Chrome complexes in rewetting and neutralizing effluents and hints for recycling post-tanning wet-process effluent

Although the treatment for recycling chrome in tanning effluents is relatively mature, the post-tanning processes have no corresponding effective technology due to the lack of knowledge about the effluents. In this work we investigated the composition and properties of rewetting and neutralizing effluents in posttanning processes. Goat skins were tanned with sulfate chrome liquor of alkalinity 33% (SC), and then conventional technologies were applied to get rewetting and neutralizing effluents (RE, NE). The effluents were separated and analyzed by ion exchange chromatography, gel filtration chromatography, ultravioletvisible absorption spectroscopy, and Fourier-transform infrared spectroscopy. Hide powder tanned by the chrome recovered from the effluents was evaluated by thermogravimetric analysis and differential scanning calorimetry. The results indicated that RE and NE showed different valent chrome complexes from SC. Released chrome mainly combined with SO₄²⁻ and HCOO⁻ through a single-point coordinate mode. Molecular weight of chrome complexes in the effluents was smaller and the tanning ability was poorer. Based on the results, the composition and properties of chrome complexes in RE and NE were understood, and some of their structures were speculated. Since these characteristics have a relationship with tanning performance, they may provide theoretical support for developing effluent-related recycling technology in the leather industry.