Current trends in modern tanning and multifunctional post-tanning as a new age tool for sustainable leather...

Skins and hides are processed into high-value leathers through tanning and post-tanning unit operations, which together consume nearly 500-600 kg of chemicals per ton of raw hide, generating large volumes of effluents. Conventional chromium tanning, which still accounts for 80-85% of global leather production, provides excellent hydrothermal stability (shrinkage temperature > 100 °C) but suffers from environmental concerns due to 2-5% residual chromium discharge in wastewater and sludge. Alternative inorganic and organic tanning systems, including polyphenols, aldehydes, and syntans, have shown comparable stabilization (shrinkage temperature 85-98 °C) but often compromise mechanical strength, softness, or color uniformity. Post-tanning operations further involve the application of high molecular weight polymers and oils, contributing to 40-50% of the total chemical load, and require extensive washing and neutralization. Recent innovations in hybrid tanning agents-such as bio-based polyphenol-aluminum complexes and polymer-aldehyde-have achieved a 30-40% reduction in total chemical input, while multifunctional post-tanning auxiliaries have improved fullness and roundness indices by 20-25%, reducing effluent load by nearly 35%. This review article exclusively focuses on outlining the recent strategies adopted in the synthesis of chromium-free tanning agents and multifunctional post-tanning agents and their mechanism of crosslinking with collagen. This review also gives insights into the merits, limitations, and futuristic perspective of the recently developed hybrid tanning and multifunctional post-tanning agents. A key outcome is the identification of hybrid tanning and multifunctional auxiliaries as scalable pathways to reduce chemical load by up to 40% and wastewater toxicity by 30-50%, aligning leather processing with green chemistry and circular economy principles.