Pressure jump adsorption-desorption

Hayes and Leckie (1986) postulate on the basis of their pressure jump relaxation experiments on the adsorption-desorption of Pb2+ at the goethite-water interface the following mechanism ... 

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Based on the pressure-jump relaxation results reported here, the following mechanism is postulated for the adsorption/desorption of Pb2+ ion at the goethite/water interface (8) ... 

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Astumian, R.D. Sasaki, M. Yasunga,T. Schelly, Z.A. (1981) Proton adsorption-desorption kinetics on iron oxides in aqueous suspensions, using the pressure jump method. J. Phys. Chem. 85 3832—3835 Atkins, P.W. (1990) Physical chemistry. 4 Ed. 

Zhang, P.C. Sparks, D.L. (1990) Kinetics and mechanism of sulfate adsorption and desorption on goethite using pressure jump relaxation. Soil Sci. Soc. Am. J. 54 1266-1273 Zhang, P.C. Sparks, D.L. (1990) Kinetics of selenate and selenite adsorption/desorption at the goethite/water interface. Environ. Sci. Technol. 24 1848-1856... 

Mikami, N., Sasaki, M., Hachiya, K., Astumian, R. D., Ikeda, T., and Yasunaga, T. (1983). Kinetics of the adsorption-desorption of P04 on the A1203 surface using the pressure-jump technique. J. Phys. Chem. 87, 1454-1458. 

Ashida, M., Sasaki, M., Kan, H., Yasunaga, T., Hachlya, K, and Inoue, T. (1978). Kinetics of proton adsorption-desorption at Ti02-H20 interface by means of pressure jump technique. J. Colloid Interface Sci. 678, 219-225. 

A number of soil chemical phenomena are characterized by rapid reaction rates that occur on millisecond and microsecond time scales. Batch and flow techniques cannot be used to measure such reaction rates. Moreover, kinetic studies that are conducted using these methods yield apparent rate coefficients and apparent rate laws since mass transfer and transport processes usually predominate. Relaxation methods enable one to measure reaction rates on millisecond and microsecond time scales and 10 determine mechanistic rate laws. In this chapter, theoretical aspects of chemical relaxation are presented. Transient relaxation methods such as temperature-jump, pressure-jump, concentration-jump, and electric field pulse techniques will be discussed and their application to the study of cation and anion adsorption/desorption phenomena, ion-exchange processes, and hydrolysis and complexation reactions will he covered. 

Pressure-jump relaxation was also used by others to study anion adsorption/desorption kinetics on soil constituents. These investigations have included the study of the kinetics and mechanisms of acetic acid adsorption on a silica-alumina surface (Ikeda et al., 1982a) and phosphate (Mikami et al., 1983a) and chromate adsorption (Mikami et al., 1983b), on 7-AI2O3. Double relaxation times on the order of milliseconds were observed in each of these studies. 

Hachiya, K., M. Sasaki, T. Ikeda, N. Mikami, and T. Yasunaga. 1984. Static and kinetic studies of adsorption-desorption of metal ions on a 7-AI2O3 surface. 2. Kinetic study by means of pressure-jump technique. J. Phys. Chem. 88 27-31. 

Ikeda, T., M. Sasaki, K. Hachiya, R.D. Astumian, T. Yasunaga, and Z.A. Schelly. 1982a. Adsorption-desorption kinetics of acetic acid on silica-alumina particles in aqueous suspension, using the pressure-jump relaxation method. J. Phys. Chem. 86 3861-3866. 

Zhang, P., and D.L. Sparks. 1990. Kinetics and mechanisms of sulfate adsorption/desorption on goethite using pressure-jump relaxation. Soil Sci. Soc. Am. J. 54 1266-1273. 

The kinetics of proton adsorption/desorption on anatase was studied using a pressure jump technique in [179], and rate constants were calculated. Reference [161] presents the kinetics of proton adsorption for TiO2... 

Hachiya, K., Takeda, K., and Yasunaga, T, Pressure-jump method to adsorption-desorption kinetics. Adsorption Sci. Technol.. 4. 25, 1987. 

Proton adsorption/desorption kinetics may be studied by pressure-jump type techniques. Protonation is usually very fast deprotonation may be slower but time scales of a few tens of seconds are not exceeded For practical purposes, the oxide surface charge can be considered as being instantaneously established on contact with the metal-containing solution. 

Elucidation of the kinetics and mechanisms of mineral-fluid interactions requires high-resolution X-ray scattering measurements on rapid time scales. Time series analyses are desired for addressing the evolution of structure and composition at the interface, on time scales as small as milliseconds or less. The high brilliance of the third-generation synchrotron sources affords new opportunities for such time-resolved studies, because we can observe in real time the processes of adsorption/desorption and complex formation at mineral-fluid interfaces. For example, experiments using a pressure-jump relaxation techniques yield rates of adsorption and desorption of protons and hydroxide at the surface of metal oxides in the range of milliseconds to seconds (reviewed by Casey and... 

From sets of spectra such as those shown in Fig. 3 and uptake curves displayed by Fig. 8 not only isotherms and isosteres could be derived, using the respective plateaux for the temperatures and pressures indicated, but also from the ascending branches (measured via FTIR after an upward pressure jump) or the descending branches (determined after a downward pressure jump) the kinetics of adsorption and desorption into zeolitic pores could be derived. These processes were assumed to be diffusion controlled. Their evaluation required a fit of the appropriate solution of Tick s second law as provided by Crank [39] to the experimentally measured uptake (or removal) points, which are indicated in Fig. 6 by filled crosses for the case of ethylbenzene uptake. 

Kinetics of Selenium Adsorption. Zhang and Sparks 4G) examined selenate and selenite adsorption and desorption on goethite using pressure jump relaxation techniques. Selenate produced a single relaxation, that was interpreted as outer-sphere complexation with surface protonation based on fitting to the triple layer model. The forward rate constant was 10 L mol s Selenite adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of inner-sphere complexes of both HSeOj and SeOj, based on optimized fits using the triple layer model. The model optimized fit for the pK, of the surface species was approximately 8.7. Forward rate constants for the first step were on the order of 10 L -mor -s for HSeOj and 10 L -mor -s for SeOj. Forward rate constants for the formation of the inner-sphere complexes were 100 and 13 s respectively for HSeOj and SeOj. Agreement between the equilibrium constant obtained from batch and kinetic studies was taken as confirmation of the proposed reactions. 

---