Proton and metal ion interactions of fulvic acid solutions and of alumina and alumina/fulvic acid suspensions

Abstract

Four generally applicable ion-substrate adsorption models were selected from the literature. These (the Gouy Chapman, constant capacitance, triple layer and polyelectrolyte gel models) were examined with the aim of identifying those best suited to modelling the interactions of natural organic matter and organic coated particles in aquatic, and particularly estuarine, systems. In order to aid this investigation, data on the proton exchange behaviour of a natural fulvic acid sample (in solutions containing the major ions of seawater and in an artificial seawater solution), a model particulate (γ-alumina) and mixtures of these two (in NaCl solutions) were obtained by potentiometric titration. Modelling of the fulvic acid data was most successful with the Gouy Chapman and constant capacitance models. The use of an iterative speciation modelling procedure provided stability constant values for surface complexation reactions of the carboxylic acid site of the fulvic acid. These values were obtained from high ionic strength data and, although they gave a good prediction of fulvic acid surface charge in artificial seawater, they are unlikely to be useful under estuarine conditions as lower ionic strength data was not well modelled. The effect of solution ionic strength on fulvic acid surface charge development could only be reproduced by taking the non-planar surface geometry of the fulvic acid into account using the spherical Gouy Chapman model. The triple layer and polyelectrolyte gel models were not able to describe the behaviour of the fulvic acid successfully. Using the constant capacitance model intrinsic stability constant values were obtained for the interactions of alumina when negatively charged. Stability constants were also obtained for the interactions of the fulvic acid/alumina mixture, although the effect of fulvic acid adsorption and desorption reactions were not considered explicitly by the model. These reactions are pH dependent and contribute to the overall proton balance during titration. Polyelectrolyte gel analysis indicated that the organic coating of the mixture was impermeable to electrolyte ions.

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