Crayfish Shell-Derived Biochar Enhances Paracoccus-Mediated NMP Biodegradation Under Anoxic Conditions: A Waste-to-Wealth Strategy

N-methyl-2-pyrrolidone (NMP) is a widely used industrial solvent that can enter industrial wastewater, creating a need for efficient and low-cost treatment methods. In this study, biochar prepared from waste Procambarus clarkii shells was used as an auxiliary material to evaluate its promotion of NMP biodegradation by Paracoccus sp. strain NH1 under anoxic conditions. Among the materials tested, biochar prepared at 300 °C showed the highest mean NMP degradation efficiency after 24 h, increasing from 85.9% in the no-material control to 98.0% (p < 0.05). Mean ETSA was also significantly higher in the SSB 300 group than in the no-material control (p < 0.05). Abiotic controls with 500 mg/L NMP showed that NMP loss within 24 h was below 5% in the presence of raw crayfish shell material, SSB 300-SSB 600, or activated carbon, indicating that adsorption or other abiotic losses were insufficient to explain the observed enhancement. Surface characterization showed that low-temperature biochar retained more nitrogen-containing and oxygen-containing surface chemical structures than high-temperature biochar; at the same time, SSB 300 was also associated with higher N-methylhydantoin amidohydrolase (NMA) activity, higher extracellular polymeric substance (EPS) polysaccharide production, and more favorable cell attachment. Because the mineral salts medium contained nitrate, this study interprets ETSA as an indicator of enhanced microbial respiratory/redox activity rather than evidence of direct electron transfer through biochar to NMP. Therefore, the results support a multifactorial enhancement mechanism, including metabolic stimulation, surface-associated colonization, and potentially redox-active biochar surface chemistry. These findings provide a route with practical application potential for integrating crayfish shell waste valorization with NMP biological removal. The novelty of this work lies in linking the properties of shell waste-derived biochar to the biological removal of NMP, while distinguishing abiotic NMP loss from microbially mediated enhancement.