Complexation of zinc in organic soils--EXAFS evidence for sulfur associations

Metal stable isotopes fractionation during adsorption

Isotope technology is an ideal tool for tracing the sources of certain pollutants or providing insights into environmental processes. In recent years, the advent of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has enabled the precise measurement of various metal stable isotopes. Due to the presence of "fingerprint" properties in various environmental samples, metal stable isotopes have been applied to distinguish the source of contaminants effectively and further understand the corresponding environmental processes. The environmental fate of metal elements is strongly controlled by adsorption, an essential process for the distribution of elements between the dissolved and particulate phases. The adsorption of metal elements on mineral and organic surfaces significantly affects their biogeochemical cycles in the environment. Therefore, it is crucial to elucidate the fractionation characteristics of stable metal isotopes during the adsorption process. In this review, three typical transitional metal elements were selected, considering Mo as the representative of anionic species and Fe and Zn as the representative of cationic species. For Mo, the heavier Mo isotope is preferentially adsorbed in the solution phase, pH has a more significant influence on isotope fractionation, and temperature and ionic strength are relatively insensitive. Differences in coordination environments between dissolved and adsorbed Mo during adsorption, i.e., attachment mode (inner- or outer-sphere) or molecular symmetry (e.g., coordination number and magnitude of distortion), are likely responsible for isotopic fractionation. For Fe, The study of equilibrium/kinetic Fe isotopic fractionation in aqueous Fe(II)-mineral is not simple. The interaction between aqueous Fe(II) and Fe (hydroxyl) oxides is complex and dynamic. The isotope effect is due to coupled electron and atom exchange between adsorbed Fe(II), aqueous Fe(II), and reactive Fe(III) on the surface of Fe (hydroxyl) oxide. For Zn, the heavier Fe isotope preferentially adsorbs on the solid phase, and pH and ionic strength are essential influencing factors. The difference in coordination environment may be the cause of isotope fractionation.

Contrasted fate of zinc sulfide nanoparticles in soil revealed by a combination of X-ray absorption spectroscopy, diffusive gradient in thin films and isotope tracing

Incidental zinc sulfide nanoparticles (nano-ZnS) are spread on soils through organic waste (OW) recycling. Here we performed soil incubations with synthetic nano-ZnS (3 nm crystallite size), representative of the form found in OW. We used an original set of techniques to reveal the fate of nano-ZnS in two soils with different properties. ⁶⁸Zn tracing and nano-DGT were combined during soil incubation to discriminate the available natural Zn from the soil, and the available Zn from the dissolved nano-⁶⁸ZnS. This combination was crucial to highlight the dissolution of nano-⁶⁸ZnS as of the third day of incubation. Based on the extended X-ray absorption fine structure, we revealed faster dissolution of nano-ZnS in clayey soil (82% within 1 month) than in sandy soil (2% within 1 month). However, the nano-DGT results showed limited availability of Zn released by nano-ZnS dissolution after 1 month in the clayey soil compared with the sandy soil. These results highlighted: (i) the key role of soil properties for nano-ZnS fate, and (ii) fast dissolution of nano-ZnS in clayey soil. Finally, the higher availability of Zn in the sandy soil despite the lower nano-ZnS dissolution rate is counterintuitive. This study demonstrated that, in addition to nanoparticle dissolution, it is also essential to take the availability of released ions into account when studying the fate of nanoparticles in soil.

