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Published in Soil Sci. Soc. Am. J. 67:1935-1942 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-9—SOIL MINERALOGY

Mineralogical and Chemical Characterization of Iron-, Manganese-, and Copper-Containing Synthetic Hydroxyapatites

B. Sutter*,a,d, D. W. Mingb, A. Clearfieldc and L. R. Hossnerc

a Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
b NASA Johnson Space Center, Houston TX 77058
c Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
d SETI Institute, NASA Ames Research Center, Moffett Field, CA 94035

* Corresponding author (bsutter{at}mail.arc.nasa.gov).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
The National Aeronautics and Space Administration's (NASA) Advanced Life Support (ALS) Program is evaluating the use of Fe-, Mn-, and Cu-containing synthetic hydroxyapatite (SHA) as a slow release fertilizer for crops that might be grown on the International Space Station or at Lunar and Martian outposts. Separate Fe-, Mn-, and Cu-containing SHA materials along with a transition-metal free SHA (pure-SHA) were synthesized using a precipitation method. Chemical and mineralogical analyses determined if and how Fe, Mn, and Cu were incorporated into the SHA structure. X-ray diffraction (XRD), Rietveld refinement, and transmission electron microscopy (TEM) confirmed that SHA materials with the apatite structure were produced. Chemical analyses indicated that the metal containing SHA materials were deficient in Ca relative to pure-SHA. The shift in the infrared PO4–{nu}3 vibrations, smaller unit cell parameters, smaller particle size, and greater structural strain for Fe-, Mn-, and Cu-containing SHA compared with pure-SHA suggested that Fe, Mn, and Cu were incorporated into SHA structure. Rietveld analyses revealed that Fe, Mn, and Cu substituted into the Ca(2) site of SHA. An Fe-rich phase was detected by TEM analyses and backscattered electron microscopy in the Fe-containing SHA material with the greatest Fe content. The substitution of metals into SHA suggests that metal-SHA materials are potential slow-release sources of micronutrients for plant uptake in addition to Ca and P.

Abbreviations: ALS, Advanced Life Support • BSE, Backscatter electron • EDS, energy dispersive spectroscopy • EMPA, electron microprobe analysis • EPR, electron paramagnetic resonance • NASA, National Aeronautics and Space Administration • SHA, synthetic hydroxyapatite • TEM, transmission electronic microscopy, XRD, X-ray diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
THE National Aeronautics and Space Administration's (NASA's) Advanced Life Support (ALS) program is currently evaluating a slow-release fertilizer for growing crops that will minimize resupply costs for long duration human missions on the moon and Mars (Ming et al., 1995). Plants will be used to supply food, recycle air, and regenerate water (evapotranspiration) for the astronauts (Averner, 1989). This slow-release fertilizer (termed zeoponics) is composed of NH+4– and K+–exchanged clinoptilolite and a nutrient (Fe, Mn, Cu, Zn, Mo, B, S, Cl, Mg) containing SHA (Ming et al., 1995; Steinberg et al., 2000). Plant nutrients incorporated into the sparingly soluble SHA structure will be slowly released as the SHA dissolves. Slow release of nutrients by SHA is attractive for cropping systems that are needed to maintain life support needs for a crew on long duration missions for a year or more (e.g., Mars outpost).

Hydroxyapatite, [Ca10(PO4)6(OH)2], has hexagonal symmetry (P63/m space group). Two non-equivalent Ca positions form columns of polyhedra around a central (001) hexad (Liu and Comodi, 1993). The Ca(1) site is a polyhedron coordinated by nine oxygen atoms while the Ca(2) site is an octahedron coordinated by five oxygen atoms and a hydroxyl (OH-) group. The Ca(2) site also forms a weak seventh bond to another oxygen [O(1)] making a coordination total of seven (Hughes et al., 1989). The Ca(2) atoms form a triangle around the [001] zone axis at z = 0.25 and 0.75 that contains OH-. The Ca(1) and Ca(2) polyhedra and phosphate tetrahedra are linked by shared oxygen atoms.

