Interaction of Metal Ions with Hydrous Oxide Surfaces

In this paper, the results on the adsorption of cation on hydrous Si02, Ah03 and \u27Mn02 surfaces are presented. Robust (kinetically inert) complexes, e. g. [Co(NH3) 6]3+, contrary to metal ions, e. g. Pb · aq2+, are not able to displace the alkalimetric titration curves of hydrous oxides. In the case of robust complexes the pH-dependence of adsorption is only the function of the 1surface charge (and its pH-dependence). The interpretation of metal ion adsorption in terms of different models is also .given. In solution of monomeric ·metal species, hydrolysis need not be invoked to account for the pH-dependence of adsorpti,on to hydrous oxide surfaces; this dependence can be explained with the basicity of the Meo- gfoup and the affinity of this group to the metal ion. Polymeric or colloidal metal species are usually adsorbed strongly to surfaces; in this case the surface substrate, as long as the surface charge is opposite to the charge of the adsorbing species, has little influence upoP.. the adsoription

A Ligand Exchange Model for the Adsorption of Inorganic and Organic Ligands at Hydrous Oxide Interfaces

Specific adsorption of organic and inorganic weak acids and of anions on hydrous oxide surfaces and the concomitant influences upon surface charge can be interpreted as ligand exchange reactions at the reactive surface sites. Direct (inner sphere) binding of the ligands to the surface is postulated. The extent of adsorption and its pH dependence can be explained by considering the affinity of the surface sites and those of the ligands. Surface equilibrium constants have been determined experimentally for various surface reactions; they can be used to predict extent of adsorption and resulting surface charge. The adsorption of simple weak acids or their anions is largest around the pH value of pH = pK. The surface complex formation constants show the same trend in stability as the corresponding solute complex formation constants; thus surface coordination equilibrium constants can be estimated from the corresponding complex formation constants in solution

Surface Complexation and Its Impact on Geochemical Kinetics

The weathering of rocks, the formation of soils, the alteration and dissolution of sediments are a consequence of surface reac-. tions. Furthermore, many redox processes such as the oxidation of V02+, Mn2+ and Fe2+, the ncn-biotic degradation of organic substances and photosensitized processes are catalyzed by surfaces. The electric double layer theory, despite its efficiency in quantifying certain phenomena of colloid stability, has limitations because it neglects chemical speciation at the surface and does not provide information on the chemical structure of the interfacial region. The surfaces of naturally occurring solids are characterized by functional groups, e. g., OH- groups on the surface of hydrous oxides at on organic surfaces. Specific adsorption of - or interaction with - H+, OH-, metal ion s and ligands occurs through coordination at the surface; inner-sphere surface complexes can be formed. The form of occurrence of the individual compounds (speciation) needs to be known in order to understand their reactivity; especially the geometry of the coordination shell of surface sites or of reactants at surfaces is a prerequisite for interpreting reaction rates occuring at the particle-water interface. Some case studies on the oxidation of Mn2+ and V02+ and on the dissolution of hydrous oxides and silicates are presented. In each case, the kinetics of the processes and how it is affected by solution variables such as H+ and ligands (such as oxalate and other di- ar hydroxy-carboxylates) are explained by simple mechanistic models that involve the coordination at the mineral-solution interface. Simple rate laws are derived illustrating the rates\u27 dependence on the concentration (activity) of surface species

The Role of Surface Coordination in Precipitation and Dissolution of Mineral Phases

In precipitation and dissolution of minerals, the coordinative partners of the crystal-forming ions are changed. In heterogeneous nucleation, these ions interact coordinatively (surface complex formation and ligand exchange) with the surface sites of the heteronuclei or template. The dissolution of oxides too depends on the influence of surface complex-forming ligands and on the degree of surface protonation. A specific rate law for the dissolution of 1>-Ah03 as influenced by [H+] and [oxalate] is given. Generalized ideas on the effect of surface coordination in accelerating or inhibiting the dissolution of hydrous oxides and its relevance in the chemistry of rock weathering and metal corrosion are discussed

The Role of Surface Coordination in Precipitation and Dissolution of Mineral Phases

In precipitation and dissolution of minerals, the coordinative partners of the crystal-forming ions are changed. In heterogeneous nucleation, these ions interact coordinatively (surface complex formation and ligand exchange) with the surface sites of the heteronuclei or template. The dissolution of oxides too depends on the influence of surface complex-forming ligands and on the degree of surface protonation. A specific rate law for the dissolution of 1>-Ah03 as influenced by [H+] and [oxalate] is given. Generalized ideas on the effect of surface coordination in accelerating or inhibiting the dissolution of hydrous oxides and its relevance in the chemistry of rock weathering and metal corrosion are discussed

