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DIVISION S-9—SOIL MINERALOGY

Characterization of Iron, Manganese, and Copper Synthetic Hydroxyapatites by Electron Paramagnetic Resonance Spectroscopy

^a National Research Council, NASA Ames Research Center, Moffett Field, CA 94035
^b Dep. of Physics and Astronomy, Georgia State Univ., Atlanta, GA 30303
^c Dep. of Chemistry, Texas A&M Univ., College Station, TX 77843
^d Dep. of Soil and Crop Sciences, Texas A&M Univ., College Station, TX 77843
^e NASA Johnson Space Center, Houston TX 77058

* Corresponding author (b-sutter{at}mail.arc.nasa.gov)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
The incorporation of micronutrients (e.g., Fe, Mn, Cu) into synthetic hydroxyapatite (SHA) is proposed for slow release of these nutrients to crops in NASA's Advanced Life Support (ALS) program for long-duration space missions. Separate Fe³⁺ (Fe-SHA), Mn²⁺ (Mn-SHA), and Cu²⁺ (Cu-SHA) containing SHA materials were synthesized by a precipitation method. Electron paramagnetic resonance (EPR) spectroscopy was used to determine the location of Fe³⁺, Mn²⁺, and Cu²⁺ ions in the SHA structure and to identify other Fe³⁺-, Mn²⁺-, and Cu²⁺-containing phases that formed during precipitation. The EPR parameters for Fe³⁺ (g = 4.20 and 8.93) and for Mn²⁺ (g = 2.01, A = 9.4 mT, D = 39.0 mT and E = 10.5 mT) indicated that Fe³⁺ and Mn²⁺ possessed rhombic ion crystal fields within the SHA structure. The Cu²⁺ EPR parameters (g_z = 2.488, A_z = 5.2 mT) indicated that Cu²⁺ was coordinated to more than six oxygens. The rhombic environments of Fe³⁺ and Mn²⁺ along with the unique Cu²⁺ environment suggested that these metals substituted for the 7 or 9 coordinate Ca²⁺ in SHA. The EPR analyses also detected poorly crystalline metal-oxyhydroxides or metal-phosphates associated with SHA. The Fe-, Mn-, and Cu-SHA materials are potential slow release sources of Fe, Mn, and Cu for ALS and terrestrial cropping systems.

Abbreviations: ALS, Advanced Life Support • EPR, electron paramagnetic resonance • INAA, instrumental neutron activation analysis • NMR, nuclear magnetic resonance • SHA, synthetic hydroxyapatite • XRD, x-ray diffraction • ZFS, zero field splitting

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
PLANTS WILL SUPPLY food and recycle air and water for humans on long duration missions to the Moon and Mars (Averner, 1989; Allen et al., 1995). The ALS Program of the National Aeronautics and Space Administration (NASA) is developing a synthetic substrate that slowly releases essential nutrients for plant growth on these long-duration missions (Ming et al., 1995; Henderson et al., 2000; Steinberg et al., 2000). This synthetic plant growth substrate (which is called zeoponics) is composed of NH⁺₄- and K⁺-exchanged clinoptilolite (a natural zeolite) and a nutrient (Fe, Mn, Cu, Zn, Mo, B, S, Cl, Mg) containing synthetic hydroxyapatite (SHA) [Ca₁₀(PO₄)₆(OH)₂]. Nutrients incorporated into the sparingly soluble SHA structure are expected to be slowly released from SHA at rates suitable for plant growth.

Infrared spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, x-ray diffraction (XRD) analysis, and Rietveld refinement of XRD powder data have provided evidence that Fe, Mn, and Cu substitute for Ca in the synthetic apatite structure (Suitch et al., 1985; Tripathy et al., 1989; Golden and Ming, 1999; Sutter et al., 2002). Another useful technique for examining the substitution of Fe, Mn, and Cu into the SHA structure is EPR spectroscopy. Electron paramagnetic resonance spectroscopy has characterized the bonding and coordination environment of Fe³⁺, Mn²⁺, and Cu²⁺ associated with zeolites (Goldfarb et al., 1994; De Vos et al., 1996; Carl and Larsen, 1999), phyllosilicates (Bergaoui et al., 1995; Muller et al., 1995), glasses (Scholz et al., 1996; Stößer et al., 1996; Cozar et al., 1999), ligands (Chaves et al., 1997), and proteins (Peisach and Blumberg, 1974; DeRose et al., 1995; Horton et al., 1998).

