Costanza-Robinson Molly S, Henry Eric J
Middlebury College, Program for Environmental Studies, Department of Chemistry & Biochemistry, 276 Bicentennial Way, Middlebury, VT 05753, USA.
University of North Carolina-Wilmington, Department of Geography and Geology, 601 South College Road, Wilmington, NC 28403, USA.
Chemosphere. 2017 Mar;171:275-283. doi: 10.1016/j.chemosphere.2016.12.072. Epub 2016 Dec 21.
Surfactant miscible-displacement (SMD) column experiments are used to measure air-water interfacial area (A) in unsaturated porous media, a property that influences solute transport and phase-partitioning. The conventional SMD experiment results in surface tension gradients that can cause water redistribution and/or net drainage of water from the system ("surfactant-induced flow"), violating theoretical foundations of the method. Nevertheless, the SMD technique is still used, and some suggest that experimental observations of surfactant-induced flow represent an artifact of improper control of boundary conditions. In this work, we used numerical modeling, for which boundary conditions can be perfectly controlled, to evaluate this suggestion. We also examined the magnitude of surfactant-induced flow and its impact on A measurement during multiple SMD flow scenarios. Simulations of the conventional SMD experiment showed substantial surfactant-induced flow and consequent drainage of water from the column (e.g., from 75% to 55% S) and increases in actual A of up to 43%. Neither horizontal column orientation nor alternative boundary conditions resolved surfactant-induced flow issues. Even for simulated flow scenarios that avoided surfactant-induced drainage of the column, substantial surfactant-induced internal water redistribution occurred and was sufficient to alter surfactant transport, resulting in up to 23% overestimation of A. Depending on the specific simulated flow scenario and data analysis assumptions used, estimated A varied by nearly 40% and deviated up to 36% from the system's initial A. We recommend methods for A determination that avoid generation of surface-tension gradients and urge caution when relying on absolute A values measured via SMD.
表面活性剂混溶驱替（SMD）柱实验用于测量非饱和多孔介质中的气-水界面面积（A），这一特性会影响溶质运移和相分配。传统的SMD实验会导致表面张力梯度，进而引起水的重新分布和/或系统中水的净排出（“表面活性剂诱导流”），这违背了该方法的理论基础。尽管如此，SMD技术仍在使用，一些人认为表面活性剂诱导流的实验观测结果是边界条件控制不当的假象。在这项工作中，我们使用数值模拟（其边界条件可以得到完美控制）来评估这一观点。我们还研究了在多种SMD流动情况下表面活性剂诱导流的大小及其对A测量的影响。传统SMD实验的模拟结果显示存在大量表面活性剂诱导流，进而导致柱中的水排出（例如，饱和度从75%降至55%），实际A增加高达43%。无论是水平柱方向还是替代边界条件都无法解决表面活性剂诱导流的问题。即使对于避免柱中表面活性剂诱导排水的模拟流动情况，也会发生大量表面活性剂诱导的内部水重新分布，并且足以改变表面活性剂的运移，导致A的高估高达23%。根据具体的模拟流动情况和所使用的数据分析假设，估计的A相差近40%，与系统初始A的偏差高达36%。我们推荐避免产生表面张力梯度的A测定方法，并在依赖通过SMD测量的绝对A值时敦促谨慎行事。
