Arsenic Speciation in Mekong Delta Sediments Depends on Their

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Arsenic speciation in Mekong Delta sediments depends on their depositional environment Yuheng Wang, Pierre Le Pape, Guillaume Morin, Maria Asta, Georgina King, Barbora Bartova, Elena Suvorova, Manon Frutschi, Maya Ikogou, Vu Hoai Cong Pham, Phu Le Vo, Frédéric Herman, Laurent Charlet, and Rizlan Bernier-Latmani Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05177 • Publication Date (Web): 16 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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Environmental Science & Technology

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Arsenic speciation in Mekong Delta sediments depends on their

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depositional environment

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Yuheng Wang1, Pierre Le Pape2, Guillaume Morin2, Maria P. Asta1, Georgina King3, Barbora

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Bártová1, Elena Suvorova1, Manon Frutschi1, Maya Ikogou2, Vu Hoai Cong Pham4, Phu Le

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Vo4, Frédéric Herman5, Laurent Charlet6 and Rizlan Bernier-Latmani1*

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Ecole Polytechnique Fédérale de Lausanne (EPFL), Environmental Microbiology Laboratory (EML), EPFLENAC-IIE-EML, Station 6, CH-1015 Lausanne, Switzerland

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Centre National de la Recherche Scientifique (CNRS) – Université Pierre et Marie Curie (UPMC Paris 6),

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Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC, CNRS-UPMC-IRD-MNHN

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UMR 7590), Campus Jussieu, 4 place Jussieu, 75005 Paris, France

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University of Bern, Institute of Geological Sciences, Baltzerstrasse 1+3, CH-3012, Bern

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Ho Chi Minh City University of Technology, Faculty of Environment and Natural Resources, VNU-HCM, 268

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Ly Thuong Kiet, Ho Chi Minh City, Vietnam 5

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University of Lausanne, Institute of Earth Surface Dynamics, Géopolis Building, CH-1015 Lausanne, Switzerland

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Université Grenoble Alpes, Université Savoie Mont Blanc, CNRS, IRD, IFSTTAR, ISTerre, 38000 Grenoble, France

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*correspondence to: [email protected]

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ACS Paragon Plus Environment


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TOC Art

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Himalaya

Sea

Mekong Delta

Before 7200 BP Floodplain

transgression

Mangrove Sea level

O-As(V)

7200 - 7000 BP

transgression

O-As NOM-As(III)

7000 - 5000 BP

regression

O-As NOM-As(III)

A.er 5000 BP

Moderate NOM

30 31

2

O-As NOM-As(III) Pyrite-As

O-As(V)

O-As NOM-As(III)

High NOM




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Abstract

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Arsenic (As) contamination in groundwater is pervasive throughout deltaic regions of

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Southeast Asia and threatens the health of millions. The speciation of As in sediments

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overlying contaminated aquifers is poorly constrained. Here, we investigate the chemical and

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mineralogical compositions of sediment cores collected from the Mekong Delta in Vietnam,

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elucidate the speciation of iron and arsenic, and relate them to the sediment depositional

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environment. Gradual dissolution of ferric (oxyhydr)oxides with depth is observed down to 7

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m, corresponding to the establishment of reducing conditions. Within the reduced sediment,

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layers originating from marine, coastal or alluvial depositional environments are identified

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and their age is consistent with a late Holocene transgression in the Mekong Delta. In the

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organic matter- and sulfur-rich layers, arsenic is present in association with organic matter

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through thiol-bonding and in the form of arsenian pyrite. The highest arsenic concentration

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(34 – 69 ppm) is found in the peat layer at 16 m and suggests the accumulation of arsenic due

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to the formation of thiol-bound trivalent arsenic (40% – 55%) and arsenian pyrite (15% –

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30%) in a paleo-mangrove depositional environment (~7200 yr BP). Where sulfur is limited,

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siderite is identified, and oxygen- and thiol-bound trivalent arsenic are the predominant

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forms. It is also worth noting that pentavalent arsenic coordinated to oxygen is ubiquitous in

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the sediment profile, even in reduced sediment layers. But the identity of the oxygen-bound

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As species remains unknown. This work shows direct evidence of thiol-bound trivalent

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arsenic in the Mekong Delta sediments and provides insight to refine the current model of the

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origin, deposition, and release of arsenic in the alluvial aquifers of the Mekong Delta.

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Introduction

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Arsenic (As) contamination in aquifers of the Mekong Delta, as well as in other deltaic

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areas of South and Southeast Asia1-4 impacts the health of millions. It is widely accepted that

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arsenate, typically associated with oxidized environments, is bound to iron (oxyhydr)oxides

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that are transported in river water from the Himalayas and downstream watersheds to those

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deltaic areas. Arsenate, together with ferric (oxyhydr)oxides, is then deposited by river water

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at the floodplain sediment surface. Bioreduction of Fe(III) and As(V) during diagenetic burial

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leads to the subsequent release of As5, 6, and natural organic matter (NOM) released from the

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margins of peat layers7 fuels this microbial process by serving as an electron donor6, 8.

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Additionally, other As-bearing phases were uncovered in deltaic sediments. For instance,

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sequential extractions from sediments in Bangladesh have suggested that arsenic could also

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bind to NOM9. Micro- and bulk-scale As K-edge X-ray absorption spectroscopy (XAS) has

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provided evidence for the presence of arsenate, arsenite, and sulfide-bound arsenic in the form

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of arsenian pyrite, in sediments in the Bengal Basin10 and in the Cambodian Mekong Delta11,

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12

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these previous studies10, 11, 12, the presence of additional sulfide-bound As species could not be

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excluded. Consequently, both the nature and abundance of the sulfide-bound As pool in

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deltaic sediment layers rich in clay remain unknown7,11,13.

