| Because of petroleum spills and organic solvent inadequate disposal,groundwater NAPL(non-aqueous phase liquid)contamination is a growing problem and threat to groundwater security in China.Because of its low water solubility,low biodegradability and high interfacial tension,conventional remediation method could not remediate NAPL efficiently.In this work,cosolvent-nanoparticle foam is used for remediation.First,foam could remove NAPL in pores via mobilization,and then residual NAPL could be swept by cosolvent using solubilization.NAPL contamination could be remediated efficiently.Therefore,in this work,pore-scale solubilization mechanism of cosolvent and mobilization mechanism of nanoparticle foam are investigated respectively to provide key parameters for numerical simulation of contaminant migration in macroscopic site and process information for the actual site remediation optimization scheme.With the help of Raman spectroscopy,NAPL dissolution in static and flowing cosolvent solution is observed quantitatively.The molecular diffusion between NAPL and cosolvent solution is obtained.Moreover,the enhanced solution mechanism and effect law of cosolvent on NAPL in immobile domain pores are investigated.In addition,emerging fly ash nanoparticle is used to generate nanoparticle foam.NAPL remediation by water flooding and foam flooding in the two-dimensional porous medium is observed,and foam displacement process and optimum concentration of fly ash nanoparticle are studied.Conclusions are as following:(1)A quantitative Raman technique for in-situ observation of contaminant concentration field in pore space is established.Standard samples are prepared using mercury sealing technique and quantitative relationship of PCE(tetrachloroethylene)-ethanol-water ternary system is obtained at atmospheric pressure and different temperature.The quantitative observation of contaminant concentration field in pore space at different temperatures could be realized.(2)Using capillary tube as one-dimensional single pore,PCE-ethanol solution dissolution process in static station is observed quantitatively.In static station,dissolution is the result of molecular diffusion,so solubility and diffusion coefficient of PCE in ethanol solution and ethanol in PCE are investigated via quantitative Raman spectroscopy.The result shows with ethanol concentration increasing,both solubility and diffusion coefficient increase,and PCE and ethanol dissolution is accelerated.Increasing ethanol concentration from 50% to 80%,PCE solubility increases for 14 times and diffusion coefficient increases for 1.3 times,which means increasing ethanol concentration could improve PCE solubilization capacity and dissolution rate clearly.In later period,for 70%ethanol solution,PCE concentration near the interface could still increase for 0.02.As a result,there still exists the bigger concentration gradient of PCE in ethanol solution and PCE dissolution rate is twice higher than that in 55% ethanol solution.So,in actual use,for immobile pores,increasing ethanol concentration from 55% to 70%,ethanol use is increased by 20% and dissolution rate increase for 100% to achieve cost-effective and efficient restoration.(3)In the process of injecting cosolvent solution for soil and aquifer remediation,it is very difficult to remove residual NAPL contaminant in immobile domain pores.It is the key problem for cosolvent flushing to how to remove residual NAPL and avoid longterm contamination.For observing remediation process in immobile domain pores,transparent micromodels are utilized to simulate two-dimensional porous media.Via quantitative Raman spectroscopy,PCE dissolution process by ethanol solution flushing in immobile domain pores is observed visually,via in-situ observation of contaminant concentration field in pores.The result shows there exist advection in immobile domain pores.Advection could take cosolvent solution saturated with NAPL away from immobile domain pores and bring cosolvent solution with little NAPL to the interface of NAPL to accelerate NAPL dissolution.(4)Quantitative Raman spectroscopy is used to obtain the effect of ethanol concentration,injection rate and pore throat size on the concentration distribution in immobile domain pores under the flushing condition in dominant channels,and to reveal the pore-scale mechanism and influence law of NAPL dissolution.Increasing ethanol concentration has little effect on advection in pores,and could increase PCE solubility so that flowing ethanol solution could bring more dissolved PCE out to accelerate PCE dissolution.Increasing ethanol concentration from 50% to 70%,PCE dissolution rate is increased by 89%,which is larger than the increase rate of ethanol dosage.Increasing injection rate mainly accelerate advection in pores near the domain channel to enhance PCE dissolution but the effect is not obvious.Increasing injection rate from 1 μ L /min to5 μl/min,the dissolution rate increased only 52%,which is much smaller than the increase ratio of ethanol solution dosage.Increasing pore throat size could enhance advection in pores away from the domain channel to accelerate PCE dissolution.Increasing pore throat size from 0.3mm to 0.4mm,PCE dissolution rate increases by about 3 times,which means pore throat size played a decisive role in PCE dissolution rate in the later period and is a key parameter to simulate and predict the remediation performance.Therefore,in later period,the higher concentration cosolvent solution at a lower injection rate is better for remediation.(5)In actual sites,NAPL pollutants could be flushed by groundwater for a long time after infiltration,resulting in long-term residual NAPL pollution.Therefore,the later cleaning and restoration work is also carried out based on residual NAPL contamination.In order to simulate the distribution of residual NAPL,a micromodel is used to simulate the displacement and removal process of saturated NAPL in porous media under water flushing.The result shows after invading aquifer,NAPL could be removed partially by flowing groundwater.Before water flows through NAPL in pores to the outlet,NAPL could be swept by flowing water.But when water flow through to the outlet,domain channel form in the porous medium and water hardly enter other pores to remove NAPL effectively.As a result,residual NAPL might exist in the long-term and need to be removed using other remediation method.(6)As a kind of emerging material to realize waste resource recovery,fly ash could be used to generate foam to remove NAPL effectively in porous media to improve remediation benefit.The displacement and removal process of residual NAPL after water flooding with different concentration fly ash nanoparticle foam are observed.The result shows foam could increase sweep area and improve residual NAPL sweep efficiency.With nanoparticle concentration increasing,both foam stability and removal efficiency increase significantly at first,turn at 1000 ppm,and then decrease rapidly.Stability and remediation efficiency of 1000 ppm fly ash nanoparticle foam is the best,which could remove 44% of residual NAPL in the porous medium at a constant rate,and is the most suitable for groundwater NAPL removal.In addition,it is better to avoid using high concentration fly ash nanoparticle and otherwise nanoparticle deposit may form to block pores.Therefore,in actual use,1000 ppm surfactant and 1000 ppm fly ash nanoparticle foam could play an optimal displacement and removal effect. |
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