The mechanism for photo-promoted release of vanadium from

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The mechanism for photo-promoted release of vanadium from vanadium titano-magnetite Xingyun Hu, Xianjia Peng, and Linghao Kong Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05707 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 31, 2018

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Environmental Science & Technology

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The mechanism for photo-promoted release of vanadium from vanadium

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titano-magnetite

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Xingyun Hua,b, Xianjia Penga,b,c*, Linghao Konga,b

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a

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Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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b

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for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China

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c

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*

National Engineering Laboratory for Industrial Wastewater Treatment, Research Center for

Beijing Key Laboratory of Industrial Wastewater Treatment and Resource Recovery, Research Center

University of Chinese Academy of Sciences, Beijing 100190, China Corresponding author: Xianjia Peng, Tel: +86-10-62849198, E-mail: [email protected]

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TOC

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1 ACS Paragon Plus Environment


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Abstract

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The release of V from vanadium titano-magnetite, a predominant natural source of V, was studied

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under light irradiation. The release rate of V from vanadium titano-magnetite was accelerated by light

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irradiation, and the oxidation of V was detected. The essence of the photo-promoted release of V is

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that the immobile low valence V is transformed to the mobile V(V) by photo-induced active species

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generated from the photocatalysis process of magnetite. Among the photo-induced active species, •OH

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and H2O2 were recognized as the most important oxidizing agents. Not only can they directly convert

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the immobile low-valence V to the mobile V(V), but also initiate Fenton reaction, which produce more

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•OH and then further promotes the oxidation of low-valence V. In addition, a conceptual model of the

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photo promoted release of V was proposed. This study, as part of a broader study of the release

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behavior of V, can improve the understanding of the pollution problem about V, as well as the fate and

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environmental geochemistry cycling of V in natural environment.


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1 INTRODUCTION

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Vanadium (V) has been suggested to be a potentially hazardous pollutant in the same class as

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mercury (Hg), lead (Pb) and arsenic (As). As an emerging contaminant, the release and mobilization

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processes of V have yet to be studied as much as those of Hg, Pb and As, although its economic

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importance is growing.1,2 Consequently, the understanding of V pollution and its environmental

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geochemistry cycling is still superficial. Vanadium has three different oxidation states, V(III), V(IV) and

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V(V), and V(V) is the most toxic and mobile one among them, which means that the redox behavior of V

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can govern the mobility and geochemical cycling of V in the environment. 3-5 However, previous

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studies on the release kinetics of V from important vanadium containing ores, i.e., vanadium

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titano-magnetite (VTM) and stone coal only obtained the release rate and amount of V under different

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simulated environmental conditions, but the redox of V in the release process was not involved.

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Thus, it is a necessity to develop a better understanding of the redox of V in its release process from

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natural vanadium-containing minerals.

6-8

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Sunlight is recognized as a significant trigger factor on the redox behavior of valence-variable

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elements. Some studies have shown that light promoted the release of Sb and Cd from valentinite,

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stibnite and cadmium sulfide through the oxidation of Sb(III) and/or S(-II) by photo-induced active

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oxidants from the photocatalysis of minerals themselves.9-12 Vanadium titano-magnetite (VTM), as a

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predominant natural source of V, contained three oxidation states of V, i.e., V(III), V(IV) and V(V).

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The photochemistry of titano-magnetite and the UV-Fenton reactions mediated by titano-magnetite

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have been investigated in the catalytical degradation of environmental pollutants. Once it is exposed to

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light irradiation, the photo-induced oxidants or reductants may be generated from the photocatalysis of

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VTM. 13-16 Magnetite and titanium dioxide as the famous semiconducting materials, usually present as 3 ACS Paragon Plus Environment


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a base unit in the VTM deposits; especially magnetite is very sensitive to light irradiation. Thus, it can

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be hypothesized that under the light irradiation, the immobile V(III) and V(IV) may be oxidized to V(V)

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which is soluble and mobile, and thereby the release behavior of V from VTM may be promoted.

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The purpose of the present study is to investigate whether the simulated sunlight irradiation

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influences the release of V from VTM or not, and if so, how the light irradiation influences the release

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behavior of V. Firstly, a preliminary experiment was conducted under the simulated sunlight

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irradiation to verify the promotion effect of light on the release rate and amount of V. Subsequently,

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the mechanism of photo-prompted release of V was investigated through the identification of key

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active species, addition of H2O2 and free radical scavengers, and the removal of dissolved oxygen. The

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study on the redox and release mechanism of V from its predominant source, vanadium titanium

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magnetite, under light irradiation, as part of a broader study of the release behavior of V, can deepen

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the understanding of the fate and environmental geochemistry cycling of V in the environment, and

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then further improve the awareness of emerging pollution problem of V.

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2 MATERIALS AND METHODS

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2.1 Materials

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Vanadium titano-magnetite certified reference material [GBW(E)070130] was purchased from

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Jinan ZhongBiao Science and Technology Ltd. (Jinan, China). Its dominant component was

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characterized as magnetite (Fe3O4, PDF#19-0629) by X-ray diffraction (XRD) (Fig. S1a). No

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diffraction peak for V and Ti compounds was identified by XRD, because of relatively minor amounts

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of V and Ti (< 3% by weight percent). Magnetite (Fe3O4) with hyperfine field of 48.70 T (tetrahedral

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site A, isomer shifts 0.331 mm/s) and 45.36 T (octahedral site B, isomer shifts 0.603mm/s) and


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ilmenite with isomer shifts 1.07–1.10 mm/s were found in the VTM by Mössbauer spectra of 57Fe (Fig.

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S1b). In addition, chemical method was used to separate V(III), V(IV) and V(V) based on their

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different solubility characteristics in acidic and basic solution (Supporting information). The content

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of V(III), V(IV) and V(V) in the VTM were determined as 2.2 mg/g, 1.17 mg/g and 1.3 mg/g,

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respectively. Its specific surface area is 1.56 m2/g and the point of zero charge (PZC) is pH 6.3 (Fig.

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S1c).

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Fe3O4 with a specific surface area of 2.74 m2/g, was purchased from Shanghai Macklin

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Biochemical Co., Ltd. It has the same crystal structure as magnetite contained in the VTM (Fig. S1a).

