| With the continuous advancement of industrialization,a large amount of heavy metal wastewater has been discharged.Heavy metal pollution which is seriously harmful to the ecological environment and human health has become an important environmental problem.At the same time,heavy metal pollution gradually presented the trend of compound pollution.How to reduce and remove the heavy metal pollution in the environment has become a hot and difficult problem in the field of environmental governance.In recent years,polymer/layered silicate complexes have attracted much attention due to their unique functional properties and structures.Clay minerals,such as vermiculite,have attracted much attention in environmental remediation due to their many advantages,such as large reserves,low cost and so on.Chitosan is a widely existing alkaline polysaccharides in natural polymer materials and has been widely used in various fields due to its biofunctionality,biodegradability and biocompatibility.Chitosan has good adsorption properties for metal ions,organics and dye molecules due to the presence of a large number of amino and hydroxyl groups on its molecules.Chitosan/vermiculite composite was prepared by chitosan and vermiculite to form a green environment-friendly adsorbent.It can not only improve its adsorption properties,but also reduce the cost and repeated pollution.Therefore,the chitosan/vermiculite complex has a broad application prospect.In addition,chitosan/vermiculite composite modified by Fe?III?has excellent affinity to Cr?VI?besides its good adsorption capacity of cations,so it can be use to simultaneously remove the anion and cation from the waste water,which has a positive guiding role in the treatment for compound pollution of heavy metals.This study has obtained the following research results:1.Chitosan and vermiculite was used to prepare a novel adsorbent?CTS-VMT?with epichlorohydrin as the cross-linker.The obtained adsorbent was characterized by XRD,FTIR,SEM,BET,TG-DTG and Zeta potential.And the adsorption properties of Cd?II?and Pb?II?on CTS-VMT were also studied.The results showed that:?1?The CTS molecules did not penetrate into the layer of vermiculite but were mainly cross-linked on the vermiculite surface through the effect of hydrogen bonds;?2?The surface of the CTS-VMT composite become rougher and looser.Its porosity increases and the specific surface area increased.After the modification,the vermiculite structure underwent some changes and the thermal stability changed.In addition,the negatively charge VMT nanosheets were adsorbed by quaternized chitosan under electrostatic incorporation,forming a novel chitosan loaded CTS-VMT nanosheets.The surfaces of CTS-VMT were the negatively charged in the pH range of2.0-10.0;?3?The adsorption performances of CTS-VMT for Cd?II?and Pb?II?were higher than that of VMT,and the maximum adsorption capacities were 58.48 mg·g-1 and 166.67mg·g-1,respectively.The adsorption process fitted well the pseudo-second-order model and the Langmuir isotherm model;?4?The adsorption capacities of CTS-VMT for Cd?II?and Pb?II?increased with the increase of the solution pH,and the reaction temperature had no obvious effect on the adsorption process.The adsorption process was the spontaneous and endothermic reaction.CTS-VMT had excellent regeneration performance and stability;?5?The adsorption mechanism of CTS-VMT for Cd?II?and Pb?II?included chelation and electrostatic adsorption,in which chelation was the main adsorption mechanism.2.Chitosan and vermiculite was used to synthesize a new adsorbent?CTS-Fe-VMT?with ferric sulfate as modified material and epichlorohydrin as the cross-linker.The structure and surface properties of CTS-Fe-VMT were investigated through a series of characterization methods.And the adsorption properties of Cr?VI?and Cd?II?in the composite pollution system were mainly studied.The results showed that:?1?Fe?III?and chitosan were loaded on the surface vermiculite by electrostatic adsorption and hydrogen bonding without intercalation into the interlayer of vermiculite;?2?CTS-Fe-VMT was mesoporous material with the rough surface and the porous structure,whose surface area increased and average pore size decreased;?3?The Zeta potential values of CTS-Fe-VMT were positive at pH<5.45,indicating that the surface was positively charged.The isoelectric point appeared around pH=5.45;?4?The adsorption amounts of VMT to Cr?VI?were scarce,while the adsorption performance of CTS-Fe-VMT to Cr?VI?was greatly improved.CTS-Fe-VMT had a certain increase in the adsorption performance of Cd?II?comparing to VMT.In addition,the coexisting cations in the solution could promote the adsorption of Cr?VI?,and the adsorption of Cd?II?could also be effectively promoted by coexisting anion;?5?The adsorption of Cr?VI?and Cd?II?on CTS-Fe-VMT were very fast and could achieve balance within 5 min and 150min,respectively.The adsorption of two pollutants on CTS-Fe-VMT were in good agreement with pseudo-second-order model and Freundlich isotherm,and the maximum adsorption capacity could reach 102.74 mg·g-1?Cr?VI??and 59.524 mg·g-1?Cd?II??;?6?Coexisting Cr?VI?and Cd?II?enhanced the mutual adsorption performance.The main reason was that:Cd?II?improved the uptake of Cr?VI?on CTS-Fe-VMT by pre-adsorbing cations(Cd2+and Cr?OH?2+)as the cation bridge;Meanwhile,Cr?VI?enhanced the removal of Cd?II?by anion shielding effect and proton consumption;?7?CTS-Fe-VMT had regeneration performance and recycling performance,which had a certain application value in the treatment of environmental pollution with the coexistence of anion and cationic heavy metals. |
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