Erosion of bentonite particles at the interface of a compacted bentonite and a fractured granite

Compacted bentonite has been considered as a candidate buffer material in the underground repository for the disposal of high-level radioactive waste. An erosion of bentonite particles caused by a groundwater flow at the interface of a compacted bentonite and a fractured granite was studied experimentally under various geochemical conditions. The experimental results showed that bentonite particles could be eroded from a compacted bentonite buffer by a flowing groundwater depending upon the contact time, the flow rate of the groundwater, and the geochemical parameters of the groundwater such as the pH and ionic strength.

A gel formation of the bentonite was observed to be a dominant process in the erosion of bentonite particles although an intrusion of bentonite into a rock fracture also contributed to the erosion. The concentration of the eroded bentonite particles eroded by a flowing groundwater was increased with an increasing flow rate of the groundwater. It was observed from the experiments that the erosion of the bentonite particles was considerably affected by the ionic strength of a groundwater although the effect of the pH was not great within the studied pH range from 7 to 10. An erosion of the bentonite particles in a natural groundwater was also observed to be considerable and the eroded bentonite particles are expected to be stable at the given groundwater condition.

The erosion of the bentonite particles by a flowing groundwater did not significantly reduce the physical stability and thus the performance of a compacted bentonite buffer. However, it is expected that an erosion of the bentonite particles due to a groundwater flow will generate bentonite particles in a given groundwater condition, which can serve as a source of the colloids facilitating radionuclide migration through rock fractures.     

Sorption reversibility kinetics in the ternary system radionuclide–bentonite colloids/nanoparticles–granite fracture filling material

The kinetics of radionuclide desorption from bentonite colloids and subsequent sorption onto fracture filling material can influence colloid-facilitated radionuclide migration in ground water. To shed light on the significance of these issues batch-type experiments using a cocktail of strong and weak sorbing radionuclides as well as FEBEX bentonite colloids in the presence of fracture filling material from Grimsel (Switzerland) under Grimsel ground water conditions have been conducted. Results show that tri- and tetravalent radionuclides, ²³²Th(IV), ²⁴²Pu(IV) and ²⁴³Am(III) are clearly colloid associated in contrast to ²³³U(VI), ²³⁷Np(V) and ⁹⁹Tc(VII). Concentrations of colloid-borne ²³²Th(IV), ²⁴²Pu(IV) and ²⁴³Am(III) decrease after ∼100 h showing desorption from bentonite colloids while ²³³U(VI) and ⁹⁹Tc(VII) concentrations remain constant over the entire experimental time of 7500 h thus showing no interaction either to colloids or to the fracture filling material. ²³²Th(IV) and ²⁴²Pu(IV) data yield a slower dissociation from colloids compared to ²⁴³Am(III) indicating stronger RN–colloid interaction. In the case of ²³⁷Np(V), a decrease in concentration after ∼300 h is observed which can be explained either by slow reduction to Np(IV) and subsequent sorption to mineral surfaces in accordance with the evolution of pe/pH and/or by a slow sorption onto the fracture filling material. No influence of the different fracture filling material size fractions (0.25–0.5 mm, 0.5–1 mm and 1–2 mm) can be observed implying reaction independence of the mineral surface area and mineralogical composition. The driving force of the observed metal ion desorption from colloids is binding to fracture filling material surfaces being in excess of the available colloid surface area (76:1, 55:1 and 44:1 for the 0.25–0.5 mm, 0.5–1 mm and 1–2 mm size fraction of the FFM, respectively).     

地下水系统中镍污染和天然胶体共迁移特征

为了对地下水系统中天然胶体与Ni²⁺的共迁移特征进行研究,通过静态吸附实验和石英砂模拟含水层介质柱实验研究了土壤胶体对Ni²⁺在地下水中运移的影响,以及pH、离子强度（IS）、有机质等对土壤胶体吸附Ni²⁺的影响。结果表明：随着pH值升高,土壤胶体对Ni²⁺的吸附量增加;离子强度的增加会显著地降低土壤胶体吸附Ni²⁺的能力;腐殖酸（HA）的存在会增强胶体对Ni²⁺的吸附能力;在有胶体的情况下,Ni²⁺穿透砂柱的时间会缩短,吸附能力增强,吸附量增加,但当离子强度增加时,虽然Ni²⁺穿透砂柱的时间也被缩短,但是吸附量却降低。     

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。     

Natural colloids and suspended particles in the Whiteshell Research Area,Manitoba, Canada,and their potential effect on radiocolloid formation

