Photochemical Formation and Transformation of Birnessite: Effects of

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Environmental Processes

Photochemical Formation and Transformation of Birnessite: Effects of Cations on Micromorphology and Crystal Structure Tengfei Zhang, Lihu Liu, WenFeng Tan, Steven L. Suib, Guohong Qiu, and Fan Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06592 • Publication Date (Web): 24 May 2018 Downloaded from http://pubs.acs.org on May 24, 2018

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Environmental Science & Technology

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Photochemical Formation and Transformation of Birnessite: Effects of Cations on

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Micromorphology and Crystal Structure

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Tengfei Zhang,† Lihu Liu,† Wenfeng Tan,† Steven L. Suib,‡ Guohong Qiu,*,† Fan Liu†

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†

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Ministry of Agriculture, Hubei Key Laboratory of Soil Environment and Pollution Remediation,

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College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, Hubei

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Province, China

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‡

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06269-3060, USA

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

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

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ABSTRACT: As important components with excellent oxidation and adsorption activity in soils and

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sediments, manganese oxides affect the transportation and fate of nutrients and pollutants in natural

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environments. In this work, birnessite was formed by photocatalytic oxidation of Mn2+aq in the

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presence of nitrate under solar irradiation. The effects of concentrations and species of interlayer

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cations (Na+, Mg2+, and K+) on birnessite crystal structure and micromorphology were investigated.

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The roles of adsorbed Mn2+ and pH in the transformation of the photosynthetic birnessite were further

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studied. The results indicated that Mn2+aq was oxidized to birnessite by superoxide radicals (O2•−)

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generated from the photolysis of NO3− under UV irradiation. The particle size and thickness of

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birnessite decreased with increasing cation concentration. The birnessite showed a plate-like

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morphology in the presence of K+, while exhibited a rumpled sheet-like morphology when Na+ or Mg2+

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was used. The different micromorphologies of birnessites could be ascribed to the position of cations in

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the interlayer. The adsorbed Mn2+ and high pH facilitated the reduction of birnessite to low-valence

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manganese oxides including hausmannite, feitknechtite, and manganite. This study suggests that

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interlayer cations and Mn2+ play essential roles in the photochemical formation and transformation of 1

ACS Paragon Plus Environment


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birnessite in aqueous environments.

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Graphic for Manuscript

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INTRODUCTION

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Manganese oxides are widely distributed in soils and sediments.1 Birnessite, which consists of

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edge-sharing MnO6 octahedral layers interlayered with cations and water molecules, is one of the most

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common manganese oxides in natural environments.1,2 The Mn(IV) vacancies in MnO6 octahedral

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layers result in the negative charge of birnessite, and facilitate the adsorption of cations.3,4 As the main

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oxidant in terrestrial and aquatic environments, birnessite participates in the oxidation of inorganic (e.g.,

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Cr(III) and As(III)) and organic pollutants.5,6 The formation and transformation of birnessite are always

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accompanied by the changes in its adsorption capacity and redox activity, which affects the toxicity and

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bioavailability of trace elements and pollutants in the environments.7–10

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In recent studies, O2•− produced by the photochemical reaction of NO3− under UV irradiation was

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found to oxidize Mn2+aq to birnessite in the presence of O2, and hydroxyl radicals (OH•) generated from

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the photolysis of NO3− was not likely responsible for the photochemical formation of birnessite.11

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However, OH• can oxidize low-valence hydrated metal ions including Mn2+aq to high-valence metal

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ions or oxides.12 Hence, the effect of OH• on the photochemical formation of birnessite remains elusive.

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In addition, the oxidation of Mn2+aq by the photolysis of nitrate may occur in nitrate-rich wastewaters, 2


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eutrophic waters and sediment surface, which are almost anoxic conditions.13 Therefore, more attention

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should be paid to the effect of dissolved oxygen on the photooxidation of Mn2+aq.

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In the formation process, the coexisting cations affect the micromorphology, chemical composition,

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and crystallinity of birnessite.14–17 Hexagonal birnessites with different micromorphologies could be

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obtained in the presence of Mg2+, K+, Ca2+ or Fe3+, due to the different positions of these cations in the

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birnessite.14 As reported, with increasing content of structural Fe, the thickness of birnessite decreased

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along the c-axis due to the creation of more defects such as distortion, stress, and vacancies.15 The

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Mn(III) content in the birnessite(-like) formed through the oxidation of Mn2+aq by Pseudomonas putida

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strain GB-1 in the presence of Na+ and Ni2+ was lower than that in the presence of Ca2+.16 During the

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oxidation of Mn2+aq by Bacillus sp. strain SG-1, it was found that Na+ favors the formation of

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hexagonal birnessite, while Ca2+ facilitates the formation of orthogonal birnessite.17 When comparable

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concentrations of Ca2+ were used, there were also differences in the crystal structure of birnessite

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formed by abiotic and biotic oxidation of Mn2+aq due to the different mechanisms of Mn2+aq

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oxidation.14,17 However, little is known about the effects of coexisting cations (e.g., Na+, Mg2+, and K+)

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on the reaction of NO3− and Mn2+aq under UV irradiation.

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Birnessite is the precursor of some manganese oxides, including cryptomelane (α-MnO2), groutite

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(α-MnOOH), manganite (γ-MnOOH), feitknechtite (β-MnOOH), and hausmannite (Mn3O4).2,7,8,18–20

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The comproportionation reaction of structural Mn(IV) and Mn2+aq can induce the reduction of

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birnessite to low-valence manganese oxide minerals and the release of H+ under anoxic conditions.7,8

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The increase in Mn2+aq concentration and pH promotes the transformation of birnessite.7,8 Under oxic

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conditions, the reaction of birnessite and 7.5 mmol L−1 Mn2+aq resulted in the formation of β-MnOOH

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at pH 7.0, but little alteration was observed in the sheet structure of birnessite when the Mn2+aq

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concentration was decreased to 0.75 mmol L−1.18 In the reaction between birnessite and 0.4 mmol L−1

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Mn2+aq under anoxic conditions, only a part of birnessite was transformed into β-MnOOH after 8 days

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at pH 7.5, while birnessite was completely transformed into β-MnOOH and γ-MnOOH with pH 3



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increasing to 8.0.8 In addition, Mn(III) could promote rotationally ordered stacking in birnessite

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structure by enhancing the electrostatic repulsion between layer Mn(IV) in adjacent sheets, and Mn(III)

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content was affected by the formation process of birnessite.16,18,21 The addition of pyrophosphate can

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lead to the increase of Mn(III) content in photosynthetic birnessite.21 The effects of pH, Mn2+aq

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concentration and Mn(III) content on the transformation of photosynthetic birnessite need further

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investigation.

