Trace Metals In Natural Waters Research Articles

Improvement of Advanced Sample Preparation Systems for the Determination of Trace Ni in Seawater by Electro-Membranes.

Due to its important environmental role, the analysis of trace metals in natural waters is attracting increasing attention; consequently, faster and more accurate analytical methods are now needed to reach even lower limits of detection. In this work, we propose the use of electro-membrane extraction (EME) to improve analytical methods based on hollow fiber liquid phase micro-extraction (HFLPME). Specifically, an EME-based method for the determination of trace Ni in seawater has been developed, using an HFLPME system with di-2-ethylhexyl phosphoric acid (DEHPA) in kerosene as a chemical carrier, followed by instrumental determination by graphite furnace atomic absorption spectroscopy (GFAAS). Under optimum conditions, Ni was pre-concentrated 180 ± 17 times after 15 min, using sample pH = 5.5, the concentration of DEHPA 0.9 M in the liquid membranes, and 1.9 M HNO3 in the acceptor solution, as well as an electric potential of 25 V with the sample being stirred at 500 rpm. When compared with other HFLPME systems for pre-concentration of trace Ni in seawater in the absence of electric potential, the enrichment factor was improved 2.2 times, while the time of extraction was reduced an 89%. The limit of detection of the new method was 23.3 ng L-1, and both its applicability and accuracy were successfully evaluated by analyzing Ni concentration in a seawater-certified reference material (BCR-403), showing the reliability of EME for sample preparation in the determination of trace metals in marine water samples.

Simultaneous IСP-MS determination of trace metals in natural water and snow after their preconcentration on novel adsorbent based on Al2O3 impregnated with Alizarin Complexone

ABSTRACTA simple and available method for on-line preparation of the adsorbent and solid phase extraction procedure for multi-element determination by inductively coupled plasma mass spectrometry (ICP-MS) has been proposed. The adsorbent (Al2O3-PB-AC) was synthesised by sequential impregnation of Al2O3 with Polybrene (PB) and Alizarin Complexone (AC). Cd(II), Cr(III), Ni(II), Pb(II) and Zn(II) were quantitatively recovered using Al2O3-PВ-AC from aqueous solutions at pH 6.5 and at a flow rate of 1.0 mL min−1. All the elements can easily be eluted by 3.0 mL 0.5 M HNO3. A mini-column packed with 0.100 g Al2O3-PB-AC retained all elements quantitatively from up to 50 mL multi-element solution with an enrichment factor of 16.7. The relative standard deviation for five replicate determinations was 3.5%, 7.3%, 6.2%, 7.4% and 4.7% for Cd(II), Cr(III), Ni(II), Pb(II) and Zn(II), respectively, for determination of 5 μg L−1 level. The developed method was applied for SPE-ICP-MS determination of Cd, Cr, Ni, Pb, Zn in snow and river waters.

Key factors in electromembrane microextraction systems for metals analysis in natural waters

ABSTRACTApplication of an electric potential to hollow fibre liquid phase microextraction (HF-LPME) systems previously optimised for metals preconcentration from natural waters could improve their analytical performance increasing enrichment factors and reducing operation times. Nevertheless, the effectiveness of the direct application of an electric potential to these systems may be limited due to the effects produced by other operational parameters.In this work, the effect of a variable electric potential on the enrichment factors of four HF-LPME systems used for the analysis of trace metals in natural waters (Cd, Ni, Ag and Cu) has been studied. In addition, the effect of organic phase composition, distance between electrodes and electrodes diameter has been also studied. From the results obtained, composition of organic phase can be considered as the key factor in electromembrane (EME) systems, since its polarity determine the operational range of the applied electric potential and consequently the enrichment factor that could be achieved.EMEs have demonstrated to be a real alternative to preconcentrate Cd, Ag and Cu from natural water samples in very short times (30 min). In fact, enrichment factors increased up to one order of magnitude if compared with HF-LPME methodology without application of an electrical potential.

