| Lysozyme, which is non-toxic and has the function of eliminating inflammation and improving immunity, has a variety of applications in pharmaceutical and food industries. Therefore, separation and purification of lysozyme is of vital importance. Comparing to the traditional method, magnetic adsorption with selectivity and specificity is an effective method for lysozyme separation, and magnetic adsorbents can be separated easily by magnetic field gradient. However, pure magnetic Fe3O4 nanoparticles trend to aggregate in liquid and the biocompatibility is poor. Magnetic composite nanoparticles, which can overcome the shortcoming of pure nanoparticles, have attracted wide interest of researchers domestic or abroad.In this paper, magnetic Fe3O4 nanoparticles were synthesized by co-precipitation, and magnetic Fe3O4/chitosan nanoparticles were obtained by adding chitosan. The characteristics of Fe3O4/chitosan were studied by XRD, TEM, TGA, FTIR and determination of pH at the point of zero charge. Batch experiments were carried to study the effects of initial pH, lysozyme concentration, temperature, contact time and ionic strenth on lysozyme adsorption by Fe3O4/chitosan. The effect of NaCl concentration on desorption ratio were investigated, and the reusability of Fe3O4/chitosan were also studied. The kinetics and thermodynamics of lysozyme adsorption were investigated under different conditions. The pseudo-first-order and pseudo-second-order models were applied to describe the experimental kinetics. Langmuir, Freudlich and Sips isotherms were used to describe the experimental thermodynamics. In addition, the conformational change of lysozyme during the adsorption and desorption process were monitored by FTIR, UV-Vis, fluorescence and CD spectra.The TEM and XRD results indicated that Fe3O4/chitosan were nanosized and kept the spinel structure. TGA and FTIR confirmed that chitosan was bound to Fe3O4 successfully. The batch experiment results suggested that the adsorption capacity increased at the beginning and then decreased with the increase of initial pH. The adsorption capacity increased with increase in initial lysozyme concentration and temperature, and it reached 110.79 mg/g at 310 K. The equilibrium time was 20 h and it was independent on initial concentration. The adsorption of lysozyme showed a highly ionic strength dependent profile. With the increase of ionic strength, the adsorption capacity increased first and then decreased, which reached the maximum (143.12 mg/g) at 1.0 mol/L NaCl concentration. The desorption experiment showed that the desorption ratio was 97.0% when the concentration of NaCl was 0.5 mol/L. The adsorption-desorption cycle experiment results showed that the adsorption capacity decreased with the increase of cycle time, which was related to the decrease of desorption ratio. The adsorption process followed the pseudo-second-order model, and the initial adsorption rate increased with increase in initial lysozyme concentration. The equilibrium process could be well described by Sips model. The thermodynamic parameters,ΔG0,ΔH0 andΔS0, illustrated that the adsorption process was spontaneous, endothermic and entropy increase. The spectra results indicated that original structure of lysozyme was not damaged during the adsorption and desorption process.
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