Novel apatite-based sorbent for defluoridation: synthesis and sorption characteristics of nano-micro-crystalline hydroxyapatite-coated-limestone.
Elevated levels of fluoride (F(-)) in groundwaters of granitic and basaltic terrains pose a major environmental problem and are affecting millions of people all over the world. Hydroxyapatite (HA) has been shown to be a strong sorbent for F(-); however, low permeability of synthetic HA results in poor sorption efficiency. Here we provide a novel method of synthesizing nano- to micrometer sized HA on the surfaces of granular limestone to improve the sorption efficiency of the HA-based filter. Our experiments with granular limestone (38-63, 125-500 μm) and dissolved PO4(3-) (0.5-5.3 mM) as a function of pH (6-8) and temperature (25-80 °C) indicated rapid formation of nano- to micrometer sized HA crystals on granular limestone with the maximum surface coverage at lower pH and in the presence of multiple additions of aqueous PO4(3-). The HA crystal morphology varied with the above variables. The sorption kinetics and magnitude of F(-) sorption by HA-coated-fine limestone are comparable to those of pure HA, and the F(-) levels dropped to below the World Health Organization's drinking water limit of 79 μM for F(-) concentrations commonly encountered in contaminated potable waters, suggesting that these materials could be used as effective filters. Fluorine XANES spectra of synthetic HA reacted with F(-) suggest that the mode of sorption is through the formation of fluoridated-HA or fluorapatite at low F(-) levels and fluorite at high F(-) loadings.
Department of Geosciences, Princeton University , Princeton, New Jersey 08544, United States.
Cynthia M
CM
Kanno
Rebecca L
RL
Sanders
Steven M
SM
Flynn
Genevieve
G
Lessard
Satish C B
SC
Myneni
D000327:Adsorption; D002119:Calcium Carbonate / Q000138:chemical synthesis* / Q000737:chemistry*; D003460:Crystallization; D017886:Durapatite / Q000138:chemical synthesis* / Q000737:chemistry; D004777:Environment; D005459:Fluorides / Q000737:chemistry / Q000302:isolation & purification*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008903:Minerals / Q000737:chemistry; D053758:Nanoparticles / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry; D017550:Spectroscopy, Fourier Transform Infrared; D013696:Temperature; D013997:Time Factors; D056928:X-Ray Absorption Spectroscopy; D014961:X-Ray Diffraction
D008903:Minerals; D010710:Phosphates; D017886:Durapatite; D002119:Calcium Carbonate; D005459:Fluorides