Quaternary ammonium-functionalized magnetic chitosan microspheres as an effective green adsorbent to remove high-molecular-weight invert sugar alkaline degradation products (HISADPs)

Quaternary ammonium-functionalized magnetic chitosan microspheres (QAMCM) were fabricated for the removal of high-molecular-weight invert sugar alkaline degradation products (HISADPs), the main class of colorants in beet juice. The equilibrium adsorption capacity of QAMCM for HISADP was 98.03 mg/g at 0.6 mg/mL QAMCM dosage, 333 K temperature, pH 7, and 60 mg/L initial HISADP concentration, and the corresponding removal rate reached 98.03%. The adsorption kinetics were studied by using a pseudo-first-order and pseudo-second-order (PFO–PSO) combined model. Results revealed that HISADP adsorption by QAMCM is a complex process because of the involvement of both chemisorption and physisorption instead of only one mechanism. However, chemisorption (electrostatic attraction and electron sharing) is the dominant kinetic mechanism. Analysis of the mass transfer mechanisms showed that the dynamic characters of the adsorption capacities of the liquid film around QAMCM and the pores of QAMCM could be well described using a film–pore mass transfer model, which could be used to provide insights into the adsorption mass transfer behaviors of the system. Additionally, HISADPs with smaller sizes were preferentially adsorbed by QAMCM compared with larger ones. Research on the isotherms and thermodynamics showed that HISADP adsorption by QAMCM occurred via an endothermic multi-layer process and was spontaneous. Overall, QAMCM was found to be an efficient, green, and recyclable decolorization adsorbent for beet juice.