Removal of bisphenol compounds from leather using laccase enzymes

Introduction
Bisphenol A (BPA), Bisphenol F (BPF), and Bisphenol S (BPS) are chemicals that have been shown to exhibit estrogenic effects and reproductive toxicity. These compounds disrupt hormonal balance, leading to reproductive and developmental toxicity, neuroendocrine dysfunction, immune system suppression, DNA damage, and chromosomal abnormalities. As a result, these substances have attracted increasing attention and are subjected to stricter regulation across various industries, not only the tanning industry.
Aromatic polymer tanning agents (syntans and resins) are widely used in the post-tanning wet processing of leather production to give the leather a number of desirable properties. Typically, aromatic polymer tanning agents are synthesized through methods such as condensation and sulfonation, utilizing compounds such as phenol and methylphenol. As a result, these agents often contain significant levels of BPA, BPS, BPF, and other bisphenol compounds, resulting in high concentrations of these chemicals during the leather processing stages. This has resulted in increasing concern in the tanning industry, especially since substances such as BPA, BPS, and BPF typically exhibit low water solubility.
Laccase is a copper-containing polyphenol oxidase enzyme found in plants, fungi, bacteria, and insects and can oxidize bisphenols, polyphenols, chlorophenols, aniline, and other organic substances into nontoxic polymers or carboxylic acids. Laccase could potentially play a role in the removal of bisphenol compounds from leather. This study was carried out to evaluate laccase’s ability to degrade BPA, BPS, and BPF in leather, providing a basis for developing a biotechnological treatment strategy.

Experimental
Four laccase enzymes were initially tested to determine which enzyme was most efficient at degrading bisphenol. After the initial experimental work, Laccase Lac-NWX was selected to further investigate the effects of various conditions, including temperature, pH, reaction time, enzyme concentration, and enzyme addition methods, on the catalytic degradation of common bisphenol compounds in leather, such as Bisphenol A (BPA), Bisphenol F (BPF), and Bisphenol S (BPS).

Results
Effect of Temperature on the Degradation Efficiency of Bisphenols by Laccase
Figure 1 shows the degradation performance of Laccase LacNWX toward BPA, BPS, and BPF at different temperatures. As the temperature increased, the degradation efficiencies of all three bisphenol compounds first increased and then decreased, with the optimal temperature observed around 45°C.
At the optimal temperature, Lac-NWX displayed the highest catalytic degradation performance for BPF, achieving a degradation efficiency of approximately 93.5%. The second-highest degradation efficiency was observed for BPS (around 83.8%), while BPA exhibited the lowest degradation efficiency, approximately 65.3%.

Figure 1. Effect of temperature on laccase degradation of BPA, BPF and BPS.

Effect of pH on the Degradation Efficiency of Bisphenols by Laccase
Further studies were conducted at 45°C, keeping the same enzyme concentration and reaction time, to examine the degrading performances of laccase on various bisphenol compounds at different pH values, with the results shown in Figure 2.
As the pH gradually increased from 3.0 to 6.5, the degradation efficiency initially increased and then rapidly decreased. The highest efficiencies were observed around pH 4.5, with degradation efficiencies of approximately 92.2% for BPF, 83.3% for BPS, and 74.0% for BPA. These findings suggested that the optimal pH for the catalytic degradation of bisphenol compounds by Lac-NWX was around 4.5.

Figure 2. Effect of pH on laccase degradation of BPA, BPF and BPS.

Effect of Reaction Time and Laccase Dosage on the Degradation
Under controlled conditions of pH, temperature, and enzyme concentration, the effects of reaction time and enzyme dosage on bisphenol degradation were further investigated. The results presented in Figure 3(A) indicated that as the reaction time increased, the degradation efficiency of BPA, BPF, and BPS progressively improved, and then stabilized at approximately 72.3%, 91.7%, and 84.0%, respectively, after 30 minutes. Moreover, increasing the enzyme dosage slightly enhanced the degradation efficiency of BPF but did not significantly improve the degradation of BPA and BPS, as shown in Figure 3(B).

Figure 3. Effect of reaction time (a) and enzyme concentration (b) on laccase degradation of BPA, BPF and BPS.

Effect of Single vs Multiple Doses of Laccase on Degradation of Bisphenols in Leather Powder and Retanned Leather with Aromatic Polymer
During leather processing, laccase will need to function in a heterogeneous solid–liquid catalytic system. The enzymes will have to penetrate into the leather prior to effectively acting on bisphenol compounds. The efficacy of this process would be influenced by several factors, including the properties of the enzyme, the interaction between the enzyme and the leather, the presence of chromium ions, and other contributing elements.
To further investigate the degradation efficiency of Lac-NWX on bisphenol compounds, experiments were conducted using both leather powder and small leather samples. As shown in Figure 4(A), LacNWX exhibited high degradation efficiency toward BPA, BPS, and BPF in leather powder, with a single enzyme offer degradation efficiency ranging from approximately 58.4% to 83.0%. Increasing the number of enzyme additions enhanced the effectiveness of Lac-NWX. When the enzyme was applied in a three-step addition, the degradation efficiencies for BPA, BPS, and BPF increased to 80.1%, 89.5%, and 96.9%, respectively, and the residual bisphenol compounds in retanned leather powder decreased from above 250 mg/kg to below 50 mg/kg. These results suggest that Lac-NWX could effectively degrade bisphenol compounds in leather powder.
Similarly, in retanned wet blue leather (Figure 4(B)), the overall degradation efficiency of Lac-NWX was somewhat diminished due to its limited penetration ability within the leather. Nevertheless, it still achieved degradation efficiencies of approximately 63.0% to 80.1% for bisphenol compounds such as BPA, BPS, and BPF within the leather. The residual BPA concentration in the leather was reduced from 375 mg/kg to around 120 mg/kg, while the residual concentrations of BPS and BPF were reduced to below 75 mg/kg.

Figure 4. Laccase degradation of BPA, BPF and BPS in chrome tanned hide powder (a) and retanned leather (b) with single and multiple additions of the enzyme.

These results demonstrated that laccase could effectively degrade bisphenol compounds in leather during the leather-making process. However, improving the enzyme’s activity stability and penetration ability within the leather would facilitate more thorough removal of bisphenol compounds.

Conclusion
The results of this research showed that under the optimized conditions (pH 4.5, 45°C, 30 min) and with a three-step enzyme addition strategy, laccase achieved degradation efficiencies of approximately 63.0%–80.1% on retanned leather. The catalytic degradation efficiency of laccase followed the order of BPF > BPS > BPA and was significantly influenced by the concentration of bisphenol compounds.
These findings provide experimental support for the use of laccase in treating leather wastewater and related pollutants, and offer valuable insights for further developing enzyme-based biotechnologies to remove bisphenol compounds from leather.

This article is a summary of the paper “Evaluation of Laccase for the Degradation of Bisphenol Compounds and its Potential Application in Leather Processing”. Journal of the American Leather Chemists Association (JALCA) Vol 120, 451 - 458 (2025).