Ligand Steric Interactions Modulate Multidentate Ligand Exchange Pathways: Kinetics of Nickel(II) Ion Capture from N-Substituted IDA Complexes by CDTA.

Exchange reactions between multidentate ligands (also known as chelating agents) contribute to kinetic control of metal ion speciation in aquatic environments. However, the complexity of the stepwise reaction mechanism complicates predictions of kinetic behavior (rates, rate laws, and mechanisms). Clarity is achieved with the adjunctive-semijunctive-disjunctive paradigm, which categorizes multidentate ligand exchange pathways along a continuum according to the decreasing ease of forming intermediate mixed-ligand ternary complexes. In order to better understand how steric interaction between entering and leaving ligands affects reaction pathways and kinetic behavior, we use a capillary electrophoresis method to monitor exchange between trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetate (CDTA) and nickel(II) complexes with each of the following N-substituted iminodiacetate ligands (XIDA), iminodiacetate (IDA), methyliminodiacetate (MIDA), and benzyliminodiacetate (BIDA). Kinetic modeling indicates that reactions between CDTA and 1:1 nickel-XIDA complexes occur via parallel adjunctive and disjunctive pathways. With greater steric bulk of N-substituents on iminodiacetate, product formation via a disjunctive pathway increases while formation via the adjunctive pathway decreases. Kinetic analysis demonstrates how the shift in reaction pathways has a nonlinear effect on both the magnitude of the overall rate and the rate dependence on ligand concentrations. Furthermore, we discuss the implications of this work for understanding dynamic metal ion speciation in the environment.

Adsorption of Phosphorus Oxyanions at the FeOOH(goethite)/Water Interface: The Importance of Basicity.

PIII-containing H-phosphonate (HPO32-) and its monoethyl ester (fosetyl), essential pesticides for control of oomycete and fungal pathogens, are among the few pesticides transported by both xylem and phloem, making application as folia spray, soil spray, and trunk injection equally effective. To understand bioavailability and efficacy within soils, knowledge of adsorption to soil minerals is important. FeOOH(goethite) is often selected as an archetypal mineral surface. In the present work, H-phosphonate (with pKa values of 1.5 and 6.78) adsorption onto FeOOH is nearly complete below pH 6 and decreases to negligible amounts by pH 11, following an S-shaped curve. Fosetyl (pKa: 0.9), in contrast, does not adsorb to any significant extent, regardless of pH. To place these observations in context, adsorption of six other phosphorus oxyanions was investigated, and fitted using a CD-MUSIC model. Phosphate defines a similar S-shaped curve but adsorbs more strongly than H-phosphonate. Despite moderate differences in basicity, pH dependence and extents of adsorption for the four additional diprotic oxyanions methylphosphonate (pKas: 2.40, 8.00), benzylphosphonate (2.24, 7.93), phenylphosphonate (1.9, 7.47), and phenyl phosphate (1.1, 6.28) are quite similar to those of H-phosphonate. As with fosetyl, the other low pKa monoprotic oxyanion in our study, phenylphosphinate (pKa: 1.75), does not adsorb. Basicity, that is, pKa, is revealed to be the principal determinant of extents of adsorption.

Crochelins: Siderophores with an Unprecedented Iron-Chelating Moiety from the Nitrogen-Fixing Bacterium Azotobacter chroococcum.

Microbes use siderophores to access essential iron resources in the environment. Over 500 siderophores are known, but they utilize a small set of common moieties to bind iron. Azotobacter chroococcum expresses iron-rich nitrogenases, with which it reduces N2 . Though an important agricultural inoculant, the structures of its iron-binding molecules remain unknown. Here, the "chelome" of A. chroococcum is examined using small molecule discovery and bioinformatics. The bacterium produces vibrioferrin and amphibactins as well as a novel family of siderophores, the crochelins. Detailed characterization shows that the most abundant member, crochelin A, binds iron in a hexadentate fashion using a new iron-chelating γ-amino acid. Insights into the biosynthesis of crochelins and the mechanism by which iron may be removed upon import of the holo-siderophore are presented. This work expands the repertoire of iron-chelating moieties in microbial siderophores.

Influence of oxygenation on chromium redox reactions with manganese sulfide (MnS(s)).

