Hydrolyzable ions, adsorption

In view of the fact that no hafnium is present in the unhydrolyzed form under any of the conditions given in Table II, the ion exchange mechanism of adsorption by glass may be disregarded. This is substantiated by the fact that a different treatment of the glass had no effect upon its adsorption capacity for hafnium. This is similar to the results of Starik and Rozovskaya who found small effects upon the adsorptivity of hydrolyzed ions caused by glass modification even when drastic treatment... 

System 2. The presence of polyvalent lattice ions in the system containing minerals with PDIH+ and OH- leads to their specific adsorption in the EDL and is often accompanied with a change in IP and PZC provided the surface charge has the opposite sign of that of the adsorbing lattice ion. This leads to an inhibition or activation of the mineral surface as shown in Fig. 16179). The same is true for a hydrolytic product of lattice ions exhibiting a stronger surface activity than non-hydrolyzed ions, as a result of a combined electrostatic and chemisorptive effect. 

Properties of a° (pH) curves are basic elements in the Interpretation of more complicated systems involving oxides. One of these is the adsorption of hydrolyzable ions (Cd, Al, etc.) or anions that themselves can be titrated (HPO, etc.). In sec. 3.14 some of the relevant applications will be discussed. Another application is that of mixed oxides. The systems include mechanically mixed pure oxides and mixed crystals (such as spinels and ferrites). A number of authors have studied such mixed oxides, thereby reporting the variation of the pH° as a function of the mole fraction of the solid. Sometimes linearity was found, sometimes not. No genered rules can be given. The surface composition is not necessarily identical to that of the bulk, molecules of one oxide may leach and adsorb onto the other and lateral interactions of surface groups of the two constituents affect their pK s and pK s. Mixed oxides are important for a number of technical applications (heterogeneous catalysts with special properties, components of batteries) and also occur in clay minerals, the topic of the following subsection. 

The hydrolysis in Eqs. 3.4, 3.6, and 3.7 oversimplifies the actual mechanism of hydrolysis. The hydrolyzed ions begin to associate and form polynuclear ions, which enlarge with further hydrolysis. These polynuclear ions form in solutions and may exist in natural waters for a long time before they precipitate as a solid. Whether they exist in soil solutions, with the soil s large and adsorptive surface area and at the low concentrations in soil solutions, is less certain. When ions dissolve from solids, on the other hand, the ions formed most likely remain as mononuclear species because their concentrations are so low. 

Crosslinked styrene (St)/maleic anhydride (MA) eopolymers have been synthesized, hydrolyzed with dicarbojgrlie aeid, and converted to bear dihydrojg hosphino functionalities. On the dihydro3QT>hosphino-funetionalized styrene-methyl acrylate (20% divinylbenzene) copolymer, the adsorption of Pb showed a linear relationship with the concentrations and fitted the Langmuir isotherm. The kineties of Pb adsorption on this dihydrojg hosphino-functionalized eopolymer were studied. The metal-ion adsorption kinetics of this copolymer appeared to show particle diffusion. They explained that the moving boundary advanced from the surface of the molecule towards the center. ... 

Analogous results have been obtained with a large number of colloid dispersions and a variety of hydrolyzable ions. Since the hydrolysis products vary with the metal ion and the pH, their adsorption and charge reversal phenomena will differ in each case, as shown with ferric [15], chromium [16], zinc [17], and other ions. 

Finally, the surface charge modifications by hydrolyzed ions can be used to control the adsorption behavior of a solid. For example, if the sign is converted from negative to positive, anionic species will be attracted and vice versa. In addition, the adsorbed solutes may then act as coupling agents for other adsorbates, such as dyes, amino acids, etc. [21,22]. 

To date, potentiometric titration is still a main approach to study the surface acid base chemistry of clay minerals. Only some papers deal with the dissolution of a solid matrix resulting in various hydrolyzed aluminum species, silicic acid and their product hydrous aluminosilicates, though their interaction with a clay surface should be considered in the modeling description. The surface complexation model (SCM) was successfully applied in a recent paper [6] to interpret surface acid-base reactions involving the dissolution of illite clays during prolonged titration. Voluminous literature on ion adsorption and surface complexation... 

The adsorption of hydrolyzing ions Cr and Co on siUca gel was investigated [78], and speciation of mononucleus hydroxy species in the equiUbrium aqueous phase was also considered to calculate the equilibrium distribution between the soUd phase and bulk solution over the broad range of pH s. Although the different surface complexes of chromium and cobaltic assumed to form on surface sites of silica were not proved by any independent method, the fitting procedure was executed successfully, if only all the possible equilibria were accounted for in the SCMs. 

Adsorption of hydrolyzable ions takes places through various mechanisms in which hydrolysis by surface nucleophilic groups and hydrolysis by the solution itself (OH , water molecules) compete. Therefore, as long as the hydrolysis and precipitation pH is not reached, the surface is likely to behave as a ligand, leading to chemisorption through a true substitution reaction ... 

Structure Modification. Several types of stmctural defects or variants can occur which figure in adsorption and catalysis (/) surface defects due to termination of the crystal surface and hydrolysis of surface cations (2) stmctural defects due to imperfect stacking of the secondary units, which may result in blocked channels (J) ionic species, eg, OH , AIO 2, Na", SiO , may be left stranded in the stmcture during synthesis (4) the cation form, acting as the salt of a weak acid, hydrolyzes in aqueous suspension to produce free hydroxide and cations in solution and (5) hydroxyl groups in place of metal cations may be introduced by ammonium ion exchange, followed by thermal deammoniation. 

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. 

If a hydrolyzed metal ion is adsorbed, its OH" will be included in the proton balance similarly, in case of adsorption of protonated anions, their H+ will be included in the proton balance. 

