Conclusions And Recommendations

This review has illustrated various applications of fractal concepts to the description of adsorption/desorption processes on environmental materials. The surface fractal dimension Ds can be determined by several independent methods, among which those based on adsorption measurements are shown to be the most convenient and straightforward. However, a comparison of fractal dimensions calculated from different approaches was shown to be problematic; therefore, the need for further studies in this field is evident.

Particular attention has been paid to the discussion of the fractal approach to the adsorption of gases by clay minerals and soils. Further promising developments are expected when applying fractal concepts in new environmental fields. For example, promising research has been conducted very recently on the fractal description of ion adsorption and on the influence of surface roughness on the electric properties of the double layer [124, 125]. Processes and models leading to the preparation of adsorbents with fractal surfaces have also been investigated [126].

Adsorption processes are controlled by the energetic properties of the absorbent and absorbate, to the distribution of sites where adsorption occurs and to the geometric heterogeneity of the adsorbent. Discriminating the contribution of each of these factors to the adsorption process as a whole is particularly challenging and requires an appropriate combination of experiment and computer modeling. Despite the substantial progress in the latter field, only rather simple models of selected soil components (e.g. clay minerals) have been used in computer simulations so far [127,128]. Large-scale off-lattice simulations of gas adsorption onto soils remain extremely difficult. Therefore, the further development of methods for performing computer simulations to investigate the formation of natural adsorbents and the structure of adsorbed layers will be extremely helpful.

In the past few years, the concept of 'quenched-annealed' mixtures has been widely applied to the study of the structure and thermodynamic properties of fluids adsorbed in disordered porous matrices [129]. According to this approach, computer simulations and integral-equation theories [129] describing gas adsorption have been proposed that are based on the equilibration of adsorbate molecules in a matrix of particles frozen in a disordered configuration and sampled from a given probability distribution. The novelty of this approach is that the structure of the porous network can be implicitly incorporated into the description by using a static structure factor of the adsorbent. In particular, the porous adsorbent can be considered as being formed by processes leading to the formation of mass fractals, e.g. particle-cluster or cluster-cluster diffusion-limited aggregation processes [130]. Although the concept of 'quenched-annealed' mixtures appears to be very promising for modeling gas adsorption onto soils, to the best of our knowledge, only a single paper that describes preliminary attempts to apply such models to adsorption processes in soils [131] has been published on this topic. Developments in this direction are encouraged and expected.

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