Manganese sulfide (MnS(s)) minerals exist in sulfidic environments and can have unique reactive abilities because of sulfide, which is a known reductant, and Mn, the oxyhydroxides of which are known oxidants. This study elucidated the role of MnS(s) in controlling Cr speciation with implications on its fate and toxicity in the natural environment, specifically sulfidic sediments that undergo biogeochemical changes due to sediment resuspension during dredging, bioturbation, and flood events. In continuously mixed batch reaction experiments, aqueous CrVI reduction under anaerobic conditions occurred primarily on the surface of MnS(s) displaying a biphasic behavior- the initial rapid removal of CrVI from solution was followed by a slow decline due to surface passivation by reaction products, mainly sorbed or precipitated CrIII. The reaction progress increased with MnS(s) surface area loading but decreased on increasing CrVI concentration and pH, suggesting that surface site regeneration through product desorption was the rate-controlling mechanism. Below circum-neutral pH, higher solubility of MnS(s) resulted in additional CrVI reduction by reduced sulfur species in solution, whereas increased CrIII solubility lowered surface passivation allowing for more reactive sites to participate in the reaction. Aeration of MnS(s) at pH≥7 caused the formation of a heterogeneous MnIII(hydr)oxide that was composed of hausmanite and manganite. CrVI reoccurrence was observed on aeration of CrVI-spiked MnS(s) from the oxidation of product CrIII. The reoccurrence at pH≥7 was attributed to the oxidation of product CrIII by MnIII(hydr)oxide, whereas the reoccurrence at pH<7 was hypothesized from the oxidation of product CrIII by intermediate aqueous MnIII and/or sulfur species. Just as with Cr, MnS(s) may play an important role in speciation, fate, and transport of other environmental contaminants.

Biogeochemical controls on hexavalent chromium formation in estuarine sediments.

Predicting the aquatic and human health impacts of chromium (Cr) necessitates one to determine its speciation as either relatively nontoxic Cr(III) or toxic Cr(VI) and elucidate the influence of biogeochemical changes on its behavior and fate. In the Baltimore Harbor, Cr predominantly exists as Cr(III) associated with sediments. While reduction of Cr(VI) to Cr(III) is dominant in these anoxic sediments, the potential of Cr(III) oxidation and Cr(VI) reoccurrence during sediment resuspension and oxygenation resulting from dredging, bioturbation, and flood events poses a serious concern. In batch experiments, aqueous Cr(VI) spiked into continuously mixed anoxic suspensions was reduced to product Cr(III) under anaerobic conditions. No Cr(VI) reoccurrence was observed when conditions remained anaerobic. Aeration caused Cr(VI) reoccurrence from the abiotic oxidation of product Cr(III). Rates of aeration-driven Cr(VI) reoccurrence increased with pH, and Cr(VI) reoccurrence positively correlated with dissolved manganese (Mn) decline at pH ≥ 7. Aeration-driven oxidation of Mn(II) to Mn(III,IV)(hydr)oxides was the underlying mechanism causing product Cr(III) oxidation. Cr(VI) reoccurrence decreased with sediment loading and negatively correlated with the acid volatile sulfide (AVS) concentration. Although sediment resuspension and oxygenation may create temporary conditions conducive to Cr(VI) formation, long-term Cr(VI) persistence is unlikely in the presence of sediment reductants. While such natural attenuation in reducing environments mitigates the risk associated with Cr toxicity, this risk may still persist in Mn-rich and reductant-deficient environments.

Chemical speciation of trace metals in mammalian cell culture media: looking under the hood to boost cellular performance and product quality.

Upstream process development seeks to optimize media formulations to promote robust cell culture conditions and regulate product quality attributes such as glycosylation, aggregation, and charge variants. Transition metal ions Mn, Fe, Cu, and Zn present in cell culture media have a significant impact on cell growth, metabolism and product quality. These metals and other media components can have different chemical associations or speciation in media that are poorly characterized but may significantly impact their properties and effect on cellular performance. Computer-based equilibrium models are a good starting point for exploring metal speciation, bioavailability and conditions where precipitation may occur. However, some equilibrium constants, especially for newly introduced medium components, have not been experimentally determined. Owing to concurrent physical and biological processes, speciation may also be controlled by reaction kinetics rather than by equilibrium. These factors highlight the importance of analytically interrogating medium speciation to gain insights into the complex interconnections between media components and bioprocess performance.

