Low Molecular Weight Organic Ligands Research Articles

Infinite coordination polymer networks: metallogelation of aminopyridine conjugates and in situ silver nanoparticle formation.

Herein we report silver(i) directed infinite coordination polymer network (ICPN) induced self-assembly of low molecular weight organic ligands leading to metallogelation. Structurally simple ligands are derived from 3-aminopyridine and 4-aminopyridine conjugates which are composed of either pyridine or 2,2'-bipyridine cores. The cation specific gelation was found to be independent of the counter anion, leading to highly entangled fibrillar networks facilitating the immobilization of solvent molecules. Rheological studies revealed that the elastic storage modulus (G') of a given gelator molecule is counter anion dependent. The metallogels derived from ligands containing a bipyridine core displayed higher G' values than those with a pyridine core. Furthermore, using single crystal X-ray diffraction studies and 1H-15N two-dimensional (2D) correlation NMR spectroscopy, we show that the tetracoordination of silver ions enables simultaneous coordination polymerization and metallosupramolecular cross-linking. The resulting metallogels show spontaneous, in situ nanoparticle (d < 2-3 nm) formation without any additional reducing agents. The silver nanoparticle formation was followed using spectroscopic studies, and the self-assembled fibrillar networks were imaged using transmission electron microscopy (TEM) imaging.

Quantum Chemical Approach for Calculating Stability Constants of Mercury Complexes

Stability constants are central to the multiscale modeling of the thermodynamic speciation, cycling, and transport of mercury (Hg) and other contaminants in aquatic environments. However, for Hg, experimental values for many relevant complexes are not available, and for others can span ranges in excess of 10 log units. The missing data and large uncertainties lead to significant knowledge gaps in predictions of thermodynamic speciation. As an alternative to experimental measurements, thermodynamic quantities can be calculated with quantum chemical methods. Among these, density functional theory (DFT) with a polarizable continuum solvent combines accuracy with practicability. Here, we present an accurate and quick approach in which we use DFT with continuum solvation to calculate stability constants of Hg complexes with inorganic and low molecular-weight organic ligands in aqueous solution. Specifically, we use the M06/[SDD]6-31+G(d,p) level of theory in combination with a modified version of the SMD solve...

Uptake, Distribution, and Transformation of Zerovalent Iron Nanoparticles in the Edible Plant Cucumis sativus.

Here, we investigated the fate of nanoscale zerovalent iron (nZVI) on the Cucumis sativus under both hydroponic and soil conditions. Seedlings were exposed to 0, 250, and 1000 mg/L (or mg/kg soil) nZVI during 6-9 weeks of a growth period. Ionic controls were prepared using Fe-EDTA. None of the nZVI treatments affected the plant biomass. On the basis of the total iron contents and the superparamagnetic property of nZVI-exposed roots, there was no evidence of pristine nZVI translocation from the roots to shoots. Electron microscopy revealed that the transformed iron nanoparticles are stored in the root cell membrane and the vacuoles of the leaf parenchymal cells. X-ray absorption spectroscopy identified ferric citrate (41%) and iron (oxyhydr)oxides (59%) as the main transformed products in the roots. The shoot samples indicated a larger proportion of ferric citrate (60%) compared to iron (oxyhydr)oxides (40%). The 1.8-fold higher expression of the CsHA1 gene indicated that the plant-promoted transformation of nZVI was driven by protons released from the root layers. The current data provide a basis for two potential nZVI transformation pathways in Cucumis sativus: (1) interaction with low molecular weight organic acid ligands and (2) dissolution-precipitation of the mineral products.

Intramolecular deactivation processes of electronically excited Lanthanide(III) complexes with organic acids of low molecular weight