. However, although arsenian pyrite has been evidenced in organic-rich sediment layers in

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The geological formation history of the Mekong Delta is recorded in the stratigraphic

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structure of the sediments, which is dominated by alluvial deposition, tidal events and marine

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deposition14-16. The latter is mainly due to the Holocene transgression, by which the

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postglacial sea-level rise turned alluvial depositional systems into estuaries and shallow

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marine sediments17. Mangrove forests followed the migration of the coastline and intruded

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into more elevated areas in the Mekong Delta18. At about 6,000 – 5,000 yr BP, the sea water

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level reached its maximum, and marine deposition prevailed throughout the Mekong Delta17,

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18

. Later, due to the slow regression of the sea level and high fluvial sediment load over the 4


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last 4,550 yr BP, the Mekong delta extensively prograded to its current form14, 16. While the

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stratigraphic structure of Mekong Delta sediments and its relation to the depositional

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environment are well documented, systematic studies of As speciation along sediment

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stratigraphic profiles are scarce in the literature. Nonetheless, previous studies have

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documented that geogenically-derived As in aquifers appears to be associated with sediments

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deposited during the Holocene marine transgression.19, 20

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Here, we retrieved sediment cores from the Mekong Delta in Vietnam in an attempt to

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obtain a comprehensive view of the As-bearing phases in the sediments, to investigate arsenic

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speciation in more detail and to link it to the sediment depositional environment. We probed

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the redox-preserved sediments at a range of spatial scales, including bulk mineralogical and

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chemical analyses, electron microscopy, and XAS at Fe and As K-edges. We uncovered the

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complex speciation of Fe and As in the sediment relative to its stratigraphic structure and

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dated the sediment to relate its composition to its depositional environment. More specifically,

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As is present in a combination of four species: sulfur-bound As(III) associated with NOM,

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arsenic in pyrite, oxygen-bound As(III) and oxygen-bound As(V). Their fractions in the total

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As pool varied with depth and thus with depositional environment. This work shows direct

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evidence of thiol-bound trivalent arsenic in the Mekong Delta sediments and is the first

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integrated and systematic study of As speciation along a sediment stratigraphic profile in a

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deltaic environment.

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Materials and Methods

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Sampling. Sampling trips to the Quoc Thai Commune, An Giang Province, Vietnam took

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place in January 2014 and January 2015 (Figure S1). The cores (QTC2 and QTC3) were

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sampled and cut into sections as a function of depth (Figure S2) under an Argon stream or in

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an N2-atmosphere field anoxic chamber, and the sections were sealed in individual MYLAR®

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bags. A portion of each segment was dried under vacuum using a desiccator in an anoxic 5



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chamber for a few days, ground and homogenized to a fine powder, and stored in sealed

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serum bottles until analysis. The sediment samples were analyzed in a similar way to that

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described by Wang et al.21, 22.

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Analysis of sediment chemistry. Water content of the sediment samples was obtained by

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weighing the samples before and after drying. Sediment total organic carbon (TOC) content

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was measured in dried homogenized samples with a Shimadzu® TOC-V CPH/CPN analyzer.

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To study the sediment chemical composition and As concentration, sediment samples were

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analyzed with a PANalytical® Axios-mAX X-ray fluorescence (XRF) spectrometer using

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wax-fused pellets for both major and trace elements. The detection limits are 0.01% for major

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elements and 1 to 5 ppm for trace elements. Accuracy of the analyses was assessed by

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analysis of standard reference materials.

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Electron microscopy. Scanning electron microscopy (SEM) observation on gold-coated

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finely ground dry sediment powder was either conducted using a Carl Zeiss® Merlin

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microscope with a GEMINI II column at 1 kV or using a Gemini Zeiss® Ultra55 microscope

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at 15 kV. For scanning transmission electron microscopy and X-ray energy dispersive

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spectroscopy (STEM-EDS), finely ground dry sediment powder was loaded onto C-coated Cu

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or Ni TEM grids and transferred under anoxic conditions to the microscope (an FEI TecnaiTM

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Osiris microscope at 200 kV equipped with four windowless Super-X SDD EDX detectors).

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Quantitative EDS results were obtained with the Bruker® Esprit software. The morphology

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were studied by high-resolution TEM (HRTEM).

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X-ray diffraction (XRD). XRD analysis was conducted under anoxic conditions. Finely

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ground dry sediment samples preserved in serum bottles were loaded onto a Si single crystal

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“Zero Diffraction Plate” in a Jacomex® anaerobic chamber and then inserted in an anaerobic 6


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sample chamber equipped with a Kapton® window. XRD measurements were performed

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under anoxic conditions using CoKa (wavelength = 1.79 Å) radiation on a Panalytical®

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X’Pert Pro MPD diffractometer mounted in Bragg-Brentano configuration and equipped with

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an X'Celerator® detector. Data were recorded in the continuous-scan mode in the 3 - 80° 2θ

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range with a step of 0.033°.

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Iron K-edge X-ray absorption spectroscopy. Iron K-edge extended X-ray absorption fine

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structure (EXAFS) spectra were collected in transmission detection mode at 80 K using liquid

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N2 cryostat on the XAFS beamline at ELETTRA (Trieste, Italy) and on the 4-1 beamline at

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the Stanford Synchrotron Radiation Lightsource (SSRL, CA, USA) using a Si(111) double

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crystal monochromator. The raw spectra were normalized and background subtracted using

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the Athena Code45. Linear combination fitting (LCF) of EXAFS data was performed on k3-

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weighted EXAFS data over a k-range of 3-14 Å-1 and E0 was set to 7,125 eV. Respective

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signals of the model compounds used for the LCF are presented in Figure S3. Details of the

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data collection and processing are provided in the Supplementary Information.

