Purpose

Clay-humic substance complexes play a major role in controlling the mobility of elements and colloids in natural soils. The purpose of the present study is to explore the different reaction mechanisms induced by cations on the humic acid (HA) and montmorillonite (Mt) heteroaggregation and to analyze the binding mechanism of HA-Mt.

Materials and methods

HA is extracted from soil and colloidal Mt are prepared as K⁺-saturated. The aggregation kinetics of HA-Mt composite nanoparticles in Ca(NO₃)₂, Mg(NO₃)₂, and Cu(NO₃)₂ solutions were detected by dynamic light scattering. Furthermore, FT-IR spectroscopy was used to characterize the reactive sites involved in interaction with the metal ions.

Results and discussion

The results revealed that the order of reaction of these three metal cations with the HA-Mt composite was Cu²⁺ > Ca²⁺ > Mg²⁺, as evident from the total average aggregation rate, critical coagulation concentration, and activation energy. The heteroaggregation process was sensitive to the 1% mass percentage of HA; however, more HA (4% mass percentage) did not significantly affect this process compared with 1% HA. Higher cation concentration and higher HA content (10% mass percentage) were two necessary conditions for promoting HA-Mt heteroaggregation. The vibration peak intensities of the carboxyl group C-O bonds and hydroxyl group O-H bonds of HA were affected by the formation of coordinate bonds with different metal ions.

Conclusions

Metal cations were preferentially complexed by the carboxyl groups of HA, and due to its polarization-induced and electric field–enhanced oxidizing properties, Cu²⁺ has the strongest aggregation ability for HA-Mt, followed by Ca²⁺ and Mg²⁺. The HA-Mt heteroaggregation is partially reversible by adjusting electrostatic repulsion. The results of this study improve our understanding of the roles of cations and HA in clay-humic substance interactions.

中文翻译：

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腐殖酸与蒙脱石在二价电解质中的异质聚集：腐殖酸含量和离子浓度的影响

目的

粘土腐殖质复合物在控制天然土壤中元素和胶体的迁移中起主要作用。本研究的目的是探讨阳离子对腐殖酸（HA）和蒙脱土（Mt）杂聚体诱导的不同反应机理，并分析HA-Mt的结合机理。

材料和方法

HA从土壤中提取，胶态Mt制备为K ^+-饱和。通过动态光散射检测HA-Mt复合纳米粒子在Ca（NO ₃）₂，Mg（NO ₃）₂和Cu（NO ₃）₂溶液中的聚集动力学。此外，FT-IR光谱用于表征与金属离子相互作用的反应位点。

结果与讨论

结果表明，这三种金属阳离子与HA-Mt复合材料的反应顺序为Cu ²⁺ > Ca ²⁺ > Mg ²⁺，这从总平均聚集率，临界凝结浓度和活化能可以看出。杂聚过程对1％的HA敏感；但是，与1％HA相比，更多的HA（质量百分比为4％）对该过程没有显着影响。较高的阳离子浓度和较高的HA含量（质量百分比为10％）是促进HA-Mt异质聚集的两个必要条件。HA的羧基CO键和羟基OH键的振动峰强度受不同金属离子配位键形成的影响。

结论

金属阳离子优先被HA的羧基络合，并且由于其极化诱导和电场增强的氧化特性，Cu ²⁺对HA-Mt的聚集能力最强，其次为Ca ²⁺和Mg ²⁺。通过调整静电排斥力，HA-Mt异质聚集可部分逆转。这项研究的结果提高了我们对阳离子和HA在粘土与腐殖质相互作用中的作用的理解。