Labile Organic Complexes Research Articles

Use of diffusive gradients in thin films (DGT) to measure potentially bioavailable metals in southeastern USA blackwater streams

We used diffusive gradients in thin films (DGT) to measure potentially bioavailable metals in coastal plain streams in the southeastern USA that exhibited strong to moderate blackwater characteristics. Metals were partitioned into particulate metals, DGT-inert metals (i.e., colloidal and refractory organic complexes not accumulated by DGT), and DGT-labile metals (i.e., free metal ions, small inorganic complexes, and labile organic complexes). We also examined the influence of different DGT deployment times using data collected from the field and a follow-up laboratory study. The DGT-measured fraction of dissolved metals in the streams was 15% for Cd, 21% for Zn, 33% for Cu, 37% for Pb, and 98% for Mn. Metals bound to particulates predominated only for Pb. Most of the Cd, Pb, Zn, and Cu were associated with colloids, refractory organic complexes, or particles. Relatively small amounts were in free ion or labile complexes likely to be bioavailable through respiratory surfaces. Modeled concentrations of free and inorganically bound Cu and Pb were lower than the DGT fraction indicating that DGT accumulated some organically bound Cu and Pb that might not have been bioavailable. DGT-exposure times in excess of 5days may have contributed to the accumulation of partly labile organic-metal complexes and were associated with substantial biofouling that caused metal uptake by DGT to depart from linearity.

Distribution character of localized iron microniche in lake sediment microzone revealed by chemical image.

DGT (diffusive gradients in thin films) technique and LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) for heterogeneous distribution of the soluble labile iron (Fe) at submillimeter resolution in lake sediment porewater are reported. The soluble labile Fe species include ion and labile organic complexes. The chemical images in two dimensions (2D) for DGT concentration of Fe (CDGT(Fe)) are investigated for Fe remobilization character. There are 902 CDGT(Fe) values between 1000 and 2000 μg L-1, 463 values between 2000 and 3000 μg L-1, and 112 values over 3000 μg L-1 in all chemical maps. Based on the linear correlation relationships between CDGT (Fe) and total Fe (TFe), total organic carbon (TOC), acid-volatile sulfide (AVS), Eh, concentrations of the soluble reactive phosphorus (P) (SRP), and soluble labile trace metals (Zn, Cu, Pb, and Zn) in a vertical 1D profile of sediment or porewater, Fe release mechanisms are mainly due to the reductive Fe release from iron oxyhydroxides and the decomposition of organic matter in algae biomass and deep sediment layer. It can be used to explain the formation mechanisms of Fe microniches in chemical maps with heterogeneous character to a great extent. CDGT(Fe) peak flux in the center of Fe microniche and the low CDGT (Fe) at the edge of a microniche are due to the formation of the insoluble iron sulfide and the abundant acid-volatile sulfide (AVS) in sediment. The verified co-remobilization of the soluble labile Fe and trace metals or SRP in sediment porewater can be used to predict their simultaneous release from Fe microniches with the large CDGT (Fe) peaks. The different kinds of Fe microniche zones and hot spots from sediment/water interface (SWI) to deep sediment correspond to the formation mechanisms of microniches mentioned above. Moreover, some narrow Fe microniche zones with the large CDGT (Fe) across chemical maps are due to the desorption of Fe(II) from the freshly formed oxide on Myriophyllum verticiilatur roots, which are located at sites of microniche zones.

Determination of labile inorganic and organic species of Al and Cu in river waters using the diffusive gradients in thin films technique

The diffusive gradients in thin films (DGT) technique, using a diffusive gel or a restrictive gel, was evaluated for the determination of labile inorganic and organic species of Al and Cu in model synthetic solutions and river water samples. Experiments were performed both in situ and in the laboratory. In the solutions containing Al ions, the major labile fraction consisted of inorganic species. The organic complex fractions were mainly kinetically inert. For the model Cu solutions, the most labile fraction consisted of inorganic species; however, significant amounts of labile organic complexes of Cu were also present. A comparison was made between the results obtained using restrictive gel DGT and tangential flow ultrafiltration (TF-UF). The Cu fraction determined by restrictive gel DGT (corresponding to the "free" ions plus the labile fraction of small molecular size complexes) was larger than that determined by TF-UF (corresponding to all small molecular size ions), suggesting that the techniques exhibited different porosities for discrimination of inorganic species. For the river water samples analyzed in the laboratory, less than 45% of the analytes were present in labile forms, with most being organic species. For the in situ measurements, the labile inorganic and organic fractions were larger than those obtained in the laboratory analyses. These differences could have been due to errors incurred during sample collection and storage. All results were consistent with those found using two different methods, namely, solid-phase extraction and the DGT technique employing the apparent diffusion coefficient.

