Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from

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Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water: Thermodynamic and Spectroscopic Studies Cheng-Hua Liu, Ya-Hui Chuang, Tsan-Yao Chen, Yuan Tian, Hui Li, Ming-Kuang Wang, and Wei Zhang Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 9, 2015

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Environmental Science & Technology

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Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from Water:

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Thermodynamic and Spectroscopic Studies Cheng-Hua Liu,†,‡ Ya-Hui Chuang,† Tsan-Yao Chen,§ Yuan Tian,† Hui Li,† Ming-Kuang

3

Wang,# and Wei Zhang†,‡,*

4 5 6 7 8 9 10

†

Department of Plant, Soil and Microbial Sciences, Michigan State University, East Lansing,

Michigan 48824, United States ‡

Environmental Science and Policy Program, Michigan State University, East Lansing,

Michigan 48824, United States §

Department of Engineering and System Science, National Tsing Hua University, Hsinchu,

30013, Taiwan

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#

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*Corresponding author: Dr. Wei Zhang, Address: 1066 Bogue ST RM A516, East Lansing,

Department of Agricultural Chemistry, National Taiwan University, Taipei 10617, Taiwan

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MI 48824, United States; Tel: 517-353-0471; Fax: 517-355-0270; Email: [email protected]

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TOC/ABSTRACT ART

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ACS Paragon Plus Environment


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ABSTRACT

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Removal of arsenic (As) from water supplies is needed to reduce As exposure through

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drinking water and food consumption in many regions of the world. Magnetite nanoparticles

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(MNPs) are promising and novel adsorbents for As removal, due to their great adsorption

20

capacity for As and easy separation. This study aimed to investigate adsorption mechanism of

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arsenate As(V) and arsenite As(III) on MNPs by macroscopic adsorption experiments in

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combination with thermodynamic calculation and microspectroscopic characterization using

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synchrotron radiation-based X-ray absorption spectroscopy (XAS) and X-ray photoelectron

24

spectroscopy (XPS). Adsorption reactions are favorable endothermic processes as evidenced by

25

increased adsorption with increasing temperatures, and high positive enthalpy change. EXAFS

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spectra suggested predominant formation of bidentate binuclear corner-sharing complexes (2C)

27

for As(V), and tridentate hexanuclear corner-sharing (3C) complexes for As(III) on MNP surfaces.

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The macroscopic and microscopic data conclusively identified the formation of inner-sphere

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complexes between As and MNP surfaces. More intriguingly, XANES and XPS results revealed

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complex redox transformation of the adsorbed As on MNPs exposed to air: Concomitant with the

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oxidation of MNPs, the oxidation of As(III) and MNPs was expected, but the observed As(V)

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reduction was surprising, likely due to the role played by the reactive Fe(II).

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INTRODUCTION

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Contamination of natural waters by inorganic arsenic (As) has dire consequences to public

35

health worldwide, due to potential toxic and carcinogenic effects from As exposure via

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consumption of As-tainted drinking water and food.1 Short-term exposure to high level of As is 2


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fatal, whereas chronic exposure to trace levels of As can cause skin, bladder, and lung cancers.2, 3

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As can be released from both natural and anthropogenic sources such as rock and soil weathering,

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wood preservation, pesticides, mining, and industrial wastewater discharge, and is therefore

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commonly found in natural waters as inorganic oxyanions of trivalent arsenite (AsO33−, As(III))

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and pentavalent arsenate (AsO43−, As(V)).4 Crop irrigation with As-contaminated water allows

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for trophic transfer of As through food chain.5 Exposure through contaminated drinking water is

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another direct pathway to humans. Many regulatory authorities thus impose a maximum

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contamination level of 10 µg L−1 in drinking water.6, 7 However, human populations in certain

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regions of the world are constantly over-exposed to As, especially in developing countries,8, 9 due

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to high As levels in water supplies, insufficient water treatment, or lack of water quality

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surveillance and public awareness.10, 11 Therefore, effective As removal from water supplies is

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essential to protecting environmental and human health.

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Among many techniques currently available for As removal from water,12,

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adsorption

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process is considered one of most promising techniques because it is economical, effective, and

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socially acceptable.14,

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nanoparticles, zero-valent iron nanoparticles, carbon nanotube, and iron oxide nanoparticles are

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novel adsorbents, due to greatly enhanced As removal efficiency at nanometer scale.16

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Nonetheless, significant challenges remain regarding post-treatment separation of adsorbent

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nanoparticles from treated water.16 Because superparamagnetic magnetite (Fe3O4) nanoparticles

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(MNPs) can be easily separated from aqueous solution with a low external magnetic field, there

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have been growing interest in As removal by MNPs.17, 18 This is particularly well-suited for

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In particular, engineered nanomaterials such as titanium oxide

3



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applications in water treatment facilities or in-situ groundwater remediation. To better develop

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the magnetite-based As removal technique, more fundamental knowledge on both macroscopic

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and microscopic aspects of As adsorption by MNPs is needed.

