| With the rapid development of industry,heavy metal pollution of the water body has become an increasingly serious problem,which affects the ecological environment as well as the human health.Lead,as a heavy metal extensively used in industrial production,is one of the most hazardous elements to human beings.Pectin contains a large amount of-COOH,which is considered to be the most promising biological adsorbent for removing Pb2+ from wastewater.Pectin compounds are effective substances for the collection of metal ions in high metal concentrations,however,it can't achieve the desired removal rate in low Pb2+ concentration wastewater.In addition,pectin is soluble in water,difficult to be separated and recycled,drawbacks such as low mechanical strength limit its application.Based on the two main defects of pectin,this paper modified the pectin molecule with ethylenediamine by amidation,so as to improve the adsorption capacity of pectin in low Pb2+ concentration wastewater.Then,the modified pectin of strong adsorption capacity combined with sodium alginate of strong mechanical properties were compUunded,using Ca2+ as the cross-linking agent and embed with Fe3O4 to prepare a new modified pectin-based magnetic microsphere.The main results of this paper are as follows:1.Ethylenediamine-modified pectins with different degrees of amidation(DA were 31.62%,38.94%,43.13%and 48.31%,respectively,named as EP32,EP39,EP43 and EP48)were prepared and characterized by Fourier transform infrared spectroscopy(FTIR),elemental analysis and X-ray photoelectron spectroscopy(XPS).These results indicated that ethylenediamine was grafted on pectin through amide bonds.The rheological analysis showed that the modification of ethylenediamine increased the viscosity of pectin.The Zeta potential analysis showed that the modification of ethylenediamine increased the alkalinity of pectin.The adsorption properties of Pb2+ in aqueous solution by modified pectins were investigated.The modified pectins obtained optimum adsorption performance when pH was 5.0.The adsorption rate of Pb2+ by all the modified pectins were very high,and the adsorption equilibrium was reached after 1 h.It was found EP48 exhibited great removal efficiency of Pb2+at low Pb2+concentrations of 40-80 mg/L,and the removal rate was much higher than that of original pectin.The zeta potential analysis showed that EP48 had the fastest increase in zeta potential when Pb2+was continuously added and was the first to be electroneutralized.Particle size analysis further confirmed that EP48 was the first precipitated and formed a larger EP48-Pb2+complex.The FTIR and XPS analyses indicated that the mechanism of Pb2+ adsorption by original pectin is the ion exchange of carboxylic acid groups,while the adsorption of Pb2+by modified pectin involves not only the ion exchange of carboxylic groups but also the chelation with acylamino and amino groups.2.A novel modified pectin-based magnetic microsphere adsorbent was prepared by combining the EP48 with sodium alginate and embedding the Fe3O4 with Ca2+as the cross-linking agent.The samples were characterized by FTIR,texture profile analysis(TPA)and thermogravimetric analysis(TGA).The FTIR analysis showed that amide groups introduced by ethylenediamine modification still existed after cross-linking reaction.The result of TPA and TGA showed that the addition of alginate and Fe3O4 nanoparticles improved the mechanical strength and thermal stability of the microspheres.The adsorption properties of Pb2+by modified pectin-based magnetic microspheres were studied under different adsorbent dose,pH value,initial Pb2+concentration and contact time.The results showed that it required 8 h for the modified pectin-based magnetic microspheres to reach the adsorption equilibrium when pH was 5.0 and the adsorbent does was 1 g/L.Kinetics of Pb2+ adsorption process was better described by the pseudo-second order model,indicating that the adsorption is controlled by chemical adsorption.Moreover,according to the parameters of the diffusion equation,the intraparticle diffusion in the second stage is the decisive step of the absorption rate.Langmuir model can better fit the experimental results,and the maximum adsorption capacity is 175.19 mg/g.The desorption and regeneration experiment showed that the removal rate of modified pectin-based magnetic microspheres maintained over 98.9%after 10 desorption cycles.3.Modified pectin-based magnetic microsphere adsorbents of different sizes(750?m,540 ?m,400?m and 100 ?m)were prepared by microcapsule encapsulator.The adsorption properties of Pb2+ by four sizes of modified pectin-based magnetic microspheres were studied under different adsorbent dose,pH value,initial Pb2+concentration and contact time.The results showed that the time to reach adsorption equilibrium for 750 ?m,540 ?m,400 ?m and 100 ?m of the modified pectin-based magnetic microspheres was 8 h,6 h,4 h and 1 h,respectively,when pH was 5.0 and the adsorbent does was 1 g/L.The time required for adsorption equilibrium was greatly decreased by the reduction of microsphere size The Langmuir model fitted the experimental results well and the maximum adsorption capacities of the modified pectin-based magnetic microspheres at 750 ?m,540 ?m,400 ?m and 100 ?m are 175.43 mg/g,185.19 mg/g,232.56 mg/g and 256.41 mg/g,respectively.Desorption and regeneration experiments showed that after the 9th desorption,the 100 ?m modified pectin-based magnetic microspheres had been partially swelling and broken,while the other three sizes of modified pectin-based magnetic microspheres kept their structure and morphology,and the removal rate of Pb2+ remained above 99.2%.In this study,a modified pectin-based magnetic microsphere adsorbent with high adsorption performance was prepared,which solved the problem that pectin adsorbent could not be reused.This study provided a feasible way for the prevention and treatment of heavy metal pollution in water. |
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