The transformation of two-line ferrihydrite into crystalline products

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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]

1

ACS Paragon Plus Environment

ACS Earth and Space Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

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

3

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

4

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

5

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

6

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

7

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

9

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

15

(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

17

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

18

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

19

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

21

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

25

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

44

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

80

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

85

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

88

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

95

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

100

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

101

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

104

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

105

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

106

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

107

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

132

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

139

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

146

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

152

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,

154

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

8


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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

157

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

158

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

159

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

160

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

162

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

164

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

165

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

167

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

168

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

169

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

170

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

171

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

173

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

174

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

175

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

176

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

179

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

180

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

183

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.

187

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

188

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

189

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

191

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

192

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

193

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

194

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

195

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

196

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

197

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

198

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

10


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

200

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

201

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

202

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

203

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

204

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

207

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

208

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

209

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

211

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

212

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

213

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

214

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

217

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

218

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

219

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

220

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

222

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

223

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

224

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

225

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

226

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

227

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

228

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

229

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

231

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

232

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

233

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

234

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

235

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

236

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

237

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

238

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

239

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

240

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

241

sulfate and nitrate media.

242

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

12


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

244

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

245

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

246

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

247

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

248

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

249

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

250

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

251

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

252

also the transformation rate.

253

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

254

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

255

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

256

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

257

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

258

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

259

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

260

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

261

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

262

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

263

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

264

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

266

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

267

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

268

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

269

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

270

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

271

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

272

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

273

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

274

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

275

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

276

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

277

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

278

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

279

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

280

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

281

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

282

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

283

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

284

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

285

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

286

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

14


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287

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

288

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

289

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

290

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

291

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

292

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

293

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

294

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

295

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

296

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

297

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

298

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

299

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

300

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

301

untransformed

302

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

303

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

304

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

305

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

306

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

307

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

308

presence of Ca under slightly alkaline conditions.

amorphous

ferrihydrite

left,

compared

with

∼1.1%

for

that

of

15



Page 16 of 34

309

Detailed transformation process for solids precipitated from sulfate media by using

310

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

311

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

312

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

313

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

314

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

315

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

316

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

317

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

318

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

319

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

320

Figures S1 – S6 of supporting information.

321

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

322

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

323

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

324

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

325

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

326

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

327

decreased from >80% to

The transformation of two-line ferrihydrite into crystalline products

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