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The influence of cadmium (Cd) on internal and external defense mechanisms of Sorghum bicolor (L.) Moench. x Sorghum sudanense was studied by electrochemical methods to infer the type of detoxification processes developed by the plant under environmental stress conditions. Hydroponic experiments with sorghum were conducted in nutrient solutions in the absence and presence of 0.1 mg Cd L−1. Plant exposure to Cd stimulated the release of root exudates with metal-binding affinity. However, their presence should not affect significantly the bioavailability of Cd, since the complex was dissociated within a very short period (≤ 50 milliseconds) in response to the consumption of the free metal ion at the root interface. The presence of Cd in the solution also stimulated a very significant increase of thiolic groups inside the plant, even at the very low concentration of 0.1 mg Cd L−1. These results suggest that the main defense mechanism developed by sorghum against metal toxicity is an internal process,...

The aim of this work is to study the influence of the Tagus estuarine sediment on lead speciation in the overlying natural water. The water sample in the presence of the sediment was contaminated three times with Pb(II) in a laboratory experiment. In different periods of time, at 1–7 days after each contamination, small volumes of water were titrated with lead. The titration was followed by anodic stripping voltammetry in differential pulse mode. Before and after contamination systematic analysis of the voltammetric parameters (peak current, peak potential and peak width) were carried out to get a clear picture of Pb(II) complexation in the soluble fraction in contact with the sediment. Two main types of organic ligands, macromolecular ligands and small compounds, were detected before contamination. Both of them form labile complexes (degree of lability within the timescale of some milliseconds). The small compounds, with a diffusion coefficient similar to that of the free metal ion, present a homogeneous behaviour in terms of Pb complexation. On the other hand macromolecular ligands, with a diffusion coefficient of 1.2×10 −6 cm 2 s −1 , can be described by two different binding groups, which might be of phenolic and carboxylic type as presented by humic matter. The sediment eliminated lead contamination (10 −6 mol dm −3 was the maximum concentration added) from 12 dm 3 of water (surface of 8 dm 2 ) within 2 days. It was also found that the sediment released organic ligands responsible for both labile and inert Pb complexes “seen” by voltammetry. The release of organic ligands that decreases the bioavailability of Pb(II) was clearly detected 1 week after contamination. Therefore, the sediment acts as a buffer for lead through two mechanisms against lead contamination: removing lead ions from the solution and releasing organic ligands to the water column. In a short period of time the sediment responds as a self-cleaning system for Pb(II) contamination in the estuarine water, which may have a very important influence in environmental pollution.

Hydroponic experiments with sorghum were carried out in nutrient solution medium, under controlled environmental conditions for different periods of time. Cadmium (Cd ) was added at a final concentration of 0.1, 1 and 10 mg dm−3. The effect of time and cadmium concentration on the content of thiolic groups (TSH) in the “cell sap” of sorghum tissues are discussed in relation to plant resistance/tolerance to metal stress. The increase in Cd level in the nutrient solution was associated with a higher total Cd accumulation in the plant (root and shoot), which produced a significant enhancement on the thiolic groups content. Furthermore, higher Cd concentrations in plant tissues induced not only an increase in sulphur-rich compounds but also enhanced the binding capacity of these compounds. This work is under the Research Project PRAXIS/P/QUI/10034/98 of FCT. It has also the financial support supplied by the Portuguese Education Ministry through the PRODEP II – PhD fellowship. info:eu-repo/semantics/publishedVersion

