Review: Natural oxalates and their analogous synthetic complexes

Study of Micro-Samples from the Open-Air Rock Art Site of Cueva de la Vieja (Alpera, Albacete, Spain) for Assessing the Performance of a Desalination Treatment

In this work, some micro-samples belonging to the open-air rock art site of Cueva de la Vieja (Alpera, Albacete, Spain) were analysed. These samples were collected after and before a desalination treatment was carried out, with the aim of removing a whitish layer of concretion that affected the painted panel. The diagnostic study was performed to study the conservation state of the panel, and to then confirm the effectiveness of the treatment. Micro energy dispersive X-ray fluorescence spectrometry, Raman spectroscopy, and X-ray diffraction were employed for the characterization of the degradation product as well as that of the mineral substrate and pigments. The micro-samples analysis demonstrated that the painted layer was settled on a dolomitic limestone with silicon aggregates and aluminosilicates as well as iron oxides. The whitish crust was composed by sulfate compounds such as gypsum (CaSO4·2H2O) with a minor amount of epsomite (MgSO4·7H2O). An extensive phenomenon of biological activity has been demonstrated since then in almost all of the samples that have been analysed, and the presence of calcium oxalates monohydrate (CaC2O4·H2O) and dehydrate (CaC2O4·2H2O) were found. The presence of both calcium oxalates probably favoured the conservation of the pictographs. In addition, some carotenoids pigments, scytonemin (C36H20N2O4), and astaxanthin (C40H52O4) were characterized both by Raman spectroscopy and by X-ray diffraction. Hematite was found as a pigment voluntarily used for the painting of the panels used in a mixture with hydroxyapatite and amorphous carbon. The results of the analyses of the samples taken after the cleaning treatment confirmed a substantial decrease in sulphate formation on the panel surface.

Crystal Chemistry of the Copper Oxalate Biomineral Moolooite: The First Single-Crystal X-ray Diffraction Studies and Thermal Behavior

Moolooite, Cu(C₂O₄)·nH₂O, is a typical biomineral which forms due to Cu-bearing minerals coming into contact with oxalic acid sources such as bird guano deposits or lichens, and no single crystals of moolooite of either natural or synthetic origin have been found yet. This paper reports, for the first time, on the preparation of single crystals of a synthetic analog of the copper-oxalate biomineral moolooite, and on the refinement of its crystal structure from the single-crystal X-ray diffraction (SCXRD) data. Along with the structural model, the SCXRD experiment showed the significant contribution of diffuse scattering to the overall diffraction data, which comes from the nanostructural disorder caused by stacking faults of Cu oxalate chains as they lengthen. This type of disorder should result in the chains breaking, at which point the H₂O molecules may be arranged. The amount of water in the studied samples did not exceed 0.15 H₂O molecules per formula unit. Apparently, the mechanism of incorporation of H₂O molecules governs the absence of good-quality single crystals in nature and a lack of them in synthetic experiments: the more H₂O content in the structure, the stronger the disorder will be. A description of the crystal structure indicates that the ideal structure of the Cu oxalate biomineral moolooite should not contain H₂O molecules and should be described by the Cu(C₂O₄) formula. However, it was shown that natural and synthetic moolooite crystals contain a significant portion of "structural" water, which cannot be ignored. Considering the substantially variable amount of water, which can be incorporated into the crystal structure, the formula Cu(C₂O₄)·nH₂O for moolooite is justified.

Expanding the taxonomic and environmental extent of an underexplored carbon metabolism-oxalotrophy

