Effect of Iron(II) on Arsenic Sequestration by Î´-MnO2: Desorption

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Effect of Iron(II) on Arsenic Sequestration by #-MnO2: Desorption Studies Using Stirred-Flow Experiments and X-ray Absorption Fine Structure Spectroscopy Yun Wu, Wei Li, and Donald L. Sparks Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04087 • Publication Date (Web): 18 Oct 2015 Downloaded from http://pubs.acs.org on October 28, 2015

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Environmental Science & Technology

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Revision to ES&T

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Effect of Iron(II) on Arsenic Sequestration by δ-MnO2: Desorption Studies Using

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Stirred-Flow Experiments and X-ray Absorption Fine Structure Spectroscopy

5 6 Yun Wu1, Wei Li1,2*, and Donald L. Sparks1

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1 Environmental Soil Chemistry Research Group, Delaware Environmental Institute, University of Delaware, Newark, DE, 19716, United States

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Sciences and Engineering, Nanjing University, Nanjing 210046, People’s Republic of China

2 Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth

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* Corresponding author:

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Wei Li, Phone: +86(25) 836-86042; e-mail: [email protected]

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ACS Paragon Plus Environment


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Abstract

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Arsenic (As) mobility in the environment is greatly affected by its oxidation state

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and the degree to which it is sorbed on metal oxide surfaces. Manganese (Mn) and iron

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(Fe) oxides are ubiquitous solids in terrestrial systems and have high sorptive capacities

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for many trace metals, including As. Although numerous studies have studied the effects

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of As adsorption/desorption onto Fe and Mn oxides individually, the fate of As within

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mixed systems representative of natural environments has not been resolved. In this

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research, As(III) was initially reacted with a poorly crystalline phyllomanganate (δ-MnO2)

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in the presence of Fe(II), prior to desorption. This initial reaction resulted in the sorption

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of both As(III) and As(V) on mixed Fe/Mn-oxides surfaces. A desorption study was

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carried out using two environmentally significant ions, phosphate (PO43-) and calcium

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(Ca2+). Both a stirred-flow technique and X-ray absorption fine structure spectroscopy

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(XAFS) analysis were used to investigate As desorption behavior. Results showed that

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when As(III):Fe(II)=1:1 in the initial reaction, only As(V) was desorbed, agreeing with a

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previous study showing that As(III) is not associated with the Fe/Mn-oxides. When

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As(III):Fe(II)=1:10 in the initial reaction, both As(III) and As(V) can be desorbed from

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the Fe/Mn-oxide surface and more As(III) is desorbed than As(V).

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desorbents used in this study completely removed As(III) or As(V) from the Fe/Mn-

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oxides surface. However, the As desorption fraction decreases with increasing Fe(II)

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concentration in the initial reactions.

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Keywords: Arsenic; oxidation; stirred-flow technique; X-ray absorption fine structure

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(XAFS) spectroscopy

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Neither of the

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1. Introduction

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Elevated levels of As can be widely present in the environment as a result of

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mineral weathering and dissolution, geothermal activity, and numerous anthropogenic

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sources including mining wastes, coal ash, and arsenical pesticides, posing a severe

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health threat to millions of people worldwide.1-4 In the environment, As primarily exists

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in an inorganic form and in two oxidation states, As(V) and As(III). Arsenate [As(V)]

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generally predominates in oxic waters and soils, whereas arsenite [As(III)] can be

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prevalent in suboxic groundwater5 and is considered more mobile and more toxic in

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natural environments.4,6-8 The two inorganic species together likely represent more than

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99% of total As in soils, sediments and waters.9,10

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In the earth’s surface environment, the mobility of As is governed largely by its

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chemical speciation, as well as mineral-water interfacial reactions.2,4 Adsorption or

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coprecipitation with metal oxides, particularly Fe-(hydr)oxides, is an important process

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controlling the fate and transport of As in natural aquatic systems due to their abundance

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in the environment and the high affinity for As species.2,11,12

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adsorption mechanisms of arsenate and arsenite on a range of natural and synthetic

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minerals (e.g., metal oxides) have been extensively investigated using macroscopic and

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spectroscopic techniques including synchrotron X-ray absorption spectroscopy (XAS)

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and FTIR.13-18 According to these studies, both arsenate and arsenite can be strongly

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adsorbed on polyvalent metal (hydr)oxide surfaces, forming predominantly inner-sphere

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surface complexes via ligand exchange reactions, resulting in either monodentate or

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binuclear bidentate complexes depending on surface loading and solution chemistry.14-18

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The molecular-scale


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Only a few recent studies reported the formation of outer-sphere As surface complexes

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on ferric (hydr)oxide surfaces,16,19 which might be responsible for As desorption.

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While sorption mechanisms of As on mineral surfaces have been extensively

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studied and well-established, mechanisms for As desorption remain poorly understood.