Cadmium Isotope Fractionation during Complexation with Humic Acid

Cadmium (Cd) isotopes are known to fractionate during complexation with various environmentally relevant surfaces and ligands. Our results, which were obtained using (i) batch experiments at different Cd concentrations, ionic strengths, and pH values, (ii) modeling, and (iii) infrared and X-ray absorption spectroscopies, highlight the preferential enrichment of light Cd isotopes bound to humic acid (HA), leaving the heavier Cd pool preferentially in solution (Δ^114/110Cd_HA-Cd(aq) of -0.15 ± 0.01‰). At high ionic strengths, Cd isotope fractionation mainly depends on its complexation with carboxylic sites. Outer-sphere complexation occurs at equilibrium together with inner-sphere complexation as well as with the change of the first Cd coordination and its hydration complexes in solution. At low ionic strengths, nonspecific Cd binding induced by electrostatic attractions plays a dominant role and promotes Cd isotope fractionation during complexation. This significant outcome elucidates the mechanisms involved in HA-Cd interactions. The results can be used during (i) fingerprinting the available Cd in soil solution after its complexation with solid or soluble natural organic matter and (ii) evaluating the contribution of Cd complexation with organic ligands and phytoplankton-derived debris versus Cd assimilation by phytoplankton in seawater.

Simulated Aquifer Heterogeneity Leads to Enhanced Attenuation and Multiple Retention Processes of Zinc

Alluvial aquifers serve as one of the main water sources for domestic, agricultural, and industrial purposes globally. Groundwater quality, however, can be threatened by naturally occurring and anthropogenic metal contaminants. Differing hydrologic and biogeochemical conditions between predominantly coarse-grained aquifer sediments and embedded layers or lenses of fine-grained materials lead to variation in metal behavior. Here, we examine processes controlling Zn partitioning within a dual-pore domain-reconstructed alluvial aquifer. Natural coarse aquifer sediments from the Wind River-Little Wind River floodplain near Riverton, WY, were used in columns with or without fine-grained lenses to examine biogeochemical controls on Zn concentrations, retention mechanisms, and transport. Following the introduction of Zn to the groundwater source, Zn preferentially accumulated in the fine-grained lenses, despite their small volumetric contributions. While the clay fraction dominated Zn retention in the sandy aquifer, the lenses supported additional reaction pathways of retention-the reducing conditions within the lenses resulted in ZnS precipitation, overriding the contribution of organic matter. Zinc concentration in the groundwater controlled the formation of Zn-clays and Zn-layered double hydroxides, whereas the extent of sulfide production controlled precipitation of ZnS. Our findings illustrate how both spatial and compositional heterogeneities govern the extent and mechanisms of Zn retention in intricate groundwater systems, with implications for plume behavior and groundwater quality.

Organic compounds alter the preference and rates of heavy metal adsorption on ferrihydrite

The availability of heavy metals in terrestrial environments is largely controlled by their interactions with minerals and organic matter, with iron minerals having a particularly strong role in heavy metal fate. Because soil organic matter contains a variety of compounds that differ in their chemical properties, the underlying impact organic matter-soil mineral associations bestow on heavy metal binding is still unresolved. Here, we systematically examine the binding of Cd, Zn and Ni by a suite of organic-ferrihydrite assemblages, chosen to account for various compound chemistries within soil organic matter. We posited that organic compound functionality would dictate the extent of association with the organic-ferrihydrite assemblages. Increased heavy metal binding to the assemblages was observed and attributed to the introduction of additional binding sites by the organic functional groups with differing metal affinities. The relative increase depended on the metal's Lewis acidity and followed the order Cd > Zn > Ni, whereas the reverse order was obtained for metal binding by pristine ferrihydrite (Ni > Zn > Cd). Citric acid-, aspartic acid- and cysteine-ferrihydrite assemblages also enhanced the metal binding rate. X-ray absorption spectroscopy revealed that the organic coating contributed significantly to Zn binding by the assemblages, despite relatively low organic surface coverage. Our findings provide valuable information on the nature of heavy metal-organic-mineral interactions and metal adsorption processes regulating their bioavailability and transport.

