Metal speciation dynamics and bioavailability: Zn(II) and Cd(II) uptake by mussel (Mytilus edulis) and carp (Cyprinus carpio)

In the analysis of metal biouptake from complexing environments, both chemical speciation and biological uptake characteristics have to be taken into account. The commonly used free ion activity model is based on equilibrium speciation and implies that diffusion of the bioactive free metal toward the organism is not rate-limiting. In the presence of complexes, however, sufficiently labile species might contribute to the biouptake via preceding dissociation. Coupling of the ensuing diffusional mass transfer flux of metal with the biouptake flux of free metal, the supposedly bioactive species, shows under which conditions labile metal complexes can contribute to the uptake. The goal of the present paper is to apply this type of analysis to experimental data on metal uptake by mussel (Mytilus edulis) and carp (Cyprinus carpio) in complexing environments. These biosystems have fairly well-characterized uptake parameters, but the uptake fluxes cannot be fully explained by considering equilibrium speciation only. For Zn(II) uptake by mussel, evidence was found for diffusional limitation at low concentrations, whereas for Cd(III) uptake by carp, diffusion is not limiting at all. The analysis provides an example of how a more comprehensive treatment of complex systems can be applied to real experimental data.     

Stability of lead(II) complexes of alginate oligomers

The current work reports on the Pb(ll) complexes formed with oligomeric uronic acids (carboxylated saccharide residues) found polymerized in the cell walls and envelopes of algae and bacteria alike. The application of partial acid hydrolysis, size-exclusion chromatography (SEC), 1H NMR, and scanned deposition stripping chronopotentiometry (SSCP) has permitted the determination of stability constants for Pb(II) with both mannuronic (M) and guluronic (G) acid oligomers ranging from the dimer to the pentamer. The determined logarithm of the stability constants range between 4.11 +/- 0.05 and 5.00 +/- 0.04 mol(-1) x dm3 for the eight oligomers studied (pH 6; I = 0.1 mol x dm(-3)). Additional experiments under the same experimental conditions employing galacturonic and glucuronic acid oligomers yielded slightly lower values (2.19 +/- 0.10 to 4.02 +/- 0.07 mol(-1) x dm3) that were expected based on their structure, whereby the monomers which were not included in the alginate oligomer series (unavailable by SEC), yielded the lowest stability constants. This work demonstrates the applicability of the SSCP technique for the determination of stability constants for metal-ligand complexes in which the ligands display relatively low molecular mass. Previous studies on heavy metal interaction with the matrix polysaccharide alginate have largely been restricted to the whole polymer that forms a gel upon binding to network bridging ions such as calcium. The results will be discussed in this context with the emphasis being placed on the relevance of these findings to processes occurring at the biointerface and results from the relevant literature.     

Similarities and differences between alginic acid gels and ionically crosslinked alginate gels

A Na-alginate solution can, under given conditions, undergo a sol–gel transition at pH below pKa for the uronides. These gels are presumably stabilised by intermolecular hydrogen bonds. This paper summarises the effect of alginate variables such as chemical composition and sequence and molecular weight on mechanical properties and network fine structure. Alginic acid gels resemble ionically crosslinked alginate gels in the sense that guluronic acid blocks are the building elements of most importance with respect to gel formation. The main difference is that mannuronic acid blocks, although with less efficiency compared polyguluronate, also support alginic acid gel formation. With the exception strictly alternating alginates made by treating mannuronan with the AlgE4 epimerase, the so-called alternating (random) sequence of the alginate molecule seems to destabilise alginic acid gel formation due to its lack of repeating structure. Small Angle X-ray Scattering (SAXS) reveal a scattering intensity for alginic acid gels being typically and order of magnitude higher than Ca-alginate gels, suggesting junction zones composed of random aggregates with a high degree of multiplicity. A molecular understanding of alginic acid gels is of importance in the general understanding of polyuronides at low pH as well as for the utilisation of alginates in drug delivery systems.     

