Competitive Incorporation of Perrhenate and Nitrate into Sodalite

Nuclear waste storage tanks at the Hanford site in southeastern Washington have released highly alkaline solutions, containing radioactive and other contaminants, into subsurface sediments. When this waste reacts with subsurface sediments, feldspathoid minerals (sodalite, cancrinite) can form, sequestering pertechnetate (⁹⁹TcO₄^–) and other ions. This study investigates the potential for incorporation of perrhenate (ReO₄^–), a chemical surrogate for ⁹⁹TcO₄^–, into mixed perrhenate/nitrate (ReO₄^–/NO₃^–) sodalite. Mixed-anion sodalites were hydrothermally synthesized in the laboratory from zeolite A in sodium hydroxide, nitrate, and perrhenate solutions at 90 °C for 24 h. The resulting solids were characterized by bulk chemical analysis, X-ray diffraction, scanning electron microscopy, and X-ray absorption near edge structure spectroscopy (XANES) to determine the products’ chemical composition, structure, morphology, and Re oxidation state. The XANES data indicated that nearly all rhenium (Re) was incorporated as Re­(VII)­O₄^–. The nonlinear increase of the unit cell parameter with ReO₄^–/NO₃^– ratios suggests formation of two separate sodalite phases in lieu of a mixed-anion sodalite. The results reveal that the sodalite cage is highly selective toward NO₃^– over ReO₄^–. Calculated enthalpy and Gibbs free energy of formation at 298 K for NO₃- and ReO₄-sodalite suggest that NO₃^– incorporation into the cage is favored over the incorporation of the larger ReO₄^–, due to the smaller ionic radius of NO₃^–. Based on these results, it is expected that NO₃^–, which is present at significantly higher concentrations in alkaline waste solutions than ⁹⁹TcO₄^–, will be strongly preferred for incorporation into the sodalite cage

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 [Hg­(II)] uptake and methylation remain unclear. We report that the presence of a small amount of copper ions [Cu­(II), <100 nM] enhances Hg­(II) uptake and methylation by washed cells of ND132, while Hg­(II) methylation is inhibited at higher Cu­(II) concentrations because of the toxicity of copper to the microorganism. The enhancement or inhibitory effect of Cu­(II) is dependent on both time and concentration. The presence of nanomolar concentrations of Cu­(II) facilitates rapid uptake of Hg­(II) (within minutes) and doubles MeHg production within a 24 h period, but micromolar concentrations of Cu­(II) completely inhibit Hg­(II) methylation. Metal ions such as zinc [Zn­(II)] and nickel [Ni­(II)] also inhibit but do not enhance Hg­(II) methylation under the same experimental conditions. These observations suggest a synergistic effect of Cu­(II) on Hg­(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

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%)

Contrasting Effects of Dissolved Organic Matter on Mercury Methylation by <i>Geobacter sulfurreducens</i> PCA and <i>Desulfovibrio desulfuricans</i> ND132

Natural dissolved organic matter (DOM) affects mercury (Hg) redox reactions and anaerobic microbial methylation in the environment. Several studies have shown that DOM can enhance Hg methylation, especially under sulfidic conditions, whereas others show that DOM inhibits Hg methylation due to strong Hg–DOM complexation. In this study, we investigated and compared the effects of DOM on Hg methylation by an iron-reducing bacterium <i>Geobacter sulfurreducens</i> PCA and a sulfate-reducing bacterium <i>Desulfovibrio desulfuricans</i> ND132 under nonsulfidic conditions. The methylation experiment was performed with washed cells either in the absence or presence of DOM or glutathione, both of which form strong complexes with Hg via thiol-functional groups. DOM was found to greatly inhibit Hg methylation by <i>G. Sulfurreducens</i> PCA but enhance Hg methylation by <i>D. desulfuricans</i> ND132 cells with increasing DOM concentration. These strain-dependent opposing effects of DOM were also observed with glutathione, suggesting that thiols in DOM likely played an essential role in affecting microbial Hg uptake and methylation. Additionally, DOM and glutathione greatly decreased Hg sorption by <i>G. sulfurreducens</i> PCA but showed little effect on <i>D. desulfuricans</i> ND132 cells, demonstrating that ND132 has a higher affinity to sorb or take up Hg than the PCA strain. These observations indicate that DOM effects on Hg methylation are bacterial strain specific, depend on the DOM:Hg ratio or site-specific conditions, and may thus offer new insights into the role of DOM in methylmercury production in the environment

Influence of Structural Defects on Biomineralized ZnS Nanoparticle Dissolution: An in-Situ Electron Microscopy Study

