Metal-ligand Species Research Articles

Interface Synergistic Effect from Hierarchically Porous Cu(OH)2@FCN MOF/CF Nanosheet Arrays Boosting Electrocatalytic Oxygen Evolution

The electrolysis of water is an efficient and environmentally friendly technology for large-scale hydrogen production. However, the oxygen evolution reaction (OER) involves a multi-electron–proton coupling transfer step that limits the efficiency of water splitting. Therefore, there is an urgent need to develop electrocatalysts with expected activity and stability to accelerate the kinetics of the oxygen evolution reaction. In this paper, hierarchically porous Cu(OH)2@(Fe, Co, Ni)MOF/CF nanosheet (denoted as Cu(OH)2@FCN MOF/CF) arrays were successfully prepared by the hydrothermally induced in situ growth of FCN MOF nanosheets using modified Cu(OH)2 nanowires as carriers; herein, the tuned active species of metal ligands in the FCN MOF composition structure are used as the main catalytic reaction size in the OER. The synergistic effect of a unique porous structure and the active metal-ligand species in the MOF render the catalyst a large electrochemically active surface area and more active species. Then, the active material is fully contacted with the electrolyte to expose more electrochemically active sites, thus greatly improving the electrocatalytic activity and durability of the OER. Specifically, the Cu(OH)2@FCN MOF/CF delivers a minimum overpotential of 290 mV and low Tafel slope of 96.15 mV·dec−1 at 10 mA·cm−2 as well as ultra-long cycling stability. The resulted OER performance is superior to most reported MOF-based electrocatalysts. This novel structural design not only provides a new strategy for the facile preparation of low-cost and high-efficiency OER electrocatalysts but also paves an avenue for the development of other MOF-based composite electrocatalysts with excellent electrocatalytic performances.

Bifunctional 3-Hydroxy-4-Pyridinones as Potential Selective Iron(III) Chelators: Solution Studies and Comparison with Other Metals of Biological and Environmental Relevance.

The binding ability of five bifunctional 3-hydroxy-4-pyridinones towards Cu2+ and Fe3+ was studied by means of potentiometric and UV–Vis spectrophotometric measurements carried out at I = 0.15 mol L−1 in NaCl(aq), T = 298.15 K and 310.15 K. The data treatments allowed us to determine speciation schemes featured by metal-ligand species with different stoichiometry and stability, owing to the various functional groups present in the 3-hydroxy-4-pyridinones structures, which could potentially participate in the metal complexation, and in the Cu2+ and Fe3+ behaviour in aqueous solution. Furthermore, the sequestering ability and metal chelating affinity of the ligands were investigated by the determination of pL0.5 and pM parameters at different pH conditions. Finally, a comparison between the Cu2+ and Fe3+/3-hydroxy-4-pyridinones data herein presented with those already reported in the literature on the interaction of Zn2+ and Al3+ with the same ligands showed that, from the thermodynamic point of view, the 3-hydroxy-4-pyridinones are particularly selective towards Fe3+ and could therefore be considered promising iron-chelating agents, also avoiding the possibility of competition, and eventually the depletion, of essential metal cations of biological and environmental relevance, such as Cu2+ and Zn2+.

Single-Molecule Observation of the Intermediates in a Catalytic Cycle.

The development of catalysts benefits from knowledge of the intermediate steps that accelerate the transformations of substrates into products. However, key transient species are often hidden in ensemble measurements. Here, we show that a protein nanoreactor can sample the intermediate steps in a catalytic cycle by the continuous single-molecule observation of a stoichiometric reaction in solution. By monitoring changes in the flow of ionic current through an α-hemolysin protein pore, we observed three intermediate metal-ligand complexes in a gold(I)-catalyzed reaction that converts an acetylenic acid to an enol lactone, revealing a transitional coordination complex that had been previously unobserved. A kinetic isotope effect helped assign the various metal-ligand species. Measurements of the lifetimes of the intermediates allowed a full kinetic analysis of the metal-catalyzed reaction cycle.

