Arsenate and Selenate Scavenging by Basaluminite: Insights into

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Arsenate and selenate scavenging by basaluminite: Insights into the reactivity of aluminum phases in acid mine drainage Sergio Carrero, Alejandro Fernandez-Martinez, Rafael Perez-Lopez, Agnieszka Poulain, Eduardo Salas-Colera, and Jose Miguel Nieto Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03315 • Publication Date (Web): 30 Nov 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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Environmental Science & Technology

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Arsenate and selenate scavenging by basaluminite: Insights into the

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reactivity of aluminum phases in acid mine drainage

3 Sergio Carrero a,*, Alejandro Fernandez-Martinez b,c, Rafael Pérez-López a, Agnieszka

4

Poulain d, Eduardo Salas-Colera e,f, José Miguel Nieto a

5 6 a

7

Department of Geology, University of Huelva, Campus ‘El Carmen’, 21071, Huelva, Spain b

8 c

9

Université Grenoble Alpes, ISTerre, F-38041 Grenoble, France d

10 e

11

ESRF, 71 avenue des Martyrs, F-38043 Grenoble, France

SpLine Spanish CRG Beamline, ESRF, 6 Rue Jules Horowitz, BP 220, Grenoble Cedex 09, France f

12 13 14

CNRS, ISTerre, F-38041 Grenoble, France

Instituto de Ciencia de Materiales de Madrid, CSIC, Sor Juana Inés de la Cruz 3, 28049, Cantoblanco Madrid, Spain.

15 16 Version to be submitted to Environmental Science & Technology, October 11, 2016.

17

*

Corresponding author. Tel.: +34-95-921-9682; fax: +34-95-921-9810

E-mail address: [email protected] (S. Carrero)

1 ACS Paragon Plus Environment


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Abstract

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Basaluminite precipitation may play an important role in the behavior of trace elements in water and

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sediments affected by acid mine drainage and acid sulfate soils. In the present study the affinity of

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basaluminite and schwertmannite for arsenate and selenate is compared, and the coordination geometries

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of these oxyanions in both structures are reported. Batch isotherm experiments were conducted to

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examine the sorption capacity of synthetic schwertmannite and basaluminite and the potential competitive

25

effect of sulfate. In addition, synchrotron-based techniques such as differential pair distribution function

26

(d-PDF) analysis and extended X-ray absorption fine structure (EXAFS) were used to determine the local

27

structure of As(V) and Se(VI) complexes. The results show that oxyanion exchange with structural sulfate

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was the main removal mechanism for selenate, whereas arsenate was removed by a combination of

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surface complexes and oxyanion exchange. Arsenate adsorption capacity by basaluminite was twice

30

higher than by schwertmannite and three times higher than selenate in both phases. The exchange ratios

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were 1:2 and 1:1 sulfate with respect to arsenate and selenate, respectively. High sulfate concentrations in

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the solutions did not show a competitive effect for arsenate sorption capacity, but had strong impact in the

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selenate uptake, suggesting some kind of specific interaction for arsenate. Both d-PDF and EXAFS

34

results indicated that bidentate binuclear inner-sphere was the most probable type of ligand for arsenate

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on both phases and for selenate on schwertmannite, whereas selenate forms outer-sphere complexes in the

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aluminum octahedral interlayer of basaluminite. Overall, these results show a strong affinity of poorly-

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crystalline aluminum phases such as basaluminite towards As(V) and Se(VI) oxyanions, with adsorption

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capacities on the same order of magnitude as iron oxides. The results obtained in this study are relevant to

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the understanding of trace element behavior in environments affected by acid water, potentially opening

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new research lines focused on remediation by natural attenuation processes or engineered water treatment

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systems.

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Keywords: basaluminite, adsorption, arsenate, selenate, EXAFS, d-PDF

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Table of Contents (TOC) art

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1. Introduction

47 48

Basaluminite [Al4OH10(SO4)·3-5H2O] is a nanocrystalline Al-hydroxysulfate commonly found in

49

Al-bearing acid mine drainage (AMD) systems.1–3 Farkas & Pertlik4 defined basaluminite as the

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nanocrystalline variety of felsöbányaite [Al4OH10(SO4)·5H2O]. However, basaluminite can be found in

51

AMD systems and its thermodynamic properties have been broadly described in the geochemical

52

literature and thermodynamic databases, whereas felsöbányaite is considered a rare mineral.4

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Basaluminite precipitates from acid SO4-Al-rich solutions with low pH values, such as water and

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sediments affected by (1) AMD in coalfields areas5,6 and massive sulfide ore deposits;7 and (2) acid

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sulfate soil (ASS) waters.8 This precipitation releases protons and is an effective buffer for the system at a

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pH range from 4.5 to 5.5.2,9,10 High Fe concentrations are also present in these polluted environments,

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which are controlled by the precipitation of oxy-hydroxides and oxy-hydroxysulfates such as

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schwertmannite [Fe8O8(OH)(8-x)(SO4)x·nH2O; with x varying from 1 to 1.75], goethite [FeOOH] or

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jarosite [KFe3(OH)6(SO4)2].2,11 Schwertmannite is described as the nanocrystalline Fe-oxyhydroxysulfate