Mechanisms of Sb(III) oxidation mediated by low molecular weight phenolic acids

Quinone-like moieties are active components of natural organic matter (NOM), whose redox reactions may exert substantial influence on the behavior of trace metals in natural waters. The interactions between NOM and antimony (Sb) influence the mobility and toxicity of Sb, but the mechanism of this process is poorly understood. In this study, batch oxidation experiments were conducted to evaluate the effects of model quinone-like moieties, i.e., 1,4-hydroquinone and low molecular weight phenolic acids (LMWPAs), including gallic acid, caffeic acid, and protocatechuic acid, on the oxidation of Sb(III) in the absence of light. The results indicated that Sb(III) was rapidly oxidized to Sb(V) in the presence of different phenolic acids in neutral and alkaline conditions. The oxidative reactions were pH dependent and the Sb(III) oxidation rate constant (kobs) increased by one or two orders of magnitude when the pH shifted from 7.0 to 9.0. The combination of electron paramagnetic resonance and free radical quenching studies elucidated that reactive oxygen species (hydrogen peroxide) and quinone species (the semiquinone radical and oxidized forms of LMWPAs) were the main contributors to the Sb(III) oxidation. According to density functional theory based calculations, the bond dissociation enthalpy of the phenolic hydroxyl group is well correlated with kobs, which is in line with its pro-oxidant property towards Sb(III) through the production of semiquinone radical. The findings of this study provided new insights into the mechanism of the oxidation of Sb(III) by NOM.

Apparent Quantum Efficiency Spectra for Superoxide Photoproduction and Its Formation of Hydrogen Peroxide in Natural Waters

Superoxide (O2−) is a key intermediate in the cycling of organic matter and trace metals in natural waters but production rates are difficult to determine due to low steady-state concentrations, rapid decay rates, and unstable standards. On the other hand, superoxide’s dismutation product, hydrogen peroxide (H2O2), is relatively stable in filtered water. Thus, if the stoichiometry between O2− and H2O2 is known, one can derive superoxide data from H2O2 measurements. The relationship between O2− and H2O2 remains uncertain in seawater but work by Petasne and Zika (1987) presented a method for examining the relationship between O2− and H2O2 during irradiations of coastal seawater using superoxide dismutase (SOD), which forces a 2:1 stoichiometry between O2− and H2O2. Here we report the first O2− apparent quantum yield (AQY) spectra following their approach; performing irradiations of various fresh and seawater samples and measuring H2O2 accumulation with and without added SOD. For all but a single riverine sample, H2O2 AQY spectra fell in a narrow range, but O2− AQY spectra varied such that O2−:H2O2 ratios were always greater than 2 and were highest for the clear waters of the Gulf Stream (~3.4 O2− per H2O2 generated). Because this approach eliminates the need to measure O2− production rates directly, it represents a simple way to refine the stoichiometric relationships that would potentially allow global estimates of O2− photoproduction rates, O2− steady-state concentrations ([O2−]ss), and related surface ocean redox reactions based on more manageable H2O2 photochemical studies.

