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high pH values favor the formation of goethite, while pH values near neutral favor the. 38 formation of ... product at all pHs (2 – 10) and temperat...
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The transformation of two-line ferrihydrite into crystalline products: effect of pH, and media (sulfate vs. nitrate) Danni Zhang, Shaofeng Wang, Ying Wang, Mario A. Gomez, Yihang Duan, and Yongfeng Jia ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00001 • Publication Date (Web): 29 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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ACS Earth and Space Chemistry

The transformation of two-line ferrihydrite into crystalline products: effect of pH, and media (sulfate vs. nitrate)

Danni Zhang1, Shaofeng Wang*,1, Ying Wang1,2, Mario A. Gomez3, Yihang Duan1,2, Yongfeng Jia*,1 1

Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences, Shenyang, 110016, China 2 University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China 3 Institute of Environmental Protection, Shenyang University of Chemical Technology, Shenyang, 110142, China

TOC Art

Goe

pH 4 Ca

Goe

Goe Goe

2+

Fh SO4

2-

Ca

2+

Fh 2-

SO4

pH 8 hem

hem

hem hem

hem

Keywords: Two-line ferrihydrite; Crystallization; Sulfate; Media; pH * Corresponding author phone and fax:+86-24-8397-0503; e-mail: [email protected] * Corresponding author phone and fax:+86-24-8397-0502; e-mail: [email protected]

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ABSTRACT

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Two-line ferrihydrite (Fh), ubiquitous in soils, groundwater, and aquatic sediments, may

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serve as an important sink for sequestering trace metals, metalloids, and organic matter via

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adsorption/coprecipitation due to its high surface area and reactivity. Although considerable

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attention has been paid to the transformation process of this thermodynamically metastable

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solid, little is known about the transformation products, the crystallization rates, or the

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transformation routes of two-line Fh in sulfate- and calcium-rich environments. This work

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systematically investigates the transformation of 2-line ferrihydrite produced by using

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different neutralization reagents (CaO vs. NaOH) at different pHs (4 and 8), temperatures

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(25 °C, 40 °C, and 80 °C), and media (sulfate vs. nitrate). X-ray diffraction (XRD), Raman,

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Fourier transform infrared spectroscopy (FTIR), and chemical extraction were employed to

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characterize the transformed solids as well as the crystallization rate of two-line Fh. The

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results show that the crystallization products in nitrate media include hematite (major) and

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goethite (minor) under acidic conditions (pH 4) and only hematite under alkaline conditions

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(pH 8) and are independent of the neutralization reagent used. By contrast, in sulfate media,

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goethite is the dominant product under slightly acidic conditions (pH 4) with 6-line

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ferrihydrite as the intermediate product, whereas under slightly alkaline conditions the

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formation of hematite is favored. Chemical extraction results indicate that the transformation

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process of those CaO-neutralized solids precipitated at pH 8 is intensely retarded by calcium

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ion. The results suggest that neutralization reagents (calcium ion), as well as the reaction

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media (sulfate ion), play an important role in the 2-line ferrihydrite crystallization process.

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INTRODUCTION

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Ferrihydrite (Fh) is an X-ray amorphous form of iron(III) oxyhydroxide commonly

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occurring in soils, surface, and groundwater sediments.1 There are two types of Fh ("2-line"

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and "6-line") on the basis of the number of diffuse XRD peaks. Due to its high surface area

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and reactivity, it usually shows large adsorptive capacities for trace elements such as As, Co,

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and Cd,2-4 hence playing an important role in the sequestration of contaminants in the

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environment. In hydrometallurgical operations, 2-line Fh is considered the dominant

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arsenic-bearing phase in the FeIII-AsV coprecipitates generated during arsenic removal and

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immobilization processes.5

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Ferrihydrite is also a thermodynamically metastable solid and may transform to more

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crystalline and stable forms of iron(III) oxyhydroxide. Although the transformation process of

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Fh has been widely studied under various conditions and a vast quantity of related papers is

