Proton Binding Characteristics of Dissolved Organic Matter Extracted from the North Atlantic.

Marine dissolved organic matter (DOM) presents key thermodynamic properties that are not yet fully constrained. Here, we report the distribution of binding sites occupied by protons (i.e., proton affinity spectra) and parametrize the median intrinsic proton binding affinities (log K̅H) and heterogeneities (m), for DOM samples extracted from the North Atlantic. We estimate that 11.4 ± 0.6% of C atoms in the extracted marine DOM have a functional group with a binding site for ionic species. The log K̅H of the most acidic groups was larger (4.01-4.02 ± 0.02) than that observed in DOM from coastal waters (3.82 ± 0.02), while the chemical binding heterogeneity parameter increased with depth to values (m1= 0.666 ± 0.009) ca. 10% higher than those observed in surface open ocean or coastal samples. On the contrary, the log K̅H for the less acidic groups shows a difference between the surface (10.01 ± 0.08) and deep (9.22 ± 0.35) samples. The latter chemical groups were more heterogeneous for marine than for terrestrial DOM, and m2 decreased with depth to values of 0.28 ± 0.03. Binding heterogeneity reflects aromatic carbon compounds' persistence and accumulation in diverse, low-abundance chemical forms, while easily degradable low-affinity groups accumulate more uniformly in the deep ocean.

Seasonal Variations in Proton Binding Characteristics of Dissolved Organic Matter Isolated from the Southwest Baltic Sea.

The physicochemical characteristics of dissolved organic matter (DOM) strongly influence its interactions with inorganic species such as protons and trace elements in natural waters. We collected water samples at Boknis Eck, a time series station in the Baltic Sea with a low exposure to freshwater inputs, to investigate how seasonal fluctuations impact the proton binding properties of the isolated DOM. We used potentiometric titrations to assess the binding properties of solid-phase extracted DOM (SPE-DOM) over a seasonal cycle. We report and critically analyze the first NICA parameters estimates of carboxylic-like and phenolic-like sites for brackish water SPE-DOM. The total amount of functional groups (QmaxH,tot) showed no seasonal fluctuations and an average value of 136 ± 5.2 mmol·mol C-1. The average proton affinity (logKH) and binding site heterogeneity (m) showed a relatively minor variability for samples obtained between April and September, when the water remained stratified. These results contribute to a better understanding of the ion binding characteristics of DOM in natural brackish waters.

Speciation of Inorganic Compounds in Aquatic Systems Using Diffusive Gradients in Thin-Films: A Review.

The speciation of trace metals in an aquatic system involves the determination of free ions, complexes (labile and non-labile), colloids, and the total dissolved concentration. In this paper, we review the integrated assessment of free ions and labile metal complexes using Diffusive Gradients in Thin-films (DGT), a dynamic speciation technique. The device consists of a diffusive hydrogel layer made of polyacrylamide, backed by a layer of resin (usually Chelex-100) for all trace metals except for Hg. The best results for Hg speciation are obtained with agarose as hydrogel and a thiol-based resin. The diffusive domain controls the diffusion flux of the metal ions and complexes to the resin, which strongly binds all free ions. By using DGT devices with different thicknesses of the diffusive or resin gels and exploiting expressions derived from kinetic models, one can determine the labile concentrations, mobilities, and labilities of different species of an element in an aquatic system. This procedure has been applied to the determination of the organic pool of trace metals in freshwaters or to the characterization of organic and inorganic complexes in sea waters. The concentrations that are obtained represent time-weighted averages (TWA) over the deployment period.

Acid-base properties of dissolved organic matter extracted from the marine environment.

Marine dissolved organic matter (DOM) plays a key role in the current and future global carbon cycle, which supports life on Earth. Trace metals such as iron, an essential micronutrient, compete with protons and major ions for the binding to DOM. These competitive effects and the DOM binding capacity are related to the DOM acid-base properties, which also influence DOM transport and reactivity in marine waters. Here we present the results of a complete set of acid-base titration experiments of a pre-concentrated marine DOM sample in the range 0.01 ≤ I ≤ 0.7 M and 3 ≤ pH ≤ 10. We characterize the obtained proton binding curves using a combination of the non-ideal competitive adsorption (NICA) isotherm and Donnan electrostatic model. Within the main chemical groups of marine DOM, the carboxylic distribution was accurately characterized from the obtained data (QmaxH, 1=2.52 mol·kg-1, logk¯H,1 = 3.26, m1 = 0.69 and b = 0.70). This carboxylic mode was found to be less acidic and more homogeneous than a generic fulvic acid, but the differences are consistent with the reported variability of fulvic acids of freshwater and terrestrial origin. We find that changes in temperature (down to 5.5 °C), and the presence of calcium or magnesium (at 0.01 M) resulted in no significant modification of the proton ion binding curves obtained at 25 °C and 0.7 or 0.1 M ionic strength, respectively. We demonstrate the relevance of proton binding parameters for the modelling of the system iron/marine DOM throughout a wide range of salinity and acidity conditions in the context of different future ocean scenarios.

