title: Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland: Speciation Changes of Iron, Sulfur, and Arsenic.
abstract: Iron-rich organic flocs are frequently observed in surface waters of wetlands and show a high affinity for trace metal(loid)s. Under low-flow stream conditions, flocs may settle, become buried, and eventually be subjected to reducing conditions facilitating trace metal(loid) release. In this study, we reacted freshwater flocs (704-1280 mg As/kg) from a minerotrophic peatland (Gola di Lago, Switzerland) with sulfide (5.2 mM, S(-II)spike/Fe = 0.75-1.62 mol/mol) at neutral pH and studied the speciation changes of Fe, S, and As at 25 ± 1 °C over 1 week through a combination of synchrotron X-ray techniques and wet-chemical analyses. Sulfidization of floc ferrihydrite and nanocrystalline lepidocrocite caused the rapid formation of mackinawite (52-81% of Fesolid at day 7) as well as solid-phase associated S(0) and polysulfides. Ferrihydrite was preferentially reduced over lepidocrocite, although neoformation of lepidocrocite from ferrihydrite could not be excluded. Sulfide-reacted flocs contained primarily arsenate (47-72%) which preferentially adsorbed to Fe(III)-(oxyhydr)oxides, despite abundant mackinawite precipitation. At higher S(-II)spike/Fe molar ratios (≥1.0), the formation of an orpiment-like phase accounted for up to 35% of solid-phase As. Despite Fe and As sulfide precipitation and the presence of residual Fe(III)-(oxyhydr)oxides, mobilization of As was recorded in all samples (Asaq = 0.45-7.0 μM at 7 days). Aqueous As speciation analyses documented the formation of thioarsenates contributing up to 33% of Asaq. Our findings show that freshwater flocs from the Gola di Lago peatland may become a source of As under sulfate-reducing conditions and emphasize the pivotal role Fe-rich organic freshwater flocs play in trace metal(loid) cycling in S-rich wetlands characterized by oscillating redox conditions.
authors.affiliation: Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich , 8092 Zurich, Switzerland.
authors.forename: Laurel K
authors.initials: LK
authors.lastname: ThomasArrigo
authors.affiliation: Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich , 8092 Zurich, Switzerland.|Section for Environmental Chemistry and Physics, Department of Plant and Environmental Sciences, University of Copenhagen , DK-1871 Frederiksberg C, Denmark.
authors.forename: Christian
authors.initials: C
authors.lastname: Mikutta
authors.affiliation: Department of Environmental Geochemistry, Bayreuth Center for Ecology and Environmental Research (BAYCEER), Bayreuth University , 95440 Bayreuth, Germany.
authors.forename: Regina
authors.initials: R
authors.lastname: Lohmayer
authors.affiliation: Department of Environmental Geochemistry, Bayreuth Center for Ecology and Environmental Research (BAYCEER), Bayreuth University , 95440 Bayreuth, Germany.
authors.forename: Britta
authors.initials: B
authors.lastname: Planer-Friedrich
authors.affiliation: Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich , 8092 Zurich, Switzerland.
authors.forename: Ruben
authors.initials: R
authors.lastname: Kretzschmar
mesh_terms: D001151:Arsenic / Q000302:isolation & purification*; D005427:Flocculation; D005618:Fresh Water / Q000737:chemistry*; D007501:Iron / Q000302:isolation & purification*; D008903:Minerals / Q000737:chemistry*; D009930:Organic Chemicals / Q000737:chemistry*; D010084:Oxidation-Reduction; D012996:Solutions; D013440:Sulfides / Q000737:chemistry*; D013455:Sulfur / Q000737:chemistry / Q000302:isolation & purification*; D053833:Wetlands; D056928:X-Ray Absorption Spectroscopy
chemical_list: D008903:Minerals; D009930:Organic Chemicals; D012996:Solutions; D013440:Sulfides; D013455:Sulfur; D007501:Iron; D001151:Arsenic