Preparation And Adsorption Properties Of Tannic Acid-based Heavy Adsorbents

With the rapid development of global industry,heavy metal water pollution is becoming more and more serious.In particular,heavy metals such as Pb（Ⅱ）and Cr（Ⅵ）pose a serious threat to human health due to their high toxicity and difficulty in removal.Therefore,it is urgent to find efficient removal methods.Adsorption method is considered as one of the most potential methods to remove heavy metal pollution in the future because of its convenient operation,wide application range and good adsorption effect.In recent years,the preparation and adsorption properties of heavy metal adsorbents such as chelated resins,porous carbon,metal-organic frame materials and organic polymers have been deeply explored.Tannic acid,as a biomass material,has attracted wide attention in the adsorption field because of its green physical properties and the structural characteristics of polyphenol hydroxyl group.The results showed that the adsorbents developed with tannic acid as raw materials had universal chelation ability and good adsorption effect on Cd（Ⅱ）,Cu（Ⅱ）,Pb（Ⅱ）,Ni（Ⅱ）,Cr（Ⅵ）and other heavy metal ions.Based on the high designability of tannic acid,this paper designed and constructed a series of heterogeneous tannic acid heavy metal adsorbents from the molecular level,and studied their adsorption properties for Pb（Ⅱ）and Cr（Ⅵ）.The specific contents are as follows:（1）Melamine microspheres（MF）with uniform particle size（2μm）and melamine tannic acid polymer（MF-TA）were prepared by hydrothermal method.The composition and structure of the adsorbent were analyzed by FT-IR,SEM,XPS,TG and other characterization methods.It was proved that MF-TA has good stability and metal adsorption ability.The adsorption processes of Cr（Ⅵ）and Pb（Ⅱ）were studied by kinetic and isothermal adsorption models.The results showed that the saturated adsorption capacity Qe of Cr（Ⅵ）and Pb（Ⅱ）on MF was 398.5mg/g（C₀=350 mg/L,p H=2）and 225.8 mg/g（C₀=200 mg/L,p H=5）at 200 rpm and 303 K,respectively.The Qe of MF-TA on Pb（Ⅱ）and Cr（Ⅵ）reached 567.6 mg/g（C₀=350 mg/L,p H=2）and 195.6 mg/g（C₀=200 mg/L,p H=5）,respectively.The adsorption processes of MF and MF-TA conform to the quasi-second-order kinetics and Langmuir isothermal adsorption model.Through comparative experiments,it is found that with the increase of TA:MF ratio,the adsorption capacity of Cr（Ⅵ）increases gradually,while the adsorption capacity of Pb（Ⅱ）decreases slightly.By analyzing the adsorption principle,it is found that the phenolic hydroxyl group on TA surface are protonated under acidic conditions,which increases the electrostatic attraction and reduces Cr（Ⅵ）to Cr（III）through its own oxidation reducibility.However,tannic acid also consumes a certain amine adsorption site while loading,resulting in a slight decrease in the performance of Pb（Ⅱ）adsorption,which indicates that amine is more attractive to Pb（Ⅱ）.Through the above experiments,the adsorption characteristics of amine group and phenolic hydroxyl group on different metals were understood,which provided the direction and ideas for the study of tannic acid heavy metal adsorbent.（2）In order to further develop the adsorbent of tannic acid based heavy metals with high adsorption performance,polyethylenimide（PEI）was selected as the molecular ligand,and the tannic acid-polyethylenimide polymer（TA-PEI）was prepared by one-step hydrothermal method,and aluminum was selected as the metal source.A high specific surface area polyphenol metal network（MPN）was constructed for the adsorption of Cr（Ⅵ）by tannin-polyethylenimine-aluminum polymer（TPAl）.It was found that p H was the key factor in determining the specific surface area of TPAl.The maximum surface area of TPAl-2 was 220.6m²/g at p H 8.5.The results show that the Qe of TPAl-2 on Cr（Ⅵ）reaches 757.4 mg/g under the conditions of p H=2,temperature 303 K and initial concentration C₀ 400 mg/L.Under the conditions of p H=2,temperature 303 K and C₀ 300 mg/L,the removal rate of Cr（Ⅵ）in the solution was 68%for TA-PEI and 97%for TPAl-2 for 2 h,and the adsorption rate was increased by about 1.4 times.The composition and structure of the adsorbent were analyzed by SEM,XPS,EDS and other characterization methods,and it was found that Al element could be evenly distributed on the surface of TPAl-2,which was speculated to be due to the formation of Al-O and Al-N bonds,and the introduction of metal Al would not affect the performance of Cr（Ⅵ）adsorption.The kinetic model is used to study the adsorption process of TPAl-2,which fits the quasi-second-order kinetic model better.The isothermal adsorption line and thermodynamic study show that with the increase of temperature,the adsorption performance of TPAl-2 on Cr（Ⅵ）is also improved,and the adsorption process is continuously heat-absorbing and more consistent with the Langmuir model.The results of this chapter provide a way to construct tannic acid based adsorbents with high specific surface area.（3）In order to further improve the actual use efficiency of the adsorbent,inspired by multilayer functional materials and combined with the self-assembly ability between TA and PEI,a multilayer assembly method（LBL）was proposed to prepare PEI-TA multilayer decorative melamine massive adsorbent（PEI-TA）_n-MS.In this study,TA and PEI were alternately deposited on the MS frame by LBL assembly driven by the Michael addition and Schiff base reaction.The physicochemical structure of（PEI-TA）_n-MS was characterized and its adsorption properties for Pb（Ⅱ）were studied.The optimum adsorption conditions,kinetics,thermodynamics and isothermal adsorption processes were studied in detail.The results show that MS decorated with two layers of PEI-TA has higher adsorption capacity than MS decorated with one layer.The maximum removal rate of Pb（Ⅱ）by（PEI-TA）₂-MS（257.4 mg/g）was higher than that of most PEI-based adsorbents reported in the past,which reflected the rationality of the design strategy of this adsorbent.