Browsing by Author "Domingos, R. F."
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- Application of permeation liquid membrane and scanned stripping chronopotentiometry to metal speciation analysis of colloidal complexesPublication . Domingos, R. F.; Benedetti, M. F.; Pinheiro, J. P.The potential of permeation liquid membrane (PLM) to obtain dynamic metal speciation information for colloidal complexes is evaluated by measurements of lead(II) and copper(II) complexation by carboxyl modified latex nanospheres of different radii (15, 35, 40 and 65 nm). The results are compared with those obtained by a well characterized technique: stripping chronopotentiometry at scanned deposition potential (SSCP). Under the PLM conditions employed, and for large particles or macromolecular ligands, membrane diffusion is the rate-limiting step. That is, the flux is proportional to the free metal ion concentration with only a small contribution from labile complexes. In the absence of ligand aggregation in the PLM channels, good agreement was obtained between the stability constants determined by PLM and SSCP for both metals.
- Comparison of AGNES (absence of gradients and Nernstian equilibrium stripping) and SSCP (scanned stripping chronopotentiometry) for trace metal speciation analysisPublication . Domingos, R. F.; Huidobro, C.; Companys, E.; Galceran, J.; Puy, J.; Pinheiro, J. P.The free metal ion concentrations obtained by SSCP (stripping chronopotentiometry at scanned deposition potential) and by AGNES (absence of gradients and Nernstian equilibrium stripping) techniques have been compared and the usefulness of the combination of both techniques in the same electrochemical cell for trace metal speciation analysis is assessed. The free metal ion concentrations and the stability constants obtained for lead(II) and cadmium(II) complexation by pyridinedicarboxylic acid, by 40 nm radius carboxylated latex nanospheres and by a humic acid extracted from an ombrotrophic peat bog were determined. Whenever possible, the free metal ion concentrations were compared with the theoretical predictions of the code MEDUSA and with the free metal ion concentrations estimated from ion selective electrodes (ISE). SSCP values were in agreement with the ones obtained by AGNES, and both of them agreed reasonably with the ISE values and the theoretical predictions. For the lead(II)-humic acid, it was not possible to obtain the stability constants by SSCP due to the heterogeneity effect. However, using AGNES it is possible to obtain, for these heterogeneous systems, the free bulk metal concentration, which allows us to retrieve the stability constant at bulk conditions.
- Copper removal by algal biomass: biosorbents characterization and equilibrium modellingPublication . Vilar, V. J. P.; Botelho, C. M. S.; Pinheiro, J. P.; Domingos, R. F.; Boaventura, R. A. R.The general principles of Cu(II) binding to algal waste fromagar extraction, composite material and algae Gelidium, and different modelling approaches, are discussed. FTIR analyses provided a detailed description of the possible binding groups present in the biosorbents, as carboxylic groups (d-glucuronic and pyruvic acids), hydroxyl groups (cellulose, agar and floridean starch) and sulfonate groups (sulphated galactans). Potentiometric acid–base titrations showed a heterogeneous distribution of two major binding groups, carboxyl and hydroxyl, following the quasi-Gaussian affinity constant distribution suggested by Sips, which permitted to estimate the maximum amount of acid functional groups (0.36, 0.25 and 0.1mmolg−1) and proton binding parameters (pKH= 5.0, 5.3 and 4.4; mH = 0.43, 0.37, 0.33), respectively for algae Gelidium, algal waste and composite material. A non-ideal, semi-empirical, thermodynamically consistent (NICCA) isotherm fitted better the experimental ion binding data for different pH values and copper concentrations, considering only the acid functional groups, than the discrete model. Values of pKM (3.2; 3.6 and 3.3), nM (0.98, 0.91, 1.0) and p (0.67, 0.53 and 0.43) were obtained, respectively for algae Gelidium, algalwaste and composite material. NICCA model reflects the complex macromolecular systems that take part in biosorption considering the heterogeneity of the biosorbent, the competition between protons and metals ions to the binding sites and the stoichiometry for different ions.
