Carlos Rey-Castro

Al(III) and Fe(III) binding by humic substances in freshwaters, and implications for trace metal speciation

Published experimental data for Al(III) and Fe(III) binding by fulvic and humic acids can be explained approximately by the Humic Ion-Binding Model VI. The model is based on conventional equilibrium reactions involving protons, metal aquo ions and their first hydrolysis products, and binding sites ranging from abundant ones of low affinity, to rare ones of high affinity, common to all metals. The model can also account for laboratory competition data involving Al(III), Fe(III) and trace elements, supporting the assumption of common binding sites. Field speciation data (116 examples) for Al in acid-to-neutral waters can be accounted for, assuming that 60–70 % (depending upon competition by iron, and the chosen fulvic acid : humic acid ratio) of the dissolved organic carbon (DOC) is due to humic substances, the rest being considered inert with respect to ion binding. After adjustment of the model parameter characterizing binding affinity within acceptable limits, and with the assumption of equilibrium with a relatively soluble form of Fe(OH)3, the model can simulate the results of studies of two freshwater samples, in which concentrations of organically complexed Fe were estimated by kinetic analysis. The model was used to examine the pH dependence of Al and Fe binding by dissolved organic matter (DOM) in freshwaters, by simulating the titration with Ca(OH)2 of an initially acid solution, in equilibrium with solid-phase Al(OH)3 and Fe(OH)3. For the conditions considered, Al, which is present at higher free concentrations than Fe(III), competes significantly for the binding of Fe(III), whereas Fe(III) has little effect on Al binding. The principal form of Al simulated to be bound at low pH is Al3+, AlOH2+ being dominant at pH >6; the principal bound form of Fe(III) is FeOH2+ at all pH values in the range 4–9. Simulations suggest that, in freshwaters, both Al and Fe(III) compete significantly with trace metals (Cu, Zn) for binding by natural organic matter over a wide pH range (4–9). The competition effects are especially strong for a high-affinity trace metal such as Cu, present at low total concentrations (1 nM). As a result of these competition effects, high-affinity sites in humic matter may be less important for trace metal binding in the field than they are in laboratory systems involving humic matter that has been treated to remove associated metals.

Systematic investigation of the physicochemical factors that contribute to the toxicity of ZnO nanoparticles.

ZnO nanoparticles (NPs) are prone to dissolution, and uncertainty remains whether biological/cellular responses to ZnO NPs are solely due to the release of Zn(2+) or whether the NPs themselves have additional toxic effects. We address this by establishing ZnO NP solubility in dispersion media (Dulbeccos modified Eagles medium, DMEM) held under conditions identical to those employed for cell culture (37 °C, 5% CO2, and pH 7.68) and by systematic comparison of cell-NP interaction for three different ZnO NP preparations. For NPs at concentrations up to 5.5 μg ZnO/mL, dissolution is complete (with the majority of the soluble zinc complexed to dissolved ligands in the medium), taking ca. 1 h for uncoated and ca. 6 h for polymer coated ones. Above 5.5 μg/mL, the results are consistent with the formation of zinc carbonate, keeping the solubilized zinc fixed to 67 μM of which only 0.45 μM is as free Zn(2+), i.e., not complexed to dissolved ligands. At these relatively high concentrations, NPs with an aliphatic polyether-coating show slower dissolution (i.e., slower free Zn(2+) release) and reprecipitation kinetics compared to those of uncoated NPs, requiring more than 48 h to reach thermodynamic equilibrium. Cytotoxicity (MTT) and DNA damage (Comet) assay dose-response curves for three epithelial cell lines suggest that dissolution and reprecipitation dominate for uncoated ZnO NPs. Transmission electron microscopy combined with the monitoring of intracellular Zn(2+) concentrations and ZnO-NP interactions with model lipid membranes indicate that an aliphatic polyether coat on ZnO NPs increases cellular uptake, enhancing toxicity by enabling intracellular dissolution and release of Zn(2+). Similarly, we demonstrate that needle-like NP morphologies enhance toxicity by apparently frustrating cellular uptake. To limit toxicity, ZnO NPs with nonacicular morphologies and coatings that only weakly interact with cellular membranes are recommended.

