Influence of the ligand concentration and pH on the complexation of Cu(II) by a soil fulvic acid

Abstract

The interaction between heavy metals and organic matter in natural ecosystems influences markedly their availability to organisms, because the different chemical species of the metal are not all equally acceptable for animals and plants [1]. Humic substances (fulvic and humic acids) play an important role in this type of reactions owing to their complexation capacity and high concentration in the environment [2]. Moreover, the relative proportions of a metal ion bound to natural organic compounds depend on the acidity/basicity of the ligands, which also is influenced by the nature of the vegetal cover [3][4]. Therefore, the knowledge of the extent of the interactions between metal ions and organic matter is important to provide information on their states in soils and water.Due to the inherent complexity of natural polyelectrolytes like fulvic and humic acids or humic-like substances, different techniques have been used for the analysis of their complexation reactions, depending on the information required [1]. Indeed, no technique alone provides a complete description of the behavior of fulvic or humic acids in presence of metal ions. Among other, electrochemical techniques have been extensively used in this type of analysis, and anodic stripping voltammetry (ASV) could be an useful choice to obtain information for the study of complex chemical interactions, due to its very low detection limit and the small changes induced in the equilibria. Therefore, it was decided to include ASV in a project that has been in progress in our Departments, to obtain data that can complete information about humic and humic-like substances and their interaction with metal ions obtained by other techniques [5][6][7][8][9][10][11][12][13][14]. In the present paper, the results of a voltammetric study about the influence of the concentration of a soil fulvic acid (FAH n ) on the stability of its complexes with Cu(II), and on the equivalent weight (EW) of the ligand are presented. The objective of the work was to test ASV for the implementation of a non-traditional pHvariable methodology used before with Cu(II) ion selective potentiometry [12], to analyze the interaction of metal ions with soil fulvic acids. Experimental Reagents and materialsAll reagents were p.a. quality, and were used without further purification. For the preparation of the solutions, deionized water with resistivity > 16 MΩ cm -1 was employed.The sample of FAH n used in the present work was isolated from a forest soil collected in Ermesinde (near Oporto, NW of Portugal), using a procedure similar to that of IHSS [15]. For that, the solid soil sample was dried at room temperature and treated with HCl 2 M until pH ≈ 2 was reached. After 15 days of agitation, the mixture was decanted and the soil was discarded. The liquid was then passed through an Amberlite XAD-8 column to concentrate the fulvic acids, washed with water until chloride ions were not detected, and then eluted with NaOH 0.1 M. The liquid obtained, which Abstract. Anodic Stripping Voltammetry has been ...

Effect of soil properties and sample preparation on extractable and soluble Pb and Cd fractions in soils

“…It is also suitable for the determination of stability constants on a wide pH scale [10]. Differential pulse anodic stripping voltammetry (DPASV) was applied for measurement of complexation of heavy metals with humic and fulvic acids in model soil solutions [15][16][17] as well as in extracted soil solutions [18,19]. Moreover, voltammetric methods are used for the study of changes in metal solubility and speciation in agricultural soils treated by sewage sludge or other organic matter such as pig manure [20,21].…”