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Hypetfine Interactions 151/152: 283-290, 2003. 9 2004 Khtwer Academic Pttblishers. Printed in the Netherlands.

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Ferrihydrite Modification by Boron Doping JOHN G. STEVENS, AIRAT M. KHASANOV and MELISSA S. GRASETTE WHITE MiJssbauer E/feet Data Center, University of'North Carolina at ksheville, One University Heights, Asheville, NC, 28804, USA; e-mail: [email protected]

On the occasion o f the 80th birth~hty o f Hendrik de Waard

Abstract. Modification of ferrihydrite nanoparticles with boron is investigated using M6ssbauer spectroscopy and powdered X-ray diffraction. These materials were heated to temperatures up to 600~ The addition of the boron in the fabrication of [errihydrite nanoparticles raises the transition temperature of the phase transformation of ferrihydrite to hematite. Evidence is that the boron does not penetrate the iron oxihydroxide structure, but attaches itself to the surface of the particles. Key words: M6ssbauer, fcrrihydrite, boron, nanoparticles, iron oxides.

1. I n t r o d u c t i o n Details of the ferrihydrite crystal structure, its evolution, and its relation to other iron oxide and hydroxide phases remain an object of intensive investigation and discussion. In early studies ferrihydrite is described as an amorphous ferric hydroxide [1,2] or a colloidal ferric hydroxide reflecting its poor crystallinity [3, 4]. Later it was established that a considerable variety of crystal structure exists between the two extreme cases, labeled "two-line ferrihydrite" and "six-line ferrihydrite" after their characteristic X-ray patterns. Ferrihydrite is abundant in nature and is crucial to the process of iron oxidation; it is known to be involved in biological systems. Due to small particle size and high surface area it has active sorbent properties, which also attracts research attention. Ferrihydrite with different amounts of Si, C, A1, As, U, and a range of organics were studied. The ability of ferrihydrite to retain these inclusions and their effect on its properties were investigated. (See, for example, [5].) Doping ferrihydrite with Si and AI results in small decreases of the iron hyperfine field [6]. Si and P doping of geothite synthesized through ferrihydrite has been attempted, and ordering temperature was found to be affected by the amounts of these impurities [7]. Retention of As on the ferrihydrate surface has been investigated, which results in a decrease of the iron magnetic hyperfine interaction [8].

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Recently, substantial attention has been given to the role of boron in biological functions in animals as well as redox metabolism in plants [9, 10]. The objective of this study is to investigate the effect of boron on the properties of ferrihydrite.

2. Experimental Samples were prepared by combining a 1.48 M solution of FeC13 with variable amounts of a heated 1.48 M H3BO3 solution and addition of I. 1 M sodium carbonate solution. In most samples, sodium hydroxide was added to raise the solution pH to between 8 and 9 to cause precipitation. The precipitate was then heated to 100~ for two hours to evaporate the water. The substance was then cooled and rinsed with isopropanol and deionized water to remove the sodium chloride. Samples coded B0, B 1, B2, and B3 correspond to 0, 25, 50, and 75 molar percent of B in the initial solution. A series of four samples were later simultaneously heated in an oven. Heat treatments lasted three hours at temperatures 200, 400, 500, and 600~ X-ray analysis was performed with a Philips X'pert diffractometer. MOssbauer spectra were obtained at 100 K and 300 K with a 57Co(Rh) source calibrated and referenced with o~-Fe. The MOssbauer data tk)r these samples are summarized in Table I.

