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Manganese (Mn) contamination in groundwater poses significant challenges for drinking water treatment. This study explores the mechanisms of Mn removal in a long-term oxygenated groundwater biofilter. The filter media coating primarily consists of abiotic disordered birnessite (δ-MnO 2 ) with a microglobular structure and an average oxidation state of approximately 3.45. This material plays a key role in the effective adsorption and oxidation of Mn(II) dissolved in groundwater. The results indicate that Mn removal is predominantly abiotic, with biofilm activity contributing to less than 10% of Mn(II) oxidation. Biological colonization is minimal, as evidenced by the low microbial activity and protein-to-polysaccharide ratio. However, Mn-oxidizing and Mn-reducing bacteria were identified under aerobic conditions, suggesting that they play facultative or complementary roles in Mn cycling. The unexpected coexistence of the two types of bacteria highlights the need for further investigation into their role in δ-MnO 2 transformation and regeneration. The study provides foundational insights into the dynamics of Mn(II) removal in biofilters and proposes an initial framework for understanding the Mn(II) biogeochemical cycle within such common engineered systems.

The purpose of this study was to investigate the relationship between the chemical form and localization of zinc (Zn) in plant leaves and their Zn accumulation capacity. An interspecific cross between Arabidopsis halleri sp. halleri and Arabidopsis lyrata sp. petrea segregating for Zn accumulation was used. Zinc (Zn) speciation and Zn distribution in the leaves of the parent plants and of selected F1 and F2 progenies were investigated by spectroscopic and microscopic techniques and chemical analyses. A correlation was observed between the proportion of Zn being in octahedral coordination complexed to organic acids and free in solution (Zn–OAs + Znaq) and Zn content in the leaves. This pool varied between 40% and 80% of total leaf Zn depending on the plant studied. Elemental mapping of the leaves revealed different Zn partitioning between the veins and the leaf tissue. The vein: tissue fluorescence ratio was negatively correlated with Zn accumulation. The higher proportion of Zn–OAs + Znaq and the depletion of the veins in the stronger accumulators are attributed to a higher xylem unloading and vacuolar sequestration in the leaf cells. Elemental distributions in the trichomes were also investigated, and results support the role of carboxyl and/or hydroxyl groups as major Zn ligands in these cells.

Deep-sea mud is rich in rare-earth elements, primarily found in fluorapatite, a mineral deposit that forms over hundreds of thousands to millions of years through the accumulation of fish remains. After fish die, biogenic apatite captures rare earth elements from seawater on the seafloor and from pore waters during the diagenesis process. The conventional model for rare earth element enrichment suggests that they are incorporated into the bioapatite crystal structure through solid-state diffusion. However, our data reveal that cerium atoms are instead precipitated within an amorphous layer surrounding bioapatite nanocrystals, as shown by high-energy-resolution X-ray absorption spectroscopy and transmission electron microscopy. Computational simulations further support this finding, predicting that cerium atoms cluster on the surface of fluorapatite. These results suggest that the fluorapatite-water interface plays a crucial role in the enrichment of cerium, as well as other rare earth elements, in marine sediments. Cerium atoms are not incorporated into the crystal structure of bioapatites as host grains, but precipitate on the surface, according to high-energy-resolution X-Ray absorption spectroscopy data from bioapatites of fish bones and teeth recovered in deep-sea sediments, and computational simulations.

The chemical forms of zinc (Zn) in the Zn-tolerant and hyperaccumulator Arabidopsis halleri and in the non-tolerant and nonaccumulator Arabidopsis lyrata subsp. petraea were determined at the molecular level by combining chemical analyses, extended x-ray absorption spectroscopy (EXAFS), synchrotron-based x-ray microfluorescence, and μEXAFS. Plants were grown in hydroponics with various Zn concentrations, and A. halleri specimens growing naturally in a contaminated site were also collected. Zn speciation in A. halleri was independent of the origin of the plants (contaminated or non-contaminated) and Zn exposure. In aerial parts, Zn was predominantly octahedrally coordinated and complexed to malate. A secondary organic species was identified in the bases of the trichomes, which contained elevated Zn concentrations, and in which Zn was tetrahedrally coordinated and complexed to carboxyl and/or hydroxyl functional groups. This species was detected thanks to the good resolution and sensitivity of synchrotron-based x-ray microfluorescence and μEXAFS. In the roots of A. halleri grown in hydroponics, Zn phosphate was the only species detected, and is believed to result from chemical precipitation on the root surface. In the roots of A. halleri grown on the contaminated soil, Zn was distributed in Zn malate, Zn citrate, and Zn phosphate. Zn phosphate was present in both the roots and aerial part of A. lyrata subsp. petraea. This study illustrates the complementarity of bulk and spatially resolved techniques, allowing the identification of: (a) the predominant chemical forms of the metal, and (b) the minor forms present in particular cells, both types of information being essential for a better understanding of the bioaccumulation processes.

