Study On The Removal Of Hexavalent Chromium And Trichloroethylene In Groundwater By Nano Zero-valent Iron

Nanoscale zero-valent iron(nZVI)particles have been widely applied in the remediation of contaminated soils and groundwater over the past two decades,primarily due to its small size,large specific surface and high degrees of reactivity.In this paper,the research on the nano zero-valent iron technology can be divided into two parts:The one study investigated the correlation between the colloidal stability and reactivity(toward Cr(?))of surface-modified nano zero-valent iron(SM-nZVI)as affected by the surface coating(i.e.,PA A and starch)under various geochemical conditions.The other one study investigated the degradation of trichloroethene(TCE)by nanoscale zero-valent iron(nZVI)w/o persulfate(PS)in the absence and presence of ethylenediaminetetraacetic acid(EDTA).The first part of the study found that:Generally,the colloidal stability of nZVI was enhanced with increasing loading of surface coating,while there is an optimum loading of surface coating for the most efficient Cr(?)removal by SM-nZVI.At lower loadings than the optimum loading,the surface coating could enhance the particle stabilization,facilitating the Cr(?)reduction by providing more available surface sites.However,the over-loaded surface coating on the surface of nZVI particles could not only occupy the reactive sites but also inhibit the mass transfer of Cr(?)ions from water to the particle surface by providing the electrostatic or steric repulsion.The presence of Ca2+ exerted no obvious influence on the colloidal stability of PAA-nZVI,but enhanced the Cr(?)removal,which should be due to the reduced electrostatic repulsion between PAA-nZVI and the negatively charged Cr(VI)ions.The presence of Ca2+ decreased the Cr(?)removal by starch-nZVI,which should be due to the higher degree of particle settling caused by the presence of Ca2+ Although the presence of HA increased the colloidal stability of P-nZVI,the Cr(?)removal by P-nZVI was decreased with increasing concentration of HA due to the occupation of the reactive sites and the provided electrosteric repulsion effect.In the case of starch-nZVI,the Cr(?)removal was decreased with increasing concentration of HA.The second part of the main findings:In the absence of EDTA,the degradation of TCE obeyed the pseudo-first-order kinetics,and the rate constants increased exponentially with increasing molar ratio of PS and nZVI.Ethene was found to be the main reaction products in either the single nZVI system or nZVI/PS system.The presence of EDTA increased TCE degradation by nZVI in a short reaction period(within 360 min),whereas the TCE degradation was almost ceased after 1 day in a long-term(6 days)experiment,resulting in a lower TCE degradation than that in the absence of EDTA.However,EDTA drastically decreased TCE degradation in nZVI/PS system.The positive effect of EDTA in the nZVI system should be attributed to its possible inhibition of the precipitation of Fe2+/Fe3+5 which could reduce the surface passivation of nZVI.On the other hand,the adverse effect should be due to the rapid corrosion of nZVI by EDTA and the generation of a large amount of Fe2+ ions in short time,which could possibly lead to a rapid consumption of sulfate radicals originally for the degradation of TCE.