Natural colloids (1–450 nm) and suspended particles (>450 nm) were characterized in groundwaters of the Whiteshell Research Area of southern Manitoba to evaluate their potential role in radionuclide transport through fractured granite. Data on particle concentrations, size distributions, compositions and natural radionuclide content were collected to predict radionuclide formation and to provide a database for future colloid migration studies. The concentrations of colloids between 10 and 450 nm ranged between 0.04 and 1 mg/l. The concentrations of suspended particles, which require higher groundwater velocities for transport, varied from 0.04 to 14 mg/l. Colloid (10–450 nm) concentrations as low as these observed in Whiteshell Research Area groundwater would have a minimal effect on radionuclide transport, assuming that radionuclide sorption on colloids is reversible. If radiocolloid formation is not reversible, and radionuclide-containing colloids cannot sorb onto fracture walls, the importance of natural colloids in radionuclide transport will depend upon particle migration properties.     

膨润土中离子扩散特征试验研究

膨润土在填埋场中有较为广泛的应用。膨润土材料作为衬垫时,扩散是污染物运移的重要方式。为深入了解不同应力状态下膨润土阻滞污染物运移的能力,对Ca2＋,Cl－和Zn2＋在不同孔隙率膨润土中的扩散进行了试验研究,得到的结果与已有文献有较好的一致性。随着孔隙率的增加,离子扩散系数增加,验证了二者之间的指数关系。讨论了在一定孔隙率范围内离子有效扩散系数的估算方法,估算结果与实测值接近     

Effet de l'agrégation sur le transport de la kaolinite recouverte d'acide humique à travers un sable de quartz d'origine naturelleAggregation effect on the transport of humic-coated kaolinite colloids through a natural quartz sand

To evaluate the risk of contaminant transport by mobile colloids, it is necessary to understand how colloids and associated pollutants behave during their migration through uncontaminated soil or groundwater. In this study, we investigated the influence of aggregation induced by Ca²⁺ and trace metals (Pb²⁺, Cu²⁺) concentrations on the transport of humic-coated kaolinite colloids through a natural quartz sand at pH=4. Adsorbed divalent cations reduce the colloids surface charge and thereby induce aggregation and deposition in porous media. To cite this article: R. Ait Akbour et al., C. R. Geoscience 334 (2002) 981–985.     

铜离子在非饱和土中迁移的离心模型试验研究

利用土工离心机考察了铜离子泄漏后6个月至3年的时间里非饱和土中铜离子的变化和污染物的迁移机理。通过4种黏性上的离心模型试验研究，总结出铜离子在这类土体中的扩散规律。铜离子在一年内，主要受重力影响和水力梯度的作用，随渗流运移，存在一个舌状竖向扩散峰，污染范围近似呈三角形；一年后扩散范围逐渐转变为近似的正方形。竖向扩散速度高于水半向。土体密实度对铜离予的扩散有较为明显的影响，有效的击实或者碾压有助下防止污染物迁移。黏粒含量对铜离子的迁移阻碍非常迅速而且有效，黏粒含量很小的变化都会对迁移范围和速度有较大的影响。     

Formation of iron-containing colloids by the weathering of phyllite

The formation of colloids during the weathering of phyllite was investigated by exposing ground phyllite to Milli-Q water. Secondary mineral colloids of 10¹–10² nm were detected in significant concentrations. At pH of about 8.5, the solution concentration of these colloids reached up to 10 mg/L (however, acidification to pH 4.0 prevented the formation of the colloids). The mineralogical composition of the secondary mineral colloids is assumed to be a mixture of ferrihydrite, manganese oxyhydroxides, aluminosilicates, amorphous Al(OH)₃ and gibbsite with possible additions of iron silicates and␣iron-alumino silicates. The colloids were stable over longer periods of time (at least several weeks), even in the presence of suspended ground rock. Direct formation of iron-containing secondary mineral colloids at the rock–water interface by the weathering of rock material is an alternative to the well-known mechanism of iron colloid formation in the bulk of water bodies by mixing of different waters or by aeration of anoxic waters. This direct mechanism is of relevance for colloid production during the weathering of freshly crushed rock in the unsaturated zone as for instance crushed rock in mine waste rock piles. Colloids produced by this mechanism, too, can influence the transport of contaminants such as actinides because these colloids have a large specific surface area and a high sorption affinity.     

