Synthesis and characterization of functional calcium-phosphate-chitosan adsorbents for fluoride removal from water

Functional calcium-phosphate-chitosan adsorbents for fluoride (F^-) removal from water with different proportions of calcium (0.7 or 1.4 % w/v) were synthesized by: i) ionotropic gelation technique followed by drying in a convection oven (IGA) or freeze drying (FDA); ii) freeze-gelation followed by drying in a convection oven (FGA). Adsorbents were analyzed by SEM-EDX and FTIR- ATR. F^- removal percentages higher than 45 % were obtained with calcium-phosphate-chitosan adsorbents for an initial F^- concentration of 9.6 mg L^-1. Optimal conditions for F^- removal were attained, using calcium-phosphate- chitosan adsorbents synthesized by ionotropic gelation with 0.7 % of Ca (IGA0.7). Under these conditions, initial F^- concentration of 5 mg L^-1, was reduced below the maximum limit of 1.5 mg L^-1 established by WHO. Regeneration of IGA0.7 was achieved in acid media. The performance of IGA0.7 was slightly reduced in the presence of coexisting anions (nitrate, carbonate, arsenate). Adsorption kinetics was represented satisfactorily by the pseudo-second order equation; Langmuir isotherm provided the best fit to the equilibrium data and IGA0.7 exhibited a maximum F^- adsorption capacity q_L = 132.25 mg g^-1. IGA0.7 particles were characterized by thermogravimetry coupled to FTIR, XRD, XPS and SEM-EDX. The calcium-phosphate-chitosan adsorbents constitute a suitable and emerging material for water defluorination.