Study On Removal Of Nitrobenzene In Groundwater With Porous Silicon Modified Zero-valent Iron Composite

With the rapid socioeconomic development,the demands of mankind for material has seen a clear increase.The unreasonable treatment and improper disposal,even the chemical accidents in the process of industrial production and application lead to increasingly severe groundwater contamination and made a great threat to man's living,production and the safety of water use.Since N itrobenzene(N B)is widely applied in agriculture,industry,medicine and national defence,its multi-scope application has created an increasing demand.And therefore NB has been found as a common groundwater contaminant in the last few years.Owing to its carcinogenicity,teratogenicity,mutagenicity and stubborn in environment,p revention and remediation of groundwater contamination by nitrobenzene is urgently needed.The technology of nanoscale zero valent iron in-situ reactive zone(IRZ)is considered an effective and economic process for grounwater remediation.The inject ability and strong reducing capacity of nanoscale zero valent iron make it likely to build an effective IRZ,and thus to achive the purpose of groundwater remediation.At present,the nanoscale zero valent iron IRZ technology has been applied in field demonstration abroad and China's Taiwan Region,and achieved better remediation effects,which demonstrate its ability in addressing practical problems.Nevetheless,the agglomeration and oxidization of bare nanoscale zero valent iron particles limits its application in IRZ.Porous silicon,as a common environmental benign and good biological compatibility carrier material that can be sysnethied with controlled structure and morphology,can greatly improve the dispersion stability and antioxidant capacity of nanoscale zero valent iron.However,the current research on porous silicon modified nanoscale zero valent iron focuses mainly on surface water treatment,few researches focuses on in-situ grounwater remediation.The effect of hydrogeochemical constituents of the groundwater on NB reduction by porous silicon modified nanoscale zero valent iron,the mechanism and behavior of porous silicon modified nanoscale zero valent iron transport in aquifer porous media,and the ability of porous silicon modified nanoscale zero valent iron building an effective IRZ are not clear,which needs a further systematic research.This paper aimed to develop a porous silicon modified nanoscale zero valent iron composite to be suitable for being applied in IRZ,with the original design intention of anti-agglomeration,antioxidation and pro-migration.Firstly,two kinds of zero valent iron composite modified by porous silicon Fe0@MSN and Fe0@p-Si O2 were prepared using “a bottle around-a-ship method” and “one step metod” respectively.Transmission electron microscopy(TEM),X-ray diffraction(XRD),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared(FTIR),and N2 adsoption-desorption were employed to characterize the sythesized materials,and the compostions and properties of these two materials were analyzed to optimized the preparation.Secondly,the effect and influence factors of these two sythesized materials on NB reducing were investigated systematically,and then the micromechanism of N B reduction by Fe0@p-Si O2 was emphatically analyzed.Through the comparison of these two sythesized materials,the Fe0@p-Si O2 prepared by “one step synthesis” was supposed to be more suitable to be applied in the in-situ remediation of NB contaminated groundwater.Thirdly,the migration capability and behavior of the Fe0@p-Si O2 were conducted by column experiments,and the transportation mechanisms of Fe0@p-Si O2 in the porous aquifer was revealed.Finally,in the two-dimension simulation experiment,the ability of Fe0@p-Si O2 forming reacted zone,and the reparing of N B contaminated aquifer were primarily explored.The results showed that:(1)Structure,composition and properties of Fe0@MSNSmall size zero-valent iron particles can be generated in the pore channels of monodisperse mesoporus silica(MSN)in the presence of two solvents,and can be immobilized through chemical bonding of Si-OH on the MSN surface,which effectively prevents the zero-valent iron particles agglomeration and promotes the dispersibility and stability in water at the same time.The optimum loading of iron in Fe0@MSN is 4.3%,zero-valent iron particles are dispersed uniformly among the pores of MSN.Fe0@MSN is a spherical particle about 100 nm,with higher monodispersity,largerer specific surface area(743.49 m2/g)and a negative surface potential(-39.6 m V),which can effectively enhace the reduction of nanoscale zero valent iron to NB and the transportation in the aquifer porous media.(2)Researches on the influence factors of NB reduction by Fe0@MSN compositeThe reduction of NB by Fe0@MSN composite conforms to first order kinetics equation.Increasing the amount of zero valent iron,decreasing the initial NB concentration and decreasing the initial p H of reaction system could promote the reduction efficiency of NB by Fe0@MSN composite.When the p H value of reaction system is less than 5,the iron was barely leaching from Fe0@MSN composite in the process of NB reduction.n Fe2+/n NB=11:1 is the optimal ratio of NB reduction by Fe0@MSN composite,at which,the generation rate of anline was 95.57%,the maximum reduction capacity for converting NB to aniline(AN)was approximately 191.14 mg/g,which was 1.78 times higher than that of unmodified nanoscale zero valent iron.Na+,K+,Cl-,NO3-and SO42-presented no obvious effect on NB reduction,Ca2+,Mg2+ and HCO3-showed promoted inhibitory effect on NB reduction with the order of HCO3->Mg2+>Ca2+.