Speciation of trace metals in natural waters: the influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper

How do certain atmospheric aerosols affect Cu-binding organic ligands in the oligotrophic coastal sea surface microlayer?

It is still unclear how the chemical speciation of Cu in surface seawater is impacted by aerosols from various sources deposited on the sea surface, which is surprising, considering the environmental importance of Cu. Therefore, we used voltammetry to investigate Cu complexing capacity (CuCC) in the sea surface microlayer (SML) and in the underlying water (ULW) of the oligotrophic middle Adriatic Sea during February-July 2019. The focus was on the impacts of specific atmospheric processes such as open-fire biomass burning (BB), pollination season and Saharan dust intrusion. The presence of ligand class L2 (19.9-392.0, average 63.8, median 43.1) nM; log K2 (8.3-10.2, average 9.6, median 9.6) was observed in all samples, while ligand class L1 (40.5-76.1, average 53.6, median 48.9) nM; log K1 (10.3-11.1, average 10.6, median 10.5) was found in only 25% of SML samples. Throughout the period, the SML was enriched with organic ligands by a factor of up to 9.1 compared to the ULW, mainly due to the high sensitivity of the SML to specific atmospheric depositions. In addition, measurements with corresponding specific model aerosols were conducted to analyse their impacts on CuCC. Pollen directly affected CuCC in the SML by increasing the concentration of allochthonous ligands such as proteins. The deposition of BB aerosols rich in nutrients and trace metals stimulated the biological production of organic ligands, showing an indirect effect on CuCC delayed by up to two weeks. Finally, Saharan dust had a negligible impact on CuCC. This study illustrates the susceptibility of oligotrophic coastal area to the effects of pollen and open-fire BB aerosols in altering the Cu-binding organic ligands in the SML.

Effect of dissolved organic matter on copper bioavailability to a coastal dinoflagellate at environmentally relevant concentrations

The speciation and bioavailability of copper (Cu) in the marine environment are affected by the presence of dissolved organic matter (DOM). Previous studies conducted at dissolved Cu concentrations >100 nM confirmed that Cu bioavailability depends on the concentration of labile Cu, as measured by anodic stripping voltammetry (ASV), which aligns with the expectations of the biotic ligand model (BLM). However, ambient Cu concentrations in coastal waters are generally lower, ranging between 1 and 80 nM, and the effect of DOM on the bioavailability of Cu to marine organisms has not been tested within that range of Cu concentrations. The present study aims to assess the impact of two types of DOM, a commercially available fulvic acid, and marine DOM extracted by ultrafiltration, on Cu bioavailability to phytoplankton using short-term 65Cu internalisation by the marine dinoflagellate Prorocentrum micans. Results showed that Cu internalisation decreases with DOM additions as expected according to the BLM and in agreement with ASV measurements of labile Cu, at the highest tested Cu concentration (100 nM). On the contrary, at a lower Cu concentration (20 nM), organic complexes appear to be partially bioavailable, thereby challenging the general applicability of the BLM model at environmentally relevant concentrations in coastal areas.

Organic Copper Speciation by Anodic Stripping Voltammetry in Estuarine Waters With High Dissolved Organic Matter

The determination of copper (Cu) speciation and its bioavailability in natural waters is an important issue due to its specific role as an essential micronutrient but also a toxic element at elevated concentrations. Here, we report an improved anodic stripping voltammetry (ASV) method for organic Cu speciation, intended to eliminate the important problem of surface-active substances (SAS) interference on the voltammetric signal, hindering measurements in samples with high organic matter concentration. The method relies on the addition of nonionic surfactant Triton-X-100 (T-X-100) at a concentration of 1 mg L^-1. T-X-100 competitively inhibits the adsorption of SAS on the Hg electrode, consequently 1) diminishing SAS influence during the deposition step and 2) strongly improving the shape of the stripping Cu peak by eliminating the high background current due to the adsorbed SAS, making the extraction of Cu peak intensities much more convenient. Performed tests revealed that the addition of T-X-100, in the concentration used here, does not have any influence on the determination of Cu complexation parameters and thus is considered "interference-free." The method was tested using fulvic acid as a model of natural organic matter and applied for the determination of Cu speciation in samples collected in the Arno River estuary (Italy) (in spring and summer), characterized by a high dissolved organic carbon (DOC) concentration (up to 5.2 mgC L^-1) and anthropogenic Cu input during the tourist season (up to 48 nM of total dissolved Cu). In all the samples, two classes of ligands (denoted as L₁ and L₂) were determined in concentrations ranging from 3.5 ± 2.9 to 63 ± 4 nM eq Cu for L₁ and 17 ± 4 to 104 ± 7 nM eq Cu for L₂, with stability constants logK _Cu,1 = 9.6 ± 0.2-10.8 ± 0.6 and logK _Cu,2 = 8.2 ± 0.3-9.0 ± 0.3. Different linear relationships between DOC and total ligand concentrations between the two seasons suggest a higher abundance of organic ligands in the DOM pool in spring, which is linked to a higher input of terrestrial humic substances into the estuary. This implies that terrestrial humic substances represent a significant pool of Cu-binding ligands in the Arno River estuary.

