Clay Migration Intensity Driven by Water Flow as Influenced by Cation-Surface Interactions

Si-han MIAO; Xin-min LIU; Song LI; Fei-nan HU; Hang LI

doi:10.13718/j.cnki.xdzk.2018.01.016

2018 Volume 40 Issue 1

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Si-han MIAO, Xin-min LIU, Song LI, et al. Clay Migration Intensity Driven by Water Flow as Influenced by Cation-Surface Interactions[J]. Journal of Southwest University Natural Science Edition, 2018, 40(1): 107-114. doi: 10.13718/j.cnki.xdzk.2018.01.016

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Si-han MIAO, Xin-min LIU, Song LI, et al. Clay Migration Intensity Driven by Water Flow as Influenced by Cation-Surface Interactions[J]. Journal of Southwest University Natural Science Edition, 2018, 40(1): 107-114. doi: 10.13718/j.cnki.xdzk.2018.01.016

Clay Migration Intensity Driven by Water Flow as Influenced by Cation-Surface Interactions

School of Resources and Environment/Chongqing Key Laboratory of Soil Multi-Scale Interfacial Process, Southwest University, Chongqing 400716, China

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Corresponding author: Hang LI
Received Date: 31/10/2016
Available Online: 20/01/2018
MSC: X53

Abstract

Soil particle migration often causes soil erosion, agricultural non-point source pollution and other environmental problems. It is generally acknowledged that soil particle migration during rainfall is mainly attributed to raindrops and other external effects. However, its internal mechanism is still not clear. A study reported herein proposes a new perspective, i. e. how does the internal force between particles influence clay migration intensity? In order to accurately calculate the force between clay particles, pure montmorillonite aggregates were chosen in this experiment as the study material and two monovalent cation species (Na⁺ and K⁺) were employed to characterize the difference of ion-surface interactions. The results obtained were as follows. First, the clay migration intensity for Na⁺ system was higher than that for K⁺ system under the same electrolyte concentration. Second, the electrostatic field strength for K⁺ system was significantly lower than that for Na⁺ system under the same concentration. Correspondingly, the electrostatic repulsive pressure between clay particles for Na⁺ system was much greater than that for K⁺ system. Third, the electrostatic repulsive pressure between particles and the clay migration intensity increased with increasing electrostatic field intensity of montmorillonite. The study indicated that the clay migration intensity was mainly influenced by the electrostatic repulsive pressure from the electrostatic field strength of aggregates, but at low electrolyte concentrations, the electrostatic field also affected the clay migration intensity through the hydration repulsive force. In conclusion, this study suggested that the electrostatic effect generated by the electrostatic field of clay aggregates may be the internal driving force of the clay migration under the water flow, and as we considered only the effect of water flow and did not consider the factor of raindrops in this experiment, the importance of raindrops on soil migration should not be ignored.
- particle migration,
- ion-surface interaction,
- internal mechanism,
- specific ion effect

References

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土壤颗粒随水迁移常引发土壤退化、农田面源污染等一系列生态与环境问题^[1].其中土壤团聚体的破碎被认为是土壤颗粒随水迁移发生的第一步，也是关键一步^[2]，因此，控制土壤团聚体破碎程度是控制土壤颗粒水迁移强度直接而有效的手段^[3].已有研究表明，土壤颗粒间的相互作用是团聚体破碎的内在驱动力^[3].土壤颗粒间的水合斥力^[4]、静电斥力和分子引力产生的颗粒间的压强可达到10²~10³atm^[5]，而雨滴打击力等外部作用力只能产生1~3 atm的作用压强^[6].因此，相比于经典侵蚀理论中的各种外力作用，土粒间的内部作用力才应是土壤团聚体破碎的决定因素^[3].