Mandelic acid and phenyllactic acid "Reaction Sets" for exploring the kinetics and mechanism of oxidations by hydrous manganese oxide (HMO).

At pH 4.0, hydrous manganese oxide (HMO) oxidizes mandelic acid by two mole-equivalents of electrons, yielding phenylglyoxylic acid and benzaldehyde. These intermediates, in turn, are oxidized by two mole-equivalents of electrons to the same ultimate oxidation product, benzoic acid. The four compounds of the "reaction set" just described are conveniently monitored using capillary electrophoresis (CE) and HPLC. Extents of adsorption are negligible and their sum exhibits mass balance. Concentrations of phenylglyoxylic acid, benzaldehyde, and benzoic acid can therefore be used to calculate mole-equivalents delivered to HMO for comparison with experimentally-determined dissolved MnII concentrations. Semi-log plots (ln[substrate] versus time) and numerical analysis can also be used to explore rates of oxidation of the functional groups represented, i.e. an α-hydroxycarboxylic acid, an α-ketocarboxylic acid, and an aldehyde. Inserting a -CH2- group between the benzene ring and the functional groups just described yields a new reaction set comprised of phenyllactic acid, phenylpyruvic acid, and phenylacetaldehyde, plus the C-1 ultimate oxidation product, phenylacetic acid. At pH 4, mass balance for phenyllactic acid oxidation fell short by ∼9%. Phenyllactic acid was oxidized 2.7-times more slowly than mandelic acid, while phenylpyruvic acid was oxidized 12.7-times faster than phenylglyoxylic acid. Unlike benzaldehyde, oxidation rates for phenylacetaldehyde were too fast to measure. Under pH 4.0 conditions, this reaction set approach was used to explore the acceleratory effects of increases in HMO loading and inhibitory effects of 500 µM phosphate and pyrophosphate additions. Mandelic acid and phenyllactic acid were oxidized by HMO far more slowly at pH 7.0 than at pH 4.0. At pH 7.0, 2 mM MOPS and phosphate buffers did not yield appreciable dissolved MnII, despite oxidation of organic substrate. 2 mM pyrophosphate, in contrast, solubilized HMO-bound MnII and MnIII.

Competitive adsorption of phosphate and phosphonates onto goethite.

Phosphate and phosphonates are both strongly adsorbed onto mineral surfaces and their removal during wastewater treatment is mainly due to adsorptive processes. We have conducted experiments to study the mutual influence of phosphate and six different phosphonates on each other in buffered medium at pH 7.2. We have used phosphonates having one to five phosphonic acid groups (HMP, IDMP, HEDP, NTMP, EDTMP and DTPMP). The presence of phosphonates suppressed the adsorption of phosphate. The monophosphonate HMP had the smallest and the polyphosphonates the largest effect on phosphate adsorption. The presence of phosphate lowered phosphonate adsorption. The competition in the multicomponent system can reasonably well be predicted using a surface complexation model developed for single component systems. The competitive model only failed in systems containing the polyphosphonate DTPMP. With this approach we can predict the behavior of both compounds during wastewater treatment. The calculations show that phosphonates have a small effect on phosphate adsorption at the actual concentrations in observed wastewater. Adsorption of low concentrations of phosphonates was calculated to be significantly reduced by phosphate concentrations as observed in wastewater.

PbO2(s, plattnerite) reductive dissolution by aqueous manganous and ferrous ions.

Pb(IV)O2(s, plattnerite) nanoparticle aggregrates in aqueous suspension are readily reduced by Mn2+(aq) and Fe2+(aq) between pH 3.0 and 8.5, yielding pb2+(aq) and adsorbed Pb". Fe2+(aq) oxidation generates Fe(III) (hydr)oxides that impede Pb(IV) reduction, especially at pH > or =5. Under acidic conditions, production of dissolved Fe(III) may also be significant. Mn2+(aq) oxidation generates mixed Mn(III)/Mn(IV) (hydr)oxides that are less of an impediment to Pb(IV) reduction. Adding both Fe2+(aq) and Mn2+(aq) can set in motion a Mn redox cycle that catalyzes PbO2(s) reduction by Fe2+(aq). Reaction with Fe2+(aq) and Mn2+(aq) can affect subsequent reactions with natural organic matter. Hydroquinone was employed as a representative organic reductant. Both hydroquinone and its two-electron oxidation product p-benzoquinone are readily quantified by HPLC. Adding Fe2+(aq) before hydroquinone greatly diminishes p-benzoquinone production. Adding Mn2+(aq) before hydroquinone has little effect on p-benzoquinone production. Free chlorine residual variations in premise plumbing can setthe stage for the reactions documented here.

