蔗糖改性纳米铁原位反应带修复硝基苯污染地下水研究
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摘要
硝基苯作为一种重要的化工基础原料被广泛应用于医药、农业、染料等行业，产量巨大，并且逐年递增。在硝基苯的生产和运输过程中可能会发生硝基苯的排放、泄漏，会导致地下水的硝基苯污染。由于硝基苯为高毒性、持久性有机污染物，且具有“三致”特性，因而其造成的地下水污染会极大威胁饮用水的安全和人类健康。因此，寻求有效的硝基苯污染地下水修复技术，并对修复机理、影响因素、修复效果等方面做系统性的研究，对于地下水硝基苯污染的控制与治理具有重要现实意义。
目前，针对地下水有机污染的修复技术较多，主要分为原位修复技术和异位修复技术两大类，相比于地下水异位修复，原位修复可以不对污染的地下水进行抽取，在地下对污染进行修复。该方法具有直接快速、经济高效的特点，因而被认为具有广阔的发展前景。现阶段，可用于控制和治理地下水硝基苯污染的原位修复技术主要有泥浆阻截墙法（cut-off wall）、原位生物修复技术（in-situbioremediation）、原位化学修复技术（in-situ chemical remediation）和可渗透反应墙技术（Permeable Reactive Barrier，PRB）。不同的修复技术，其适用条件和限制因素不同，如周期、费用、效果等等。原位反应带技术（In-situ Reaction Zone，IRZ）是由可渗透反应墙技术演变发展而来的一种新兴原位修复技术，该技术具有施工设备简单、修复费用低、效果好且修复范围不受污染羽深度限制的优点。在发达国家，原位反应带技术在氯代烃、重金属等污染场地的治理中已经有一定的应用并取得了良好的修复效果；但在国内的研究刚刚起步，尤其是对于反应带的形成及变化机制、修复区域影响因素的研究、反应带渗透性变化的分析等方面尚需加强，对这些方面的深入研究对于原位反应带技术的实际应用具有重要意义。
原位反应带修复技术的关键是选择一种合适的反应试剂，能够在地下环境中形成有效反应带，对污染物进行高效去除。对于地下水硝基苯污染，可以选择的反应试剂较多，如芬顿试剂、铁粉和硝基苯降解菌等，而纳米铁及改性纳米铁在环境修复领域越来越多的应用为此提供新的思路。在国外，利用纳米铁原位还原反应带修复地下水有机物污染的成功案例已有报道，但对于纳米铁颗粒在地下环境中的迁移规律及其对反应带形成的影响研究还很少见，需要进行深入探讨。
本研究在综合国内外文献的基础上，利用蔗糖改性纳米铁（Sucrose ModifiedNanoscale Zero-valent Iron，SM-NZVI）作为硝基苯污染修复的反应试剂，通过对蔗糖用量、环境初始pH、铁投加量、共存离子等影响因素的研究，确定还原反应最佳进行条件；并对SM-NZVI迁移机理及特性进行分析，掌握SM-NZVI迁移规律及对反应带形成的影响；在此基础上，通过注入井注入SM-NZVI，对反应带修复范围进行研究，得到反应带形成规律；最后通过模拟实验，创建蔗糖改性纳米铁原位还原反应带，并对反应带修复效果的影响因素、反应带渗透性变化及污染含水层修复效果等方面进行全面系统的研究。具体研究内容和成果如下：
（1）蔗糖改性纳米铁还原硝基苯效果研究
利用液相化学还原法制备纳米铁及蔗糖改性纳米铁并进行表征，研究了蔗糖改性纳米铁还原硝基苯影响因素，对硝基苯还原动力学规律及铁氧化产物组分进行了分析，主要研究成果为：
①纳米铁和蔗糖改性纳米铁平均粒径分别为70~80nm和100~150nm，通过蔗糖改性，可使纳米铁的分散性明显提高。
②与未改性的纳米铁相比，蔗糖改性纳米铁对硝基苯的还原能力明显增强。反应10h，硝基苯去除效率提高44.2%，苯胺生成率提高46.4%。
③蔗糖添加量会影响改性纳米铁性能，研究表明，当蔗糖添加比例为0.8时，改性纳米铁对硝基苯的还原效率最高。
④反应体系初始pH和铁投加量会对还原反应产生重要影响。环境初始pH越低，铁投加量越大，硝基苯还原效率越高。HCO3-、Cl-、SO42-和NO3-均对硝基苯的还原起促进作用，促进作用顺序为HCO3-＞Cl-＞SO42-＞NO3-；Ca2+、Mg2+在不同程度上对硝基苯的还原起抑制作用，且Ca2+的抑制作用大于Mg2+；地下水常见离子在共存状态下对硝基苯还原过程影响较小，可以不予考虑。
