Environmental NanoChemistry EECE 534 Environmental NanoChemistry Arsenic Species Formed from Arsenopyrite Weathering along a Contamination Gradient in Circumneutral River Floodplain Soils Petar N. Mandaliev,†,§ Christian Mikutta,† Kurt Barmettler,† Tsvetan Kotsev,‡ and Ruben Kretzschmar †Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, CHN, 8092 Zurich, Switzerland ‡Department of Geography, National Institute of Geophysics, Geodesy and Geography, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Xuanhao Wu 2016.2.15

Guidelines Introductions Scientific Questions and Objectives Materials and Methods Results and Discussions Conclusions Implications

Introduction Natural riverine floodplains An area of land adjacent to a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge. Toxic trace metals: Cd, Zn, Pb, Cu, Hg Metalloids: As, Sb Mainly sources? Mine waste discharge Ore processing Tailings dam failures www.nature.org

Arsenic Sulfoarsenide and sulfide minerals: Arsenopyrite (FeAsS) Realgar (α-As4S4) Orpiment (As2S3) Predominant forms of dissolved As in well-aerated soils: Arsenate species: H2AsO4, HAsO42− Reducing conditions in anaerobic soils: Arsenite species: H3AsO30, H2AsO3 www.nrdc.org

As-Fe Inner-sphere surface complexes Siderite (FeCO3) Magnetite (Fe3O4) Carbonate green rust (Fe2+6(1−x)Fe3+6xO12H2(7−3x)CO3·3H2O; 0 ≤ x ≤ 1) Goethite (α-FeOOH) Bacteria Poorly crystalline Fe(III) oxyhydroxides As Secondary As minerals arsenic.tamu.edu

Scientific Questions Objectives What is the biogeochemistry of As in strongly As-polluted, circumneutral floodplain soils? How is As and Fe spatially correlated in soils? Objectives Examine the solid-phase speciation and distribution of As (and Fe) in floodplain soils. Detection of secondary As species in soils by X-ray fluorescence (XRF) spectrometry, powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and selective chemical extraction of poorly crystalline mineral phases.

Materials and methods Field Site Ogosta River Historic mining A large tailing dam failure in 1964

Soil Sampling and Soil Characterization Flash-frozen using liquid N2 X-ray diffraction (XRD): soil mineralogy X-ray fluorescence spectrometry (XRF): total element contents Poorly crystalline As extraction: sodium citrate, ascorbic acid, sodium bicarbonate (pH 7.5) Inductively coupled plasma-optical emission spectrometry (ICP-OES)

Arsenic and Iron XAS X-ray absorption near-edge structure (XANES) Extended X-ray absorption fine structure (EXAFS) As K-edge (11867 eV) and Fe K-edge (7112 eV) www.chemwiki.ucdavis.edu

X-ray absorption near-edge structure (XANES) X-rays are ionizing electromagnetic radiation that have sufficient energy to excite a core electron of an atom to an excitonic state. Different core electrons have distinct binding energies. Absorption edge Energies of absorption edges in X-ray absorption spectra reveal the identity of the corresponding absorbing elements. As(V) (edge energy: 11873 eV) As(-I) (edge energy: 11866 eV) Trivalent As (edge energy: 11869 eV) As K-edge (11867 eV) Fe K-edge (7112 eV) Atoms with a higher oxidation state require more energetic X-ray to excite its core electron because the nucleus is less-shielded and carries a higher effective charge.

Extended X-ray absorption fine structure (EXAFS) within 50 eV extend to 1000 eV or more above the absorption edges The oscillatory structure caused by the interference between the outgoing and the back-scattered photoelectron waves. Properties analyzed include coordination number, disorder of neighboring atoms, and distance of neighboring atoms.

Results and Discussions Soil Characterization pH: 5.7 - 7.1 Inorganic C: 41 g/kg Organic C: mostly less than 20 g/kg XRD results: samples contained quartz, calcite, siderite, phyllosilicates and feldspars.

Spatial Distribution and Extractability of As and Fe Pearson correlations (ρ) between As and Fe or S along the transect: ρAs−Fe = 0.75, P < 0.001 ρAs−S = 0.45, P < 0.05 As was deposited along with Fe and S in the floodplain.

Spatial Distribution and Extractability of As and Fe Selective chemical extractions with citrate-ascorbate solution at circumneutral pH >0.25 As associated with poorly crystalline Fe oxyhydroxides P1 and P2: 19−78% (average 53%) of total As 9−27% (average 17%) of total Fe P3: 54−62% of total As 10−13% of total Fe No Ca arsenates

Arsenic XANES As K-edge XANES spectroscopy As(V) (edge energy: 11873 eV) As(-I) (edge energy: 11866 eV) Trivalent As (edge energy: 11869 eV) Reference compounds As(V)-adsorbed ferrihydrite Arsenopyrite(-I) Amorphous ferric arsenate (AFA) As(III)-adsorbed ferrihydrite

As K-edge EXAFS spectroscopy

Iron XAS

Iron XAS As-rich hydrous ferric oxide (As-HFO-0.39)

Structure of As-HFO-0.39 As(V) effectively suppressed the polymerization of Fe(III) via corner sharing octahedral linkages and that the As-HFO-0.39 reference consists of small polymers of edge-sharing FeO6 octahedra to which As(V) is bound in a 2C complex. Fe-As distance: 3.32 ± 0.02 Å Complies with those found for monodentate binuclear (2C) surface complexes of As(V) on Fe(III) oxyhydroxides. Slightly longer (∼0.03−0.08 Å) compared to distances of edge-sharing FeO6 octahedra in the structure of common crystalline Fe(III) oxyhydroxides Fe-Fe distance: 3.09 ± 0.01 Å.

Conclusions Arsenic and Fe were found to be spatially correlated and both elements were strongly enriched in the fine soil particle size fractions (<2 μm and 2−50 μm). Between 14 and 82% of the total As was citrate-ascorbate extractable. Molar As/Fe ratios were as high as 0.34 in the bulk soil extracts and increased up to 0.48 in extracts of the fine particle size fractions. Arsenic K-edge XAS spectra showed the predominance of As(V) and were well fitted with a reference spectrum of As(V) adsorbed to ferrihydrite. Whereas no As(III) was detected, considerable amounts of As(-I) were present and identified as arsenopyrite originating from the mining waste. Iron K-edge XAS revealed that in addition to As(V) adsorbed to ferrihydrite, X-ray amorphous As(V)-rich hydrous ferric oxides (“As-HFO”) was the dominating secondary As species in the soils.

Environmental Implications The extremely high concentrations of As in the fine particle size fractions (up to 214 g/kg) and its association with poorly crystalline Fe(III) oxyhydroxides and As-HFO phases suggest a high As mobilization potential under both oxic and anoxic conditions, as well as a high bioaccessibility of As upon ingestion, dermal contact, or inhalation by humans or animals. Information on the particle-size dependent As speciation must therefore be taken into account for evaluating the health risks associated with As in strongly polluted soils used for agriculture and gardening, and for the development of appropriate soil remediation or risk minimization strategies. The speciation method of As species in this place may also be used in other contamination spots.

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