| Lignite is rich in organic matter,the main part of which is humic acid.Humic acid with more functional groups,can effectively remove organic dye molecules and heavy metal ions in the solution,as a kind of efficient green adsorbent.However,in the process of adsorption,part of humic acid will be dissolved in the solution,making the loss of adsorbent and thereby reducing the effect of sewage treatment.The method,that using chitosan to modify humic acid,achieves the purpose of improving the stability and adsorption performance of the adsorbent,which has a certain research significance for the clean and environmental use of lignite,but also can provide theoretical support for the development and utilization of printing and dyeing wastewater adsorption materials.In this study,the commercial humic acid was modified by chitosan,and the humic acid/chitosan compound adsorbent was successfully prepared for removing the organic dye methylene blue(MB)from aqueous solution.The influence of synthesis conditions on the adsorption performance of the adsorbent was investigated.The effects of time,temperature and p H value on the adsorption performance also were investigated.The isothermal adsorption model and adsorption kinetics model were analyzed,and the adsorption mechanism was deduced.Humic acid was extracted from lignite in different regions by means of alkali-soluble acid separation.The lignite-based humic acid/chitosan compound adsorbent was prepared and its adsorption capacity for methylene blue was investigated.The physical and chemical properties of the composites were characterized by X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FT-IR),specific surface area and aperture analyzer(BET)and thermogravimetric analyzer(TG).The results show that the immobilized modification of humic acid by chitosan can effectively reduce the water solubility of humic acid and significantly improve its capability to adsorb methylene blue,and this preparation method can be widely applied the lignite humic acid to prepare the composite adsorbent.The carboxyl functional groups in the humic acid/chitosan adsorbent may play a key role in the adsorption of methylene blue.Methylene blue is a cationic dye,so lower p H value in the adsorption environment is not conducive to the adsorption of methylene blue by the compound adsorbent.The composite adsorbent has good stability and can be effectively desorbed by HCl solution after adsorbing methylene blue.After 5 cycles of adsorption-desorption,the composite adsorbent can still show good adsorption effect.Combined with the analysis of characterization,kinetics and thermodynamics,it is considered that the adsorption of methylene blue on the composite adsorbent is a spontaneous monolayer adsorption process,which conforms to the Langmuir isothermal adsorption model,and the adsorption follows the pseudo-second-order kinetics model,and there is a chemical adsorption process.Interactions between adsorbent and methylene blue may include ion exchange,hydrogen bonding and π-π bonding.In conclusion,the humic acid/chitosan adsorbent has good adsorption performance for methylene blue,and can be separated from the solution after adsorption,without causing secondary pollution.The adsorbent is green and biodegradable,which avoids the pollution caused to the environment after the scrap of the adsorbent.The study on the preparation of green and environment-friendly adsorbent from humic acid extracted from lignite for printing and dyeing wastewater provides a feasible idea for the clean functional utilization of lignite. |
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