The Speciation of Metals in Natural Waters

The equilibria and rates of reactions of trace metals in natural waters are affected by their speciation or the form of the metal in the solution phase. Many workers have shown, for example, that biological uptake (Anderson and Morel in Limnol Oceanogr 27:789–813, 1982), the toxicity (Sunda and Ferguson in Trace metals in seawater, Plenum Press, New York, 1983) as well as the solubility (Millero et al. in Mar Chem 50:21–39, 1995; Liu and Millero in Geochim Cosmochim Acta 63:3487–3497, 1999) are affected by the speciation. For example, Fe(II) and Mn(II) are biologically available for marine organisms, while Fe(III) and Mn(IV) are normally not available. The speciation of metals also affects the rates of oxidation (Millero in Geochim Cosmochim Acta 49:547–553, 1985, Res Trends Curr Top Sol Chem 1:141–169, 1994; Sharma and Millero in Geochim Cosmochim Acta 53:2269–2276, 1989; Vazquez et al. in Geophys Res Lett 16:1363–1366, 1989) and reduction (Res Trends Curr Top Sol Chem 1:141–169, 1994; Millero et al. in Mar Chem 36:71–83, 1991) of metals in natural waters. The ionic interactions of metals are controlled by interactions with inorganic (Cl−, OH−, CO3 2−, etc.) and organic ligands (e.g., Fulvic and Humic acids). The speciation of metals is also affected by the oxidation potential (Eh) and the pH in the solution. In this paper we have developed a Pitzer Model (Pitzer in J Phys Chem 77:268–277, 1973, Activity coefficients in electrolyte solutions, 2nd edn, CRC Press, Boca Raton, 1991) that can be used to determine the speciation of trace metals in seawater and other natural waters. It is based upon the Miami Pitzer Model (Millero and Pierrot in Aquatic Geochem 4:153–199, 1998) that has been shown to predict reliable activity coefficients for the major components of seawater. The computer code (Pierrot in Ph.D. Thesis, University of Miami, Miami, Florida, 2002) for these calculations is described in detail, in this paper. It has been used in an earlier paper (Millero and Pierrot in Chemistry of marine water and sediments, Springer, Berlin, 2002) and more recently used to examine the effect of pH on the speciation of metals in seawater (Millero et al. in Oceanography 22(4):72–85, 2009).

Miniaturized and direct spectrophotometric multi-sample analysis of trace metals in natural waters

Trends in the analysis of trace metals in natural waters are mainly based on the development of sample treatment methods to isolate and pre-concentrate the metal from the matrix in a simpler extract for further instrumental analysis. However, direct analysis is often possible using more accessible techniques such as spectrophotometry. In this case a proper ligand is required to form a complex that absorbs radiation in the ultraviolet–visible (UV–Vis) spectrum. In this sense, the hydrazone derivative, di-2-pyridylketone benzoylhydrazone (dPKBH), forms complexes with copper (Cu) and vanadium (V) that absorb light at 370 and 395 nm, respectively. Although spectrophotometric methods are considered as time- and reagent-consuming, this work focused on its miniaturization by reducing the volume of sample as well as time and cost of analysis. In both methods, a micro-amount of sample is placed into a microplate reader with a capacity for 96 samples, which can be analyzed in times ranging from 5 to 10 min. The proposed methods have been optimized using a Box–Behnken design of experiments. For Cu determination, concentration of phosphate buffer solution at pH 8.33, masking agents (ammonium fluoride and sodium citrate), and dPKBH were optimized. For V analysis, sample (pH 4.5) was obtained using acetic acid/sodium acetate buffer, and masking agents were ammonium fluoride and 1,2-cyclohexanediaminetetraacetic acid. Under optimal conditions, both methods were applied to the analysis of certified reference materials TMDA-62 (lake water), LGC-6016 (estuarine water), and LGC-6019 (river water). In all cases, results proved the accuracy of the method.

Laura Sigg: Investigating the Speciation, Bioavailability and Ecotoxicology of Trace Metals in Natural Waters

Laura Sigg is an internationally renowned expert in the aquatic geochemistry and ecotoxicology of trace metals. During her outstanding career as a researcher at Eawag, the Swiss Federal Institute of Aquatic Science and Technology, and adjunct professor at the Department of Environmental Systems Science of ETH Zurich, she pioneered the development of new theoretical concepts as well as practical methods for the assessment of the speciation, fate and effect of trace metals in the hydrosphere (Fig. 1). During his stay in Toulouse as invited professor some years ago, George Luther III proposed us to edit a special issue of Aquatic Geochemistry to honor our colleague Laura Sigg, who serves as a long-term associate editor of this journal. We were very happy to take up this challenge and invited people to submit original manuscripts to a special issue focusing on the fields of speciation, bioavailability and ecotoxicology of trace metals in natural waters. After being educated in chemistry with Paul W. Schindler at the University of Bern, where she learned the precision and rigor of experimental work and how to assess and synthetize ideas, Laura Sigg moved to Zurich and worked for her PhD with Werner Stumm. She started the first part of her career with ground-breaking research on thermodynamic models for adsorption processes at the solid–water interface and published two highly influential papers which became classics in this field (Stumm et al. 1980; Sigg and Stumm 1981).

