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