Kinetics of Isotope Exchange on Hydroxyapatite

Kukura, M.; Bell, L. C.; Posner, A. M.; Quirk, J. P.

doi:10.2136/sssaj1973.03615995003700030016x

Cited by 10 publications

(4 citation statements)

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“…On the basis of the pH 5.0 and 7.3 isotherm regions below sorbed [Zn] surf ≈ 220 and 450 μmol/g, respectively, the calculated maximum Zn sorption densities range from 3.9 μmol/m 2 at pH 5.0 to 7.9 μmol/m 2 at pH 7.3 for the HAP surface. The calculated sorption density at pH 7.3 is similar to a theoretically calculated Ca site density of 7.2 μmol/m 2 for the HAP surface ( , ).…”

Section: Resultssupporting

confidence: 81%

Sorption Mechanisms of Zinc on Hydroxyapatite: Systematic Uptake Studies and EXAFS Spectroscopy Analysis

Lee

Elzinga

Reeder

2005

Environ. Sci. Technol.

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The systematics and mechanisms of Zn uptake by hydroxyapatite (HAP) in preequilibrated suspensions open to PCO2 were characterized using a combination of batch sorption experiments, X-ray diffraction (XRD), and extended X-ray absorption fine structure spectroscopy (EXAFS) over a wide range of pH and Zn concentrations. Sorption isotherms of Zn(II) on HAP at pH 5.0 and 7.3 show an initial steep slope at low Zn(II) concentrations, followed by a plateau up to [Zn] < approximately 750 microM, suggesting Langmuir-type behavior. At [Zn] > 750 microM, a sharp rise in the pH 5.0 isotherm suggests precipitation, whereas slight continued uptake in the pH 7.3 isotherm is suggestive of an additional uptake mechanism. The sorption isotherm at pH 9.0 shows a steep uptake step at [Zn] < or = 0.8 microM, followed by an increasing linear trend up to [Zn] = 5 microM, without any indication of a maximum, suggesting that precipitation is an important uptake process at this pH. Zn K edge EXAFS results show a first oxygen shell at 1.96-1.98 +/- 0.02 A in sorption samples with [Zn]tot < or = 250 microM at pH 5.0, 7.3, and 9.0, consistent with tetrahedral coordination. EXAFS results reveal additional P and Ca neighbors that support formation of an inner-sphere Zn surface complex where the Zn is coordinated to surface P04 tetrahedra in a corner-sharing bidentate fashion, bridging a Ca atom. In contrast, EXAFS and XRD data indicate that precipitation of Zn3(PO4)2-4H2O (hopeite) dominates the mode of Zn uptake at [Zn]tot > or = 3 mM at pH 5.0. Principal component analysis and linear combination fits of EXAFS data reveal a mixture of inner-sphere Zn surface complexation and precipitation of Zn5(OH)6(CO3)2 (hydrozincite) in sorption samples for [Zn]tot = 5 mM at pH 7.3 and for [Zn]tot = 1 mM at pH 9.0.

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Section: Resultssupporting

confidence: 81%

Sorption Mechanisms of Zinc on Hydroxyapatite: Systematic Uptake Studies and EXAFS Spectroscopy Analysis

Lee

Elzinga

Reeder

2005

Environ. Sci. Technol.

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“…The uptake level at the onset of chernikovite precipitation corresponds to a surface coverage of 0.44 µmol/m 2 or about 6% monolayer coverage relative assuming a Ca site density of 7.2 µmol/m 2 for the HA surface. The Ca site density was determined from Ca isotopic exchange at pH of the point of zero charge that was in good agreement with site density calculated from crystallographic parameters (43). The low surface coverage at the onset of precipitation may result from only a fraction of surface sites with sufficient binding energy to out-compete dissolved phosphate for complexation of U(VI) ions and/or accumulation of positive surface charge.…”

Section: Resultssupporting

confidence: 57%

Mechanisms of Uranium Interactions with Hydroxyapatite: Implications for Groundwater Remediation

Fuller

Bargar

Davis

et al. 2001

Environ. Sci. Technol.

310

256

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The speciation of U(VI) sorbed to synthetic hydroxyapatite was investigated using a combination of U LIII-edge XAS, synchrotron XRD, batch uptake measurements, and SEM-EDS. The mechanisms of U(VI) removal by apatite were determined in order to evaluate the feasibility of apatite-based in-situ permeable reactive barriers (PRBs). In batch U(VI) uptake experiments with synthetic hydroxyapatite (HA), near complete removal of dissolved uranium (>99.5%) to <0.05 microM was observed over a range of total U(VI) concentrations up to equimolar of the total P in the suspension. XRD and XAS analyses of U(VI)-reacted HA at sorbed concentrations < or = 4,700 ppm U(VI) suggested that uranium(VI) phosphate, hydroxide, and carbonate solids were not present at these concentrations. Fits to EXAFS spectra indicate the presence of Ca neighbors at 3.81 A. U-Ca separation, suggesting that U(VI) adsorbs to the HA surfaces as an inner-sphere complex. Uranium(VI) phosphate solid phases were not detected in HA with 4700 ppm sorbed U(VI) by backscatter SEM or EDS, in agreement with the surface complexation process. In contrast, U(VI) speciation in samples that exceeded 7000 ppm sorbed U(VI) included a crystalline uranium(VI) phosphate solid phase, identified as chernikovite by XRD. At these higher concentrations, a secondary, uranium(VI) phosphate solid was detected by SEM-EDS, consistent with chernikovite precipitation. Autunite formation occurred at total U:P molar ratios > or = 0.2. Our findings provide a basis for evaluating U(VI) sorption mechanisms by commercially available natural apatites for use in development of PRBs for groundwater U(VI) remediation.

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“…Studies of the sorption of metals on hydroxyapatite surfaces have been conducted for Zn2+ (21,22), Cd2+ (1, 23,24), Sr2+ (25), Ni2+, and Cu2+ (26) and for 45Ca (27,28).…”

Section: Introductionmentioning

confidence: 99%