Remediation studies of trace metals in natural and treated water using surface modified biopolymer nanofibers

In this study, remediation results of trace metals in natural water and treated water using three functionalized nanofiber mats of cellulose and chitosan are reported. The nanofiber materials, packed in mini-columns, were employed for the remediation of five toxic trace metals (Cd, Pb, Cu, Cr and Ni) from natural water samples. Trace metals in real water samples were undetectable as the concentrations were lower than the instrument’s detection limits of 0.27×10−3 (Cd) and 4.2×10−2 (Pb)μgmL−1, respectively. However, after percolation through the functionalised biosorbents in cartridges, detectability of the metal ions was enhanced. The starting volume of the natural water sample was 100mL, which was passed through a column containing the nanofibers sorbent and the retained metals eluted with 5mL of 2.0M nitric acid. The eluate was analyzed for metals concentrations. An enrichment factor of 20 for the metals was realized as a result of the pre-concentration procedure applied to handle the determination of the metals at trace levels. The order of remediation of the studied metals using the nanofibers was as follows: chitosan/PAM-g-furan-2,5-dione<cellulose-g-furan-2,5-dione<cellulose-g-oxolane-2,5-dione. The modified biopolymer nanofibers were able to adsorb trace metals from the river water and treated water, thereby confirming their capability of water purification. These materials are proposed as useful tools and innovative approach for improving the quality of drinking for those consumers in small scale households.

Development of a solid phase extraction method for the multielement determination of trace metals in natural waters including sea-water by FI-ICP-MS

An automated solid phase extraction method by flow analysis on-line inductively coupled plasma mass spectrometry (FI-ICP-MS) for the determination of cobalt, chromium, nickel, cadmium, manganese, zinc, copper and lead in sea-water and other natural waters is described. The system is based on retention of the analytes onto a minicolumn packed with a chelating resin, 1,5-bis (2-pyridyl)-3-sulphophenyl methylene thiocarbonohydrazide immobilized on aminopropyl-controlled pore glass, placed in the injection valve of a simple flow manifold. The effects of chemical and flow variables were investigated and selected as a compromise between sensitivity and sampling frequency. So, the sample solutions (adjusted to pH 8.0±0.5) were passed through the column. After washing the column with water, the adsorbed metals were subsequently eluted into the plasma with 4% m/m nitric acid. Detection limits of the trace metals (180s sample loading time at a sample flow rate of 0.7mLmin−1; sampling frequency 8.6h−1) were 0.002μgL−1 for Co, 0.057μgL−1 for Cr, 0.117μgL−1 for Ni, 0.004μgL−1 for Cd, 0.210μgL−1 for Mn, 0.260μgL−1 for Zn, 0.030μgL−1 for Cu and 0.020μgL−1 for Pb, with enrichment factors between 2.2 and 6.8. The accuracy of the proposed method was checked with certified reference materials (CRMs) of sea-water SLEW 3, LGC6016 and CASS-5, river water SLRS-5 and fortified lake water TMDA-54.4. The results from the determination of these metals were in agreement with the certified values and recovery values ranged between 92.2 and 110.6%. The method was applied to the determination of these metal ions in sea-water samples collected in the Málaga Bay in order to realize a pilot study necessary to generate preliminary information on which to base a more detailed pollution study by heavy metals of the Bay.

