Preparation Of Modified Cyclodextrin Microspheres And Their Adsorption Properties For Heavy Metal-organic Dyes Compound Pollution

The heavy metal-organic dye composite contaminated wastewater has the characteristics of great damage and difficult treatment,which poses a greater challenge to the treatment of composite contaminated wastewater.The adsorption method has the characteristics of high efficiency,simple operation,and low cost,and has been widely used in the treatment of heavy metal-organic dye composite polluted wastewater.The key lies in the development of cost-effective adsorption materials.Therefore,the development of new and efficient adsorption materials is of great significance for the treatment of heavy metal-organic dye composite pollution.In recent years,the unique structure and properties ofβ-cyclodextrin（β-CD）can graft reactive groups,which can not only remove organic dyes,but also improve the ability to adsorb heavy metals,and can be used to deal with complex composite pollution.In this study,a novel modified cyclodextrin microsphere adsorption material（DTC-CDMS）was designed to study adsorption performance for heavy metal ions,organic dyes and heavy metal-organic dyes,and to explore its adsorption mechanism.Using β-CD,epichlorohydrin（EPI）,triethylenetetramine（TETA）as raw material,liquid paraffin as oil phase,and span-80 as emulsifier,cyclodextrin microspheres（CDMS）were synthesized by reverse microemulsion method.Modified cyclodextrin microspheres（DTC-CDMS）were obtained by modifying the dithiocarbamate to the surface of the CDMS under alkaline conditions.The optimal conditions for the synthesis of CDMS are a molar ratio ofβ-CD to TETA of 1:20,a molar ratio ofβ-CD to EPI of 1:65,a volume of deionized water of 6 m L,a volume ratio of water to oil of 1:4 and stirring speed of 400 r·min^-1.The optimum TETA to CS₂molar ratio for the synthesis of DTC-CDMS is 1:4.Compared with CDMS,the removal efficiency of MO by DTC-CDMS is slightly reduced,but the removal efficiency of Cu²⁺is increased by about 40%.DTC-CDMS is a microsphere with a surface diameter of approximately 140μm and a rough surface.The pore size of DTC-CDMS is mainly distributed at 10-100 nm,and DTC-CDMS is stable below 521 K.The molar contents ofβ-CD,N element and CS₂in DTC-CDMS are calculated to be 0.11,8.05 and 1.72 mmol·g^-1,respectively.The grafting ratio of CS₂is 0.214,which provide abundant adsorption sites for heavy metal ion adsorption.With the increase of DTC-CDMS dosage,the removal efficiency（R）of Cu²⁺and Pb²⁺increase first and then balance,and the R values reached 99.70%and 99.83%,respectively.The optimum p H range for removing Cu²⁺and Pb²⁺from wastewater is 4-7,and the equilibrium time for adsorption is 240 min.The isothermal adsorption process of Cu²⁺and Pb²⁺by DTC-CDMS accords with the Freundlich model,the adsorption kinetic process can be described by pseudo-second order kinetic model,and the chemisorption spontaneous reaction of chemisorption and physical adsorption in the adsorption process is driven by entropy increase.Adsorption mechanism of heavy metal ions by DTC-CDMS:electrostatic interaction of N-CS₂^-groups with heavy metal ions and coordination of-C-N with heavy metal ions.However,due to the reducibility of Cu²⁺,a part of C-S is oxidized to-SO₃^-with Cu（Ⅱ）being reduced to Cu（Ⅰ）.With the increase of DTC-CDMS dosage,the R values of MO and TB increase first and then balance,and the R values reached 91.95%and 99.14%,respectively.The optimum p H range for removal of MO and TB from wastewater is 3-4 and 2-3,respectively,and the equilibrium time for adsorption is 240 min.The isotherm adsorption process of DTC-CDMS for MO and TB accords with the Langmuir model,and the maximum adsorption capacity of DTC-CDMS for MO and TB at 293 K is 159.91 and 271.10 mg·g^-1,respectively.The adsorption kinetic process can be described by pseudo-second order kinetic model.During the adsorption process,chemisorption and physical adsorption coexist and absorb spontaneous reactions,which are driven by entropy.Adsorption mechanism of DTC-CDMS for organic dyes:β-CD cavity in DTC-CDMS,electrostatic attraction of protonated amine groups to MO or TB.In the binary system,with the increase of DTC-CDMS dosage,the removal efficiency of Cu²⁺,Pb²⁺,MO and TB increase first and then balance,and the R value could reach 98.81%,94.91%,98.96%and 99.08%.The optimal p H range for removing TB is 2-6,while the optimum p H range for removing Cu²⁺,Pb²⁺and MO is 3-6.The isothermal adsorption process of Cu²⁺and Pb²⁺by DTC-CDMS accords with the Freundlich model,and the isotherm adsorption process for MO and TB conforms to the Langmuir model.During the adsorption process,the chemical adsorption and physical adsorption coexisted spontaneously,and the spontaneous reaction was driven by entropy,the adsorption kinetics process can be described by pseudo-second order kinetic model.In Cu²⁺-MO,Cu²⁺-TB,Pb²⁺-MO and PB²⁺-TB binary systems,Cu²⁺and MO/TB have synergistic effects,and Cu²⁺and MO can promote mutual removal;MO has a certain inhibitory effect on the removal of Pb²⁺by DTC-CDMS,but Pb²⁺promotes the removal of MO by DTC-CDMS;Pb²⁺and TB have antagonistic effects;Due to the large steric hindrance of TB,the amount of TB adsorbed on DTC-CDMS reached the maximum,and Cu²⁺/Pb²⁺did not promote the removal of TB by DTC-CDMS.The adsorption process of Cu²⁺by DTC-CDMS includes the electrostatic attraction and coordination chelation of strong chelating group N-CS₂^-to Cu²⁺in DTC-CDMS,and the coordination of partially deprotonated amine groups with Cu²⁺,electrostatic attraction of Cu²⁺by-SO₃^-after DTC-CDMS adsorbs MO/TB.The adsorption process of Pb²⁺by DTC-CDMS includes electrostatic attraction and coordination chelation of N-CS₂^-to Pb²⁺.The adsorption process of DTC-CDMS on MO/TB includes theβ-CD cavity to the MO/TB host-guest inclusion,electrostatic attraction of partially protonated amine groups with MO/TB,electrostatic attraction of MO/TB after adsorption of Cu²⁺/Pb²⁺by DTC-CDMS.