Adsorption of fluoride from water using Al–Mg–Ca ternary metal oxide-coated sand

Abstract

An aluminum–magnesium–calcium-coated sand (AMCCS) sorbent was evaluated as a low-cost ternary metal oxide adsorbent for adsorption and removal of fluoride from water with potential application in flow-through water filtration systems. The AMCCS sorbent coating contained mostly amorphous oxides of aluminum, magnesium, and calcium. The adsorption of fluoride occurred in less than an hour, while fluoride adsorption followed pseudo-second-order kinetics. The favorable adsorption of fluoride onto the AMCCS sorbent occurred according to the Langmuir and Freundlich adsorption equations, whereas the physical adsorption of fluoride onto the AMCCS sorbent occurred based on the Dubinin–Radushkevich adsorption equation. The adsorption of fluoride occurred over a broad pH range from 2 to 10.5 with a decrease in adsorption at pH above 10.5, indicative of the adsorption of fluoride onto the positively charged surface of AMCCS sorbent at pH below the AMCCS sorbent pHPZC of 10.4. The AMCCS sorbent was able to remove fluoride from natural waters with appreciable alkalinity, total hardness and total dissolved solids, indicative of AMCCS sorbent selectivity for fluoride adsorption. The AMCCS sorbent was re-coated and reused after several adsorption cycles, rendering the AMCCS sorbent a recyclable and sustainable adsorbent for effective and rapid removal of fluoride from water.