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available,6−12 the transformation products and the crystallization process of formation of the

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more crystalline iron(III) oxyhydroxide from the precursor Fh in sulfate and calcium-rich

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environments is still not fully understood.8,9 Some investigations have reported that low and

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high pH values favor the formation of goethite, while pH values near neutral favor the

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formation of hematite.8,10 However, recent research reported that hematite was the major

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product at all pHs (2 – 10) and temperatures (50 – 100 °C) used when 2-line Fh was

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precipitated from nitrate media.11 As for the transformation routes, it is widely accepted that

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2-line Fh undergoes a dissolution–crystallization process if it is transformed to goethite or it

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may directly be converted to hematite via a solid-state mechanism.8,12 By contrast, a recent

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investigation suggested that the transformation of 2-line Fh (precipitated in nitrate media)

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went through a two-stage crystallization process, with goethite as an intermediate.11 In

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addition, the effects of some common cations (e.g. nickel and lead) and anions (e.g.

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phosphate and arsenic) on the transformation process of 2-line Fh have also been studied.2,13,14

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It has been found that different divalent metal ions exert different effects on the

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transformation products and the crystallization rate of Fh.13 Additionally, results showed that

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calcium may influence the release of other contaminant substances such as arsenate or

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uranium from Fh.15‒18 Therefore, calcium as a widely occurring and used divalent metal ion,

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may also have some effects on the transformation process of Fh itself.

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As for the effects of anions on the transformation process of Fh, Baltpurvins et al.19

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investigated the effect of anion composition (Cl-, SO42-, NO3-) on the aging of freshly

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precipitated iron(III) hydroxide sludge (NaOH as a neutralization reagent and room

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temperature for a period of 1 year) only under neutral to alkaline conditions. The results

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showed that hematite formation was favored at pH 7 – 9, whereas goethite formation was

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favored at pH 10 – 11 no matter what reaction media was used and the transformation rate

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increased in the order sulfate < chloride < nitrate. Sulfate may form bidentate-binuclear

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adsorption complexes on Fh surfaces, especially under acidic conditions.20 Therefore, the

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transformation products, rates, and the routes of Fh may be different in sulfate and

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calcium-rich environments compared with those in sodium and nitrate systems. In brief, most

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of the conclusions mentioned above were proposed based on the data obtained from nitrate

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media. Little is known about the transformation process and the crystallization products from

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sulfate systems, especially those that use slaked lime as the neutralization reagent in the

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acidic pH range, which are more representative of conditions occurring in hydrometallurgical

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and acid mine drainage (AMD) operations.5,21

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The objectives of this work were (1) to investigate the influence of the reaction media

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(nitrate vs. sulfate) and the neutralization reagent (CaO vs. NaOH) on the transformation

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process of 2-line Fh as well as the crystallization product at various pHs and temperatures; (2)

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to study the redistribution of Fe between the amorphous and more crystalline phases during

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the transformation process.

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EXPERIMENTAL SECTION

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All chemicals were of analytical grade and deionized (DI) water was used for all

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experiments. All glassware was cleaned by soaking in dilute HNO3 (5%) overnight and rinsed

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three times with DI-water.

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Preparation and aging treatment of 2-line ferrihydrite. Two-line Fh was prepared via

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fast hydrolysis of ferric iron (from Fe2(SO4)3·5H2O or Fe(NO3)3·9H2O). The acidic FeIII

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solutions were neutralized to pH 4 or 8 with 1 M NaOH solution or slaked lime while the

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mixtures were stirred mechanically. After stabilization for 1 hour at room temperature, the

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slurries were transferred to tightly capped conical flasks, with the total volume of each slurry

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at 500 mL with an Fe concentration of 3000 mg/L. The slurries were allowed to age in a

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water bath maintained at 25 °C, 40 °C, or 80 °C ± 2 °C by an immersion circulator. A

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multi-stage (two or three) neutralization process (first at pH 4 and then raised to alkaline pH

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conditions) is an industrial method of choice for treating toxic acidic solutions.16 This is the

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reason why pH 4 and 8 were used in this work.

