Metal Oxalate Complexes Research Articles

Lead Solubility and Mineral Structures of Coprecipitated Lead/Calcium Oxalates.

Low-molecular-weight organic acids such as oxalate, which are ubiquitous in the environment, can control the solubility and bioavailability of toxic metals such as Pb in soils and water by influencing complexation and precipitation reactions. Here, we investigated Pb solubility in relation to Pb-oxalate precipitation at pH 5.0 in the absence and presence of calcium (Ca), a common cation in environmental matrices. At Pb mole fractions less than 0.10, sequestration of Pb into Ca oxalate to form a solid solution substantially lowered Pb solubility relative to that of pure Pb oxalate to an extent inversely proportional to the Pb mole fraction. Small Pb/Ca solid-solution distribution coefficients at these low mole ratios was largely attributed to the stronger complexation of Pb compared to Ca with oxalate to form soluble metal-oxalate complexes, which in turn limited Pb incorporation into the Ca-oxalate crystal lattice. Characterization of the Pb/Ca-oxalate coprecipitates by X-ray diffraction, optical microscopy, and Fourier transform infrared spectroscopy revealed that the whewellite (Ca-oxalate monohydrate) structure was destabilized by substitution of small amounts of Pb into the lattice, and thus, the formation of the Ca-oxalate dihydrate (weddellite) was favored over the monohydrate. At Pb mole fractions above 0.20, discrete crystallites of Pb oxalate were identified. These new findings imply that Pb/Ca-oxalate coprecipitates in the presence of Ca could reduce the solubility of Pb in Pb-contaminated acid soils.

Rare earth elements concentrations and speciation in rainwater from Guiyang, an acid rain impacted zone of Southwest China

Rare earth elements (REEs) and major ions were determined in rainwater samples collected from Guiyang in the acid rain impacted zone of Southwest China. The concentrations of total dissolved and acid-soluble REEs in rainwater are higher than those of most of the world's cities as well as the local surface water. The dissolved REE concentrations are negatively correlated to pH in rainwater. This is consistent with acidification of rainwater resulting in increasing REE concentrations. Speciation calculations using the PHREEQC-Model predict that the free REE metal ion (i.e., Ln3+), sulfate (LnSO4+), oxalate (LnOx+ and LnOx2−) and fluoride (LnF2+) complexes were relatively important forms of dissolved REE. Although the contribution to rainwater acidity of oxalic acid is much lower than that of sulfuric acid, the proportion of REE-oxalate complexes is not lower than that of REE-sulfate complexes in rain waters. The rainwater is enriched in the middle REEs (MREE) compared to both the light REEs (LREE) and heavy REEs (HREE). REE-phosphate complexes, phosphatic minerals and a Fe-Mn-oxyhydroxide coating are more likely controls on the development of MREE enrichment in the rainwater. Complexation of sulfate and REE played little role in the development of MREE enrichment, even though sulfate is the most important anion in acidic rainwater. In the shale-normalized REE pattern plot, La showed a clear positive anomaly. La might have two main anthropogenic sources in rainwater. One of the important sources could be automobile emission; another main source of excess La in rainwater could be the REE fertilizer that has been widely used in the agriculture of China for approximately 30years.

Synthesis, spectroscopic, thermal and XRD studies of aminoguanidinium copper and cadmium oxalates

New aminoguanidinium metal oxalate complexes with the formulae (AmgH)2[Cu(C2O4)2] and (AmgH)2[Cd(C2O4)2(H2O)2] have been synthesized and characterized. The copper compound loses aminoguanidine exothermically at 240 °C in DTA, whereas aminoguanidine is lost endothermically at 200 °C in cadmium. Both cases decompose exothermically via their respective metal oxalate intermediates to give metal oxide as the end product. It was also observed that hydrazine is lost exothermically in the hydrazinium copper oxalate hydrate. The single-crystal X-ray diffraction study of the copper complex revealed that aminoguanidinium ions are not involved in coordination but act as charge-compensating cations. It is interesting to note that both oxalates act as bidentate chelating ligands. One oxalate bridges the neighbouring copper atom through its carbonyl oxygen with a bond length of 2.561 A to form square pyramidal geometry around the copper atom.

