3 research outputs found
Effects of Manganese Oxide on Arsenic Reduction and Leaching from Contaminated Floodplain Soil
Reductive release
of the potentially toxic metalloid As from FeÂ(III)
(oxyhydr)Âoxides has been identified as an important process leading
to elevated As porewater concentrations in soils and sediments. Despite
the ubiquitous presence of Mn oxides in soils and their oxidizing
power toward AsÂ(III), their impact on interrelated As, Fe, and Mn
speciation under microbially reducing conditions remains largely unknown.
For this reason, we employed a column setup and X-ray absorption spectroscopy
to investigate the influence of increasing birnessite concentrations
(molar soil Fe-to-Mn ratios: 4.8, 10.2, and 24.7) on As speciation
and release from an As-contaminated floodplain soil (214 mg As/kg)
under anoxic conditions. Our results show that birnessite additions
significantly decreased As leaching. The reduction of both As and
Fe was delayed, and AsÂ(III) accumulated in birnessite-rich column
parts, indicating the passivation of birnessite and its transformation
products toward AsÂ(III) oxidation and the precipitation of FeÂ(III)Â(oxyhydr)Âoxides.
Microbial Mn reduction resulted in elevated soil pH values, which
in turn lowered the microbial activity in the birnessite-enriched
soil. We conclude that in Mn-oxide-rich soil environments undergoing
redox fluctuations, the enhanced As adsorption to newly formed FeÂ(III)
(oxyhydr)Âoxides under reducing conditions leads to a transient stabilization
of As
Impact of Birnessite on Arsenic and Iron Speciation during Microbial Reduction of Arsenic-Bearing Ferrihydrite
Elevated
solution concentrations of As in anoxic natural systems
are usually accompanied by microbially mediated AsÂ(V), MnÂ(III/IV),
and FeÂ(III) reduction. The microbially mediated reductive dissolution
of FeÂ(III)-(oxyhydr)Âoxides mainly liberates sorbed AsÂ(V) which is
subsequently reduced to AsÂ(III). Manganese oxides have been shown
to rapidly oxidize AsÂ(III) and FeÂ(II) under oxic conditions, but their
net effect on the microbially mediated reductive release of As and
Fe is still poorly understood. Here, we investigated the microbial
reduction of AsÂ(V)-bearing ferrihydrite (molar As/Fe: 0.05; Fe<sub>tot</sub>: 32.1 mM) by <i>Shewanella</i> sp. ANA-3 (10<sup>8</sup> cells/mL) in the presence of different concentrations of
birnessite (Mn<sub>tot</sub>: 0, 0.9, 3.1 mM) at circumneutral pH
over 397 h using wet-chemical analyses and X-ray absorption spectroscopy.
Additional abiotic experiments were performed to explore the reactivity
of birnessite toward AsÂ(III) and FeÂ(II) in the presence of MnÂ(II),
FeÂ(II), ferrihydrite, or deactivated bacterial cells. Compared to
the birnessite-free control, the highest birnessite concentration
resulted in 78% less Fe and 47% less As reduction at the end of the
biotic experiment. The abiotic oxidation of AsÂ(III) by birnessite
(<i>k</i><sub>initial</sub> = 0.68 ± 0.31/h) was inhibited
by MnÂ(II) and ferrihydrite, and lowered by FeÂ(II) and bacterial cell
material. In contrast, the oxidation of FeÂ(II) by birnessite proceeded
equally fast under all conditions (<i>k</i><sub>initial</sub> = 493 ± 2/h) and was significantly faster than the oxidation
of AsÂ(III). We conclude that in the presence of birnessite, microbially
produced FeÂ(II) is rapidly reoxidized and precipitates as As-sequestering
ferrihydrite. Our findings imply that the ability of Mn-oxides to
oxidize AsÂ(III) in water-logged soils and sediments is limited by
the formation of ferrihydrite and surface passivation processes
Mineralogical Controls on the Bioaccessibility of Arsenic in Fe(III)–As(V) Coprecipitates
X-ray
amorphous FeÂ(III)–AsÂ(V) coprecipitates are common
initial products of oxidative As- and Fe-bearing sulfide weathering,
and often control As solubility in mine wastes or mining-impacted
soils. The formation conditions of these solids may exert a major
control on their mineralogical composition and, hence, As release
in the gastric tract of humans after incidental ingestion of As-contaminated
soil. Here, we synthesized a set of 35 FeÂ(III)–AsÂ(V) coprecipitates
as a function of pH (1.5–8) and initial molar Fe/As ratio (0.8–8.0).
The solids were characterized by synchrotron X-ray diffraction, FT-IR
spectroscopy, and electrophoretic mobility measurements, and their
As bioaccessibility (BA<sub>As</sub>) was evaluated using the gastric-phase
Solubility/Bioavailability Research Consortium in vitro assay (SBRC-G).
The coprecipitates contained 1.01–4.51 mol kg<sup>–1</sup> As (molar Fe/As<sub>solid</sub>: 1.00–8.29) and comprised
varying proportions of X-ray amorphous hydrous ferric arsenates (HFA<sub>am</sub>) and AsÂ(V)-adsorbed ferrihydrite. HFA<sub>am</sub> was detected
up to pH 6 and its fraction decreased with increasing pH and molar
Fe/As ratio. Bioaccessible As ranged from 2.9 to 7.3% of total As
(<i>xÌ…</i> = 4.8%). The BA<sub>As</sub> of coprecipitates
formed at pH ≤ 4 was highest at formation pH 3 and 4 and controlled
by the intrinsically high solubility of the HFA<sub>am</sub> component,
possibly enhanced by sorbed sulfate. In contrast, the BA<sub>As</sub> of coprecipitates dominated by AsÂ(V)-adsorbed ferrihydrite was much
lower and controlled by As readsorption and/or surface precipitation
in the gastric fluid. Bioaccessible As increased up to 95% with increasing
liquid-to-solid ratio, indicating an enhanced solubility of these
solids due to interactions between Fe and the glycine buffer. We conclude
(i) that natural FeÂ(III)–AsÂ(V) coprecipitates exhibit a particularly
high solubility in the human gastric tract when formed at pH ∼
3–4 in the presence of sulfate, and (ii) that the in vitro
bioaccessibility of As in FeÂ(III)–AsÂ(V) coprecipitates as assessed
by tbe SBRC-G assay depends critically on their solid-phase concentration
in As-contaminated soil and mine-waste materials