Aquatic colloids as chemical reactants: surface structure and reactivity

Abstract

Colloids are ubiquitous; they occur in natural waters, even in seawater, groundwater and interstitial soilwater, in relatively large concentrations (more than 10 9 colloids per liter). We consider first an idealized α-Fe 2O 3 (hematite) colloid and consider how its surface chemistry, surface speciation and surface charge is affected by its interaction H +, OH −, metal ions and ligands. The interaction is modeled with the help of the surface complex formation theory; effects of electrostatic interaction are taken care of with the Gouy—Chapman diffuse double layer theory. The surface charge of a particle can be estimated from the extent of isomorphic substitution, and from H +, OH −, metal ions and ligands bound to the surface. Competitive surface complex formation equilibria can be used to estimate surface charge and, in turn, surface potential. Steric stabilization by polymer segments needs to be considered when the thickness of the polymer layer is larger than the thickness of the electric double layer, e.g. in seawater, δ h(polymer) > δ D(debye length). Most surface-controlled processes depend on the identity of the surface species and the geometry of the coordinating shell. The overlapping orbital of the inner-sphere surface complex interconnects the solid phase (ionic or covalent solid, polymer) with the aqueous solution phase. Surface complex formation concepts have been extended to carbonates, sulfides, phosphates and organic particles (cells). The surface structure can be modified by hydrophobic adsorption and the sorption of polymers. Colloid surfaces can mediate electron transfer (including light-induced) processes. Electron cycling mediated by surfaces often complements or substitutes for an enzymatic mechanism.