Adsorbent-adsorbate interactions in the adsorption of Cd(II) and Hg(II) on ozonized activated carbons

The present work investigated the effect of surface oxygenated groups on the adsorption of Cd(II) and Hg(II) by activated carbon. A study was undertaken to determine the adsorption isotherms and the influence of the pH on the adsorption of each metallic ion by a series of ozonized activated carbons. In the case of Cd(II), the adsorption capacity and the affinity of the adsorbent augmented with the increase in acid-oxygenated groups on the activated carbon surface. These results imply that electrostatic-type interactions predominate in this adsorption process. The adsorption observed at solution pH values below the pH(PZC) of the carbon indicates that other forces also participate in this process. Ionic exchange between -C pi-H3O+ interaction protons and Cd(II) ions would account for these findings. In the case of Hg(II), the adsorption diminished with an increase in the degree of oxidation of the activated carbon. The presence of electron-withdrawing groups on oxidized carbons decreases the electronic density of their surface, producing a reduction in the adsorbent-adsorbate dispersion interactions and in their reductive capacity, thus decreasing the adsorption of Hg(II) on the activated carbon. At pH values above 3, the pH had no influence on the adsorption of Hg(II) by the activated carbon, confirming that electrostatic interactions do not have a determinant influence on Hg(II) adsorption.     

Role of transition metals,Fe(II), Cr(II), Pb(II), and Cd(II) in lipid peroxidation

Cod liver oil was oxidized with Fenton-like systems containing transition metals Fe(II), Cr(II), Pb(II), and Cd(II). Malonaldehyde (MA) formed from 10 μl cod liver oil oxidized by a Fenton-like system containing each metal at levels of 0.25, 0.5, 1 and 4 μmol was analyzed by a gas chromatograph equipped with a nitrogen phosphorus detector. The MA production exhibited dose response and the greatest amount (837.0 ± 19.1 nmol) was obtained by the Fe(II) system at the level of l μmol. Generally, higher MA formation is observed in the lower the third ionization potential of the metal. The decreasing order of MA formed in the metal systems at the level of 1 μmol is Fe(II) > Cr(II)(274.1 ± 20.1 nmol) > Pb(II)(150.7 ± 13.0 nmol) > Cd(II)(95.4 ± 6.7 nmol). The amounts of MA formed in Cr(II), Pb(II), and Cd(II) systems were considerably lower than those in the Fe(II) system. The relative formations of MA in the Cr(II) and Pb(II) systems were similar to those in the Fe(II) system. The results suggest that trace amounts of metals contribute oxidative effects to lipid peroxidation followed by various diseases.     

Removal of Cd(II) ions by oxidized coconut coir

Biosorption potential of oxidized coconut coir (OCC) for removal of Cd(II) from aqueous medium at batch and column level was studied. Lignin and cellulose groups of coir were modified to acidic groups. Optimum biosorption was observed at pH 6. Isotherm data revealed that Langmuir gives best fit for experimental results with maximum adsorption capacity of 12.35 mg/g compared to unmodified coconut coir (5.29 mg/g). The column experiments were conducted as a function of flow rate, bed height, and influent Cd(II) concentration. Biosorption has best performance at 10 mg/L inlet concentration, 5 ml/min flow rate and 7 cm bed height.     

Measurement of cadmium(II) and calcium(II) complexation by fulvic acids using 113Cd NMR

Aquatic and terrestrial fulvic acids are environmentally important in pollution transport because they affect the bioavailability and transport of metal ions. The complexation of the metal ions, Cd(II) and Ca(II), with several fulvic acids is examined in this study using 113Cd NMR. Our results indicate that Cd(II) predominately binds to the oxygen containing functional groups of the fulvic acids. A single 113Cd NMR resonance is observed in NMR spectra of Cd(II)-fulvic acid solutions indicating fast exchange between free and complexed cadmium species. An average association equilibrium constant, K(Cd), is determined from NMR spectra measured for the titration of fulvic acid with Cd(II). The K(Cd) values determined for the four fulvic acids studied range between 1.2 and 3.5 x 10(3) M(-1). Competitive binding between Ca(II) and Cd(II) is used to indirectly determine an average association equilibrium constant, K(Ca), for Ca(II) with each fulvic acid. Overall K(Ca) values range from 4.6 to 7.8 x 10(2) M(-1).     

