Please use this identifier to cite or link to this item: http://hdl.handle.net/10773/18278
Title: Interacção de substâncias húmicas estuarinas com os iões hidrogénio, cobre(II) e chumbo(II)
Author: Palheiros, Isabel Maria Boal
Keywords: Ácidos fúlvicos
Substâncias húmicas
Defense Date: 1997
Publisher: Universidade de Aveiro
Abstract: Neste trabalho foi estudado o comportamento ácido-base de substâncias húmicas de águas estuarinas, bem como a sua interacção com iões de metais vestigiários, nomeadamente os iões cobre(l1) e chumbo(ll). As substâncias húmicas foram extraídas e isoladas da água de um estuário, por adsorção a resina XAD-8, segundo o método adoptado pela Intemational Humic Substances Society (IHSS). Os ácidos fúlvicos e os ácidos húmicos obtidos foram posteriormente fraccionados segundo a carga, por eluição com solventes de pH crescente, após a adsorção na resina X4D-8. As substâncias húmicas recolhidas foram caracterizadas por análise elementar e espectroscopias de infravemelho e de ressonância magnética nuclear de "C. A composição das substâncias obtidas é típica das amostras estuarinas, verificando-se, como é usual, uma maior aromaticidade dos ácidos húmicos relativamente aos ácidos fúlvicos. O fraccionamento segundo a carga separa as fracções obtidas essencialmente no que respeita ao teor em grupos carboxílicos, verificando-se que este diminui com o pH do solvente utilizado na eluição. O grau de aromaticidade, pelo contrário, aumenta com o pH da eluição. A quantificação e a caracterização dos grupos funcionais, muito heterogéneos, das substâncias húmicas foi efectuada através de titulações com base, seguidas potenciometricamente. A determinação da concentração hidrogeniónica é o passo decisivo nos estudos de protonação e foi efectuada com o máximo rigor. Nesse sentido, foi proposto e utilizado um método de calibração inovador, em que os parâmetros da resposta do eléctrodo são retirados da própria titulação das substâncias húmicas. Os parâmetros de protonação, nomeadamente as concentrações e afinidades hidrogeniónicas, foram estimados utilizando modelos de distribuição discreta e de distribuição contínua para as afinidades, tendo-se concluído que a heterogeneidade das substâncias húmicas pode ser modelada satisfatoriamente dos dois modos, desde que seja utilizado um número suficientemente elevado de parâmetros. (etc.)
The aim of this work was to study the proton and metal ion interactions with estuanne humic matter. The humic substances were isolated from an estuanne water by adsorption onto an XAD-8 resin, accordingly to the procedure proposed by the Intemational Humic Substances Society. The fulvic and humic acids thus obtained were further fraccionated on a charge base, by desorption from XAD-8 resin with solvents of sequencially higher pH values. The substances were characterized by elemental analysis, IR and %-NMR spectroscopies, and were found to be typical of estuanne humic matter. The carboxylic content was higher for fulvic than for hurnic acids, and for the aromaticity the reverse was true. The charge fraccionation separated the substances mainly for their carboxylic content, which varied inversely with the pH of extraction. The acid-base behaviour of the substances was studied by potentiometnc titrations. A new method of electrode calibration was developed to determine the proton conmtration, that withdraws the electrode parameters directly from the humic substances titrations. The method proved to be both consistent and reliable. Base titrations were periormed at three electrolyte concentrations, as well as electrolyte titrations at constant pH. The dissociation of the functional groups is enhanced when raising ionic strength values, and their heterogeneity is diminished. The electrostatic effect was modelled using both a Donnan rnodel and a thermodynamic double layer model. After correcting the base titrations data for this efíect, an intrinsic distnbution for proton affinities was obtained. The estimated parameten for the rnolecular size show that the humic molecules radius is about twice the fulvic one. Hysteresis was observed for proton binding to the humic substances, probabiy due to confomational changes of the particles, since revenibility of hysteresis was dependent on ionic strength. . The binding of copper to the hurnic and fulvic acids was studied by potentiornetric titrations using a copper ion selective electrode. The binding of the fulvic acids was the sarne for pH values 4, 6 and 8, but for the hurnic acids it was higher for the highest pH. The affinity of copper(l1) was slightly stronger for phenolic than for carboxylic groups. The cornplexation of lead with the humic substances was studied by voltarnrnetric techniques. The experiments were perfomed at high ligandlrnetal ion ratios as usually found in the aquatic environrnent, and so a sensitive technique such as anodic stripping voltamrnetry was used to determine the complexing pararneters. The binding of lead to the hurnic substances was mainly due to carboxylic goups, and was stronger for the lowest ionic stregth. The influente of secondary phenomena, such adsorption and surface electrode concentrations, was studied and proved to affect voltamrnetric determinations. The conforrnation of hurnic substances particles in solution was found to be dependent on the procedure followed for the solution preparation.
Description: Doutoramento em Química
URI: http://hdl.handle.net/10773/18278
Appears in Collections:DQ - Teses de doutoramento

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