Defluoridation by a Mg–Al–La triple-metal hydrous oxide: synthesis, sorption, characterization and emphasis on the neutral pH of treated water

Abstract

A study has been designed for a fluoride adsorbent synthesized by a co-precipitation approach of Mg(II), Al(III) and La(III) salts (MAL) in 2 : 1 : 2 ratios. The prepared material illustrated relatively stable fluoride adsorption potential and an amorphous structure at temperatures lower than 600 °C. The defluoridation capacity of a 2 g L⁻¹ adsorbent dose at room temperature was observed to be between 24.08 to 98.28% for a 10 and 20.66 mg L⁻¹ fluoride solution in a broad pH range (2 to 12), while for a 45.45 mg L⁻¹ solution, it varied between 26.49 to 80.42%. The adsorption process on MAL was endothermic in nature (−ΔG), follows pseudo second order kinetics (R² = 0.999) and best fitted with a Langmuir sorption isotherm model (R² = 0.9985). The prepared material was characterized by XRD, SEM, BET, EDX, FTIR and XPS, whereas contact times, sorption kinetics, dosage and the effect of major co-existing anions were carried out for the optimization of the adsorbent. In addition to amphoteric behavior of Al(III) in the composite material, the effect of fluoride concentration and sorbent quantity in a batch process appeared to be important factors in maintaining the near neutral pH of the treated water. Similarly, one other factor, flow rate was found to affect the pH in the column adsorption process. Regeneration of up to 95.71% of the fluoride loaded adsorbent was achieved with a methanol and HCl mixture. The results showed that Al(III) acted as amphoteric reagent and keeps the pH within drinking water range or near neutral.