Modification of pumice with HCl and NaOH enhancing its fluoride adsorption capacity: Kinetic and isotherm studies

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Abstract
Fluoride concentration, if exceeds the standard levels in drinking water, can lead to adverse effects on human health. Adsorption is one of the widely considered appropriate technologies for water defluoridation. In this study, modified pumice with HCl and NaOH were used for the fluoride removal from aqueous solution, and two linear forms of the Langmuir and Freundlich isotherms model were applied to determine the best fit of equilibrium expressions. Several experimental parameters including pH (2–10), temperature (20–60°C), adsorbent dosage (2–10 g/L), fluoride concentration (5–20 mg/L), and contact time were studied. Results showed that removal efficiency improved with increase in contact time, initial fluoride concentration, and adsorbent dosage. The maximum removal efficiency (96%) was observed at pH 6, 10 g/L adsorbent dosage, 20°C solution temperature, 90-min reaction time, and 20 mg/L of fluoride concentration. The kinetic studies indicated that the adsorption equilibrium and kinetic data were in good agreement with the Langmuir Model (R2 = 0.95) with qmax = 65.5 mg/g and pseudo-second-order reaction (R2 = 0.99). According to the results, the modified pumice was found to be able to remove fluoride from aqueous environments with good removal efficiency and adsorption capacity.

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