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The aging process lasted up to about 280 days (for those CaO-neutralized solids

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precipitated from sulfate media and aged at room temperature, the aging time was extended to

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about 1800 days) and, at prescribed time intervals, the solid phase was separated by filtration,

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washed at least three times with DI-water and vacuum-dried. All solid samples were ground

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into a fine powder before spectroscopic characterization and chemical extraction. Aliquots of

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weighed solid samples were dissolved in 1 M HCl solution and diluted with DI-water for the

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measurement of the total concentrations of Fe, Ca and SO4 in the solid phase. Some samples,

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especially those aged for a long time at elevated temperatures, were digested in 6 M HCl

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solution in order for the solid to completely dissolve.

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Chemical extraction procedure. Single-step 0.4 M HCl extraction was conducted for

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the selected samples to determine the fraction of Fh transformed to the crystalline phase. The

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solid/solution ratio was 1:1000 (0.02 g to 20 mL). The suspensions were shaken for 1 hour at

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room temperature and then filtered through a 0.22-µm membrane for the analysis of the

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concentrations of Fe in the filtrates. Amorphous iron(III) oxyhydroxide in the aged solid was

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the target phase for this extraction process.13,22

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Determination of the iron, calcium and sulfate concentrations. The Fe concentration

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in solutions as well as in the solid phase was determined on an atomic absorption

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spectrometer (AA240, Varian, Inc. Palo Alto, CA). The detection limit of the instrument for

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iron was 0.5 mg/L. The concentration of Ca and SO4 was determined by using inductively

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coupled plasma–atomic emission spectroscopy (ICP-AES, Thermo-6300) with the detection

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limit of 0.02 and 0.03 mg/L, respectively.

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XRD analysis. The mineralogical characteristics of the ferrihydrite precipitated at

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different pHs and aged at different temperatures were determined on a Rigaku D/max 2400

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X-ray diffractometer equipped with a copper target (CuKα1 radiation, λ = 1.5418 Å), a crystal

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graphite monochromator and a scintillation detector. The equipment was run at 56 kV and

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182 mA by step scanning from 10° to 80°2θ with increments of 0.02°2θ. Phase identification

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and semi-quantitative Rietveld refinement for phase analysis was conducted with X'pert

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HighScore Plus software using the PAN-Inorganic and Mineral Crystal Structure Database

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version 2.0.23 In all cases, the iron oxyhydroxide used for analysis (with JCPDS #) were:

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2-line Fh (98‒011‒1015), 6-line Fh (98‒007‒6750), goethite (98‒011‒7368), and hematite

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(98‒011‒9589). The semi-quantitative Rietveld phase conducted with the X'pert HighScore

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Plus software was tested for validity by analyzing a reagent grade hematite standard and

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refining the XRD data with unlikely multiple phases with diffraction peaks in all regions.

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Rietveld analysis (accuracy of ≤10%) confirmed that hematite was the major phase present as

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expected. 23

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Raman analysis. Raman spectra were obtained on a Thermo DXR Raman microscope

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equipped with a solid laser diode operating at 780 nm and a 400 lines/mm grating.

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Approximately 0.1 g of the ground sample was put on a glass slide using a metal spatula. The

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data were obtained in the range of 60 to 1800 cm-1 with an energy resolution of 4 cm-1. The

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laser beam produced a spot size of approximately 3 µm in diameter using the 10×short

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distance objective. The scans were collected at 3.33% (0.8 mW) of the laser output at the

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microscope exit to avoid phase transformation.

