Bio-Based Antimicrobial Nanocomposites Derived from Tannery Waste for Functional Leather Coating: Circular Valorization of Chrome Shaving Dust

Abstract

This study aimed to establish a waste valorization approach by developing bio-based antimicrobial nanocomposites and exploring their potential reuse within a circular economy framework. Protein was extracted from chrome-tanned shaving dust through eco-friendly microbial degradation using Bacillus thuringiensis SRL4A. Four types of metal-protein-based nanocomposites, namely Protein-ZnO, Ag-Protein, polyaniline (PANI)/Protein-ZnO, and PANI/Ag-Protein, were synthesized using the protein hydrolysate derived from tannery solid waste and incorporated into leather finishing formulations. Characterization techniques, including TKN, amino acid analysis, UV-vis spectroscopy, FT-IR, XRD, zeta potential, TGA, SEM, and EDX, were employed to determine the functional groups, crystallographic structure, thermal stability, morphology, and composition of the nanocomposites. TKN analysis confirmed that the protein content in the sample obtained from shaving dust was 73.08% and other analyses confirmed the successful incorporation of metal particles into the protein matrix. The particle sizes of the synthesized nanocomposites were 32 nm (Protein-ZnO), 34 nm (Ag-Protein), 44 nm (PANI/Protein-ZnO), and 45 nm (PANI/Ag-Protein). Cytotoxicity tests revealed that all nanocomposites were non-toxic to living cells. The minimum inhibitory concentration (MIC) values for Protein-ZnO were 0.325 mg/mL against gram-positive B. cereus and 1.25 mg/mL against gram-negative E. coli. For Ag-Protein, PANI/Protein-ZnO, and PANI/Ag-Protein, the MIC was 0.625 mg/mL against both B. cereus and E. coli. All nanocomposites exhibited antimicrobial activity, with Protein-ZnO showing the highest zone of inhibition (ZOI = 20 mm) against B. cereus, and Ag-Protein showing the highest ZOI (22 mm) against E. coli. Leather samples coated with varying amounts of nanocomposites in different finishing formulations demonstrated effective antibacterial activity and met the standard requirements for finish film quality and mechanical performance. Notably, Protein-ZnO-treated leather exhibited the highest ZOI (18 mm) against B. cereus, while Ag-Protein-treated leather achieved the highest ZOI (14 mm) against E. coli. This work presents a sustainable approach to repurposing tannery waste into high-performance antimicrobial materials, with potential applications in the leather, textile, packaging, and biomedical industries, as well as the valorization of protein derived from tannery waste.