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Other chemicals which had a purity of analytical grade or better were purchased from Sinopharm

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Chemical Reagent Co. Ltd. (Shanghai, China). All aqueous solutions were prepared using deionized

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water (18.2 MΩ/cm at 25 °C) from a Milli-Q water purification device.

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2.2 Experimental Methods

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A 500-mL beaker with 1.5 g VTM powder and 350 mL of aqueous solution was used to study the

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release of V in the VTM solution under the light irradiation. A 500W long-arc xenon lamp (35 cm×1

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cm) (Shanghai Jiguang Special Lighting Appliance Factory, China) with a lamp cover was placed 50

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cm above the beaker to irradiate light. Its spectral power distribution mainly ranged from 400 nm to

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800 nm (Fig. S2). The suspension was exposed to visible light irradiation with an ultraviolet cutoff

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glass filter (λ > 400 nm) above the beaker. The light intensity received by the liquid level was detected

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as 43,000±1000 Lux, closing to the received solar light intensity on sunny days on the earth. Because

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the lamp was calorigenic, the reactions require the use of cooling water to keep it at room temperature.

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At selected time intervals, 0.5 mL aqueous samples were taken and filtered through 0.22 µm

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polyethersulfone membrane filters (ANPEL Laboratory Technologies lnc., China) for analysis. The



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sorption of V on the membrane filters can be negligible. As controls, the dissolution of Fe3O4 both in

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the dark and under light irradiation at pH 4.0 and 6.5 were also investigated.

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To test the release of V from VTM at different pH values, solutions with pH 2.5, pH 4.0, pH 6.5

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and pH 9.0 were prepared using diluted hydrochloric acid and/or 5 mmol/L carbonates-bicarbonate

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buffers. The different pH values were selected to simulate the pH of acid mine drainage (pH 2-4)

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and natural waters (pH 6-9). The ionic strength of the solution was adjusted to 0.05 mol/L using NaCl.

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The pH of the solutions was measured at the beginning and at the end of the experiments. To check the

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effect of dissolved oxygen, anoxic environment was created for the feed solution using a gas purging

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tube. To exclude oxygen (O2) from the solutions, high purity nitrogen (N2, 99.99%) was input into the

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feed solution for approximately 2 h before the reaction and insufflation was sustained throughout the

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irradiation. To determine the effect of H2O2 on the release of V, 30 µmol/L H2O2 was added in the feed

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solution.

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To identify whether precipitation between aqueous Fe3+ and V5+ occurred in the VTM suspension

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at pH 2.5, dissolved Fe3+ (25 mg/L) and V5+ (20 mg/L) were prepared individually and then blended in

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a series of 10 mL centrifuge tubes. At selected time intervals, 5 mL aqueous samples were taken from

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each centrifuge tube and filtered using 0.22 µm polyethersulfone membrane filters for analysis.

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2.3 Analytical Methods

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2.3.1 Characterization of vanadium titano-magnetite

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The characterization methods of VTM including X-ray diffraction (XRD),

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Fe Mössbauer

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measurement, zero zeta potential, Brunauer-Emmett-Teller (BET) N2-adsorption and ultraviolet visible

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diffuse reflection spectrum (UV-vis DRS) are described in detail in the Supporting Information.

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2.3.2 Analysis of V, Fe and Ti in the aqueous solution


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The total V, Fe and Ti were determined by inductively coupled plasma optical emission

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spectroscopy (ICP-OES, NexION300, PerkinElmer Inc., USA) with a detection limit of 10 µg/L.

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Some aqueous samples need to be diluted with Milli-Q water to fit in the working range (0 – 20 mg/L,

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R2≥0.9990). Dissolved V(V) was measured by phosphortungstate spectrophotometric method on an

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ultraviolet visible light spectrophotometer (DR600, HACH Co. Ltd., USA).

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analyzed in acid solution at pH 1 by electron paramagnetic resonance (EPR) on an A300 electron spin

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resonance spectrometer (Bruker Corporation, Germany) at approximately 9.40 GHz at room

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temperature. The concentration of Fe(II) was measured colorimetrically using 1,10-phenanthroline on

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an ultraviolet visible light spectrophotometer (DR600, HACH Co. Ltd., USA).

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2.3.3 Analysis of hydroxyl free radical

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Vanadyl (IV) was

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The measurement of hydroxyl free radical was performed on a Bruker EleXsys E500 electron

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paramagnetic resonance spectrometer (Germany) at room temperature (25 °C ± 1) using 300 mmol/L

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dimethyl pyridine N-oxide (DMPO) as a capture reagent. Typical instrument settings were: sweep

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width 100.0 G, power attenuation 13.0 dB, modulation amplitude 2 G and sweep time 40 s. The EPR

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signals of the irradiated samples for 2 min at pH 2.5, pH 4.0, pH 6.5 and pH 9.0 were recorded. The

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results for samples in the dark at the above corresponding pH values were obtained as the controls.

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2.3.4 Analysis of hydrogen peroxide

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Concentration of hydrogen peroxide (H2O2) was measured by an ultraviolet visible light

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spectrophotometer (DR600, HACH Co. Ltd., USA) at 400 nm with the help of the yellow complex

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formed with potassium titanyl oxalate dehydrate and H2O2 in quartz cuvettes of 1 cm path length.19

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The method has a 0.1 mg/L (approximately 3.0 µmol/L) detection limit.