The luminescence of Lanthanide(III) complexes with different model ligands was studied under direct as well as sensitized excitation conditions. The research was performed in the context of studies dealing with deep-underground storages for high-level nuclear waste. Here, Lanthanide(III) ions served as natural analogues for Actinide(III) ions and the low-molecular weight organic ligands are present in clay minerals and furthermore, they were employed as proxies for building blocks of humic substances, which are important complexing molecules in the natural environment, e.g., in the far field of a repository site. Time-resolved luminescence spectroscopy was applied for a detailed characterization of Eu(III), Tb(III), Sm(III) and Dy(III) complexes in aqueous solutions. Based on the observed luminescence the ligands were tentatively divided into two groups (A, B). The luminescence of Lanthanide(III) complexes of group A was mainly influenced by an energy transfer to OH-vibrations. Lanthanide(III) complexes of group B showed ligand-related luminescence quenching, which was further investigated. To gain more information on the underlying quenching processes of group A and B ligands, measurements at different temperatures (77K≤T≤353K) were performed and activation energies were determined based on an Arrhenius analysis. Moreover, the influence of the ionic strength between 0M≤I≤4M on the Lanthanide(III) luminescence was monitored for different complexes, in order to evaluate the influence of specific conditions encountered in host rocks foreseen as potential repository sites.

Effect of desferrioxamine B and Suwannee River fulvic acid on Fe(III) release and Cr(III) desorption from goethite

Siderophores are biogenic chelating ligands that facilitate the solubilisation of Fe(III) and form stable complexes with a range of contaminant metals and therefore may significantly affect their biogeochemical cycling. Desferrioxamine B (DFOB) is a trihydroxamate siderophore that acts synergistically with fulvic acid and low molecular weight organic ligands to release Fe from Fe(III) oxides. We report the results of batch dissolution experiments in which we determine the rates of Cr(III) desorption and Fe(III) release from Cr(III)-treated synthetic goethite as influenced by DFOB, by fulvic acid, and by the two compounds in combination. We observed that adsorbed Cr(III) at 3% surface coverage significantly reduced Fe(III) release from goethite for all combinations of DFOB and fulvic acid. When DFOB (270μM) was the only ligand present, dissolved Fe(III) and Cr(III) increased approximately 1000-fold and 16-fold, respectively, as compared to the ligand-free system, a difference we attribute to the slow rate of water exchange of Cr(III). Suwannee River fulvic acid (SRFA) acts synergistically with DFOB by (i) reducing the goethite surface charge leading to increased HDFOB+ surface excess and by (ii) forming aqueous Fe(III)–SRFA species whose Fe(III) is subsequently removed by DFOB to yield aqueous Fe(III)–DFOB complexes. These observations shed new light on the synergistic relationship between DFOB and fulvic acid and reveal the mechanisms of Fe(III) acquisition available to plants and micro-organisms in Cr(III) contaminated environments.

Accessibility of Humic-Associated Fe to a Microbial Siderophore: Implications for Bioavailability

Microorganisms in aerobic, circum-neutral environments are challenged to acquire sufficient nutrient Fe due to low solubilities of Fe oxides. To overcome this challenge, many aerobic microbes produce low molecular weight (MW) organic ligands, or siderophores, with extremely high Fe-binding affinities. This research expands the existing understanding of siderophore-mediated Fe acquisition from minerals by examining the effects of the siderophore desferrioxamine B (DFOB) on Fe removal from aquatic humic substances (XAD-8-isolated) and other organic matter (OM) isolates (reverse osmosis, RO; and "transphilic", XAD-4) from several rivers including the Suwannee River (GA, USA). Analysis of samples by asymmetrical flow field-flow fractionation (AsFlFFF) with in-line ICP-MS and UV-vis detectors showed that Fe was naturally abundant and primarily associated with intermediate to high MW OM. An excess of DFOB (relative to naturally present Fe) removed ∼ 75% of Fe and shifted the OM MW distribution to lower MWs, perhaps due to removal of "bridging" Fe, although additional mechanistic study of MW shifts is needed. Removal of other OM-associated metals (e.g., Al, Cu, Zn) by DFOB was minimal for all but a few samples. Fe bound to humic substances and other more "transphilic" organic components therefore should be considered readily bioavailable to aerobic, siderophore-producing microorganisms.