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In this work, the influence of reactive oxygen species (O2•− and OH•) and coexisting cations (Na+,

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Mg2+ and K+) on the formation of birnessite was investigated, and the effects of adsorbed Mn2+ and H+

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(pH) on the transformation of photosynthetic birnessite were further examined. The study was expected

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to facilitate a better understanding of the formation and transformation of manganese oxides in

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supergene environments.

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MATERIALS AND METHODS

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Birnessite Formation. In this work, the reaction solutions used in anoxic experiments were prepared

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using deoxygenated deionized water and sealed in an anaerobic glove box (YQX-II) protected by

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high-purity nitrogen gas. In a typical experiment, a mixed solution of 0.25 mmol L−1 MnSO4 and 10

86

mmol L−1 NaNO3 was prepared. The initial pH of the mixed solution was adjusted to 6.0 using NaOH

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(0.1 mol L−1) and H2SO4 (0.1 mol L−1) under continuous magnetic stirring. The mixed solution was

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transferred into 150 mL quartz tubes and sealed. Then, the sealed quartz tubes were exposed to solar

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irradiation for 12 h. The experiments under solar irradiation were conducted on the rooftop of College

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of Resources and Environment building (E 114°21′12′′, N 30°28′34′′), Huazhong Agricultural

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University. The light intensity was 0.24–1.78 mW cm−2 at 320–400 nm and the outdoor temperature

92

was 23–38 °C in the experiments (Aug. 15th, 2016). After reaction, the precipitates were collected, and

93

the detailed collection process is presented in the Supporting Information.

94

In order to investigate the effect of light source on the formation of manganese oxide minerals, a 4


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mixed solution of 0.25 mmol L−1 MnSO4 and 10 mmol L−1 NaNO3 was placed into a photoreactor

96

(PL-03) and exposed to UV and Vis irradiation for 12 h, respectively. The details of the photoreactor

97

are described in Figure S1. A 1000-W high pressure Hg lamp was used as UV light source, and the

98

light (λ > 420 nm) was cut by light filter. The spectral curve of the high-pressure mercury lamp is

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shown in Figure S2. The light intensity was 4.6 mW cm−2 at 320–400 nm. A 1000-W Xe lamp was

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used as Vis light source, and the UV light (λ < 400 nm) was also cut by light filter. The light intensity

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was 4.2 mW cm−2 at 400–1000 nm. The results were recorded by taking photos. The product obtained

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under UV irradiation was named as 0.01Na-HB. To examine the possible reactive oxygen species, OH•

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and O2•− were scavenged by 20 mmol L−1 of benzoate (BA) and 20 mg L−1 of superoxide dismutase

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(SOD), respectively, in the photochemical processes. This method is detailedly described in the

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Supporting Information. In order to investigate the effect of dissolved oxygen on the photooxidation of

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Mn2+, air was purged into the solution under UV irradiation.

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To investigate the effects of Na+, Mg2+ and K+ on the product compositions, MSO4 (M = Na+, Mg2+

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and K+) was added to 10 mmol L−1 MNO3 with 0.25 mmol L−1 MnSO4 solutions. The concentrations of

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Na+, Mg2+ and K+ were adjusted at 0.1 or 1.0 mol L−1 in the reaction systems. Then, the mixed

110

solutions were exposed to UV irradiation. The products were respectively named as 0.1Na-HB,

111

0.1Mg-HB, 0.1K-HB, 1.0Na-HB, 1.0Mg-HB, and 1.0K-HB when 0.1 and 1.0 mol L−1 Na+/Mg2+/K+

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were used in the aqueous systems. There was no obvious difference in the pH change under different

113

conditions (Figure S3).

114

Birnessite Transformation. Briefly, 40.0 mg 0.01Na-HB was added to 200 mL deionized water.

115

The pHs of the suspensions were controlled at 4.0 and 6.0 by manual addition of NaOH (0.5 mol L−1)

116

and H2SO4 (0.5 mol L−1) with continuous magnetic stirring in air atmosphere. There was no obvious

117

change in pH when pH was adjusted to 4.0, and 6.0, while a rapid decrease was observed at pH 8.0 in

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the reaction processes. The suspension pH was controlled at 8.0 using 20 mmol L−1 HEPES

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(4-(2-hydroxyethyl)-1-piperazineethanesulfonicacid and 0.5 mol L−1 NaOH. The reaction was 5



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performed at pH 8.0 under anoxic conditions to reduce the effect of O2 on the transformation of

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birnessite. During the transformation of birnessite, the pH of all suspensions was adjusted 3 times a day

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and the volume of added NaOH/H2SO4 solution was less than 50 µL each time. The suspensions were

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magnetically stirred at room temperature for 30 days. After a period of reaction time, about 20 mL

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suspension in the reaction system was drawn off and filtered through a 0.22 µm microporous

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membrane. The concentration of Mn2+aq in the filtrate was determined by atomic absorption

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spectroscopy (AAS). The solid was washed with deoxygenated deionized water for 5 times and stored

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in a vacuum oven at room temperature.

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To study the effect of adsorbed Mn2+ on the transformation of birnessite, the initial concentration of

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Mn2+aq was controlled at 0.7, 3.5 and 7.0 mmol L−1 at pH 8.0 under oxic and anoxic conditions,

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respectively. All the anoxic transformation experiments were conducted in the anaerobic glove box to

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exclude the interference of O2.

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Characterization and Analysis. The crystal structure of the samples was analyzed by powder X-ray

133

diffraction (XRD Cu Kα, λ = 0.15418 nm). Fourier transform infrared spectroscopy (FTIR) spectra

134

were collected using a Bruker VERTEX 70 spectrometer. Field emission scanning electron microscopy

135

(FESEM) and high-resolution transmission electron microscopy (HRTEM) were used to characterize

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the micromorphology of the samples. The molar ratios of cations (Na+, Mg2+, and K+) to Mn in the

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birnessites were determined by dissolving 10.0 mg samples in 50 mL NH2OH·HCl (0.2 mol L−1). The

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concentrations of Mg2+ and Mn2+ were determined by AAS, and those of Na+ and K+ were determined

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by flame photometer. Mn K-edge X-ray absorption spectroscopy (XAS) spectra were measured at room

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temperature on the 1W1B beamline at the Beijing Synchrotron Radiation Facility, China. The detailed

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parameters for analyses of XAS spectra are described in the Supporting Information.

142 143 144

RESULTS Formation of birnessite. The XRD pattern and HRTEM image of the sample suggested that 6


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gauze-like birnessite (JCPDS No. 86-0666) was obtained through solar irradiation under anoxic

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conditions (Figure S4). The mixed solutions were respectively exposed to dark, UV and Vis irradiation

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to identify the excitation band of NO3−. No precipitate was formed in the absence of UV irradiation or

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NO3− (Figure S5). These results indicated that excited NO3− was responsible for the oxidation of Mn2+aq

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to birnessite under UV irradiation.

