A Review of Colloid Transport in Fractured Rocks

Recent recognition of colloid and colloidassociated transport of strongly sorbing contaminants in fractured rocks highlights the importance of exploring the transport behavior of colloids under conditions prevailing in the field.The rapid transport of colloids through fractured rocks-as affected by the hydraulic properties of the flow system,the properties of fracture surface and the geochemical conditionshas not been sufficiently elucidated,and predictions of colloid transport through fractures have encountered difficulties,particularly at the field scale.This article reviews the current understanding of the mechanisms and modeling of colloid transport and retention in fractured rocks.Commonly used experimental techniques and approaches for conducting colloid transport experiments at different scales,ranging from the laboratory to the field scale,are summarized and commented upon.The importance of various interactions(e.g.,dissolution,colloid deposition,generation,mobilization and deposition of filling materials within fractures) between the flowing solution and the fracture walls(in many cases,with skin or coating on the host rock at the liquid-solid interface) has been stressed.Colloid transport through fractures of high heterogeneity has not yet been well understood and modeled at the field scale.Here,we summarize the current knowledge and understanding accumulated in the last two decades in regard to colloid and colloidassociated transport through fractures.Future research needs are also discussed.

Surfactant-enhanced flushing enhances colloid transport and alters macroporosity in diesel-contaminated soil

Soil contamination by diesel has been often reported as a result of accidental spillage,leakage and inappropriate use. Surfactant-enhanced soil flushing is a common remediation technique for soils contaminated by hydrophobic organic chemicals. In this study, soil flushing with linear alkylbenzene sulfonates（LAS, an anionic surfactant） was conducted for intact columns（15 cm in diameter and 12 cm in length） of diesel-contaminated farmland purple soil aged for one year in the field. Dynamics of colloid concentration in column outflow during flushing, diesel removal rate and resulting soil macroporosity change by flushing were analyzed. Removal rate of n-alkanes（representing the diesel） varied with the depth of the topsoil in the range of 14%–96% while the n-alkanes present at low concentrations in the subsoil were completely removed by LAS-enhanced flushing. Much higher colloid concentrations and larger colloid sizes were observed during LAS flushing in column outflow compared to water flushing. The X-ray micro-computed tomography analysis of flushed and unflushed soil cores showed that the proportion of fine macropores（30–250 μm in diameter）was reduced significantly by LAS flushing treatment. This phenomenon can be attributed to enhanced clogging of fine macropores by colloids which exhibited higher concentration due to better dispersion by LAS. It can be inferred from this study that the application of LAS-enhanced flushing technique in the purple soil region should be cautious regarding the possibility of rapid colloid-associated contaminant transport via preferential pathways in the subsurface and the clogging of water-conducting soil pores.

Evolution of fracture permeability due to co-colloidal bacterial transport in a coupled fracture-skin-matrix system

A numerical model is developed for investigating the evolution of fracture permeability in a coupled fracture-matrix system in the presence of fracture-skin with simultaneous colloidal and bacte- rial transport, by taking into account the effects of thermal stress and silica precipitation/dissolution, which is computed using linear reaction kinetics. The non-linear coupled equations are numerically modeled using the fully implicit finite difference method and a constant continuous source is adopted while modeling thermal, contaminant, colloidal and bacterial transport. Due to co-colloid bacterial trans- port under non-isothermal conditions, in a coupled fracture-skin-matrix system, the fracture apertures vary spatially, with a corresponding pressure variation for a constant discharge. A series of numerical experiments were conducted for analyzing the spatial variation of fracture aperture in response to the combined effects of thermal stress, silica precipitation/dissolution, and simultaneous colloidal and bacte- rial transport in the presence of the fracture-skin. The simulation results suggest that temperature and contaminant concentration of the mobile fluid within the fracture increases with reduction in initial frac- ture aperture. The pattern of variation followed by the fracture aperture is nearly the same in the presence and absence of bacterial transport but the magnitude of the fracture aperture is low under the influence of bacterial transport. The variation in the fracture aperture resulting from precipitation-dissolution and thermoelastic stress is significant when the fracture aperture is very low and reduces with increment in fracture aperture. The variation in fracture aperture and pressure remains the same for both undersaturated and supersaturated fluid entering the fracture due to the influence of bacterial transport at the inlet of the fracture.