Stability and structure of pentavalent antimony complexes with aqueous organic ligands

Despite the rapidly growing concern about antimony pollution of waters and soils, the effect of organic matter on the behavior of this toxic trace element is poorly understood because of a lack of data on Sb V-organic ligand interactions in aqueous solution. We used in situ potentiometry and X-ray absorption spectroscopy (XAS) to measure in aqueous solution at ambient conditions the stability and structure of aqueous complexes formed by pentavalent antimony (Sb V) with low molecular weight organic ligands, such as carboxylic acids (acetic, adipic, malonic, lactic, oxalic, citric and salicylic), phenols (catechol), polyols (xylitol and mannitol), and amines (glycine), which have O- and N-functional groups typical of natural organic matter. Potentiometric titrations from pH 2 to 10 demonstrate negligible Sb V complexing with amine and carboxylic acids with single functional group (acetic acid) or non-adjacent functional groups (adipic acid). In contrast, Sb V forms stable complexes with poly-carboxylic, hydroxy-carboxylic acids, and with aliphatic and aromatic polyol ligands in the pH range typical of natural waters. XAS measurements show that in these species the Sb V atom has a distorted octahedral geometry composed of 6 oxygen atoms forming a five- or six-membered bidendate cycle. Stability constants of Sb V-organic complexes, generated for the first time in this study, were used to model Sb V binding with natural humic acids containing the same functional groups as those used in this work. Our predictions of Sb V binding with natural humic acids indicate that in an aqueous organic-rich solution of 1 μg L − 1 Sb and 20 mg L − 1 dissolved organic carbon (DOC) up to 40% of total Sb binds to aqueous organic matter via carboxyl and hydroxy-carboxyl functional groups at pH ≤ 4, whereas at neutral-to-basic pH this amount does not exceed 5%. These estimations are in agreement with direct dialysis measurements conducted with a purified commercial humic acid. The low affinity of Sb V to organic matter at near-neutral pH contrasts with that of Sb III whose organic complexes may account up to 80% of total Sb in DOC-rich waters. The large differences in Sb III versus Sb V binding to organic matter may be used for tracing in organic-rich sediments and waters the two main Sb oxidation states, which have different toxicities for aquatic organisms.

Incongruent weathering of Cd and Zn from mine tailings: A column leaching study

The weathering of discharged mine tailings can contaminate groundwaters, rivers and floodplains with potentially toxic Cd and Zn, depending on tailings mineralogy, storage, dispersal and climatic conditions. The mechanisms of long-term tailings weathering and its influence on waste piles and floodplain environments were assessed by a column leaching experiment that incorporated tailings and soil from Potosí, Bolivia, and modelled 20 cycles of wet and dry season conditions over three calendar years. Chemical analysis of the leachate and column solids, optical mineralogy, XRD, SEM, EPMA, BCR and water-soluble chemical extractions and speciation modelling were carried out to determine the processes responsible for the leaching of Cd, Fe, S and Zn. Over this period, approximately 50 to 95% of the original Cd and 50 to 60% of the Zn were leached from the columns. Large amounts of leached Cd and Zn at the beginning of the experiment are attributed to the dissolution of soluble sulphate minerals present in the original tailings and formed after the first wetting of the columns. The Zn/Cd mass ratios of the tailings and soil, initially 429 and 400, respectively, vary considerably over the course of the experiment. Low values (between 220 and 300) in the early cycles are attributed to preferential weathering of Cd-rich wurtzite [Zn,Fe)S] and sequestration of Zn in preference to Cd in secondary Fe phases forming in the columns. In the middle cycles, dissolution of secondary Fe(OH)3 under low pH (<3) conditions, and of ferroan (Cd-poor) sphalerite [Zn,Fe)S], releases Zn and raises the Zn/Cd ratio to 550–600 in the tailings-only columns and up to 1500 in the mixed tailings-soil columns. The very high ratios in the latter are also ascribed to the formation of low molecular weight organic ligands that have high affinity for Zn over Cd. In the later column-cycles, Zn/Cd ratios return to near-initial values, due to the weathering of Fe-poor sphalerite and secondary Fe phases, and the declining preference of Zn over Cd in the soil organic acids under the strongly acidic conditions prevailing in the columns. The formation and dissolution of secondary soluble sulphate minerals also play a role in Cd and Zn cycling, especially at the beginning of the experiment.