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To confirm the role of O2•− and OH•, Mn2+aq photooxidation experiments were performed by

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respectively adding SOD and BA under UV irradiation. No oxidation of Mn2+aq was observed in the

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presence of SOD, and the consumption of Mn2+aq slightly decreased in the presence of BA, which

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suggested that O2•− is mainly responsible for the oxidation of Mn2+aq (Figure 1a).11,22 The concentration

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of p-HBA decreased with the addition of Mn2+aq into the reaction system (Figure 1b), and the

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reductions of OH• were respectively calculated to be 116.2 and 118.7 µmol L−1 under oxic and anoxic

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conditions in the presence of Mn2+aq.

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Figure S6 shows the XRD patterns of the wet and dried samples obtained in the photochemical

158

systems with different Na+, Mg2+, and K+ concentrations. A mixture of buserite and birnessite was

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formed in the presence of Mg2+ (Figure S6a). Single-phased birnessite was generated in the presence of

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Na+ or K+, and the birnessite presented a smaller d001 in the presence of K+. After drying, all samples

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were observed as single-phased birnessites, and no obvious difference was found in the d001 (Figure

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S6b). These results indicated that the coordination of cations in wet birnessite was different from that in

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dried birnessite.

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In the dried samples of 0.01Na-HB, 0.1Na-HB, and 1.0Na-HB, the ratio of d100 to d110 was about

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1.73, suggesting that hexagonal birnessite was formed.15 With increasing Na+ concentration, the full

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width at half maximum (FWHM) of (001) diffraction peak increased, indicating a decrease in

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crystallinity of the birnessite.15 When Mg2+ and K+ were used instead of Na+, similar change trends

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were observed in the FWHM of (001) diffraction peak with increasing cation concentration. In the

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systems with the same concentration, the presence of K+ facilitated the formation of birnessite with 7



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higher crystallinity. According to the FWHM of (001) diffraction peak, the average thickness of

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birnessite along the c-axis was calculated by the Scherrer formula.23 The average thickness of

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0.01Na-HB, 0.1Na-HB and 1.0Na-HB was 17.3, 16.3 and 11.2 nm, that of 0.1Mg-HB and 1.0Mg-HB

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was 16.7 and 11.3 nm, and that of 0.1K-HB and 1.0K-HB was 25.7 and 14.9 nm, respectively. These

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results demonstrated that the thickness of birnessite decreased with increasing concentration of cations.

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The Na/Mn molar ratio in 0.01Na-HB, 0.1Na-HB and 1.0Na-HB was 2.3:100, 4.3:100 and 7.0:100,

176

the Mg/Mn molar ratio in 0.1Mg-HB and 1.0Mg-HB was 0.7:100 and 3.7:100, and the K/Mn molar

177

ratio in 0.1K-HB and 1.0K-HB was 3.6:100 and 7.2:100, respectively. The cation content in the

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birnessite increased with increasing cation concentration. The Mn average oxide state and relative

179

contents of Mn(II/III/IV) in 0.01Na-HB, 0.1Na-HB, 0.1Mg-HB, and 0.1K-HB were obtained from Mn

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K-edge XANES spectra fitted by the Combo method (Figure S7).24 With increasing Na+ concentration

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in the reaction system, the relative content of Mn(IV) in 0.1Na-HB was slightly higher than that in

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0.01Na-HB (Table S1).

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The crystal structure and bond length of the birnessite were further characterized by Mn K-edge

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k3-weighted EXAFS spectra (Figure 2). Consistent with the XRD results, a single positive amplitude

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was observed at 8.0 Å–1, suggesting that the birnessites were of hexagonal symmetry (Figure 2a).25–27

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In the Fourier transformed spectra, two strong backscattering peaks at R + δR ~1.5 Å and ~2.5 Å were

187

observed, which correspond to the distances of the first Mn−O shell (Mn−O1, R ~1.90 Å) and

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edge-sharing Mn−Mn shell (Mn−Mnedg, R ~2.85 Å), respectively (Figure 2b).27 The peak at R + δR

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~3.1 Å is assigned to the second Mn−O shell (Mn−O2) and corner-sharing Mn−Mn shell

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(Mn−Mncor).26–28 The Mn−Mncor shell results from the coordination of Mn2/3+ with the unsaturated O

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atoms around Mn(IV) vacancies.26–28 The Mn−O and Mn−Mn bond lengths in the birnessites were

192

calculated from EXAFS spectra using single scattering model with 1.0 < R + δR < 3.5 Å. As presented

193

in Table S2, no evident differences were observed in the Mn−O1 and Mn−Mnedg bond lengths among

194

0.01Na-HB, 0.1Na-HB, 0.1Mg-HB, and 0.1K-HB. However, the Mn−Mncor bond length slightly 8


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increased with increasing radius of interlayer cations.

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The micromorphology of the birnessite was characterized by HRTEM and FESEM (Figures 3 and

197

S8). 0.01Na-HB exhibited a gauze-like morphology, and the lattice fringes separated by ~0.71 nm

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correspond to the (001) planes of birnessite (Figure S8a). 0.1Na-HB, 1.0Na-HB, 0.1Mg-HB, and

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1.0Mg-HB presented a rumpled sheet-like morphology, while 0.1K-HB and 1.0K-HB showed a

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plate-like morphology.

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Transformation of photosynthetic birnessite. The concentration of Mn2+aq is one important factor

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to induce the change of the crystal structure and chemical compositions of birnessite.7–9 The fitting

203

results of Mn K-edge XANES spectra suggested that the relative content of Mn(II) in the

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photosynthetic birnessite was within 12–16%, which was higher than that in the birnessite (~4%)

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prepared by traditional methods (Table S1).15 In addition, the photosynthetic birnessite showed a

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gauze-like morphology (Figure S8a), which was remarkably different from the three-dimensional

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hierarchical microsphere-like morphology of the birnessite prepared by traditional methods.15 Because

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high relative content of Mn(II) and morphology might influence the transformation process of

209

birnessite, the transformation experiments of photosynthetic birnessite were carried out at constant pH

210

of 4.0–8.0 under oxic and anoxic conditions.

211

Figure 4 shows the XRD patterns of the transformation products of birnessite at pH 4.0–8.0 under

212

oxic conditions. There was no obvious change in the diffraction peaks of birnessite at pH 4.0 after 30

213

days. When the pH was controlled at 6.0 and 8.0, the characteristic peaks of hausmannite (Mn3O4,

214

JCPDS No. 89-4837) were observed after 20 days and the peak intensity increased with reaction time.

