T H Phan

Rizlan Bernier-Latmani

Delphine Tisserand

Fabrizio Bardelli

Pierre Le Pape

Manon Frutschi

Antoine Géhin

Raoul-Marie Couture

Laurent Charlet

Van T H Phan
email: phanhaivan@gmail.com

Antoine Gehin

As release under the microbial sulfate reduction during redox oscillations in the upper Mekong delta aquifers, Vietnam: A mechanistic study

Keywords: Redox oscillations, Arsenic release, Microbial sulfate reduction, Elemental sulfur

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Arsenic, a well-known environmental carcinogen, is subject to a variety of mobility-altering processes (WHO IARC, 2004). The World Health Organization's provisional guideline value for total As in drinking water is 10 μg/L. Concentrations of arsenic in groundwater higher than 50 μg/L have been reported in several Southeast Asian countries including Bangladesh, India, Cambodia, China, and Vietnam (e.g. [START_REF] Wang | Arsenic speciation in Mekong Delta sediments depends on their depositional environment[END_REF][START_REF] Phan | Arsenic in Shallow Aquifers Linked to the Electrical Ground Conductivity: the Mekong Delta Source Example[END_REF][START_REF] He | A review of arsenic presence in China drinking water[END_REF][START_REF] Fendorf | Arsenic chemistry in soils and sediments[END_REF][START_REF] Kocar | Integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the Mekong delta[END_REF][START_REF] Van Geen | Comparison of arsenic concentrations in simultaneously-collected groundwater and aquifer particles from Bangladesh, India, Vietnam, and Nepal[END_REF][START_REF] Zheng | Redox control of arsenic mobilization in Bangladesh groundwater[END_REF][START_REF] Buschmann | Contamination of drinking water resources in the Mekong delta floodplains: Arsenic and other trace metals pose serious health risks to population[END_REF]. In particular, the Mekong Delta floodplain, which stretches over 62100 km 2 in Southern Vietnam and South Cambodia, harbors numerous wells with elevated As concentrations [START_REF] Buschmann | Contamination of drinking water resources in the Mekong delta floodplains: Arsenic and other trace metals pose serious health risks to population[END_REF][START_REF] Nguyen | Late holocene depositional environments and coastal evolution of the Mekong River Delta, Southern Vietnam[END_REF]. A national survey, conducted by the Department of Water Resources Management monitored the As concentration in tube wells from 2002 to 2008, showed that large areas of the Mekong Delta were contaminated by arsenic, in nearly 900 wells [START_REF] Erban | Release of arsenic to deep groundwater in the Mekong Delta, Vietnam, linked to pumping-induced land subsidence[END_REF].

Areas with As concentrations in groundwater exceeding 1600 ppb are located along the main Mekong river branches, from An Giang to Can Tho City [START_REF] Wang | Arsenic speciation in Mekong Delta sediments depends on their depositional environment[END_REF][START_REF] Phan | Arsenic in Shallow Aquifers Linked to the Electrical Ground Conductivity: the Mekong Delta Source Example[END_REF][START_REF] Hanh | Community exposure to arsenic in the Mekong river delta, Southern Vietnam[END_REF][START_REF] Hoang | Arsenic in groundwater and sediment in the Mekong River delta, Vietnam[END_REF][START_REF] Nguyen | Source and release mechanism of arsenic in aquifers of the Mekong Delta , Vietnam[END_REF]. The origin of As in groundwater of the Mekong Delta is naturally produced by microbial reduction of arsenic-bearing iron oxides in the alluvial sediments [START_REF] Polya | Arsenic hazard in shallow Cambodian groundwaters[END_REF][START_REF] Rowland | The control of organic matter on microbially mediated iron reduction and arsenic release in shallow alluvial aquifers[END_REF][START_REF] Dongen | Hopane, sterane and n-alkane distributions in shallow sediments hosting high arsenic groundwaters in Cambodia[END_REF][START_REF] Kocar | Arsenic Release and Transport in Sediments of the Mekong Delta[END_REF][START_REF] Polizzotto | Near-surface wetland sediments as a source of arsenic release to ground water in Asia[END_REF][START_REF] Stuckey | Peat formation concentrates arsenic within sediment deposits of the Mekong Delta[END_REF][START_REF] Winkel | Arsenic pollution of groundwater in Vietnam exacerbated by deep aquifer exploitation for more than a century[END_REF]. Following river sediment transport, As bound to Fe-(oxyhydr)oxides is delivered to the delta from the Himalayan mountain range. Arsenic is then released to pore water due to the establishment of reducing conditions [START_REF] Berg | Magnitude of arsenic pollution in the Mekong and Red River Deltas -Cambodia and Vietnam[END_REF][START_REF] Harvey | Arsenic mobility and groundwater extraction in Bangladesh[END_REF][START_REF] Polizzotto | Processes conducive to the release and transport of arsenic into aquifers of Bangladesh[END_REF][START_REF] Kocar | Integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the Mekong delta[END_REF][START_REF] Nguyen | Source and release mechanism of arsenic in aquifers of the Mekong Delta , Vietnam[END_REF][START_REF] Stuckey | Peat formation concentrates arsenic within sediment deposits of the Mekong Delta[END_REF]. Natural organic matter (NOM) supplied from surface sedimentary deposits feeds the dissimilatory reducing bacteria by serving as electron donors that play the important role on As mobilization in this area [START_REF] Kocar | Integrated biogeochemical and hydrologic processes driving arsenic release from shallow sediments to groundwaters of the Mekong delta[END_REF][START_REF] Lawson | Pond-Derived Organic Carbon Driving Changes in Arsenic Hazard Found in Asian Groundwaters[END_REF][START_REF] Stuckey | Peat formation concentrates arsenic within sediment deposits of the Mekong Delta[END_REF][START_REF] Lawson | Tracing organic matter composition and distribution and its role on arsenic release in shallow Cambodian groundwaters[END_REF][START_REF] Magnone | Biomarkerindicated extent of oxidation of plant-derived organic carbon ( OC ) in relation to geomorphology in an arsenic contaminated Holocene aquifer , Cambodia[END_REF].

In nature, the seasonal reversal of flow direction -rivers to aquifers in the wet season and aquifers to rivers in the dry season -influences the redox balance of the aquifers [START_REF] Benner | Groundwater flow in an arsenic-contaminated aquifer, Mekong Delta, Cambodia[END_REF][START_REF] Stuckey | Peat formation concentrates arsenic within sediment deposits of the Mekong Delta[END_REF]. Redox oscillations occurring across the oxic-anoxic interfaces, which are frequent in periodically inundated swamplands due to the changes of the water levels, may affect the release of metal (loid)s (e.g., As, Fe) into the groundwater [START_REF] Karimian | Effect of cyclic redox oscillations on water quality in freshwater acid sulfate soil wetlands[END_REF]. The behavior and transport of As are closely related to the biogeochemical cycling of Fe, S and C (e.g. [START_REF] Bostick | Arsenite sorption on troilite (FeS) and pyrite (FeS2)[END_REF][START_REF] O'day | The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions[END_REF][START_REF] Root | Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer[END_REF]. Microbial sulfate (SO4 2-) reduction, couples with the oxidation of organic matter (OM) or H2, may also affect As behavior via the production of sulfide (S(-II)) [START_REF] Barton | Biotechnology Handbooks 8 -Sulfate-Reducing Bacteria[END_REF], Poulton et al., 2004;[START_REF] Rochette | Kinetics of arsenate reduction by dissolved sulfide[END_REF][START_REF] Muyzer | The ecology and biotechnology of sulphate-reducing bacteria[END_REF]. Under different conditions, HS -can immobilize As by the precipitation of As sulfides (e.g., orpiment (As2S3(s)) and realgar (AsS(s))) if the As concentration is high enough [START_REF] O'day | The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions[END_REF][START_REF] Root | Speciation and natural attenuation of arsenic and iron in a tidally influenced shallow aquifer[END_REF], or form reduced inorganic sulfur such as FeS2(s) and FeSm(s) [START_REF] Berner | Sedimentary Pyrite Formation[END_REF][START_REF] Rickard | The solubility of FeS[END_REF][START_REF] Rickard | Chemistry of Iron Sulfides[END_REF] onto which As can adsorb [START_REF] Wolthers | Arsenic mobility in the ambient sulfidic environment: Sorption of arsenic(V) and arsenic(III) onto disordered mackinawite[END_REF][START_REF] Couture | Sorption of arsenite, arsenate, and thioarsenates to iron oxides and iron sulfides: A kinetic and spectroscopic investigation[END_REF].

Arsenic can also co-precipitate with iron sulfide minerals to form arsenic-substituted pyrite, known as "arsenian pyrite" (e.g. Rittle et al., 1995;Newman et al., 1998;[START_REF] Charlet | Reactivity at (nano)particle-water interfaces, redox processes, and arsenic transport in the environment[END_REF][START_REF] Langner | Arsenic sequestration by organic sulphur in peat[END_REF][START_REF] Pape | Arsenic Incorporation in Pyrite at Ambient Temperature at Both Tetrahedral S-I and Octahedral FeII Sites: Evidence from EXAFS-DFT Analysis[END_REF][START_REF] Wang | Arsenic speciation in Mekong Delta sediments depends on their depositional environment[END_REF]. Lastly, arsenite is known to bind strongly onto sulfhydryl groups (thiols) of OM [START_REF] Thomasarrigo | Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland: Speciation Changes of Iron, Sulfur, and Arsenic[END_REF][START_REF] Langner | Spatial Distribution and Speciation of Arsenic in Peat Studied with Microfocused Xray Fluorescence Spectrometry and Xray Absorption Spectroscopy[END_REF]Couture et al., 2013a;[START_REF] Langner | Arsenic sequestration by organic sulphur in peat[END_REF]. The dominance of one mechanism over the other is determined by variety factors (e.g., aqueous As speciation, pH, Fe and S mineralogy) whose identification is complicated by dynamic conditions.

