Surface complex ternary

Pathway (d) in Fig. 9.3 provides a possible explanation for the efficiency of a combination of a reductant and a complex former in promoting fast dissolution of Fe(III) (hydr)oxydes. In this pathway, Fe(II) is the reductant. In the absence of a complex former, however, Fe2+ does not transfer electrons to the surface Fe(III) of a Fe(III) (hydr)oxide to any measurable apparent extent. The electron transfer occurs only in the presence of a suitable bridging ligand (e.g., oxalate). As illustrated in Fig. 9.3d, a ternary surface complex is formed and an electron transfer, presumably inner-sphere, occurs between the adsorbed Fe(II) and the surface Fe(III). This is followed by the rate-limiting detachment of the reduced surface iron. In this pathway, the concentration of Fe(U)aq remains constant while the concentration of dissolved Fe(III) increases thus, Fe(II)aq acts as a catalyst to produce Fe(II)(aq) from the dissolution of Fe(III)(hydr)oxides. 

Schindler, P. W. (1990), "Co-Adsorption of Metal Ions and Organic Ligands Formation of Ternary Surface Complexes", in M. F. Jr.. Hochelia and A. F. White, Eds., Mineral-Water Interface Geochemistry, Mineralogical Soc. of America, Washington, DC, 281-307. 

The presence of anions in solution may enhance cation adsorption by formation of mixed metal/ligand surface complexes (Schindler, 1990). This effect is termed ternary adsorption. Two forms of ternary adsorption have been identified ... 

Dissolution of goethite and ferrihydrite at pH 6 by M-EDTA (M = Pb, Zn, Cu, Co, Ni) is slower than that by EDTA alone (Nowack Sigg, 1997). Dissolution was considered to involve the formation of a ternary surface complex which then dissociated releasing M into solution after which Fe was detached from the oxide as Fe-EDTA. For ferrihydrite, the rate of dissolution depended on the nature of M, because the rate determining step was dissociation of M-EDTA. For goethite, on the other hand, this step was fast, hence the rate of dissolution was independent of M. 

Reductive dissolution may be more complex than the two previous mechanisms in that it involves electron transfer processes. Formation of Fe" via reductive dissolution can be effected by adsorption of an electron donor, cathodic polarization of an electrode supporting the iron oxide and by transfer of an electron from within a ternary surface complex to a surface Fe ". Addition of Fe" to a system containing a ligand such as EDTA or oxalate promotes electron transfer via a surface complex and markedly accelerates dissolution. 

Lenhart, J.J. Bargar, J.R. Davis, J.A. (2001) Spectroscopic evidence for ternary surface complexes in the lead(ll)-malonic acid-hematite system. J. Coll. Int. Sci. 234 448-452 Lenhart, J.L. Honeyman, B.D. (1999) Uranium (VI) sorption to hematite in the presence of humic acid. Geochim. Cosmochim. Acta 63(19/20) 2891-2901 Leussing, D.L. Kolthoff, I.M. (1953) Iron-thio-glycolate complexes. J. Am. Chem. Soc. 75 3904-3911... 

EPR parameters of Cu(II) ternary surface complexes at 77 K bpy, 2,2 -bipyridine pic, a-picolinic acid. 

A comparative study of the Cu(II)-edta-Ti02 ternary surface complexes by potentiometry, EPR and electrochemical methods showed that the adsorptive properties of the Cu(II)-edta complexes are very similar to those of individual edta species [201]. The Cu(II)-edta adsorption ratio, equal to 1 1, indicated that the complexes were adsorbed intact. The Cu(II)-edta-Ti02 surface complex with a distorted structure of the trigonal bipyramid had not been previously observed in solutions. It was revealed that Cu(II)-edta complexes could be electroreduced at a glassy carbon electrode in the same potential region, where the nano-Ti02 electrodes were inactive [201]. 

Here, the adsorption of valine on different cation-exchanged montmorillonites is described (Nagy and Konya 2004). A discussion of the kinds of interactions that are possible in the ternary system of montmorillonite/valine/metal ions will be presented, and a description how the metal ions can affect these interactions. The interlayer cations (calcium, zinc, copper ions) were chosen on the basis of the stability constants of their complexes with valine. The adsorption of valine on montmorillonite is interpreted using a surface-complexation model. 

The extrapolation of experimental studies on trace-metal adsorption to natural waters is difficult a particular problem is the formation of both ternary surface complexes involving dissolved organic matter and the aqueous complexation of trace elements by dissolved organic matter. The number of studies trying to shed light on the complexity of these interactions by combined studies of organic matter in the suspended phase and the dissolved phase is incredibly low. 

Cations. Cations like Zn2+ form binuclear complexes and reduce surface protonation. Such ions can also form ternary surface complexes with inhibitors. 

Figure 4.6 shows uptake curves for cations, and analogous curves for anions are their mirror images. There is no generally accepted explanation why the uptake does not reach 0% at sufficiently unfavorable, or the uptake does not reach 100% at sufficiently favorable electrostatic conditions, even at low concentrations of the solute. Formation of very stable ternary surface complexes involving impurities on the one hand and formation of complexes with products of dissolution of the adsorbent on the other have been discussed as possible rationale, but some examples of unusual results can be also due to experimental errors (inadequate phase separation). Figure 4.6(C) shows two types of uptake curves with a maximum. Such uptake curves are observed for cations in the presence of carbonates (or other weak acids), which form stable complexes with a metal cation of interest. At low pH these ligands are fully protonated, and they do not compete with the surface for metal...

E. Ternary Surface Complexes and Multiple Surface Species... 

Considering ternary surface complexes, i,e. surface species involving counterions (in addition to surface site(s) and the specifically adsorbed ions of interest). 

Adsorption of metal cations that form stable water soluble complexes with anions present in solution has been often interpreted in terms of formation of multiple surface species that differ in the number of anions coadsorbed with one adsorbed cation. When this number is greater than zero these species are termed ternary surface complexes. This can be interpreted as complexation of the adsorbed cation. This type of multiple surface species can be combined with the discussed above species that differ in the number of protons released per one specifically adsorbed cation. Less common (but also physically reasonable for ions that tend to form polymeric species in solution) is the idea of multiple polymeric surface species. For example, adsorption of cobalt on alumina was modeled in terms of formation of =A10Co2(OH)2 and =A10Co4(OH)5 surface species [106],... 

Strut lire of ternary surface complexes with bipyridme and dimethylglioxymc is proposed... 

Ternary Pb-Cl surface complexes are formed at pH < 6 with Pb bound to a surface oxygen and Cl bound to surface Fe... 

NaaOj 8-aminoqujnolme. and (to less extent) 1. 2-phcnylenediamme ([ligand]-[Cuj or 2 [CuJ) Formation of tliTce different ternary surface complexes for each ligand ... 

Ludwig, Ch. and Schindler, P.W., Surface complexation on TiOj. II. Ternary surface complexes Coadsorption of Cu(II) and organic ligands (2,2 -bipyridyl, 8-aminoqui-noline, and o-phenylenediamine) onto TiOj (anatase), 7. Colloid Interf. Sci., 169, 291, 1995. 

Given the immense number of metal-ligand pairs possible in systems as complex as soils, the question arises How do we decide in which cases a ternary complex might influence metal solubility It is reasonable to expect that those particular cations and anions that are predisposed to form ion pairs in solution will display this same tendency to pair on adsorptive surfaces, forming ternary complexes. This is a statement of the general principle that ... 

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