Solar Irradiation Induced Transformation of Ferrihydrite in the

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Environmental Processes

Solar Irradiation Induced Transformation of Ferrihydrite in the Presence of Aqueous Fe2+ Zhipeng Shu, Lihu Liu, Wen-Feng Tan, Steven L. Suib, Guohong Qiu, Xiong Yang, Lirong Zheng, and Fan Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b02750 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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Environmental Science & Technology

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Solar Irradiation Induced Transformation of Ferrihydrite in the Presence

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of Aqueous Fe2+

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Zhipeng Shu,†,ξ Lihu Liu,†,ξ Wenfeng Tan,† Steven L. Suib,‡ Guohong Qiu,*,† Xiong Yang,† Lirong

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Zheng,§ Fan Liu†

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†

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Ministry of Agriculture and Rural Affairs, Hubei Key Laboratory of Soil Environment and

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Pollution Remediation, College of Resources and Environment, Huazhong Agricultural University,

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Wuhan 430070, Hubei Province, China

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‡

Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtse River),

Department of Chemistry, University of Connecticut, Storrs, 55 North Eagleville Road, Storrs,

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Connecticut, 06269-3060, USA

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§

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Sciences, Beijing 100039, China

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ξ Shu

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*

Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of

Z.P. and Liu L.H. contributed equally to this work and shared the first author

Corresponding author: Qiu G.H., [email protected]

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Abstract: Ferrihydrite commonly occurs in soils and sediments, especially in acid mine drainage

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(AMD). Solar irradiation may affect Fe(II)-catalyzed transformation of metastable ferrihydrite to

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more stable iron oxides on AMD surface. We investigated the Fe(II)-catalyzed transformation

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process and mechanism of ferrihydrite under light irradiation. In nitrogen atmosphere, Fe2+aq could

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be oxidized to goethite and lepidocrocite by hydroxyl radical (OH•), superoxide radical (O2•−) and

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hole (hvb+) generated from ferrihydrite under ultraviolet (UV) irradiation (300–400 nm) at pH 6.0,

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and O2•− and hvb+ were mainly responsible for Fe2+aq oxidation. In addition, the ligand-to-metal

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charge-transfer (LMCT) process between Fe(II) and ferrihydrite could be promoted by UV

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irradiation. Goethite proportion increased with increasing Fe2+aq concentration. Both visible (vis) 1

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and solar irradiation could also lead to the oxidation of Fe2+aq to goethite and lepidocrocite, and the

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proportion of lepidocrocite increased with increasing light intensity. Fe2+aq was photochemically

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oxidized to schwertmannite at pH 3.0 and 4.5, and the oxidation rate was higher than that under

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dark conditions in air atmosphere. The photochemical oxidation rate of Fe2+aq decreased in the

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presence of humic acid. This study facilitates a better understanding of the formation and

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transformation of iron oxides in natural environments and ancient Earth.

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Graphic for Manuscript

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INTRODUCTION

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Iron is ubiquitous and the redox reactions between Fe(II) and Fe(III) are important reactions in

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natural environments.1,2 The iron oxides (including oxides, oxyhydroxides and hydroxides)

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containing Fe(II) and Fe(III) are commonly found in sediments and soils.3 Ferrihydrite is usually

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the first iron oxide mineral formed in the hydrolysis process of Fe3+,4,5 and is a necessary precursor

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for the formation of hematite through solid-state transformation.6 Ferrihydrite widely occurs in acid

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mine drainage (AMD),2,5–7 and forms complexes with Cu(II), Cr(VI), Pb(II) and As(V) owing to its

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large specific surface area and high adsorption capacity.2,7,8 The iron oxides with different crystal

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structures show various adsorption capacities for heavy mental ions.9 Therefore, the fate of toxic

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heavy metal ions in the environment is affected by the transformation of ferrihydrite. 2


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The transformation process of metastable ferrihydrite to more stable iron oxides has been

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extensively studied.4–6 Fe2+aq is a common product formed during various biological and abiotic

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processes, and plays an important role in the transformation of ferrihydrite.10 As reported, Fe2+aq

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concentration could reach 70–100 mg L−1 in the AMD with pH 2.9–4.8.11 Fe2+aq is absorbed on

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ferrihydrite in the form of inner-sphere complex.12 The surface property of ferrihydrite can be

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changed by the adsorption of Fe2+aq, which affects the transformation of ferrihydrite.6 After the

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adsorption of Fe2+aq on ferrihydrite surface, electron transfer from the adsorbed Fe2+ (Fe2+ads) to

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ferrihydrite takes place, which is known as a LMCT process.10 The LMCT process between Fe2+ads

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and ferrihydrite can accelerate the transformation of ferrihydrite to iron oxides including goethite

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(α-FeOOH), lepidocrocite (γ-FeOOH), magnetite (Fe3O4) and hematite (α-Fe2O3).4,13,14 The

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Fe(II)-catalyzed transformation process of ferrihydrite is affected by the reaction conditions

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including Fe2+aq concentration, temperature and pH. For example, a low ratio of Fe2+aq to

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ferrihydrite (< 1 mmol g−1) leads to the generation of goethite and lepidocrocite, while a higher ratio

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of Fe2+ to ferrihydrite results in the generation of magnetite.2,5 With increasing reaction temperature

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and pH, the rate of transformation from ferrihydrite to magnetite increases.7 When the newly

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formed products are goethite and lepidocrocite, low concentration of Fe2+ads results in an increased

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proportion of lepidocrocite, while high concentration of Fe2+ads and fast adsorption of Fe2+aq on

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ferrihydrite surface facilitate the formation of more goethite.1,4,15 Therefore, the transformation rate

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and products of ferrihydrite catalyzed by Fe2+aq may be different under various reaction conditions.