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precursor of the other minerals.2,12–15 Schwertmannite precipitates at a lower pH than basaluminite (2.5-

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3.2).10

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As(V) and Se(VI) represent a serious environmental problem in watercourses affected by oxidation

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of iron sulfides ores.16,17 Both toxic elements coexist in streams, groundwater and sediments affected by

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AMD and ASS.16–18 The acid waters in these environments contribute to the high mobilization of both

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elements. Precipitation of nanocrystalline Al and Fe minerals in AMD- and ASS-affected environments

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has a significant role in the behavior of trace elements.2,13 Several laboratory experiments have reported

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that schwertmannite has a high affinity by As, where the sorption capacity of As(V) is higher than that of

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As(III) at low pH,15,19,20 and Se(VI).21 Much less is known about the capacity of basaluminite for

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removing trace elements. Basaluminite has been found to be an efficient adsorbent of elements such as Cu

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and Si in polluted waters.2,9,22 Recently, a significant affinity of basaluminite towards As(V) was

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described in neutralization experiments using Fe(II)-rich AMD solutions under reducing conditions.10 In

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oxic systems, upon AMD neutralization, the precipitation of schwertmannite removes As(V) and Se(VI)

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from the solution at a pH ~3.5, which is lower than that of the formation of basaluminite (pH ~ 5),

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masking the basaluminite affinity towards these trace elements. However, basaluminite could be an

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important scavenger of these elements in areas where Fe concentration in solution is much lower than Al,


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or where all iron is as Fe(II), in solution at pH around 5, avoiding the precipitation of Fe-phases. Such

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conditions often occur in the bottom waters of stratified reservoirs affected by AMD,23 in ASS with low

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redox potenctial,8 or in acid waters with low or negligible Fe concentrations in solution.24

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The high affinity by trace elements of both Al- and Fe-mineral phases seems to be related to mineral

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properties such as low crystallinity, small particle size (in the nanometer scale) and large specific surface

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areas. However, both phases have been defined as metastable minerals which can undergo mineralogical

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transformation during ageing processes, and thus potentially releasing previously sorbed oxyanions, and

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inhibiting their uptake due to the higher crystallinity of the resulting phases.4,13,15,25 Trace element

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sorption onto Fe-phases has been largely studied using wet chemistry and synchrotron radiation-based

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techniques such as extended X-ray absorption fine structure (EXAFS) and Fourier transform of total X-

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ray scattering (pair distribution function; PDF).19,26,27 Whereas As(V) surface adsorption, coprecipitation

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and other complex sorption processes onto Fe-oxide phases present in AMD are well documented,26,28–31

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the reactivity of poorly–crystalline Al minerals in these environments remains less understood. To our

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knowledge, there are no geochemical and structural studies on the adsorption capacity and other retention

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mechanisms of As(V) and Se(VI) by basaluminite.

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To fill this gap in our knowledge of the reactivity of Al-phases in acid waters, the present study

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examines the adsorption capacity of As(V) and Se(VI) onto synthetic basaluminite and schwertmannite as

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a function of arsenate and selenate loadings. The solid phases resulting from these experiments were

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examined using high energy X-ray diffraction (HEXD) and EXAFS. These results were compared with

95

previous values reported in scientific literature.

96 97

2. Materials and methods

98 99

2.1. Solid synthesis

100 101

Both As(V) and Se(VI) adsorption experiments were carried out with synthetic basaluminite and

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schwertmannite. Basaluminite was synthesized by addition of 214 mL of a 0.015 mol L-1 Ca(OH)2

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solution to 30 mL of 0.05 mol L-1 Al2(SO4)3·18H2O, according to the method described by Prietzel and

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Hirsch.32 The titration was conducted by drop-by-drop addition and continuous stirring at room

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temperature. Synthetic schwertmannite was precipitated using the procedure described by Adams &



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Rawajfih,8 where 2.506 g of Fe2(SO4)3, previously dehydrated, was added to 1 L of Milli-Q water,

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preheated at 85º C, and stirred for 1 h. Both precipitates were recovered filtering the suspension through a

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0.45 µm nylon membrane filter and the solids were washed several times with pure water.

109 110

2.2. Adsorption experiments

111 112

Adsorption isotherms were performed following the procedure previously described by Asta et al.25

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Batch experiments were performed by the reaction of 20 mL of a As(V) or Se(VI)-doped solutions with

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0.05 g of solid at pH 3.5 ± 0.1 and 5.0 ± 0.1 for schwertmannite and basaluminite, respectively, in high-

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density polyethylene plastic vials. The suspensions were continuously stirred at room temperature (24 ºC)

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and in dark during 72 h. As(V) and Se(VI) uptake by basaluminite and schwertmannite were

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characterized by: (1) the adsorption kinetics at initial concentrations of 3.0 mmol L-1 of As(V) or Se(VI)

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at different times (5, 15, 30 min, 1, 3, 5, 10, 24, 48 and 72 h); (2) the adsorption capacities at different

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initial As(V) and Se(VI) concentrations (3·10-2, 5·10-2, 0.1, 0.4, 0.8, 1.0, 3.0, 5.0, 7.0 and 10 mmol L-1)

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without competitive effect; and (3) same as (2) but in the presence of different SO42- concentrations (5.0,

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8.0, 10, 50 and 100 mmol L-1) for constant As(V) and Se(VI) concentrations (1.0 mmol L-1). Details of

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the adsorption experiment and isotherm models are described in the Section S1 of the Supporting

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Information (SI).