3. Results After repeated washings, a series of fine powdered samples was obtained with the color changing from dark brownish-red to a lighter color with a substantial light gray hue with increasing boron concentration. The X-ray diffraction pattern of B0 shows a pattern consistent with 6-line ferrihydrite. Samples B l, B2, and B3 are closer to the 2-line ferrihydrite pattern. Diffractograms of B0, B 1, and B2 heated to 400~ and higher and B3 heated to 600~ show the presence of the crystalline hematite phase. Typical X-ray diffraction patterns are shown in Figure l. Room-temperature MOssbauer spectra of pure ferrihydrite consist of two distinct absorption peaks. This is shown in Figure 2. Initial attempts to fit these spectra with a single doublet resulted in an unsatisfactory match of the line shape. The characteristic parameter of goodness-of-fit, X2, was between 2.5 and 3.5 in all attempts to fit the spectra with a single doublet model. The main discrepancy between experimental and theoretical spectra in this approach was in the area of absorption peaks, which indicates a more complex structure of the spectra. The next step of fitting was the application of a two-doublet model. This approach succeeded in a c h i e v i n g X 2 parameter values around 1.0. Both doublets have practically identical values of isomer shift, 8 = 0.36 mm/s and ~ = 0.35 mm/s. The inner doublet has quadrupole splitting A = 0.54 mm/s and the outer doublet, A = 0.87 mm/s. The linewidth (F) of the inner doublet is 0.32 mm/s, which is not much more than the linewidth of the oe-Fe calibration spectrum, of 0.25 mm/s. The linewidth of the outer doublet is considerably larger at 0.42 mm/s. The relative areas of each of

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the components are nearly the same at 41% and 59% - the accuracy of estimation of this parameter is always low, usually 4-5% due to small separation of the two spectral components. This makes most conclusions about the spectral areas rather unreliable. The addition of boron with increasing concentration does not produce a significant change in the spectra. One does note a small increase in A from 0.71 mm/s to 0.76 mm/s and F from 0.45 mm/s to 0.48-0.52 mm/s in the single doublet fit. The same tendency is observed for the two-doublet fit. Both components show increase in A from 0.54 mm/s to 0.62 mm/s and 0.87 mm/s to 0.91 mm/s. Linewidths also increase for both components from 0.32 mm/s to 0.36 mm/s and 0.42 mm/s to 0.59 mm/s. The area changes are within accuracy limits. Heating of the samples up to temperatures of phase transformation (ferrihydrite to hematite) causes some increase in A and F in the single doublet fit, with 3 staying the same. The two-doublet approach confirms this general tendency. Inner doublet a increases from 0.59 mm/s to 0.65 ram/s, and the outer a from 0.90 mm/s to 1.10 ram/s, or 0.91 mm/s to 1.05 ram/s, while the spectral area ratio stays the same within the limits of accuracy. Most obvious is the shift in the phase transformation temperature. The spectrum of the pure ferrihydrite after heating at 350~ is a sextet with H = 50.0 T. Sample B I shows a sextet with H = 50.4 T only after heating to 400~ Sample B2 has only 10% magnetically ordered component after 400~ and 44% magnetic after 500~ heating, while B3 is purely paramagnetic even after 500~ heating. The resulting magnetic component is well resolved in all cases, with H from 49.3 to 50.6 T. Spectra at T = 100 K do not change the proportion of paramagnetic phase compared to room temperature.

4. Discussion Interpretation of the spectra varies considerably in the literature for ferrihydrite. Single-doublet fit, though very crude and producing a statistically unsatisfactory fit, still remains as a first step in fitting and is believed to produce averaged values for characterization of ferrihydrite [11]. Two-doublet fit gives a much better description of the spectrum and has some logical connection with the structural properties [12]. The latter was questioned in several studies [13-15] from both crystallographic site assignment and statistical ambiguity points of view. A threedoublet fit was attempted to explore possible tetrahedral coordination of Fe [16]. An alternative approach is fitting with a distribution of hyperfine parameters - 3, A, and H - which can also vary from single restricted distribution to multiple unrestricted distribution [13, 15, 17-19]. The basic structural element of ferrihydrite is believed to be a FeO3(OH)3 octahedron. These octahedra are linked by edge into dioctahedral chains. Two-line ferrihydrite particles consist of chains of different length. The next step of crystallization is cross linkage of dioctahedral chains by double corner sharing, forming

