[论文解读] A simple criterion and experiments for onset of flocculation in kaolin clay suspensions

Nicholas Rommelfanger, Bernhard Vowinckel+4 more|arXiv (Cornell University)|Mar 29, 2022

Soil and Unsaturated Flow被引用 9

一句话总结

本研究提出了一种简单且基于物理原理的判据，利用无量纲参数Φ来预测高岭土悬浮液中絮凝的起始。实验表明，絮凝在极低盐浓度下即发生——NaCl为0.6 mM（≈0.04 ppt），CaCl₂为0.04 mM（≈0.004 ppt）——表明高岭土在几乎所有天然水体环境中均会发生絮凝，且对于单价离子，絮凝阈值与黏土浓度无关；而对于二价离子，由于表面电荷中和作用，阈值随浓度略有升高。

ABSTRACT

Cohesive effects between fine-grained sediment particles greatly influence their effective settling rate and erodibility. Many studies have observed a qualitative difference in settling dynamics between clays in freshwater, where particles remain dispersed, and in saltwater, where aggregates form and settle rapidly. The critical coagulation concentration (CCC) of salt that separates the two regimes however remains under-investigated, even though knowledge of the CCC is crucial to understanding aggregation in settings such as estuaries, where large salt concentration gradients occur. Furthermore, no simple criterion exists to predict the CCC for clay suspensions. In this study, systematic experiments are performed to determine the CCC, by measuring transmitted light intensity through clay suspensions. To investigate the effect of ion valence, sodium chloride (NaCl) and calcium chloride (CaCl$_2$) are used. For kaolin clay, the results show a CCC of 0.6mM NaCl ($\approx 0.04$ppt NaCl $=$ 0.04 PSU), and of 0.04mM CaCl$_2$ ($\approx 0.004$ppt CaCl$_2$). Because these salinities are lower than those commonly observed in nature, these findings indicate that kaolin clay should flocculate in nearly all natural aquatic environments. Furthermore, due to the fact that tap water often has salinities higher than this threshold, these results imply that great care is needed in experiments, especially in large facilities where using distilled water is not feasible. In addition, a simple criterion to estimate the CCC for a kaolin clay suspension is derived. This criterion predicts that flocculation occurs at extremely low salt concentrations and is approximately independent of clay concentration, in agreement with the experimental observations and consistent with experimental evidence from the literature.

研究动机与目标

• 确定高岭土悬浮液在淡水和咸水环境中的临界絮凝浓度（CCC）。
• 开发一种简单且易于应用的判据，以预测絮凝起始，而无需依赖复杂的DLVO理论。
• 研究离子价态（Na⁺与Ca²⁺）对高岭土悬浮液中絮凝阈值的影响。
• 解决现场观测与实验室结果在天然水体中盐诱导絮凝方面存在的矛盾。
• 为实验设计提供实用指导，特别是在无法使用去离子水的实验设施中。

提出的方法

• 通过测量高岭土悬浮液中透射光强度的系统实验，以检测絮凝起始点。
• 使用NaCl和CaCl₂研究离子价态对临界絮凝浓度（CCC）的影响。
• 在不同黏土浓度（4–40 ppt）下测量CCC，以评估其浓度依赖性。
• 基于物理参数推导出一个无量纲参数Φ，用于预测絮凝起始。
• 将实验结果与文献数据及理论预期（如Schulze-Hardy规则）进行比较。
• 利用环境扫描电子显微镜（ESEM）和离心分离技术进行颗粒表征和悬浮液制备。

实验结果

研究问题

• RQ1 高岭土悬浮液在何种盐浓度下开始絮凝？
• RQ2 离子价态（Na⁺与Ca²⁺）如何影响高岭土悬浮液中的临界絮凝浓度？
• RQ3 临界絮凝浓度是否依赖于悬浮液中的黏土浓度？
• RQ4 能否提出一种简单且具有物理解释的判据，以预测絮凝起始而无需进行完整的DLVO计算？
• RQ5 为何现场观测有时与实验室中关于盐诱导絮凝的发现相矛盾？

主要发现

• 高岭土在NaCl溶液中的临界絮凝浓度（CCC）为0.6 mM（≈0.04 ppt NaCl），显著低于典型自来水的盐度。
• 在CaCl₂溶液中，CCC为0.04 mM（≈0.004 ppt CaCl₂），约为NaCl的1/15，与Schulze-Hardy规则一致。
• NaCl的CCC与黏土浓度无关，表明Stern层中Na⁺吸附可忽略。
• CaCl₂的CCC随黏土浓度升高而略有增加，可能归因于Ca²⁺在Stern层的吸附及表面电荷中和。
• 所提出的无量纲参数Φ在Φ ≈ 10⁻³时可预测絮凝起始，为DLVO理论提供了一种简单替代方案。
• 由于CCC值极低，高岭土悬浮液在几乎所有天然水体环境中均会发生絮凝，这挑战了实验流体动力学中的既有假设。