The hyperaccumulator Alyssum murale uses complexation with nitrogen and oxygen donor ligands for Ni transport and storage

The Kotodesh genotype of the nickel (Ni) hyperaccumulator Alyssum murale was examined to determine the compartmentalization and internal speciation of Ni, and other elements, in an effort to ascertain the mechanism used by this plant to tolerate extremely high shoot (stem and leaf) Ni concentrations. Plants were grown either hydroponically or in Ni enriched soils from an area surrounding an historic Ni refinery in Port Colborne, Ontario, Canada. Electron probe micro-analysis (EPMA) and synchrotron based micro X-ray fluorescence (μ-SXRF) spectroscopy were used to determine the metal distribution and co-localization and synchrotron X-ray and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopies were used to determine the Ni speciation in plant parts and extracted sap. Nickel is concentrated in the dermal leaf and stem tissues of A. murale bound primarily to malate along with other low molecular weight organic ligands and possibly counter anions (e.g., sulfate). Ni is present in the plant sap and vasculature bound to histidine, malate and other low molecular weight compounds. The data presented herein supports a model in which Ni is transported from the roots to the shoots complexed with histidine and stored within the plant leaf dermal tissues complexed with malate, and other low molecular weight organic acids or counter-ions.

Siderophore sorption to clays

Siderophores are low molecular weight organic ligands exuded by some aerobic organisms and plants to acquire Fe under Fe-limited conditions. The hydroxamate siderophores may sorb to aluminosilicate clays through a variety of mechanisms depending upon the nature of the clay and of the siderophore along with solution conditions such as pH, ionic strength, and presence of metal cations. They may also affect metal binding to clays. Here, we review previous studies of siderophore sorption to aluminosilicate clays; briefly discuss how the techniques of X-ray diffractometry, Fourier-transform infrared spectroscopy, and X-ray absorption spectroscopy may be applied to such studies; review effects of siderophores on metal sorption to clays; and highlight some areas for future research.

Adsorption of insecticidal toxin from Bacillus thuringiensis subsp. Kurstaki by some Chinese soils: effects of organic acid ligands addition

We have investigated the effects of low molecular weight organic acid ligands on the adsorption of the insecticidal toxin from Bacillus thuringiensis (Bt) by the colloidal ( 0.97). When present at low concentrations, organic acids (acetate, oxalate, citrate) had an inhibitory effect on toxin adsorption. Uptake, however, was promoted when the organic acid concentration exceeded 10 mM. The toxin was very strongly bound by the soils but the soil-toxin interaction weakened in the presence of organic acids. A small portion of the toxin was adsorbed by electrostatic and ligand exchange interactions. The addition of organic acids appeared to enhance these interactions.

Adsorption of the insecticidal protein of Bacillus thuringiensis subsp. kurstaki by soil minerals: Effects of organic acid ligands

The introduction of the toxin of Bacillus thuringiensis (Bt) into soil, as a result of the rapid increase in the planting of commercial Bt-transformed crops worldwide, may constitute a hazard to the soil ecosystem. The present investigation is concerned with the effect of low molecular-weight organic acid ligands (acetate, oxalate and citrate) on the adsorption of the toxin of B. thuringiensis subsp. kurstaki by kaolinite, montmorillonite, goethite, and silicon dioxide (SiO 2) and its desorption from the surface of these minerals. At the same time, we measured the desorption of bound toxin by NaCl and phosphate buffer. Low concentrations (< 10 mmol L − 1 ) of organic acid anions inhibited toxin adsorption by kaolinite, goethite, and silicon dioxide, whereas high concentrations promoted adsorption. For montmorillonite, however, an increase in the concentration of oxalate or citrate inhibited the adsorption of toxin. Less than 15% of the adsorbed amount was desorbed by each of the three organic acid anions. A small proportion (< 11%) of the toxin was adsorbed via electrostatic forces and ligand exchange, and this percentage increased when organic acid ligands were present. Our results indicate that low molecular-weight organic acid ligands can markedly influence the adsorption of Bt toxin by soil minerals, and result in the binding of the toxin on the minerals getting looser.