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Arsenic K-edge X-ray absorption spectroscopy. Arsenic K-edge X-ray near edge

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spectrosopcy (XANES) and EXAFS spectra were collected in fluorescence detection mode at

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10 - 15 K using a liquid He cryostat at the 11-2 wiggler beamline at SSRL (California, USA)

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with a Si(220) double crystal monochromator. The raw spectra were normalized and

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background subtracted using the Athena code. LCF of XANES data was performed from -20

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to 60 eV with E0 set to 11,868 eV. LCF of EXAFS data were performed on k3-weighted

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EXAFS data over a k-range of 3-12 A-1 and E0 was set up at 11,868 eV. Model compound

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spectra are presented in Figure S4. Details of the data collection and processing are provided

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in the Supplementary Information. 7



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Additional analyses. Sediment porosity and hydraulic conductivity analyses, and sediment

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dating using either 14C for the NOM-rich layer at 16 m or optically stimulated luminescence

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(OSL) measurements for selected silicate-rich layers were carried out. The experimental

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details of these analyses are reported in the Supplementary Information.

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Results

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Chemical and mineralogical composition, iron speciation and age of the sediments. The

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sampling area is located in the upper Mekong Delta in Vietnam (Figure S1), in a rice paddy

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200 m away from the Bassac River, one of the main branches of the Mekong River, in an area

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where heavy As groundwater contamination was reported

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collected in January 2014, covers the depth of 0-20 m. A second core (QTC3), collected in

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January 2015, extends down to 40 m. The two cores, collected from immediately adjacent

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locations, possess very similar lithologies and chemical compositions as a function of depth

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(Figure 1 and S5, Tables S1 and S2). XRD analyses reveal that the major minerals are quartz,

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mica/illite, chlorite, and albite, and that they are distributed largely uniformly in the sediment

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cores (Figure S6). Additionally, Fe(II)-bearing minerals—pyrite and siderite—were identified

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at specific depths below 7 m using XRD (Figure S6) and scanning electron microscopy

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(SEM) (Figure S7).

2-4

. A sediment core (QTC2),

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In order to determine Fe speciation in the sediment cores, bulk Fe K-edge XAS

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analysis was carried out for QTC2 samples and the results are shown in Figure S8. LCF of the

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EXAFS data was performed using relevant model compounds described in the Supplementary

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Material and Methods section, and the results are reported in Figure 2a and Table S3. Ferric

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oxyhydroxides—goethite and ferrihydrite—were detected in the depth range of 0-6 m with a

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decreasing abundance with depth. Phyllosilicate minerals, e.g., illite and chlorite, also host a

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significant fraction of total Fe. Below 7 m, Fe-reducing conditions begin to prevail and an

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alternation of pyrite and siderite is observed. Biotite is the main Fe-bearing phyllosilicate

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mineral in the reduced sediment layers, where about 9%-19% of Fe is also found to be

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associated with NOM.

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Based on Fe K-edge XAS data, sediment chemical and mineralogical composition,

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and visual observation of the cores, we describe five distinct sediment types henceforth

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referred to as type I through V (Figure 1, Table S4). Type I (orange layer in Figure 1a) is a

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clay layer in the depth range of 0-7 m, with a color transitioning from brown-orange, to light

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green and to light gray. It is NOM-poor (TOC = 0.2 – 0.6% w/w) (Figure 1d, Table S4). In

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this sediment layer, the Fe-bearing phases are phyllosilicates (illite and chlorite) and ferric

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oxyhydroxides (goethite and ferrihydrite). The latter two account for ca. half of the Fe pool at

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0 - 6 m, but disappear at 7 m where traces of pyrite are detected (Figure 2a). Thus, this layer

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(type I) represents oxic sediments undergoing a redox transition from 6 to 7 m. The layer at 2

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m was dated to 2,560 ± 360 yr (Table 1). Type II (dark gray layers in Figure 1a) is

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represented by two gray clay layers at 8 – 11 m and 14 m, respectively, and includes moderate

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to high NOM (TOC = 1.4 - 7.5% w/w) (Figure 1d, Table S4), and high pyrite content (1.2 -

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1.9% w/w) (Figure 2a, Table S4). The 8 m layer is at the shallowest depth at which pyrite

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precipitation is observed with XRD, suggesting that Fe(III)- and sulfate-reducing conditions

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were extant at the time of formation of this mineral. This sediment type contains pyrite and

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biotite as the main Fe-bearing components and Fe in NOM as a minor component. The 10 m

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layer was dated to 7,830 ± 890 yr (Table 1). Type III (light gray layer in Figure 1a) is a gray

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clay layer at 12 m and is moderate in NOM (TOC = 1.0-1.4% w/w) (Figure 1d, Table S4) but

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rich in siderite (1.6% w/w) (Figure 2a, Table S4). The sulfur content is significantly lower

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than in adjacent sediment layers (Figure 1c). This sediment contains biotite and siderite as the

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main Fe components, and pyrite as a minor phase (Figure 2a, Table S4). This suggests that

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sulfate was less abundant during the deposition than for the adjacent type II sediment. This 9



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layer (at 12 m) was dated to 6,480 ± 790 (Table 1). Type IV (black layer in Figure 1a) is at 16

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m and is a peat layer with a very high NOM (TOC = up to 33.9% w/w) (Figure 1d, Table S4)

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and pyrite contents (2.6% w/w) (Figure 2a, Table S4). Consistent with this composition, Fe

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speciation is dominated by pyrite and NOM-associated Fe (Figure 2a). The composition also

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underscores an abundant sulfate supply during sediment deposition. This layer was dated

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using 14C to 8,079 ± 708 cal. yr BP (Table 1). Type V (green layer in Figure 1a) is a gray clay

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layer at 17 - 33 m, that is relatively poor in NOM (TOC = 0.1 – 0.9% w/w, except for the

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layer at 17 m where TOC = 2.7 – 4.8% w/w, as it lies immediately beneath type IV NOM-rich

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layer) and pyrite-free (Table S4), and in which siderite content varies with depth (Figure 2a,

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Figure S6). Fe is mainly present in biotite and siderite and, to a much lesser extent, associated

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with NOM (Figure 2a). This suggests that sulfate was scarce during the deposition of this

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layer. This layer was dated at two depths: to 8,840 ± 1,670 at 18m (near the top of the layer)

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and at 16,870 ± 2,190 yr at 33 m (near the bottom of the layer). Taking into account the 2σ

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uncertainties of the determined ages, we confirm a younger-to-older chronological sequence

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from the top to the bottom of the sediment profile (Table 1).