There is significant interest in elucidating the processes driving As mobility in sediments under sulfate reducing conditions and to understand hydrological transport controls on As fluxes from sulfate-rich sediments in the Mekong Delta [START_REF] Métral | Comparison of dissolved and particulate arsenic distributions in shallow aquifers of Chakdaha, India, and Araihazar, Bangladesh[END_REF][START_REF] Buschmann | Impact of sulfate reduction on the scale of arsenic contamination in groundwater of the Mekong, Bengal and Red River deltas[END_REF].

Nevertheless, experimental studies accurately isolating biogeochemical processes during the oscillating redox conditions in Vietnamese paddy fields and shallow aquifers are still scarce. In particular, the role of sulfate-reducing bacteria (SRB) and sulfate-oxidizing bacteria (SOB) in As immobilization in the sediment during redox dynamic conditions remain poorly defined at such sites. To determine the mechanisms controlling As mobility in sulfate-rich sediment of seasonally flooded soils, we investigated the As, Fe, and S species generated by oscillating redox conditions in laboratory experiments. We used automated bioreactors containing sediments with equal total As concentration and variable sulfate contents. A 16S rRNA amplicon analysis was performed to unravel the role of bacteria in the sediment through redox cycles and X-ray absorption near-edge spectroscopy (XANES) was exploited to reveal solid-phase As species.

Materials and methods

Field site characterization

The study area and sediment sampling

The seasonally flooded sediment was collected at the Quoc Thai commune, An Giang province, Vietnam, in a paddy soil near the Mekong River during the dry season, in 2014 (Fig. SI-1) where was contained a high As concentration in groundwater [START_REF] Wang | Arsenic speciation in Mekong Delta sediments depends on their depositional environment[END_REF][START_REF] Phan | Arsenic in Shallow Aquifers Linked to the Electrical Ground Conductivity: the Mekong Delta Source Example[END_REF]. A sediment core was drilled from a depth of 0 -20 m and separated into sections as a function of depth under an argon (Ar) flux and then immediately placed in the heat sealed Mylar ® bags under N2 atmosphere [START_REF] Wang | Arsenic speciation in Mekong Delta sediments depends on their depositional environment[END_REF]. The sediment samples were shipped in a cooler with ice back to France and stored at +4 o C until processing. Peat-rich sediment from 16m -deep layers was obtained for the batch experiments. All preparation and samples conditioning were performed in a Jacomex® under N2 atmostphere (O2 < 10 ppm) to ensure anoxic conditions.

Mineralogy and elemental composition analyses

The elemental composition was measured using the total acidic digestion (e.g. HNO3 + H2O2 + HF, H3BO3 + HF) followed by Inductively Coupled Plasma -Optical Emission Spectrometry (ICP-OES) (Agilent 720-ES, Varian) [START_REF] Cotten | Arsenic binding to organic and inorganic sulfur species during microbial sulfate reduction: a sediment flow-through reactor experiment[END_REF]. X-Ray diffraction (XRD) measurements were performed with CoKα radiation on a Panalytical ® X'Pert Pro MPD diffractometer using a Debye-Scherrer configuration using an elliptical mirror to obtain a high flux and parallel incident beam, and an X'Celerator ® detector to collect the diffracted beam. All sediment samples were transferred into glass capillaries inside a glovebox Jacomex® (O2 ≤ 10 ppm) to avoid oxidation by air.

Experimental design and redox oscillation set-up

Two batch bioreactors, R1 and R2, were prepared with 1L of sediment suspension containing 100g of dry sediment (<1 mm fraction), arsenite (As(III)), and two different concentrations of sulfate (SO4 2-). The contaminants were prepared by dissolving of sodium (meta)arsenite (NaAsO2, ≥ 98%, Sigma-Aldrich) and sodium sulfate (Na2SO4(s), ≥ 99%, anhydrous, granular, Sigma-Aldrich), to obtain a solution with final concentration of 50 μM of As(III), 0.1 and 1 mM of SO4 2-for R1 and R2, respectively. The suspensions were pre-equilibrated inside the glove box for 2 weeks before doing the redox experiments. More details of the reactor system were described in the previous studies (e.g. [START_REF] Parsons | The impact of oscillating redox conditions: Arsenic immobilisation in contaminated calcareous floodplain soils[END_REF][START_REF] Markelova | Speciation dynamics of oxyanion contaminants (As, Sb, Cr) in argillaceous suspensions during oxic-anoxic cycles[END_REF][START_REF] Phan | Interplay of S and As in Mekong Delta sediments during redox oscillations[END_REF]. At the start of the redox experiment (t = 0), two mixture solutions containing the same concentration of As(III) and two different concentrations of SO4 2-were added in the two reactors (R1 and R2). Cellobiose (C12H22O11 -Sigma-Aldrich), a byproduct of microbial hydrolysis of cellulose [START_REF] Lynd | Microbial Cellulose Utilization: Fundamentals and Biotechnology[END_REF][START_REF] Schellenberger | Functionally redundant cellobiose-degrading soil bacteria respond differentially to oxygen[END_REF], was manually replenished at the start of each anoxic half-cycles to obtain the final concentration of DOC equal to 8.33 mM including the DOC in sediments. All input solutions were adjusted for ionic strength (I = 30 mM) using sodium chloride (NaCl -Sigma-Aldrich). Reactors were covered with aluminum foil to avoid the photo-degradation of organic matter and contaminants.

Reduced -oxidized oscillations were carried out by automatically modulating the influx gas between the mixture of N2 and 300 ppm CO2 in anoxic half-cycle (7 days) except for the first one with 5 days, and compressed air of 1% CO2, 79% N2 and 20% O2 in oxic half-cycle (7 days) using an Agilent switching unit and a system of solenoid valves. Redox cycles were monitored for approximately 40 days. Gas flow rate and temperature were kept constant at 30 mL/min and 30 o C. Suspension samples were collected five days/week of each half-cycle for aqueous, solid phase and microbial analyses through a connection on the top of the reactors. Except for the days collecting the plus 5 mL slurry for microbial analysis, a 15 mL of suspension was collected in the other days.

These suspensions were centrifuged at 14000 rpm for 20 min (Sigma 3-30 KS) to separate the solid phase (e.g., XRD and XANES analysis), while the supernatant was then filtered through cellulose hydrophilic 0.22μm membrane (Chromafil RC, Roth). A part of supernatant was acidified with HNO3 (2%, Sigma-Aldrich) or HCl (0.1 M, Roth) and preserved at 4 o C for total element concentration and DOC analysis, respectively. A 2 mL of supernatant was stored at 4 o C without acidification for anion measurement. The rest of supernatant, slurry samples were flash frozen using liquid nitrogen and kept at -80 o C for another aqueous and microbial analysis.

Aqueous phase characterization

All standards and reagents, were prepared with ultra-pure water, and were of analytical grade from Fluka, Sigma-Aldrich or Merck. The glass and plastic parts were firstly washed with 5% HNO3 then with ultra-pure water (18 MΩ.cm -1 ). The Eh and pH data were recorded automatically every 10 seconds using an Aligent 34970A BenchLink Data Logger through the Eh and pH electrodes (Mettler-Toledo Xerolyt Solid). Measured Eh value was corrected for the reference electrode's voltage (+207 mV) relative to the standard hydrogen electrode (SHE), and corresponded to the Ag/AgCl (3 M KCl) reference electrode. The pH electrodes were calibrated using the measured data of three-point using pH buffers of 4, 7 and 10 at 25 o C at the start and the end of experiment displaying that the electrode response were not shifted significantly. The real pH values were then calculated based on the calibration curves.

Analysis of total cations, including trace and major elements, were performed with ICP-OES (Agilent 720-ES, Varian) after dilution and acidification by ultrapure HNO3 (2%, Sigma-Aldrich) with a detection limit of 0.02 and 0.1 mg. L -1 , respectively and a precision better than 5%. DOC was determined using a Shimazu VCSN analyzer (TOC-5000, Shimadzu) with a detection limit of 0.3 mg/L and a precision better than 2.5% for most samples. Nevertheless, it is important to note that for concentrations close to the detection limit, the precision can be significantly lower (50-100%). DOC tubes were heated at 400 o C for 3 hours avoiding organic carbon contamination. Non-acidified samples were used to analyze major anions (e.g. F -, Cl -, NO3 -, NO2 -, Br -, SO4 2-, PO4 3-) and acetate by ion chromatography using a Metrohm 761 compact ion chromatography with a detection limit of 0.1 mg. L -1 and a precision better than 5%. [Fe 2+ ] and [S(-II)] were determined photometrically on the sample filtered using the Ferrozine method [START_REF] Lovley | Organic Matter Mineralization with Reduction of Ferric Iron in Anaerobic Sediments[END_REF][START_REF] Viollier | The ferrozine method revisited: Fe(II)/Fe(III) determination in natural waters[END_REF][START_REF] Stookey | Ferrozine-a new spectrophotometric reagent for iron[END_REF] and Cline method [START_REF] Cline | Spectrophotometric determination of hydrogen sulfide in natural waters[END_REF], respectively. Standard stock solutions were prepared using Na2S*9H2O for the calibration curve of S(-II) and FeCl2*4H2O for the calibration curve of Fe 2+ . The standards and samples were then measured immediately using UV-Vis spectroscopy (Lambda 35, Perkin Elmer) at 562 nm and 664 nm absorbance for Fe 2+ and S(-II), respectively. Aqueous As species (e.g. As(III), As(V), MMA and DMA) were analyzed at Plateforme AETE -HydroScience/OSU OREME, Montpellier, France, using the coupling LC-ICP-MS system [START_REF] Bohari | Improvements of hydride generation for the speciation of arsenic in natural freshwater samples by HPLC-HG-AFS[END_REF]. The detail condition was described in [START_REF] Phan | Interplay of S and As in Mekong Delta sediments during redox oscillations[END_REF]. The detection limit was 0.09 μg/L for As(III), 0.06 μg/L for DMA, 0.04 μg/L MMA and 0.41 μg/L for As(V), with a precision better than 5%.