Electron paramagnetic resonance spectroscopy of synthetic fluorapatite single crystals and powders indicated that Mn²⁺ mostly occurred in the Ca(1) site (Kasai, 1962; Ohkubo, 1968; Warren, 1970; Warren and Mazelsky, 1974); however, Mn²⁺ substitution into the Ca(2) site has also been shown by EPR (Ohkubo and Mizuno, 1966; Ohkubo, 1968). Electron paramagnetic resonance spectroscopy has shown that Fe³⁺ was "homogeneously dissolved in hydroxyapatite" (Hornung and Engel, 1992), Fe³⁺ "entered into hydroxyapatite crystals" (Kohen et al., 1984), and that Fe³⁺ was adsorbed onto SHA (Meguro and Ikeya, 1992). Misono and Hall (1973) used EPR to examine the oxidizing and reducing environments of Cu²⁺ in SHA and observed two sets of Cu²⁺ parameters (g_z = 2.43, A_z = 9.0 mT and g_z = 2.36, A_z = 12.5 mT) that they attributed to Cu²⁺ in the Ca(1) and Ca(2) sites.

Several EPR parameters are used when discussing EPR results and they include the g, A, D, and E parameters. The g parameter is a measure of the coupling between the unpaired electron's spin angular momentum (S) with its orbital angular momentum (L) (Symons, 1978). The competition between the ligand(s) competing to remove L and the metal to maintain L causes variations in g values for metal-ligand complexes. This allows EPR to be a valuable tool in delineating between different metal-ligand environments.

The unpaired electron interacts (couples) with the nuclear spin (I) to form a 2I + 1 line hyperfine structure centered on g and spaced with the distance quantified by the hyperfine coupling parameter A. The coupling between the nuclear and electron spins becomes stronger as the A parameter becomes larger. The combination of g and A parameters can be utilized to differentiate between electron environments of Fe³⁺, Mn²⁺, and Cu²⁺ ions.

The EPR zero field splitting (ZFS) parameters D and E measure the deviation of the ion crystal field from ideal tetrahedral or octahedral symmetries and they apply to ions with more than one unpaired electron, e.g., low field Fe³⁺ and Mn²⁺. However, the broad nature of the Fe³⁺ EPR spectra makes determining D and E difficult (De Vos et al., 1996). Copper(II) has only one unpaired electron; therefore, D and E do not apply to Cu²⁺.

The examples above illustrate that information regarding the structural environments of Fe³⁺, Mn²⁺, and Cu²⁺ in the Fe-, Mn- and Cu-SHA materials can be obtained from EPR spectroscopy. While EPR has shown that metals can be incorporated into apatite, no EPR analyses have been conducted on synthetic apatite prepared by the method described here. The SHA materials in this study were produced similarly to those in Golden and Ming (1999), who utilized XRD and infrared spectroscopy to confirm substitution of Fe, Mn, and Cu into SHA. However, the SHA materials examined in this study possess a lower concentration range (0.01–2.5 wt.%) of metals than the SHA materials of Golden and Ming (1999) (1.5–5.0 wt.%). The objectives of this research were to use EPR spectroscopy (i) to show that Fe³⁺, Mn²⁺, and Cu²⁺ were incorporated into the SHA, (ii) to characterize their structural environments, and (iii) to characterize other associated phases that might have precipitated during SHA synthesis.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Iron-, Mn-, and Cu-containing synthetic hydroxyapatites were produced by a procedure similar to Golden and Ming (1999). Calcium nitrate monohydrate [Ca(NO₃)₂·H₂O] (235 g) was dissolved in 420 ml of 20% (v/v) NH₄OH (Solution A) while (NH₄)₂HPO₄ (72.2 g) was dissolved in 380 ml of deionized water (Solution B). After the (NH₄)₂HPO₄ was completely dissolved, 30 ml of 20% (v/v) NH₄OH was added. Each transition metal reagent (Table 1) was independently dissolved in 100 ml of deionized water and then added to the B solution. Each mixture of the transition metal and B solutions was mixed for 5 min. The metal-B solution was then added to the A solution and mixed by a propeller stirrer for 24 h. After mixing, the precipitate was allowed to age for 48 h. Subsequently, the supernatant was decanted and then washed with 2.5 L of deionized water three times to remove excess NH₄OH and NO^-₃. The washing procedure was repeated three times. After washing, the SHA precipitate was separated from the liquid by filtering through a Whatman #41 filter paper (Whatman Inc., Clifton, NJ). The SHA precipitate was placed into a cold oven and the temperature was raised to 400°C and maintained for 24 h. The Cu0.1- and Mn0.5-SHA materials were mixed for 5 min not 24 h as were the other SHA materials. When referring to these SHA materials collectively, they will be termed metal-SHA. The number associated with each metal-SHA label is the approximate metal concentration (g kg^-1) in SHA.