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Currently, there are three widely proposed mechanisms for As release from contaminated

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sediments and soils: (1) oxidation of arsenic rich sulfide materials, (2) reductive

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dissolution of iron oxides and concurrent release of adsorbed As, and (3) competitive

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adsorption/desorption.20

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adsorption/desorption has received the least attention. Sorption/desorption is a dynamic

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process, which is determined by a number of geochemical factors, including pH,

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structural changes in the solid phases, and the presence of competing ions.21,22 The

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presence of other naturally occurring ions in waters may compete for sorption sites on the

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mineral surface and thereby affect the stability and mobility of adsorbed As species.22,23

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Of

these

proposed

mechanisms,

competitive

Previous research indicated that individual anion species exhibit widely different For example, Goh and Lim24 reported that the

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affinities for mineral surface sites.

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capabilities of common anions to increase As mobility in subsurface environments

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follows the order: phosphate > carbonate > sulfate ≈ chloride. Phosphate (PO43-) is one

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of the most common anions in groundwater, with concentrations ranging from 0 to 5

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mg/L, owing to the use of phosphate fertilizers.25

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effective competitor with arsenate for sorption sites due to their similarities in

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coordination geometry and geochemical behavior.11,22,24 Calcium is one of the dominant

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cations in both groundwater and soil solutions.26 It has been reported that calcium (Ca2+)

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can compete with many metals for sorption sites on soil mineral surfaces.4,26,27 Therefore,

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Phosphate is widely known as an


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identifying the effects of phosphate and calcium on arsenic desorption is essential to

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understand the arsenic mobility in the environment.

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In previous studies,28 it was found that both As(III) and As(V) sorption on δ-

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MnO2 increased in the presence of Fe(II), due to the formation of Fe(III)-(hydr)oxides.

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Although the removal of total As increases with increasing Fe(II) addition, the stability

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and mobility of the adsorbed arsenic remain unclear. In this study, we investigated the

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As desorption behavior from δ-MnO2 using a stirred-flow technique and characterized the

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produced Fe(III)-(hydr)oxides and As speciation using XAFS. The objective was to

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determine the stability and of sorbed As and the geochemical coupling among As, Mn

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and Fe under competition with an anion (PO43-), and a cation (Ca2+), both of which are

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commonly found in the environment.

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2. Experimental Section 2.1. Chemicals

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All chemicals used in this study met or exceeded American Chemical Society

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standards. 18.2 MΩ deionized (DI) water was used for all solutions. NaAsO2 and

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FeSO4·7H2O were used as sources of As(III) and Fe(II), respectively. As(III) stock

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solution (100 mmol/L) was stored at 4 ºC and prepared every month. Fe(II) solution was

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freshly prepared every time in the glove box. The DI water used to make the Fe(II) and

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As(III) solutions were degassed using N2 (g) for 2 hours before use. Ca2+ and PO43-

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desorptive solutions were made by dissolving certain amounts of NaH2PO4 and CaCl2 in

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DI water, respectively.

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arsenate are provided in the Supporting Information.

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Details of the synthesis of δ-MnO2, ferrihydrite, and ferric

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2.2. Stirred-Flow Experiments

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Stirred flow experiments were conducted using the same apparatus as described

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previously. 28 The reactor is a modified version of the stirred-flow reactor described in a

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previous study.28 All stirred-flow reactions were conducted by pumping a solution of

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As(III) and Fe(II) in a 0.01 mol/L NaNO3 background electrolyte buffered at a certain pH

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at a rate of 1.0 mL/min into a stirred-flow chamber containing 2.0 g/L δ-MnO2. For

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reactions involving Fe(II), nitrogen gas was pumped into the solution to maintain anoxic

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conditions. All suspensions in the reaction chamber were mixed at 100 rpm via a

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magnetic stir bar. Effluent solutions were filtered with a 0.22-µm pore size filter and then

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collected by a fraction collector.

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immediately transferred into a glove box until analysis, and all other solutions were

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stored at 4 ºC in the dark until analysis. All stirred-flow experiments were conducted at

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least in duplicate.

Effluent solutions which contained Fe(II) were

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In all desorption reactions, 2 g/L δ-MnO2 was reacted with 100 µmol/L As(III)

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and 100 or 1000 µmol/L Fe(II) at pH 6 for 36 hours, followed immediately by reaction

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with a desorptive solution. Three desorptive solutions were used: (i) 0.01 mol/L NaNO3

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background electrolyte alone, (ii) 100 µmol/L calcium chloride in background electrolyte,

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and (iii) 100 µmol/L sodium phosphate in background electrolyte.

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experiments, all desorptive solutions were adjusted to pH 6 with HCl and NaOH and

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pumped with N2 gas for 4 hours to maintain anoxic conditions. Background electrolyte

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Prior to the

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was introduced into the stirred flow reactor (containing δ-MnO2) for at least 2 hours at a

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rate of 1 mL/min prior to each experiment. In each reaction, influent flow of As(III) and

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Fe(II) solutions were stopped after 36 hours, and then desorptive solution was introduced

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into the reactor, thus beginning the desorption phase of the reaction. Changing the

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influent solution from As(III) and Fe(II) solution to the desorbent solution took less than

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5 seconds, and thus flow of solution into the reactor was effectively constant throughout

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each experiment (oxidation followed by desorption). A plot showing the full data, As(III)

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and Fe(II) oxidation followed by desorption from one experiment, is presented in Figure

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1.