Microbial community analysis

The 16S rRNA gene diversity was accessed through paired-end Illumina MiSeq at Research and Testing Laboratory (Texas, USA) following the method of [START_REF] Zhang | PEAR: A fast and accurate Illumina Paired-End reAd mergeR[END_REF]. Slurry samples were collected at the end of the reduced cycle and in the middle of the oxidized cycle in both reactors (Fig. 1a-b). The overall approach entails 16S rRNA gene amplicon sequencing as a function of time and geochemical conditions. A 0.25 g suspension sample was used for total geonomic DNA (gDNA) extraction using the PowerSoil ® DNA Isolation kit (MO BIO Laboratories, Carlsbad, USA). The DNA quantification was determined using a Nanodrop 1000 Spectrophotometer (Thermo Fisher Scientific, USA). After the DNA quantification, the amplification of the 16S rRNA gene was followed by the polymerase chain reaction (PCRs) using the forward (28F, GAGTTTGATCNTGGCTCAG) and reverse (519R, GTNTTACNGCGGCKGCTG) primers that amplify the V1-V3 region [START_REF] Fan | Reconstruction of ribosomal RNA genes from metagenomic data[END_REF] PCR products were conceived on an agarose gel, and the 16S rRNA band excised and purified using the Wizard® SV Gel and PCR Clean-Up System (Promega, USA). The sequencing process of PCR 16S rRNA amplicons were merged, denosed, quality trimmed and demultiplexed using USEARCH [START_REF] Edgar | UPARSE: Highly accurate OTU sequences from microbial amplicon reads[END_REF]. These sequences were then clustered into operational taxanomic units (OTUs) using the UPARSE algorithm with a similarities higher than 97% [START_REF] Edgar | UPARSE: Highly accurate OTU sequences from microbial amplicon reads[END_REF], and then checked for Chimera detection using UCHIME (Edgar, 2011). A summarized database of high quality sequences derived from the NCBI database was described in the OTUs table (Table 1), and the full data shown in Table S-3 (Supporting Information).

Solid-phase characterization

X-ray absorption near edge structure (XANES)

Solid-phase samples were collected from the two reactors at the end of the redox cycles indicated in Fig. 1a-b. X-ray absorption near edge structure (XANES) was used to characterize both As and S speciation. S K-edge XANES data were collected at the XAFS beamline of the Elettra synchrotron (Trieste, Italy), whereas As K-edge XANES data were done at the bending magnet beamline BM08-LISA of the European Synchrotron Radiation Facility (ESRF, Grenoble, France).

Sulfur K-edge (2472 eV) measurements were performed using a Si(111) monochromator calibrated relative to the white line of a Na2S2O3(s) standard (Sigma-Aldrich) at 2471.64 eV. Spectra were collected in fluorescence mode using a solid-state Si detector. A commercial elemental sulfur reference was measured in fluorescence mode before each scan for accurate energy calibration. For the linear combination fitting (LCF) of XANES spectra at the S K-edge, the following suite of sulfur references was used: (i) FeS (mackinawite), synthesized under strict anoxic conditions with solutions obtained after dissolution of FeCl2*6H2O and Na2S*9H2O salts, (ii) pure FeS2 (pyrite) was synthesized according to the protocol described in Le [START_REF] Pape | Arsenic Incorporation in Pyrite at Ambient Temperature at Both Tetrahedral S-I and Octahedral FeII Sites: Evidence from EXAFS-DFT Analysis[END_REF], (iii) S8 (elemental sulfur, Sigma -Aldrich), and (iv) commercial K2SO4 (sulfate). Some of the reference compounds spectra (e.g. FeS, FeS2 and K2SO4) were acquired on the 4-3 beamline at SSRL in the fluorescence mode.

For LCF of XANES spectra at the As K-edge, the following set of arsenic references compounds was considered: (i) NaAsO2 and NaHAsO4 (Sigma-Aldrich), MTA(V) (monothioarsenate), DTA(V) (dithioarsenate), and TeTA(V) (tetrathioarsenate), prepared using the method of [START_REF] Schwedt | Separation of thio-and oxothioarsenates by capillary zone electrophoresis and ion chromatography[END_REF] and optimized by [START_REF] Suess | Discrimination of thioarsenites and thioarsenates by X-ray absorption spectroscopy[END_REF]; (ii) As-bearing minerals from natural deposits were considered such as orpiment (As2S3), realgar (AsS), arsenian pyrite (As-substituted FeS2), and arsenopyrite (FeAsS) (Le [START_REF] Pape | Arsenic Incorporation in Pyrite at Ambient Temperature at Both Tetrahedral S-I and Octahedral FeII Sites: Evidence from EXAFS-DFT Analysis[END_REF], (iii) As(III)-and As(V)-sorbed ferrihydrite were used as proxies for As(III)-O and As(V)-O local molecular environments [START_REF] Hohmann | Molecular-level modes of As binding to Fe (III) (oxyhydr) oxides precipitated by the anaerobic nitrate-reducing Fe(II)oxidizing Acidovorax sp. strain BoFeN1[END_REF][START_REF] Ona-Nguema | EXAFS Analysis of Arsenite Adsorption onto Two-Line Ferrihydrite, Hematite, Goethite, and Lepidocrocite[END_REF] and (iv) glutamyl-cysteinyl-glycinyl-thiolarsenite (As(III)-Glu) synthesized using the method of [START_REF] Miot | XAS Study of Arsenic Coordination in Euglena gracilis Exposed to Arsenite[END_REF] was used as representative of thiol bound As(III) organic matter (OM) species (see in Table S-4 in Supporting information for more details on the references used). Some of the reference compounds spectra (e.g. arsenian pyrite and thiol-bound As(III)) were collected at 15K in fluorescence mode at the SAMBA beamline at SOLEIL synchrotron [START_REF] Phan | Interplay of S and As in Mekong Delta sediments during redox oscillations[END_REF] Both of As and S XANES data analysis were performed using the Athena software [START_REF] Ravel | Athena, Artemis, Hephaestus: Data analysis for X-ray absorption spectroscopy using Ifeffit[END_REF] and the relative contributions of the different compounds to the sample spectra were achieved by linear combination fitting (LCF). A homemade program based on a Lenvenberg-Marquardt alogorithm was used to improve the fit quality following the method of Resongles et al.

(2016), which was described in detail in [START_REF] Phan | Interplay of S and As in Mekong Delta sediments during redox oscillations[END_REF].

Acid-volatile sulfide and elemental sulfur analysis

To complement the S K-edge XANES results, we measured acid-volatile sulfide (AVS) [START_REF] Hsieh | Analysis of sulfides in the presence of ferric minerals by diffusion methods[END_REF][START_REF] Burton | A simple and inexpensive chromium-reducible sulfur method for acid-sulfate soils[END_REF][START_REF] Couture | Non-steady state diagenesis of organic and inorganic sulfur in lake sediments[END_REF]. AVS extraction was based on a purge and trap method [START_REF] Allen | Analysis of acid-volatile sulfide (AVS) and simultaneously extracted metals (SEM) for the estimation of potential toxicity in aquatic sediments[END_REF], which relies on the conversion of sulfur compounds within sediments after centrifuging and keep dry in the glovebox that are first purged with HCl to generate volatile H2S, and secondly trapped with NaOH. Finally, a complexation is done by adding a diamine reagent to the trapped solution to form a methylene blue molecule [START_REF] Cline | Spectrophotometric determination of hydrogen sulfide in natural waters[END_REF], which is quantified by UV spectroscopy at 670 nm. The calibration curve was performed by the same method using Na2S solution at 10mM. AVS purge and trap measurements were carried out within airtight Teflon reactors under continuous flow of nitrogen to avoid oxidation of sulfide. A quantity of 0.18 to 0.50 g of previously freeze-dried sediment was used for AVS extractions and quantification.

The amount of sulfur that remained in the solid phase at the surface in the form of elemental sulfur (S8) was analyzed using a perchloroethylene extraction combined with High Performance Liquid Chromatography (HPLC) following the method described in [START_REF] Mcguire | Extraction and Quantitative Analysis of Elemental Sulfur from Sulfide Mineral Surfaces by High-Performance Liquid Chromatography[END_REF].

Perchloroethylene-extractable sulfur was obtained after pretreatment of unfiltered samples with 250 μL of ZnAc (5%) to precipitate free sulfide [START_REF] Wan | Occurrence of surface polysulfides during the interacrion betwwen Ferric (hydr)Oxides and Aqueous Sulfide[END_REF]. Sulfide fixation allows ZnAc to react with S(-II) as well as with Sn 2-(n ≥ 2, polysulfides) leading to the precipitation of ZnS. After 10 minutes, 4 mL of perchloroethylene was injected into 0.5 mL of suspensions. The samples were shaken for 3 hours and filtered through 0.22μm pore size membranes. The supernatants were then analyzed by HPCL (PerkinElmer 2000 pump and auto-sampler, UV-vision detectors and software AZUR V6.0 software) using a C18 column (Nucleosil 100-5PAH) and isocratic elution in methanol 95% at a flow rate of 0.4 mL/min. The detection was performed at a wavelength of 265 nm.

Quantitative total As concentration

To determine the bulk As concentration, sediment samples, collecting in the end of each anoxic an oxic half-cycles, were dissolved completely using a hot plate digestion [START_REF] Cotten | Arsenic binding to organic and inorganic sulfur species during microbial sulfate reduction: a sediment flow-through reactor experiment[END_REF]. About 50 mg of dried and homogenized sample were first added with 0.84 mL of distilled HNO3 (14 N) and 0.2 mL of H2O2 to dissolve organic matter, then followed by mixing of 0.4 mL of distilled HF (48%) under a fume hood, and put on a hot plate at 80 o C for 3 days. This solution was let cool down before adding 20 mL of H3BO3 (4.5%). The final extract was diluted with MilliQ water to 250 mL and analysed for total As concentration using ICP-AES.

