Ion adsorption by hydrous metal oxides

Machesky, M.L (1990) Influence of temperature on ion adsorption by hydrous metal oxides. ACS Symp. Ser. 416 282-292... 

Machesky M. L. (1989) Influence of temperature on ion adsorption by hydrous metal oxides. In Chemical Modeling of Aqueous System II (eds. D. C. Melchior and R. L. Bassett). American Chemical Society, Washington, DC, vol. 416, pp. 282-292. 

Influence of Temperature on Ion Adsorption by Hydrous Metal Oxides... 

Cation and anion adsorption by hydrous metal oxides influence several processes of environmental concern including contaminant transport, nutrient availability, and mineral dissolution rates (i,2). Various factors influence the amount of a particular ion adsorbed including solution pH, type of oxide and its surface area and crystallinity, time, ionic strength, properties and concentration of the adsorbing species, and competing species. These factors have received various degrees of scrutiny in previous studies. Temperature is another potentially important variable but has not to date received as much... 

Metal hydroxides in general are anion-selective in acid solution and turn to be cation-selective beyond a certain pH, called the point of the iso-selectivity, pHpjS it is pHpjS = 10.3 for ferric oxide and pHpis = 5.8 for ferric-ferrous oxide [72]. Adsorption of multivalent ions may also control the ion selectivity of hydrous metal oxides because of its effect on the fixed charge in the oxides. For instance, hydrous ferric oxide, which is anion-selective in neutral sodium chloride solution, turns to be cation-selective by the adsorption of such ions as divalent sulfate ions, divalent molybdate ions, and trivalent phosphate ions [70,73]. It is worth emphasizing that such an ion-selectivity change due to the adsorption of multivalent ions frequently plays a decisive role in the corrosion of metals. 

Mechanisms of Sorption Processes. Kinetic studies are valuable for hypothesizing mechanisms of reactions in homogeneous solution, but the interpretation of kinetic data for sorption processes is more difficult. Recently it has been shown that the mechanisms of very fast adsorption reactions may be interpreted from the results of chemical relaxation studies (25-27). Yasunaga and Ikeda (Chapter 12) summarize recent studies that have utilized relaxation techniques to examine the adsorption of cations and anions on hydrous oxide and aluminosilicate surfaces. Hayes and Leckie (Chapter 7) present new interpretations for the mechanism of lead ion adsorption by goethite. In both papers it is concluded that the kinetic and equilibrium adsorption data are consistent with the rate relationships derived from an interfacial model in which metal ions are located nearer to the surface than adsorbed counterions. 

Two types of solid substrate surfaces were used in this study amorphous hydrous iron(lll) oxide (HFO) and amorphous hydrous chromium(III) oxide (HCO). Both substrates were prepared by slowly increasing the acidic pH of either an iron(III) nitrate or chromium(IIl) nitrate solution of concentration 250 ppm with respect to the metal ion in question. Metal ion adsorption or coprecipitation experiments using amorphous hydrous metal oxide substrates are generally described in terms of the concentration (ppm) of metal ion that was used to form the colloidal adsorbent rather than its corresponding specific surface area (m / L) [64,65]. 

The type of substrate the metal is bound to, determines its propensity to be mobilized under completely specific conditions - the metal co-precipitated with carbonates can reach the water environment due to the decrease of a pH, while ions sorbated onto hydrous ferric oxides will be mobilized if there is a decrease in the redox potential, etc. On the other hand, the strength of the connection which the ion of a heavy metal establishes with its substrate will directly determine its mobility - the ion bound by weak, adsorptive connections to some substrate will easily convert into a solution, while the huminous associated metals will be firmly bound to... 

The main mechanism of ligand adsorption is ligand exchange the surface hydroxyl is exchanged by another ligand. This surface complex formation is also competitive OH ions and other ligands compete for the Lewis acid of the central ion of the hydrous oxide (e.g., the Al(iii) or the Fe(III) in aluminum or ferric (hydr)oxides). The extent of surface complex formation (adsorption) is, as with metal ions, strongly... 

In Fig. 3.5 we illustrated generally that an alkalimetric or acidimetric titration curve of a hydrous oxide dispersion becomes displaced by the adsorption of a metal ion or, - in opposite direction - by the adsorption of an anion (ligand). 

Metal oxides selectively adsorb divalent cations even at solution pH values lower than the PZC of metal oxides. The mechanism of metal ion association with hydrous-oxide surfaces involves an ion-exchange process in which the adsorbed cations replace bound protons. Specifically, adsorbed cations raise the value of PZC of oxides. pH affects adsorption of metal cations, either by changing the number of sites available for adsorption or by changing the concentration of the cation species (Me +, MeOH+, Me(OH)2) that are preferentially adsorbed (Jackson, 1998). 