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2.3. Determination of Metal Concentrations

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Speciation and concentration of As(III) and As(V) were quantified using liquid

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chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). As(III)

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and As(V) were separated using a Phenomenex Prodigy 5 μm ODS-3 100 Å (150 x 4.6)

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column with a flow rate of 1 mL/min, using a 10 μm sample injection before

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quantification by LC-ICP-MS. The mobile phase consisted of 5% methanol in 5 mM

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tetra-butyl ammonium hydroxide, and the pH was adjusted to 5.9 using malonic acid

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(final concentration of about 3 mM).

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ferrozine method.29 In order to eliminate the interference of Fe(III) in Fe(II)

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measurements, 0.05 mol/L NaF was added to mask Fe(III) in the Fe(II) analysis. Total

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Fe was determined by reducing total Fe(III) to Fe(II) and then Fe(II) was measured by the

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ferrozine method without addition of NaF solution. The Fe(III) content in the liquid

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solution was obtained by difference. Mn, Ca, and P concentrations were quantified by

Fe(II) concentration was determined by the

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inductively coupled plasma optical emission spectrometry (ICP-OES), where 5 mL of

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sample were acidified using 1% (w/v) nitric acid mixed with 0.5% (w/v) hydrochloric

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acid.

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calculating the difference between the mass of As (μmol) introduced into the reactor and

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the mass of As (μmol) removed from the reactor over time (more details can be found in

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the Supporting Information).

The amount of As sorbed/desorbed during the reaction was determined by

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2.4. XAS Analysis

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X-ray absorption spectroscopic analysis was performed on reacted δ-MnO2 after

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36 hours oxidation followed by 24 hours desorption. When the reaction was completed,

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the suspension in the reaction chamber was immediately filtered (0.22-µm membrane) to

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remove any background electrolyte.

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immediately covered with Kapton tape and stored under anoxic conditions for less than 3

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days prior to spectroscopic analysis. Several sorption standards, including As(V)-δ-

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MnO2, As(V)- and As(III)-ferrihydrite, and Fe(II)-δ-MnO2, were prepared. Details of

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sample preparation are provided in the Supporting Information.

After filtration, the residual wet paste was

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Extended X-ray absorption fine structure (EXAFS) and X-ray absorption near

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edge (XANES) spectroscopic data for As were collected at the National Synchrotron

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Light Source (NSLS) at Brookhaven National Laboratory (Upton, NY) on beamline

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X11A. The monochromator consisted of two parallel Si-(111) crystals with a vertical

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entrance slit separation of 0.5 mm. As K-edge EXAFS spectra were collected. All

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samples were oriented at 45° to the incident beam and a Lytle detector was used to collect

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As spectra in fluorescence mode. The monochromator angle was calibrated to the As(V)

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K-edge (11.874 keV) using diluted Na2HAsVO4 as a standard (10% Na2HAsVO4 with

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90% boron nitride). This standard was monitored in transmission mode simultaneous to

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sample collection to check for potential energy shifts. Multiple scans were collected and

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averaged at room temperature for each sample to improve the spectral quality.

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EXAFS and XANES spectroscopic data for Fe were collected at beamline 4-1 at

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the Stanford Synchrotron Radiation Lightsource (SSRL) and beamline 14W at the

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Shanghai Synchrotron Radiation Facility (SSRF). The monochromator consisted of two

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parallel Si-(220) crystals with a vertical entrance slit separation of 0.5 mm. Fe (7.112

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keV) K-edge EXAFS spectra were collected in fluorescence mode. The monochromator

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crystals were detuned 40% for Fe in I0 to reject higher order harmonics. A 3-path length

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Mn filter was used, and Sollier slits were used for signal optimization and removal of

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elastically scattered radiation. The monochromator angle was calibrated to the Fe(0) K-

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edge (7112 eV ) using a Fe metal foil.

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described in the Supporting Information.

Details of the EXAFS data analysis were

198 199 200 201

3. Results and Discussion 3.1. As(III) Oxidation and Sorption

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Prior to the desorption experiments in this study, sorption experiments were

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conducted by reaction of 100 µmol/L As(III) with 2 g/L δ-MnO2 in the presence of either

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100 or 1000 µmol/L Fe(II) for 36 hours at pH 6. To help interpret the desorption process,

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we briefly summarize the As(III) oxidation and sorption behavior and mechanisms, based

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on the results from the stirred-flow experiments.28

9

The initial reaction among As(III),



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Fe(II) and δ-MnO2 involves three key steps: (i) As(III) and Fe(II) are adsorbed onto the

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δ-MnO2 surface where they are rapidly oxidized to As(V) and Fe(III), respectively,