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2.4 Estimation of release rate of V



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For all the experiments, the mobilized mass of V in aqueous solutions is plotted as a function of

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time. The average release rates of V are calculated from the cumulative migrated amount (mg) of Vtot

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in the solution at certain reaction time (Eq. 1). Note that we do not convert these release rates of V

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into dissolution rates of the VTM, since the ideal chemical equation of V and the precise stoichiometry

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of V in the VTM are difficult to be determined. Specifically, the secondary precipitation of V at pH 2.5

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was recognized as undissolved V, which is not involved in the calculation of release kinetics. r=

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[ ]

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3 RESULTS AND DISCUSSION

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3.1 The release of V from VTM under light irradiation

(1)

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To determine the effect of light irradiation on the release behavior of V from VTM, preliminary

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experiments were conducted in the dark and under light irradiation, respectively, for 120 min. The

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release amount of V with the time in the VTM suspension are showed in Fig. 1. It can be clearly seen

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that light irradiation accelerates the release of total V at pH 2.5, pH 4.0 and pH 6.5. Accordingly, the

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average release rates of V increased from 6.1×10−3 mg/min in the dark to 9.1×10−3 mg/min under light

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irradiation at pH 2.5, from 5.3×10−3 mg/min to 9.9×10−3 mg/min at pH 4.0, and from 7.6×10−3 mg/min

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to 1.4×10−2 mg/min at pH 6.5. While, little promotion effect of the light irradiation at pH 9.0 was

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found on the release of V. Above results indicated that the light irradiation promoted the release rate

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and amount of V in the VTM suspension at pH 2.5-6.5.

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Furthermore, at pH 4.0-9.0, only V(V) was detected in the VTM suspension in the dark and under

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light irradiation, while, at pH 2.5, both V(IV) and V(V) were detected. It is known that V occurs in III,

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IV and V oxidation states in the VTM, and the higher oxidation states of V corresponds to the more


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soluble and mobile ability. Thus, it can be speculated that the light irradiation can accelerate the

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release of V through the oxidation of immobile V(III) or V(IV) to more soluble and mobile V(V). To

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verify that light irradiation can promote the oxidative dissolution of V(III) and V(IV), an approximate

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dissolution experiment of V2O3 and VO2 in the presence of pure Fe3O4 at pH 4.0 and pH 6.5 in the

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dark and under light irradiation were performed. It can be found that light irradiation greatly promoted

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the release of V from VO2 and V2O3 at pH 4.0 and pH 6.5 (Fig. S3). The average release rate of V in

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the VO2 suspension increased from 9.8×10−4 mg/min at pH 4.0 and 8.4 × 10−4 mg/min at pH 6.5 in the

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dark to 3.8×10−3 mg/min and 3.4×10−3 mg/min under light irradiation, respectively. The average

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release rate of V from the dissolution of V2O3 increased from 5.9×10−4 mg/min at pH 4.0 and 3.5×10−4

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mg/min at pH 6.5 in the dark to 2.2×10−3 mg/min and 2.0×10−3 mg/min under light irradiation,

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respectively, and all released V in the solution existed in the form of V(V). The result indicated that

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light irradiation could promote the oxidative dissolution of V(III) and V(IV) in the presence of Fe3O4.

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Probably, the specific chemical states of V in VTM are different from that in V2O3 and VO2, or they

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may not be the good surrogates for simulating the reactions of V in VTM, but it still valued for

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understanding the photo-promoted release mechanism of V in VTM.

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Thus, analogically, in the VTM suspension under light irradiation, the essence of photo-promoted

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release of V is assumed to be the oxidation of immobile V(IV) or V(III) to soluble and mobile V(V) by

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some photo-induced active species generated from the photocatalysis process of magnetite or titanium

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dioxide. The UV-Vis diffuse reflection spectrum of pure Fe3O4 in Fig. S4 showed the same excellent

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optical absorptivity as VTM in ultraviolet and visible region (200-800 nm). As we have known,

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magnetite can be regarded as a compound containing ferrous oxide and ferric oxide (FeO·Fe2O3). The

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energy gap (Eg) of FeO and Fe2O3 are 2.4 eV and 2.2 eV, respectively, while corresponding valence



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band edges are 2.23 eV and 2.48 eV, respectively, with respect to the normal hydrogen electrode

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(NHE), 16 which are above the valence band edges of semiconductor that could react with H2O/OH- to

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form hydroxyl radical (1.99 eV). In addition, in the photochemical experiment of the pure Fe3O4

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suspension at pH 2.5, 4.0, 6.5 and 9.0, obvious signals of •OH were detected (Fig. S5), suggesting the

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occurrence of photocatalysis process of magnetite. While, when compared with Fe3O4, the effect of

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photochemistry of titanium dioxide on the oxidation of V(IV) or V(III) can be neglected in the studied

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system, because of its low content in the VTM and the optical absorptivity in ultraviolet region

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(190-380 nm). Besides, the standard electrode potential of Fe(III)/Fe(II), V(IV)/V(III) and V(V)/V(IV)

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at 298 K are 0.771 eV, 0.337 eV and 0.991 eV individually, so the redox reaction between Fe and V of

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different oxide state will happen, further influence the release behavior of V. According to above

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speculation, major reactions that may occur in the VTM suspension under light irradiation have been

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listed in Table 1. The photo-oxidants described above may be in the form of •OH, •O2−, •OOH and

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H2O2. Furthermore, the generated H2O2 can trigger the homogeneous or heterogeneous Fenton reaction,

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producing more •OH to oxidize V(III) or V(IV). 20, 21 Subsequently, to verify above hypothesis, the key

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active species were identified and their effects on the release behavior of V were discussed.

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3.2 Identification and evaluation of major photo-oxidants

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3.2.1 •OH and H2O2

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The signal of •OH-DMPO in the VTM suspension at pH 2.5-9.0 was detected by EPR under light

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irradiation in Fig. 2. However, a slight signal of •OH-DMPO was also detected in control groups,

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which is attributed to the strong light adsorption ability of VTM. In the measured process by EPR,

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some light induced the photochemical reaction. The redox potential of •OH was concluded as 1.9-2.8

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eV in acidic and basic solution, so it can oxidize V(III) and V(IV) to V(V) (the redox potential of


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V(IV)/V(III) and V(V)/V(IV) are 0.337 eV and 0.991 eV, respectively). The •OH probably generated

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from three ways: (1) The photo-induced (hvb+) can be captured on the magnetite surface undergoing a

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charge transfer with adsorbed water molecules (H2O) or with surface-bound hydroxide species (OH−)

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to generate active hydroxyl free radicals (•OH) as shown in Eq. 3. (2) Fenton reaction initiated by

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H2O2 generated under light irradiation, which resulted in more •OH producing (Eqs. 6, 8). Because the

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magnetite contains both Fe(II) and Fe(III), which are the crucial cations for the initiation of Fenton

