“Since all methods [to remove fluoride] produce a sludge with very high concentration of fluoride that has to be disposed of, only water for drinking and cooking purposes should be treated, particularly in the developing countries.”
Reference: Fluorosis (see Interventions), World Health Organization
- Hydrous oxides of aluminium, iron and their mixture immobilized in water hyacinth.
- Adsorption capacities >4 mg/g achieved, with hydrous oxide giving the highest.
- Adsorbent achieved>80% fluoride uptake in 3 sorption-desorption cycles.
- Immobilized adsorbent showed high potential for use in continuous flow systems.
In this study, the effectiveness of water hyacinth beads doped with hydrous aluminium oxide (WH-HAO), hydrous iron oxide (WH–HFO) and hydrous aluminium oxide-iron oxide mixture (WH-HAO-HFO) was investigated for the removal of fluoride from aqueous solution in batch and fixed bed column systems. The effect of solution pH, initial fluoride concentration, flow rate, bed depth, and competing ions were studied. The adsorption capacities of the adsorbents were 4.43, 4.25, and 4.18 mg/g F for WH-HAO, WH-HAO-HFO and WH-HFO, respectively. Increasing solution pH had the effect of reducing fluoride adsorption, with the effect less pronounced for adsorption using WH-HFO adsorbent. Equilibrium modelling showed that the data better fitted the Freundlich isotherm compared to the Langmuir isotherm, which was consistent with the heterogeneous, rough and porous physical surface indicated by scanning electron microscope (SEM) characterization. Column sorption studies revealed that the breakthrough point was reached earlier at higher inlet fluoride concentration, lower flow rate and lower adsorbent bed depth. The maximum fluoride adsorption capacity of the adsorbent decreased with increasing flow rate and increasing inlet fluoride concentrations while the volume of fluoride solution treated before saturation was found to increase with increasing bed depth and flow rate. Competition to fluoride adsorption followed the order HCO3->PO43->SO42-. Adsorbent regeneration studies showed that at least 80% F adsorption was achieved in 3 sorption-desorption cycles using WH-HAO beads, and NaOH was the most effective desorption agent, achieving >80% desorption of previously adsorbed fluoride in 3 h. Water hyacinth doped with hydrous metal oxides and immobilized in alginate showed great potential for further exploration as adsorbent for the removal of fluoride from water in continuous flow systems.
Break through curves for the effect of flow rate on fluoride adsorption.