Water exchange adsorption rate constants

The term F2/CsRT is obtained from the constant capacitance model (Chapter 3.7). Fig. 4.6 gives a plot of the linear free energy relation between the rate constants for water exchange and the intrinsic adsorption rate constant, kads. 

Figure 13.3. Linear free energy relation between the rate constants for water exchange (s" ) and the intrinsic adsorption rate constants adsom) (M s ) from the pressure-jump experiments of Hachiya et al. The intrinsic constants refer to an uncharged surface. The linear free energy relations based on the experimental points are extended to some ions with lower H2O exchange rate in order to predict absorption rates. (Adapted from Wehrii et al., 1990.)...

If water exchange from the adsorbate is relatively rapid (k 10 s ), adsorption rates will correlate with the rates of exchange of waters around the corresponding hydrated metal. In other words, Pb(II) will adsorb more rapidly than Mn(n) because the rates of exchange of inner-sphere water in Pb H20)1 (aq) is much more rapid than for Mn(H20)l (aq) (see 4, 5). Conversely, if rates of elimination of the ti -OH2 site at the surface are more rapid than rates of elimination of a water from the inner-coordination sphere of the hydrated metal adsorbate, then the overall rate of reaction will be constant. 

The transport of disulfoton from water to air can occur due to volatilization. Compounds with a Henry s law constant (H) of <10 atm-m /mol volatilize slowly from water (Thomas 1990). Therefore, disulfoton, with an H value of 2.17x10" atm-m /mol (Domine et al. 1992), will volatilize slowly from water. The rate of volatilization increases as the water temperature and ambient air flow rate increases and decreases as the rate of adsorption on sediment and suspended solids increases (Dragan and Carpov 1987). The estimated gas- exchange half-life for disulfoton volatilization from the Rhine River at an average depth of 5 meters at 11 °C was 900 days (Wanner et al. ] 989). The estimated volatilization half-life of an aqueous suspension of microcapsules containing disulfoton at 20 °C with still air was >90 days (Dragan and Carpov 1987). 

It is often desirable, where applicable, to use the local equilibrium assumption when predicting the fate of subsurface solutes. Advantages of this approach may include 1) data such as equilibrium constants are readily available, as opposed to the lack of kinetic data, and 2) for transport involving ion exchange and adsorption, the mathematics for equilibrium systems are generally simpler than for those controlled by kinetics. To utilize fully these advantages, it is helpful to know the flow rate below which the local equilibrium assumption is applicable for a given chemical system. Few indicators are available which allow determination of that critical water flux. 

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