A.M. Mota

Complexation study of humic acids with cadmium (II) and lead (II)

A set of experiments on the complexation of cadmium and lead with humic acids were performed at pH 5.0 using normal- and reverse-pulse polarography and d.c. and differential-pulse anodic stripping voltammetry. The concentration of deprotonated groups at pH 7.0, the average stability parameter, K, the differential equilibrium function, f(KDEF), and a parameter, Γ, proportional to the metal buffer capacity of the system and related to the heterogeneity of the ligand, were calculated. It was concluded that the complexes are labile within the experimental error for the time scale of the techniques; that lead complexes are more stable than cadmium complexes and that the ligand has a more heterogeneous behaviour with respect to lead than to cadmium.

Dynamic properties of humic matter by dynamic light scattering and voltammetry

The diffusion coefficients of humic matter samples (Purified Peat (PP) Moss and Fluka) in solution were obtained by both dynamic light scattering (DLS) and voltammetric methods. The diffusion coefficients from voltammetric measurements are greater than those determined by DLS, due to the different way each technique responds to the size distribution of polydisperse samples. Large aggregates are present in both PP humic acid (HA) and Fluka samples (diameters larger than 30 nm), their size depending on the origin and/or preparation procedure of the sample, as well as on the experimental conditions of pH and ionic strength. The influence of ionic strength on the diffusion coefficient of PPHA was analysed. Similar results were obtained by DLS and voltammetry in samples with low polydispersity up to ionic strength of 0.1 M. For higher ionic strengths aggregation may occur, leading to an increase in the apparent molecular size. The influence of pH on the diffusion coefficient of PPHA shows one population formed by large aggregates (185 nm diameter) at pH 5 in the absence of added salt. When pH is lowered below 2.5, coagulation occurs quickly leading to the precipitation of PPHA. Some irreversible disagreggation (120 nm diameter) is promoted by pH increase up to 10. PPHA samples, initially set to pH 10 and then adjusted to pH between 3 and 5, present somehow different characteristics from samples directly set to the required pH, which stresses the influence of the preparation method on final sample properties.

Adsorption of fulvic-like organic ligands and their Cd and Pb complexes at a Mercury electrode

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.

Ultrasonic-assisted enzymatic digestion (USAED) for total elemental determination and elemental speciation: A tutorial

Due to its potential as sample treatment for Analytical Chemistry, the Ultrasonic-Assisted Enzymatic Digestion (USAED) for total elemental determination and elemental speciation is described under the most recent achievements published in literature, focusing on the variables that critically affect the performance of this relatively new sample treatment, such as the type of enzymes or the type of ultrasonic system used for the acceleration of the solid-liquid extraction process. Moreover, analytical chemists are aware of common errors produced in data interpretation concerning USAED. In addition, a guide for the rapid application of this methodology is also provided along with detailed explanations. Finally, future trends regarding USAED are also given and commented.

Determination of Cd and Pb in biological reference materials by electrothermal atomic absorption spectrometry: A comparison of three ultrasonic-based sample treatment procedures

Three different ultrasonic-based sample treatment approaches, the automated ultrasonic slurry sampling, the ultrasonic assisted acid solid-liquid extraction (ASLE) and the enzymatic probe sonication (EPS) were compared and discussed for the determination of Cd and Pb by ET-AAS in biological reference materials. The sample mass chosen to perform the analysis was 10mg and the liquid volume was 1ml of nitric acid 1M. The best results were obtained with the slurry procedure with which it was possible accurate and precise determination of the Cd and Pb content in four of the five reference materials studied. Optimum performance (total metal extraction) of ASLE assisted by ultrasound for Cd was only achieved in two of the four materials assessed whereas total Pb recovery was only possible in three of the five samples. Total extraction with the enzymatic probe sonication was only obtained for Cd in oyster tissue. Neither ASLE nor EPS were able to extract Cd or Pb from spruce needles. Pb concentration obtained after EPS was found to be highly dependent from sample centrifugation speed and time.

Influence of metal concentration at the electrode surface in differential pulse anodic stripping voltammetry in the presence of humic matter

Abstract 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 i calculated from the saturation of th 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.

Tandem focused ultrasound (TFU) combined with fast furnace analysis as an improved methodology for total mercury determination in human urine by electrothermal-atomic absorption spectrometry.

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 (6mm 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 (3mm 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 (8.5ml). Low volume and low concentration reagents are used. The sample treatment is rapid (less than 3min 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 0.9mugl(-1). The relative standard deviation of aqueous standards (n=10) was 4% for a concentration of 100mugl(-1) and 5% for a concentration of 400mugl(-1). Recoveries from spiked urine with inorganic mercury, methyl-mercury, phenyl-mercury and diphenyl-mercury ranged from 86 to 98%.

Advanced oxidation processes for degradation of organomercurials: determination of inorganic and total mercury in urine by FI-CV-AAS

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.

ON LABILITY OF CHEMICALLY HETEROGENEOUS SYSTEMS. COMPLEXES BETWEEN TRACE METALS AND HUMIC MATTER

Abstract Complexation of trace metals by humic matter is an important issue, as it determines to a large extent their bioavailability, toxicity and mobility. In a complexing medium the supply of metal species towards an interface is always related to the kinetics of the complex association/dissociation reactions, irrespective of whether one is dealing with living organisms in natural media, or with an electrode in voltammetry, or with a membrane in a separation process. In voltammetric studies lability criteria have been developed for steady-state and dynamic experiments, for homogeneous and chemically heterogeneous systems. In this work these criteria are reformulated on a more rigorous level and used to investigate the lability of cadmium and lead complexes in different samples of fulvic and humic matter (chemically heterogeneous systems). It is explained how lability varies with the stability of the chemically heterogeneous complex. The discussion further shows that, amongst other things, an increasing degree of chemical heterogeneity leads to a decrease of the lability.

Voltammetric analysis of the competition between calcium and heavy metals for complexation by humic material

Abstract The competition between calcium and the heavy metals zinc(II) and cadmium(II) in their interaction with humic substances has been studied by differential pulse anodic stripping voltammetry at constant ionic strength. Two experimental methodologies were employed: (1) addition of humic acid to a solution of heavy metal; and (2) addition of heavy metals to a humic acid solution, both using various concentrations of calcium. The former method leads to complexation curves from which an overall mean stability for the heavy metal complex can be obtained. In the latter procedure, mean stability values are obtained at each titration point from the fractions of free and bound heavy metal ions. In the presence of competing ions like calcium, the association of heavy metals with the humic material can be well understood by incorporation of an additional equilibrium reaction for calcium with the humate polyanion. Hence, we are able to obtain information successfully on heavy metal speciation in more complex systems like natural waters, which contain competing cations such as calcium.