Effect of Clay Minerals on Transport of Surfactants Dispersed Multiwalled Carbon Nanotubes in Porous Media

Clay minerals can hinder the transport of various contaminants in soil and aquifer, but how clay minerals affect the transport of nanoparticles in aquifers has not been investigated in depth. In this paper, the transport of surfactants dispersed multi-walled carbon nanotubes（MWCNTs） in welldefined quartz sand and mixtures of quartz sand and clay minerals（kaolinite and montmorillonite） with varying ionic strengths was studied. Sodium dodecyl benzenesulfonate（SDBS） and octyl-phenolethoxylate（TX100） MWCNT suspensions can migrate through quartz sand easily, but the presence of less than 2% w/w clay minerals in quartz sand can significantly hinder the transport of MWCNT suspensions, especially at high ion strength（0.6 m M CaCl2）. The inhibition mechanism of clay minerals for surfactant-dispersed MWCNTs in porous media is the interception of MWCNTs. Kaolinite has stronger inhibition effect for MWCNTs transport than montmorillonite because more kaolinite can be retained in the quartz sand. Adsorption of surfactants by clay minerals does not affect the transport of MWCNTs significantly. This finding is important for the environmental assessment of MWCNT transport risks in soils and aquifers.

Enhancing Groundwater Quality through Computational Modeling and Simulation to Optimize Transport and Interaction Parameters in Porous Media

There is a shortage of high quality drinking water caused by the introduction of contaminants into aquifers from various sources including industrial processes and uncontrolled sewage. Studies have shown that colloids, collections of nanoparticles, have the potential to remediate polluted groundwater. For such applications of nanoparticles, it is important to understand the movement of colloids. This study aims to enhance the previously developed MNM1D (Micro- and Nanoparticle transport Model in porous media in one-dimensional geometry) by making more realistic assumptions about physical properties of the groundwater-porous medium system by accounting for a non-constant flow velocity and the presence of electromagnetic interactions. This was accomplished by coupling the original model with the Darcy-Forchheimer fluid model, which is specific to transport in porous media, coupled with electromagnetic effects. The resulting model also accounts for attachment and detachment phenomena, both of the linear and Langmuirian type, as well as changes to hydrochemical parameters such as maximum colloidal particle concentration in the porous medium. The system of partial-differential equations that make up the model was solved using an implicit finite-difference discretization along with the iterative Newton’s method. A parameter estimation study was also conducted to quantify parameters of interest. This more realistic model of colloid transport in porous media will contribute to the production of a more efficient method to counteract contaminants in groundwater and ultimately increase availability of clean drinking water.

Tracing Water Flow and Colloidal Particles Transfer in an Unsaturated Soil

In recent years, many studies have been carried out on colloidal particle transfer in the unsaturated zone because they can be a risk to the environment either directly or as a vector of pollutants. A study was conducted on the influence of porous media structure in unsaturated conditions on colloidal particle transport. Three granular materials were set up in columns to replicate a fluvio-glacial soil from the unsaturated zone in the Lyon area (France). It is a sand, a bimodal mixture in equal proportion by weight of sand and gravel, and a fraction of bimodal mixture. Nanoparticles of silica (SiO2-Au-FluoNPs), having a hydrodynamic diameter between 50 and 60 nm, labeled by organic fluorescent molecules were used to simulate the transport of colloidal particles. A nonreactive tracer, bromide ion (Br-) at a concentration of C0,s = 10-2 M was used to determine the hydrodispersive properties of porous media. The tests were carried out first, with a solution of nanoparticles (C0,p = 0.2 g/L) and secondly, with a solution of nanoparticles and bromine. The transfer model based on fractionation of water into two phases, mobile and immobile, MIM, correctly fits the elution curves. The retention of colloidal particles is greater in the two media of bimodal particle size than that in the sand, which clearly demonstrates the role of textural heterogeneity in the retention mechanism. The increase in ionic strength produced by alimenting the columns with colloidal particle suspension in the presence of bromide, increases retention up to 25% in the sand. The total concentration profile of nanoparticles collected at the end of the experiment shows that the colloidal particles are retained primarily at the entrance of the columns. Hydrodispersive calculated parameters indicate that flow is more heterogeneous in bimodal media compared to sand.