Study of metal–ligand species by ESI-MS: The case of La, Nd and Th complexes with EDTA

Electrospray ionization-mass spectrometry (ESI-MS) has been used to investigate the speciation of La, Nd and Th in the presence of the complexing agent EDTA. Two methods of sample introduction were optimized and compared: direct infusion and flow injection, without chromatographic column. The species found were independent of the sample introduction method applied. The dependence of the species response with ESI parameters and focusing of ions was studied, and a significant change in relative intensities was observed while no differences in the species found were detected. The predominant species were always the same. System behavior was evaluated by varying the pH and the metal/ligand ratio. The results are consistent with literature data. Although the linear range is rather small (10−4–10−5M), this technique could be useful to identify the species in equilibrium in different chemical environments. System behavior for mixtures of metals and for a mineral water sample was studied. The characterization of formed complexes will contribute to understand their environmental mobility.

Metal species involved in long distance metal transport in plants.

The mechanisms plants use to transport metals from roots to shoots are not completely understood. It has long been proposed that organic molecules participate in metal translocation within the plant. However, until recently the identity of the complexes involved in the long-distance transport of metals could only be inferred by using indirect methods, such as analyzing separately the concentrations of metals and putative ligands and then using in silico chemical speciation software to predict metal species. Molecular biology approaches also have provided a breadth of information about putative metal ligands and metal complexes occurring in plant fluids. The new advances in analytical techniques based on mass spectrometry and the increased use of synchrotron X-ray spectroscopy have allowed for the identification of some metal-ligand species in plant fluids such as the xylem and phloem saps. Also, some proteins present in plant fluids can bind metals and a few studies have explored this possibility. This study reviews the analytical challenges researchers have to face to understand long-distance metal transport in plants as well as the recent advances in the identification of the ligand and metal-ligand complexes in plant fluids.

Ozonation of phosphonate antiscalants used for reverse osmosis desalination: Parameter effects on the extent of oxidation

Abstract The recovery of usable product water in brackish water reverse osmosis (RO) desalination is limited by the potential for membrane scaling. This limited recovery results in high costs of disposal of the concentrate (waste) stream. Concentrate treatment to improve recovery is vital, and treatment that includes antiscalant oxidation prior to precipitation and solid/liquid separation might allow increased precipitation through antiscalant deactivation. Ozone and hydrogen peroxide were used to oxidize phosphonate-type antiscalants under varying conditions of water composition, pH, ozone dose, and antiscalant type; orthophosphate (P) is an oxidation product and was used to measure the extent of oxidation. Antiscalant oxidation increases with ozone dose (1–10 mg/L O 3 ) and in the presence of calcium, from 1 mg/L P to 7 mg/L P for an antiscalant concentration of 27 mg/L as P. The addition of hydrogen peroxide causes only minor increases in antiscalant oxidation. The extent of oxidation varies with pH as a function of metal–ligand speciation, with the doubly-protonated metal–ligand species dominating the reactivity of the antiscalant. In the presence of calcium, the primary oxidation pathway for phosphonate antiscalants is through direct reaction with ozone (70–80% of measured P). In the absence of calcium, ozone and hydroxyl radicals appear to contribute relatively equally to oxidation. These results suggest phosphonate antiscalants can be oxidized with relatively low ozone doses, thereby deactivating the chelating behavior of the antiscalant and allowing increased precipitation of scaling salts such as calcium and magnesium.

Generation of metal–ligand cluster ion beams through pulsed discharge ionization and ablation

Pulsed capacitor discharge ionization in supersonic expansions was investigated for the production of intense beams of molecular cluster ions from seeded and ablated compounds. A pulsed discharge based on a triggered spark gap switch was designed and used as a method for ionization and ablation. Several combinations of nozzle geometry and electrode arrangement in front of a pulsed valve, were made to optimize the intensity of the ion beam as well as its composition. The cationic metal–ligand complexes Cu +–(methanol) n , Cu +–(acetone) n , Cu +–(toluene) n , Cu +–(water) n , and Al +–(water) n were synthesized by ablation of the metal from metallic discharge electrodes in a discharge gas mixture of helium seeded with the ligand of choice. The cluster mass spectra of the expanded plasmas show little background ion signal besides the metal–ligand species. Charge exchange processes in the expansion guarantee high ionization yields of the desired species and account for low backgrounds. Changes in the successive binding energy of Cu +–(water) n clusters n = 1–4 are clearly observed in the cluster mass spectra as step formation. A similar pattern found in Cu +–(acetone) n suggests the same trend in the successive binding energy as known for water.

