Cleaning Up Leather Processing With Salty Solvents

Turning animal hides into the supple, rich-hued leather in shoes, handbags, coats, and myriad other products is a complex, multistep process that creates a lot of wastewater, some of it tainted by heavy metals. Now, researchers have developed a leather processing approach that eliminates most of that waste by replacing water with a deep eutectic solvent, a combination of a solid anion and a solid cation that produces a salt in a liquid state (ACS Sustainable Chem. Eng. 2015, DOI: 10.1021/acssuschemeng.5b00226).

Leather production is a “filthy process” that involves tanning, fatliquoring, and dyeing, among other steps, says Andrew P. Abbott of the University of Leicester, in England. Tanning a hide normally involves chemically cross-linking its collagen by soaking it in a solution containing Cr(III), which could be oxidized in the environment to produce Cr(VI), a potent carcinogen. Tanning removes the hide’s fat, diminishing its flexibility and sheen. Fatliquoring then replaces lost oil but creates greasy wastewater that is difficult to treat. Dyeing adds to the wastewater burden.

Abbott realized that deep eutectic solvents, which are similar to ionic liquids, could potentially replace the water in leather processing. Because the proteins in leather are charged, the leather sucks the solvent into its structure, he says. To test the approach, the researchers added tanning or dyeing agents to a gellike deep eutectic solvent consisting of choline chloride and ethylene glycol and then painted the material onto cow hides. They found that they could then skip the fatliquoring step, Abbott says, because the solvents themselves create a sumptuous feel when absorbed into a pelt. Abbott’s team found that their leather shrunk at a lower temperature, but its strength and flexibility matched those of conventional leathers. “It looks like any other piece of leather,” Abbott says.

Currently, dyeing requires an additional step, but the researchers are planning to combine the tanning and dyeing reagents into one batch of solvent to streamline their leather production process.