Polyethyleneimine/polydopamine-functionalized self-floating microspheres for caramel adsorption: Interactions and phenomenological mass transfer kinetics

A polyethyleneimine/polydopamine-functionalized self-floating microsphere (EHGM/PDA/PEI) bears an etched hollow glass microsphere as a support, polydopamine as an interfacial cross-linking “glue”, and polyethyleneimine as a functional monomer was fabricated and used as a decolorization sorbent for sugarcane juice. The advantages of EHGM/PDA/PEI for sugarcane juice decolorization were attributed to its richness of target polyethyleneimine monomers, well-connected and abundant three-dimensional network-like aperture structure, and nontoxicity. Moreover, EHGM/PDA/PEI has excellent self-floating performance to allow separation from the adsorption system after decolorization and may have a faster separation speed and lower energy consumption than conventional solid–liquid separation methods. Synthetic caramels were used as model colorants of sugarcane juice to assess EHGM/PDA/PEI performance. The adsorption capacity of EHGM/PDA/PEI for caramels was 18.36 mg/g with a removal efficiency of 91.80% in the adsorption equilibrium state. The interaction mechanisms revealed that the caramel adsorption on EHGM/PDA/PEI was mostly mediated by protonated amine groups, followed by electron sharing. Two layers of caramels adhered onto EHGM/PDA/PEI were mainly mediated by electrovalent bonds between carboxylate and protonated amine. On average, one caramel molecule can be shared by 3–4 protonated amine groups of EHGM/PDA/PEI, which suggests a parallel sorption orientation. Four phenomenological mass transfer models [external diffusion resistance (EXTDR), internal diffusion resistance (INTDR), EXTDR–INTDR mixed, and adsorption onto active sites (AOAS)] proposed new and unique insights into the dynamic properties of the adsorption process.