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After the equilibrium period, the samples were centrifuged and the supernatant was filtered through

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0.2 µm nylon membrane filters, acidified with HNO3 (65 %) to pH < 1 and stored at 4º C for further

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chemical analysis. The composition of the starting solid was calculated by acid digestion of 0.05 g of

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solid in HNO3 (65%) and recovered with 50 mL of Milli-Q ultrapure water for further chemical analysis.

128

After adsorption experiments each solid was washed several times with Milli-Q ultrapure water and dried

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at 35 ºC for 48 h for subsequent PDF and EXAFS analysis.

130 131

2.3. Analytical techniques

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Both As(V) and Se(VI) solutions, before and after each adsorption experiment, and solid digestions

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of both starting precipitates, were analyzed for Fe, Al, As, Se and S by inductively coupled plasma atomic

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emission spectrometry (ICP-AES Thermo Jarrel-Ash) in the laboratories of the IDAEA (CSIC) in


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Barcelona. Three blanks and three duplicates were analyzed every 20 samples to check the analytical

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accuracy. Detection limits were: 140 µg L-1 for Al and Fe; 15 µg L-1 for As and Se; 300 µg L-1 for S; and

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the analytical error was lower than 5%. In addition, the saturation index of the solid phases and aqueous

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speciation of solutions was calculated by using the PHREEQC code33 with the Mintq.v4 thermodynamic

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database34, which was enlarged with data from Bigham et al.11 to account for schwertmannite solubility.

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Schwertmannite and basaluminite precipitates were lyophilized using a VirTis Benchtop freeze-

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dryer (Hucoa-Erlöss, Spain) in order to obtain a dry powder. The water proportion in both phases was

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calculated by thermogravimetric analysis (TGA) using a TGA92-12 SETARAM, with a N2 flow of 1.8

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l/h. The BET-determined surface area of schwertmannite and basaluminite was measured using 5-point

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N2 adsorption isotherms with a Micromeritics ASAP 2000 surface area analyzer. HEXD and PDF were

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performed at the beamline ID31 at the European Synchrotron Radiation Facility (ESRF). Differential pair

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distribution functions (d-PDF) were obtained by subtracting a reference PDF of the pure material

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(synthetic As(V)- or Se(VI)-free basaluminite and schwertmannite) from the PDFs of the samples

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recovered after adsorption experiments. Retention models of As(V) and Se(VI) adsorbed onto both

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structures were constructed from the structure proposed by Fernandez-Martinez et al.14 and Farkas &

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Pertlik4 for schwertmannite and basaluminite, respectively. Partial pair distribution functions of As(V)

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and Se(VI) located in different structural positions were calculated using PDFgui software.35

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EXAFS data were collected at the Spanish CRG beamline BM25A at the ESRF. Same structural

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models used for PDF analyses were refined in the EXAFS data analysis. Debye-Waller factors,

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interatomic distances, coordination numbers and Fermi energy levels were fitted using a least square

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refinement algorithm. Statistical F-tests36,37 were applied to determine the statistical significance of

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different tested hypothesis involving different number of shells added to the models. Only those models

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which improved the fit between theory and experimental EXAFS at the 90% level of confidence were

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selected. Details of the synchrotron experiment are described in the Section S2 of the SI.

160 161

3. Results and discussion

162 163

3.1. Phase characterization

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The HEXD analysis of the starting solids confirmed the nature of schwertmannite and basaluminite

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(see Fig. S1 of the SI). The diffraction pattern of schwertmannite was identical to that described in

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previous studies (e.g. Fernandez-Martinez et al., 2010). Also, the basaluminite pattern presented broad

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diffraction peaks at 1.80º, 3.58º and 5.5º 2θ angle, as those described by Farkas & Pertlik.4 Surface areas

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for schwertmannite and basaluminite were of 42.3 and 80.6 m2 g-1 respectively. The value for

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schwertmannite was within the values reported by Burton et al.,19 Paikaray et al.,38 and Antelo et al.20 The

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value for basaluminite matches values found for similar Al-hydroxysulfates.39

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Chemical analysis and TGA data reveal a unit cell formula of schwertmannite of

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Fe8O8(OH)4.15(SO4)1.92-8.95H2O and Al4(OH)9.02(SO4)1.49-4.56H2O for basaluminite. In both cases, sulfate

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concentration was slightly higher than in previously reported chemical formulas of synthetic

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schwertmannite11 and basaluminite,3 and within the sulfate concentration range of natural samples.40–42

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High sulfate concentration in schwertmannite have been related to a significant amount of outer-sphere

177

complexes.14 Although less is known about basaluminite, this phase has been described as a

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nanocrystalline variety of felsöbányaite, where all the sulfate is located in the interlayer, in an outer-

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sphere position (i.e., keeping its surrounding hydration layer).4

180 181

3.2. Sorption kinetics

182 183

Speciation analyses with the PHREEQC code result in H2AsO4- (arsenate) and SeO42- (selenate) as

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the dominant species with proportions higher than 90% within the 3-5 pH range. The time necessary to

185

reach equilibrium between the solid phases and As(V)- or Se(VI)-rich solutions is shown in Figure S2 of

186

the SI. The adsorption kinetics of both oxyanion onto schwertmannite and basaluminite show that, in all

187

cases, the equilibrium was reached before 72 h. In addition, Fe and Al concentrations in solution during

188

equilibrium were always lower than 0.15 mmol L-1 and 0.99 mmol L-1, respectively, indicating a

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maximum dissolution of solid phases of 2% and 4.5% for schwertmannite and basaluminite, respectively.