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releasing Mn(II) to solution. Competition for oxidative power on δ-MnO2 exists between

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As(III) and Fe(II); (ii) Fe(III) precipitates as crystalline Fe(III)-(hydr)oxides and poorly

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crystalline ferrihydrite and ferric arsenate, coating on the δ-MnO2 surface and causing

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surface passivation, which lowers the oxidation rates of both As(III) and Fe(II); and (iii)

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the remaining As(III), As(V), Mn(II) and Fe(II) are partially adsorbed and/or associated

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onto the newly formed Fe(III) minerals. Also, some As(V) and Mn(II) can be adsorbed

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on δ-the MnO2 surface. The oxidation of As(III) and Fe(II) proceeds in two distinct

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phases, a fast oxidation phase during the initial few hours followed by a slow oxidation

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phase due to the surface passivation of δ-MnO2. After 24 hours, the reaction reaches a

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steady state phase, where concentrations of all metals in solution stay constant.28

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Therefore, desorption experiments were started after a 36 hour reaction period (Figure 1).

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3.2. As(III) Desorption Kinetics

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At an As(III) to Fe(II) ratio of 1 : 1 in the initial reaction, As(III) concentration in

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the effluent solution follows the dilution curve very well, no matter what kind of

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desorptive was used (Figure 2). This suggests that no As(III) was desorbed from the

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solids, as the desorbed As(III) was calculated by the difference between the desorption

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curve of As(III) and the dilution curve.

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When the As(III) to Fe(II) ratio was 1 to 10 in the initial reaction, As(III) was

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desorbed by PO43-, Ca2+, and the background electrolyte (BE) from the solid phase

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(Figure 3). Among the three desorptives, PO43- desorbed the most As(III), with 20.56

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µmol/L in the effluent, followed by Ca2+, which desorbed 16.55 µmol/L As(III), and the

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background electrolyte (BE) which desorbed the least As(III), i.e., 13.52 µmol/L (Table

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1). This trend was also supported by the As XANES spectra, where the As(III) peak

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intensity decreased in the order of BE > Ca > P (Figure 4a). Regardless of the desorptive,

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As(III) desorption was greater than As(V) desorption (Table 1), which agrees with the

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lower affinity of As(III) to either Fe(III)-(hydr)oxides and δ-MnO2 compared with As(V).

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15,16,18,30-34

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XANES spectra (Figure 4a), which means not all As(III) was desorbed from the solid

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phases, consistent with the changes of As(III) in effluent concentration based on Table 1.

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This is further supported by the EXAFS fitting results (Table 2) that the relatively longer

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As-O distance (> 1.69 Å) and smaller coordination number ( < 4) are both indications of

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the presence of As(III) in the solids after desorption. These non-detachable As(III)

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species are probably trapped in the micropores of iron hydroxide aggregates or possibly

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incorporated in the crystal structure of the solid iron phases.

However, after 24 hours of desorption, the As(III) peak was observed in the

244 245

3.3. As(V) Desorption Kinetics

246

Desorption of As(V) from the solid phase was observed for both initial reactions,

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with either As(III):Fe(II)=1:1 or 1:10 (Figures 2 and 3). For all three desorptives, more

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As(V) was desorbed at an As(III):Fe(II) of 1:10 (Table 1). This is ascribed to the

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formation of more ferrihydrite at higher Fe(II) concentration, which resulted in more

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As(V) adsorbed to the solids that can be readily desorbed.

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Similar to As(III) desorption, PO43- desorbed the As(V) about 2 to 4 times the

252

amount desorbed by Ca2+ or the background electrolyte (Table 1). Due to the similarities

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in acid dissociation constants and coordination geometry, phosphate (pKa1=2.1, pKa2=7.2

254

and pKa3=12.3) behaves much like arsenate (pKa1=2.2, pKa2=6.9 and pKa3=11.4).11 It has

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been reported that phosphate could desorb arsenate more readily and more efficiently

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from metal oxide minerals, compared to other ions such as carbonate, silicate, calcium,

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magnesium, potassium, and sodium, etc.4,11,12,20 In terms of the percentage of total As,

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44.3 % of the sorbed As can be desorbed by PO43- when the As(III):Fe(II)=1:1 in the

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initial reaction, while only 21.4 % of the sorbed As can be desorbed by PO43- when the

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As(III):Fe(II)=1:10 in the initial reaction. These values are much smaller than found in a

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desorption study by Lafferty et al.4 that showed roughly 67% of the sorbed As was

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mobilized from the δ-MnO2 surface after 24 hours of desorption by PO43-. Comparison

263

of these desorption behaviors reveals that more As tends to be sequestrated on the solid

264

phase in the presence of Fe(II)/Fe(III)-(hydr)oxides and the fraction of desorbed As

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decreases as the Fe(II) to As(III) ratio increases. This indicates that As sorbed on the

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Fe(III)-(hydr)oxides surface is less mobile than As sorbed on the δ-MnO2 surface. And

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the decreased As desorption rate at higher Fe concentration could be attributed to the

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possible formation of amorphous ferric arsenate precipitates, which are more resistant

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than surface sorbed As species to the phosphate desorptive.