- Determination of diffusion coefficients of nanoparticles and humic substances using scanning stripping chronopotentiometry (SSCP)Publication . Pinheiro, J. P.; Domingos, R. F.; Lopez, R.; Brayner, R.; Fiévet, F.; Wilkinson, K.A methodology, based on a labile metal ion probe using stripping chronopotentiometry at scanned deposition potential (SSCP), is presented for the determination of the diffusion coefficients of nanoparticles and humic matter. The novel methodology was successfully applied to the determination of diffusion coefficients (and thus hydrodynamic diameters) of eight standard nanoparticles with radii ranging from 5 to 129 nm and two samples of colloidal humic substances with hydrodynamic radii of ca. 1 nm. Good agreement was found between the SSCP determinations and results obtained by dynamic light scattering (DLS), transmission electron microscopy (TEM) and fluorescence correlation spectroscopy (FCS). The SSCP technique is critically analysed with respect to its use for the determination of diffusion coefficients of colloidal complexes.
- Electrochemical methodology to study labile trace metal/natural organic matter complexation at low concentration levels in natural watersPublication . Domingos, R. F.; Benedetti, M. F.; Croué, J. P.; Pinheiro, J. P.A new electrochemical methodology to study labile trace metal/natural organic matter complexation at low concentration levels in natural waters is presented. This methodology consists of three steps: (i) an estimation of the complex diffusion coefficient (DML), (ii) determination at low pH of the total metal concentration initially present in the sample, (iii) a metal titration at the desired pH. The free and bound metal concentrations are determined for each point of the titration and modeled with the non-ideal competitive adsorption (NICA-Donnan) model in order to obtain the binding parameters. In this methodology, it is recommended to determine the hydrodynamic transport parameter, α, for each set of hydrodynamic conditions used in the voltammetric measurements. The methodology was tested using two fractions of natural organic matter (NOM) isolated from the Loire river, namely the hydrophobic organic matter (HPO) and the transphilic organic matter (TPI), and a well characterized fulvic acid (Laurentian fulvic acid, LFA). The complex diffusion coefficients obtained at pH 5 were 0.4 ± 0.2 for Pb and Cu/HPO, 1.8 ± 0.2 for Pb/TPI and (0.612 ± 0.009) × 10−10 m2 s−1 for Pb/LFA. NICA-Donnan parameters for lead binding were obtained for the HPO and TPI fractions. The new lead/LFA results were successfully predicted using parameters derived in our previous work.
- Impact of spherical diffusion on labile trace metal speciation by electrochemical stripping techniquesPublication . Pinheiro, J. P.; Domingos, R. F.The impact of the spherical diffusion contribution in labile trace metal speciation by stripping techniques was studied. It was shown that the relative error in the calculation of the stability constants caused by assuming linear diffusion varies with the efficiency of stirring, the diffusion coefficient of the complex and the value of the stability constant itself. A model system (lead/carboxyl modified latex nanospheres) was studied to quantify the effect of these parameters in the error. The results showed that under the usual experimental conditions the relative error in the calculation of the stability constant is never smaller than 10% when using a Metrohm 663VA stand. A new equation, considering spherical diffusion, was applied to the model system under study and successfully calculated the correct stability constants. Therefore, it is clear that in trace metal speciation determinations using stripping techniques at spherical electrodes the linear approximation should not be used.
- Metal speciation dynamics in colloidal ligand dispersions. Part 3: Lability features of steady-state systemsPublication . Pinheiro, J. P.; Domingos, R. F.; Minor, M.; Van Leeuwen, H. P.A lability criterion is developed for dynamic metal binding by colloidal ligands with convective diffusion as the dominant mode of mass transport. Scanned stripping chronopotentiometric measurements of Pb(II) and Cd(II) binding by carboxylated latex core-shell particles were in good agreement with the predicted values. The dynamic features of metal ion binding by these particles illustrate that the conventional approach of assuming a smeared-out homogeneous ligand distribution overestimates the lability of a colloidal ligand system. Due to the nature of the spatial distribution of the binding sites, the change in lability of a metal species with changing ligand concentration depends on whether the ligand concentration is varied via manipulation of the pH (degree of protonation) or via the particle concentration. In the former case the local ligand density varies, whereas in the latter case it is constant. This feature provides a useful diagnostic tool for the presence of geometrically constrained binding sites.