⑤改性前后纳米铁对硝基苯的还原过程均符合准一级动力学方程，改性前后的反应速率常数分别为2.04×10-3min-1和7.79×10-3min-1。
⑥蔗糖改性纳米铁氧化产物主要为Fe3O4和Fe2O3。
（2）蔗糖改性纳米铁在饱和多孔介质中的迁移特性及机理研究
通过一维模拟实验，研究SM-NZVI在多孔介质中的迁移特性；利用滤床理论和对流弥散方程对SM-NZVI最大迁移距离和沉积速率常数进行定量计算；并通过T-E理论模型对SM-NZVI迁移主要控制机理进行分析，得到如下实验结论：
①改性后纳米铁迁移能力提高，SM-NZVI的迁移能力是未改性纳米铁的1.6倍。
②含水层介质粒径越大越有利于SM-NZVI的迁移，SM-NZVI在粗砂中的迁移能力是在细砂中的1.63倍。
③注入速度对SM-NZVI的迁移能力影响存在临界值，当注入速度低于该临界值时，SM-NZVI的迁移能力随着注入速度的增大而提高，当注入速度高于该临界值时，注入速度对SM-NZVI迁移能力的影响不显著。本实验条件下，影响SM-NZVI迁移能力的临界注入速度约为0.06cm/s。
④SM-NZVI的迁移能力随注入浓度的增加而下降。因此，在实际应用中，应综合考虑反应带形成情况与反应试剂浓度对修复效果的影响，选择合适的注入浓度，保证有效反应带的创建。
⑤利用滤床理论创建计算模型对SM-NZVI在饱和多孔介质中的最大迁移距离进行预测，改性前后纳米铁在粗砂中的最大迁移距离分别为79.8cm和232.9cm。通过与场地中测得的实际数据比较，表明该模型预测结果具有较好的准确性，这为实际场地中注入井的布设提供了理论参考。
⑥SM-NZVI在多孔介质中的沉积速率k值随着注入速度的增加先减小后增大，随着纳米铁注入浓度的增大而减小，这与由穿透曲线得到的结论一致，说明利用一级沉积动力学描述蔗糖改性纳米铁在饱和多孔介质中的沉积过程是可行的。
（3）蔗糖改性纳米铁原位反应带形成机理研究
通过二维模拟实验，对SM-NZVI反应带地水流方向和含水层垂直方向修复区域进行研究，得到的主要实验结论有：
①地下水流速一定时，蔗糖改性纳米铁反应带宽度（地下水流方向修复范围）随含水层介质粒径的增大而增加。
②反应带形成初始阶段，地下水流速对反应带宽度的影响不显著。反应带扩展过程中，扩展速率随地下水流速的增加而增大，随时间的增加而减小。扩展后的反应带在水流方向上具有不均匀性。
③较高的SM-NZVI浆液浓度不利于反应带宽度的扩展，但与介质粒径和地下水流速相比，浆液浓度对反应带宽度的影响较小。
④浆液分次注入的方式可以提高反应带宽度；地下水流速一定时，反应带的扩展速率与注入方式无关。
⑤SM-NZVI浆液注入体积一定时，浆液浓度越大，反应带厚度（含水层垂直方向修复范围）越大；注入的SM-NZVI颗粒质量一定时，浆液浓度越小，反应带厚度越大；因此，对于一定量的SM-NZVI颗粒，较小的浆液浓度有利于反应带含水层垂直方向和地下水流方向修复范围的扩展。
（4）蔗糖改性纳米铁原位反应带还原硝基苯效能研究
创建SM-NZVI原位反应带，考察介质类型、地下水流速、SM-NZVI浓度及注入方式对硝基苯还原效果的影响，同时研究反应带渗透性变化规律，得到的主要实验结论有：
①SM-NZVI在介质上的附着会降低硝基苯的去除效果，且介质粒径越小对还原反应影响越大。
②在粗砂、细砂介质中，反应带对硝基苯的平均去除率分别72.3%和56.8%，含水层介质粒径越大，反应带对硝基苯的去除效果越好，且反应带渗透性变化相对较小，即较大粒径介质中反应带的可持续修复能力较好。
③SM-NZVI浓度为6.62g/L和13.45g/L时，反应带对硝基苯的平均去除率分别为72.3%和80.8%。注入的SM-NZVI浓度较高时，反应带短期内对硝基苯的还原能力更强，但反应带渗透性下降较快，反应带对污染物的持续修复能力降低。
④地下水流速为0.1m/d和0.5m/d时，反应带对硝基苯的平均去除率分别为72.3%和36.5%。地下水流速较高时，反应带对污染物的去除能力降低，但较大的水流剪切力促进了产物的迁移，使得反应带渗透性变化不大。
⑤污染物浓度为181.7mg/L和352.2mg/L时，反应带对硝基苯的平均去除效率分别为72.3%和45.0%。较高的污染物浓度会加速蔗糖改性纳米铁表面钝化层的形成不利于污染物的还原，反应带渗透性下降较快，反应带可持续修复能力较差。