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As a semiconductor, ferrihydrite can produce hole−electron pairs (hvb+−ecb−) under light

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irradiation.16 O2•− can be generated from the reaction between ecb− and O2.16,17 The release of Fe2+aq

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from the photoreduction of ferrihydrite can promote photo-Fenton reaction at low pH (< 3.0),

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resulting in the generation of OH•.16–18 Ferrihydrite has been widely used in the photochemical 3



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oxidation of organic contaminants and toxic elements including Sb(III) and As(III) due to the high

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oxidation activity of reactive oxygen species (ROS), while the transformation of ferrihydrite has

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been seldom concerned in these processes.16,19,20 The ROS generated under light irradiation may

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affect the Fe(II)-catalyzed transformation of ferrihydrite. Our previous results indicated that Fe2+aq

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can be oxidized to iron oxides including goethite, lepidocrocite and schwertmannite by the ROS

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produced from NO3− photolysis under solar irradiation, and the composition of products depends on

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the pH and anion species.21 Solar irradiation can penetrate mineral particles with a thickness of

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about 0.3 mm and water of several meters depth, depending on the characteristics of the particles

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and waters and light intensity.22 Therefore, the transformation process of ferrihydrite in AMD under

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solar irradiation may be different from that under dark conditions in previous reports.

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In this work, we investigated the Fe(II)-catalyzed transformation of ferrihydrite under dark and

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light irradiation conditions. The effects of Fe2+aq concentration, pH, light source and natural organic

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matter on the transformation process and products were also studied. Powder X-ray diffraction

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(XRD), field emission scanning electron microscopy (FESEM), Fourier transform infrared

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spectroscopy (FTIR) and X-ray absorption spectroscopy (XAS) were used to characterize the

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species and proportion of the iron oxides formed in the system. The reaction mechanism was

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analyzed by determining the Fe2+aq concentration and possible ROS. The findings may help to better

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understand the transformation of ferrihydrite and cycling of related elements in natural

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environments.

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METHODS

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Transformation of Ferrihydrite. Ferrihydrite was synthesized by dropwise adding NaOH

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solution into a Fe(NO3)3 solution.23 Deoxygenated water was used in the experiment. The 4


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suspension containing ferrihydrite (0.1 g L−1) and FeSO4 (0–5 mmol L−1) was prepared using

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3-(N-morpholino) propanesulfonic acid (MOPS) (50 mmol L−1) at pH 6.0 ± 0.05 in the YQX-II

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anoxic glove box (Shanghai CIMO Medical Instrument Manufacturing Co. Ltd, China). Although

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the oxygen had been removed using nitrogen in the glove box, 0.3 ppm of dissolved oxygen was

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determined by a JPB-607A dissolved oxygen meter. 150-mL quartz tubes were used to hold the

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suspension with a controlled volume of 100 mL in each quartz tube. The sealed quartz tubes were

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taken out from the glove box and placed into a PL-03 photochemical reactor equipped a 1000-W

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mercury lamp (Beijing Precise Technology Co., Ltd.) for 12 h. The wavelength irradiated on the

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reaction system was controlled within 300–400 nm by a filter. The spectral curve of the mercury

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lamp was shown in our previous paper,24 and the transmittance of the filter was shown in Figure

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S1a. The light intensity was determined to be 1.68 × 10−6 Einstein L−1 s−1 using ferrioxalate

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actinometry.25,26 The reaction under dark conditions was performed by wrapping the quartz tubes

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with aluminum foil in the PL-03 photochemical reactor.

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The pH of ferrihydrite suspension (0.1 g L−1) was respectively adjusted to 4.5 ± 0.05 and 3.0 ±

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0.05 using 1.0 mol L−1 NaOH and 1.0 mol L−1 H2SO4 solutions to investigate the effect of pH on the

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transformation of ferrihydrite in nitrogen atmosphere. FeSO4 solution was added into the

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ferrihydrite suspension after adjusting pH, and the concentration of FeSO4 was controlled at 1.0

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mmol L−1 in the reaction system. The pH respectively decreased to 4.24 and 2.91 in the system with

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initial pH of 4.5 and 3.0 after UV irradiation (300–400 nm) for 12 h. The solutions of FeSO4 (1.0

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mmol L−1) in the absence of ferrihydrite were exposed to nitrogen atmosphere under dark and UV

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irradiation conditions for 12 h. The reactions of ferrihydrite (0.1 g L−1) and Fe2+aq (1.0 mmol L−1)

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were performed at different pHs under dark conditions in air atmosphere to study the effect of

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dissolved oxygen, and the dissolved oxygen concentration was determined to be 6.5 ppm. The 5



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reactions of ferrihydrite (0.1 g L−1) and FeSO4 (1.0 mmol L−1) at pH 6.0 were conducted under

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light-dark cycle, vis, simulated solar and solar light in nitrogen atmosphere to investigate the effect

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of light source on the transformation. The reactions under vis and simulated solar irradiation were

