Research On The Structure Control And Performance Of A Series Of Chitosan-based Adsorbents

Large amounts of resources and energy are to be consumed during the process of synthesis,processing,application and post-treatment of polymer materials,which further causes pollution to the environment.Green polymer materials are low toxic or non-toxic to human health and the environment,and they can fully degrade in the environment.For example,chitosan is a kind of polymer materials with amino groups and carboxyl groups,which are fully biodegradable.It also has a great application in water treatment.Since the practical application of chitosan itself is limited due to several weak points,modified chitosan-based adsorbents with improved performance can be prepared by combining different modification components with chitosan.On the one hand,the performance of each component can be improved by the synergist effects among different components.On the other hand,it can also avoid the problems of nanoparticles aggregation and difficulty in recycling.Consequently,a series of modified chitosan-based composite adsorbents were prepared;their structures and morphologies were investigated;the adsorption-desorption mechanism was explored through batch adsorption tests,column adsorption experiments,and instrumental characterization before and after adsorption.p H and salt ion-resistence properties and reusability of the adsorbents were investigated.The main conclusions are as follows:?1?The chitosan-based magnetic composite adsorbent?CS-MCP?were prepared by combining chitosan with Fe₃O₄ particles composite,and then grafting polyacrylic-derivatives branches with different surface charges onto chitosan backbones,including electropositivepoly(2-methyl acryloyl oxygen ethyl trimethyl ammonium chloride?PDMC?,electronegative polyacrylic acid?PAA?and electroneutral poly(methyl methacrylate?PMMA?.The modified adsorbents were labeled as CD-MCP,CA-MCP and CM–MCP,respectively.Adsorption performance evaluation,including adsorption capacities from single-and binarysolute solutions at different p Hs,influences of coexisting salts,desorption and reusability,demonstrated CD-MCP with polycationic brushes was a cost-effective adsorbent for DCF removal:Compared to CS-MCP,all the modified MCPs exhibited higher removal efficiencies,due to the enhanced surface areas resulting from core-brush topology;Among the modified MCPs with different surface charges,CD-MCP displayed the highest adsorption capacities,attributed to the electrostatic attraction between the positively charged brushes and the anionic contaminant's species.Adsorption mechanism was investigated from both the macroscopic?thermodynamics and kinetics?and microscopic?changes of solution p H and spectral analyses of the adsorbent after adsorption?viewpoints:Charge attraction was the intrinsic driving force;A monolayer coverage of pharmaceuticals were adsorbed onto the brushes of CD-MCP;OH^-exerted competitive adsorption effect;After adsorption,the originally extended polycationic brushes turned collapsed.?2?To further enhance adsorption capacity?Qe?and p H-resistance,novel representative core-brushes shaped aromatic rings-functionalized CS-MCPs were used for adsorption of typical pharmaceuticals?norfloxacin,tylosin and diclofenac sodium?DCF??.For norfloxacin and tylosin,modified CS-MCP with polyanions?poly?sodium p-styrenesulfonate??branches exhibited high Qe?165 mg/g and 134 mg/g,respectively?;whereas for DCF,modified CS-MCP with polycations?poly?p-vinylbenzyl trimethylammonium chloride??branches is favored?151 mg/g?.Due to existence of phenyl groups on branches,Qe remained high levels in a wide p H range,proving improved p H-resistance.Considerable resistance against coexisting ions,as well as reusability,of the adsorbents,were found.Physico-chemical analyses,instrumental characterizations and theoretical calculations demonstrated that electrostatic attraction and p-electron-containing interactions contributed to adsorption:charged ends of contaminants were fixed to oppositely charged part of branches;whereas other fragments of pharmaceutical molecules bound with p-electrons of phenyls on branches.The novel core-brush shaped biopolymer-based MCPs showed potential in water treatment and guidance for designing the brush characteristics was provided according to pharmaceutical characteristics.?3?Hydrogel adsorbents were prepared by the combination of metal hydroxide?La,Zr and Fe?with chitosan,and the polymer containing quaternary ammonium functional groups was introduced into the chitosan-based hydrogel composite adsorbent by graft copolymerization.Thus,a series of metal hydroxide loaded quaternary ammonium modified chitosan composite adsorbent was prepared.According to the different types of metal hydroxides loaded on the adsorbent,three kinds of metal hydroxides loaded quaternary ammonium modified chitosan composite adsorbent materials were obtained?La?OH?₃:CS-La-N;Zr?OH?₂:CS-Zr-N;Fe?OH?₃:CS-Fe-N?.Performance of three kinds of adsorbents was investigated.Among them,the adsorbent CS-La-N-20%?20%:dosage of quaternary ammonium monomer?showed a good adsorption effect?up to160mg/g?and maintained good adsorption performance in the range of p H 3-11.The main forces of adsorption are ligand exchange and electrostatic.The experiments of static and dynamic desorption cycles show that the adsorbent CS-La-N-20%can be reused for many times and the adsorption property is basically unchanged.?4?Metal organic frameworks?MOFs?/chitosan composite beads were prepared by combining different MOFs?with different metal centers?zirconium,iron and aluminum??with chitosan.According to the different MOF types,three kinds of chitosan composite adsorbents were obtained?named CS-Ui O-66;CS-MIL-101?Al?;CS-MIL-101?Fe??.The phosphate adsorption capacities were investigated.The adsorbent CS-Ui O-66 showed a good adsorption performance for phosphate,and its maximum adsorption capacity was up to 52 mg/g.The main interaction in the adsorption process was electrostatic attraction,and the Zr sites on Ui O-66 formed Zr-O bonds with phosphate to achieve the improved removal efficiency.In summary,chitosan-based composite adsorbents with appropriate chemical modification showed good adsorption effect on a variety of pollutants in different waters.These adsorbent materials have good application potentials in the treatment of real water.