Evaluation of diffusive gradients in thin films (DGT) technique for speciation of trace metals in estuarine waters - A multimethodological approach

Understanding the potential bioavailability of trace metals (TM) in marine systems is of prime importance to implement adapted regulations and efficiently protect our coastal and estuarine waters. In this study Diffusive Gradients in Thin films (DGT) technique with two different pore size was used to evaluate the potentially bioavailable fractions (DGT-labile) of Cd, Co, Cu, Ni, Pb and Zn at various depths of a highly stratified estuary (the Krka River estuary, Croatia) both in winter and summer. DGT-labile concentrations were compared to (1) total dissolved concentrations, (2) concentrations of labile species measured by anodic stripping voltammetry (ASV-labile) for Cu and (3) concentrations derived by chemical speciation modelling. High correlation between dissolved and DGT-labile concentrations was found for all metals, except for Zn where contamination problems prevented reliable conclusions. Percentages of DGT-labile fractions over total dissolved concentrations were (AVG ± SD): 92 ± 3%, 64 ± 2%, 23 ± 5%, 61 ± 3% and 57 ± 6% for Cd, Pb, Cu, Ni and Co, respectively. No significant difference was found between trace metal concentrations measured with an open pore and restricted pore devices, implying the predominance of kinetically labile metal complexes smaller than 1 nm. For Cu, ASV-labile and DGT labile concentrations were highly correlated (0.97) with ASV-labile concentration being around 35% lower than that of the DGT-labile. Modelling of chemical speciation reliably predicted dynamic (free, inorganic and part of organic complexes) concentration of Cd, whereas dynamic concentrations of Cu and Pb were underestimated by 32% and 65%, respectively. In view of the relative simplicity of DGT devices, they are well suited for the monitoring effort of coastal waters, informing on potentially bioavailable concentrations of TM and thereby, helping to achieve good environmental status of coastal waters, as stipulated within the EU Water Framework Directive.

Improved voltammetric methodology for chromium redox speciation in estuarine waters

Chromium is a toxic element naturally present in natural waters whose chemical speciation regulates its cycling, mobility and bioavailability. We present here: 1- an improved analytical method for chromium speciation (Cr(VI) vs Cr(III)) in estuarine samples by catalytic adsorptive cathodic stripping voltammetric (cat-AdCSV) and 2- a study highlighting a significant change of redox speciation during summer and winter. Initial measurements first revealed that surface-active substances (SAS) present in estuarine samples strongly influenced the analytical determination of Cr by partially masking the Cr peak through an increase of the background current. We found that the application of a low negative accumulation potential (-1.65 V) resulted in much better voltammograms compared to those obtained using the usual accumulation potential of -1.0 V. Using humic acid (HA) as a model SAS of natural origin, we show that this negative potential clearly prevents adsorption of SAS on the Hg-electrode surface, which in turns benefits the adsorption of the in-situ formed Cr(III)-DTPA complex and the resulting signal. The optimised method was applied to determine chromium redox speciation and distribution along the 23 km long salinity gradient, well oxygenated, Krka River estuary (Croatia). Cr(VI) was found to be the dominant redox species in both summer and winter, with Cr(III) contribution being lower in summer (up to ∼30%, average of ∼5%) than in winter (up to ∼50%, average of ∼30%). In summer, lower concentrations of Cr(VI) were found in the freshwater end-member (2.5 nM) than in the seawater end-member (4-5 nM), while the opposite trend was found in winter. Hexavalent chromium exhibited a non-conservative behaviour along the salinity gradient for both seasons. Chromium predominantly exists in dissolved phase, and contribution of particles reactive Cr(III) was minor.

