Simple Cation Adsorption from Solution

Figure 6.1 A simple electrostatic adsorption mechanism illustrating the protonation-deprotonation chemistry of surface hydroxyl groups on oxide surfaces (which are neutral at the PZC) and the corresponding uptake of anionic or cationic complexes. Proton transfer to or from the surface can significantly affect the solution pH.

Beside the reaction of the simple ions with surface groups of the metal oxide, the adsorption process may go through the adsorption of hydrolyzed forms or aqua complexes from the solution, and finally by hydrolysis of such formed surface compounds. When the cation adsorption takes place from the solution nearly saturated by the oxide or hydroxide, then, the process may be accompanied with surface precipitation of the appropriate compound. James and Healy proved, that at edl, the precipitation of the metal oxide or hydroxide may happen at the concentrations lower than the saturation of the bulk, as a result of the lowering of the dielectric constant [138]. 

In simple adsorption from aqueous solution, Hg has features in contrast and in common with the base metals. The hydroxy-cation is the active species in the model for heavy-metal adsorption and this also appears to be true for Hg. However, in contrast with Cu, Pb and Zn, the adsorption is less efficient and is strongly inhibited by the formation of halide complexes, as has been shown by Forbes et al. (1974) (Fig. 12-1). These authors also demonstrate that the adsorption of Hg to goethite is effective at pH as low as 4, allowing it to be trapped subsequent to sulphide oxidation. Whilst many minerals in weathered rocks and soils may each adsorb Hg, the relative efficiency of the hydrous iron oxides (Andersson, 1979) implies that these phases will be the dominant host in most exploration samples. However, the soil organic matter is also of importance and, although the association with Hg has been described as adsorption, it seems more... 

Simple ion exchange describes the competitive adsorption onto clays of most metal cations present in solution at concentrations from about 10 to 10 mol/kg. It has most often been used.to describe the sorption of alkaline earth and alkali metal cations onto clays. In the case of minerals having pH-dependent surface charge (e.g., kaolinite, metal oxyhydroxides) simple ion exchange or the power-exchange function (see below) may also fit the adsorption data mea.sured in systems at constant pH. 

The influence of the cations and anions has been discussed separately with the solution properties and reactions in the main focus. It has, however, been known over 100 years that anions play a crucial role for the stabilization and coagulation of colloids. More recently, the contribution of anions on the stabilization of particles, biocolloids, and bubbles has received renewed attention. - In these papers, it has been pointed out that there exists a collaborative interaction between cations and anions upon adsorption of one of the complexes from solution. At high concentrations this effect renders the simple indifferent ions specific and selective to each other. It is also seen as a dependency on the acid-base pair chosen for the regulation of the pH. This effect certainly needs to be added as an extension to (correction of) the DLVO theory. However, as shown in this paper, it is just as probable that the anion and cation collaborate during the adsorption and formation of gels and precipitates at the surface. The presence of such mixed phases has been confirmed experimentally, e.g., during the formation of hydroxoapatite in silica gel layers. ... 

The clay mineral bentonite (sodium montmorillonite) has an excellent ion exchange and adsorption capacity. Films can be applied to electrode surfaces from colloidal clay solutions by simple dip or spin coating that become electroactive after incorporation of electroactive cations or metal particles 136-143)... 

A Stern-Volmer plot obtained in the presence of donors for the stilbene isomerization has both curved and linear components. Two minimal mechanistic schemes were proposed to explain this unforeseen complexity they differ as to whether the adsorption of the quencher on the surface competes with that of the reactant or whether each species has a preferred site and is adsorbed independently. In either mechanism, quenching of a surface adsorbed radical cation by a quencher in solution is required In an analogous study on ZnS with simple alkenes, high turnover numbers were observed at active sites where trapped holes derived from surface states (sulfur radicals from zinc vacancies or interstitial sulfur) play a decisive role... 

For purification of aqueous solutions the use of adsorption processes for cationic impurities is also common. As economical adsorbents, montmoriUonite, tober-morite, magnetite and silica gel were found sufficient for the removal of Cd(II), Cr(VI) and Cu(II) in rinsing wastewater from a plating factory [104], From this investigation, it was found that the removal efficiency tended to increase with increasing pH and decrease with increasing metal concentration. This method allows the realization of a rapid, simple and cheap rinse water treatment system for the removal of heavy metals. 

Kanungo, S.B., Adsorption of cations on hydrous oxides of iron. III. Adsorption of Mn, Co, Ni, and Zn on p-FeOOII from simple electrolyte solutions as well as from a complex electrolyte solution resembling seawater in major ion content, J. Colloid Interf. Sci.. 162, 103, 1994. 

Figure 2 A very simple model of electrostatic adsorption on a negatively charged oxide surface with formation of a "double layer" (surface + diffuse layer). Small dosed drcles are cations, larger open drcles are anions, oq" surface charge density x distance from the surface into the solution k thickness of double layer < ) electric potential c ix) and c (x) local concentrations in cations and anions, respectively. The shaded area represents the excess of cations over anions in the diffuse layer, and therefore the amount of cations that are electrostatically adsorbed.

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