Objective Conventional aromatic syntans are crucial retanning materials in the leather industry. However, their production mainly relies on non-renewable petrochemical resources and is associated with formaldehyde release issues. To address the unsustainable feedstock and potential environmental/health risks, this study aimed to develop a green, biomass-based retanning agent using lignin (LG), a naturally abundant and renewable aromatic resource. The direct application of lignin is hindered by its key drawbacks, namely its deep color and poor water solubility. Therefore, an efficient oxidative modification method was employed to overcome these limitations, producing high-performance retanning materials that have the potential to replace petroleum-based products.

Methods A hydrogen peroxide−ozone (H₂O₂−O₃) composite oxidation system was used to controllably modify LG under mild pH conditions, yielding a series of oxidized lignins (OLGs). The evolution of product color, chemical structure, molecular size, and morphology was systematically analyzed via chromaticity measurement, functional group titration, nuclear magnetic resonance, molecular weight determination, and scanning electron microscope. The coordination mechanism between OLG and chromium ions (Cr³⁺) was investigated using X-ray photoelectron spectroscopy. OLGs with different oxidation degrees were applied in the retanning process of bovine wet-blue leather. LG and a commercial aromatic synthetic tanning agent (BTL) served as controls. The retanning performance was evaluated through comprehensive testing of the crust leather’s physical properties (thickening rate, shrinkage temperature, tensile strength, tear strength, burst strength, softness, and fullness) and its microstructure.

Results Oxidative modification successfully optimized LG's structure and properties. OLGs showed significant lightening, with solution chromaticity decreasing from 350.0 to 63.2 and solid color difference ΔE from 56.6 to 41.4. Oxidation induced benzene ring opening. The content of phenolic hydroxyl groups decreased from 1.5 mmol/g to 0.5 mmol/g. Methoxy groups were converted to carboxyl groups, whose content increased from 0 to 2.3 mmol/g. Concurrently, the weight-average relative molecular mass (M_w) decreased markedly (from 11 769 to 1 652), and the aggregate structure became finer and more dispersed. OLG coordinated with Cr³⁺ via carboxyl/phenolic hydroxyl groups, forming -C-O-Cr and -COO-Cr bonds, which is the key mechanism for its retanning action. The moderately oxidized product OLG−1.5 (carboxyl: 1.8 mmol/g, phenolic hydroxyl: 0.9 mmol/g, M_w: 3 308) exhibited the best performance balance. Compared to BTL, crust leather retanned with OLG−1.5 demonstrated improved performance: thickness increase rose from 16.2% to 32.8%, shrinkage temperature increased by 1.6 ℃, tensile strength increased from 14.4 N/mm² to 20.3 N/mm², and tear strength increased from 92.1 N/mm to 95.3 N/mm, while possessing comparable organoleptic properties. Microstructural analysis revealed that OLG−1.5 uniformly penetrated and effectively filled the collagen fiber network. In contrast, LG exhibited poor penetration and filling effects due to its excessively high M_w and severe aggregation.

Conclusions The H₂O₂−O₃ oxidation system efficiently produced high-performance biomass-based retanning agents. By regulating the oxidation degree of LG, OLG−1.5 was obtained, featuring a retained aromatic skeleton (ensuring filling ability) and abundant carboxyl groups (enhancing hydrophilicity and coordination capacity). This product uniformly penetrated into leather and was firmly fixed via its coordination with Cr³⁺, delivering excellent filling, thickening, and mechanical enhancement. Its overall performance surpassed that of the traditional petroleum-based product. This study provides a novel pathway for the valorization of LG and an effective solution for developing green and sustainable leather chemicals.