Heavy metals in benthic foraminifera from the highly polluted sediments of the Naples harbour (Southern Tyrrhenian Sea, Italy)

A systematic investigation evaluated the concentrations of a selected number of trace elements (Cd, Co, Cu, Li, Ni, Pb, V and Zn) in carbonates of the benthic foraminifera A mmonia tepida collected from surface sediments of the highly polluted harbour of Naples. Application of cleaning procedures, combined with Scanning Electron Microscopy investigation (SEM) of the analysed shells allowed reliable quantification of the elements in the carbonate lattice. Adoption of biogenic carbonate/seawater distribution coefficients reported in the literature provided the ranges of variability of total dissolved trace elements in the studied marine environment. Very high concentrations of Zn, Cd, and Cu calculated in seawater (from 100 to 10,000 times higher than those reported for uncontaminated Mediterranean seawaters) testify to intense effects of anthropogenic impact on the harbour mainly related to the industrial and commercial activities carried out in the neighbouring area. The ensemble of the obtained results emphasizes the high potential of measurements of trace elements in the biogenic carbonates of benthic foraminifera as tracers of anthopogenic pollution of seawater and reliable proxies of potentially bioavailable forms (as free ions and/or more labile organic complexes) of seawater dissolved metals.

Evaluation of DGT as a metal speciation tool in wastewater

This paper aims to evaluate the performance of the diffusive gradient in thin film technique (DGT) as a speciation tool for metals in wastewater. The validity of metal sampling by DGT in wastewater was checked. DGT was used in parallel with the Daphnia magna acute toxicity test in order to obtain information on the speciation of copper and cadmium in diluted and spiked filtered wastewater (raw and treated) from two treatment plants. Combining the chemical (DGT) and the biological methods ( D. magna toxicity test) allowed metal to be fractionated into inorganic, labile organic and inert organic metal. Copper was mainly found as inert organic complexes, whereas the major part of cadmium was found to be labile organic complexes. The proportion of inert organic copper complexes was higher in the presence of treated wastewater than in raw wastewater. The use of restricted gels in DGT devices discriminated more labile organic cadmium than labile organic copper, indicating that cadmium weak ligands have more complex structures than copper weak ligands. In our experimental conditions (i.e. a high metal to ligand ratio), DGT, even equipped with restricted gels, was able to accumulate labile organic complexes. This result highlights that the ecotoxicological interpretation of DGT measurement should be considered carefully. DGT is a reliable tool to assess the chemical characteristics of metals (i.e. reactivity) in wastewater, but it does not ensure that only inorganic metal is measured.

Etude de L'effet du Ver de Terre Lumbricus Terrestris sur la Speciation des Metaux Lourds du Sol Study of the Effect of Earthworm Lumbricus Terrestris on the Speciation of Heavy Metals in Soils

Evaluation of the effect of earthworm Lumbricus terrestris on the speciation of copper and cadmium was carried out on two types of soils with a high metallic contamination due to municipal wastes spreading. The concentrations of total dissolved metals were higher in the soil containing earthworms. This increase was larger for the soil submitted to disturbance by earthworms for a long time (3 months). The main chemical species in the lixiviates of all type of soils including controls, were labile forms of cadmium with 52 to 87% and stable forms of copper which represents 67 to 95% of total concentration of dissolved metal. In the solid phase, there was a slight transfer of cadmium and copper from the oxidizable fraction into the exchangeable and acid soluble fractions. This suggests that soil disturbance by earthworms increases the mobility of these metals particularly cadmium. On the contrary copper appears in lixiviates as non labile organic complexes. Analysis of the whole results showed differences between soils as regards the mobility of the metals studied, which reflected the role of the mains physico-chemical characteristics (pH, C.E.C. and total calcareous content).

Comparison of diffusive gradients in thin films and equilibrium dialysis for the determination of Al, Fe(III) and Zn complexed with dissolved organic matter

The distinction between ‘free’ metals and organically complexed metals in aqueous solutions is important for research involving the mobility or bioavailability of metals in the environment. In this study, the applicability of equilibrium dialysis (molecular weight cut-off=1000 Da) and diffusive gradients in thin films (DGT) to determine ‘free’ Al, Fe(III) and Zn in four forest soil solutions was compared. The ‘free’ metals as measured by both methods, consist of hydrated metal cations and soluble inorganic metal complexes. In addition, dialysis measures any organic complexes<1000 Da and DGT measures a portion of smaller labile organic complexes. The four soil solutions were prepared by water extraction of an organic soil horizon (H) from a Fimic Anthrosol, and contained either 20 or 75 mg C/l dissolved organic matter at pH 4.0 and pH 7.0. To test the performance of both methods and optimize experimental volume and time, experiments using metal nitrate solutions were carried out. In the solutions at pH 4.0, no significant differences in average ‘free’ metal contents were found for Al, Fe(III) or Zn. This makes DGT a viable alternative for equilibrium dialysis for the study of the complexation of all three metals in acidic soils. At pH 7.0, again no significant difference between both methods were found for Al and Fe(III), but the DGT results for Zn were significantly higher. This was likely caused by labile organic complexes that were only detected by DGT.

