Fluoride removal by aluminum-modified pine sawdust: Effect of competitive ions

The removal of fluoride from aqueous solutions by pine sawdust chemically modified with aluminum in batch systems was investigated. The loading process of aluminum in this biomaterial was achieved by the hydrolysis of metal salts. The kinetics andsorption capacities of the unmodified and modified pine sawdust were evaluated. The effect of different parameters such initial fluoride concentration, pH and contact time was studied on the sorption process. Since the presence of other ions may affect the sorption of fluoride, some studies of binary or multiple systems have simulated a real natural water effluent. The aim of this study was the evaluation of competitive ions in the sorptionof fluoride using pine sawdust in fluoride (F−) single systems and F−/HCO3− and F−/SO42− binary systems. Batch-type contact experiments were performed to evaluate the kinetics and equilibrium fluoride sorption, all at 25 °C, 100 rpm and pH values adjusted to 6, 7 and 8 depending on the conditions of the binary or single systems. It was found that the Lagergren model described the F− sorption kinetic data and the Langmuir and Freundlich equations described the equilibria of the sorption binary systems. These results suggested that the sorption mechanism was chemisorption on a heterogeneous material. The pH effects governing sorption capacities were also evaluated, showing a decrease as pH value rises, indicating that this sorption process is highly pH-dependent. The physical characterization of the biosorbent after fluoride removal by infrared analyses showed fluoride removal in different functional groups on the surface of the modified and unmodified biosorbents. The point of zero charge and co-existing ions affected the efficiency of defluoridation significantly. The pine sawdust showed a high affinity for fluoride ions compared with conventional adsorbents; therefore, it can be considered as a good low cost biosorbent for the defluoridation of water.