Removal of direct dyes by coagulation: Adaptability and mechanism related to the molecular structure

The textile industry uses a variety of synthetic dyes for dyeing and discharges a large amount of colored wastewater. It is difficult to achieve good decolorization effect for textile wastewater by conventional biological treatment methods, because it contains various synthetic dyes with complex structures. At present, coagulation is an economical and effective treatment method. However, in dye removal by coagulation research, the adaptability and mechanisms of coagulation removal of many dyes are still unknown due to the wide variety of dyes in use. In this study, ferric chloride (FeCl3), aluminum chloride (AlCl3) and magnesium chloride (MgCl2) were selected as coagulants, and direct orange 26 (DO), direct yellow 11 (DY) and direct black 19 (DB) were used as the dye samples to conduct a series of coagulation experiments; in order to understand the coagulation adaptability and mechanism for high removal efficiency of direct dyes. In this system without particles, the direct dyes were dissolved directly in pure water and maintained in molecular state. The results showed that 1) AlCl3, FeCl3 and MgCl2 exhibited superior coagulation efficiency on the three dyes. Direct dyes which contain more amino functional groups are more easily removed by coagulation. 2) The influence of hydraulic conditions on the residual turbidity was more significant compared with the color removal. 3) Zeta potential increased with the increase of the coagulant dosage, but the colloidal particles did not re-stabilize. This proved that charge neutralization is not the dominant mechanism of coagulation. The FT-IR and XRD patterns indicated that the direct dyes were removed by the chemical combination of the -SO3H on the dye molecule and the hydrolysate of the coagulant.

Automated summary

The textile industry generates a large amount of colored wastewater containing various synthetic dyes. Conventional biological treatment methods are not effective in achieving good decolorization due to the complex structures of these dyes. Coagulation has been identified as an economical and effective treatment method. This study investigated the adaptability and mechanisms of coagulation in removing direct dyes through a series of experiments using different coagulants. The results revealed that ferric chloride, aluminum chloride, and magnesium chloride exhibited superior coagulation efficiency on direct dyes, with dyes containing more amino functional groups being more easily removed.