Giancarla Alberti

Analytical methods for determination of free metal ion concentration, labile species fraction and metal complexation capacity of environmental waters: a review.

Different experimental approaches have been suggested in the last few decades to determine metal species in complex matrices of unknown composition as environmental waters. The methods are mainly focused on the determination of single species or groups of species. The more recent developments in trace elements speciation are reviewed focusing on methods for labile and free metal determination. Electrochemical procedures with low detection limit as anodic stripping voltammetry (ASV) and the competing ligand exchange with adsorption cathodic stripping voltammetry (CLE-AdCSV) have been widely employed in metal distribution studies in natural waters. Other electrochemical methods such as stripping chronopotentiometry and AGNES seem to be promising to evaluate the free metal concentration at the low levels of environmental samples. Separation techniques based on ion exchange (IE) and complexing resins (CR), and micro separation methods as the Donnan membrane technique (DMT), diffusive gradients in thin-film gels (DGT) and the permeation liquid membrane (PLM), are among the non-electrochemical methods largely used in this field and reviewed in the text. Under appropriate conditions such techniques make possible the evaluation of free metal ion concentration.

AGNES: A technique for determining the concentration of free metal ions. The case of Zn(II) in coastal Mediterranean seawater

Absence of Gradients and Nernstian Equilibrium Stripping (AGNES) is a recently suggested electroanalytical technique designed for the determination of the free concentration of heavy metals (such as Zn, Cd or Pb) which is here developed and applied to seawater samples. A key improvement for the implementation of AGNES with complex matrices is the development of a new blank, called the shifted blank (presented in this work for the first time), which can be applied to the same sample where the measurement is intended. The careful selection of the required parameters for the determination of the free Zn concentration (or activity) at the nanomolar level is described in detail. The methodology has been validated with a synthetic solution containing Zn and nitrilotriacetic acid (NTA) and then applied, as a first case, to two coastal seawater samples taken close to Barcelona and Tarragona (Catalonia, North-Eastern Spain) finding values in the range of 1-3nM, representing around 25% of total Zn. This technique can, in the near future, be crucial in helping to elucidate the role of the free zinc(II) concentration in natural waters.

Separation of vanadium(V) and -(IV) by sorption on an iminodiacetic chelating resin

Abstract The distribution of vanadium(IV) and vanadium(V) between an aqueous solution and the chelating resin Chelex 100 was investigated on the basis of the Gibbs-Donnan model. As expected from this model the thermodynamic equilibrium constant in aqueous solution and in the resin phase were found to be equal. The following values for the complexation constant and for the exchange coefficients in NaNO 3 0.1 M were found: log K = − 1.93 and log β 10 ex = −1.89 for vanadium(IV) and log K = 0.81 and log β 11 ex = 0.70 for vanadium(V). A complex VO 2 L (log β 10 ex = −1.48) was found to be formed inside Chelex 100 at pH higher than 3.5. The competition by the hydrolysis reactions in aqueous solution was quantitatively considered to explain the sorption curves of vanadium(V) in the acidity range up to pH 9. As vanadium(IV) is not desorbed at pH higher than 8 a separation method was proposed, based on a batch technique. It consists of sorbing both vanadium(V) and vanadium(IV) at a pH of about 4.5, and separating them by stripping vanadium(V) at a pH of about 10 and then vanadium(IV) at pH about 0.8. A preconcentration factor of 40 could be obtained in this way. The model was also used to predict conditions for separation by a column method. The interference of fulvic acid on the oxidation state of vanadium and on its sorption on Chelex 100 was examined.

Determination of the total concentration and speciation of Al(III) in tea infusions.

The aluminium species in different tea infusions were investigated, by determining their stability constants and concentration. This was done for some particular samples using a simple experimental method based on the sorption of aluminium on the strongly sorbing resin Chelex 100, by a batch procedure. From the thermodynamic information obtained it is possible to calculate the concentration of the different species, and in particular that of the free metal ion, which is very important for evaluating the adsorption of aluminium on biological membranes. It was found that aluminium in the tea infusions here considered is present at high total concentration, approximately 0.1 mM, but mainly linked to strong complexes, for instance with side reaction coefficient higher than 10(5.11) at pH 3.95 in one case (tea 1). This could be the reason for the low toxicity of aluminium in tea. These strong complexes were not dissociated even in the presence of Chelex 100. In this case only a limiting value of the reaction coefficient could be evaluated. The presence of the very strong complexes was found in all the tea sample here considered. In two of the considered samples (one black and one green tea) a part of Al(III) was linked to less strong complexes, for example with a reaction coefficient 10(4.14) (tea 2, pH 4.20). The presence in the considered tea infusions of other substances able to complex aluminium was also detected, by the well known ligand titration procedure, at concentration ranging from 0.65 to 3.37 mM in three tea infusions, and at somewhat higher concentration in the case of the ready drink, which was also considered for comparison.

