Aquatic Chemistry

Functional groups on the interface of natural solids (minerals and particles) with water provide a diversity of interactions through the formation of coordinate bonds with H+, metal ions, and ligands. The concept of active surface sites is essential in understanding the mechanism of many surface-controlled processes (nucleation and crystal growth, biomineralization, dissolution and weathering of minerals, soil formation, catalysis of redox processes, and photochemical reactions). The enhancement of the dissolution rate by a ligand implies that surface complex formation facilitates the release of ions from the surface to the adjacent solution. These ligands bring electron density within the coordinating sphere of the central ion. Surface species thus destabilize the bonds in the surface lattice; they are especially efficient in the dissolution of iron and aluminum oxides and of aluminum silicates. Ascorbate, phenols, and S(-II) compounds, including H2S, readily form surface complexes with Fe(III) or Mn(III,IV) (hydr)oxides that subsequently undergo electron transfer and the release of Fe(II) or Mn(II) into solution. Reductive and nonreductive dissolutions are markedly inhibited by competitive (ligand exchange ) adsorption of inorganic oxoanions. These oxoanions can form bi- or multinuclear surface complexes. A better understanding of the electronic structure of the interface of solids and aquatic solutes would push the boundaries of aquatic surface chemistry.