40 research outputs found
Mercury Reduction and Oxidation by Reduced Natural Organic Matter in Anoxic Environments
Natural organic matter (NOM)-mediated redox cycling of elemental mercury Hg(0) and mercuric Hg(II) is critically important in affecting inorganic mercury transformation and bioavailability. However, these processes are not well understood, particularly in anoxic water and sediments where NOM can be reduced and toxic methylmercury is formed. We show that under dark anoxic conditions reduced organic matter (NOM<sub>re</sub>) simultaneously reduces and oxidizes Hg via different reaction mechanisms. Reduction of Hg(II) is primarily caused by reduced quinones. However, Hg(0) oxidation is controlled by thiol functional groups via oxidative complexation, which is demonstrated by the oxidation of Hg(0) by low-molecular-weight thiol compounds, glutathione, and mercaptoacetic acid, under reducing conditions. Depending on the NOM source, oxidation state, and NOM:Hg ratio, NOM reduces Hg(II) at initial rates ranging from 0.4 to 5.5 h<sup>–1</sup>, which are about 2 to 6 times higher than those observed for photochemical reduction of Hg(II) in open surface waters. However, rapid reduction of Hg(II) by NOM<sub>re</sub> can be offset by oxidation of Hg(0) with an estimated initial rate as high as 5.4 h<sup>–1</sup>. This dual role of NOM<sub>re</sub> is expected to strongly influence the availability of reactive Hg and thus to have important implications for microbial uptake and methylation in anoxic environments
Thiol-Facilitated Cell Export and Desorption of Methylmercury by Anaerobic Bacteria
Methylmercury (MeHg) toxin, formed
by anaerobic bacteria, is rapidly
excreted from cells, but the mechanism of this process is unclear.
We studied the factors affecting MeHg export and its distribution
in cells, on cell surfaces, and in solution by two known mercury methylators, Geobacter sulfurreducens PCA and Desulfovibrio
desulfuricans ND132. Thiols, such as cysteine, were
found to greatly facilitate desorption and export of MeHg, particularly
by PCA cells. In cysteine-free assays (4 h), less than 10% of the
synthesized MeHg was found in solution and greater than 90% was associated
with PCA, of which about 73% was sorbed on the cell surface and 19%
remained inside the cells. In comparison, 77% of MeHg was in solution,
leaving about 13% of MeHg sorbed and about 10% inside the ND132 cells.
Our results demonstrate that MeHg export is bacteria specific, time
dependent, and influenced by thiols, implicating important roles of
ligands, such as natural organic matter, in MeHg production and mobilization
in the environment
Cysteine Inhibits Mercury Methylation by <i>Geobacter sulfurreducens</i> PCA Mutant Δ<i>omcBESTZ</i>
Cysteine enhances Hg uptake and methylation
by the <i>Geobacter
sulfurreducens</i> PCA wild-type (WT) strain in short-term assays.
The prevalence of this enhancement in other strains remains poorly
understood. We examined the influence of cysteine concentration on
time-dependent HgÂ(II) reduction, sorption, and methylation by PCA
WT and its <i>c</i>-type cytochrome-deficient mutant (Δ<i>omcBESTZ</i>) in phosphate-buffered saline. Without cysteine,
the mutant methylated twice as much HgÂ(II) as the PCA WT, whereas
addition of cysteine inhibited Hg methylation, regardless of the reaction
time. PCA WT, however, exhibited both time-dependent and cysteine
concentration-dependent methylation. In a 144 h assay, nearly complete
sorption of the HgÂ(II) by PCA WT occurred in the presence of 1 mM
cysteine, resulting in our highest observed level of methylmercury
production. The chemical speciation modeling and experimental data
suggest that uncharged HgÂ(II) species are more readily taken up and
that this uptake is kinetically limiting, thereby affecting Hg methylation
by both the mutant and WT
Photochemical Oxidation of Dissolved Elemental Mercury by Carbonate Radicals in Water
Photochemical
oxidation of dissolved elemental mercury, Hg(0),
affects mercury chemical speciation and its transfer at the water–air
interface in the aquatic environment. The mechanisms and factors that
control Hg(0) photooxidation, however, are not completely understood,
especially concerning the role of dissolved organic matter (DOM) and
carbonate (CO<sub>3</sub><sup>2–</sup>) in natural freshwaters.