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Arsenic redox states in the sediment. The As K-edge XANES spectra (Figure 3a) of type II

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(8 and 14 m), type III (12 m), type IV (16 m), and type V (18 m, 20 m and 35 m) sediments

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display two absorption maxima at about 11,869 eV and 11,875 eV. The first absorption peak

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(11,869 eV) corresponds to a mixture of As in arsenian pyrite (at 11,868.7 eV), S-bound

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As(III) (at 11,869.5 eV) and O-coordinated As(III) (arsenite) (at 11,871.4 eV)23-25. The

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second absorption maximum corresponds to O-coordinated As(V) (arsenate) (at 11,875.0

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eV)24-26 (Figure S4). The XANES spectra of the type I (2 m and 7 m) sediment exhibit a main

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absorption maximum at 11,875 eV, corresponding to O-bound As(V), along with a

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contribution from O-bound As(III) at 11,871.4 eV. Thus, in sediment types I to V, mixed As

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redox states prevail and their composition is quantified using LCF analysis (Figure 3a, Table 10


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S5, Figure S9). Even in the oxidized sediment (type I), we detect a contribution from As(III),

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presumably from As(V) reduction. In the sediment types that include NOM (types II, III, IV),

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the XANES spectra are dominated by As in arsenian pyrite, and S-bound As(III) with minor

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O-bound As species. In type V sediments, XANES data indicate contributions from S-bound

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As(III) and O-bound As(III) and As(V) species. LCF analysis of the As K-edge EXAFS data

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shows the same results and thus reinforces the reliability of the fitting components (Figure 2b,

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Table S5). It is worth noting that the LCF analysis of both the As K-edge XANES and

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EXAFS data must invoke As(V) oxyanions to obtain acceptable fits for all samples including

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reduced sediments of type II to V (Figure 2b, Figure 3, Table S5).

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Arsenian pyrite in the sediment. LCF results for As EXAFS and XANES data (Figure 2b,

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Figure 3, Table S5) underscore the significant contribution of arsenian pyrite to sediment

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types II to IV, while evidencing its absence in types I and V. Scanning Electron Microscopy

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(SEM) observation of type II, III and IV samples revealed the presence of both framboidal

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and euhedral pyrite particles of varying size (Figure S7). STEM coupled to EDS and SAED

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analysis of sediment types II (8, 10 and 14 m), type III (12 m), and type IV (16 m) confirms

249

that both As-bearing and As-free pyrite particles occur and that the former group contains As

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atomic concentrations of 0.06 - 0.4%. (Figure S10-S14, Table S6). Pyrite particles in the

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sediment samples were readily identified during the STEM observation due to their

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distinctive morphology. Moreover, using a positive correlation between the abundance of

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pyrite-associated arsenic and pyrite abundance in the sediment, we estimated an average As

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concentration of ~280 ppm wt. in pyrite in these sediments (Figure S15).

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Thiol-bound arsenic(III) in the sediment. Arsenian pyrite alone does not account for the

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entire pool of S-bound As in sediment types II (8 and 14 m), III (12 m), IV (16 m), and V (18

258

m) according to LCF results for As K-edge XAS data (Figure 3b). A sulfur-bound As(III) 11



259

species lacking second shell coordination is needed to fit the EXAFS data. This As species

260

corresponds rather to As(III) bound to thiol groups27,28 than to amorphous As2S3

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precipitates29,30, as the former exhibits a shorter As-S distance and yields a better fit (Figure

262

S16). Neither amorphous As2S3 nor NOM-associated As could be observed by STEM-EDS,

263

likely because NOM and S are 3-4 orders of magnitude more abundant than As, diluting the

264

pool of As associated with either. Thiol-bound As(III) is particularly abundant in type IV

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organic-rich sediment (16 m, peat layer) as shown in Figure S17, which supports the finding

266

that As(III) is bound to NOM thiol groups.

267 268

Discussion

269

Iron speciation in relation to sediment stratigraphic structure and depositional

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environment. The sediment core structure evidences the alternation of pyrite-rich (type II (8

271

– 11 m, 14 m) and type IV (16 m)) and pyrite-poor but siderite-rich (type III (12 m) and type

272

V (17 - 37 m)) sediments (Figure 2a, Figure S6). We hypothesize that this originates from

273

varying sulfate abundance in the depositional environment, which resulted from a late

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Holocene transgression of the Mekong delta. The Mekong delta has prograded from the

275

Vietnam-Cambodia border to the South China Sea in the last 20,000 yrs17. Thus, sediment

276

layers at the bottom of the cores (type V) represent an alluvial depositional environment

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where limited sulfate was available, and siderite, rather than pyrite, was formed during

278

diagenetic burial. The dating of the sediment layer at 33 m to 16,870 ± 2,190 yr (Table 1) is

279

broadly consistent with this progradation phase. However, a transgressive phase that lasted

280

from ~15,000 to ~5,000 yr BP14, 17, 18, and during which the sea water level rose by ~75 m17, is

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consistent with the pyrite-bearing middle of the core (type II to IV). The sediment layer at 18

282

m (type V) dated at 8,840 ± 1,670 yr does not include pyrite but rather siderite, and represents

