Microbial degradation behavior and mechanism of polyamide 6 fibers in seawater

Polyamide 6 (PA6) is a widely used thermoplastic polymer. Compared to other nylons, it offers good mechanical properties as well as chemical and thermal stability. Difficult to degrade and coupled with its massive annual global production, it has rapidly become a significant ecological pollutant. Therefore, developing environmentally friendly degradation methods is an urgent need, among which microbial degradation represents a promising solution. This study conducted laboratory-simulated seawater degradation experiments on polyamide 6 (PA6) fibers. The structural changes of PA6 over a 190-day degradation period were analyzed using characterization techniques, including scanning electron microscopy, relative viscosity, and differential scanning calorimetry. Results demonstrated significant and irregular degradation of PA6. Prolonged degradation led to decreases in relative viscosity, number-average molecular weight, melting temperature, and melting enthalpy. FTIR and XPS analyses revealed cleavage of C–C, C–N, and C = O bonds, accompanied by intensified signals of C–OH and O = C–O–C groups, indicating hydrolysis of amide bonds to generate terminal amino and carboxyl groups. Degradation product analysis suggested that the initial step of PA6 degradation primarily produced tetrameric and cyclic dimeric forms of 6-aminohexanoic acid. Further microbial community analysis and functional annotation identified Proteobacteria as the dominant microbial population responsible for PA6 degradation. This study provides new insights into the degradation behavior of polyamide fibers in marine environments and contributes substantially to solving the problem of marine polyamide waste.