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reaction. (3) The photolysis of Fe(OH)2+ (Eq. 7). At pH 2.5, approximately 50 µg/L Fe(III) was

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detected in the VTM suspensions. While, in the individual light irradiation experiment for 50 µg/L

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Fe(III) aqueous solution at pH 2.5, no signal of •OH-DMPO was detected. Thus, the sources of •OH

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from the photolysis of Fe(OH)2+ could be excluded. The first two ways were deemed to be the

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important sources of •OH, and they were co-occurring in the VTM suspension under light irradiation,

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especially the photo-induced Fenton process. The content of Fe(II) in the VTM solid after the reaction

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under light irradiation and/or H2O2 conditions dropped by 2%-5% than that in the fresh VTM solid,

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meanwhile, the content of Fe(III) in the VTM solid after the reaction increased proportionately (Table

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S1). These results indicated that heterogeneous Fenton reaction was initiated under light irradiation

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(Eq. 8). From Fig. S6, the added H2O2 at pH 2.5-9.0 was decomposed at different levels, so it can be

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argued that extra •OH would be generated rapidly because of the Fenton reaction (Eq. 8). In addition,

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at pH 2.5, 4.7 mg/L Fe(II) ions was detected under light irradiation, while no Fe ions were detected at

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pH 4.0-9.0, which illustrated that Fenton reaction also happened at pH 2.5. That is why the amount of

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•OH at pH 2.5 was much higher than that at pH 4.0-9.0. Thus, the photo-induced Fenton reaction was

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identified as the most important source of •OH.

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So, in the VTM suspension under light irradiation, •OH and H2O2 are always concomitant



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because of the Fenton reaction, it is almost impossible to distinguish their effect on the release of V. To

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verify their effect on the release of V at pH 2.5-9.0, H2O2 was added at a dosage of 30 µmol/L in the

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dark. Most H2O2 was transformed to •OH by Fe(II) quickly, meaning that both the concentration of

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•OH and H2O2 increased correspondingly (Fig. S6). The EPR patterns for the addition of H2O2

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indicated that extra •OH was generated (Figs. 2a-2d). From Figs. 3a-3d, after adding H2O2, the average

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release rate of V increased from 6.1×10-3 mg/min to 9.9×10-3 mg/min at pH 2.5, from 4.6×10−3 mg/min

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to 1.4×10−2 mg/min at pH 4.0 and from 7.6×10−3 mg/min to 1.4×10−2 mg/min at pH 6.5, respectively.

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The release rate of V at pH 9.0 was accelerated from 3.5×10−2 to 3.9×10−2 mg/min at the first 30 min

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and then reached a solubility equilibrium. In this situation, there still were undecomposed H2O2 in the

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solution, thus, the result verified the combined importance of •OH and H2O2 on the promotion of V

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release. It is known that redox potential of ·OH/H2O and H2O2/H2O are 2.27 eV and 1.35 eV,

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respectively. Therefore, hydroxyl radical will provide stronger oxidizability than H2O2 in a

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concomitant system. However, in the VTM suspension under light irradiation, no H2O2 was detected.

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It is probable that the generated H2O2 was transformed to •OH instantly through Fenton reaction

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(Eqs.8, 9, 11, 12). It should be noted that the importance of individual •OH on the promoted release of

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V was verified through the addition of H2O2 instead of free-radical scavengers, that is because the

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common free radical masking agents were found to had a remarkable effect on the release of V in the

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VTM suspension. The previous studies have verified the importance of •OH and H2O2 on the oxidation

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of SbIII, and promotion release of Sb from Sb2O3 and Sb2S3 through photocatalytic process of Sb2O3

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and Sb2S3 themselves. 9-11

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3.2.2 •O2− and •OOH

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In the photocatalysis process of magnetite, the photo-induced electrons (ecb−) can be captured by the


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dissolved oxygen (O2) to produce superoxide free radicals (•O2−) (Eq. 4). Furthermore, the •O2−

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becomes •OOH through a protonation process, and then two of the •OOH can form H2O2 (Eqs. 5, 6).

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From the EPR patterns in Fig. 2, the signals of •OOH-DMPO were found at pH 4.0-9.0, which

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indirectly indicated that there were •O2− or H2O2 in the VTM suspension ((Eqs. 5, 6, 9, 11, 15). Due to

247

the existence of abundant Fe(II) and Fe(III) in the VTM suspension, the generated H2O2 in this

248

reaction pathway will transform into •OH again through Fenton reaction. The end-product in this

249

pathway is still •OH. It was found that the removal of dissolved oxygen from the VTM suspension

250

under light irradiation slightly lowered the release rate and amount of V (Fig. 4). The release rate of V

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decreased from 9.2×10-3 mg/min to 8.1×10-3 mg/min at pH 2.5, from 9.9×10-3 mg/min to 8.4×10-3

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mg/min at pH 4.0 and from 1.3×10-2 mg/min to 1.2×10-2 mg/min at pH 6.5, respectively. The result

253

suggests that this reaction pathway also played a role in the release of V. And many previous studies

254

have also verified the generation of •O2− and its oxidizing ability to metals or refractory organics in the

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heterogeneous photocatalysis reaction system. 9, 11, 22, 23

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3.3 The mechanism of photo promoted release of V

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Based on above experimental results, •OH and H2O2 are the most significant intermediate products

258

from the photocatalytic and Fenton reactions. Thus, the effect mechanism of •OH and H2O2 on the

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release of V was discussed emphatically in the subsequent section. Because of the different solution

260

characteristics in the VTM suspension at pH 2.5 and pH 4.0-9.0, the promotion mechanism at pH 2.5

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and pH 4.0-9.0 was discussed individually.