283

a time at which sea level was rising but had not reached this particular location. The peat layer

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at 16 m (type IV) is dated to 8,079 ± 708 cal. yr BP (Table 1) and corresponds to the short 12


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period during which a mangrove forest covered the current sampling location18. Mangrove

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forests usually thrive at marine coastlines and promote Fe(III) and sulfate reduction and pyrite

287

precipitation31 as well as NOM accumulation in sediments32. The common presence of

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framboidal and euhedral pyrite (Figure S7) accompanied by high NOM content (Figure 1,

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Table S4) observed in these layers is reminiscent of present-day mangrove sediments33. At

290

about 6,000 – 5,000 yr BP, the sea water level reached its maximum17, 18 and the depositional

291

environment turned to a shallow marine depositional environment implying lower NOM input

292

under sulfate-reducing conditions. Such conditions explain the presence of pyrite in the

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sediment layers at 12 m (type III) and at 10 and 14 m (type II) above the type IV peat layer.

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Consistent with these findings, Stuckey et al.10 observed a peat layer at a shallower depth (6

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m) at a location upstream from our site along the Mekong River11 that was dated at ~5,500 yr

296

BP, commensurate with continued transgression until about 5,000 BP. Finally, type I

297

sediment represents the most recent alluvial deposition in the latest 5,000 yrs18, and formed

298

above sea level after marine regression. The layer at 2 m is dated to 2,560 ± 360 yr (Table 1),

299

which is consistent with that interpretation.

300

Therefore, we propose that type IV sediments represent deposition in a paleo-

301

mangrove environment and type II and III represent shallow marine environments (Figure 4).

302

The lower pyrite and S content in type III sediment and the presence of siderite resulting from

303

Fe (oxyhydr)oxides reduction could be due to a transient high input of alluvial sediment or to

304

a lower sea level between two transgression events. A previous study in the Ganges-

305

Brahmaputra Delta in Bangladesh has evidenced the lack of pyrite in a 20-100 m depth

306

interval and similarly surmised a low sulfur flux during deposition10. Overall, the dating

307

results correspond well to the iron speciation, providing strong support to the interpretation of

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the depositional environment for each sediment type.

309

13



310

Evidence of thiol-bound arsenic associated with NOM. Both NOM- and pyrite-associated

311

As are predominant S-bound As species found in the sediment cores investigated here. Thiol-

312

bound As(III) associated with NOM is particularly abundant in the organic-rich peat layer

313

(type IV) but also occurs in lower amounts in most of the other reduced layers (Figure 2b.

314

Table S4). This species identified here with As K-edge XAS analysis likely accounts for a

315

significant part of the As-sulfide pool reported in a peat layer in the Cambodian Mekong

316

Delta by Stuckey et al.11, 12, in addition to arsenian pyrite. Arsenic binding to high affinity

317

NOM-thiol groups was recently reported in As-rich peat from a wetland27 but was never

318

demonstrated in deltaic environments. Here, accumulation of thiol-bound As(III) in the

319

organic-rich layer at 16 m could have occurred in the paleo-mangrove that was extant around

320

7,200 yr BP and could continue to be active in trapping dissolved arsenic released by As(V)

321

bioreduction6 at the sediment redox transition boundary. Alternatively, it could serve as a

322

local source of organic carbon and As to the aquifer in addition to the reductive dissolution of

323

iron (oxyhydr)oxides and this possibility remains to be ascertained.

324 325

Authigenic origin of pyrite-associated arsenic. Arsenian pyrite was previously identified in

326

a peat layer in sediments from the Mekong delta in Cambodia11. Here, we show that arsenian

327

pyrite is present in the NOM-rich peat layer (type IV) interpreted as paleo-mangrove deposits,

328

as well as in less NOM-rich clay layers (types II and III) interpreted as shallow marine

329

deposits. In these environments, both the formation of authigenic arsenian pyrite and the

330

preservation of detrital arsenian pyrite are possible. The latter could originate from arsenic-

331

rich coal deposits in the upper Mekong regions34, where it is the dominant arsenic host35,36.

332

However, it is generally expected to undergo oxidation during erosional processes37. Thus,

333

arsenian pyrite is likely to have formed authigenically in the mangrove sediments,

334

incorporating As into the pyrite structure38, 39, 40. Indeed, both the framboidal and euhedral

14


Page 14 of 25

Page 15 of 25


335

pyrite particles found in the sediments are reported to form in low-temperature sulfate-

336

reducing environments40, such as marine41 and mangrove sediments33.

337 338

Oxygen-bound arsenic in the sediment. In the present study, LCF analysis of both the As

339

K-edge XANES and EXAFS data must invoke sorbed As(III) and As(V) oxyanions to obtain

340

acceptable fits (Figure 2b, Figure 3, Table S5). This is consistent with previous studies, which

341

have shown that bulk As XAS analyses of reduced deltaic sediments exhibited As(III) and

342

As(V) along with arsenian pyrite10,

343

(oxyhydr)oxides that were not bioavailable in the sediment. However, this interpretation has

344

been controversial due to the need to implicate the persistence of ferric minerals in reduced

345

sediments42. In the present cores, at depths below 7 m (i.e., in sediment types II-V), ferric

346

oxyhydroxides are under the detection limit of EXAFS (~10 % of total Fe) and were not

347

observed by STEM, suggesting that As(V) could be associated with another phase in these

348

reduced layers. Moreover, STEM-EDS analysis shows that siderite43 or phyllosilicate

349

minerals44, 45 known to potentially bear As(III) and As(V) did not show detectable amount of

350

As in any of the reduced sediment samples (data not shown). Therefore, the identities of the

351

oxygen bound As(III) and As(V) pools in the reduced sediment remain unknown but are

352

unlikely to be ferric (oxyhydr)oxides.

