Chrome-free synergistic tanning system based on biomass-derived hydroxycarboxylic acid–zirconium complexes

Reducing or eliminating chrome pollution from leather processing is an essential task for the sustainable development of leather industry. Herein, we established a chrome-free tanning system based on biomass-derived hydroxycarboxylic acid (BHCA)–zirconium complexes. The synergistic tanning mechanism of BHCA and zirconium salt was investigated by exploring the interaction among BHCA, zirconium salt, and collagen fiber (CF). Results show that the hydroxyl and carboxyl groups in BHCA coordinated with zirconium ion to form Zr complexes, while BHCA caused changes in the morphology and conformation of collagen through ionic and hydrogen bonding. The order of the contribution of active groups in CF involved in the coordination reaction in BHCA–Zr tanning presented as amino group > carboxyl group > hydroxyl group. Moreover, BHCA with multiple functional groups and large molecular size provided indispensable non-covalent binding sites for BHCA–Zr tanning system, resulting in the formation of multi-point crosslinking network structure among CFs. Thus, BHCA–Zr exhibited an obvious synergistic effect, and its tanning performance was remarkably better than that of conventional lactic acid–zirconium complexes. This work provides new insights into the development of novel chrome-free eco-tanning system.