Study On Enhanced Remediation Of NAPLs Contaminated Vadose Zone With Surfactant Foams

Non-aqueous phase liquids (NAPLs) are typical organic pollutants inunderground environment. With development of industrial, NAPLs are widely used inagriculture, fuel and medicine as important industrial materials. Due to largeproduction and widely used, it could lead to leaks and emissions occur during theproduction and transport, therefore resulting in poured into the undergroundenvironment and causing serious contamination of soil and groundwater. Therefore, itis significant to search for effective remediation technologies of soil contaminated byNAPLs and study the remediation mechanisms, factors and restorative effects.At present, there are many remediation technologies for organic contaminationof soil, which can be divided into two main types of in-situ and ex-situ remediationtechnology. Compare with the latter, in-situ remediation technology can achieve theremediation underground, without excavation of contaminated soil. In-situremediation technology has features of directing and cost-effective, so it was widelyused in last few years. At present, in-situ remediation technologies used in NAPLscontaminated soil include soil vapor extraction technology, in-situ bioremediation,in-situ chemical flushing technology and in-situ chemical oxidation-reductiontechnology. In-situ chemical flushing technology, especially in-situ surfactant solutionflushing technology is one of the commonly used methods to repair soil contaminatedby NAPLs. The technology was favored by domestic and foreign scholars for thereasons of high pollutant removal efficiency, low cost and simple operation. In-situchemical oxidation-reduction technology was mainly relies on injecting reaction drugto occur redox reaction with NAPLs in soil to achieve soil restoration. In recent years,nano zero valent iron (nZVI) slurry was injected to repair NAPLs pollutants in siol. However, the practice has proved that traditional surfactant flushing technology waseasily affected by the low permeability and non-uniformity of porous media. Theliquid eluent is easy to produce channel effect and it is difficult to cover the entirecontaminated area, therefore resulting in inefficient elution. At the same time, theeluent solution and drug are largely influenced by gravity, leading to more difficult tocontrol the transport of them. Therefore, the eluent solution is easy to enter the aquiferand lead to expanding of the contaminated area. In response to these problems,foreign scholars put forwarded surfactant foam technology to enhance in-situremediation. The core idea of this technique is that using gas-liquid mixing systemfoam as a flushing agent instead of traditional surfactant solution to flushing thecontaminated soil or using foam as a delivery vehicle to transport nZVI particles tothe target nZVI contaminated area. Foam is a multi-phase dispersion with goodliquidity, so it is more evenly distributed in the medium. At the same time, the foamflow in porous media is controlled mainly by the injection pressure instead of gravity,thus avoiding the expanding of contaminated area. Therefore, the characteristics andadvantages of in situ foam flushing technology determine it having a good prospect insoil remediation polluted by NAPLs. The paper intends to develop foam as an eluentto replace the traditional liquid eluent to strengthen the remediation of NAPLscontaminated soil. In addition, this paper also discusses the feasibility and impactfactors of foam as a delivery vehicle to transport nZVI particles in porous media.This paper chooses the typical NAPLs pollutant nitrobenzene as the targetpollutant. On the basis of detailed analysis of foaming ability, foam stability,solubilization of pollutant, desorption of contaminants in soil, adsorption in soil ofvarious types of surfactants, it was identified the type of surfactant suitable for soilflushing. Then, the delivery characterastics of foam flow in homogeneous andheterogeneous were analyzed in this work. Besides, the spatial distribution of pressureduring foam migration process and the main factors affecting foam injection pressurewere explored in this study. This Study compared the restoration efficiency ofdifferent types of eluent (water, surfactant solution, foam). On this basis, the foam injection rate, foam quality, and medium size on soil remediation efficiency andpollutant removal mechanism were analyzed in this work. In addition, the paper alsoexamines the feasibility of foam as a delivery vehicle to transport nZVI particles inporous media. The impact of surfactant concentration and medium particle size on thedelivery of nZVI particles carried by foam were carried out in this paper. Finally,foam injection and extraction methods were studied, providing theoretical support forthe application of this technology in actual site. The concrete research contents andresults are as follows:（1）Research on surfactant selection and affecting factors on foam stability①Solubilization capacity for nitrobenzene, desorption capacity for nitrobenzenecontaminated soil, adsorption in the medium of the four surfactants, named SDS,SDBS, Triton X-100, Tween80were examined in this paper.②The main factors affecting foam stability are surfactant concentration, foamquality, foam morphology, and contaminants. Study concluded that foam stabilityincreased with increasing of surfactant concentration within a certain range ofresearch. When the foam quality was between90%and93%, foam stability increasedwith the increasing of foam quality. Colloidal microbubbles stability was strongerthan the stability of regular foam. Foam stability was reduced in the present ofnon-aqueous phase liquids.③Solubilization capacity for nitrobenzene of the four kinds of surfactantssatisfies the following order: SDS<SDBS<Triton X-100<Tween80; desorptionefficiency for nitrobenzene contaminated soil satisfies: SDBS> SDS> Tween80>Triton X-100; the maximum amount of adsorption in the medium for SDS, SDBS,Triton X-100were3,3.7,74.8mg/g, respectively. The adsorption amount of nonionicsurfactant Triton X-100is much greater than the adsorption amount of anionicsurfactant SDS, SDBS. Foam stability formed by SDS solution was greater thanfoams stability formed by SDBS and Triton X-100surfactants solution. Therefore,select SDS as the surfacant for subsequent experiments.（2）Transport properties and influencing factors of foam flow in porous media ①The separation between the foaming gas front, the solution wetting front, andthe foam front was significant. The foam transport had a retardation of2.0relative tothe foaming gas.