New insights into the mechanism of coupled photocatalysis and Fenton-based processes using Fe surface-modified TiO2 nanotube layers: The case study of caffeine degradation

Controversial mechanism of simultaneous photocatalysis and Fenton-based processes: additional effect or synergy?

“…In our research group, we have recently published new insights into the coupling of photocatalysis and Fenton-based processes where we have attempted to describe the synergy in the degradation of caffeine using Fe-modified TiO 2 nanotube layer. 51 These Fe–TiO 2 nanomaterials were producing few μM of in situ H 2 O 2 , which is consistent with other published reports. 11,50 Fe–TiO 2 also exhibited few μM of Fe leaching after 3 h under UVA light at pH = 3, thus highlighting the potential importance of homogeneous reactions.…”

“…11,50 Fe–TiO 2 also exhibited few μM of Fe leaching after 3 h under UVA light at pH = 3, thus highlighting the potential importance of homogeneous reactions. 51 The degradation efficiency of caffeine in the synergistic system was more than the additional effect of (i) H 2 O 2 photolysis, i.e. , under UVA light without Fe–TiO 2 , (ii) Fe–TiO 2 photocatalysis (along with H 2 O 2 activation by photogenerated e − ), and (iii) Fenton-based processes.…”

“…11,50 Fe-TiO 2 also exhibited few mM of Fe leaching after 3 h under UVA light at pH = 3, thus highlighting the potential importance of homogeneous reactions. 51 The degradation efficiency of caffeine in the synergistic system was more than the additional effect of (i) H 2 O 2 photolysis, i.e., under UVA light without Fe-TiO 2 , (ii) Fe-TiO 2 photocatalysis (along with H 2 O 2 activation by photogenerated e À ), and (iii) Fenton-based processes. Although the synergy between these processes was obvious, its assessment was not trivial to clarify: heterogeneous Fenton reaction, i.e., Fe-TiO 2 with H 2 O 2 in the dark showed no efficiency, while under UVA light, iron leaching occurred, thus suggesting the involvement of both photo-Fenton and homogeneous Fenton processes.…”

“…11 It should be noticed that in none of these reported works, 11,13,46,49,50 the analysis of leached iron (or any other Fenton active element) has been performed and we recently reported that the leached iron from Fe-TiO 2 nanotube layer has a significant role in the degradation of organic pollutants via homogeneous Fenton-based processes. 51 Indeed, the role of dissolved iron might be significant in the performance of the system. [57][58][59][60][61] Therefore, the contribution of such homogeneous reactions should be systematically assessed in future investigations of synergistic systems.…”

“…7). 51 Therefore, we have assessed for the first time the individual contribution of each identified process, along with in situ generation of H 2 O 2 and iron leaching. 51 We have proven that these latter features play a crucial role in the synergy.…”