The dissolution of metal sulfides, such as ZnS, is an important biogeochemical process affecting fate and transport of trace metals in the environment. However, current studies of in situ dissolution of metal sulfides and the effects of structural defects on dissolution are lacking. Here we have examined the dissolution behavior of ZnS nanoparticles synthesized via several abiotic and biological pathways. Specifically, we have examined biogenic ZnS nanoparticles produced by an anaerobic, metal-reducing bacterium <i>Thermoanaerobacter</i> sp. X513 in a Zn-amended, thiosulfate-containing growth medium in the presence or absence of silver (Ag), and abiogenic ZnS nanoparticles were produced by mixing an aqueous Zn solution with either H₂S-rich gas or Na₂S solution. The size distribution, crystal structure, aggregation behavior, and internal defects of the synthesized ZnS nanoparticles were examined using high-resolution transmission electron microscopy (TEM) coupled with X-ray energy dispersive spectroscopy. The characterization results show that both the biogenic and abiogenic samples were dominantly composed of sphalerite. In the absence of Ag, the biogenic ZnS nanoparticles were significantly larger (i.e., ∼10 nm) than the abiogenic ones (i.e., ∼3–5 nm) and contained structural defects (e.g., twins and stacking faults). The presence of trace Ag showed a restraining effect on the particle size of the biogenic ZnS, resulting in quantum-dot-sized nanoparticles (i.e., ∼3 nm). In situ dissolution experiments for the synthesized ZnS were conducted with a liquid-cell TEM (LCTEM), and the primary factors (i.e., the presence or absence structural defects) were evaluated for their effects on the dissolution behavior using the biogenic and abiogenic ZnS nanoparticle samples with the largest average particle size. Analysis of the dissolution results (i.e., change in particle radius with time) using the Kelvin equation shows that the defect-bearing biogenic ZnS nanoparticles (γ = 0.799 J/m²) have a significantly higher surface energy than the abiogenic ZnS nanoparticles (γ = 0.277 J/m²). Larger defect-bearing biogenic ZnS nanoparticles were thus more reactive than the smaller quantum-dot-sized ZnS nanoparticles. These findings provide new insight into the factors that affect the dissolution of metal sulfide nanoparticles in relevant natural and engineered scenarios, and have important implications for tracking the fate and transport of sulfide nanoparticles and associated metal ions in the environment. Moreover, our study exemplified the use of an in situ method (i.e., LCTEM) to investigate nanoparticle behavior (e.g., dissolution) in aqueous solutions

Anaerobic Mercury Methylation and Demethylation by <i>Geobacter bemidjiensis</i> Bem

Microbial methylation and demethylation are two competing processes controlling the net production and bioaccumulation of neurotoxic methylmercury (MeHg) in natural ecosystems. Although mercury (Hg) methylation by anaerobic microorganisms and demethylation by aerobic Hg-resistant bacteria have both been extensively studied, little attention has been given to MeHg degradation by anaerobic bacteria, particularly the iron-reducing bacterium <i>Geobacter bemidjiensis</i> Bem. Here we report, for the first time, that the strain <i>G. bemidjiensis</i> Bem can mediate a suite of Hg transformations, including Hg­(II) reduction, Hg(0) oxidation, MeHg production and degradation under anoxic conditions. Results suggest that <i>G. bemidjiensis</i> utilizes a reductive demethylation pathway to degrade MeHg, with elemental Hg(0) as the major reaction product, possibly due to the presence of genes encoding homologues of an organomercurial lyase (MerB) and a mercuric reductase (MerA). In addition, the cells can strongly sorb Hg­(II) and MeHg, reduce or oxidize Hg, resulting in both time and concentration-dependent Hg species transformations. Moderate concentrations (10–500 μM) of Hg-binding ligands such as cysteine enhance Hg­(II) methylation but inhibit MeHg degradation. These findings indicate a cycle of Hg methylation and demethylation among anaerobic bacteria, thereby influencing net MeHg production in anoxic water and sediments

Effect of Hg exposure on acidic compartmentalization of <i>A</i>. <i>parkinsoniana</i> labeled with AO.

<p>Epifluorescence micrographs of single optical sections showing overlay of AO green and red fluorescence for (A) T1-control, (B,C) T1-100 ppm, and (D) T2-100 ppm, Bars: 20μm. (E) Histogram of maximum dimension (diameter) of acidic vesicles. (F) Histogram of red Mean Fluorescence Intensity (MFI) expressed in arbitrary units (A.U.) for control and 100 ppm at both T1 and T2. Error bars indicate ± standard error of the mean.</p

Effect of Hg exposure on lipid distribution of <i>A</i>. <i>parkinsoniana</i> labeled with NR.

<p>Epifluoresecence micrographs of single optical sections showing overlay of NR yellow and red fluorescence for (A) T1- control, (B) T1-1 ppm, (C) T1-100 ppm and (D) T2-100 ppm, Bars: 20 μm. (E) Histogram of yellow Mean Fluorescence Intensity (MFI) expressed in arbitrary units (A.U.) for the three treatments over time (control, 1 ppm, and 100 ppm at both T1 and T2). Error bars indicate ± standard error of the mean.</p

Micrographs showing presence of Hg in <i>A</i>. <i>parkinsoniana</i> specimens (T2-100 ppm).

<p>(A) young chamber containing vacuoles; (B) high magnification of a young chamber; (C,D) basal part of pores; (E) foramen/septum. (F) Example EDS spectrum taken with a spot size of 4 on cross (E). Arrows mark the occurrence of Hg. Scale bar: (A) 10 μm; (B) 1.5 μm; (C) 2.5 μm; (D) 0.8 μm; (E) 5 μm.</p