Surface Complex Formation with Metal Ions

Surface Complex Formation with Metal Ions... 

Organic material can strongly adsorb metal ions the functional groups on their surfaces act as ligands (carboxyl, amino groups etc.) for metal ions. All these functional groups favor the surface complex formation with metals the adsorption reactions are favored at higher pH (Fig. 11.11). 

The native conformation of proteins is stabilized by a number of different interactions. Among these, only the disulfide bonds (B) represent covalent bonds. Hydrogen bonds, which can form inside secondary structures, as well as between more distant residues, are involved in all proteins (see p. 6). Many proteins are also stabilized by complex formation with metal ions (see pp. 76, 342, and 378, for example). The hydrophobic effect is particularly important for protein stability. In globular proteins, most hydrophobic amino acid residues are arranged in the interior of the structure in the native conformation, while the polar amino acids are mainly found on the surface (see pp. 28, 76). 

Another interesting approach used to bond wood involves the swelling (66) or dissolution (with cellulose solvents) of the carbohydrate matrix (67) at the surface. Bonding then takes place when two surfaces are brought into contact and the volatile components evaporated by heating. Crosslinks form through reformation of H-bonds or possible complex formation with metal ions, in the case of cellulosic solvents. 

Figure 2. These curves were calculated with the help of experimentally determined equilibrium constants. Part a Extent of surface complex formation as a function of pH (measured as mole percent of the metal ions in the system, adsorbed or surface-bound). Total ion concentration [TOTFe] = HP3 M (2 X 10 mol/L of reactive sites metal concentrations in solution = 5 X 10 M I = 0.1 M NaNOs. (The curves are based on data compiled by Dzombak and Morel in reference 5. ) Part b Surface complex formation with ligands (anions) as a function of pH. Binding of anions from dilute solutions (5 X 10 M) to hydrous ferric oxide [TOTFe] = 10 M. I = 0.1. (Curves are based on data from Dzombak and Morel in reference 5.) Part c Binding of phosphate, silicate, and fluoride on goethite (a-FeOOH) the species shown are surface species (6 g/L of FeOOH, PT = 10 M, SiT = 8 X JO- M). (Reproduced with permission from reference...

VIBRATIONAL SPECTROSCOPY Infrared and Raman spectroscopies have proven to be useful techniques for studying the interactions of ions with surfaces. Direct evidence for inner-sphere surface complex formation of metal and metalloid anions has come from vibrational spectroscopic characterization. Both Raman and Fourier transform infrared (FTIR) spectroscopies are capable of examining ion adsorption in wet systems. Chromate (Hsia et al., 1993) and arsenate (Hsia et al., 1994) were found to adsorb specifically on hydrous iron oxide using FTIR spectroscopy. Raman and FTIR spectroscopic studies of arsenic adsorption indicated inner-sphere surface complexes for arsenate and arsenite on amorphous iron oxide, inner-sphere and outer-sphere surface complexes for arsenite on amorphous iron oxide, and outer-sphere surface complexes for arsenite on amorphous aluminum oxide (Goldberg and Johnston, 2001). These surface configurations were used to constrain the surface complexes in application of the constant capacitance and triple layer models (Goldberg and Johnston, 2001). 

A well-known example of the synergistic effect is the inhibition of steel corrosion in acidic media by a mixture of iodide ions and amines or imines. The synergism was mainly explained by coulombic attraction between the charges of the adsorbed ions (Aramaki and Hackerman, 1969 Kordesch and Marko, 1960 McKee, 1967 Kemball, 1959). The strong chemisorption of iodide ions on the metal surface yields coulombic repulsion. Stabilization of the adsorbed iodide ions by means of electrostatic interaction with amines leads to enhanced adsorption and a higher inhibition effect. Insoluble surface complex formation between iodide ions and amines was also assumed and verified (Syed Azin et al., 1995 Donahue and Nobe, 1967). Potassium iodide also improves the inhibition efficiency of trans-cinnamaldehyde and alkynols on steel corrosion in 20% HCl solution (Rozenfeld, 1981). 

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 ... 

Surface complex formation of cations by hydrous oxides involves the coordination of the. metal ions with the oxygen donor atoms and the release of protons from the surface, e.g.,... 

The main mechanism of ligand adsorption is ligand exchange the surface hydroxyl is exchanged by another ligand. This surface complex formation is also competitive OH ions and other ligands compete for the Lewis acid of the central ion of the hydrous oxide (e.g., the Al(iii) or the Fe(III) in aluminum or ferric (hydr)oxides). The extent of surface complex formation (adsorption) is, as with metal ions, strongly... 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Surface Complex Formation. Metal ions form both outer and inner sphere complexes with solid surfaces, e.g. hydrous oxides of iron, manganese, and aluminium. In addition, metal ions, attracted to charged surfaces, may be held in a diffuse layer, which, depending upon ionic strength, extends several nanometres from the surface into solution. 

Diffuse layer metal retention and outer sphere complex formation involve electrostatic attractive forces, which are characteristically weaker than co-ordinative interactions leading to inner sphere surface complex formation. A number of factors influence metal interactions with surfaces, including the chemical composition of the surface, surface charge, and the nature and speciation of the metal ion. The importance of the pH of the aqueous phase in these interactions will be discussed further in Section 3.2.4.1. 

Differences in the speciation of other ions at the surface can be noted. Using an analysis similar to that above for metal ions, one finds that adsorbed anions are less acidic than in bulk solution. For example, it was shown in Figure 6 that protolysis of adsorbed sulfate ions becomes significant in the pH range 4-5, whereas in solution bisulfate is formed at much more acidic conditions (pH 2). Complexes formed by supporting electrolyte, e.g. Na" ,. with oxide surface sites have greater stability constants (logK 0.5-1.7) than observed for complex formation with oxyanions in solution (log K 0.0) (J.). ... 

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