Chromium release from new stainless steel, recycled and nickel-free orthodontic brackets.

OBJECTIVE To test the hypothesis that there is no difference in the amounts of chromium released from new stainless steel brackets, recycled stainless steel brackets, and nickel-free (Ni-free) orthodontic brackets. MATERIALS AND METHODS This in vitro study was performed using a classic batch procedure by immersion of the samples in artificial saliva at various acidities (pH 4.2, 6.5, and 7.6) over an extended time interval (t(1) = 0.25 h, t(2) = 1 h, t(3) = 24 h, t(4) = 48 h, t(5) = 120 h). The amount of chromium release was determined using an atomic absorption spectrophotometer and an inductively coupled plasma atomic emission spectrometer. Statistical analysis included a linear regression model for repeated measures, with calculation of Huber-White robust standard errors to account for intrabracket correlation of data. For post hoc comparisons the Bonferroni correction was applied. RESULTS The greatest amount of chromium was released from new stainless steel brackets (0.52 +/- 1.083 microg/g), whereas the recycled brackets released 0.27 +/- 0.38 microg/g. The smallest release was measured with Ni-free brackets (0.21 +/- 0.51 microg/g). The difference between recycled brackets and Ni-free brackets was not statistically significant (P = .13). For all brackets, the greatest release (P = .000) was measured at pH 4.2, and a significant increase was reported between all time intervals (P < .002). CONCLUSION The hypothesis is rejected, but the amount of chromium released in all test solutions was well below the daily dietary intake level.

Nickel release from new conventional stainless steel, recycled, and nickel-free orthodontic brackets: An in vitro study

INTRODUCTION The aim of this study was to compare the nickel released from 3 kinds of orthodontic brackets: new conventional stainless steel, recycled stainless steel, and nickel-free brackets. METHODS This in-vitro study was performed by using a classic batch procedure. Samples were immersed in artificial saliva at various acidities (pH 4.2, 6.5, 7.6) over an extended time interval (0.25, 1, 24, 48, and 120 hours). The amount of nickel released was determined by using an atomic absorption spectrophotometer and an inductively coupled plasma atomic emission spectrometer. Statistical analysis included a linear regression model for repeated measures, with calculation of Huber White robust standard errors to account for intrabracket correlation of data. For post-hoc comparisons, the Bonferroni correction was applied. RESULTS The recycled brackets released the most nickel (74.02 +/- 170.29 microg per gram); the new stainless steel brackets released 7.14 +/- 20.83 microg per gram. The nickel-free brackets released the least nickel (0.03 +/- 0.06 microg per gram). All the differences among the groups were statistically significant (P = 0.000). CONCLUSIONS Reconditioned brackets released the most nickel. Moreover, the highest nickel release was recorded in the 2 experiments performed at pH 4.2; it was lower at pH 6.5 and 7.6. Conversely, no relevant differences were observed overall between the maxillary and mandibular arches.

Application of the new electroanalytical technique AGNES for the determination of free Zn concentration in river water.

Absence of gradients and Nernstian equilibrium stripping (AGNES) is a recently developed electroanalytical technique specifically designed for the direct determination of free concentrations of metal ions. AGNES is applied here to the determination of free Zn concentration in a river water sample. The method has been validated with synthetic solutions of low ionic strengths containing Zn and 2,6-pyridinedicarboxylic acid and then applied to synthetic river waters and to a natural sample collected from Besòs River in Montcada i Reixac (Catalonia, North-Eastern Spain). In the river sample, an average free Zn concentration of 12.8(4) nM was obtained, while the total dissolved Zn concentration was 0.51(8) μM. To control and maintain pH and pCO2 constant during AGNES measurements, a novel device for N2/CO2 mixed purging has been developed.

