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

Abstract

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.