Preparation Of Magnetic Polymeric Adsorbents And Its Adsorption Properties For Cd(?)

The emission of various Cd(?)-containing industrial wastewater exceeding the emission standard will bring long-term threat to the environment and human health.The Cd(?)concentrations of most of Cd(?)-containing industrial wastewater reported are 0-80 mg/L.Conventional methods for removing heavy metal ions from wastewater such as chemical precipitation and solvent extraction are difficult to dispose low-concentration wastewater.By contrast,adsorption process is efficient,easy to operate,and with less sludge disposal problems for the low-concentration systems,thus it has been considered as one promising strategy for the treatment of low-concentration wastewater.Superparamagnetic adsorbents have tunable structure and size,and they can move,enrich,coagulate and transport under external magnetic field.Superparamagnetic adsorbents are easy to maneuvre under external magnetic field,which can avoid the problems of high pressure,high energy consumption,channeling and not easy for continuous operation in traditional fixed bed adsorption process.Magnetic carrier based separation technology has been reported to remove Cd(?).However,the reported magnetic adsorbents for Cd(?)removal showed low adsorption capacity and selectivity.Moreover,the large-scale and continuous separation of magnetic adsorbents is difficult.Aim to deal with the issues mentioned above,this dissertation carries out a research on the effects of functional group type and density at the interface on the capacity and selectivity of adsorbents.Magnetic adsorbents with high adsorption capacity and enhanced selectivities towards Cd(?)were prepared.Besides,gas-assisted magnetic separation process and devices suitable for the large-scale and continuous separation of magnetic adsorbents for Cd(?)were developed.Firstly,to address the problem of low capacity,the effects of immobilized functional groups density on Cd(?)adsorption capacity were studied.A series of ethanediamine-modified magnetic poly(glycidyl methacrylate)(m-PGMA-EDA)microspheres with different amine density were synthesized as the adsorbents for Cd(?).The results showed that Cd(?)saturation adsorption capacity increased with the immobilized amine density.While absolute linear relation was not observed in the whole range of amine density.The molar ratio of N atom to adsorbed Cd ions decreased with the increase of amine density and reached a minimum value about 4 when the amine density reached 1.25 mmol/m2.These studies demonstrated that Cd(II)adsorption capacity could be significantly improved by increasing the immobilized functional groups density,and the density of immobilized functional groups could affect the complex ratio of functional group to Cd.The maximum Cd(II)adsorption capacity of m-PGMA-EDA was 189.89 mg/g.The Cd(II)adsorption capacity and rate of m-PGMA-EDA improved significantly when compared with the reported magnetic adsorbents.However,Zn(II)had significant impact on Cd(II)adsorption capacity and the Cd/Zn selectivity factor was about 0.5.This indicated that m-PGMA-EDA had stronger affinity for Zn(II)than Cd(II).FT-IR and XPS results demonstrated that the coordination between amine groups and Cd(II)was the primary adsorption mechanism,and hydroxyl groups had synergistic coordination effect.Next,to solve the problem of selective separation of Cd(II),dithiocarbamate(DTC)containing-SH and-C=S groups was introduced onto polyethyleneimine(PEI)modified magnetic PGMA microspheres based on the HSAB(Hard and soft acids and bases)theory.The obtained adsorbent was designated as mPGMA-PEI-S.The Cd/Zn selectivity factor was 3.36-4.00 at pH 4-7 for mPGMA-PEI-S.The equilibrium time was 10 min,which was much shorter than that of the adsorbent prepared by encapsulating PEI-based dithiocarbamate in alginate matrix(24 h).EDX,FT-IR and XPS results suggested that cation exchange and coordination interactions existed in the adsorption process of Cd(II)for mPGMA-PEI-S.To further enhance the Cd/Zn selectivity and recyclability of the adsorbent,two layers of di(2-ethylhexyl)dithiophosphoric acid(D2EHDTPA)were innovatively impregnated into PGMA microspheres through chemical and physical interaction respectively,and the obtained adsorbent was designated as PGMA-D2EHDTPA.The results showed that the Cd/Zn selectivity factor for PGMA-D2EHDTPA was greater than 80,even up to 330 at pH 3-7.Cd/Zn separation with PGMA-D2EHDTPA consumed less organic solvent and the operation condition was milder,compared with Cd/Zn separation using D2EHDTPA-toluene extraction system with 1-4 mol/L H2SO4 in the aqueous phase,the volume ratio of oil to water was 1:2 in the reference.The adsorbents could be reused after desorption by 6 mol/L HCl solution,and 90%of the adsorption capacity maintained after 6 adsorption-desorption cycles.Adsorption mechanisms included cation-exchange and coordination interactions,elucidated by FT-IR and nuclear magnetic resonance(NMR).In the last part of the study,gas-assisted magnetic separation process and devices suitable for the large-scale and continuous separation of magnetic adsorbents for Cd(?)were developed,aiming at solving the difficult scaling up of traditional magnetic separation.In the interval experiments,the recovery of Cd(?)-loaded magnetic sorbents could reach 90%within 5 min.The continuous gas-assisted magnetic separation experiments proved the feasibility of scalable and continuous separation of Cd(?)-loaded magnetic PGMA microspheres.