Here, we evaluate Hg(0) photooxidation rates affected by reactive
ionic species (e.g., DOM, CO<sub>3</sub><sup>2–</sup>, and
NO<sub>3</sub><sup>–</sup>) and free radicals in creek water
and a phosphate buffer solution (pH 8) under simulated solar irradiation.
The Hg(0) photooxidation rate (<i>k</i> = 1.44 h<sup>–1</sup>) is much higher in the presence of both CO<sub>3</sub><sup>2–</sup> and NO<sub>3</sub><sup>–</sup> than in the presence of CO<sub>3</sub><sup>2–</sup>, NO<sub>3</sub><sup>–</sup>, or
DOM alone (<i>k</i> = 0.1–0.17 h<sup>–1</sup>). Using scavengers and enhancers for singlet oxygen (<sup>1</sup>O<sub>2</sub>) and hydroxyl (HO<sup>•</sup>) radicals, as
well as electron paramagnetic resonance spectroscopy, we found that
carbonate radicals (CO<sub>3</sub><sup>•–</sup>) primarily
drive Hg(0) photooxidation. The addition of DOM to the solution of
CO<sub>3</sub><sup>2–</sup> and NO<sub>3</sub><sup>–</sup> decreased the oxidation rate by half. This study identifies an unrecognized
pathway of Hg(0) photooxidation by CO<sub>3</sub><sup>•–</sup> radicals and the inhibitory effect of DOM, which could be important
in assessing Hg transformation and the fate of Hg in water containing
carbonate such as hard water and seawater
Oxidation of Dissolved Elemental Mercury by Thiol Compounds under Anoxic Conditions
Mercuric ion, Hg<sup>2+</sup>, forms strong complexes with thiolate compounds that commonly
dominate HgÂ(II) speciation in natural freshwater. However, reactions
between dissolved aqueous elemental mercury (Hg(0)<sub>aq</sub>) and
organic ligands in general, and thiol compounds in particular, are
not well studied although these reactions likely affect Hg speciation
and cycling in the environment. In this study, we compared the reaction
rates between Hg(0)<sub>aq</sub> and a number of selected organic
ligands with varying molecular structures and sulfur (S) oxidation
states in dark, anoxic conditions to assess the role of these ligands
in Hg(0)<sub>aq</sub> oxidation. Significant Hg(0)<sub>aq</sub> oxidation
was observed with all thiols but not with ligands containing no S.
Compounds with oxidized S (e.g., disulfide) exhibited little or no
reactivity toward Hg(0)<sub>aq</sub> either at pH 7. The rate and
extent of Hg(0)<sub>aq</sub> oxidation varied greatly depending on
the chemical and structural properties of thiols, thiol/Hg ratios,
and the presence or absence of electron acceptors. Smaller aliphatic
thiols and higher thiol/Hg ratios resulted in higher Hg(0)<sub>aq</sub> oxidation rates than larger aromatic thiols at lower thiol/Hg ratios.
The addition of electron acceptors (e.g., humic acid) also led to
substantially increased Hg(0)<sub>aq</sub> oxidation. Our results
suggest that thiol-induced oxidation of Hg(0)<sub>aq</sub> is important
under anoxic conditions and can affect Hg redox transformation and
bioavailability for microbial methylation
Time-Dependent Density Functional Theory Assessment of UV Absorption of Benzoic Acid Derivatives
Benzoic acid (BA) derivatives of environmental relevance
exhibit
various photophysical and photochemical characteristics. Here, time-dependent
density functional theory (TDDFT) is used to calculate photoexcitations
of eight selected BAs and the results are compared with UV spectra
determined experimentally. High-level gas-phase EOM-CCSD calculations
and experimental aqueous-phase spectra were used as the references
for the gas-phase and aqueous-phase TDDFT results, respectively. A
cluster-continuum model was used in the aqueous-phase calculations.
