Preparation Of Chitin/polyethylenimine Biosorbents And Their Adsorption Behaviors For Cr(?) And U(?) As Well As The Adsorption Mechanisms

With the rapid development of industry,the problem of heavy metal pollution in water is becoming more and more serious.Adsorption is an important method for removing heavy metal pollution from water bodies,which has the advantages of low cost,high efficiency and simple operation.There are many types of adsorbent materials,among which,natural polymer materials have many advantages,such as a wide range of sources,a rich variety of functional groups,environmental friendliness,and renewability.They have great potential for application in the field of adsorption.Chitin(CH)is a biological macromolecule abundant in nature.Due to the strong hydrogen bondings in its molecular structure,chitin molecules are difficult to be dissolved and utilized.Therefore,the preparation of chitin-based adsorbents and their performances are both greatly limited.In order to expand the application of chitin in the field of heavy metal adsorption,this research used the sodium hydroxide/urea aqueous solution developed in recent years as the green solvent to dissolve chitin powder by the freeze-thawing method and then polyethylenimine(PEI),which is rich in amine groups,was introduced by the cross-linking reaction to fabricate the chitin/polyethylenimine composite adsorbents.Fourier transform infrared spectra(FT-IR),scanning electron microscopy(SEM),nitrogen adsorption and desorption experiments as well as thermogravimetric analysis-differential scanning calorimetry were used to explore the composition and structural characteristics of the composite materials,after which the composite sorbents'adsorption behaviors and the adsorption mechanisms were explored.In this study,two kinds of chitin/polyethylenimine composites were prepared using epichlorohydrin and ethylene glycol diglycidyl ether as the crosslinkers for the adsorption of Cr(?)and U(?),respectively,and the main results of the study are as follows:(1)The optimal mass ratio of polyethylenimine to chitin was 1.0 when the composite(CH-PEI(ECH))was prepared by a low-temperature dissolution-crosslinking-freeze-drying process using epichlorohydrin as the crosslinker.The pore structure of this material(CH-1.0P(ECH))is predominantly macro-and mesoporous.The experimental results of the adsorption of Cr(?)in water by CH-1.0P(ECH)showed that the adsorption performance of the material was best at the solution p H=4.The adsorption process was well fitted by the pseudo-second-order kinetic model and the Langmuir model.The maximum adsorption capacity was calculated to be487.56 mg/g.In addition,the interference effects of other anions on Cr(?)adsorption are related to the amount of negative charge and ion radius of them,and the interference effects of SO₄^2-,NO₃^-and Cl^-decreased sequentially.The XPS results of CH-1.0P(ECH)before and after the adsorption indicate that a large number of protonated amines on the material adsorb Cr(?)anions in solution by electrostatic attraction,while some of Cr(?)anions can be reduced to Cr(III)ions by the secondary amines and hydroxyl groups,which is less toxic.In this study,the use of polyethylenimine resulted in a significant increase in the number of adsorption sites and therefore the adsorption capacity of the sorbent,while reductive groups such as hydroxyl groups on the chitin structure were able to reduce some of the Cr(?)anions to Cr(III)ions.(2)The optimum mass ratio of polyethylenimine to chitin was 1.5 when the film-like CH-PEI(EGDE)composite was prepared by the casting-crosslinking-freeze-drying process,and the macroscopic morphology and microstructure of the material(CH-1.5P(EGDE))remained intact during the adsorption-desorption processes,indicating that it has a relatively stable structure and is easy for solid-liquid separation.The adsorption experiments of CH-1.5P(EGDE)for U(?)removal from bicarbonate solution showed that the adsorption performance of the adsorbent was best at the solution p H=6,and the time to reach the adsorption equilibrium was 2,3 and 4 h for 2,10 and 50 mg/L uranium solutions,respectively.The maximum adsorption capacity of the adsorbent could reach 1023.74 mg/g at 25?.The results of desorption experiments showed that 0.5 mol/L nitric acid aqueous solution could reach a desorption rate of 100%within 9 min.Besides,the adsorption and desorption efficiency of the adsorbent decreased by less than 5%during the five cycles,indicating that the sorbent possesses good reusability.Based on the analysis of the FT-IR and XPS characterization results of CH-1.5P(EGDE)before and after the adsorption,it can be inferred that the nitrogen-containing groups of the composite adsorb the anionic uranyl-carbonate complexes in solution via anion exchange and electrostatic attraction,and they play a major role in the adsorption process.The above results indicate that the film-like CH-1.5P(EGDE)is an environmentally-friendly adsorbent,which has a stable structure,is easy to separate,has good reusability and can remove uranyl-carbonate complexes efficiently.In summary,this work has conducted a series of studies on the development of chitin/polyethylenimine composites and the adsorption of heavy metals from water bodies.The preparation of the two kinds of chitin/polyethylenimine composite adsorbents and their outstanding performances in Cr(?)and U(?)adsorption fields respectively provide the possibility for the extensive application of chitin in the field of heavy metal adsorption,and also provide important experience for the development of chitin composites with high adsorption capacity,easy separation and good recycling performances.