Surface protonation isotherms

Surface protonation isotherms. Dots represent experimental data from titration curves at ionic strength I = 0.1 (Hematite, I = 0.2). References are indicated in Table 3.1. The concentration of protonated sites MOH is given in moles nr2. BET surface data were used to calculate the surface concentration. 

Schulthess, C. P., and D. L. Sparks. 1989. Back titration technique for proton isotherm modeling of oxides surface. Soil Sci. Soc. Am. J. 50 1406-1411. 

Uptake of methylphosphonic, aminomelhyl-phosphoniCs hydroxymethyl-phosphonic. l-hydro ycthane-(l, 1-diphosphonic), iminodi-(methylphosphonic). nitnlotris-(methylene-phosphonic). elhylcncdimtnlotetrakis-(methylcnephosphonic), and diethylenetruu-tnlopentakis-(methylenephosphonic) acids was interpreted in terms of formation of 2-9 different surface species whose stability constants are interrelated, namely, log K = (1145 + 7.31 nH2.53 + 0 46n>Z where n is the surface protonation level and Z IS the surface complex charge, and fully deprotonated anions are componenls. Adsorplion isotherms at constant pH were also obtained in the presence of buffers lEP at pH 7 2 in dispersion titrated with Na COj. pristine lEP at pH 8.5. 

Figure 17 shows the chemical structures of anionic amphiphile sodium-1,2-bis (tetradecylcarbonyl)ethane-l-sulfonate (2Cj4SNa)[34] and poly(ethyleneimine)(PEI). A benzene/ethanol (9 1)(WV) solution of anionic amphiphile was spread on the pure water surface or the PEI-water solution (lxlO5 unit M in monomer unit, pH=3.2) surface at a subphase temperature, Tsp of 293 K. At this pH, ca. 70 % of nitrogen atom in PEI molecule was protonated[35]. Surface pressure-area(ji-A) isotherms were measured with a microprocessor controlled film balance system. 

The fact that Tdhpz on Au is identical to that on Pt at pH 7 is evidence that ri1-N surface-coordinated DHPz is also formed on Au at this same pH. Since (i) the DHPz isotherm on Au at pH 0 is not stepwise unlike those exhibited by compounds attached in multiple orientational states, and (ii) it has already been shown above that hydroquinone, the homoaromatic analogue of DHPz, is not chemisorbed on Au, it can be argued that i -N surface-coordination of DHPz occurs on Au at pH 0 even at coverages below the saturation value. It can be inferred further that the driving forces for protonation and Au-surface-coordination of the N heteroatom are equally competitive in molar acid. 

Preparation of the PILC. As seen in Table 1, two factors determine the extent of A1 fixation (% Al O ) by the clay the final pH of the solution and the size of the clay particles. The influence of pH is readily explained by the equilibrium of formation of the polymer and by a competitive exchange w th the protons. The surface area increases from 42 to 180-360m /g upon intercalation, as reported on Table 1, and seems to be determined by the amount of A1 fixation. It appears that on sample G the extent of A1 fixation reaches a plateau at Al/clay=5. After this, diffusional limitations control the exchange on the large particles.The N2 adsorption gives a typical type IV isotherm, with 70% of the surface area localized in micropores smaller than 20A, after dehydration at 300°C. 

Acid/hase potentiometry enables the surface charge density to be measured. This involves comparison of the titration curves obtained for the suspension of oxide at several different ionic strengths (10 10" M) with that of the electrolyte alone, followed by calculation of the net consumption of protons or hydroxyl ions (mol g ) at each pH. The data is presented as a plot of excess of acid or base (Fh - Toh ) mol g or mol m ) vs pH (adsorption isotherm) or as a plot of surface charge, cr, (coulombs m ) vs pH (charging curve) (Figure 10.5). 

Perrone et al. (2001) modelled Ni(II) adsorp-tion to synthetic carbonate fluoroapatite (CaI0 ((P04)5(C03))(0H,F). The solid phase had a pHIEP of 6.3 and a ZPC of 6.4 with an SSA of 8.8m2/g, an estimated sorption site density of 3.1 sites/nm2. They conducted 8-day isotherms in closed vessels at Ni concentrations of 5 x 10-10 to 1 x 10 8 M, constant I (0.05, 0.1 or 0.5 M), constant solid phase concentrations of 10 g/dm3 at pH values of 4 to 12. As Ni sorption occurred, no significant release of Ca was seen. Sorption was reversible. Rather than precisely characterize surface functional groups, they elected to describe their sorbent surfaces using acid-base reactions for the average behaviour of all sites involved in protonation and deprotonation. Potentiametric titration data were used to estimate the constants with the FTTEQL computer code ... 

The model had been substantiated by measuring the potential dependent electrosorption isotherms of all species involved that show that the proto-nated alcaloids are those species that are by far most strongly adsorbed, whereas the acetyl pyridines are least strongly adsorbed, especially at lower pH relative to the electrosynthesis, which is performed at pH 4 to 4.5. The optically inductive reduction of 2- and 4-acetyl pyridine to the optically active carbinols demands the formation of a dense but not too densely packed surface layer of adsorbed protonated alcaloid, which still allows for insertion of the oxo-compound or the ketyl radical, respectively. Performing the reaction with too high alcaloid concentrations leads to compaction on the adsorbate layer, the ketyl radical is squeezed out, and optimal induction is no longer observed. 

Martin et al. (1996) studied the surface structures formed when 4-chloro-catechol adsorbs onto Ti02. These surface interactions were studied to gain a better understanding of how these surface structures affect photoreactivity. Adsorption isotherms of 4-chlorocatechol demonstrate that the compound adsorbs to a greater extent at pH values 7 to 9. The interactions of protons and 4-chlorocatechol with the Ti02 surface are explained by the double layer theory (Martin et al., 1996). 

Aluminum oxides As(V) and As(lll) adsorption on activated alumina pH dependence, kinetics, and column breakthrough. Regeneration by desorbing with NaOH. Modeling with pH-dependent Langmuir isotherm (for As) and surface complexation model (for protons) Ghosh and Yuan (1987)... 

A significant problem in surface complexation models is the definition of adsorption sites, The total number of proton-exchangeable sites can be determined by rapid tritium exchange with the oxide surface (25). Although surface equilibria are usually written in terms of one surface site, e.g. Equations 5, 6, 8, 9, adsorption isotherms for many ions show that the number of molecules adsorbed at maximum surface coverage (fmax) is less than the total number of surface sites. For example, uptake of Se(VI) and Cr(VI) ions on Fe(0H)3(am) at T ax 1/3 and 1/4 the total... 

In Eqs. 31J and 32J we used Ihe same value of the size parameter n. This assumption may not be generally correct, since the species being adsorbed are not identical. Even if they do not differ in actual size (the removal of one proton does not change the size of a phenol molecule significantly), their orientation on the surface could be quite different, and the number of water molecules replaced may not be the same. This is not relevant to the derivation of the combined adsorption isotherm, since we are dealing with the process shown in Eq.32J only. 

Figure 4 Comparison of sorption models. Several commonly used sorption models are compared with respect to the independent constants they require. These constants are vahd only under specific conditions, which must be specified in order to properly use them. In other words, the constants are conditional with respect to the experimental variables described in the third column of the figure. is the radionuclide distribution constant K and n are the Freundlich isotherm parameters and are surface complexation constants for protonation and deprotonation of surface sites K-, are surface complexation constants for sorption of cations and anions in the constant...

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