Pb(II) binding to humic substances: an equilibrium and spectroscopic study

The binding of Pb(II) to humic acids is studied through an approach combining equilibrium and spectroscopic measurements. The methods employed are potentiometric and fluorometric titrations, fluorescence excitation−emission matrices (EEM) and IR spectroscopy. Potentiometric titration curves are analyzed using the NICA equations and an electrostatic model treating the humic particles as an elastic polyelectrolyte network. EEMs are analyzed using parallel factor analysis, decomposing the signal in its independent components and finding their dependence on Pb(II) activity. Potentiometric results are consistent with bimodal affinity distributions for Pb(II) binding, whereas fluorometric titrations are explained by monomodal distributions. EEM analysis is consistent with three independent components in the humic fluorescence response, which are assigned to moieties with different degree of aromaticity. All three components show a similar quenching behavior upon Pb(II) binding, saturating at relatively low Pb(II) concentrations. This is attributed to metal ion induced aggregation of humic molecules, resulting in the interaction between the aromatic groups responsible for fluorescence; this is also consistent with IR spectroscopy results. The observed behavior is interpreted considering that initial metal binding (observed as strongly binding sites), correspond to bi- or multidentate complexation to carboxylate groups, including binding between groups of different humic molecules, promoting aggregation; further metal ions (observed as weakly binding sites) bind to single ligand groups.Fil: Orsetti, Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de Los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; ArgentinaFil: Marco Brown, Jose Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de Los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; ArgentinaFil: Andrade, Estela M.. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de Los Materiales, Medioambiente y Energía; ArgentinaFil: Molina, Fernando Víctor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de Los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; Argentin

Study of the heterogeneity of humic through cation binding

Las sustancias húmicas son componentes fundamentales del suelo y aguas naturales, siendo mezclas complejas y heterogéneas de materiales polidispersos formados por reacciones químicas y bioquímicas durante la descomposición y transformación de restos de plantas y microorganismos. En este trabajo de Tesis se estudia el fenómeno de enlace de cationes (protones y metales) a dichas sustancias, en particular ácidos húmicos, dado que el mismo es fundamental en cuanto a la biodisponibilidad y transporte de xenobióticos en ambientes naturales. En las reacciones de enlace a otras especies como cationes se manifiesta claramente la heterogeneidad de las sustancias húmicas, por lo tanto el estudio de la naturaleza de estos enlaces es un paso fundamental para comprender la heterogeneidad de dichas sustancias. Para ello, se comenzó utilizando un método numérico de regularización para obtener distribuciones de constantes de enlace condicionales para el enlace de protones y metales pesados. Luego, se desarrolló un modelo para describir el enlace inespecífico, denominado Modelo de Red Polielectrolítica Elástica (EPN), que describe las sustancias húmicas como geles polielectrolíticos, en el marco de la teoría de redes poliméricas de Flory. Este modelo fue combinado con el modelo NICA para describir las uniones específicas, ajustando con el resultante modelo NICA-EPN datos de titulaciones ácido-base y con Pb(II). De esta manera se obtuvieron las distribuciones “verdaderas” que describen el enlace de estos cationes a los ácidos húmicos estudiados, y que luego fueron comparadas con las distribuciones condicionales obtenidas a partir de medidas de fluorescencia. Así, se observa que por fluorescencia solamente se detecta un tipo de sitio de enlace para el metal, mientras que la distribución “verdadera” es bimodal. Se complementaron estos estudios con el análisis por métodos quimiométricos de matrices de emisión-excitación fluorescente de los ácidos húmicos en ausencia y presencia de Pb(II). Se obtuvieron así los espectros de emisión y excitación de los 3 componentes que contribuyen al comportamiento fluorescente, coincidiendo con lo reportado en literatura. Por otro lado, se obtuvo la concentración relativa de cada componente en muestras con distinta cantidad de Pb(II) agregado, mostrando que la desactivación fluorescente es más marcada cuanto mayor es el grado de aromaticidad de la sustancia. La complementación con estudios de espectroscopía IR-ATR mostró un aumento en la dimerización de grupos –COOH en presencia del metal. En base a estos resultados se plantea la formación de un enlace AH-Pb en el cual el metal está puenteando dos moléculas de ácido húmico, lo cual favorece la agregación intermolecular.Humic substances are major components of soils and groundwaters, being complex and heterogeneous mixtures of polydisperse materials formed by chemical and biochemical reactions during decomposition and transformation of plants and microorganism remains. In this Thesis, the binding of cations (protons and metals) to such substances is studied, particularly to humic acids, since it is fundamental regarding bioavailability and transport of xenobiotics in natural environments. It is in these binding reactions to other species as cations that the heterogeneity of humic substances is clearly manifested, therefore the study of the nature of these bonds is a crucial step to understand the heterogeneity of these substances. To do so, in a first step a numerical constrained method was used to obtain conditional binding constant distributions for binding of protons and heavy metals. Then, a model to describe the unspecific binding was developed, named Elastic Polyelectrolyte Network model (EPN), which describes humic substances as polyeletrolitical gels in the framework of the Flory polymer network theory. This model was combined with the NICA model to describe the specific binding, adjusting acid-base and Pb(II) tritation data to the resulting NICA-EPN model. By doing so, “true” distributions were obtained, which describe the binding of these cations with the studied humic acids, and then they were compared to the conditional distributions obtained from fluorescence measurements. It was observed that by fluorescence techniques only one type of binding site is detected, while the “true” distribution is bimodal. These studies were complemented with the analysis by chemometric tools of emission-excitation fluorecence maps of the humic acids in the presence and absence of Pb(II). Emission and excitation spectra of 3 distinct components that contribute to fluorescence behavior were obtained, in good agreement to the results reported in literature. On the other hand, the relative concentration of those components in samples with different amounts of Pb(II) was also obtained, showing that the fluorescence quenching is more marked as the aromaticity degree of the component increases. The complementation with IR-ATR spectroscopic studies showed an increase in the dimerization of –COOH groups in the presence of the metal. Based on these results, the formation of a AH-Pb bond where the metal is acting as a bridge between two humic acid molecules, which promotes intermolecular aggregation, is proposed. Thus, the binding of Pb(II) to humic acids has two contributions: one due to single humic-Pb binding and another one due to humic-Pb- humic binding.Fil:Orsetti, Silvia. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Modelling proton and metal binding to humic substances with the NICA-EPN model