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conducted in the PL-03 photochemical reactor equipped with a 1000-W xenon lamp. The vis light

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was obtained using a filter, and the transmittance of the filter and spectral curve of the xenon lamp

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was respectively shown in Figure S1b and Figure S1c. The light intensity was measured to be 2.12

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× 10−8 Einstein L−1 s−1 using ferrioxalate actinometry.25,26 The light intensity of simulated solar

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irradiation was controlled at 1.84 × 10−8, 2.82 × 10−8 and 6.78 × 10−8 Einstein L−1 s−1. Little of

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UV-B (280–315 nm), most UV-A (315–400 nm) and vis light of solar irradiation can reach the

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earth’s surface.18 Therefore, the intensity of solar irradiation was determined at 320–400 nm and

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400–1000 nm using UV-A irradiatometer and FZ-A irradiatometer (Photoelectric Instrument Factor

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of Beijing Normal University), respectively. In order to investigate the effect of natural organic

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matter on the Fe(II)-catalyzed transformation process of ferrihydrite, the reactions of ferrihydrite

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(0.1 g L−1), FeSO4 (1.0 mmol L−1) and humic acid (10 mg L−1) (1S102H, ESHA, purchased from the

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International Humic Substances Society) were performed at pH 6.0 under UV irradiation or dark

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conditions in nitrogen atmosphere for 12 h. The solid products formed in the reaction were collected

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using 0.22-µm filter membrane, and washed by water until the conductivity of solution was below

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20 μS cm−1. The obtained solid was freeze-dried and stored in a refrigerator at the temperature of 4

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°C.

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Characterization and Analysis. In order to determine the presence of OH•, benzoic acid (BA)

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(10 mmol L−1) was added to the system of ferrihydrite (0.1 g L−1) and FeSO4 (1.0 mmol L−1) at pH

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6.0 under UV irradiation (300–400 nm) in nitrogen atmosphere. A high-performance liquid

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chromatography (HPLC, Agilent 1200) was used to analyze the concentration of p-hydroxybenzoic 6


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acid (p-HBA) generated from the reaction between OH• and BA, and the concentration of

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accumulated OH• was about 5.87 times that of p-HBA.21,24,27,28 The hvb+ formed in the

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photochemical reaction was scavenged using methanol (600 mmol L−1), and the concentration of

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formaldehyde (HCHO) generated from the oxidation of methanol was quantified by HPLC using

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2,4-dinitrophenylhydrazine (DNPH).29 The role of O2•− was investigated using superoxide

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dismutase (SOD) (50 mg L−1) and deactivated SOD (50 mg L−1).30,31 The deactivation of SOD was

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conducted in an autoclave at 120 °C for 0.5 h.32 The UV−vis spectrophotometry at 551 nm using

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N,N-diethyl-p-phenylenediamine (DPD) was used to determine the instant concentration of H2O2.33

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Fe2+aq concentration was determined using the 1,10-phenanthroline analytical method at 510 nm by

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a UV−vis spectrophotometer.14 The data determination and analysis of the UV−vis adsorption

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spectra of Fe2+aq solution, ferrihydrite, and ferrihydrite and Fe2+aq suspension at pH 6.0 were

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included in the Supporting Information.

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Power XRD with λ of 0.15418 nm (Bruker D8 ADVANCE, Cu Kα) was used to characterize the

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crystal structure of products. Evaluation software was used to evaluate the molar ratio of goethite to

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lepidocrocite based on the (110) peak area of goethite and (020) peak area of lepidocrocite.14

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FESEM (SU8000, Hitachi) and transmission electron microscopy (TEM, FEI, Talos F200C) were

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used to observe the micromorphology. FTIR spectra analyses were performed on a Bruker

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VERTEX 70 spectrometer. The species and proportion of iron oxides in the products were

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determined from the fitting of Fe K-edge extended X-ray absorption fine-structure (EXAFS) spectra

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using the references of goethite, lepidocrocite and ferrihydrite. Goethite and lepidocrocite were

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synthesized according to the method reported in the literature.34 XAS data were collected on 1W1B

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beamline at the Beijing Synchrotron Radiation Facility, China in transmission mode over the energy

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range of 6912–7864 eV. The detailed collection and analysis of XAS were presented in the 7



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Supporting Information.

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RESULTS

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Transformation of Ferrihydrite under UV Irradiation. The suspension of ferrihydrite (0.1 g

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L−1) and FeSO4 (1.0 mmol L−1) at pH 6.0 under UV irradiation and dark conditions was centrifuged

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and the solid products were obtained at different times. Under UV irradiation, the diffraction peaks

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of lepidocrocite occurred after 2 h, and a mixture of goethite and lepidocrocite was generated after

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12 h. Under dark conditions, the main product was goethite (Figure S2). The XRD results indicated

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that the transformation process and products were affected by UV irradiation. The species and

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proportion of iron oxides were also determined from the linear combination fits of Fe K-edge

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EXAFS spectra (Figure 1).1 As presented in Table 1, after irradiation under UV light, the proportion

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of goethite showed little change, while that of ferrihydrite decreased from 75.9% to 36.6%, and that

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of lepidocrocite increased from 3.8% to 42.0% compared with those under dark conditions. The

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FESEM and TEM images showed that the pristine ferrihydrite was nanoparticles with a diameter

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size of 4 nm and was transformed to massive goethite under dark conditions. Under UV irradiation,

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massive goethite and lamellar lepidocrocite were observed (Figure S3). These results showed that

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UV irradiation resulted in the generation of more lepidocrocite in the system of ferrihydrite and

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Fe2+aq.

