Introduction

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Further information

 

Effects of mineral humic interactions in relation to modelling metal retentions by soils.

The contribution of total absorption graph

The application of surface complexation models to predict the metal binding behaviour of geocolloids (heterogeneous geological materials) is less well advanced than the use of these models to describe metal binding by pure mineral and humic particles. A contributory factor to this is the presence of adsorbed humic substances associated with the mineral surfaces found in geocolloids. Therefore an important advance in the surface complexation modelling concept is the development of models to account for the binding of humic substances on mineral surfaces.

In collaboration with soil scientists at the University of Wageningen a surface complexation model has been developed that describes the binding of fulvic acid by the Fe (hydr)oxide mineral goethite. The development of such a model requires several essential components. These include:

  • A source of purified and well characterized fulvic acid
  • A model to describe the chemistry of the goethite surface
  • Experimental data for the binding of fulvic acid by goethite
  • A computational modelling framework to allow the implementation of models which cannot be done using existing geochemical models.

All these criteria were met in the project. A source of well-purified fulvic acid was available at the Institute, together with an experimental set up to obtain data on the adsorption of fulvic acid by goethite. The adsorption of fulvic acid by goethite was determined in a background electrolyte of LiClO4 as a function of fulvic acid concentration, pH, and ionic strength. For the modelling work Wageningen had experience on the use of models for both humic and mineral surfaces, whilst at the Macaulay the ORCHESTRA modelling framework provided a means to implement new modelling ideas to describe mineral humic interactions.

For the modelling the protonation behaviour of the fulvic acid was described using a discrete functional group approach. The adsorption of the fulvic acid on the goethite was described using the CD-MUSIC model. This model allows the charge of the bound fulvate molecule to be distributed over the region of the goethite solid-solution interface. Simultaneously, the concentration, pH, and salt dependency as well as the basic charging behaviour of the goethite could be described well. The surface species used in the model (Figure 1) indicate that at low pH, inner sphere coordination of carboxylic groups of the fulvate molecule are important, whereas at high pH the outer sphere coordination with reactive groups of the fulvate molecule with high log KH is important. Ion pair formation between non coordinated reactive groups of the bound fulvate molecule and Li+ ions becomes more important with increasing pH.

The model has been applied to datasets of the binding of Strichen fulvic acid by goethite. It is shown that the model can describe the basic charging behaviour of both goethite and organic acid, and the concentration, pH and salt dependency of the adsorption satisfactorily (Figure 2).

Further developments will test the usefulness of such a modelling approach to model metal binding by the humic/fulvic/mineral mixtures.