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Shell-by-shell fits of the As EXAFS spectra of solids after 24 hours desorption

271

showed that only one second As-Fe/Mn shell was found between 3.31 Å and 3.33 Å, with

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a coordination number of about 2. These distances and coordination numbers are in good

273

agreement with the local structure in an As bidentate binuclear corner-sharing complex

274

on Fe oxides14,35-37 as well as an As(V)-Fe(III) precipitate (e.g. ferric arsenate),37

275

suggesting that the un-desorbed As exists either in a precipitate phase or in the form of a

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bidentate binuclear complex on Fe(III)-(hydr)oxides. A previous study by Sherman and

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Randall38, using density functional theory calculations, showed the bidentate binuclear

278

complex was more favored energetically and more stable than the monodentate complex.

279

It was also observed that As in monodentate complexes with iron oxides were more

280

readily desorbed compared to bidentate complexes, which was due to the weaker binding

281

strength of the Fe-As bond in the monodentate complexes.39 Both of these findings

282

explain why the bidentate binuclear complex is the dominant species after desorption.

283 284

3.4. Mn(II) Desorption Kinetics

285

Desorption of Mn(II) from the solid phase was observed in both initial reactions,

286

with either As(III):Fe(II)=1:1 or 1:10 (Figure 2 and 3). For all three desorptives, more

287

Mn(II) was desorbed from an initial reaction with As(III):Fe(II)=1:10 than

288

As(III):Fe(II)=1:1 (Table 1), since more δ-MnO2 was reduced in the presence of the

289

higher Fe(II) concentration, resulting in more Mn(II) adsorbed on the solids. Of the three

290

desorptives used in this study, Ca2+ is expected to react with sorption sites on Fe/Mn-

291

oxides that are most similar to Mn(II). It has been reported that Ca2+ can compete with

292

many transition metals for sorption sites on metal oxide surfaces.4,26,27 Lafferty et al.

293

postulated that Ca2+ could bind in triple corner sharing complexes at the vacancy sites of

294

δ-MnO2, which is the primary location of Mn(II) sorption on δ-MnO2. Similar to a

295

previous study,4 we found that Ca2+ desorbed the most Mn(II), twice as much as the

296

amount desorbed by PO43- or the background electrolyte for both initial reactions with

297

either As(III):Fe(II) = 1:1 or 1:10 (Table 1).

298

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3.5. Fe(II) Desorption and Fe(III)-(hydr)oxide Formation after Desorption

300

When the As(III) to Fe(II) ratio is 1:1 in the initial reaction, no matter what kind

301

of desorptive was used (Figure 2), the difference between the Fe(II) concentration curve

302

and the dilution curve is negligible. This suggests that no Fe(II) is desorbed from the

303

solid phase using any of the three desorptives, which agrees with the Fe K-edge XANES

304

analysis (Figure 5a and Figure S6) that the Fe phases produced from oxidation of 100

305

µmol/L As(III) and 100 µmol/L Fe(II) by δ-MnO2 exhibit similar features with Fe(III)

306

oxyhydroxides (e.g., ferrihydrite, goethite, lepidocrocite), indicative of the presence of

307

Fe(III). The absence of a peak at ~ 7120 ev, a characteristic feature for Fe(II) in the Fe

308

K-edge derivative XANES for either Fe(OH)2 or green rust (Figure S6b),30 suggested

309

that no Fe(II) existed in the solid phases after 24 hours of desorption (Figure 5a).

310

When the As(III) to Fe(II) ratio was 1:10 in the initial reaction, a very small

311

amount of Fe(II) was desorbed from the solid phase (Figure 3).

Similar to Mn(II)

312

desorption, Ca2+ desorbed the largest amount of Fe(II), 13.27 µmol/L, since Ca2+ reaction

313

with sorption sites on Fe/Mn-oxides was most similar to Fe(II).

314

background electrolyte desorbed almost equal amounts of Fe(II), 9.7 and 9.19 µmol/L,

315

respectively (Table 1). But it should be noted that these values are roughly around 1% of

316

the initial Fe(II) concentration (i.e., 1000 µmol/L). This finding is consistent with the Fe

317

XANES spectra for all three desorptives, that no characteristic Fe(II) peaks were

318

observed after 24 hours of desorption (Figure 5a and Figure S6).30

PO43- and the

319

Fe K-edge EXAFS spectra of the solid phases after 24 hours desorption were also

320

analyzed by linear combination fitting in order to investigate the composition of the

321

Fe(III)-(hydr)oxides. The fitting results are shown in Table 3 and data fits are plotted in

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Figure 5b. The mineralogical composition of the solids before desorption was presented

323

in Table 3. Four major Fe(III) compounds were found in the solids after desorption,

324

including ferric arsenate, ferrihydrite, goethite, and lepidocrocite. For the initial reaction

325

with As(III):Fe(II)=1:1, the ferric arsenate fraction decreased from 56.1% before

326

desorption to 41.2 ~ 51.6% after desorption, while the fractions of ferrihydrite, goethite

327

and lepidocrocite increased slightly. This indicates that the As associated with Fe(III)

328

(hydr)oxides during the initial reaction was not immobile and could be partially dissolved

329

during the desorption reaction, resulting in the release of As. For the initial reaction with

330

As(III):Fe(II)=1:10, the fractions of each Fe(III)-(hydr)oxide do not change much after

331

desorption.

