Nanoscale Zerovalent Iron@mesoporous Silica Core-shell Particles To Remove 1,1,1-trichloroethane In Groundwater

Chlorinated hydrocarbons,the typical organic pollutant in soil and groundwater environments,have caused serious harm to human health and ecological security.Nanoscale zerovalent iron(NZVI)can effectively remove chlorinated hydrocarbons in soil and groundwater environments due to its strong electronegative property,large surface area and high reaction activity.However,NZVI particles tend to aggregate and form clusters due to their inherent magnetic property and high surface energy,and the aggregation will greatly reduce their dispersibility in water,transportability in soil porous media,and performance of removing chlorinated hydrocarbons.In this study,the nanoscale zerovalent iron@mesoporous silica(NZVI@mSiO2)core-shell particles were prepared by a combination of chemical reduction and improved Stober method.The effects of the reaction time after adding silicon source and the amount of silicon source added on the structure of NZVI@mSiO2 were studied.The structure,morphology,surface property,thermal stability,dispersibility in water,and suspension stability in polar and nonpolar solvents of NZVI@mSiO2 and NZVI were compared.A model soil environment was built to study and compare the transportability of NZVI@mSiO2 and NZVI in soil.A model groundwater environment was built to study and compare the removal of 1,1,1-trichloroethane(1,1,1-TCA)in solution and chlorinated hydrocarbons in groundwater samples from a pesticide factory site.The degredation pathway of 1,1,1-TCA by using NZVI@mSiO2 and NZVI as reductive reagents was studied.Using 1,1,1-TCA as a model chlorinated hydrocarbon,the effects of operation and solution chemistry conditions including clay,pH value,ion concentration,and natural organic matter(NOM)on the adsorption of chlorinated hydrocarbons by mSiO2 were studied.Using 1,1,1-TCA as a model chlorinated hydrocarbon,the effects of operation and solution chemistry conditions including pH value,NZVI@mSiO2 dosage,ion concentration,and NOM on the removal of chlorinated hydrocarbons by NZVI@mSiO2 were studied,too.The results are as follows:(1)NZVI@mSiO2 can be successfully prepared by using a combination of chemical reduction and improved Stober method.The thickness of the mSiO2 shell can be controlled by adding different amounts of TEOS,reaching about 18 run when the TEOS amount is 1000 ?L,forming NZVI@mSiO2 with a size of about 91 nm.The stable structure of NZVI@mSiO2 can be obtained after adding TEOS and stirring at 300 rpm for 4 h.mSiO2 shell reduces the aggregation of NZVI particles and keeps the sites on NZVI surface for reacting with target pollutants.The specific surface area of NZVI@mSiO2 is 71 m2/g,higher than 12 m2/g for NZVI,which can promote the adsorption and degradation of target pollutants.The higher oxidation resistance of NZVI@mSiO2 in favour of preserving NZVI@mSiO2,avoiding the fast exhausting of NZVI core resulting from the reaction with water.(2)The repulsive force between NZVI@mSiO2 particles resulting from the-12.11 mV surface Zeta potential at neutral pH,the hydration shell formed because of the hydrophilicity of the mSiO2 shell,and the Brownian movement of the smaller aggregates with sizes less than 1.3 ?m,contribute to the better dispersibility of NZVI@mSiO2 in water compared with NZVI.The higher suspension stability after settling for 1 h,the transport of small aggregats with sizes less than 1.3 ?m in soil porous media,and the repulsion between NZVI@mSiO2 and clay particles because of their negative surface Zeta potential,make NZVI@mSiO2 have higher transportability in soil compared with NZVI.NZVI@mSiO2 achieved higher degradation rates than NZVI for the dechlorination of chlorinated hydrocarbons,due to the large specific surface area,the porous network,and the concentration gradient formed inside and outside the mSiO2 shell during the reaction between chlorinated hydrocarbons and NZVI core.The dechlorination pathway of 1,1,1-TCA was studied in detail,revealing the formation of 1,1-dichloroethane.ethylene and ethane as the main products.(3)Both clay and mSiO2 have negative surface Zeta potential in suspension at neutral pH,make clay co-exist with tnSiO2 in the suspension,contributing to the negligible effect of clay on the adsorption of chlorinated hydrocarbons by mSiO2.Low initial pH value can reduce the surface Zeta potential of mSiO2,lead to the aggregation of mSiO2 particles,reduce the stability of mSiO2 suspension,and restrain the adsorption of chlorinated hydrocarbons by mSiO2.As the initial pH decreased from 9.2 to 4.0,the surface Zeta potential of mSiO2 decreased from-25.0 to-5.3 mV,the transmittance of supernatant after 1 h settling increased from 1.6%to 15.2%,the quantity of 1,1,1-TCA adsorbed by mSiO2 after 2 h decreased from 3.52 to 2.94 mg/g.Cations can decrease the surface Zeta potential of mSiO2 and make the aggregation among mSiO2 particles,reduce the stability of mSiO2 suspension,decrease the number of sites for adsorption,so as to restrain the adsorption of chlorinated hydrocarbons by mSiO2-As the concentrations of CaCl2 increased from 0 to 1 g/L,the surface Zeta potential of mSiO2 decreased from-12.11 to 2.1 mV,the transmittance of supernatant after 1 h settling increased from 3.2%to 70.4%,the quantity of 1,1,1-TCA adsorbed by mSiO2 after 2 h decreased from 3.16 to 1.91 mg/g.NOM can reduce the stability of mSiO2 suspension through bridging and trapping mSiO2 particles,while the introduced new sites of it increases the adsorption of chlorinated hydrocarbons.(4)Low initial pH can accelerate the corrosion of the NZVI core and the spalling of the passivation layer on NZVI surface,thus improve the removal of chlorinated hydrocarbons by NZVI@mSiO2.As the initial pH decreased from 9.2 to 4.0,the removal efficiency of 1,1,1-TCA by NZVI@mSiO2 increased from 61%to 91%,the pseudo-second-order rate constant increased from 0.0022 to 0.0113 L/mmol/min.Higher NZVI@mSiO2 dosage provides more active sites for the reaction,which is beneficial to the removal of chlorinated hydrocarbons.As the NZVI dosage increased from 0.5 to 4 g/L,the removal efficiency of 1,1,1-TCA increased from 46%to 99%,the pseudo-second-order rate constant increased from 0.0013 to 0.0280 L/mmol/min.Cations can reduce the surface Zeta potential of NZVI@mSiO2,reduce the stability of NZVI@mSiO2 suspension,form precipitates on the NZVI@mSiO2 surface,thus restrain the removal of chlorinated hydrocarbons by NZVI@mSiO2.As the concentration of CaCl2 increased from 0 to 1 g/L,the removal efficiency of 1,1,1-TCA by NZVI@mSiO2 decreased from 72%to 31%,the pseudo-second-order rate constant decreased from 0.0041 to 0.0007 L/mmol/min.Although NOM can reduce the stability of NZVI@mSiO2 suspension,the increased adsorption sites and the reduced pH value of the solution finally promote the removal of chlorinated hydrocarbons.The results of this study show that NZVI@mSiO2 can remove target contaminants fastly and efficiently,proving the good prospect of NZVI@mSiO2 as a reagent for environmental remediation.