| Nitrobenzene is a kind of basic material of chemical industry, which is widelyapplied in medicine, agriculture, dyes and other fields. It has a great output which isstill increasing year by year. The release and the leakage of nitrobenzene occurring inthe process of production and transportation will cause nitrobenzene pollution ofgroundwater. Due to the highly toxic, perdurability and the “carcinogenesis,teratogenesis and mutagenesis” of nitrobenzene, it can lead to a great threaten to thesafety of drinking water and people’s health once the groundwater is polluted.Therefore, it has important practical significance for exploring effective remediationtechnologies for nitrobenzene pollution of groundwater, and making systematicstudies on its remediation mechanism, influence factors, remediation effects and soon.At present, there are many remediation technologies for organic contaminationof groundwater, which can be divided into two main types of in-situ and ex-situremediation technology. Compare with the latter, in-situ remediation technology canachieve the remediation underground, without pumping polluted groundwater water.It is a potential technology with advantages of immediate and cost-effective,therefore, it considered to have broad development prospects. At the present stage, themain in-situ remediation technologies for nitrobenzene pollution of groundwater arecut-off wall, in-situ bioremediation, in-situ chemical remediation and permeablereactive barrier (PRB). Different remediation technologies have different applicationconditions and limiting factors, such as remediation period, cost, effect and so on. Asa kind of in-situ remediation technology of new emerging, in-situ reaction zone (IRZ)is developed from PRB, with the advantages of more simple construction equipment, lower cost, better effect and no limitation to the depth of contaminant plume. IRZ hasbeen applied to the management of the fields which are contaminated by chlorinatedhydrocarbons and heavy metal in many developed countries; However, in our country,the study about IRZ is just starting on so that the researches about the formation andchange mechanism, the influence factors on its remediation area and permeabilitychanging rules of IRZ need to be strengthen, which signify that it has importancemeaning to make more explorations in all these aspects for the practical application ofIRZ.The key of IRZ is choosing an appropriate reaction reagent to form an effectivereaction zone in the underground to achieve the removal of pollutants. There are manyreagents can be selected for nitrobenzene pollution, such as Fenton’s reagent, ironpower and nitrobenzene degradation bacteria, and nanoscale zero-valent iron (NZVI)and modified NZVI offer a new thought due to their more and more applications inenvironmental remediation field. Some successful remediation cases about organiccontamination in groundwater by injecting NZVI particles to form IRZ have beenreported, but there are rare researches on the transfer rules of NZVI and its influenceon the formation of IRZ, which aspects need more deep-going work.Based on the research domestic and abroad, sucrose was chosen as reactionreagent in this paper, and a series of influence factors such as sucrose ratio, initial pH,iron dosage and coexisting ions were investigated for obtaining optimum conditionsfor reduction reaction. Moreover, the transport mechanism and characteristics ofSM-NZVI were analyzed in order to understand its impact on the formation of thereaction zone, and then the remediation area of IRZ was studied by injectingSM-NZVI through injection well to approach the formation rules. Finally, under thebasement of comprehensive analysis of all the mechanisms, IRZ of SM-NZVI wasestablished through simulation experiments to study the influence factors ofremediation ability, permeability variation of the reaction zone and remediationeffects of the aquifer contaminated by NB. The concrete research content and resultsare as follows:(1) Researches on the reduction effect of nitrobenzene by SM-NZVI SM-NZVI was prepared by liquid chemical reduction method before itscharacterization. The influence factors on nitrobenzene reduction by SM-NZVI werestudied, and both kinetics of the reaction and the component of iron oxide productwere analyzed, the experimental results as follows:①The mean diameter of NZVI and SM-NZVI was70~90nm and100~150nm，respectively，and the sucrose could effectively improve the dispersity of NZVI as adispersant.②Compared with non-modified NZVI, the nitrobenzene reduction ability ofSM-NZVI obviously enhanced so that the removal efficiency of nitrobenzene wasincreased44.2%and the production rate of aniline increased46.4%after10h.③Sucrose dosage would affect the performance of modified NZVI, andSM-NZVI showed the best reduction ability when the sucrose ratio was0.8%.④The initial pH and iron dosage also had an effect on the removal ofnitrobenzene, lower pH and higher iron dosage brought higher removal efficiency ofnitrobenzene. Moreover, HCO3-, Cl-, SO42-and NO3-could promote the reductionreaction with the order of HCO3-＞Cl-＞SO42-＞NO3-; conversely, Ca2+, Mg2+wouldinhibited the reaction with the order of Ca2+＞Mg2+; coexistence of these commonions in groundwater had little effect on nitrobenzene reduction and could be ignored.