| Heavy metals,such as Cu(Ⅱ)and Cr(Ⅵ),pose a serious worldwide threat not only to the public health but also to the whole food chain due to their non-biodegradability and high carcinogenicities.Adsorption is one of the most common and economical water treatment methods because of its advantages of simple operation,low cost,high efficiency,and can even be used to remove heavy metals with low concentration.The adsorptive removal of heavy metals has been widely studied,but the in-depth study on the adsorption mechanism is rare and seldom involves the interaction between metal and material surface.To deal with the issues mentioned above,the removal of heavy metal ions from water by transition metal oxygen/sulfide and their composites was studied in this paper,and the surface complex formation model(SCFM)coupled with adsorption kinetics,thermodynamics,and isotherms was used to explore the performance and mechanism of adsorbents for removing Cu(Ⅱ)and Cr(Ⅵ).Understanding the adsorption mechanism might provide a technical basis for the processual design of heavy metal adsorbents.The specific work as follows:(1)Nano-MoS2 was synthesized by one-step hydrothermal method.It exhibited excellent adsorption performance for Cu(Ⅱ)removal from water,and the maximum adsorption capacity(qmax)was 221.3 mg-g-1.The surface complex formation model was used to describe the mechanism of Cu(Ⅱ)adsorption on the surface of MoS2 adsorbent.It indicated that there involved physical interactions and chemical complexation in Cu(Ⅱ)removal by MoS2 from aqueous solution.Chemical complexation interaction was an important driving force to determine the removal efficiency of Cu(Ⅱ)from water.(2)MnO2/TA composites were prepared by a simple hydrothermal method and used to remove Cu(Ⅱ)from water.The qmax was 194.3 mg·g-1.Adsorption kinetics and isotherm investigations showed that pseudo-second-order kinetics and the langmuir isotherm model fit the experimental data well.There were two steps in the adsorption process,including film diffusion and pore diffusion,according to intra-particle diffusion kinetics.Further investigations of the activation energy and thermodynamics of the adsorption process provided insight into Cu(Ⅱ)adsorption on MnO2/TA,which is an endothermic and spontaneous physico-chemisorption process.Coupled with the study based on SCFM,it could be concluded that the physical electrostatic interaction played an important role in the initial stage of the adsorption reaction,and chemical complexation interaction was the dominant driving force to reach the completion of the adsorptive process.The decline of adsorption capacity was about 10%for MnO2/TA after being reused five times,and this indicates that MnO2/TA is a potential material for use in environmental treatment.(3)MoS2 and MnO2 were modified by tannic acid with Fe2+ as coupling agent to prepare MoS2/MnO2/TA composites for removing Cu(Ⅱ)from aqueous solution.The qmax was 175.5 mg·g-1.The investigations on adsorption isotherms and kinetics showed that Langmuir isotherm and pseudo-second-order kinetic model could well fit the experimental data.The adsorption rate was controlled by both film diffusion and pore diffusion.Further investigations of the thermodynamics of the adsorption process provided insight into Cu(Ⅱ)adsorption on MoS2/MnO2/TA,which is an endothermic and spontaneous physico-chemisorption process.Coupled with the study based on SCFM,we found that there are electrostatic,solvation,lateral interactions and complexation in the adsorption process,in which electrostatic interaction and complexation are the main reasons for adsorption,whereas solvation and lateral interactions are not conducive to adsorption.Physical action was dominant in the initial stage of the adsorption reaction,whereas chemical action is an important power to promote adsorption and reach equilibrium.The experimental data show that MoS2/MnO2/TA adsorbent can be reused.(4)MnO2 was modified by tannic acid and polyethylenimine to prepare MnO2/PEI/TA composites for removing Cu(Ⅱ)and Cr(Ⅵ)from aqueous solution.MnO2/PEI/TA composites exhibited a dendritic structure of spatial network,which provided more active sites to adsorb or react with metal ions.Due to the combination of TA and PEI,the oxygen/nitrogen-containing functional groups were abundant on the surface of composite,which greatly increased the enrichment of cationic and anionic metals by complexation.The investigation on adsorption kinetics and isotherms illustrated there existed multiple interactions in addition to physical force in the adsorption process of metal ions.We described the adsorption process of Cu(Ⅱ)and Cr(Ⅵ)adsorption on MnO2/PEI/TA by using the surface complexation formation model to elucidate the physical-chemical interaction in the adsorption mechanism.It showed that the adsorption free energies for both Cu(Ⅱ)and Cr(Ⅵ)were negative,which indicated that both Cu(Ⅱ)and Cr(Ⅵ)adsorption on MnO2/PEI/TA were spontaneous process.There involved electrostatic force,solvation,lateral interaction,and complexation in the adsorption process.The complexation was favorable for Cu(Ⅱ)and Cr(Ⅵ)adsorption,whereas solvation and lateral interactions were not conducive to adsorption.The effect of electrostatic action on adsorption is related to the solution pH.It should be noted that,Cr(Ⅵ)can not only be adsorbed on the surface of MnO2/PEI/TA,but also react with the reduction functional group of the surface and convert to Cr(Ⅲ). |
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