Previous studies have confirmed that Fe, Mn, and Cu were incorporated into the SHA structure (Tripathy et al., 1989; Golden and Ming, 1999). The PO4–{nu}3 IR absorption wave number values of substituted SHA have been reported to be shifted relative to unsubstituted SHA (Tripathy et al. 1989; Golden and Ming, 1999). The substitution of metals into SHA has been shown to cause a shift in the d[002] XRD peak relative to pure-SHA as predicted by Vegard's rule for isomorphic substitution (Golden and Ming, 1999). The lower level of metal substitutions in this study were not expected to cause a significant shift in the d[002] peak, but Rietveld refinement was anticipated to indicate metal substitution. The Rietveld procedure examines the entire XRD pattern (not just one peak), which in addition to providing a wealth of structural information, results in a sensitive analysis of X-ray data.

Single-crystal structure refinements of natural Mn- and Sr-bearing apatites have shown that Mn2+ and Sr2+ occur in the Ca(1) and Ca(2) sites, respectively (Hughes et al., 1991). Rietveld refinement indicated that Mn2+ occupies the Ca(1) site (Suitch et al., 1985) while Mg2+, Sr2+, Na+, and Sb3+ occupy the Ca(2) site (Sudarsanan and Young, 1980; DeBoer et al., 1991; Bigi et al., 1996; Bigi et al., 1998; Feki et al., 1999).

Nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy have shown that Fe3+, Mn2+, and Cu2+ were incorporated into the SHA structure, and possibly substituted for Ca2+ in the SHA materials of this study (Sutter et al., 2002a, b). Electron paramagnetic resonance spectroscopy also indicated that amorphous/short-order Fe-, Mn-, and Cu-phases were associated with Fe-, Mn-, and Cu-SHA (Sutter et al., 2002b). The objectives of this research were to characterize the mineralogical and chemical properties of Fe-, Mn-, and Cu-containing SHA and to utilize Rietveld refinement to determine the structural locations of Fe3+, Mn2+, and Cu2+ in SHA. This study builds on the work of Golden and Ming (1999) by examining lower concentration ranges of Fe3+ and Mn2+ in SHA (1.1–2.5 wt. % versus 4.9–5.1 wt. %) and by assessing the location of the Fe3+, Mn2+, and Cu2+ in the SHA structure. Furthermore, we report on the first Rietveld analyses of Fe3+ and Cu2+ substituted SHA.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 
Hydroxyapatite Synthesis
Pure-SHA was synthesized by a procedure similar that described by Golden and Ming (1999). Calcium nitrate [Ca(NO3)2 · H2O] (235 g) was dissolved in 420 mL of 20% (v/v) NH4OH and 72.2 g of (NH4)2HPO4 was dissolved in 380 mL of deionized water. After the (NH4)2HPO4 was completely dissolved, 30 mL of 20% (v/v) NH4OH was added to the (NH4)2HPO4 solution. The P solution was combined with the Ca solution and mixed by a propeller stirrer for 24 h. After mixing, the precipitate was allowed to age for 48 h. Subsequently, the solution was decanted and the SHA precipitate was washed four times with 2.5 L of deionized water to remove excess NH4OH and NO-3. After washing, the SHA precipitate was separated from solution by filtering through Whatman #41 filter paper (Whatman, Clifton, NJ). The washed pure-SHA precipitate was heated in an oven at 400°C for 24 h.

Transition-metal substituted SHAs were produced by first dissolving the appropriate transition metal (see Table 1) in 100 mL of deionized water. The metal solution was then added to the P solution (as described above) and mixed for 5 min. The metal-P solution was then added to the Ca solution (as described above), and the remaining procedures for mixing, washing, and drying were followed as outlined above. A total of six transition-metal containing SHAs were synthesized using these procedures (Table 1). The six transition-metal containing SHAs will collectively be termed metal-SHA.


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  		Table 1. Total amounts of transition metal salts used to prepare synthetic hydroxyapatite (SHA).

