Preparations And Adsorption Properties Of As(Ⅴ)-imprinted Diatom-based Adsorption Materials

Arsenic,as an internationally recognized carcinogen,causes a serious harm to aquatic organism and human health.Arsenic pollution in water mainly comes from the emissions from mineral mining,metal smelting,fossil fuel combustion,arsenic-containing drug development and other activities.The adsorption method is one of the most economical and feasible methods of arsenic removal at present.With the rapid development of social economy and industry,the composition of arsenic-containing wastewater has become increasingly complex,so that the traditional adsorption materials have been difficult to achieve the expected effect of arsenic removal.In order to obtain adsorbents with high adsorption capacity,rapid adsorption kinetics and high selectivity,the amino functionalized As（Ⅴ）-imprinted diatom-based adsorbent（As（Ⅴ）-ⅡAD）and quaternary ammonium functionalized magnetic As（Ⅴ）-imprinted diatom-based adsorbent（As（Ⅴ）-ⅡQD）were prepared by copolymerization of functional monomers and crosslinkers and surface ion-imprinting technology with diatom（Dt）as imprinting substrate and As（Ⅴ）ion as template.As（Ⅴ）-ⅡAD and As（Ⅴ）-ⅡQD were characterized,and the behavior,performance and specific recognition mechanism of As（Ⅴ）adsorbed by As（Ⅴ）-ⅡAD and As（Ⅴ）-ⅡQD were systematically studied.The main findings obtained are as follows:（1）The optimum conditions for the preparation of As（Ⅴ）-ⅡAD were determined by single factor and response surface method（RSM）:1 g of Dt,solution p H=4.0,polymerization at 35℃for 12 h,and the mole ratio of TEOS and APTES=1:1.The characterization of Zeta,SEM-EDS,TG,XRD,FTIR and XPS of As（Ⅴ）-ⅡAD shows that when p H<4.12,the amide nitrogen（-CO-NH-）modified on the surface of As（Ⅴ）-ⅡAD is easy to protonate（-CO-NH₂⁺-）,which is conducive to the adsorption of As（Ⅴ）;As（Ⅴ）-ⅡAD has the characteristics of large specific surface area and porous structure,which is easy to expose a large number of imprinted sites to the material surface and improve the mass transfer rate;The increase of the percentage of C and N elements on the surface of As（Ⅴ）-ⅡAD relative to Dt,the high-temperature segmental loss of adsorbent quality,the weakening of the diffraction peak intensity of quartz（101）,the appearance of amide II band（N-H）absorption peak(1536 cm^-1),and the appearance of N 1s signal peaks（401.79 e V and 399.54 e V）of-CO-NH₂⁺-and pyridine ring（C-N=C）all indicate APTES,TEOS and 4-PA successfully modified Dt,and As（Ⅴ）-ⅡAD was successfully prepared.（2）The adsorption capacity of As（Ⅴ）-ⅡAD to As（Ⅴ）（54.05 mg/g）is significantly higher than that of NAD to As（Ⅴ）（21.95 mg/g）,indicating that the application of ion imprinting technology is helpful to improve the adsorption capacity of target ions.It was found that the closer the p H value of the adsorption solution was to the p H value of the imprinting process,the better the adsorption selectivity of As（Ⅴ）-ⅡAD for As（Ⅴ）.When the p H value of the adsorption solution and the p H value of the imprinting process are both 4.0,the adsorption selectivity coefficients KAs（Ⅴ）/Cr（Ⅵ）and KAs（Ⅴ）/Mo（Ⅵ）are as high as 12.14 and 27.79 respectively.The binding process was well described by the pseudo-second-order kinetic,Weber-Morris and Langmuir models,and the adsorption is a spontaneous and endothermic process.SEM-EDS,FTIR and XPS analysis showed that the N of-CO-NH₂⁺-and C-N=C on As（Ⅴ）-ⅡAD surface was the