Tobacco (Nicotiana tabacum L. cv Xanthi) plants were exposed to toxic levels of zinc (Zn). Zn exposure resulted in toxicity signs in plants, and these damages were partly reduced by a calcium (Ca) supplement. Confocal imaging of intracellular Zn using Zinquin showed that Zn was preferentially accumulated in trichomes. Exposure to Zn and Zn + Ca increased the trichome density and induced the production of Ca/Zn mineral grains on the head cells of trichomes. These grains were aggregates of submicrometer-sized crystals and poorly crystalline material and contained Ca as major element, along with subordinate amounts of Zn, manganese, potassium, chlorine, phosphorus, silicon, and magnesium. Micro x-ray diffraction revealed that the large majority of the grains were composed essentially of metal-substituted calcite (CaCO₃). CaCO₃ polymorphs (aragonite and vaterite) and CaC₂O₄ (Ca oxalate) mono- and dihydrate also were identified, either as an admixture to calcite or in separate grains. Some grains did not diffract, although they contained Ca, suggesting the presence of amorphous form of Ca. The presence of Zn-substituted calcite was confirmed by Zn K-edge micro-extended x-ray absorption fine structure spectroscopy. Zn bound to organic compounds and Zn-containing silica and phosphate were also identified by this technique. The proportion of Zn-substituted calcite relative to the other species increased with Ca exposure. The production of Zn-containing biogenic calcite and other Zn compounds through the trichomes is a novel mechanism involved in Zn detoxification. This study illustrates the potential of laterally resolved x-ray synchrotron radiation techniques to study biomineralization and metal homeostasis processes in plants.

Metal sulfide minerals are assumed to form naturally at ambient conditions via reaction of a metallic element with (poly)sulfide ions, usually produced by microbes in oxygen-depleted environments. Recently, the formation of mercury sulfide (β-HgS) directly from linear Hg(II)-thiolate complexes (Hg(SR) 2 ) in natural organic matter and in cysteine solutions was demonstrated under aerated conditions. Here, a detailed description of this non-sulfidic reaction is provided by computations at a high level of molecular-orbital theory. The HgS stoichiometry is obtained through the cleavage of the S-C bond in one thiolate, transfer of the resulting alkyl group (R’) to another thiolate, and subsequent elimination of a sulfur atom from the second thiolate as a thioether (RSR’). Repetition of this mechanism leads to the formation of RS-(HgS) n -R chains which may self-assemble in parallel arrays to form cinnabar (α-HgS), or more commonly, quickly condense to four-coordinate metacinnabar (β-HgS). The mechanistic pathway is thermodynamically favorable and its predicted kinetics agrees with experiment. The results provide robust theoretical support for the abiotic natural formation of nanoparticulate HgS under oxic conditions and in the absence of a catalyst, and suggest a new route for the (bio)synthesis of HgS nanoparticles with improved technological properties.

Most frequently arsenic is nominally monovalent (As1-) in pyrite (FeS2) and substituted for S. Nominally trivalent arsenic (As3+) has been reported previously in hydrothermal Peruvian pyrite and was considered to be substituted for Fe based on the negative correlation between the concentrations of the two elements. Here, we provide the first observation of the incorporation of As3+ in goldfieldite [Cu12(As,Sb,Bi)2Te2S13] and As5+ in colusite [Cu26V2(As,Sb)4Sn2S32] inclusions in As1--pyrite from high-sulfidation deposits in Peru. This information was obtained by combining spatially resolved electron probe (EPMA), synchrotron-based X-ray fluorescence (SXRF), and absorption spectroscopy (micro-XANES and micro-EXAFS) with new high energy-resolution XANES spectroscopy (HR-XANES). The two Cu sulfide inclusions range from several to one hundred micrometers in size, and the As3+/As5+ concentration varies from a few parts per million (ppm) to a maximum of 17.33 wt% compared to a maximum of 50 ppm As1- in pyrite. They also contain variable amounts of Sn (18.47 wt% max), Te (15.91 wt% max), Sb (8.54 wt% max), Bi (5.53 wt% max), and V (3.25 wt% max). The occurrence of As3+/As5+-containing sulfosalts in As1--containing pyrite grains indicates that oxidizing hydrothermal conditions prevailed during the late stage of the mineralization process in the ore deposits from Peru. From an environmental perspective, high concentrations of potentially toxic As, contained in what appear to be non-As-bearing pyrite, may pose a heretofore unrecognized threat to ecosystems in acid mine drainage settings. More generally, the combination of techniques used in this study offers a new perspective on the mineralogy and crystal chemistry of hazardous elements in pyrite, such as highly toxic and little studied thallium.