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As shown in Figure 2, the Fe2+aq concentration in the reaction system showed no obvious changes

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under dark conditions, while decreased with reaction time under UV irradiation and even could not

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be detected after 12 h. In single Fe2+aq solution (1.0 mmol L−1) with pH 6.0, an 8% decrease in

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Fe2+aq concentration was observed and a mixture of goethite, lepidocrocite and iron oxide hydroxide

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was formed after 12 h under UV irradiation in nitrogen atmosphere (Figure S4). The concentrations 8


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of the iron oxides formed in the system of ferrihydrite (0.1 g L−1) and FeSO4 (1.0 mmol L−1) under

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UV irradiation and dark conditions for 12 h could be calculated based on the consumed Fe2+aq and

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the linear combination fits of Fe K-edge EXAFS spectra (Figure 2 and Table 1). Fe5HO8·4H2O,

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α-FeOOH and γ-FeOOH were respectively used as the formulas of ferrihydrite, goethite and

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lepidocrocite for the calculation in this work.6 The concentrations of ferrihydrite were 71.8 and 75.9

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mg L−1, those of goethite were 38.8 and 18.8 mg L−1, and those of lepidocrocite were 76.2 and 3.5

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mg L−1, under UV irradiation and dark conditions, respectively. These results showed that UV

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irradiation promoted the transformation of ferrihydrite and the oxidation of Fe2+aq to lepidocrocite

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and goethite, and lepidocrocite was the predominant species of product from the oxidation of Fe2+aq.

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BA, SOD, deactivated SOD and methanol were respectively added into the photochemical system

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to evaluate the roles of OH•, O2•− and hvb+ in the oxidation of Fe2+aq.21,24,30,31 The decrease in

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consumed Fe2+aq in the presence of BA or deactivated SOD indicated that OH• contributed to the

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oxidation of Fe2+aq. The consumed Fe2+aq showed a significant decrease after the addition of SOD

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and methanol, indicating that O2•− and hvb+ also play important roles in the oxidation of Fe2+aq.

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Effect of Fe2+ Concentration. Fe2+aq concentration affects the transformation products of

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ferrihydrite.1,4 Figure S5 shows the XRD patterns of the products formed in the suspension of

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ferrihydrite and FeSO4 at different concentrations under UV irradiation and dark conditions for 12 h.

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Under UV irradiation, no obvious transformation was observed in the absence of Fe2+aq, while

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goethite and lepidocrocite were generated after the addition of Fe2+aq. The molar ratio of goethite to

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lepidocrocite calculated from the analyses of XRD pattern was 0.71, 0.91 and 1.46 when the Fe2+aq

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concentration was 1.0, 3.0 and 5.0 mmol L−1, respectively (Table 2). Under dark conditions, no

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obvious transformation was observed in the absence of Fe2+aq as well. After the addition of Fe2+aq,

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the main product was goethite. Figure S6 shows the corresponding FESEM images of the products. 9



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Compared with dark conditions, UV irradiation resulted in more lamellar lepidocrocite and less

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goethite. These results indicated that the proportion of goethite increased with increasing Fe2+aq

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concentration under UV irradiation.

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Effect of pH. Figure S7 shows the XRD patterns of the products formed in the suspension of

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ferrihydrite and FeSO4 at different pHs under UV irradiation and dark conditions for 12 h in

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nitrogen atmosphere. Under UV irradiation and dark conditions, no obvious change was observed in

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the diffraction peaks of ferrihydrite at pH 3.0 and 4.5, while goethite and lepidocrocite were formed

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at pH 6.0. The increase in pH facilitated the transformation of ferrihydrite, which is consistent with

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the previous report.4 The diffraction intensity of goethite and lepidocrocite formed under UV

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irradiation was stronger than that formed under dark conditions (Figure S7). The micromorphology

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of ferrihydrite changed little at pH 3.0 and 4.5 under dark conditions; while an urchin-like

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architecture was observed under UV irradiation, which is the typical micromorphology of

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schwertmannite (Figure S8).21

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Both ferrihydrite and schwertmannite are poorly crystallized iron oxides, and it is difficult to

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differentiate schwertmannite from ferrihydrite in XRD pattern. The FTIR was used to further

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investigate the possibility of schwertmannite formation at pH 3.0 and 4.5 under UV irradiation

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(Figure S9). The bands at 1150, 1029 and 466 cm−1 are the characteristic peaks of lepidocrocite.35

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The bands at 892 and 790 cm−1 are the typical mode of Fe–OH–Fe in goethite.35 The bands at 3405

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and 1621 cm−1 are assigned to the stretching and bending vibrations of water, respectively.35 The

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bands at 1132 and 610 cm−1 are related to the stretching vibrations of S−O in SO42−.36 These results

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indicated the generation of schwertmannite at pH 3.0 and 4.5 under UV irradiation.

