Multinuclear surface complexes, mineral

Reviews (9, 63, 64) of the reactions between hydroxylated mineral surfaces and aqueous solutions brought out the richness of variety found in surface phenomena involving natural particles. Isolated surface complexes, the principal topic of this chapter, are expected when reaction times are short and the adsorbate content is low [Figure 6, inspired by Schindler and Stumm 63)]. Thus, surface complexes occupy a reasonably well-defined domain in the tableau of reaction time scale versus sorbate concentration. Localized clusters of adsorbate (47, 48, 65, 66) that contain two or more adsorbate ions bonded together can form if the amount sorbed is increased by accretion or bv the direct adsorption of polymeric species (multinuclear surface complexes). Surface clusters can erase the hyperfine structure in the ESR spectrum of an immobilized adsorbate (33, 67) or produce new second-neighbor peaks from ions like the absorber in its EXAFS spectrum (47, 66). 

What are some of the most important findings from these XAFS studies of metal ion sorption processes One is the discovery that metal ion complexes at mineral/water interfaces are often different from those in bulk aqueous solutions. These differences include higher degrees of hydrolysis and different first-shell coordination environments (e.g., ( ) surface complexes Bargar et al. 1997a), and a higher proportion of multinuclear complexes (e.g., ( ) and ( ) surface complexes on alumina Chisholm-Brause et al. 1990a Fitts et al. 2000) for surface complexes vs. solution complexes. These differences are likely related to differences in the properties of water at interfaces vs. in bulk solutions (see earlier... 

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