Adsorption time

A useful approach to the phenomenon of adsorption is from the point of view of the adsorption time, as discussed by de Boer [4]. Consider a molecule in the... 

The rate constants k and ki may be related to the concepts of the preceding section as follows. First, k is simply the reciprocal of the adsorption time, that is. 

Fig. 5.10. Series of sequential in-situ AFM images of the growth of ODS on silicon. The numbers indicate the adsorption time in minutes.

Current use of statistical thermodynamics implies that the adsorption system can be effectively separated into the gas phase and the adsorbed phase, which means that the partition function of motions normal to the surface can be represented with sufficient accuracy by that of oscillators confined to the surface. This becomes less valid, the shorter is the mean adsorption time of adatoms, i.e. the higher is the desorption temperature. Thus, near the end of the desorption experiment, especially with high heating rates, another treatment of equilibria should be used, dealing with the whole system as a single phase, the adsorbent being a boundary. This is the approach of the gas-surface virial expansion of adsorption isotherms (51, 53) or of some more general treatment of this kind. 

FIGURE 3-17 Adsorptive stripping potentiogranis for 0.5 ppm calf-thymus DNA following different adsorption times of 1-150 s (curves a-f) (Reproduced with permission from reference 38.)... 

Table 8. Influence of Au C ratio on particle size and adsorption time.

IRRAS spectrum collected at 0.4 V. In this way the build-up of C=Onds with adsorption time was monitored, as shown in Figure 3.36(b). From Figures 3.36(a) and (b) it is clear that the deactivation of the platinum electrode is closely related to the increase in C=Oads. 

Adsorption Kinetics. Figure 1 shows the adsorbance, Ap, of NaPSS-3 as a function of adsorption time for two NaCl concentrations at the NaPSS concentration of 0.04 g/lOOml. Hie Ap first increases with adsorption time and then the equilibrium adsorbance is attained after 1.5 x 10J minutes. 

The thickness, t, of the adsorbed layer for NaPSS-3 is plotted against adsorption time as shown in Figure 2. Hie t also increases with increasing adsorption time and becomes a constant value. Similar time dependence was obtained in the other experiments. Therefore, both Ap and t determined after 1.5 x 103 minutes were taken as the equilibrium values. 

Figure 2. Thickness, t, of the adsorbed layer for NaPSS-3 as a function of adsorption time. Symbols are the same as in Figure 1.

In Figures 3 and 4, plots of Ap and t against the square root of adsorption time, T, are displayed, respectively. Before attainment of adsorption equilibrium both plots are linear within experimental error and therefore, adsorption process of NaPSS is governed by diffusion. 

It is reasonable to assume, at least for oppositely charged polymers and particles, that aA > ag, which means that the adsorption time is always expected to be shorter than the coagulation time under perikinetic conditions. Consequently, perikinetic flocculation rates are very likely not to be adsorption rate limited. The ratio of orthokinetic adsorption time to orthokinetic coagulation time is... 

Van den Berg [131] used this technique to determine nanomolar levels of nitrate in seawater. Samples of seawater from the Menai Straits were filtered and nitrite present reacted with sulfanilamide and naphthyl-amine at pH 2.5. The pH was then adjusted to 8.4 with borate buffer, the solution de-aerated, and then subjected to absorptive cathodic stripping voltammetry. The concentration of dye was linearly related to the height of the reduction peak in the range 0.3-200 nM nitrate. The optimal concentrations of sulfanilamide and naphthyl-amine were 2 mM and 0.1 mM, respectively, at pH 2.5. The standard deviation of a determination of 4 nM nitrite was 2%. The detection was 0.3 nM for an adsorption time of 60 sec. The sensitivity of the method in seawater was the same as in fresh water. 

Absorptive cathodic stripping voltammetry has been used [151,152] to determine nanomolar levels of nitrite in seawater. The nitrite is derivatised by diazotisation with sulfanilamide and coupled with 1-naphthylamine to form an azo dye. The dye adsorbs onto a mercury drop electrode and its reduction is fully reversible. The concentration of dye is linearly related to concentration of nitrite in the range 0.3-200 nM. Down to 0.3 nM nitrite can be determined in seawater for an adsorption time of 60 seconds. 

The predicted effects of NO adsorption time on the time delay between the maxima in NH3 and N production and the maxima in H2O and N2 production are shown in Fig. 17. It is seen that the predictions bound the experimental observations and show the the proper trend with increasing NO exposure. 

Abstract Recently, the interest on biomaterials and especially in tannins was growing and some attractive results were obtained in the adsorption of some metals by tannin adsorbents. Tannins are widely distributed in nature and have multiple adjacent polyhydroxyphenyl groups in their chemical structure which have extremely high afiSnity for heavy metal ions. This study will describe how tannin can be used as an effective zinc and lead sorbent by the use of tannin resins. Batch method was used in the experiments in which pH profde, adsorption time, adsorbent/liquid ratios, initial concentration of metal ions, adsorbent amount and temperature were investigated to determine binding properties of adsorbent for the Zn(II) and Pb(ll) ions. 

Abstract In this study, a new natural adsorbent (sumae leaves) for removing Cu (II) ion from the aqueous solutions has been investigated. Leaves of sumae were obtained from Siirt, Tmkey. The tannins were extraeted with acetone water (70 30, v/v) mixture from the leaves of sumac. For the total tannin determination Folin-Ciocalteu method was used and tannin content was found 27%. In batch experiments, pH profile, adsorption time, adsorbent/hquid ratio, initial concentration of metal ions, adsorbent amoimt, particle size of adsorbent and temperature were performed to determine binding properties of adsorbent for the Cu(II) ions. The concentrations of the metal ions in solutions before and after adsorption were measured with an atomic absorption spectrophotometer. 

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