Competitive photocatalytic oxidation of Cu(II)--EDTA and Cd(II)--EDTA with illuminated TiO2

Competitive photocatalytic oxidation (PCO) of mixtures of Cu(II)-EDTA and Cd(II)-EDTA was studied with variation of molar ratio of these two complexes (1 x 10(-4):0, 8 x 10(-5): 2 x 10(-5), 5 x 10-5:5 x 10(-5), 2 x 10-5:8 x 10(-5), 0:1 x 10(-4) M) and in the pH range of 4-8. PCO rates for each compound can be described using a combined aqueous + adsorbed pathway: -dC/dt = k1Caq(1+ k2Caq)+ kadsCads. This expression is valid under both noncompetitive and competitive conditions. Differences in rates under competition result from differences in the partitioning of the two species between the TiO2 surface and the aqueous phase. Total initial complex degradation rates (rTT), obtained by summation of the total destruction rates for Cu(II)-EDTA and Cd(II)-EDTA, were relatively constant at pH 4 and 5 for all ratios. At these pH values, contribution of adsorbed pathways to rTT was important, and rates were similar to those of the aqueous phase pathways. From pH 6 to 8, the degree of adsorption, and thus the adsorbed pathway rate, diminished. Through the adsorbed pathway, no difference in rate constants was found between Cu(II)-EDTA and Cd(II)-EDTA; Cd(II)-EDTA is somewhat more reactive through the aqueous phase pathway.     

Solution speciation controls mercury isotope fractionation of Hg(II) sorption to goethite

The application of Hg isotope signatures as tracers for environmental Hg cycling requires the determination of isotope fractionation factors and mechanisms for individual processes. Here, we investigated Hg isotope fractionation of Hg(II) sorption to goethite in batch systems under different experimental conditions. We observed a mass-dependent enrichment of light Hg isotopes on the goethite surface relative to dissolved Hg (ε(202)Hg of -0.30‰ to -0.44‰) which was independent of the pH, chloride and sulfate concentration, type of surface complex, and equilibration time. Based on previous theoretical equilibrium fractionation factors, we propose that Hg isotope fractionation of Hg(II) sorption to goethite is controlled by an equilibrium isotope effect between Hg(II) solution species, expressed on the mineral surface by the adsorption of the cationic solution species. In contrast, the formation of outer-sphere complexes and subsequent conformation changes to different inner-sphere complexes appeared to have insignificant effects on the observed isotope fractionation. Our findings emphasize the importance of solution speciation in metal isotope sorption studies and suggest that the dissolved Hg(II) pool in soils and sediments, which is the most mobile and bioavailable, should be isotopically heavy, as light Hg isotopes are preferentially sequestered during binding to both mineral phases and natural organic matter.     

Spectroscopic evidence for Ni(II) surface speciation at the iron oxyhydroxides-water interface