Stability of Soil and Biosolid Nanocolloid and Macrocolloid Particles in the Absence and Presence of Arsenic, Selenium, Copper and Lead

Due to their enhanced stability and contaminant transport potential, environmental nanoparticles derived from soil and biosolid materials may pose a considerable risk to groundwater quality. Very little information exists on the stability and transportability of environmental or natural nanocolloids in the presence of As, Se, Pb and Cu contaminants, all of which are considered to represent substantial threats to human and animal populations through groundwater contamination. This study involved stability settling experiments of nanocolloids (NCs) (<100 nm) and macrocolloids (MCs) (100 - 2000 nm) fractionated from Bt horizons of three Kentucky soils and one biosolid waste material in water suspensions of 0, 2, and 10 mg·L-1 of As, Se, Pb and Cu. The results indicated greater stability in the mineral than the biosolid colloid fractions, and enhanced stability of NCs over corresponding MCs in the presence or absence of contaminants at low contaminant loads. At high contaminant loads nearly all colloids were unstable except for the bio-nanocolloids which still sustained considerable stability. At low contaminant loads, the MC fraction stability sequence was smectitic > mixed > kaolinitic > biosolid. Among the nano-fractions, the smectitic and kaolinitic colloids demonstrated lower stability than the MCs, but higher than those of the mixed and biosolid fractions. Physicochemical characterizations indicated that extensive organic carbon surface coatings and higher Al/Fe:Si ratios may have induced higher stability in the NC fractions, but their overall stability may also have been hindered in some cases by nano-aggregation phenomena.

多孔介质中多环芳烃与细菌共运移数值模拟

地下水中微生物等胶体对多环芳烃的运移具有促进作用,为进行多环芳烃污染的精准、高效修复,需要建立准确、可靠的多环芳烃与细菌胶体共运移数值模拟模型。研究基于室内砂柱荧蒽运移系列实验,采用Hydrus中Colloid-Facilitated Solute Transport(C-Ride)模块构建荧蒽与细菌FA1共运移数值模型,并采用马尔科夫链蒙特卡洛方法(Markov Chain Monte Carlo,MCMC)进行模型参数不确定性分析,定量刻画荧蒽在水动力和微生物胶体作用下的运移过程。结果表明:基于Hydrus C-Ride模块和MCMC参数不确定性分析,能够准确地刻画荧蒽和细菌FA1的共运移过程;细菌FA1促进了荧蒽在多孔介质中的迁移速度,且导致荧蒽在多孔介质中运移回收率的增加,即由55.06%提升至76.16%,其中吸附至可移动胶体迁移和随水流迁移贡献的回收率分别为41.46%、34.69%。研究成果对于指导地下水污染微生物修复方案的优化设计具有重要的理论和现实意义。

胶体颗粒在多孔介质中的迁移沉积行为研究进展

胶体在地下环境中的迁移沉积行为是其重要环境过程之一。胶体穿过土壤层进入地下水环境后,其携带污染物会随地下水流而迅速扩散迁移。由于地下环境相对复杂,地下水污染通常较难治理和控制,其会直接或间接地影响生态环境以及人类健康。因此,研究胶体在多孔介质中的迁移和沉积机制,对维护地下水安全有重要的实际意义。从五个方面综述了胶体在多孔介质中迁移沉积的影响因素,包括多孔介质特性、土壤类型结构和化学因素,并对未来研究的发展进行了展望。

猪粪源溶解性有机质对锰矿区耕地土壤中镉迁移的影响

镉(Cd)是迁移潜力最大的重金属元素之一。土壤中Cd迁移过程及影响因素的研究对于土-水生态系统中Cd的环境风险评估及修复治理有重要意义。为探求外源富碳有机质对金属矿区污染耕地土壤中Cd迁移的影响,开展原状土柱淋洗试验,考察猪粪源溶解性有机质(DOM)的引入(232 mg/L)对胶体结合态Cd(C-Cd)和水溶态Cd(D-Cd)迁移过程的差异及影响机制。结果表明:①猪粪DOM引入前,土壤胶体是Cd迁移的主要载体,胶体浓度是影响总镉(T-Cd)和C-Cd浓度动态变化的主要因素;猪粪DOM引入后,原状土柱淋洗过程中T-Cd和C-Cd的迁移量分别是对照阶段的1.8倍和2.2倍,pH成为影响Cd迁移的关键因素。②猪粪DOM的引入主要通过提升溶液pH,促进土壤胶体的迁移(迁移量较对照阶段增加了68.4%)进而增加C-Cd的迁移量(较对照阶段增加了124.4%)。同时,pH的增加也抑制了Cd^(2+)的解吸,导致D-Cd迁移量较对照阶段减少了41.2%。③猪粪DOM引入后的淋洗过程中,DOM通过提升土壤胶体Cd富集系数(由32.6mg/kg增至43.4mg/kg),进一步提升了土壤胶体对Cd迁移的支配作用,T-Cd中C-Cd的占比高达90.1%。因此,在农用地Cd污染土壤或场地污染土壤修复过程中涉及外源富碳有机质(如畜禽粪便、城市污泥、堆肥/生物质炭)应用时,应重点关注外源DOM促进胶体结合态Cd迁移进入浅层地下水的环境风险。