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Over last several years, macroscopic adsorption studies have provided valuable insight on the

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effects of adsorbent particle size, solution pH and ionic strength on As adsorption by MNPs.17,

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19-21

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by magnetite have been sparse with conflicting reports in the literature.22-25 As shown by Figure

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S1 in Supporting Information, four types of As surface complexation with iron oxide surface

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have been proposed: 22-37 (i) bidentate mononuclear edge-sharing (2E), (ii) bidentate binuclear

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corner-sharing (2C), (iii) monodentate mononuclear corner-sharing (1V), and (iv) tridentate

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hexanuclear corner-sharing (3C) complexes. Jönsson and Sherman22 suggested that both As(III)

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and As(V) form 2C complexes on the {100} surfaces of magnetite at As–Fe distance (RAs-Fe) of

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3.39 and 3.42 Å, respectively, using extended X-ray absorption fine structure (EXAFS)

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spectroscopy. However, Morin et al.23 and Wang et al.24 reported formation of 3C complexes of

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As(III) on {111} surfaces of magnetite at RAs–Fe of 3.47–3.52 Å. More recently, Wang et al.25

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suggested that As(V) forms 2C complexes on the {100} surfaces of magnetite at RAs–Fe of 3.35–

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3.39 Å with a lower coordination number of 0.8–1.3 than the expected value of 2, explained by

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the presence of outer-sphere complexes of As(V). Thus, the type of As inner-sphere complexes

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formed on the magnetite surface has yet to be settled.

77 78

However, convincing and indisputable evidences on molecular mechanism of As adsorption

Furthermore, since both mobility and toxicity of As(III) are greater than As(V),1,

3

any

potential reductive transformation of As(V) to As(III) should be given due consideration. Drying 4


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is an important sludge management process in water treatment facilities to substantially reduce

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the volume and mass of sludge for easier transport and disposal. During drying, the MNPs (a

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mixed-valence iron (II, III) oxide) and adsorbed As species are exposed to oxygen, which may

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result in complex surface redox reactions. As expected, Ona-Nguema et al.38 reported rapid

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oxidation of As(III) to As(V) at magnetite surface by dissolved oxygen in aqueous solution.

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However, the reduction of adsorbed As(V) to As(III) on magnetite/maghemite nanoparticles

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reported by Chowdhury et al.39 was rather surprising, but unfortunately no mechanistic

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explanation was provided. Therefore, the potential redox transformation of adsorbed As on

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MNPs and underlying mechanisms need to be further elucidated. This information is required for

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more effective and safer disposal of the MNP sludge produced in the As removal processes.

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This study aimed to elucidate As adsorption mechanisms by MNPs through spectroscopic

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techniques involving X-ray absorption near edge structure (XANES), EXAFS, and X-ray

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photoelectron spectroscopy (XPS), in addition to batch sorption experiments and thermodynamic

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calculations. The redox transformation of As adsorbed on MNPs during the drying process was

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also investigated. We chose arsenate As(V) and arsenite As(III) as our model As species, and

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performed As adsorption experiments under a range of solution pH, ionic strength, and

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temperature. Thermodynamic parameters were calculated using As adsorption isotherm data at a

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temperature range of 10–55 °C. Synchrotron radiation-based XANES, EXAFS and XPS analyses

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were employed to probe As molecular binding mechanism and redox reactions on MNP surfaces.

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Specifically, As-adsorbed MNPs samples with and without drying were analyzed to study the

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transformation of As species and MNPs upon exposure to oxygen. 5



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MATERIALS AND METHODS

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General. All chemicals used were of analytical grade. Since As(III) and MNPs are

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oxygen-sensitive, special care was required to maintain anoxic conditions to minimize the

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potential effect of atmospheric and dissolved oxygen. In this study, all deionized (DI) water was

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deaerated by purging with N2 gas overnight prior to use. All containers for solutions, suspensions,

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or solid samples were purged with N2 gas for 10 seconds before they were tightly capped or

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sealed so as to remove oxygen. Transferring of solutions, suspensions, or solid samples during

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the experiments was performed as quickly as possible to reduce air exposure time. Unless noted

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otherwise, the above steps were taken during the experiments. These anti-oxidation measures

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appeared to be effective, as minimal oxidation of MNPs and As(III) were observed during the

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adsorption experiments demonstrated by subsequent spectroscopic analyses.

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Magnetite Synthesis and Characterization. Magnetite nanoparticles (MNPs) were

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synthesized using a modified iron (II) and iron (III) co-precipitation method under N2

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protection.40 The methods for MNP synthesis and characterization were detailed in Supporting

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Information S2. X-ray diffraction (XRD) pattern confirmed the presence of standard magnetite

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(JCPDS 19-0629) without other crystalline iron oxide phase (Figure S2). Transmission electron

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microscopy images showed near-spherical primary particles with an average diameter of 34 nm

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(Figure S3). However, MNPs existed as larger aggregates under adsorption experimental

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conditions (Figure S3), and the aggregate size was 2.57 µm following a 30-min sonication, and

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increased to 5.1 µm after 24-hr shaking (Figure S4).41 Additionally, the MNPs had a N2-BET

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specific surface area of 39 m2 g−1 and an isoelectric point (IEP) of 7.2 (Figure S5). 6


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Adsorption Experiments. Batch adsorption experiments were conducted under anoxic

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conditions to measure As adsorption by MNPs in triplicate. As(V) and As(III) solutions were

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freshly prepared in DI water from disodium hydrogen arsenate (Na2HAsO4·7H2O, J.T. Baker)

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and sodium arsenite (NaAsO2, J.T. Baker), respectively. Sodium nitrate (NaNO3) was added as

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background electrolyte to control solution ionic strength. Dilute HNO3 and NaOH solutions were

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used to adjust pH of As solutions and MNP suspensions. Adsorption kinetics of As(V) and As(III)

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described in Supporting Information S2 was first examined to determine the equilibration time

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needed for adsorption isotherm experiments. As(V) and As(III) adsorption on MNPs occurred

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rapidly and reached equilibrium around 120 min (Figure S6). Consequently, the adsorption

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equilibration time was set at 24 hours to ensure complete reactions.

