An approach on chromium discharge reduction: Effect and mechanism of ketone carboxylic acid as high exhaustion chrome tanning agent

Chrome tanning agent is the most widely used tanning agent with the best performances of finished leather, but the traditional chrome tanning process poses severe challenges because of the serious chromium pollution, thus the upgrading of cleaner chrome tanning process is imminent. In order to reduce the residual chromium in wastewater and increase the utilization rate of chrome tanning agent, 4-acetyl-heptanediate (AHA), a high exhaustion chrome auxiliary, was developed and applied in chrome retanning process. The results showed that AHA chrome retanning processes (AHA-6 and AHA-4) significantly reduced the chromium content in wastewater by 48.14% and 47.57%, and increased the chromium content in leather by 102.14% and 82.43%. COD and TDS values decreased by 43.82% and 31.46% with AHA-4 method. The addition of AHA obviously reduced the chromium concentration of wastewater, improved the chromium absorption rate, and promoted the cleaner chrome retanning process, which satisfied with the overall requirements of modern sustainable leather industry. The formation of AHA and chromium (III) was discussed using Gaussian 16 program to explain the high exhaustion chrome tanning agent mechanism. The work will guide on design and preparation of new high exhaustion chromium tanning agents for clean leather production. • AHA, containing carboxylic acid and ketone carbonyl groups, was developed for high exhaustion chromium retanning process. • Chromium content values of effluent liquors reduced sharply with AHA addition. • Total chromium content values of leather samples confirmed that AHA had a positive coordination effect on chromium ions. • Physical and mechanical properties clarified successful collagen modification and enhanced chromium coordination sites. • [Cr (AHA)] + and [Cr (AHA) 2 ] - coordinations were demonstrated by molecular simulation.