Photo-Fenton and Electro-Fenton performance for the removal of pharmaceutical compounds in real urban wastewater

Abstract

The objective of this study was to evaluate the performance of Photo-Fenton (PF) and Electro-Fenton (EF) processes and find the best option for the removal of pharmaceutical compounds in a real urban effluent of wastewater treated by a chemically-enhanced primary treatment (CEPT). In both processes was used the iron residual (average 2.4 mg L⁻¹) resulting from CEPT avoiding further iron salts addition. Central composite designs (CCD) with response surface methodology (RSM) were applied to select the operational conditions. Humic acid (HA) was the chelating agent in the PF process, where Fe³⁺:HA molar ratios and H₂O₂ concentrations were the factors. While current density and the electrolysis time were the factors in the EF process. The operational conditions selected for evaluating the removal of pharmaceutical compounds were a 1:6.8 for Fe³⁺:HA molar ratio and 100 mg L⁻¹ for H₂O₂ in PF, and 80 mA cm⁻² and 100 min for EF. EF was evaluated with a Boron Doped Diamond electrode anode and cathode (BDD/BDD cell) without the addition of supporting electrolyte. In PF, the removal efficiencies were between 24.2% and 56% for gabapentin, ibuprofen, naproxen, pentoxifylline, and venlafaxine. Compounds such as carbamazepine and fenofibric acid increased their concentration after the PF process. This increase was related to the enzymatic cleavage of glucuronides and the possible presence of •OH radical scavengers that decreased the process efficiency. By contrast, EF showed an average removal efficiency of between 53.3% and 99.9% for all identified compounds, resulting in the best option for treating this real effluent of wastewater. These results in EF could be related to a synergic effect between homogeneous •OH and BDD(•OH) that contributed to improving the oxidation of the contaminants. The electrical energy per order (EEO) was determined with values between 11.5 and 144.9 kWh log⁻¹ m⁻³, suggesting a need to apply other preliminary treatments that reduce the current density (i.e., coupled treatments with the biological process).