Exchange from surface bound

In addition to exchanging structural electrons, ferric oxyhydroxide minerals also act to mediate electron exchange from surface bound Fe to several reducible pollutants of environmental concern (52). In this case, the redox capacity of the mineral is not limited by the formation of a passivating layer because the bulk reductant is aqueous ferrous iron and reactive surface sites are continually regenerated. Haderlein and Pecher (55) review the environmental factors affecting the reactivity of surface bound Fe(II) in heterogeneous systems. 

Fig. 2. Schematic representation of the surface structure of one polytype of (VO)2P20 . The arrows represent the possible pathways for facile exchange of surface bound oxygen, either monoatomic or diatomic, between the active sites. The site-isolation due to the diffusion barrier created by the pyrophosphate groups is clearly shown by these arrows (from [12]).

Zinc adsorption can occur via exchange of Zn2+ and Zn(OH)+ with surface-bound Ca2+ on calcite (Zachara et al., 1988). Zinc and Ni form surface complexes on calcite as hydrate until they are incorporated into the structure via recrystallization (Zachara et al., 1991). The selectivity of metal sorption on calcite is as follows Cd > Zn > Ni (Zachara et al., 1991). The easily reducible oxide bound metals are primarily from Mn oxides (Chao, 1972 Shuman, 1982 and 1985a). At pH > 6, Zn sorption on Mn oxide abruptly increases because of hydroxylation of the ions (Loganathan et al., 1977), and a high soil pH in arid soil may favor Zn sorption on Mn oxides due to a great... 

However, it has become clear that protons removed from a substrate to a basic group in a protein need not exchange rapidly with solvent (see Eq. 9-102). In fact, the proton removed by fumarate hydratase from malate is held by the enzyme for relatively long periods of time. Its rate of exchange between malate and solvent is slower than the exchange of a bound fumarate ion on the enzyme surface with another substrate molecule from the medium.59 Thus, the overall rate is determined by the speed of dissociation of products from the enzyme and we cannot yet decide whether removal of a proton precedes or follows loss of OH. ... 

Some work on sediments is reported here in the belief that it may also be useful in the analysis of soil samples. Thus Asikainen and Nikolaides [33] have carried out a sequential extraction study of chromium from contaminated aquifer sediments and found that 65% of the chromium was extractable. Of this amount 25% was exchangeable, 11% was bound to organic matter and 30% was bound to iron and manganese oxide surfaces. Thomas et al. [34] also investigated the use of BCR sequential extraction procedures for river sediments, and found the method to work well. Real et al. [35] improved sequential extraction by optimising microwave heating. 

The vapor phase synthesis of methacrylic acid from propionic acid and formaldehyde was studied [42]. In particular, the choice of alkali metal cation and loading were evaluated for their effect on the activity and selectivity of silica supported catalysts. Experiments were carried out in 0.5 in. (o.d.) quartz reactors equipped with 0.125 in. thermowells. Alkali metal cations supported on silica are effective base catalysts for the production of methacrylic acid. Silica surfaces exchanged with alkali metal cations are capable of chemisorbing propionic acid yielding surface-bound silyl propionate esters and metal propionate salts. The alkali metal cation influences the temperature at which desorption of the ester occurs (Cs < Na < Li < support). For silica catalysts of equimolar cation loading, activity and selectivity to methacrylic acid show the opposite trend, Cs > K. > Na > Li. Methacrylic acid selectivity reaches a maximum at intermediate cation loadings where interaction of adjacent silyl esters is minimized [42]. 

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