332 333 334

4. Environmental Implications

335

The mobility of As in the environment is governed largely by its chemical

336

speciation, as well as the aqueous sorption-desorption and oxidation-reduction reactions

337

at the mineral-water interface. For example, some groundwater in Bangladesh often

338

contains co-existing As(III) (up to 13.3 µmol/L) and Fe(II) (up to 540 µmol/L),40 where

339

oxidation by either oxygens or Mn oxide would enhance the As mitigation.41,42 This

340

study indicated that Mn oxide could oxidize Fe(II) to form mixed Fe/Mn-oxide solids

341

when Fe(II) is present, which can sequester a large amount of As via complicated

342

oxidation, sorption, and co-precipitation reactions. Thus, in a natural setting where As,

343

Fe, and Mn coexists (e.g. Cape Cod, MA, USA),41 As availability would be controlled by

344

the geochemical interactions between these constituents42 as well as the redox potential

15



Page 16 of 33

345

mediated by microbial activities.43

346

sequesterted As phase can be mobilized by environmentally competing ions (e.g. Na+,

347

Ca2+ , NO3-, and PO43-), and As(III) is more prone to be desorbed than As(V), due to the

348

weaker binding of As(III) with Fe/Mn-oxides. Even the background electrolyte (NaNO3)

349

was able to desorb As to some degree, indicating that a small portion of As sorbed on

350

Fe/Mn-oxides is potentially quite mobile in the environment. Although some sorbed As

351

can be desorbed from the Fe/Mn-oxide surface, there is still a certain amount of As that

352

was resistant to the desorptive ions. These findings emphasize that the mobility of As

353

needs to be carefully monitored for contaminated sites after the remediation using Fe/Mn-

354

containing chemical agents, especially in those environment with abundant competitive

355

aqueous ions.

However, this research also reveals that the

356 357 Supporting Information

358 359 360

Additional data are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

361 362 363 364

Acknowledgements The authors thank four anonymous reviewers for their insightful and constructive

365

comments, which significantly strengthened this manuscript.

366

Hendricks for laboratory assistance and Caroline Golt at the Soil Testing Lab at the

367

University of Delaware for assistance with the HPLC-ICP-MS As speciation analyses.

16


We thank Gerald

Page 17 of 33


368

This research was funded by the National Science Foundation (NSF) through the

369

Delaware EPSCoR program (Grant No. EPS0814251) and the Institute of Soil

370

Environmental Quality (ISEQ) fellowship awarded to Dr. Yun Wu. Wei Li is grateful

371

to the Dengfeng Young Faculty Career Development Program at Nanjing University.

372

We are also grateful to the Shanghai Synchrotron Radiation Facility (SSRF) for use of

373

the synchrotron radiation facilities at beamline 14W.

374

carried out at the SSRL, a national user facility operated by Stanford University on

375

behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. Use of the

376

National Synchrotron Light Source, Brookhaven National Laboratory, was supported

377

by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences,

378

under contract no. DE-AC02-98CH10886.

Portions of this research were

379

17



380 381 382

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Page 23 of 33


529 530

Tables and Figures

531 532 533

Table 1. The amount (µmol) of As(III), As(V), Fe(II) and Mn(II) desorbed by Ca2+, PO43-, and background electrolyte (BE) for 24 hours after 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of either 100 or 1000 µmol/L Fe(II) for 36 hours at pH 6. Desorption Amount (µmol) Metals

As(III):Fe(II)=1:10

BE

Ca2+

PO43-

BE

Ca2+

PO43-

As(III)

-

-

-

13.52

16.55

20.56

As(V)

2.40

3.12

8.98

4.70

6.32

14.53

Mn(II)

4.45

8.53

4.07

11.91

21.78

12.76

Fe(II)

-

-

-

9.19

13.27

9.70

11.8%

15.4%

44.3%

11.1%

14.0%

21.4%

% of As desorbed

534 535 536 537 538 539

As(III):Fe(II)=1:1

Note: The total As sorbed (As(III) and As(V)) calculated from previous stirred-flow experiments is 20.26 µmol and 163.63 µmol for reaction of 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of either 100 or 1000 µmol/L Fe(II), respectively. The percent of As desorbed is calculated as the quantity of As desorbed divided by the quantity of As sorbed.

