Defluoridation of water through the transformation of octacalcium phosphate into fluorapatite

Abstract

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 260 million people worldwide. The octacalcium phosphate (OCP), a mineralogical precursor of bio-apatite, is here tested as a fluoride remover. A new two-step method for the synthesis of OCP is proposed: 1) synthesis of brushite from calcium carbonate and phosphoric acid; 2) subsequent hydrolysis of brushite. 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 2 h of all experiments, and the drinkable limit of 1.5 mg/L is reached within a minimum of 3 h 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.

Mineralogical-geochemical study of the anionic competition effect on the octacalcium phosphate reaction into fluorapatite

“…The OCP was synthesized following the method described by Idini et al. (2019) , which briefly consists of the synthesis of dicalcium phosphate dihydrate (DCPD, CaHPO 4 · 2H 2 O) at room temperature using H 3 PO 4 and CaCO 3 , and the subsequent hydrolysis of DCPD into OCP in a solution of ultrapure water with a starting pH of 7.2.…”

“…(1) , a slightly higher F − amount was used (0.368 mmol in 50 mL, that is 7.37 mmol/L corresponding to 140 mg/L), and the molarity of the other competing anions in solution was set up to that value. The time required because the OCP → FAp reaction goes close to completion in such conditions is less than 16 h, therefore every experiment was repeated, and pH and F − concentrations monitored, after 16, 20 and 24 h ( Idini et al., 2019 ) in order to verify the homogeneity of the measurements, for a total amount of 30 samples (Supplemental Data Table SD1). The complete chemical composition of solutions and solids was determined on the samples of the first trial after 24 h.…”

“…Recent studies have shown that OCP reacts in the presence of fluoride in solution to form FAp ( Idini et al., 2019 ), and this reaction occurs also in biological conditions. The formation of FAp, instead of bioapatite, in mammal hard tissues causes different pathologies called skeletal and dental fluorosis ( Everett, 2011 ).…”

“…The World Health Organization (WHO) indicates the fluoride-rich groundwaters as the main source of the excessive intake of fluorine by people, and the WHO guidelines for drinking-water quality (GDWQ) adopt a fluoride concentration of 1.5 mg/L as the permissible limit for human consumption ( Fawell et al., 2006 ). As reported in the literature ( Idini et al., 2019 , 2020 ), in the frame of the FLOWERED project ( www.floweredproject.org , Horizon2020 program funded by the European Commission) the use of OCP has been applied to uptake high dissolved fluoride from groundwater through the mineralogical transformation into FAp.…”

Application of Octacalcium Phosphate with an Innovative Household‐scale Defluoridator Prototype and Behavioral Determinants of its Adoption in Rural Communities of the East African Rift Valley

“…The underpinning key concept of the method for water defluoridation is to transfer the mechanism of absorption by the human body to a defluoridator device based on the same mineralogical principle. A previous research paper showed that OCP has empirical removal capacity of 25.7 mg/g; the water treatment lasts a few hours and is free of collateral effects (Idini et al 2019). To meet the requirement “easy to use” targeted for rural communities of the East African Rift Valley (EARV), a standardization of the defluoridation procedure and use of the prototype named “FLOWERED Defluoridator Device” (FDD) is presented.…”