Interaction of Metal Ions with Hydrous Oxide Surfaces

In this paper, the results on the adsorption of cation on hydrous Si02, Ah03 and \u27Mn02 surfaces are presented. Robust (kinetically inert) complexes, e. g. [Co(NH3) 6]3+, contrary to metal ions, e. g. Pb · aq2+, are not able to displace the alkalimetric titration curves of hydrous oxides. In the case of robust complexes the pH-dependence of adsorption is only the function of the 1surface charge (and its pH-dependence). The interpretation of metal ion adsorption in terms of different models is also .given. In solution of monomeric ·metal species, hydrolysis need not be invoked to account for the pH-dependence of adsorpti,on to hydrous oxide surfaces; this dependence can be explained with the basicity of the Meo- gfoup and the affinity of this group to the metal ion. Polymeric or colloidal metal species are usually adsorbed strongly to surfaces; in this case the surface substrate, as long as the surface charge is opposite to the charge of the adsorbing species, has little influence upoP.. the adsoription

The Coordination Chemistry of the Oxide-Electrolyte Interface; The Dependence of Surface Reactivity (Dissolution, Redox Reactions) on Surface Structure

Many heterogeneous processes (formation and dissolution of solid phases, redox and photochemical processes at the solid- -water interface are \u27kinetically controlled by a reaction step at the surface (and not by a transport step). Obviously the surface reactivity depends on the surface species and their structural identity, which — in turn — depend on the coordination chemical interactions that occur at the solid water interface. We discuss these processes for oxide-water interfaces in terms of a unifying rate law: R = kxa P} S, where R is the reaction rate [imolm\u272 s\u271], xa denotes the mole fraction of the reaction-active surface sites [—], Pj represents the probability to find a specific site in a suitable coordinative arrangement [—], and S is the surface concentration of sites [molm\u272]. Thus, for example, the dependency of the dissolution rate of an oxide mineral on pH can be explained in terms of surface protonation (or surface deprotonation). Similarly the effect of ligands such as oxalate on the dissolution can be accounted for by the concentration of ligand surface complexes. We extend this concept to the reductive dissolution of iron(III) hydroxides, to the oxidation of transition metal ions and other redox- and photorerox-reactions occurring at hydrous oxide surfaces and illustrate the dependence of reaction rates on specifically adsorbed oxidants and feductants. The cycling of iron as it occurs in natural systems (water, sediments, soils and atmosphere), dis used to illustrate the various redox processes, including photocatalyzed reactions, that are mediated by surfaces. Furthermore, we try to illustrate that the concepts of surface reactivity should be applicable to the interpretation of corrosion reactions, specyiifically the passivity of iron oxide films

The Coordination Chemistry of the Oxide-Electrolyte Interface; The Dependence of Surface Reactivity (Dissolution, Redox Reactions) on Surface Structure

Many heterogeneous processes (formation and dissolution of solid phases, redox and photochemical processes at the solid- -water interface are \u27kinetically controlled by a reaction step at the surface (and not by a transport step). Obviously the surface reactivity depends on the surface species and their structural identity, which — in turn — depend on the coordination chemical interactions that occur at the solid water interface. We discuss these processes for oxide-water interfaces in terms of a unifying rate law: R = kxa P} S, where R is the reaction rate [imolm\u272 s\u271], xa denotes the mole fraction of the reaction-active surface sites [—], Pj represents the probability to find a specific site in a suitable coordinative arrangement [—], and S is the surface concentration of sites [molm\u272]. Thus, for example, the dependency of the dissolution rate of an oxide mineral on pH can be explained in terms of surface protonation (or surface deprotonation). Similarly the effect of ligands such as oxalate on the dissolution can be accounted for by the concentration of ligand surface complexes. We extend this concept to the reductive dissolution of iron(III) hydroxides, to the oxidation of transition metal ions and other redox- and photorerox-reactions occurring at hydrous oxide surfaces and illustrate the dependence of reaction rates on specifically adsorbed oxidants and feductants. The cycling of iron as it occurs in natural systems (water, sediments, soils and atmosphere), dis used to illustrate the various redox processes, including photocatalyzed reactions, that are mediated by surfaces. Furthermore, we try to illustrate that the concepts of surface reactivity should be applicable to the interpretation of corrosion reactions, specyiifically the passivity of iron oxide films