Insights into the sustainable design of engineered hydrochar co-doped with cobalt and nitrogen as peroxymonosulfate activator for fluoroquinolones removal

The exploration of waste-derived hydrochar as peroxymonosulfate (PMS) activator for fluoroquinolones removal remains limited. Herein, various Co, N-co-doped hydrochars (Co-N-HCs) were designed via different fabrication pathways (i.e., one-, two-, and three-step pathways) and their characteristics were investigated, revealing the variation in surface chemistry due to different fabrication approaches. These Co-N-HCs with different surface chemistry were employed to remove fluoroquinolone antibiotics, namely ciprofloxacin (CIP), via PMS activation and the results show that the performance of one-step catalyst (HC-1S-A-2) was the highest with k_app = 0.026 min⁻¹ compared to the two-step and three-step catalysts (0.008 – 0.022 min⁻¹). The better performance of the HC-1S-A-2 was due to its highest I_D/I_G ratio (2.20) and relatively higher electronic conductivity (6.21 × 10^-4 S m⁻¹) of the catalyst, which could enhance the PMS activation and reactive species (RS) generation for CIP removal. The catalyst was further optimized by varying Co content, and the 2.0 wt% Co content (HC-1S-A-3) emerged as the most effective, demonstrating efficient CIP removal across various operational conditions. The chemical scavenging and electrochemical studies revealed that the hydroxyl radical (major ROS), sulfate radical, singlet oxygen, and other nonradical pathways were involved in CIP degradation while the major active site was Co coupled with pyrrolic N and pyridinic N. Additionally, based on the identified CIP intermediates during the degradation process, the CIP degradation pathways were proposed, and the intermediates were subjected to a toxicity assessment. The results showed that CIP and its intermediates can be successfully mineralized and detoxified by increasing the catalytic reaction time. Overall, this work provides a sustainable approach to transform waste into engineered hydrochar for pollutants removal.