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3.3.1 The release of V in the VTM suspension at pH 2.5

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In the dark, 0.5 mg/L V(IV), 2.0 mg/L V(V), 7.0 mg/L Fe(II) and 0.05 mg /L Fe(III) were detected

264

at 120 min in the VTM suspension. While under light irradiation, the corresponding concentration of



265

Fe(II) and V(V) decreased to 4.5 mg/L and 0.2 mg/L, respectively, but the concentration of V(IV)

266

increased to 3.5 mg/L. It can be inferred in the VTM suspension that the redox reaction between Fe(II)

267

and V(V) and the precipitation reaction between Fe(III) and V(V) occurred. The XRD of solid product

268

at pH 2.5 apart from VTM (Fig. S1a) verified the generation of ferric vanadate. Thus, the decrease of

269

V(V) concentration was ascribed to the redox reaction with Fe(II) and precipitation reaction with

270

Fe(III) (Eqs. 8, 10, 21). However, the concentration of V(IV) in the solution was higher than the

271

computation value according to Eqs. 19 and 21. It can be inferred that, under light irradiation, the

272

photo-induced active species restricted the precipitation of Fe(III) and V(V) or increased the

273

dissolution of ferric vanadate precipitation, and then improved the release rate and amount of V.

274

To verify the inference, the precipitation experiment between Fe(III) and V(V), and dissolution

275

experiment of ferric vanadate precipitation at pH 2.5 were conducted individually under light

276

irradiation. From Figs. 5a and 5b, the concentration of Fe(III) and V(V) decreased with time, and the

277

decreased mole ratio of them were estimated to be approximately 1, indicating that precipitation

278

reaction between them occurred (Fig. S7a). However, in the presence of light or/and H2O2, the

279

concentration of Fe(III) and V(V) was higher than those in the absence of light and H2O2 and some

280

Fe(II) ions were detected at different levels (Fig. 5c), which indicated that the generation of H2O2

281

partly induced the reductive of Fe(III) (Eqs. 11, 12), inhibiting the formation of ferric vanadate

282

precipitation under light irradiation (Fig. S7b).

283

In the as-prepared ferric vanadate aqueous solution, it can be found that the concentration of Fe(III)

284

and V(V) increased during the first 60 min and then decreased from 60 min to120 min (Figs. 5d and

285

5e). This illustrated that in the presence of light or/and H2O2, ferric vanadate dissolved during the first

286

60 min because of the reduction of part of Fe(III) (Fig. 5f) contained in the ferric vanadate


Page 14 of 30

Page 15 of 30


287

precipitation by H2O2 (Eqs. 11, 12), releasing V and Fe gradually to a certain degree. Subsequently,

288

dissolved Fe(II) was oxidized to Fe(III) again by H2O2 or the other oxidants, and then V(V) and Fe(III)

289

precipitated again from 60 min to120 min. Anyway, under light irradiation, the dissolution of ferric

290

vanadate precipitation was initiated.

291

In summary, above two groups of experiments can be ascribed to a series of heterogeneous Fenton

292

reactions among photo-induced H2O2, Fe(II) and Fe(III) after receiving the light irradiation. Under

293

light irradiation, continuous •OOH and H2O2 generated from the photocatalysis process of magnetite

294

could convert Fe(III) to Fe(II), not only inhibiting the formation of ferric vanadate precipitation but

295

also promoting the dissolution of pre-existing ferric vanadate precipitation, thereby the release rate and

296

amount of V at pH 2.5 increased under light irradiation.

297

3.3.2 The release of V in the VTM suspension at pH 4.0-9.0

298

In the VTM suspension at pH 4.0-9.0, neither Fe(III) nor Fe(II) was detected in the dark and

299

under light irradiation. And in the magnetite suspension at pH 4.0 - 9.0, Fe was not detected as well.

300

That is to say, in the VTM suspension at pH 4.0-9.0, only undissolved VTM solid and dissolved V(V)

301

were in the reaction system.

302

In this situation, the promotion effect of light irradiation on the release of V was attributed to the

303

oxidation of immobile V(III) or V(IV) in the VTM to soluble and mobile V(V) by the photo-induced

304

active species. The individual dissolution experiment of V2O3 and VO2 at pH 4.0 and pH 6.5 in the

305

dark and under light irradiation could indirectly verify the hypothesis. The dissolution of VO2 at pH

306

6.5 under light irradiation was selected to further investigate the importance of •OH and O2 on the

307

release of V. 0.5 mol/L isopropyl alcohol (IPA) was used to scavenge •OH; at this concentration, 99%

308

of the·•OH radicals can be scavenged by IPA.24, 25 From Fig. S8, the presence of a scavenger slowed



309

the release rate of V from 3.4×10−3 mg/min to 1.6 ×10−3 mg/min. The removal of dissolved oxygen

310

reduced the release rate of V from 3.4×10−3 mg/min to 2.5×10−3 mg/min. Furthermore, based on the

311

release rate data of V under corresponding condition, the participation ratio of •OH in the oxidative

312

release of V(IV) was estimated to be 50%, and the participation ratio of •O2− and •OOH was estimated

313

to be 26.5%. The results suggested that •OH, •O2− and •OOH played a major role in the oxidative

314

release of V(IV), and implied that other than •OH, •O2− and •OOH, the other oxidants existed.

315

According to the redox potential law, another oxidant may be Fe(III) (Eq. 20).

316

In the VTM suspension at pH 9.0, the release amount of V was much higher than those in acidic

317

solutions (Fig. 1). That is because the thermodynamic property of V(V) made it easier to be dissolved

318

in basic solution. From Figs. 1, 3 and 4, under light irradiation, H2O2 and O2 had little effect on the

319

release of V at pH 9.0 and the concentration of V reached to an equilibrium after 30 min, maybe

320

because the release amount of V under this condition had reached saturation.

321

3.3.3 Conceptual model of the photo-promoted release of V

322

The essence of photo-promoted release of V is to transform the immobile low-valence V to the

323

soluble and mobile V(V) by the photo-induced active species generated from the photocatalysis

324

process of magnetite. According to the mentioned results, a postulated mechanism of photo-promoted

325

release of V was proposed in Fig. 6 and the main reaction pathways were marked with bold solid lines.

326

The magnetite contained in the VTM received light with sufficient energy irradiation (hv > Eg),

327

and an electron (ecb−) from its valence band jumped to the conduction band with the concomitant

328

generation of a hole (hvb+) in the valence band (Eq. 2). Then the hvb+ was captured on the VTM surface

329

undergoing a charge transfer with adsorbed water molecules (H2O) to generate active hydroxyl free

330

radicals (•OH) (Eq. 3). The ecb− was captured by the dissolved oxygen to produce •O2− (Eq. 4).