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After 30 days, the birnessite retained the gauze-like morphology at pH 4.0, while Mn3O4 particles were

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generated at pH 6.0 and 8.0 (Figure S9a–c). The FTIR spectra of the transformed products further

217

indicated the formation of Mn3O4 after 30 days (Figure S9d). The concentration of Mn2+aq gradually

218

increased with reaction time and decreased with increasing pH, and reached 243.5, 51.4 and 5.5 µmol

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L−1 at pH 4.0, 6.0 and 8.0 after 30 days, respectively (Figure 5). Therefore, the adsorbed Mn2+ was 9



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released into the solutions at low pH, and drove the transformation of birnessite to Mn3O4 at high pH.8

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To study the effect of adsorbed Mn2+ on the transformation of birnessite, the initial concentration of

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Mn2+aq was controlled at 0.7, 3.5 and 7.0 mmol L−1 at pH 8.0 under oxic conditions, respectively.

223

Mn3O4 and feitknechtite (β-MnOOH, JCPDS No. 18-0804) were formed in the reaction system of

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birnessite and 0.7 mmol L−1 Mn2+aq after 1 day, and the intensity of their corresponding XRD peaks

225

increased with reaction time (Figure S10a). There was no obvious change in the crystallinity of

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birnessite after 30 days. When the Mn2+aq concentration was increased to 3.5 mmol L−1, birnessite was

227

transformed into Mn3O4, β-MnOOH and manganite (γ-MnOOH, JCPDS No. 88-0649) after 1 day

228

(Figure S10b). After 3 days, the corresponding XRD peaks of birnessite could not be detected, and the

229

intensity of the corresponding peaks of Mn3O4 and γ-MnOOH increased. A mixed phase of Mn3O4 and

230

γ-MnOOH was observed after 30 days. When Mn2+aq concentration was increased from 3.5 to 7.0

231

mmol L−1, less β-MnOOH and more γ-MnOOH were formed after 1 day, and the same final products

232

were obtained after 30 days (Figure S10c). Mn3O4 particles and lath-shaped β-MnOOH were observed

233

at low Mn2+aq concentration (0.7 mmol L−1), and rod-shaped γ-MnOOH and Mn3O4 particles were

234

formed at high Mn2+aq concentration of 3.5 and 7.0 mmol L−1 (Figure S11a–c). These results suggested

235

that the increase of Mn2+aq concentration could promote the transformation of birnessite into

236

γ-MnOOH.

237

The reaction of birnessite and 0–7.0 mmol L−1 Mn2+aq was performed at pH 8.0 under anoxic

238

conditions to investigate the influence of O2 on the transformation of birnessite. The transformation

239

products of birnessite under anoxic conditions were similar to those under oxic conditions (Figure S12).

240

However, when birnessite reacted with 0.7 mmol L−1 Mn2+aq, the crystallinity of birnessite decreased

241

with reaction time under anoxic conditions (Figure S12b). These results suggested that the crystal

242

stability of birnessite is promoted at low Mn2+aq concentration in the presence of O2.

243 244

DISSCUSION 10


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Formation of birnessite. In this work, birnessite was generated from the photochemical oxidation of

246

Mn2+aq in the presence of NO3− at the initial pH of 6.0 with UV irradiation under anoxic and oxic

247

conditions, respectively (Figures S6 and S13). UV light at 240 nm can directly photo-oxidize Mn2+aq.29

248

No obvious decrease in the concentration of Mn2+aq was observed in the solution of MnSO4 (0.25 mmol

249

L−1) at initial pH 6.0 under UV irradiation after 12 h, suggesting that the Mn2+aq oxidation was not

250

affected by the UV light at 240 nm possibly due to the weak light intensity at 240 nm in this work

251

(Figures 1a and S2).

252

O2•− is mainly responsible for the oxidation of Mn2+aq. NO2− can be generated from the

253

photochemical reaction of NO3− under UV irradiation.30–34 Under oxic conditions, the reaction between

254

NO2− and dissolved oxygen leads to the formation of O2•−.11,30 Although the mixed solutions were

255

saturated by high-purity N2, low dissolved oxygen may exist in the system. In addition, oxygen could

256

be formed in the photolysis of NO3− with UV irradiation.31,34 In this work, dissolved oxygen was

257

detected to be 0.28 ppm in the anoxic solution. The consumption of Mn2+aq remarkably decreased in the

258

presence of SOD, and NO2− was also detected in the reaction photochemical processes of NO3− and

259

Mn2+aq (Figures 1a and S14a). To further confirm the formation of O2•− from NO2− photolysis, the

260

mixed solution of MnSO4 (0.25 mmol L−1) and NaNO2 (10 mmol L−1) was exposed to UV irradiation.

261

Mn2+aq was oxidized as indicated by the decrease in its concentration and the formation of birnessite

262

precipitate (Figures S13 and S14b). The consumption of Mn2+aq in the presence of NO2− was less than

263

that in the presence NO3−, which may be attributed to the efficient trapping of O2•− by excess NO2−.35

264

These results suggested that O2•− is responsible for the oxidation of Mn2+aq to birnessite, which was

265

further indicated by the increased consumption of Mn2+aq under oxic conditions (Figure 1a).

266

Both O2•− and OH• can be produced by the photochemical reaction of NO3− under UV irradiation.30,31

267

The consumption of Mn2+aq decreased in the presence BA, possibly because the pathway of O2•−

268

generation was inhibited by the consumption of OH•.31 In the photochemical system of NO3− and BA,

269

the concentration of p-HBA decreased in the presence of Mn2+aq (Figure 1b). The reaction between O2•− 11



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and Mn2+aq inhibited the formation of OH•,31 which was further indicated by the generation of more

271

NO2− after the addition of Mn2+aq (Figure S14a). The consumption of Mn2+aq in the presence of BA was

272

higher than that in the presence of SOD, further suggesting that O2•− is responsible for the oxidation of

273

Mn2+aq.11,22

274

Effects of Cations. The chemical composition of manganese oxides is affected by the coexisting

275

cations in the reaction system.14–17 Na+, Mg2+ and K+ are located in the interlayer to neutralize the

276

negative charges in MnO6 layer.3 In this work, a mixture of buserite and birnessite was formed in the

277

presence of Mg2+, while single-phased birnessite was generated in the presence of Na+ or K+ (Figure

278

S6a). Na+, Mg2+ and K+ are present in the form of hydrated cations in aqueous solution, and are

279

absorbed into the interlayer of manganese oxides during the formation process.36 The hydrated radii of

280

Na+, Mg2+ and K+ are 3.58, 4.28 and 3.31 Å, respectively.37 Due to the larger hydrated radius of Mg2+,

281

buserite was firstly formed and gradually transformed into birnessite during the dehydration process.1,38

282

It is possible that birnessrite was directly formed due to the smaller hydrated radii of Na+ and K+. The

283

higher charge density of hydrated K+ could also contribute to the smaller distance between adjacent

284

layers.