PbO2(s, plattnerite) reductive dissolution by natural organic matter: reductant and inhibitory subfractions.

Natural organic matter (NOM) is a diverse collection of molecules, each possessing its own reductant, complexant, and adsorption properties. Here, we are interested in the ability of NOM to bring about the reductive dissolution of Pb(IV)O2(s). Adding the coagulants FeCl3 or Al2(SO4)3 followed by membrane filtration is one way to remove a subset of NOM molecules from surface water samples. Another is to pass water samples through a granular activated carbon (GAC) column. Results from applying these treatments to Great Dismal Swamp water (DSW) and Nequasset Bog Water (NBW) can best be explained as follows: (i) GAC column treatment is more efficient at removing the NOM fraction most responsible for reductive dissolution. (ii) Coagulation/filtration, with either coagulant, is most efficient at removing a second, inhibitory fraction. Inhibition may arise from (i) adsorption at the mineral/water interface, which blocks approach of reductant molecules and (ii) a micelle-like aggregate nature, which provides hydrophobic pockets that capture reductantmolecules, again keeping them away from the mineral/water interface. Hypotheses regarding reductant and inhibitory fractions are further evaluated using representative low-molecular-weight compounds. Substituted hydroquinones are used as mimics of the reductant fraction, and malonic acid, quinic acid, trehalose, alginic acid, and polygalacturonic acid are used as mimics of the inhibitory fraction.

Reversible redox chemistry of quinones: impact on biogeochemical cycles.

The role of quinone biomolecules and quinone moieties of natural organic matter (NOM) as the electron transfer mediator in essential biogeochemical processes such as iron bioreduction and contaminant degradation has received considerable interests in the past decade. Hypothesized electron shuttling mechanism must be evaluated based on the availability and stability of quinones under a given environmental setting. The goal of this review is to examine the source, reactivity, and fate of potential quinone catalysts with respect to chemical interactions (e.g., with other quinones and nucleophiles) that will inevitably occur in complex environmental media. We will first discuss natural and anthropogenic sources of quinones in aqueous environments, and fundamental transformation pathways including cross reaction, autoxidation, and addition reactions. We will then assess how the described sources (molecular structure) and transformation pathways (stability) will impact the ability of a quinone molecule to catalyze a biogeochemical process. Thermodynamics and kinetics of electron transfer reactions with both the electron donor (e.g., hydrogen sulfide as a bulk reductant) and the terminal electron acceptor (e.g., nitroaromatic explosives in contaminant degradation), and stability towards irreversible side reactions are the key factors determining the geochemical conditions under which the catalysis by a quinone molecule will be operative.

Phosphonate- and carboxylate-based chelating agents that solubilize (hydr)oxide-bound MnIII.

Recent field studies suggest that dissolved MnIII should be ubiquitous at oxic/anoxic interfaces in all natural waters and may play important roles in biogeochemical redox processes. Here, we uncovered environmentally relevant synthetic phosphonate-based chelators that solubilize (hydr)oxide-bound MnIII via ligand-promoted dissolution at circum-neutral pHs and that their ability to release aqueous MnIII can be predicted based on the chemical structure. For two (hydr)oxides (manganite and birnessite) reacting with excess concentrations of pyrophosphoric acid (PP), methylenediphosphonic acid (MDP), and phosphonoacetic acid (PAA), ligand-promoted dissolution is predominant from pH 6--8, initial dissolution rates and plateau concentrations for dissolved MnIII decrease in the order PP > MDP > PAA, and at pH 5, MDP reacts equally well (with birnessite) or more efficiently (with manganite) than PP, and PAA remains the least reactive chelator. For manganite reacting with an excess concentration of aminophosphonate/carboxylate-based chelators, the aminophosphonate-containing iminodimethylenephosphonic acid and glyphosate yield appreciable amounts of dissolved MnIII, but the aminocarboxylate-based methyliminodiacetic acid yields solely dissolved MnII via MnIII reduction.

