Formation and Morphology Evolution from Ferrihydrite to Hematite in

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Formation and Morphology Evolution from Ferrihydrite to Hematite in the Presence of Tartaric Acid Mingxia Wang, Zhengxing Tao, Juan Xiong, Xiaoming Wang, Jingtao Hou, Luuk Koopal, and Wenfeng TAN ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00186 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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ACS Earth and Space Chemistry

Formation and Morphology Evolution from Ferrihydrite to Hematite in the Presence of Tartaric Acid

Mingxia Wanga, Zhengxing Taoa, Juan Xionga, Xiaoming Wanga, Jingtao Houa, Luuk K. Koopala,b and Wenfeng Tana*

aKey

Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze

River), Ministry of Agriculture; College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, P.R. China. bPhysical

Chemistry and Soft Matter, Wageningen University and Research,

Stippeneng 4 (Helix), 6708 WE, Wageningen, The Netherlands.

*Corresponding author: Wenfeng Tan Tel: +8613871431976 Email: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: Hematite, with ferrihydrite as the common precursor, is the most stable iron oxide in the soils and sediments and has many applications in the environmental systems. As a common reducing agent in soils, tartaric acid (L-TA) can reduce Fe3+ to Fe2+ and template the formation of hematite from ferrihydrite. Here, the formation of hematite in the presence of L-TA was investigated under different L-TA concentrations, initial suspension pH, and aging time. The products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high- resolution transmission electron microscopy (HRTEM). Both the transformation process and the particle morphology of hematite were affected by the initial suspension pHi at which the L-TA was added to the suspension. Optimal pHi values at a L-TA/Fe(III) molar ratio of 1.0 % and an aging time of 10 h at 100 oC were pHi 7 and pHi 11. At pHi 7, the optimal L-TA/Fe(III) molar ratio for the transformation was 1.0 % and aging at 100 oC was completed after about 20 h. The transformation occurred through a dissolutioncrystallization process. Crystalline corn-like particles (84 m2/g) were obtained through oriented attachment mechanism. At a L-TA/Fe(III) molar ration of 3.0 % the ferrihydrite surface was saturated with L-TA and the transformation was inhibited. At pHi 11, L-TA/Fe(III) of 1.0 % and aging for 10 h (100 oC), sub-rounded crystalline particles (24 m2/g) were obtained by solid-phase transformation, oriented attachment and Ostwald ripening mechanism. KEYWORDS: iron oxide, hematite, transformation, morphology, low molecular organic acids, dissolution-crystallization


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1. INTRODUCTION Iron is the fourth most abundant element in the Earth’s surface. It occurs in diverse host rock lithologies, sediments, and soils as accessory oxide and hydroxide minerals and nanoparticles that can dominate the reactive mineral/water interfacial area. Poorly crystalline ferrihydrite, which commonly exist in soils and sediments, are thermodynamically unstable and can transform to more crystalline Fe(III)-oxides, e.g., goethite and/or hematite,1 resulting in a loss of ability to scavenge other trace metals from solution.2 Hematite (α-Fe2O3) is one of the most common iron oxides in tropical and subtropical soils, and has a wide range of applications in environment protection, catalysis, magnetic detector, sensor, clinical diagnosis and lithium-ion batteries.3-6 The synthesis of hematite has therefore attracted considerable attention in recent years and by using different methods and reactants, nanoparticles,7,

8

nanowires,9 cubes,10

spindles,11 nanorods,12 flowerlike3 and hollow structure11 of hematite have been produced. Within the synthesis of hematite dissolved organic acids can be used as reducing agent and template. During particle growth, organic acids can be adsorbed on the crystal plane through the selective adsorption or specific adsorption and influence the minerals’ morphology by the mineral’s self-assembly,3 oriented attachment12 or Ostwald ripening mechanism.13 Ferrihydrite is the precursor with the formation of hematite, and high temperature and pH value closed to the PZC of ferrihydrite have been reported to be beneficial to the formation of hematite.14-16 It is known that adding Fe(II) to the reaction system promotes the transformation from ferrihydrite to hematite



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through two kinds of mechanism, i.e., dissolution-reprecipitation and solid-state transformation,15,