Oxalate serves various functions in the biological processes of plants, fungi, bacteria, and animals. It occurs naturally in the minerals weddellite and whewellite (calcium oxalates) or as oxalic acid. The environmental accumulation of oxalate is disproportionately low compared to the prevalence of highly productive oxalogens, namely plants. It is hypothesized that oxalotrophic microbes limit oxalate accumulation by degrading oxalate minerals to carbonates via an under-explored biogeochemical cycle known as the oxalate-carbonate pathway (OCP). Neither the diversity nor the ecology of oxalotrophic bacteria is fully understood. This research investigated the phylogenetic relationships of the bacterial genes oxc, frc, oxdC, and oxlT, which encode key enzymes for oxalotrophy, using bioinformatic approaches and publicly available omics datasets. Phylogenetic trees of oxc and oxdC genes demonstrated grouping by both source environment and taxonomy. All four trees included genes from metagenome-assembled genomes (MAGs) that contained novel lineages and environments for oxalotrophs. In particular, sequences of each gene were recovered from marine environments. These results were supported with marine transcriptome sequences and description of key amino acid residue conservation. Additionally, we investigated the theoretical energy yield from oxalotrophy across marine-relevant pressure and temperature conditions and found similar standard state Gibbs free energy to "low energy" marine sediment metabolisms, such as anaerobic oxidation of methane coupled to sulfate reduction. These findings suggest further need to understand the role of bacterial oxalotrophy in the OCP, particularly in marine environments, and its contribution to global carbon cycling.

Solid Solutions of Lindbergite-Glushinskite Series: Synthesis, Ionic Substitutions, Phase Transformation and Crystal Morphology

To clarify the crystal chemical features of natural and synthetic oxalates Me2+(C2O4)∙2H2O (Me2+ = Fe, Mn, Mg, Zn), including minerals of the humboldtine group, solid solutions of lindbergite Mn(C2O4)∙2H2O−glushinskite Mg(C2O4)∙2H2O were precipitated under various conditions, close to those characteristic of mineralization in biofilms: at the stoichiometric ratios ((Mn + Mg)/C2O4 = 1) and non-stochiometric ratios ((Mn + Mg)/C2O4 < 1), in the presence and absence of citrate ions. Investigation of precipitates was carried out by powder X-ray diffraction, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Thermodynamic modelling was performed in order to evaluate the lindbergite−glushinskite equilibrium. It was shown that glushinskite belongs to the orthorhombic β-modification (sp. Gr. Fddd), while lindbergite has a monoclinic α-modification (sp. gr. C2/c). Mg ions incorporate lindbergite in much higher quantities than Mn ions incorporate glushinskite; moreover, Mn glushinskites are characterized by violations of long-range order in their crystal structure. Lindbergite−glushinskite transition occurs abruptly and can be classified as a first-order isodimorphic transition. The Me2+/C2O4 ratio and the presence of citric acid in the solution affect the isomorphic capacity of lindbergite and glushinskite, the width of the transition and the equilibrium Mg/Mn ratio. The transition is accompanied by continuous morphological changes in crystals and crystal intergrowths. Given the obtained results, it is necessary to take into account in biotechnologies aimed at the bioremediation/bioleaching of metals from media containing mixtures of cations (Mg, Mn, Fe, Zn).

Selective Extraction of Transition Metals from Spent LiNix Coy Mn1-x-y O2 Cathode via Regulation of Coordination Environment

The complexity of chemical compounds in lithium-ion batteries (LIBs) results in great difficulties in the extraction of multiple transition metals, which have similar physicochemical characteristics. Here, we propose a novel strategy for selective extraction of nickel, cobalt, and manganese from spent LiNi_x Co_y Mn_1-x-y O₂ (NCM) cathode through the regulation of coordination environment. Depending on adjusting the composition of ligand in transition metal complexes, a tandem leaching and separation system is designed and finally enables nickel, cobalt, and manganese to enrich in the form of NiO, Co₃ O₄ , and Mn₃ O₄ with high recovery yields of 99.1 %, 95.1 %, and 95.3 %, respectively. We further confirm that the combination of different transition metals with well-designed ligands is the key to good selectivity. Through our work, fine-tuning the coordination environment of metal ions is proved to have great prospects in the battery recycling industry.

Inorganic phosphate in growing calcium carbonate abalone shell suggests a shared mineral ancestral precursor

The presence of phosphate from different origins (inorganic, bioorganic) is found more and more in calcium carbonate-based biominerals. Phosphate is often described as being responsible for the stabilization of the transient amorphous calcium carbonate phase. In order to specify the composition of the mineral phase deposited at the onset of carbonated shell formation, the present study investigates, down to the nanoscale, the growing shell from the European abalone Haliotis tuberculata, using a combination of solid state nuclear magnetic resonance, scanning transmission electron microscope and spatially-resolved electron energy loss spectroscopy techniques. We show the co-occurrence of inorganic phosphate with calcium and carbonate throughout the early stages of abalone shell formation. One possible hypothesis is that this first-formed mixed mineral phase represents the vestige of a shared ancestral mineral precursor that appeared early during Evolution. In addition, our findings strengthen the idea that the final crystalline phase (calcium carbonate or phosphate) depends strongly on the nature of the mineral-associated proteins in vivo.

