Modification of melamine sponge by solid-phase esterification of citric acid-polyvinyl alcohol and its selective adsorption for cationic dyes

In the work, the functionalized melamine sponge has been successfully prepared by an in-situ solid esterification reaction of poly (vinyl alcohol) (PVA) and citric acid (CA) monomers. The esterification products were evenly coated onto the surface of the sponge framework of the melamine sponge (MS) by the mild heat treatment. Characterization results showed that the as-prepared PVA/CA@MS was provided with a rich hydroxyl group and carboxyl groups due to the as-formed cross-linked hydrogel and the suspending carboxyl group derived from the citric acid monomer. Therefore, the as-prepared 3D PVA/CA@MS was employed as the specific adsorbent to remove cationic dyes due to its oxygen-containing group. The adsorption performance of 3D PVA/CA@MS for cationic methylene blue and methyl violet was systemically carried out by kinetic and isothermal adsorption investigation. The data analyses suggested that the adsorption for PVA/CA@MS for MB and MV followed Langmuir isotherm adsorption and the maximum experimental adsorption capacities were 137.6 and 262.5 mg•g−1, respectively. The pseudo-second-order adsorption kinetic model fitted well with the experimental data of the time-dependent adsorption process. Mechanistic studies indicated that the electrostatic interaction was the main driving force for the adsorption of cationic dyes. These results demonstrated that the as-fabricated 3D PVA/CA@MS with easy separation performance was an efficient adsorbent for the selective removal of cationic dyes.