Surface coordination reaction

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

Various theories have been proposed to describe and interpret the adsorption of metal ions at hydrous oxide interfaces (10). Most models have stressed either the double layer structure and ion-solvent interactions (11, 12, 13) or surface coordination reactions with amphoteric functional groups (14, 15, 16). Recently Davis (17) and Davis and Leckie (18, 19)... 

However, hydrogen ion attack on minerals (hydrolysis) is not the only means of mineral dissolution. Other ligands, such as organics in soil solution, are known to accelerate the dissolution of minerals. This process, too, can be viewed as a surface-coordination reaction with the following mechanism. For a hypothetical diprotic acid (H2A) and a central metal ion (Me) of valence z, we obtain... 

The specific adsorption of H, OH, cations, and anions on hydrous oxides and the concomitant establishment of surface charge can be interpreted in terms of the formation of surface complexes at the oxide-water interface. The fixed charge of the solid surface and the pH of its isoelectric point can be measured experimentally by determining the proton balance at the surface (from alkalimetric titration curve) and by the analytical determination of the extent of adsorbate adsorption. Equilibrium constants established for the surface coordination reactions can be used to predict pHiEp, to calculate adsorption isotherms, and to estimate concentration-pH regions for which the hydrous oxide dispersions are stable from a colloid-chemical point of view. 

Specific adsorption of cations and anions on hydrous oxide surfaces may be interpreted as a surface coordination reaction (JO) (Figure 1). 

A graphical representation of the potential energy surface or reaction coordinate. The transition state occurs at the saddle point. ( Adapted from Ref. 18.)... 

Therefore the electron-transfer reaction from Fe2+ to Fe3+ proceeds along a reaction path like the one indicated in the figure. Note that the electron-transfer step itself occurs practically at a constant distance from the metal surface the reaction coordinate is given by the solvent coordinate. This is the reason why the simple treatment presented in Chapter 6 is valid. 

In the first sequence the dissolution reaction is initiated by the surface coordination with H+, OH, and ligands which polarize, weaken, and tend to break the metal-oxygen bonds in the lattice of the surface. Since reaction (5.7) is rate limiting and using a steady state approach the rate law on the dissolution reaction will show a dependence on the concentration (activity) of the particular surface species, Cj [mol nr2] ... 

Schematic representation of the various reaction modes for the dissolution of Fe(III)(hydr)oxides a) by protons b) by bidentate complex formers that form surface chelates. The resulting solute Fe(III) complexes may subsequently become reduced, e.g., by HS c) by reductants (ligands with oxygen donor atoms) such as ascorbate that can form surface complexes and transfer electrons inner-spheri-cally d) catalytic dissolution of Fe(III)(hydr)oxides by Fe(II) in the presence of a complex former e) light-induced dissolution of Fe(III)(hydr)oxides in the presence of an electron donor such as oxalate. In all of the above examples, surface coordination controls the dissolution process. (Adapted from Sulzberger et al., 1989, and from Hering and Stumm, 1990.)...

Sulzberger, B. (1990), "Photoredox Reactions at Hydrous Metal Oxide Surfaces a Surface Coordination Chemistry Approach", in W. Stumm, Ed., Aquatic Chemical Kinetics, Wiley-lnterscience, New York, pp. 401-429. 

Table I. Examples of Sub-Reactions Needed to Describe Overall Metal Adsorption Using Surface Coordination Models.

Effect of Oxide Mineralogy on Reductive Dissolution. Oxide/hydrox-ide surface structures and the coordinative environment of metal centers may change substantially throughout the course of a reductive dissolution reaction. Nonstoichiometric and mixed oxidation state surfaces produced during surface redox reactions may exhibit dissolution behavior that is quite different from that observed with more uniform oxide and hydroxide minerals. 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

Mn(II) adsorption on metal oxide surfaces. The binding of Mn(II) on y-FeOOH can be understood in a surface coordination chemical framework. The surface groups on a metal oxide are amphoteric and the hydrolysis reactions can be written ... 

EXAFS and FT-IR spectra of the supported V complexes reveal the unique surface-attaching reaction promoted by surface Si-OH groups. The decrease in CN of V-O(N) bonds indicates that the V center becomes unsaturated on the surface. The large shift of V(ph-o) and the change in intensity ratio of Vjph), together with a comparison of FT-IR spectra between Ph-OH and Ph-ONa, demonstrate that the PhO- coordination is dispatched from the V center by the reaction of the... 

It is the proton gradient that causes the enzyme to release the ATP formed on its surface. The reaction coordinate diagram of the process (Fig. 19-22) illustrates the difference between the mechanism of ATP synthase and that of many other enzymes that catalyze ender-gonic reactions. 

Most popular schemes used to collect analytes are based on coordination reactions and electrostatic attraction. Common examples include the accumulation of nickel onto dimethylglyoxime-containing surfaces [39], the uptake and voltammetry of mercury on a diphenylcarbazide-carbon paste electrode [40], the use of surface-bound crown ethers for the collection and measurements of lead [41], or of trioctylphosphine oxide for the preconcentration of uranium [42], and the utility of polyelectrolyte-coated electrodes for the electrostatic collection of counterionic reactants [43,44], Bioaccumulation through binding to surface-bound microorganisms [45] or biocatalytic processes [46] can also offer the desired sensitivity and selectivity enhancements. 

While low coordination number sites, steps, and kinks, are the active sites for bond breaking in platinum, the atomic terrace sites with larger coordination numbers may also become active sites with unique chemistry for other elements. It will perhaps become possible to identify the bond-breaking ability of various coordination number sites of a given metal in breaking H—H, C—H, C —C, 0=0, N=N, etc., chemical bonds. By varying the atomic surface structure, which would change the relative concentrations of the different coordination number surface sites, the product distribution in surface chemical reactions may be markedly varied. 

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