Degradation of trimethoprim antibiotic by UVA photoelectro-Fenton process mediated by Fe(III)–carboxylate complexes

•First study of UVA photoelectro-Fenton mediated by Fe(III)–carboxylate complexes.•Inferior efficiency of PEF with Fe(III)–malate complexes on TMP degradation.•Similar ability of PEF with Fe(III)–oxalate, –citrate and –tartrate to degrade TMP.•1:3, 1:6 and 1:9 Fe(III)-to-oxalate molar ratios required at pH 4.5, 5.0 and 5.5.•Influence of iron speciation on the classical photo-Fenton at different temperatures.

A UVA photoelectro-Fenton (PEF) process mediated by Fe(III)–carboxylate complexes was applied to the removal of trimethoprim (TMP) antibiotic from water using a 2.2 L lab-scale flow plant equipped with a double compound parabolic collector (CPC) and an electrochemical cell composed of a boron-doped diamond (BDD) anode and a carbon–PTFE air-diffusion cathode. The presence of Fe(III)–carboxylate complexes enhances the regeneration of Fe3+ to Fe2+, allows to maintain iron in solution at higher pH values and can decrease the formation of Fe(III)–sulfate, Fe(III)–chloride and some Fe(III)–pollutants complexes. First, the efficiency of different carboxylate ligands like oxalate, citrate, tartrate and malate was assessed, followed by the application of various initial Fe(III)-to-carboxylate molar ratios and pH values. The PEF process with Fe(III)–oxalate, Fe(III)–citrate and Fe(III)–tartrate complexes revealed similar ability to degrade the antibiotic solution with the employment of 1:3, 1:1 and 1:1 Fe(III)-to-carboxylate molar ratios, respectively, and using pH of 4.5, Fe3+ concentration of 2.0 mg L−1 (total iron emission limit for the discharge of treated effluents according to the Portuguese legislation), current density of 5 mA cm−2 and 20 °C. The PEF process mediated by Fe(III)–malate complexes was much less effective. 1:6 and 1:9 Fe(III)-to-oxalate molar ratios were required to yield similar TMP removal kinetics at pH 5.0 and 5.5 compared to pH 4.5, respectively. Additionally, the influence of initial TMP content and solution temperature on the PEF process with Fe(III)–carboxylate complexes was assessed and the role of the different reactive oxidizing species was clarified by the addition of scavenging agents. Generated low-molecular-weight carboxylic acids were monitored by ion-exclusion HPLC.

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