Iron-functionalized biochar in raceway pond reactors for quaternary treatment of municipal WWTP effluents

This paper presents a comprehensive study on the use of iron-functionalized biochar (FeBC) as a catalyst for microcontaminant removal from municipal wastewater treatment plant (MWWTP) effluents by solar photo-Fenton in raceway pond reactors (RPRs). To this end, the structural characteristics of raw biochar (RBC) obtained from woodchips and FeBC were determined, and the adsorption and oxidation processes of acetamiprid (ACTM, 100 μg/L) were evaluated in 5-cm deep RPRs containing 500 g of RBC or FeBC (2.7 g/L of iron) supported in the channels. Afterwards, the effects of hydraulic residence time (HRT) and FeBC load on microcontaminant degradation in continuous flow were assessed. Batch experiments conducted in tap water and validated in MWWTP effluent showed fast ACTM adsorption kinetics with RBC and FeBC, with more than 90 % of ACTM adsorbed in 60 min. In the presence of H₂O₂ (150 mg/L), the ACTM oxidation percentages were 15 % with RBC in the dark, 13 % with RBC under sunlight, 50 % with FeBC in the dark (Fenton-like), and 56 % with FeBC under sunlight (solar photo-Fenton), confirming the efficacy of iron-loaded biochar as a suitable catalyst for microcontaminant degradation through Fenton-like processes. Continuous flow solar photo-Fenton enhanced ACTM degradation, achieving 88 % of ACTM oxidation with a 60-min HRT, 2.7 g/L of iron and 50 mg/L of H₂O₂. Halving the HRT increased the treatment capacity from 4.0 mg ACTM/m^2․h to 6.91 mg ACTM/m^2․h. These results highlight the potential of FeBC as a heterogeneous catalyst for the quaternary treatment of MWWTP effluents in continuous flow reactors.