不同环境因子对黑土胶体在饱和多孔介质中运移特性的影响

为评价不同环境条件下黑土胶体在地下水多孔介质中的运移,采用离心法对采自于黑龙江海伦黑土样品中的胶体颗粒进行提取.通过胶体在石英砂填充柱中的淋溶试验研究了pH、离子强度和溶解性有机质等环境因子对胶体在多孔介质中运移特性的影响.结果表明:当pH在4～9的范围内变化时,MR(回收率)和k(沉积速率系数)没有发生显著变化,即其对胶体运移能力影响不显著.随着离子强度从0.001 mmol/L升至1 mmol/L,黑土胶体穿透曲线MR降低,k值升高,说明高无机离子强度使胶体易于沉积在石英砂表面,运移能力降低.当溶液中含有溶解性有机质HA(胡敏酸)和FA(富里酸)时,MR降低,k值增加,表明胶体运移能力受到抑制,并且FA对胶体颗粒运移的抑制作用比HA强.溶液离子强度、HA和FA含量对黑土胶体在饱和多孔介质中的运移过程影响较大,是影响地下水中胶体运移过程的关键环境因子.

孔隙水流速对胶体在饱和多孔介质中运移的影响

采用室内石英砂柱出流实验和数学模型,研究了流速对自然土壤胶体、蒙脱石胶体和腐殖酸钠胶体在饱和多孔介质中运移的影响。结果表明:孔隙水流速影响着胶体在多孔介质中的运移过程,流速的提高可加速土壤胶体的出流过程,减小蒙脱石胶体和腐殖酸钠胶体的沉积率。胶体的运移过程除受孔隙水流速的影响外,胶体特性的不同亦决定着胶体运移过程的差异,由于胶体表面性质不同,与介质之间的作用力不同,决定了胶体运移过程中的沉积和释放。

地下水系统中胶体的形成机理及其对污染物迁移的影响

胶体能有效地吸附地下水中的污染物并对其迁移距离及速度产生显著的影响 ,因此 ,研究胶体在地下水中的性质具有重要的意义。鉴于这一研究工作所存在的困难 ,叙述了在地下水中胶体的取样方法及分析技术 ,且以分析胶体在地下水系统中的形成机理为基础 ,对胶体的迁移与富集规律的定量模拟预测作了简要的介绍。

土壤胶体在不同饱和度土壤介质中的释放与淋溶行为研究

采用饱和与非饱和填充土柱纵向淋溶研究方法,结合对流弥散模型方程(CDE)对穿透曲线的拟合计算,全面考察了土壤介质水饱和度、土壤水pH/离子强度、土壤孔隙水流速和土壤胶体颗粒大小对天然土壤胶体在实际土壤介质中释放、沉积迁移行为的影响。分别获取胶体扩散系数和阻滞因子值,定量说明实验中水化学、水动力学等条件的作用影响力。结果显示,介质不饱和条件不利于胶体的释放和淋溶;高pH和低离子强度条件对土壤胶体释放与迁移有利;淋溶过程的间断干扰,可以促使土壤胶体的增量淋溶释放;淋溶强度及胶体颗粒粒径大小,能够影响胶体穿透时间和穿透浓度峰值大小。

高pH环境对胶体在饱和多孔介质中迁移的影响

以超纯水漂洗和经酸处理过的玻璃微珠作为对照,通过比较在不同p H环境下胶体在两种多孔介质中的吸附解吸行为来探讨高p H环境对胶体迁移的影响.结果表明,不利条件下通过提高环境的p H可以使吸附在初级势阱中的胶体解吸下来,从而降低胶体的吸附能力.不利条件下化学杂质有利于胶体在初级势阱中的吸附.然而随着环境p H的增加,这些因化学杂质而发生吸附的胶体会有部分被释放出来.当溶液的p H增加到10或更高后,化学杂质对胶体吸附过程的影响可以忽略.胶体在多孔介质表面的吸附数量与胶体的吸附解吸过程和溶液的环境有关,尤其是高p H值环境.研究证实了传统DLVO理论存在一定局限性.本研究为高p H环境在胶体迁移中发挥的作用提供了进一步的认识.