- Stability of core/shell quantum dots-role of pH and small organic ligandsPublication . Domingos, R. F.; Franco, C.; Pinheiro, J. P.The improvement of knowledge about the toxicity and even processability, and stability of quantum dots (QD) requires the understanding of the relationship between the QD binding head group, surface structure, and interligand interaction. The scanned stripping chronopotentiometry and absence of gradients and Nernstian equilibrium stripping techniques were used to determine the concentration of Cd dissolved from a polyacrylate-stabilized CdTe/CdS QD. The effects of various concentrations of small organic ligands such as citric acid, glycine, and histidine and the roles of pH (4.5–8.5) and exposure time (0–48 h) were evaluated. The highest QD dissolution was obtained at the more acidic pH in absence of the ligands (52 %) a result of the CdS shell solubility. At pH 8.5 the largest PAA ability to complex the dissolved Cd leads to a further QD solubility until the equilibrium is reached (24 % of dissolved Cd vs.4 % at pH 6.0). The citric acid presence resulted in greater QD dissolution, whereas glycine, an amino acid, acts against QD dissolution. Surprisingly, the presence of histidine, an amino acid with an imidazole functional group, leads to the formation of much strong Cd complexes over time, which may be non-labile, inducing variations in the local environment of the QD surface.
- Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 6: features of irreversible complex systemsPublication . Town, R. M.; Pinheiro, J. P.; Domingos, R. F.; Van Leeuwen, H. P.The features of SSCP waves for complex species involving a nonreversible electron transfer process are described. For quasireversible systems, with electron transfer rate constants, k0, between O(10 4) to O(10 6) m s 1, the shape of the SSCP wave is dependent on the value of k0, the deposition time and the electrode size. Under these conditions, the stability of a metal complex can be determined from the shift in half-wave deposition potential, Ed,1/2, as compared to the metal-only case. This is true even when the system is not fully labile, so long as the shape of the SSCP wave is not altered in the presence of the complexant, i.e., k0 remains the same. The experimental parameters, notably deposition time and electrode size, can be optimised to achieve this condition. For irreversible systems, k0 < O(10 7) m s 1, the SSCP wave shape is independent of k0, deposition time, and electrode size. Further decreasing k0 only leads to additional shift in the wave from E0. In such cases the contribution to the Ed,1/2 from the stability constant cannot be a priori deconvoluted from that due to k0 if its value changes due to the presence of ligand. In irreversible systems, direct reduction of electroactive complexes is not uncommon, nor is its appearance at potentials more positive, i.e., more reversibly, than that for the metal itself.
- Trace metal dynamic speciation studied by scanned stripping chronopotentiometry (SSCP)Publication . Domingos, R. F.; Lopez, R.; Pinheiro, J. P.Abstract. The ability of scanned stripping chronopotentiometry (SSCP) to obtain dynamic information for metal complexation with heterogeneous colloidal ligands was evaluated by measurements of lead(II) and cadmium(II) complexation by humic and fulvic acids extracted from an ombrotrophic peat bog. Average stability constants were calculated, using a first order chemical heterogeneity approach, and compared with those obtained by an ion selective electrode (ISE). SSCP average stability constants were overestimated in comparison to those obtained by the ISE, which suggests that the first order heterogeneity approach did not fully account for the ligand heterogeneity. However, the comparison of the stability constants obtained from the two SSCP signals (the shift of the half-wave potential and the decrease of transition time) provides information about the dynamic nature of the metal complexes formed with the humic matter. These results were in reasonable agreement with the theoretical predictions of the dynamic theory for colloidal systems.