Humic acids as complexing agents to drive photo-Fenton at mild pH in saline matrices: Process performance and mechanistic studies

The role of iron complexation on (photo)-Fenton process at mild pH has been investigated; in particular, the effect of humic acids, their interaction with chlorides and the involvement of the target pollutants has been studied. For this purpose, solutions consisting in mixture of 6 pollutants (acetaminophen, acetamiprid, amoxicillin, caffeine, carbamazepine and clofibric acid) in different matrices ( a) distilled water, b) low salinity water (1 g⋅L-1 of Cl-), c) high salinity water (30 g⋅L-1 of Cl-), d) tap water and e) real sea water) have been irradiated with simulated sunlight with and without humic acids (10 mg L-1). The ability of humic acids to complex Fe(III) resulted in an enhancement of pollutants removal as prevented iron inactivation, but in highly saline waters were they were not able to displace completely Cl- as ligands for Fe(III) and in sea waterwhere other species could also contribute to inhibit photo-Fenton. Furthermore, pollutants with phenolic and/or carboxylic moieties also contribute to keep iron active at pH = 5, most probably by complexation. This is in line with dissolved iron at the end of the experiment and hydrogen peroxide consumption. Electron spin resonance measurements of ⋅OH show a very good correlation between the radical formation and pollutants removal, indicating that in the presence of complexing agents, generation of ⋅OH is favoured. These results indicate that HA could be used as auxiliary to apply photo-Fenton in low salinity environments rather than in real seawater.