Adsorption divalent metal ions

This may be due to ion—oxide surface interactions or to ions already present on the surface. Alternatively, this deviation may reflect varying sihcate polymer acidity. Similar behavior has been observed for the adsorption of aqueous sihca to the surface of y-Al O (67). Divalent metal ions tend to reduce sihcate adsorption. 

Cotton, F.A. Wilkinson, G. (1988) Advanced inorganic chemistry. 5 ed., Wiley, New York Coughlin, B.R. Stone, A.T. (1995) Non reversible adsorption of divalent metal ions (Mnll, Coll, Nill, Cull and Pbll) onto goethite. Effects of acidification. Fell addition and picoli-nic acid addition. Environ. Sci. Techn. 29 2445-2455... 

Honaker and Preiser reported the first fundamental kinetic mechanism of chelate extraction in 1962 [1]. They elucidated that the rate-determining step for the extraction of divalent metal ions with dithizone was the formation of their 1 1 complexes in the aqueous phase. They proposed that a simple batch extraction method could be used as an alternative method of the complicated stopped-flow method, which was the only method available at the time, to measure such a fast reaction rate. Since the 1970s, hydrometallurgy has been developed in many countries, and extensive kinetic studies on the metal extraction have been conducted in an effort to improve the extraction rate as well as to develop effective and reusable extractants. The extractants used in hydrometallurgy are required to be highly hydrophobic and readily coordinative with various metal ions. On the basis of the interfacial adsorptivity of the extractant, Flett et al. [2] expected an interfacial reaction mechanism in the chelate extraction process. There was, however, no experimental evidence to prove the interfacial mechanism directly [3]. 

Coprecipitation with the hydrous oxides, such as of iron(III) and aluminum, occurs by adsorption and possibly also by compound formation. The precipitates, coming down in either amorphous or finely crystalline form with extensive surface, adsorb large amounts of water and adsorb hydroxide ions as potential-determining ions. Figure 9-1 illustrates the effect of varying the concentration of ammonium chloride and ammonium hydroxide on the amount of coprecipitation of divalent metal ions with hydrous iron(III) oxide. When the concentration of ammonium chloride is increased at a constant ammonia concentration, the adsorption is decreased... 

FIGURE 9-1 Adsorption of divalent metal ions on hydrous PefUI) oxide /I, as a function of NH C1 concentration with a NH3 concentration of 0.9 M, except 0.7 M for zinc S, as a function of NH3 concentration with a NHtQ concentration of 1 M. (From Kolthoff andOverholser. )... 

Divalent metal ion chelating agents (e.g. tetracyclines) and heavy metals accumulate in bone by adsorption onto the bone-crystal surface and eventual incorporation into the crystal lattice. 

Suzuki, A., Seki, H., and Maruyama, H., An equilibrium study of adsorption of divalent metal ions onto a metal oxide adsorbent, J. Chem. Eng. Jpn, TI, 505, 1994. 

Figure 3.10. The 10-A interlayer of smectite created by the adsorption of hydrated divalent metal ions, M(H20)6. Shaded circles represent water molecules open circles depict silicate oxygens.

An extensive numerical study on such heterogeneous three-step processes has been given by Stone and Morgan (1987). The adsorption-desorption kinetics of divalent metal ions is fast Yasunaga and Ikeda (1986) report relaxation times in the order of milliseconds to seconds. The pH as a master variable governs the adsorption of Fe(II) in the preceding example. The elucidation of adsorption equilibria and the structure of precursor complexes such as (=Fen,-0-Fen)+ at the mineral surface is therefore a prerequisite for the study of heterogeneous redox kinetics. 

In general, pHpzc values for silica and manganese oxides are below pH 7 and values for aluminum and iron oxides are greater than pH 7. Therefore, the electrostatic attraction of a divalent metal ion for a particular mineral will vary with pH. The adsorption of divalent cations is expected to increase with increasing pH as the surface becomes less positively charged. For example, Fig. 7-1 demonstrates the shift in the fractional pH adsorption edge of cadmium (Cd(II)) with pHpzc of the sorbent. Reported pHpzc values of rutile, ferrihydrite and corundum (a-Al203) are 5.8, 8.5 and 8.9, respectively (Davis Kent, 1990 Stumm, 1992 Hayes et al., 1990). Adsorption of Cd(II) for all of the minerals shown occurs below its reported pHpzc-... 

Silica adsorbs on gamma-AI2O3 in a broad pH range (5, 6), which might be explained by assuming that the silica species in solution have a similarly broad range of pKa values (between 6 and 10). The silica adsorption does not exhibit the sharp maximum value normally seen for weak acids at the pH value equal to the acid s pKa value. Silica adsorption was reduced by the presence of divalent metal ions, possibly as a result of reduced silicate species activity. 

Many biopolymers arc known to bind metal ions strongly. Alginic acid used as the immobilizing agent also has a complexing ability with divalent metal ions. Many studies have been carried out on the application of AA to the aqueous phase separation of heavy metals, and the possibility of AA as the adsorbent material has been suggested [1-5]. HA-M comprises 20% AA on a dry weight basis. Therefore, it can be expected that AA plays an important role in metal adsorption by HA-M. 

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