Applications in environmental bioinorganic: Nutritional and ultrastructural evaluation and calculus of thermodynamic and structural properties of metal-oxalate complexes

Lead (Pb) is known by its toxicity both for animals and plants. In order to evaluate its toxicity, plants of Brachiaria brizantha were cultivated on nutritive solution of Hoagland during 90days and submitted to different concentrations of Pb. The content of macro and micronutrients was evaluated and there was a reduction on root content of Ca, besides the lowest dosages of Pb had induced an increase of N, S, Mn, Cu, Zn and Fe. The cell ultrastructure of leaves and roots were analyzed by transmission electronic microscopy (TEM). Among the main alterations occurred there were invaginations on cell walls, the presence of crystals on the root cells, accumulation of material on the interior of cells and vacuolar compartmentalization. On the leaves the degradation of chloroplasts was observed, as well as the increase of vacuoles. Structures for the formation of oxalate crystals were proposed through molecular modeling and thermodynamic stability. Calculi suggest the formation of highly stable metal-oxalate complexes.

Non-invasive identification of metal-oxalate complexes on polychrome artwork surfaces by reflection mid-infrared spectroscopy

In this work a reflection mid-infrared spectroscopy study of twelve metal-oxalate complexes, of interest in art conservation science as alteration compounds, was performed. Spectra of the reference materials highlighted the presence of derivative-like and/or inverted features for the fundamental vibrational modes as result of the main contribution from the surface component of the reflected light. In order to provide insights in the interpretation of theses spectral distortions, reflection spectra were compared with conventional transmission ones. The Kramers–Kronig (KK) algorithm, employed to correct for the surface reflection distortions, worked properly only for the derivative-like bands. Therefore, to pay attention to the use of this algorithm when interpreting the reflection spectra is recommended. The outcome of this investigation was exploited to discriminate among different oxalates on thirteen polychrome artworks analyzed in situ by reflection mid-infrared spectroscopy. The visualization of the νs(CO) modes (1400–1200cm−1) and low wavenumber bands (below 900cm−1) in the raw reflection profiles allowed Ca, Cu and Zn oxalates to be identified. Further information about the speciation of different hydration forms of calcium oxalates were obtained by using the KK transform. The work proves reflection mid-infrared spectroscopy to be a reliable and sensitive spectro-analytical method for identifying and mapping different metal-oxalate alteration compounds on the surface of artworks, thus providing conservation scientists with a non-invasive tool to obtain information on the state of conservation and causes of alteration of artworks.

Uranyl–copper(ii) heterometallic oxalate complexes: coordination polymers and frameworks

Five oxalate complexes containing both uranyl and copper(II) ions and 2,2′-bipyridine (bipy), 2,2′-bipyrimidine (bipym) or 1,10-phenanthroline (phen) as co-ligands have been synthesized under hydrothermal conditions and their crystal structures were determined. [UO2Cu(C2O4)(NO3)2(bipy)(H2O)] (1) is a molecular complex in which the two cations are connected by the bis-chelating oxalate. A one-dimensional uranyl oxalate ribbon is formed in [UO2Cu(C2O4)2(bipym)(H2O)] (2), with uranyl ions bound to two chelating anions and to a third, monodentate oxalate which holds the decorating [Cu(bipym)(H2O)]2+ groups. Complex [(UO2)2Cu2(C2O4)2(bipym)3(OH)2]·2NO3 (3) is a two-dimensional assembly in which uranyl oxalate chains are linked to one another by [Cu2(bipym)3]4+ dinuclear units through bridging hydroxide ions. Finally, [UO2Cu(C2O4)2(bipy)] (4) and [UO2Cu(C2O4)2(phen)] (5) display identical three-dimensional arrangements containing uranyl oxalate sheets connected to one another by copper oxalate dinuclear units. The magnetic properties of compounds 2 and 4 have been investigated. Weak inter-chain antiferromagnetic interactions between copper(II) atoms are present in 2, while 4 displays strong oxalate-mediated antiferromagnetic exchange (J = -335 cm(-1)). These complexes are the first uranyl-d block metal oxalate complexes to be reported.