Time weighted average concentrations measured with Diffusive Gradients in Thin films (DGT).

Time weighted average (TWA) concentrations can improve the assessment of water quality. DGT (Diffusive Gradients in Thin films) devices have been suggested as simple tools to measure TWA metal concentrations, but the connection of TWA with cDGT has not been rigorously discussed. It is shown here that cDGT is the average DGT-labile concentration along the deployment, which suggests that it is well suited to correlate with toxicity effects. In terms of real species, cDGT is a good estimator of the TWA concentration for simple metal solutions (no ligands are present) when the accumulation takes place under perfect sink conditions. Differences between cDGT and the TWA concentration for short pulses (<40 min), when the transient regime becomes relevant, are reported. In the presence of complexes, cDGT contains the TWA of the product of the labile fraction times the diffusivity of the complex (relative to that of the free metal). This means that cDGT can underestimate the TWA of the total metal concentration due to the presence of complexes less mobile than the free metal or not fully labile. These findings are illustrated with Cd, Ni, Mg or Ni + nitrilotriacetic acid (NTA) solutions. When only one complex is relevant, as in the Ni + NTA system, a simple correction factor can yield the TWA concentration from cDGT.

Dissolution and Phosphate-Induced Transformation of ZnO Nanoparticles in Synthetic Saliva Probed by AGNES without Previous Solid-Liquid Separation. Comparison with UF-ICP-MS.

The variation over time of free Zn2+ ion concentration in stirred dispersions of ZnO nanoparticles (ZnO NPs) prepared in synthetic saliva at pH 6.80 and 37 °C was followed in situ (without solid-liquid separation step) with the electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping). Under these conditions, ZnO NPs are chemically unstable due to their reaction with phosphates. The initial stage of transformation (around 5-10 h) involves the formation of a metastable solid (presumably ZnHPO4), which later evolves into the more stable hopeite phase. The overall decay rate of ZnO NPs is significantly reduced in comparison with phosphate-free background solutions of the same ionic strength and pH. The effective equilibrium solubilities of ZnO (0.29-0.47 mg·L-1), as well as conditional excess-ligand stability constants and fractional distributions of soluble Zn species, were determined in the absence and presence of organic components. The results were compared with the conventional ultrafiltration and inductively coupled plasma-mass spectrometry (UF-ICP-MS) methodology. AGNES proves to be advantageous in terms of speed, reproducibility, and access to speciation information.

Effect of polymer coating composition on the aggregation rates of Ag nanoparticles in NaCl solutions and seawaters.

The aggregation behaviour of polymer-coated silver nanoparticles (AgNPs) was characterized in NaCl solutions, and in two seawaters of different salinities and dissolved organic matter (DOM) contents. Representative organic coatings i.e. tannic acid (TA), alginic acid (ALG), two gum Arabic samples (GAL and GAH), branched polyethylenimine (BPEI), and non-ionic surfactants (reference material NM-300K) were selected to cover a wide range of zeta-potentials. The stability in NaCl solutions, as determined from the rate of variation in hydrodynamic size within a timeframe of one hour, followed the order BPEI≫NM-300K≈GAL≫ALG≈TA≫GAH. In the seawater samples the order was NM-300K≈GAL≫ALG>GAH>TA≈BPEI, and only TA, GAL and NM-300K batches behaved as expected from the NaCl experiments. Remarkably, the BPEI sample showed the largest aggregation rate in the seawater sample with the highest DOM concentration (277µM C). The GAH sample displayed a non-monotonic variation in aggregation rate with NaCl concentration, apparently due to concomitant precipitation of AgCl. The results indicate that non-electrostatic stabilization mechanisms and DOM-coating interactions are important for the prediction of stability and persistence of polymer-coated AgNPs in seawater.

Extending the Use of Diffusive Gradients in Thin Films (DGT) to Solutions Where Competition, Saturation, and Kinetic Effects Are Not Negligible.

DGT (Diffusion Gradients in Thin films) was designed to sample trace metals in situ at their natural concentrations. The setup and the experimental deployment conditions were established to allow interpretation of a linear accumulation of metal with time, using a simple expression based on a steady-state flux under perfect sink conditions. However, the extension of DGT to a wide range of analytes and its use under varied conditions has shown that, in some situations, these conditions are not fulfilled, so that accumulations with time are nonlinear. Previously, when such curvature was observed, concentrations in solution could not be reliably calculated. Here, we present fundamentally derived equations that reproduce the time accumulation for three situations: (i) kinetic limitations in the binding to the resin, (ii) saturation or equilibrium effects, or (iii) non-negligible competitive effects. We show how the accumulations can be quantified, in terms of the required kinetic and thermodynamic constants, and provide practical guidance for their use to obtain reliable estimates of solution concentrations. Solutions containing Mg or Mn, where all three situations can prevail, are used as examples. Calculated concentrations show reasonable agreement with the experimentally known values and with the results of a numerical model of the system, significantly improving the estimations based on perfect sink conditions. Such an approach opens up the possibility of using DGT more widely in challenging systems and allows DGT data to be interpreted more fully.