17, 18

but the excessive addition of Fe(II) may generate

magnetite or maghemite.19, 20 Fe(II) in the open system of the earth's topsoil layer is unstable, and it can be oxidized into Fe(III) by the co-contribution of biological processes and chemical processes.21-23 Meanwhile, the reductive microorganisms and plant root exudates, which widely exist in soil, can reduce Fe(III) to Fe(II).21, 24, 25

Questions remain about the effects of biogenic or organic Fe(II) on the rate

and extent of iron mineral formation and the importance of biogenic or organic Fe(II)-induced crystallization processes. Ferric oxalate is an example of an organic acid that can be used as template to promote the formation of hematite. The dFe-Fe value in ferric oxalate is similar to the lattice parameter d = 5.041 Å in hematite, so oxalate may act as a template for the nucleation of hematite.26, 27 Also the molecular skeleton of

polysaccharide alginic

acid can be used as a template to promote hematite formation.28 The chemical composition, numbers and properties of functional groups and the special structure of organic acids plus steric effects determine together the regulation of the morphology of hematite. Amino acids with different groups have regulation effects on the morphology of hematite which strongly depend on the complexation ability to Fe3+ ions or primary particles.13 Carboxylic acids, with different chain length or different number of -OH and –COOH groups, result in different hydrophilicity or hydrophobicity, thus influenced particle size, shape and specific surface area of the product.29-31 Some organic additives can also induce iron oxide anisometric growth.32 For


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example, ascorbic acid (AA) is a good candidate for the reduction of Fe(III) to Fe(II) as electron donor.33, 34 The molecule of L-ascorbic acid (AA), with molecular formula C6H8O6, contains four -OH groups in positions 2, 3, 5 and 6. Our previous result showed that AA catalysed the reductive dissolution of ferrihydrite to hematite, and affected the shape of the hematite particles from spheres to ellipsoids and then to elongated with increasing molar ratio of AA/Fe(III).33 Tartaric acid (TA) with L-TA type is also a common low molecular fatty acid in the environment, which has two -COOH and two α-OH groups.26 Fe3+ ions is reduced by L-TA to generate Fe2+ ions which could promote the formation of hematite, due to the strong reducibility of α-OH group.35-37 It is unclear whether L-TA can be used as a template to promote the formation of hematite at relatively mild conditions, and whether L-TA can induce anisometric growth of hematite by selective adsorption. The present study aims to provide an answer to these questions by studying the transformation of ferrihydrite to hematite under normal pressure condition with different L-TA concentrations, initial suspension pH, and aging time. The products were analyzed by X-ray diffraction and electron microscopy to ascertain the effect of reaction conditions on the morphology of the produced hematites, and to propose the growth mechanisms of hematite. 2. MATERIALS AND METHODS The ultrapure water used throughout the experiments, produced by an Echo Pu AWL4001-U pure water system, had a conductivity less than 2.0 μS/cm. All chemicals of analytical grade were purchased from Sinopharm Chemical Reagent Co., Ltd and used without further purification. The chemical name of L-(+)-tartaric acid (L-TA) is 2,3-



dihydroxybutanedioic acid with molecular formula C4H6O6 and molar mass 150.09 g/mol. The proton dissolution constants of its carboxylic groups are pKa1=2.6 and pKa2=4.4. 2.1 Sample preparation In general, the synthesis involved the formation of the precursor ferrihydrite suspension by adding a NaOH solution to a FeCl3 solution till a desired pH. Then, LTA was added and the suspension was stirred for a given time at room temperature and subsequently aged at 100 oC to facilitate the formation of hematite. Effect of the L-TA /Fe(III) molar ratio. At room temperature, 6 mol/L NaOH were dropwise added to a series of 100 ml 0.1 mol/L FeCl3 solutions in a 250 ml polyethylene flask under vigorous magnetic stirring until pH 7 and a suspension of ferrihydrite was formed. Then a certain amount ( 0-0.045 g ) of L-TA was added into the suspension to reach a given molar ratio L-TA/Fe(III) (0%, 0.25%, 0.5%,1.0%, 1.5%, 2.0%, and 3.0%), and the pH of the suspension was readjusted to pH7 with 0.1 mol/L NaOH solution. After stirring for 30 min, the suspension was tightly sealed and aged in a glycerin bath preheated to 100 oC for 10 h. After cooling down, the product was dialyzed until a conductivity of the dialyzate less than 20 μS/cm. The suspensions were centrifuged and the wet product was freeze dried, ground to pass a 100-mesh sieve, and stored in a desiccator. Effect of the aging time. The effect of aging on the transformation of ferrihydrite to hematite was studied. Based on the above experiments, the precursor synthesis were carried out at the same conditions (0.1 mol/L FeCl3, 6 mol/L NaOH, pH 7 and 1.0 % L-