Thermodynamic modeling

Thermodynamic calculations were performed using the PHREEQC program version 3.3.10 [START_REF] Parkhurst | Description of input and examples for PHREEQC version 3 -A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations[END_REF]. The following calculations were performed: (i) thermodynamic aqueous species distribution during the anoxic and oxic half cycles, and (ii) the saturation indices (SI = log IAP/Ksp) at each sampling point with respect to solid arsenic, sulfur and iron phases. The WATEQ4F database for the As species was updated according to the data reported in the literature [START_REF] Helz | Thermodynamic model for arsenic speciation in sulfidic waters : A novel use of ab initio computations[END_REF]. The added species included As(III) species (HnAsO3 n-3 (As(III) -arsenite), HnAsO4 n-3 , HnAsSO2 n-3 (MTA(III) -monothioarsenite), HnAsS2O n-3 (DMAs(III) -dithioarsenite), HnAsS3 (TTA(III) -trithioarsenite), and As(V) species (HnAsO4 n-3 (As(V) -arsenate), HnAsSO3 n-3 (MTA(V) -monothioarsenate), HnAsS2O2 n-3 (DTAs(V) -dithioarsenate), HnAsS3O n-3 (TTA(V)trithioarsenate) and HnAsS4 n-3 (TeTA(V) -tetrathioarsenate)). Their reactions and equilibrium constants are summarized in Table S-1 in Supporting Information.

Results

Aqueous chemistry

The aqueous phase results of the redox experiments carried out in reactors R1 and R2, containing respectively with 0.1 mM and 1 mM of sulfate, are summarized in Fig. 1.

Eh and pH cycling. The reducing and oxidizing steps can be clearly identified monitoring the Eh and pH values as a function of time (Fig. 1a-b). Eh ranges from -300 mV, in anoxic half-cycles, to +500 mV, in oxic half-cycles, and pH varied in the range between 5.2 and 7.8 for both reactors. Eh and pH underwent repeated cycling in the low sulfate reactor (R1). However, in the high sulfate reactor (R2), oxidizing conditions were not fully attained in all cycles. Reducing half cycles were characterized by a decrease in Eh and an increase in both pH and in the Fe 2+ concentration, while the oxidizing half cycles showed opposite trends (Fig. 1a-b). Intra-cycle Eh changes were similar to the Eh -monitoring studies of flooded soils and previous redox oscillation experiments [START_REF] Couture | On-Off Mobilization of Contaminants in Soils during Redox Oscillations[END_REF][START_REF] Parsons | The impact of oscillating redox conditions: Arsenic immobilisation in contaminated calcareous floodplain soils[END_REF][START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF]. In the reductive processes, the decrease in Eh may be driven by the consumption of successive terminal electron acceptors, such as Fe(III) and SO4 2-coupled to DOC oxidation by the microbial community [START_REF] Essington | Soil and water chemistry an integrative approach[END_REF], which resulted in hydroxyl (OH -) production and pH increase. DOC cycling. A 8.33 mM of DOC was manually replenished after adding cellobiose at the start every anoxic half-cycle (Fig. 1c-d). During the oxic half-cycle, in both reactors, DOC decreased significantly together with the low concentration of acetate during each oxic half-cycles probably due to respiration oxidation of Fe 2-, S(-II) and As(III). During the anoxic half-cycle, in R1 DOC also decreased slightly, while in R2 with higher SO4 2-, DOC increased significantly together with an increase production of acetate. This is probably due to the release of DOC from sediment was greater in amount of DOC consumption by heterotrophic bacteria [START_REF] Parsons | The impact of oscillating redox conditions: Arsenic immobilisation in contaminated calcareous floodplain soils[END_REF].

Total Fe and Fe(II) cycling. In both reactors, [Fe] and [Fe 2+ ] increased in the reducing cycles, consistently with the reduction reaction of labile Fe-oxides [START_REF] Essington | Soil and water chemistry an integrative approach[END_REF][START_REF] Thompson | Iron-oxide crystallinity increases during soil redox oscillations[END_REF]. During the oxic cycles, [Fe 2+ ] dropped when Eh values increased indicating that Fe 2+ originating from initial pyrite oxidation and Fe-(oxyhydr)oxides was oxidized to Fe(III), which precipitated as poorly soluble phases, e.g., ferrihydrite and goethite. Furthermore, thermodynamic predictions suggest that in anoxic sulfidic conditions, mackinawite (FeS(s)) is the first iron sulfide to precipitate, and it constitutes a major component of the empirically defined "acid volatile sulfides" (AVS) [START_REF] Rickard | Acid volatile sulfide (AVS)[END_REF]. The saturation index of FeS (SI > 0) was calculated using PHREEQC in both reactors and is reported in Fig. 1e-f. S cycling. [SO4 2-] decreased during the anoxic half-cycles and increased during the oxic half-cycles and reached about 1.1 mM in R1 and 2 mM in R2, respectively, both were > 1 mM, compared to the added SO4 2-of 0.1 mM at R1 and 1 mM at R2. This difference is probably due to the oxidation of pyrite from the sediment (Fig. 1g-h). Additionally, complete [S(-II)] depletion was observed during the oxidation cycles, while in reduction cycles, [S(-II)] varied in the range between 0.8 -8.7 μM in R1, and 1.4 -4.8 μM in R2 (Fig. 1g-h). Low [S 0 ] was detected in supernatants, which is consistent with the fact that elemental sulfur is typically associated with the solid phase (S8) [START_REF] Mcguire | Extraction and Quantitative Analysis of Elemental Sulfur from Sulfide Mineral Surfaces by High-Performance Liquid Chromatography[END_REF][START_REF] Wan | Occurrence of surface polysulfides during the interacrion betwwen Ferric (hydr)Oxides and Aqueous Sulfide[END_REF]. [S8] was found in the suspension at concentration ranges between 0.06 -8.7 μM and 0.1 -2.7 μM in R1 and R2, respectively. This finding suggests that sulfate reduction were followed by sulfide oxidation to sulfur and sulfate.

As cycling. Intra-cycle mobilization of aqueous As was observed during the anoxic half cycles.

Particularly, As was released in the initial anoxic half-cycle, reaching up to 32 μM and 27 μM in R1 and R2, respectively, and then sequestered in the solid phase in subsequent anoxic cycles.

Conversely, oxidizing conditions re-immobilized As, returning to a base-level of approximately 1μM in both R1 and R2. The As decreasing trend and the concomitant increasing in Eh were observed (Fig. 1k-l). When the oxidizing condition prevail, the most thermodynamically favorable As species was As(V) (H2AsO4 -and HAsO4 2-). The reducing condition was established during the anoxic half-cycles, which favors the formation of arsenite (H3AsO3). Successive redox cycles resulted in 92% and 83% removal of total dissolved As in R1 and R2, respectively.

Microbial community analysis

Operational taxonomic unit (OTU) libraries were generated for the samples from day 19 (anoxic, R1-or R2-19) and 22 (oxic, R1-and R2-22), as shown by the red arrows in the Fig. 1a-b. Numerous bacterial phyla were identified in the microbial community during the reducing and oxidizing phases mainly consist of Proteobacteria including Alpha-, Beta-, Gamma-, Delta-and Epsilonproteobacteria classes, and the phyla Bacteroidetes and Firmicutes. The full data are presented in Table S-3, Supporting Information.

In reactor R1, there were only three phylotypes that we more abundant under anoxic condtions than under oxic conditions and present at greater than 1% of OTUs: Arcobacter sp., represented the largest group in R1, with more than 60% of the OTUs, Klebsiella p., representing ~6% of the OTUs, and Acidovorax sp., representing 2% of the OTUs in R1. The first two organisms were also more abundant under anoxic conditions in R2 (Table 1).

In contrast, and somewhat surprisingly, taxa related to SRB and iron-reducing bacteria were detected under both oxic and anoxic conditions. Geobacter species (Geobacter sp. and Geobacter bremensis), known iron and sulfur reducers, were enriched in the two experiments (R1 and R2) during both the oxic and anoxic cycles, representing ~2.7 and ~6.7% (anoxic and oxic, respectively) of the OTUs in R1 and ~6 and ~25% of the OTUs in R2 (anoxic and oxic, respectively). SRB, including Desulfobulbus sp., Desulfomicrobium sp., and Desulfovibrio sp., were detected in both reactors with higher contributions in reactor R2 than in reactor R1 (Table 1).

In the oxic half-cycles, taxa including iron-and sulfur-oxidizing bacteria such as Thiobacillus sp.

were detected suggesting the microbial oxidation of Fe 2+ , S 2-or sulfur [START_REF] Hedrich | The iron-oxidizing proteobacteria[END_REF]. This was particularly true for R1 (Thiobacillus sp. represented 3.3% of OTUs) and less so for R2 (0.4% of OTUs). Iron oxidizers, e.g., Rhodobacter sp., potentially able to use reduced sulfur compounds (S 0 , HS -, S2O3 2 ), H2, or organic compounds as electron donors, were also identified, particularly in R2 (17% and 14.3% of OTUs in the anoxic and oxic half-cycle, respectively).

Additionally, the genus Hydrogenophaga sp., are facultatively autotrophic aerobic hydrogenoxidizing bacteria [START_REF] Willems | Hydrogenophaga, a New Genus of Hydrogen-Oxidizing Bacteria that Includes Hydrogenophaga flava comb. nov. (Formerly Pseudomonas flava), Hydrogenophaga palleronii (Formerly Pseudomonas palleronii), Hydrogenophaga pseudoflava (Formerly Pseudomonas pseudofla[END_REF] and OTUs related to this genus were predominantly identified in the oxic phase in R1 (11.8% of OTUs in oxic and 3.6% in anoxic) and in the anoxic phase in R2 (7.2% of OTUs in oxic and 9.3% in anoxic).

Finally, the presence of many heterotrophic bacteria, capable of operating under both aerobic and anaerobic conditions, such as Rhizobium sp. and Klebsiella sp., may account for the oxidation of OM during both phases [START_REF] Eller | Changes in Activity and Community Structure of Methane-Oxidizing Bacteria over the Growth Period of Rice[END_REF][START_REF] Franciscon | Microaerophilic-aerobic sequential decolourization/biodegradation of textile azo dyes by a facultative Klebsiella sp. strain VN-31[END_REF]. Other heterotrophic bacteria, OM respiring bacteria, take a prominent part of the overall community in both reactors representing from 17 -45% of total OTUs (Table 1).

