ED process was modeled based on basic electrochemistry rules and copper ion separation experimental data (Mohammadi et al., 2005). The experiments were performed for zinc, lead and chromium ions. It was found that performance of an ED cell is almost independent on the type of ions and only depends on the operating conditions and the cell structure. In spite of its limitation, ED offers advantages for the treatment of wastewater laden with heavy metals such as the ability to produce a highly concentrated stream for recovery and the rejection of undesirable impurities from water. Moreover, valuable metals such as Cr and Cu can be recovered. Since ED is a membrane process, it requires clean feed, careful operation, periodic maintenance to prevent any stack damages.
6. Photocatalysis
In the recent years, photocatalytic process in aqueous suspension of semiconductor has received considerable attention in view of solar energy conversion. This photocatalytic process was achieved for rapid and efficient destruction of environmental pollutants. Upon illumination of semiconductor–electrolyte interface with light energy greater than the semiconductor band gap, electron–hole pairs (e−/h+) are formed in the conduction and the valence band of the semiconductor, respectively (Herrmann, 1999). These charge carriers, which migrate to the semiconductor surface, are capable of reducing or oxidizing species in solution having suitable redox potential. Various semiconductors have been used: TiO2, ZnO, CeO2, CdS, ZnS, etc. As generally observed, the best photocatalytic performances with maximum quantum yields are always obtained with titanium dioxide. Fig. 14 shows the conceptual reaction path of photocatalysis over titanium dioxide particle.