Collagen modification and coordination reaction of diketone dicarboxylic acid as high exhaustion chromium tanning agent: Experimental and DFT simulations

Abstract

High exhaustion chromium tannage is an important method to curb the environmental pollution of traditional chrome tanning process. One diketone dicarboxylic acid, 3,3-diacetyl-pentanedioic acid (DAPA) was proposed and applied for high exhaustion chromium tanning process. The chromium content of effluent liquor from DAPA-4 method (394.8 mg·L⁻¹) was much lower than that of Control method (1223.7 mg·L⁻¹). Chromium uptake ratio of DAPA-4 method (90.13 %) was much higher than that of Control method (69.41 %). The tensile strength (Ts), tearing strength, and bursting strength values of leather sample from DAPA-4 method were higher than those of Control method. COD and TDS of effluent liquor from DAPA-4 method was 2193.0 mg/L and 30154.0 mg/L. The high exhaustion chromium tanning mechanism of DAPA including collagen modification and coordination reaction was analyzed from experimental and Density Functional Theory (DFT) simulations. Chemical reaction between diethyl-3,3-diacetylpentanedioate (DAPD) and guanidine carbonate resulted as pyrimidine derivative (4,6-dimethyl-2-imino-5,5-diethyl-acetate-pyrimidine, MDES). Collagen modification of DAPA upon white hide powder was determined. The effect of molar ratio, temperature, and pH value on coordination products formed between DAPA and trivalent chromium ion was analyzed. The β-dicarbonyl group of DAPA will react with guanidine group of arginine (Arg) to form a pyrimidine derivative, which result in collagen modification and introduce two carboxylic acid (-COOH) groups into collagen fiber. The introduction of -COOH group into collagen enhances the chromium uptake. This work will shed light on design and preparation of high exhaustion chromium tanning agent, also show practical application for sustainable chrome-less leather production.