Undrained cyclic behavior of bentonite–sand mixtures and factors affecting it

In this work, the cyclic behavior of bentonite–sand mixtures and factors affecting it were studied by means of a ring-shear apparatus and a scanning electron microscope. It was found that bentonite content had a significant influence on the liquefaction potential of the studied soils. A small amount of bentonite in the mixtures would cause the formation of “loose” microstructures, resulting in the occurrence of rapid liquefaction under cyclic loading, while a high bentonite content would cause the formation of clay matrixes, thus raising the soil resistance to liquefaction. In addition, the effect of pore water chemistry on the cyclic behavior of a high plasticity bentonite–sand mixture was carefully examined. It was also found that the presence of ions in pore water would change the clay microfabric, making it more open and thus more vulnerable to liquefaction. Finally, the effects of loading frequency on the cyclic behavior of mixtures with different amounts of bentonite were investigated. It was found that as the bentonite content increased, the influence became more pronounced.     

基于膨润土凝胶分形结构的屈服强度理论

在核废料处置库安全使用的设计年限（数万年至数十万年）内,膨润土遇水侵蚀,导致缓冲/回填层致密性降低、渗透性增加,危及核废料处置库安全。膨润土侵蚀是以膨润土凝胶形式迁移,膨润土凝胶的屈服强度就是侵蚀的临界剪切应力。膨润土凝胶颗粒之间的联结靠颗粒间的长程作用,即van der Waals力,颗粒间的联结作用取决于凝胶的结构。本文基于膨润土凝胶结构的分形模型,假设凝胶的屈服强度等于冲刷面上单位面积的van der Waals力的总和,导出了膨润土凝胶的屈服强度（σ_y）的表达式,表示为凝胶的固体体积率（φ_s）的幂函数,即σ_y=σ_y0φ_sm,幂函数的指数是凝胶结构分维的函数。膨润土凝胶的屈服强度理论得到了试验数据的验证。     

苯酚溶液作用下CMC改性膨润土化学相容性试验研究

膨润土的聚合物改性可有效提升竖向阻隔屏障化学相容性。有机污染物、聚合物种类对改性膨润土防渗性能研究鲜有报道。通过膨胀指数、改进滤失试验,研究苯酚溶液作用下羧甲基纤维素钠(CMC)改性膨润土的膨胀和渗透性能,结合傅里叶红外光谱分析,评价CMC分子链的特征基团对提升化学相容性的作用机理。相同苯酚浓度条件下CMC改性膨润土的膨胀指数(SI)较母土测定结果高1.5~2.5倍。同一孔隙比范围内,改性作用使膨润土渗透系数(k)降低一个数量级。CMCⅢ改性膨润土防渗性能相对最优。研究范围内,聚合物分子量和取代度未显示对SI和k的影响规律。CMC改性提升化学相容性的主因有:(1)聚合物水凝胶对膨润土颗粒间的大孔隙具有堵塞作用;(2)特征基团(羧酸根、羧基)与苯酚的酯化反应等相互作用,以及苯酚与水凝胶中水分子的氢键作用阻碍苯酚迁移,从而减轻其对压缩蒙脱石黏土矿物双电层的不利影响。     

Sorption of selenite in a multi-component system using the “dialysis membrane” method

⁷⁹Se is a potentially mobile long-lived fission product, which may make a dominant contribution to the long-term radiation exposure resulting from deep geological disposal of radioactive waste. Its mobility is affected by sorption on minerals. Selenium sorption processes have been studied mainly by considering interaction with a single mineral surface. In the case of multi-component systems (e.g. soils), it is difficult to predict the radioelement behaviour only from the mineral constituents. This study contributes to the understanding of multi-component controls of Se concentrations towards predicting Se behaviour in soils after migration from a disposal site. This goal was approached by measuring selenite sorption on mono and multi-phase systems physically separated by dialysis membranes. To the best of the authors’ knowledge, very few studies have used dialysis membranes to study the sorption competition of selenite between several mineral phases. Other workers have used this method to study the sorption of pesticides on montmorillonite in the presence of dissolved organic matter. Indeed, this method allows measurement of individual K_d in a system composed of several mineral phases. Dialysis membranes allowed (i) determination of the competition of two mineral phases for selenite sorption (ii) and determination of the role of humic acids (HAs) on selenite sorption in oxidising conditions. Experimental results at pH 7.0 show an average Se(IV) sorption distribution coefficient (K_d) of approximately 125 and 9410 L kg⁻¹ for bentonite and goethite, respectively. The average K_d for goethite decreases to 613 L kg⁻¹ or 3215 L kg⁻¹ in the presence of bentonite or HA, respectively. For bentonite, the average K_d decreases slightly in the presence of goethite (60 L kg⁻¹) and remains unchanged in the presence of HA. The experimental data were successfully modelled with a surface complexation model using the PHREEQC geochemical code. The drastic decrease in Se(IV) sorption on goethite in a multi-phase system is attributed to competition with dissolved silica released by bentonite. As with Si the HA compete with Se for sorption sites on goethite.     