Phosphate involvement in calcium carbonate biominerals raises questions on biomineralisation pathways. Here, the authors explore the presence of phosphate in the growing shell of the European abalone and suggest a shared mixed mineral ancestral precursor with final crystal phase being selected by mineral-associated proteins.

Abiotic Formation of Calcium Oxalate under UV Irradiation and Implications for Biomarker Detection on Mars

A major objective in the exploration of Mars is to test the hypothesis that the planet has ever hosted life. Biogenic compounds, especially biominerals, are believed to serve as biomarkers in Raman-assisted remote sensing missions. However, the prerequisite for the development of these minerals as biomarkers is the uniqueness of their biogenesis. Herein, tetragonal bipyramidal weddellite, a type of calcium oxalate, is successfully achieved by UV-photolyzing pyruvic acid (PA). The as-prepared products are identified and characterized by micro-Raman spectroscopy and field emission scanning electron microscopy. Persistent mineralization of weddellite is observed with altering key experimental parameters, including pH, Ca²⁺ and PA concentrations. In particular, the initial concentration of PA can significantly influence the morphology of weddellite crystal. Oxalate acid is commonly of biological origin; thus calcium oxalate is considered to be a biomarker. However, our results reveal that calcium oxalate can be harvested by a UV photolysis pathway. Moreover, prebiotic sources of organics (e.g., PA, glycine, alanine, and aspartic acid) have been proven to be available through abiotic pathways. Therefore, our results may provide a new abiotic pathway of calcium oxalate formation. Considering that calcium oxalate minerals have been taken as biosignatures for the origin and early evolution of life on Earth and astrobiological investigations, its formation and accumulation by the photolysis of abiological organic compounds should be taken into account.

Biofilm Medium Chemistry and Calcium Oxalate Morphogenesis

The present study is focused on the effect of biofilm medium chemistry on oxalate crystallization and contributes to the study of the patterns of microbial biomineralization and the development of nature-like technologies, using the metabolism of microscopic fungi. Calcium oxalates (weddellite and whewellite in different ratios) were synthesized by chemical precipitation in a weakly acidic environment (pH = 4-6), as is typical for the stationary phase of micromycetes growth, with a ratio of Ca2+/C2O42- = 4.0-5.5, at room temperature. Additives, which are common for biofilms on the surface of stone in an urban environment (citric, malic, succinic and fumaric acids; and K+, Mg2+, Fe3+, Sr2+, SO42+, PO43+ and CO32+ ions), were added to the solutions. The resulting precipitates were studied via X-ray powder diffraction (XRPD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). It was revealed that organic acids, excreted by micromicetes, and some environmental ions, as well as their combinations, significantly affect the weddellite/whewellite ratio and the morphology of their phases (including the appearance of tetragonal prism faces of weddellite). The strongest unique effect leading to intensive crystallization of weddellite was only caused by the presence of citric acid additive in the medium. Minor changes in the composition of the additive components can lead to significant changes in the weddellite/whewellite ratio. The effect of the combination of additives on this ratio does not obey the law of additivity. The content of weddellite in the systems containing a representative set of both organic acids and environmental ions is ~20 wt%, which is in good agreement with natural systems.

A review of plant metabolites with metal interaction capacity: a green approach for industrial applications

Rapid industrial development is responsible for severe problems related to environmental pollution. Many human and industrial activities require different metals and, as a result, great amounts of metals/heavy metals are discharged into the water and soil making them dangerous for both human and ecosystems and this is being aggravated by intensive demand and utilization. In addition, compounds with metal binding capacities are needed to be used for several purposes including in activities related to the removal and/or recovery of metals from effluents and soils, as metals' corrosion inhibitors, in the synthesis of metallic nanoparticles and as metal related pharmaceuticals, preferably a with minimum risks associated to the environment. Plants are able to synthesize an uncountable number of compounds with numerous functions, including compounds with metal binding capabilities. In fact, some of the plants' secondary metabolites can bind to various metals through different mechanisms, as such they are excellent sources of such compounds due to their high availability and vast diversity. In addition, the use of plant-based compounds is desirable from an environmental and economical point of view, thus being potential candidates for utilization in different industrial activities, replacing conventional physiochemical methods. This review focuses on the ability of some classes of compounds that can be found in relatively high concentrations in plants, having good metal binding capacities and thus with potential utilization in metal based industrial activities and that can be involved in the progressive development of new environmentally friendly strategies.