Sequential injection anodic stripping voltammetry with monosegmented flow and in-line UV digestion for determination of Zn(II), Cd(II), Pb(II) and Cu(II) in water samples

A cost-effective sequential injection system incorporating with an in-line UV digestion for breakdown of organic matter prior to voltammetric determination of Zn(II), Cd(II), Pb(II) and Cu(II) by anodic stripping voltammetry (ASV) on a hanging mercury drop electrode (HMDE) of a small scale voltammetric cell was developed. A low-cost small scale voltammetric cell was fabricated from disposable pipet tip and microcentrifuge tube with volume of about 3mL for conveniently incorporated with the SI system. A home-made UV digestion unit was fabricated employing a small size and low wattage UV lamps and flow reactor made from PTFE tubing coiled around the UV lamp. An in-line single standard calibration or a standard addition procedure was developed employing a monosegmented flow technique. Performance of the proposed system was tested for in-line digestion of model water samples containing metal ions and some organic ligands such as strong organic ligand (EDTA) or intermediate organic ligand (humic acid). The wet acid digestion method (USEPA 3010a) was used as a standard digestion method for comparison. Under the optimum conditions, with deposition time of 180s, linear calibration graphs in range of 10–300μgL−1 Zn(II), 5–200μgL−1 Cd(II), 10–200μgL−1 Pb(II), 20–400μgL−1 Cu(II) were obtained with detection limit of 3.6, 0.1, 0.7 and 4.3μgL−1, respectively. Relative standard deviation were 4.2, 2.6, 3.1 and 4.7% for seven replicate analyses of 27μgL−1 Zn(II), 13μgL−1 Cd(II), 13μgL−1 Pb(II) and 27μgL−1 Cu(II), respectively. The system was validated by certified reference material of trace metals in natural water (SRM 1640 NIST). The developed system was successfully applied for speciation of Cd(II) Pb(II) and Cu(II) in ground water samples collected from nearby zinc mining area.

Solid-phase extraction and pre-concentration of trace metals in natural waters using indigenously synthesized chelating resin and their subsequent determination by ICP-OES

A laboratory made chelating resin based on immobilization of fluorinated β-diketone group on styrene divinyl benzene was employed for the extraction and pre-concentration of trace metals in natural water samples before their determination by inductively coupled plasma optical emission spectrometry (ICP-OES). Extraction efficiency of the resin toward metal ions was examined by column procedure. Various experimental parameters consisting pH, sample loading flow rate, eluent concentration/volume and breakthrough limits were investigated for the effective uptake of metals on the resin. The resin capacity at pH (9) was found to be 10.20, 9.13, 7.47, 8.67, 3.12, 6.45 and 9.80 mg g−1 for Cd (Cadmium), Co (Cobalt), Fe (Iron), Mn (Manganese), Mo (Molybdenum), Ni (Nickel) and Zn (Zinc), respectively. The method was applied for the extraction/pre-concentration of Cd, Co, Fe, Mn, Mo, Ni and Zn in natural water samples and their subsequent determination was carried out by ICP-OES. Percentage recoveries obtained for the selected metals except Mo were ≥ 95% under similar experimental conditions with < 5% relative standard deviation. Alkali/alkaline earth metals, the major matrix ions were not retained on the resin, the resin is selective for trace metals, therefore it is suitable for the extraction of these metals in natural water samples for environmental and health-related studies. © 2011 Curtin University of Technology and John Wiley & Sons, Ltd.

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

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4 V. Accordingly, an application of negative potential (−1.6 V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.

A Binary Aqueous Component Model for the Sediment−Water Partitioning of Trace Metals in Natural Waters