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TA/Fe(III) molar ratio) except the volume of the FeCl3 solution was adjusted to 500 ml to allow sampling. Then L-TA was added to the stirred suspension to achieve a 1.0% L-TA/Fe(III) molar ratio, the pH was adjusted to 7 and the reaction was allowed to proceed for 30 min at room temperature. Subsequently, the product was aged in a erlenmeyer with screw cap at 100 oC up to 20 h. At given times (3, 4, 5, 6, 10 or 20 h) a sample (50 ml) was withdrawn from the suspension under stirring. After cooling the product was washed and dried as described above. Effect of the initial pH (pHi). The precursor was made at room temperature, by adding dropwise 6 mol/L NaOH to 100 ml 0.1 mol/L FeCl3 solutions in a 250 ml polyethylene flask under vigorous stirring until the desired pHi (initial pH = 4, 5, 6, 7, 8, 9, 10 or 11) was reached. After addition of L-TA with a L-TA/Fe(III) molar ratio of 1.0 %, the pH was readjusted to the desired pHi with 0.1mol/L NaOH solution. Then, the suspension was aged at 100 oC for 10h after stirring for 30 min. At pHi 7 and pHi 11 the pH course during aging was followed in time (for pHi 7 up to 20h aging time). The finial product was treated the same as above. 2.2 Identification and analysis of the synthesized samples Mineralogical identification of the synthesized samples was carried out by a Bruker D8 ADVANCE X-ray diffractometer (XRD), which equipped with mono-chromated Cu Kα radiation (λ = 0.15418 nm) at a tube voltage of 40 kV and a tube current of 40 mA. The scan speed was 10 °/min with a step size of 0.02 ° in the 2ϴ range of 5 - 85 °. Micromorphologies of the samples coated with a gold evaporated ﬁlm were investigated by JSM-6700F field emission scanning electron microscopy at a test



acceleration voltage of 15 kV, emission current of 5 μA. And the particle micromorphologies were also obtained with a JEM 2010HT and JEM-2010FEF (UHR) operated at 200 kV. Before tested, the samples were firstly dispersed in absolute alcohol by ultrasonic concussion, and deposited on a holey copper grid and then air dried. The specific surface area (SSA) of the samples was obtained by N2 adsorption at 77 K with Quantachrome Autosorb-1 analyzer. Prior to adsorption, the samples were degassed in vacuum at 80 °C for approximately 3 h to remove water and other physically adsorbed substances. The result was calculated by multipoint BrunauerEmmett-Teller (BET) method and a N2 molecular area of 0.162 nm2. 3. RESULTS AND DISCUSSION 3.1 Effect of the L-TA /Fe(III) Molar Ratio XRD results (Fig. 1a) demonstrate that the amount of hematite increased firstly and then decreased with the increase of L-TA. In the absence of L-TA, the product was ferrihyfrite. When the L-TA/Fe(III) molar ratio increases from 0.5 % to 1.0 %, the products diffraction peaks all matched well with hematite (JCPDS 72-0469: hematite, α-Fe2O3). The intensities of characteristic diffraction peak for crystal planes (012), (104), (110), (113), (024), (116), (122), (214) and (300) increased gradually with increasing the L-TA amount. While the L-TA/Fe(III) molar ratio increased from 1.0 % to 3.0 %, the peaks of α-Fe2O3 weakened gradually and even disappeared. With the LTA/Fe(III) molar ratio of 3.0 % the product remained ferrihydrite. The formation percentage of hematite were quantified by using whole pattern fitting of X-ray diffraction pattern with Materials Data Inc. (MDI) Jade 6.0 software.38 The fitting