Porewater chemistry in compacted bentonite: Application to the engineered buffer barrier at the Olkiluoto site

Compacted bentonite is used as sealing and buffer material in engineered barrier systems (EBS) of high-level radioactive waste repositories. The chemical characteristics of this clay and its porewater affect the migration of radionuclides eventually released from the waste. They also determine the integrity and long-term performance of the clay barriers. Key features are the structural negative charge and the large proportion of structural (interlayer) water of the main mineral montmorillonite, which leads to exclusion of anions and a surplus of cations in a large part of the porosity space. The objective of this contribution was to assess the impact of different porosity model concepts on porewater chemistry in compacted bentonite in the context of the planned Finnish spent nuclear fuel repository at Olkiluoto. First, a structural model based on well-established crystallographic and electrostatic considerations was set up to estimate the fractions of the different porosity types. In view of the uncertainty related to the chemical properties of the interlayer water, two very different model concepts (anion-free interlayer, Donnan space), together with a well-established thermodynamic model for bentonite, were applied to derive the porewater composition of the bentonite buffer at Olkiluoto. The simulations indicate very similar results in the “free” water composition for the two models and thus support the validity of the reference porewater concept commonly used in performance assessment of waste repositories. Differences between the models are evident in the composition of the water affected by the surface charge (i.e. diffuse double layer and interlayer). These reflect the conceptual uncertainty in current multi-porosity diffusion models.     

Stability assessment and characterization of colloids in coastal groundwater aquifer system at Kalpakkam

Natural colloids have a potential role in facilitating the transport of radionuclides in groundwater. To assess the role of mobile colloidal phases in radionuclide transport, characterization of colloids for size, size distribution, zeta potential, surface charge and elemental composition is required. Groundwater samples were collected from 12 borewells in the study area and were characterized with respect to physicochemical parameters. Water quality parameters such as pH, temperature, specific conductance, TDS and dissolved O₂ were measured in situ. Based on salinity, two groundwater types were identified in the study area: (1) freshwater type and (2) brackish type. Laboratory and field-scale turbidity measurements in addition to quantitative analysis of major ions were carried out. It was observed that the colloid concentrations are reduced at higher salinity. Zetasizer, particle size analyzer, scanning electron microscopy, energy dispersive X-ray analysis and X-ray diffraction techniques were used to characterize the colloidal particles in groundwater. Colloids were present in all the groundwater samples, the concentration varying between 0.05 and 6 mg/L. Colloid concentration was greater in wells with low ionic strength and the number of colloidal particles varied between 3 × 10⁹ and 4 × 10¹¹ particles/L. The average colloid size varied between 200 and 350 nm for various samples. The zeta potential of the colloidal particles varied between −25.5 and –34.0 mV. SEM analysis of colloidal particles revealed the presence of clays particularly kaolinite and that the mineral composition of colloids reflects the mineralogy of the aquifer. For proper risk assessment and remediation studies, the role of colloids in radionuclide transport assumes greater importance. This study highlights the need for, and relevance of, detailed colloid characterization to assess its role in the migration of radionuclides from near surface disposal facilities.     

Feasibility of using fly ash,lime, and bentonite to neutralize acidity of pore fluids

Acidic groundwater resulting from the poorly planned use of acid sulfate soils has become a major environmental issue in coastal Australia over the last several years. Use of permeable reactive barriers (PRBs) designed to generate alkalinity by promoting sulfate reduction has recently become popular as an alternative solution to this problem. However, recent studies have also revealed that the long-term performance of such PRBs can be significantly undermined by chemical precipitation and clogging of pore space, which would decrease the buffer capacity and hydraulic conductivity of the reactive material. This study seeks to explore the feasibility of using bentonite in addition to lime and fly ash to form mixtures with a high buffer capacity and permeability that would enable groundwater flow through PRBs over a substantial period of time. A series of laboratory experiments, including buffer capacity and leaching tests, were performed on different mixtures of fly ash with lime and bentonite using acidic fluids of low pH. It was found that the ability of such mixtures to neutralize acidic fluids was mostly controlled by the content of lime. Laboratory data also showed that an addition of bentonite to lime—fly ash mixtures could decrease the buffer capacity of soil. Compaction tests indicated that the presence of bentonite would increase the dry density of mixtures at the optimum moisture content. A series of hydraulic conductivity tests were carried out to study changes in the coefficient of permeability of lime—fly ash mixtures with different contents of bentonite permeated with acidic liquids. The obtained results revealed that the coefficient of permeability of the specimens tended to increase over a period of time, likely due to the changes in the diffuse double layer of bentonite particles.     