Page 16 of 30

Page 17 of 30


331

Furthermore, the •O2− became •OOH through a protonation process, and then two •OOH formed

332

hydrogen peroxide (H2O2) (Eqs. 5, 6). Furthermore, the generated H2O2 could trigger Fenton reactions,

333

producing more •OH (Eq. 8).

334

In the VTM suspensions at pH 2.5, the ultimate photo-induced active species •OH and/or H2O2

335

improved the release rate and amount of V through inhibiting the formation or promoting the

336

dissolution of ferric vanadate precipitation (Eqs. 11, 12). In the VTM suspension at pH 4.0-9.0, the

337

promotion effect was attributed to the transformation of low-valence V in the VTM to the soluble and

338

mobile V(V) by above species. In a word, it is the combined effect of photocatalytic process and

339

Fenton reaction of VTM that promoted the release of V.

340

4 ENVIRONMENTAL SIGNIFICANCE

341

Titanomagnetite deposits are the primary source of V in the world. In China, an estimated 40% of

342

V production is from vanadiferous slag from the smelting of vanadium titanium magnetite. As the

343

most important base of V production in the world, the giant Panzhihua vanadium titanium magnetite

344

deposit provides 64% of V for China. Extensive mining and smelting activities have had major

345

environmental impacts in mines or nearby regions, resulting in abnormally elevated concentrations of

346

V in the water and soil near mines or smelting sites.26-30 When these minerals are exposed to water

347

environment under solar light irradiation, the release rate of V can be accelerated and the release

348

amount of V increased by the photocatalysis process of magnetite. Low toxic and immobile V(III) and

349

V(IV) are oxidized to high toxic and mobile V(V) during the process, which enhanced the

350

environmental risk of V. As the starting point of biogeochemistry cycle of V, this study on the

351

dissolution of the most important V containing minerals helps us to understand the pollution problem



352

of V. This study, as a part of broader study on the release behavior of V, is a premise that helps us to

353

understand the migration and transformation behavior of V in the presence of the other important

354

factors, such as natural organic ligands, in natural environment. To clarify the fate and environmental

355

geochemistry cycle of V in the environment, further studies are needed.

356

In addition, magnetite naturally doped with V is an immense resource of (photo)catalyst and

357

greatly influence the geochemical processes of organic pollutants through initiating the homogeneous

358

or/and heterogeneous Fenton reaction, producing more free radicals. Thereby, a wide range of element

359

or compounds are oxidized and some geochemically and environmentally important reactions,

360

including the photooxidation of phenol, polychlorinated biphenyls, sulfur-containing organic

361

compounds, and nitro gen-containing organic compounds are initiated.16, 25, 31

362

ACKNOWLEDGEMENTS

363

This work was supported by the National Natural Science Foundation of China (No. 21607166,

364

41641034 and 41473113) and the National Water Pollution Control and Treatment Science and

365

Technology Major Project (No. 2015ZX07205-003).

366

Supporting Information

367

Eight figures and one table relate to characterization of VTM, the spectral power distribution of light

368

source, the release amount of V from VO2 and V2O3 in the presence of Fe3O4, EPR signals intensity

369

for free radicals in the pure Fe3O4 suspension, the decomposition efficiency of 30 µmol/L H2O2 at

370

different pH values and the relative content of Fe(II) and Fe(III) in the fresh and reacted VTM

371

supplied as Supporting Information.


Page 18 of 30

Page 19 of 30


372

REFERENCES

373

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374 375

2.

Moskalyk, R.R.; Alfantazi, A.M. Processing of vanadium: a review. Miner. Eng. 2003, 16 (9), 793-805.

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Imtiaz, M.; Rizwan, M.S; Xiong, S.; Li, H.; Ashraf, M.; Shahzad, S.M.; Shahzad, M.; Rizwan, M.; Tu,

3.

Crans, D.; Amin, S.; Keramidas, A. Chemistry of relevance to vanadium in the environment. In:

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Nriagu, J. (Ed.), Vanadium in the Environment. Part 1: Chemistry and Biochemistry. John Wiley

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Lett. 1984, 23, 227-231.

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Nechay, B.R.; Nanning, L.B.; Nechay, P.S.E.; Post, R.L.; Branthan, J.J.; Macara, I.G.; Kubena, L.F.; Philip, T.D.; Nielsen, F.H. Role of vanadium in biology. Fred Proc. 1986, 45, 123-132.

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Llobet, J.M.; Domingo, J.L. Acute toxicity of vanadium compounds in rats and mice. Toxicol.

6.

Hu, X.; Yue, Y.; Peng, X. Release kinetics of vanadium from vanadium titano-magnetite: The

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effects of pH, dissolved oxygen, temperature and foreign ions. J. Environ. Sci. 2017,

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doi.org/10.1016/j.jes.2017.07.001. In Press.

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7.

Hu, X.; Yue, Y.; Peng, X. Release kinetics of vanadium from vanadium (III, IV and V) oxides:

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Hu, X.; Kong, L.; He, M. Kinetics and mechanism of photopromoted oxidative dissolution of antimony trioxide. Environ. Sci. Technol. 2014, 48(24), 14266-14272. 19 ACS Paragon Plus Environment


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10. Kong, L.; He, M. Mechanisms of Sb(III) photooxidation by the excitation of organic Fe(III) complexes. Environ. Sci. Technol. 2016, 50 (13), 6974–6982. 11. Hu, X.; He, M; Kong, L. Photopromoted oxidative dissolution of stibnite. Appl. Geochem. 2015, 61, 53-61. 12. Davis, A.P.; Hsieh, Y.H.; Huang, C.P. Photo-oxidative dissolution of CdS(s): the effect of complexing agents. Chemosphere, 1995, 31(4), 3093-3104.

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13. Zhong, Y.; Liang, X.; Zhong, Y.; Zhu, J.; Zhu, S.; Yuan, P.; He, H.; Zhang, J. Heterogeneous

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UV/Fenton degradation of TBBPA catalyzed by titanomagnetite: Catalyst characterization,

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performance and degradation products. Water Res, 2012, 46(15), 4633-4644.