 
Electron Microprobe Analysis
Electron microprobe analysis (EMPA) was used to determine the chemical compositions (i.e., Ca, P, S, Fe, Mn, Cu) of SHA materials. The SHA materials were pressed into pellets (103.5 MPa) and analyzed with a Cameca SX-50 electron microprobe (Cameca, Trumbull, CT) at 15 kV and 10 nA with an electron beam diameter of 20 µm. The electron microprobe sample stage was moved back and forth at 20-µm steps over a length of 200 µm to obtain an average chemical analysis. Polished C.M. Taylor (Gold Beach, OR) and Smithsonian standards were used for calibration. Backscatter electron (BSE) images of pressed pellets of all di-ethylene-triamine-pentaacetic acid (DTPA) treated SHA materials were collected at 15 kV and 3 nA with acquisition times of 60 s.

Results of the EMPA analyses were used to determine the Ca/P and (Ca + metal)/P mole ratios. The S in the Fe- and Mn-SHA materials was assumed to be SO2-4 substituting for PO3-4 (Khorari et al., 1994; Golden and Ming, 1999). For samples with S, (Ca + metal)/(P + S) mole ratio was utilized.

Transmission Electron Microscopy
Morphology and size of SHA crystallites were examined with a JEOL JEM 2010 TEM (JEOL, Peabody, MA). Synthetic hydroxyapatite material was added to acetone until a milky appearance was noticed in the SHA + acetone suspension. The suspension was further dispersed in a sonicator for 30 s. A drop of the SHA + acetone suspension was then evaporated onto a holey-carbon film and examined by TEM.

Infrared Spectroscopy and X-ray Diffraction Analysis
A Perkin Elmer 2000 Fourier transform infrared (FT-IR) spectrometer (Perkin Elmer, Norwalk, CT) was used for transmission FT-IR with a 1-cm-1 resolution and a scan range of 4000 to 450 cm-1. The sample was ground in Nujol and placed between two KBr discs for analysis. X-ray diffraction data of powdered samples were collected with a Rigaku 200 powder diffractometer (Rigaku, The Woodlands,TX). Copper K{alpha} X-rays were produced with a rotating anode operated at 50 kV and 180 mA. The samples were analyzed from 10 to 100° 2{theta} using 0.02° steps and 10-s integration times.

Rietveld Refinement
Rietveld refinement was performed using the General Structure Analysis System (GSAS) (Larson and Von Dreele, 1998). Initial atomic positions, temperature factors, and unit cell dimensions were obtained from the Holly Springs, GA single-crystal hydroxyapatite (Hughes et al., 1989). The SHA materials were refined in the P63/m space group. The background was calculated using the Chebyschev polynomial, whereas the profile was fitted using the pseudo-Voight function. Rietveld refinement of lanthanum hexaboride (LaB6) (National Institute of Standards and Technology, Gaithersburg, MD) provided the Gaussian profile coefficients U, V, W, and P with values of 0.1, -0.1, 1.4, and 0, respectively. The X-ray diffractometer contributes to the Gaussian component of the profile coefficients; therefore, the Gaussian profile coefficients were not varied (Delhez et al., 1993; Baig et al., 1999). Profile coefficients that were varied included the Lorentizian microstrain (Ly), Lorentizian microstrain anisotropy (ptec), Lorentizian particle size (Lx), Lorentizian particle-size anisotropy (stec), and asymmetry (asym) and shift (shft) coefficients.

The refinement process began with refining the background, scale factor and the a and c lattice parameters. Subsequently, the profile coefficients were refined followed by the atomic positions and Ca/transition metal occupancy factors. The estimated standard deviations presented are double the original values in the Rietveld refinement output. This was done to ensure that the estimated standard deviations were not underestimated.

The Lorentizian particle strain and particle-size values were used to determine particle size and percentage of strain for SHA materials. The following equations were used to determine particle size and strain.





where K (Scherrer constant) is equal to one, {lambda} is the Cu K{alpha} radiation wavelength (1.5406 Å), Lyi is the strain broadening value due to the instrument and Lx and Ly are as defined above (Larson and VonDreele, 1998). The Lyi value for strain broadening due to the instrument was determined by refining standard LaB6 (National Institute of Standards and Technology, Gaithersberg, MD). The parallel and perpendicular particle size refers to the average sizes parallel and perpendicular to the elongated c-axis of the SHA particles. The strain is given as a percentage of a unit cell dimension (Von Dreele personal communication, 2000). For example, 0% indicates that there is no variation in the unit cell dimensions. Strain values of 0.1 to 2.0% indicate significant variation in the unit cell sizes caused by defects (Von Dreele personal communication, 2000).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSION
 REFERENCES
 