Natural organic matter-cations complexation and its impact on water treatment: A critical review

The quality and quantity of natural organic matter (NOM) has been observed to evolve which poses challenges to water treatment facilities. Even though NOM may not be toxic itself, its presence in water has aesthetic effects, enhances biological growth in distribution networks, binds with pollutants and controls the bioavailability of trace metals. Even though NOM has heterogeneous functional groups, the predominant ones are the carboxyl and the phenolic groups, which have high affinities for metals depending on the pH. The properties of both the NOM and the trace elements influence the binding kinetics and preferences. Ca²⁺ prefers to bind with the carboxylic groups especially at a low pH while Zn²⁺ prefers the amine groups though practically, most cations bind to several functions groups. The nature of the chemical environment (neighboring ligands) the ligand finds itself equally influences its preference for a cation. The presence of NOM, cations or a complex of NOM-cations may have significant impact on the efficiency of water processes such as coagulation, adsorption, ion exchange resin and membrane filtration. In coagulation, the complexation between the coagulant salts and NOM helps to remove NOM from solution. This positive influence can further be enhanced by the addition of Ca²⁺. A negative influence is however, observed in lime-softening method as NOM complexes with Ca²⁺. A negative influence is also seen in membrane filtration where divalent cations partially neutralize the carboxyl functional groups of NOM thereby reducing the repulsion effect on NOM and increasing membrane fouling. The formation of disinfection by-products could either be increased or reduced during chlorination, the speciation of products formed is modified with generally the enhancement of haloacetic acid formation observed in presence of metal cations. This current work, presents in details the interactions of cations and NOM in the environment, the preference of cations for each functional group and the possible competition between cations for binding sites, as well as the possible impacts of the presence of cations, NOM, or their complex on water treatment processes.

Pheomelanin pigment remnants mapped in fossils of an extinct mammal

Recent progress has been made in paleontology with respect to resolving pigmentation in fossil material. Morphological identification of fossilized melanosomes has been one approach, while a second methodology using chemical imaging and spectroscopy has also provided critical information particularly concerning eumelanin (black pigment) residue. In this work we develop the chemical imaging methodology to show that organosulfur-Zn complexes are indicators of pheomelanin (red pigment) in extant and fossil soft tissue and that the mapping of these residual biochemical compounds can be used to restore melanin pigment distribution in a 3 million year old extinct mammal species (Apodemus atavus). Synchotron Rapid Scanning X-ray Fluorescence imaging showed that the distributions of Zn and organic S are correlated within this fossil fur just as in pheomelanin-rich modern integument. Furthermore, Zn coordination chemistry within this fossil fur is closely comparable to that determined from pheomelanin-rich fur and hair standards. The non-destructive methods presented here provide a protocol for detecting residual pheomelanin in precious specimens.

High Energy Resolution-X-ray Absorption Near Edge Structure Spectroscopy Reveals Zn Ligation in Whole Cell Bacteria

Identifying the zinc (Zn) ligation and coordination environment in complex biological and environmental systems is crucial to understand the role of Zn as a biologically essential but sometimes toxic metal. Most studies on Zn coordination in biological or environmental samples rely on the extended X-ray absorption fine structure (EXAFS) region of a Zn K-edge X-ray absorption spectroscopy (XAS) spectrum. However, EXAFS analysis cannot identify unique nearest neighbors with similar atomic number (i.e., O versus N) and provides little information on Zn ligation. Herein, we demonstrate that high energy resolution-X-ray absorption near edge structure (HR-XANES) spectroscopy enables the direct determination of Zn ligation in whole cell bacteria, providing additional insights lost from EXAFS analysis at a fraction of the scan time and Zn concentration. HR-XANES is a relatively new technique that has improved our understanding of trace metals (e.g., Hg, Cu, and Ce) in dilute systems. This study is the first to show that HR-XANES can unambiguously detect Zn coordination to carboxyl, phosphoryl, imidazole, and/or thiol moieties in model microorganisms.

Accumulation, interaction and fractionation of fluoride and cadmium in sierozem and oilseed rape (Brassica napus L.) in northwest China