(3)Preparation and properties of Fe0@p-Si O2 compositesThe hydrolysis and condensation of tetraethyl orthosilicate(TEOS)under base catalysis can effectively coat the formed zero valent iron particles,the obtained Fe0@p-Si O2 is a core-shell type composite with zero valent iron as the core and porous silicon as the shell.The synthesized composite has disordered mesoporous pore structure and large specific surface area.The coordinate bridging behavior between Si-OH and zero valent iron particles is the main mechanism of zero valent iron stabilization by Fe0@p-Si O2 composite.The coating layer(Si O2 shell)not only effectively prevents the aggregation and oxidation of the zero valent iron particles,but improves its dispersion stability in water.The optimum molar ratio of Fe0@p-Si O2 composite preparation is n Fe3+:n TEOS:n Na OH=1:1.85:1.19,which can effectively enhance the reduction activity of zero valent iron on N B and the transport ability in saturated porous media.(4)Micromechanism and influence factors of NB reduction by Fe0@p-Si O2The micro-mechanisms of N B reduction by composite materials Fe0@p-Si O2 can be descripted as followed: N B gets through the pores of a porous silicon coating by adsorption and diffusion,and involves in the reduction reaction on the surface of the zero valent iron,by using zero valent iron as the elctron donor.The reductive pathways of NB are direct reactions and coupled reaction.Intermediate products mainly include nitrosobenzene and azobenzene oxide.The reduction efficiency of NB by Fe0@p-Si O2 composite is proportional to the amount of zero valent iron.Considering the reduction efficiency,the optimum additional ratio of Fe0@p-Si O2 to NB was 6:1(n Fe/n NB).NB can completely reduced to AN within 30 min under optimum conditions and the reductive capability of Fe0@p-Si O2 was 366.3 mg g-1 which was 3.41 times higher than unmodified nanoscale zero valent iron.A higher initial concentration of NB accelerated the reduction of NB to nitrosobenzene(reaction I),however,it was no significant effect on subsequent reduction reactions(the reduction of nitrosobenzene to hydroxy aniline and then to AN).There were adverse,negligible and advantageous influences on the reduction of N B by Fe0@p-Si O2 at initial p H=11,7-9 and under acidic condition,respectively.The dissolution o f iron ions to aqueous phase could be effectively prevented by the porous silicon shell(iron ions did not dissolve to aqueous phase at p H>5),which decreased the biotoxicity of Fe0@p-Si O2 in application to contaminated groundwater remediation.Na+,K+,Cl-,NO3-and SO42-had almost no effect on the reduction of N B by Fe0@p-Si O2,while Ca2+ and Mg2+ had inhibiting effect.Moreover,HCO3-facilitated the direct reduction of NB to AN and the coupled reactions to a certain degree as well.Compared to Fe0@MSN,Fe0@p-Si O2 had a simpler preparation technics,a higher production,a better ability of reducing NB,and a stronger transport ability in saturated porous media.Therefore,the application of Fe0@p-Si O2 as the remediant was more advantageous to in-situ remediation of NB contaminated groundwater.(5)Researches on the transportation of Fe0@p-Si O2 in saturated porous mediaPorous silicon coating technology is a very effective nanoscale zero valent iron modification method which can effectively enhance the transport ability of nanoscale zero valent iron.The injection velocity,zero valent iron concentration and ionic strength show greater impact on the transportation of Fe0@p-Si O2.The results of one-dimension column experiments indicated that there are two kinds of transport behavior,i.e.,“agglomeration-straining” and “desorption-re-migration” in the process of particle transportation,and the convection and difusion under hydrodynamic force dominated on the transportation of Fe0@p-Si O2 in saturated porous media.In the practical engineering application,proper injection velocity and concentration should be selected depending on the pollution condition of groundwater and geochemical background,to ensure the establishment of effective IRZ.(6)Researches on the construction of Fe0@p-Si O2 composite IRZ in two-dimensional simulated tank and its remediation effect for NB contaminated aquiferFe0@p-Si O2 composite can be successfully used in the construction of IRZ in the two-dimensional simulated tank.The construction of IRZ by Fe0@p-Si O2 composite is affected by the particle size of the aquifer medium and the injection condition.The area,concentration and uniformity of IRZ can be adjusted by changing the injection condition.Fe0@p-Si O2 composite can be used as the effective remediant for NB contaminated groundwater after being injected into the aquifer.In the initial stage of injection(about 24 hours),the contact between injected composite and NB,which resulted in the rapid partial oxidation of Fe0@p-Si O2 composite,leading the increased transportability and wider remediation range.As the time gose on,the scope of remediation decreased,the remediation area was mainly concentrated in the vicinity of the injection well and its downstream parts.During the runing of IRZ,the p H value of the aquifer rapidly increased,DO decreased significantly,and ORP rapidly changed from positive to negative.With the deactivation of IRZ,the hydrochemical environment of aquifer can gradually return to the initial level.Above all,this study establishes the theoretical foundation for the application of porous modified nanoscale zero valent iron in the in-situ remediaion of NB contaminated groundwater,and provides a new option for nanoscale zero valent iron IRZ,which has important guiding significance for practical engineering application.