Construction of fulvic acid-based chrome-free tanning agent with on-demand regulation and wastewater recycling: A novel tanning-dyeing integrated strategy

High-performance chrome-free colored leather is an obstacle for the leather industry. Mineral sourced fulvic acid (MFA) is characterized by excellent water solubility and abundant functional groups, such as auxochrome carboxyl, phenolic hydroxyl and chromophore quinonyl. So MFA is commonly used as plant growth regulator and dyeing. In view of this, MFA was firstly extracted from low-value weathered coal, and then a novel chrome-free tanning agent (MFAA) with tanning-dyeing integration was prepared by the coordination of MFA with Al (Ⅲ). Quantitative analysis showed that the contents of carboxyl and quinonyl in MFA were 9.88 mmol/g and 1.52 mmol/g, respectively. The coordination mechanism between MFA and Al (Ⅲ) was analyzed by FTIR and XPS. The shrinkage temperature of leather tanned by MFAA was more than 90 ℃. The tensile strength and elongation at break reached 33.9 MPa and 114.1 %, respectively, which were higher than those tanned by commercially available organic tanning agents. Meanwhile, the UV–Vis absorption spectrum analysis showed that the dyeing performance of MFAA was different under different coordination rate of MFA with Al (Ⅲ), so a series of leathers with different colors from dark to brown can be produced on demand. Finally, the recycled MFA from tanning wastewater was applied in the cultivation of rice seeds. The results showed that it could promote the germination rate of seed and the growth of root and stem. Therefore, this research is expected to simultaneously reduce the pollution of Cr6+ and organic dyes.