介质粗糙度和铅污染对粘土矿物胶体运移的影响

土壤胶体在多孔介质中的运移受多种环境因素的影响,其中胶体特性和介质条件是决定胶体运移过程的关键因素。选取土壤体系中不同结构类型的粘土矿物胶体高岭石和蒙脱石作为实验材料,通过胶体对铅的吸附实验,制得被铅污染的胶体,利用室内砂柱模拟实验,分别研究了水洗和烘烤处理得到的表面粗糙度不同的石英砂介质条件下,污染与未污染的四种特性胶体在多孔介质中的迁移规律。结果表明:铅的存在降低了高岭石胶体和蒙脱石胶体在饱和多孔介质中的流动性;介质表面粗糙度会显著影响蒙脱石胶体的运移过程,而对高岭石胶体基本无影响;胶体粒径大小是决定胶体运移过程是否受介质表面影响的主要因素。

稠油胶体体系的研究与应用进展

综述了稠油胶体体系理论在国内外研究应用的进展,分析了稠油胶体体系具有的双电层效应、稳定性与流变特征等性质,介绍了稠油胶体体系在稠油开采、输送和加工中的应用,在此基础上提出稠油胶体体系理论研究过程中存在的问题,并探讨了今后稠油胶体体系的研究方向。

咸水中胶体迁移-沉积对砂介质渗透性损失的数学模型

采用室内土柱试验,研究咸水中胶体迁移引起砂介质渗透性变化的特征,通过胶体迁移-沉积作用的分析对砂介质渗透性损失的机理进行了探讨.结果表明:砂介质渗透性随孔隙体积数变化的动态特征符合Boltzmann函数;砂介质渗透性总的损失率为37.4%;渗透系数的损失量随孔隙体积数的增加而增加.胶体在砂介质中的穿透曲线呈"S"型,其中的2个拐点将曲线分成开始穿透、快速穿透和平稳穿透3个阶段,但并没有完全穿透,其迁移过程中发生了沉积.胶体沉积作用是砂介质渗透性降低的主要原因.

地下环境中胶体促使下的污染物运移研究进展

大量的研究证据表明,胶体能促使地下环境中污染物的运移。胶体的沉积、释放和运移等动力学过程受许多因素的控制,如胶体的表面性质(亲水或疏水)、胶体的稳定性、水溶液的化学组分、离子强度、pH值和水的流速等。许多研究者已提出了考虑不同影响因素的模型从理论上或用实验室土柱试验来模拟胶体的运移行为。在未来应加强野外条件下不同类型的天然介质中胶体促使下污染物运移的影响因素、作用机制和运移过程的研究,这有助于人们提出有效的策略或发展新的治理技术以防止污染的扩大或修复污染,最终达到保护地下环境的目的。

紫色土坡地泥岩裂隙潜流中的胶体迁移

为探求自然胶体迁移进入地下水的潜力,建立原位坡地径流观测场,研究了2013年夏季3场不同雨型降雨事件下,紫色土坡地（1 500 m2）泥岩裂隙潜流中自然胶体的迁移规律。结果表明：裂隙潜流中胶体对降雨的响应时间为30~90 min,比潜流对降雨的响应更迅速,且取决于坡地雨前干旱情况及降雨强度;胶体浓度峰早于潜流流量峰,峰值浓度相对背景浓度可增加1~2个数量级,最大雨强及雨型决定潜流流量和胶体浓度峰型。气液界面是胶体初始迁移响应的主要驱动因素,雨水混合土壤前期水对土壤介孔和大孔内壁胶体的剪切、裹携是胶体释放、分散与迁移的主要机制。因此,胶体辅助运移可能成为紫色土地区吸附性较强的污染物（如磷、疏水性农药等）的重要迁移方式。