imprinted site for selective adsorption of As（Ⅴ）.Through DFT calculation of the constructed intuitive coordination geometry,it is further confirmed and summarized that the recognition mechanism of As（Ⅴ）-ⅡAD selective adsorption of As（Ⅴ）is mainly constructed by the cooperation of steric hindrance,complexation and electrostatic attraction.The regeneration test results show that after 5 adsorption-desorption cycles,the adsorption capacity of As（Ⅴ）decreases from 61.03 mg/g to 58.04 mg/g,and the loss rate is only 4.9%,indicating that As（Ⅴ）-ⅡAD has good regeneration performance and adsorption stability.（3）The optimum conditions for the preparation of As（Ⅴ）-ⅡQD were determined by single factor and RSM:1 g of Dt,solution p H=6.0,polymerization at 35℃for 15 h,and the mole ratio of TEOS and Si-QAS=1:1.The characterization of VSM,Zeta,SEM-EDS,TG,XRD,FTIR and XPS of As（Ⅴ）-ⅡQD shows that As（Ⅴ）-ⅡQD has superparamagnetism and is conducive to rapid recovery and utilization;when p H<7.24,the surface of As（Ⅴ）-ⅡQD is positively charged,which is conducive to the adsorption of As（Ⅴ）;As（Ⅴ）-ⅡQD has the characteristics of large specific surface area and porous structure,which is easy to expose a large number of imprinted sites to the material surface and improve the mass transfer rate;The increase of the percentage of C,N,O and Fe elements on the surface of As（Ⅴ）-ⅡQD relative to Dt,the high-temperature segmental loss of adsorbent quality,the appearance of diffraction peak of Fe₃O₄,the appearance of IR absorption peaks of Fe-O(573 cm^-1),-CH₂(2926 cm^-1),-CH₃(2856 cm^-1)and C-N(1289 cm^-1),and the appearance of Fe 2p_1/2（724.01e V）and Fe 2p_3/2（710.21 e V）signal peaks of Fe₂O₃,Fe 2p_1/2（722.01 e V）and Fe 2p_3/2（708.61e V）signal peaks of Fe O and N 1s（400.63 e V）signal peak of（CH₃）₃N⁺-all indicate Fe₃O₄,Si-QAS and TEOS successfully modified Dt,and As（Ⅴ）-ⅡQD was successfully prepared.（4）The adsorption capacity of As（Ⅴ）-ⅡQD to As（Ⅴ）（53.09 mg/g）is significantly higher than that of NQD to As（Ⅴ）（22.38 mg/g）,indicating that the application of ion imprinting technology is helpful to improve the adsorption capacity of target ions;The binding process was well described by the pseudo-second-order kinetic,Weber-Morris and Langmuir models,and the adsorption is a spontaneous and endothermic process;The selectivity coefficients_As（V）/SO₄^2-,As（V）/F^-,As（V）/NO₃^-,As（V）/Pb²⁺,As（V）/Cd²⁺andAs（V）/Zn²⁺of As（Ⅴ）-ⅡQD adsorption of As（Ⅴ）are as high as 13.83,17.84,29.11,138.25,184.33 and276.50 respectively,and the relative selectivity coefficients′_{As（V）/SO4}2^-,′As（V）/F^-,′As（V）/NO₃^-,′_As（V）/Pb²⁺,′_As（V）/Cd²⁺and′_As（V）/Zn²⁺relative to NQD are 10.89,10.94,10.62,10.63,10.64 and 10.63 respectively,indicating that As（Ⅴ）-ⅡQD adsorption of As（Ⅴ）has specific selectivity.SEM-EDS,FTIR and XPS analysis showed that-N⁺（CH₃）₃ on the surface of As（Ⅴ）-ⅡQD was the imprinted site for selective adsorption of As（Ⅴ）.Through DFT calculation of the constructed intuitive coordination geometry,it is further confirmed and summarized that the recognition mechanism of As（Ⅴ）-ⅡQD selective adsorption of As（Ⅴ）is mainly constructed by the cooperation of steric hindrance,complexation and electrostatic attraction.The regeneration test results show that after 6 adsorption-desorption cycles,the adsorption capacity of As（Ⅴ）decreases from 57.28 mg/g to 55.02 mg/g,and the loss rate is only 3.9%,indicating that As（Ⅴ）-ⅡQD has good regeneration performance and adsorption stability.