James—Healy model predictions

Figure 5. Comparison of lead adsorption data (points and solid connecting lines) with best-fitting James-Healy model predictions (broken lines)...

In summary, the fact that the VSC-VSP model predicts adsorption edge shift and the effect of charge reversal on the pH-dependence of adsorption, neither of which can be accounted for on the basis of the model of James and Healy ( ), lends support to its use in accurately describing heavy metal adsorption. The agreement between theory and experiment shown in Figure 7 additionally supports the use of the VSC-VSP model. 

In the VSC-VSP model the charge density Og of adsorbed cations exerts an effect on the surface and Gouy potentials which can be quite profound where relatively large amounts of cations are adsorbed. This effect, not considered in the classical double-layer theory as presented by James and Healy ( ), can help us.to explain, for example, a rise in the increase in adsorption with increased pH that is different from that otherwise predicted. 

Like the James and Healy ( ) model, the VSC-VSP model also requires an estimate of the magnitude of chemical interactions, which usually must be determined by comparison of experimental data with adsorption predicted at various values of AGpi. ... 

Figure 9 shows the pH-dependence of lead adsorption which would be predicted using the chemical free-energy term that gives the best fit in the James and Healy ( ) model with the data in Table I. It can be seen that the adsorption edge positions are rather similar for total lead concentrations which differ by an order of magnitude or more. Only when the total lead concentration closely approaches the CEC is it predicted that it becomes... 

Nickel Adsorption onto Silica and Goethite. Figure 4 contains adsorption isotherms for nickel onto -Si02 and a-FeOOH as compared with precipitation which occurs in the absence of oxide surfaces. The lines in Figure 4, as with all figures in this chapter showing sorptive behavior, are predicted using the James and Healy model in conjunction... 

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