R. M. McKenzie, The adsorption of lead and other heavy metals on oxides of manganese and iron, Aust. J. Soil Res. 18 61 (1980). H. Kerndorf and M. Schnitzer, Sorption of metals on humic acid, Geochim. Cosmochim. Acta 44 1701 (1980). D. G. Kinniburgh, M. L. Jackson, and J. K. Syers, Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminium, Soil Sci. Soc. Am. J. 40 796 (1976). H. Farrah and W. F. Pickering, Influence of clay-solute interactions on aqueous heavy metal ion levels, Water, Air and Soil Pollution 8 189 (1977). 

J.T.G. Overbeek, Electrokinetic phenomena, in Colloid Science, Vol. I (H. R. Kruyt, ed.). Elsevier, Amsterdam, 1952. R. O. James and T. W. Healy, Adsorption of hydrolyzable metal ions at the oxide-water interface. II Charge reversal of SiOa and Ti02 colloids by adsorbed Co(II), La(III), and Th(IV) as model systems, J. Colloid Interface Science 40 53 (1972). C.-P. Huang and W, Stumm, Specific adsorption of cations on hydrous a-Al203, /. Colloid Interface Science 43 409 (1973). S. L. Swartzen-Allen and . Matijevid, Colloid and surface properties of clay suspensions. II Electrophoresis and cation adsorption of montmorillonite, /. Colloid Interface Sci. 50 143(1975). G. R. Wiese, R. O. James, D. E. Yates, and T. W. Healy, Electrochemistry of the colloid-water interface, in Electrochemistry (J. O M. Bockris, ed.). Butterworths, London, 1976. D, W. Fuerstenau, D. Man-mohan, and S. Raghavan, The adsorption of alkaline-earth metal ions at the rutile/aqueous solution interface, in P. H. Tewari, op. cit. ... 

Hydrolysis and Adsorption. Some years ago, a theory was advanced, that hydrolyzed metal species, rather than free metal ions, are adsorbed to hydrous oxides. The pH-dependence of adsorption (the pH edge for adsorption is often close to the pH for hydrolysis) was involved to account for this hypothesis. As Figs. 2.7b and c illustrate, there is a correlation between adsorption and hydrolysis but this correlation is caused by the tendency of metal ions to interact chemically with the oxygen donor atoms with OH, and with S-OH. The kinetic work of Hachiya et al. (1984) and spectroscopic information are in accord with the reaction of (free) metal ions with the surface. 

Effect of ligands and metal ions on surface protonation of a hydrous oxide. Specific Adsorption of cations and anions is accompanied by a displacement of alkalimetric and acidimetric titration curve (see Figs. 2.10 and 3.5). This reflects a change in surface protonation as a consequence of adsorption. This is illustrated by two examples ... 

Waste constituents may be immobilized in a soil system mainly by sorption and/or partitioning. Adsorption on soil particles is competitive, pH-dependent and, usually, inversely proportional to the solubility of the compound in water. Dry soils are better adsorbents than wet ones. HSs are able to form complexes with metal ions and hydrous oxides and also interact with minerals and a variety of organic compounds, including alkanes, fatty acids, dialkyl phthalates, pesticides and herbicides, and may therefore increase the concentration of these constituents in soil and natural waters. 

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... 

F ure 9.21. The net charge at the hydrous oxide surface is established by the proton balance (adsorption of or OH and their complexes) at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidi-metric titration curve and from a measurement of the extent of adsorption of specifically adsort)ed ions. Specifically adsorbed cations (anions) increase (decrease) the pH of the point of zero charge (pzc) or the isoelectric point but lower (raise) the pH of the zero net proton condition (pznpc). Addition of a ligand, at constant pH, increases surface protonation while the addition of a metal ion (i.e., specifically adsorbed) lowers surface protonation. (Adapted from Hohl et al., 1980.)... 

Figure 7. The effect of ligands and metal ions on surface protonation of a hydrous oxide is illustrated by two examples (1). Part a Binding of a ligand (pH 7) to hematite, which increases surface protonation. Part h Adsorption of Pb2+ to hematite (pH 4.4), which reduces surface protonation. Part c Surface protonation of hematite alone as a function of pH (for comparison). All data were calculated with the following surface complex formation equilibria (1 = 5 X 10"3 M >. Electrostatic correction was made by diffuse double layer model.

Since the charge of many polar surfaces (especially of metal (hydrous) oxides) is determined by the concentration of H,0+ and OH" ions, the surfactant adsorption is strongly influenced by the solution pH. Changes in the pH may drastically affect the value of T, the concentration ranges corresponding to different regions on the isotherm (Fig. III-8), and the shape of the isotherm itself. 

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