Study On Modification Of Nano-zero Valent Iron Supported By Porous Materials For Hexavalent Chromium Removal From Water

With the development of modern industry,since the wastewater from factories has not been effectively controlled,the discharge of Cr(?)wastewater has caused serious pollution of surface water,groundwater and soil.Meanwhile,Cr(?)can cause serious harm to human skin,ears,nose,respiratory tract and gastrointestinal tract through bio-chain accumulation,acute or short-term exposure.Therefore,solving the pollution problem of Cr(?)wastewater has become a focus in the field of water treatment.Currently,nZVI can be used for in situ remediation of Cr(?)wastewater due to its high reducibility,high specific surface area,good transport and migration capabilities.nZVI can adsorb Cr(?)and reduce it to Cr(III),and then precipitate Cr(III)ions to reduce the toxicity and mobility of Cr(?).However,the easy agglomeration and oxidation properties of nZVI limit its practical application.Therefore,in this study,aiming at the characteristics of nZVI being easy to agglomerate and easy to oxidize,nZVI was modified to improve its dispersibility and stability,and to provide a theoretical basis for the removal of Cr(?)wastewater.In this work,we synthesized biochar-supported iron-nickel bimetallic(BC@nZVI/Ni),resin-supported sulfided nano-zero-valent iron(D201@S-nZVI)and resin-supported carboxymethylcellulose-stabilized nano-zero-valent iron(CMC-D201@nZVI)composites were used to remove Cr(?)simulated wastewater,to explore their Cr(?)removal performance,and to investigate the adsorption mechanism and adsorption behavior of the adsorbent.The specific research contents and results are as follows:(1)BC@nZVI/Ni composites were prepared by the liquid-phase reduction method and used to remove Cr(?)simulated wastewater.BC@nZVI/Ni had a good removal effect in a wide p H range(2.11-9.22).The removal efficiency for 20 mg·L^-1 Cr(?)reached 100%at p H?3.08.The maximum adsorption capacity was 55.52 mg·g^-1 at 298 K.Its excellent removal performance of BC@nZVI/Ni is mainly attributed to the abundant surface functional groups of BC and its prevention effect on nZVI agglomeration,and the reduction effect of Ni(0)on Fe(II).The adsorption process fitted well with Freundlich model and pseudo-second-order kinetic model.It was a spontaneous endothermic process.The main mechanisms of Cr(?)removal by BC@nZVI/Ni are electrostatic attraction,ligand exchange,surface complexation,reduction and co-precipitation.(2)D201@S-nZVI composites were prepared by the liquid-phase reduction method and used to remove Cr(?)simulated wastewater.D201@S-nZVI obtained higher Cr(?)removal efficiency than nZVI,S-nZVI,D201 and D201@nZVI.Its removal efficiency and maximum adsorption capacity solution reached 89.67%and 137.35 mg·g^-1 for 100 mg·L^-1Cr(?),respectively.D201@S-nZVI has excellent removal performance,recycling performance and longevity.This is because sulfide-modified nZVI forms multi-component nanoparticles with nZVI as the core and Fe S_x as the shell.The Fe S_x layer prevents the leaching of iron,increases the surface roughness of the nZVI particles,and weakens the magnetic attraction between the nZVI particles,and has excellent electron transport and electron donating capability.In addition,the functional groups of D201 can electrostatically attract Cr(?),and its unique skeleton structure well disperses S-nZVI nanoparticles.The adsorption process is the single-layer adsorption controlled by chemical adsorption.It is also a spontaneous exothermic process.(3)CMC-D201@nZVI composites were prepared by the liquid-phase reduction method and used to remove Cr(?)simulated wastewater.CMC-nZVI with nZVI as the core and CMC as the shell loaded on the surface of D201 possesses excellent stability,longevity and regeneration performance.This is because the special framework structure of D201 weakens the agglomeration of nZVI,and the positively charged quaternary amine groups on its surface can electrostatically adsorb Cr(?);nZVI enhances its stability through the combination of bidentate bridging with CMC;The CMC layer further improves the distribution and mobility of nZVI by electrostatic repulsion and steric hindrance,and simultaneously can complex the products(Fe(III),Cr(III),Cr_xFe_1-xOOH or Cr_xFe_1-x(OH)₃)in the reaction system to promote the adsorption process.Under the conditions of p H 4.90,adsorption dose 50 mg,temperature 298 K,and shaker speed 150 rpm,the maximum adsorption capacity of 0.05 CMC-D201@nZVI is 169.24 mg·g^-1.Its adsorption behavior is spontaneous single-layer chemical adsorption.(4)In this paper,the Cr(?)removal performance was investigated by three different nZVI-based composites,BC@nZVI/Ni,D201@S-nZVI and CMC-D201@nZVI,and their maximum adsorption capacities were 55.52,137.35 and 169 mg·g^-1,where CMC-D201@nZVI has a higher maximum adsorption capacity than most nZVI-based composites.