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what is known as six-line ferrihydrite [20]. This makes the ferrihydrite structure related to goethite, where cross-linked dioctahedral chains form a three-dimensional network. It is still unclear if the two-line ferrihydrite is strictly analogous to six-line ferrihydrite or goethite. From this point of view the following local iron environments are possible: Fe located in an octahedral environment edge-linked to 2, 3, and 4 other octahedra, plus having corner links to 0, 1, or 2 octahedra depending on the location at the chain ends, in the body of the chain, or adjacent to or opposite a cross-linked chain. The exact effect of certain environments on total distortion of the Fe coordination polyhedron is rather difficult to predict. The relative abundance of different situations and particle size can be estimated within the framework of this model using a Fe-Fe interchain/across-chain distance of 0.345 nm from [20]. Assuming two dioctahedral chains cross-linked at corners as a basic structural element, one can distinguish an "inner" part of a particle formed by octahedra having four edge neighbors and two corner neighbors. The "outer" part is formed by octahedra that do not have corner links. In this model, the ratio of inner Fe sites to outer sites is about 1 : 1. The width of the particle is limited to about 1.4 nm, with particle length depending on the chain length. More advanced models involving three or four cross-linked chains change this ratio to 2:1 and 4:1 with particle widths of 2 nm and 2.8 nm. If there is a correlation between Fe site location and distortion of local environment, then it is reasonable to expect the spectral area ratio in two-component interpretation varying accordingly. Coherent scattering domain size for 2-line ferrihydrite is 2-3 nm [20], which restricts further complication of the model. Heat treatment results in weight loss, which is attributed to gradual elimination of OH groups and H20 molecules without noticeable phase transformation [211. This process can cause additional distortion of the Fe environment, at least for the outer part of the particle. Observed increase of quadrupole splitting and linewidth with heating fits this picture and agrees with previous observations [l, 3]. Murad e t al. also mentioned that poor crystallinity correlates with larger A and linewidth [14]. All this indicates that heating results in a more distorted and less stable structure compared to original ferrihydrite. Increased concentration of B immediately after precipitation causes a certain increase in A. This can be interpreted as frustration of the regular structure by either entrapment of B on the ferrihydrite surface or close contact with elements of a different crystal structure. A somewhat higher value of A = 0.99 mm/s of the outer doublet observed by Murad in the presence of calcite, quartz, and dolomite could be of the same nature [22]. Samples with a higher concentration of B show slower transformation to hematite. Sample B3 is paramagnetic at room temperature after heating to 500~ Sample B2 show 53% paramagnetic component in the spectrum, while B 1 and B0 are 100% magnetic. A of the outer doublet in samples B2 and B3 show values considerably higher than characteristic values of pure ferrihydrite. This indicates that the crystal structure is strained even more just before phase transformation to hematite, but still the main ferrihydrite feature is preserved. This

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observation is consistent with the summary of Jambor and Dutrizac that adsorption of various ions generally retards the transformation of ferrihydrite to goethite or hematite [20]. B adsorption raises the possibility of the formation of elements of F e - B - O structures even when none are visible in an X-ray diffraction pattern. Several known Fe-B-O structures contain an Fe 3+ ion in octahedral sites affected by B in the next nearest environment. FeBO3 shows a single component with 3 = 0.23 mm/s and A = 0.19 mm/s [23]. FezBO4 (warwickite) contains iron in the 3+, 2+, and 2.5+ charge states with 3 (Fe 3+) = 0.39 mm/s and A = 0.38 mm/s and 0.55 mm/s [24]. Fe2MgBO5 (ludwigite) contains Fe z+, Fe 3+, and Fe zS+ with 6 (Fe 3+) = 0.41 mm/s and A = 1.06 mm/s [25]. In Fe3BO5 (vonsenite) the iron is present in 3+, 2+, and 2.5+ with 3 (Fe 3+) = 0.45 mm/s and A = 0.91 mm/s [25]. From a comparison of isomer shift values for these phases and our results, there is no immediate reason to propose the idea of B diffusion into the ferrihydrite structure and its modification in the direction of the mentioned crystals. The magnetic component in the spectra always have a well-developed structure with H = 49.3 to 50.6 T and relatively narrow linewidth. This contribution is characteristic tor hematite. Smaller than the usual H = 51.5 T is explained by poor crystallinity of newly formed hematite and is consistent with other observations [ I-3]. No change in the relative area of magnetic fraction at 100 K compared to room temperature indicates that the hematite phase is present in the form of fairly large particles and makes a hypothesis of admixture of superparamagnetic contribution of hematite to the characteristic ferrihydrite spectrum rather unlikely.

Acknowledgements The authors appreciate the assistance received from Macy Little and David Losure, who assisted in this project. The University of North Carolina is acknowledged for providing a student grant to support this investigation.

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