Impact of the Salt Concentration and Biophysical Cohesion on the Settling Behavior of Bentonites
Impact of the Salt Concentration and Biophysical Cohesion on the Settling Behavior of Bentonites
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DOI：
10.3389/feart.2022.886006
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发表时间：
2022-07
期刊：
影响因子：
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通讯作者：
Ellen Krahl;B. Vowinckel;L. Ye;T. Hsu;A. Manning
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文献类型：
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作者：
Ellen Krahl;B. Vowinckel;L. Ye;T. Hsu;A. Manning
粘土矿物在水环境中的絮凝行为是河口和河流动力学中的一个重要过程，在那里可以局部发生强烈的盐度梯度。关于盐浓度对粘性沉积物沉降过程的影响，文献中报道了各种相互矛盾的观察结果。为了系统地解决这个问题，我们研究了粘土矿物的沉降行为作为环境水的盐浓度的函数。具体来说，我们专注于蒙脱石作为一种原型粘土矿物具有高阳离子交换容量（CEC）。为此，我们研究了怀俄明州膨润土（Volclay SPV）作为许多建筑和工业用途的一个非常重要的组成部分的悬浮液。我们进行了一项实验活动，以研究中等浑浊的蒙脱石浓度在不同盐度（氯化钠）的单价盐溶液中的沉降行为，以代表不同的环境，从去离子到海水，分别。随后的沉降过程通过在长达48 h的总观察时间内以规则的时间间隔用相机拍照来监测。此外，进行改进的比重计分析以确定絮凝的粘土在盐水中的悬浮液的粒度分布（就当量直径而言）。尽管相当高的阳离子交换容量的调查粘土（CEC=88.1），我们的研究结果表明，沉降速度急剧增加的范围内的0.6-1.0 PSU和保持近似恒定的较高的盐度。这个临界盐浓度在这里被定义为临界凝聚浓度（CCC），并且远低于天然开放水体的盐度。比重计分析表明，60%的团聚体超过20 μm的等效粒度。最后，本研究的结果补充实验研究胞外聚合物（EPS）对膨润土在盐水中的絮凝行为的影响。我们的研究结果表明，盐度是絮凝的原始触发器，而EPS允许更大的絮凝体尺寸，但它并没有发挥显着的作用，在河口环境中的膨润土的沉降过程。
The flocculation behavior of clay minerals in aquatic environments is an important process in estuarine and riverine dynamics, where strong gradients in salinity can locally occur. Various contradicting observations have been reported in the literature on the impact of salt concentration on the settling process of cohesive sediments. To address this issue in a systematic manner, we investigate the settling behavior of clay minerals as a function of the salt concentration of the ambient water. Specifically, we focus on montmorillonite as a prototype clay mineral with a high cation exchange capacity (CEC). To this end, we study suspensions of Wyoming bentonite (Volclay SPV) as a very important constituent for many constructional and industrial purposes. We perform an experimental campaign to study the settling behavior of moderately turbid montmorillonite concentrations in monovalent salt solutions with different salinities (sodium chloride) to represent different environments ranging from deionized to ocean water, respectively. The subsequent settling process was monitored by taking pictures by a camera in regular time intervals over a total observation time up to 48 h. In addition, a modified hydrometer analysis is conducted to determine the grain size distribution (in terms of an equivalent diameter) of the flocculated clay suspension in salt water. Despite the rather high cation exchange capacity of the investigated clay (CEC=88.1), our results show that the settling speed drastically increases within a range of 0.6–1.0 PSU and stays approximately constant for higher salinities. This critical salt concentration is defined here as the critical coagulation concentration (CCC) and lies well below the salinity of natural open water bodies. The hydrometer analysis revealed that 60% of the agglomerates exceed the equivalent grain size of 20 μm. Finally, the findings of this study are supplemented with experiments studying the effect of Extracellular Polymeric Substances (EPS) on the flocculation behavior of bentonite in salt water. Our results demonstrate that salinity is the original trigger for flocculation, whereas EPS allows for even larger floc size but it does not play a significant role for the settling processes of bentonite in estuarine environments.