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

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

Production of Black Carbon-like and Aliphatic Molecules from Terrestrial Dissolved Organic Matter in the Presence of Sunlight and Iron

Photochemical processing of dissolved organic matter (DOM) in natural waters can alter its composition and structure, supply particulate organic matter (POM) to sediments, and deliver modified terrestrial DOM to the ocean. Our studies show that terrestrial DOM exposed to simulated sunlight is altered to produce POM with a markedly different molecular composition enriched with newly formed aliphatic and condensed aromatic molecules. This process is closely tied to the chemistry of iron, which primarily exists as dissolved Fe­(II) and Fe­(III)–organic complexes in initial DOM and photochemically matures to Fe­(III) oxyhydroxides before coprecipitating out with POM. The newly formed condensed aromatic compounds resemble black carbon, which until now was thought to be produced by only combustion. These new molecules contribute a pool of Fe-rich, aliphatic, and black carbon-like organic matter to sediments as the terrestrial DOM is transported through rivers. We estimate that the annual global flux of this photoproduced black carbon, most of which may be preserved in sediments, is nearly equivalent to the estimated flux of dissolved black carbon to the ocean from all other sources