Soil fluoride (F) and cadmium (Cd) pollution are of great concern in recently years, due to the fact that considerable amounts of wastewater, gas and residue, containing F and Cd, have been discharged into the environment through ore smelting. Soil F and Cd contamination may result in their interaction in soil and plant, which affects their fractionation distribution in soil and accumulation in oilseed rape. Oilseed rape, which is widely planted and consumed as a popular vegetable in arid and semi-arid land of northwest China, has been believed to a hyperaccumulator for Cd. However, there is limited information about the accumulation, interaction and fractionation of F and Cd in soil-oilseed rape system under F-Cd stresses. A pot-culture experiment, with single (F or Cd) or double elements (F-Cd) being added to soil, was carried out study the accumulation, interaction and fractionation of F and Cd in sierozem and oilseed rape. We found that soil F applications increased the contents of Cd in exchangeable fraction (EX-Cd), the bound to carbonate fraction (CAB-Cd) and the bound to iron and manganese oxides fraction (FMO-Cd) in soil and also increased plant Cd accumulation. Therefore, we suggest that the permitted level of F should be confined within soil quality standards for farmland of China in order to upset the effect of high F concentration on bioavailability of soil Cd. However, soil Cd applications showed negative effects on the content of F in water soluble fraction (Water-F), hence decreased plant F accumulation. A better understanding of the accumulation, interaction and fractionation of F and Cd in sierozem-oilseed rape system are of great importance for environmental protection and for human health. The present study may serve as a basic understanding of the accumulation, interaction and fractionation of F and Cd in sierozem-oilseed rape system, and provide a suggestion for the environmental management.

Metal (Pb, Cd, and Zn) Binding to Diverse Organic Matter Samples and Implications for Speciation Modeling

This study evaluated the influence of dissolved organic matter (DOM) properties on the speciation of Pb, Zn, and Cd. A total of six DOM samples were categorized into autochthonous and allochthonous sources based on their absorbance and fluorescence properties. The concentration of free metal ions ( C_M²⁺) measured by titration using the absence of gradients and Nernstian equilibrium stripping (AGNES) method was compared with that predicted by the Windermere humic aqueous model (WHAM). At the same binding condition (pH, dissolved organic carbon, ionic strength, and total metal concentration) the allochthonous DOM showed a higher level of Pb binding than the autochthonous DOM (84- to 504-fold C_Pb²⁺ variation). This dependency, however, was less pronounced for Zn (12- to 74-fold C_Zn²⁺ variation) and least for Cd (2- to 14-fold C_Cd²⁺ variation). The WHAM performance was affected by source variation through the active DOM fraction ( F). The commonly used F = 1.3 provided reliable C_Pb²⁺ for allochthonous DOMs and acceptable C_Cd²⁺ for all DOM, but it significantly under-predicted C_Pb²⁺ and C_Zn²⁺ for autochthonous DOM. Adjusting F improved C_M²⁺ predictions, but the optimum F values were metal-specific (e.g., 0.03-1.9 for Pb), as shown by linear correlations with specific optical indexes. The results indicate a potential to improve WHAM by incorporating rapid measurement of DOM optical properties for site-specific F.

Correlations between aromaticity of dissolved organic matter and trace metal concentrations in natural and effluent waters: A case study in the Sagami River Basin, Japan

Chemical speciation, reactivity, and bioavailability of trace metals in aqueous systems arestrongly influenced by dissolved organic matter (DOM). DOM is a mixture of diverse components, so a range of organic molecules potentially participates in the occurrence of dissolved trace metals. In this study, we investigated water quality variables that influence dissolved trace metal concentrations in natural and effluent water systems with a particular attention given to the relationship between DOM optical properties and dissolved copper and iron concentrations. We found that specific UV absorbance (SUVA₂₅₄: an indicator of DOM aromaticity) has a significant correlation with dissolved trace metal to dissolved organic carbon concentration ratios ([Me]_T/[DOC]) for copper and iron in natural freshwaters and treated municipal wastewater in the Sagami River basin, Japan. This trend was also prevalent for other freshwaters in temperate climates except for Fe-rich waters. Our findings indicate that the concentrations of dissolved copper and iron in natural and effluent waters are significantly influenced not only by DOM concentration, but also by aromaticity of DOM, and that this DOM property can be inferred from spectrophotometric measurements.