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Figure S10 shows the Fe2+aq concentrations in the suspension of ferrihydrite and FeSO4 at

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different pHs under UV irradiation and dark conditions in nitrogen atmosphere. The Fe2+aq 10


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concentration changed little at pH 3.0–6.0 under dark conditions. The consumed Fe2+aq increased

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with increasing pH under UV irradiation. Figure S11 shows the Fe2+aq concentrations in the system

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of ferrihydrite (0.1 g L−1) and Fe2+aq (1.0 mmol L−1) at different pHs under dark conditions in air

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atmosphere. At pH 6.0, the oxidation rate of Fe2+aq under dark conditions in air atmosphere was

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higher than that under UV irradiation in nitrogen atmosphere, while the oxidation rate of Fe2+aq

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under UV irradiation in nitrogen atmosphere was higher than that under dark conditions in air

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atmosphere at pH 3.0–4.5. The concentration of OH• determined at pH 6.0 was 1.8 μmol L−1 after

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12 h, which was remarkably lower than that determined at pH 3.0 (81.2 μmol L−1) (Figure S12a).

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H2O2 was not detected at pH 6.0 (Figure S12b), and the instant concentration of H2O2 reached about

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6.0 μmol L−1 at pH 3.0.

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Effect of Light Source. The reaction between ferrihydrite (0.1 g L−1) and FeSO4 (1.0 mmol L−1)

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at pH 6.0 in nitrogen atmosphere was also conducted under light-dark cycle, vis, simulated solar

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and solar irradiation for 12 h. No Fe2+aq was detected, and no obvious difference was observed in

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the XRD patterns for the solid products obtained under light-dark cycle and dark conditions after 12

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h of reaction under UV irradiation (Figure S13). The XRD patterns indicated the formation of

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goethite and lepidocrocite under vis, simulated solar and solar irradiation (Figure S14). The molar

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ratio of goethite to lepidocrocite in the products formed under UV, vis and solar irradiation was

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calculated to be 0.71, 1.18 and 1.71, respectively (Table 2). When the light intensity was controlled

247

at 1.84 × 10−8, 2.82 × 10−8 and 6.78 × 10−8 Einstein L−1 s−1 under simulated solar irradiation, the

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molar ratio of goethite to lepidocrocite in the products was calculated to be 2.68, 1.36 and 0.30,

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respectively (Table 2). These results indicated that the proportion of lepidocrocite increased with

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increasing light intensity.

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Effect of Natural Organic Matter. The reaction systems of ferrihydrite (0.1 g L−1), FeSO4 (1.0 11



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mmol L−1) and humic acid (10 mg L−1) were exposed to UV irradiation and dark conditions in

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nitrogen atmosphere for 12 h (Figure S15). Under dark conditions, more lepidocrocite was formed

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in the presence of humic acid compared with the case in the absence of humic acid, which was

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consistent with the results in the literature.23 Under UV irradiation, goethite and lepidocrocite were

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formed, and the photochemical oxidation rate of Fe2+aq decreased in the presence of humic acid.

257 258

DISCUSSION

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Formation Mechanism of Goethite and Lepidocrocite. Goethite and lepidocrocite were

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respectively the predominant species of products under dark and light irradiation conditions (Table

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1), and the oxidation rate of Fe2+aq was faster under light irradiation than under dark conditions

262

(Figure 2). In single Fe2+aq solution, the oxidation of Fe2+aq to iron oxides was observed at pH 6.0

263

under UV irradiation in nitrogen atmosphere (Figure S4). As reported, FeOH+ can absorb the UV

264

light at the wavelength of 300–450 nm at pH > 6.5, leading to the oxidation of Fe2+aq to iron oxides.

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At pH 4.8–6.1, the FeOH+ concentration is low and Fe2+aq can be oxidized to Fe(III) under the

266

irradiation of UV light at the wavelength of 100–280 nm.37,38 In this work, the pH was controlled at

267

3.0–6.0, leading to the oxidation of a small amount of Fe2+aq (8%) in the absence of ferrihydrite.

268

In the presence of ferrihydrite under light irradiation, the oxidation rate of Fe2+aq was increased

269

with the formation of goethite and lepidocrocite (Figure 2). hvb+−ecb− can be formed on the surface

270

of iron oxides under light irradiation, which is similar to the case of TiO2.39,40 OH• can be produced

271

from the reaction between hvb+ and hydroxyl group in water.39,41 In addition, Fe2+aq is released from

272

the photoreduction of iron oxides, and Fe(OH)2+ formed from the oxidation of Fe2+aq can produce

273

Fe2+aq and OH•, resulting in the photo-Fenton reaction.1,17,19 In this work, the low concentration of

274

OH• at pH 6.0 under UV irradiation can be ascribed to the precipitation of Fe3+, which hinders the 12


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formation of OH• through photo-Fenton reaction.16,19 Therefore, there was only a 14% decrease in

276

consumed Fe2+aq in the presence of BA (Figure 2).

277

The steady state concentration of OH• ([OH•]ss) was used to study the role of OH• in the

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photochemical oxidation of Fe2+aq. The reaction system of ferrihydrite (0.1 g L−1) and FeSO4 (1.0

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mmol L−1) at pH 6.0 in the presence of BA (3.4 μmol L−1) was exposed to UV irradiation in

280

nitrogen atmosphere, and the corresponding concentrations of BA at different time points were

281

determined to calculate [OH•]ss and Fe2+aq concentrations (Figure S16). The detailed calculation

282

process is shown in the Supporting Information. The [OH•]ss was calculated to be 3.5 × 10−15 mol

283

L−1, and the decrease in Fe2+aq concentration calculated from the whole reaction with OH• was

284

significantly lower than that determined in the system, further indicating that OH• was produced but

285

it was not responsible for the oxidation of most Fe2+aq ions.

