2 research outputs found
Chromium(III) Complexation to Natural Organic Matter: Mechanisms and Modeling
Chromium is a common soil contaminant,
and it often exists as chromium(III).
However, limited information exists on the coordination chemistry
and stability of chromium(III) complexes with natural organic matter
(NOM). Here, the complexation of chromium(III) to mor layer material
and to Suwannee River Fulvic Acid (SRFA) was investigated using EXAFS
spectroscopy and batch experiments. The EXAFS results showed a predominance
of monomeric chromium(III)-NOM complexes at low pH (<5), in which
only Cr···C and Cr–O–C interactions were
observed in the second coordination shell. At pH > 5 there were
polynuclear
chromium(III)-NOM complexes with Cr···Cr interactions
at 2.98 Å and for SRFA also at 3.57 Å, indicating the presence
of dimers (soil) and tetramers (SRFA). The complexation of chromium(III)
to NOM was intermediate between that of iron(III) and aluminum(III).
Chromium(III) complexation was slow at pH < 4: three months or
longer were required to reach equilibrium. The results were used to
constrain chromium-NOM complexation in the Stockholm Humic Model (SHM):
a monomeric complex dominated at pH < 5, whereas a dimeric complex
dominated at higher pH. The optimized constant for the monomeric chromium(III)
complex was in between those of the iron(III) and aluminum(III) NOM
complexes. Our study suggests that chromium(III)-NOM complexes are
important for chromium speciation in many environments
Monothioarsenate Transformation Kinetics Determining Arsenic Sequestration by Sulfhydryl Groups of Peat
In
peatlands, arsenite was reported to be effectively sequestered
by sulfhydryl groups of natural organic matter. To which extent porewater
arsenite can react with reduced sulfur to form thioarsenates and how
this affects arsenic sequestration in peatlands is unknown. Here,
we show that, in the naturally arsenic-enriched peatland <i>Gola
di Lago</i>, Switzerland, up to 93% of all arsenic species in
surface and porewaters were thioarsenates. The dominant species, monothioarsenate,
likely formed from arsenite and zerovalent sulfur-containing species.
Laboratory incubations with sulfide-reacted, purified model peat showed
increasing total arsenic sorption with decreasing pH from 8.5 to 4.5
for both, monothioarsenate and arsenite. However, X-ray absorption
spectroscopy revealed no binding of monothioarsenate via sulfhydryl
groups. The sorption observed at pH 4.5 was acid-catalyzed dissociation
of monothioarsenate, forming arsenite. The lower the pH and the more
sulfhydryl sites, the more arsenite sorbed which in turn shifted equilibrium
toward further dissociation of monothioarsenate. At pH 8.5, monothioarsenate
was stable over 41 days. In conclusion, arsenic can be effectively
sequestered by sulfhydryl groups in anoxic, slightly acidic environments
where arsenite is the only arsenic species. At neutral to slightly
alkaline pH, monothioarsenate can form and its slow transformation
into arsenite and low affinity to sulfhydryl groups suggest that this
species is mobile in such environments