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For adsorption isotherm experiments, MNPs (50 mg) were mixed with 25 mL of DI water in

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polypropylene bottles, and the pH of MNP suspensions was adjusted to 5.0. After 30-min

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sonication in an ultrasonic bath, 25 mL of As(V) or As(III) solution was added into the MNP

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suspensions to reach initial As concentrations of 0.05, 0.125, 0.25, 0.5, 0.75, or 1.0 mM, ionic

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strength of 0.01 M NaNO3 and solution pH 5.0. The mixtures were horizontally shaken at 180

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rpm in a shaking water bath (BT-350 PID, Yih Der, Taiwan) for 24 h at 10, 25, and 40°C for

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As(III) experiments, and at 10, 25, 40, and 55 °C for As(V) experiments. After equilibration, the

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suspensions were filtered through a 0.22-µm syringe filter with mixed cellulose esters membrane

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(Millipore, USA), and the final As concentrations in the filtrates were determined by inductively

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coupled plasma optical emission spectrometry (ICP-OES, Perkin Elmer Optima 2000DV,

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Waltham, MA, USA). The difference between the initial and final As concentrations was 7



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assumed to be adsorbed by MNPs. The surface coverage were further calculated by dividing the

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adsorbed As concentration with the BET specific surface area of MNPs.25, 35 The adsorption

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isotherms were plotted as function of the adsorbed As concentrations vs the equilibrium As

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concentrations in the solution.

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More adsorption experiments were conducted following the similar procedure as described

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above. To examine the effect of solution pH on As adsorption, the MNP suspensions were mixed

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with 1 mM As(V) or As(III) solutions at solution pH of 5.0, 6.0, 7.0, 8.0, and 9.0, and ionic

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strength of 0.01 M NaNO3. The MNP suspensions were also mixed with 1 mM As(V) or As(III)

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solution at ionic strength of 0, 0.005, 0.01, 0.05, or 0.1 M NaNO3 and pH 5.0 to examine ionic

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strength effect.

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Thermodynamic Calculations. The MNPs were homogeneous crystalline solids shown by

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the XRD pattern (Figure S2), and As(V) and As(III) each formed only one type of monolayer

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complexes on the MNP surface revealed by the XAS study next. By assuming no lateral

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movement of adsorbed As and no interactions between neighboring adsorbed As, the Langmuir

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equation could be derived for the As adsorption on the MNPs in the aqueous solution.42, 43 The

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isotherm data were fitted with the linearized Langmuir equation as follows:

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Ce 1 1 = Ce + qe qmax K L qmax

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where Ce (mmol L−1) is the equilibrium As concentration in solution, qe (mmol g−1) is the

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adsorbed As amount on MNPs, qmax (mmol g−1) is the maximum As adsorption capacity, and KL

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(L mmol−1) is the Langmuir constant. Thermodynamic parameters for As adsorption were first

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estimated using the Gibbs free energy equation and the linearized Van′t Hoff equation (i.e., the

(1)

8


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Van′t Hoff plot) as follows:44-47

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∆G 0 = − RT ln K

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∆H 0 ∆S 0 ln K = − + RT R

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where ∆G0 (kJ mol−1) is the change of free energy, ∆H0 (kJ mol−1) is the change of enthalpy, ∆S0

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(kJ mol−1) is the change of entropy, T (K) is the absolute temperature, R is the ideal gas constant

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(0.008314 kJ mol−1 K−1), and K is the dimensionless equilibrium coefficient. K can be estimated

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from the Langmuir constant (KL) as:48

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K = K L × Cw

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where Cw is the water concentration (5.56 × 104 mmol L−1).

(2) (3)

(4)

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Since adsorption enthalpy changes with surface coverage of adsorbed As, assessing the

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enthalpy changes with increasing As adsorption (qe) would be useful to understand the

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interaction between As and MNPs. Thus, the observed molar differential enthalpies (∆Hobs) of As

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adsorption on MNPs were further estimated using the differential Van′t Hoff equation:49 qe Ce = −R 1 d T d ln

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∆H obs

(5)

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At any given qe, Ce was computed using the Langmuir parameters obtained at different

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temperatures. ∆Hobs was then calculated from the slope of the linear plots of ln(qe/Ce) versus 1/T.

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Synchrotron Radiation-based XAS and XPS Analyses. XAS was used to investigate the

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oxidation state and local molecular environment of adsorbed As(V) or As(III) on MNPs, and

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XPS to study the oxidation state of adsorbed As and Fe on the outer surface of MNPs. The 9



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As-adsorbed MNPs for the XAS and XPS analyses were prepared at pH 5.0 under anoxic

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conditions, as described in Supporting Information S2. The As-adsorbed MNP samples were

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either kept in the anoxic wet-paste or oven-dried at 40 °C overnight. The anoxic wet-pasted

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samples labeled as As(V)-MWP and As(III)-MWP were used for investigating actual adsorption

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mechanisms in aqueous solution, and the oven-dried samples labeled as As(V)-MOD and

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As(III)-MOD for studying surface redox reactions upon exposure to oxygen under simulated

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drying process.

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Detailed XAS and XPS analysis procedures were provided in Supporting Information S2.

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Briefly, As K-edge XAS spectra of As-adsorbed MNP samples were acquired at the beamline

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BL17C1 in the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan.