Experimental evidences for a new model in the description of the adsorption-coupled reduction of Cr(VI) by protonated banana skin

This work reports experimental evidences, not previously considered, to evaluate the Cr(VI) removal by protonated banana skin biomass. Variations in the number of hydroxyl groups, quantified by potentiometric titrations, and CO2 evolution during experiments, were attributed mainly to the oxidation of hydroxylic entities present in the studied material. The results indicate that these groups together with the carboxylic moieties are the main functionalities involved on the adsorption-coupled reduction process. The column experiment carried out provides a new approach to obtain the maximum reduction capacity of the material (3.72 mmol g(-1)). Moreover, we hereby propose a model that reports the first evidence for the instant bound of Cr(III) species to the material used, formed after the reduction of Cr(VI) present in solution. The removal process was quantified carrying out experiments under various pHs, biomass doses and Cr(VI) concentrations, and the mechanism underlying chromium removal was identified.

Suitability of analytical methods to measure solubility for the purpose of nanoregulation

Abstract Solubility is an important physicochemical parameter in nanoregulation. If nanomaterial is completely soluble, then from a risk assessment point of view, its disposal can be treated much in the same way as “ordinary” chemicals, which will simplify testing and characterisation regimes. This review assesses potential techniques for the measurement of nanomaterial solubility and evaluates the performance against a set of analytical criteria (based on satisfying the requirements as governed by the cosmetic regulation as well as the need to quantify the concentration of free (hydrated) ions). Our findings show that no universal method exists. A complementary approach is thus recommended, to comprise an atomic spectrometry-based method in conjunction with an electrochemical (or colorimetric) method. This article shows that although some techniques are more commonly used than others, a huge research gap remains, related with the need to ensure data reliability.

Surface charge and permeable gel descriptions of the ionic strength influence on proton binding to seaweed biomass

Abstract The development of physicochemical models for the description of cation binding to algal biomass is an essential issue for the practical application of seaweeds as biosorbents in waste water treatment. These models should be able to account for the polyelectrolytic properties of these materials, and to reasonably describe and predict the ionic strength effect. Data regarding the proton binding equilibria of Sargassum muticum, Cystoseira baccata and Saccorhiza polyschides in NaCl and KNO3 were reported on a preceding paper. These data are re-analysed in this work, in order to compare two different electrostatic models (surface charge and Donnan models), representing two opposite idealised interface structures (planar surface and permeable gel). Simple analytical equations relating the apparent pK with both the dissociation degree and the ionic strength are derived for each case. The experimental data are reproduced with similar accuracy by the surface charge and the simplest Donnan model (rigid volume), although the latter seems to yield slightly better results. The comparison of the geometric parameters involved in each model (Donnan volume and specific area) with independent experimental measurements is not straightforward. Therefore, both models seem to be almost equivalent, within their respective assumptions. Nevertheless, Donnan has the advantage that provides a simple way to account for activity coefficient effects and non-specific binding.

Model-independent link between the macroscopic and microscopic descriptions of multidentate macromolecular binding: Relationship between stepwise, intrinsic, and microscopic equilibrium constants

The binding of ions or other small molecules to macromolecules and surfaces can be macroscopically characterized by means of the stepwise (or stoichiometric) equilibrium constants, which can be obtained experimentally from coverage versus concentration data. The present work presents a novel, simple, and direct interpretation of the stepwise constants in terms of the microscopic, site-specific, stability constants. This formalism can be applied to the most general case, including the heterogeneity of the sites, interactions among them, multicomponent adsorption, and so forth, and, in particular, to chelate complexation. We show that the stepwise equilibrium constants can be expressed as a product of two factors, (i) the average number of free potential sites (per bound ion) of the microscopic species to be complexed (stoichiometric factor) and (ii) the average of the microscopic stability constants of their free potential sites. The latter factor generalizes the concept of the intrinsic equilibrium constant to systems with chelate complexation and reduces to the standard definition for monodentate binding. However, in the case of heterogeneous multidentate complexation, the stoichiometric factor cannot be known a priori, so that the finding of the intrinsic constants is not trivial. One option is to approximate the stoichiometric factor by the value that would correspond to identical active centers. We investigate the accuracy of this assumption by comparing the resulting approximate intrinsic constants to those obtained by Monte Carlo simulation of several binding models. For the cases investigated, it is found that the assumption is quite accurate when no correlated structures (typical of short-range interactions) are formed along the chain. For adsorption of particles attached to a large number of active centers, the formalism presented here leads to the Widom particle insertion method.

