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

Abstract

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 SO₄²⁻ 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.