Protection of Metal Artifacts with the Formation of Metal–Oxalates Complexes by Beauveria bassiana

Several fungi present high tolerance to toxic metals and some are able to transform metals into metal–oxalate complexes. In this study, the ability of Beauveria bassiana to produce copper oxalates was evaluated. Growth performance was tested on various copper-containing media. B. bassiana proved highly resistant to copper, tolerating concentrations of up to 20 g L−1, and precipitating copper oxalates on all media tested. Chromatographic analyses showed that this species produced oxalic acid as sole metal chelator. The production of metal–oxalates can be used in the restoration and conservation of archeological and modern metal artifacts. The production of copper oxalates was confirmed directly using metallic pieces (both archeological and modern). The conversion of corrosion products into copper oxalates was demonstrated as well. In order to assess whether the capability of B. bassiana to produce metal–oxalates could be applied to other metals, iron and silver were tested as well. Iron appears to be directly sequestered in the wall of the fungal hyphae forming oxalates. However, the formation of a homogeneous layer on the object is not yet optimal. On silver, a co-precipitation of copper and silver oxalates occurred. As this greenish patina would not be acceptable on silver objects, silver reduction was explored as a tarnishing remediation. First experiments showed the transformation of silver nitrate into nanoparticles of elemental silver by an unknown extracellular mechanism. The production of copper oxalates is immediately applicable for the conservation of copper-based artifacts. For iron and silver this is not yet the case. However, the vast ability of B. bassiana to transform toxic metals using different immobilization mechanisms seems to offer considerable possibilities for industrial applications, such as the bioremediation of contaminated soils or the green synthesis of chemicals.

Oxalate metal complexes in aerosol particles: implications for the hygroscopicity of oxalate-containing particles

Abstract. Atmospheric aerosols have both a direct and an indirect cooling effect that influences the radiative balance at the Earth's surface. It has been estimated that the degree of cooling is large enough to weaken the warming effect of carbon dioxide. Among the cooling factors, secondary organic aerosols (SOA) play an important role in the solar radiation balance in the troposphere as SOA can act as cloud condensation nuclei (CCN) and extend the lifespan of clouds because of their high hygroscopic and water soluble nature. Oxalic acid is an important component of SOA, and is produced via several formation pathways in the atmosphere. However, it is not certain whether oxalic acid exists as free oxalic acid or as metal oxalate complexes in aerosols, although there is a marked difference in their solubility in water and their hygroscopicity. We employed X-ray absorption fine structure spectroscopy to characterize the calcium (Ca) and zinc (Zn) in aerosols collected at Tsukuba in Japan. Size-fractionated aerosol samples were collected for this purpose using an impactor aerosol sampler. It was shown that 10–60% and 20–100% of the total Ca and Zn in the finer particles (<2.1 μm) were present as Ca and Zn oxalate complexes, respectively. Oxalic acid is hygroscopic and can thus increase the CCN activity of aerosol particles, while complexes with various polyvalent metal ions such as Ca and Zn are not hygroscopic, which cannot contribute to the increase of the CCN activity of aerosols. Based on the concentrations of noncomplexed and metal-complexed oxalate species, we found that most of the oxalic acid is present as metal oxalate complexes in the aerosols, suggesting that oxalic acid does not always increase the hygroscopicity of aerosols in the atmosphere. Similar results are expected for other dicarboxylic acids, such as malonic and succinic acids. Thus, it is advisable that the cooling effect of organic aerosols should be estimated by including the information on metal oxalate complexes and metal complexes with other dicarboxylic acids in aerosols.

The hydrogen bonds in transition metal oxalate complexes: influence of Jahn–Teller distortion

The hydrogen bonds in transition metal oxalate complexes: influence of Jahn–Teller distortion

Reactivity of metal oxides: Thermal and photochemical dissolution of MO and MFe 2O 4 ( M=Ni, Co, Zn)

Dissolution rates of NiO, CoO, ZnO, α-Fe 2O 3 and the corresponding ferrites in 0.1 mol dm −3 oxalic acid at pH 3.5 were measured at 70 °C. The dissolution of simple oxides proceeds through the formation of surface metal oxalate complexes, followed by the transfer of surface complexes (rate-determining step). At constant pH, oxalate concentration and temperature, the trend in the first-order rate constant for the transfer of the surface complexes ( k Me ; Me=Ni, Co, Zn, Fe) parallels that of water exchange in the dissolved metal ions ( k −w). Thus, the most important factor determining the rates of dissolution of metal oxides is the lability of Me–O bonds, which is in turn defined by the electronic structure of the metal ion and its charge/radius ratio. UV (384 nm) irradiation does not increase significantly the dissolution rates of NiO, CoO and ZnO, whereas hematite is highly sensitive to UV light. For ferrites, the reactivity order is ZnFe 2O 4>CoFe 2O 4⪢NiFe 2O 4. Dissolution is congruent, with rates intermediate between those of the constituent oxides, Fe 2O 3 and MO ( M=Co, Ni, Zn), reflecting the behavior of very thin leached layers with little Zn and Co, but appreciable amounts of Ni. The more robust Ni 2+ labilizes less the corresponding ferrite. The correlation between log k M and log k −w is somewhat blurred and displaced to lower k M values. Fe(II), either photogenerated or added as salt, enhances the rate of Fe(III) phase transfer. A simple reaction mechanism is used to interpret the data.

