Coupled iron-modified biochar and myriophyllum for nutrient and oxytetracycline removal from aquaculture wastewater

Aquaculture wastewater commonly contains excess nutrients and residual antibiotics, whereas conventional plant-only ecological ponds may provide insufficient removal of these coexisting contaminants. To enhance system performance through substrate engineering, iron-modified oyster shell biochar (IOBC) was developed as a functional medium and coupled with Myriophyllum in an ecological purification system. Response surface methodology identified optimal IOBC preparation conditions of 700 °C, a modification ratio of 1:1, and 3.5% iron addition, under which oxytetracycline (OTC) removal reached 89.37%. The optimized IOBC was subsequently incorporated into a continuous-flow system treating simulated aquaculture wastewater. In the continuous-flow system treating simulated aquaculture wastewater, pollutant removal increased with hydraulic retention time, and the coupled IOBC-Myriophyllum treatment consistently outperformed the filler-only and plant-only systems. At 72 h, the coupled system achieved removal efficiencies of 38.89% for NH4+-N, 38.79% for TN, 72.81% for TP, 67.67% for CODMn, and 59.46% for OTC. Sequencing and mechanistic analyses indicated that the enhanced performance resulted from coordinated adsorption, plant uptake, microbial nitrification and denitrification, Ca- and Fe-mediated phosphorus precipitation, and oxidative and biological transformation of OTC. These findings demonstrate that functional substrate modification can strengthen ecological treatment of nutrient- and antibiotic-containing aquaculture wastewater.