Effects of Siderophores on Pb and Cd Adsorption to Kaolinite

AbstractSiderophores are low molecular weight organic ligands synthesized by aerobic microorganisms to acquire Fe. In addition to Fe(III), siderophores may complex other metals such as Pb and Cd. This study compared the effects of the trihydroxamate siderophores desferrioxamine-B (DFO-B), desferrioxamine-D1 (DFO-D1), desferrioxamine-E (DFO-E), and the monohydroxamate siderophore-like ligand acetohydroxamic acid (aHA) on Pb and Cd (except for DFO-E) adsorption to kaolinite (KGa-1b) at pH 4.5 to 9, in 0.1 M NaClO4, at 22°C, in the dark. At pH &gt;6, all of the studied ligands decreased Pb adsorption to kaolinite: aHA by 5–40% and DFO-B, DFO-D1 and DFO-E by 30–75%; the greater effects were at higher pH. The studied ligands decreased Cd adsorption to kaolinite at pH &gt;8: aHA by 5–20% and the trihydroxamates by as much as 80%. We also observed enhancement of Pb adsorption in the presence of DFO-B at pH ≈5–6.0, probably due to adsorption of the doubly positively charged H3Pb (DFO-B)2+ complex, although spectroscopic evidence is needed.

Degradation of low molecular weight organic acids complexed with heavy metals in soil

Soils contaminated with heavy metals also contain a number of organic ligands, particularly in the rhizosphere and thus, a fraction of the bioavailable metals in soils likely exists in a complexed form. The presence of soluble, metal-complexing organic ligands can influence the fate and transport of metals as well as mineralisation of organic compounds. Oxalate and citrate complexed with Cd, Cu, Mg, Pb and Zn were used as model metal–organic complexes possibly occurring in heavy metal contaminated soils. Soil respiration was used to determine the biodegradation of these compounds in a clay alkaline soil kept under different management regimes. Overall, the results indicated that metal complexes with citrate generally were more degradable than oxalate–metal complexes. For each organic acid, biodegradability of the metal–organic complexes varied for different metals, following the ranking order: Mg>Zn>Cu≅Pb>Cd. Addition of Cd complexes decreased the soil respiration. These results indicate that the formation of the complexes between heavy metals and low molecular weight organic ligands might affect the soil functionality, especially in the rhizosphere of contaminated soils, as determined by soil respiration.

Formation and disappearance of Al tridecamer in the presence of low molecular weight organic ligands

Formation and disappearance of phytotoxic Al tridecamer (Al13) in the rhizosphere can be affected by root exudates such as organic ligands. We investigated the effects of organic ligands on the formation and disappearance of Alla as a function of pH and COOH / Al ratio. The aqueous Al species were analyzed by using a liquid state 27Al-NMR and an atomic absorption spectrometer and separated into four fractions, as follows; 1) Al13, 2) Al monomer and dimer (AlSYM), 3) Al complexed with organic ligands (AlORG), and 4) other undefined species (AlNON). Formation of Al13 was suppressed by acetate, oxalate, and citrate. In addition, Al13 that preexisted in a hydroxyaluminmn solution was disappeared by the introduction of acetate, oxalate, and citrate. Decrease in the concentration of Al13 at the early stage of the reaction was caused by the decomposition of Al13 along with the increase in the concentrations of AlSYM and AlORG. After 7 d of reaction, it was likely that Al13 was transformed into Al(OH)3 0 and other undefined species that could not be detected by 27Al-NMR. The ability of the carboxylic groups to suppress the occurrence of Al13 (molar-basis) was in the order of acetate << oxalate < citrate, corresponding to the ability of complexation with Al. In conclusion, the phytotoxic effects of Al13 were less pronounced because of the ubiquitous presence of organic ligands in the rhizosphere.

Comparative evaluation of the activity of some homogeneous and polymeric catalysts for the epoxidation of alkenes by organic hydroperoxides

Attempts have been made to immobilize some molybdenum-containing complexes with low molecular weight organic ligands, including newly synthesized ones, on commercial ion-exchange resins. The catalytic activity of these complexes in the epoxidation reaction of cyclohexene with tert-butyl hydroperoxide was evaluated and compared to that of the molybdenyl acetylacetonate homogeneous catalyst as well as to some polymeric molybdenum- containing catalysts derived from ion-exchange resins and obtained previously.

