7 research outputs found
New Clues to the Local Atomic Structure of Short-Range Ordered Ferric Arsenate from Extended Xāray Absorption Fine Structure Spectroscopy
Short-range
ordered ferric arsenate (FeAsO<sub>4</sub>Ā·<i>x</i>H<sub>2</sub>O) is a secondary As precipitate frequently
encountered in acid mine waste environments. Two distinct structural
models have recently been proposed for this phase. The first model
is based on the structure of scorodite (FeAsO<sub>4</sub>Ā·2H<sub>2</sub>O) where isolated FeO<sub>6</sub> octahedra share corners
with four adjacent arsenate (AsO<sub>4</sub>) tetrahedra in a three-dimensional
framework (framework model). The second model consists of single chains
of corner-sharing FeO<sub>6</sub> octahedra being bridged by AsO<sub>4</sub> bound in a monodentate binuclear <sup>2</sup>C complex (chain
model). In order to rigorously test the accuracy of both structural
models, we synthesized ferric arsenates and analyzed their local (<6
Ć
) structure by As and Fe K-edge extended X-ray absorption fine
structure (EXAFS) spectroscopy. We found that both As and Fe K-edge
EXAFS spectra were most compatible with isolated FeO<sub>6</sub> octahedra
being bridged by AsO<sub>4</sub> tetrahedra (<i>R</i><sub>FeāAs</sub> = 3.33 Ā± 0.01 Ć
). Our shell-fit results
further indicated a lack of evidence for single corner-sharing FeO<sub>6</sub> linkages in ferric arsenate. Wavelet-transform analyses of
the Fe K-edge EXAFS spectra of ferric arsenates complemented by shell
fitting confirmed Fe atoms at an average distance of ā¼5.3 Ć
,
consistent with crystallographic data of scorodite and in disagreement
with the chain model. A scorodite-type local structure of short-range
ordered ferric arsenates provides a plausible explanation for their
rapid transformation into scorodite in acid mining environments
Speciation of Zn in Blast Furnace Sludge from Former Sedimentation Ponds Using Synchrotron X-ray Diffraction, Fluorescence, and Absorption Spectroscopy
Blast furnace sludge (BFS), an industrial waste generated in pig iron production, typically contains high contents of iron and various trace metals of environmental concern, including Zn, Pb, and Cd. The chemical speciation of these metals in BPS is largely unknown. Here, we used a combination of synchrotron X-ray diffraction, micro-X-ray fluorescence, and X-ray absorption spectroscopy at the Zn K-edge for solid-phase Zn speciation in 12 BPS samples collected on a former BFS sedimentation pond site. Additionally, one fresh BPS was analyzed for comparison. We identified five major types of Zn species in the BFS, which occurred in variable amounts: (1) Zn in the octahedral sheets of phyllosilicates, (2) Zn sulfide minerals (ZnS, sphalerite, or wurtzite), (3) Zn in a KZn-ferrocyanide phase (K2Zn3[Fe-(CN)(6)](2)center dot 9H(2)O), (4) hydrozincite (Zn-5(OH)(6)(CO3)(2)), and (5) tetrahedrally coordinated adsorbed Zn. The minerals franklinite (ZnFe2O4) and smithsonite (ZnCO3) were not detected, and zincite (ZnO) was detected only in traces. The contents of ZnS were positively correlated with the total S contents of the BPS. Similarly, the abundance of the KZn-ferrocyanide phase was closely correlated with the total CN contents, with the stoichiometry suggesting this as cyanide phase. This study provides the first quantitative Zn speciation in BFS deposits, which is of great relevance environmental risk assessment, the development of new methods for recovering Zn and Fe from BPS, and potential applications of BFS as sorbent materials in wastewater treatment
Arsenic Species Formed from Arsenopyrite Weathering along a Contamination Gradient in Circumneutral River Floodplain Soils
