Study On The Adsorption And Degradation Of Organic Pollutants By Modified Nanometer Zero-valent Iron

Nano-zero-valent iron（nZVI）has the properties of low toxicity,low cost and high reduction ability.It is a promising reductive material for the removal of many common pollutants,including heavy metal ions,inorganic anions,halogenated organic matter,drugs,phenolic compounds,dyes and other common contaminants.But it is easy to agglomerate and oxidize,so its application is limited.Therefore,how to avoid the agglomeration of nZVI particles and improve their reactivity has become a research hotspot in this field.In this study,the natural polymer material Chitosan（CS）,porous supporting material Diatomite（DA）and anionic surfactant Sodium Dodecylbenzene Sulfonate（SDBS）are used as the stable modifier,loading agent and surface modifier in the preparation of nZVI,three kinds of adsorption materials were prepared:Chitosan stabilized Nano-zero-valent iron（CS/nZVI）,Diatomite supported chitosan modified Nano-zero-valent iron（CS/DA/nZVI）,sodium dodecyl benzene sulfonate modified chitosan modified Nano-zero-valent iron（SDBS/CS/nZVI）.And they were used for the adsorption and degradation of Tetracycline hydrochloride（TC）,Congo Red（CR）and Crystal Violet（CV）organic pollutants,respectively.The main research contents are as follows:（1）Using chitosan（CS）as stabilizer,CS modified nZVI was prepared by liquid phase reduction method and used for the removal of antibiotic organic pollutant tetracycline hydrochloride（TC）.The effects of mass ratio of CS and nZVI,p H and initial concentration of solution on the adsorption performance were investigated,and the fitting analysis of thermodynamics and kinetics was carried out.The results showed that CS could prevent the agglomeration of nZVI.When the mass ratio of CS to nZVI was 1:7,the amount of adsorbent was 50 mg,and the initial concentration of solution was 60 mg/L,the removal rate and adsorption capacity were 98.31%and117.97 mg/g,respectively.The adsorption kinetics conforms to the Pseudo-First-Order kinetic equation（PFO）,and the calculated initial rate is 93.31mg·g^-1·min^-1,which is higher than that of nZVI.The adsorption isotherm is consistent with Liu’s isotherm equation,which indicates that the undetermined energy of the surface activity of the composites is different.Meanwhile,the process is a spontaneous process of exothermic and entropy reduction.The mechanism analysis showed that due to the stabilizing effect of CS,the removal of TC by the composite material was mainly completed through electrostatic attraction,hydrogen bonding and the reduction and degradation of nZVI.Meanwhile,the introduction of CS improved the long-term stability of nZVI,and the removal rate was still 86.23%after 120 days,much higher than that of nZVI（56.65%）.（2）Using chitosan（CS）as stabilizer and diatomite（DA）as loading agent,CS/DA/nZVI composites were prepared by liquid phase reduction method and used to remove the anionic dye Congo red（CR）.The effect of mass ratio of CS,DA and nZVI on adsorption performance was studied.The results of single factor investigation were as follows:when the amount of adsorbent was 40mg and the initial concentration of solution was 100 mg/L,the adsorption and removal of CR by CS/DA/nZVI（1:7:7）reached an equilibrium at 60 min.At this time,the removal rate and adsorption capacity were 98.85%and 247.14 mg/g,respectively.The adsorption kinetics and thermodynamic fitting results show that the adsorption of CR by CS/DA/nZVI（1:7:7）conforms to the Pseudo-Second-Order kinetic equation（PSO）and the Freundlich adsorption isotherm equation.According to the analysis of thermodynamic parameters,the adsorption of CR by CS/DA/nZVI（1:7:7）is a spontaneous process of exothermic and entropy increase.The adsorption results of different anionic and cationic dyes show that electrostatic attraction and the existence of hydrogen bonds between–NH₂and-SO₃functional groups can improve the removal ability of the composites.（3）CS/nZVI（1:7）was further modified by different anionic surfactants such as sodium lignosulfonate（SLS）,sodium dodecyl benzene sulfonate（SDS）and sodium dodecyl benzene sulfonate（SDBS）,and then the cationic dye crystal violet（CV）was removed from the solution by them.0.2 g SDBS was selected as modifier according to the removal rate,and SDBS/CS/nZVI was prepared by liquid phase reduction method.SEM and BET analysis showed that the nZVI was well dispersed under the stabilization of CS and modification of SDBS,and the particle size became smaller and the specific surface area doubled.The single factor study showed that at 298 K,when the amount of adsorbent was 50 mg and the initial concentration of solution was100 mg/L,the adsorption removal of CV by SDBS/CS/nZVI reached equilibrium in 30min,and the removal rate and adsorption capacity were 98.35%and 196.69 mg/g,respectively.In the p H range of 3～11,the removal rate is always above 96%.Kinetic studies show that the adsorption of CV by SDBS/CS/nZVI conforms to the PFO,while the degradation of CV conforms to the PSO.The adsorption process conforms to Liu’s adsorption isotherm model,and the parameter fitting results of thermodynamic research show that the process is a spontaneous process with exothermic and entropy reduction.Due to the introduction of SDBS,the surface of the composite has negative charge,electrostatic attraction and the degradation of nZVI are the main reasons for the high adsorption rate（4）Considering the application of engineering practice,dynamic adsorption experiments were carried out on CS/nZVI（1:7）to remove TC,CS/DA/nZVI（1:7:7）to remove CR and SDBS/CS/nZVI to remove CV.The results are as follows:when the peristaltic pump rotation speed is 100 r/min,the dosage of adsorbent is 0.2 g,and the initial concentration of solution is 5 mg/L,the TC removal rate and adsorption capacity of CS/nZVI（1:7）are 94.53%and 7.09 mg/g,respectively.When the speed of peristaltic pump was 100 r/min,the dosage of adsorbent was 0.2 g,and the initial concentration of solution was 15 mg/L,the CR removal rate and adsorption capacity of CS/DA/nZVI（1:7:7）were 97.00%and 21.83 mg/g,respectively.When the speed of peristaltic pump was 100 r/min,the dosage of adsorbent was 0.2 g,and the initial concentration of solution was 15 mg/L,the CV removal rate and adsorption capacity of SDBS/CS/nZVI were 90.23%and 20.30 mg/g,respectively.It can be seen that,compared with the static experiment,on the premise of satisfying the high removal rate,the dynamic adsorption experiment requires more adsorbent dosage,is suitable for the treatment of pollutants with smaller initial concentration,and it takes more time to reach the adsorption equilibrium.