Hydrous oxides adsorption

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

Strong adsorption on Fe and Mn oxides and hydrous oxides Precipitation... 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

Silica used as a filler for rubbers is silicon dioxide, with particle sizes in the range of 10-40 nm. The silica has a chemically bound water content of 25% with an additional level of 4-6% of adsorbed water. The surface of silica is strongly polar in nature, centring around the hydroxyl groups bound to the surface of the silica particles. In a similar fashion, other chemical groups can be adsorbed onto the filler surface. This adsorption strongly influences silica s behaviour within rubber compounds. The groups found on the surface of silicas are principally siloxanes, silanol and reaction products of the latter with various hydrous oxides. It is possible to modify the surface of the silica to improve its compatibility with a variety of rubbers. 

Some emphasis is given in the first two chapters to show that complex formation equilibria permit to predict quantitatively the extent of adsorption of H+, OH , of metal ions and ligands as a function of pH, solution variables and of surface characteristics. Although the surface chemistry of hydrous oxides is somewhat similar to that of reversible electrodes the charge development and sorption mechanism for oxides and other mineral surfaces are different. Charge development on hydrous oxides often results from coordinative interactions at the oxide surface. The surface coordinative model describes quantitatively how surface charge develops, and permits to incorporate the central features of the Electric Double Layer theory, above all the Gouy-Chapman diffuse double layer model. 

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... 

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. 

The net charge at the hydrous oxide surface is established by the proton balance (adsorption of H or OH" and their complexes at the interface and specifically bound cations or anions. This charge can be determined from an alkalimetric-acidimetric titration curve and from a measurement of the extent of adsorption of specifically adsorbed 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). 

We consider here first, the kinetics of the adsorption of metal ions on a hydrous oxide surface. 

Adsorption of Metai Ions to a Hydrous Oxide Surface... 

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). 

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 ... 

The oxidation of Fe(II), V02+, Mn2+, Cu+ by oxygen is favored thermodynamically and kinetically by hydrolysis and by specific adsorption to hydrous oxide surfaces. As suggested in formula (IV) of Fig. 9.1 Fe(II) and the other transition elements Mn(II), VO(II), Cu(I) may more readily associate (probably outer-spherically) with... 

Distribution coefficients based on adsorption equilibria are independent of the total concentrations of metal ions and suspended solids, as long as the metal concentrations are small compared with the concentration of surface groups. Examples of the Kd obtained from calculations for model surfaces are presented in Fig. 11.1. A strong pH dependence of these Kd values is observed. The pH range of natural lake and river waters (7 - 8.5) is in a favorable range for the adsorption of metal ions on hydrous oxides. 

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. 

Relaxation studies have shown that the attachment of an ion to a surface is very fast, but the establishment of equilibrium in wel1-dispersed suspensions of colloidal particles is much slower. Adsorption of cations by hydrous oxides may approach equilibrium within a matter of minutes in some systems (39-40). However, cation and anion sorption processes often exhibit a rapid initial stage of adsorption that is followed by a much slower rate of uptake (24,41-43). Several studies of short-term isotopic exchange of phosphate ions between aqueous solutions and oxide surfaces have demonstrated that the kinetics of phosphate desorption are very slow (43-45). Numerous hypotheses have been suggested for this slow attainment of equilibrium including 1) the formation of binuclear complexes on the surface (44) 2) dynamic particle-particle interactions in which an adsorbing ion enhances contact adhesion between particles (43,45-46) 3) diffusion of ions into adsorbents (47) and 4) surface precipitation (48-50). 

As stated previously, it is difficult to assign ions to a few discrete "mean planes of adsorption" and have these mean planes correspond in every case to the location of ions expected from hypothetical structure and bonding at the hydrous oxide surface. 

P=f(Xp. pH). As described above, the magnitude of P is inexorably linked to the variations of x with pH and adsorption density. However, the response of x (and P) to T and pH varies among hydrous oxides. For example, Figure 9a shows the instantaneous (isotherm) proton coefficient (xp) "zones" determined for Cd ion adsorption onto (am)Fe20o O, a-A O and oc-TiC. The zones are defined by the calculated proton coefficients determined for a range of pH values and adsorption density. The "thickness" of each zone gives a qualitative comparison of the pH dependency of Xp at each adsorption... 

Anderson, P.R. Malotky, D.T. (1979) The adsorption of protolyzable anions on hydrous oxides at the isoelectric pH. J. Colloid Interface Sci. 72 413-427... 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

In Skirmer, H.G.W. Fitzpatrick, R.W. (eds.) Biomineralization processes of iron and manganese. Catena Verlag, Cremhngen-Destedt, Catena Suppl. 21 75—99 Ghoneimy, H.F. Morcos.T.N. Misak, N.Z. (1997) Adsorption of Co and Zn ions on hydrous Fe(lll), Sn(lV) and mixed Fe(lll)/ Sn(IV) oxides. Part 1. Characteristics of the hydrous oxides, apparent capacity and some equilibria measurements. Colloids Surfaces A. 122 13-26... 

Hayes, KF. Papelis, C. Leckie, J.O. (1988) Modeling ionic strength Effects on anion adsorption at hydrous oxide/solution interfaces. J. Colloid Interface Sd. 78 717—726 Hayes, KF. Roe, A.L. Brown, G.E. Hodgson, KO. Leckie, J.O. Parks, G.A. (1987) In-situ X-ray absorption study of surface complexes Selenium oxyanions on a-FeOOH. Sdence 238 783-786... 

Anion adsorption by goethite and gibbsite. II. Desorption of anions from hydrous oxide surfaces. J. Soil Sci. 25 16-26 Hiradate, S. Inoue, K. (1998) Interaction of mugineic acid with iron (hydr)oxides Sulfate and phosphate influences. Soil Sci. Soc. 

Adsorption of alkaline earth, transition, and heavy metal cations by hydrous oxide gels of iron and aluminum. Soil Sci. Soc. Am. J. 40 796-799... 

Specific adsorption of zinc and cadmium by iron and aluminum hydrous oxides. In Proc. 15 Hanford Life Sci. Symp. on Biol. Implications of Metals in the Environment, Hanford,Washington, USERDA-NTIS, 231-239... 

Kraemer, S.M. Hering, J.G. (1997) Influence of solution saturation state on the kinetics of ligand-controlled dissolution of oxide phases. Geochim. Cosmochim. Acta 61 2855-2866 Kraemer, S.M., Xu, J., Raymond, K.N. Spo-sito, G. (2002) Adsorption of Pb(II) and Eu(III) by oxide minerals in the presence of natural and synthetic hydroxamate sidero-phores. Environ. Sd. Technol. 36 1287-1291 Kraemer, S.M. Cheah, S.-F. Zapf, R. Xu, J. Raymond, KN. Sposito, G. (1999) Effect of hydroxamate siderophores on Fe release and Pb(II) adsorption by goefhite. Geochim. Cosmochim. Acta 63 3003—3008 Kratohvil, S. Matijevic, E. (1987) Preparation and properties of coated uniform colloidal partides. I. Aluminum (hydrous) oxide on hematite, diromia, and titania. Adv Ceram. Mater. 2 798-803... 

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