Direct Fabrication of Chitin-nZnO Antibacterial Hydrogels from Crab Shells via a Hot-Water Freeze–Thaw Method and Associated Life Cycle Assessment

Conventional chitin hydrogel preparation from crustacean shells involves complex procedures, consuming large amounts of strong acids/alkalis and high energy, causing severe environmental burdens. Herein, a simple, green hot-water freeze–thaw (HWFT) method for fabrication of chitin/nano-ZnO (chitin-nZnO) composite antibacterial hydrogels directly from crab shells was developed. Under optimal conditions (150 °C, 3 h, liquid-to-solid ratio 100:8, 11% NaOH, two freeze–thaw cycles at –25 °C), the chitin-nZnO hydrogel was successfully prepared. The composite hydrogel exhibited excellent antibacterial activity against Staphylococcus aureus, Escherichia coli, and Bacillus subtilis, with best inhibition zone diameters of 18.0, 17.0, and 10.0 mm, respectively. Life cycle assessment (LCA) demonstrated that, compared with the conventional process, the HWFT method could reduce global warming potential by 36.6%, cumulative energy demand by 35.1%, and multiple toxicity indicators (marine ecotoxicity, freshwater ecotoxicity, human carcinogenic toxicity, and human noncarcinogenic toxicity) by 29.9–32.5%. This study provides a green and efficient route for high-value utilization of crustacean shell wastes and highlights the importance of integrating LCA into sustainable process design.