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FTIR analysis. The infrared spectra of the solids precipitated and aged at pH 4 before

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and after sequential extraction processes were obtained on a Thermo Nicolet 6700 Fourier

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transform infrared spectrometer. The solid was first extracted by 0.4 M HCl solution at the

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solid solution ratio (S/L) of 1:1000 (0.02 g to 20 mL) for 1 hour in order to remove the

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untransformed amorphous Fh and then subjected to a 24-hour-extraction process by 1 M

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KH2PO4 solution (pH 5, S/L = 1:100) in order to minimize the effect of the adsorbed form of

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sulfate in the solid on the FTIR results.13,22,24 Finally, a 0.01 M NaOH solution was used as the

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extraction reagent (at least for 12 hours, S/L = 1000) in order to remove some unwanted

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adsorbed phosphate during the second extraction process.25

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The reference materials (6-line Fh, goethite, and hematite) were synthesized according

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to the methods described by Cornell and Schwertmann26 and the characteristic data for these

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samples were also obtained. For the preparation of the SO4 adsorbed on goethite sample, 0.02

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g goethite was added to 20 mL 1 M Na2SO4 solution and the pH of the suspension was

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adjusted to 4. The sample was washed with DI-water and collected by centrifuging after

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being equilibrated at 25 °C for 24 hours. This solid was also subjected to FTIR analysis.

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The KBr/sample discs were prepared by mixing 0.5% of finely ground samples in KBr.

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The samples were scanned in the mid-IR range (400 − 4000 cm-1) with the resolution of 4

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cm-1.

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RESULTS AND DISCUSSION

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Transformation products. The XRD patterns of the fresh and aged iron(III)

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oxyhydroxides at 80 °C and different pHs in sulfate and nitrate media by using CaO and

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NaOH as neutralization reagents are compared in Figure 1 and Figure 2. Elevated

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temperature was applied to accelerate the transformation process.11,27 The transformation

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products were the research foci in this section, as for the transformation intermediates and the

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transformation rates, they will be discussed in the following sections of this paper,

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respectively. All the freshly prepared solids showed characteristic 2-line Fh XRD peaks at

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∼34º and ∼61º.28 For the NaOH-neutralized solid precipitated from sulfate media at pH 4,

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goethite was the only crystalline solid in the transformed product even after the solid was

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aged for about 300 days (Figure 1). However, at pH 8, hematite (major) and goethite (minor)

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coexisted in the 110 day-aged solid. These results indicate that goethite is more stable under

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acidic conditions in the presence of sulfate, while hematite may be the final transformation

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product under weakly alkaline conditions. When the neutralization reagent was changed to

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CaO, for the solid precipitated at pH 4 from sulfate media, the transformation product was the

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same as that of NaOH-neutralized solid (goethite was the only end product). This means the

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influence of the presence of Ca on the transformation product of Fh is weak under acidic

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conditions. Whereas, when the pH was changed to 8, hematite was the only transformation

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product for this solid. The formation of goethite seems to be suppressed for this solid (Figure

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2) in comparison with the products of the NaOH-neutralized solids precipitated at pH 8 from

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sulfate media. By contrast, in nitrate media, the transformation products for those samples

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formed at pH 4 were hematite as the major phase and goethite as the minor phase, while

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hematite was the only end product for those samples precipitated at pH 8. These observations

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were irrespective of what neutralization reagent was used (Figure 1 and Figure 2). The reason

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why a rather long time of aging was used for some solids such as those precipitated from

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sulfate media at pH 4 is that we wanted to confirm whether goethite is the end transformation

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product under these conditions, or if it is just the intermediate product during the

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transformation process of Fh since some authors believed that goethite is the intermediate

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product during the two-stage crystallization process of Fh.11

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The Raman spectra of the freshly precipitated 2-line Fh and the aged samples are

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compared with those of the reference materials in Figure 3 and Figure 4. Two-line Fh shows

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characteristic Raman bands at ∼362 cm-1, ∼490 cm-1, and ∼719 cm-1 which are ascribed to

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Fe-O in plane, out-of-plane, as well as symmetric stretching vibrations, respectively (Figure

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3), consistent with previous results.29 Six-line Fh has similar Raman bands with that of 2-line

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Fh. The strong bands located at ∼302 cm-1, ∼385 cm-1, and ∼483 cm-1 are due to the Fe-OH

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symmetric bending, Fe-O-Fe/-OH symmetric stretching, and Fe-OH asymmetric stretching

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vibrations of goethite, respectively. The characteristic hematite bands appear at ∼225 cm-1,

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∼294 cm-1 and ∼407 cm-1 representing Fe-O symmetric stretching and bending vibrations.30

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For the NaOH-neutralized solids precipitated at pH 4 from sulfate media, the Raman

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spectra of the aged solid (for at least 10 hours) are in agreement with the spectrum of goethite.