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Accordingly, a reaction time of 72 h was considered enough to reach the equilibrium between solid and

191

liquid phases without excessive solid dissolution, in agreement with other kinetic studies previously

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reported by Burton et al.,19 and Antelo et al.20

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To check for possible dissolution of the two solids and precipitation of other phases, saturation

194

indexes of solutions with respect to As(V), Se(VI), Al and Fe(III)-bearing phases were determined by


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using the PHREEQC code (see Table S1 of the SI). The results show that the solutions were

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undersaturated with respect to all As(V) and Se(VI)-bearing phases and oversaturated with respect to

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schwertmannite and basaluminite. High-energy X-ray diffraction patterns confirmed the purity of the

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systems under study (Fig. S1), with no noticeable peaks from other phases.

199 200

3.3. Oxyanion uptake from solution

201 202

Adsorption isotherms for arsenate and selenate in schwertmannite and basaluminite are shown in

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Figure 1 (see all fitted parameters Langmuir isotherms in the Table S2 of the SI). The experimental data

204

was fitted using a non-competitive Langmuir isotherm. Sulfate concentrations in equilibrium can be

205

related to the substitution of structural sulfate by both oxyanions and OH- ions. Given that anion exchange

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was considered as a possible scenario in the adsorption processes (exchange isotherms are showed in

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Figure S3 of the SI),19 As(V)- and Se(VI)-free blank solutions were also prepared in order to account for

208

sulfate substitution by OH- ions. The correlation between corrected sulfates concentration in solution with

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respect to oxyanion adsorbed on the solid are shown in Figures 2.

210 211

3.3.1. Arsenate adsorption onto solid phases

212 213

The adsorption isotherms of arsenate in both phases were fitted using a Langmuir model, with molAl-1) than in schwertmannite (128

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higher arsenate concentrations in basaluminite (326 mmol

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mmolH2AsO4 molFe-1) (Fig. 1). The values obtained in this study for arsenate adsorbed onto schwertmannite

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were within the range previously reported for this phase,19,20 and were higher than values reported for Fe-

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phases such as hydrous ferric oxide, magnetite,43 goethite, jarosite,25 and Al-phases such as ɣ-alumina.44

H2AsO4

218

Ion exchange seems to be one of the main processes controlling the oxyanion adsorption in both

219

solid phases. Exchange isotherms show that, at equilibrium, around 35% and 50% of initial structural

220

sulfate in schwertmannite and basaluminite, respectively, was substituted by arsenate (Fig. S3). In

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addition, exchange coefficients (Rex) were obtained as the slope of the linear regression of the relation

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between desorbed sulfate (mmolSO4 mol-1Fe/Al) and adsorbed arsenate (mmolH2AsO4 mol-1Fe/Al) (Fig. 2). A

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value of Rex close to 1 mmolSO4 mmol-1XO4 can be interpreted as complete substitution of the structural

224

sulfate by an equivalent charged oxyanion through an ion exchange mechanism. Values lower than 1



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mmolSO4 mmol-1XO4 would indicate only a partial substitution. The Rex values for arsenate were 0.48 and

226

0.41 in schwertmannite and basaluminite, respectively. These Rex values are in agreement with the anion

227

charge: arsenate (H2AsO4-) has exactly half the negative charge than sulfate (SO42-), therefore, two

228

arsenates were adsorbed per every sulfate removed from the solid phases.

229 230

3.3.2. Selenate adsorption onto solid phases

231 232

Equilibrium selenate concentrations removed by both solid phases were lower than for arsenate (Fig.

233

1). The Langmuir model shows a good agreement with the experimental data and maximum selenate

234

adsorption capacity in basaluminite (122 mmolSeO4 molAl-1) was lower than in schwertmannite (153

235

mmolSeO4 molFe-1). Sorption capacity of selenate onto schwertmannite was higher than previous values

236

reported for other iron phases for both selenite and selenate.30,45,46 Furthermore, basaluminite shows also

237

higher affinity for selenate than other Al-oxyhydroxides.47

238

The exchange isotherm revealed that structural sulfate was replaced by selenate up to a 65% and

239

45% in schwertmannite and basaluminite, respectively, without reaching a steady state in the maximum

240

exchange percent for the concentration range used in this experiment (Fig. S3). Moreover, the Rex value

241

was around 1 mmolSO4 mmol-1SeO4 in both phases, indicating that all selenate adsorbed from the solution

242

was removed by ionic exchange mechanisms in a stoichiometric proportion (Fig. 2). Relative high sulfate

243

content in the mineral structure has been associated with a greater presence of outer-sphere complexes

244

onto the structure of schwertmannite14 and basaluminite,4 where sulfate could be easily removed due to its

245

low energetic stability.