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14. Liang, X.; Zhong, Y.; Zhu, S.; Zhu, J.; Yuan, P.; He, H.; Zhang, J. The decolorization of Acid

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Orange II in non-homogeneous Fenton reaction catalyzed by natural vanadium–titanium

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magnetite. J. Hazard. Mater., 2010. 181(1-3), 112-120.

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15. Costa, R.C.C.; Lelis, M.F.F.; Oliveira, L.C.A.; Fabris, J.D.; Ardisson, J.D.; Rios, R.R.V.A.; Silva,

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C.N.; Lago, R.M. Novel active heterogeneous Fenton system based on Fe3−xMxO4 (Fe, Co, Mn,

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Ni): the role of M2+ species on the reactivity towards H2O2 reactions, J. Hazard. Mater. 2006,

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129,171-178.

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16. Xu, Y.; Schoonen, M. A.A. The absolute energy positions of conduction and valence bands of selected semiconducting minerals. Am. Mineral. 2000, 85, 543-556. 17. Akcil, A.; Koldas, S. Acid Mine Drainage (AMD): causes, treatment and case studies. J. Clean. Product. 2006, 14, 1139-1145.

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18. Yue, Y.; Wang, X.; Hu, X.; Peng, X. Improvement determination of the oxidovanadium (Ⅳ) and

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total vanadium in the solution by modified phosphorus-tungsten-vanadium spectrophotometry.


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Chinese Journal of Analysis Laboratory, 2017, 36, 362-367. 19. Sellers, R.M. Spectrophotometric determination of hydrogen-peroxide using potassium titanium (IV) oxalate. Analyst, 1980, 105, 950-954.

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20. Liang, X.; Zhu, S.; Zhong, Y.; Zhu, J.; Yuan, P.; He, H.; Zhang, J. The remarkable effect of

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vanadium doping on the adsorption and catalytic activity of magnetite in the decolorization of

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methylene blue. Appl. Catal. B: Environ. 2010, 97(1-2), 151-159.

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21. Wu, R.C.; Qu, J.H. Removal of azo dye from water by magnetite adsorption–Fenton oxidation, Water Environ. Res. 2004, 76, 2637–2642.

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22. Hansard, S. P., Easter, H. D., Voelker, B. M. Rapid reaction of nanomolar Mn(II) with superoxide

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radical in seawater and simulated freshwater. Environ. Sci. Technol., 2011, 45 (7), 2811–2817.

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23. Huang, W., Brigante, M., Wu, F., Mousty, C., Hanna, K., Mailhot, G. Assessment of the Fe(III)–

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EDDS complex in Fenton-like processes: from the radical formation to the degradation of

428

bisphenol A. Environ. Sci. Technol., 2013, 47 (4), 1952-1959.

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24. Brezonik, P. L.; Fulkerson-Brekken, J. Nitrate-induced photolysis in natural waters: Controls on

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concentrations of hydroxyl radical photo intermediates by natural scavenging agents. Environ. Sci.

431

Technol. 1998, 32, 3004-3010.

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25. Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. Critical-review of rate constants for

433

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

434

solution. J. Phys. Chem. Ref. Data 1988, 17, 513-886.

435 436

26. Huang, J.H.; Huang, F.; Evans, L.; Glasauer, S. Vanadium: Global (bio)geochemistry. Chem. Geol. 2015, 417, 68-89.

437

27. Teng, Y.; Ni, S.; Zhang, C.; Wang, J.; Lin, X.; Huang, Y. Environmental geochemistry and

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ecological risk of vanadium pollution in Panzhihua mining and smelting area, Sichuan, China. 21 ACS Paragon Plus Environment


439

Chin. J. Geochem. 2006, 25(4), 379-385.

440

28. Teng, Y.; Yang, J.; Wang, J.; Song, L. Bioavailability of V extracted by EDTA, HCl, HOAC,

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and NaNO3 in topsoil in the Panzhihua Urban Park, located in Southwest China. Biol. Trace

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Elem. Res. 2011, 144, 1394-1404.

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29. Panichev, N.; Mandiwana, K.L.; Moema, D.; Molatlhegi, R.; Ngobeni, P. Distribution of

444

vanadium (V) species between soil and plants in the vicinity of vanadium mine. J. Hazard. Mater.

445

2006, 137, 649-653.

446

30. Połedniok, J.; Buhl, F. Speciation of vanadium in soil. Talanta 2003, 59, 1-8.

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31. Spikes, J.D. Selective photooxidation of thiols sensitized by aqueous suspensions of cadmium

448

sulfide. Photochem. Photobio. 1981, 34, 549-556.

449


Page 22 of 30

Page 23 of 30


450

The captain of figures

451

Fig.1 The release amount of V from the dissolution of vanadium titano-magnetite at different pH: (a)

452

At pH 2.5, (b) At pH 4.0, (c) At pH 6.5 and (d) At pH 9.0.

453

Fig. 2 Electron spin resonance signals intensity for free radicals under different conditions: (a) At pH

454

2.5, (b) At pH 4.0, (c) At pH 6.5 and (d) At pH 9.0.

455

Fig. 3 The release amount of V in the presence of 30 µmol/L H2O2 at different pH: (a) At pH 2.5, (b)

456

At pH 4.0, (c) At pH 6.5 and (d) At pH 9.0.

457

Fig.4 The release amount of V under the anoxic condition at different pH: (a) At pH 2.5, (b) At pH 4.0,

458

(c) At pH 6.5 and (d) At pH 9.0.

459

Fig. 5 The effect of H2O2 on the concentration of V, Fe(III) and Fe(II) at pH 2.5: (a-c) in the formation

460

of ferric vanadate: the initial concentration of V(V) and Fe(III) are 20 and 25 mg/L, respectively; (d-f)

461

in the dissolution of ferric vanadate

462

Fig. 6 Conceptual model for photo-promoted release of V from vanadium titano-magnetite

463



464

Page 24 of 30

The list of figures

465 4.0

5.0 Control Light

V Concentration (mg/L)

3.5

(a) pH 2.5

Control Light

4.5

(b) pH 4.0

4.0

3.0

3.5 2.5

3.0

2.0

2.5

1.5

2.0

1.0

r=9.1x10 mg/min

1.5

0.5

r=6.1x10-3 mg/min

1.0

-3

r=9.9x10-3 mg/min r=5.3x10-3 mg/min

0.5 0

20

40

60

80

100

120

0

20

40

60

80

100

120

100

120


6 Control Light

5

(c) pH 6.5

(d) pH 9.0

Control Light

10 9

4 8 3 7 2 6

r=1.4x10-2 mg/min 1

r=7.6x10-3 mg/min 5 0

466

20

40

60

80

100

120

0

20

40

60

80

Time (min)

Time (min)

467

Fig.1 The release amount of V from the dissolution of vanadium titano-magnetite at different pH: (a)

468

At pH 2.5, (b) At pH 4.0, (c) At pH 6.5 and (d) At pH 9.0.