Size distribution of copper complexing ligands in tropical freshwaters

The size distribution of soluble copper complexing ligands (<0.45 μm) present in water samples collected from a tropical river system was characterised using a combination of equilibrium dialysis and differential pulse anodic stripping voltammetry. The proportion of copper complexing capacity associated with small molecular weight (<1,000 Dalton) ligands increased progressively from 40% in the upper catchment to 79% at a lowland floodplain/wetland location. This increase indicated the dominant role of the floodplain as a source of low molecular weight ligands to the river system. Asignificant proportion of copper complexation was also associated with the >14,000 Dalton fraction. As the majority of soluble iron was also associated with this fraction, it is likely that this complexing pool contains organic ligands adsorbed to the surface of inorganic colloids as well as large molecular weight ligands. Destruction of dissolved organic matter by sample pre-treatment with UV light resulted in a complete loss in copper complexation capacity detectable by anodic stripping voltammetry. This confirmed that the observed complexation of copper was associated with organic ligands and not inorganic components of the sample. A statistically significant linear correlation was observed between copper complexation capacity and fluorescence (340 nm excitation/440 nm emission). This indicated that the ligands were most likely to be associated with humic and fulvic compounds having conjugated molecular structures. Overall, the study emphasises the role of low molecular weight organic ligands in controlling copper speciation in tropical freshwater systems.

Kinetics and mechanisms of kaolinite dissolution: effects of organic ligands

The effect of protons, low molecular weight organic ligands, soil humic acid (HA), and stream water dissolved organic matter (DOM) on the rate of dissolution of kaolinite was examined. In acid solution (no ligands present) the rate of dissolution increased with increasing [H +] and the rate of Si dissolution was generally faster than Al. Low molecular weight organic ligands markedly increased the dissolution rates of both Al and Si in the following order: oxalate > malonate ≈ salicylate > o-phthalate. In the presence of organic ligands, the rate of Al dissolution was generally much greater than Si. Soil HA and stream water DOM did not promote the dissolution of kaolinite under the experimental conditions examined in this study. The dissolution kinetics of Al were interpreted in terms of a surface complexation model and the rate equations described in terms of the concentrations of specific (i.e. inner sphere) surface complexes.

Red Spruce Rhizosphere Dynamics: Spatial Distribution of Aluminum and Zinc in the Near-Root Soil Zone

Abstract Red spruce roots, associated with mature, healthy trees occupying canopy positions, were located in three 8.5 m² forest floor plots at 250 m elevation in the Hubbard Brook Experimental Forest, White Mountain National Forest, New Hampshire. Roots, 0.25-1.0 cm diam, were severed, and cut ends were placed in soil in plastic bags. In 14 months, new roots developed behind cut ends and were cultured in wooden trays containing screened forest floor material. Forest floor concentrations of aluminum, calcium, chlorine, iron, lead, and zinc were established. Trays containing roots were supplied with a distilled water treatment or with distilled water containing chloride salts of aluminum, lead or zinc sufficient to increase substrate cation concentration by 500 ppm. Eight weeks following treatment, roots (largely ectomycorrhizal, diam 0.4-1.0 mm) with associated rhizosphere soil were harvested by impregnating specimens in agar and freezing in liquid nitrogen. Cross sections of root-rhizosphere soil samples were freeze dried and examined with a scanning electron microscope and energy dispersive x-ray spectrometer. Gradients of element concentrations were established along transects from root interiors (cortex) through the inner and outer rhizosphere zones (2 mm from root) and in bulk soil (devoid of roots). The pH of rhizosphere soil was approximately 0.5 unit less than bulk soil. Aluminum concentrations exhibited a strongly descending gradient from bulk soil through the rhizosphere to the root. Estimated concentrations ranged from 1000 ppm within 200μ of the root to 10 X this amount in soil beyond 2000μ. Calcium distribution, without aluminum amendment, was relatively constant through the rhizosphere, but with aluminum amendment, calcium exhibited a sharply decreasing gradient near the root. In the unamended treatment, the Al:Ca ratio dropped from 7 to 0.2 across the rhizosphere. Chlorine was constant in the rhizosphere but exhibited pronounced accumulation inside the root. Iron counts were variable in the rhizosphere, but on average suggested constant concentrations across the rhizosphere. Zinc concentrations were marginally detectable and exhibited slight accumulation in cortex tissue. Aluminum uptake appeared to be strongly mediated by rhizosphere processes. Solubility, and more importantly exchangeability, of aluminum may be enhanced in the rhizosphere by lowered pH; greater availability of low molecular weight organic ligands, or some other unique chemical characteristic of the rhizosphere. Judgments made concerning potential uptake of aluminum from bulk soil characteristics of forest soils, without consideration of rhizosphere dynamics, could lead to improper conclusions. For. Sci. 35(4):1114-1124.