Exchange of varying quantities of the oxovanadium(IV) ion on Mg hectorlte resulted In hydrolysis of V at low levels of adsorption (53). The hydrolyzed product that was adsorbed on the clay surface was interpreted as having a ligand environment that was partially aqueous and partially hydroxide in nature. With increasing V... 

R.O. James, T.W. Healy, Adsorption of hydrolyzable metal ions at the oxide — water interface. II. Charge reversal of Si02 and Ti02 colloids by adsorbed Co(II), La(III), and Th(IV) as model systems, J. Colloid Interface Sd. 40 (1972) 53-64. 

In the adsorption of some metal ion by tannin adsorbents [16], tannins are widely distributed in nature and have multiple adjacent phenolic hydroxyl groups and exhibit specific chelation ability toward metal ions [17]. According to the chemical stractures of tannins, they can usually be classified into hydrolyzable tannins, condensed tannins and complex tannins. Hydrolyzed tannins yield galhc acid or ellagic acid when hydrolyzed by acid, base or some enzymes [18]. Turkish sumac tannin (hydrolyzable tannin) is illustrated in Fig. 28.1 whose basic stracture is of flavan-3-ols. 

Jambor, J.L. Dutrizac, J.E. (1998) Occurrence and constitution of natural and synthetic fer-rihydrite, a widespread iron oxyhydroxide. Chem. Rev. 98 2549-2585 James, R.O. ElealyT.W. (1972) Adsorption of hydrolyzable metal ions at the oxide-water interface. Ill A thermodynamic model of adsorption. J. Colloid Interface Sci. 40 65-81 James, R.O. Parks, G.A. (1982) Characterization of aqueous colloids by their electrical double layer and intrinsic surface chemical properties. Surface Colloid Sci. 12 119-126... 

Before concentration, acid hydrolyzates are neutralized, most commonly with barium carbonate, although such organic bases as methyldioctylamine has been used.81 This step normally causes little loss, except by adsorption on, for example, barium sulfate,82 but the following points are of interest. Neutralization with ammonia has been recommended,83 as the neutral solution may be evaporated directly to dryness without filtration, and the ammonium sulfate formed is insoluble in methyl sulfoxide, a solvent used for trimethyl-silylation. The authors83 also found that, when hydrolyzates are neutralized with ion-exchange resins, the pH of the concentrated solutions may differ by as much as 2 units of pH. D-Fructose has been found to be epimerized by barium carbonate or pyridine, and lead... 

Analysis of glycosaminoglycan hydrolyzates is often complicated by the presence of basic and acidic glycoses, in addition to neutral monosaccharides. Adsorption chromatography on silica gel was used to classify such a hydrolyzate, and the neutral fraction was shown to contain D-glucose, D-galactose, and D-mannose.288 Separation on ion-exchange resins may also be used.51,149 The resolution of the acidic and basic fractions is discussed in Sections IX (p. 71) and X (p. 78). 

Several attempts have been made to correlate the adsorptivity of hydrolyzable cations to the composition of the species in aqueous solution (1, 2, 20). In particular, the adsorption of thorium on silver halides indicated a very close relationship between the change in the amount of thorium adsorbed and the concentration of the hydrolyzed soluble species in solution (19). The major difficulty in this type of work is the lack of quantitative data on the hydrolysis of various metal ions. The other uncertainty is with regard to the knowledge of the true surface area of the adsorbent in aqueous solution. This latter information is needed if surface coverages are to be evaluated. 

At least some of these difficulties have been overcome in the work to be reported in this study, which deals with the adsorption of hafnium hydrolyzed species on powdered glass as a function of the acidity of the medium. The adsorption of hafnium species from aqueous solution has apparently never been investigated, yet this ion lends itself conveniently to studies of the problems discussed above. The chemistry of the hafnium ion in water is fairly well understood (23) and a suitable isotope, 181Hf, is available for adsorption studies. What makes hafnium a particularly interesting system is the fact that it forms the entire series of hydrolyzed species Hf(OH)n(4 r )+ where n 4. At intermediate acidities (pH > 4) the solutions of low concentrations contain only the neutral, soluble species Hf(OH)4. It should be emphasized that there is a pH and a concentration range over which this species is present without simultaneous formation of hafnium hydroxide. Thus, it is possible to elucidate the effect of the ionic charge upon the adsorption of hydrolyzed species in systems void of colloidal hydroxides. The glass powder was used in... 

Another advantage of hafnium is that, if the adsorption of the neutral species takes place, a close packed monolayer should eventually result owing to the absence of electrostatic repulsion. Knowing the surface area of the adsorbent this would enable one to evaluate the cross-sectional area of the hydrolyzed complex ion. This information has not been available. 

Precipitation of Hafnium Hydroxide. In order to interpret the adsorption data it was necessary to determine the conditions which lead to the precipitation of hafnium hydroxide. It is not usually advisable to depend on the solubility product because the information on this quantity is often unreliable for hydroxides of polyvalent metal ions. In addition, "radiocolloids may apparently form much below saturation conditions in radioactive isotope solutions. In the specific case of hafnium hydroxide only two measurements of the solubility seem to have been reported. According to Larson and Gammill (16) K8 = [Hf(OH)22+] [OH ]2 — 4 X 10"26 assuming the existence of only one hydrolyzed species Hf(OH)22+. The second reported value is Kso = [Hf4+] [OH-]4 = 3.7 X 10 55 (15). If one uses the solubility data by Larson and Gammill (Ref. 16, Tables I and III) and takes into consideration all monomeric hafnium species (23) a KBO value of 4 X 10 58 is calculated. 

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