Aging Study On Carboxymethyl Cellulose-coated Zero-valent Iron Nanoparticles In Aqueous Environment

More and more groundwater resources were polluted in recent years.Nanoscale iron particles(nZVI)is effective for treatment of heavy metal ions,chlorinated organic compounds and inorganic compounds in groundwater,primarily due to its small size,large surface areas and high reactivity.Moreover,the nZVI has strong mobility in porous media,so nZVI can be directly injected in situ remediation,and then widely applied to environmental remediation.Although nZVI degrades the pollutants in the environment quickly,however,the basic scientific problems such as the long-term effectiveness of the nanometer zero-valent iron in the environment and the risks of environment and health are still to be solved.nZVI in the soil and groundwater environment will occur in physical and chemical conversion,thereby changing its reactivity,migration and fate.Therefore,it is of scientific significance and application value to study the behavior of nZVI in the environment,that is,physical and chemical conversion.A great deal of research has been done on the corrosion of nano zero valent iron,but the corrosion of modified nZVI has not been paid much attention.To overcome the above drawback,nano-zero-valent iron(nZVI)and carboxymethyl cellulose-coated nano-zero valent iron(CMC-nZVI)suspensions were prepared and subjected to corrosion aging for 0-90 days in Three different kinds of water environment(oxygen containing deionized water,simulated groundwater,humic acid solution).Fourier transform infrared spectroscopy(FTIR),scanning electron microscopy(SEM),X-ray diffraction(XRD)and Raman spectroscopy were used to characterize the corrosion products.The main contents of the paper are divided into the following three parts:The experiments were carried out in deionized water in an open vial,which represents the static aquatic media exposed to the air.The results indicated that both the structures and the compositions of the corrosion products change with the time of aging,but the coating of CMC could slow down the aging rate of nZVI.For the bare nZVI,magnetite(Fe3O4)and/or maghemite(y-Fe2O3)are the dominant corrosion products after 90 days of aging.However,for the CMC-nZVI,the core-shell spheres collapses to acicular-shapedstructures after aging with crystalline lepidocrocite(γ-FeOOH)as the primary end product.Moreover,more lepidocrocite present in the corrosion products of CMC-nZVI with higher loading of CMC,which reveals that the CMC coating could influence the transformation of iron oxides.The experiments were collected after 0-90 days of aging in DI water/simulated groundwater in sealed reactors placed in dark room.The results indicated that both structure and chemical composition of the aging products of nZVI/CMC-nZVI transformed over time.The SEM images show that the original spherical particles transformed into the sheet-like and needle-like materials for the aged nZVI,but formed bulk aggregates with some flaky-shaped structures for the aged CMC-nZVI.Compare with the aging products of nZVI and CMC-nZVI(i.e.,magnetite/lepidocrocite)in the deionized water under anaerobic conditions,the formed secondary products in the simulated groundwater were complicated.The composition analysis confirm that magnetite,lepidocrocite,feroxyhyte and iron carbonate hydroxide hydrate/iron oxide hydroxide chloride were the main corrosion products for both bare nZVI and CMC-nZVI,except that the amount of each secondary mineral varied for different particles.Besides,small amount of calcium carbonate was also identified in the aging products of nZVI.This indicates that the groundwater ions could be involved in the formation of secondary minerals during nZVI corrosion.The experiments were collected after 0-90 days of aging in humic acid solution in sealed reactors placed in dark room.The results indicated that both the structures and the compositions of corrosion products changed during the time of aging,but the coating of CMC could slow down the aging rate of nZVI in the presence of HA solution.Compared to nZVI in the presence of HA,the adsorption of HA on the surface of CMC-modified nZVI via complexation with nZVI reduced aggregation of the particles by enhanced the electrosteric repulsion effect.The magnetite and/or maghemite are the dominant corrosion products for bare nZVI and CMC-nZVI in both HA solution.The loss of Fe(0)from nZVI and an increase in the maghemite to magnetite ratio in nZVI,while the corrosion products has not found the maghemite for CMC-nZVI in the presence of HA from Od to 90d,which reveals that the CMC coating could influence the transformation of iron oxides in HA solution.Furthermore,the presence of HA does not appear to inhibit the oxidation of nZVI and CMC-nZVI in the long-term.