Effect of Leather Chemicals on Cr(III) Removal from Post Tanning Wastewater

The effect of typical leather chemicals on removal of Cr(III) from tannery post tanning wastewater by PAC coagulation was investigated. It was found that all the leather chemicals investigated could form complexes with Cr(III) through their anionic hydrophilic groups. The determinations of fluorescence quenching titration indicated that the Cr(III)-complexing ability of these anionic hydrophilic groups was in the sequence of phenolic hydroxyl of vegetable tannins and syntans > carboxyl of acrylic resins > sulfonic acid group of fatliquors and dyes. The complexes formed had higher hydrophobicity because a part of anionic hydrophilic groups of leather chemicals were blocked. The Cr(III) complexes with vegetable tannins, syntans and dyes were in the form of hydrophobic colloid or particle due to both the blocking effect of hydrophilic groups and the strong hydrophobicity of benzene ring, which favored the removal of Cr(III) from post tanning wastewater by coagulation (removal extent 82%-99%). But the Cr(III) complexes with acrylic resins existed as water-soluble pieces, probably because a part of free carboxyl group still kept on the polymer chain at reaction equilibrium, and acrylic resins had no structure/group with hydrophobicity as strong as benzene ring. As a result, Cr(III) was difficult to be removed from wastewater when acrylic resins existed (removal extent <60%). That is, the formation of Cr(III)acrylic resin complexes is the major obstacle in removing Cr(III) effectively from post tanning wastewater.