X-ray Diffraction
The XRD pattern (Fig. 1) indicated that the synthesized materials were hydroxyaptatite. No other phases were detected by XRD analysis. X-ray diffraction peaks became broader as the Fe, Mn, and Cu concentration increased in comparison with pure-SHA (Fig. 1). The peak broadening is attributed to smaller particle size and greater structural strain in the metal-SHA relative to the pure-SHA (Bigi et al., 1996). This is supported by the Rietveld calculations of Lorentzian particle size and strain and will be discussed below. Furthermore, increased peak broadening of the metal-SHAs suggested that the incorporation of Fe, Mn, and Cu into SHA also decreased the crystallinity of SHA (Bigi et al., 1996; Hidaka et al., 1996; Golden and Ming, 1999). X-ray diffraction peak positions of the metal-SHA materials did not shift relative to those of pure-SHA. The concentrations of Fe, Mn, and Cu in the metal-SHAs were too low to cause measurable shifts in XRD peak positions, which would have indicated substitution of Ca by the metals in SHA.



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  		Fig. 1. X-ray diffraction patterns (CuK{alpha} radiation) of Pure-, Fe12-, Fe25-, Mn11-, Mn24-, Cu12-, and Cu20-synthetic hydroxyapatite (SHA) materials (d-spacings in nanometers).

 
Electron Microprobe Analysis
The measured Ca/P ratios for pure-SHA were slightly less than the ideal stochiometric ratio for hydroxyapatite (1.67) (Table 2). Calcium deficient hydroxyapatites are known to form during precipitation from solution and usually have the following formula


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  		Table 2. Total chemical compositions and molar (Ca + metal)/P ratios of the synthetic hydroxyapatite (SHA) materials. Standard deviations are from three replicates.

 
(LeGeros et al., 1978; Mortier et al., 1989; Morgan et al., 2000). Heating Ca deficient hydroxyapatite to 400°C results in the following reaction.


(Mortier et al., 1989)

A small feature at 735 cm-1 (infrared data not shown) for the SHA materials in this study indicated that a small amount of P2O4-7 was present in all SHA materials and was the result of heating SHA (see Materials and Methods) (LeGeros et al., 1978). Previous work has shown that Ca-deficient hydroxyapatite retain a hydroxyapatite structure with some structural defects (Mortier et al., 1989; Morgan et al., 2000). Rietveld refinement indicated that the SHA materials in this work also retained a hydroxyapatite structure.

The stochiometry of Fe-SHA and Mn-SHA materials deviated from that of pure-SHA (Table 2). This deviation in stochiometry suggested that Fe and Mn may have modified the SHA structure and inhibited Ca incorporation. Other ions substituting for Ca into apatite are known to induce modifications of crystallinity, morphology, lattice parameters, and stability of the apatite structure (Bigi et al., 1996). The Cu-SHA materials possessed (Ca + Cu)/P ratios close to pure-SHA suggesting that Cu was less effective than Fe and Mn at inhibiting Ca incorporation.

Backscattered electron images of Fe25-SHA illustrated small (3–10 µm diameter) white particles (Fig. 2) . White particles were not observed in Fe12-, Mn11-, Mn24, Cu12, and Cu20-SHA. The white particles in Fe25-SHA were enriched in Fe and deficient in Ca relative to areas without the white particles (Tables 2 and 3). Electron paramagnetic resonance (EPR) spectrum of Fe25-SHA suggested the presence of Fe-phosphate in this sample (Sutter et al., 2002b). A 5-µm electron beam diameter was used to analyze these 5- to 7-µm diam. Fe-enriched particles. The interaction volume of the beam no doubt also sampled SHA materials surrounding the Fe-enriched particle and resulted in the detection of some Ca (Table 3). The presence of Fe-phosphate particles had no effect on the total chemical analyses of Fe25-SHA because the analyses were performed on areas free of the Fe-phosphate particles (i.e., white particles found in BSE).