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Electron storage ring was operated at 1.5 GeV with 360 mA at a top-up mode. The As-adsorbed

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MNP samples were scanned in an energy range of 11667 to 12867 eV to obtain a full As K-edge

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(11867 eV) XAS spectrum. The XAS data analysis was performed using the ATHENA and

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ARTEMIS interfaces to the IFEFFIT version 1.2.11 program package.50 The XPS spectra of

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As-free and As-adsorbed MNPs were collected at the beamline BL24A1 in NSRRC.

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High-resolution scans were carried out in an energy range of 70 to 35 eV for Fe3p and As3d XPS

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spectra. The XPS data analyses were performed using the XPSpeak 4.1 software.

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RESULTS AND DISCUSSION

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Adsorption and Thermodynamics Calculations. As(V) and As(III) adsorption occurred

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rapidly within the first 5 minutes and reached a plateau at 120 minutes, which fitted well with a

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pseudo second order kinetic model (Table S1 and Figure S6). The estimated initial adsorption 10


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rate was 7.7 mmol g−1 h−1 for As(V) and 6.7 mmol g−1 h−1 for As(III), indicating that As

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adsorption by MNPs was much faster than bulk magnetite that reached As adsorption

205

equilibrium around two days.51 The observed and fitted adsorption isotherms of As(V) and As(III)

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at various temperatures and pH 5.0 are presented in Figure 1, and the fitted isotherm parameters

207

in Table 1. The Langmuir equation fitted the isotherm data well (R2 ≥ 0.97), suggesting

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monolayer As adsorption onto the MNP surface. The maximum adsorption capacity (qmax)

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slightly increased from 0.20 mmol g−1 at 283 K to 0.25 mmol g−1 at 328 K for As(V), and from

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0.21 mmol g−1 at 283 K to 0.23 mmol g−1 at 313 K for As(III), respectively, demonstrating As

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adsorption as an endothermic process.

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The calculated thermodynamic parameters including ∆G0, ∆H0, and ∆S0 are given in Table 1,

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and ∆Hobs as a function of qe in Figure 2. The negative ∆G0 values suggest spontaneous

214

adsorption reactions.52 The positive ∆S0 likely resulted from the release of orderly structured

215

hydration water from and subsequent increase in randomness with increased concentration of

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adsorbed As on the solid surface.52, 53 The positive ∆H0 again indicates the endothermic nature of

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As adsorption (Table 1). An increase of ∆Hobs with increasing qe (i.e., more energy was needed

218

for As adsorption at greater qe) could be attributed to the fact that greater repulsion between the

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adsorbed As and the free As needs to be overcome at higher qe. Overall, the large positive

220

enthalpy change and the entropy increase collectively contribute to the strong spontaneous As

221

adsorption by MNPs.

222

Solution pH and Ionic Strength Effects and IEP Measurements. As(V) adsorption

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decreased monotonically with increasing pH from 0.21 ± 0.01 mmol g−1 at pH 5.0 to 0.14 ± 0.00 11



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mmol g−1 at pH 9.0; As(III) adsorption started to decrease at pH greater than 7.0 (Figure S7a).

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These results are consistent with the pH-dependent As speciation and surface charge of MNPs. In

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the experimental pH range, the predominate species of As(V) is H2AsO4− and HAsO42−, and that

227

of As(III) is H3AsO30. As(V) adsorption would be facilitated by electrostatic attraction between

228

negatively charged As(V) species and positively charged MNP surface at pH below the IEP of

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MNPs (i.e., 7.2, Figure S5), but inhibited by electrostatic repulsion at higher pH. Conversely,

230

neutral As(III) species rendered electrostatic interaction insignificant at pH below 7.2. At pH 8.0

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and 9.0, As(III) adsorption slightly decreased due to competitions from hydroxyls (OH−) for the

232

adsorption sites as well as the deprotonation of MNP surfaces.

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The above observations agreed with the previously reported adsorption of multi-protonated

234

oxyanion species on metal oxides.54-56 The adsorption of oxyanions such as As(V) and As(III)

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involves two-step ligand exchange reaction: the hydroxyl group of metal hydroxide is first

236

protonated, and then the H2O ligand is replaced with the oxyanion. Consequently, the adsorption

237

is affected by the pH-dependent surface protonation of metal hydroxides. Since the differences of

238

adsorption affinity among oxyanion species are usually small, this trend is primarily attributed to

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deprotonation of metal hydroxide surface with increasing pH, 56 but As speciation is expected to

240

play a role in electrostatic interaction that modulates As flux toward the MNP surface, a crucial

241

step prior to ligand exchange. While ligand exchange forms inner-sphere complexes, this

242

evidence alone cannot rule out the possibility of outer-sphere surface complexation between As

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species and protonated hydroxyl group at acidic pH.

244

Nonetheless, given that As(V) and As(III) adsorption remained relatively unchanged from 0 12


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to 100 mM NaNO3 (Figure S7b), the outer-sphere complexation was likely to be minor because

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outer-sphere complexation is typically suppressed with increasing ionic strength.57 Compared to

247

that of As-free MNPs, the IEP of As(V)-adsorbed MNPs decreased from 7.2 to 4.9 and that of

248

As(III)-adsorbed MNPs from 7.2 to 6.1, respectively (Figure S5). This significant IEP shift to

249

lower pH after As adsorption could result from shielding of positively charge sites on the MNP

250

surface by the formation of inner-sphere complexes, with a greater IEP shift from As(V) than

251

As(III) due to greater negative charge number of As(V).

