| The molecule of chitosan contains high contents of amino groups, which can be protonated by integrating H+ in solution. So it has favorable adsorbability to some inorganic acid, organic acid, acor compound. Chitosan can be dissolved in acid solution. It limits its application as absorbent.The references about the chitosan structure characteristics, properties and modification research actualities and its practical application had been investigated sufficiently. On the basis of this, using glutaradehyde as the cross-linker, magnetic chitosan microspheres (MCTS) were prepared by the suspension cross-linking technique.The experiments had been carried out to study the adsorption behavior of MCTS for three kinds of phytohormone. In the medium of isopropanol, using perchloric acid as catalyst, hydroxypropyl chloride magnetic chitosan microspheres (ECH-MCTS) were prepared. Furthermore, aminated magnetic chitosan microspheres (EN-MCTS) were prepared by grafting ethylenediamine on the microsphere surface. The adsorption efficiency of aminated magnetic chitosan microspheres were investigated in removal of anoinic dyes acid rhodamine B(AR),fuchsin acid (FA) and reactive brilliant blue X-BR (RBB) compared with MCTS and ECH-MCTS. The adsorption thermodynamics, kinetics and mechanism were discussed. Chitosan powder had been prepared to be magnetic resin. To the contribution of magnetic field, the chitosan and the resin can be easily isolated. This expanded the application of chitosan consumedly. The major study works and results were as follows:1. The preparation and characterization of magnetic chitosan microspheres.The effects of the dosage of crosslinker glutaraldehyde on microspheres morphology and yield had been observed intensively. The modality observed by optical microscope show that, the yield of microsphere increased, but the aggregation of the microspheres became more serious as the quantity of crosslinker increasing and the adsorption values to abscisic acid decreased correspondingly. The reason was that the amino groups on the surface of the microspheres had been occupied by overmany crosslinker. Therefore, when the crosslinker concentration was 7% and the qantity of the chitosan was 0.5 g, the magnetic microspheres with good dispersion property and uniformity the particle size diffusion were obtained.Scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to investigate the morphology and size of the microspheres. The results indicated that the microspheres had well shaped spherical form with smooth surface and the particle size was 3~5μm. The fourier transfor IR (FT-IR) spectrameter , X-ray diffraction (XRD) , differential scanning calorimetry (DSC) and thermogravinetry (TG) were used to analyze the molecular structure and the alteration of the microspheres'thermal character. The results showed that Fe3O4 nanoparticles remained its spinel structure, which packed in the microspheres to form core-shell structure, and the weight percentage of Fe3O4 nanoparticles was estimated to be about 10.9 wt%. The thermal stability of MCTS was lower than chitosan, but was as high as chitosan before 225℃, and also can be easily separated from aqueous solution under the magnetic field.2. The studies of adsorption behaviour and mechanism for acidic phytohormone onto magnetic chitosan mocrospheresMCTS was utilized to adsorb abscisic acid (ABA), gibberellin (GA3) ,3-indoleacetic acid (IAA) by static adsorption. The effects of adsorbent dosage, contact time, pH and the original concentration of ABA, GA3, IAA on the adsorption rate were investigated and optimized. In order to explore the adsorption mechanism of the modified microspheres, the experimental data were fitting by dynamics model including pseudo-second-order equation and pseudo-first-order equation, and isotherms Langmuir and Freundlich isotherms. The results showed that the adsorbent dosage was appropriated at 1.25 g L-1. In 20 minutes ,the adsorption rate to ABA(c0 = 32.28mg L-1),IAA(c0 = 43.80mg L-1),GA3(c0 = 200.0mg L-1)raised up to maximum, and the one step adsorption rate was 89%,93% and 81% respectively. Furthermore, the adsorption efficiencies were greatly influenced by the acidity and ionic strength of solution. A possible explanation for pH effect on adsorption may be related to the surface charge of magnetic microspheres