A review of the green chemistry approaches to leather tanning in imparting sustainable leather manufacturing

Leather is made of collagen protein polymer, which is used in the manufacture of a variety of products including footwear, automotive upholstery, garments, and sports equipment. Animal skins/hides are converted into leather using a series of chemical processes. Of them, the tanning process is the most important chemical process that converts animal skins into leather by stabilising collagen fibre so that they do not putrefy. However, it is a hazardous process because of the use of various toxic chemicals in tanning, re-tanning and fatliquoring treatments producing toxic effluent. Over the years many tanning treatments based on chromium sulphate, and vegetable and synthetic tannins in combination with heavy metals, have been developed but tanning with chromium sulphate (known as Cr-tanning) is still the most effective, cheap, and widely used tanning process in the leather industry. Although the development of various improved Cr-tanning methods highly reduced the chemical and water consumption in leather tanning, it is still under scrutiny because of the production of effluent containing a harmful level of Cr and there is strong evidence that when the treated leather is disposed into the environment, part of the released trivalent chromium is converted into carcinogenic hexavalent chromium. Many sustainable alternatives to Cr-tanning based on chemical and enzymatic crosslinking, various bio-derived polymers, enzymes, modified zeolites, and nanostructured materials have been developed over the years with limited success. The alternative methods are either not as effective as Cr-tanning, affect the dyeability and other functional and organoleptic properties of leather, and or are cost-prohibitive. In this comprehensive review article, various tanning methods used in industry or studied in the laboratory are critically reviewed, and their advantages and disadvantages are outlined. The consumption of tanning agents, total chemicals including various auxiliaries and fatliquoring agents, and water in tanning, and the tanning performance and mechanical properties of the processed leather are compiled and compared. The reaction mechanisms of novel tanning agents with leather collagens and the future directions to make leather tanning more sustainable are outlined. This review article will be a guide for academicians/researchers/manufacturers involved in leather processing to develop more sustainable leather materials.