NiFe2O4/biochar decorated porous polymer membranes for the flow-through photo-Fenton degradation of tetracycline

Herein, coffee grounds derived biochar was used as support for the fabrication of NiFe2O4/biochar composite particles. The composites achieved up to 10x higher turnover frequencies (TOF) in the photo-Fenton degradation of tetracycline (20 mg/L) in batch compared to the pure NiFe2O4 particles. This could be correlated to a decreased agglomeration, lower band gap energy and electronic interaction with the biochar. The hydrophobic support further enabled the incorporation of the NiFe2O4/biochar powders into porous polyethersulfone (PES) membranes via film casting cum phase separation. The resulting about 150 µm thick porous flow-through membrane reactors achieved high degrees of conversion (95–99 %) at a flux of 100 L/m2h in the continuous photo-Fenton degradation of tetracycline (20 mg/L) in water. We could further show that the TOF values are 5-20x times higher in flow-through the catalytic membranes compared to the NiFe2O4/biochar catalyst particles used in batch mode. We also demonstrated that our photocatalytic membranes can remove low concentrations of tetracycline (2 and 0.2 mg/L) below the detection limit of the used mass spectrometry analysis. Monitoring the leaching of Ni and Fe from one membrane over 28 h of flow-through revealed that Ni predominantly leaches from the incorporated NiFe2O4/biochar, which, however, did not lead to any loss of the catalytic activity. Moreover, we found that the residence time in the membrane has a direct influence on the tetracycline degradation pathways and the resulting product profile, which represents a novel approach for tuning Fenton-like reactions towards specific products.