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  		Fig. 2. Backscattered electron image of Fe25-synthetic hydroxyapatite (SHA). The dominant gray area contains Fe25-SHA spotted with white particles high in iron and low in calcium (Arrow 1). See Table 3 for chemistry of white particles. Pellet surface depressions are indicated by Arrow 2.

 

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  		Table 3. Total elemental compositions of two representative white particles in the back scattered electron image (Fig. 2) of Fe25-SHA. Standard deviations are from three replicates.

 
Transmission Electron Microscopy
All SHA materials exhibited elongated crystallites with rounded ends (e.g., Mn11-SHA, Fig. 3a) . No differences in SHA crystallite morphology were observed between SHA materials. The elongated particles were confirmed to be apatite due to the observance of the apatite (100) d-spacing of 0.81 nm in high resolution TEM (HRTEM) images of all SHA materials (Fig. 3a). The elongated SHA particles were also similar to synthetic apatite particles reported in the literature (Nelson et al., 1989; Golden and Ming, 1999; Yasukawa et al., 1999). The SHA particles had lengths of 20 to 126 nm and widths of 5 to 22 nm. Aggregation of the SHA particles did not allow for an accurate statistical analysis of lengths and widths of the SHA materials. Therefore, it was difficult to determine if any relationships existed between crystallite size and substitution levels of metals in SHAs.



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  		Fig. 3. Transmission electron microscopy (TEM) of SHA samples: (a) TEM of Mn11-SHA, Arrow 1 indicates elongated crystallite with rounded head, Arrows 2 indicate oval features, and Number 3 indicates energy dispersive spectrum site (Fig. 4). Particle with apatite (100) d-spacing 0.81 nm indicates Mn11-SHA crystallites have apatite structure; (b) TEM of Fe-enriched particle found in Fe25-SHA, Arrow 1 indicates energy dispersive spectrum site (Fig. 4).

 


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  		Fig. 4. Energy dispersive spectra of Mn11-synthetic hydroxyapatite (SHA) particles shown in Fig 3a (Arrow 3) and of an Fe-enriched particle shown in Fig. 3b (Arrow 1).

 
The oval features similar to those shown for Mn11-SHA in Fig. 3a were observed for all of the SHA materials (data not shown). Oval features have been observed in other synthetic apatites and were attributed to variations in crystal thickness (Nelson, 1981; Nelson et al., 1983; Nelson et al., 1989).

Energy dispersive spectroscopy (EDS) spectra of the Mn11-SHA crystallites all possessed low Mn peak intensities similar to that shown in the Mn11-SHA EDS spectra in Fig. 4 . This indicated that no phases other than Mn11-SHA were detected in the Mn11-SHA sample. Similarly, no phases other than metal-SHA were detected in Fe12-SHA, Mn24-SHA, and Cu-SHA materials (data not shown). The amorphous/short-order phases detected by EPR for these samples (Sutter et al., 2002b) apparently did not exist as individual particles or were at concentrations too low to be detected by TEM. The Fe-rich phase that was detected by BSE microscopy and determined to be a Fe phase separate from Fe25-SHA was detected by TEM and EDS analyses (Fig. 3b and 4). The mineralogy of this Fe-enriched phase could not be assessed because of its instability in the electron beam.

Infrared Spectroscopy
The Fe- and Mn-SHA v3–PO4 vibration were shifted to higher wavenumbers relative to v3–PO4 vibration for the pure-SHA which indicated that Fe and Mn substituted for Ca in SHA (Table 4). The v3–PO3-4 vibrational wavenumbers increase because Fe and Mn have a shorter ionic radius than Ca causing an increase in the P-O bond strength (Golden and Ming, 1999). The difference between Cu-SHA and pure-SHA v3–PO4 vibrational wavenumbers was less than the differences observed for the Fe- and Mn-SHA materials relative to pure-SHA. Two possibilities may explain the Cu-SHA IR results, (i) Cu did not substitute into SHA as much as did Fe and Mn, or (ii) Cu substitution did not cause the P-O bond strength to increase as much as Mn and Fe substitution.