12

and invoked As(V) adsorbed onto ferric

353 354

Environmental implications. The correlation of As speciation with both Fe speciation and

355

the chronostratigraphic record is consistent with the distribution of As species being governed

356

by depositional history. These processes occurring during diagenesis include NOM

357

accumulation and early diagenetic pyritization resulting from Holocene transgression in the

358

Mekong Delta. Consequently, the distribution of As species in these sediments, shown to

359

result from sedimentary processes, helps constrain the conceptual model of the origin of As

360

species. The model for As release from sediment to groundwater in South and Southeast Asia 15



361

that has emerged from previous work invokes the reduction of As-bearing Fe(III)

362

(oxyhydr)oxides in the near surface environment of flooded soils6, 8, 46. Here, we confirm the

363

presence of Fe(III) (oxyhydr)oxides in the first 7 m of the sediment from the Mekong Delta in

364

Vietnam and their gradual disappearance at depth. Further, we show that arsenian pyrite and

365

thiol-bound trivalent arsenic in NOM are the predominant As species in the reduced sediment.

366

Accumulation of both pyrite-associated and NOM-bound As in the reduced sediment layers

367

are proposed to have occurred in the paleo-mangrove environment and in shallow marine

368

sediments. Complexation of As by NOM could also be a relevant process for the Bengal

369

Delta as arsenic enrichment by NOM during marine transgression has also been proposed for

370

sediments there47.

371

Immobilization of As by pyrite and NOM in the reduced sediment layers could remain

372

an active mechanism to date, trapping dissolved arsenic9, 11, 27 released by As(V) bioreduction6

373

at the sediment redox boundary, and advected down the sediment profile48 (which is

374

supported by hydraulic conductivity data for the sediments (Table S7)). On the other hand,

375

pyrite-associated and NOM-bound As could serve as local sources of As. To date, no study

376

has specifically tackled this potential and, at this time, we cannot exclude that possibility.

377

Further investigations, potentially laboratory-based experiments on these sediments and

378

sediment porewaters analyses, are needed to evaluate the reactivity of the As-rich sediment

379

layers inherited from depositional processes with respect to their ability to trap or release As

380

to advected porewaters.

381 382 383 384 385 386 387 388 389 390

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxx AUTHOR INFORMATION Corresponding author *E-mail: [email protected] 16


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391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441


Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This study is funded by the Swiss National Science Foundation under grant number 200021_157007. It was conducted under the framework of a CARE-RESCIF initiative. We acknowledge funding from the CODEV-EPFL Seed Money Fund and from RESCIF-EPFL. We also acknowledge funding for the OSL dating from the Swiss National Science Foundation under grant number PZ00P2_167960. We are grateful to CARE for access to the laboratory facility at HCMUT. We thank the following individuals in Vietnam for help with fieldwork: Mr. Nguyen Huu Loc (DONRE of An Giang), Mr. Pham Van Si (Division of Natural Resources & Environment, An Phu district, An Giang province), Mr. Hieu (staff of Quoc Thai Commune), Mr. Huynh (Land owner at Quoc Thai), and Vu Viet Anh, Le Hoang Anh, Nguyen Thi Bao Tu, Pham Kim Bao Ngan, Hua Bao Anh and Le Khanh (students of Faculty of Environment & Natural Resources, HCMUT - VNU). We thank Elena Rossel from EPFL for help with TOC analyses, CEAL of EPFL for analytical support, Jean-Claude Lavanchy from University of Lausanne for assistance during XRF analysis, and CIME at EPFL for access to electron microscopes. We also thank Ludovic Delbes and Benoit Baptiste for their help in anaerobic XRD measurements as well as Imène Esteve for her help in SEM data acquisition at IMPMC. The Fe K-edge XAS experiments were performed on the XAFS beamline at ELETTRA, Trieste, Italy and on the 4-1 beamline at the Stanford Synchrotron Radiation Lightsource (SSRL). The As K-edge XAS experiments were performed on the 11-2 beamline at SSRL. We are grateful for the technical assistance received during the synchrotron-based analyses from Ryan Davis and John Bargar at SSRL, and from Luca Olivi, Clara Guglieri Rodriguez and Giuliana Aquilanti at ELETTRA. Use of the SSRL, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515.