252

Arsenic Adsorption and Transformation Mechanisms Studied by XAS. The

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abovementioned macroscopic adsorption and thermodynamic evidences present a convincing

254

case for specific chemical adsorption of As(V) and As(III) on MNPs, but does not provide insight

255

on coordination and configuration of formed inner-sphere complexes, i.e, whether inner-sphere

256

complexes are formed through monodentate, bidentate, or tridentate bonds (Figure S1). Also, the

257

oxidation state of As after adsorption on MNPs has yet to be determined. Therefore, we would

258

use the spectroscopic data to indisputably pinpoint the As adsorption and redox transformation

259

mechanisms.

260

XANES spectroscopy was used to investigate the oxidation state of adsorbed As(V) and

261

As(III) on MNPs. According to the As K-edge XANES spectra, the absorption edge of

262

As(III)-MWP and As(V)-MWP were located at the same absorption edge of the As(V) and As(III)

263

reference spectra with an adsorption maximum at 11875.0 eV and 11871.8 eV, respectively

264

(Figure 3). The Linear Combination Fitting (LCF) analysis revealed only trace levels of As(V)

265

(i.e., 2%) in As(III)-MWP and As(III) (i.e., 4%) in As(V)-MWP (Table S2). Thus, no major redox 13



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reaction occurred for the As(V) or As(III) adsorbed on MNPs under anoxic experimental

267

conditions. Conversely, dramatic redox reactions occurred upon exposure to air during the

268

overnight drying process, as evidenced by the white line energy shifts in the XANES spectra of

269

As(III)-MOD and As(V)-MOD (Figure 3), and the pronounced increases of As(V) fraction for

270

As(III)-MOD (i.e., 34%) and As(III) fraction for As(V)-MOD (i.e., 12%) (Table S2). Since

271

minimal redox transformation of adsorbed As on MNPs was observed in anoxic waste-paste

272

samples, the direct redox reaction of As by MNPs could be excluded. Thus, the redox reaction of

273

As in oven-dried samples more possibly occurred during the drying process, due to the exposure

274

to atmospheric oxygen. These As redox reactions were further corroborated with the XPS results

275

in next section.

276

Previously, four types of As surface complexes forming on FeO6 octahedral sites have been

277

proposed (Figure S1);22-37 this is instrumental to analyzing the EXAFS data for determining local

278

molecular coordination geometry surrounding the adsorbed As atoms. The k3-weighted As

279

K-edge EXAFS spectra and Fourier-transformed radial distribution function (RDF) profile for

280

As-adsorbed MNPs samples are shown in Figure 3, and the fitted structural parameters including

281

the coordination number (CN), interatomic distance (R), and Debye–Waller factor (σ2) in Table

282

S3. The Fourier-transformed RDF profile of all samples showed one domain signal of As–O

283

first-neighbor contribution and relatively weak signals of As–Fe second-neighbor contributions.

284

The As–O first coordination shell was attributed to 3.9 and 2.9 oxygen atoms at 1.69 and 1.79 Å

285

in As(V)-MWP and As(III)-MWP, corresponding to the molecular structure of AsO4 tetrahedron

286

and AsO3 trigonal-pyramid, respectively. In addition, the As–O shell was fitted by 3.5 and 3.2 14


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oxygen atoms at 1.69 and 1.74 Å in As(V)-MOD and As(III)-MOD, respectively. In agreement

288

with the XANES results, the changes of R and CN reflect the partial reduction of As(V) to As(III)

289

in As(V)-MOD and oxidation of As(III) to As(V) in As(III)-MOD.

290

In the As-Fe second coordination shells of As(V)-adsorbed samples, the As atom was

291

surrounded by Fe atoms at 3.35 Å with a CN of 1.5 for As(V)-MWP and at 3.39 Å with a CN of

292

1.4 for As(V)-MOD, suggesting the formation of the bidentate binuclear corner-sharing (2C)

293

complexes. These As(V)–Fe distances of 2C complexes are close to the values reported in the

294

previous studies of As(V) adsorption on magnetite,22,

295

lepidocrocite.27, 34 In agreement with Jönsson and Sherman22 and Wang et al.25, the dominant 2C

296

surface complexes were formed between one adsorbed AsO4 tetrahedron and two FeO6 octahedra

297

on {100} surfaces of magnetite (Figure 4). The As–Fe distance of As(V)-MOD is slightly longer

298

than that of As(V)-MWP, due to the sum of contributions from 2C complexes of both As(V) and

299

As(III). Importantly, the transformation of adsorbed As(V) to As(III) on MNPs surface would not

300

change the ligand complexation type.

25

maghemite,32 goethite,27,

34

and

301

Conversely, the EXAFS fitting on the As–Fe shells of As(III)-adsorbed samples revealed

302

different surface complexes, compared with that of the As(V)-adsorbed sample. The As–Fe shells

303

of As(III)-MWP were contributed from 3.8 Fe atoms at 3.49 Å and 1.0 Fe atoms at 3.76 Å, which

304

does not match that of 2C complexes (~3.3–3.4 Å) and monodentate mononuclear corner-sharing

305

(1V) complexes (~3.5–3.6 Å) of As(III) adsorption on iron oxides.29-31, 33 The EXAFS results of

306

As(III)-MWP essentially corroborated with that of Morin et al.23 and Wang et al.,24 indicating the

307

formation of tridentate hexanuclear corner-sharing (3C) complexes between adsorbed As(III) and 15



Page 16 of 32

308

2 to 6 FeO6 octahedra on the vacancy and defect sites of {111} magnetite surfaces (Figure 4).

