Process enhancement at near neutral pH of a homogeneous photo-Fenton reaction using ferricarboxylate complexes: Application to oxytetracycline degradation

•Formation of iron–antibiotic complexes interferes with conventional photo-Fenton.•Carboxylate anions minimize the formation of iron–antibiotic complexes.•Degradation of OTC by a photo-Fenton process was achieved at near-neutral pH values.•Process variables, interfering species and two real wastewaters were assessed.•Remaining DOC after OTC removal is quickly converted to biodegradable compounds.

This work demonstrates the application at near neutral pH of a photo-Fenton reaction mediated by ferricarboxylates on the treatment of aqueous solutions containing the antibiotic Oxytetracycline (OTC) under solar irradiation. The formation of a Fe:OTC complex after Fe2+ oxidation to Fe3+, in the presence of H2O2, showed the inconvenience of using the conventional Fe2+/H2O2/UV–Vis process at near neutral pH levels, as the complex is retained in the filter. To overcome this, a Fe3+/Oxalate/UV–Vis or Fe3+/Citrate/H2O2/UV–Vis process was proposed. The higher tendency of Fe3+ to form complexes with carboxylates avoids the formation of Fe:OTC complexes and allows for proper OTC detection along reaction times. The photo-Fenton process itself is improved by the extension of the iron solubility to higher and more practical pH values, by the increase of the quantum yield of Fe2+ production and by presenting stronger radiation absorption at wavelengths up to 580 nm. In this way, process efficiency was evaluated for different variables such as Fe3+ concentration, pH, temperature and irradiance, using a compound parabolic collector (CPC) photoreactor at lab-scale under simulated solar radiation. Reaction rates were compared in the presence of different inorganic anions and humic acids, and in two different real wastewater matrixes. Results obtained in a CPC pilot-scale plant under natural solar light, using an iron/oxalate molar ratio of 1:3 ([Fe3+] = 2 mg L−1, the maximum allowable discharge limit imposed by Portuguese regulations), at an initial pH of 5.0, showed that the antibiotic is quickly removed from solution, with consequent loss of activity on Escherichia coli (DSM 1103). The original DOC was decreased by 51%, with a remaining high percentage of low-molecular-weight carboxylate anions, and the final pH is within the legal discharge limits.

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