285

After drying, hexagonal birnessites with different relative contents of Mn(IV) were prepared with

286

different concentrations and species of cations. In the crystal structure of birnessite, the negative

287

charges resulting from Mn(IV) vacancies are balanced by cations located in the interlayer.3,14,15 The

288

incorporation of cations into birnessite structure is hindered by other coexisting cations in the system.39

289

As reported, there is almost no Mn2+ present in the structure of hexagonal birnessite.38,40 In addition,

290

the increase in the relative content of Mn2+ above/below Mn(IV) vacancies can result in the increase in

291

Mn−Mncor bond length.26 The fitted Mn−Mncor bond length of 0.1Na-HB (3.45 Å) was slightly longer

292

than that of 0.01Na-HB (3.44 Å), and the Mn(II) relative content of 0.1Na-HB (15%) was slightly

293

higher than that of 0.01Na-HB (12%) (Tables S1 and S2). Therefore, Mn2+ is located above/below

294

Mn(IV) vacancies by coordinating with three unsaturated O atoms around Mn(IV) vacancies. The 12


Page 13 of 28


295

molar ratio of Na to Mn was higher in 0.1Na-HB (4.3:100) than in 0.01Na-HB (2.3:100); besides, the

296

relative content of Mn(IV) in 0.1Na-HB (76%) was higher than that in 0.01Na-HB (72%), suggesting

297

that more Na+ and less Mn2/3+ were located in the interlayer of birnessite. Thus, the absorbed Na+ may

298

inhibit the coordination of Mn2/3+ with unsaturated O atoms around Mn(IV) vacancies by neutralizing

299

the negative charges of MnO6 layer. In the formation process of birnessite, the presence of Mg2+ could

300

facilitate the oxidation of low-valence manganese oxides in the reaction system.41 Hence, the increase

301

in cation concentrations may also lead to the generation of more Mn(IV) and less Mn(II,III) in

302

birnessite.

303

In addition, the micromorphology of birnessite was affected by the interlayer cations for the dried

304

samples (Figure 3b–d). The interlayer cations bind with unsaturated O atoms around Mn(IV) vacancies

305

through electrostatic interaction, double-sharing surface complexation, triple-corner-sharing interlayer

306

complexation or edge-sharing layer complexation, which depends on the radius and charge of the

307

cations.14,42,43 Na+ and K+ are located in the interlayer by coordinating with two O atoms in the MnO6

308

layer and four O atoms from interlayer H2O; Mg2+ is located in the interlayer by coordinating with

309

three O atoms in the MnO6 layer and three O atoms from interlayer H2O.3,44 The radii of Na+ (1.17 Å),

310

Mg2+ (0.72 Å) and K+ (1.49 Å) are obviously different.37 The O atoms from MnO6 layer have different

311

electrostatic interactions with Na+, Mg2+ and K+, leading to the different positions of cations in the

312

interlayers of birnessites.3,14,43 The radius of K+ is larger than that of Na+ and Mg2+, and thus K+ has a

313

relatively lower charge density. K+ coordinates with the O atoms from MnO6 layer through weak

314

electrostatic interactions and thus it tends to stay in the middle of interlayers.14,43 K+ might provide a

315

balanced supporting force to the adjacent MnO6 layers, which could explain the plate-like morphology

316

of 0.1K-HB and 1.0K-HB. However, Na+ and Mg2+ tend to shift to the more negative surface between

317

the adjacent layers because of their smaller radius and higher charge density,14,43 resulting in their

318

unbalanced supporting force to adjacent MnO6 layers and thus the rumpled sheet-like morphology of

319

birnessite. In the structure of birnessite, Mn2/3+ coordinates with three unsaturated O atoms around 13



Page 14 of 28

320

Mn(IV) vacancies through triple-corner sharing interlayer complexation.26,27,42 The interaction intensity

321

of Mn2/3+ with unsaturated O atoms around Mn(IV) vacancies may be decreased with increasing

322

interaction intensity of interlayer cations with the O atoms from MnO6 layer, which leads to the

323

increase in Mn−Mncor bond length.43 Thus, the Mn−Mncor bond length in birnessite increased with

324

decreasing radius of interlayer cations (Table S2).

325

The thickness and particle size of birnessite decreased while the relative contents of cations (Na+,

326

Mg2+, or K+) in birnessite increased with increasing cation concentration. The adsorption of more

327

cations on the edge sites of MnO6 octahedra facilitates the nucleating of birnessite particles, which

328

inhibits the growth of the sheets.45 Moreover, Na+, Mg2+, and K+ are alien cations relative to Mn2/3/4+ in

329

birnessite structure, and are obviously different from Mn2/3/4+ in valence, radius, and electronegativity.

330

These alien cations in birnessite could induce the formation of more defects (e.g., distortion, stress,

331

vacancies and repulsion) in the layers and inhibit the crystal growth.15 The changes in the

332

micromorphology and crystal structure during the dehydration processes for the wet phyllomanganites

333

will be further studied.

334

Transformation of Photosynthetic Birnessite. Previous studies have suggested that the crystal

335

structure and mineral phase of birnessite are stable at low pH (< 7.0) in aqueous systems.7,8 In this

336

work, a part of birnessite was transformed to Mn3O4 at pH 6.0 and 8.0 without the addition of Mn2+aq

337

(Figure 4b, c). In the reduction process of birnessite, the added Mn2+aq was absorbed above/below the

338

Mn(IV) vacancies, and then reacted with structural Mn(IV) by interfacial electron transfer.7,8 The Mn2+

339

adsorbed above/below the Mn(IV) vacancies caused the reduction of birnessite into Mn3O4.8 The

340

release of Mn2+aq decreased with the increase of pH in the suspension, implying that more Mn2+ was

341

adsorbed above/below the Mn(IV) vacancies in the birnessite and participated in the reduction of

342

birnessite at higher pH. The relative content of Mn(II) in the photosynthetic birnessite structure was

343

about 12%, which is higher than that of the birnessite (~4%) prepared by traditional methods (Table

344

S1).15 The Mn2+ adsorbed above/below the Mn(IV) vacancies facilitates the transformation of 14


Page 15 of 28


345

birnessite into Mn3O4. In this study, Mn3O4 might be formed in the surface-catalyzed oxidation of Mn2+

346

by O2 on the birnessite surface.46 However, Mn3O4 was also observed in anaerobic environment

347

without the addition of Mn2+aq (Figure S12a). These results suggested that the Mn2+ adsorbed

348

above/below the Mn(IV) vacancies result in the transformation of birnessite into Mn3O4 without the

349

addition of Mn2+aq under anoxic conditions.

