Transfiguration of discarded PVDF ultrafiltration membranes: Optimization of pyrolysis parameters for high-value char production

Disposing vast amounts of non-biodegradable wastes presents engineering and economic challenges to environmental management globally. Concomitantly, recycling and upcycling of non-biodegradable materials can play a crucial role in sustainable waste management, reducing the need for raw material extraction and energy-intensive manufacturing. Accordingly, this study aimed to explore the utilization of discarded polyvinylidene fluoride (PVDF) ultrafiltration membranes for char production via optimized pyrolysis. The Generalized Reduced Gradient (GRG) optimization method was applied to predict the optimal final temperature (T) and residence time (t) to achieve the maximum membrane char (MC) iodine number under reasonable conditions. Factorial analysis revealed curvature in the model with a p-value of 0.001 at a 95 % confidence interval, indicating the potential applicability of the response surface method (RSM). The optimized pyrolysis (584 °C; 111 min) achieved an average yield of 31.3 ± 2.6 % of MC with an iodine number of 242.35 mg/g. The structural, surface, and textural properties of PVDF fibers and MC were extensively characterized. Thermal analysis of discarded PVDF confirmed the oxidative breakdown of the fluorinated polymer and the revocation of the C-F bond. Moreover, the Raman and FTIR spectral analysis revealed the coexistence of β- and γ- crystalline phases in the PVDF fiber. Consequently, the produced MC demonstrated superior carbon properties and high potential application for various industrial and environmental purposes, aligning with the circular economy approach for reutilizing discarded PVDF membranes.