Batch and column adsorption and desorption of fluoride using hydrous ferric oxide: Solution chemistry and modeling

Elevated intake of fluoride (F), mainly through drinking water, is a major threat to human health worldwide. A study was conducted to remove F from aqueous solution by adsorption onto hydrous ferric oxide (HFO) in batch and fixed-bed column experiments. Of the seven adsorbents (four anion exchange resins, three multivalent metal oxides) tested, HFO had the highest adsorption capacity. Fluoride adsorption on HFO fitted well to Langmuir and Freundlich models with a Langmuir adsorption maximum of 6.71 mg F/g at pH 6.5. Fluoride adsorption continuously decreased from pH 3 to 7. Point of zero charge of HFO was pH 5 which fell to pH 4 in the presence of 10 mg F/L and 1 g HFO/L, indicating that F was specifically adsorbed on HFO. Fluoride was not desorbed by 0.1 M NaCl and 0.1 M Na2SO4 but effectively desorbed by 0.1 M NaOH. The F adsorption capacity in column experiments (10% or 20% HFO + 90% or 80% anthracite) was progressively reduced with increasing number of adsorption/NaOH desorption cycles up to three cycles reaching a final value of 3.26 mg F/g HFO. The breakthrough data from column studies at different bed heights, inlet concentrations, and pHs were fairly well described by Thomas model, but using an artificial neural network approach improved the model capability.