A model defining the overall sediment-water partitioning of a chemical, K(D), and the partitioning of its conservative components, (K(D))i, is presented. With respect to many trace metals in natural waters it is proposed that, through strong and perhaps specific complexation, two independent aqueous components coexist and a binary form of the model is appropriate. For two components of a metal that exhibit unequal partitioning, an inverse relationship between K(D) and particle concentration is predicted. Published experimental measurements of K(D) for metals in river waters, derived under conditions which exclude variable concentrations of preexistent colloidal particles, displayed either an inverse dependence (Cu, Ni, and Pd) or little dependence (Cs) on particle concentration. Regarding the former, iterative fits with the binary model were better than empirical fits based on a third (colloidal) phase model, and suggested the presence of between about 10 and 75% of a particle-reactive component ((K(D))1 approximately 5 x 10(4) to 10(10) mL g(-1)) and 25 and 90% of a less reactive (e.g., strongly complexed) component ((K(D))2 < or = 2.5 x 10(3) mL g(-1)). Regarding Cs, data indicated the presence of a single component whose K(D) was on the order of 10(3) mL g(-1). These observations challenge the conventional means by which sediment-water partitioning is considered and modeled, and imply that a third phase is not always a prerequisite for the particle concentration effect frequently observed in laboratory and field studies.

Trace metal adsorption onto urban stream suspended particulate matter (Auckland region, New Zealand)

Trace metal adsorption to suspended particulate matter (SPM) influences bioavailability and toxicity of trace metals in natural waters. For highly contaminated urban catchments in the greater Auckland (New Zealand) area, trace metal adsorption to SPM was assessed and compared to similar data from non-urban catchments in the Auckland region, to determine whether there was any difference in the ability of the SPM to adsorb Cu, Pb and Zn. The degree of trace metal adsorption onto the SPM was assessed by way of adsorption edge experiments. It was found that the ability of the Auckland urban SPM to adsorb trace metals decreased in the order Pb > Cu > Zn. Little difference in adsorption was observed between the non-urban Waikato and Kaipara River SPM and urban SPM, or between urban SPM from different flow regimes and seasons, despite some compositional differences in the SPM. This suggests that on the basis of a single surface-binding site, metal adsorption onto SPM could be readily predicted across a range of urban and non-urban catchments in the Auckland region. Adsorption edges were modelled with a diffuse layer, surface complexation model to assess the role of Fe-oxide in adsorption. The MINTEQA2 model was used, assuming Fe-oxide (as HFO) was the only adsorbing surface. There was generally good agreement between observed and modelled adsorption for Pb, indicating the importance of Fe-oxide surfaces for Pb adsorption. However, the model did not predict Zn or Cu adsorption as well. The TOC content of the SPM, and presence of dissolved ligands and organic matter in the water column, appeared to play an important role in Cu adsorption to the SPM. For Zn, the presence of adsorbing surfaces other than HFO appeared to influence adsorption.

Kinetic Limitations in Measuring Stabilities of Metal Complexes by Competitive Ligand Exchange-Adsorptive Stripping Voltammetry (CLE-AdSV)

The kinetic limitations of Competitive Ligand Exchange-Adsorptive Stripping Voltammetry, CLE-AdSV, for the determination of very stable metal complexes are explained in detail. For a given type of metal, from a certain lower limit of the complex stability constant, K, the usual simple equilibrium interpretation of CLE-AdSV measurements is not generally valid. By critical assessment of data for natural waters we show that in many cases the reported stability constants appearto derive from nonequilibrium conditions in the bulk sample and hence overestimate the real values. Fe(II) is a special case due to the particular kinetic features of hydroxide as a ligand. Our results call for validation of such data by analysis on the basis of the kinetics involved and/or by independent kinetic-free experimental approaches. Earlier speculations from CLE-AdSV results on very strong ligands and derived features such as the potential bioavailability of trace metals in natural waters require reconsideration.

Evaluation of paper-based diffusive gradients in thin film samplers for trace metal sampling

