Adsorption metal cations

Metal cation adsorption processes include exchange, Coulombic, and site-specific adsorption. Heavy metal cations exhibit exchange reactions with negatively charged surfaces of clay minerals. Cationic adsorption is affected by the pH and in an acid environment (pH < 5.5), and some heavy metals do not compete with Ca " (a ubiquitous constituent in the subsurface) for mineral adsorption sites. At a higher pH, heavy metal adsorption increases abruptly and becomes irreversible. 

Stumm et al. (5) ended their paper with a variety of remarks that, taken as a whole, implied that the adsorbed metal products in equations 5a and 6a are inner-sphere surface complexes. Their suggestion reflected observations of specific metal cation adsorption and comparisons of aqueous metal complexes with the corresponding surface complexes. They cautioned, however, that this kind of interpretation could not be made unequivocally without direct molecular evidence for inner-sphere complex formation. 

Conditional equilibrium constants analogous to those in equations 5b and 6b were also defined, with aOH instead of aH, and aL instead of aM. Graphical methods of estimating reaction stoichiometry and thermodynamic equilibrium constants were illustrated similarly to what was shown for metal cation adsorption [see also Kummert and Stumm (16) and Sigg and Stumm (17)]. 

Criscenti and Sverjensky (1999, 2002) continued to build the internally consistent set of triple layer model equilibrium constants developed by Sverjensky and Saliai (1996) and Sahai and Sverjensky (1997a,b) by reexamining sets of adsorption edge and isotherm data for divalent metal cation adsorption onto oxide surfaces. In contrast to previous investigations, they found tliat the adsorption of transition and heavy metals on solids such as goethite, y-ALOs, corundum, and anatase, which have dielectric constants between 10 and 22, was best described by surface complexes of the metal with the electrolyte anion. Metal (M +j adsorption from NaNOs solutions is described by... 

Kosmulski, M., Selectivity of alkali metal cations adsorption on controlled pore glasses, J. Radianal. Nucl. Chem., 118, 209, 1987. 

Figure 4.3. Metal cation adsorption versus pH on hematite. (Adapted from R. M. McKenzie. 1980. The adsorption of lead and other heavy metals on oxides of manganese and iron. Aust /. Soil Res. 18 61-73.)...

Whether an anion or ligand enhances metal cation adsorption by ternary complex formation or suppresses it by competition with the surface may depend on the anion/ metal ratio. A large mole excess of the anion generally suppresses metal cation adsorption, while molar parity with the metal favors adsorption by ternary complex formation. 

Fewer data, particularily calorimetric, are available for evaluating the influence of temperature on metal cation adsorption. The enthalpy of Cd(II) adsorption onto rutile was determined using isoperibol solution calorimetry and a value of +10 kJ/mole was found (6). A recent variable temperature study (25) allows enthalpies for Cd(II), Zn(II), and Ni(II) adsorption onto hematite (synthesized in the presence of 0.86% Si) to be calculated using equation (8). These data are summarized in Table IV. 

Rule JH (1999) Trace metal cation adsorption in sods selective chemical extractions and biological availability. In Dabrowski A, ed. Adsorption and its applications in industry and environmental protection, pp. 319—349. Elsevier, Amsterdam. 

Table 4.1. Categories of ligand effects on metal cation adsorption...

It is often observed that metal cation adsorption data obtained over a range of pH values can be described mathematically with the equation" ... 

METAL CATION ADSORPTION. The formation of inner-sphere surface complexes involving metal cations is typically described in the constant capacitance model with the chemical reactions ... 

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