Study of innovative materials for the removal of high concentrations of fluoride from water for drinking and domestic use

Natural fluoride contamination of drinking water is a serious issue that affects several countries of the world. The consumption of water with fluoride concentration higher than 1.5 mg/L (WHO recommended limit) is recognized to cause serious diseases, and fluoride removal from natural contaminated waters is a health priority for more than 200 million people worldwide. The octacalcium phosphate (OCP), a mineralogical precursor of bio-apatite, is here tested as a fluoride remover. Fluoride removal experiments are performed in batch-mode using different initial concentrations of fluoride (from 40 to 140 mg/L) and reaction times. Most of fluoride is removed within the first two hours of all experiments, and the drinkable limit of 1.5 mg/L is reached within a minimum of 3 hours for an initial fluoride concentration of 40 mg/L. The experimental fluoride removal capacity of OCP is 25.7 mg/g, and 4 g of OCP can effectively treat 1 L of water with fluoride concentration up to 50 times higher than the drinking limit of 1.5 mg/L. XRD and chemical characterization of the solid phases, before and after the removal experiments, indicate that OCP transforms into fluorapatite (FAP) uptaking fluoride from solution. The property of OCP to be the crystalline precursor of apatite has been also investigated using equal molar amounts of fluoride, chloride, hydroxyl and carbonate anions dissolved in aqueous solution, in order to asses if and how the anionic competition can influence the formation of the different apatite end-members. The investigation, conducted through XRD, SEM and ICP-OES analyses, shows that fluoride is the main anion removed from solution during the OCP transformation, and consequently fluorapatite is the main resulting apatitic phase, followed by hydroxyapatite. With the aim to perform the field trial, a new device based on the application of OCP has been developed. The prototype (FLOWERED Defluoridator Device [FDD]) essentially is composed of a 20‐L tank and a recirculating pump that guarantees the interaction between water and OCP. The device is powered by a car battery for a fixed pumping working time using a fixed amount of OCP for every defluoridation cycle. The results of tests performed in the rural areas of Tanzania show that a standardized use of the prototype can lower the dissolved fluoride from an initial concentration of 21 mg/L to below the World Health Organization (WHO) drinkable limit of 1.5 mg/L in 2 h without secondary negative effects on water quality. The approximate cost of this device is around US$220, whereas that of OCP is about $0.03/L of treated water. As with any device, acceptance requires a behavioural change on behalf of rural communities that needed to be investigated. To this end, we piloted a survey to explore how psychological and socioeconomic factors influence the consumption of fluoride‐free water. Results show that the adoption of FDD and OCP is more appealing to members of the rural communities who are willing to pay more and have a high consumption of water. Moreover, we suggest that given the low level of knowledge about fluorosis diseases, the government should introduce educational programs to make rural communities aware of the negative health consequences.