Full description of copper uptake by algal biomass combining an equilibrium NICA model with a kinetic intraparticle diffusion driving force approach

In this work kinetic and equilibrium studies related to copper binding to the protonated macroalga Sargassum muticum are reported. An intraparticle-diffusion linear driving force (LDF) model has been chosen for the quantitative description of the kinetics at several initial metal concentrations. Copper intraparticle homogeneous diffusion coefficient (D(h)) obtained is in the range 0.2-0.9×10(-10) m(2) s(-1). NICA isotherm is demonstrated to constitute a substantial improvement with respect to a simpler Langmuir competitive equation. The binding parameters were chosen to provide the best simultaneous description of the equilibrium experiments. Values of log K(Cu) (4.3), n(Cu) (1) and p (0.31) in NICA isotherm, and log K(Cu) (3.5-5) in Langmuir competitive model, have been obtained. These parameters have been also used to predict the competition between copper and cadmium for binding sites. Two acids, HNO(3) and HCl, have been tested to evaluate their effectiveness to release copper from the metal-laden biomass.

Acid-base equilibria of phthalic acid in saline media: ion association from Pitzer equations.

The protonation constants of 1,2-benzenedicarboxylic acid (phthalic acid) at 25 degrees C in NaCl and CaCl(2) (ionic strengths ranging from 0.1 to 2 mol kg(-1)) were determined potentiometrically, and the results were analyzed by means of Pitzer equations. The values of the logarithm of the first and second dissociation constants ranged from 2.730 and 4.948 (in 0.101 mol kg(-1) NaCl) to 2.449 and 3.984 (in 0.624 mol kg(-1) CaCl(2)), respectively. Tabulated interaction parameters for the inorganic species involved along with salting coefficients estimated from literature allowed the calculation of new interaction parameters for the phthalate-Na(+)/Ca(2+) systems. The specific ion interaction model considered seems to account for the influence of the formation of weak complexes of phthalate with calcium ions on the equilibrium constants.

Competitive Cd2+/H+ complexation to polyacrylic acid described by the stepwise and intrinsic stability constants.

Stepwise constants can be used to describe competitive proton and metal binding to macromolecules with a large number of sites. With the aim of accessing information on the microscopic binding model, we report an expression that connects the stepwise constants to the site-specific metal constants. This expression holds for a very general complexation model including heterogeneity, interactions, and chelate complexation. Assuming bidentate binding of the Cd ions to adjacent carboxylate groups in poly(acrylic acid), stepwise and intrinsic stability constants for proton and cadmium binding were estimated from the experimental data. Intrinsic values were split into specific and electrostatic contributions (by means of the Poisson-Boltzmann equation under cylindrical geometry). Free of the electrostatic contribution, the remaining Cd binding energy showed almost no dependence on the coverage and ionic strength, and the corresponding average values allowed for a reasonable reproduction of raw binding data. Small systematic discrepancies from the homogeneous behavior are critically discussed.

Influence of the settling of the resin beads on diffusion gradients in thin films measurements

Binding resin beads used in DGT (diffusion gradients in thin films) tend to settle to one side of the resin during casting. This phenomenon might be relevant for metal accumulation when partially labile complexes dominate the metal speciation, especially after recognizing the important role played by complex dissociation in the resin domain. The influence of the inhomogeneity of the binding agent distribution on metal accumulation is here assessed by numerical simulation of DGT devices with binding beads in only one half of the resin disc, as a reasonable model of the standard resin discs. Results indicate that a decrease in mass accumulation of less than 13% can arise in these inhomogeneous devices (as compared with an ideal disc with homogeneous dispersion of the resin beads) when complexes with stability constant K<10(2)m(3)mol(-1) (K<10(5)Lmol(-1)) dominate the metal speciation. The loss increases as K increases, but the percentage of mass loss always remains lower than the volume fraction of resin disc without beads. For very labile or inert complexes, the impact of the inhomogeneous distribution of binding resin beads is negligible. As kinetic dissociation constants of complexes can be estimated from the distribution of the metal accumulation in a DGT device with a stack of two resin discs, the influence of the inhomogeneity on the recovered kinetic constant is also assessed. For the cases studied, the recovered kinetic dissociation constant, kd,recovered, retains the correct order of magnitude, being related to the true kd by kd≈f(-1)kd,recovered, quite independently of K and kd values, being f the fraction of volume of the resin disc where resin beads are dispersed.