Solid sulfur and arsenic dynamics

The initial anoxic sediments contained 32.1 g/kg dry sediment organic C, 0.7 g/kg total S, 15.6 g/kg total Fe (Table S-1). Smectite, quartz, muscovite, chlorite, albite, and pyrite were identified in the input sediment (Table S-1). The total As content was lower than detection limit as determined by total digestion and ICP-MS, but As K-edge XANES indicated that As was initially present in the Obound As(V) and As(III) oxidation states, S bound As(III), and arsenian pyrite.

Solid S speciation

LCF of the XANES spectra at the S K-edge revealed that the majority of S accumulated in R1 and R2 as FeS2(s), sulfate (SO4) and S8, (Fig. 2 andTable 2). Most S was in the form of FeS2(s) during the last anoxic cycles (e.g. samples R1 33, R2 33). While S8 mostly deposited in R1 during the anoxic half-cycles, about 11% of S8 rested in the second oxic half-cycle, and increased to 22% in the last anoxic half-cycle (Table 2). The AVS values indicated of 1.8 and 0.17 µM.g -1 dry sediment during the second anoxic cycles (e.g., samples R1 19, R2 19), respectively (Table 2), while S spectra show that the fitting procedure with FeS(s) was not obtained a good result.

Solid As speciation

To determine the effect of redox cycling on solid arsenic speciation, X-ray absorption spectra were recorded at the As K-edge and the bulk As concentration was analyzed using total digestion on sediments sampled at the end of each half-cycle for both reactors, and on the initial sediment. It is worth noting that Linear Combination Fitting XANES analysis is able to detect only species that are present in amounts higher than 5 -10%, and to distinguish only species with sufficiently different spectral features. Arsenic K-edge XANES data suggested the presence of four distinct As species (Fig. 3, Fig. 4). The initial sediment included O-bound As(V), O-bound As(III), S-bound As(III), and arsenian pyrite (i.e arsenic-substituted pyrite) (Fig. 3). The bulk As concentrations in the same samples were analyzed using total digestion and ICP. The total concentration of As during the oxic conditions were higher than during the anoxic conditions in both reactors (Fig. 4). An increasing As trend in the sediment during the anoxic half-cycles were also observed. The presence of an admixture of O-bound As(III) and O-bound As(V) species in all samples, with an increase in the proportion of As(V) over As(III) during the oxidizing cycles (Fig. 4). In the last oxic cycle, the difference between As(V) and As(III) is quite small: 57% (or 53%) of As(V) and 44% (or 47%) of As(III) (Table 3, Fig. 4). In R1, in the anoxic half cycles, and partly also in oxic half cycles, 8 -15% of the total As solid phase was attributed to thiol-bound As(III) (Fig. 3, Fig. 4a). In R2, LCF suggested that S-bound As compounds contributed to 8 -16% of the total As, but they were not found to contribute in the last cycles, both R1 and R2 (Fig. 3; Fig. 4b).

Discussion

Microbial sulfate reduction and sulfide oxidation

The analytical results of pore water and microbial analysis in both reactors suggest the occurrence of Fe and SO4 2-reduction during the anoxic half-cycles. Sulfate-reducing microorganisms (e.g., Desulfobulbus sp., Desulfomicrobium sp. and Desulfovibrio sp.) and iron-reducing ones (e.g., Geobacter sp., Dechloromonas sp.) were detected, supporting the occurrence of microbial SO4 2-and Fe reduction processes are ongoing. The detected species claimed to be involved in arsenaterespiration in the mobilization of As in Mekong Delta region [START_REF] Héry | Molecular and cultivation-dependent analysis of metal-reducing bacteria implicated in arsenic mobilisation in South-East Asian aquifers[END_REF][START_REF] Héry | Microbial ecology of arsenic-mobilizing Cambodian sediments: lithological controls uncovered by stable-isotope probing[END_REF][START_REF] Lear | Molecular Analysis of Arsenate-Reducing Bacteria within Cambodian Sediments following Amendment with Acetate[END_REF][START_REF] Rizoulis | Microbially mediated reduction of Fe(III) and As(V) in Cambodian sediments amended with 13Clabelled hexadecane and kerogen[END_REF]. It was surprising that Geobacter species were more abundant during oxic half-cycles than anoxic half-cycles. Geobacter species were previously considered strict anaerobes [START_REF] Caccavo | Geobacter sulfurreducens sp.nov, a hydrogen-and acetate-oxidizing dissimilatory metal-reducing microorganism[END_REF]Lin, et al., 2004). However, evidence of the use of oxygen as a terminal electron acceptor by some Geobacter species have emerged as has their ability to reduce O2 under microaerophilic conditions (Lin et al., 2004;[START_REF] Parsons | The impact of oscillating redox conditions: Arsenic immobilisation in contaminated calcareous floodplain soils[END_REF]. Sulfate-reducing bacteria (SRB), such as Desulfovibrio sp., were also observed in oxic half-cycles. Geobacter species, can survive oxygen exposure and also contain enzymes to reduce oxygen [START_REF] Cypionka | Periplasmic oxygen reduction by Desulfovibrio species[END_REF].

Nonetheless, these findings suggest the persistence of anaerobic micro-environments in which these organisms are able to grow during the nominally oxic phase. It can be hypothesized that the duration of the half-cycles was sufficient to lead to bulk geochemical parameters, indicating oxic conditions but probably not to fully oxidize microenvironments in the reactor.

Typically, carbon compounds are incompletely oxidized to acetate by SRB, which may explain the accumulation of acetate under anoxic conditions in R2 and, to lesser extent in R1. Coincidentally, Fe(II) and S(-II) were released repeatedly during three anoxic half-cycles for both high and low sulfate contents. These results seem to indicate an increasing trend in iron and sulfate reduction activities with the increase in sulfate concentration.

Arcobacter sp. represents 60% of the OTUs in the anoxic half-cycle in R1. There are several possible interpretations for the predominance of this genus. One is that the Arcobacter species is a microaerophilic sulfide oxidizer and has grown in the reactor during the anoxic phase, as dissolved oxygen concentrations were decreasing. The time point at day 19, simply might represent the anoxic end point of prolonged microaerobic conditions. Another explanation may be that this particular species is capable of reducing sulfur or other intermediate valence sulfur compounds. The prevalence of Hydrogenophaga sp., which are typically aerobic hydrogen oxidizers, during the anoxic half-cycle of R1 reactor would support the first interpretation.

Typical aerobic sulfide oxidizing bacteria, such as Thiobacillus sp.. Rhodobacter sp., were also relatively abundant in oxic cycles, particularly in R2. These organisms are typically phototrophic, anaerobic iron oxidizers. However, since the reactors were wrapped in aluminum foil, phototrophy seems unlikely. They are also chemotrophs and it is conceivable that they could oxidize sulfide anaerobically with nitrate as an electron acceptor [START_REF] Cytryn | Sulfide-Oxidizing Activity and Bacterial Community Structure in a Fluidized Bed Reactor from a Zero-Discharge Mariculture System[END_REF].

Mechanism of As mobility during redox oscillation

Control on As release under oxic condition

During the oxic half-cycles, 92% and 83% of dissolved As was removed in R1 and R2, respectively. Moreover, As K-edge XANES spectra show that O-bound As(V) and As(III), which can sorb onto Fe-(oxyhydr)oxides were the main As solid phase species in R1 and R2 (Fig. 4; Table 2). Other forms of As species, present in the initial sediment (e.g. arsenian pyrite and thiol bound As(III)), were absent in the oxic half-cycles, probably due to their oxidation. The ratio of As(V)/As(III) increased in the first two oxic half-cycles, and remained constant in the last oxic half-cycle. The precipitation of arsenic pentoxide (As2O5) may occur during the oxidation process. Although, calculations performed at pH values of 5.5 -5.7 (SI of arsenic pentoxide (As2O5) ≥ 4.5), indicated that As2O5 may form in both R1 and R2 during the oxic half-cycles, As2O5 is relatively easy to dissolve to As(V) in the aqueous phase. Accordingly, As2O5 was not detected by As K-edge XANES analysis. Adsorption onto freshly formed Fe-(oxyhydr)oxides is the most likely mechanism to explain its removal from the aqueous phase in nature or through the redox cycling batch experiments [START_REF] Nickson | Mechanism of arsenic release to groundwater, Bangladesh and West Bengal[END_REF][START_REF] Van Geen | Comment on "Arsenic Mobility and Groundwater Extraction in Bangladesh[END_REF][START_REF] Root | Arsenic sequestration by sorption processes in high-iron sediments[END_REF][START_REF] Couture | On-Off Mobilization of Contaminants in Soils during Redox Oscillations[END_REF][START_REF] Parsons | The impact of oscillating redox conditions: Arsenic immobilisation in contaminated calcareous floodplain soils[END_REF]. Altogether, these results suggest that As immobilization during the oxidizing conditions is controlled by As adsorption on Fe-(oxyhydr)oxides.

Control on As release under anoxic condition

Under anaerobic conditions, microbially driven oxidation of organic matter coupled to the dissimilatory reductive dissolution of As-bearing Fe-(oxyhydr)oxides is commonly accepted to cause the transfer of As from the solid to the aqueous phase [START_REF] Anawar | Geochemical occurrence of arsenic in groundwater of Bangladesh: sources and mobilization processes[END_REF][START_REF] Hoque | Near surface lithology and spatial variation of arsenic in the shallow groundwater: Southeastern Bangladesh[END_REF][START_REF] Horneman | Decoupling of As and Fe release to Bangladesh groundwater under reducing conditions. Part I: Evidence from sediment profiles[END_REF][START_REF] Nickson | Mechanism of arsenic release to groundwater, Bangladesh and West Bengal[END_REF]. As observed in a previous study performed on the same core of sediment collected in An Giang (Vietnam), under the effect of pyrite oxidation, which inhibits the sulfate-reducing bacteria, dissolved As was backfilled cycle after cycle [START_REF] Phan | Interplay of S and As in Mekong Delta sediments during redox oscillations[END_REF]. In contrast, this study shows that the successive anoxic half-cycles exhibit a decreased concentration of dissolved As (e.g. 92% in R1 and 83% in R2), and that As does not desorb at the end of the experiment, at both different sulfate concentrations (Fig. 1). This demonstrates that microbial SO4 2-reduction led to the sequestration of a portion of aqueous As.

