An eco-friendly metal-less tanning process: Zr-based metal-organic frameworks as novel chrome-free tanning agent

Chrome-free tanning agent is of great significance because it can eliminate the notable environmental pollution of chrome tanning. Metal-organic frameworks (MOFs) are the products of the desired combination of metal ions and organic ligands, which make them the born self-masking tanning agent. Herein, a series of Zr-MOFs were prepared by a solvothermal method, and then applied in leather tanning process. Among the Zr-MOFs family, UiO-66 tanned leather exhibited the highest shrinkage temperature (Ts) of 75.3 °C. Meanwhile, the smaller particle size and self-masking structure of Zr-MOFs allowed for the uniform distribution in collagen fibers matrix, thus giving improved physical-mechanical and organoleptic properties. It was noted that 80% metal usage reduced could be achieved with Zr-MOF as tanning agent comparing with conventional zirconium sulfate. It was found that Zr4+ of Zr-MOFs could mainly coordinate with -COOH and -NH2 groups in collagen fibers to form a stable cross-linked structure. In addition, the BOD5/COD value (>0.3) of Zr-MOF tanning waste liquor indicated good biodegradability, further confirming the eco-friendly metal-less tanning process. All results indicate that the proposed Zr-MOFs will promote the eco-friendly metal-less tanning process and open a new chapter for the chrome-free tannage.