Interactions between trace elements and dissolved organic matter in the stagnant anoxic deep layer of a meromictic lake

About 80% and more than 90% of the dissolved organic carbon (DOC) in the bottom water of Lake Pavin were isolated, respectively, on Amberlite XAD and Spherodex DEAE resins. Trace element concentrations in the fractions isolated were analyzed by using ICP‐MS. Uranium, molybdenum, and antimony were found to be about 40% associated with fulvic and hydrophilic acids adsorbed on XAD resins at pH 2, the metal‐organic association being not dissociated. Much higher percentages of the same elements, plus vanadium, were adsorbed on the DEAE resin at the pH of natural water, possibly because of the better preservation of acid labile organic complexes or of the supplementary adsorption of inorganic compounds. Ultrafiltration and dialysis made it possible to exclude colloidal fractions together with much U (78%), Mo (80%), V (55%), and DOC (65%). These elements and many others (including barium) were previously found to be quite reactive at the bottom of the lake, (apparently being scavenged by settling particles, which are mostly diatoms, then dissolved at the sediment‐water interface). Conversely, trace elements with a conservative behavior in the bottom layer (such as lithium and cesium) were not found associated with dissolved organic mater (DOM). Barium was not strongly associated with the extractable DOM, which may argue for a direct interaction with inorganic particles or the existence of very labile complexes. Our work suggests the existence of relatively stable (nonacid labile) U and Mo–DOM colloidal associations in the anoxic bottom waters of the lake and their importance in the scavenging of those metals.

Speciation of dissolved copper and nickel in South San Francisco Bay: a multi-method approach

A multi-method approach was used to investigate the chemical speciation of dissolved copper and nickel in South San Francisco Bay. Dissolved copper speciation was determined by four different analytical approaches: competitive ligand equilibration-cathodic stripping voltammetry [CLE-CSV], differential pulse anodic stripping voltammetry using a thin mercury film rotating glassy carbon disk electrode [DPASV(TMF-RGCDE)], DPASV using a hanging mercury drop electrode [DPASV(HMDE)], and chelating resin column partitioning-graphite furnace atomic absorption spectrometry [CRCP-GFAAS]. Dissolved nickel speciation was determined by CLE-CSV and CRCP-GFAAS. Each of the methods employed provided useful insight into the chemical speciation of dissolved copper and nickel. CLE-CSV provided the best characterization of the stronger (but less predominant) of the two organic copper-complexing ligands detected, while DPASV(TMF-RGCDE) provided the truest measurement of inorganic copper, and therefore, the best characterization of the weaker organic copper-complexing ligand which exerted the strongest influence on dissolved copper speciation. These methods provide complementary results which can be combined to provide a more complete understanding of the chemical speciation of copper. DPASV(HMDE) and CRCP-GFAAS both provide an operational measurement of labile copper which includes some fraction of labile organic complexes in addition to inorganic copper species. CLE-CSV and CRCP-GFAAS both yielded similar results for nickel speciation indicating the presence of a single class of extremely strong nickel-complexing ligands. The dissolved copper in South San Francisco Bay is found to exist predominantly as organic complexes (80–92%). About 27% of dissolved copper was complexed with the stronger (L1; [CuL1 ≈ 13 nM, log K′CuL1 > 13.5) of the copper-complexing ligands detected, while copper complexed with the weaker ligand L2 comprised the greatest fraction of total dissolved copper species (52–65%; [CuL2] = 20–30 nM; log K′CuL2 = 9.0–9.6). A third to a half of total dissolved nickel was complexed by one, extremely strong class of organic ligands ([NiL] = 17–28 nM; log K′NiL > 17); the remaining total dissolved nickel existed as inorganic or labile forms.

Characterization of Iron and Zinc in Albuquerque Sewage Sludge

AbstractChemical forms of Fe and Zn present in anaerobically digested sewage sludge from Albuquerque, NM were identified and quantified. Water‐soluble Fe and Zn were speciated based on charge and stability of metal complexes, and on their degree of association with soluble organics. Chemical forms of Fe and Zn present in the solid phase of the sludge were characterized with a sequential extraction procedure. Soluble Fe was predominantly present as negatively charged slowly labile organic complexes of molecular weights &gt; 1000 daltons. Zinc was associated with negative and neutral species that were very or moderately labile, according to the techniques used. The contribution of soluble organics to the chemistry of Fe and Zn was evaluated using gel chromatography. Iron was associated with a larger portion of sludge organics and with larger molecular weight fractions than Zn complexes. The percentage of total sludge Fe and Zn found in readily soluble forms was very small (&lt;1%). The amounts may nevertheless represent significant pools of metal forms available for plants. Zinc was found in larger concentrations than Fe in the potentially more labile, organic, and carbonate fractions of the sludge. The Albuquerque sludge is an excellent source of Fe and Zn to plants because it contains readily soluble forms of these metals that may be maintained in soil solution through organic complexation.