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Similar transformation products were obtained for those CaO-neutralized solids (Figure 4).

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However, for the NaOH-neutralized solids precipitated from the same media at pH 8,

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hematite was the dominant product as indicated from Raman results after the sample was

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aged for at least 12 hours and this is also the case for the sample neutralized by CaO under

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the same reaction conditions. By contrast, for the solids precipitated from nitrate media at pH

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4, hematite (major) and goethite (minor) coexisted in the crystallization product no matter

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what neutralization reagent was applied. The conspicuous peaks at ∼225 cm-1, ∼294 cm-1 and

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∼407 cm-1 correspond to hematite, and a weak peak at ∼385 cm-1 also appears for the

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transformed product which could be ascribed to goethite. When the reaction pH increased to

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8, hematite was the only conversion product, and this observation was also independent of the

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neutralization reagent used. The results obtained from Raman spectra are in agreement with

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XRD characterizations (Figure 1 and Figure 2). Our findings are to some extent in agreement

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with previous work which suggested that hematite is the dominant product for a wide range

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of pHs and temperatures if 2-line Fh was precipitated from nitrate media by using NaOH as

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the neutralization reagent.11 However, it has to be mentioned that the crystallization products

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in this work were totally different from the results of that investigation, especially when the

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solid was precipitated and aged under acidic conditions from sulfate media (goethite was the

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only product).

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Crystallization rate. Since the conversion product is commonly a heterogeneous

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mixture consisting of the untransformed 2-line Fh and crystalline phases (Figure 1), chemical

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extraction procedures as well as Rietveld refinement calculations based on XRD data were

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used in this work to determine the percentage of each form of iron oxyhydroxide in the solid

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phase during the transformation process of Fh.

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The percentage of Fe extracted by 0.4 M HCl from the fresh and aged Fh precipitated

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from sulfate and nitrate media by using NaOH and CaO as neutralization reagents,

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respectively, at various pHs and at 80 °C was plotted as a function of aging time in Figure 5.

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The data indicate that for all the samples studied, the percentage of 0.4 M HCl extractable

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iron(III) oxyhydroxide decreased to less than 2% after the samples aged for 21 days, except

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for those CaO-neutralized solids precipitated and aged at pH 8. The Fe extractable curves

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totally overlap with each other for those NaOH-neutralized solids precipitated and aged at pH

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8 from both sulfate and nitrate media. However, at pH 4, a different crystallization rate is

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observed for those NaOH-neutralized solids precipitated from nitrate and sulfate media. For

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example, there is still about 82.4% untransformed Fh left in the solid phase even after 36

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hours of aging at pH 4 and at 80 °C for the NaOH-neutralized solid precipitated from nitrate

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media. By contrast, the amount of Fe extracted by 0.4 M HCl accounts for less than 4% of the

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total Fe for the NaOH-neutralized solid precipitated from sulfate media at the same pH,

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temperature and aging time. A similar trend is also observed for those CaO-neutralized

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samples. These results are consistent with XRD and Raman characterization. For example,

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some crystalline forms of iron oxyhydroxide appeared after it was aged at 80 °C for 6 hours

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for the NaOH-neutralized solid precipitated at pH 4 from sulfate media, whereas, the solid

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was still amorphous for the NaOH-neutralized solid precipitated at the same pH and aging

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time from nitrate media. (Figures 1 – 4). Similar results can also be observed for those

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CaO-neutralized solids precipitated at pH 4 from sulfate and nitrate media.

