Bidentate-binuclear

Bidentate complexes Refers to Stem inner-sphere bidentate-mononuclear or bidentate-binuclear complexes. 

Bidentate-binuclear complex A Stem inner-sphere adsorption complex that consists one adsorbed atom bonding to two separate metal oxides. As discussed in Chapter 2, an arsenate may adsorb onto two separate pairs of iron and oxygen atoms. 

Inner-sphere complex A type of Stern layer, where a chemical species bonds directly onto the surface of a solid material (adsorbent) in an aqueous solution. The formation of inner-sphere complexes is called chemisorption. Stern inner-sphere adsorption complexes are further divided into monodentate, bidentate-mononuclear, and bidentate-binuclear types (compare with outer-sphere complex). 

Figure 24. Possible stable Co2+ surface complexes on the (A) (001) and (B) (110) surfaces of rutile indicated by the GI-EXAFS data of Towle et al. (1999). In (A), site (1) is a bidentate polynuclear complex, while site (2) is a bidentate mononuclear complex. In (B), site (1) is a monodentate complex, while site (2) is a bidentate binuclear complex.

Among the current spectroscopic methods extended x-ray absorption fine structure spectroscopy (EXAFS) has received the most attention, however other methods such as FTIR, may be equally as promising. EXAFS is considered to provide definitive information on inner- vs. outer-sphere bonding and is suitable for determining mode of attachment to the surface (monodentate, bidentate, binuclear) but does not resolve questions of surface speciation since it is not sensitive to H atoms. In addition, examination of the same system by different researchers has in some instances resulted in different conclusions. 

Sun and Doner (31) examined the OH stretching modes of deuterated goethite in the presence of arsenate using FTIR. Based on transmission and ATR modes in dry systems, the authors concluded that arsenate oxyanions replaced singly coordinated surface hydroxyl groups to form bidentate binuclear complexes. Reactions presented by Sun and Doner (31) indicate HAs04 as the surface species, bound to metal centers via two O bonds. 

FIGURE 9.17 Calculated Zn(II)-Ti02 surface complexes from density functional theory (a) dissolved Zn(II) with six outer-sphere water molecules (b) monodentate mononuclear (c) bidentate binuclear (BB) (d) bidentate mononuclear (BM). Gray, black, large white, and small white circles denote Zn, O, Ti, and H atoms, respectively. (Reprinted with permission from He et al. 2011, 1873-1879. Copyright 2011 American Chemical Society.)... 

The effect of pH on heavy metal ion adsorption capacity was studied by previous researchers using the shake flask experiments. Eric and Roux used the shake flask experiment to study the influence of pH on the heavy metal ion binding onto a fimgus-derived bio-sorbent in the year, 1992. Also the evaluation of the effect of the hydrochloric acid concentration on the adsorption of platinum group metal ions onto chemically modified chitosan was done by Inoue et al., using the shake flask experiment [85]. Depending upon the type of P complexation with the surface such as monodentate, bidentate mononuclear, and bidentate binuclear the phosphorus desorption is potentially controlled. These complexes can be either non protonated or protonated depending on the suspension pH [184]. 

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