Investigation of the speciation of aluminium in drinking waters by sorption on a strong anionic-exchange resin AG1X8

The sorption properties of the strong anion exchange resin AG1X8 towards aluminium(III) were investigated in view of the study of the speciation of this metal ion in drinking waters. This was done by determining the side reaction coefficients of the metal ion in the sample solution in the presence of the resin. It was found that aluminium(III) is sorbed on AG1X8 by the formation of Al(OH)3 and Al(OH)−4. The exchange coefficients were determined at different conditions and the intrinsic complexation constants were evaluated on the basis of the Gibbs–Donnan model. From the exchange coefficients the ratio of metal ion sorbed on the resin to the free metal ion in solution, K* was evaluated. It is a measure of the sorbing properties of a resin towards a metal ion at a given pH. AG1X8 was used for the determination of total aluminium(III) concentration in drinking water samples by the resin titration (RT) method at pH 7.5 and 6.2. In the first case the total concentration obtained by RT was equal to that determined directly, showing that all aluminium is either quantitatively sorbed or in equilibrium with AG1X8. The side reaction coefficient αM(I) was evaluated to be around 1012 in the considered drinking water samples at this pH. Not any aluminium(III) was sorbed on AG1X8 from the considered drinking water samples at pH=6.2 suggesting that aluminium is present here as species with αM(I)>107. These results were compared with those obtained using Chelex 100, which is able to sorb aluminium(III) also at pH 6.2. More information about the speciation of aluminium can be obtained by comparing the results from two different sorbing resins.

Investigation on sorption equilibria of Mn(II), Cu(II) and Cd(II) on a carboxylic resin by the Gibbs–Donnan model

Sorption mechanism of bivalent metal ions on a weak cationic resin containing the carboxylic group is studied. The Gibbs-Donnan model is used to describe and then to predict the sorption through the determination of the intrinsic complexation constants. These quantities characterize the sorption being independent of experimental conditions. They are determined according to a well established procedure and using a recently proposed iterative method for calculation of counter ion concentration in the resin phase. Sorption mechanisms are also studied adding appropriate soluble ligands whose complexing properties are exactly known to the solution containing the resin and the metal ion. Competing with the resin for the complexation of the metal, they shift the sorption curve to higher pH and often this allows detecting other complexes between the metal and the resin. In this way for Mn(II), besides the 1:1 complex formed in the more acidic solution, with logbeta(110)=-4.55, the complex ML(2), characterized by logbeta(120)=-9.80, is found; for Cd(II), besides the ML complex, with logbeta(110)=-3.01, at pH higher than 7, the specie MLOH with logbeta(11-1)=-8.28. For Cu(II) the complex ML(2) is detected, confirming previous findings, with logbeta(120)=-7.24. In the presence of two different ligands, sulphosalicylic and malonic acid, a different complex, ML(2)OH, is identified, with the same intrinsic complexation constant for the two ligands, logbeta(12-1)=-13.35. As expected from the model, the intrinsic complexation constants, especially for the 1:1 complex, are in a good agreement with the complexation constants of acetic acid.

Determination of the complexing properties of drinking waters toward copper(II) and aluminium(III) by ligand titration

Abstract The complexing capacity of some drinking waters for aluminium(III) and copper(II) is determined by a ligand titration with metal ions based on the use of complexing resins. The resins used in the titration are the iminodiacetic resin Chelex 100, the carboxylic resin Amberlite CG50 and the anionic exchange resin AG1X8. They allow the detection of ligands forming complexes of different stability with the metal ions used for the titration, since they have different sorbing properties. After equilibration with the resin, the concentration of the free metal ion in solution is evaluated from the concentration of sorbed metal ion and from the quantity K∗, which is the ratio of the concentration of the metal ion sorbed on the resin to the free metal ion in solution. It strongly depends on the conditions, but it can be evaluated, at the considered conditions, from the sorption equilibria of the metal ion on the resin. The concentration of the ligands in solution and the conditional stability constant are obtained from the Ruzik linearization procedure. Very strong ligands of copper(II) and aluminium(III) were detected in a tap water sample at concentrations ranging from 10−7 to 10−6 mol kg−1, and forming complexes having conditional complexation constants KcI=2.3×1017 (pH=6.77) and 4.5×1016 (pH=6.24), respectively, for copper(II) and aluminium(III). Weaker ligands were detected using the less strongly sorbing resins Amberlite CG50 and AG1X8, but at a concentration equal to that of the strong ligands. This was ascribed to the presence of competing metals in solution, not sorbed by the weak resins. Two other drinking waters had completely different complexing properties both towards copper(II) and aluminium(III), containing much weaker ligands.