Sustainable leather processing: A critical review of emerging green technologies and practices

The leather processing industry, particularly tanning, generates substantial amounts of solid and liquid waste, which are major contributors to environmental pollution. Tanning effluents and solid wastes contain hazardous chemicals like chromium, acids, alkalis, salts, organic substance, nitrogen, sulfides etc. and result in increasing biochemical oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solid (TDS), total nitrogen, and heavy metal content in soil and water. Different air pollutants such as H2S, NH3, volatile organic compounds (VOCs) are also emitted from leather processing steps. Mitigation of these problems is important to protect the environment. In recent years, several technologically developed and green approaches are brought to light to mitigate these issues such as zero discharge approach, salt-free preservation, enzymatic soaking, enzymatic liming and unhairing, carbon dioxide based deliming, phyto-based tanning to replace chromium, high chrome exhaustion methods etc. Following these methods the pollution loads such as chlorides, BOD, COD and TDS can be reduced up to ∼98 %, ∼75 %, ∼80 % and ∼97 %, respectively while chromium uptake can be increase up to ∼99 %. This review focuses on the recent developments of environment friendly approaches of leather processing from preservation to finishing. It also covers the traditional leather manufacturing process and its potential impacts on the environment. The key technical challenges of the current methods are presented and potential directions for future research are also suggested.