540 541 542 543

23



Page 24 of 33

Table 2. Structural parameters derived from shell-by-shell fits of k3-weighted As EXAFS data of 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of either 100 or 1000 µmol/L Fe(II) for 36 hours at pH 6, and then desorbed by Ca2+, PO43-, and background electrolyte (BE) for 24 hours. Fitting parameters for sorption standards of As(V) on δ-MnO2, As(III) and As(V) on ferrihydrite (Fh), as well as a standard of a synthetic ferric arsenate precipitate are also provided. As-O Sample

Label

Desorbent

As-O-O

As-Mn/Fe

CN

R

σ2

CN

R

σ2

CN

R

σ2

∆Eo

R factor

36hr, As1Fe1

Control

4.3

1.69

0.002

12

3.12

0.002

2.0

3.33

0.010

6.06

0.01

BE, As1Fe1

BE

4.1

1.69

0.003

12

3.12

0.003

1.9

3.32

0.011

5.88

0.01

Ca, As1Fe1

Ca2+

4.0

1.69

0.003

12

3.12

0.003

1.9

3.32

0.011

5.57

0.01

P, As1Fe1

PO43-

3.9

1.69

0.003

12

3.11

0.003

1.4

3.33

0.007

5.72

0.01

36hr, As1Fe10

Control

3.2

1.72

0.004

12

3.15

0.004

1.9

3.35

0.007

4.89

0.02

BE, As1Fe10

BE

3.4

1.71

0.004

12

3.11

0.004

1.8

3.32

0.009

4.68

0.02

Ca, As1Fe10

Ca2+

3.5

1.71

0.004

12

3.12

0.004

1.9

3.32

0.011

3.82

0.02

P, As1Fe10

PO43-

3.7

1.71

0.003

12

3.12

0.003

1.3

3.31

0.009

4.08

0.02

As(V)-δ-MnO2

4.1

1.69

0.002

12

3.08

0.002

0.5

3.21

0.005

6.44

0.02

As(V)-Fh

4.4

1.69

0.002

12

3.09

0.002

0.9

3.29

0.006

5.30

0.01

As(III)-Fh

2.8

1.79

0.004

6

3.20

0.004

1.0

3.35

0.006

9.05

0.01

Ferric Arsenate

4.5

1.69

0.002

12

3.11

0.002

1.8

3.33

0.003

6.33

0.01

As(III):Fe(II)=1:1

As(III):Fe(II)=1:10

Standards

Note: CN, coordination number, uncertainty for As-O is ± 0.1-0.47, for As-Mn/Fe is ± 0.18-0.54, CNAs-O-O is fixed to be 12, σ2As-O-O is constrained to be σ2As-O; R(Å), interatomic distance, uncertainty for As-O is ± 0.002-0.003, for As-Mn/Fe is ± 0.01-0.03; σ2, Debye-Waller factor, uncertainty for As-O is ± 0.001-0.002, for As-Mn/Fe is ± 0.001-0.013; ∆E0 (eV), difference between experimentally determined threshold energy and the FEFF calculated threshold energy, uncertainty is ±

24


Page 25 of 33


0.3-2.1; S02, amplitude reduction factor, is fixed at 0.95, with uncertainty of ± 0.02-0.15; R factor, goodness of fit, R=∑(data-fit)2/∑data2.

25



Page 26 of 33

Table 3. Mineralogical composition of Fe(III)-(hydr)oxides derived from linear combination fits to k3-weighted Fe EXAFS data of 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of either 100 or 1000 µmol/L Fe(II) for 36 hours at pH 6, and then desorbed by Ca2+, PO43-, and background electrolyte (BE) for 24 hours. Standards Sample

As(III):Fe(II)=1:1

As(III):Fe(II)=1:10

Desorbent

R factor Ferric Arsenate

Ferrihydrite

Goethite

Lepidocrocite

Control

56.1 ± 2.5%

43.9 ± 4.3%

N/A

N/A

0.05

BE

51.6 ± 2.8 %

42.3 ± 3.2 %

6.1 ± 0.9 %

N/A

0.08

Ca2+

43.7 ± 4.1 %

46.8 ± 2.3 %

5.3 ± 2.0 %

4.2 ± 0.9 %

0.07

PO43-

41.2 ± 2.1 %

47.3 ± 1.8 %

7.0 ± 1.4 %

4.5 ± 0.5 %

0.06

Control

21.5 ± 2.6%

62.3 ± 2.7%

N/A

16.2 ± 3.8%

0.06

BE

23.1 ± 1.1 %

60.3 ± 4.3 %

8.3 ± 0.7 %

8.3 ± 1.3 %

0.05

Ca2+

21.3 ± 1.7 %

58.1 ± 2.8 %

10.7 ± 0.9 %

9.9 ± 1.2 %

0.05

PO43-

16.3 ± 1.3 %

60.8 ± 3.5 %

17.1 ± 1.6 %

5.8 ± 0.4 %

0.05

Note: R factor, goodness of fit, R=∑(data-fit)2/∑data2.