Metal oxalate complexes as novel inorganic dopants: Studies on their effect on conducting polyaniline

Doped polyaniline materials with metal oxalate complexes of Cr, Fe, Mn, Co and Al were synthesized byin situ chemical oxidative polymerization of aniline using potassium perdisulphate as oxidant in aqueous sulphuric acid medium. These polymer materials were characterized by chemical analyses, spectral studies (UV-visible and IR), X-ray diffraction and thermal techniques and also by conductivity measurements by four-probe technique. The presence of complex anion in polyaniline material was confirmed by chemical and spectral analyses. The yield and conductivity of metal oxalate doped polyanilines were found to be high when compared to the simple sulphate ion doped polyaniline prepared under similar condition. UV-visible and IR spectral features not only confirmed the polyaniline doping by complex anions but also substantiated their facilitating effect on conductivity. The X-ray diffraction patterns indicated some crystalline nature in metal oxalate doped polyaniline and amorphous in polyaniline sulphate salt. The conductivity of the polymer samples strongly depended on the degree of crystallinity induced by complex counter anions as dopant. All the polymer materials, as evident from TGA curves, were observed to undergo three-step degradation of water loss, de-doping and decomposition of polymer. Further, the thermal stability of polyaniline was found to improve on doping with metal oxalate complex.

Characteristic mechanisms of the homogeneous and heterogeneous oxidation of aromatic amines with transition metaloxalate complexes

The oxidation–reduction reactions of aromatic amines { o-aminophenol ( o-AP), o-phenylenediamine ( o-PDA) and p-phenylenediamine ( p-PDA)} with Co(III), Mn(III), and Cu(II)oxalate complexes have been investigated in homogeneous and heterogeneous systems. The kinetics of the redox reaction of both systems have been studied spectrophotometrically. The redox products were identified and characterised by diffuse reflectance spectroscopy (DRS), and mass spectrometry. The redox reactions follow first-order kinetics with respect to each of the reactants and first order in [amines] in homogeneous and heterogeneous (supported complexes on Amberlite IRA 904 anion-exchange resin) phases, respectively. However, the specific oxidation rate of the amines is found to follow the order, p-PDA> o-AP> o-PDA. The effect of the oxalate anion on the reaction rate of both systems was examined. There is a significant difference in the behaviour of the oxidation of o-AP and o-, p-PDA with all the metal complexes in both systems. Moreover, the oxidation of o-AP and o-, p-PDA appears to follow the outer- and inner-sphere mechanistic classification, respectively.

Retention behaviour of metal ions on tri- n-butyl phosphate- and tri- n-butyl amine-impregnated silica gel-G layers in oxalic acid–oxalate mobile phases

The chromatographic behaviour of thirty metal ions was studied on tri- n-butyl phosphate- (TBP) and Tri- n-butyl amine- (TBA) impregnated silica gel-G layers in oxalic acid–oxalate mobile phases. The effects of TBP impregnation, pH of the mobile phase and p K 1 of metal oxalate complexes on R F values are discussed. For almost all metal ions, there is a sudden change in their chromatographic behaviour at 60% TBP impregnation. The mechanism of migration is explained in terms of adsorption, precipitation and complex formation.

Separation of metal-oxalate complexes on an anion-exchange column

Divalent metal cations (Mn 2+ , Co 2+ , Ni 2+ and Cu 2+ ) were separated on Q-L anion exchanger in the form of their oxalate complexes. The mechanism of the retention of anionic complexes on an anion-exchange column is proposed using the general theory of side equilibria in liquid chromatography. Dependences of the analyte retention on the mobile phase composition were measured and evaluated with the aid of the derived relationships.