⑥反应带运行过程中渗透性会呈下降趋势，但下降后的渗透系数仍与初始渗透系数在同一数量级上，因此，非极端条件下，蔗糖改性纳米铁反应带的渗透性不会因为还原反应的进行而发生极端改变，堵塞现象较难发生。
（5）蔗糖改性纳米铁原位反应带修复硝基苯污染含水层
开展污染含水层修复模拟实验，研究不同含水层介质中SM-NZVI反应带对硝基苯的修复效果，考察含水层渗透性变化，具体研究成果为：
①反应带形成范围及硝基苯污染的修复效果受含水层介质影响显著。含水层介质粒径越大，反应带修复范围越大，对污染的修复效果越好。在粗砂、细砂介质中，反应带分别运行至50d、40d时进入拖尾修复期，拖尾浓度分别为87.24mg/L和170.24mg/L，60d后，苯胺的平均生成率为40.1%和20.8%。
②还原反应的进行使得槽内的pH显著升高，ORP不断下降，反应带运行结束后，粗砂、细砂介质中，pH和ORP平均值分别为8.79和-262.4mv、7.86和-235.7mv，模拟装置内呈现碱性还原环境。
③SM-NZVI的注入会使含水层出水量减少，粗砂、细砂介质中，SM-NZVI注入后，出水量分别减少了13.8%和11.9%；反应带运行过程中，出水量相对稳定，说明还原反应对含水层渗透性影响较小。
④随着SM-NZVI的注入及还原反应的进行，含水层水头变化增加，且离注入井越近水头变化越明显。
⑤SM-NZVI原位反应带的渗透系数低于原含水层介质渗透系数，但渗透系数仍为同数量级；反应运行60d后，粗砂介质中反应带渗透系数下降了60.9%，细砂介质中反应带渗透系数下降了70.6%，但均未出现含水层堵塞现象。
⑥反应带的形成和修复能力具有不均一性，注入井处SM-NZVI浓度最高，污染物还原效率也最高，随着与注入井距离的增加，SM-NZVI浓度不断减小，污染物还原效率下降; SM-NZVI随水流迁移，同时缓慢向含水层深部沉淀，因此反应带在含水层垂直方向上也具有不均一性，反应带下部SM-NZVI浓度较高，污染物还原效率更好。
本研究的创新点体现在：
（1）研究了蔗糖改性纳米铁在饱和多孔介质中的迁移机理及特性，利用滤床理论建立了SM-NZVI在地下含水层中迁移的预测模型，通过与场地工程应用中测得的实际数据比较，表明该模型预测结果具有较好的准确性，模型的建立解决了传统原位反应带修复技术注入井只能依据经验布设的问题。通过实验，得到了SM-NZVI注入速度和注入浓度与最大迁移距离之间的关系方程；采用对流弥散方程和滤床理论对SM-NZVI沉淀过程进行动力学分析；通过T-E（Tufenkji-Elimelech）理论模型计算迁移过程相关参数，阐明了SM-NZVI在饱和多孔介质中迁移的控制机理，为蔗糖改性纳米铁在地下水修复中的实际应用提供理论参考。
（2）通过模拟实验，研究了SM-NZVI反应带渗透系数的变化及其影响因素，发现含水层介质类型、地下水流速和污染物浓度是影响反应带渗透性变化的主要因素，获得了修复过程中含水层反应带渗透系数变化规律；通过对不同条件下反应带修复硝基苯效果的评估，揭示了反应带渗透性变化对硝基苯污染修复效果的影响，为提高反应带可持续修复能力奠定了基础。
（3）对SM-NZVI原位反应带技术进行了研究，通过实验室二维模拟实验创建了SM-NZVI原位还原反应带，考察了不同含水层介质中反应带对硝基苯的修复效果及地下环境pH和ORP变化情况，并进一步阐明了反应带形成及变化规律，为反应带的实际应用提供理论指导。
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 media
The 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-NZVI
The 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 IRZ
IRZ 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-NZVI
Simulation 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.
