Study On The Synthesis Of Micro/Nano-sized Iron Materials And The Removal Of Heavy Metal Pollution In Groundwater

In recent years,the synthesis and application of micro/nano-sized iron materials is the focus and the advanced subject in the field of environmental remediation.In this paper,pyrite?FeS₂?and magnetite?Fe₃O₄?were synthesized and tested for immobilization of Cr?VI?,As?III?/As?V?,and Re?VII??a non-radioactive surrogate of Tc?VII??.The chemical compositions and crystallographic structures of the prepared micro/nano-sized materials were analyzed using various analytical instruments.The synthesis reaction mechanism,influence of dispersant and electrochemical properties were discussed.The removal efficiency of heavy metal ions by synthetic materials was tested by batch and column experiments.The effects of different factors on the removal of heavy metals by synthetic materials were investigated.The experimental results were analyzed by chemical kinetics,thermodynamics,surface analysis and computer simulations.The main findings are summarized as follows:1)Na Ac acted as a pH buffer as well as a dispersant in the synthesized process of pyrite particles.The pyrite particles had larger specific surface area,smaller particle size and more active surface sites.At the Na Ac/Fe molar ratio 1.2,mean hydrodynamic size of pyrite particles was 1.6±0.2?m,and the SSA was 7.42 m²/g.2)The reduction and immobilization of Cr?VI?by pyrite followed the pseudo-second order kinetic model.Physical adsorption and chemical reduction are both effective ways to remove chromium by pyrite.The pyrite particles can effectively immobilize Cr?VI?over a broad pH range from 5.0-9.0.The most effective Cr removal was observed at pH 6.0,where equilibrium was reached after 1 h and>99%of Cr was rapidly removed.3)The pyrite particles can effectively immobilize Cr?VI?in model Chinese loess soil.The distribution coefficient?K_d?values of the pyrite?0.48 g/L as Fe?amended soil were 306and 21 times higher than those for the untreated soil under pH?6.0 and?9.0.Installation of a 3-cm reactive layer of FeS₂in a soil column was able to capture the leachable Cr?VI?from the soil,and the retardation factor?R_d?for the 3-cm FeS₂layer sample was 381 times higher than that for the plain soil.The synthetic pyrite particles have the potential to be used as PRB reactive materials.4)Re?VII?reduction rate increased with increasing SSA of the pyrite particles.More acidic pH progressively accelerated the Re?VII?-Re?IV?reduction.The major reaction product was Re O₂.S₂^2-and Fe²⁺both react as reducing agents,and S₂^2-may be the major reducing agent.5)Under anoxic conditions,the desorption/reoxidation of Re?IV?was not obvious.The immobilized Re was vulnerable to oxygen oxidation,and about 78%of Re was remobilized after 40 days of exposure to air regardless of the initial pH.The presence of HA,EDTA and Chinese loess soil retarded/inhibited the reoxidation of Re under oxic condition.Structural Re?IV?species can effectively prevent the Re reoxidation process.6)The pyrite particles can effectively immobilize As?V?over a broad pH range of 3.0?8.0.Under neutral and acidic conditions,As₂O₃and As₂S₃may be the main solid products.As the pH goes up,As?III?products mainly existed as As?OH?₂^-and As O₂OH₂^-,etc.7)Starch effectively prevents Fe₃O₄particles from aggregation through steric stabilization mechanisms.The reactions between As?III?/As?V?and Fe₃O₄nanoparticles are mainly surface chemisorption.On the surface of Fe₃O₄nanoparticles,arsenic mainly exists as the As-O/Fe-O-As species.Fe₂O₃-like products promote the adsorption of arsenic on the surface.The Fe₃O₄nanoparticles can effectively immobilize As?III?and As?V?under different pH values.The existence form of As?III?and As?V?determines the adsorption amount on the Fe₃O₄surface.8)Fe₃O₄nanoparticles can effectively remove As in underground water of Shanyin.At the As/Fe₃O₄molar ratio 0.013,the total As removal rate of shanyin groundwater could reach94.67%.