title: Iron and arsenic speciation and distribution in organic flocs from streambeds of an arsenic-enriched peatland.
abstract: Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. To date, spectroscopic speciation analyses of Fe and trace elements in these mineral-organic matter (OM) associations are missing. In this study, we investigated the speciation and distribution of Fe and As in flocs collected from low-flow streams (pH 5.3-6.3) of the naturally As-enriched peatland Gola di Lago (Switzerland) using (57)Fe Mössbauer spectroscopy and synchrotron X-ray techniques. The flocs were rich in acid carbohydrates and contained up to 22.1 wt % Fe, 34.9 wt % C, and 2620 mg/kg As. Mössbauer analyses revealed small quantities (<5%) of Fe(II) and Fe(III)-OM complexes and the predominance of ferrihydrite (∼ Fe(5)HO(8) · 4H2O, 51-59%) and lepidocrocite (γ-FeOOH, 34-46%). The latter was not observed by synchrotron X-ray diffraction, implying a coherent scattering domain size of <10 nm. Iron X-ray absorption spectroscopy (XAS) confirmed the Mössbauer results, and bulk As XAS indicated the prevalence of arsenate (71-84%) in the flocs. Shell-fit analyses showed that As was entirely sorbed to Fe(III)-(oxyhydr)oxides and that both arsenate and arsenite exclusively formed monodentate-binuclear ("bridging") complexes (R(As-Fe) = 3.31-3.34 Å). Microfocused X-ray fluorescence spectrometry documented a strong correlation between As and Fe in the flocs. These analyses also revealed intense As hotspots coinciding with abundant freshwater green algae (Closterium spp.). Microfocused As X-ray absorption near-edge structure spectra collected at algae-specific points identified up to 29% As(III), which, in combination with ∼ 5% As(III) detected at Fe-rich points, suggests As(V) bioreduction in the algae. Our findings imply that floc (bio)organics serve primarily as nucleation sites for the precipitation of nanocrystalline Fe(III)-(oxyhydr)oxides, rendering flocs effective sorbents for trace metal(loid)s. Thus, Fe-rich freshwater flocs likely play a pivotal role for the speciation and cycling of trace elements in wetlands.
authors.affiliation: Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, CHN, ETH Zurich , 8092 Zurich, Switzerland.
authors.forename: Laurel K
authors.initials: LK
authors.lastname: ThomasArrigo
authors.forename: Christian
authors.initials: C
authors.lastname: Mikutta
authors.forename: James
authors.initials: J
authors.lastname: Byrne
authors.forename: Kurt
authors.initials: K
authors.lastname: Barmettler
authors.forename: Andreas
authors.initials: A
authors.lastname: Kappler
authors.forename: Ruben
authors.initials: R
authors.lastname: Kretzschmar
mesh_terms: D001151:Arsenic / Q000302:isolation & purification*; D004777:Environment; D005427:Flocculation; D007501:Iron / Q000302:isolation & purification*; D009930:Organic Chemicals / Q000737:chemistry*; D045483:Rivers / Q000737:chemistry*; D013052:Spectrometry, X-Ray Emission; D015204:Spectroscopy, Mossbauer; D013557:Switzerland; D014874:Water Pollutants, Chemical / Q000302:isolation & purification*; D053833:Wetlands; D056928:X-Ray Absorption Spectroscopy
chemical_list: D009930:Organic Chemicals; D014874:Water Pollutants, Chemical; D007501:Iron; D001151:Arsenic