目前，针对地下水有机污染的修复技术较多，主要分为原位修复技术和异位修复技术两大类，相比于地下水异位修复，原位修复可以不对污染的地下水进行抽取，在地下对污染进行修复。该方法具有直接快速、经济高效的特点，因而被认为具有广阔的发展前景。现阶段，可用于控制和治理地下水硝基苯污染的原位修复技术主要有泥浆阻截墙法（cut-off wall）、原位生物修复技术（in-situbioremediation）、原位化学修复技术（in-situ chemical remediation）和可渗透反应墙技术（Permeable Reactive Barrier，PRB）。不同的修复技术，其适用条件和限制因素不同，如周期、费用、效果等等。原位反应带技术（In-situ Reaction Zone，IRZ）是由可渗透反应墙技术演变发展而来的一种新兴原位修复技术，该技术具有施工设备简单、修复费用低、效果好且修复范围不受污染羽深度限制的优点。在发达国家，原位反应带技术在氯代烃、重金属等污染场地的治理中已经有一定的应用并取得了良好的修复效果；但在国内的研究刚刚起步，尤其是对于反应带的形成及变化机制、修复区域影响因素的研究、反应带渗透性变化的分析等方面尚需加强，对这些方面的深入研究对于原位反应带技术的实际应用具有重要意义。
原位反应带修复技术的关键是选择一种合适的反应试剂，能够在地下环境中形成有效反应带，对污染物进行高效去除。对于地下水硝基苯污染，可以选择的反应试剂较多，如芬顿试剂、铁粉和硝基苯降解菌等，而纳米铁及改性纳米铁在环境修复领域越来越多的应用为此提供新的思路。在国外，利用纳米铁原位还原反应带修复地下水有机物污染的成功案例已有报道，但对于纳米铁颗粒在地下环境中的迁移规律及其对反应带形成的影响研究还很少见，需要进行深入探讨。
本研究在综合国内外文献的基础上，利用蔗糖改性纳米铁（Sucrose ModifiedNanoscale Zero-valent Iron，SM-NZVI）作为硝基苯污染修复的反应试剂，通过对蔗糖用量、环境初始pH、铁投加量、共存离子等影响因素的研究，确定还原反应最佳进行条件；并对SM-NZVI迁移机理及特性进行分析，掌握SM-NZVI迁移规律及对反应带形成的影响；在此基础上，通过注入井注入SM-NZVI，对反应带修复范围进行研究，得到反应带形成规律；最后通过模拟实验，创建蔗糖改性纳米铁原位还原反应带，并对反应带修复效果的影响因素、反应带渗透性变化及污染含水层修复效果等方面进行全面系统的研究。具体研究内容和成果如下：
（1）蔗糖改性纳米铁还原硝基苯效果研究
利用液相化学还原法制备纳米铁及蔗糖改性纳米铁并进行表征，研究了蔗糖改性纳米铁还原硝基苯影响因素，对硝基苯还原动力学规律及铁氧化产物组分进行了分析，主要研究成果为：
①纳米铁和蔗糖改性纳米铁平均粒径分别为70~80nm和100~150nm，通过蔗糖改性，可使纳米铁的分散性明显提高。
②与未改性的纳米铁相比，蔗糖改性纳米铁对硝基苯的还原能力明显增强。反应10h，硝基苯去除效率提高44.2%，苯胺生成率提高46.4%。
③蔗糖添加量会影响改性纳米铁性能，研究表明，当蔗糖添加比例为0.8时，改性纳米铁对硝基苯的还原效率最高。
④反应体系初始pH和铁投加量会对还原反应产生重要影响。环境初始pH越低，铁投加量越大，硝基苯还原效率越高。HCO3-、Cl-、SO42-和NO3-均对硝基苯的还原起促进作用，促进作用顺序为HCO3-＞Cl-＞SO42-＞NO3-；Ca2+、Mg2+在不同程度上对硝基苯的还原起抑制作用，且Ca2+的抑制作用大于Mg2+；地下水常见离子在共存状态下对硝基苯还原过程影响较小，可以不予考虑。
⑤改性前后纳米铁对硝基苯的还原过程均符合准一级动力学方程，改性前后的反应速率常数分别为2.04×10-3min-1和7.79×10-3min-1。
⑥蔗糖改性纳米铁氧化产物主要为Fe3O4和Fe2O3。
（2）蔗糖改性纳米铁在饱和多孔介质中的迁移特性及机理研究
通过一维模拟实验，研究SM-NZVI在多孔介质中的迁移特性；利用滤床理论和对流弥散方程对SM-NZVI最大迁移距离和沉积速率常数进行定量计算；并通过T-E理论模型对SM-NZVI迁移主要控制机理进行分析，得到如下实验结论：