246 247

3.4. Sulfate competition

248 249

Percentages of arsenate and selenate adsorbed on both solid phases at different solution initial

250

sulfate concentrations are given in Table S3 of the SI. Arsenate sorption on both solids was complete

251

regardless of the sulfate concentration. On the other hand, selenate sorption onto both phases decreased as

252

sulfate concentration increased in solution (Table S3). These results indicate that both phases present

253

higher affinity for arsenate than for selenate. A similar competitive effect for arsenate on schwertmannite

254

has been reported by Paikaray et al.38 and Song et al.48 where arsenate removal from the solution showed


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a weak dependence with the sulfate concentration in solution. On the contrary, other studies have shown

256

that when arsenate and sulfate containing solutions are put in contact with minerals with exchangeable

257

structural sulfate groups (e.g. jarosite), the total adsorption was lower than in minerals without structural

258

sulfate groups (e.g. goethite). This is due to sulfate in solution competing with the exchangeable sites of

259

jarosite, and to the high affinity of arsenate to specific binding sites on goethite surface sites.25,27,49

260

Nevertheless, the isotherm adsorption carried out in this study indicated that arsenate could enter in the

261

structure by oxyanion exchange and surface complexation mechanisms, where the ion exchange

262

accounted for the 50% of the total arsenate removed. On the other hand, the isotherms showed that

263

selenate uptake can be explained by just an ion exchange process.

264 265

3.5. Oxyanion adsorption mechanisms

266 267

3.5.1. Differential PDF

268 269

Differential PDFs (d-PDFs) showing the short-range order around both oxyanions and semi-

270

quantitative structural models carried out for arsenate and selenate in the nanostructure proposed by

271

Farkas and Pertlik4 for felsöbányaite and by Fernandez-Martinez et al.14 for schwertmannite are shown in

272

Figures 3 and 4, respectively. Experimental d-PDFs were compared with modeled ones generated from

273

different molecular models, including surface complexes with (i) a monodentate ligand, (ii) a bidentate

274

binuclear ligand, (iii) a bidentate mononuclear ligand, and (iiii) electrostatic complex in outer-sphere

275

position (Fig. 3c and 4c).

276

Basaluminite As(V) d-PDF showed a peak at 1.67 Å corresponding to As-O distances in arsenate26,27

277

(Fig. 3a). In addition, a second shell at 3.15 Å was observed, which is in agreement with previous As-Al

278

distances described in adsorption experiments on Al-phases44,50 (Fig. 3a). This second neighbor was less

279

clear, with a background close to the signal intensity, due to the low atomic number of Al. The agreement

280

between the relative intensities of As-O and As-Al distances indicates that arsenate was bound through a

281

bidentate binuclear inner-sphere ligand (Fig 3d). On the other hand, the Se(VI) d-PDF showed only a

282

clear first shell at 1.63 Å. (Fig.3b). This was attributed to the formation of electrostatically bound outer-

283

sphere complexes. A weak peak at 4 Å was associated with the Se-O distance with O located in the

284

octahedral layer (Fig. 3e). Finally, both As(V) and Se(VI) on basaluminite d-PDFs showed two negative



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285

peaks at 1.46 Å and 2.38 Å, whose intensities increased concomitantly with the oxyanion concentration

286

into the solids. These peaks can be attributed to S-O and O-O distances of exchanged sulfate tetrahedral,

287

confirming that anion exchange is one of the mechanisms leading to the retention of both oxyanions (Fig.

288

3a and b). Note that surface complexation is also observed for arsenate from isotherm experiments.

289

Two main peaks can be observed in the arsenate on schwertmannite d-PDF at 1.67 Å and 3.28 Å

290

(Fig. 4a). The first As(V) correlation can be assigned to the As-O distance. The second As(V) peak was at

291

similar distance to the As-Fe pair found in previous d-PDF experiments of As(V) sorption onto Fe-

292

phases,26 and it matches As-Fe interatomic distances obtained from EXAFS studies of arsenate sorption

293

onto Fe oxyhydroxides.27,28,49 The relative intensity between As-O and As-Fe peaks indicated that, at low

294

As(V) concentration in solution, arsenate formed bidentate binuclear inner-sphere ligands, whereas at

295

concentration higher than 10 mmol L-1, 50% of oxyanions were located in bidentate binuclear inner-

296

sphere and 50% in outer sphere position (Fig 4d). In the case of selenate, two peaks at 1.63 Å and 3.25 Å

297

relative to the Se-O and Se-Fe pairs respectively can be observed (Fig. 4b).21,30 Analysis of the relative

298

peak intensities of the modeled and experimental d-PDFs yield a 25% of selenate with a bidentate

299

binuclear inner-sphere ligand and a 75% in outer-sphere position (Fig. 4e). Similarly to basaluminite, the

300

oxyanion adsorption was concomitant to sulfate desorption (negative peaks at 1.46 Å and 2.38 Å). In

301

addition, d-PDFs from schwertmannite showed negatives peaks at 1.98 Å, 3 Å (Fig 4a and b), 4.73 Å, 5.4