Understanding in situ metal surface speciation on mineral surfaces is critical to predicting the natural attenuation of metals in the subsurface environment. In this study, we have demonstrated the novel Ni K-edge X-ray absorption spectroscopy (XAS) measurements needed to understand Ni(ll) surface speciation in three synthetic iron oxyhydroxides (ferrihydrite, goethite, and hematite). The adsorption of Ni gradually increases with increasing pH from 5 to 8, and the adsorption edge appears at near the point of zero salt effect (PZSE) of the solids. The results of XAS analysis indicate four different Ni inner-sphere surface species are present. While total Ni surface species in hematite at pH 6.85 surfaces consist of approximately 63% face-sharing (interatomic distance of Ni-Fe (R(Ni-Fe)) approximately 2.9 A) and approximately 37% corner-sharing (R(Ni-Fe) approximately 4.0 A) surface species on iron octahedra, a combination of two different edge-sharing (between NiO6 and FeO6 octahedra, in chains or in rows) and corner-sharing surface species are observed in goethite and ferrihydrite at pH 5.09-6.89. In ferrihydrite, approximately 70% of surface species are edge-sharing surface species (in chains) (R(Ni-Fe) approximately 3.0 A), followed by approximately 30% of edge-sharing species (in rows) (R(Ni-Fe) approximately 3.2 A) and approximately 3-5% of corner-sharing surface species (R(Ni-Fe) approximately 4.0 A). Goethite contains approximately 54% edge-sharing (R(Ni-Fe) approximately 3.0 A), approximately 26% edge-sharing (R(Ni-Fe) approximately 3.2 A), and 20% corner-sharing surface species. These findings indicate that the reactivity and surface speciation of Ni are sensitive to the crystallinity of iron oxyhydroxides. The spectroscopic evidence for multi-Ni surface speciation should be factored into predictions of the transport of Ni in soil-water environments.     

Comparison of Cd(II), Cu(II), and Pb(II) biouptake by green algae in the presence of humic acid

The present study examines the role of humic acid, as a representative of dissolved organic matter, in Cd(II), Cu(II), and Pb(II) speciation and biouptake by green microalgae. Cellular and intracellular metal fractions were compared in the presence of citric and humic acids. The results demonstrated that Cd and Cu uptake in the presence of 10 mg L(-1) humic acid was consistent with that predicted from measured free metal concentrations, while Pb biouptake was higher. By comparing Cd, Cu, and Pb cellular concentrations in the absence and presence of humic acid, it was found that the influence of the increased negative algal surface charge, resulting from humic acid adsorption, on cellular metal was negligible. Moreover, the experimental results for all three metals were in good agreement with the ternary complex hypothesis. Given that metal has much higher affinity with algal sites than humic acid adsorbed to algae, the contribution of the ternary complex to metal bioavailability was negligible in the case of Cd (II) and Cu (II). In contrast, the ternary complex contributed to over 90% of total cellular metal for Pb(II), due to the comparable affinity of Pb to algal sites in comparison with humic acid adsorbed to algae. Therefore, the extension of the biotic ligand model by including the formation of the ternary complex between the metal, humic acid, and algal surface would help to avoid underestimation of Pb biouptake in the presence of humic substances by green algae Chlorella kesslerii.     

Monitoring of Zn(II), Cd(II), Pb(II) and Cu(II) During Refining of Some Vegetable Oils Using Differential Pulse Anodic Stripping Voltammetry

In this study, a novel approach has been developed using differential pulse anodic stripping voltammetry (DPASV) for the simultaneous determination of Zn(II), Cd(II), Pb(II) and Cu(II) in edible oils at hanging mercury drop electrode. The microwave digestion of oil samples was carried out with concentrated HNO₃ and H₂O₂. KNO₃ was used as a supporting electrolyte. The experimental conditions optimized such as deposition time, stirring rate and size of mercury drop were 300 s, 600 rpm and 10 mm², respectively. The method was applied to quantify Zn(II), Cd(II), Pb(II) and Cu(II) in crude and refined hazelnut, corn, sunflower and olive oils. During refining of different vegetable oils, the removal efficiencies of Zn(II), Cd(II), Pb(II) and Cu(II) were calculated as 98.20–99.91, 98.50–99.90, 95.26–99.76 and 95.93–99.92 %, respectively. The limits of detection for Zn(II), Cd(II), Pb(II) and Cu(II) were 2.1?×?10^?8, 8.7?×?10^?10, 7.1?×?10^?9 and 3.4?×?10^?9 and the limits of quantification were 6.8?×?10^?8, 2.9?×?10^?9, 2.3?×?10^?8 and 1.1?×?10^?8 M with linear regression coefficients (R ²) of 0.9930, 0.9928, 0.9893 and 0.9931, respectively. It was observed that the above metals in crude and refined vegetable oils could be determined simultaneously by the DPASV method.     

