fluoride Adsorption from Water Using Activated Carbon Modified with Nitric Acid and Hydrogen Peroxide

Adsorption on activated carbon is one of the most commonly used methods for removing excess amounts of fluoride from water. This research has tested the parameters of fluoride adsorption on four commercial activated carbons modified with nitric acid and hydrogen peroxide. The adsorption properties included the initial fluoride concentrations (2–40 mg/L), pH (4–9), dosage of activated carbon (2–20 g/L), contact time (15–360 min) and temperature (25–45 °C). The research results showed a decrease in the proportion of adsorbed fluoride by increasing its initial concentration, while an increase in temperature had a positive effect on the proportion of removed fluoride. The highest fluoride adsorption capacity of 1 mg/g belonged to HN-H2O2 active carbon at a temperature of 45 °C. The maximum adsorption capacities were recorded at pH 4 for all adsorbents oxidized with H2O2 (0.041–0.168 mg/g) and at pH 6 for all adsorbents oxidized with HNO3 (0.065–0.134 mg/g). An increase in the temperature to 45 °C resulted in the maximum increase in the adsorption capacity for all adsorbents, and the adsorbent HN-HNO3 (0.158 mg/g) had the highest adsorption capacity. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich isotherms were used to determine the adsorption mechanism. The experimental data showed the best match with the Freundlich model at all temperatures, while the pseudo-second-order kinetic model described the adsorption rate the best.