26


Page 27 of 33


Figure captions Figure 1. As(III), As(V), Mn(II) and Fe(II) concentrations from one replicate of 100 µmol/L As(III) oxidation by 2 g/L δMnO2 in the presence of 100 µmol/L Fe(II) for 36 hours at pH 6, followed by desorption by 100 µmol/L sodium phosphate in a background electrolyte for another 24 hours, starting from 36 h to 60 h. Figure 2. As(III), As(V), Fe(II) and Mn(II) desorbed by Ca2+, PO43-, and background electrolyte (BE) after 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of 100 µmol/L Fe(II) for 36 hours at pH 6. As(III) oxidation data prior to desorption are not shown and data shown are the first 12 h of the 24 h desorption experiments. Figure 3. As(III), As(V), Fe(II) and Mn(II) desorbed by Ca2+, PO43-, and background electrolyte (BE) after 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of 1000 µmol/L Fe(II) for 36 hours at pH 6. As(III) oxidation data prior to desorption are not shown and data shown are for the first 12 h of 24 h desorption experiments. Figure 4. (a) Arsenic K-edge derivative XANES; (b) As K-edge EXAFS; and (c) Fourier transformed EXAFS of samples before and after desorption by Ca2+, PO43-, and background electrolyte (BE) for 24 hours after 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of either 100 or 1000 µmol/L Fe(II) for 36 hours at pH 6. EXAFS experimental data are presented as solid lines, and fits are presented as dashed lines. Figure 5. (a) Fe K-edge derivative XANES; and (b) Fe K-edge EXAFS of selected samples before and after desorption by background electrolyte (BE), Ca2+, and PO43- for 24 hours after 100 µmol/L As(III) oxidation by 2 g/L δ-MnO2 in the presence of either 100 or 1000 µmol/L Fe(II) for 36 hours at pH 6. Experimental XAS data are presented as solid lines, and fits are presented as dashed line.

27



Page 28 of 33

TOC Graph

Concentration (μmol/L)

100

80

60

As(III) 40

As(V) Mn(II)

20

Fe(II)

0 0

12

24

36 Time (h)


48

60

Page 29 of 33



100

80

60

As(III) As(V)

40

Mn(II) Fe(II)

20

0 0

12

24

36 Time (hr)

Figure 1.


48


(a) As(III) Desorption, As(III):Fe(II)=1:1

60

50 40

BE

30

Ca

P

20

(b) As(V) Desorption, As(III):Fe(II)=1:1

40



70

Dilution Curve

10

35 30 25 20 15 10 5 0

0 0

2

4

6

8

10

0

12

2

4

(c) Fe(II) Desorption, As(III):Fe(II)=1:1

8

10

12

8

10

12

(d) Mn(II) Desorption, As(III):Fe(II)=1:1

60


60

6

Time (hr)

Time (hr)


Page 30 of 33

50

40 30 20 10

0

50

40 30 20 10

0 0

2

4

6

8

10

12

0

2

Time (hr)

4

6

Time (hr)

Figure 2.


Page 31 of 33


(a) As(III) Desorption, As(III):Fe(II)=1:10

35 30 BE

25

Ca

20

P

15

Dilution Curve

10

(b) As(V) Desorption, As(III):Fe(II)=1:10

30



40

5 0

25 20 15

10 5 0

0

2

4

6

8

10

0

12

2

4

(c) Fe(II) Desorption, As(III):Fe(II)=1:10

10

12

8

10

12

(d) Mn(II) Desorption, As(III):Fe(II)=1:10

20



8

Time (hr)

Time (hr)

1200

6

1000 800

600 400 200 0

15

10 5 0

0

2

4

6

8

10

12

0

2

Time (hr)

4

6

Time (hr)

Figure 3.



(b)

36hr, As1Fe1

Fourier Transform Magnitude

Relative Intensity

36hr, As1Fe1 BE, As1Fe1 Ca, As1Fe1 P, As1Fe1 36hr, As1Fe10 BE, As1Fe10 Ca, As1Fe10 P, As1Fe10 As(3) Std

(c)

k3χ(k)

(a)

Page 32 of 33

BE, AS1Fe1 Ca, As1Fe1 P, As1Fe1

36hr, As1Fe10 BE, As1Fe10 Ca, As1Fe10

P, As1Fe10

As(5) Std

11860 11870 11880 11890

Energy (eV)

3

5

7

9

k (Å -1)

11

13

0

Figure 4.


1

2

3

R + ΔR (Å)

4

5

Page 33 of 33


(a)

(b) 36hr, As1Fe10

Relative Intensity

BE, As1Fe10

P, As1Fe10 36hr, As1Fe1

k3χ(k)

Ca, As1Fe10

BE, As1Fe1 Ca, As1Fe1 P, As1Fe1 7100

7120

7140

3

Energy (eV)

6

9

k (Å -1)

Figure 5.


12

Effect of Iron(II) on Arsenic Sequestration by Î´-MnO2: Desorption

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