Effect of pH and H2O2 dosage on catechol oxidation in nano-Fe3O4 catalyzing UV–Fenton and identification of reactive oxygen species

• Nano-Fe3O4 catalyzing UV–Fenton presented a wide effective initial pH range and less requirement of H2O2 dosage.
• Evolution of formic acid, acetic acid, oxalic acid, and maleic acid were investigated to explain solution pH change.
• HO, O21 and O2- were identified and mechanism for nano-Fe3O4 catalyzing UV–Fenton was preliminarily explored.

This laboratory scale batch study examined catechol oxidation by UV–Fenton with commercial nanosized Fe3O4 as catalyst, focusing on influence of initial pH and H2O2 dosage on oxidation efficiency (represented by COD removal) and H2O2 utilization efficiency. In a wide initial pH range (2.0–8.0), this heterogeneous UV–Fenton process was powerful, especially reaction at pH 7.0 obtaining the highest COD removal of 93%. The remarkably high oxidation efficiency under neutral and slightly basic conditions was due to fast decrease of solution pH to be strongly acidic. Solution pH decreased until 120 min and then increased, which was ascribed to the formation and destruction of some carboxylic acids. During the degradation, formic acid, acetic acid, oxalic acid, and maleic acid were detected. The values of H2O2 utilization efficiency at 240 min near 1.30 in reactions with 11.80 mM H2O2 under initial pH from 5.0 to 8.0 indicated this process would consume 23% less H2O2 dosage than the theoretical value for obtaining the same oxidation efficiency. Increasing H2O2 dosage accelerated catechol oxidation rate, but decreased the H2O2 utilization efficiency when H2O2 dosage enhanced from 0.50 × ΔH2O2 (ΔH2O2: theoretical H2O2 dosage of 11.80 mM for complete oxidation of 100 mg L−1 catechol) to 1.00 × ΔH2O2. Radical identification experiments based on inhibition of methylene blue degradation rate under respective scavenger for HO, O21 and HO2HO2 showed HO, O21 and O2- were involved in nano-Fe3O4 catalyzing UV–Fenton, and total scavenging of HO would completely inhibit degradation. This result implied only HO was the direct product, while O21 and O2- were secondary oxidants coming from HO involved reactions. This finding increases insight into the mechanism for nano-Fe3O4 catalyzing UV–Fenton. Maintenance of catalytic ability of nano-Fe3O4 was also evaluated in six repeated runs.