Raman study of complexation in aqueous solutions of magnesium acetate

Raman spectra of aqueous solutions of magnesium acetate at 293 K have been studied in the concentration range 0.65–3.24 M, as have the Raman spectra of the crystallohydrate Mg(CH 3COO) 2·4H 2O. Fourier deconvolution and band fitting techniques have been used to analyse the spectra in three characteristic regions. The gradual evolution of two new components, in addition to the component corresponding to the CC vibration of the free acetate ion, is observed in the CC region of the solutions. These are identified as belonging to mono- and bisacetato complexes. In saturated solutions the bisacetato complexes are the dominating metal-ligand species. Their presence is confirmed also by Fourier deconvolution of the in-plane rocking CO 2 vibration.

Effects of chemical speciation in growth media on the toxicity of mercury(II).

The toxicity of metals, including mercury, is expressed differently in different media, and the addition of soluble organics to the growth medium can have a significant impact on bioassay results. Although the effect of medium composition on metal toxicity is generally attributed to its effect on metal speciation (i.e., the chemical form in which the metal occurs), the importance of individual metal-ligand species remains largely unclear. Here, we report the results of a study that investigated, both experimentally and from a modeling perspective, the effects of complex soluble organic supplements on the acute toxicity (i.e., 50% inhibitory concentration [IC50]) of mercury to a Pseudomonas fluorescens isolate in chemically well-defined synthetic growth media (M-IIX). The media consisted of a basal inorganic salts medium supplemented with glycerol (0.1%, vol/vol) and a variety of common protein hydrolysates (0.1%, vol/vol), i.e., Difco beef extract (X = B), Casamino Acids (X = C), peptone (X = P), soytone (X = S), tryptone (X = T), and yeast extract (X = Y). These were analyzed to obtain cation, anion, and amino acid profiles and the results were used to compute the aqueous speciation of Hg(II) in the media. Respirometric bioassays were performed and IC50s were calculated. Medium components varied significantly in their effects on the acute toxicity of Hg(II) to the P. fluorescens isolate. IC50s ranged from 1.48 to 14.54 micrograms of Hg ml-1, and the acute toxicity of Hg(II) in the different media decreased in the order M-IIC >> M-IIP > M-IIB >> M-IIT > M-IIS >>> M-IIY.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical Composition and Metal Speciation in Porewaters from the Upper Qu’Appelle River Basin, Saskatchewan

Abstract Porewater samples from the upper Qu’Appelle River basin in Saskatchewan, Canada, were analyzed to obtain metal, inorganic ligand and amino add profiles. These data were used to compute the aqueous speciation of the metals in each porewater using the computer program GEOCHEM-PC. The porewaters were classified as slightly to moderately saline. Metal concentrations reflected both the geology of the drainage basin and the impact of anthropogenic activities. Whereas K and Na were present almost entirely as the free aquo ions, carbonate equilibria dominated the speciation of Ca. Mg and Mn (the predominant metal ligand species were of the type MCO3 (s). MCO30. and MHCO3+). Trace metal concentrations were generally within the ranges reported for non-polluted freshwater systems. Whereas the speciation of the trace metals Cr(III) and Co(II) was dominated by carbonate equilibria, Hg(II)-, Zn(II)- and Fe(II)-speciation was dominated by hydroxy-metal complexes of the type M(OH)+ and M(OH)2°. The speciation of Fe(III) was dominated by Fe(OH)3 (s). In porewaters with high chloride concentrations (> 2 mM), however, significant amounts of Hg(II) were bound as HgCl20 and HgClOH0. The aqueous speciation of Al was dominated by Al(OH)4− and Al2Si2O4(OH)6 (s). Total concentrations of dissolved free amino acids varied from 15.21 to 25.17 umole L−1. The most important metal scavenging amino acids were histidine (due to high stability constants for the metal-histidine complexes) and tryptophan (due to its relatively high concentration in the porewaters. i.e., 5.96 to 7.73 umole L−1). Secondary concentrations of various trace metal-amino add complexes were computed for all the porewaters, but metal-amino acid complexes dominated the speciation of Cu(II) in all the porewaters and Ni(II) in two of the porewaters.

