Competitive adsorption mechanisms of pigments in sugarcane juice on starch-based magnetic nanocomposites

The pigments in sugarcane result the crystallised sucrose appears unsatisfactorily yellow. In this study, cationic tapioca starch (CTS)-functionalized magnetic nanoparticles (CTS@Fe3O4) were synthesized and used as adsorbents for the removal of undesirable pigments. The adsorption properties of CTS@Fe3O4 were investigated by a sugarcane juice colorant model consisting of caffeic acid (CA), gallic acid (GA) and melanoidin (ME). The equilibrium adsorption capacities of CTS@Fe3O4 for CA, GA, and ME were 185, 160 and 580 mg g(-1) at the optimal conditions (60, 60 and 180 mg L-1 initial concentrations, respectively; 0.3 mg mL-1 CTS@Fe3O4 dosage, 313 K temperature, and pH value of 7). The adsorption process was described well by second-order kinetic and Langmuir isotherm models with a high fitting correlation coefficient approaching 1, suggesting that the pigments formed a surface monolayer with a homogenously distributed adsorption energy and was mainly dominated by chemisorption. The thermodynamic parameters (Gibbs free energy <0, enthalpy >0, and entropy >0) revealed that the adsorption process was endothermic and spontaneous. For the binary system, the competitive adsorption between pigments was primarily antagonistic. The speed of adsorption was the main factor affecting competitive adsorption, and the additional adsorption force reduced the effects of coexisting adsorbates.