Effect of dye structure on color removal efficiency by coagulation

Abstract The color removal efficiencies by coagulation of two groups of laboratory-prepared dye solutions were compared; one group comprised dyes with the same chromophores and auxochromes, and the other group consisted of dyes with different chromophores and the same or different auxochromes. Several coagulants, polyaluminum chloride (PAC), cationic polyacrylamide (CPAM), anionic polyacrylamide (APAM), aluminum sulfate (alum as Al2(SO4)3), ferric chloride (FeCl3), aluminum chloride (AlCl3), and magnesium chloride (MgCl2), were selected to cover a large range of experimental variations in order to obtain sufficient data to predict and model the color removal efficiency. The effect of coagulant dosage on the color removal efficiency was determined, and the dye removal mechanism by coagulation was further investigated using the best performing coagulants for each dye group (disperse, acid, reactive, and basic). It was shown that the best dye–coagulant combinations that achieved superior color removal efficiency for each dye group with the same structures and functional groups were disperse dye–MgCl2 at a pH of 12, acid dye–MgCl2 at a pH of 12, reactive dye–PAC at a pH of 7.5, and basic dye–APAM at a pH of 8.3, in descending order. When treated with the same coagulant, dyes with the same auxochromes and charge exhibited very similar color removal efficiency. The state of the dye molecules in water was shown to have a significant influence on the coagulation effect. The experimental results indicated that the dye color removal efficiency by coagulation is a function of the dye structures.