A new sample preparation procedure based on tandem (that is, different diameter probe sonicators used in the same sample treatment) focused ultrasound (TFU) for mercury separation, preconcentration and back-extraction in aqueous solution from human urine has been developed. The urine is first oxidized with KMnO 4 /HCl/focused ultrasound (6 mm probe). Secondly, the mercury is extracted and preconcentrated with dithizone and cyclohexane. Finally, the mercury is back-extracted and preconcentrated again with the aid of focused ultrasound (3 mm probe). The procedure allows determining mercury by electrothermal atomic absorption spectrometry with fast furnace analysis and calibration against aqueous standards. Matrix modification is provided by the chemicals used in the sample treatment. The procedure is accomplished with low sample volume (8.5 ml). Low volume and low concentration reagents are used. The sample treatment is rapid (less than 3 min per sample) and avoids the use of organic phase in the graphite furnace. The preconcentration factor used in this work was 14. The limit of detection and the limit of quantification in urine were, respectively, 0.27 and 0.9 μg l −1 . The relative standard deviation of aqueous standards ( n =10) was 4% for a concentration of 100 μg l −1 and 5% for a concentration of 400 μg l −1 . Recoveries from spiked urine with inorganic mercury, methyl-mercury, phenyl-mercury and diphenyl-mercury ranged from 86 to 98%.

This article describes an experiment, carried out under controlled environment conditions, to investigate the effects of a fulvic acid fraction of soil organic matter on growth, cadmium (Cd) uptake and redistribution by sorghum. In addition the uptake of copper (Cu), zinc (Zn) and iron (Fe) was also determined. Sorghum was grown in nutrient solutions with 0, 0.1, 1 and 10 mg Cd dm(-3), in the absence and presence of organic matter (32 mg C dm(-3)), for various periods up to 20 days. A decrease in sorghum biomass due to Cd toxicity was observed at 10 mg Cd dm(-3), but for concentrations of 0.1 and 1 mg Cd dm(-3) the biomass was increased compared with control, without visual toxicity symptoms. The presence of organic matter (OM) further increased biomass production. Cadmium was mainly retained in sorghum roots, as usually found in tolerant plants, but Cd accumulation in sorghum was greater than in other Gramineae, or even more tolerant plants such as lettuce. The presence of OM decreased the bioavailability of Cd that was partially retained in solution by the OM ligands. However, OM promoted the translocation of Cd to shoots, an effect that may pose a risk to public health because plant-animal transfer of Cd could be enhanced. The presence of OM decreased the uptake of Cu, Zn and Fe. The presence (vs. absence) of 0.1 mg Cd dm(-3) enhanced the uptake of Fe, both in the absence and presence of OM.

International audience; The influence of metal concentration in bulk solution and of deposition time on voltammograms obtained by differential pulse anodic stripping voltammetry in the presence of humic matter is discussed based on the slow diffusion of complexes, adsorption of the ligand on the electrode and ligand to metal concentration ratio during the stripping step. The conditional stability parameters at pH 5, in the bulk solution and in the absorbed layer, are determined. The concentration of adsorbed sites is calculated from the saturation of the sites at the electrode surface with metal ions, estimated either from the critical deposition time or from the height of the shoulder presented in the voltammogram.

A new methodology, based on advanced oxidation processes, for the determination of inorganic and total mercury in human urine is proposed. Ultrasound in conjunction with ozone (sonozone) was used to avoid urine dilution for inorganic mercury determination. A new oxidation method in urine based on KMnO4–HCl–focused ultrasound is proposed for conversion of methylmercury into inorganic mercury and subsequent determination by flow injection-cold vapour atomic absorption spectrometry (FI-CV-AAS). The methodology was applied to the determination of inorganic and total mercury in spiked urine from different non-exposed volunteers.

The uranyl complexes of n-propanediaminetetra-acetic acid, n-butanediaminetetra-acetic acid and n-hexanediaminetetra-acetic acid have been studied by potentiometry, with computer evaluation of the titration data by the MINIQUAD program. Stability constants of the 1:1 and 2:1 metal:ligand chelates have been determined as well as the respective hydrolysis and polymerization constants at 25° in 0.10 M and 1.00 M KNO 3 . The influence of the length of the alkane chain of the ligands on the complexes formed is discussed.

The nature of the EDTA complex of uranium(VI) is discussed, and it is concluded that there is no need to postulate stabilization of the complex by hydrogen-bonding between a protonated nitrogen atom and the uranyl ion.

