Characterisation and tanning effects of purified chestnut and sulfited quebracho extracts

Vegetable tannins are environmentally friendly tanning agents. However, they generally impart a dark colour to the tanned leather and highly contribute to the organic load in wastewaters. In this study, we employed a purification protocol separately on chestnut tannin (CT) and sulfited quebracho tannin (QT) to obtain the purified fractions (PCT and PQT). These samples were characterised by GPC, 1H NMR, 13C NMR, FT-IR, and HPLC-DAD techniques and applied for tanning tests. Through the purification process, non-tannin components and smaller molecules such as gallic acid, glucopyranose, and catechin were effectively removed from CT and QT, which consequently led to the reduced moisture content, pH value, and lighter colour of purified fractions. The crust leathers processed with PCT and PQT showed desirable light shades. Moreover, the organic loads in PCT and PQT tanning wastewater were reduced by 13.5% and 19.1%, respectively, when compared to those in traditional CT and QT tanning wastewater. Additionally, the physical and mechanical characteristics of crust leathers processed with PCT and PQT were comparable to those processed with CT and QT. Thus, purification of vegetable tannins may serve as a feasible strategy for producing light-colored vegetable-tanned leather while minimizing organic pollutant discharge during the vegetable tanning process.