286

Both hvb+ and OH• can be scavenged by methanol, and the reaction rate constant for the reaction

287

of OH• and BA (5.9 × 109 L mol−1 s−1) was about 6 times that for the reaction of OH• and methanol

288

(9.7 × 108 L mol−1 s−1).31 However, the concentration of methanol was 60 times that of BA in this

289

work. Therefore, almost all of OH• can be scavenged by methanol. After the addition of methanol, a

290

significant decrease (64%) in consumed Fe2+aq was observed (Figure 2). In addition, the

291

concentration of HCHO generated from the oxidation of methanol was higher than that of OH•

292

(Figure S17). These results indicated the oxidation of Fe2+aq by hvb+.

293

O2•− can be produced from the reaction between ecb− and dissolved O2.39,41 The consumed Fe2+aq

294

was respectively decreased by 50% and 22% in the presence of SOD and deactivated SOD (Figure

295

2), indicating that the intermediate of O2•− was formed and participated in the photochemical

296

oxidation of Fe2+aq. Although the photochemical reaction was conducted in nitrogen atmosphere,

297

there was still a small amount of dissolved oxygen (0.3 ppm) remaining in the suspension, which 13



Page 14 of 30

298

facilitated the generation of O2•−. These results indicated that O2•− is also responsible for the

299

oxidation of Fe2+aq under UV irradiation.

300

As reported, H2O can also react with ecb− to form H2. However, the reaction rate constant for the

301

reaction of ecb− and O2 (1.9 × 1010 L mol−1 s−1) is significantly larger than that for the reaction of

302

ecb− and H2O (1.9 × 101 L mol−1 s−1).31 Therefore, dissolved oxygen was the main electron acceptor

303

in this work, which was further supported by the obvious decrease (50%) in consumed Fe2+aq in the

304

presence of SOD.

305

During the transformation of ferrihydrite, the products including goethite and lepidocrocite can

306

also produce hvb+, O2•− and OH• under simulated solar light.39,42 The reaction systems of

307

goethite/lepidocrocite (0.1 g L−1) and FeSO4 (1.0 mmol L−1) with pH 6.0 were also exposed to UV

308

light or dark conditions for 12 h in nitrogen atmosphere to study the roles of goethite and

309

lepidocrocite in the photochemical oxidation of Fe2+aq. The decrease in Fe2+aq concentration and the

310

corresponding XRD patterns of solid products indicated that goethite and lepidocrocite also

311

contribute to the photochemical oxidation of Fe2+aq (Figures S18 and S19).

312

After 12 h of reaction, the concentration of ferrihydrite under UV irradiation (71.8 mg L−1) was

313

lower than that under dark conditions (75.9 mg L−1), indicating that UV irradiation promotes the

314

transformation of ferrihydrite to goethite and lepidocrocite. During the photochemical oxidation of

315

As(III) and Sb(III) by ferrihydrite, As(III) and Sb(III) can form complexes with ferrihydrite, and

316

electrons are transferred from As(III) and Sb(III) to Fe(III) in ferrihydrite through the LMCT

317

process.16,19 The ligand-to-metal electron transfer in LMCT process can be indicated by UV−vis

318

absorption spectra.43,44 Fe2+aq can be adsorbed on ferrihydrite surface to form Fe(II)-ferrihydrite

319

complex.45 The UV−vis absorption spectra of the ferrihydrite suspension at pH 6.0 were collected

320

to analyze the role of Fe(II)-ferrihydrite complex in the oxidation of Fe2+aq (Figure 3). No obvious 14


Page 15 of 30


321

absorption of Fe2+ was observed (1.0 mmol L−1) at 350–400 nm. Ferrihydrite (0.1 g L−1) presented

322

absorption over the UV and vis region, which is consistent with the previous report.17,19 The

323

absorption intensity of ferrihydrite suspension in the presence of Fe2+aq was significantly higher

324

than that in the absence of Fe2+aq, and increased with increasing Fe2+aq concentration. The UV−vis

325

absorption spectra of the suspension containing ferrihydrite and Fe2+aq presented a deflection point

326

at 366 nm. Benesi−Hildebrand equation (Equation S1) was used to fit the absorption data of the

327

ferrihydrite suspension in the presence of Fe2+aq at 366 nm (Figure 3). The good linear relationship

328

indicated the occurrence of a LMCT process between Fe2+ and ferrihydrite.16,19 Therefore, the

329

photochemical activity of Fe(II)-ferrihydrite complex facilitates the transformation of ferrihydrite.