Optimization of the anion-exchange separation of metal-oxalate complexes

Anion exchangers, prepared by the sorption of cetylpyridinium chloride onto a reversed phase, have been evaluated for the separation of the transition metal ions Mn 2+, Co 2+, Ni 2+, Cu 2+, and Zn 2+. Parameters examined included nature of complexing reagent, eluent pH, eluent concentration, speciation of the metal ion in the complexation system, and column capacity. The separations obtained with an oxalate eluent were compared to those obtained with a cation exchanger prepared by the addition of n-octanesulfonate to the eluent. Theoretical plate heights were in the range of 0.010 mm to 0.025 mm, and were in many instances, slightly better than those obtained with cation separation systems. Metal ion speciation in the eluent appeared to be a dominant factor determining the efficiency of the anion separation; metal ions with one dominant complexed ion generally gave more efficient separations. Compared to cation separations, analysis times were similar, but separation selectivities were considerably different.

Biogeochemistry of oxalate in the antarctic cryptoendolithic lichen-dominated community

Cryptoendolithic (hidden in rock) lichen-dominated microbial communities from the Ross Desert of Antarctica were shown to produce oxalate (oxalic acid). Oxalate increased mineral dissolution, which provides nutrients, creates characteristic weathering patterns, and may ultimately influence the biological residence time of the community. Oxalate was the only organic acid detectable by HPLC, and its presence was verified by GC/MS. Community photosynthetic metabolism was involved in oxalate production since rates of (14)C-oxalate production from (14)C02 were higher in light than in dark incubations. Flaking of the sandstone at the level of the lichen-dominated zone a few millimeters beneath the rock surface can be explained by dissolution of the sandstone cement, which was enhanced by Si, Fe, and Al oxalate complex formation. Added oxalate was observed to increase the solubility of Si, Fe, Al, P, and K. Oxalate's ability to form soluble trivalent metal-oxalate complexes correlated with the observed order of metal oxide depletion from the lichen-dominated zone (Mn > Fe > Al). Thermodynamic calculations predict that Fe oxalate complex formation mobilizes amorphous Fe oxides (ferrihydrite) in the lichen-dominated zone, and where oxalate is depleted, ferrihydrite should precipitate. Hematite, a more crystalline Fe oxide, should remain solid at in situ oxalate concentrations. Oxalate was not a carbon source for the indigenous heterotrophs, but the microbiota were involved in oxalate mineralization to CO2, since oxalate mineralization was reduced in poisoned incubations. Photooxidation of oxalate to C02 coupled with photoreduction of Fe(Ill) may be responsible for oxalate removal in situ, since rates of (14)C-oxalate mineralization in dark incubations were at least 50% lower than those in the light. Removal of oxalate from Si, Fe, and Al complexes should allow free dissolved Si, Fe, and Al to precipitate as amorphous silicates and metal oxides. This may explain increased siliceous crust (rock varnish or desert varnish) formation near the surface of colonized rocks were light intensity is greatest.

Thermoanalytical investigations on the decomposition of transition metal oxalate complexes using selective CO, CO2 and H2O monitors

Simultaneous TG, DTG, DTA measurements along with the continuous and selective monitoring of carbon monoxide, carbon dioxide and water evolved were carried out on K3[M(C2O4)3].3H2O-type transition metal complexes (whereM=Cr, Fe and Co). Based on the comparison of the recorded curves a detailed description of the decomposition mechanism was possible. In the case of the cobalt complex an exothermic process corresponding to modification of electron configuration is superimposed on the endothermic dehydration reaction.The reduction of the cobalt central atom — dissimilarly to that of chromium and iron — takes place in two steps.The carbon monoxide detector developed recently proved to be an important tool in the study of the decomposition of these complexes.

Photo- and Thermosensitivity of Metal Oxalate Complexes

Photo- and Thermosensitivity of Metal Oxalate Complexes

Crystal field aspects of vibrational spectra: III. First transition series metal(III) oxalate and cyanide complexes and second transition series metal(III) acetyl-acetonates: effect of spin state

The metal-ligand stretching frequencies of first transition series metal(III) complex oxalates, K 3[M(C 2O 4] 3·3H 2O, have been determined and their crystal field stabilization energies (CFSE's) calculated. The frequencies follow the order Sc < V < Cr > Mn > Fe > Co < Ga predicted by the CFSE data for a series of complexes in which the d 4 and d 5 ions have high spin configuration. For complexes in which these ions are low spin, a different order is predicted by the theory. It is shown that the metal-ligand frequencies of the first transition series metal(IIl) complex cyanides and the second transition series metal(III) acetylacetonates follow this order.

Raman study of aqueous metal oxalate complexes and related oxalates

Raman study of aqueous metal oxalate complexes and related oxalates