In the road context, adsorption/desorption phenomena greatly influence the fate of pollutants entering the road construction, present therein or transported through road-construction layers and further down. Sorption phenomena are also of importance regarding pollutants possibly leached (dissolved) from some road materials (e. g. alternative materials) under the effect of infiltration, and adsorbed on a surface downstream. Sorption/desorption sequences (under the effect of surface characteristics and seepage pH, for example) can lead to a progressive downward transfer of substances.
Adsorption can be defined as the attachment or adhesion of a molecule or an ion in the gaseous or liquid phase to the surface of another substance (an adsorbent) in the solid phase or to the surface of a soil particle. Desorption describes the process by which molecules or ions move in the opposite direction. Adsorption/desorption is a universal surface phenomenon. It can occur at any surface, e. g. surfaces formed by any type of opening, capillary, crack, depression or other type of physical irregularity. The nature of the adsorbing surface plays an essential role in the process. The smaller the size of the soil particles, or the greater the porosity, the more efficiently the adsorption will occur because of the increase in surface area provided. Road pollutants are therefore leached much more quickly through a coarse-textured soil than through a clayey soil (Brencic, 2006). This feature is of particular relevance when traffic accidents involve cargoes of harmful or toxic compounds.
The adsorption/desorption of substances between the liquid form and the surface of solid-state materials, such as soil particles, is one of the processes of greatest importance for the behaviour of inorganic and organic substances in the soil. The degree of adsorption increases with the concentration of the substance in the solution outside the adsorbent until a maximum is gradually approached. As the reaction kinetics depend on temperature (adsorption decreases with higher temperature because the molecules are more energetic and less easily held by their potential sorbent), the quantitative assessment of adsorption is done by means of so — called isotherms. Various models can be used to interpret isotherms, e. g. Langmuir, Freundlich or Brunauer-Emmet-Teller (BET) (Fig. 6.4) a variant of which is given inEq. 6.16.
S = Qt в C/(1 + в C)
Fig. 6.4 Variation of the sorbed quantity (S) as a function of the concentration of sorbate (C) for different temperatures (T1 >T2>T3) — Langmuir isotherm (adapted from Bontoux, 1993 and Selim & Sparks, 2001)
where S = mass of sorbate sorbed per mass of sorbent (typically in units of mg/kg); QT = maximum sorption capacity of the sorbent at temperature, T(°), в = a variable that is only a function of the temperature, T, and C = aqueous concentration of sorbate (typically in units of mg/l).
Desorption can occur when a “new” ion (or other chemical) arrives at a sorption site and is sorbed, preferentially, over a previously sorbed ion of a different type. Less readily, sorbed species can be desorbed if the concentration of that species decreases in the groundwater around the sorbent.
Time is required for sorption/desorption reactions to become complete. Therefore the approach adopted both in analysis and in testing is to allow sufficient time for equilibrium to develop. Often this will take hours, perhaps days, to complete. Care is required when the input or output condition is changing due, for example, to flow bringing more contaminant. Then, true equilibrium may not be possible. The use of an isotherm approach necessitates the assumption of equilibrium conditions.
More important, though, is that the adsorption is also pH dependent; cations such as most metal ions are more strongly adsorbed at increasing pH. The degree of adsorption rises sharply in a short interval of increasing pH. This is due to the fact that the charge of the particle surfaces is greatly pH dependent. The pH of the soil thus largely regulates the mobility of heavy metals occurring in the soil. With the exception of some amphoteric compounds (e. g. some metal hydroxides) and some oxyanions (e. g. MoO42-, AsO43-), the general rule is that many heavy metals are more mobile at lower pH (Berggren Kleja et al., 2006).
