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_re) 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^–1, 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_re can be offset by oxidation of Hg(0) with an estimated initial rate as high as 5.4 h^–1. This dual role of NOM_re is expected to strongly influence the availability of reactive Hg and thus to have important implications for microbial uptake and methylation in anoxic environments

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 Hg­(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 Hg­(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 Hg­(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 Hg­(II) species are more readily taken up and that this uptake is kinetically limiting, thereby affecting Hg methylation by both the mutant and WT

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

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 (CO₃^2–) in natural freshwaters. Here, we evaluate Hg(0) photooxidation rates affected by reactive ionic species (e.g., DOM, CO₃^2–, and NO₃^–) 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^–1) is much higher in the presence of both CO₃^2– and NO₃^– than in the presence of CO₃^2–, NO₃^–, or DOM alone (<i>k</i> = 0.1–0.17 h^–1). Using scavengers and enhancers for singlet oxygen (¹O₂) and hydroxyl (HO^•) radicals, as well as electron paramagnetic resonance spectroscopy, we found that carbonate radicals (CO₃^•–) primarily drive Hg(0) photooxidation. The addition of DOM to the solution of CO₃^2– and NO₃^– decreased the oxidation rate by half. This study identifies an unrecognized pathway of Hg(0) photooxidation by CO₃^•– 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, Hg²⁺, forms strong complexes with thiolate compounds that commonly dominate Hg­(II) speciation in natural freshwater. However, reactions between dissolved aqueous elemental mercury (Hg(0)_aq) 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)_aq 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)_aq oxidation. Significant Hg(0)_aq 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)_aq either at pH 7. The rate and extent of Hg(0)_aq 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)_aq 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)_aq oxidation. Our results suggest that thiol-induced oxidation of Hg(0)_aq 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

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 Hg­(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 Hg­(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^–19 to 25 × 10^–19 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 Zn²⁺ 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

Rapid Removal of Hg(II) from Aqueous Solutions Using Thiol-Functionalized Zn-Doped Biomagnetite Particles

The surfaces of Zn-doped biomagnetite nanostructured particles were functionalized with (3-mercaptopropyl)­trimethoxysilane (MPTMS) and used as a high-capacity and collectable adsorbent for the removal of Hg­(II) from water. Fourier transform infrared spectroscopy (FTIR) confirmed the attachment of MPTMS on the particle surface. The crystallite size of the Zn-doped biomagnetite was ∼17 nm, and the thickness of the MPTMS coating was ∼5 nm. Scanning transmission electron microscopy and dynamic light scattering analyses revealed that the particles formed aggregates in aqueous solution with an average hydrodynamic size of 826 ± 32 nm. Elemental analyses indicate that the chemical composition of the biomagnetite is Zn_0.46Fe_2.54O₄, and the loading of sulfur is 3.6 mmol/g. The MPTMS-modified biomagnetite has a calculated saturation magnetization of 37.9 emu/g and can be separated from water within a minute using a magnet. Sorption of Hg­(II) to the nanostructured particles was much faster than other commercial sorbents, and the Hg­(II) sorption isotherm in an industrial wastewater follows the Langmuir model with a maximum capacity of ∼416 mg/g, indicating two −SH groups bonded to one Hg. This new Hg­(II) sorbent was stable in a range of solutions, from contaminated water to 0.5 M acid solutions, with low leaching of Fe, Zn, Si, and S (<10%)

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 (Zn²⁺, Cd²⁺, and Hg²⁺) together with Cu²⁺ and the anions OH^–, SH^–, Cl^–, and F^–. 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^–1 and MSE of 0.9 kcal mol^–1 between theoretical and experimental hydration free energies, which range from −72.4 kcal mol^–1 for SH^– to −505.9 kcal mol^–1 for Cu²⁺. Using B3PW91 with DFT-D3 dispersion corrections (B3PW91-D) and SMD yields a STDEV of 3.3 kcal mol^–1 and MSE of 1.6 kcal mol^–1, 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

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 (Zn²⁺, Cd²⁺, and Hg²⁺) together with Cu²⁺ and the anions OH^–, SH^–, Cl^–, and F^–. 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^–1 and MSE of 0.9 kcal mol^–1 between theoretical and experimental hydration free energies, which range from −72.4 kcal mol^–1 for SH^– to −505.9 kcal mol^–1 for Cu²⁺. Using B3PW91 with DFT-D3 dispersion corrections (B3PW91-D) and SMD yields a STDEV of 3.3 kcal mol^–1 and MSE of 1.6 kcal mol^–1, 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