Diffusive gradients in thin films (DGT) samplers rely on diffusion of metals through a permeable diffusion layer of known thickness for in situ measurement of trace metals in natural waters, with the principal diffusion layer used to date being based on polyacrylamide hydrogels. Alternative diffusion layers based on commercially available chromatography paper have been investigated and applied as a diffusion layer in DGT samplers, with Cd diffusion coefficients being within 20% of the value in polyacrylamide hydrogels. Cd diffusion coefficients were, however, found to increase with decreasing ionic strength at ionic strengths ≤5 × 10 −4 mol L −1 NaNO 3, but unlike the polyacrylamide diffusion layer the chromatography paper showed reproducible behaviour at these low ionic strengths. Samples of the chromatography paper equilibrated with various ionic strength solutions containing Cd showed that the Cd was weakly bound by the paper, providing a potential explanation for the enhanced diffusion. The chromatography paper diffusion layer was also tested with a commercially available phosphate ion exchange binding phase where diffusion coefficients were determined for Cd, Cu, Pb, Zn, Co, Ni and Mn. The paper-based DGT samplers were found to be easier to prepare and handle than the polyacrylamide hydrogel based samplers, making them ideal for application as a routine monitoring technique.

Hydrophobicity and octanol-water partitioning of trace metals in natural waters.

The hydrophobicities of dissolved Al, Cu, Mn, and Pb have been determined in various contaminated natural water samples by 1-octanol extraction and C18 column retention. Octanol extraction varied among the metals studied and between the environments sampled but, in general, was greatest for Pb, whose conditional octanol-water partition coefficient, Dow, exceeded unit value in some samples. In most cases, metal partition into octanol either increased with increasing pH or exhibited a maximum under near-neutral conditions. Although the order and pH-dependence of metal retention by the C18 columns was consistent with these observations, the extent of retention was generally greater than the extent of metal extraction by octanol, possibly because of interferences effected by the C18 column matrix. Speciation calculations and results of controlled experiments employing metals in the presence of model ligands suggest that metals may become hydrophobic either by neutralizing relatively hydrophilic ligands or by combining with ligands that are intrinsically hydrophobic themselves. Given that octanol solubility affords an upper estimate of lipophilicity, the results of this investigation may have important implications regarding our understanding of metal bioavailability and toxicity in natural waters.

Competition between alga ( Pseudokirchneriella subcapitata), humic substances and EDTA for Cd and Zn control in the algal assay procedure (AAP) medium

The chemical speciation of trace metals in natural waters has important implications for their biogeochemical behavior. Trace metals are present in natural waters as dissolved species and associated with colloids and particles. The complexation of one trace metal (Cd and Zn at 200 and 390 μg/l respectively) with a green alga Pseudokirchneriella subcapitata in colloid-free algal culture medium and in presence of colloidal humic substances (HS) is presented. The influence of the nature of colloids was also addressed using three “standard” HS: fulvic acid (FA) and, soil (SHA) and peat humic acids (PHA). The chemical speciation model, MINTEQA2, was used to simulate the influence of pH and standardized culture medium on metal association with humic substances. The model was successfully modified to consider the differences in the metal complexation with fulvic (FA) and humic acids (HA). The deviations of concentrations of metals associated with HS between experimental results and model predictions were within a factor of ≈2. The results of speciation model highlight the influence of the experimental conditions (pH, EDTA) used for alga bioassay on the behavior of Cd and Zn. The computed speciation suggests working with a pH buffered/EDTA-free mixture to avoid undesirable competition effects. The behavior of Cd and Zn in solution is more strongly influenced by HS than by alga. Metal–HS associations depend on metal and humic substance nature and concentration. Cd is complexed to a higher extent than Zn, in particular at larger HS concentration, and the complexation strength is in the order FA < HA.

Métodos de baixo custo para purificação de reagentes e controle da contaminação para a determinação de metais traços em águas naturais

Probably one of the most difficult and challenging aspects of measuring trace metals in natural waters is to avoid contamination during sampling, manipulation and analysis. This work discusses how to avoid contamination using simple procedures, and considers alternative methods to purify deionised water and low grade reagents to enable accurate determination of trace metals in natural waters in a common laboratory. Measurements were performed by cathodic stripping voltammetry and copper was used as a model metal to test the procedures. It was possible to evaluate copper speciation in natural waters even when total dissolved copper concentration was as low as 1.5 nmol L-1. The methods' accuracy was confirmed by analysis of certified seawater.