The formation of As sulfide precipitations (e.g. orpiment (As2S3(s)) and realgar (AsS(s))) is one of the possible mechanisms to remove As from the aqueous phase by direct reaction with dissolved S(-II) (Couture et al. 2013a;[START_REF] O'day | The influence of sulfur and iron on dissolved arsenic concentrations in the shallow subsurface under changing redox conditions[END_REF]. In this study, thermodynamic calculations indicate that AsS(s), As2S3(s), and amorphous As sulfide (As2S3(s)) were slightly oversaturated by SI of 0.7, where HS -is estimated at the maximum aqueous sulfide concentration (e.g. 8 and 5 μM in R1 and R2, respectively), and at slightly acidic to quasi-neutral pH (Fig. 5). In contrast, As K-edge data suggest no evidence for the formation of AsS(s) and As2S3(s) during each 7-day anoxic half-cycles. This is probably due to low sulfide concentration and slow kinetics (Burton et al., 2013).

Another mechanism is linked to thiols formed during OM sulfurization (Couture et al., 2013a).

Recently, [START_REF] Wang | Arsenic speciation in Mekong Delta sediments depends on their depositional environment[END_REF] have reported the accumulation of thiol-bound As(III) in an organicrich sediment of the Mekong Delta in Vietnam. Our finding of S-bound As(III) species in the initial samples are consistent with this. However, over successive redox cycles, As K-edge XANES data suggest that S-bound As(III) gradually decreased and remained negligible in the last anoxic halfcycle in both reactor (Fig. 4). Since the high porewater Fe(II) concentration favorably buffers aqueous S(-II) to very low concentration, it subsequently limits the sulfurization of organic matter [START_REF] Burton | Arsenic Mobility during Flooding of Contaminated Soil: The Effect of Microbial Sulfate Reduction[END_REF]. Consequently, in the successive cycles, the thiol bound As(III) complex formation is unlikely to be a dominant mechanism of aqueous As immobilization.

Arsenic may also sorb onto mackinawite (FeS(s)) under the effect of microbial SO4 2-reduction [START_REF] Burton | Arsenic Mobility during Flooding of Contaminated Soil: The Effect of Microbial Sulfate Reduction[END_REF]. The thermodynamic calculation also indicates that FeS(s) was oversaturated during anoxic half-cycles (Fig. 1e-f). However, the S K-edge XANES data indicated the presence of S(-II) and S 0 , which suggests that the experimental conditions might be favorable for pyrite formation rather than FeS precipitation [START_REF] Berner | Sedimentary Pyrite Formation[END_REF][START_REF] Raiswell | Pyrite formation in euxinic and semi-euxinic sediments[END_REF][START_REF] Hurtgen | Anomalous enrichments of iron monosulfide in euxinic marine sediments and the role of H2S in iron sulfide transformations: Examples from Effingham inlet, Orca Basin, and the Black Sea[END_REF]. In agreement, As K-edge results showed no evidence for As(V) or As(III) adsorption onto Fe sulfides during redox cycling experiments. Hence, the As sequestration on FeS(s) may be marginal.

The actual As-sequestration mechanism could be the sorption of As on FeS2(s), a dominant Fe sulfide detected by S K-edge XANES investigation. The As(III) and As(V) species often sorb more strongly than the thioarsenates onto Fe-(oxyhydr)oxides and FeS(s), whereas MTA(V) sorbs most strongly on FeS2 and leads to O-bound As(III), because thiorarsenics donate S atoms to FeS2 [START_REF] Couture | Sorption of arsenite, arsenate, and thioarsenates to iron oxides and iron sulfides: A kinetic and spectroscopic investigation[END_REF]. Based on the thermodynamic modeling constants, As species are expected to be dominated by As(III), MTA(V), TTA(V), and TeTA(V) both at low (R1) and high (R2) sulfate contents (Fig. 5). Bath experiments revealed a mix of O-bound As(III) and As(V) species, with a ratio of As(III)/As(V) considerably increasing during the reducing half-cycles, and higher O-bound As(III) content than O-bound As(V) one, was observed in the last anoxic half-cycles, probably due to the enrichment of thioarsenate on FeS2(s) (Fig. 4, Table 3). However, the mechanism of thioarsenate sequestration in the Fe-As-S system requires further investigation to be confirmed, e.g., by performing EXAFS analysis to reveal the coordination of O-bound As(III) on Fe sulfides (e.g., FeS(s), FeS2(s)) at low aqueous As concentrations. The fact that during anoxic half-cycles, the proportion of O-bound As(III) increases, and that aqueous As is removed during anoxic cycles could be due to the formation of aqueous thioarsenate sorbed on FeS2. This would explain the observed increment of the O-bound As(III) species during the last anoxic half-cycles.

Conclusion

This study was aimed to better understand whether the high As concentrations detected in the aquifer in An Giang (Vietnam) can be attributed to redox dynamics. It has been shown that S and Fe biogeochemistry play a key role in determining the fate of As in deltaic sediments during redox oscillations. In particular, the results of this study indicate that: (i) As is first released during Fe reducing conditions but is then sequestered during SO4 2-reduction; (ii) As adsorbs/desorbs on Fe-(oxyhydr)oxides, (iii) aqueous thioarsenic can form and be adsorbed on Fe sulfide minerals (e.g., andR2 Table 2. Proportion of the different S solid phases in the sediment samples by applying a linear combination fitting procedure (LCF) on XANES spectra at the S K-edge (Fig. 3). The acid volatile sulfide (AVS, µmol/g of dry soil) was calculated from sequential extractions. Goodness of fit is estimated by both the classical R-factor (Rf) and the reduced chi-square (Redχ 2 ) indicators.

Table 3. Proportion of the different As solid phases in the initial sediment sample (R0) and in sediments extracted from bioreactors (R1 and R2) as revealed by applying a linear combination fitting (LCF) performed on the corresponding XANES spectra. The numbers in parenthesis give the error on percentages for each fitting component. Goodness of fit is estimated by both the classical R-factor (Rf) and the reduced chi-square (Redχ 2 ) indicators.

  (Biometra T3 Thermocycler, Germany). The PCR reaction mix for amplification contained 5 μL 10×PCR buffer, 1 μL dNTPs (10 mM), 2 μL each of the forward and reverse primer (5 μM) and 0.8 μL Taq polymerase and balance water with the total volume of 50 μL. The thermal program, set up of a 5 min at 95 o C, 1 min at 50 o C, 1 min at 68 o C, 34 cycles at 95 o C for 1 min/cycle, 10 min at 68 o C, was performed with the Lightcycler® 480 (Roche).

  FeS2); and (iv) the reactions leading to the decrease of aqueous As concentrations over subsequent cycles are driven by microbial activity, which induces fermentation during anoxic periods, and respiratory consumption during oxic ones. While previous studies have shown that As mobility is controlled by reductive dissolution of Fe-(oxyhydr)oxides in the reducing cycles, the results of the present work suggests that As sequestration in seasonally saturated sediments in the Mekong DeltaVietnam is controlled by the combined impact of surface hydrology, and by the reaction of Fe sulfide minerals and soluble As compounds (e.g. thioarsenic). This mechanism requires further investigation, e.g. by using techniques able to reveal the coordination of O-bound As(III) species on FeS2, such as As K-edge EXAFS analysis.

Figure Caption Figure 1 .

 Caption1 Figure Caption

Figure 2 .

 2 Figure 2. Normalized S K-edge XANES spectra of sediment samples collected from reactors R1 (0.1 mM SO4 2-) and R2 (1 mM SO4 2-) at the end of each anoxic or oxic half-cycles, as shown in Fig. 1. The spectra of the samples (open circles) are reported together with the LCF curves superimposed (solid lines). The spectra of the reference compounds used for LCF (K2SO4, S8 and FeS2) are also shown. All spectra are vertically shifted for clarity.

Figure 3 .

 3 Figure 3. The spectra of the reference compounds used for LCF (As(III/V) goethite, FeAsS, As2S3, thiol-bound As(III) are shown. Normalized As K-edge XANES spectra of sediment samples collected from reactors R1 (0.1 mM SO4 2-) and R2 (1 mM SO4 2-) at the end of each anoxic or oxic half-cycle, as shown in Fig. 1. The spectra of the samples (open circles) are reported together with the LCF curves superimposed (solid lines). All spectra are vertically shifted for clarity.

Figure 4 .Figure 5 .

 45 Figure 4. Arsenic quantitative speciation in reactors (a) R1 (0.1 mM SO4 2-) and (b) R2 (1 mM SO4 2 )at the end of each anoxic or oxic half-cycles. The As speciation is estimated by multiplying total As concentration sediments samples analyzed using the total digestion (the fraction of As as each of four species was obtained by LCF of As K-edge XANES data)

Figure 1 .

 1 Figure 1. Aqueous chemistry measured Eh (SHE) and pH (a-b), DOC and acetate (c-d), Fe(tot) and Fe 2+ (e-f), S(tot) and SO4 2-, HS -and S8 in suspension (g-h), As(tot), As(V) and As(III) (i-j) data with time in reactor R1 (0.1 mM SO4 2-) (left) and R2 (1.0 mM SO4 2-) (right). Blue and white shaded areas indicate the anoxic and oxic half-cycles, respectively. Sampling points for S K-edge XANES and microbial community analysis are shown on the Eh curve (As and S K-edge XANES = open purple circles, 16S rRNA = red arrows), cellobiose (OC) adding points = green arrows, and saturation index of porewaters (e-f) with respect to FeS.

Figure 2 .