Instrumental neutron activation analysis (INAA) was performed following a procedure outlined by Golden and Ming (1999) to examine the low concentrations of Mn and Cu in Mn0.5- and Cu0.1-SHA. Samples were encapsulated in pure SiO₂ glass tubes and irradiated at the Texas A&M University nuclear reactor facility for 2 h at a neutron flux of 2.8 x 10¹² n cm^-2 s^-1. Counts were performed at 2 h for Mn and 12 h for Cu. Copper data were corrected for interference from positrons produced by pair production from ²⁴Na gamma rays using data from Cu-free samples. Positrons from pair production from other nuclides were insignificant relative to the positrons for ⁶⁴Cu. The INAA data reduction procedure has been described by Mittlefehldt and Lindstrom (1993), and references therein.

Diethylene-triamine-penta-acetic acid (DTPA) complexes with soluble and exchangeable Fe, Mn, and Cu and mobilizes these metals from Fe-, Mn-, and Cu-containing solid phases (e.g., metal-oxyhydroxides) (Loeppert and Inskeep, 1996). To determine if any nonstructural transition metal phases contributed to the EPR signal, each metal-SHA (0.2 g) was treated with 75 ml of 0.001 M DTPA for 24 h on an orbital shaker. After 24 h, the DTPA solution was filtered (0.2-m millipore filter), and fresh DTPA solution was added. This procedure was repeated for a total of three extractions. After the final extraction, the treated metal-SHA materials were dried at 65°C and then examined by EPR along with the untreated metal-SHA materials. After each extraction all solutions were analyzed for Fe, Mn, and Cu using a Perkin Elmer 3100 flame atomic absorption spectrometer (Perkin Elmer, Shelton, CT).

A Bruker ESR 300 (Bruker Biospin Corp., Billerica, MA) was used for X-band (9.43 GHz) EPR at 298 K and 100 K using a variable temperature Oxford Instruments Ltd. liquid nitrogen cryostat (Oxford Instruments Ltd., Santa Clara, CA. A Hewlett Packard HP 5342A frequency counter (Hewlett Packard, Palo Alto, CA) measured the microwave frequency. Iron X-band spectra were collected at 20 mW with modulation amplitude of 2.9 mT. Manganese and Cu X-band spectra were collected at 5 mW with modulation amplitude of 0.5 mT. Q-band (34.0 GHz) spectra were recorded on a Bruker ER200 console and magnet with a Varian E110 Q-band microwave bridge (Varian, Palo Alto, CA) equipped with a tunable TE₀₁₁ Q-band EPR/ENDOR resonance cavity (Sienkiewicz et al., 1996). Copper Q-band spectra were collected using a Janis variable temperature liquid helium cyrostat (Janis Research Company, Inc., Wilmington, MA) at 70 K, 20 mW, and modulation amplitude 0.1 mT. Manganese Q-band spectra were collected at 245 K, 6.3 mW, and modulation amplitude of 0.059 mT. The modulation frequency for all X and Q band EPR analyses was 100 kHz. The Mn0.5-SHA and Cu3-SHA Q- and X-band spectra were simulated with WINEPR SimFonia using second-order perturbation theory, 200 theta angles, and 200 phi angles (Bruker Analytische Messtechnik GmbH, 1996).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Total Chemical Analyses and DTPA Extraction
X-ray diffraction confirmed that the metal-SHA materials examined in this study were apatite and that no other crystalline phases were detected (Sutter, 2000). Elemental compositions of metal-SHA are listed in Table 2. The DTPA-extraction data showed that larger amounts of metal were released after the first extraction relative to subsequent extractions (Table 2). The initial high release of metals after the first DTPA extraction suggested the existence of poorly crystalline metal phases along with SHA. This data will be discussed below along with the EPR results, which suggested that Fe³⁺, Mn²⁺, and Cu²⁺ were substituted into the SHA structure. The similarity in the amounts of metals released after the second and third DTPA extractions suggested that the metals were from metal-SHA dissolution (Table 2).