XAS study of iron and arsenic speciation during Fe(II) oxidation in the presence of As(III)

The speciation of As and Fe was studied during the oxidation of Fe(II)-As(III) solutions by combining XAS analysis at both the Fe and As K-edges. Fe(II) and As(III) were first hydrolyzed to pH 7 under anoxic conditions; the precipitate was then allowed to oxidize in ambient air for 33 h under vigorous stirring. EXAFS analysis at the As K-edge shows clear evidence of formation of inner-sphere complexes between As(III) and Fe(II), i.e., before any oxidation. Inner-sphere complexes were also observed when Fe became sufficiently oxidized, in the form of edge-sharing and double-corner linkages between AsIIIO3 pyramids and FeIIIO6 octahedra. XAS analyses at the Fe K-edge reveal that the presence of As(III) in the solution limits the polymerization of Fe(II) and the formation of green rust and inhibits the formation of goethite and lepidocrocite. Indeed, As(III) accelerates the Fe(II) oxidation kinetics and leads to the formation of nanosized Fe-As subunits of amorphous aggregates. These observations, rather than a presumed weaker affinity of As(III) for iron oxyhydroxides, might explain why As(III) is more difficult to remove than As(V) by aerating reducing groundwater.     

Kinetic speciation of Co(II), Ni(II), Cu(II), and Zn(II) in model solutions and freshwaters: lability and the d electron configuration

The kinetic speciation of Co(II), Ni(II), Cu(II), and Zn(II) in model solutions of a well-characterized fulvic acid (Laurentian fulvic acid), freshwater samples from the Rideau River (Ottawa, Ontario), and freshwater samples from the Sudbury (Ontario) area were investigated by the competing ligand exchange method using Chelex 100 as the competing ligand and by inductively coupled plasma-mass spectrometry to measure the dissociation kinetics. The metal species were quantitatively characterized by the rate coefficient for the first-order dissociation of metal complex to free metal ion. This technique can be applied to almost all elements and represents an important advance in our ability to investigate the kinetic availability of metal species in the freshwater environment. The order of the lability of the metal complexes, Co(II) > Ni(II) > Cu(II) < Zn(II), follows the reverse order of the ligand field stabilization energy with the exception of Cu(II); the behavior of Cu(II) is also due to the Jahn-Teller effect, which shortens the equatorial bonds and lengthens the axial bonds of a tetragonally distorted Cu(II)-L6 complex. This study has demonstrated a relationship between the lability of metal-DOM complexes of the 3d transition metals in freshwaters and their d electron configuration. This is the first time that the importance of the d electron configuration on the lability of metal complexes in the freshwater environment has been demonstrated. The slow complexation kinetics of both Ni(II) and Cu(II) suggestthatthe usual equilibrium assumption for freshwaters may be invalid.     

Application of an in situ bismuth-coated glassy carbon electrode for electroanalytical determination of Cd (II) and Pb (II) in Korean polished rices

Rapid on-site monitoring of heavy metals in plants is necessary to minimize the exposure of humans to the contaminated food. The potential of using anodic stripping voltammetry (ASV) for simultaneous determination of cadimium (Cd) and lead (Pb) in rice was evaluated. Eight different Korean polished rices were digested using concentrated HNO₃ solution in a microwave oven. The peak currents measured with an in situ bismuth-coated electrode were linearly proportional to Cd and Pb concentrations (1.5 to 200 ppb) in 0.1 M HNO₃ (R² ⩾0.98). The electrode was feasible for measuring Pb in acid digests of the tested polished rices with measurement errors of <16.3 ppb. However, even though there was acceptable agreement in 4 of the 8 samples tested for Cd between the ASV and inductively coupled argon plasma (ICP) methods, further studies would be needed to improve the predictive capability of the electrode while reducing the variability in measurement and lowering the detection limit.     