The uranyl and vanadyl complexes formed with salicylic, phthalic and 3,4-dihydroxybenzoic acids have been studied by potentiometry in order to determine the stability constants of the M(m)L(n) species formed in solution, and the constants for the hydrolysis and polymeric complexes, at 25.0 degrees , in 0.10, 0.40 and 0.70M sodium perchlorate. MINIQUAD was used to process the data to find the best models for the species in solution, and calculate the formation constants. The uranyl-salicylic acid system was also studied by spectrophotometry and the program SQUAD used to process the data obtained. The best models for these systems show that co-ordination of the uranyl ion by carboxylate groups is easier than for the vanadyl ion, whereas the vanadyl ion seems to form more stable complexes with phenolate groups. Both oxo-cations seem to tend to hydrolyse rather than form complexes when the L:M ratios are greater than unity. Although the change in the constants with ionic strength is small, the activity coefficients of the salicylate and phthalate species have been calculated at ionic strengths 0.40 and 0.70M, along with the interaction parameters with Na(+), from the stability constants found for the species ML and H(2)L, according to the Brønsted-Guggenheim expression.

Benzyliminodiacetic acid, pyridine-2-carboxylic acid and pyridine 2,6-dicarboxylic acid were used as model compounds of the components of natural pedogenic fulvic acids to study the most important factors related to their adsorbing and complexing properties which may affect the behaviour of their complexes at a mercury electrode. In the absence of metal ions, their adsorption properties were studied by alternating current polarography, at different pHs and ligand concentrations. The adsorption of their Pb and Cd complexes was studied by normal pulse polarography, in a low concentration range. Comparison of all the experimental data shows that adsorption of the free ligand and the complexes is always primarily due to the hydrophobic part of the ligand which tends to accumulate on the Hg surface. Any electrically charged group of the molecule (either positive or negative) increases its hydrophilicity and decreases its adsorbability. However, this effect plays a significant role only when the distance between the charged and hydrophobic groups is small. For instance, it was found to be much less important with the benzyliminodiacetic ligand than with the other two ligands. For all the ligands and complexes, 10−6M is the approximate limit below which the adsorption is small in the normal time-scale of a dropping mercury electrode, due to the small adsorption equilibrium constants of free or protonated ligands and to the slowness of the overall adsorption rate of the complexes. These results are compared to those obtained with pedogenic fulvic compounds.

The adsorption on a mercury/water interface of a sample of humic acids from an Irish peat has been analyzed by a.c. voltammetry, using a hanging mercury drop electrode in an unstirred and/or stirred medium, at different pH values (pH = 2.5, 5 and 8) and two potential values (E = −0.5 and −0.9 V), in order to have a better insight into the kinetics and equilibrium adsorption which may drastically affect the voltammetric signal in trace metal speciation of natural waters. The results at pH 2.5 present a slower kinetic of adsorption but a higher coverage degree at the equilibrium when compared with pH 5 and/or 8 due to the charge of the molecules. At −0.5 V, close to the potential of zero charge, the kinetics of adsorption is discussed in terms of a diffusion mechanism, but at −0.9 V the experimental curves clearly show that the global adsorption process cannot be explained by a pure diffusion mechanism based on Frumkin isotherm.

The heterogeneity of natural organic ligands in the presence of a metal ion affects its concentration profile in the diffusion layer. In fact, owing to heterogeneity, the ratio of complexed metal to free metal increases in the diffusion layer with proximity to the electrode surface. The diffusing species are then decelerated because the diffusion coefficient of the macromolecular complex is lower than that of the metal ion, and so the total amount of reduced metal is decreased compared with that obtained in a homogeneous system with the same bulk speciation. Therefore, the voltammetric current, which is directly related to the concentration profile in the diffusion layer, is affected by heterogeneity. In the work reported in this paper the concentration profile in the diffusion layer was determined on the basis of the diffusion process and a heterogeneity model describing the concentration of the different complexing sites and their complexing affinity for the metal. The mean stability parameters for the complexation of natural organic ligands with a metal ion were determined from this profile, simulating the value that should be determined from voltammetric currents. The difference between the simulated value and the mean stability parameter related to bulk speciation was analysed.