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23. Wang, Y.; Morin, G.; Ona-Nguema, G.; Menguy, N.; Juillot, F.; Aubry, E.; Guyot, F.; Calas, G.; Brown Jr., G. E. Arsenite sorption at the magnetite-water interface during aqueous precipitation of magnetite: EXAFS evidence for a new arsenite surface complex. Geochim. Cosmochim. Acta 2008, 72, 2573-2586. 24. Wang, Y.; Morin, G.; Ona-Nguema, G.; Juillot, F.; Guyot, F.; Calas, G.; Brown Jr., G. E. Evidence for Different Surface Speciation of Arsenite and Arsenate on Green Rust: An EXAFS and XANES Study. Environ. Sci. Technol. 2010, 44, 109-115. 25. Wang, Y.; Morin, G.; Ona-Nguema, G.; Brown, G. E. Arsenic(III) and Arsenic(V) Speciation during Transformation of Lepidocrocite to Magnetite. Environmental Science & Technology 2014, 48, 14282-14290. 26. Wang, Y.; Morin, G.; Ona-Nguema, G.; Juillot, F.; Calas, G.; Brown Jr., G. E. Distinctive Arsenic(V) Trapping Modes by Magnetite Nanoparticles Induced by Different Sorption Processes. Environ. Sci. Technol. 2011, 45, 7258-7266. 27. Langner, P.; Mikutta, C.; Kretzschmar, R. Arsenic sequestration by organic sulphur in peat. Nature Geosci 2012, 5, 66-73. 28. Miot, J.; Morin, G.; Skouri-Panet, F.; Férard, C.; Aubry, E.; Briand, J.; Wang, Y.; Ona-Nguema, G.; Guyot, F.; Brown, G. E. XAS Study of Arsenic Coordination in Euglena gracilis Exposed to Arsenite. Environ. Sci. Technol. 2008, 42, 5342-5347. 29. Newman, D. K.; Beveridge, T. J.; Morel, F. M. M. Precipitation of arsenic trisulfide by Desulfotomaculum auripigmentum. Applied and Environmental Microbiology 1997, 63, 2022-2028. 30. Burton, E. D.; Johnston, S. G.; Kocar, B. D. Arsenic Mobility during Flooding of Contaminated Soil: The Effect of Microbial Sulfate Reduction. Environmental Science & Technology 2014, 48, 13660-13667. 31. Otero, X. L.; Ferreira, T. O.; Huerta-Díaz, M. A.; Partiti, C. S. M.; Souza Jr, V.; VidalTorrado, P.; Macías, F. Geochemistry of iron and manganese in soils and sediments of a mangrove system, Island of Pai Matos (Cananeia — SP, Brazil). Geoderma 2009, 148, 318335. 32. Chmura, G. L.; Anisfeld, S. C.; Cahoon, D. R.; Lynch, J. C. Global carbon sequestration in tidal, saline wetland soils. Global Biogeochemical Cycles 2003, 17, 1111. 33. Noël, V.; Marchand, C.; Juillot, F.; Ona-Nguema, G.; Viollier, E.; Marakovic, G.; Olivi, L.; Delbes, L.; Gelebart, F.; Morin, G. EXAFS analysis of iron cycling in mangrove sediments downstream a lateritized ultramafic watershed (Vavouto Bay, New Caledonia). Geochimica et Cosmochimica Acta 2014, 136, 211-228. 34. Zhou, Y.; Ren, Y. Distribution of arsenic in coals of Yunnan Province, China, and its controlling factors. International Journal of Coal Geology 1992, 20, 85-98. 35. Huggins, F. E.; Shah, N.; Zhao, J.; Lu, F.; Huffman, G. P. Nondestructive determination of trace element speciation in coal and coal ash by XAFS spectroscopy. Energy & Fuels 1993, 7, 482-489. 36. Yudovich, Y. E.; Ketris, M. P. Arsenic in coal: a review. International Journal of Coal Geology 2005, 61, 141-196. 37. Nickson, R. T.; McArthur, J. M.; Ravenscroft, P.; Burgess, W. G.; Ahmed, K. M. Mechanism of arsenic release to groundwater, Bangladesh and West Bengal. Appl. Geochem. 2000, 15, 403-413. 38. Savage, K. S.; Tingle, T. N.; O’Day, P. A.; Waychunas, G. A.; Bird, D. K. Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California. Applied Geochemistry 2000, 15, 1219-1244. 39. Le Pape, P.; Blanchard, M.; Brest, J.; Boulliard, J.-C.; Ikogou, M.; Stetten, L.; Wang, S.; Landrot, G.; Morin, G. Arsenic Incorporation in Pyrite at Ambient Temperature at Both Tetrahedral S–I and Octahedral FeII Sites: Evidence from EXAFS–DFT Analysis. Environmental Science & Technology 2017, 51, 150-158. 19



543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567

40. Blanchard, M.; Alfredsson, M.; Brodholt, J.; Wright, K.; Catlow, C. R. A. Arsenic incorporation into FeS2 pyrite and its influence on dissolution: A DFT study. Geochimica Et Cosmochimica Acta 2007, 71, 624-630. 41. Wilkin, R. T.; Barnes, H. L. Formation processes of framboidal pyrite. Geochimica et Cosmochimica Acta 1997, 61, 323-339. 42. Polizzotto, M. L.; Harvey, C. F.; Li, G.; Badruzzman, B.; Ali, A.; Newville, M.; Sutton, S.; Fendorf, S. Solid-phases and desorption processes of arsenic within Bangladesh sediments. Chem. Geol. 2006, 228, 97-111. 43. Jönsson, J.; Sherman, D. M. Sorption of As(III) and As(V) to siderite, green rust (fougerite) and magnetite: Implications for arsenic release in anoxic groundwaters. Chem. Geol. 2008, 255, 173-181. 44. Goldberg, S. Competitive adsorption of arsenate and arsenite on oxides and clay minerals. Soil Science Society of America Journal 2002, 66, 413-421. 45. Chakraborty, S.; Wolthers, M.; Chatterjee, D.; Charlet, L. Adsorption of arsenite and arsenate onto muscovite and biotite mica. Journal of Colloid and Interface Science 2007, 309, 392-401. 46. Islam, F. S.; Gault, A. G.; Boothman, C.; Polya, D. A.; Charnock, J. M.; Chatterjee, D.; Lloyd, J. R. Role of metal-reducing bacteria in arsenic release from Bengal delta sediments. Nature 2004, 430, 68-71. 47. Chatterjee, D.; Roy, R. K.; Basu, B. B. Riddle of arsenic in groundwater of Bengal Delta Plain—role of non-inland source and redox traps. Environmental Geology 2005, 49, 188-206. 48. Polizzotto, M. L.; Kocar, B. D.; Benner, S. G.; Sampson, M.; Fendorf, S. Near-surface wetland sediments as a source of arsenic release to ground water in Asia. Nature 2008, 454, 505-508.