Arsenic is a toxic trace element,
which commonly occurs as contaminant
in riverine floodplains and associated wetlands affected by mining
and ore processing. In this study, we investigated the solid-phase
speciation of As in river floodplain soils characterized by circumneutral
pH (5.7ā7.1) and As concentrations of up to 40.3 g/kg caused
by former mining of arsenopyrite-rich ores. Soil samples collected
in the floodplain of Ogosta River (Bulgaria) were size-fractionated
and subsequently analyzed using a combination of X-ray fluorescence
(XRF) spectrometry, powder X-ray diffraction (XRD), X-ray absorption
spectroscopy (XAS), and selective chemical extraction of poorly crystalline
mineral phases. Arsenic and Fe were found to be spatially correlated
and both elements were strongly enriched in the fine soil particle
size fractions (<2 Ī¼m and 2ā50 Ī¼m). Between
14 and 82% of the total As was citrate-ascorbate extractable. Molar
As/Fe ratios were as high as 0.34 in the bulk soil extracts and increased
up to 0.48 in extracts of the fine particle size fractions. Arsenic <i>K</i>-edge XAS spectra showed the predominance of AsĀ(V) and
were well fitted with a reference spectrum of AsĀ(V) adsorbed to ferrihydrite.
Whereas no AsĀ(III) was detected, considerable amounts of AsĀ(-I) were
present and identified as arsenopyrite originating from the mining
waste. Iron <i>K</i>-edge XAS revealed that in addition
to AsĀ(V) adsorbed to ferrihydrite, X-ray amorphous AsĀ(V)-rich hydrous
ferric oxides (āAs-HFOā) with a reduced number of corner-sharing
FeO<sub>6</sub> octahedra relative to ferrihydrite were the dominating
secondary As species in the soils. The extremely high concentrations
of As in the fine particle size fractions (up to 214 g/kg) and its
association with poorly crystalline FeĀ(III) oxyhydroxides and As-HFO
phases suggest a high As mobilization potential under both oxic and
anoxic conditions, as well as a high bioaccessibility of As upon ingestion,
dermal contact, or inhalation by humans or animals
Bioaccessibility of Arsenic in Mining-Impacted Circumneutral River Floodplain Soils
Floodplain
soils are frequently contaminated with metalĀ(loid)Ās
due to present or historic mining, but data on the bioaccessibility
(BA) of contaminants in these periodically flooded soils are scarce.
Therefore, we studied the speciation of As and Fe in eight As-contaminated
circumneutral floodplain soils (ā¤21600 mg As/kg) and their
size fractions using X-ray absorption spectroscopy (XAS) and examined
the BA of As in the solids by in-vitro gastrointestinal (IVG) extractions.
Arsenopyrite and AsĀ(V)-adsorbed ferrihydrite were identified by XAS
as the predominant As species. The latter was the major source for
bioaccessible As, which accounted for 5ā35% of the total As.
The amount of bioaccessible As increased with decreasing particle
size and was controlled by the slow dissolution kinetics of ferrihydrite
in the gastric environment (pH 1.8). The relative BA of As (% of total)
decreased with decreasing particle size only in a highly As-contaminated
soil ā which supported by Fe XAS ā suggests the formation
of As-rich hydrous ferric oxides in the gastric extracts. Multiple
linear regression analyses identified Al, total As, C<sub>org</sub>, and P as main predictors for the absolute BA of As (adjusted <i>R</i><sup>2</sup> ā¤ 0.977). Health risk assessments for
residential adults showed that (i) nearly half of the bulk soils may
cause adverse health effects and (ii) particles <5 Ī¼m pose
the highest absolute health threat upon incidental soil ingestion.
