Attraction behavior induces the aggregation and precipitation of organic dyes: Improving efficiency through co-treatment strategy

Abstract

Organic dyes harm the environment through bioaccumulation, toxicity, water contamination, and persistence. Organic dyes are commonly charged species, while limited works reported the interaction behavior between the charges. In this study, seven typical organic dyes are selected to investigate the general rules of the aggregation-precipitation behaviors between cationic and anionic organic dyes at molecular scale. The maximum precipitation rates of organic dyes reached 96.0 % when the amount of positive charges of cationic dyes was stoichiometric to the negative charges of anionic dyes. The aggregation process is reversible in ethanol solvent and insensitive to temperature, indicating the aggregation behavior is a physical process. The aggregation behavior of cationic and anionic dyes in water and ethanol solvents was simulated at molecular scale. 94.9 % of water molecules dispersed out of the dye clusters within 20 picoseconds in the simulation, while only 0.8 % of ethanol dispersed under the same conditions. The inter-molecule van der Waals force of the ethanol-solvent system was higher than that of the water-solvent system, resulting in the failure of dyes aggregation in ethanol. This study proposed a concise strategy of mixing cationic and anionic dyes in charge balance to cause aggregation and co-precipitation of dyes to improve removal efficiency.