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results of X-ray diffraction pattern from 0 to 3% L-TA/Fe(III) molar ratio showed that the percentage of hematite in the products was 0, 17.7%, 25.9%, 100%, 64.7%, 25.4% and 0, respectively. The results indicate that the transformation of ferrihydrite to hematite in the presence of L-TA is optimal at a L-TA/Fe(III) molar ratio of about 1.0 % and inhibited at 3.0 %. SEM images (Fig. 1b-1f) of the products in the absence and presence of L-TA (0 to 3.0 %) show that the morphologies of the particles changed from irregular block aggregate particles to short rod-like particles, and finally to irregular block particles again. The product (Fig. 1b) appeared as irregular block aggregate without L-TA. And the product at 0.5 % L-TA/Fe(III) (Fig. 1c) was rod-like with 50-100 nm in diameter, 100-150 nm in length. At the optimum ratio for the phase transformation (1.0 %, Fig. 1d) the product was uniform, corn-like particles with 100-150 nm in diameter, 200-300 nm in length. But at 2.0 % L-TA/Fe(III) (Fig. 1e), the form was irregular again and changed into flake aggregate at the 3.0 % L-TA/Fe(III) (Fig.1f). The results demonstrate that the amount of L-TA controls both phase transformations and the morphologies of the products. Addition of L-TA to ferrihydrite suspension may have several effects: (1) adsorption on ferrihydrite, (2) complexation with Fe3+ released from ferrihydrite dissolution, (3) nucleation of hematite through a template molecular effect, and (4) adsorption on hematite crystals. With the low molar ratio of L-TA (CL-TA 9 (i.e., > pHPZC,ferrihydrite) ferrihydrite is negatively charged and L-TA anion adsorption is minimal or absent due to the electrostatic repulsion. L-TA adsorption on hematite will also be small because hematite is also negative at pH > 9, so L-TA affects the transformation, but has little direct effect on the morphology of hematite. Fe3+ ions in the solution and L-TA anions produce the main species of Fe(OH)2 to catalyze the formation of hematite by a solid-state transformation process. The catalysis by Fe(OH)2, implies that aggregation and rearrangement of ferrihydrite are involved with the formation of hematite. Aggregation and rearrangement are relatively fast processes compared to nucleation and growth and this explains why the transformation of ferrihydrite to hematite is relatively fast at pHi > 9.15 Hematite has the corundum structure which is based on a hexagonal close-packed


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(hcp) anion packing. And the hydroxy coordination forms of different crystal planes are different. For (001) plane which is perpendicular to c axis, surface hydroxyl groups on it can form a bidentate ligand with organic acids. Whereas, for (110), (100), (012) and (104) planes which are parallel to c axis, surface hydroxyl groups on them form a monodentate ligand.41 L-TA contains two carboxylic and two hydroxy groups, which have high affinity with surface hydroxy groups of iron oxides.17 Organic acid could have stronger affinity and adsorption ability in (110), (100) planes compared to (001) plane.39 The crystal cell parameters c of synthetic product with 0.50% and 1.00% L-TA added are 13.785 Å and 13.803 Å, respectively (Table 1). The results indicate that with increasing L-TA/Fe(III) molar ratio the growth of the crystal extends continuously along c axis. The increase of the length-to-diameter ratio of product reveals that L-TA may restrain the crystal growth of plane which is perpendicular to c axis. The effects of L-TA on the morphology are mainly in the following aspects: (1) crystal planes of hematite which are parallel to the c axis adsorb L-TA strongly, thus inhibits growth of the plane which is perpendicular to the c axis and induces the anisometric growth; (2) The dFe-Fe distance of iron tartrate is similar to the lattice parameter of hematite (d = 5.041 Å), so adsorbed L-TA can act as template for hematite nucleation. Moreover, below the PZC of ferrihydrite L-TA will be adsorbed on ferrihydrite through electrostatic and specific interaction and form L-TA- ferrihydrite complexes and increase the orderliness of the ferrihydrite structure, which contributes to the formation of hematite.35 The likely formation mechanism of hematite with different morphologies was as



