Diuron adsorption isotherms
The data corresponding to the adsorption of diuron onto AC at
different temperatures without pH control are shown in Fig. 2a–c.
As can be seen, they are well defined in the range studied, covering
up to around 55 lmol L1. The uptake of diuron increases significantly
with temperature in the whole range of concentration tested,
thus the observed effect of temperature on the adsorption capacity
of diuron does not follow the most common trend for adsorption.
This apparent endothermic behaviour has been interpreted in
terms of an increased planarity and diffusivity of the diuron molecules
linked to the easier desolvation of diuron molecules in solution
as temperature increases [10,11,27]. In a first approach, the
equilibrium isotherms of Fig. 2a can be considered as L-type, more
specifically L-3 subtype of Giles classification [12], indicating
favourable adsorption related to the low water solubility of diuron
and the occurrence of multilayer adsorption. The ever decreasing
slope and the plateau of the first part of the isotherm are found in
most cases of adsorption from dilute solutions. This pattern is representative
of a progressive occupation of the surface available for
adsorption. The saturation shown by the plateau has been interpreted
as the result of monolayer completion, although it does
not necessarily imply ordering in a close-packed layer of adsorbate.
The layer can also contain solvent molecules and adsorbate clusters,
and the filling of the sites available on the original surface can coexist
in some extent with the formation of multilayer or the filling of pores. The solute concentration at which the plateau of the isotherm
is fully developed decreases as temperature increases, thus
it varies from around 45 lmol L1 to around 30 lmol L1 for the
adsorption at 15 and 45 C, respectively. In a previous paper, Fontecha-
Cámara et al. [10] reported L-type isotherms for the adsorption
of diuron on activated carbon fiber and cloth at pH 7, whereas Bouras
et al. [9] reported S-type isotherms for the adsorption of diuron
on surfactant-modified pillared clays at pH 6. Thus, the formation of
multilayer seems to be conditioned by the nature of the adsorbent
surface and the effect of pH on the adsorbent-solute and adsorbatesolute
interactions.
The observation of the isotherms within the low concentration
range (Fig. 2b) shows that at 15–35 C they have a continuous concave
shape (L-3) whereas at 45 C the shape of the isotherm could
be even considered of S-3 type with an inflection at diuron concentration
around 2 lmol L1. Such shift at high temperatures would
suggest a higher contribution of cooperative adsorption, promoted
by the interactions between the solute and the adsorbate.
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