Fluoride removal by Ca-Al-CO₃ layered double hydroxides at environmentally-relevant concentrations

In this study, F^- removal by Ca-Al-CO₃ layered double hydroxides (LDHs) was investigated at environmentally-relevant concentration ranges (2-12 mg/L) to below the WHO guideline, with an emphasis on the effect of LDHs' modification, as well as the effects of initial F^- concentration, adsorbent dose, pH, temperature and co-existing ions. Ca-Al-CO₃ LDHs, either untreated, calcined or microwave treated, showed affinity for the removal of F^- from synthetic groundwater with capacities of 6.7-8.4 mg F^-/g LDHs at groundwater-relevant pH, with a higher F^- removal capacity at lower pH (<8) and lower temperature (12 °C, as compared to 25 °C & 35 °C). Since calcination and microwave treatment resulted in only marginal defluorination improvements, using untreated LDHs appears the practically most feasible option. For the untreated LDHs, competition with Cl^- and NO₃^- was not observed, whereas at higher HCO₃^- and SO₄^2- concentrations (>250 mg/L) a slight reduction in F^- removal was observed. This study indicates the potential of Ca-Al-CO₃ LDHs as a cost-effective F^- removal technology, particularly when locally sourced and in combination with low-cost pH correction.