and phytohormone. Amino groups on the surface of microspheres played a major role in adsorption proces. It could be protoned in low pH, combining with acid radical ion dissociated by ABA, IAA or GA3 to complete the adsorption process. So when the pH is too low, amino group couldn't be protonated easily, but the high acidity impeded the dissociation of ABA, IAA and GA3. And aslo as the increase of ionic strength, high concentrations of NaCl ions could cover the particle surface and form an ion shield, which can decrease the diffusivity of phytohormone and enlarge the absorbed molecules, and therefore reduce the adsorption rate.The results for the experimental data applying pseudo-first-order equation and pseudo-second-order equation showed that the adsorption for phytohormone conformed to the pseudo-second-order equation. It confirmed that the adsorption rate was controlled by chemical sorption. The isotherms model of ABA and GA3 of the modified microspheres indicated that Langmuir isotherm equation gave better fit than the Freundlich isotherm. It confirmed that the adsorption on microspheres was in a monolayer form. The regeneration of the adsorbents could be implemented by 0.1 mol L-1 sodium chloride , only the 5% decrease of adsorption rate could be obtained after regenerating absorbents for five times. These results indicated that the magnetic chitosan microspheres can be recycled.3. The preparation and characterization of modified magnetic chitosan microspheres.The adsorption efficiency of chitosan closely related to its alkalinity. Cross-linking reaction weakened the alkalinity of absorbent and reduced the adsorption efficiency of absorbent. Thus, two kinds of modified microspheres were prepared in order to increase the adsorption efficiency.①Preparation of hydroxypropyl chloride magnetic chitosan microspheres (ECH-MCTS) by using epichlorohydrin modified magnetic chitosan microspheres②Preparation of a new adsobent aminated magnetic chitosan microspheres (EN-MCTS) by using ethylenediamine grafted on the surface of microspheres.The content of amino groups on microspheres were determined by titrimetric method. IR , XRD ,DSC and TG were used to analyze the alteration of ECH-MCTS and EN-MCTS'molecular structure and characte. The results showed that the amino group content of ECH-MCTS decreased obviously. It indicated that both amino groups and hydroxy groups participated in the reaction with ethylenediamine. However, the amino group content of EN-MCTS was twice than that of MCTS. So the alkalinity of EN-MCTS had been strengthened and the adsorbability had been enhanced .4. The studies on adsorption behaviour and mechanism for anionic dye onto MCTS, ECH-MCTS and EN-MCTSThe adsorption behaviour for the three anionic dyes acid rhodamine B, fuchsin acid and reactive brilliant blue X-BR were studied, the experimental factors contact time, pH and temperature had also been explored and optimized. The results showed that within 60 minutes, the adsorption rate for AR and FA on EN-MCTS can reach to 95%, when the original concentrations of AR were chosen at 0.50 mmol L-1, FA were at 0.80 mmol L-1, and the adsorbent dosage was at 0.5 g L-1 and 0.8 g L-1 respectively. However, within 4 hours, the adsorption rate for RBB can reach to 98% on EN-MCTS when the original concentrations were chosen at 0.30 mmol L-1 and the adsorbent dosage was at 0.8 g L-1. The adsorption capability could be enhanced as the concentration of dyes increasing, while the adsorption rate decreased as the acidity increasing on the three magnetic microspheres. But the adsorption on MCTS was affected greatly by the solution acidity compared with ECH-MCTS and EN-MCTS. The results indicated that the adsorption for anionic dyes were affected by the structure of dyes and the steric hindrance of amino groups on microspheres .The results for the experimental data applying dynamics equations and isotherm equations showed that the adsorption for anionic dyes conformed to the pseudo-second-order equation and Langmuir isotherm equation. It confirmed that the adsorption process was in monolayer controlled by chemical sorption. The regeneration of the adsorbents could be implemented by 0.5 mol L-1 NaOH + 2 mol L-1NaCl (V/V=1:1) , the second-regenerated ratios were all > 95%. |
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