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  		Table 4. Numerical criteria of fit (Rwp), unit cell lattice parameters, unit cell volumes, particle size, and strain parallel and perpendicular to the (001) axis, and occupancy factors (fraction occupied) for transition metals in Ca(2) site of the hydroxyapatite structure as determined by Rietveld refinement. Estimated standard deviations are from Rietveld refinement. The PO4–v3 infrared peak in wavenumbers (1 cm-1 resolution) for all synthetic hydroxyapatites (SHA) are also listed.

 
Rietveld Analysis
The Rietveld criteria of fit (Rwp), lattice parameters, particle strain, particle size, and occupancy values of the SHA materials are presented in Table 4. The low Rwp values for this work compare well with literature values of successful Rietveld refinement of cation substituted apatites (Rwp = 0.052 - 0.161) (Deboer et al., 1991; Bigi et al., 1996; Bigi et al., 1998; Baig et al., 1999; Feki et al., 1999). The mostly flat difference curve between the calculated and observed XRD patterns for pure-SHA (Fig. 5) and the metal-SHA materials (data not shown) are further evidence that the Rietveld refinements were successful in describing the SHA materials. The smaller Rwp values of the metal-SHA relative to the pure-SHA were caused by the higher X-ray background in the metal-SHA compared with pure-SHA. The XRD patterns become easier to model as the XRD background intensity increases (Young, 1993).



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  		Fig. 5. Overlay of experimental X-ray diffraction pattern (+) and Rietveld refinement calculated X-ray diffraction pattern (—) of pure-synthetic hydroxyapatite using CuK{alpha} radiation. The lower line is the difference pattern between the experimental and calculated X-ray diffraction patterns.

 
Metal-SHAs had shorter a and c lattice parameters and unit cell volumes than pure-SHA suggesting that Fe, Mn, and Cu were incorporated into the SHA structure and possibly substituted for Ca (Table 4). The substitution of Cu, Zn (Tripathy et al., 1989), and Mg (Bigi et al., 1996) into SHA structure and the substitution of Fe into fluorapatite (Okazaki et al., 1986) have been previously shown to shorten the a and c parameters relative to a and c parameters of unsubstituted apatite materials.

The metal-SHAs had greater XRD peak broadening than pure-SHA because the metal-SHAs had smaller parallel and perpendicular particle sizes than pure-SHA (Table 4). The parallel particle size decreased with increasing metal concentration for Fe- and Cu-SHA (Table 4). The Mn-SHA materials had an increase in parallel and a decrease in perpendicular particle size with increasing Mn concentration. The presence of Mn, Fe, and Cu in SHA might have retarded crystal growth, which resulted in smaller particles because not enough time was allowed for growth during synthesis (Hartman, 1982).

All SHA materials showed significant structural strain (Table 4). However, as the metal concentration increased, the parallel and perpendicular strain increased relative to pure-SHA. The greater strain in the metal-SHAs also contributed to X-ray peak broadening. Iron(III), Mn2+, and Cu2+ ionic radii are smaller (0.064, 0.080, and 0.072 nm, respectively) than Ca2+ (0.099 nm). Strain occurs when the other atoms (e.g., Ca, P, and O) in the SHA structure adjust their positions to accommodate the smaller metal ions. Low concentrations of Mg2+, Ni2+, Fe2+, and Co2+ in SHA have been known to strain the SHA structure to the point of collapse (Le Geros et al., 1980). X-ray diffraction peak broadening has been previously reported for Fe2+ (Okazaki et al., 1986; Tanizawa et al., 1990), Zn2+ (Bigi et al., 1995), and CO2-3 (Chickerur et al., 1980; Baig et al., 1999) incorporated into SHA.

