Role of carbohydrases in minimizing use of harmful substances: leather as a case study

Abstract

Leather processing is an important industrial activity. Globally about 2.0 billon sqmt of leather is produced annually. Conventional cleansing operations carried out prior to tanning generate large amounts of waste. Among them dehairing and fibre opening process (reliming) generate large amount of effluent containing hazardous substances and alkaline sludge, resulting in high negative impact on the environment. In this study, both these pre-tanning process steps have been combined using a cocktail of carbohydrases along with optimum quantity of chemicals to minimize the environmental concerns. Carbohydrate and proteoglycan removal were chosen as the parameters of study for efficacy of unhairing and fibre opening. The morphology features of skins were analysed using scanning electron microscopy and histology. Pollution load of the enzyme aided process effluent was determined and compared with conventional process. Findings of the study indicate complete elimination of reliming process step is possible when both unhairing and fibre opening is carried out simultaneously using carbohydrases as an adjunct. Reduction in use of harmful sulphide and lime up to 40% apart from substantial saving in time and water input is the major outcome of the present work.

Appendices

Appendix 1

The pelts were washed with 200% water for 10 min. Subsequently, the pelts were delimed by adding 100% water and 1% (w/w) ammonium chloride for 45 min in a drum. Deliming was ascertained by checking the cross section of the delimed pelts for colourlessness due to phenolphthalein indicator. After deliming, bating process were carried out in the same bath for 30 min by the addition of bating enzyme. The pelts were washed with 100% water for 10 min. Pickling was carried out. 1% sulphuric acid (w/w) was added in 4 feeds at 10-min interval and tumbled in a drum for 60 min to obtain pickled skin at pH of 2.8. The pickled skins were tanned using 8% (w/w) basic chromium sulphate (BCS) in 50% pickle water for 90 min. Then 50% (w/w) water was added and the drum was run further for 30 min. To the running drum, 1% (v/w) sodium formate (mixed with 10% w/v water) was added. After 30 min, 1% (w/w) sodium bicarbonate (mixed with 10% w/v water) was added in 3 feeds at 10-min interval and continued the tumbling for 60 more min to bring the pH to 3.8.

Appendix 2

Post-tanning operations comprise of rechroming of semi-finished wet blue leather, neutralization, dyeing, fat liquoring and finishing. The wet blue leathers obtained by the procedure under Appendix 1 to were shaved to 1.0 mm thickness. All the samples were washed in 100% (w/v) water in a drum for 10 min. After draining, the wet blue leathers were treated with 1.0% (w/w) neutralizing syntan with 100% water for 20 min. Sodium formate 0.5% (w/w) and sodium bicarbonate were then added to the drum in 3 feeds at 10-min interval, while the drum was in running mode. After ensuring the pH of the cross sections at 5.0, the leather samples were washing twice with 200% (w/v) water for 10 min. The neutralized skins were washed with water followed by treatment with resin syntan (3% w/w) and allowed to run in the drum for 20 min. After this, dying (2% w/w acid dye) and fat liquoring (4% w/w synthetic fat liquor) were carried out by drumming for 30 min. Subsequently melamine- and naphthalene-based retanning syntans 4% (w/w) was added and run for 40 min followed by the addition of synthetic fat liquor 4% (w/w), polymeric fat liquor 3% (w/w) and natural fat liquor oil 4% (w/w) and further running the drum for 40 min. Finally the auxiliaries were fixed using 2% (v/w) formic acid diluted with 20% (v/w) water and added at 3 feeds at every 10-min interval and the drum was further run for 30 min and piled overnight. The leathers were set, conditioned, again set with reversible setting machine and hooked for drying. After drying, leather was staked and buffed using 400-grit emery paper.