Danilo Rudello

Trace Iron Determination by Cyclic and Multiple Square-Wave Voltammetry at Nafion Coated Electrodes. Applicationto Pore-Water Analysis

Nafion coated glassy carbon electrodes (NCE) are employed for preconcentrating and detecting Fe2+ and Fe3+ cations from aqueous solutions. The influence of the supporting electrolyte composition and of the redox state on the analyte partitioning within the Nafion coating are examined. By using cyclic voltammetry, the ion-exchange voltammetric determination of iron in the μM concentration range is achieved from voltammetric peak currents relevant to the reversible redox process: Fe3++e⇋Fe2+. Depending on the starting potential of the voltammetric scan, all the iron is initially converted to the Fe(III) or Fe(II) redox state, so that, from the voltammogram, the overall concentration of iron is always obtained. However, the simple measurement of the open circuit potential at the NCE before starting the scan allows one to get information on the ratio between Fe(III) and Fe(II) incorporated in the Nafion coating. From relevant ion-exchange distribution coefficients, open circuit potential values can be related to the Fe(III)/Fe(II) concentration ratio in the sample. The use at the NCE of a new electroanalytical technique named multiple square-wave voltammetry in the double differential mode allows the significative lowering of detection limits pushing the operative range of the method in the nM range. Application to analysis of Fe(II) in the pore-waters of sediments of the lagoon of Venice (Italy) is presented.

Seasonal and depth variability of reduced sulphur species and metal ions in mud-flat pore-waters of the Venice lagoon.

An in-situ sampler was employed to monitor seasonal changes in the composition of pore-waters at different depths in a mud-flat located in the central part of the Lagoon of Venice, in proximity of the industrial area of Porto Marghera. The use of voltammetric methods of analysis allowed the simultaneous determination of concentrations of reduced S( − II), thiosulphate, Fe(II) and Mn(II) as well as zinc, lead, cadmium and copper ions. Other parameters such as the concentration of sulphate, Eh, pH and conductivity were also measured. Seasonal increases of S( − II) and Fe(II) concentrations in summer and autumn samples were observed, with the highest concentrations located at about 25 and 55 cm depth, respectively. An opposite seasonal dependence was observed for Mn(II) and S2O32− concentrations, which displayed higher values in winter: the highest concentrations were observed at a depth of 45 cm for Mn(II) and of 65 cm for thiosulphate. The concentrations of Cu, Cd, Pb and Zn ions showed a less evident seasonal variability. Top and bottom samples revealed high concentration values for zinc and copper ions, with the highest values in the 1,000–1,600 nM and 100–300 nM ranges, respectively. Cadmium and lead ions were present at lower concentrations, always less than 70 nM and 30 nM, respectively.

A new device for in-situ pore-water sampling

A new prototype pore-water sampler is proposed which allows temporal sampling of pore-water from intertidal sediments. The system consists of a nylon device provided with regularly spaced chambers, double filtering ports and pipes for in-situ sample recovering. The field use of this apparatus for determining sulfur species in pore-waters is described; a satisfactory agreement between data obtained by using the proposed in-situ sampler and by core-squeezing in proper experimental conditions is found. Finally, advantages and differences with respect to other in-situ samplers are critically evaluated.

Sulfide as a confounding factor in toxicity tests with the sea urchin Paracentrotus lividus: Comparisons with chemical analysis data

Sperm cell and embryo toxicity tests with the sea urchin Paracentrotus lividus were performed to assess the toxicity of sulfide, which is considered a confounding factor in toxicity tests. For improved information on the sensitivity of these methods to sulfide, experiments were performed in the same aerobic conditions used for testing environmental samples, with sulfide concentrations being monitored at the same time by cathodic stripping voltammetry. New toxicity data for sulfide expressed as median effective concentration (EC50) and no-observed-effect concentration (NOEC) are reported. The EC50 value for the embryo toxicity test (total sulfide at 0.43 mg/L) was three times lower than for the sperm cell test (total sulfide at 1.20 mg/L), and the NOEC values were similar (on the order of total sulfide at 10(-1) mg/L) for both tests. The decrease in sulfide concentration during the bioassay as a consequence of possible oxidation of sulfide by dissolved oxygen was determined by voltammetric analysis, indicating a half-life of about 50 min in the presence of gametes. To check the influence of sulfide concentrations on toxicity effects in real samples, toxicity (with the sperm cell toxicity test) and chemical analyses also were performed in pore-water samples collected with an in situ sampler in sediments of the Lagoon of Venice (Italy). A highly positive correlation between increased acute toxicity and increased sulfide concentration was found. Examination of data revealed that sulfide is a real confounding factor in toxicity testing in anoxic environmental samples containing concentrations above the sensitivity limit of the method.