350

Mn2+aq concentration affected the transformation product of birnessite (Figure S10). β-MnOOH and

351

Mn3O4 were formed in the reaction between birnessite and 0.7 mmol L−1 Mn2+aq, and γ-MnOOH and

352

Mn3O4 were formed when the Mn2+aq concentration was increased to 3.5 and 7.0 mmol L−1. Birnessite

353

could be reduced to β-MnOOH and γ-MnOOH by Mn2+aq.7,8 The transformed products of birnessite

354

with Mn2+aq were dependent on the structural stability of manganese oxides. The structural stability of

355

manganite (γ-MnOOH) and Mn3O4 is higher than that of β-MnOOH.8 The thermodynamic equilibria

356

also suggests that γ-MnOOH is the expected transformation product in the reaction system of birnessite

357

and 0.7–7.0 mmol L−1 Mn2+aq at pH 8.0 (Figure S15). However, γ-MnOOH was not observed after 30

358

days of reaction between 0.7 mmol L−1 Mn2+aq and birnessite, and β-MnOOH was formed as an

359

intermediate during the reduction of birnessite to γ-MnOOH at higher Mn2+aq concentration. The

360

electron transfer from Mn2+aq to structural Mn(III) of β-MnOOH facilitated the transformation of

361

β-MnOOH into γ-MnOOH.7,8 The transformation rate of β-MnOOH into γ-MnOOH was slow at low

362

Mn2+aq concentration. The amount of absorbed Mn2+ increased with increasing Mn2+aq concentration,

363

which facilitated the transformation of β-MnOOH into γ-MnOOH.

364

The presence of O2 also affected the transformation of birnessite (Figures S10 and S12b–d). The

365

crystal structure of birnessite was more stable under oxic conditions compared with under anoxic

366

conditions at low Mn2+aq concentration (0.7 mmol L−1). More Mn2+aq was consumed under oxic

367

conditions than under anoxic conditions, suggesting that O2 accelerates the oxidation of Mn2+aq (Figure

368

S16). At high Mn2+aq concentration (3.5 and 7.0 mmol L−1), birnessite was completely transformed to

369

γ-MnOOH and Mn3O4. Under oxic conditions, manganese and iron oxides would present catalytic 15



Page 16 of 28

370

oxidation activity, and Mn2+aq could be catalytically oxidized to β-MnOOH and Mn3O4 by O2 on the

371

surface.47,48 Therefore, at low Mn2+aq concentration under oxic conditions, the catalytic oxidation of

372

Mn2+aq by O2 leads to the decrease of its concentration in the solution, which hinders the reduction of

373

Mn2+aq and birnessite.

374

Environmental Implications. In this work, hexagonal birnessite was obtained through the oxidation

375

of Mn2+aq by O2•− under UV and solar irradiation. In natural aqueous systems, many substances such as

376

dissolved organic matter, NO3− and NO2− can generate reactive oxygen species.49 Mineralization

377

mediated by microorganisms is regarded as the major formation pathway of manganese oxides in

378

natural environments, and little research attention has been paid to the pathway of photochemical

379

mineralization.1,50 The study of photochemical formation of birnessite contributes to a better

380

understanding of the formation process of manganese oxides in supergene environments. The present

381

work shows that the particle size, micromorphology, and chemical composition of birnessite are

382

affected by the coexisting cations, and absorbed Mn2+ promotes the transformation of the

383

photosynthetic birnessite into Mn3O4 at near neutral pH. In addition, the pH, Mn2+aq concentration, and

384

O2 also influence the transformation of the photosynthetic birnessite into low-valence manganese oxide

385

minerals. These results may facilitate a better understanding of the diversity of natural manganese

386

oxides. Natural manganese oxides formed under different geologic conditions always contain different

387

metal ions in their structures, such as Cu, Zn, Pb, and Ni.15,42,51 The transformation of manganese

388

oxides is always accompanied by the changes in their adsorption capacity and redox activity, which

389

affect the desorption−resorption behaviors and redox of the absorbed substances.9 Therefore, the

390

formation and transformation processes of manganese oxides affect the toxicity and bioavailability of

391

trace elements and pollutants in environments. Further studies are needed to elucidate the factors

392

controlling and driving the photochemical formation and transformation of manganese oxides, such as

393

adsorbed metal ions and organic pollutants.

394 16


Page 17 of 28


395

ASSOCIATED CONTENT

396

Supporting Information

397

Description of collection and characterization of products, and photoreactor (PL-03); HRTEM and

398

FESEM images, XRD patterns, FTIR spectra, Mn K-edge XANES and EXAFS fitting results of

399

Mn-oxide solids; Changes in pH, Mn2+ and NO2− concentrations. This material is available free of

400

charge via the Internet at http://pubs.acs.org.

401 402

AUTHOR INFORMATION

403

Corresponding Author

404

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

405

ORCID

406

Guohong Qiu: 0000-0002-1181-3707

407

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

408

Wenfeng Tan: 0000-0002-3098-2928

409

Notes

410

The authors declare no competing financial interest.

411 412

ACKNOWLEDGMENTS

413

The authors thank the National Natural Science Foundation of China (Grant Nos. 41571228 and

414

41425006), the National Key Research and Development Program of China (Grant No.

415

2017YFD0801000), the Fok Ying-Tong Education Foundation (No. 141024), and the Fundamental

416

Research Funds for the Central Universities (No. 2662015JQ002) for financial support. Steven L. Suib

417

acknowledges support of the US Department of Energy, Office of Basic Energy Sciences, Division of

418

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

419

greatly acknowledge Dr. Lirong Zheng and Dr. Shengqi Chu at beamline 1W1B at Beijing Synchrotron 17



Page 18 of 28

420

Radiation Facility (BSRF) for the technical assistance with data collection and analyses. The authors

421

thank Dr. Lihong Qin and Dr. Jianbo Cao at Public Laboratory of Electron Microscopy in Huazhong

422

Agricultural University for the help of FESEM and HRTEM characterization.

423 424 425 426 427 428 429 430

REFERENCES (1) Post, J. E. Manganese oxide minerals: Crystal structures and economic and environmental significance. Proc. Natl. Acad. Sci. U.S.A. 1999, 96 (7), 3447–3454. (2) Wang, Q.; Yang, P.; Zhu, M. Q. Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions. Environ. Sci. Technol. 2018, 52 (4), 1844–1853. (3) Post, J. E.; Veblen, D. R. Crystal structure determinations of synthetic sodium, magnesium, and potassium birnessite using TEM and the Rietveld method. Am. Mineral. 1990, 75 (5–6), 477–489.

431

(4) Liu, L. H.; Luo, Y.; Tan, W. F.; Zhang, Y. S.; Liu, F.; Qiu, G. H. Facile synthesis of birnessite-type

432

manganese oxide nanoparticles as supercapacitor electrode materials. J. Colloid Interface Sci. 2016,

433

482, 183–192.

