| Nanoscale zero-valent iron(nZVI)has strong reductive ability and good adsorption performance,showed high removal efficiency to organic and inorganic contaminants.nZVI particles are easy to aggregate due to high surface energy and magnetic force,and is prone to be passivated due to the side reaction with environmental media and formation of iron(hydr)oxides outside the FeG core,which significantly reduce the reactivity.Its aggregation and passivation limit its application.Sulfidation of nZVI has attracted increasing interest for improving the reactivity,preventing surface passivation and prolonging the longevity of nZVI towards various contaminants.In this study,sulfidated nanoscale zero-valent iron(S-nZVI)was synthesized through a one-step method that sodium dithionite was used as sulfidation reagents during the nZVI synthesis.S-nZVI has higher specific surface area and better dispersibility,providing more active site for pollutant removal.In this study,S-nZVI was used to remove heavy metals from aqueous solution.Two typical heavy metals,Cr(Ⅵ)in the form of oxyanions and Cd(Ⅱ)in the form of metal cation were chosen as the target pollutants.This study revealed that sulfide modification promoted Cr(Ⅵ)removal by nZVI.Cr(Ⅵ)could be removed by S-nZVI via fast adsorption and reduction,and the removal rate and capacity of Cr(Ⅵ)were mainly governed by the specific surface area,electron transfer ability,surface-bounded Fe(Ⅱ)species,and surface charges.The specific surface of S-nZVI with low(0.1 S/Fe)and high(0.5 S/Fe)sulfide content was about 2.4-fold lower and 1.9-fold higher than unmodified nZVI,respectively,while the Cr(Ⅵ)removal efficiency by S-nZVI was 0.8-fold lower and 2.6-fold higher that that by unmodified nZVI,respectively.High specific surface area would provide more active site for Cr(Ⅵ)removal,and as an efficient electron conductor,flake-like FeSx phase would also favor electron transfer from Fe0 core to Cr(Ⅵ).FeSx directly participated in the reduction of Cr(Ⅵ)due to its reducibility.S-nZVI maintained high Cr(Ⅵ)removal efficiency and low Fe dissolution after aging for 12 days.S-nZVI possesses more excellent longevity and stability in the removal of Cr(Ⅵ).Other influence factors were also investigated for potential application,including intial pH,temperature,initial Cr(Ⅵ)concentration,ionic strength,and co-existed ions.The removal mechanism of Cr(Ⅵ)by S-nZVI involved sulfide modification to increase the specific surface area and provide more active sites,the corrosion of Fe0 to produce surface-bounded Fe(Ⅱ)species to adsorb Cr(Ⅵ)species,followed by the favored reduction of Cr(Ⅵ)to Cr(Ⅲ)due to the electron transfer ability of FeSx,then the formation of Cr(Ⅲ)/Fe(Ⅲ)hydroxides precipitates.Sulfide modification effectively enhanced nZVI reactivity for Cd(Ⅱ)removal,since sulfide modification prevented the nZVI particles from aggregation and partial Cd(Ⅱ)precipitated as CdS.Mechanism analysis indicated that Cd(Ⅱ)was immobilized through complexation and precipitation.Cd(Ⅱ)removal rate on nZVI was only 32%in 2 h,while complete immobilization could be achieved in 15 min on S-nZVI,and S-nZVI with an optimal S/Fe molar ratio of 0.3 offered a cadmium removal capacity of about 150 mg g-1 at pH 7 and 303 K.The process of Cd(Ⅱ)immobilization on S-nZVI was fitted well with pseudo-second-order kinetic model,and the increase of temperature favored Cd(Ⅱ)immobilization,suggesting an endothermic process.The presence of Mg2+ and Ca2+ hindered Cd(Ⅱ)removal while Cu2+ did the opposite,which led to the order as Cu2+>control>Mg2+>Ca2+.The removal rate of Cd(Ⅱ)maintained a high level(>85%)with the fluctuation of environmental conditions such as pH,ion strength and presence of HA.This study demonstrated that S-nZVI could be a promising adsorbent for Cd(Ⅱ)immobilization from cadmium-contaminated water. |
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