Department of Earth, Environmental and Physical Sciences, University of Siena, Pian dei Mantellini 44, 53100 Siena, Italy.
Department of Chemical, Physical, Mathematical and Natural Sciences, University of Sassari, Via Vienna 2, 07100 Sassari, Italy.
Water Res. 2023 Mar 1;231:119608. doi: 10.1016/j.watres.2023.119608. Epub 2023 Jan 14.
The use of surfactants represents a viable strategy to boost the removal yield of Dense Non-Aqueous Phase Liquids (DNAPLs) from groundwater and to shorten the operational timing of the remediation process. Surfactants, in general, help in reducing the interfacial tension at the DNAPL/water interface and enhance the solubility of the pollutant in the water phase through the formation of dispersed systems, such as micelles and emulsions. In this paper, we show that a suitable choice of a surfactant, in this case belonging to the bio-degradable class of ethoxylated alcohols, allows for the formation of hydrodynamic interfacial instabilities that further enhances the dissolution rate of the organic pollutant into the water phase. In a stratified configuration (denser organic phase at the bottom and lighter water phase on top), the instabilities appear as upward-pointing fingers that originate from the inversion of the local density at the interface. This inversion stems from the synergetic coupling of two effects promoted by the ethoxylated surfactant: i) the enhanced co-solubility of the DNAPL into the water (and viceversa), and (ii) the differential diffusion of the DNAPL and the surfactant in the aqueous phase. By dissolving into the DNAPL, the surfactant also reduces locally the surface tension at the liquid-liquid interface, thereby inducing transversal Marangoni flows. In our work, we carefully evaluated the effects of the concentration of different surfactants (two different ethoxylated alcohols, sodium dodecylsulphate, cetyltrimethyl ammonium bromide, N-tetradecyl-N, N-dimethylamine oxide and bis(2-ethylhexyl) sulfosuccinate sodium salt) on the onset of the instabilities in 3 different DNAPLs/water stratifications, namely chloroform, trichloroethylene and tetrachloroethylene, with a special emphasis on the trichloroethylene/water system. By means of a theoretical model and nonlinear simulations, supported by surface tension, density and diffusivity measurements, we could provide a solid explanation to the observed phenomena and we found that the type of the dispersed system, the solubility of the DNAPL into the water phase, the solubility of the surfactant in the organic phase, as well as the relative diffusion and density of the surfactant and the DNAPL in the aqueous phase, are all key parameters for the onset of the instabilities. These results can be exploited in the most common remediation techniques.
表面活性剂的使用是一种可行的策略，可以提高地下水中密集非水相液体（DNAPLs）的去除率，并缩短修复过程的操作时间。通常，表面活性剂通过形成分散体系（如胶束和乳液）来帮助降低 DNAPL/水界面的界面张力，并提高污染物在水相中的溶解度。在本文中，我们表明，选择合适的表面活性剂（在这种情况下属于可生物降解的乙氧基化醇类）可以形成流体动力界面不稳定性，从而进一步提高有机污染物在水相中的溶解速率。在分层配置（底部较致密的有机相和顶部较轻的水相）中，不稳定性表现为向上指向的指状物，这些指状物源自界面局部密度的反转。这种反转源于乙氧基化表面活性剂促进的两种效应的协同耦合：i）DNAPL 更易溶于水（反之亦然），以及 ii）DNAPL 和表面活性剂在水相中的扩散差异。通过溶解在 DNAPL 中，表面活性剂还会局部降低液-液界面的表面张力，从而诱导横向马兰戈尼流。在我们的工作中，我们仔细评估了不同表面活性剂（两种不同的乙氧基化醇、十二烷基硫酸钠、十六烷基三甲基溴化铵、十四烷基-N，N-二甲基氧化胺和双（2-乙基己基）磺基琥珀酸钠）的浓度对 3 种不同的 DNAPLs/水分层中不稳定性的起始的影响，即氯仿、三氯乙烯和四氯乙烯，特别关注三氯乙烯/水系统。通过理论模型和非线性模拟，并辅以表面张力、密度和扩散率测量，我们可以对观察到的现象提供可靠的解释，并且我们发现分散体系的类型、DNAPL 在水相中的溶解度、表面活性剂在有机相中的溶解度、表面活性剂和 DNAPL 在水相中的相对扩散和密度，都是不稳定性起始的关键参数。这些结果可以在最常见的修复技术中得到利用。
