Structural characteristics and collagen reaction ability of polyphenols in larch tanning wastewater – an important hint for vegetable tanning wastewater recycling

Wastewater recycling technologies have been widely used in tanneries to reduce environmental impacts. However, only a few recycling methods have been developed for vegetable tanning processes due to limited knowledge of their wastewaters. In this study, the structures of the polyphenols in the larch tanning wastewater (LTW) and commercial larch tannin (CLT) were characterized with spectrographic and chromatographic analysis. Structures of these polyphenols were illustrated accordingly; their collagen reaction abilities were then tested with thermal analysis and visualized with a molecular docking software. Results showed that polyphenols in LTW and CLT were all procyanidins. However, average molecular weight of the polyphenols in LTW was 1104Da, in contrast to an obviously higher value for the CLT sample (3334Da). Meanwhile, the LTW presented obvious lower reactivity in the collagen-polyphenol reaction, including lower thermal stabilities and docking energies, fewer hydrogen bonds, and lower interaction energies. These results provide a hint for vegetable tanning wastewater recycling: because of these structural differences, reactivity of the polyphenols in wastewater are lower than the ones in commercial tannins. Therefore, before the recycling processes, polyphenols must be chemically modified in accordance with these structural differences.