How do low doses of desferrioxamine B and EDTA affect the phytoextraction of metals in sunflower?

The aim of this study was to compare the efficiency of siderophore desferrioxamine B (DFOB) and EDTA in increasing the phytoextraction of metals in sunflower. A 28-day pot experiment was conducted in a metal-contaminated soil supplied with 200μmolkg^-1 of DFOB or EDTA. Pore water was collected and pseudo-polarographic analyses were conducted to assess the impact of the two chelators on the mobility and speciation of metals in the liquid phase. Our results showed that DFOB is not an efficient mobilizing agent of divalent metals in soil. Adding DFOB selectively increased the mobility of trivalent metals while the supply of EDTA simultaneously increased the mobility of both trivalent and divalent metals. EDTA significantly reduced the labile fractions of Cd, Cu, (Pb) and Zn measured in the porewater. The labile concentration of Cd and Zn measured in presence of EDTA was even less than that measured in the control. As expected from the pore water analysis, the addition of DFOB did not affect the phytoextraction of any divalent metals. In contrast, the addition of EDTA enhanced Cu and Ni phytoextraction in sunflower 2.0 to 2.8 fold for Cu and 1.3 to 2.3 fold for Ni, depending on the cultivar. This result supports different hypotheses regarding the forms and the related pathways in which metals are taken up in presence of EDTA. Based on the results obtained for Ni, whose uptake is rate limited by its internalization across the cell membrane, the direct uptake of metal-EDTA complexes via the non-selective apoplastic pathway is hypothesized to contribute the most to the overall uptake of metals in presence of EDTA, even added at "low" concentrations.

Implications of effluent organic matter and its hydrophilic fraction on zinc(II) complexation in rivers under strong urban pressure: aromaticity as an inaccurate indicator of DOM-metal binding

The zinc binding characteristics of dissolved organic matter (DOM) fractions from the Seine River Basin were studied after being separated and extracted according to their polarity: hydrophobic, transphilic, and hydrophilic. The applied experimental methodology was based on a determination of labile zinc species by means of differential pulse anodic stripping voltammetry (DPASV) at increasing concentrations of total zinc on a logarithmic scale and at fixed levels of: pH, ionic strength, and temperature. Fitting the DOM fractions with two discrete classes of ligands successfully allowed determining the conditional zinc binding constants (Ki) as well as total ligand density (LiT). The binding constants obtained for each DOM fraction were then compared and discussed with respect to the hydrophobic/hydrophilic nature and sample origin. Results highlighted a strong complexation of zinc to the effluent organic matter and especially the most hydrophilic fraction, which also displayed a very low specific UV absorbance. Although the biotic ligand model takes into account the quality of DOM through UV absorbance in the predictions of metal bioavailability and toxicity, this correction is not efficient for urban waters.

Electrochemical sensors and devices for heavy metals assay in water: the French groups' contribution

A great challenge in the area of heavy metal trace detection is the development of electrochemical techniques and devices which are user-friendly, robust, selective, with low detection limits and allowing fast analyses. This review presents the major contribution of the French scientific academic community in the field of electrochemical sensors and electroanalytical methods within the last 20 years. From the well-known polarography to the up-to-date generation of functionalized interfaces, the different strategies dedicated to analytical performances improvement are exposed: stripping voltammetry, solid mercury-free electrode, ion selective sensor, carbon based materials, chemically modified electrodes, nano-structured surfaces. The paper particularly emphasizes their advantages and limits face to the last Water Frame Directive devoted to the Environmental Quality Standards for heavy metals. Recent trends on trace metal speciation as well as on automatic "on line" monitoring devices are also evoked.

