Study On The Preparation Of Functionalized MXene Nanocomposites And Their Adsorption Properties

Organic dyes and heavy metal ions in wastewater are the main polluting components,which are extremely active,stable,diffusive and carcinogenic,with significant hazards to human health and ecological environment.Compared with traditional methods such as chemical precipitation and ion exchange,adsorption has the advantages of simple operation,low cost and high removal efficiency,which is widely used in the wastewater treatment area.The key of adsorption method is the selection of adsorbent.The composite adsorbents composed of organic and inorganic adsorbents combined has a large specific surface area and abundant active functional groups,thus presenting a better adsorption performance.However,the adsorption performance of currently developed composite adsorbents is still low and cannot satisfy the actual demand.Moreover,these composite adsorbents usually can only remove one kind of pollutant and cannot adsorb organic dyes and heavy metal ions at the same time,which also limits their practical application.To solve the problem of poor adsorption capacity of composite adsorbents and the fact that they can only remove one kind of pollutants,In this thesis,MXene was selected as the adsorbent.Polypyrrole（PPy）,polydopamine（PDA）and polyethyleneimine（PEI）were used as modifiers to prepare MXene nanocomposites with excellent adsorption properties for methylene blue（MB）,methyl orange（MO）and hexavalent chromium（Cr（VI））were prepared.The main results are as follows.（1）After etching titanium aluminum carbide（Ti₃AlC₂）with lithium fluoride（LiF）and hydrochloric acid（HCl）,few-layered MXene（f-MXene）nanosheets were prepared by ultrasonication,hand shaking and centrifugation.Then,PPy nanoparticles were loaded on the surface of f-MXene nanosheets by in situ polymerization to prepare MXene/PPy composite particles.Adsorption test results show that MXene/PPy composite particles exhibit a good adsorption capacity for MB,and the adsorption behavior is consistent with the Langmuir isothermal adsorption model and the pseudo-second-order kinetic model,with a maximum theoretical adsorption capacity of 553.6 mg/g,and the adsorption process is spontaneous and heat absorbing.Zeta potential shows a negative charge on the surface of MXene/PPy composite particles in a solution p H of 1-6,making the adsorption of MB by MXene/PPy composite particles through electrostatic forces,hydrogen bonding andπ-πinteractions.In addition,comparing the adsorption amount for each sample before and after two months of storage,it is found that the adsorption amount of f-MXene nanosheets decreases significantly after two months of storage under air and room temperature conditions,while the adsorption capacity of MXene/PPy composite particles increases slightly instead,indicating that MXene/PPy composite particles have better stability.The mechanism of improving the stability of MXene is that PPy nanoparticles are loaded on the surface of MXene,forming a protective film,which prevents the contact between MXene and air and inhibits the oxidation of MXene,thus improving the stability of MXene.（2）To address the problem that the negative charge on the surface of MXene is not favorable for the adsorption of Cr（VI）,the MXene/PDA（MPDA）composite particles were prepared by coating PDA nanoparticles on f-MXene nanosheets using dopamine（DA）self-polymerization.The adsorption test results show that the greater PDA coating degree,the better the adsorption performance of MPDA composite particles on Cr（VI）.The best adsorption performance of Cr（VI）is MPDA-10 composite particles,and its adsorption behavior is in accordance with Langmuir isothermal adsorption model and pseudo-second-order kinetic model,and the adsorption process is spontaneous and heat absorption,with the maximum theoretical adsorption amount of 862.32 mg/g.The adsorption mechanism is attributed to the fact that the oxygen-containing functional group of MXene and the amino and phenolic hydroxyl groups on the PDA chain are protonated in acidic solution,which makes the MPDA composite particles positively charged and thus adsorbs Cr（VI）by electrostatic forces.Besides,the functional groups of MPDA composite particles are able to provide electrons and protons to reduce Cr（VI）to trivalent chromium（Cr（III））.The study shows that PDA nanoparticles are also able to improve the stability of MXene and their stability improvement mechanism is the same as that of MXene/PPy composite particles.（3）Wastewater usually contains both organic dyes and heavy metal ions,so MXene materials that can remove only one of these pollutants cannot meet the practical needs.To address these problems,a series of MXene/PEI（MPEI）composite particles were prepared by grafting PEI of different relative molecular masses to f-MXene nanosheets using glutaraldehyde（GA）as a cross-linking agent.The adsorption test results demonstrate that MPEI composite particles have excellent adsorption capacity for both MO and Cr（VI）.The best adsorption performance for MO is MPEI600 composite particles,and the best adsorption capacity for Cr（VI）is MPEI70000 composite particles.The adsorption behaviors of both are in accordance with Langmuir isothermal adsorption model and pseudo-second-order kinetic model,and the calculated maximum adsorption amounts are 910.4 mg/g and 870.5 mg/g,respectively,with the adsorption process being spontaneous heat absorption.The adsorption mechanism is that the oxygen-containing functional group of MXene and the amino group of PEI are protonated in acidic solution to make the MPEI composite particles positively charged,thus adsorbing MO and Cr（VI）by electrostatic forces,and the functional group of MPEI composite particles can also adsorb MO by forming hydrogen bonds with the azo structure of MO.Further study indicates that Cr（VI）is reduced to Cr（III）under the action of functional groups of MPEI composite particles,and the reduction rate of MPEI 70000 composite particles is 52.2%.The stability test find that PEI can improve the stability of MXene to some extent,and the stability improvement mechanism is the same as MXene/PPy and MPDA composite particles.Through the research of this thesis,MXene composites with high efficiency removal of MB,MO and Cr（VI）and good stability were successfully prepared,which is significant for the functionalized modification of MXene and the expansion of MXene applications in wastewater treatment.