Dye-Mediated Interactions in Chitosan-Based Polyelectrolyte/Organoclay Hybrids for Enhanced Adsorption of Industrial Dyes.

Noncovalent, dye-mediated interactions between organo-montmorillonites ("organoclays") and a chitosan-based polyelectrolyte are exploited for highly effective and fast removal of different, industrially important anionic dyes (single-azo, double-azo, and anthraquinone) from aqueous solutions. The addition of only 10 wt % of the polyelectrolyte to conventional organoclay results in a 100% increase in absolute dye uptake capacity, an acceleration of dye uptake kinetics by up to 500%, and the flocculation of large, easily separable sorbent aggregates. These substantial improvements in adsorption performance are driven by the mediating effect of the anionic dyes (acting as the electrostatic mediator between the positively charged polyelectrolyte chains and organoclays), enabling the formation of true hybrid sorbent structures without the need for covalent cross-linking chemistry. The dye-mediated sorption and hybrid formation mechanism is further evidenced by structural and chemical characterization of the hybrid sorbents (small-angle X-ray diffraction and IR mapping) as well as by analysis of dye sorption kinetics according to the intraparticle diffusion model. Importantly, the organoclay/polyelectrolyte hybrid system provides a highly interesting adsorbent for the treatment of dye mixtures. Our study shows that structurally different anionic dyes localize at different sites within the hybrid structure (organoclay intergallery spaces vs polyelectrolyte/organoclay interface), enabling the simultaneous adsorption of different dyes with high efficiency. Consequently, the total uptake capacity for dye mixtures was 50% larger than that of individual dyes, demonstrating the enormous potential of the hybrids for industrial wastewater purification, where dye mixtures are ubiquitous.