Effects of Organic Ligands on Tanning Performance and Environmental Benefits of Aluminum-Zirconium Complexes

Oxidized sodium lignosulfonate (OSL), tartaric acid (TA), and oxidized starch (OST) were used as the ligands of aluminum-zirconium salt (AZ) for leather tanning. These complex tanning systems were compared with conventional chrome (Cr) tanning system in terms of the performance of tanned leather, the performance of crust leather, and the environmental benefits. OSL-AZ combines with collagen mainly by forming coordination bonds. Meanwhile, OSL forms ionic, hydrogen, and hydrophobic bond interactions with collagen through its sulfonic acid group, carboxyl group, phenolic hydroxyl group, and hydrophobic benzene ring. Thus, OSL-AZ forms a rigid macromolecule crosslinking network among collagen fibers. The OSL-AZ tanned leather has a shrinkage temperature high up to 88.4 ℃, an isoelectric point up to 7.54, and an uptake rate for post-tanning chemicals over 84%, thereby resulting in excellent physical properties of OSL-AZ crust leather, which is second only to Cr crust leather. TA and OST without sulfonic acid group, phenolic hydroxyl group and rigid benzene ring exhibit relatively weak interaction with leather collagen. As a result, the tanning performance of TA-AZ and OST-AZ is worse than that of OSL-AZ. The biodegradability of OSL-AZ tanned leather and wastewater is obviously better than that of traditional Cr tanned leather and wastewater. Therefore, OSL-AZ tanning technology is a practical and environmentally friendly leather manufacturing technology.