Accumulation of Mg to Diffusive Gradients in Thin Films (DGT) Devices: Kinetic and Thermodynamic Effects of the Ionic Strength.

Availability of magnesium is a matter of concern due to its role in many environmental and biological processes. Diffusive Gradients in Thin Films (DGT) devices can measure Mg availability in situ. This work shows that Mg accumulation in water largely increases when ionic strength (I) decreases. This phenomenon can be explained from (i) the increase of both the association equilibrium (K) and rate (ka,R) constants for the reaction between Mg cations and resin sites, and (ii) the growing contribution of the partitioning of Mg cations at the resin-gel interface, as I decreases. Two theoretical models that take into account electrical interactions among Mg cations, background electrolyte, and resin sites can successfully be used to determine ka,R and K at each I. Both models yield similar ka,R values, which fulfill an expression for the kinetic salt effect. For freshwater (with a typical salinity of 10 mM and circumneutral pH), the binding of Mg is so fast and strong that the simplest perfect-sink DGT expression can be helpful to predict (overestimation lower than 5%) the accumulation in solutions with Mg concentrations up to 1 mM whenever the deployment time is below 9 h. Perfect sink conditions can still be applied for longer times, in systems with either a lower I or a lower Mg concentration.

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

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.

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.

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 (Dulbecco's modified Eagle's 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.

Limits of the linear accumulation regime of DGT sensors.

A key question for the practical application of DGT (Diffusive Gradients in Thin films) as dynamic sensors in the environmental monitoring of trace metals is the influence of pH and dissolved ligands over the linear accumulation regime. Protons compete with metal ions for the binding to the DGT resin sites at relatively low pH, whereas high affinity dissolved ligands compete with resin sites for the binding of metals. Any of the two phenomena can lead to a departure from the linear accumulation regime and an underestimation of the actual species concentration in solution. These effects are studied here through numerical simulation of the diffusion-reaction processes in both gel and resin domains using a detailed chemical model of metal ions and protons interacting with resin sites. Results were tested successfully against experimental data of the Cd-NTA representative system. Charts to delimitate the range of experimental conditions (pH, ligand concentration and strength) where the linear accumulation regime prevails, can be helpful for designing sampling strategies in field conditions. For example, it is foreseen that perturbations of linear regime within 10 h of deployment are negligible above pH 5 and weak complexation (log K' < 0) or above pH 7 and strong complexation (log K' < 3), where K' is the effective stability constant. These plots can also be approximately used for partially labile systems whenever the time is replaced with the product lability degree times t.

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.

A semi-grand canonical Monte Carlo simulation model for ion binding to ionizable surfaces: proton binding of carboxylated latex particles as a case study.

In this paper, we present a computer simulation study of the ion binding process at an ionizable surface using a semi-grand canonical Monte Carlo method that models the surface as a discrete distribution of charged and neutral functional groups in equilibrium with explicit ions modelled in the context of the primitive model. The parameters of the simulation model were tuned and checked by comparison with experimental titrations of carboxylated latex particles in the presence of different ionic strengths of monovalent ions. The titration of these particles was analysed by calculating the degree of dissociation of the latex functional groups vs. pH curves at different background salt concentrations. As the charge of the titrated surface changes during the simulation, a procedure to keep the electroneutrality of the system is required. Here, two approaches are used with the choice depending on the ion selected to maintain electroneutrality: counterion or coion procedures. We compare and discuss the difference between the procedures. The simulations also provided a microscopic description of the electrostatic double layer (EDL) structure as a function of pH and ionic strength. The results allow us to quantify the effect of the size of the background salt ions and of the surface functional groups on the degree of dissociation. The non-homogeneous structure of the EDL was revealed by plotting the counterion density profiles around charged and neutral surface functional groups.

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.

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.

Effective affinity distribution for the binding of metal ions to a generic fulvic acid in natural waters.

The effective distribution of affinities (Conditional Affinity Spectrum, CAS) seen by a metal ion binding to a humic substance under natural water conditions is derived and discussed within the NICA-Donnan model. Analytical expressions for the average affinity of these distributions in general multi-ion mixtures are reported here for the first time. These expressions enable a simple evaluation of the effect of all interfering cations on the affinity distribution of a given one. We illustrate this methodology by plotting the affinity spectra of a generic fulvic acid for 14 different cations in the presence of major inorganic ions and trace metals at pH and concentration values representative of a river water. The distribution of occupied sites and their average affinity at the typical freshwater conditions are also reported for each ion. The CAS allows usto distinguish three groups of cations: (a) Al, H, Pb, Hg, and Cr, which are preferentially bound to the phenolic sites of the fulvic ligand; (b) Ca, Mg, Cd, Fe(II), and Mn, which display a greater effective affinity for carboxylic sites, in contrast to what would be expected from their individual complexation parameters; and (c) Fe(III), Cu, Zn, and Ni, for which phenolic and carboxylic distributions are overlapped.

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.