On the relevant role of iron complexation for the performance of photo-Fenton process at mild pH: Role of ring substitution in phenolic ligand and interaction with halides

In this work, a systematic study on the effect of Fe(III) complexation by phenolic derivatives with different substitution on photo-Fenton has been carried out. Solutions containing a mixture of six pollutants (30 mg·L−1) were treated by solar simulated photo-Fenton ([Fe(II)] = 0.09 mmol·L−1, [H2O2] = 4.29 mmol·L−1, pH = 5, [Ligand] = 0.227 mmol·L−1. The efficiency of ligand increased with the presence of -OH groups (mainly in ortho- position), while the reverse was true for -COOH and -OCH3. This was correlated with the ability of the ligands to reduce Fe(III) to Fe(II). Catechol (CAT) supressed the interference of chlorides at [NaCl] = 30 mg/L−1 by complexing iron(III) thus preventing formation of FeCl2+, as 85% of Fe(III) was found as Fe(CAT)3 according to mathematical calculations. Hence, effluent matrix, including anions and organic pollutants, has a remarkable effect on iron speciation and a it should be considered when determining reaction conditions.