Feasibility of metabolite-mediated bioleaching on composite-mineral waste from FRP strengthening: experimental insights under realistic debonding

The present study investigates the end-of-life management of fiber-reinforced polymer (FRP) strengthening systems, which are widely adopted to enhance structural safety, by exploring metabolite-mediated bioleaching as a sustainable treatment strategy for composite–mineral waste. In particular, the study focuses on residues resulting from debonding phenomena, which reflect the actual condition of FRP systems removed from existing structures. To this aim, single shear-lap tests were performed on concrete and masonry specimens strengthened with externally bonded carbon FRP strips, with the objective of reproducing realistic detachment mechanisms and generating representative waste samples. The results showed a nonlinear response with progressive stiffness degradation up to peak load, followed by sudden debonding. Failure modes were governed by mixed mechanisms involving both interfacial debonding and cohesive failure in the substrate, resulting in highly heterogeneous residues. These materials were subsequently subjected to metabolite-mediated bioleaching experiments through mesophilic dark fermentation. The results highlighted the key role of cementitious phases in regulating biological processes, enhancing volatile fatty acid production, and promoting calcium dissolution, thereby contributing to pH buffering and improving hydrogen yields. Maximum biohydrogen production reached approximately 113 mL H 2 /g glucose in cement-based conditions, while FRP components showed no inhibitory effects. Significant mass losses (i.e., up to −22%) were observed in cementitious samples, whereas fiber-based materials exhibited limited degradation, indicating potential fiber recovery. The results demonstrate that metabolite-mediated bioleaching can effectively treat realistic FRP-derived waste, supporting both resource recovery and the sustainability of strengthening technologies in urban environments.