①改性后纳米铁迁移能力提高，SM-NZVI的迁移能力是未改性纳米铁的1.6倍。
②含水层介质粒径越大越有利于SM-NZVI的迁移，SM-NZVI在粗砂中的迁移能力是在细砂中的1.63倍。
③注入速度对SM-NZVI的迁移能力影响存在临界值，当注入速度低于该临界值时，SM-NZVI的迁移能力随着注入速度的增大而提高，当注入速度高于该临界值时，注入速度对SM-NZVI迁移能力的影响不显著。本实验条件下，影响SM-NZVI迁移能力的临界注入速度约为0.06cm/s。
④SM-NZVI的迁移能力随注入浓度的增加而下降。因此，在实际应用中，应综合考虑反应带形成情况与反应试剂浓度对修复效果的影响，选择合适的注入浓度，保证有效反应带的创建。
⑤利用滤床理论创建计算模型对SM-NZVI在饱和多孔介质中的最大迁移距离进行预测，改性前后纳米铁在粗砂中的最大迁移距离分别为79.8cm和232.9cm。通过与场地中测得的实际数据比较，表明该模型预测结果具有较好的准确性，这为实际场地中注入井的布设提供了理论参考。
⑥SM-NZVI在多孔介质中的沉积速率k值随着注入速度的增加先减小后增大，随着纳米铁注入浓度的增大而减小，这与由穿透曲线得到的结论一致，说明利用一级沉积动力学描述蔗糖改性纳米铁在饱和多孔介质中的沉积过程是可行的。
（3）蔗糖改性纳米铁原位反应带形成机理研究
通过二维模拟实验，对SM-NZVI反应带地水流方向和含水层垂直方向修复区域进行研究，得到的主要实验结论有：
①地下水流速一定时，蔗糖改性纳米铁反应带宽度（地下水流方向修复范围）随含水层介质粒径的增大而增加。
②反应带形成初始阶段，地下水流速对反应带宽度的影响不显著。反应带扩展过程中，扩展速率随地下水流速的增加而增大，随时间的增加而减小。扩展后的反应带在水流方向上具有不均匀性。
③较高的SM-NZVI浆液浓度不利于反应带宽度的扩展，但与介质粒径和地下水流速相比，浆液浓度对反应带宽度的影响较小。
④浆液分次注入的方式可以提高反应带宽度；地下水流速一定时，反应带的扩展速率与注入方式无关。
⑤SM-NZVI浆液注入体积一定时，浆液浓度越大，反应带厚度（含水层垂直方向修复范围）越大；注入的SM-NZVI颗粒质量一定时，浆液浓度越小，反应带厚度越大；因此，对于一定量的SM-NZVI颗粒，较小的浆液浓度有利于反应带含水层垂直方向和地下水流方向修复范围的扩展。
（4）蔗糖改性纳米铁原位反应带还原硝基苯效能研究
创建SM-NZVI原位反应带，考察介质类型、地下水流速、SM-NZVI浓度及注入方式对硝基苯还原效果的影响，同时研究反应带渗透性变化规律，得到的主要实验结论有：
①SM-NZVI在介质上的附着会降低硝基苯的去除效果，且介质粒径越小对还原反应影响越大。
②在粗砂、细砂介质中，反应带对硝基苯的平均去除率分别72.3%和56.8%，含水层介质粒径越大，反应带对硝基苯的去除效果越好，且反应带渗透性变化相对较小，即较大粒径介质中反应带的可持续修复能力较好。
③SM-NZVI浓度为6.62g/L和13.45g/L时，反应带对硝基苯的平均去除率分别为72.3%和80.8%。注入的SM-NZVI浓度较高时，反应带短期内对硝基苯的还原能力更强，但反应带渗透性下降较快，反应带对污染物的持续修复能力降低。
④地下水流速为0.1m/d和0.5m/d时，反应带对硝基苯的平均去除率分别为72.3%和36.5%。地下水流速较高时，反应带对污染物的去除能力降低，但较大的水流剪切力促进了产物的迁移，使得反应带渗透性变化不大。
⑤污染物浓度为181.7mg/L和352.2mg/L时，反应带对硝基苯的平均去除效率分别为72.3%和45.0%。较高的污染物浓度会加速蔗糖改性纳米铁表面钝化层的形成不利于污染物的还原，反应带渗透性下降较快，反应带可持续修复能力较差。
⑥反应带运行过程中渗透性会呈下降趋势，但下降后的渗透系数仍与初始渗透系数在同一数量级上，因此，非极端条件下，蔗糖改性纳米铁反应带的渗透性不会因为还原反应的进行而发生极端改变，堵塞现象较难发生。