302

Å and 6.35Å (not show in Fig. 4), mainly in the case of arsenate adsorption. These peaks are coincident

303

with interatomic distances of Fe-O (first shell), Fe-Fe (first shell), Fe-O (second shell), Fe-Fe (second

304

shell) and Fe-Fe (third shell), respectively.14 The presence of these peaks indicates that structural changes

305

(e.g., Fe-vacancies, angular and/or longitudinal Fe-O bond distortions) are taking place in schwertmannite

306

concomitantly to oxyanion adsorption. Similar type of structural distortions have been observed in

307

manganese oxide nanoparticles as a result of ion adsorption.51

308 309

3.5.2. As(V) and Se(VI) K-edge EXAFS

310 311

As(V) and Se(VI) K-edge EXAFS in high loaded samples, and the parameters of the best structural

312

models tested are shown in Figures 5 and Table 1. As(V) EXAFS in basaluminite was characterized by

313

the presence of two shells at 1.69 ± 0.01 Å and 3.19 ± 0.02 Å, that are attributed to As-O and As-Al

314

distances (Fig. 5a). Peaks attributed to As-O distance were consistent with undistorted arsenate


Page 13 of 29


315

tetrahedron with coordination number of 4.06 ± 0.03. Three different models were considered during the

316

fitting process: (i) an arsenate in outer-sphere position (just one shell); (ii) a surface complex with a

317

monodentate ligand; and (iii) a surface complex with a bidentate binuclear ligand. This third model, with

318

an As-Al coordination number of 1.64 ± 0.05 (bidentate binuclear) yields the best fit, and a 99.8% of

319

confidence in the F-test (see all parameters used for the EXAFS models and F-tests in the Table S4 to S7

320

of the SI) (Table 1). On the other hand, Se(VI) EXAFS in basaluminite was best described as an outer

321

sphere complex, where only a peak at 1.65 ± 0.04 Å was observed and associated to the Se-O distance

322

with coordination number of 4.01 ± 0.03 (Fig. 5a). None of the complexes involving inner-sphere ligands

323

gave F-test values that were above the confidence level (Table S4 and S6). The low As-Al coordination

324

number does not totally agree with the d-PDF data for arsenate, suggesting that arsenate could also be

325

partially retained in hydrogen-bonded outer-sphere complexes. It is worth noting that a broad, medium-

326

range correlation can be observed in the d-PDF data of selenate in basaluminite at d ~ 4 Å, which matches

327

with the distance from the center of the interlayer to the Al-hydroxide layer (distance not visible in

328

EXAFS). This reinforces the conclusion that sulfate is exchanged by selenate occupying the same

329

structural position.

330

As(V) EXAFS in schwertmannite shows similar results to the d-PDF: A first interatomic distance

331

corresponding to an As-O shell at 1.69 ± 0.01 Å with a coordination number of 3.94 ± 0.04 and a second

332

shell corresponding to an As-Fe distance at 3.29 ± 0.02 Å (Fig. 5b). Again, three different structural

333

models were considered, including one in outer-sphere position, a complex with a 100% and 50% of

334

binuclear bidentate inner sphere ligand, and another one with monodentate inner sphere ligand. In this

335

case, the F-test yields similar values of statistical significance for the second neighbor shells, meaning

336

that both the 100% and 50% binuclear bidentate ligands are equally significant (Fig 5b) (Table S5 and

337

S7). In view of past investigations, the formation of binuclear complexes is privileged,28 although the

338

possibility that a monodentate ligand is present49 or even that different types of complexes (inner and

339

outer-sphere) are present simultaneously cannot be ruled out. Finally, selenate absorbed in

340

schwertmannite shows a similar distribution of interatomic distances in both EXAFS and d-PDF data,

341

with two shells centered at 1.65 ± 0.01 Å and 3.34 ± 0.02 Å, related to the pairs Se-O and Se-Fe,

342

respectively. A coordination number for the Se-Fe shell of 2 (fixed value) yielded a good fit, indicating

343

that selenate is covalently bonded through a bidentate binucleate inner-sphere ligand. Results from d-PDF

344

and EXAFS models of arsenate and selenate in schwertmannite were coincident in their structural



Page 14 of 29

345

location at high oxyanion loading. Both samples present around 50% of arsenate in bidentate binuclear

346

inner sphere position, whereas selenate shows different coordination with weaker bonds in basaluminite

347

than schwertmannite. In addition, the relative intensities of the peaks from the d-PDFs give information

348

about the proportion of complexes with inner and outer-sphere ligands. As(V) d-PDF and EXFAS models

349

showed that schwertmannite hosts arsenate in two different positions, with inner-sphere ligands forming

350

in first place and outer-sphere ligands forming subsequently.

351 352

3.6. Environmental implication

353 354

The formation of inner-sphere complexes with strong covalent bonds has important environmental

355

implications. Both basaluminite and schwertmannite are metastable solid phases which are exposed to

356

changes in field conditions and aging towards more stables phases with less retention capability. Covalent

357

bonding results in an increase stability of the oxyanion complexes, retarding release and enhancing

358

colloidal transport in streams affected by AMD and ASS, as well as delaying the solid transformation into

359

more stable phases.