follows. When L-TA was present in solution and the pHi was 7, Fe (II) ions (mainly Fe (OH)+ and Fe (OH)2) were generated and these could catalyze the transformation of weakly crystalline ferrihydrite via dissolution/crystallization into small crystalline hematite particles. The newly formed particles with high surface energy were unstable. L-TA was, therefore, selectively adsorbed onto the small particles of hematite to reduce its surface energy. The small hematite particles with L-TA adsorbed on the surface formed corn-like hematite to achieve thermodynamic stability structure through oriented attachment along (001) plane and a dissolution-crystallization process.40 The specific surface area of hematite was 84.0 m2/g. When the pHi was 11, less Fe (Ⅱ) (mainly Fe(OH)3- and Fe (OH)2) was generated due to the less amount L-TA adsorbed onto ferrihydrate, which could not induce anisometric growth of hematite. Under this condition, hematite formed mainly through the solid-phase transformation process, and initial particles of hematite were subround plate. Then the small particles grew to large subround particles through Ostwald ripening mechanism.40, 41 The formed hematite had a better crystallinity than hematite synthesized at pH 7 and the specific surface area was relatively small, which is only 24.0 m2/g. 4. CONCLUSIONS Reductive organic matter in soil can affect the formation and transformation of iron oxides. L-TA as additive, which is one of the plant root exudates, can easily induce the formation of hematite using fresh ferrihydrite as precursor. The initial pH of the ferrihydrite suspension is a key factor for the L-TA catalyzed transformation of


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ferrihydrite to hematite as it affects the adsorption of L-TA to ferrihydrite and hematite. Both the transformation process and the morphology of hematite are affected by the initial pH. Optimal initial pH values at a L-TA/Fe(III) molar ratio of 1.0 % are pHi 7 and pHi 11. At pHi 7 the optimal L-TA/Fe(III) molar ratio for the transformation was 1.0% and at L-TA/Fe(III) 3.0% the transformation was inhibited. L-TA acted both as reducing agent and as template to control particle size and morphology of hematite and the transformation occurred through dissolution-crystallization. The crystallinity of the hematite increased with increasing aging time and the particle morphology was cornlike through oriented attachment and dissolution-crystallization. At pHi 11 and a molar ratio L-TA/Fe(III) of 1.0%, sub-rounded crystalline hematite particles are obtained. The process is solid-phase transformation and the morphology of the hematite particles is caused by oriented attachment and Ostwald ripening. This study provides a systematic understanding of the roles of Fe(II) induced by a low molecule organic acid in the crystal transformation of ferrihydrite to hematite. Supporting Information, the detailed description, XRD patterns of the formation products with or without L-TA at different aging temperatures. ACKNOWLEDGEMENTS The authors gratefully thank the National Key Research and Development Program of China (No. 2016YFD0800403) and the National Natural Science Foundation of China (No. 41571229 and No. 41330852). REFERENCES (1) Zachara, J. M.; Kukkadapu, R. K.; Fredrickson, J. K.; Gorby, Y. A.; Smith, S. C. Biomineralization of poorly crystalline Fe(III) oxides by dissimilatory metal reducing bacteria



