Biomass-derived polycarboxylate–aluminum–zirconium complex tanning system: a sustainable and practical approach for chrome-free eco-leather manufacturing

Chrome-free eco-leather production plays an important role in the sustainable development of the leather industry. Here, we established an eco-leather tanning system based on biomass-derived polycarboxylate–aluminum–zirconium complexes (PAZ). The tanning conditions, storage stability, and tanning mechanism of PAZ tanned leather were investigated, and then the industrial-scale application performance of the PAZ tanning system was compared with that of a conventional Cr tanning system from the aspects of collagen microstructure, leather properties, and environmental impacts. The PAZ tanning agent showed a high uptake rate of 95%, endowed the tanned leather with a shrinkage temperature of over 80 °C, and allowed the tanned leather to be stably stored for more than one year. The PAZ penetrated uniformly into the hierarchical structure of collagen fibers and mainly coordinated with the amino and carboxyl groups of collagen, thereby achieving intermolecular and intrafibril cross-linking in the leather matrix. The PAZ tanned leather possessed strong absorption capacity for post-tanning chemicals (over 85%) due to its high isoelectric point (7.2). Thus, the physical properties of PAZ crust leather were comparable to those of the Cr -tanned crust leather. The wastewater of the PAZ tanning system exhibited a reduction of 43.4% in total organic carbon load and better biodegradability than the Cr tanning system. The acute oral toxicity and acute dermal toxicity tests confirmed that the PAZ crust leather has low biotoxicity. This work not only offers insights into the PAZ tanning mechanism at the molecular level but also provides a sustainable and practical approach for manufacturing chrome-free eco-leather.