⑤The processes of nitrobenzene reduction by non-modified NZVI andSM-NZVI were according with pseudo-first order reaction kinetic model, and thereaction rate constant was2.04×10-3min-1and7.79×10-3min-1, respectively.⑥The oxidation products of SM-NZVI were mainly Fe3O4and Fe2O3.(2) Researches on the transport characteristic and mechanism of SM-NZVI in thesaturated porous mediaThe transport characteristic of SM-NZVI in saturated porous media was studiedthrough one-dimensional simulation experiments; the largest transport distance anddeposition rate constant of SM-NZVI were calculated by the theory of filter bed andconvective-dispersive equation, respectively; control mechanism of transport behaviorwas analyzed by T-E model, and the experimental results are: ①The transport ability of SM-NZVI was better than non-modified NZVI andthe transport ability of the former was1.6times than the latter.②Larger particle diameter of aquifer media was good for the transport ofSM-NZVI, and the transport ability of SM-NZVI in coarse sand was1.63times thanin the fine sand.③There was a critical value for injection speed of SM-NZVI to affect itselftransport. When injection speed was lower than the value, the transport ability wouldincrease with increasing injection rate; when injection speed was higher than the value,the effect of injection speed was not significant. In our tests, this critical value wasapproximately0.06cm/s.④The transport ability of SM-NZVI increased with declining concentration ofthe slurry. Therefore, in a practical application, the appropriate concentration shouldbe determined by considering both the effect of formation of reaction zone and theconcentration of reaction reagent on remediation ability of IRZ, for making sure thateffective reaction zone can be created in the contaminated plume.⑤A calculation model was created using the theory of filter bed to predict thelargest transport distance (Lmax) of SM-NZVI in saturated porous media. The Lmaxofnon-modified NZVI and SM-NZVI was79.8cm and232.9cm, respectively. Thepredicted Lmaxwas compared with the measured data in the fields and the comparisonresults indicated that the model was responsible and could provide theoreticalreference for the layout of the injection wells in the actual fields.⑥Deposition rate coefficient of SM-NZVI in porous media represented by kincreased first and then decreased with the increasing injection speed, as well asdecreased with the descending concentration of SM-NZVI. The above results wereagreed with the breakthrough curve, which means that first-order deposition kineticscan well describe the deposition behavior of SM-NZVI in saturated porous media.(3) Researches on the formation mechanism of IRZ of SM-NZVIThe remediation area of IRZ in the direction of groundwater flow and thevertical direction of aquifer was studied by two-dimensional simulation experiments,and the results are: ①When the groundwater velocity was constant, the width of SM-NZVIreaction zone (the remediation area in the direction of groundwater flow) wasincreased with a increasing size of aquifer media.②In initial formative stage of IRZ, the effect of groundwater velocity on itswidth was non-significant. However, in the extending process of the reaction zone, thegrowth rate of the width increased with the increasing groundwater velocity butdecreased with time, and the extended reaction zone was uniformity.③Higher SM-NZVI slurry concentration went against the width growth ofreaction zone, but its influence was limited compared to media size of aquifer and thegroundwater velocity.④The manner of slurry injection by several times could improve the width ofthe reaction zone, but its growth rate had nothing to do with the injection manner ofthe slurry with a constant groundwater velocity.⑤The higher slurry concentration brought the thicker reaction zone (or largerremediation area in depth direction) with a constant injection volume, but it would bereverse condition when the quality of SM-NZVI was definite. Thus, for a certainquality of SM-NZVI particles, lower concentration of the slurry was good forexpending the remediation area in both directions of depth and groundwater flow.(4) Researches about the reduction efficiency of nitrobenzene by IRZIRZ of SM-NZVI was created to study the influence factors on nitrobenzenereduction, such as the type of aquifer media, the groundwater velocity and theconcentration, injection manner of SM-NZVI slurry, moreover, the permeabilitychange of IRZ was analyzed and the results are as follows:①The attachment of SM-NZVI on media led to the decrease of nitrobenzeneremoval efficiency, and the smaller size of the media brought greater impact.②In coarse sand and fine sand, the mean removal efficiency of nitrobenzenewas72.3%and56.8%, respectively, which signified that the IRZ had better reductionability in the aquifer of lager size media. Moreover, in the coarse sand, thepermeability change of IRZ was comparatively lesser, thus the IRZ had bettersustainable remediation ability. ③When the concentration of SM-NZVI was6.62g/L and13.45g/L, the meanremoval efficiency of nitrobenzene was72.3%and80.8%, respectively. Higherinjection concentration of SM-NZVI could accelerate nitrobenzene reduction in theshort term, however, the permeability of IRZ would fall more quickly, which was notgood for pollutant persistent reduction.