Spectacular Selectivity in the Capture of Methyl Orange by Composite Anion Exchangers with the Organic Part Hosted by DAISOGEL Microspheres

There is a paramount need in finding sorbents endowed with selectivity in sorption of certain dyes from their mixture with other dyes from the same family. In this context, novel composite anion exchangers (CANEXs) were fabricated here by an innovative approach using silica DAISOGEL as the host for an anion exchanger (ANEX) bearing vinylbenzyl N,N-diethyl 2-hydroxyethyl ammonium moieties. Information about the outer surface versus in-pore generation of ANEX as a function of silica morphology was acquired by scanning electron microscopy. It was demonstrated that the CANEX microspheres were able to selectively capture methyl orange (MO) in binary mixtures with either methylene blue (MB) as the cationic dye or Chicago Sky Blue 6B (CSB) as the competing azo dye. The adsorption kinetics of MO and CSB were well-fitted by a pseudo-second-order model, indicating that chemisorption controlled the sorption process. Isotherms of "H" type characterized the sorption of MO, whereas "L" type isotherms described the sorption of CSB. Langmuir and Sips isotherms were the most suitable models to describe the sorption process at equilibrium. Even if only about 10 wt % of the CANEX sorbents was involved in the sorption process, the maximum sorption capacity was 180.25 mg MO/g composite and 153.86 mg CSB/g sorbent. Moreover, the CANEX sorbents exhibited a spectacular preference for MO molecules in competition with CSB at pH 5.5. Selectivity coefficient for MO in the mixture with either MB or CSB was 370 and 38.4, respectively. Removal efficiency of MO remained up to 100% after 10 consecutive sorption/desorption cycles.