309

The As–Fe distance of 3.76 Å was attributed to the adjacent FeO4 tetrahedra around 3C

310

complexes (Figure 4). Although Jönsson and Sherman22 suggested that the adsorbed As(III)

311

would form 2C complexes at As-Fe distance of 3.4 Å on the {100} magnetite surfaces, this was

312

not observed in this study and that of Morin et al.23 and Wang et al.24 In As(III)-MOD, the As–Fe

313

shells of were fitted by 3.8 Fe atoms at 3.39 Å and 1.3 Fe atoms at 3.67 Å. These As–Fe

314

distances of As(III)-MOD were shorter than that of As(III)-MWP, but the coordination numbers

315

were similar. Interestingly, these As-Fe distances were close to the 3C complexes (~3.4–3.5 Å

316

and ~3.6–3.7 Å) of As(V)-magnetite co-precipitates on the {111} surface,25 which was not

317

observed in As(V) adsorption in our study. Thus, the As-Fe shells at 3.39 and 3.67Å was more

318

likely to due to the mixed contribution from the 3C complexes of both As(V) and As(III) instead

319

of the 2C and 1V complexes, implying the transformation of adsorbed As(III) to As(V) would not

320

change the ligand complexation type.

321

Arsenic Redox Transformation on MNPs Studied by XPS. The XAS analysis of

322

As-adsorbed MNP samples showed the oxidation of adsorbed As(III) and reduction of adsorbed

323

As(V) occurred during the drying process. Since Fe atoms in magnetite are in Fe(II) and Fe(III)

324

mixed valence states, the possible role of reactive Fe(II) atoms in the redox transformation of

325

adsorbed As should not be ignored. Because As K-edge XAS analysis cannot provide

326

information on Fe oxidation state, the XPS analysis was further conducted to elucidate the

327

change of both Fe and As oxidation status on the outer surface of MNPs.

328

The XPS spectra of Fe3p and As3d peaks for As-free MNPs, As(III)-MOD, and As(V)-MOD 16


Page 17 of 32


329

samples are shown in Figure 5. By deconvoluting the Fe3p peak into Fe(III) and Fe(II)

330

component peaks and the As3d peak into As(V) and As(III) component peaks, as well as

331

analyzing each component peak area (Table S4), the changes in the oxidation state of As and Fe

332

atoms after exposure to air could be assessed. The As-free MNPs sample had 67% of Fe(III) and

333

33% of Fe(II), which agrees well with theoretical value of magnetite (Fe(III)/Fe(II) = 2.0). For

334

the As(V)-MOD sample, the Fe(III)/Fe(II) ratio of Fe3p peak was 3.2, indicating a significant

335

oxidation of Fe(II) to Fe(III) on magnetite surface. Interestingly, approximately 32% of adsorbed

336

As(V) was reduced to As(III). For the As(III)-MOD sample, the Fe(III)/Fe(II) ratio of Fe3p peak

337

was 2.9, and about 49% of adsorbed As(III) was oxidized to As(V), suggesting a parallel

338

oxidation of As(III) and Fe(II).

339

Also noted were greater fractions of As(V) in As(III)-MOD and As(III) in As(V)-MOD

340

detected by the XPS analysis than the XANES analysis (Table S2 and Table S4). This likely

341

resulted from the differential probing regions by these two techniques. The XPS analysis focuses

342

on the outer surface (< 10 nm) of MNPs, but the XANES analysis detects the signals from the

343

adsorbed As on the external and interior surfaces of MNP aggregates. Thus, the redox signature

344

in the XANES analysis was dampened because the adsorbed As in the interior of MNP

345

aggregates were protected from redox reactions occurring on the external surfaces.

346

The XPS and XANES analyses suggest that complex redox reactions occurred on the outer

347

surface of As-adsorbed MNPs after exposure to air, and both As(III) oxidation and As(V)

348

reduction were concomitant with magnetite surface oxidation, whereas minimal redox reactions

349

of adsorbed As on MNPs occurred during the anoxic adsorption experiments (Table S2 and S4). 17



Page 18 of 32

350

These surface redox reactions are illustrated in Figure 6. The oxidation of magnetite to

351

maghemite (γ-Fe2O3) and As(III) to As(V) was expected in the presence of oxygen. Moreover,

352

the oxidation of adsorbed As(III) was not only caused by direct reaction with oxygen, but also

353

facilitated by highly oxidizing species (i.e. O2●−) produced from the oxidation of Fe(II) in

354

magnetite surface due to a Fenton-type reaction (Figure 6b).38, 58, 59 While As(III) oxidation is

355

expected, the reduction of adsorbed As(V) to As(III) was surprising. A previous XPS study

356

reported the reduction of adsorbed As(V) to As(III) on magnetite/maghemite nanoparticles, but

357

provided no mechanistic explanation.39 Here we propose a mechanism responsible for this novel

358

surface redox reaction, illustrated in Figure 6a. Based on the XPS results, we postulate that a thin

359

maghemite layer is formed when magnetite surface is exposed to oxygen. During this process,

360

the octahedral center Fe(II) atoms are oxidized to Fe(III), and cation vacancies are subsequently

361

created at the octahedral sites. Then, electroneutrality must be balanced by either the diffusion of

362

Fe(II) or electron migration from internal bulk magnetite to oxidized surface.60, 61 Consequently,

363

the adsorbed As(V) could be reduced to As(III) via reacting with the migrated Fe(II) or electrons.

