Transport Mechanism And Remediation Of Surface Modified Nanoscale Zero-valent Iron In Aquifer

As the development of urbanization,disposal of various contaminants are progressively increasing,and groundwater pollution in many districts is more and more serious.In-situ reactive zone with sample facilities has little destruction of aquifer and is cost-effective.Meanwhile,it can remediate contaminants at various depths by adjusting parameters.Nanoscale zero-valent iron(NZVI),as a kind of promising environmental materials,has relatively high surface area and high reactivity,thus has potential of application in site remediation.Because of the intrinsic magnetic forces,NZVI is subject to aggregation,inducing poor stability and limited transport in aquifer,so surface modifications are adopted to facilitate the application of NZVI.In this paper,based on surface modification of NZVI,we conducted elaborate characterization of surface modification of NZVI,and then investigated three important aspects evaluating NZVI in-situ reactive zone performance: transport,reactivity and hydrochemical effect,to provide insights for practical application.Main conclusions are as follows:(1)Surface modification mediated NZVI property and thus controlling its relative environmental performance.Firstly,the substance that added into NZVI synthesis process as surface modification may control the size of NZVI.Secondly,surface modification substance is adsorbed on NZVI surface.Moreover,electrokinetic potential of NZVI was manipulated by modification substance.When adding 500 mg/L PV3 A into NZVI synthesizing process,NZVI size was drastically reduced,and zeta potential changed from positively charged value of 14.3 to negatively charged value of-26.2 mV,and an adsorbed layer with thickness of 86.2±9.2 nm was formed on NZVI surface.So we can regulate properties of NZVI to facilitate application by using various modification substances.(2)This paper established and optimized elaborate characterizations of surface modified NZVI,including size,main components,zeta potential,surface functional groups and surface layer properties.We applied Ohshima soft particle theory coupled electrokinetic potential of surface modified NZVI at various ionic strengths to calculate the adsorbed layer thickness,charged density and softness parameter.Results indicated that the adsorbed layer thickness was positively related with modification substance concentration.NZVI modified with 100,200,350 and 500 mg/L PV3 A yielded layer thickness of 40.3±3.2,46.4±5.7,79.7±5.6 and 86.0±9.2 nm,respectively.As the increase of thickness,NZVI are better dispersed with higher steric repulsive energy contribution.(3)In this paper,we investigated the mechanism of NZVI transport and deposition in aquifer.For NZVI at lower concentration,when modified NZVI slurry was pulse injected into aquifer,NZVI migrated with water as piston flow,and the transport velocity of NZVI was controlled by water velocity but with delay at some extent.NZVI was deposited on the aquifer materials during transport,and the mass of deposited NZVI exhibited a non-exponential decline.Mathematical model considering advection-diffusion-deposition fitted well with NZVI breakthrough curves.(4)Transport and deposition of surface modified NZVI were affected by water velocity,grain size,ionic strength and NZVI concentration.With increasing water velocity,fewer particles got contact with porous media,so transport of NZVI increased with higher breakthrough concentration and broaden radius of influence.As the deceasing grain size,NZVI transport was limited.Ionic strength affected NZVI transport by screening double layer,yielding zeta potential of NZVI and aquifer media to a lower value in the negative direction,or inducing aggregation,so the transport decreased with increasing ionic strength.NZVI concentration exhibited significant influence on transport,generally,the higher concentration,the lower transportability.Higher concentration was with shorter separated distance,resulting higher magnetic attractive energy and triggering NZVI aggregation,showed limited transport.(5)Equivalent diameter of NZVI in porous media was calculated in this paper by conducting column experiments to obtain breakthrough curves and coupling the elaborate characterization of modified NZVI and porous media,to qualitatively and quantitatively investigate NZVI aggregation.The measurement of nanogold as controlled particles confirmed the feasibility and accuracy of calculation.At higher concentration of 0.5 and 2 g/L,and 0.06 g/L at ionic strength of 100 mM,aggregation occurred.The aggregated size was negatively linear related with shear rate.NZVI affected transport and deposition by controlling single collector efficiency and/or triggering straining phenomenon.(6)The reactivity of NZVI to degrade contaminants was related with the property of both modified NZVI and contaminants.The surface layer affected reactivity mainly by directly influencing electron transfer,blocking reactive sites on NZVI surface and controlling contact efficiency.The concentration of PV3 A decreased NZVI reactivity by lowering the degradation rate but not influencing the pathway and efficiency.At specific conditions,the rate constant for nitrobenzene by PV3 A modified NZVI was negatively non-linear related with adsorbed layer thickness.The thicker layer,the lower rate.So the existence of surface layer decreased degradation rate,and dissolution and partition may be the key factor.(7)We proposed design scheme of NZVI in-situ reactive zone.Firstly,the property of modified NZVI and aquifer are characterized.Meanwhile,1D or 2D experiment should be conducted to obtain the breakthrough curves of NZVI.Then coupling the results with single collector efficiency and attachment efficiency,the equivalent diameter of NZVI in porous media is calculated.If the ratio of equivalent diameter and grain size exceed 0.01,straining occurs,which violate colloidal filtration theory,we conclude that this kind of modified NZVI is not suitable to be applied as in-situ reactive zone at this specific geochemical condition.On the other hand,if the ratio of equivalent diameter and grain size is lower than 0.01,we could use the advection-dispersion-deposition model to simulated transport and predict the engineering parameters such as influencing radius,time to establish zone and deposited mass on grains.(8)We used the proposed scheme to design the remediation of nitrobenzene contaminated aquifer,results showed that the injection of NZVI could effectively remediated the range of 10-15 cm downgradient injection wells,introducing significant nitrobenzene reduction,with aniline formation as degradation product,which is more easily biodegraded.Injection of NZVI produced reducing conditions with pH increase,DO drop and ORP decrease to negative values.Ferrous iron was generated in groundwater,as indicator and oxidation product of NZVI.Nitrate in groundwater was consumed and decreased by reduction of NZVI.As the consumption of NZVI by degrading nitrobenzene,the reactive zone formed,developed and decayed,the hydrochemical parameters in groundwater gradually recovered to initial values.