360

Basaluminite was shown to be an effective scavenger of both oxyanions. Compared to

361

schwertmannite, it showed higher total retention values and proportions of As(V) inner-sphere ligands

362

and lower values of these two parameters for Se(VI). Schwertmannite adsorption capacity for arsenate

363

and selenate has been largely studied. It is considered one of the solid phases with higher capacity to

364

remove these species from environments affected by acid waters. However, this and other recent studies

365

have pointed to an important role of Al-phases in the metal behavior, due to Al-phases precipitation and

366

Al incorporation in other Fe-phases (i.e. ferrihydrite),52–54 with higher arsenate adsorption capacities than

367

the values previously reported for schwertmannite and Al-free ferrihydrite.19,20,26 It is expected that a

368

deeper understanding of the As and Se adsorption/coprecipitation mechanisms (including reduced

369

species) and of the basaluminite structure will ensure a better prediction of the transport and retention of

370

adsorbed elements during the aging of this still relatively unknown mineral.

371


Page 15 of 29


372

Associated information

373

Supporting Information. The supporting information contains 1) details of arsenate and selenate

374

adsorption experiments, indicating the equation employed to calculate the isotherm models (S1); 2)

375

details of EXAFS and HEXD synchrotron experiments (S2); 3) tables with the saturation index calculated

376

with PHREEQC code, the fitted parameters in Langmuir models, the results of arsenate and selenate

377

competition with sulfate, and the parameter and F-test results of the different models considered during

378

the EXAFS refinement; 4) five figures illustrating the details of the solid phases characterization,

379

adsorption and exchange processes and complementary information of EXAFS results.

380



Page 16 of 29

381

Acknowledgments

382

We would like to thank the Spanish Ministry of Economic and Competitiveness (EMPATIA,

383

CGL2013-48460-C2-R), the Regional Government of Andalucía (Spain; P12-RNM-2260), and a grant

384

from Labex OSUG@2020 (investissements d’avenir – ANR10 LABX56) for financial research support.

385

S. Carrero was supported by a research predoctoral fellowship AP2010-2117 (Spanish Ministry of

386

Education, Spain). R. Pérez-López also thanks the Spanish Ministry of Science and Innovation and the

387

‘Ramón y Cajal Subprogramme’ (MICINN-RYC 2011). ESRF data were acquired through inhouse

388

research program at ID31 and during the experiment 25-01-976 at BM25A respectively. Chemical

389

analyses were performed at the CIDERTA Research Institute of the University of Huelva and at the

390

laboratories of IDAEA (CSIC) in Barcelona. We would also like to thank Dr. Li Xiang-Dong (Associate

391

Editor) and three anonymous reviewers for comments that significantly improved the quality of the

392

original paper.

393


Page 17 of 29


394

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541 542



543

Page 22 of 29

Table captions

544 545

TABLE1: Modeling parameter for As(V) K-edge EXAFS in 100% bidentate binucleate inner-sphere

546

ligand (Bas-As) and Se(VI) K-edge EXAFS 100% outer-sphere ligand (Bas-Se) in basaluminite; and

547

As(V) (Sch-As) and Se(VI) (Sch-Se) K-edge EXAFS in 100% bidentate binucleate inner-sphere ligand

548

schwertmannite.

549


Page 23 of 29


550

Figure captions

551 552

FIGURE 1: Adsorption isotherm of arsenate (□) and selenate (∆) (mmolXO4 molFe/Al-1) onto (a)

553

schwertmannite and (b) basaluminite. Curves obtained using the Langmuir equations are drawn for

554

comparison with the experimental data. The experiment was conducted at ionic strength of 100 mmol L-1,

555

3·10-2 to 10 mmol L-1 oxyanion concentration and 2.5 g L-1 of solid phase.

556 557 558

FIGURE 2: Relationship between sulfate release from the solid phases (mmolSO4 molFe/Al-1) and adsorbed arsenate (□) or selenate (∆) from the solution (mmolXO4 molFe/Al-1).

559 560

FIGURE 3: The d-PDF of (a) arsenate and (b) selenate onto basaluminite in the sample loaded with

561

1, 10, 30, 50 mmol L-1 of As(V) and Se(VI). Models of (c) basaluminite structure reported by Farkas and

562

Pertlik (1997) doped with arsenate or selenate located in: I) monodentate inner-sphere, II) bidentate inner-

563

sphere, III) bidentate mononuclear inner-sphere and VI) outer-sphere theoretical position; and the most

564

probable structural location of (d) arsenate and (e) selenate onto basaluminite. The signal intensity was

565

normalized by the maximum of Al-O distance.

566 567

FIGURE 4: The d-PDF of (a) arsenate and (b) selenate onto schwertmannite in the sample loaded

568

with 1, 10, 30, 50 mmol L-1 of As(V) and Se(VI). Models of (c) schwertmannite structure by Fernandez-

569

Martinez et al. (2010) doped with arsenate or selenate located in I) monodentate inner-sphere, II)

570

bidentate inner-sphere, III) bidentate mononuclear inner-sphere and VI) outer-sphere theoretical position;

571

and the most probable structural location of (d) arsenate and (e) selenate onto schwertmannite. The signal

572

intensity was normalized by the maximum of Fe-O distance.

