Preparation Of Nanoscale Zero-Valent Iron By High Gravity Method And Their Application For Heavy Metal Lead And Chromium Removal

As a common material for water treatment,nanoscale zero-valent iron?nZVI?can effectively deal with heavy metal ions containing wastewater.However,the main problems during the preparation and application of nZVI are as follows:?1?easy to be oxidized,difficult to preserve and burn spontaneity after drying because the high reactivity of nZVI;?2?difficult to continuously and largely prepare in a batch reactor,and the as-prepared particles often have large particle size with obvious agglomeration;?3?coexisting ions exhibited different promotion or inhibition effect on the removal efficiency of heavy metal ions due to the complex components of practical heavy metal wastewater.In order to solve the shortcoming of oxidative deactivation,the as-prepared nZVI were directly preserved in liquid phase instead of drying method,and the reactivity of nZVI under different preservation methods was investigated by using the Pb???removal efficiency as an evaluation index.Then the obtained nZVI were characterized by SEM,TEM,XRD and XPS,the SEM and TEM results showed that the nZVI particle was a spherical chain-like agglomerate with a particle size of about 55 nm,the XRD and XPS results suggested the obtained nZVI contains Fe0,Fe²⁺ and Fe³⁺ components.The study of Pb???removal performance showed that the optimum pH was 6.0.Under this condition,both undried nZVI and dried nZVI showed an monolayer adsorption for Pb???.Compared with dried nZVI,the maximum adsorption capacity of undried nZVI was increased by about 1.20 times which was 807.23 mg·g^-1.The kinetic study results showed that the removal efficiency of Pb???by both undried nZVI and dried nZVI were followed pseudo-second-order kinetics model,and the adsorption rate of undried nZVI was faster than that of dried nZVI,indicating that the liquid phase preservation could decrease thedegree of oxidation of nZVI and maintain high reactivity.On the basis of liquid phase preservation,nZVI particles were prepared by high gravity method with simultaneously modified with carboxymethyl cellulose?CMC?.The yield of CMC-nZVI prepared by the high gravity method under laboratory conditions was calculated to be 336 gh^-1.The TEM and SEM results showed that the size of CMC-nZVI particles with 15 nm was smaller than that of unmodified nZVI,the particle distribution was uniform and the agglomeration phenomenon was not obvious.The XRD results showed that the main component of CMC-nZVI was Fe0,and,the FTIR,TG and DSC results indicated the CMC component existed on the surface of CMC-nZVI particles and the content was about 1.61%.The study of Pb???removal performance showed that the optimum pH was 6.0.Under this condition,unmodified nZVI and CMC-nZVI showed an monolayer adsorption for Pb???,Compared with unmodified nZVI,the maximum adsorption capacity of CMC-nZVI was increased by about 1.48 times which was 1237.32 mg·g^-1The kinetic study results showed that the removal of Pb???by both unmodified nZVI and CMC-nZVI followed pseudo-second-order kinetics model,and the adsorption rate of CMC-nZVI was faster than that of unmodified nZVI.The removal mechanism was investigated by characterizing the CMC-nZVI particle after adsorption,the presence of Pb²⁺ and Pb0 on the surface of CMC-nZVI after adsorption indicated that the removal mechanism involved adsorption and reduction interaction.The study of Cr???removal performance indicated that the adsorption of Cr???by CMC-nZVI was monolayer adsorption,and the maximum adsorption capacity was 170.14 mg·g-1 under optimal condition.The kinetic study results showed that the removal efficiency of Cr???by CMC-nZVI followed pseudo-second-order kinetics model.However,the XPS results of Cr???removal showed that Cr???only existed on the surface of CMC-nZVI after adsorption,indicating that the removal mechanism was mainly the reduction process.Finally,the effects of coexisting ions on removal efficiency of Pb???and Cr???were also investigated.As a result,Na⁺ had no significant effect on the removal of Pb???and Cr???.Cu²⁺ and Ni²⁺had a certain inhibited effect on the removal of Pb???,the inhibitory effects followedthe sequence of Cu²⁺>Ni²⁺.However,both Cu²⁺ and Ni²⁺ exerted promoted influence on the removal efficiency of Cr???.The Cd²⁺ had a suppressed influence at low concentration levels and then posed promoted influence at high concentration levels for the removal efficiency of Pb???,while Cd²⁺ posed promoted influence on the removal efficiency of Cr???.The NO₃^-had no significant effect and Cl-had a promoted effect on Cr???removal efficiency.The presence of SO₄^2--exhibited gradually obvious promoted effect on Cr???removal with the increase of SO₄^2--concentration.