Study On Enhanced Remediation Of Nitrobenzene-contaminated Heterogeneous Aquifer With XG/VO-ZVI

With the rapid economic development,groundwater pollution problem has become increasingly prominent.In particular,nitrobenzene spill has occurred frequently for many years,which led to significant economic losses and polluted the surrounding environment.Nitrobenzene?NB?is listed as one of"priority pollutants"because of its bioaccumulative and has three effects.It needs harder conditions to degraded nitrobenzene by microorganisms,due to the strong electron affinity,stable structure of nitrobenzene.The remediation of NB-contaminated groundwater has become one of the hot issues in recent years.The emulsified oil modified zero-valent iron?EZVI?in-situ reactive zone technology has been widely used because of its high-efficiency and economical characteristics.However,heterogeneity is often encountered in underground aquifers.Contaminants migrating from source zones will diffuse from transmissive regions into low permeable zones.The residual buildup of contaminant in low permeable zones over tine becomes particularly challenging for in-situ remediation because remedial amendments leaves the less permeable areas untouched?bypassed?.Such a heterogeneity-induced bypassing phenomenon inhibits the success of remedial operation greatly.Moreover,stored contaminant can serve as a long-term source and eventually diffuse back into the surrounding higher permeability flow zones under concentration difference via matrix diffusion.The adverse effect can significantly increase the cost or prolong remediation operations.In this paper,a stable,anti-oxidative xanthan gum/vegetable oil modified zero-valent iron?XG/VO-ZVI?suspension with shear thinning properties was prepared.The effects of concentration of XG and various groundwater components on the degradation of NB were studied.Tank experiment aimed to demonstrate the feasibility of XG/VO-ZVI using in remediation of NB-contaminated heterogeneous aquifer.It is greatly significant for practical engineering applications.The main conclusions are as follows:?1?NZVI was prepared using a liquid phase reduction method in the anaerobic environment under laboratory conditions,and then was modified with emulsified vegetable oil and a mixture of emulsified vegetable oil and xanthan gum,respectively.The results show that the XG/VO-ZVI greatly reduced the agglomeration and sedimentation of NZVI,and hardly any settlement in 60 min was seen.The zeta potential of XG/VO-ZVI was negative,implying particles could have good migration in aquifer media.?2?Compared with XG-VO/ZVI and VO-XG/ZVI,XG/VO-ZVI had better removal efficiency on NB,and the reaction quickly reached equilibrium.When the concentration of XG was less than 100 mg L^-1,the degradation of NB by XG/VO-ZVI was similar to that of EZVI.However,when the concentration of XG was greater than100 mg L^-1,the degradation efficiency of NB was reduced.The greater the XG concentration,the smaller the aniline?AN?production,the smaller the change in the concentration of azoxybenzene?AZOX?,and the greater azobenzene?AZO?production.?3?Cl^-could promote the regeneration of active sites of NZVI and was beneficial for the degradation of NB by XG/VO-ZVI.HCO₃^-promoted iron corrosion and adhered to the surface of iron particles,hindering the degradation of NB by XG/VO-ZVI.Precipitation of Ca²⁺/Mg²⁺adhered on the surface of NZVI particles,suppressing the reduction reaction.However,HCO₃^-and Ca²⁺/Mg²⁺only affected the active sites of iron and had no inhibition on the electron transfer between NB and its degradation intermediates,so they had no effect on formation of AZO and AZOX.The initial Fe²⁺participated in the reaction,which could promote the electron transfer between the solid and liquid phases and accelerate the reduction reaction.The total content of AZO and AZOX,which are by-products of the degradation,decreased with increasing Fe²⁺concentration.?4?The XG/VO mixture had the shear thinning property that the solution viscosity decreases with the increase of the shear stress.Theoretically,it can be used as a carrier of NZVI to promote the migration of iron particles in the heterogeneous media.Compared with EZVI,XG/VO-ZVI had better mobility had higher sweep efficiency and more uniform distribution in low-permeability zones.Correspondingly,a large number of degradation products AN were detected.Therefore,XG/VO-ZVI can enhance remediation of NB-contaminated heterogeneous aquifers.