Influence of the phenols on the biogeochemical behavior of cadmium in the mangrove sediment

Phenols exert a great influence on the dynamic process of Cd in the soil-plant interface. We investigated the influence of phenols on the biogeochemical behavior of cadmium in the rhizosphere of Avicennia marina (Forsk) Vierh. All combinations of four levels of cadmium (0, 1, 2 and 4 mg/kg DW) and two levels of phenol (0 and 15 mg/kg DW) were included in the experimental design. We found that phenols facilitated increasing concentrations of exchangeable cadmium (Ex-Cd), acid volatile sulfide (AVS) and reactive solid-phase Fe (II) in sediments, and iron in plants, but inhibited Cd accumulation in iron plaque and roots. The concentrations of AVS and reactive solid-phase Fe (II) were significantly positively correlated with Cd treatment. As for the biogeochemical behavior of Cd in mangrove sediments, this research revealed that phenols facilitated activation and mobility of Cd. They disturbed the "source-sink" balance of Cd and turned it into a "source", whilst decreasing Cd absorption in A. marina. Additionally, phenols facilitated iron absorption in the plant and alleviated the Fe limit for mangrove plant growth.

Fate of cadmium in the rhizosphere of Arabidopsis halleri grown in a contaminated dredged sediment

In regions impacted by mining and smelting activities, dredged sediments are often contaminated with metals. Phytotechnologies could be used for their management, but more knowledge on the speciation of metals in the sediment and on their fate after colonization by plant roots is needed. This work was focused on a dredged sediment from the Scarpe river (North of France), contaminated with Zn and Cd. Zn, Cd hyperaccumulating plants Arabidopsis halleri from metallicolous and non-metallicolous origin were grown on the sediment for five months in a pot experiment. The nature and extent of the modifications in Cd speciation with or without plant were determined by electron microscopy, micro X-ray fluorescence and bulk and micro X-ray absorption spectroscopy. In addition, changes in Cd exchangeable and bioavailable pools were evaluated, and Cd content in leachates was measured. Finally, Cd plant uptake and plant growth parameters were monitored. In the original sediment, Cd was present as a mixed Zn, Cd, Fe sulfide. After five months, although pots still contained reduced sulfur, Cd-bearing sulfides were totally oxidized in vegetated pots, whereas a minor fraction (8%) was still present in non-vegetated ones. Secondary species included Cd bound to O-containing groups of organic matter and Cd phosphates. Cd exchangeability and bioavailability were relatively low and did not increase during changes in Cd speciation, suggesting that Cd released by sulfide oxidation was readily taken up with strong interactions with organic matter and phosphate ligands. Thus, the composition of the sediment, the oxic conditions and the rhizospheric activity (regardless of the plant origin) created favorable conditions for Cd stabilization. However, it should be kept in mind that returning to anoxic conditions may change Cd speciation, so the species formed cannot be considered as stable on the long term.

Zinc in house dust: speciation, bioaccessibility, and impact of humidity

Indoor exposures to metals arise from a wide variety of indoor and outdoor sources. This study investigates the impact of humid indoor conditions on the bioaccessibility of Zn in dust, and the transformation of Zn species during weathering. House dust samples were subjected to an oxygenated, highly humid atmosphere in a closed chamber for 4 to 5 months. Zinc bioaccessibility before and after the experiment was determined using a simulated gastric acid extraction. Bulk and micro X-ray absorption structure (XAS) spectroscopy was used to speciate Zn in dust. Exposure to humid conditions led to a significant increase in Zn bioaccessibility in all samples, which was due to a redistribution of Zn from inorganic forms toward the organic pools such as Zn adsorbed on humates. ZnO readily dissolved under humid conditions, whereas ZnS persisted in the dust. Elevated humidity in indoor microenvironments may sustain higher Zn bioaccessibility in settled dust compared to drier conditions, and part of this change may be related to fungal growth in humid dust. These results help to explain the greater bioaccessibility of certain metals in house dust compared to soils.