364

Finally, the presence of inner-sphere complexes, evidenced by EXAFS data, may further

365

facilitate the electron transfer process between As and Fe through binding ligand. Nonetheless,

366

this proposed mechanism needs to be further studied.

367

IMPLICATIONS

368

The findings of this study demonstrate that MNPs have high affinity to both As(V) and As(III)

369

from water. This is distinctively advantageous over conventional technologies (i.e., coagulation,

370

ion-exchange, and activated alumina adsorbent), for which the pre-oxidation of As(III) to As(V) 18


Page 19 of 32


371

is needed to enhance efficiency.14, 62 Moreover, the application of MNPs for remediation of

372

As-contaminated waters is further facilitated by easy-separation of MNPs from treated water by

373

applying a low external magnetic field. Therefore, the As removal by the MNPs could be used

374

for in-situ groundwater remediation, or retrofitted into existing water treatment facilities during

375

the coagulation process or one of final clean-up processes. Subsequently, the As-adsorbed MNP

376

sludge produced during the As removal will likely undergo a typical sludge drying process.

377

Therefore, the redox transformation of adsorbed As, especially the reduction of As(V) to As(III),

378

likely occur on the MNP surfaces. Consequently, the release of adsorbed As, especially As(III),

379

from the MNPs under fluctuating redox conditions needs to be further evaluated in order to

380

design safe disposal strategies for post-treatment MNP sludge.

381

ASSOCIATED CONTENT

382

Supporting Information

383

S1. Schematic of As inner-sphere complexes; S2. Supplemental Materials and Methods,

384

including MNP synthesis and characterization, adsorption kinetics experiments, and XAS and

385

XPS sample preparation, data collection, and analyses; S3. Supplemental Results and Discussion,

386

including MNP characterization, adsorption kinetics, solution pH and ionic strength effects, and

387

XAS and XPS results. This material is available free of charge via the Internet at

388

http://pubs.acs.org.

389

ACKNOWLEDGMENT

390

This research was partly supported by the AgBioResearch of Michigan State University (MSU)

391

through the Hatch Act Formula Grant from the US Department of Agriculture–National Institute 19



Page 20 of 32

392

of Food and Agriculture (Project No. MICL02248). We express our special thanks to Dr. Jyh-Fu

393

Lee and Dr. Yaw-Wen Yang for providing the beamtime of BL17C1 and BL24A1 for XAS and

394

XPS data collection at NSRRC, Dr. Volodymyr V. Tarabara at MSU for providing the

395

measurement of aggregate size distribution of magnetite nanoparticles, and Dr. Brian J. Teppen

396

at MSU for assisting in spectroscopic analyses and interpretations. We would also like to thank

397

the three anonymous reviewers for their insightful and constructive comments that helped us

398

improve an earlier version of this paper.

20


Page 21 of 32


399 400 401

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(2) Hughes, M. F.; Beck, B. D.; Chen, Y.; Lewis, A. S.; Thomas, D. J., Arsenic exposure and

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Implications for rice contribution to arsenic consumption. Environ Sci Technol 2003, 37, (2), 229-234. (6) Smith, A. H.; Lopipero, P. A.; Bates, M. N.; Steinmaus, C. M., Public health - Arsenic

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epidemiology and drinking water standards. Science 2002, 296, (5576), 2145-2146. (7) WHO, Guidelines for Drinking-water Quality. 4th ed.; World Health Organization: Geneva, Switzerland, 2011.

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(8) Smith, A. H.; Lingas, E. O.; Rahman, M., Contamination of drinking-water by arsenic in

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Bangladesh: a public health emergency. B World Health Organ 2000, 78, (9), 1093-1103. (9) Berg, M.; Tran, H. C.; Nguyen, T. C.; Pham, H. V.; Schertenleib, R.; Giger, W., Arsenic contamination of groundwater and drinking water in Vietnam: A human health threat. Environ

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(26) Auffan, M.; Rose, J.; Proux, O.; Borschneck, D.; Masion, A.; Chaurand, P.; Hazemann, J.-L.;

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Chaneac, C.; Jolivet, J.-P.; Wiesner, M. R.; Van Geen, A.; Bottero, J.-Y., Enhanced adsorption of arsenic onto maghemites nanoparticles: As(III) as a probe of the surface structure and

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heterogeneity. Langmuir 2008, 24, (7), 3215-3222. (27) Farquhar, M. L.; Charnock, J. M.; Livens, F. R.; Vaughan, D. J., Mechanisms of arsenic

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pyrite: An X-ray absorption spectroscopy study. Environ Sci Technol 2002, 36, (8), 1757-1762. (28) Fendorf, S.; Eick, M. J.; Grossl, P.; Sparks, D. L., Arsenate and chromate retention

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mechanisms on goethite. 1. Surface structure. Environ Sci Technol 1997, 31, (2), 315-320. (29) Guo, H. M.; Ren, Y.; Liu, Q.; Zhao, K.; Li, Y., Enhancement of arsenic adsorption during

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mineral transformation from siderite to goethite: Mechanism and application. Environ Sci

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Technol 2013, 47, (2), 1009-1016. (30) Manning, B. A.; Fendorf, S. E.; Goldberg, S., Surface structures and stability of arsenic(III)

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on goethite: spectroscopic evidence for inner-sphere complexes. Environ Sci Technol 1998, 32, (16), 2383-2388. (31) Manning, B. A.; Hunt, M. L.; Amrhein, C.; Yarmoff, J. A., Arsenic(III) and arsenic(V)