④When the groundwater flow velocity was0.1m/d and0.5m/d, the meanremoval efficiency of nitrobenzene was72.3%and36.5%, respectively. Highergroundwater flow velocity reduced the nitrobenzene reduction, but greater shear forcefrom the flow enhanced the transport of the products, so the permeability of IRZ hadnot changed much.⑤When the concentration of nitrobenzene was181.7mg/L and352.2mg/L, themean removal efficiency of nitrobenzene was72.3%and36.5%, respectively. Higherpollutant concentration could accelerate the formation of the passivation layer on thesurface of SM-NZVI, which inhibited nitrobenzene reduction. The passivation layeralso led the permeability of IRZ decrease significantly and the IRZ could not showgood remediation ability for long-term.⑥Although the permeability of IRZ trended to decreased in its running process,the declined permeability coefficient was still in the same order of magnitude with theinitial value, therefore, except the extreme conditions, the IRZ permeability ofSM-NZVI would not remarkably change, let alone brought blocking problem.(5) Researches on the remediation of nitrobenzene contaminated aquifer widthIRZ of SM-NZVISimulation experiments about the remediation of nitrobenzene contaminatedaquifer width IRZ of SM-NZVI were carried out to study the removal efficiency ofnitrobenzene in different aquifer media, as well as the permeability change, and thespecific research results are:①The aquifer media had important influence for IRZ on the remediationefficiency of nitrobenzene pollution. Running to the50and40days in coarse sandand fine sand, respectively, IRZ entered the trailing period with the trailingconcentration of87.24mg/L and170.24mg/L. After60d, the mean removal efficiency of nitrobenzene was79.0%and58.8%while the production rate of aniline was40.06%and20.78%, respectively, which manifested that larger size of aquifer mediawas conducive to the organic pollution remediation by IRZ.②In both of coarse and fine sand, pH value of the aquifer increased and ORPdecreased significantly resulting from the reduction reaction, and the final averagedwere8.79,-262.4mv and7.86,-235.7mv, showing an alkaline and reductionenvironment in the aquifer.③The injection of SM-NZVI caused the water yield declined, and the averagedreduction rate was13.8%and11.9%in coarse and fine sand; in the remediationprocess, the water yield was relatively stable which meant the reduction reaction hadless effect on aquifer permeability.④The variation of water head increased as a result of the injection ofSM-NZVI and reduction reaction, and the more obviously variation could be seennearby the wells.⑤The IRZ permeability was lower than initial aquifer permeability, but theirpermeability was in the same order of magnitude; after60days, the permeability ofcoarse and find sand media decreased60.9%and70.6%, respectively, but there wasno clogging occurred.⑥Both of IRZ formation and its remediation ability were heterogeneous，thatwas, the concentration of SM-NZVI and the nitrobenzene reduction efficiency nearthe wells was the highest, and they decreased with the increase of the distance fromthe wells; the nanoiron particles would sink to the deep aquifer while transportingwith the flow, thus IRZ was heterogeneous in the vertical direction of the aquifer, andthe concentration of SM-NZVI in the bottom of the zone was more higher whichcorresponded to better remediation efficiency.The innovations of this research are:(1) The transport mechanism and characteristic of SM-NZVI in saturated porousmedium was studied and the prediction model was established for the transportdistance of SM-NZVI using filter bed theory. The predicted Lmaxwas compared withthe measured data in the fields and the comparison results indicated that the model was responsible and solved the problem that the layout of injection wells only reliedon experience of the tradition IRZ. The relationship between Lmaxand the injectionspeed and injection concentration of SM-NZVI was definite through the experiments;the process of precipitation of SM-NZVI was described by convective-dispersiveequation (CDE) and filter bed theory; the correlation parameters about the migrationwere calculated by T-E model for illuminating the control mechanism of the transportbehavior of SM-NZVI, which provide the theoretical reference for the practicalapplication of SM-NZVI in groundwater remediation.(2) The change of the permeability coefficient of IRZ and its influence factorswere studied through simulation experiments, which found that the type of aquifermedia, groundwater flow velocity, the pollutant concentration were the main influencefactors affected the permeability of IRZ and thus the permeability changing rules wasconfirmed; furthermore, the removal efficiency of nitrobenzene by IRZ in differentaquifer media was assessed to reveal the effect of the permeability of IRZ on theremediation ability itself. All these research mentioned lay the foundation foroptimizing performance and improve sustainable remediation ability of IRZ in filedapplication.(3) The study on IRZ was conducted through IRZ of SM-NZVI being created intwo-dimensional simulated setup, from which the removal efficiency of nitrobenzene,as well as pH and ORP variation, can be confirmed; in addition, the formation andchanging rules of IRZ were further discussed, all of which provide theoreticalguidance for the application of IRZ. |
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