Among the 15 exchange–correlation (XC) functionals assessed,
five functionals, including the meta-GGA hybrid M06-2X, double hybrid
B2PLYPD, and range-separated functionals CAM-B3LYP, ωB97XD,
and LC-ωPBE, were found to be in excellent agreement with the
EOM-CCSD gas-phase calculations. These functionals furnished excitation
energies consistent with the pH dependence of the experimental spectra
with a standard deviation (STDEV) of ∼0.20 eV. A molecular
orbital analysis revealed a πσ* feature of the low-lying
transitions of the BAs. The CAM-B3LYP functional showed the best overall
performance and therefore shows promise for TDDFT calculations of
processes involving photoexcitations of benzoic acid derivatives
Prediction of Aluminum, Uranium, and Co-Contaminants Precipitation and Adsorption during Titration of Acidic Sediments
Batch and column recirculation titration
tests were performed with
contaminated acidic sediments. A generic geochemical model was developed
combining precipitation, cation exchange, and surface complexation
reactions to describe the observed pH and metal ion concentrations
in experiments with or without the presence of CO<sub>2</sub>. Experimental
results showed a slow pH increase due to strong buffering by Al hydrolysis
and precipitation and CO<sub>2</sub> uptake. The cation concentrations
generally decreased at higher pH than those observed in previous tests
without CO<sub>2</sub>. Using amorphous AlÂ(OH)<sub>3</sub> and basaluminite
precipitation reactions and a cation exchange selectivity coefficient <i>K</i><sub>Na\Al</sub> of 0.3, the model approximately described
the observed (1) pH titration curve, (2) Ca, Mg, and Mn concentration
by cation exchange, and (3) U concentrations by surface complexation
with Fe hydroxides at pH < 5 and with liebigite (Ca<sub>2</sub>UO<sub>2</sub>(CO<sub>3</sub>)<sub>3</sub>·10H<sub>2</sub>O)
precipitation at pH > 5. The model indicated that the formation
of
aqueous carbonate complexes and competition with carbonate for surface
sites could inhibit U and Ni adsorption and precipitation. Our results
suggested that the uncertainty in basaluminite solubility is an important
source of prediction uncertainty and ignoring labile solid phase Al
underestimates the base requirement in titration of acidic sediments
Coupled Mercury–Cell Sorption, Reduction, and Oxidation on Methylmercury Production by <i>Geobacter sulfurreducens</i> PCA
<i>G. sulfurreducens</i> PCA cells have been shown to
reduce, sorb, and methylate HgÂ(II) species, but it is unclear whether
this organism can oxidize and methylate dissolved elemental Hg(0)
as shown for <i>Desulfovibrio desulfuricans</i> ND132. Using
HgÂ(II) and Hg(0) separately as Hg sources in washed cell assays in
phosphate buffered saline (pH 7.4), we report how cell-mediated Hg
reduction and oxidation compete or synergize with sorption, thus affecting
the production of toxic methylmercury by PCA cells. Methylation is
found to be positively correlated to Hg sorption (<i>r</i> = 0.73) but negatively correlated to Hg reduction (<i>r</i> = −0.62). These reactions depend on the Hg and cell concentrations
or the ratio of Hg to cellular thiols (−SH). Oxidation and
methylation of Hg(0) are favored at relatively low Hg to cell–SH
molar ratios (e.g., <1). Increasing Hg to cell ratios from 0.25
× 10<sup>–19</sup> to 25 × 10<sup>–19</sup> moles-Hg/cell (equivalent to Hg/cell–SH of 0.71 to 71) shifts
the major reaction from oxidation to reduction. In the absence of
five outer membrane <i>c</i>-type cytochromes, mutant Δ<i>omcBESTZ</i> also shows decreases in Hg reduction and increases
in methylation. However, the presence of competing thiol-binding ions
such as Zn<sup>2+</sup> leads to increased Hg reduction and decreased
methylation. These results suggest that the coupled cell-Hg sorption
and redox transformations are important in controlling the rates of
Hg uptake and methylation by <i>G. sulfurreducens</i> PCA
in anoxic environments