②The pressure drop in the sediment primarily occurred and evenly distributed inthe foam-occupied region. The high foam injection pressure ascribed the higheffective viscosity of foam in porous media.③The influence of sediment permeability on the foam injection pressuredepended on the foam injection Darcy velocity. When the Darcy velocity is relativelylarge, a decrease in sediment permeability resulted in increased foam injectionpressure. When Darcy velocity is small, an increase in sediment permeability resultedin increased foam injection pressure. In the experiment range, an increase in foamquality and decrease in foam injection rate resulted in decreased foam injectionpressure. Effect of foam injection rate on foam injection pressure was larger than theeffect of foam quality on injection pressure.④Transport characteristics of foam in homogeneous and hetergeneous mediawere studied through two-dimensional simulation tank. Compare with surfactantsolution, Foam injection not only effectively increased the sweep efficiency ofsurfactant solution in porous media, but also strengthened foam migration in thehorizontal direction. Foam delivery improved the uniformity of surfactant solution inhetergeneous medium, especially strengthened the migration of surfactant solution inhigh permeability medium.（3）Transport characteristics of nanoscale zero valent iron carried by foam inporous media①SDS foam and solution effectively strengthened the nZVI migration in porousmedia. SDS solution strengthened the nZVI migration in high permeability media(0.9~1.4mm,0.5~0.9mm) and the strengthen result was not obvious in lowpermeability media (0.25~0.5mm). The foam reinforced nZVI migration in lowpermeability media (0.25~0.5mm,0.5~0.9mm) and the strengthen result was not significant in high permeability media (0.9~1.4mm). Therefore, it provide theoreticalguidance for using SDS solution and foam as tranmission fluids in different stratummedia.②nZVI stability in SDS solution and foam was greater than that in water. SDSfoam has a good carrying capacity for nZVI particles and nZVI particles had littleeffect on foam stability. The strengthen migration of nZVI carried by SDS solutionwas mainly reflected in the vertical direction and the horizontal migration of nZVIwas limited. However, foam not only strengthened nZVI transport in the horizontaldirection, but also increased distribution uniformity of nZVI particles.（4）Research on remediation efficacy of nitrobenzene contaminated soil by foamflushing technology①The total removal efficiency of nitrobenzene contaminated soil were34.86%,32.80%,97.35%by water, SDS solution, and SDS foam for0.5~1mm media. Theremediation efficiency of nitrobenzene was enhanced by foam flushing.②The factors affecting remediation effciency of nitrobenzene contaminated soilby foam flushing mainly are foam injection rate, foam quality, medium particle size.The total removal efficiency of nitrobenzene for low permeability is greater than thatin high permeability medium. Within the scope of the study, the foam quality andfoam injection rate has little effect on the total removal efficiency of nitrobenzene insoil. However, the volatile efficiency and foam liquid removal efficiency weredifferent through impacting foam stability and the contact time between foam fluidand contaminants for foam quality and foam injection rate.（5）The remediation efficacy of contaminated soil with heterogeneous media byfoam flushing and the running process analysis for foam flushing①The main removal mechanism of nitrobenzene in soil was mobilization,solubilization, and volatilization. The removal contribution rate of pure nitrobenzenewas69.5%and the majority of nitrobenzene was diaplaced by foam fluid in freephase. Solubilization, and volatilization removal efficiency of nitrobenzene were21.6%,1.5%, respectivley. The remaining rate of nitrobenzene in soil was4.3%. Due to interfacial effects, the flushing efficiency for low permeability with high resistancewas lower than high permeability media. In order to achieve completely removal ofnitrobenzene in low-permeability media, it will take a long time to remediation.②The feasibility of foam injection and extraction through vertical injection wellsand vertical+horizontal injection well combined were explored by two-demensionaltank experiments. Study results concluded that the foam flushing liquid can besuccessfully extracted out by vacuum extraction wells when the foam was injectedthrough the vertical injection wells and vertical wells+horizontal wells combinedinjection and thus avoiding expanding of contamination scope. For both injectionmodes, the amount of liquid injected was0.201PV,0.202PV when the foam coversthe entire area of tank. The liquid volume was greatly reduced and more economicalfor foam flushing.The innovations of this research are:（1）Foam flow can effectively avoid the tunnel effect and preferentially flowbecause of the low density and strong liquidity characteristics. Therefore, foam flowcan strengthen the removal efficacy of contaminants in porous media. The transportand distribution characteristics of foam flow in homogeneous and heterogeneousporous media，enhanced remediation experiment of nitrobenzene contaminated soil byfoam flushing technology were studied in this paper. The removal mechanism ofNAPLs by foam flushing and the contribution rates of mobilization, solubization, andvolatile removal for NAPLs were quantitatively studied in this work. Foam candisplace NAPLs contaminants in low permeability medium, especially can removeNAPLs as the Liberal phases from vadose zone media. The remediation efficacy ofNAPLs in low permeability and heterogeneity medium was stronger than theconventional flushing technology. The study provides theoretical reference forpractical application of foam flushing technology.（2）The strengthen transport of nZVI particles carried by foam in porous mediawas explored in this study. Especially, the transport of nZVI was enhancedsignificantly in high permeability and low permeability medium for surfactant solution and foam, respectively. The study provides data and theoretical support forusing surfactant solution and foam as vehicles to transport repairing agent to the targetarea in different stratum conditions.（3）The crafts process was improved by studing the feasibility of foam injectionand extraction methods. So this study will provide technical support for using foamflushing technology in the actual contaminated site.