434 435

(5) Manning, B. A.; Fendorf, S. E.; Bostick, B.; Suarez, D. L. Arsenic(III) oxidation and arsenic(V) adsorption reactions on synthetic birnessite. Environ. Sci. Technol. 2002, 36 (5), 976–981.

436

(6) Di Leo, P.; Pizzigallo, M. D. R.; Ancona, V.; Di Benedetto, F.; Mesto, E.; Schingaro, E.; Ventruti,

437

G. Mechanochemical degradation of pentachlorophenol onto birnessite. J. Hazard. Mater. 2013, 244,

438

303–310.

439 440 441 442 443 444

(7) Elzinga, E. J. Reductive transformation of birnessite by aqueous Mn(II). Environ. Sci. Technol. 2011, 45 (15), 6366–6372. (8) Lefkowitz, J. P.; Rouff, A. A.; Elzinga, E. J. Influence of pH on the reductive transformation of birnessite by aqueous Mn(II). Environ. Sci. Technol. 2013, 47 (18), 10364–10371. (9) Lefkowitz, J. P.; Elzinga, E. J. Structural alteration of hexagonal birnessite by aqueous Mn(II): Impacts on Ni(II) sorption. Chem. Geol. 2017, 466, 524–532. 18


Page 19 of 28


445

(10) Cui, H. J.; Qiu, G. H.; Feng, X. H.; Tan, W. F.; Liu, F. Birnessites with different average

446

manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric

447

pressure. Clays Clay Miner. 2009, 57 (6), 715–724.

448

(11) Jung, H.; Chadha, T. S.; Kim, D.; Biswas, P.; Jun, Y. S. Photochemically assisted fast abiotic

449

oxidation of manganese and formation of δ-MnO2 nanosheets in nitrate solution. Chem. Commun. 2017,

450

53 (32), 4445–4448.

451

(12) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical review of rate constants

452

for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅OH/⋅O−) in aqueous

453

solution. J. Phys. Chem. Ref. Data 1988, 17 (2), 513–886.

454 455

(13) Cloern, J. E. Our evolving conceptual model of the coastal eutrophication problem. Mar. Ecol. Prog. Ser. 2001, 210, 223–253.

456

(14) Wang, J. L.; Li, D. D.; Li, P. L.; Zhang, P. Y.; Xu, Q. L.; Yu, J. G. Layered manganese oxides for

457

formaldehyde-oxidation at room temperature: the effect of interlayer cations. Rsc Adv. 2015, 5 (122),

458

100434–100442.

459

(15) Yin, H.; Liu, F.; Feng, X. H.; Hu, T. D.; Zheng, L. R.; Qiu, G. H.; Koopal, L. K.; Tan, W. F.

460

Effects of Fe doping on the structures and properties of hexagonal birnessites – Comparison with Co

461

and Ni doping. Geochim. Cosmochim. Acta 2013, 117, 1–15.

462 463

(16) Zhu, M. Q.; Gindervogel, M.; Parikh, S. J.; Feng, X. H.; Sparks, D. L. Cation effects on the layer structure of biogenic Mn-oxides. Environ. Sci. Technol. 2010, 44 (12), 4465–4471.

464

(17) Webb, S. M.; Tebo, B. M.; Barger, J. R. Structural influences of sodium and calcium ions on the

465

biogenic manganese oxides produced by the marine Bacillus sp., strain SG-1. Geomicrobiol. J.

466

2005, 22 (3–4), 181–193.

467 468 469

(18) Hinkle, M. A.; Flynn, E. D.; Catalano, J. G. Structural response of phyllomanganates to wet aging and aqueous Mn(II). Geochim. Cosmochim. Acta 2016, 192, 220–234. (19) Tu, S.; Racz, G. J.; Goh, T. B. Transformations of synthetic birnessite as affected by pH and 19



470

Page 20 of 28

manganese concentration. Clays Clay Miner. 1994, 42 (3), 321–330.

471

(20) Zhang, Q.; Xiao, Z. D.; Feng, X. H.; Tan, W. F.; Qiu, G. H.; Liu, F. α-MnO2 nanowires

472

transformed from precursor δ-MnO2 by refluxing under ambient pressure: the key role of pH and

473

growth mechanism. Mater. Chem. Phys. 2011, 125 (3), 678–685.

474

(21) Jung, H.; Chadha, T. S.; Min, Y.; Biswas, P.; Jun, Y. S. Photochemically-Assisted Synthesis of

475

Birnessite Nanosheets and Their Structural Alteration in the Presence of Pyrophosphate. ACS

476

Sustainable Chem. Eng. 2017, 5 (11), 10624–10632.

477 478 479 480 481 482 483 484

(22) Ryu, J.; Choi, W. Effects of TiO2 surface modifications on photocatalytic oxidation of arsenite: the role of superoxides. Environ. Sci. Technol. 2004, 38 (10), 2928–2933. (23) Patterson, A. L. The Scherrer formula for X-ray particle size determination. Phys. Rev. 1939, 56 (10), 978–982. (24) Manceau, A.; Marcus, M. A.; Grangeon, S. Determination of Mn valence states in mixed-valent manganates by XANES spectroscopy. Am. Mineral. 2012, 97 (5–6), 816–827. (25) Grangeon, S.; Lanson, B.; Miyata, N.; Tani, Y.; Manceau, A. Structure of nanocrystalline phyllomanganates produced by freshwater fungi. Am. Mineral. 2010, 95 (11–12), 1608–1616.

485

(26) Silvester, E.; Manceau, A.; Drits, V. A. Structure of synthetic monoclinic Na-rich birnessite and

486

hexagonal birnessite: II. Results from chemical studies and EXAFS spectroscopy. Am. Mineral.

487

1997, 82 (9–10), 962–978.

488 489 490 491 492 493 494

(27) Webb, S. M.; Tebo, B. M.; Bargar, J. R. Structural characterization of biogenic Mn oxides produced in seawater by the marine bacillus sp. strain SG-1. Am. Mineral. 2005, 90 (8–9), 1342–1357. (28) Villalobos, M.; Lanson, B.; Manceau, A.; Toner, B.; Sposito, G. Structural model for the biogenic Mn oxide produced by Pseudomonas putida. Am. Mineral. 2006, 91 (4), 489–502. (29) Anbar, A. D.; Holland, H. D. The photochemistry of manganese and the origin of banded iron formations. Geochim. Cosmochim. Acta 1922, 56 (7), 2595–2603. (30) Kim, D. H.; Lee, J.; Ryu, J.; Kim, K.; Choi, W. Arsenite oxidation initiated by the UV photolysis 20


Page 21 of 28

495 496 497


of nitrite and nitrate. Environ. Sci. Technol. 2014, 48 (7), 4030–4037. (31) Mack, J.; Bolton, J. R. Photochemistry of nitrite and nitrate in aqueous solution: a review. J. Photochem. Photobiol. A 1999, 128 (1), 1–13.