573 574

FIGURE 5: Fourier transform amplitude of EXAFS spectra at the As(V) and Se(VI) K-edge

575

adsorbed onto (a) basaluminite and (b) schwertmannite in the samples loaded with 10 mmol L-1 of As(V)

576

and Se(VI). The surface coverage in each case was 2.27·10-2 mmol m-2; 2.25·10-2 mmol m-2; 3.34·10-2

577

mmol m-2 and 1.22·10-2 mmol m-2 for As-sch, Se-sch, As-bas and Se-bas, respectively. Experimental and

578

fitted curves are displayed in black and red colors, respectively.



Page 24 of 29

TABLE1: Modeling parameter for As(V) K-edge EXAFS in 100% bidentate binucleate inner-sphere ligand (Bas-As) and Se(VI) K-edge EXAFS 100% outer-sphere ligand (Bas-Se) in basaluminite; and As(V) (Sch-As) and Se(VI) (Sch-Se) Kedge EXAFS in 100% bidentate binucleate inner-sphere ligand schwertmannite.

σ2

R

ΔE0

V

4.064 ± 0.031 12.19 ± 0.031

0.0022 ± 0.0002 0.0033 ± 0.0003

1.688 ± 0.002 3.119 ± 0.085

6.931 ± 1.027

10 1909

Shell 3

1.639 ± 0.524

0.0063 ± 0.0029

3.191 ± 0.024

Se-OT

One shell

4.004 ± 0.032

0.0019 ± 0.0002

1.648 ± 0.036

8.061 ± 0.614

13 1273

As-OT

Shell 1

3.936 ± 0.035

0.0020 ± 0.0002

1.696 ± 0.002

7.444 ± 0.625

10 419.7

As-O-O Shell 2 As-Febi Shell 3

11.81 ± 0.035 1.792 ± 0.031

0.0029 ± 0.0003 0.0070 ± 0.0019

3.158 ± 0.048 3.295 ± 0.017

Se-OT

3.704 ± 0.031

0.0017 ± 0.0002

1.648 ± 0.002

8.114 ± 0.677

10 442.3

2 (fixed)

0.0098 ± 0.0026

3.341 ± 0.024

Model

Path

bas-As

As-OT Shell 1 As-O-O Shell 2 As-Albi

bas-Se Sch-As

Sch-Se

Neighbor N

Shell 1

Se-Febi Shell 2

Δχ2

V = independent variables 579

24

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FIGURE 1: Adsorption isotherm of arsenate (□) and selenate (∆) (mmolXO4 molFe/Al-1) onto (a) schwertmannite and (b) basaluminite. Curves obtained using the Langmuir equations are drawn for comparison with the experimental data. The experiment was conducted at ionic strength of 100 mmol L-1, 3•10-2 to 10 mmol L-1 oxyanion concentration and 2.5 g L-1 of solid phase. 197x91mm (300 x 300 DPI)



FIGURE 2: Relationship between sulfate release from the solid phases (mmolSO4 molFe/Al-1) and adsorbed arsenate (□) or selenate (∆) from the solution (mmolXO4 molFe/Al-1). 192x90mm (300 x 300 DPI)


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FIGURE 3: The d-PDF of (a) arsenate and (b) selenate onto basaluminite in the sample loaded with 1, 10, 30, 50 mmol L-1 of As(V) and Se(VI). Models of (c) basaluminite structure reported by Farkas and Pertlik (1997) doped with arsenate or selenate located in: I) monodentate inner-sphere, II) bidentate inner-sphere, III) bidentate mononuclear inner-sphere and VI) outer-sphere theoretical position; and the most probable structural location of (d) arsenate and (e) selenate onto basaluminite. The signal intensity was normalized by the maximum of Al-O distance. 164x183mm (300 x 300 DPI)



FIGURE 4: The d-PDF of (a) arsenate and (b) selenate onto schwertmannite in the sample loaded with 1, 10, 30, 50 mmol L-1 of As(V) and Se(VI). Models of (c) schwertmannite structure by Fernandez-Martinez et al. (2010) doped with arsenate or selenate located in I) monodentate inner-sphere, II) bidentate inner-sphere, III) bidentate mononuclear inner-sphere and VI) outer-sphere theoretical position; and the most probable structural location of (d) arsenate and (e) selenate onto schwertmannite. The signal intensity was normalized by the maximum of Fe-O distance. 165x182mm (300 x 300 DPI)


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FIGURE 5: Fourier transform amplitude of EXAFS spectra at the As(V) and Se(VI) K-edge adsorbed onto (a) basaluminite and (b) schwertmannite in the samples loaded with 10 mmol L-1 of As(V) and Se(VI). The surface coverage in each case was 2.27•10-2 mmol m-2; 2.25•10-2 mmol m-2; 3.34•10-2 mmol m-2 and 1.22•10-2 mmol m-2 for As-sch, Se-sch, As-bas and Se-bas, respectively. Experimental and fitted curves are displayed in black and red colors, respectively. 278x163mm (300 x 300 DPI)


Arsenate and Selenate Scavenging by Basaluminite: Insights into

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