Oxidative unhairing vs sulphide use. A critical comparison

The conventional unhairing process is one of the most pollutant steps in the leather tanning process, which usually involves great amounts of sulphides, lime and soda ash, generating high BOD and COD values in the wastewater. In this work, we have investigated different oxidative agents, combined or not with enzymes, as a possible replacement for sulphides in unhairing, focused on the possible industrial application of the method developed. Peroxymonocarbonate anion, persulfate anions (peroxymonosulfate and peroxydisulfate anion) and peroxynitrite anion have been chosen and the dehairing of bovine leather has been performed. The performance has been compared in terms of dehairing ability, time, suspended solids, pH, conductivity, COD and total nitrogen (Kjeldahl). All the tests were compared with a pilot test (hair-saving sulphide unhairing) and with a hydrogen peroxide dehairing. The results show the great ability of percarbonate, combined with enzymes, to give complete unhairing in 16 hours, no grain damage in absence of sulphides, and great homogeneity after dyeing tests. The results, also, showed a greater reduction of pollutants in the wastewater.