Speciation and bioavailability of dissolved copper in different freshwaters: comparison of modelling, biological and chemical responses in aquatic mosses and gammarids

Biological and chemical measurements were performed in mesocosms to investigate the bioavailability of copper, with a greater emphasis on the effects of competing ions and copper speciation. Measurements were achieved in three different natural waters for two aquatic species (Gammarus pulex and Fontinalis antipyretica) along a copper gradient concentration: natural concentration, spiked at 5 and 15 μg L(-1). Aquatic mosses exhibited high enrichment rates that were above the background levels compared to gammarids. The accumulation of copper in F. antipyretica is better correlated to the weakly complexed copper concentrations measured using differential pulse anodic stripping voltammetry (DPASV) and diffusive gradient in thin film (DGT) than to the free copper concentration measured using an ion selective electrode (ISE). In unspiked natural waters, the presence of dissolved organic ligands strongly controls the metal speciation and consequently largely minimised the impact of competing cations on the accumulation of Cu in mosses. Furthermore, the BioMet Biotic Ligand Model (BLM) successfully describes the site-specific copper bioaccumulation for the freshwater mosses studied. However, the comparison of the results with a previous study appears to indicate that the adsorption/desorption of Cu in mosses is impacted by seasons. This highlights a limit of the BioMet model in which the physiological state of aquatic organisms is not considered. No toxic effect of Cu exposure on lipid peroxidation was observed in the mosses and gammarids regardless of the site and the concentration considered. However, the oxidative stress measured in the mosses via their guaiacol peroxidase (GPX) activity increased in the case where internalised Cu reached maximal values, which suggests a threshold effect on the GPX activity.

Pseudopolarography of copper complexes in seawater using a vibrating gold microwire electrode

Copper (Cu) in seawater can be determined by anodic stripping voltammetry using a vibrating gold microwire electrode (VGME) with a much lower limit of detection than using a mercury electrode, enabling detection of labile Cu at trace level. The possibility of pseudopolarography of Cu using the VGME is investigated here and is calibrated against known chelating agents. The sensitivity much (15-fold) improved by application of a desorption step to remove adsorbed organic substances and excess anions. The notorious tendency of solid electrodes to be affected by memory effects was overcome by a conditioning interval between measurements that stabilized the electrode response. Model ligands, including EDTA, humic substances (HS), and glutathione (examples of natural ligands) were analyzed to calibrate the half-wave shift to complex stability. The half-wave shift on the VGME is much greater (~2×) than that on the mercury drop electrode which is attributed to several parameters including a much (5-fold) thinner diffusion layer on the VGME. Experiments showed that the same procedure is suitable for pseudopolarography of zinc. Application of the new method to samples from the Irish Sea showed Cu occurring in several complexes, all strongly bound, and some occurring in the electrochemically reversible region of the pseudopolarogram. The humic substance complex of Cu was also found to occur in the reversible region of the pseudopolarogram. The pseudopolarograms of Cu in seawater were unaffected by sample filtration and did not require purging to remove dissolved oxygen, suggesting that this method can be readily used as part of an in situ measuring system.

Assessment of low-level metal contamination using the Mediterranean mussel gills as the indicator tissue

PURPOSE

The aim of this study was to compare the level of metal contamination in two bays in the middle part of the Eastern Adriatic coastal zone in Croatia using the gills of mussels Mytilus galloprovincialis as indicator tissue. Despite the existing sources of contamination, previous studies with caged mussels only indicated moderate metal contamination of the Kastela Bay, contrary to the Trogir Bay in which marina and shipyard present a probable source of Cu- and Zn-contamination.

METHODS

The measurements of metallothioneins (MTs) and metals that induce MT synthesis (Cu, Zn, and Cd) were performed in the heat-treated gill cytosol and total proteins (TPs) in the untreated gill cytosol. MTs were determined by differential pulse voltammetry, Cu and Zn by flame atomic absorption spectrometry (AAS), Cd by graphite furnace AAS, and TPs by Bradford spectrophotometric procedure.

RESULTS

The results collected in four sampling campaigns (autumn periods from 2001 to 2004) indicated that MT levels in mussel gills (expressed on dry mass basis 2.3+/-0.3 mg g(-1)) were comparable with basal levels reported in the literature (2.5+/-0.8 mg g(-1)). Observed interindividual, temporal, and spatial MT variability could be associated with different confounding factors, such as the time of sampling, total protein concentration, and mussel size rather than cytosolic levels of Cu and Zn. Metal levels, expressed on wet mass basis, in the heat-treated gill cytosol ranged from 1.33 to 11.31 microg g(-1) for Zn, from 0.72 to 2.96 microg g(-1) for Cu, and from 0.036 to 0.100 microg g(-1) for Cd. The highest Zn level was measured at Vranjic (Kastela Bay)-the site influenced by untreated domestic wastewater, while somewhat increased Zn and the highest Cu levels were found at marina and shipyard locations (Trogir Bay). The highest Cd level was measured at Inavinil (Kastela Bay).