根据胶体颗粒相互作用的DLVO理论，颗粒间的静电斥力受颗粒周围电场的控制^[7-8]，而离子界面反应特征决定着土壤土粒周围的电场强度^[3].经典DLVO理论只考虑了离子界面反应中离子电荷价的贡献，该理论中同价离子对土壤颗粒相互作用力的影响相同，但大量实验表明同价态不同离子在土壤颗粒相互作用中表现出了强烈的离子特异性效应^[9].例如，K⁺体系团聚体稳定性强于同浓度下的Na⁺体系，而后者的分散性更强；含交换性Na⁺较多的盐碱土结构差，肥力低，而富含K⁺的紫色土肥力较高.

现有理论认为离子特异性效应的产生机制主要来自于离子大小、离子水合和色散力等作用^[10].但这些因素都只有在较高浓度(大于0.1 mol/L)电解质条件下才能发生作用，并且随着电解质浓度的降低而逐渐减弱^[11].然而，有研究发现低浓度电解质条件下仍然存在着非常强的离子特异性效应，且该效应随电解质浓度的降低变得更加明显^[12-13]. Liu等人^[14]发现，在土壤胶体颗粒强外电场作用下(10⁸~10⁹ V/m)，离子的量子涨落被急剧放大而发生强烈的非经典极化作用(其极化率可达到经典极化率的成百甚至上千倍).离子间非经典极化的差异导致这些离子与土壤颗粒界面反应间的差异，从而导致土壤颗粒间相互作用的差异. Li等人^[3]研究发现，离子的非经典极化会对土壤流失强度产生影响. Du等人^[15]定量计算了一些一价金属阳离子的非经电极化率，发现这些一价金属阳离子的非经典化率差异巨大.比如在10^-4 mol/L电解质浓度下，Cs⁺，K⁺和Na⁺的非经电极化率分别为2209，1702和417.9Å³.非经电极化率的巨大差异本质上反映了这些同价离子核外电子的能量和量子状态的巨大差异，而核外电子的能量和量子状态的巨大差异将使这些同价金属离子在土粒表面表现出不同的界面反应特征.

本研究通过K⁺和Na⁺两种离子在蒙脱石上的界面反应特征来表征金属离子界面反应如何影响粘土的水迁移.本研究的主题不是土壤中的离子构成如何影响土壤物质迁移，而是离子界面反应如何影响了土壤物质的水迁移.基于此，这里仅以K⁺和Na⁺两种离子作为本研究的指示离子.由于土壤组成十分复杂，使得包括静电作用力在内的各种作用力的定量计算都非常困难，因此，本研究采用蒙脱石作为供试材料.

1. 材料与方法

1.1. 材料及蒙脱石团聚体制备

本研究所用蒙脱石购买自内蒙古物华天宝矿物资源有限公司.采用Li等^[16]人所建立的物质表面电化学性质参数联合测定方法测得蒙脱石矿物阳离子交换量和比表面积分别为84.8 cmol/kg和716 m²/g.

为定量表征不同浓度与类型电解质条件下，粘土颗粒相互作用中的离子特异性效应，本研究将蒙脱石分别制成Na⁺饱和样和K⁺饱和样.制备方法如下：称取1 kg蒙脱石，加入0.5 mol/L的NaNO₃溶液5 L，搅拌分散24 h后，离心弃去上清液(4 000 r/min，5 min)，重复操作3次；之后，加入去离子水5 L搅拌24 h，离心弃去上清液以洗去多余盐分，重复2~3次. 70 ℃下烘干，获得Na⁺饱和样，过筛取0.5~2 mm团聚体用于模拟降雨实验. K⁺饱和样的制取采用KNO₃溶液，方法同上.