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of ferric oxyhydroxides, Corros. Sci., 1998, 40, 1239-1251. (20) Cwiertny, D. M.; Handler, R. M.; Schaefer, M. V.; Grassian, V. H.; Scherer, M. M. Interpreting nanoscale size-effects in aggregated Fe-oxide suspensions: reaction of Fe (II) with goethite, Geochim. Cosmochim. Ac., 2008, 72, 1365-1380. (21) Han, R.; Liu, T.; Li, F.; Li, X.; Chen, D.; Wu, Y. Dependence of secondary mineral formation on Fe(II) production from ferrihydrite reduction by shewanella oneidensis MR‑1, ACS Earth Space Chem., 2018, 2, 399-409. (22) Liu, T.; Chen, D.; Luo, X.; Li, X.; Li, F. Microbially mediated nitrate-reducing Fe (II) oxidation: Quantification of chemodenitrification and biological reactions, Geochim. Cosmochim. Ac., 2018. (23) Pedersen, H. D.; Postma, D.; Jakobsen, R.; Larsen, O. Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe (II), Geochim. Cosmochim. Ac., 2005, 69, 3967-3977. (24) Creutz, C. Complexities of ascorbate as a reducing agent, Inorg. Chem., 1981, 20, 4449-4452. (25) Suter, D.; Banwart, S.; Stumm, W. Dissolution of hydrous iron (III) oxides by reductive mechanisms, Langmuir, 1991, 7, 809-813. (26) Cornell, R. M.; Schwertmann, U. Influence of organic anions on the crystallization of ferrihydrite, Clays Clay Miner, 1979, 27, 402-410. (27) Cornell, R. M. Comparison and classification of the effects of simple ions and molecules upon the transformation of ferrihydrite into more crystalline products, J. Plant Nutr. Soil Sci., 1987, 150, 304-307. (28) Sreeram, K. J.; Indumathy, R.; Rajaram, A.; Nair, B. U.; Ramasami, T. Template synthesis of highly crystalline and monodisperse iron oxide pigments of nanosize, Mater. Res. Bull., 2006, 41, 1875-1881. (29) Barton, T. F.; Price, T.; Becker, K.; Dillard, J. G. The effects of dicarboxylic acids on the crystal growth of α-FeOOH in aqueous media, Colloids Surf., 1991, 53, 209-222. (30) Ishikawa, T.; Kataoka, S.; Kandori, K. The influence of carboxylate ions on the growth of βFeOOH particles, J. Mater. Sci., 1993, 28, 2693-2698. (31) Zhang, Q.; Cheng, X.; Feng, X.; Qiu, G.; Tan, W.; Liu, F. Large-scale size-controlled synthesis of cryptomelane-type manganese oxide OMS-2 in lateral and longitudinal directions, J. Mater. Chem., 2011, 21, 5223-5225. (32) Kandori, K.; Hori, N.; Ishikawa, T. Preparation of mesoporous hematite particles by a forced hydrolysis reaction accompanying a peptide production reaction, Colloids Surf. A: Physicochemical Eng. Asp., 2006, 290, 280-287. (33) Larsen, O.; Postma, D.; Jakobsen, R. The reactivity of iron oxides towards reductive dissolution with ascorbic acid in a shallow sandy aquifer (Rømø, Denmark), Geochim. Cosmochim. Ac., 2006, 70, 4827-4835. (34) Ananthan, P.; Venkateswaran, G.; Manjanna, J. Enhanced dissolution of hematite in reductivecomplexing formulation under regenerative mode, Chem. Eng. Sci., 2003, 58, 5103-5109. (35) Mikutta, C. X-ray absorption spectroscopy study on the effect of hydroxybenzoic acids on the formation and structure of ferrihydrite, Geochim. Cosmochim. Ac., 2011, 75, 5122-5139. (36) Liu, H.; Guo, H.; Li, P.; Wei, Y. The transformation of ferrihydrite in the presence of trace Fe (II): The effect of the anionic media, J. Solid State Chem., 2008, 181, 2666-2671. (37) Cornell, R. M.; Schwertmann, U. The iron oxides: structure, properties, reactions, occurrences



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Figure captions Fig. 1 XRD patterns (a) of the products obtained at different molar ratios of L-TA/Fe(III) ranging from 0 to 3.0 % and SEM images of the products (b) 0 %, (c) 0.50 %, (d) 1.00 %, (e) 2.00 %, (f) 3.00 %. The initial solution concentration of FeCl3 and NaOH were 0.1 and 6 mol/L, respectively, and the pH at L-TA addition was 7. The aging time was 10 h at 100 oC. Fig. 2 XRD patterns of the product obtained after aging times ranging from 3 h to 20 h. Precursor ferrihydrite was produced in the typical way. For the transformation L-TA was added at pH 7 (L-TA/Fe(III) ratio 1.0 %) and the reaction was allowed 30 min at room temperature before the aging at 100 oC started. Fig. 3 SEM images of the products at different aging times ranging from 3 h to 20 h: (a) 3 h, (b) 4 h, (c) 5 h, (d) 6 h, (e) 10 h and (f) 20 h. Red circles marked in (a) and (c) highlight the hematite particles. Precursor ferrihydrite was produced at typical conditions; L-TA (L-TA/Fe(III) 1.0%) was added at pH 7 and the suspension was stirred for 30 min at 25 oC before the aging at 100 oC started. Fig. 4. Particles formed at different initial pH of the transformation of ferrihydrite at 1.0% L-TA/Fe(III) molar ratio. Panel (a) XRD patterns; panels b to f SEM images: (b) pHi 4, (c) pHi 7, (d) pHi 8, (e) pHi 9, (f) pHi 11; panels g and h HETEM images: (g) pHi 7 and (h) pHi 11. The initial pH (pHi) is the pH at which L-TA is added to the precursor. The precursor is made in the usual way, but the pH at which the NaOH addition is stopped is varied and called pHi (initial pH of the transformation catalyzed by L-TA). The reaction time at 25 oC was 30 min; the aging time at 100 oC was 10 h. The TEM



