Freshwater flocs consisting primarily of microorganisms, natural organic matter, and oxyhydroxides of iron (Fe) are commonly found in organic-rich wetlands, and they may significantly affect the cycling and bioavailability of trace element in such environments. To study the reactivity of iron minerals present in such flocs with respect to Fe(II) and sulfide, we sampled floc material from streambeds of the geogenically As-enriched peatland <i>Gola di Lago</i> (Switzerland). Characterization of the floc materials by X-ray diffraction, Fe K-edge EXAFS spectroscopy, and Mössbauer spectroscopy showed that Fe (49−221 g/kg) was mainly present as ferrihydrite and nano-crystalline lepidocrocite. The flocs were rich in C (214−349 g/kg) and As (0.7−1.28 g As/kg), with the latter present as bidentate surface complexes of As(V) on Fe phases. In the first experiment, we reacted the floc materials under anoxic conditions with sulfide (5.2 mM, S(-II)/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 as well as solid-phase associated S(0) and polysulfdes. Ferrihydrite was preferentially reduced over lepidocrocite. The sulfide-reacted flocs still contained primarily As(V) (47−72%), despite abundant mackinawite precipitation. Only at high S(-II)/Fe molar ratios (≥1.0) did the formation of an orpiment-like phase account for up to 35% of solid-phase As. Significant mobilization of dissolved As was observed in all samples, with up to 33% of the aqueous As present as thioarsenates. In the second experiment, we reacted floc materials with isotopically enriched <sup>57</sup>Fe(II) solutions at pH 5.5 or 7 and followed the kinetics of Fe atom exchange and solid phase Fe speciation over one week. We observed 21.9-29.7 % Fe atom exchange (at pH 7) and slight changes in lepidocrocite and ferrihydrite contents, but all phases remained X-ray amorphous. Control experiments with pure ferrihydrite and ferrihydrite-PGA coprecipitates suggested that the formation of more crystalline Fe phases such as goethite was effectively hindered by organic matter present in the floc materials. No significant mobilization of dissolved As was observed in these experiments. The implications of our findings for As cycling in the peatlands will be discussed.

Iron mineral transformations in organic-rich freshwater flocs induced by Fe(II) and sulfide

253rd National Meeting (2017)

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