View larger version (23K): [in this window] [in a new window]   Fig. 1. X-band spectra (298 K) of Fe12-synthetic hydroxyapatite (SHA) and Fe12-SHA treated with DTPA and of Fe25-SHA and Fe25-SHA treated with DTPA. Dashed lines indicates where the derivative of the absorption curve is zero.

The broad feature in the g 2.0 region has been attributed to separate ferric oxide phases (Muller et al., 1995), variation of structural Fe³⁺ species with overlapping signals (Catana et al., 1995), surface Fe oxide or oxyhydroxides (Bahranowski et al., 1996), Fe-O-Fe clusters (Stößer et al., 1996), and adsorbed Fe³⁺ on SHA (Meguro and Ikeya, 1992) (Fig. 1). The high concentration of Fe³⁺ associated with SHA caused electron spin-spin interactions between neighboring Fe³⁺ nuclei which yielded the broad g 2.0 signal (Ashtekar et al., 1996; Bessergenev et al., 1996; Bogomolova et al., 1996; Bogumil et al., 1993). Spin-spin interaction is caused by small magnetic fields from neighboring paramagnetic ions that alter the total magnetic field of each ion. This caused the energy levels of the unpaired electrons to be shifted (Drago, 1992) which led to a variation of energies resulting in a broadened EPR signal. Poorly crystalline Fe-oxyhydroxides and Fe-phosphates would be expected to have a high concentration of neighboring Fe³⁺ ions. The broadness of the g 2 peak supports the existence of poorly crystalline Fe phase(s) associated with Fe-SHA.

The Fe12- and Fe25-SHA treated with DTPA had a lower g 2.0 peak intensity relative to the g 2.0 peak intensity of Fe12- and Fe25-SHA not treated by DTPA (Fig. 1). The DTPA treatment was not expected to remove large amounts of structural Fe from SHA; hence, the structural Fe³⁺ was not responsible for the large decrease of the g 2 peak in samples treated with DTPA. The reduction of the g 2 peak with DTPA treatment further suggests that nonstructural poorly crystalline Fe-oxyhydroxides or Fe-phosphates phase(s) were associated with the SHA crystallites. When exposed to DTPA, the poorly crystalline Fe-phase(s) were easily dissolved and removed from the SHA system.

Isolated areas enriched in Fe and P were found associated with SHA as indicated by electron microprobe analysis (data not shown). This suggested that Fe-phosphate may be the cause of the g = 2 peak that appeared when the broad g 2.0 peak was mostly removed in the Fe25-SHA treated by DTPA (Fig. 1). Sharp g = 2 peaks are known to occur with tetrahedral-coordinated Fe³⁺ in FePO₄ (Bordiga et al., 1996; Shevade et al., 1997). Kohen et al. (1984) reported that a similar sharp g = 2 peak was observed when SHA was exposed to a Fe³⁺ solution, but no explanation was given as to its origin.

Manganese Electron Paramagnetic Resonance
The central M_s = +1/2 -1/2 transition (300–360 mT) caused by the Mn²⁺ nuclei (I = 5/2) was observed in the Mn0.5- and Mn3-SHA X-band spectra (Fig. 2 and 3) . X-band spectra collected at 100 K did not provide any improvement in spectral interpretation; therefore, they were not shown. Manganese(IV) may be the cause of the low field X-band hyperfine patterns (centered on 63, 143, and 214 mT) in Mn0.5- and Mn3-SHA (Fig. 2). Manganese(IV) would have originated from Mn²⁺ oxidation during Mn-SHA synthesis. However, the pink color of the Mn-SHA materials indicated that Mn²⁺ was the dominant oxidation state in the g = 2 region of Fig. 2 and 3.

