Study On Removal Of Cadmium Ions By Polyethyleneimine Modified Adsorbent

The rapid development of industry has made heavy metal wastewater pollution a worldwide environmental problem.Among them,Cd(?)is a highly toxic heavy metal ion,which can't be decomposed,so that they accumulate in the environment and enter the human body through the enrichment of the food chain,which seriously endangers people's health.Commonly used methods for removing heavy metal ions include chemical precipitation,ion exchange,membrane filtration,and adsorption et.al.Among these methods,the adsorption method is widely used because of its low cost,simple operation,and high removal efficiency,and the key to the adsorption method is to select a suitable adsorbent.Polyethylenimine(PEI)is a typical polyamine with high adsorption capacity and adsorption selectivity for metal ions.The amine group has good reactivity,and is easy to modify the function of the common adsorbent,which and can greatly increase the adsorption capacity of the metal ion.Therefore,this paper separately applies PEI modified activated carbon and PEI modified magnetic starch microspheres.The preparation and adsorption of cadmium ions have been studied.The main research contents are as follows:(1)The activated carbon was modified by insoluble linear polyethyleneimine(PEI),and an amine functional group having complexation with Cd(?)was introduced;and then used to adsorb and remove Cd(?)in the wastewater.The BET analysis showed that the PEI was successfully loaded on the AC surface,and the specific surface area of the material decreased from 305.8 m2·g-1 to 113.3 m2·g-1.The positively charged amine functional group on the PEI molecule changes the Zeta potential of the AC surface from-38.3 to 11.1 mV.XPS analysis showed that the adsorption reaction mainly occurred on the-NH and-NH2 groups on the surface of PEI/AC.Compared with unmodified activated carbon,the adsorption of Cd(?)by PEI/AC increased from 11 mg·g-1 to 45 mg·g-1,indicating that the modification of AC by PEI greatly improves the adsorption capacity of adsorbent for Cd(?).The adsorption of Cd(?)by PEI/AC is a pH-dependent process,and the maximum adsorption capacity is 45 mg·g-1 at pH 6.The adsorption process accords with the Langmuir isotherm adsorption model and the pseudo second-order kinetic equation,which indicates that the adsorption process of Cd(?)by PEI/AC is monolayer coverage and controlled by chemical interaction.After 5 adsorption-desorption cycles,PEI/AC still showed a relatively stable adsorption capacity for Cd(?),indicating that PEI/AC adsorbent has good recyclability.(2)The tapioca starch was mechanically activated,and then magnetic starch microspheres(MSM)and methyl methacrylate magnetic starch microspheres(MMA-MSM)were prepared by inverse emulsion polymerization.Then,the branched polyethyleneimine is cross-linked to the surface of the two kinds of microspheres using glutaraldehyde,respectively,to prepare polyethyleneimine modified magnetic starch microspheres(PEI/MSM)and polyethyleneimine modified methyl methacrylate magnetic starch microspheres(PEI/MMA-MSM).It can be observed by scanning electron microscopy that the average particle size of PEI/MSM microspheres is small,distributed between 100 and 200 nm,while the PEI/MMA-MSM microspheres have a larger average particle size and are distributed between 200 and 500 nm.The PEI/MSM microspheres are irregularly granular and the adhesion between the spheres is serious;and the PEI/MMA-MSM microspheres have uniform size,smooth surface,regular shape and good dispersion.The introduction of methyl methacrylate successfully solved the problem of blocking between magnetic starch microspheres,but also increased the average particle size of the microspheres,and decreased the specific surface area from 22.31 m2·g-1 to 14.74 m2·g-1.It is known from the thermogravimetric analysis that the PEI/MMA-MSM microspheres exhibit better thermal stability.The magnetizations of PEI/MSM and PEI/MMA-MSM microspheres are 9.8 emu·g-1 and 14.6 emu·g-1,respectively,which have good magnetic responsiveness and are convenient for magnetic separation.(3)The PEI/MSM and PEI/MMA-MSM microspheres were used to adsorb Cd(?)in wastewater,respectively,and the adsorption properties of PEI/MSM and PEI/MMA-MSM microspheres on Cd(?)were investigated.pH is an important factor affecting the adsorption of PEI/MSM and PEI/MMA-MSM microspheres.And the adsorption capacity of the two microspheres reached the maximum at pH 6 and 5,respectively,at 121 mg·g-1 and 187 mg·g-1.The adsorption process of Cd(?)by PEI/MSM and PEI/MMA-MSM accords with Langmuir isotherm adsorption model and quasi-secondary kinetic equation,which indicates that the adsorption process of Cd(?)by two kinds of microspheres is monolayer coverage,and controlled by chemical interactions;and the-NH2 and-OH functional groups in the adsorbent interact with Cd(?)in solution primarily by sharing electron pairs.When the PEI/MSM and PEI/MMA-MSM undergo 5 adsorption-desorption cycles,the adsorption capacity of Cd(?)remains 73%and 81%of the initial adsorption capacity,respectively.The adsorption-desorption cycle experiment results show that the adsorbent has good recyclability.