Nanomolar Copper Enhances Mercury Methylation by <i>Desulfovibrio desulfuricans</i> ND132
Methylmercury
(MeHg) is produced by certain anaerobic microorganisms,
such as the sulfate-reducing bacterium <i>Desulfovibrio desulfuricans</i> ND132, but environmental factors affecting inorganic mercury [HgÂ(II)]
uptake and methylation remain unclear. We report that the presence
of a small amount of copper ions [CuÂ(II), <100 nM] enhances HgÂ(II)
uptake and methylation by washed cells of ND132, while HgÂ(II) methylation
is inhibited at higher CuÂ(II) concentrations because of the toxicity
of copper to the microorganism. The enhancement or inhibitory effect
of CuÂ(II) is dependent on both time and concentration. The presence
of nanomolar concentrations of CuÂ(II) facilitates rapid uptake of
HgÂ(II) (within minutes) and doubles MeHg production within a 24 h
period, but micromolar concentrations of CuÂ(II) completely inhibit
HgÂ(II) methylation. Metal ions such as zinc [ZnÂ(II)] and nickel [NiÂ(II)]
also inhibit but do not enhance HgÂ(II) methylation under the same
experimental conditions. These observations suggest a synergistic
effect of CuÂ(II) on HgÂ(II) uptake and methylation, possibly facilitated
by copper transporters or metallochaperones in this organism, and
highlight the fact that complex environmental factors affect MeHg
production in the environment
Cluster-Continuum Calculations of Hydration Free Energies of Anions and Group 12 Divalent Cations
Understanding aqueous phase processes involving group
12 metal
cations is relevant to both environmental and biological sciences.
Here, quantum chemical methods and polarizable continuum models are
used to compute the hydration free energies of a series of divalent
group 12 metal cations (Zn<sup>2+</sup>, Cd<sup>2+</sup>, and Hg<sup>2+</sup>) together with Cu<sup>2+</sup> and the anions OH<sup>–</sup>, SH<sup>–</sup>, Cl<sup>–</sup>, and F<sup>–</sup>. A cluster-continuum method is employed, in which gas-phase clusters
of the ion and explicit solvent molecules are immersed in a dielectric
continuum. Two approaches to define the size of the solute–water
cluster are compared, in which the number of explicit waters used
is either held constant or determined variationally as that of the
most favorable hydration free energy. Results obtained with various
polarizable continuum models are also presented. Each leg of the relevant
thermodynamic cycle is analyzed in detail to determine how different
terms contribute to the observed mean signed error (MSE) and the standard
deviation of the error (STDEV) between theory and experiment. The
use of a constant number of water molecules for each set of ions is
found to lead to predicted relative trends that benefit from error
cancellation. Overall, the best results are obtained with MP2 and
the Solvent Model D polarizable continuum model (SMD), with eight
explicit water molecules for anions and 10 for the metal cations,
yielding a STDEV of 2.3 kcal mol<sup>–1</sup> and MSE of 0.9
kcal mol<sup>–1</sup> between theoretical and experimental
hydration free energies, which range from −72.4 kcal mol<sup>–1</sup> for SH<sup>–</sup> to −505.9 kcal mol<sup>–1</sup> for Cu<sup>2+</sup>. Using B3PW91 with DFT-D3 dispersion
corrections (B3PW91-D) and SMD yields a STDEV of 3.3 kcal mol<sup>–1</sup> and MSE of 1.6 kcal mol<sup>–1</sup>, to which
adding MP2 corrections from smaller divalent metal cation water molecule
clusters yields very good agreement with the full MP2 results. Using
B3PW91-D and SMD, with two explicit water molecules for anions and
six for divalent metal cations, also yields reasonable agreement with
experimental values, due in part to fortuitous error cancellation
associated with the metal cations. Overall, the results indicate that
the careful application of quantum chemical cluster-continuum methods
provides valuable insight into aqueous ionic processes that depend
on both local and long-range electrostatic interactions with the solvent