Emergence of non-aqueous phase liquids redistribution driven by freeze-thaw cycles in porous media based on low-field NMR
Emergence of non-aqueous phase liquids redistribution driven by freeze-thaw cycles in porous media based on low-field NMR
复制标题
基于低场核磁共振的多孔介质中冻融循环驱动的非水相液体重新分布的出现
DOI：
10.1016/j.jhydrol.2022.128106
复制
发表时间：
2022-06-28
影响因子：
6.4
通讯作者：
Wang, Jinguo
中科院分区：
文献类型：
--
作者：
Dou, Zhi;Chen, Yongqiang;Wang, Jinguo
寒冷地区人类活动的增加导致了非水相液体污染的严重环境问题。然而，目前对冻融循环驱动的NAPLs再分布还不是很清楚。在本研究中，在温度范围为20 ？到15吗?采用低场核磁共振（NMR）技术对其进行了研究。利用横向自旋-自旋弛豫时间（T-2）分布和磁共振成像（MRI）证实了NAPLs在饱和范围为13.56% ~ 72.05%的制备样品中的再分布。实验结果表明，大孔（T-2大于300 ms）和中孔（60 ms < T-2 < 300 ms）之间发生了大量的NAPLs再活化和分布，而微孔（T-2 < 60 ms）中的NAPLs在30 FT周期内几乎保持不变。其主要原因是微孔中的再分配速率远低于大孔和中孔。此外，NAPLs含量与FT循环次数在大孔中呈正相关，在中孔中呈负相关。对于低NAPLs饱和度的样品（例如S-N=13.56%和13.56%），NAPLs的再分配率与FT循环在大孔和中孔中呈现线性关系，而在微孔中呈现非线性关系。对于所有孔隙类型，S-N的增加都会引起显著的偏差程度。通过新引入的无量纲压力比δ P来评估毛细压力和冻结诱导压力之间的平衡，这有助于确定在FT周期中NAPLs的最终动员和再分配。实验结果表明，大孔中的δ P比中孔和微孔中的δ P大得多。这可以解释不同孔隙类型之间的再分配速率的巨大差异。这一基础性研究有助于了解寒区NAPLs的动员和再分配机制。
Increasing human activities in cold regions have led to serious environmental problems due to non-aqueous phase liquids (NAPLs) contamination. However, the NAPLs redistribution driven by freeze-thaw (FT) cycles is not well understood. In this study, the redistribution of NAPLs in porous media subjected to FT cycles in a temperature range from 20 ? to-15 ? was investigated using the low-field Nuclear Magnetic Resonance (NMR) technique. The redistribution of NAPLs in the prepared samples with NAPLs saturation ranging from 13.56% to 72.05% was confirmed using the transverse spin-spin relaxation time (T-2) distribution and the magnetic resonance imaging (MRI). Experimental results revealed that substantial remobilization and distribution of NAPLs occurred between the macropores (T-2 greater than 300 ms) and the mesopores (60 ms < T-2 < 300 ms) while the NAPLs in the micropores (T-2 < 60 ms) remained almost unchanged during 30 FT cycles. The main reason is attributed to the fact that the redistribution rate in micropores is much lower than in macropores and mesopores. Furthermore, the NAPLs content and the number of FT cycles were positively correlated for macropores while negatively correlated for mesopores. For the samples with low NAPLs saturation (e.g., S-N=13.56% and 13.56%), the redistribution rate of NAPLs versus FT cycles exhibited an overall linear relationship for macropores and mesopores while a non-liner relationship for micropores. For all pore categories, the increment in S-N can cause a remarkable deviation extent. The balance between capillary pressure and freezing-induced pressure was evaluated by the newly introduced dimensionless pressure ratio delta P, which could help determine the ultimate mobilization and redistribution of NAPLs during FT cycles. Experimental tests suggested that the delta P was much larger in macropores than in mesopores and micropores. This can explain the dramatic difference in the redistribution rate among different pore categories. This fundamental study is helpful in understanding the mobilization and redistribution mechanisms of NAPLs in cold regions.