（5）蔗糖改性纳米铁原位反应带修复硝基苯污染含水层
开展污染含水层修复模拟实验，研究不同含水层介质中SM-NZVI反应带对硝基苯的修复效果，考察含水层渗透性变化，具体研究成果为：
①反应带形成范围及硝基苯污染的修复效果受含水层介质影响显著。含水层介质粒径越大，反应带修复范围越大，对污染的修复效果越好。在粗砂、细砂介质中，反应带分别运行至50d、40d时进入拖尾修复期，拖尾浓度分别为87.24mg/L和170.24mg/L，60d后，苯胺的平均生成率为40.1%和20.8%。
②还原反应的进行使得槽内的pH显著升高，ORP不断下降，反应带运行结束后，粗砂、细砂介质中，pH和ORP平均值分别为8.79和-262.4mv、7.86和-235.7mv，模拟装置内呈现碱性还原环境。
③SM-NZVI的注入会使含水层出水量减少，粗砂、细砂介质中，SM-NZVI注入后，出水量分别减少了13.8%和11.9%；反应带运行过程中，出水量相对稳定，说明还原反应对含水层渗透性影响较小。
④随着SM-NZVI的注入及还原反应的进行，含水层水头变化增加，且离注入井越近水头变化越明显。
⑤SM-NZVI原位反应带的渗透系数低于原含水层介质渗透系数，但渗透系数仍为同数量级；反应运行60d后，粗砂介质中反应带渗透系数下降了60.9%，细砂介质中反应带渗透系数下降了70.6%，但均未出现含水层堵塞现象。
⑥反应带的形成和修复能力具有不均一性，注入井处SM-NZVI浓度最高，污染物还原效率也最高，随着与注入井距离的增加，SM-NZVI浓度不断减小，污染物还原效率下降; SM-NZVI随水流迁移，同时缓慢向含水层深部沉淀，因此反应带在含水层垂直方向上也具有不均一性，反应带下部SM-NZVI浓度较高，污染物还原效率更好。
本研究的创新点体现在：
（1）研究了蔗糖改性纳米铁在饱和多孔介质中的迁移机理及特性，利用滤床理论建立了SM-NZVI在地下含水层中迁移的预测模型，通过与场地工程应用中测得的实际数据比较，表明该模型预测结果具有较好的准确性，模型的建立解决了传统原位反应带修复技术注入井只能依据经验布设的问题。通过实验，得到了SM-NZVI注入速度和注入浓度与最大迁移距离之间的关系方程；采用对流弥散方程和滤床理论对SM-NZVI沉淀过程进行动力学分析；通过T-E（Tufenkji-Elimelech）理论模型计算迁移过程相关参数，阐明了SM-NZVI在饱和多孔介质中迁移的控制机理，为蔗糖改性纳米铁在地下水修复中的实际应用提供理论参考。
（2）通过模拟实验，研究了SM-NZVI反应带渗透系数的变化及其影响因素，发现含水层介质类型、地下水流速和污染物浓度是影响反应带渗透性变化的主要因素，获得了修复过程中含水层反应带渗透系数变化规律；通过对不同条件下反应带修复硝基苯效果的评估，揭示了反应带渗透性变化对硝基苯污染修复效果的影响，为提高反应带可持续修复能力奠定了基础。
（3）对SM-NZVI原位反应带技术进行了研究，通过实验室二维模拟实验创建了SM-NZVI原位还原反应带，考察了不同含水层介质中反应带对硝基苯的修复效果及地下环境pH和ORP变化情况，并进一步阐明了反应带形成及变化规律，为反应带的实际应用提供理论指导。
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 media
The 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-NZVI
The 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 IRZ
IRZ 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-NZVI
Simulation 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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