Fluoride removal from water by Ca-Al-CO₃ layered double hydroxides and simultaneous acidification

Millions of people worldwide are exposed to excessive concentrations of fluoride (F−) from groundwater sources. Ca-Al-CO3 layered double hydroxides (LDHs) have shown promising defluoridation efficiency; however, defluoridation by Ca-Al-CO3 LDHs is highly pH sensitive. This study showed that simultaneous acidification by conventional acids, such as HCl and CO2 substantially increased the performance of Ca-Al-CO3 LDHs for F- removal at environmentally relevant concentrations (e.g., 10 mg/L) to below the WHO guideline value (1.5 mg/L), while, in comparison to other acids (HNO3, H2SO4, H3PO4), the use of HCl and CO2 does not lead to the introduction of potentially harmful or undesired anions. The addition of HCl and CO2 to LDHs suspensions did lead to changes to the LDHs structure. Leaching experiments, supported by PHREEQC modelling and characterization (SEM-EDX, XRD and FTIR), strongly suggest that the main mechanism of F- removal by Ca-Al-CO3 LDHs was F− adsorption or complexation onto/into various rehydrated mixed metal oxides which re-precipitated upon partial LDHs dissolution when acidifying.