The mathematical modelling of metal cation–natural organic matter interactions is a fundamental tool in predicting the state and fate of pollutants in the environment. In this work, the binding of protons and metal cations to humic substances is modelled applying the Elastic Polyelectrolyte Network (EPN) electrostatic model with the Non-Ideal Competitive Adsorption (NICA) isotherm as the intrinsic part (NICA–EPN model). Literature data of proton and metal binding to humic substances at different pH and ionic strength values are analysed, discussing in depth the model predictions. The NICA–EPN model is found to describe well these phenomena. The electrostatic contribution to the Gibbs free energy of adsorbate–humic interaction in the EPN model is lower than that predicted by the Donnan phase model; the intrinsic mean binding constants for protons and metal cations are generally higher, closer to independent estimations and to the range of acid–base and complexation equilibrium values for common carboxylic acids. The results for metal cations are consistent with recent literature findings. The model predicts shrinking of the humic particles with increased metal binding, as a consequence of net charge decrease.Fil: Montenegro, Andrea Constanza. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Orsetti, Silvia. Universität Tübingen; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Molina, Fernando Víctor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin

Application of a constrained regularization method to extraction of affinity distributions: Proton and metal binding to humic substances

The binding of proton and metal cations to humic substances has been analyzed with a regularized fitting procedure (using the CONTIN software package) to extract conditional affinity distributions, valid at a given ionic strength, from binding (titration) curves. The procedure was previously tested with simulated titration curves using a simple bi-Gaussian model, the NICA-Donnan model, and the Stockholm humic model. Application to literature data for proton binding shows that in several cases the affinity distribution found is bimodal (carboxylic and phenolic sites) as usually assumed; however in other cases, specially for fulvic acids, a trimodal distribution is clearly discerned, with a smaller peak between the two noted above attributed to the presence of vicinal carboxylic groups. The analysis of metal binding curves has been performed in a few cases where the available data could be reliably processed, separating the proton affinity distribution and obtaining the conditional affinity spectra. For Cd(II) and Pb(II) a bimodal distribution is found, attributed in principle to mono- and bidentate binding, based on spectroscopic data. In the case of Cu(II), a more complex affinity distribution is found showing 3-4 peaks; this is consistent with spectroscopic studies, where different binding modes, up to tetradentate, have been observed. © 2009 Elsevier Inc. All rights reserved.Fil: Orsetti, Silvia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Andrade, Estela María. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Molina, Fernando Víctor. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentin