High-performance quaternary ammonium-functionalized chitosan/graphene oxide composite aerogel for remelt syrup decolorization in sugar refining

The decolorization of remelt syrup is the most critical step in the sugar refining process as it directly affects the quality of refined sugar. Adsorption is a useful method that can be applied to decolorize the syrup. A quaternary ammonium-functionalized chitosan/graphene oxide composite aerogel (GO-QACSA) was developed as an effective adsorbent for the removal of high-molecular-weight reducing sugar alkaline degradation products (HRSADPs), the main class of colorants in remelt syrup. The advantages of GO-QACSA to remove HRSADPs can be attributed to its well-connected 3-D network-like porous structure, nontoxicity, high hydrophilicity, and the richness of quaternary ammonium groups. The equilibrium adsorption capacity of GO-QACSA for HRSADPs reached 364.09 mg/g and the removal rate was > 90%. GO-QACSA exhibited an ultra-fast adsorption rate. Recycling experiments indicated that GO-QACSA exhibited good renewability and reusability. The interaction mechanism for the adsorption of HRSADP on GO-QACSA was studied, and the results revealed that two layers of HRSADPs adhered to the surface of GO-QACSA via electrovalent bonds formed between the quaternary ammonium and carboxylate groups. On average, 2–3 carboxylate ions in one HRSADP molecule bonded with the quaternary ammonium cations of GO-QACSA. Four phenomenological mathematical models, namely, External mass transfer resistance (EMTR), Internal mass transfer resistance (IMTR), combined EMTR–IMTR, and Adsorption on active sites (AAS), were presented based on the best-fit equilibrium isotherms. These models could be used to provide new insights into the adsorption mass transfer behaviors of the system.