Determination of Trace Amounts of Cd(II), Cu(II), and Ni(II) in Food Samples Using a Novel Functionalized Magnetic Nanosorbent

This work describes a novel sorbent based on functionalization of magnetic nanoparticles by 2-aminobenzothiazole and its application in the extraction and preconcentration of trace amount of Cd(II), Cu(II), and Ni(II) ions. This nanosorbent was characterized by Fourier transfer infrared spectroscopy, thermal analysis, X-ray powder diffraction, elemental analysis, and scanning electron microscopy. The effects of various factors such as pH value, sorption time, sorbent dosage, type, volume, and concentration of the eluent as well as the elution time were investigated. Following the sorption and the elution of target analytes, the Cd(II), Cu(II), and Ni(II) ions were determined by flame atomic absorption spectrometry. Under the optimal conditions, the limits of detection (LODs) were 0.03, 0.009, and 0.1 μg L^?1 for Cd(II), Cu(II), and Ni(II), respectively. Linearity was within the range of 0.1–75 ng mL^?1 for Cd(II), 0.03–50 ng mL^?1 for Cu(II), and 0.5–100 ng mL^?1 for Ni(II) in the initial solution with r ² values greater than 0.9978. The relative standard deviations of the method were less than 8.4 %. The preconcentration factor of the method was 277. The sorption capacity of this new sorbent was 65, 78, and 49 mg g^?1 for Cd(II), Cu(II), and Ni(II), respectively. The proposed method was validated using two certified reference materials (LGC 6010 hard drinking water and NIST SRM 1515 apple leaves) in order to exhibit its applicability. Ultimately, this method was applied to the rapid extraction of the trace quantities of Cd(II), Cu(II), and Ni(II) ions in different food samples, and satisfactory results were obtained.     

Sorption studies of Zn(II)- and Cd(II)ions from aqueous solution on treated sawdust of sissoo wood

Sawdust, an inexpensive material, has been utilized as an adsorbent for the removal of Zn(II)- and Cd(II)ions from aqueous solution. The effects of time of equilibrium, pH, temperatures and dosage of the adsorbent on the removal of Zn(II)- and Cd(II)ions have been studied. The equilibrium nature of Zn(II)- and Cd(II)ions adsorption at different temperatures (25–60 °C) has been studied. The percent adsorption of Zn(II)- and Cd(II)ions increased with an increase in pH, temperature and dosage of treated sawdust. The applicability of the Langmuir isotherm suggests the formation of monolayer coverage of Zn(II)- and Cd(II)ions at the surface of the adsorbent. The thermodynamic parameters like free energy, enthalpy and entropy changes for the adsorption of Zn(II)- and Cd(II)ions has also been computed and discussed. The heat of adsorption [ΔH=17.706 kJmole^-1 for Zn(II) and ΔH=16.949 kJmole^-1 for Cd(II)] implied that the adsorption was an endothermic adsorption. The sawdust was found to be a metal adsorbent as effective as activated carbon.     

Determination of Cd (II), Cu (II), and Pb (II) in Some Foods by FAAS after Preconcentration on Modified Silica Gels with Thiourea

Abstract: This study describes preconcentration method for determination of Cd (II), Cu (II), and Pb (II) in some food samples. Silica gel was modified with thiourea and characterized by IR and C, H, N, S elemental analysis. Modified silica gel was used as a solid phase extraction for determination of Cd (II), Cu (II) and Pb (II) in tuna fish, biscuit, black tea, rice, ka?ar cheese, honey, tomato paste, and margarine samples. The analytical conditions including eluent type, pH of sample solutions, flow rates of sample and eluent solutions, etc. were optimized. The influence of the matrix ions on the involvement of the Cd (II), Cu (II), and Pb (II) were also studied. GBW 07605 tea standard reference material was used for validation of method. The method was successfully applied for the determination of Cd (II), Cu (II), and Pb (II) ions in food samples. The detection limits were in the range of 0.81 μg/L, 0.38 μg/L, and 0.57 μg/L for cadmium, copper and lead, respectively. The relative standard deviations of the procedure were below 10%. The sorption capacities were found as 92 μmol/L Cd (II), 286 μmol/L Cu (II), and 121 μmol/L Pb (II).