From Fig. 6 it is noted that mechanical action induced an initial particle size refinement followed, after 30min, by an increase in particles mean size. This effect may be explained by considering that aggregation phenomena take probably place as the result of electrostatic interaction between small particles produced during the first instants of milling process. On the other hand, it is worth noting that, according to the Rumpf’s theory [22], Van der Waals forces play a significant role only at particle size lower than 1m. Moreover, the corresponding bond energy increases as the size of the aggregating particles is reduced. Finally, the weak aggregates formed are then compacted by the impacts being so able to entrap heavy metals according to the mechanisms proposed byMontinaro et al. [14,16]. This behaviour probably plays a very important role