Gas migration behavior in saturated bentonite under flexible conditions with consideration of temperature effects

Acta Geotechnica - Investigation of thermal effects on gas migration behavior in saturated bentonite materials is of great significance for the assessment of long-term safety and performance...     

Minimal alteration of montmorillonite following long-term interaction with natural alkaline groundwater: Implications for geological disposal of radioactive waste

Bentonite is one of the more safety-critical components of the engineered barrier system in the disposal concepts developed for many types of radioactive waste. Bentonite is utilised because of its favourable properties which include plasticity, swelling capacity, colloid filtration, low hydraulic conductivity, high retardation of key radionuclides and stability in geological environments of relevance to waste disposal. However, bentonite is unstable under the highly alkaline conditions induced by Ordinary Portland Cement (OPC: initial porewater pH > 13) and this has driven interest in using low alkali cements (initial porewater pH9-11) as an alternative to OPC. To build a robust safety case for a repository for radioactive wastes, it is important to have supporting natural analogue data to confirm understanding of the likely long-term performance of bentonite in these lower alkali conditions. In Cyprus, the presence of natural bentonite in association with natural alkaline groundwater permits the zones of potential bentonite/alkaline water reaction to be studied as an analogy of the potential reaction between low alkali cement leachates and the bentonite buffer in the repository. Here, the results indicate that a cation diffusion front has moved some metres into the bentonite whereas the bentonite reaction front is restricted to a few millimetres into the clay. This reaction front shows minimal reaction of the bentonite (volumetrically, less than 1% of the bentonite), with production of a palygorskite secondary phase following reaction of the primary smectites over time periods of 10⁵–10⁶ years.     

Hydrothermal transformations in an aluminophosphate glass matrix containing simulators of high-level radioactive wastes

The interaction of aluminophosphate glass with water at 95°C for 35 days results in glass heterogenization and in the appearance of a gel layer and various phases. The leaching rate of elements is low owing to the formation of a protective layer on the glass surface. It is shown that over 80% of uranium leached from the glass matrix occurs as colloids below 450 nm in size characterized by high migration ability in the geological environment. To determine the composition of these colloids is a primary task for further studies. Water vapor is a crystallization factor for glasses. The conditions as such may appear even at early stages of glass storage because of the failure of seals on containers of high-level radioactive wastes. The examination of water resistance of crystallized matrices and determination of the fraction of radionuclide in colloids are also subjects for further studies.     

Potential for the formation and migration of colloidal material from a near-surface waste disposal site

Organic material typically constitutes a substantial volume (∼ 90%) of the low-level radioactive wastes (LLRW) intended for near-surface disposal at Chalk River Laboratories (CRL), Ontario, Canada. These wastes can contain a large variety of organic materials, including paper, cardboard, plastic bags, used clothing, and mop heads. After emplacement in a disposal facility, leaching of the LLRW by water can mobilize inorganic and organic substances, ranging from small molecules such as acetic acid to unidentifiable material of colloidal size range. This study determined the potential for colloid formation produced by LLRW degradation, because colloid-facilitated transport of contaminants could affect the safety performance of a disposal facility.The decomposition of compacted LLRW was simulated by recirculating water in a closed system over several compacted bales of waste to determine the potential composition and colloid content of leachates. Size fractionation of organic matter was performed on leachate samples that had been aged for 18 months to simulate the microbial degradation of organic matter within leachates during migration out of the LLRW disposal facility. The aged leachates contained high concentrations of dissolved organic matter, ranging between 74 and 5074 mg/l as C. In most of the leachates, volatile fatty acids accounted for a significant fraction (up to 81%) of the dissolved organic carbon. Although 5–110 mg/l of organic colloids were observed in leachates, in most cases, the organic colloids made up a very small fraction of the total leached organic carbon. Therefore, since the complexation properties of dissolved and colloidal organics are probably similar, contaminants complexed to organics are most likely to be dissolved and not affected by colloid transport. The leachates also contained significant quantities of Fe and Al, which could potentially precipitate Fe and Al as colloids after oxidation. Although a significant portion of the dissolved Fe may have been produced by the corrosion of the ☐es used to contain the bales, the high Fe concentrations could be representative of leachates from LLRW that contain metallic Fe components. If Fe and Al colloids are stable, stable concentrations in LLRW leachates could be high enough to affect contaminant transport. Therefore, the Fe and Al content of LLRW should be minimized. The concentrations of natural colloids in sandy aquifers, such as those found at CRL are too low to affect contaminant migration significantly.