Positive metal occupancy factors indicated that Fe, Mn, and Cu substituted into the Ca(2) site (Table 4). Negative metal occupancy factors resulted when Fe, Mn, and Cu were refined in the Ca(1) site; this indicated that Fe, Mn, and Cu did not occur in the Ca(1) site. The Fe12-, Mn11-, and Cu20-SHA materials had metal Ca(2) site occupancies that were within their estimated standard deviations indicating that the amounts of the metals were too low for the refinement procedure to show statistically significant substitution. Nevertheless, significantly positive Ca(2) occupancy results for the Fe25-, Mn24-, and Cu12-SHA materials demonstrated that metals were substituted into the Ca(2) site and coupled with all the other analyses presented in this work suggest that the metals also substituted into Ca(2) site of the Fe12-, Mn11-, and Cu20-SHA materials. Rietveld refinement of other substituted SHA has reported Mg2+, Sr3+, Na+, and Sb3+ substituted in the Ca(2) site (Sudarsanan and Young, 1980; DeBoer et al., 1991; Bigi et al., 1996; Bigi et al., 1998; Feki et al., 1999).

Rietveld refinement determined that Cu occupancy in Cu20-SHA was lower than in the Cu12-SHA (Table 4). The Cu-SHA materials had similar parallel strain values, but lower perpendicular strain values relative to the other SHA materials. Thus the Rietveld refinement provided additional support for less Cu substitution in Cu-SHA than Fe and Mn substitution in the Fe- and Mn-SHA. Electron paramagnetic analyses indicated that Cu-oxyhydroxide or Cu-phosphate phases resistant to DTPA extraction are associated with the Cu-SHA materials (Sutter et al., 2002b). The higher concentration of Cu during synthesis of Cu20-SHA may have caused formation of more Cu-oxyhydroxide or Cu-phosphate phase(s) and less Cu substitution into SHA. Iron(III) and Mn2+ (d5) have electronic configurations with ideal octahedral symmetry, and Cu2+ (d9) possesses a distorted octahedral symmetry (Jahn-Teller effect). The distorted octahedral symmetry of Cu2+ may cause Cu2+ substitution in SHA to be more difficult than substitution of Fe3+ and Mn2+ into SHA.


   CONCLUSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
REFERENCES
 
X-ray diffraction and TEM analyses indicated that SHA materials produced in this work had hydroxyapatite mineralogy. Total chemical analyses indicated that SHA materials were deficient in Ca. The XRD (i.e., peak broadening) and infrared analyses suggested that Fe, Mn, and Cu substituted into the SHA structure. Rietveld refinement of the metal-SHA materials demonstrated that the Fe, Mn, and Cu substituted into the Ca(2) site of SHA, however, Cu substitution appeared to be less than that of Fe and Mn. The amorphous/short-order metal phases (i.e., non hydroxyapatite phases) detected previously by EPR analyses (Sutter et al., 2002b) were apparently too low in concentration to be detected by XRD, TEM, and EDS analyses. However, an Fe-rich phase, was detected by TEM and BSE analyses in the highest Fe-substituted sample (Fe25-SHA). The Fe-rich phase was not detected by XRD because of its low concentration or because it was amorphous. Greater strain and lower particle sizes in metal-substituted SHA may result in different solubilities and dissolution rates compared with SHA without substituted metals. Dissolution studies are needed to assess the solubilities and dissolution rates of metal-SHA to further assess the potential of metal-SHA as a nutrient source for plants.


   ACKNOWLEDGMENTS
 
This research was partially funded by NASA's Graduate Student Researchers Program (NGT51229) and NASA's ALS Program. The authors acknowledge Ray Guillimette, for assisting with the electron microprobe work; Carl Dufner, Texas A&M Electron Microscopy Center for access and guidance with the transmission electron microscopy; Franz Gingl, Bob Von Dreele, and Robert Young for assistance with GSAS and Rietveld refinement; Jaan Laane and Daniel Autry for infrared spectroscopy assistance; and Joe Dixon for help with transmission electron microscopy interpretation. We also appreciate DC Golden, the three anonymous reviewers and the associate editor David Laird for their critical review of this manuscript.

Received for publication September 3, 2002.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSION
 REFERENCES
 




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B. Sutter, L. R. Hossner, and D. W. Ming
Dissolution Kinetics of Iron-, Manganese-, and Copper-Containing Synthetic Hydroxyapatites
Soil Sci. Soc. Am. J., March 1, 2005; 69(2): 362 - 370.
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