Effects of natural organic matter on the binding of arsenate and copper onto goethite

Goethite is a common iron oxide present in the environment. It is used as a model system to represent the mineral fraction of the soil, whereas humic acid is used as a model system to represent natural organic matter (NOM). In this study, we tried to understand the behaviour of anionic and cationic pollutants in a model system, which could easily represent the organic-mineral interaction in the environment. Specifically, the retention of arsenate and copper over different organo-mineral systems was studied. The obtained results show an increase in the adsorption of copper as the pH and the C content in the organo-mineral composites was raised, whereas in the case of the arsenate, the opposite behaviour was observed. The adsorption behaviour was compared with that of the end-members goethite and humic acid. Furthermore, the NOM-CD model (natural organic matter-charge distribution model) was used to describe the reactivity of the pollutants in the solid/solution interphase and to predict the surface and aqueous speciation. To simulate the behaviour of the organo-mineral composites, a surface component was included to describe the chemical and electrostatic interactions of the NOM with the goethite in the modelling of the solid/solution interphase. This approximation allows the quantification of the effective organic matter, which can be used to describe the copper and arsenate reactivity in organo-mineral systems.