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The percentages of various forms of iron oxyhydroxides in the solids which were

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precipitated at pH 4 and 8 from sulfate and nitrate and aged at 80 °C for different times are

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shown in Figure 6 and Figure 7. For all of the solids in this work, the amorphous phase

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decreased with increasing aging time, while the crystalline form of iron oxyhydroxides

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increased. The data obtained by using X'pert HighScore Plus software with the Rietveld

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method was in agreement with the data obtained by chemical extraction as well as XRD and

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Raman characterization. For example, goethite is the only product in the solids precipitated

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from sulfate media at pH 4, while goethite (minor) and hematite (major) coexisted in the

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products for the solids precipitated from nitrate media at pH 4. In addition, the data presented

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in Figure 6 and Figure 7 further confirm that the transformation process was almost finished

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after aging for 21 days except for those CaO-neutralized solids precipitated at pH 8 from

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sulfate and nitrate media.

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The results obtained in this study are partially in agreement with the conclusions

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proposed by Das et al.11 who believed that the crystallization rate of NaOH-neutralized Fh

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precipitated from nitrate media was faster with increasing pH. Similar results were also

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obtained in our study for those NaOH-neutralized solids precipitated at pH 4 and 8 (Figure 5).

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However, for those solids precipitated from sulfate media, the chemical extraction data as

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well as the XRD and Raman characterization results indicate that the crystallization rate

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follows the order pH 4 > pH 8 and this phenomenon was more evident for those

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CaO-neutralized solids. Furthermore, as discussed above, different crystallization products

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were also obtained in different reaction media (goethite in sulfate vs. hematite plus goethite in

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nitrate) which means that sulfate does not only affect the products of the Fh at acidic pH but

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also the transformation rate.

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Therefore, the amounts of calcium and sulfate in the solid phase which were precipitated

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at pH 4 and 8 were analyzed in order to determine whether the crystallization rate and the

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products were affected by the presented calcium or sulfate ion (Figure 8). For the solids

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precipitated and aged at pH 4, the calcium content in the solid phase was much lower than

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those solids precipitated and aged at pH 8. Therefore, the adsorbed and/or incorporated

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calcium ion in the solid phase was not the reason why a different crystallization rate and

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products were observed for those solids. By contrast, large quantities of sulfate were adsorbed

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and/or incorporated in the solid phase which was precipitated at pH 4. The data showed that

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the content of sulfate in the solid phase could reach up to ∼172 mg/g for those

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NaOH-neutralized freshly precipitated solids. The content of sulfate in the solid phase was

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much larger for those NaOH-neutralized solids than those CaO-neutralized solids due to the

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reason that there exists a gypsum equilibrium for the CaO-neutralized samples.

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The FTIR spectra of the freshly precipitated 2-line Fh at pH 4 and the aged samples at

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80 °C before and after extraction processes are compared with those of the reference

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materials in Figure 9. Only the frequency region of interest (500 − 1200 cm-1) is displayed

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where the stretching and bending vibrations of S−O bonds show characteristic infrared active

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bands. Goethite shows characteristic O−H deformation-2 and -1 vibration, which located at

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∼791 cm-1 and ∼885 cm-1, respectively, and the results are comparable to previous data.31 The

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band at ∼980 cm-1 on the FTIR spectrum of the freshly prepared 2-line Fh is assigned to S−O

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symmetric (ν1), and the bands at ∼1049 cm-1 and ∼1122 cm-1 are assigned to the asymmetric

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stretching vibrations (ν3).20,32 These three peaks can also be seen for the 3 and 6 hour-aged

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samples without significant position change. The presence of the ν1 band and the splitting of

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the ν3 band indicate that the sulfate ion may form outer and/or inner complex sphere

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adsorption complexes on those freshly precipitated 2-line Fh or the aged solids.20,32 However,

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when the solid is aged longer than 12 hours, an additional peak at ∼1090 cm-1 as well as a ν3

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shift from ∼1122 to ∼1134 cm-1 occurs. The FTIR spectra of sulfate adsorbed on goethite