View larger version (25K): [in this window] [in a new window]   Fig. 2. X-band spectra (298 K) Mn0.5-, Mn3-, and Mn11-synthetic hydroxyapatite (SHA) and Mn0.5-, Mn3-, and Mn11-SHA treated with DTPA. The components of the Mn0.5 and Mn3-SHA spectra marked A indicate a low concentration of another Mn phase. Dashed lines represent where the derivative of the absorption curve is zero. Spectral intensities between samples were scaled for ease of comparison while spectral intensities within samples are similarly scaled. The Mn3-SHA material is not marked DTPA and Untreated because the difference between the DTPA spectra and untreated spectra was difficult to separate.

View larger version (23K): [in this window] [in a new window]   Fig. 3. X-band spectra (298K) of Mn0.5-, Mn3-, and Mn11-synthetic hydroxyapatite (SHA) that were all treated with DTPA. The Mn0.5-SHA simulated spectrum is also presented. Spectral intensities were scaled for ease of comparison.

The central sharp peak observed in Mn0.5-SHA was eliminated after DTPA treatment, and was possibly from a small concentration of an inorganic or organic contaminant in the synthesis reagents that was removed by DTPA (Fig. 2 and 3). The high intensity of the Mn²⁺ peaks in Mn3- and Mn11-SHA dominated the spectrum and masked any contamination peak.

Despite the decrease in peak resolution of the X-band hyperfine pattern with increasing Mn concentration, similar Mn spectra were obtained for all Mn concentrations (Fig. 3). This suggested that Mn was occurring in similar structural environment(s) for all Mn-SHA materials. The occurrence of multiple peaks suggested that more than one Mn²⁺ environment existed in the SHA structure. The X-band spectra were analogous to Mn²⁺ X-band spectra in which Mn²⁺ had rhombic crystal field symmetry in Mn²⁺-containing Xyl-isomerase (Bogumil et al., 1993).

Q-band EPR improved the quality of the Mn0.5-SHA spectrum by providing a well-resolved six-line hyperfine pattern (2I + 1) with double peaks caused by one Mn²⁺ nuclei (I = 5/2) (Fig. 4) . Q-band EPR utilizes higher magnetic field and microwave frequency, which reduces ZFS effects on the EPR spectrum and decreases the intensity of forbidden transitions (M_l = ±1) leading to higher spectral resolution relative to X-band EPR (Bogumil et al., 1993). The Q-band simulation of one Mn²⁺ nuclei environment in Mn0.5-SHA was similar to the experimental spectrum (Fig. 4). The Q-band simulation of the Mn0.5-SHA treated by DTPA spectrum yielded the following EPR parameters g = 2.01; A = 9.4 mT; D = 39.0 mT and E = 10.5 mT with x-, y-, and z-line widths of 1.3 mT. The E/D ratio was 0.27, which indicated that a single Mn²⁺ nuclei had a nearly full rhombic crystal field. High spin Mn²⁺ with a full rhombic crystal field will have an E/D ratio of 1/3. The similarity of the Mn0.5-SHA Q-band spectrum to the Q-band rhombic crystal field spectra of Mn²⁺-containing Xyl-isomerase further supports one Mn²⁺ nuclei environment in the Mn-SHA (Bogumil et al., 1993). Our EPR parameters were near to EPR parameters (g = 2.00, A = 9.6 mT, D = 50.4 mT, and E = 11.4 mT) for Mn²⁺ in the Ca(2) (rhombic symmetry) site in a single crystal apatite (Warren, 1970). Mn0.5-SHA parameters did not have all the same parameters as Mn²⁺ in the Ca(1) site (axial symmetry) of powdered flourapatite (g = 2.00, A = 9.44 mT, D = 42.8 mT and E = 0 mT) (Kasai, 1962). Parameters similar to flourapatite were obtained for Mn²⁺ in the Ca(1) site of single crystal natural apatite (Burley, 1964; Ohkubo and Mizuno, 1966; Ohkubo, 1968). Substituting the smaller Mn²⁺ (0.80 Å) for the larger Ca²⁺ (0.99 Å) may cause distortion of the Mn-O bonds in the Ca(1) site and lead to rhombic symmetry. Crystalline Mn-oxyhydroxides would have axial symmetry (E = 0); therefore, inclusion of crystalline Mn-oxyhydroxides in Mn-SHA was unlikely. The rhombic symmetry parameters suggested that Mn²⁺ substituted in the Ca(2) site or in a strained Ca(1) site in the Mn0.5-SHA.