Aqueous oxidation of substituted dihydroxybenzenes by substituted benzoquinones.

Strict anaerobic techniques, HPLC, and spectrophotometry are employed to explore rates of reaction between a series of substituted benzoquinone oxidants and substituted dihydroxybenzene reductants, which represent important redox-active moieties within natural organic matter (NOM). Benzoquinones and dihydroxybenzenes that lack electron-withdrawing substituents exhibit reversible reactions within the acidic range of natural waters. Initial rates for reversible reactions are proportional to [H+]-1, attributable to the greater reactivity of monoprotonated versus diprotonated dihydroxybenzene molecules. Reversible reactions are generally faster for pairs having higher thermodynamic driving force. Concentrations in reversible reactions eventually reach plateau values, which coincide with expected values calculated using standard reduction potentials. If a reactant benzoquinone possesses an electron-withdrawing substituent, reaction progress falls short of expected values. If a product benzoquinone possesses an electron-withdrawing substituent, reaction progress extends beyond what is thermodynamically predicted. Electron-withdrawing substituents raise the susceptibility of benzoquinones to side reactions such as the Michael addition reaction.

Speciation of chromium(III) and cobalt(III) (amino)carboxylate complexes using capillary electrophoresis.

Capillary electrophoresis (CE) can very efficiently resolve different dissolved metal ion species as long as rates of ligand exchange are slow relative to time scales required for electromigration. Here, we detail the separation of several Cr(III) and Co(III) complexes with the multidentate chelating agents iminodiacetic acid, nitrilotriacetic acid, trans-1,2-cyclohexanediaminetetracetic acid, N-(2-hydroxyethyl)ethylenediaminetriacetic acid, trimethylenediaminetetraacetic acid, and ethylenediaminetetraacetic acid. Successes in speciating some Ni(II) and Co(II) complexes are also reported. For Cr(III) and Co(III), subtle differences in metal ion-chelating agent stereochemistry, chelating agent denticity, and number of bridging ligands are discernible due to the high resolving power of CE. New peaks and heightened baselines were encountered when a pH buffer with strong complexing properties (orthophosphate) was employed in the background electrolyte. For this reason, we recommend using pH buffers with very weak or negligible complex properties (e.g., MES and MOPS).

Influence of calcite and dissolved calcium on uranium(VI) sorption to a hanford subsurface sediment.

The influence of calcite and dissolved calcium on U(VI) adsorption was investigated using a calcite-containing sandy silt/clay sediment from the U. S. Department of Energy Hanford site. U(VI) adsorption to sediment, treated sediment, and sediment size fractions was studied in solutions that both had and had not been preequilibrated with calcite, at initial [U(VI)] = 10(-7)-10(-5) mol/L and final pH = 6.0-10.0. Kinetic and reversibility studies (pH 8.4) showed rapid sorption (30 min), with reasonable reversibility in the 3-day reaction time. Sorption from solutions equilibrated with calcite showed maximum U(VI) adsorption at pH 8.4 +/- 0.1. In contrast, calcium-free systems showed the greatest adsorption at pH 6.0-7.2. At pH > 8.4, U(VI) adsorption was identical from calcium-free and calcium-containing solutions. For calcite-presaturated systems, both speciation calculations and laser-induced fluorescence spectroscopic analyses indicated that aqueous U(VI) was increasingly dominated by Ca2UO2(CO3)3(0)(aq) at pH < 8.4 and thatformation of Ca2UO2(CO3)3(0)(aq) is what suppresses U(VI) adsorption. Above pH 8.4, aqueous U(VI) speciation was dominated by UO2(CO3)3(4-) in all solutions. Finally, results also showed that U(VI) adsorption was additive in regard to size fraction but not in regard to mineral mass: Carbonate minerals may have blocked U(VI) access to surfaces of higher sorption affinity.

Reduction of oxamyl and related pesticides by FeII: influence of organic ligands and natural organic matter.

