Sustainable leather tanning: Enhanced properties and pollution reduction through crude protease enzyme treatment

In this study, proteolytic bacteria, particularly Pseudomonas aeruginosa strain SM4 (OQ349573), were isolated from tannery solid waste dumping yard soil and employed to produce extracellular protease enzymes. The bacteria exhibited optimal growth after 30 h of incubation at 37 °C and pH 7. Under conditions of 55 °C, pH 8, and a substrate concentration of 2 %, the crude enzyme displayed its highest activity at 105 UmL^-1. Notably, the produced crude enzyme showed no discernible inhibitory effects on detergents, metal salts, or organic solvents. Application of the crude protease at concentrations of 3 % and 2 % in chrome tanning of goatskins (GS) and cowhides (CH), respectively, yielded significant reductions of 35 % and 30 % in chromium and other post-tanning chemicals compared to conventional processes. Despite the 30 to 35 % reduction in tanning and post-tanning chemicals, the uptake of chrome and associated chemicals by crust leather was higher than observed in conventional processes. Chromium content analysis of the effluent revealed an 81 % reduction during piloting in real industrial operations, accompanied by reductions of about 46 % in BOD and COD pollution loads. The finished leather obtained from the enzymatic process exhibited superior mechanical properties, including higher tensile strengths (210 and 195 kg cm^-2), stitch tear (92 and 165 kg cm^-1), grain crack load (28 and 32 kg), and distension (73 and 62 mm) for GS and CH, respectively, surpassing or closely aligning with standard values and those obtained in conventional processes.