View larger version (25K): [in this window] [in a new window]   Fig. 4. Q-band spectra (245K) of Mn0.5-synthetic hydroxyapatite (SHA) along with its corresponding simulated spectra.

The similarity of the g = 2.01 peak positions for the X-band spectra of the Mn-SHA materials indicated that the EPR parameters derived from the Mn0.5-SHA Q- and X-band spectra could be used to describe the Mn3- and Mn11-SHA materials. This also suggested that Mn²⁺ substituted into one of the Ca sites of Mn3- and Mn11-SHA.

Copper Electron Paramagnetic Resonance
The X-band g_z hyperfine peaks at 100 K had improved peak resolution over the g_z peaks collected at 298 K; therefore, only the 100-K spectra are presented. X-band spectra of the Cu-SHA materials (Fig. 5) and the Cu3-SHA Q-band spectrum (Fig. 6) were similar to spectra reported in the literature with rhombic spin symmetry (g_z g_y g_x) (Reinhammar et al., 1980; Olivia et al., 1997). The interaction of unpaired electrons with the ^63,65Cu nuclei (I = 3/2 for both isotopes; 61 and 39% natural abundance, respectively) resulted in the X- and Q-band spectra having g_z regions with a four (2I + 1) peak hyperfine pattern (Fig. 6 and 7) .

View larger version (29K): [in this window] [in a new window]   Fig. 5. X-band spectra (100K) of Cu0.1-, Cu3-, and Cu12-synthetic hydroxyapatite (SHA) and Cu0.1-, Cu3-, and Cu12-SHA treated with DTPA. The dashed lines represent where the derivative of the absorption curve equals zero. Spectral intensities between samples were scaled for ease of comparison. Spectral intensities within samples are similarly scaled.

View larger version (24K): [in this window] [in a new window]   Fig. 6. Q-band spectra (70K) of Cu3-synthetic hydroxyapatite treated with DTPA. Inset graph is expansion of the gz(a) and gz(b) region along with their corresponding simulated spectra.

View larger version (23K): [in this window] [in a new window]   Fig. 7. X-band spectra (100K) of Cu3-synthetic hydroxyapatite (SHA) and Cu3-SHA treated with DTPA. Simulated spectra for the gz(a) and gz(b) regions are presented.

The Q-band simulation of Cu3-SHA yielded g_z(a) = 2.488, g_z(b) = 2.425, g_y = 2.17, g_x = 2.08 with A_z(a) = 5.2 mT, A_z(b) = 9.2 mT, and x-, y-, and z-line widths of 2 and 5 mT for Cu²⁺ and Cu²⁺, respectively (Fig. 6). Simulations of the Cu3-SHA X-band spectrum yielded g_z(a) = 2.485, g_z(b) = 2.420, g_y = 2.17, g_x = 2.08 with A_z(a) = 5.2 mT, A_z(b) = 9.2 mT, and x-, y-, and z-line widths of 1 and 3 mT for Cu²⁺ and Cu²⁺, respectively (Fig. 7). The X-band parameters were similar to the Q-band parameters indicating that the calculated simulated parameters adequately described Cu3-SHA. The g_z(a) parameters (g_z(a) = 2.488; A_z(a) = 5.2 mT) were not typical of those found in the literature for Cu-O complexes. Copper bonded to lattice oxygens in montmorillonite {[Cu(AlO)_n(H₂O)_4-n]^x+} possessed g_z parameters no higher than 2.41 to 2.37, and A_z parameters no lower than 10.4 to 14.3 mT (Bahranowski et al., 1996). Similarly, six coordinate Cu²⁺ in physisorbed or frozen Cu²⁺₆ had g_z and A_z parameters that ranged from 2.42 to 2.38 and 11.4 to 13.7 mT, respectively (Duval et al., 1995; Bahranowski et al., 1996; Francois et al., 1997; Schosseler et al., 1997; Carl and Larsen, 1999). Copper(II) substituted into ß-Ca₃(PO₄)₂ was octahedrally coordinated and had g_z = 2.35 and A_z = 10.24 mT (Romdhane et al., 1981). The change from five-coordinate to six-coordinate Cu-O complexes on zeolite exchange sites has been associated with concomitant increase in g_z(a) and decrease in A_z(a) parameters (Kucherov et al., 1985; Schoonheydt, 1993; Carl and Larsen, 1999). The higher g_z(a) and lower A_z(a) parameters relative to six coordinated Cu²⁺₆ and Cu²⁺ in ß-Ca₃(PO₄)₂ suggested substitution of Cu²⁺ into the seven coordinated Ca(2) site or the nine coordinated Ca(1) site. The decreased intensity of the g_z(a) peaks with DTPA treatment suggested that some of the structurally incorporated Cu²⁺ was leached out of SHA (Fig. 7).