The reduction of oxamyl and related oxime carbamate pesticides (OCPs; methomyl and aldicarb) by FeII is an important pathway for the degradation of these compounds in soil and groundwater. A series of batch kinetic experiments was carried out to assess the effects that selected carboxylate and aminocarboxylate ligands have on these reactions. In the absence of FeII, no OCP reduction by the ligands is observed. In the presence of FeII, the rate of OCP reduction varies by several orders of magnitude and can be described by the expression k(red) = [FeII]sigma(i)k(i)alpha(i) where k(red) is the observed pseudo-first-order rate constant for OCP reduction, [FeII] is the total FeII concentration, alpha(i) is the fraction of each FeII species in solution, and k(i) is the second-order rate constant for OCP reduction by each FeII species. The reactivity of individual FeII species is dependent upon the standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (E(H)o) and the availability of inner-sphere FeII coordination sites for bonding with Lewis base donor groups within the OCP structure. A linear free energy relationship is proposed. Kinetic measurements demonstrate that natural organic matter from the Great Dismal Swamp facilitates OCP reduction by FeII in the same manner as the individual organic ligands. Results from this study improve our understanding of the pathways and rates of pesticide degradation in reducing subsurface environments, especially those rich in organic matter.

Reduction of the pesticides oxamyl and methomyl by FeII: effect of pH and inorganic ligands.

This work examines the effect that pH and selected inorganic ligands have on the kinetics of reactions between FeII and two structurally related oxime carbamate pesticides, oxamyl and methomyl. In anoxic solutions containing FeII, these compounds degrade by parallel elimination and reduction pathways. Rates of FeII-independent carbamate elimination (EIcb mechanism) are proportional to [OH-], increasing 10-fold for each unit increase in pH. In homogeneous solution, rates of carbamate reduction by 0.5 mM FeII are relatively constant at pH <7, but increase dramatically between pH 7 and pH 8.3. At pH >8.3, Fe(OH)2(s) precipitation occurs, and carbamates react with both solution-phase and solid-phase FeII. Carbamate reduction by FeII is not significantly affected by the presence of chloride, bromide, nitrate, perchlorate, and sulfate. In contrast, increased rates of carbamate reduction are observed in solutions containing fluoride, carbonate, and phosphate. Kinetic measurements are interpreted in terms of changing FeII speciation according to the expression kred = [FeII]sigmaikialphai, where k(red) is the pseudo-first-order rate constant for carbamate reduction, [FeII] is the total FeII concentration, and ki and alphai are the second-order rate constant and fractional concentration of each FeII species, respectively. It follows that the overall kinetics of carbamate reduction is a function of the identity and concentration of individual FeII species present in solution as well as the inherent reactivity of each species with carbamates. The magnitude of ki is related to the standard one-electron reduction potential (E(H) degrees) of the corresponding FeIII/FeII redox couple.

Transformation of the plant growth regulator daminozide (Alar) and structurally related compounds with CuII ions: oxidation versus hydrolysis.

As part of a study of metal ion effects on chemical transformations of nitrogen-containing agrochemicals, conversion of daminozide to succinate via cleavage of the hydrazide C-N bond was examined in the presence and absence of divalent metal ions. No conversion was observed in metal ion-free solutions or in the presence of 1.0 mM NiII, ZnII, and PbII. CuII, in contrast, markedly increased rates of daminozide to succinate conversion. Halide ions (CI-, Br-) had no effect on daminozide conversion in the absence of metal ions but markedly increased conversion rates observed in the presence of CuII. The nitrogen-donor ligands ethylenediamine, N-(2-hydroxyethyl)ethylenediamine, and 1,4,7,10-tetraazacyclododecane decreased rates of CuII-facilitated conversion, while 1,5,9-triazacyclododecane actually increased rates of conversion. H NMR and UV spectroscopy provide evidence for the formation of 1:1 CuII-daminozide complexes. Halide ion effects and nitrogen-donor ligand effects point to an oxidative mechanism for CuII-facilitated daminozide breakdown, rather than hydrolysis. The structurally related compound butyric acid 2,2-dimethylhydrazide (BH) is subject to the same CuII-facilitated breakdown via an oxidative mechanism. N,N-Dimethylsuccinamic acid (SA), in contrast, breaks down via a hydrolytic mechanism.