Inhibition mechanisms of Zn precipitation on aluminum oxide by glyphosate: a 31P NMR and Zn EXAFS study

In this research, the effects of glyphosate (GPS) on Zn sorption/precipitation on γ-alumina were investigated using a batch technique, Zn K-edge EXAFS, and (31)P NMR spectroscopy. The EXAFS analysis revealed that, in the absence of glyphosate, Zn adsorbed on the aluminum oxide surface mainly as bidentate mononuclear surface complexes at pH 5.5, whereas Zn-Al layered double hydroxide (LDH) precipitates formed at pH 8.0. In the presence of glyphosate, the EXAFS spectra of Zn sorption samples at pH 5.5 and 8.0 were very similar, both of which demonstrated that Zn did not directly bind to the mineral surface but bonded with the carboxyl group of glyphosate. Formation of γ-alumina-GPS-Zn ternary surface complexes was further suggested by (31)P solid state NMR data which indicated the glyphosate binds to γ-alumina via a phosphonate group, bridging the mineral surface and Zn. Additionally, we showed the sequence of additional glyphosate and Zn can influence the sorption mechanism. At pH 8, Zn-Al LDH precipitates formed if Zn was added first, and no precipitates formed if glyphosate was added first or simultaneously with Zn. In contrast, at pH 5.5, only γ-alumina-GPS-Zn ternary surface complexes formed regardless of whether glyphosate or Zn was added first or both were added simultaneously.

Heavy metal sorption at the muscovite (001)-fulvic acid interface

The role of fulvic acid (FA) in modifying the adsorption mode and sorption capacity of divalent metal cations on the muscovite (001) surface was evaluated by measuring the uptake of Cu(2+), Zn(2+), and Pb(2+) from 0.01 m solutions at pH 3.7 with FA using in situ resonant anomalous X-ray reflectivity. The molecular-scale distributions of these cations combined with those previously observed for Hg(2+), Sr(2+), and Ba(2+) indicate metal uptake patterns controlled by cation-FA binding strength and cation hydration enthalpy. For weakly hydrated cations the presence of FA increased metal uptake by approximately 60-140%. Greater uptake corresponded with increasing cation-FA affinity (Ba(2+) ≈ Sr(2+) < Pb(2+) < Hg(2+)). This trend is associated with differences in the sorption mechanism: Ba(2+) and Sr(2+) sorbed in the outer portion of the FA film whereas Pb(2+) and Hg(2+) complexed with FA effectively throughout the film. The more strongly hydrated Cu(2+) and Zn(2+) adsorbed as two distinct outer-sphere complexes on the muscovite surface, with minimal change from their distribution without FA, indicating that their strong hydration impedes additional binding to the FA film despite their relatively strong affinity for FA.

Enhancement of copper availability and microbial community changes in rice rhizospheres affected by sulfur

The role of sulfur on the availability of Cu and the bacterial community in rice rhizospheres was investigated by pot experiments. With sulfur addition, pH in rhizosphere soil decreased and Mg(NO₃)₂ extractable Cu increased significantly. The bacterial community composition also changed with sulfur addition. Some specific clones having high similarity to Thiobacillus, which indicated that sulfur oxidation in the rice rhizosphere could increase the availability of Cu. These results suggested that sulfur source which could provide substrate to sulfur oxidizing bacteria and enhance the availability of Cu was not a suitable sulfur fertilizer for Cu polluted soil.

Copper isotope fractionation during complexation with insolubilized humic acid

The bioavailability, mobility, and toxicity of Cu depend on Cu speciation in solution. In natural systems like soils, sediments, lakes, and river waters, organo-Cu complexes are the dominating species. Organo-complexation of Cu may cause a fractionation of stable Cu isotopes. The knowledge of Cu isotope fractionation during sorption on humic acid may help to better understand Cu isotope fractionation in natural environments and thus facilitate the use of Cu stable isotope ratios (delta(65)Cu) as tracer of the fate of Cu in the environment. We therefore studied Cu isotope fractionation during complexation with insolubilized humic acid (IHA) as a surrogate of humic acid in soil organic matter with the help of sorption experiments at pH 2-7. We used NICA-Donnan chemical speciation modeling to describe Cu binding on IHA and to estimate the influence of Cu binding to different functional groups on Cu isotope fractionation. The observed overall Cu isotope fractionation at equilibrium between the solution and IHA was Delta(65)Cu(IHA-solution) = 0.26 +/- 0.11 per thousand (2SD). Modeled fractionations of Cu isotopes for low- (LAS) and high-affinity sites (HAS) were identical with Delta(65)Cu(LAS/HAS-solution) = 0.27. pH did not influence Cu isotope fractionation in the investigated pH range.