The Cu²⁺ g_z(b) (2.425) and A_z(b) (9.2 mT) parameters were near to six coordinated Cu²⁺ in frozen Cu²⁺₆ and Cu bonded to lattice oxygens in montmorillonite {[Cu(AlO)_n(H₂O)_4-n]^x+} as discussed above. The g_z(b) peak intensities were higher than the g_z(a) peak intensities before DTPA treatment and had lower peak intensities than g_z(a) peak intensities after DTPA treatment (Fig. 7). This indicated that the Cu²⁺ was more readily removed from SHA than Cu²⁺. The Q-band spectra also showed that after DTPA treatment, g_z(b) peak intensity decreased more than the g_z(a) peaks (data not shown). This observation confirmed the assignment of the g_z and A_z parameters values in the X- and Q-band spectra and suggests that two types of Cu²⁺ ions with different g_z values were present in Cu3-SHA. Furthermore, the g_z(b) peaks remained indicating that Cu²⁺ could be crystalline Cu-oxyhydroxide or Cu-phosphate associated with SHA that was partially solubilized by DTPA. Understanding the exact nature of the Cu²⁺ is difficult; however, Cu²⁺ remained after DTPA treatment suggesting that it has potential to serve as a long-term source of Cu²⁺ for plants. The similar peak positions of the g_z, g_y, and g_x regions of the Cu-SHA X-band spectra indicated that the EPR parameters derived from the Cu3-SHA Q- and X-band spectra could be used to describe the Cu0.3 and Cu12-SHA materials.

	CONCLUSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 
Electron paramagnetic resonance spectroscopy was useful in characterizing the Fe³⁺, Mn²⁺, and Cu²⁺ environments in the metal-SHA materials and in the precipitated phases associated with the metal-SHA materials. The EPR spectra suggested that Fe³⁺, Mn²⁺, and Cu²⁺ substituted for Ca in SHA. Poorly crystalline metal-oxyhydroxide or metal-phosphate phase(s) were found to be associated with SHA by EPR. Iron phosphate was found associated with the higher Fe containing SHA. The Cu-SHA materials appeared to also possess either a crystalline Cu-oxyhydroxide or a Cu-phosphate component. Recommended future work would utilize a sequential extraction procedure such as that outlined by La Force and Fendorf (2000) coupled with EPR to estimate the extent of the metals present in each phase. The incorporation of Fe, Mn, and Cu into SHA suggests that the metal-SHA materials are promising slow-release sources of micronutrients in the ALS and terrestrial cropping systems.

	ACKNOWLEDGMENTS

 
This research was partially funded by NASA's Graduate Student Researchers Program (NGT51229). The authors would like to thank Ray Guillemette for his assistance with the electron microprobe analysis and Charlie Galindo, Jr for performing the INAA analysis. Special thanks goes to Huay Keng Loke, Matt Vogt, Dan Fraser, and Paul Lindahl for their assistance in EPR analyses and spectral interpretation. The authors gratefully acknowledge Richard Morris for his critical review of this paper.

Received for publication August 6, 2001.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSION REFERENCES

 

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