Humic substances effect on the adsorption of phosphate on goethite
Juan Antelo Martínez (2004)
Directores: Florencio Arce Vazquez y Sarah Fiol López

Abstract

The objective of this work was the study of the adsorption properties of a typical iron (hydr)oxide as goethite. Anion and humic substances adsorption process on goethite was realized at different pHs and ionic strength. The final objective of this work was to improve the knowledge of the competitive adsorption between  phosphate and humic substances. First of all is necessary to carry out an extraction and characterization of one fulvic and humic acid from a soil of Galicia (NW Spain).

A structural analysis give us information about the functional groups of the humic substances. The study of the acid-base properties of this kind of substances is useful for the interpretation of the adsorption process of the humic substances on goethite.

The samples of synthetic goethite were characterized just to compare them with other samples of the bibliography. The two most important parameters to determine are the point of zero charge (PZC) and the specific surface area.

The anion adsorption study show us that this process is pH-dependent and more favourable at low pHs. At high ionic strength phosphate and arseniate adsorption is almost identical, whereas at low ionic strength arseniate shows a stronger affinity for the surface than phosphate.

The humic substance adsorption process on goethite is pH and ionic strength dependent, showing a bigger adsorption at lower pHs and higher ionic strengths. Humic acid adsorption is higher than fulvic acid adsorption.

Fulvic and humic acid decreases phosphate adsorption on goethite and the decrease is larger when the concentration of humic substance is higher. The previous adsorption of the humic substances produces a bigger reduction in the phosphate adsorption on goethite. Moreover the presence of phosphate produces a decrease in the humic substance adsorption on the mineral surface. This is indicative that phosphate can provoke a desorption process of the adsorbed humic substance.

All the experimental results were interpreted with a surface complexation model, which, furthermore, allow us to obtain the complexation constants for anions and humic substances.

 

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