330

Influencing Factors of Ferrihydrite Transformation under Light Irradiation. XRD patterns

331

indicated that the product composition was affected by Fe2+aq concentration, pH, light intensity and

332

light source (Table 2). After 12 h of reaction under UV irradiation, the proportion of goethite in the

333

products and the consumption rate of Fe2+aq increased with increasing Fe2+aq concentration (Table 2

334

and Figure S20). During the oxidation of Fe2+aq to goethite and lepidocrocite by air, the proportion

335

of lepidocrocite increased with increasing flow rate of air.46 In our previous work, the increase in

336

oxidation rate also led to the formation of more lepidocrocite in the system of FeCl2 and NO3−

337

under solar irradiation.21 The products formed in the suspension of ferrihydrite and FeSO4 (3.0 and

338

5.0 mmol L−1) under UV irradiation at different times were characterized by XRD to further study

339

the effect of Fe2+aq concentration on the formation mechanism of iron oxides (Figure S21). The

340

molar ratio of goethite to lepidocrocite was respectively 0.08, 0.09, 0.12, 0.17 and 1.46 after 1, 2, 4,

341

8 and 12 h of reaction when the Fe2+aq concentration was controlled at 5 mmol L−1. At the early

342

stage of reaction, the Fe2+aq concentration and oxidation rate were high, leading to the formation of

343

more lepidocrocite. With increasing reaction time, the concentration of dissolved oxygen decreased 15



Page 16 of 30

344

from 0.3 to 0.1 ppm at 12 h, leading to a decrease in the oxidation rate of Fe2+aq. The lepidocrocite

345

can be transformed to goethite under the catalyzation of Fe2+aq in the solution.4 A higher initial

346

concentration of Fe2+aq would facilitate the transformation of lepidocrocite to goethite, which can

347

explain the result that the proportion of goethite increased with increasing Fe2+aq concentration.

348

Under UV irradiation, a mixture of goethite and lepidocrocite was generated in the suspension of

349

ferrihydrite and Fe2+aq at pH 6.0, while schwertmannite was produced at pH 3.0 and 4.5 (Figures S8

350

and S9). These results are consistent with the effect of pH on the oxidation products of FeSO4

351

solution by the ROS generated from NO3− under solar irradiation in our previous work.21

352

Schwertmannite is ubiquitous in acid-sulfate waters and AMD.47–49 Some oxyanions including

353

As(III,V), Cr(VI) and P(V) can be adsorbed and coprecipitated by schwertmannite.47–49 The

354

metastable schwertmannite can be also transformed to goethite and lepidocrocite.21,50 Therefore, the

355

water quality of AMD is affected by the formation and transformation of schwertmannite.47–49 The

356

desertion of SO42− from the tunnel structure and increase in pH can lead to the transformation of

357

schwertmannite to goethite and lepidocrocite.50 The concentration of OH− increased with increasing

358

pH, making it easier for OH− to bind Fe3+ than SO42−.51 Therefore, the products were goethite and

359

lepidocrocite at pH 6.0. The consumed Fe2+aq increased with increasing pH in the photochemical

360

reaction (Figure S10). The occurrence of photo-Fenton reaction hindered the oxidation of Fe2+aq to

361

iron oxides at low pH,16,19 as indicated by the higher concentrations of OH• and H2O2 at pH 3.0 than

362

at pH 6.0 (Figure S12).

363

Light source and intensity affect the transformation of ferrihydrite. The UV−vis absorption

364

spectra indicated that ferrihydrite can be excited by the light with wavelengths of 200–800 nm

365

(Figure 3), which is consistent with the previous report.17 Therefore, the UV (300–400 nm), vis

366

(400–1000 nm) and solar irradiation used in the experiment could facilitate the oxidation of Fe2+aq 16


Page 17 of 30


367

to goethite and lepidocrocite. More Fe2+aq was oxidized to lepidocrocite rather than to goethite.

368

Hence, the proportion of lepidocrocite increased with increasing light intensity. These results also

369

indicate that the transformation of ferrihydrite on the surface of AMD can be affected by solar

370

irradiation.

371

Environmental Implications. By comparing the reaction process between ferrihydrite and Fe2+aq

372

under light and dark conditions, it can be found that light irradiation can promote the LMCT

373

process of Fe(II)-ferrihydrite complex, which facilitates the transformation of ferrihydrite. In

374

addition, Fe2+aq can be oxidized to goethite, lepidocrocite and schwertmannite by the oxidants (OH•,

375

O2•− and hvb+) generated in the photochemical reaction. Our findings indicate that the transformation

376

processes of ferrihydrite to crystalline iron oxides in the environment may be more abundant than

377

expected (Figure 4). In previous studies of Fe(II)-catalyzed transformation of ferrihydrite, more

378

attention was paid to the influencing factors such as microorganism,13 organic matter,5 and solution

379

conditions.4,7 As reported, the photochemical activity and strong adsorption capacity of iron oxides

380

may be one of the reasons for the cyclical changes in the concentration and species of arsenic under

381

day and night conditions.52 Our previous results also indicated that the oxidation and dissolution of

382

arsenopyrite could be accelerated by OH• and H2O2, which are formed through the decomposition

383

of H2O induced by Fe(III) due to the sulfur-deficient sites on arsenopyrite surface. Under solar

384

irradiation, the generated intermediates including FeAsO4 and goethite affect the migration of

385

arsenic and sulfur released from arsenopyrite.18 In natural environments, the presence of natural

386

organic matter affects the Fe(II)-catalyzed transformation of ferrihydrite.5,23 The ROS produced

387

from organic matter in acidic waters under solar irradiation also affects the cycling of Fe at pH

388

3.0–5.0.53 In this work, Fe2+aq could still be oxidized to goethite and lepidocrocite, although the

389

oxidation rate was lower than that in the absence of humic acid (Figure S15). In addition, by 17



Page 18 of 30

390

comparing the results under UV irradiation and dark conditions in air atmosphere, it can be seen

391

that the photochemical oxidation of Fe2+aq can occur, which promotes the oxidation of Fe2+aq on

392

ferrihydrite in the presence of oxygen at low pH (pH 3.0–4.5). Therefore, the oxidation of Fe2+aq to

393

goethite, lepidocrocite and schwertmannite may occur on the surface of waters, especially in AMD

394

due to the active intermediates generated by ferrihydrite under solar irradiation.