 2 Figure 2. Normalized S K-edge XANES spectra of sediment samples collected from reactors R1 (0.1 mM SO4 2-) and R2 (1 mM SO4 2-) at the end of each anoxic or oxic halfcycles, as shown in Fig. 1. The spectra of the samples (open circles) are reported together with the LCF curves superimposed (solid lines). The spectra of the reference compounds used for LCF (SO4, S8 and FeS2) are also shown. All spectra are vertically shifted for clarity.

Figure 3 .

 3 Figure 3. The spectra of the reference compounds used for LCF (As(III/V) goethite, FeAsS, As2S3, thiol-bound As(III) are shown in the left panel. Center and right panels: Normalized As K-edge XANES spectra of sediment samples collected from reactors R1 (0.1 mM SO4 2-) and R2 (1 mM SO4 2-) at the end of each anoxic or oxic half cycle, as shown in Fig. 1. The spectra of the samples (open circles) are reported together with the LCF curves superimposed (solid lines). All spectra are vertically shifted for clarity.

Figure 4 .Figure 5 .

 45 Figure 4. Solid arsenic quantitative speciation in reactors (a) R1 (0.1 mM SO4 2-) and (b) R2 (1 mM SO4 2-) at the end of each anoxic or oxic half-cycles. The solid As speciation is estimated by multiplying total As concentration sediments samples analyzed using the total digestion (the fraction of As as each of four species was obtained by LCF of As K-edge XANES data)

Table Caption Table 1 .

 Caption1 Analysis of taxa (contribute ≥ 1% of OTUs) corresponding to potential metabolism and oxygen tolerance of selected samples at R1

1 Table 1 .

 11 Analysis of taxa (contribute ≥ 1% of OTUs) corresponding to potential metabolism and oxygen tolerance of selected samples at R1 and R2

						polysaccharides
	Rhodobacter
	capsulatus	0.198	% OTUs 0.745 15.619 10.987 Fe(II) oxidation	Microaerophilic-aerobic	Hedrich & Johnson, 2012
	Species Dechloromonas hortensis Arcobacter sp. Vogesella Klebsiella sp. indigofera Rhizobium sp. Sulfurospirillum sp. Geobacter bremensis Desulfobulbus sp. Clostridium sp. Desulfomicrobium sp. Prolixibacter sp. Acidovorax delafieldii Novosphingobium Hydrogenophaga sp.	R1-19 Anoxic 0.304 60.487 6.381 0.035 2.804 0.735 2.490 0.286 2.158 0.102 0.512 2.147 0.441	R2-19 Anoxic 0.793 2.931 10.511 0.064 3.328 0.050 5.109 0.600 2.435 0.231 0.969 0.405 0.857	R1-22 Oxic 2.837 0.862 3.704 1.993 2.725 1.178 2.690 0.615 3.253 0.820 0.399 1.735 0.586	Potential metabolism Organic matter degradation 1.077 Sulfide oxidizing R2-22 1.072 Oxic Organic matter 2.735 Fermentation 1.229 degradation 1.668 Nitrogen fixing 1.914 Sulfur reduction 5.674 Iron and sulfur reduction 1.537 Sulfate reducing 3.295 Iron and sulfur reduction 1.517 Sulfate reducing 1.412 Fermentation 0.507 Definication Organic matter 1.124 degradation	O2 tolerance Aerobic Microaerophilic, autotrophic Wirsen et al., 2002 Reference Wolterink et al., 2005 Microaerophilic-aerobic Aerobic Luan & Medzhitov, 2016 Franciscon et al., 2009 Microaerophilic Microaerophilic Tang et al., 2009 Schmeisser et al., 2009 Muyzer & Stams, 2008; Pokorna Aerotolerant Anaerobic & Zabranska, 2015 Snoeyenbos-West et al., 2000 Microaerophilic Muyzer & Stams, 2008; Pokorna Dobbin et al., 1999; Li et al., Anaerobic & Zabranska, 2015 2011; Thabet et al., 2004 Microaerophilic Holmes et al., 2007 Aerobic Willems et al., 2015 Microaerophilic Nguyen et al., 2014
	taeniospiralis	2.020	3.095	7.484	5.679 Hydrogen oxidizing	Microaerophilic-aerobic	Willems et al., 1989
	Acinetobacter sp.	1.879	0.124	2.637	0.847 Methane oxidation	Aerobic	Wang et al., 2012
	Hydrogenophaga					Hydrogen oxidation,		Willems et al., 1989; Graff et
	pseudoflava	1.624	8.728	1.582	1.255	thiosulfate oxidation	Microaerophilic-aerobic	al., 2003
	Thiobacillus
	thioparus	0.466	2.388	0.211	0.251 Iron and sulfide oxidation Microaerophilic-aerobic	Hedrich & Johnson, 2011
	Candidatus
	Accumulibacter sp.	0.788	1.690	4.689	2.986 Iron oxidizing	microaerophilic-aerobic	Hedrich et al., 2011
								Snoeyenbos-West et al., 2000,
	Geobacter sp.	0.237	1.628	3.311 19.114 Iron and sulfur reduction	Microaerophilic	Lin et al., 2004
	Rhodobacter sp.	0.095	1.447	1.371	3.263 Iron oxidation	Microaerophilic-aerobic	Poulain and Newman, 2009
	Thiobacillus sp.	0.120	1.083	0.064	0.178 Iron and sulfide oxidation Microaerophilic-aerobic	Poulain and Newman, 2009
						Carbon and nitrogen
	Gemmobacter sp.	0.343	1.076	0.955	0.847	degradation	Facultative anaerobic	Kumaresan et al., 2015
	Flavobacterium sp.	0.011	1.026	0.000	0.000 Decompose several	Aerobic	Schellenberger et al., 2011
						2
				3

Table 2 .

 2 Solid-phase Sulfur speciation. Proportion of the different S solid phases in the sediment samples by applying a linear combination fitting procedure (LCF) on XANES spectra at the S K-edge (Fig.2). The error on percentages is estimated to be in the range between 5 and 15%. The acid volatile sulfide (AVS, µmol/g of dry soil) was calculated from sequential extractions. Goodness of fits is estimated by the reduced chi-square (Redχ 2 ) values.

	Sample	AVS (µmol/g)	K2SO4	LCF (%) S8	FeS2	Red-χ 2 (× 10 2 )
	R1 19	1.817	35	38	27	3.9
	R1 26	-	15	-	85	1.8
	R1 33	-	10	13	77	2.7
	R2 19	0.167	8	-	92	1.2
	R2 26	-	19	11	70	1.5
	R2 33	-	16	22	63	3.9
	Anoxic
	Oxic

Acknowledgements

The authors acknowledge the financial support of the doctoral scholarship from University Grenoble Alpes and Geochemistry group (ISTerre), which is part of Labex OSUG@2020 (ANR10 LAB56).

This study has been conducted under the framework of CARE-RESCIF initiatives. We would like to thank Prof. Douglas Kent and Dr. Guillaume Morin for their help and advice during the writing and review process. We give special thanks to Y. Wang from EPFL for help with collecting sediment and fieldwork in Vietnam, L. Spadini and M-C Morel in LTHE for IC and S8 analysis. Aqueous As species were analyzed on Plateforme AETE -HydroSciences/OSU OREME, Montpellier, France.

We also acknowledge Dr. Francesco D'Acapito and Dr. Giovanni Ignazio Lepore for the assistance during XAS measurements at the LISA beamline at the ESRF (BM08), Dr. Giuliana Aquilanti for the assistance at the XAFS beamline at the Elettra synchrotron.

Supplementary data

Supplementary data related to this article can be found at Supporting Information part.


A Chafai

I Essaoudi

A Ainane
email: a.ainane@fs-umi.ac.ma

F Dujardin

R Ahuja

Role of a uniform magnetic …eld on the energy spectrum of a single donor in a core/shell spherical quantum dot by

Keywords: 71.55.-i,73.21.La,73.22.Dj Shallow donor, Core/shell materials, Nanostructures, Quantum dots, Magnetic …eld

Using the variational approach within the framework of the e¤ective-mass approximation (EMA), the binding energy of a centred hydrogenic donor impurity in a CdSe/ZnTe core/shell spherical quantum dot (CSSQD) in the presence of an external magnetic …eld was investigated. In this model, we have taken into account the e¤ect of the radial dependence of the dielectric constant and of the electron e¤ective mass. Our numerical results show a remarkable in ‡uence of the nanodot spatial parameters and of the external magnetic …eld strength on the shallow donor binding energy.

Introduction

As is well-known, the evolution of electronic and optoelectronic devices was always related to the improvement of material properties. Most notably, their optical and electronic properties. Hence the immense need to understand the behavior of charged particles inside materials, particularly in semiconductor nanomaterials, which present the possibility to accurately control both their electronic and optical properties by tuning their size [START_REF] Davies | The Physics of Low-dimensional Semiconductors : an Introduction[END_REF][START_REF] Ghazi | [END_REF]3] or by the application of an external electric or magnetic …eld [4,5,6] and so on.

The behavior of shallow donors con…ned in semiconductor quantum dots was extensively investigated and various papers can be found in the literature, such as the study performed by Ribeiro and Latgé [7] concerning the in ‡uence of the quantum dot volume and the impurity position on the binding energy. In their investigation, the variational approach was deployed in order to get the binding energy of a shallow donor in a spherical and cubic shaped quantum dot as a function of the nanostructure spatial parameters and the impurity position. The energy spectrum of a hydrogenic donor inside a spherical nanodot was the subject of Ref. [8] that presents a comparative study between the donor binding energy calculated variationally, and by using a perturbation method. To the same end, Li et al. [9] have studied the electronic structures and the binding energy of a hydrogenic impurity in a hierarchically self-assembled GaAs/Al x Ga 1 x As nanodot. Their paper shows that the binding energy of the acceptor depends more sensitive on the size of the nanodot than that of the donor. The e¤ects of hydrostatic pressure on the energy spectrum of a hydrogenic donor in a GaAs-Ga 0:7 Al 0:3 As cylindrical quantum dots was reported in Ref. [10]. It was found that the binding energy increases linearly with pressure in the low pressure regime, while in the high pressure regime the binding energy increases until a maximum value after what it decreases.

