James—Healy model

On the other hand, a simpler and purely physical (electrostatic) mechanism might go toward describing the uptake of common noble metal precursors onto common oxide supports. The first model of Agashe and Regalbuto [26] was patterned after that of James and Healy [17] from colloid science literature. In the original James and Healy s model, a Langmuir isotherm was employed and... 

Data are reported as percent nickel removed vs. pH isotherms. The James and Healy adsorption model as contained in the equilibrium computer model REDEQL2 was used to facilitate data analysis (20). In this way, an assessment of the combined effects of adsorption, complexation, and precipitation could be attempted. 

In the original James and Healy model, the magnitude of the adjustable chemical term usually swamped the other two terms. By using a much smaller value of the solvation energy as suggested by Levine [27], it was found that the adjustable chemical term could be eliminated in the simulation of a number of metal cation/silica systems [26],... 

R.O. James, T.W. Healy, Adsorption of hydrolyzable metal ions at the oxide — water interface. II. Charge reversal of Si02 and Ti02 colloids by adsorbed Co(II), La(III), and Th(IV) as model systems, J. Colloid Interface Sd. 40 (1972) 53-64. 

The surface complexation models quantify adsorption with experimentally determined equilibrium constants. Another, less widely used approach considers the relationship between the equilibrium constant for the adsorption reaction and the associated free energy change (James and Healy, 1972). Attempts have been made to determine the chemical contribution to the overall adsorption free energy by fitting adsorption isotherms to the experimental data values of -50, -33 and —45 kj mol were found for the change in chemical free energy associated with adsorption of Cr, Ni and Zn, respectively, on ferrihydrite (Crawford et al., 1993). Values ranging from -21 to 241 kJ mol were found for Ni on hematite the actual value depended upon the hydrolysis species that were assumed to exist (Fuerstenau and Osseo-Assare, 1987). 

Many experimental techniques of edl the measurements, as well as edl models are subject of the reviews and the whole chapters of monographs describing the phenomena or systems with its crucial role. The most important are papers by Wiese et al. [10], James and Parks [11], Healy and White [12], Westall and Hohl [13], Westall [14], James [15], Schindler [16], Blesa and Kallay [17], Koopal [18,19] Rudzinski et al. [20], Rijemsdijk and Bolt [21] and Lyklema [22]. As measuring techniques and theoretical models are still being developed, one should expect many new papers dealing with the edl of the more complicated and better defined systems. 

The major feature of the James and Healy ( ) model is the description of adsorption by a Langmuir isotherm whose equilibrium constant is based upon a free energy of adsorption made up of what are referred to as coulombic (AG9° ), solvation, and... 

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. 

Figure 1. Relationships between charge densities, potentials, and free energies in James—Healy and VSC-VSP models...

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

The data at total lead concentration of 1.0 X 10 M can by itself be fitted to the James and Healy ( ) model with a minimal average deviation of only 7.7 percent adsorption between theory and experiment. The value of AGp (-12.18 kcal mol ) used for fitting this data, however, produces very poor agreement between theory and experiment (an average deviation of 31.4 percent adsorption) for the data at 5.0 X 10 M total lead, as is shown by Figure 6. The James and Healy ( ) model therefore cannot reconcile the data obtained under these two sets of experimental conditions. 

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... 

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. 

Within these broad constraints there is considerable latitude in choosing a model which might realistically describe the in situ adsorptive process, James and Healy (28) have developed an important model in which the free energy of adsorption (AG ads) results from the difference in electrostatic contributions between coulombic attraction (AG coul) and repulsion due to change in solvation energy (AG solv). The complete process is governed by... 

James, R. 0., and Healy, T. W., Adsorption of hydrolyzable metal ions at the oxide-water interface. III. A thermodynamic model of adsorption. J. Coll. Interface Sci. 40, 65-81 (1972)... 

James, R.O., Stiglich, P.J., and Healy, T.W., Analysis of models of adsorption of metal ions at oxide water interfaces. Faraday Disc., 59, 142, 1975. 

James, R.O. and Healy T.W., Adsorption of hydrolyzable metal ions at the oxidewater interface. I. Co(II) adsorption in SiOj and TiO2 as model systems, 7. Colloid Interf. Sci., 40. 42. 1972. 

Dewar, M. J. S., Eve G. Zoebisch, Eamonn E. Healy, and James J. PStewart. 1985. Development and use of quantum mechanical molecular models. 76. AMI A new general purpose quantum mechanical molecular model. J. Am. Chem. Soc. 107(13) 3902-3909. 

James, R.O., and T.W. Healy. 1972a. Adsorption of hydrolyzable metals ions at the oxide/water interface. I. Co (II) adsorption on Si02 and Ti02 model systems. J. Colloid Interf. Sci. 40 42-52. 

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