retaining impurities. However this retention indeed happens. The impurities are linked by H-bonding to adsorbed water molecules. When the treatment is performed under vacuum and mild temperature conditions the water is removed from the surface sites of adsorption, which are then occupied by impurities. An additional adsorption by this mechanism is seen making it possible to obtain values of adsorption greater than the monolayer one.
In Table 6 our biodiesel refining results and those reported by Faccini et al. [12], are compared. These authors used different adsorbent materials (silica, Magnesol and ion exchange resins) for biodiesel dry refining. It can be seen that:
(i) Unlike our methodology, which is of one single stage treatment, that used in reference [12] involves two steps of treatment since it is necessary to remove excess methanol before refining.
(ii) The values of total adsorption capacity obtained with ourmethodology far exceed those reported by Faccini et al. With our methodology silica Trisyl 3000 retains 2.35 times its weight of impurities while silica and Magnesol retain 0.33 and 0.65 times its weight, respectively.
It can be concluded that the proposed method has the following advantages: (i) simplicity, and (ii) it greatly improves the utilization of the adsorbent, reducing the cost of the process.
At this point, one last consideration can be made regarding the regenerability of the silicas. Our group is working on this topic using selective solvents. At present we have managed to recover 80% of the adsorption capacity of the silicas. The procedure and the main conclusions will be presented in a next contribution.