Owing to their low abundance, however, health risks were primarily
associated with particles in the 5ā50 and 100ā200 Ī¼m
size ranges. These particles are easily mobilized from riverbanks
during flooding events and dispersed within the floodplain or transported
downstream
Speciation of Zn in Blast Furnace Sludge from Former Sedimentation Ponds Using Synchrotron Xāray Diffraction, Fluorescence, and Absorption Spectroscopy
Blast furnace sludge (BFS), an industrial waste generated
in pig
iron production, typically contains high contents of iron and various
trace metals of environmental concern, including Zn, Pb, and Cd. The
chemical speciation of these metals in BFS is largely unknown. Here,
we used a combination of synchrotron X-ray diffraction, micro-X-ray
fluorescence, and X-ray absorption spectroscopy at the Zn K-edge for
solid-phase Zn speciation in 12 BFS samples collected on a former
BFS sedimentation pond site. Additionally, one fresh BFS was analyzed
for comparison. We identified five major types of Zn species in the
BFS, which occurred in variable amounts: (1) Zn in the octahedral
sheets of phyllosilicates, (2) Zn sulfide minerals (ZnS, sphalerite,
or wurtzite), (3) Zn in a KZnāferrocyanide phase (K<sub>2</sub>Zn<sub>3</sub>[FeĀ(CN)<sub>6</sub>]<sub>2</sub>Ā·9H<sub>2</sub>O), (4) hydrozincite (Zn<sub>5</sub>(OH)<sub>6</sub>(CO<sub>3</sub>)<sub>2</sub>), and (5) tetrahedrally coordinated adsorbed Zn. The
minerals franklinite (ZnFe<sub>2</sub>O<sub>4</sub>) and smithsonite
(ZnCO<sub>3</sub>) were not detected, and zincite (ZnO) was detected
only in traces. The contents of ZnS were positively correlated with
the total S contents of the BFS. Similarly, the abundance of the KZnāferrocyanide
phase was closely correlated with the total CN contents, with the
stoichiometry suggesting this as the main cyanide phase. This study
provides the first quantitative Zn speciation in BFS deposits, which
is of great relevance for environmental risk assessment, the development
of new methods for recovering Zn and Fe from BFS, and potential applications
of BFS as sorbent materials in wastewater treatment
Arsenic species formed from arsenopyrite weathering along a contamination gradient in circumneutral river floodplain soils
Arsenic is a toxic trace element, which commonly occurs as contaminant in riverine floodplains and associated wetlands affected by mining and ore processing. In this study, we investigated the solid-phase speciation of As in river floodplain soils characterized by circumneutral pH (5.7ā7.1) and As concentrations of up to 40.3 g/kg caused by former mining of arsenopyrite-rich ores. Soil samples collected in the floodplain of Ogosta River (Bulgaria) were size-fractionated and subsequently analyzed using a combination of X-ray fluorescence (XRF) spectrometry, powder X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), and selective chemical extraction of poorly crystalline mineral phases. Arsenic and Fe were found to be spatially correlated and both elements were strongly enriched in the fine soil particle size fractions (<2 Ī¼m and 2ā50 Ī¼m). Between 14 and 82% of the total As was citrate-ascorbate extractable. Molar As/Fe ratios were as high as 0.34 in the bulk soil extracts and increased up to 0.48 in extracts of the fine particle size fractions. Arsenic K-edge XAS spectra showed the predominance of As(V) and were well fitted with a reference spectrum of As(V) adsorbed to ferrihydrite. Whereas no As(III) was detected, considerable amounts of As(-I) were present and identified as arsenopyrite originating from the mining waste. Iron K-edge XAS revealed that in addition to As(V) adsorbed to ferrihydrite, X-ray amorphous As(V)-rich hydrous ferric oxides (āAs-HFOā) with a reduced number of corner-sharing FeO6 octahedra relative to ferrihydrite were the dominating secondary As species in the soils. The extremely high concentrations of As in the fine particle size fractions (up to 214 g/kg) and its association with poorly crystalline Fe(III) oxyhydroxides and As-HFO phases suggest a high As mobilization potential under both oxic and anoxic conditions, as well as a high bioaccessibility of As upon ingestion, dermal contact, or inhalation by humans or animals