Synergetic removal of oppositely charged dyes by co-precipitation and amphoteric self-floating capturer: Mechanism investigation by molecular simulation.

The adsorption performances of adsorbents to dyes are hard to maintain in a wide pH range because most of the reactions are pH-dependent, developing a cost-effective strategy to break the pH-limitation is significant. In this study, an amphoteric self-floating adsorbent (Am-SA) was synthesized by hollow silica microsphere surface modification, which was useful to capture anionic acid orange 7 (AO7) and cationic crystal violet (CV) dyes, but the adsorption performances were also greatly affected by pH. Fortunately, a co-precipitation phenomenon was noticed when the AO7 and CV solutions were mixed with a 1:1 molecule ratio. The precise structures of AO7 and CV molecules were constructed and the AO7-CV-HO mixed system was structured by Materials Studio. Besides, this system was involved in a dynamic simulation to reveal the mechanism of the co-precipitation phenomenon. The simulation results showed HO molecules dispersed out of the system via thermal motions within 30 ps, but the AO7 and CV molecules aggregated to each other via electrostatic attractions. The energy calculations also demonstrated the electrostatic attraction between AO7 and CV is the dominant factor that induced the aggregation. The aggregation phenomena were also observed in various mixed cationic-anionic dyes systems. The removals of dyes significantly improved in a wide pH range in the mixed systems as the captures of the aggregated dye clusters were much easier than that of independent dye molecules, and both co-precipitation and adsorption contributed to it. Proper utilization of the aggregation behaviors between dyes can be regarded as a prospective strategy in cost-effective treatments.