Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland: Speciation Changes of Iron, Sulfur, and Arsenic

TY - JOUR

T1 - Sulfidization of Organic Freshwater Flocs from a Minerotrophic Peatland

T2 - Speciation Changes of Iron, Sulfur, and Arsenic

AU - Thomasarrigo, Laurel K.

AU - Mikutta, Christian

AU - Lohmayer, Regina

AU - Planer-Friedrich, Britta

AU - Kretzschmar, Ruben

PY - 2016/4/5

Y1 - 2016/4/5

N2 - 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. (Figure Presented).

AB - 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. (Figure Presented).

U2 - 10.1021/acs.est.5b05791

DO - 10.1021/acs.est.5b05791

M3 - Journal article

C2 - 26967672

SN - 0013-936X

VL - 50

SP - 3607

EP - 3616

JO - Environmental Science & Technology

JF - Environmental Science & Technology

IS - 7

ER -