In the recent past, more particular attention was awarded to the study of core/shell materials due to their very improved properties. For example in Ref. [11] the energy spectrum of a GaAs/Al x Ga 1 x As/AlAs spherical quantum dot, in the presence of impurity, was performed using the fourth-order Runge-Kutta method. So as to perform a comparative study between the binding energy of a shallow donor con…ned in the center of a CdSe/ZnTe core/shell nanodot and ZnTe/CdSe inverted core/shell quantum dot, Chafai et al. [12] deployed a variational study taking into account the radial dependence of the dielectric constant on radius and using a …nite con…nement potential. It was found that the energy spectrum of nanomaterials is not only a¤ected by nanomaterials size, but also by the localization of charge carriers. On the other hand, the behavior of the binding energy of a shallow donor inside core/shell ZnSe/ZnS submitted to an external magnetic …eld was the aim of the article of Bhat et al. [13]. Their study exhibits a great dependence of the binding energy on the shell thickness and the magnetic …eld strength. The impact of the core/shell nanodot size, the magnetic …eld strength and the impurity position on the binding energy was examined by Talbi et al. [14]. It was found that in addition to the control of the binding energy by tuning the spatial parameters and the magnetic …eld strength, the existence of a critical radius which depends on the heteronanodot size and on the material band o¤set. The impact of an external electric …eld on the energy spectrum of a hydrogenic donor placed at the center of a CdTe/ZnTe spherical quantum dot is the emphasis of the investigation of Stojanović and Kostić [15]. Deploying the …nite di¤erence method Yuan et al. [16] studied the nanodot size e¤ect on the energy spectrum of a donor impurity in GaAs/AlGaAs core/ shell spherical with in…nite con…nement potential. Then, they have investigated the third-harmonic generation susceptibility for this nanosystem using the compact-density-matrix approach. In Ref. [17] a theoretical study of the energy spectrum, optical absorption coe¢ cients and refractive index of a centered donor con…ned in a multilayered spherical quantum dot can be found. The authors of this paper have shown that both the electronic and the optical properties of such system can be controlled by turning the strength of the external …elds or by tailoring the nanodot size.

In this work, we present a variational study concerning the size control of the shallow donor binding energy in a spherical core/shell nanodot. The impact of the dielectric constant discontinuity at the nanostructure boundaries was also considered. Employing a trial wave function containing three variational parameters the donor binding energy in the presence of an external magnetic …eld was calculated in the framework of the EMA. the next section is an overview of the applied theory, while section 3 is devoted to the presentation of our numerical results.

Model and theory

The nanostructure under study is composed of an on-center donor inside a CdSe/ZnTe core/shell spherical nanodot submerged in a SiO 2 matrix. The global system is the subject of an external magnetic …eld directed along to the (OZ) axis. In Figure 1, a schematic description of our nanoheterostructure is sketched. Where R C and R S are, respectively, the inner and outer radii, while E c = U e is the con…nement potential of electrons. Within the framework of the (EMA), taking into account the dependence of the electron e¤ective mass on radius and the interaction between the electron and its own image charge due to the dielectric mismatch, the Hamiltonian of our structure reads:

H D 0 = P + e c A 1 2m e (r) P + e c A + U w (r) + U coul (r) + (R s ) : (1)
The vector potential of the applied magnetic …eld and the electron momentum are noted, respectively, A and P. While the electron mass is assumed to be m c = 0:13 (m s = 0:15) in core (shell) material. The electron con…nement potential, the second term of Eq.( 1), is considered to be zero in the core material, while it takes a …nite value U e = 1:27 eV [3] when the electron is hosted by the shell material. Otherwise, it becomes in…nite. The coulomb interaction and the self-energy are, respectively, given as [18]:

U coul (r) = e 2 4 " 0 " r e 2 ( " " r (Si O 2 )) 4 " 0 " " r (Si O 2 ) R s ; (2)
and

(R s ) = e 2 8 " 0 R s 1 " 1 " r (Si O 2 ) + 0:466 e 2 4 " 0 " R s " " r (Si O 2 ) " + " r (Si O 2 ) ; (3)
" =
p " r (CdSe) " r (ZnT e) = 10:14 [START_REF] Borisenko | What Is What in the Nanoworld[END_REF] is the dielectric constant inside the core/shell nanostructure and " r (Si O 2 ) = 3:9 [START_REF] Zeng | [END_REF] is the dielectric constant of the surrounding medium.

In order to examine the impact of the spatial parameters and the magnetic …eld strength on the shallow donor binding energy we adopt a variational approach to resolve the Schrodinger equation of our system. In this optic, we have used the following trial wave function: D 0 (r) = e (r) e r e r 2 ;

(4)

where e (r), is the trial wave function of the same system without impurity chosen as:

e (r) = 8 > > > > < > > > > : N 1 sin( 1 r) r e r 2 ; r R c N 2 sinh( 2 (r Rs)) r e r 2 ; R c < r R s 0; Otherwise : (5) 1 = q 2m 1 } 2 (E e (R s )), 2 = q 2m 1 } 2 (U e + (R s ) E e )
. N i (i = 1; 2) and ; ; are, respectively, the normalization constants and the variational parameters.

In this context, the energy of an electron under an external magnetic …eld is given as:

E D 0 = min
h e (r)j H e j e (r)i h e (r) j e (r)i ;

while the energy of a hydrogenic donor subject to an external magnetic …eld reads:

E D 0 = min ; h D 0 (r)j H D 0 j D 0 (r)i h D 0 (r) j D 0 (r)i : (7)
H e (H D 0 ) is the Hamiltonian of the electron (donor) under an external magnetic …eld.

Numerical results

The numerical results presented in this section have performed using 2 m e c R D 0 : In …gure 2, we plot the electron ground state energy as a function of the shell thickness in zero magnetic …eld ( = 0) for various material core sizes: R c =1 āD 0 , 1.25 āD 0 and 1.5 āD 0 . In this …gure, we can observe a great dependence of the electron energy on the nanostructure spatial parameters . In other words, the increase of the shell thickness and the material core size leads to the decrease of the electron ground state energy. It is also clear that the electron 1s energy is more sensitive to the size of the core material than to the thickness of the shell material which may be explained by the famous quantum con…nement e¤ect.

R D 0 = } 2
In …gure 3, The donor binding energy with zero magnetic …eld versus the thickness of the shell material for various core radius values: R c =1 āD 0 , 1.25 āD 0 and 1.5 āD 0 is presented. It is noticeable that the donor binding energy remarkably depends on the nanostructure spatial parameters e.g., when the inner or the outer radius values decrease the binding energy increases which is in good agreement with [21,22].

In …gure 4, we present the electron ground state energy versus the magnetic …eld strength for a …xed nanostructure size R s =2 āD 0 and various material core sizes: R c =1 āD 0 , 1.25 āD 0 and 1.5 āD 0 . This …gure shows the possibility to enhance the electron ground state energy, especially in large nanodot, by the application of a uniform magnetic …eld which might quali…ed by the existence of a competition between the two coexisting con…nement type: the spatial con…nement, more pronounced for small nanostructure, and the magnetic con…nement more pronounced for large nanostructure.

In …gure 5, we draw the donor ground state binding energy as a function of the magnetic …eld strength for R s =2 āD 0 and R c =1 āD 0 , 1.25 āD 0 and 1.5 āD 0 . We can notice from this …gure that the e¤ect of the magnetic …eld is marginalized at the center of the nanodot, on the other hand, his impact on the binding energy is very remarkable near the border of the heteronanodot. That is related to the fact that the presence of an external magnetic …eld tends to move the charge density toward the center of large nanodot which automatically increases the binding energy. For a small nanostructure and due to the spatial con…nement e¤ect, the charge density is compressed around the center of the heterostructure that enhances the binding energy and naturally the e¤ect of the magnetic …eld is marginalized. As a matter of fact, using the combination e¤ect of an external uniform magnetic …eld and the core/shell quantum dot size leads to control the donor binding energy. At this spot, we want to draw attention that our results are in good agreement with those obtained in Refs [START_REF] Ghazi | [END_REF]22].

Conclusion

In this investigation, we have studied the combination e¤ect of the spatial con…nement and the magnetic …eld strength on the binding energy behavior of a shallow donor in a CdSe/ZnTe core/shell quantum dot. In order to make our model more realistic, we have taken into account the e¤ect of the dielectric environment and considered a …nite con…nement potential. Our results provide the possibility to enhance the binding energy via the application of an external magnetic …eld or the control of the nanodot size. It is also shown that the e¤ect of the magnetic …eld is more pronounced in regions when the binding energy is low sensitive to the e¤ect of the spatial con…nement. In the other case it is marginalized.

Acknowledgments

This work has been initiated with the support of URAC: 08, the project PPR2: (MESRSFC-CNRST) and the Swedish Research Links programme dnr-348-2011-7264 . The authors would like to thank all the organizations.

Figure captions

2 m e a 2 D 0 = 18 : 2 e 2 m e = 3 :

 201823 478 meV as unit of energy, a D 0 = "r} 842 nm as unit of length, and the dimensionless magnetic …eld strength = e } B

Figure 1 :

 1 Figure 1: On-center donor inside CdSe/ZnTe Spherical heterostructure.

Figure 2 :

 2 Figure 2: The electron ground state energy as a function of the shell thickness for = 0 and R c = 1 a D 0 ; 1:25 a D 0 and 1:5 a D 0 .

Figure 3

 3 Figure 3 The donor binding energy as a function of the shell thickness for = 0 R c = 1 a D 0 ; 1:25 a D 0 and 1:5 a D 0 .

Figure 4 Figure 5

 45 Figure 4 The electron ground state energy versus the magnetic …eld strength for R s = 2 a D 0 and R c = 1 a D 0 ; 1:25 a D 0 and 1:5 a D 0 : Figure 5 The donor binding energy versus the magnetic …eld strength for R s = 2 a D 0 and R c = 1 a D 0 ; 1:25 a D 0 and 1:5 a D 0 .

Figure 1 (

 1 Figure 1