Molecular characterization of copper in soils using X-ray absorption spectroscopy

Bioavailability of Cu in the soil is a function of its speciation. In this paper we investigated Cu speciation in six soils using X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS), and synchrotron-based micro X-ray fluorescence (mu-XRF). The XANES and EXAFS spectra in all of the soils were the same. mu-XRF results indicated that the majority of the Cu particles in the soils were not associated with calcium carbonates, Fe oxides, or Cu sulfates. Principal component analysis and target transform of the XANES and EXAFS spectra suggested that Cu adsorbed on humic acid (HA) was an acceptable match. Thus it appears that Cu in all of the soils is primarily associated with soil organic matter (SOM). Theoretical fitting of the molecular structure in the soil EXAFS spectra revealed that the Cu in the soils existed as Cu atoms bound in a bidentate complex to O or N functional groups.

Zinc isotopic fractionation: why organic matters

Zinc isotopic fractionation during adsorption onto a purified humic acid (PHA), an analogue of organic matter (OM), has been investigated experimentally as a function of pH. The Donnan Membrane device (DM) was used to separate Zn bound to the PHA from free Zn2+ ions in solution and allowed us to measure the isotopic ratios of free Zn2+. Below pH 6, adsorption of Zn on the PHA resulted in no measurable isotopic fractionation, while at higher pH, Zn bound to PHA was heavier than free Zn2+ (delta66Zn(PHA-FreeZn2+) = +0.24 per thousand +/- 0.06 2sigma, n = 4). This can be explained by changes in Zn speciation with pH, with higher complexation constants and shorter bond lengths for Zn-PHA complex compared to the free Zn2+. Complexation of Zn with PHA occurred mostly through binding to high affinity sites (HAS) and low affinity sites (LAS). Fractionation factors for 66Zn/64Zn ratios determined by mass balance calculations were equal to 1.0004 for HAS (alpha(HAS-solution)) and 1.000 for LAS (alpha(LAS-soultion)). delta66Zn(OM-FreeZn2+) should then vary according to the heterogeneous nature of the OM, because of the variable relative proportions of these two types of sites. The NICA-Donnan model, along with these fractionation factors and measured delta66Zn(TotalDissolved), was used to simulate the corresponding isotopic composition of free Zn2+ in the Seine River, France: delta66Zn(TotalDissolved-FreeZn2+) varied from +0.02 per thousand to +0.18 per thousand, depending on the HAS/LAS ratio assumed for the OM. This study allows a better understanding of Zn isotope fractionation mechanisms associated with organic mater binding.

Quantitative structure-activity relationships for aqueous metal-siderophore complexes

Siderophores, biogenic chelating agents that facilitate the solubilization and uptake of ferric iron, form stable complexes with a wide range of nutrient and contaminant metals and thus may profoundly affect their fate, transport, and biogeochemical cycling. To understand more comprehensively the factors that control the stability and reactivity, as well as the potential for microbial uptake, of metal-siderophore complexes, we probed the structures of complexes formed between the trihydroxamate siderophore desferrioxamine B (DFOB) and Cu(II), Ga(III), Mn(II), Ni(II), and Zn(II) in solution by using extended X-ray absorption fine structure (EXAFS) spectroscopy. We find that all metals studied are dominantly in octahedral coordination, with significant Jahn-Teller distortion of the Cu(II)HDFOB(0) complex. Additionally, log-transformed complex stability constants correlate not only with the charge-normalized interatomic distances within the complex, affirming and expanding existing predictive relationships, but also with the Debye-Waller parameter of the first coordination shell. The derived structure-activity relationships not only quantitatively relate the measured physical architecture of aqueous complexes to their observed stability but also allow for the prediction of siderophore-metal stability constants.