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shows three bands at ∼1049 cm-1, ∼1090 cm-1 and ∼1122 cm-1. The results indicate that for

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the aged solid before extraction, the incorporated and absorbed sulfate may coexist in the

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solids phase. In order to clarify whether the sulfate is incorporated into the structure of

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goethite, and whether it may have some effect on the crystallization process of 2-line Fh, the

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aged solids underwent a three-step sequential extraction process. For the 3 and 6 hour-aged

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solids after extraction, the solids show sulfate bands in FTIR spectra similar to each other,

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however, the band at ∼980 cm-1 due to the symmetric stretching vibrations (ν1) is replaced by

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a shoulder band at ∼1004 cm-1 and this phenomenon is more evidenced for the 21-day aged

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ACS Earth and Space Chemistry

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solid after sequential extraction processes. The overall results suggest that sulfate is indeed

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incorporated into the structure of goethite during the crystallization process of 2-line Fh at

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acidic pH and that it can change the crystallization rate and products of Fh.

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In contrast, the content of calcium in the solid phase first dramatically decreases with

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increasing aging time and then its content increases. The content of calcium in the solid phase

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is much larger for those solids precipitated and aged at pH 8 than for those precipitated and

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aged at pH 4. It was found that the content of calcium in the solid could reach up to ∼30 mg/g

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in our experimental conditions after the solids were aged for 21 days. However, under slightly

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alkaline conditions, hematite is the only or major product for all of the solids involved in this

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work (Figure 1 and Figure 2).8,10,11 Therefore, the presented calcium in the solid phase is

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responsible only for the different crystallization rates observed for the solids precipitated and

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aged at different pHs from the same reaction media. The rate of crystallization was strongly

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retarded by the presence of calcium under slightly alkaline conditions. For example, for the

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CaO-neutralized solids precipitated at pH 8 from sulfate media, there is still ∼80%

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untransformed

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NaOH-neutralized solids after aging for the same period. Ford et al.13 believed that the

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influence of some divalent metal ions (Ni and Pb in their investigation) exerted on the 2-line

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Fh crystallization process was metal-specific. Under alkaline conditions, the crystallization

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process was more significantly retarded in the presence of Ni. However, under slightly acidic

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conditions, the crystallization rate was accelerated in the presence of Pb. Similar results were

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obtained in our investigation, specifically that the crystallization rate is retarded in the

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presence of Ca under slightly alkaline conditions.

amorphous

ferrihydrite

left,

compared

with

∼1.1%

for

that

of

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Page 16 of 34

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Detailed transformation process for solids precipitated from sulfate media by using

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CaO as a neutralization reagent. According to the results obtained above, the reaction

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media as well as the neutralization reagent will exert some influence on the transformation

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process of 2-line Fh to the more crystalline form of iron(III) oxyhydroxide. As mentioned in

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the Introduction section, 2-line Fh is the major arsenic-bearing solid in hydrometallurgical

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treatment operations. Sulfate and calcium are inevitably involved in the wastes due to the use

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of sulfuric acid and slaked lime as the leaching and neutralization reagents used,

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separately.5,21 Therefore, a detailed transformation process (lower temperature and longer

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aging time emphasized for those solids) was studied for this specialized system at different

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temperatures for about 1800 days. The solid characterizations, the extractable percentage of

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Fe and the sulfate content in the solid phase with respect to the aging time are presented in

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Figures S1 – S6 of supporting information.

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Obviously, it can be seen that the crystallization rate decreases with increasing pH and

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this phenomenon is independent of the aging temperature. For example, the amount of Fe

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extracted by 0.4 M HCl solution accounted for >95% of total Fe after the pH 8 solids aged for

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1 day at 25 °C, and approximately 70% after aging for 280 days, suggesting that the

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untransformed amorphous form of Fh is the dominant phase for those solids. As for the pH 4

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samples aged at room temperature, the extractable Fe after aging for 280 days drastically

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decreased from >80% to