Combination of core embedding and mycelial surface loading: Construction of an immobilized bioreactor based on polydopamine-modified composite mycelial pellets for simultaneous nitrate and copper removal

As aquaculture expands, the large‑scale discharge of wastewater containing nitrate (NO₃^--N) and copper (Cu(II)) threatens aquatic environmental safety. To simultaneously remove NO₃^--N and Cu(II), this study prepared a core-shell structured composite biocarrier integrating hydrogel embedding with mycelial surface loading to construct a bacteria-immobilized bioreactor. This carrier utilized a hydrogel core embedding biochar produced from discarded kelp, functional bacteria, and fungal spores, immobilizing bacteria and serving as the inner component of the carrier. Mycelia formed through spore germination wrapped around the hydrogel core to form a three-dimensional scaffold. Additionally, polydopamine (PDA) was used to modify the mycelial surface, promoting bacterial attachment on fungal mycelia and biofilm formation, while potentially influencing the interfacial redox activity of the composite carrier. The highest removal efficiencies were achieved at HRT = 6h, C/N = 5, and pH = 7, reaching 99.64% for NO₃^--N and 83.79% for Cu(II). Under Cu(II) stress, the reactor maintained high denitrification performance and electron transport system activity, demonstrating good Cu(II) stress resistance. Characterization suggested Cu(II) removal through precipitation and complexation involving functional groups in PDA, the mycelial surface, and extracellular polymeric substances (EPS). Microbial community structure and KEGG results indicated that denitrification-related functional microorganisms in the reactor maintained a high relative abundance, with the potential for a complete reduction pathway from NO₃^--N to N₂. This study offered a promising strategy for the simultaneous removal of NO₃^--N and Cu(II) from wastewater.