Effect of Environmental Relevant Organic Complexants on the Surface Charge and the Interaction of Clay Mineral and Metal Oxide Particles

Abstract

In aqueous systems the interactions between clay mineral and metal oxide particles are generally governed by a local electrostatic field developing around particles. An electrified interface develops due to the formation of a multitude charged surface sites and the accumulation of countercharges in order to preserve electroneutrality. The composition of aqueous solution influences both the surface charging and the charge neutralization. The surface acquires an electrical potential different from the solution in which it is immersed. The distribution of charges in the electrical double layer and the potential decay from surface to bulk solution depends on the quality and quantity of dissolved species. Ionic species in the solid-liquid interfacial layer are situated closer to or further from the surface, depending on their size, charge, and ability to form chemical bonds with the surface sites. These may be specifically adsorbed species, which can bind to the surface through covalent interactions in addition to the pure Coulombic contributions [1], The presence of these ionic species has a direct influence on the surface charge. The formation of inner sphere complexes occurs, when ions bind directly to the surface while outer-sphere complexes form through water bridges [2,3,4]. Several dissolved organic compounds occurring in most aquatic environments have ability to form various complexes on the surface active sites. A chemically heterogeneous mixture of dominantly acidic products from microbial degradation and many other processes (e.g. oxidative polymerization of phenolic compounds in plants and soils, photolytic degradation) is called as natural or dissolved organic matter (abbreviated NOM or DOM) [5]. These small and macromolecular organic compounds (e.g. non-v olatile phenolic compounds, humic and fulvic acids) are rich in anionic functional groups, can be truly dissolved, associated with immobile phases or associated with mobile particles (e.g. colloidal clays and oxides), and so have an inherent influence on the surface charge state of inorganic mineral particles. The particle aggregation in aqueous dispersions of clay minerals and metal oxides is mainly determined by the pH and ionic strength of solution as well as the presence of multivalent inorganic ions and polyionic organic compounds, like humic substances.