469


Page 25 of 30


470

Offset Y values

2.0

+ H2O2

(a) pH 2.5

+ H2O2

(b) pH 4.0 0.16

1.5

0.12 + Light + Light

1.0

0.08 0.04

0.5

Control

Control 0.00

0.0 3460

Offset Y values

0.16

3480

3500

3520

3540

+ H2O2

(c) pH 6.5

3460

3560 0.16

0.12

3480

3500

3520

3540

3560

+ H2O2

(d) pH 9.0

0.12 + Light

+ Light

0.08

0.08

0.04

Control

0.00

0.04 Control 0.00

3460

3480

3500

3520

3540

3560

3460

3480

3500

3520

3540

3560

X [G]

X [G]

471 472

Fig. 2 Electron spin resonance signals intensity for free radicals under different conditions: (a) At pH

473

2.5, (b) At pH 4.0, (c) At pH 6.5 and (d) At pH 9.0.

474 475



4.0

7 Control H2 O2

3.5 V Concentration (mg/L)

Page 26 of 30

Control H2 O2

(a) pH 2.5 6

3.0

(b) pH 4.0

5

2.5 4 2.0 3

1.5 1.0

r=9.9x10-3 mg/min

0.5

-3

2

0

20

40

60

80

100


1

r=6.1x10 mg/min 120

0

20

40

60

80

100

120


7 Control H2 O2

6

(c) pH 6.5

9

5 4

8

3

7

2

r=3.5x10-2 mg/min for 30 min

r=7.6x10-3 mg/min 0

20

40

60

80

100

r=3.9x10-2 mg/min for 30 min

6

r=1.4x10-2 mg/min 1

476

(d) pH 9.0

Control H2 O2

10

5 120

0

20

Time (min)

40

60

80

100

120

Time (min)

477 478

Fig. 3 The release amount of V in the presence of 30 µmol/L H2O2 at different pH: (a) At pH 2.5, (b)

479

At pH 4.0, (c) At pH 6.5 and (d) At pH 9.0.

480 481


Page 27 of 30


4.0

4.5 Light N2


3.5

(a) pH 2.5

3.0

3.5

2.5

3.0

2.0

2.5

1.5

2.0

1.0


0.5 0

20

40

60

Light N2

4.0

80

100

(b) pH 4.0

1.5

r=9.9x10-3 mg/min

1.0

r=8.4x10-3 mg/min

120

0

20

40

60

80

100

120


6 Light N2

5

(c) pH 6.5

Light N2

10

(d) pH 9.0

9 4 8 3 7 2 1

r=4.9x10-2 mg/min for 30 min r=4.9x10-2 mg/min for 30 min

6


5 0

482

20

40

60

80

100

120

0

20

Time (min)

40

60

80

100

120

Time (min)

483

Fig.4 The release amount of V under the anoxic condition at different pH: (a) At pH 2.5, (b) At pH 4.0,

484

(c) At pH 6.5 and (d) At pH 9.0.



Page 28 of 30

485

Control

Light

H2O2

Light+H2O2 0.5

20 Fe(III) Concentration (mg/L)


(b)

12 10 8 6

(c) Fe(II) Concentration (mg/L)

(a)

14

18 16 14 12 10

0.4 0.3 0.2 0.1 0.0

4

0

20

40

60

8

80 100 120

3.5

0

20

40

2.0

1.5

0

20

40

60

1.5

1.0

0.5

Time (min)

40

60

80 100 120

(f)

0.15 0.10 0.05 0.00

0.0

80 100 120

20

0.20 Fe(II) Concentration (mg/L)

2.0

1.0

486

Fe(III) Concentration (mg/L)


2.5

0

80 100 120

(e)

(d) 3.0

60

0

20

40

60

80 100 120

Time (min)

0

20

40

60

80 100 120

Time (min)

487

Fig. 5 The effect of H2O2 on the concentration of V, Fe(III) and Fe(II) at pH 2.5: (a-c) in the formation

488

of ferric vanadate: the initial concentration of V(V) and Fe(III) are 20 and 25 mg/L, respectively; (d-f)

489

in the dissolution of ferric vanadate.

490


Page 29 of 30


491 492

Fig. 6 Conceptual model for photo-promoted release of V from the dissolution of vanadium

493

titano-magnetite.



494

Table 1 Probable chemical reactions in the vanadium titano-magnetite solution under light irradiation

Equation Reactions for VTM in the light irradiation

2

Fe O s Fe O h + Fe O e

3

h + H O/OH →∙ OH + H

4

e + O →∙ O

5

H +∙ O →∙ OOH

6

2 ∙ OOH → O + H O

7

FeOH ∙ OH + Fe

Fenton Reactions 8

H O + FeII → FeIII + OH +∙ OH

9

∙ OH + H O → H O +∙ OOH

10

∙ OH + FeII → FeIII + OH

11

H O + FeIII → FeII + H +∙ OOH

12

∙ OOH + FeIII → FeII + H + O Redox reactions of V

13

V + O + H O → VO + H

14

VO + H O ↔ VOOH + H

15

VOOH + O → VO +∙ OOH

16

VO +∙ OOH + H O → VO + H O + H

17

VO + H O → VO +∙ OH + H

18

VO +∙ OH → VO +H

19

VO + 2H → VO + Fe + H O + Fe

20

VO + Fe + H O → VO + 2H + Fe

21

Fe + VO + H O → FeVO s

495


Page 30 of 30

The mechanism for photo-promoted release of vanadium from

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