The transverse relaxation rate of H 2 0 in AI(H20)F has been measured as a function of temperature (255 to 417 K) and pressure (up to 220 MPa) using the I70-NMR line-broadening technique, in the presence of Mn(I1) as a relaxation agent. At high temperatures the relaxation rate is governed by chemical exchange with bulk H,O, whereas at low temperatures quadrupolar relaxation is prevailing. Low-temperature fast-injection I70-NMR was used to extend the accessible kinetic domain. The samples studied contained A13+ ( mechanism. These results of H 2 0 exchange on Al(H,O)P are discussed together with the available complex-formation rate data and permit also the assignment of I, mechanisms to these latter reactions.

Activated carbons are disorganized materials with variable pore size distributions (PSD). If one assumes that the porosity consists mainly of locally slit-shaped micropores, model isotherms can be obtained by computer simulations and used to assess the PSD on the basis of experimental isotherms. In the present study, CO isotherms have been measured at 273 K on

Microporous carbon blacks can be characterized by the same techniques as activated carbons, using the classical DR equation and comparison plots based on non-porous materials. The CO adsorption isotherm at 273 K, combined with 2 computer modelling, also leads to an assessment of microporosity. The results agree with independent techniques such as immersion calorimetry into liquids of variable molecular dimensions and a modified Dubinin equation. The study also confirms that the comparison plots based on N (77 K), CO (273 K) and C H (293 K) do not necessarily lead to overlapping results for the total micropore volume and the external surface area of the carbons.

The apparent and the real micropore size distributions (PSDs) of molecular sieve carbons can be assessed by combining the adsorption of CO 2 at 273 K with immersion calorimetry into liquids of increasing molecular dimensions. On the basis of model isotherms resulting from computer simulations, the adsorption of carbon dioxide, a relatively small probe, leads to the overall PSD of the carbon (essentially the internal micropore system). Immersion calorimetry, on the other hand, reveals the distribution of the pores accessible directly from the liquid phase, that is without constrictions. Liquid CS 2 probes the same volume as CO 2 and can be used as a reference. The paper describes the case of an industrial molecular sieve carbon obtained by blocking partly the entrance to a relatively broad micropore system, thus limiting its accessibility to molecules with diameters below 0.5-0.6 nm. It is shown how activation by steam at 900°C removes the constrictions and leads to a gradual overlap of the two PSDs. The distribution of the pore widths on the surface, observed directly by scanning tunnelling microscopy, is also given.

IntroductionThe adsorption of phenol from aqueous solutions onto carbons has received a great deal of attention, and an exhaustive review has been published recently by Radovic et al. 1 At the present time, the underlying mechanism and the prediction of adsorption equilibrium remain open questions, although a number of models have been proposed. It appears that the pH of the solution, the real surface area of the solid, and functional groups play a major role. A majority of authors describe the overall adsorption equilibrium in terms of Langmuir, Freundlich, or Redlich isotherms, and correlations have recently been suggested between the basic parameters of the isotherm and more fundamental properties such as the chemistry of the surface. Some authors have also attempted to correlate adsorption equilibrium with thermodynamic properties, by using calorimetry. 2-5 However, a major drawback of the classical models (Langmuir, Freundlich, etc.) is the difficulty in predicting adsorption equilibrium based on simple physicochemical parameters. As pointed out earlier and discussed in detail in a recent study by Stoeckli et al., 2,3 it appears that an adaptation of Dubinin's theory to the solid-liquid equilibrium provides an interesting framework for the description of phenol adsorption from aqueous solutions. A major advantage of this approach is the fact that adsorption can be predicted over a temperature range of 20-30°around room temperature, on the basis of simple physicochemical parameters and structural characteristics.It has been shown 3 that in the case of carbons with low oxygen contents, adsorption of phenol from dilute aqueous solutions corresponds essentially to the coating of the external surface and of the micropore walls by a monolayer. A good correlation was also obtained with the corresponding enthalpies of immersion at 293 K, based on a specific enthalpy near -0.110 J m -2 . On the other hand, a recent study by Stoeckli and Hugi-Cleary 2 on the adsorption of phenol from concentrated solutions (phenol liquefied with 15-25% water w/w) suggests that the mechanism corresponds to the filling of the micropore volume of activated carbons. This unambiguous result is provided by immersion calorimetry at 293 K, with benzene as a reference and combined with the adsorption of phenol from the vapor phase. Both types of experiment suggest an affinity coefficient (phenol) close to unity.It appears therefore that the adsorption of phenol, and possibly of its derivatives, follows two distinct mechanisms, and the next step, presented in this study, deals with the influence of the surface chemistry on these two mechanisms. We considered essentially the effect of surface oxygen [O], which can vary between less than 1 and 10 mmol g -1 . In view of their small concentrations (usually less than 1-1.5 mequiv g -1 ), the basic surface groups, identified by HCl titration, 6,7 were not considered. As shown below, in both concentrated and dilute solutions, it appears that water is preferentially adsorbed by the oxygen-containi...