1.2. 实验方法

将粒径为0.5~2 mm的Na⁺/K⁺饱和团聚体平铺于5 cm × 5 cm的土槽内，厚度为3 cm，坡度为30°.土槽末端安放孔径2 mm的阻拦网，土槽下端导流槽可汇总并收集随水流出的粘土颗粒.将土槽安放在针滴式滴水器下，针尖出水口距离土槽中样品表面3 cm，以尽量降低水滴对样品的打击作用，水流通量为25 mL/(cm²·min).水中分别含NaNO₃或KNO₃电解质，浓度分别为0.000 1，0.001，0.01，0.1和1 mol/L，水源温度为25 ℃.从水流产生开始计时，初期每2 min收集一次流出液，10 min后，每5 min收集一次，实验时间为90 min.将收集的悬液在105 ℃条件下烘干并称重.获取粘粒各时间段的流失量和累积流失量.实验重复3次.

1.3. 考虑离子不同界面反应特征的粘粒表面电位和粘粒间静电排斥力计算

近年的相关研究已经指出，相同价数的不同阳离子由于核外电子构象的差异，使得这些离子表现出完全不同的界面反应特征，而不同的反应特征集中体现在这些离子具有不同的非经典极化效应^[16].同价离子非经典极化的差异必定引起这些离子屏蔽粘粒周围电场能力的差异，进而引起粘粒表面电位和颗粒间作用力的差异.所以，计算粘粒表面电位和粘粒间的静电作用力时，需考虑离子不同的表面反应特征.

对于1:1型电解质体系，考虑离子非经典极化效应的颗粒表面电位可由下式计算^[14]：

其中：

式中：φ₀为蒙脱石颗粒表面电位(V)；R为气体常数(J/(mol·K))；T为绝对温度(K)；F为法拉第常数(C/mol)；Z为阳离子价数；γ为因阳离子非经典极化而产生的有效电荷数^[13]；C_T为蒙脱石阳离子交换量(mol/g)；S为蒙脱石颗粒比表面积(dm²/g)；c₀为本体电解质浓度(mol/L)；κ为德拜参数(dm^-1)，其倒数1/κ表示胶体颗粒扩散双电层厚度；ε为水的介电常数，取值为8.9×10^-10 C²/(J·dm).

对于1:1型电解质体系，考虑离子非经典极化效应的颗粒间静电斥力的大小可以通过下式进行计算^[16]：

式中：P_EDL为相邻两颗粒间的静电排斥压强(atm)；d为两颗粒间的距离(dm)；φ(d/2)为相邻两个蒙脱石颗粒双电层重叠中点处的电位值(V)，其可以通过下式计算^[13]：

3. 结论

离子界面反应强烈影响着粘土颗粒随水迁移的离子特异性效应.颗粒表面电场是控制团聚体稳定性的决定因素，而表面电场则通过离子界面反应所控制.电场强度越强，颗粒间的静电排斥压越高，粘粒水迁移强度就越大.例如实验中Na⁺离子-表面相互作用弱于K⁺，因此Na⁺体系中蒙脱石颗粒表面电场强于k⁺体系，相应地Na⁺体系中粘粒间静电排斥力远强于K⁺体系，从而导致Na⁺体系中粘土颗粒迁移强度远远大于同浓度下的K⁺体系，而且两体系间粘粒水迁移强度的差异随着电解质浓度的降低而增大.

研究发现离子不同的界面反应特征决定了相同电解质浓度下粘土团聚体中的电场强度，而且正是因为团聚体中的不同电场强度决定了粘粒之间排斥压的强度的差异，最终带来了粘粒随水迁移强度的差异.只有当离子界面反应中考虑这种差异时，理论预测结果才能与实验测定值吻合.而经典颗粒相互作用理论由于没有考虑离子界面反应中的离子特异性效应而不能正确预测颗粒间的相互作用力强度，从而使粘粒水流迁移强度的实验结果与理论预测相差悬殊.另一方面，在0.000 1 mol/L的低浓度条件下，颗粒表面电场更强，水偶极子在颗粒表面出现强吸引力，颗粒表面吸附的水分子层越厚，颗粒间的水合排斥就越强.因此，在低浓度条件下，颗粒水合排斥力对颗粒间相互作用有着重要影响而不可忽略.

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Clay Migration Intensity Driven by Water Flow as Influenced by Cation-Surface Interactions