PAA/HEC-coated Nanoscale Zero-valent Iron For Removal Of Hexavalent Chromium In The Groundwater

In recent years,with the rapid development of China's industry,a large number of chromium containing waste water produced in the industrial production process such as metallurgy,electroplating,tannery,pigments,metal processing and so on.These waste water not only pollute the surface water,but also penetrate into the water body with precipitation,which seriously endangers the health of the drinking water of the residents.Nanosclae zero-valent iron?nZVI?has been used in recent years to repair serious groundwater pollution due to its large specific surface area,strong reduction ability and low preparation cost.However,most of the existing nanoscale iron particles have the disadvantages of poor stability and poor mobility in aquifers.Therefore,further modification of nanoscale zero-valent iron,which can maintain high stability and good mobility in aquifers,has become the key to effectively repair heavy metal pollution in groundwater.Polyacrylic acid?PAA?not only has a large number of hydrophilic groups and carboxyl groups that can reduce the surface tension of water,but also can adsorb,chelate or ion exchange with heavy metal chromium,while hydroxyethyl cellulose?HEC?contains hydrophobicity groups and a certain number of hydrophobic groups,and the surface tension of water can be reduced to a certain extent when it is used as surfactant.It is helpful to disperse the colloid in water.Therefore,this study uses PAA and HEC to modify nZVI to improve the stability and migration of zero-valent iron particles in aquifers.In this study,the optimum preparation conditions for the preparation of coated nanoscale zero-valent iron were explored,and the effects of different factors on the removal of Cr???were investigated by batch experiments.The main contents and conclusions of this paper are as follows:?1?The coated P/H-nZVI was prepared,and the effect of the mass ratio of PAA and HEC,the ratio of coating agent?P/H?and nanoscale zero-valent iron?nZVI?mass ratio on the Cr???removal efficiency and the settling properties of nanoscale zero-valent iron particles were investigated.The experimental results show that when the ratio of m?PAA?:m?HEC?:m?nZVI?is 4:8:40,the coated nanoscale zero-valent iron particles has the highest removal efficiency to Cr???,and it can keep no obvious settlement in a week.?2?Through the characterization of the newly prepared nZVI and P/H-nZVI transmission electron microscopy?TEM?,the particle size of P/H-nZVI prepared by PAA and HEC as the coating material is mostly in the 50-130nm range,and the agglomeration phenomenon is obviously less than that of nZVI,and the chain structure does not appear.The results of XRD show that the prepared nZVI and P/H-nZVI are all nanoscale zero-valent iron,that is,alpha-Fe particles in the body centered cubic crystal structure at the characteristic diffraction peak of 44.8,and the characteristic diffraction peak of P/H-nZVI is more obvious than that of nZVI.The result shows that PAA and HEC as coating agents can prevent the oxidation of nanoscale zero-valent iron particles very well.The result is in FT-IR atlas has also been proved.?3?The effects of adding amount,initial pH,initial concentration,common ions of groundwater and humic acid on removal of Cr???were investigated by batch test,and the changes of removal rate of six valent chromium caused by various factors were preliminarily analyzed.It is found that the increase in the dosage of P/H-nZVI can improve the removal efficiency of Cr???.When the initial concentration of Cr???is 50mg/L in the polluted liquid,the removal rate of P/H-nZVI is 96.5%when the dosage of P/H-nZVI is increased to 0.5g/L.The pH value also has an important impact on the P/H-nZVI reduction of Cr???,and the removal efficiency of P/H-nZVI for Cr???is above 96.5%in the 3-7 range,while the removal efficiency will be reduced to 40%and 33.4%when the pH value rises to 9 and 11.The increase of initial concentration of Cr???,the presence of common ions and humic acid?HA?in the groundwater will inhibit the removal of Cr???by P/H-nZVI.P/H-nZVI has a good removal effect on the Cr???solution with a concentration of less than 50mg/L,and the effect of the common ground water ions on the removal rate of Cr???is Ca²⁺>Mg²⁺>SO₄^2->NO₃^->Cl^->HCO₃^-.?4?This experiment uses the quasi first order kinetics,quasi two order kinetics and particle diffusion model to analyze the kinetic analysis of P/H-nZVI removal of Cr???.The results show that quasi two kinetics can describe the removal process of P/H-nZVI removal of Cr???better.Moreover,because the three adsorption line segments of the particle diffusion model have not passed the original point,this is the table.The adsorption process of P/H-nZVI on Cr???has different effects on the diffusion step controlled by rate.Combined with the XPS characterization results,it is known that the removal of Cr???by P/H-nZVI is mainly caused by reduction reaction,followed by physical adsorption,and the reaction product may be precipitated Cr₂O₃ and Cr?OH?₃.?5?One dimensional simulation column was used to study the influence of the modified nanoscale zero-valent iron,the particle size of the medium,the injection velocity of the slurry,the concentration of the slurry injection and the common ions of the groundwater on the mobility of the P/H-nZVI in the simulated column.When the injection amount is 1PV,the migration ability of P/H-nZVI after PAA and HEC is better than that of unmodified nZVI;the migration ability of P/H-nZVI in coarse sand is stronger than that of fine sand in the range of 0.1¹.0mm,and the migration ability of P/H-nZVI will gradually decrease with the increase of injection speed when the injection speed 1³cm/min is 1³cm/min.In the application,the small concentration of slurry injection is not conducive to the removal of contaminants.The excessive concentration of the slurry is unfavorable to the migration of P/H-nZVI,and the existence of the coexisting ions in the groundwater will have an adverse effect on the migration ability of the P/H-nZVI slurry in the quartz sand column.