Preparation Of Graphene-based Ion Imprinted Polymers And Aerogels And Their Adsorption And Photocatalysis Of Cr(?)

Hexavalent chromium?Cr????is a typical heavy metal contaminant which can exist stably in environment.It has strong solubility and mobility in water,and its biological toxicity is higher than trivalent chromium?Cr?III??over 100 times.It also has effects of carcinogenic and genetic mutation on humans and other aquatic organisms.Adsorption is a common technical method for removal of Cr???with the characteristics of simple operation,high efficiency and economic feasibility,and it can be applied to different scale processes.The development of efficient and economical adsorption materials is the key to this technique.Most of the previous nano-adsorbents have some shortcomings,such as slow adsorption rate,low adsorption capacity and difficult separation from water.Combining adsorption with photocatalysis,in situ reduction of Cr???gathered on the surface of adsorbent to Cr?III?is also an effective way to treat the Cr???pollution in water,which not only reduces the toxicity of Cr???,but also regenerates the adsorbent material.Graphene,as a new type of carbon material,has been widely used in water treatment.In order to improve the adsorption rate and capacity of graphene-based adsorbents,ion-imprinted polymers with specific recognition function are loaded on graphene sheets,and magnetic separation is facilitated by the introduction of ferric oxide.To further resolve the problem of nano-materials separation,a graphene aerogel material with three-dimensional?3D?macrostructure was prepared to remove Cr???.By introducing different semiconductors into graphene aerogel,the graphene aerogel with heterogeneous structure was synthesized as photocatalysts,and the synergistic effect of adsorption and photocatalysis was achieved for improving the removal efficiency of Cr???.Based on the above ideas,the specific research work was carried out as follows:?1?Fe₃O₄@SiO₂ particles were loaded onto the surface of graphene oxide?GO?to form a magnetic graphene oxide lamella?MGO?.To synthesize Cr???ion-imprinted polymers on the surface of MGO,Cr???was used as template,and 4-vinyl pyridine?4-VP?,2-hydroxyethyl methacrylate?HEMA?,ethylene glycol dimethacrylate?EGDMA?and N,Nazoisobisbutyronitrile?AIBN?were used as functional monomer,co-monomer,crosslinker and initiator,respectively.2 M hydrochloric acid?containing 1%thiourea?solution was used as template eluent to remove Cr???ion from polymer and then magnetic Cr???ion-imprinted polymers?MGOIIP?were formed with specific recognition site of Cr???ion.The adsorption process of Cr???by MGOIIP can be well fitted by pseudo-second-order kinetic model and Langmuir isotherm model.The maximum adsorption capacity of MGOIIP for Cr???can reach up 311.95 mg/g under pH of 2.0 at 25?.Because MGOIIP has the specific recognition sites of Cr???,the adsorption process can quickly reach the equilibrium within 3 min.MGOIIP has good selectivity for Cr???,and there was the adsorption process is almost independent of other coexisting ions.The regeneration and recycling of adsorbents can be realized by desorption of acidic thiourea solution.After five adsorption-regeneration cycles,the ideal adsorption capacity of Cr???can still be achieved.?2?Nitrogen doped three-dimensional graphene aerogel?TPGA?with stable properties was prepared by a facile one-step hydrothermal method and freeze dry technology using Tetraethylenepentamine and pyrrole as nitrogen source.The adsorption process of TPGA for Cr???can be fitted well with the pseudo-second-order kinetic model.The removal rate of TPGA is very fast within 100 min,and the adsorption equilibrium was reached within 300min.The saturated adsorption process fitted well with Langmuir adsorption isotherm model.The calculated maximum adsorption capacity reached 415.77 mg/g at 25?.The adsorption was a spontaneous endothermic process.Dynamic adsorption experiments showed that TPGA had a good continuous removal efficiency for Cr???with low concentration,and the influent velocity had a great influence on the removal efficiency.After five adsorption-regeneration cycles,the adsorption capacity of TPGA for Cr???decreased by only 7%,indicated TPGA has a good regeneration performance.The removal efficiency of TPGA for Cr???within 20mg/L in natural water can reach over 99%.?3?TiO₂ and Zn_0.5Cd_0.5S were introduced into rGO layers by a one-step hydrothermal method and formed TiO₂/Zn0.5Cd0.5S graphene aerogel?TSGA?with 3D porous network structure.The adsorption equilibrium can be reached within 40 min for 50 mg/L Cr???when TSGA dosage of 0.8 g/L,pH of 2.0,and more than 99%of Cr???can be removed by photocatalysis for further 30 min under visible light irradiation.The removal efficiency of Cr???was much higher than that of pristine TiO₂ and Zn0.5Cd0.5S.TSGA could be completely removed from the water by simple filtration because of its 3D macrostructure,which is convenient for recycling.The removal efficiency could still reach more than 90%after five cycles without extra desorption processes.The potential application of TSGA was investigated by continuous flow operation.The adsorption equilibrium and photocatalytic equilibrium of 1.0 mg/L Cr???could be achieved within 60 min and 100 min respectively under the condition of 1.0 mg/L inlet flow rate.The removal rate of 95%Cr???could be maintained during 10 h continuous operation.The radicals trapping experiments showed that the main active material in the photocatalytic process of Cr???was photogenerated electrons.As an electron transfer medium rGO transferred the photogenerated electrons generated by TiO₂ in the conduction band to the valence band of Zn0.5Cd0.5S to compound with the photogenerated holes,thus gathering a large number of electrons in the conduction band of Zn0.5Cd0.5S,and these electrons were mainly used to reduce Cr???to Cr?III?.?4?Two kinds of bismuth-based semiconductors Bi₂S₃ and BiVO₄ were introduced to graphene aerogel by one-step hydrothermal method and formed heterojunction photocatalyst Bi₂S₃/BiVO₄ graphene aerogel?SVGA?with 3D structure.The separation efficiency of photogenerated electron-hole pairs was improved effectively and the recombination rate was reduced.When the molar ratio of Bi₂S₃ to BiVO₄ was 1:2 and the mass fraction of Bi₂S₃ to SVGA was 60%,the best removal performance could be achieved for both pollutants by using SVGA to remove Cr???and BPA simultaneously through adsorption and photocatalysis.The optimum pH for simultaneous removal of Cr???and BPA was 2.0 and the dosage was 0.8g/L.SVGA had a good recycling ability,and the removal rate of Cr???and BPA can be maintained above 85%after five recycles without desorption.Cr???was reduced to low toxic Cr?III?mainly by photogenerated electrons in the photocatalytic process.The photogenerated holes played a major role for photocatalytic degradation of BPA.Photogenerated electrons were also involved in the production and transformation of oxygen radicals,such as ·O₂^-and ·OH.