Computer, Simulated Evaluation of Possible Mechanisms for Sequestering Metal Ion Activity in Plant Vacuoles

Various mechanisms have been suggested for sequestering Zn ion activity in vacuoles of Zn-tolerant plants. One of these mechanisms, complexation in the vacuole with organic acids, has received some support in the recent literature. However, the lack of experimental evidence for anticipated vacuolar compartmentation and concerning the nature of metal-ligand species occurring in the vacuole has been criticized. In this study we have used computer modeling of chemical equilibria to predict the metalligand species in vacuoles of tobacco (Nicotiana tabacum) cultured cells. Results of this thermodynamic evaluation support the conclusion that citrate in the concentration range encountered in tobacco cultured cells exposed to 300 or 2000 mum Zn has high potential for forming soluble complexes with Zn, over the entire probable range of vacuolar pH 4 to 7. Complexation of Zn with oxalate is also predicted, especially in cells exposed to high Zn levels. Malate, though the most abundant acid present, showed little potential for competing with other ligands for Zn. Overall, results suggest that vacuolar sequestration of Zn by high levels of vacuolar citrate may be a central mechanism in the accumulation of Zn in plants exposed to either low or high levels of this metal.

Nitrilotriacetic acid, thiourea and cysteine ligands immobilized on cellulose for the uptake of trace metal ions

Functionalized celluloses containing nitrilotriacetic acid, thiourea and cysteine have been prepared for the uptake of a series of trace metal ions, namely Al(III), Ca(II), Cd(II), Co(II), Cu(II), Fe(III), Hg(II), Mg(II), Ni(II), Pb(II) and Zn(II). The recovery has been evaluated as a function of pH, and the capacities determined at the pH of maximum yield. The experimental yields obtained for the metal recoveries have been compared with the ones calculated with the aid of the computed distribution of metal-ligand species. The materials obtained may be utilized for the enrichment of trace metals, for the demetalization of solutions, and for speciation studies.

SEPARATION OF HEAVY METAL CHELATES BY ACTIVATED CARBON: EFFECT OF SURFACE AND SPECIES CHARGE

An extensive experimental investigation was conducted with activated carbons to establish the adsorption capacities of heavy metals (Cd2+, Ni2+, Cu2+, and Zn2+ ) in the presence of complexing agents (EDTA, triethylenetetramine (TRIEN), citrate, etc.). Adsorption equilibria are explained by species charges and carbon surface charge characteristics. In the pH range 7.5−8.0, the active sites are positively charged. The adsorption capacities of Cd2+ (and other heavy metals), free ligands, and cadmium-ligands are also a function of feed metal concentration, types of ligands, molar ratio of ligand/metal, and pH. Complete metal-ligand species distributions are calculated by computer solutions of multiple reaction equilibria. For an equilibrium cadmium concentration of 0.1 mM (pH 7.5−8.0), the sequence in adsorption capacity for cadmium chelates is Q Cd−EDTA > Q Cd−citrate > Q Cd−TRIEN > Q Cd−tartrate. A relationship between the adsorption capacity, surface charge of activated carbon, and the average species char...

Equilibria in complexation reactions between thorium(IV) and polydentate ligands

Chelation reactions of thorium(IV) with 3,3′-bis[N,N′-di(carboxymethyl) aminomethyl]- o-cresolsulphonphthalein (xylenol orange; DCAC) and 1-( o-arsonophenylazo)-2-naphthol-3,6-disulphonic acid (thoron; APANS) have been investigated in aqueous media at 35° at an ionic strength of 0·1 M (KNO 3) employing a pH-titration technique. Evidence is presented of the formation of the monoprotonated 1:1 metal-ligand species. These species liberate hydrogen ions at pH ≈ 4·5, and finally the nonprotonated species interacts with hydroxyl ions to form mono- and dihydroxo species. The equilibrium constants have been evaluated, and the probable co-ordination sites of ligands involved in the complexes have been suggested.