Preparation of novel chemically bonded ceramics with steel slag and potassium hydrogen oxalate

A novel chemically bonded ceramic (novel-CBC) is prepared based on the acid-base reaction of alkali metals in steel slag (SS) and oxalate anion (C₂O₄^2-) in potassium hydrogen oxalate (PO). The effects of SS/PO ratio and water-solid (W/S) ratio on the setting and compressive strength of novel-CBC were studied in this paper. Reaction products and microstructure of novel-CBC were characterized by X-ray diffractometer (XRD), field emission scanning electron microscope-energy dispersive spectroscopy (FESEM-EDS) and thermogravimetric analysis/differential scanning calorimetry (TG/DSC). An optimal formula is obtained at a SS/PO ratio of 3.0 and a W/S ratio of 0.20, which starts setting at 10 min and gives the strengths of 18.0, 25.0, 39.8 and 49.0 MPa at 1, 3, 7 and 28 days, respectively. The reactants from SS are mainly Ca-bearing phases, while only a small amount of RO phase is involved in reaction. The main reaction products of novel-CBC are calcium oxalate monohydrate (CaC₂O₄·H₂O; whewellite) crystals and agglomerates consisting of K, Mg, Al, Si and O elements. The unreacted Ca-bearing phase particle and RO phase residue are embedded in a mixture of abundant CaC₂O₄·H₂O crystals with smooth surfaces and a size of 0.5-1.0 µm with large amounts of the nanoscale agglomerates.

Evaluation of handheld and portable Raman spectrometers with different laser excitation wavelengths for the detection and characterization of organic minerals

Organic minerals occur rather rarely in some types of peat bogs, sedimentary geological environments, and hydrothermal veins. Commonly, calcium oxalates are produced by several plants, terpenoids are often associated with conifers. Because of the organic precursor, these minerals, from the smallest group of the mineralogical system, are sometimes considered as biomarkers. Potential detection of these compounds has high relevance in the fields of exobiology or geobiology. Here we show the potential of four portable Raman spectrometers, using different excitation wavelengths and technologies (operating at 532, 785, and 1064nm together with an advanced spectrometer using the sequentially shifted excitation (SSE) technology), for the rapid and non-destructive identification of these phases. For the organic minerals investigated here, the most intense Raman bands are generally detected at the expected wavenumber positions ±1-4cm^-1 in the region 100-2000cm^-1 in the spectra obtained from all spectrometers. Additionally, two spectrometers (the 532nm instrument and the SSE) are capable of detecting Raman bands in the higher wavenumber shift region of 2000-3500cm^-1, allowing the more detailed characterization and differentiation of the related phases. From this work, and on the basis of the experimental data obtained, it is clear that the longer laser excitation wavelengths are more preferable for organic minerals identification due to the better mitigation of fluorescence emission. In contrast, the Raman spectrometer equipped with the shortest excitation wavelength (532nm) gives a significantly higher spectral resolution and a more detailed discrimination of the Raman bands, provided that the conditions of general lower level of fluorescence emission are met. The results presented in the current study complement the knowledge on minerals and biomarkers of relevance for Martian environments which have been measured with mobile Raman spectrometers. The outcome creates a solid base towards the use of lightweight mobile Raman systems that can be used outdoors and on terrestrial outcrops. Moreover, these results and conclusions are of use for the further development of dedicated spectrometers destined for the instrumental suites on planetary rovers, in the frame of the forthcoming exobiology focused missions to Mars to be launched by NASA and ESA.