395

The low oxygen concentration allowed the presence of high concentration of Fe2+aq on ancient

396

Earth, and the direct photochemical oxidation of Fe2+aq to Fe(III) oxides induced by UV light was

397

regarded as an important way to generate banded iron formations.37,38 In this work, the results

398

indicated that the oxidation of Fe2+aq on the surfaces of iron oxides and LMCT process of

399

Fe(II)-ferrihydrite complex under solar irradiation may also contribute to the generation of banded

400

iron formations on ancient Earth.

401 402

ACKNOWLEDGMENTS

403

This project was financially supported by the National Natural Science Foundation of China

404

(Grant Nos. 41571228, 41425006 and 41877025), the National Key Research and Development

405

Program of China (Grant Nos. 2017YFD0801000 and 2018YFD0800304) and the Fundamental

406

Research Funds for the Central Universities (Program Nos. 2662018JC055 and 2662015JQ002).

407

Steven L. Suib is grateful to the US Department of Energy, Office of Basic Energy Sciences,

408

Division of Chemical, Biological and Geological Sciences under grant DE-FG02-86ER13622.A000.

409

The authors also thank Dr. Lihong Qin and Dr. Jianbo Cao at the Public Laboratory of Electron

410

Microscopy of Huazhong Agricultural University for the help in SEM and TEM characterization.

411 412

ASSOCIATED CONTENT 18


Page 19 of 30

413


Supporting Information

414

Supplementary Information include the spectral curve of light source and the transmittance of the

415

filter, XRD patterns, FESEM and TEM images, FTIR spectra of the as-obtained products under

416

different conditions, and the concentration of Fe2+aq, p-HBA, H2O2, HCHO, log plot of the BA

417

concentrations and Fe2+aq concentration calculated based on [OH•]ss in the photochemical reaction.

418 419

AUTHOR INFORMATION

420

Corresponding Author

421

* Qiu GH, E-mail: [email protected]

422

ORCID

423

Wenfeng Tan: 0000-0002-3098-2928

424

Steven L. Suib: 0000-0003-3073-311X

425

Guohong Qiu: 0000-0002-1181-3707

426

Notes

427

The authors declare no competing financial interest.

428 429

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430

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569

Tables

570

Table 1. Fitting results of Fe K-edge EXAFS spectra of the transformation products formed in the

571

system of ferrihydrite (0.1 g L−1) and Fe2+aq (1.0 mmol L−1) at pH 6.0 under UV irradiation and dark

572

conditions for 12 h in nitrogen atmosphere. Condition

Ferrihydrite (%)

Goethite (%)

Lepidocrocite (%)

R-factor

UV

36.6 (1.3)

21.4 (1.1)

42.0 (1.7)

0.0103

Dark

75.9 (1.0)

20.3 (0.9)

3.8 (1.4)

0.0107

573 574 575 576

Table 2. Molar ratio of goethite to lepidocrocite (G/L) in the solid products formed under different

577

conditions. The molar ratio was calculated based on the (110) peak area of goethite and (020) peak

578

area of lepidocrocite in XRD patterns.14 Condition

Fe2+ concentration (mmol

Light intensity of simulated solar (Einstein L−1 s−1)

Light source

L−1)

Sample

1

3

5

UV

vis

solar

1.84 × 10−8

2.82 × 10−8

6.78 × 10−8

G/L

0.71

0.91

1.46

0.71

1.18

1.71

2.68

1.36

0.30

26


Page 27 of 30

580


Figures

581

582 583

Figure 1. Fe K-edge EXAFS spectra (solid lines) and the corresponding linear combination fitting

584

(grey dotted lines) of the solid products formed in the system of ferrihydrite (0.1 g L−1) and Fe2+aq

585

(1.0 mmol L−1) at pH 6.0 under UV irradiation and dark conditions for 12 h using references

586

(colored lines) in nitrogen atmosphere.

27



Page 28 of 30

588

589 590

Figure 2. Fe2+aq concentrations in the system of ferrihydrite (0.1 g L−1) and Fe2+aq (1.0 mmol L−1) at

591

pH 6.0 under dark conditions and UV irradiation with different scavengers in nitrogen atmosphere.

28


Page 29 of 30


593

594 595

Figure 3. UV−vis absorption spectra of Fe(II)-ferrihydrite complex at pH 6.0. The inset shows the

596

Benesi−Hildebrand plot for the Fe(II)-ferrihydrite complex at 366 nm. Equation: 1/ΔA = 0.0175 ×

597

1/CFe(II) + 0.2067, R2 = 0.9987.

29



Page 30 of 30

599

600 601

Figure 4. Formation mechanism of goethite and lepidocrocite in the system of ferrihydrite and

602

Fe2+aq at pH 6.0 under light irradiation.

30


Solar Irradiation Induced Transformation of Ferrihydrite in the

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