Effect Of Cationic Clay On The Structure Of Hydrotalcite And The Adsorption Properties Of Organic Anionic Dyes

With the rapid development of printing and dyeing industry,the discharge of dye wastewater is increasing year by year.Adsorption method is one of the most important methods to treat printing and dyeing industrial wastewater.Hydrotalcite compounds（LDH）are widely used as adsorbents to remove organic anionic pollutants from water because of their special layered structure and properties.In this paper,the cationic clay/LDH composite was prepared by adding cationic clay into the synthetic system of Magnesium aluminum hydrotalcite（Mg₃Al-LDH）and zinc chromium hydrotalcite（Zn₂Cr-LDH）.The microstructure of the adsorbed materials was characterized by XRD,FTIR and SEM.The effects of the different adsorbed materials on the adsorption properties of methyl orange（MO）solution from simulated organic wastewater were investigated.The main research results are as follows:（1）Mg Al-LDH adsorbent was prepared by co-precipitation method.The effects of synthesis conditions(Mg²⁺/Al³⁺molar ratio,solution p H value,interlayer anions of NO₃^-,CO₃^2-,SO₄^2-,Cl^-)and adsorption conditions on the microstructure and adsorption properties of Mg Al-LDH were investigated.The adsorption mechanism of Mg₃Al-Cl LDH was determined by studying the adsorption kinetics,isothermal adsorption effect and the structural changes of LDH before and after adsorption.When the molar ratio of Mg²⁺/Al³⁺is 3:1,the p H value of synthesized solution is 12,and the anion between layers is Cl^-,the crystallinity of LDH is higher,the layer spacing（d003）is larger,the size of nanosheet is smaller,and the honeycomb structure is obvious.Under the condition of 22℃and p H=7,the maximum adsorption capacity of MO by the adsorbent is 5592.0 mg/g.The adsorption behavior of MO by Mg₃Al-Cl LDH conforms to the quasi-second-order kinetic model,and MO molecule is adsorbed in a single layer.The removal of MO in solution mainly depends on ion exchange between MO molecule and Cl^-between Mg₃Al-Cl LDH layers.Mg₃Al-Cl LDH is a kind of adsorbent with high adsorption capacity and strong stability.（2）The cationic clay/Mg₃Al-Cl LDH composite was prepared by coprecipitation method.The cationic clay includes montmorillonite（MMT）and kaolin（Kaol）.The influence of the addition of cationic clay on the microstructure and adsorption performance of LDH was investigated.The adsorption mechanism of cationic clay/Mg₃Al-Cl LDH composites was determined by studying the adsorption kinetics,isothermal adsorption effect and the structural changes of LDH before and after adsorption.At 22℃and p H=7,the adsorption capacity of MMT/Mg₃Al-Cl LDH composite and Kaol/Mg₃Al-Cl LDH composite is 4483 mg/g and4206.5 mg/g,respectively,when the cationic clay content is 0.5 g.The adsorption behavior of MO by cationic clay/Mg₃Al-Cl LDH composite conforms to the quasi-second-order kinetic model,MO molecules are adsorbed on the surface of adsorbent in a monolayer,and mainly controlled by ion exchange.（3）Cationic clay/Zn₂Cr-LDH composites were prepared by coprecipitation method on the basis of previous research.The effects of the addition of cationic clay on the microstructure and adsorption properties of LDH were investigated.The adsorption mechanism of cationic clay/Zn₂Cr-LDH composites was determined by studying the adsorption kinetics,isothermal adsorption effect and structural changes before and after adsorption.The adsorption capacity of MO on MMT/Zn₂Cr-LDH composite and Kaol/Zn₂Cr-LDH composite is 4103.5 mg/g and 3801.5 mg/g at 25℃and p H=7 when the addition of MMT and Kaol is 0.3 g and 1.0 g,respectively.Compared with Zn₂Cr-LDH（2522.0 mg/g）,the adsorption capacities of the two composites are significantly improved.The adsorption behavior of MO by the two composites conforms to the quasi-second-order kinetic model,MO molecules are adsorbed in a multilayer,and mainly controlled by ion exchange.The adsorption efficiency of MO on MMT/Zn₂Cr-LDH and Kaol/Zn₂Cr-LDH composites after 4 consecutive photocatalytic regeneration is up to 90.48%and 89.66%,respectively.The cationic clay/Zn₂Cr-LDH composite is a kind of adsorbent with high adsorption capacity,high cycle stability and green regeneration for organic anionic dyes.（4）After the addition of cationic clay（MMT and Kaol）,the adsorption capacity of Mg₃Al-LDH composite system decreased from 5592.0 mg/g to 4483.0 mg/g and 4206.5 mg/g respectively,and the adsorption capacity has a slightly reduce.However,with the introduction of cationic clay,the content of Mg₃Al-Cl LDH in the composite system was reduced by 25%,which significantly reduced the preparation cost of adsorbent while retaining a high adsorption capacity.For Zn₂Cr-LDH composite system,the adsorption capacities of MMT/Zn₂Cr-LDH and Kaol/Zn₂Cr-LDH increased from 2522.0 mg/g to 4103.5 mg/g and 3801.5mg/g,respectively.The adsorption capacities were significantly improved,and the preparation cost of the composite adsorbents was significantly reduced.Cationic clay/Zn₂Cr-LDH composite is a kind of comprehensive adsorbent with high adsorption capacity,effective photocatalytic regeneration and strong cyclic stability.The cationic clay/LDH composite prepared by coprecipitation method in this paper has the characteristics of high adsorption capacity,strong cycling stability and green regeneration.It has excellent removal effect on organic anionic dyes in aqueous solution,which provides a strong theoretical basis for the removal of organic anionic pollutants.There are 48 figures,12 tables and 140 references.