498

(32) Goldstein, S.; Rabani, J. Mechanism of nitrite formation by nitrate photolysis in aqueous

499

solutions: the role of peroxynitrite, nitrogen dioxide, and hydroxyl radical. J. Am. Chem.

500

Soc. 2007, 129 (34), 10597–10601.

501 502 503 504

(33) Benedict, K. B.; McFall, A. S.; Anastasio, C. Quantum Yield of Nitrite from the Photolysis of Aqueous Nitrate above 300 nm. Environ. Sci. Technol. 2017, 51 (8), 4387–4395. (34) Bayliss, N. S.; Bucat, R. B. The photolysis of aqueous nitrate solutions. Aust. J. Chem. 1975, 28 (9), 1865–1878.

505

(35) Vione, D.; Maurino, V.; Minero, C.; Pelizzetti, E. Phenol photonitration upon UV irradiation of

506

nitrite in aqueous solution I: Effects of oxygen and 2-propanol. Chemosphere 2001, 45 (6–7), 893–902.

507

(36) Kuma, K.; Usui, A.; Paplawsky, W.; Gedulin, B.; Arrhenius, G. Crystal structures of synthetic 7 Å

508

and 10 Å manganates substituted by mono- and divalent cations. Mineral. Mag. 1994, 58 (392),

509

425–447.

510

(37) Volkov, A. G.; Paula, S.; Deamer, D. W. Two mechanisms of permeation of small neutral

511

molecules and hydrated ions across phospholipid bilayers. Bioelectrochem. Bioenerg. 1997, 42 (2),

512

153–160.

513

(38) Lanson, B.; Drits, V. A.; Silvester, E.; Manceau, A. Structure of H-exchanged hexagonal

514

birnessite and its mechanism of formation from Na-rich monoclinic buserite at low pH. Am. Mineral.

515

2000, 85 (5–6), 826–838.

516

(39) Yin, H.; Li, H.; Wang, Y.; Ginder-Vogel, M.; Qiu, G. H.; Feng, X. H.; Zheng, L. R.; Liu, F. Effects

517

of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite. Chem. Geol. 2014, 381,

518

10–20.

519

(40) Gaillot, A. C.; Flot, D.; Drits, V. A.; Manceau, A.; Burghammer, M.; Lanson, B. Structure of 21



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520

synthetic K-rich birnessite obtained by high-temperature decomposition of KMnO4. I. Two-layer

521

polytype from 800 °C experiment. Chem. Mater. 2003, 15 (24), 4666–4678.

522 523

(41) Luo, J.; Suib, S. L. Preparative parameters, magnesium effects, and anion effects in the crystallization of birnessites. J. Phys. Chem. B 1997, 101 (49), 10403–10413.

524

(42) Manceau, A.; Lanson, M.; Geoffroy, N. Natural speciation of Ni, Zn, Ba, and As in

525

ferromanganese coatings on quartz using X-ray fluorescence, absorption, and diffraction. Geochim.

526

Cosmochim. Acta 2007, 71 (1), 95–128.

527 528 529 530

(43) Lucht, K. P.; Mendoza-Cortes, J. L. Birnessite: a layered manganese oxide to capture sunlight for water-splitting catalysis. J. Phys. Chem. C 2015, 119 (40), 22838–22846. (44) Cygan, R. T.; Post, J. E.; Heaney, P. J.; Kubicki, J. D. Molecular models of birnessite and related hydrated layered minerals. Am. Mineral. 2012, 97 (8–9), 1505–1514.

531

(45) Yin, H.; Feng, X. H.; Tan, W. F.; Koopal, L. K.; Hu, T. D.; Zhu, M. Q.; Liu, F. Structure and

532

properties of vanadium(V)-doped hexagonal turbostratic birnessite and its enhanced scavenging of Pb2+

533

from solutions. J. hazard. Mater. 2015, 288, 80–88.

534 535

(46) Qiu, G. H.; Li, Q.; Yu, Y.; Feng, X. H.; Tan, W. F.; Liu, F. Oxidation behavior and kinetics of sulfide by synthesized manganese oxide minerals. J. Soils Sediments 2011, 11 (8), 1323–1333.

536

(47) Gao, T. Y.; Shi, Y.; Liu, F.; Zhang, Y. S.; Feng, X. H.; Tan, W. F.; Qiu, G. H. Oxidation process of

537

dissolvable sulfide by synthesized todorokite in aqueous systems. J. Hazard. Mater. 2015, 290,

538

106–116.

539 540 541 542 543 544

(48) Junta, J. L.; Hochella M. F. Manganese(II) oxidation at mineral surfaces: a microscopic and spectroscopic study. Geochim. Cosmochim. Acta 1994, 58 (22), 4985–4999. (49) Nico, P. S.; Anastasio, C.; Zasoski, R. J. Rapid photo-oxidation of Mn (II) mediated by humic substances. Geochim. Cosmochim. Acta 2002, 66 (23), 4047–4056. (50) Spiro, T. G.; Bargar, J. R.; Sposito, G.; Tebo, B. M. Bacteriogenic manganese oxides. Acc. Chem. Res. 2009, 43 (1), 2–9. 22


Page 23 of 28

545 546


(51) Fuller, C. C.; Bargar, J. R. Processes of zinc attenuation by biogenic manganese oxides forming in the hyporheic zone of Pinal Creek, Arizona. Environ. Sci. Technol. 2014, 48 (4), 2165–2172.

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547 548

Figures

a

b

549 550

Figure 1. Concentrations of Mn2+ (a) and p-HBA (b) under different conditions at initial pH 6.0 with

551

UV irradiation.

552

24


Page 25 of 28


553

a

b

554 555

Figure 2. Mn K-edge k3-weighted EXAFS (a) and corresponding Fourier transformed EXAFS (b)

556

spectra (solid line) and fitting results (short dash) of the 1.0 < R + δR < 3.5 Å region with

557

single-scattering model for 0.01Na-HB, 0.1Na-HB, 0.1Mg-HB, and 0.1K-HB.

25



Page 26 of 28

558

a

b

100 nm

100 nm

c

d

100 nm

100 nm

559 560

Figure 3. FESEM images of 0.01Na-HB (a), 0.1Na-HB (b), 0.1Mg-HB (c), and 0.1K-HB (d).

26


Page 27 of 28


561

a

b

c

562 563

Figure 4. XRD patterns of transformation products of birnessite at constant pH 4.0 (a), pH 6.0 (b), and

564

pH 8.0 (c) for different time periods under oxic conditions.

27



Page 28 of 28

565

566 567

Figure 5. Concentrations of Mn2+ during the transformation of birnessite at constant pH 4.0, 6.0, and

568

8.0 under oxic conditions.

28


Photochemical Formation and Transformation of Birnessite: Effects of

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