CONCLUSIONS

The observed association of gill MT levels with several biotic and abiotic factors limits its use as the biomarker of low-level metal exposure. Therefore, the use of the metal concentrations in the heat-treated gill cytosol of Mediterranean mussels should be considered for the assessment of the low-level metal contamination of coastal marine areas.

Significance of data treatment and experimental setup on the determination of copper complexing parameters by anodic stripping voltammetry

Different procedures of voltammetric peak intensities determination, as well as various experimental setups were systematically tested on simulated and real experimental data in order to identify critical points in the determination of copper complexation parameters (ligand concentration and conditional stability constant) by anodic stripping voltammetry (ASV). Varieties of titration data sets (Cu(measured)vs. Cu(total)) were fitted by models encompassing discrete sites distribution of one-class and two-class of binding ligands (by PROSECE software). Examination of different procedures for peak intensities determination applied on voltammograms with known preset values revealed that tangent fit (TF) routine should be avoided, as for both simulated and experimental titration data it produced an additional class of strong ligand (actually not present). Peak intensities determination by fitting of the whole voltammogram was found to be the most appropriate, as it provided most reliable complexation parameters. Tests performed on real seawater samples under different experimental conditions revealed that in addition to importance of proper peak intensities determination, an accumulation time (control of the sensitivity) and an equilibration time needed for complete complexation of added copper during titration (control of complexation kinetics) are the keypoints to obtain reliable results free of artefacts. The consequence of overestimation and underestimation of complexing parameters is supported and illustrated by the example of free copper concentrations (the most bioavailable/toxic specie) calculated for all studied cases. Errors up to 80% of underestimation of free copper concentration and almost two orders of magnitude overestimation of conditional stability constant were registered for the simulated case with two ligands.

Characterisation and modelling of marine dissolved organic matter interactions with major and trace cations

A two-step protocol (nano-filtration and reverse osmosis) was applied for natural organic matter (NOM) preconcentration of a seawater sample. Complexing affinities of the so concentrated marine dissolved NOM (DNOM) towards major and trace cations were studied by potentiometric and voltammetric titration techniques. The potentiometric titration experiments fitted by models describing and characterising the DNOM-cation interactions, revealed four distinct classes of acidic sites (pKa of 3.6, 4.8, 8.6 and 12). A total acidic sites density of 445meq/mol(C) was estimated, with a majority (60%) of carboxylic-like sites. Pseudopolarographic measurements revealed two distinct groups of copper complexes: labile, reducible at about -0.2V; and inert, directly reducible at about -1.4V. Simultaneous competition between copper, calcium and proton highlighted the presence of two classes of binding sites (density of 1.72 and 10.25 meq mol(C)(-1), respectively, corresponding to 3% of total acidic sites). The first class was more specific to copper (logK(CuL) 9.9, logK(CaL) 2.5, pKa 8.6), whereas stronger competition between copper and calcium occurred for the second class (logK(CuL) 6.9, logK(CaL) 5.5, pKa 8.2). The binding sites characterisation was validated by the very good matching of the non-concentrated seawater sample titration data with the simulated curves obtained using the binding parameters from the concentrated sample. Furthermore, this comparison also validated the applied preconcentration protocol, highlighting its negligible influence on organic matter properties when considering copper complexation.

Study of interactions of concentrated marine dissolved organic matter with copper and zinc by pseudopolarography

The interaction of dissolved organic matter (DOM) with copper and zinc in a concentrated seawater sample was characterised by pseudopolarography. Measurements performed at increased concentrations of copper(II) ions showed successive saturation of active DOM sites which indicate possible partition of copper between (i) free or labile complexes, (ii) reduced and released within the potential window of the method, and (iii) electroinactive copper complexes. Pseudopolarograms measured at pH 4 indicate a release of copper which was bound to the active sites of DOM that formed non-labile complexes. Variation of the peak position and half-peak width along the scanned deposition potentials and with the increasing concentration of copper bear the information about the complex electrochemical processes at the electrode surface and in the bulk of the solution. Pseudopolarograms of zinc showed a strong dependence of the peak current and the peak position along the scanned deposition potentials on pH values, indicating preferentially complexation of zinc with carboxylic-like active sites of DOM in the measured sample. Pseudopolarography is a valuable method in the trace metal complexation and speciation studies, serving as a fingerprint of the analysed sample.