568

20


Page 20 of 25

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569 570 571


Table 1. Dating results for representative depths of all layer types in core QTC3 determined using optically stimulated luminescence (OSL) dating or 14C dating (cal. yr BP). Note: σ represents standard deviation of sediment dating results. Depth (m)

Sediment type Dating method

Age ± 2σ (yr)

2

I

OSL

2,560 ± 360

10

II

OSL

7,830 ± 890

12

III

OSL

6,480 ± 790

16

IV

18

V

OSL

8,840 ± 1,670

33

V

OSL

16, 870 ± 2,190

14

C

8,079 ± 708

572

21



Page 22 of 25

573 0 5 10

Type I: clay + Fe(III) oxides

Type II: clay + OM + pyrite

25 30

c

d

10

Type III: clay + siderite + pyrite Type II Type IV: OM + pyrite

20

b

5

QTC3

Depth (m)

15

0

Depth (m)

a

QTC2

574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598

15 20 25

Type V: clay + siderite

30 35

0 20 40 60 80 0 1 2 3 4 5 Sediment As content Sediment S content (ppm) (%)

0

10 20 30 40 Sediment TOC (%)

Figure 1. (a) Scheme showing the lithology of the QTC2 and QTC3 sediment cores based on on-site observation and powder XRD analysis of sediment samples (Figure S6). (b) arsenic, (c) sulfur, and (d) total organic carbon content profiles of the QTC2 (red circles, Table S1) and QTC3 (open blue squares, Table S2) sediment samples as a function of depth. The term “clay” in the figure refers to the mineralogy.

22


Page 23 of 25


599 600 601 602 603 604 605 606 607 608 609 610 611

2m

type I: 2,560 ± 360 yr BP

3m

type I

4m

type I

6m

type I

7m

type I

7m

type I

8m

type II

8m

type II

10 m

type II

2m

b

type I As in pyrite Thiol-bound As(III) Arsenite Arsenate

Fe in clays Fe in oxides Fe in pyrite Fe in siderite Fe in NOM

Depth

Depth

a

10 m

type II: 7,820 ± 890 yr

12 m

type III: 6,480 ± 790 yr

12 m

type III

14 m

type II

14 m

type II

16 m

type IV: 8,079 ± 708 cal. yr BP

16 m

17 m

type V

18 m

type V: 8,840 ± 1,670 yr

18 m

type V

20 m

type V

20 m

type V

type IV

0 20 40 60 80 As Concentration by Speciation (ppm)

0 1 2 3 4 5 Fe Concentration by Speciation (wt. %)

Figure 2. (a) Fe quantitative speciation in the depth range of 0-20 m and age of selected sediment layers. Quantitative speciation is calculated by multiplying the Fe concentration in QTC2 sediment samples (Table S1) and the fraction of Fe as each of the five species (Table S3) obtained by LCF of Fe K-edge EXAFS data (Figure S8). The Fe-rich clay fraction combines Fe in various reference clay minerals that are reported in Table S3. The age of selected sediment layers were determined using 14C or OSL methods and the analytical details were provided in the Supporting Information. (b) Arsenic quantitative speciation in the depth range 0-20 m. The As quantitative speciation is calculated by multiplying As concentration in QTC2 sediment samples (Table S1 and the fraction of As as each of the four species (Table S5) obtained by LCF of As K-edge EXAFS data (Figure 3).

23



Page 24 of 25

612 1

a

10

b

c

Type I (2m) Type I (7m)

Type II (10m)

Type II (14m)

3

Type III (12m)

k χ(k)

Normalized absorption

Type II (8m)

Type IV (16m)

Type V (18m)

Type V (20m)

Type V (35m)

11850

613 614 615 616 617 618 619

11880 11910 Energy (eV)

4

6

8 10 -1 k (Å )

12

Figure 3. As K-edge XAS data. As K-edge XAS data collected at 10 K for selected QTC2 sediment samples and results of linear combination fit (LCF). Experimental and calculated curves are plotted as solid and dotted lines, respectively. (a) XANES data. (b) Unfiltered EXAFS k3χ(k) functions. (c) Fourier transform (FT) of unfiltered k3χ(k) functions in the k range of 3-12 Å-1. The LCF results are reported in Table S5.

24


Page 25 of 25


Fe(III)

Fe oxides

ﬂood water

O-As(V)

O-As(V)

reduc3ve dissolu3on

aqueous As

Py + NOM

S-As(III)

As

-O-As(V) + -O-As(III)

Sd + Py

S-As(III)

As


Py + NOM

S-As(III)

As


Py + NOM

S-As(III)

As


Sd + NOM

S-As(III)


Fe(II)

620 621 622 623 624 625 626 627 628 629 630 631 632 633

Py : pyrite Sd: siderite

: Fe oxides

: pyrite

: NOM

Alluvial deposi-on layer (type I)

Shallow-marine layer (types II, 7830 yr)

Shallow-marine layer (types III, 6480 yr)

Shallow-marine layer (types II)

Paleo-mangrove layer (types IV, 8079 cal. yr BP)

Pre-transgression layer (type V)

-S-As(III): sulfur-bound As(III) -O-As(V): oxygen-bound As(V) -O-As(III): oxygen-bound As(III)

Figure 4. Conceptual model for As speciation in Mekong Delta sediment. Flood water from the Mekong River brings As-bearing Fe(III) (oxyhydr)oxides. Before marine transgression, Fe (oxyhydr)oxides are buried during sediment deposition. Marine transgression at ∼8,079 yr yields a paleo-mangrove layer in which As is trapped in the form of pyrite- and NOMassociated As during diagenesis. Further extent of the marine transgression leads to shallow marine deposits in which As is trapped in arsenian pyrite and, to a lesser extent, by NOM. Arsenate and arsenite persist in these reduced paleo-layers. Marine regression after 5,000 BP resets the deposition of As-bearing Fe(III) (oxyhydr)oxides. Fe(III) (oxyhydr)oxides begin to be reduced after burial, releasing As into sediment porewater. The size of the pyrite symbol indicates its relative abundance in different sediment layers; the grayscale of the type II – IV layers represents the relative abundance of NOM. Note that the thickness of the sediment layers is not to scale.

25


Arsenic Speciation in Mekong Delta Sediments Depends on Their

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