Electroflocculation of indigo dyeing wastewater from industrial production: Flocs growth and adsorption mechanism

During the electrocoagulation of indigo wastewater, the growth and morphology of flocs are greatly affected by the electrolytic time, with flocs morphology directly related to the adsorption of indigo dye. Therefore, understanding the morphology, structure and growth of flocs is important for the effective treatment of indigo dyeing wastewater. In this study, we investigated the relationship between flocs morphology and indigo dye adsorption capacity using aluminum as anode, graphite as cathode and a plate spacing of 3 cm at 180 r/min stirring and 240 mA constant current as the basic parameters for electrocoagulation. Microscopically, the floc morphology was characterized by fractal dimension and average particle size. Next, we assessed the kinetics and thermodynamics of the adsorption process. The results showed that floc growth could aggregate, fragmentate and recombine with an increase in electrolysis time. When the flocs grew and aggregated to the maximum particle size, they were loose in shape and large in size, which was most conducive to the adsorption of indigo dye. According to the results of kinetics and thermodynamics, flocs adsorped indigo dye via a monolayer physical adsorption process, with Van Der Waals force as its binding force. Due to the weak adsorption power, the dye dissolved again when the flocs were broken, suggesting that the growth of flocs should be controlled before the crushing stage during the electrocoagulation process of indigo dyeing wastewater.