Metal sulfide minerals are assumed to form naturally at ambient conditions via reaction of a metallic element with (poly)sulfide ions, usually produced by microbes in oxygen-depleted environments. Recently, the formation of mercury sulfide (β-HgS) directly from linear Hg(II)-thiolate complexes (Hg(SR) 2) in natural organic matter and in cysteine solutions was demonstrated under aerated conditions. Here, a detailed description of this non-sulfidic reaction is provided by computations at a high level of molecular-orbital theory. The HgS stoichiometry is obtained through the cleavage of the S-C bond in one thiolate, transfer of the resulting alkyl group (R') to another thiolate, and subsequent elimination of a sulfur atom from the second thiolate as a thioether (RSR'). Repetition of this mechanism leads to the formation of RS-(HgS) n-R chains which may self-assemble in parallel arrays to form cinnabar (α-HgS), or more commonly, quickly condense to four-coordinate metacinnabar (β-HgS). The mechanistic pathway is thermodynamically favorable and its predicted kinetics agrees with experiment. The results provide robust theoretical support for the abiotic natural formation of nanoparticulate HgS under oxic conditions and in the absence of a catalyst, and suggest a new route for the (bio)synthesis of HgS nanoparticles with improved technological properties. Nucleation of metal sulfide solids typically occurs when solubility is exceeded by elevated concentration of reduced sulfur, metal cation, or both components 1,2. In environmental aquatic systems, metal ions are commonly complexed with natural organic matter or inorganic anions, including sulfide, and free sulfide ions (S(-II)) produced mainly by dissimilatory sulfate reducing microbes 3,4 are considered necessary for solid nucleation. Sulfide can also be generated in the laboratory from intracellular cysteine by photosynthetic aerobic microorganisms 5,6 and from decomposition of sulfur compounds, such as thioglycolic acid, thioglycerol, dithiocarbamate, thio-acetamide, and cystine, by hydrothermal, solvothermal, and biomimetic synthesis routes, sonochemical reaction, microwave irradiation, and hydrolysis 7–18. Recently it was shown that sulfide ions were not required to form a metal sulfide solid 19. Metacinnabar (β-HgS) precipitated directly from linear Hg-thiolate complexes (Hg(SR) 2) in natural organic matter (NOM) and from Hg-dicysteinate complexes (Hg(Cys) 2) in aerated and deaerated aqueous solutions in the dark without a catalyzing agent. These results are relevant to soil and aquatic systems, especially in cases where organo-sulfide is the dominant sulfide source. The reaction was rather slow and took several days for Hg(II) complexed to NOM at a concentration of 30–200 mg of Hg/kg of NOM dry weight (ppm). A global reaction pathway was proposed that has similarities to one suggested for β-HgS precipitation in sodium hydrosulfide (NaHS) solution 20,21. In its reaction with NaHS, Hg(II) initially forms an unstable low coordination chain-type complex (–S-Hg-S-Hg-S-) that rapidly transforms to a four-coordinate mercury sulfide with the short range ordering of β-HgS. The disordered β-HgS nanostructures eventually yield β-HgS crystals. In the case of thiolate as the source of reduced sulfur, the starting reactant is the linear Hg(SR) 2 complex (RS-Hg-SR), which is the most stable coordination of mononu-clear Hg with thiolate ligands at neutral and acidic pH 22,23. Because β-HgS nanostructures appear rapidly once –S-Hg-S-Hg-S-chains are formed in sulfidic solution 20 , we infer that formation of the chain structure limits the rate of formation of β-HgS from Hg(SR) 2. The pathway proposed 19 for chain formation in natural organic matter is the cleavage of the S-R bond according to the reaction: + → + RS Hg SR RS Hg SR RS Hg S Hg SR R S R, (1) followed by growth of the chain through the addition of new Hg(SR) 2 complexes:

The purpose of this study was to investigate the relationship between the chemical form and localization of zinc (Zn) in plant leaves and their Zn accumulation capacity. An interspecific cross between Arabidopsis halleri sp. halleri and Arabidopsis lyrata sp. petrea segregating for Zn accumulation was used. Zinc (Zn) speciation and Zn distribution in the leaves of the parent plants and of selected F₁ and F₂ progenies were investigated by spectroscopic and microscopic techniques and chemical analyses. A correlation was observed between the proportion of Zn being in octahedral coordination complexed to organic acids and free in solution (Zn-OAs + Znaq) and Zn content in the leaves. This pool varied between 40% and 80% of total leaf Zn depending on the plant studied. Elemental mapping of the leaves revealed different Zn partitioning between the veins and the leaf tissue. The vein : tissue fluorescence ratio was negatively correlated with Zn accumulation. The higher proportion of Zn-OAs + Znaq and the depletion of the veins in the stronger accumulators are attributed to a higher xylem unloading and vacuolar sequestration in the leaf cells. Elemental distributions in the trichomes were also investigated, and results support the role of carboxyl and/or hydroxyl groups as major Zn ligands in these cells.