images of Fig. 4g and Fig. 4h apply to the transformation products obtained at initial pHi 7 and pHi 11, respectively. Fig. 5 The variation curve of pH value (a) initial pH7 and (b) initial pH11 at different reaction times. The initial solution concentration of FeCl3 and NaOH were 0.1 and 6 mol/L, respectively. The addition molar ratio of L-TA/Fe(III) was 1.00 % and the aging temperature was 100 oC.


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Table caption Table 1 The crystal cell parameters and morphology characteristics of samples obtained at different conditions



Fig. 1 XRD patterns (a) of the products obtained at different molar ratios of LTA/Fe(III) ranging from 0 to 3.0 % and SEM images of the products (b) 0 %, (c) 0.50 %, (d) 1.00 %, (e) 2.00 %, (f) 3.00 %. The initial solution concentration of FeCl3 and NaOH were 0.1 and 6 mol/L, respectively, and the pH at L-TA addition was 7. The aging time was 10 h at 100 oC.


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Fig. 2 XRD patterns of the product obtained after aging times ranging from 3 h to 20 h. Precursor ferrihydrite was produced in the typical way. For the transformation L-TA was added at pH 7 (L-TA/Fe(III) ratio 1.0 %) and the reaction was allowed 30 min at room temperature before the aging at 100 oC started.



Fig. 3 SEM images of the products at different aging times ranging from 3 h to 20 h: (a) 3 h, (b) 4 h, (c) 5 h, (d) 6 h, (e) 10 h and (f) 20 h. Red circles marked in (a) and (c) highlight the hematite particles. Precursor ferrihydrite was produced at typical conditions; L-TA (L-TA/Fe(III) 1.0%) was added at pH 7 and the suspension was stirred for 30 min at 25 oC before the aging at 100 oC started.


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Fig. 4. Particles formed at different initial pH of the transformation of ferrihydrite at 1.0% L-TA/Fe(III) molar ratio. Panel (a) XRD patterns; panels b to f SEM images: (b) pHi 4, (c) pHi 7, (d) pHi 8, (e) pHi 9, (f) pHi 11; panels g and h



HETEM images: (g) pHi 7 and (h) pHi 11. The initial pH (pHi) is the pH at which L-TA is added to the precursor. The precursor is made in the usual way, but the pH at which the NaOH addition is stopped is varied and called pHi (initial pH of the transformation catalyzed by L-TA). The reaction time at 25 oC was 30 min; the aging time at 100 oC was 10 h. The TEM images of Fig. 4g and Fig. 4h apply to the transformation products obtained at initial pHi 7 and pHi 11, respectively.


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Fig. 5 The variation curve of pH value (a) initial pH7 and (b) initial pH11 at different reaction times. The initial solution concentration of FeCl3 and NaOH were 0.1 and 6 mol/L, respectively. The addition molar ratio of L-TA/Fe(III) was 1.00 % and the aging temperature was 100 oC.



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Table 1 The crystal cell parameters and morphology characteristics of samples obtained at different conditions Initial Amounts pH of L-TA 0.5% pH 7 1.0% pH 11 1.0%

Space group R-3ch R-3ch R-3ch

D: diameter; L: length.

a (Å) 5.03 9 5.04 1 5.04 4

c (Å) 13.7 85 13.8 04 13.8 08

SBET (m2/g) 228.0 84.0 24.0


Morphol ogy corn like corn like subroun ded

Dimensions D: 50-100 nm, L: 100-150 nm nm, L: D: 80-100 150-200 nmnm D: 100-200

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Formation and Morphology Evolution from Ferrihydrite to Hematite in

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