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reactions with zerovalent iron corrosion products. Environ Sci Technol 2002, 36, (24), 5455-5461. (32) Morin, G.; Ona-Nguema, G.; Wang, Y.; Menguy, N.; Juillot, F.; Proux, O.; Guyot, F.; Calas, G.; Brown Jr, G. E., Extended X-ray absorption fine structure analysis of arsenite and arsenate

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adsorption on maghemite. Environ Sci Technol 2008, 42, (7), 2361-2366. (33) Ona-Nguema, G.; Morin, G.; Juillot, F.; Calas, G.; Brown, G. E., EXAFS analysis of arsenite adsorption onto two-line ferrihydrite, hematite, goethite, and lepidocrocite. Environ Sci

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XANES evidence for rapid arsenic(III) oxidation at magnetite and ferrihydrite surfaces by

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dissolved O-2 via Fe2+-mediated reactions. Environ Sci Technol 2010, 44, (14), 5416-5422. (39) Chowdhury, S. R.; Yanful, E. K.; Pratt, A. R., Arsenic removal from aqueous solutions by

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mixed magnetite-maghemite nanoparticles. Environ Earth Sci 2011, 64, (2), 411-423. (40) Chen, C. P.; Gunawan, P.; Xu, R., Self-assembled Fe3O4-layered double hydroxide colloidal

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hematite, magnetite, and goethite. J Hazard Mater 2007, 141, (3), 575-580. (52) Velickovic, Z.; Vukovic, G. D.; Marinkovic, A. D.; Moldovan, M. S.; Peric-Grujic, A. A.; Uskokovic, P. S.; Ristic, M. D., Adsorption of arsenate on iron(III) oxide coated ethylenediamine

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functionalized multiwall carbon nanotubes. Chem Eng J 2012, 181, 174-181. (53) Gupta, K.; Basu, T.; Ghosh, U. C., Sorption characteristics of arsenic(V) for removal from water using agglomerated nanostructure iron(III)-zirconium(IV) bimetal mixed oxide. J Chem

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peroxide: pH-dependent formation of oxidants in the Fenton reaction. Environ Sci Technol 2003, 37, (12), 2734-2742. (60) Jolivet, J.-P.; Tronc, E., Interfacial electron transfer in colloidal spinel iron oxide.

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Conversion of Fe3O4-γFe2O3 in aqueous medium. J Colloid Interf Sci 1988, 125, (2), 688-701. (61) Rebodos, R. L.; Vikesland, P. J., Effects of oxidation on the magnetization of

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nanoparticulate magnetite. Langmuir 2010, 26, (22), 16745-16753. (62) Korngold, E.; Belayev, N.; Aronov, L., Removal of arsenic from drinking water by anion

563

exchangers. Desalination 2001, 141, (1), 81-84.

564 565

25



566

Table 1. Fitted Langmuir Isotherm Parameters and Thermodynamic Calculations Arsenic Temperature species (K) As(V)

As(III)

567 568

Page 26 of 32

a

Thermodynamics a

Langmuir parameters qmax

KL

R2

(mmol g−1) (L mmol−1)

∆G0

∆H0

∆S0

(kJ mol−1) (kJ mol−1) (kJ mol−1K−1)

283 298

0.195 0.214

15.5 27.8

0.979 0.995

−32.2 −35.3

313 328

0.225 0.246

32.8 36.1

0.996 0.995

−37.5 −39.6

283

0.212

6.59

0.971

−30.1

298 313

0.222 0.227

9.00 11.4

0.980 0.989

−32.5 −34.8

13.7

0.163

13.5

0.154

Calculated by the van′t Hoff plot.

26


Page 27 of 32

569 570 571 572


Figure 1. Observed and fitted arsenic adsorption isotherms at different temperature (pH = 5, ionic strength = 0.01 M NaNO3, solid-liquid ratio = 1.0 g L−1) for (a) As(V) and (b) As(III).

27



573 574 575

Page 28 of 32

Figure 2. Relationship of ln(qe/Ce) vs 1/T (inserts) and estimated enthalpy change (∆Hobs) as a function of adsorbed concentration (qe) for (a) As(V) and (b) As(III).

576

28


Page 29 of 32


577

578 579 580 581 582 583 584

Figure 3. XAS spectra of As-adsorbed MNPs samples: (a) As K-edge XANES spectra, (b) k3-weighted k-space, (c) magnitude part of Fourier transformed R-space, and (d) real part of Fourier transformed R-space, without phase shift correction. The green and blue solid lines represent the component of As(III) and As(V) obtained from linear combination fitting of XANES spectra. The black solid lines and red open circles represent experimental and fitted data, respectively. The value after labels was the surface coverage.

585

29



586 587 588 589

Page 30 of 32

Figure 4. Schematic of arsenic adsorption on magnetite with proposed surface complexes.

30


Page 31 of 32


590 591 592 593 594

Figure 5. High-resolution XPS spectra in (a) Fe3p and (b) As3d regions of As-free MNPs, As(V)-MOD, As(III)-MOD samples. The relative peak area of component peaks was presented in percentage. The As3d spin orbit-split doublet peaks were fixed a ratio of 3:2 at 0.70 eV.

31



595 596 597 598

Page 32 of 32

Figure 6. Schematic of proposed redox reactions on magnetite surface during adsorption (anoxic) and drying (oxic) processes.

32


Mechanism of Arsenic Adsorption on Magnetite Nanoparticles from

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