Synthesis, Structure, and Electrochemical Properties of Some Cobalt Oxalates

Transition-metal oxalates have wide applications in magnetics, photoemission, electrochemistry, etc. Herein, using hydrothermal reactions, five cobalt(II) oxalates, Na₂Co₂(C₂O₄)₃·2H₂O (I), Na₂Co(C₂O₄)₂·8H₂O (II), KLi₃Co(C₂O₄)₃ (III), Li₄Co(C₂O₄)₃ (IV), and (NH₄)₂Co₂(C₂O₄)F₄ (V) have been synthesized, and their structures are determined from single-crystal X-ray diffraction or Rietveld refinement of powder X-ray diffraction data. Notably, IV and V are identified for the first time. The structures of these cobalt oxalates are versatile, covering 0D, 1D, 2D, and 3D frameworks, while the coordination environments of Co²⁺ centers are uniquely distorted octahedra. As representative examples, I and III are investigated as cathode materials for secondary batteries. Both exhibited electrochemical activity despite large cell polarization. The present study enriches the transition-metal oxalate family and provides new options for energy storage materials.

Synthesis and Characterization of (Ca,Sr)[C2O4]∙nH2O Solid Solutions: Variations of Phase Composition, Crystal Morphologies and in Ionic Substitutions

To study strontium (Sr) incorporation into calcium oxalates (weddellite and whewellite), calcium-strontium oxalate solid solutions (Ca,Sr)[C2O4]∙nH2O (n = 1, 2) are synthesized and studied by a complex of methods: powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy. Two series of solid solutions, isomorphous (Ca,Sr)[C2O4]·(2.5 − x)H2O) (space group I4/m) and isodimorphous Ca[C2O4]·H2O(sp.gr. P21/c)–Sr[C2O4]·H2O(sp.gr. P 1 - ), are experimentally detected. The morphogenetic regularities of their crystallization are revealed. The factors controlling this process are discussed.

Investigations of Cu/Zn Oxalates from Aqueous Solution: Single-Phase Precursors and Beyond

The existence of a limited solid-solution series in the Cu/Zn binary metal oxalate system is reported. Coprecipitation was applied for the preparation of a comprehensive set of mixed Cu/Zn oxalates. Rietveld refinement of the XRD data revealed the formation of mixed-metal oxalate single phases at the compositional peripheries. Accordingly, the isomorphous substitution of Zn^II into Cu^II oxalate takes place at Zn contents of ≤6.6 and ≥79.1 atom %. Zn incorporation leads to a pronounced unit-cell contraction accompanied by Vegard-type trends for the lattice parameters. Morphologically, both solid solutions show close resemblance to the corresponding pure single-metal oxalates, and thus distinct differences are identified (SEM). The successful formation of solid solutions was further evidenced by thermal analysis. The decomposition temperature of the oxalate was taken as an approximation for Zn^II incorporation into the Cu^II oxalate structure. Single decomposition events are observed within the stated compositional boundaries and shift to higher temperature with increasing Zn content, whereas multiple events are present near Cu/Zn parity. Moreover, these findings are supported by IR and Raman spectroscopic investigations. This study on the Cu/Zn mixed-metal oxalate system sheds light on the important prerequisites for solid-solution formation and identifies the structural limitations that predefine its application as catalyst precursor.

Novel iron doped calcium oxalates as promising heterogeneous catalysts for one-pot multi-component synthesis of pyranopyrazoles

The co-precipitation method using a surface modifier, glutamic acid was employed in the design of iron doped calcium oxalates (Fe-CaOx).

KLi3Fe(C2O4)3: a perovskite-like compound with 1 : 3 ordering at both A and B sites

KLi₃Fe(C₂O₄)₃ adopts a perovskite-like structure with a unique 1 : 3 cation ordering pattern at both A and B sites.

Crystal structure of catena-poly[di-ammonium [di-μ-oxalato-cuprate(II)]]

The structure of the title compound, {(NH4)2[Cu(C2O4)2]} n , at 100 K has monoclinic (P21/c) symmetry with the CuII atom on an inversion center. The compound has a polymeric structure due to long Cu⋯O inter-actions which create [Cu(C2O4)2] chains along the a axis. The structure also displays inter-molecular N-H⋯O hydrogen bonding, which links these chains into a three-dimensional network.