Multi-Objective Optimization of Mechanical Properties of Banana Pseudostem Fibers Using Sludge Retting Pretreatment

In this study, sludge retting was used as a pretreatment method for extracting banana pseudostem fibers. A Box–Behnken response surface design was employed to optimize the retting conditions. Three variables— Bacillus subtilis concentration, treatment time, and pH—were selected for analysis. Their effects on the mechanical properties of the fibers were systematically evaluated. Experimental data were analyzed using ANOVA in Design-Expert 13, and a regression model was established for parameter optimization. The optimal conditions were determined to be a Bacillus subtilis concentration of 1.18%, a treatment time of 20 days, and a pH of 7. Under these conditions, the tensile strength, elastic modulus, and elongation at break of the fibers reached 1161.63 MPa, 50.68 GPa, and 2.32%, respectively—representing improvements of 46.23%, 42.48%, and 34.1% compared to untreated samples. In addition, the fibers were analyzed using SEM, TGA-DTG, FTIR, and XRD to investigate changes in surface topography, thermal behavior, chemical bonding, and crystalline structure. Results showed that sludge retting effectively removed non-cellulosic components, enhanced thermal stability and crystallinity, and significantly improved the mechanical performance of the fibers. This study demonstrates that sludge retting is a green and sustainable pretreatment technique with strong potential for banana pseudostem fiber processing.