Hydrous oxides isotherms

Fig. 6.10 shows idealized isotherms (at constant pH) for cation binding to an oxide surface. In the case of cation binding, onto a solid hydrous oxide, a metal hydroxide may precipitate and may form at the surface prior to their formation in bulk solution and thus contribute to the total apparent "sorption". The contribution of surface precipitation to the overall sorption increases as the sorbate/sorbent ratio is increased. At very high ratios, surface precipitation may become the dominant "apparent" sorption mechanism. Isotherms showing reversals as shown by e have been observed in studies of phosphate sorption by calcite (Freeman and Rowell, 1981). 

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... 

Hydrous oxides Exchange cations References (type of isotherm )... 

Meyer R., Palmer D., Arnold W., and Case F. (1984) Adsorption of nuclides on hydrous oxides. Sorption isotherms on natural materials. Geochem. Behavior Disp. Radioact. Waste, 79—94. 

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. 

The specific adsorption of H, OH, cations, and anions on hydrous oxides and the concomitant establishment of surface charge can be interpreted in terms of the formation of surface complexes at the oxide-water interface. The fixed charge of the solid surface and the pH of its isoelectric point can be measured experimentally by determining the proton balance at the surface (from alkalimetric titration curve) and by the analytical determination of the extent of adsorbate adsorption. Equilibrium constants established for the surface coordination reactions can be used to predict pHiEp, to calculate adsorption isotherms, and to estimate concentration-pH regions for which the hydrous oxide dispersions are stable from a colloid-chemical point of view. 

The constant capacitance model has been applied successfully to describe the adsorption of both inorganic and organic anions by hydrous oxides.Both adsorption isotherms and adsorption envelopes like those illustrated in Fig. 4.8 are accounted for quantitatively, except in the case of sulfate adsorption. It is possible, however, that this anion adsorbs through outer-sphere instead of inner-sphere complex formation, as discussed in Sec. 4.4. Aside from sulfate, anion adsorption envelopes are predicted in the constant capacitance model on the basis of an increase in [OH ], which causes the concentrations of the surface complexes in Eq. 5.53 to decrease, and a decrease in o-p. 

Representative nitrogen adsorption isotherms for the TH(400) series of hydrous oxides and the corresponding as plots are shown in figure 1. as is defined as (amount adsorbed)/(amount adsorbed at P/P = 0.4) [17]. Details of the construction of the as plots, as well as that of the calculation of the surface areas Sbet and Ss, the pore volumes and the effective micropore volumes are given elsewhere [17,18]. Table 1 presents these values for the TH and TH(400) series of gels. 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

Table 6.1 Type of Isotherm for the Various Hydrous Metal Oxides...

Figure 7. A log-log plot of a sorption isotherm, with an inflection indicating the transition from adsorption to surface precipitation processes. On the right, illustrative Fourier transformed EXAFS spectra for the adsorbate (solid curve) are compared with that for a precipitate (dotted curve) to show the adsorption — precipitation transition. Data are from Charlet and Manceau (48) for Cr(III) sorbed on hydrous ferric oxide.

Hehny, A.K., de Bussetti, S.G., and Ferreiro, E.A., Sorption isotherms of mono-and divalent phosphate on hydrous Al oxides. Colloids Surf., 58, 9,1991. 

This and the following paper are progress reports in our continuing study of the catalyzed conversion of ortho and parahydrogen. Last year we presented some converter design data based on experiments with a new hydrous ferric oxide gel catalyst. These data consisted of space velocity versus conversion curves for ortho to para conversions at various temperatures, two feeds, liquid and vapor phase, and gave a comparison between results obtained under approximately isothermal and adiabatic conditions. 

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