"Editor's note in January 30th 20, literature and intelligence were concerned about the research on the structure and ionic effects of the agglomeration of natural organic matter by Professor Qu Xiaolei, Professor of environmental science, Nanjing University." we invited Mr. Qu to write the highlights of this research for the official account. On the one hand, we enrich the research field of natural organic research, especially its agglomeration behavior; on the other hand, we hope that the team will be able to do so. The results of this study can inspire and inspire the researchers, and promote the research of natural organic matter. "
Guide reading
Natural organic matter is a kind of mixture with complex chemical composition, structure and source, which widely exists in soil and water environment, and has important significance in the study of pollutant environmental fate and global carbon cycle. A series of colloidal behaviors, such as agglomeration, adhesion and deposition, will occur in the process of environmental transport of natural organic matter, which will affect the dynamic relationship between dissolved and particulate organic carbon and the land sea transport flux. In this work, the structure and ion effect of the aggregation behavior of natural organic matter are studied, and the Hofmeister effect of the aggregation behavior of natural organic matter is revealed. It is pointed out that the hydrophobicity of organic matter, the interaction between organic matter and ion and the hydration energy of ion are the main factors that affect the aggregation behavior of natural organic matter. A simple evaluation method of the aggregation behavior of organic matter is proposed.
In this work, a series of ions including common cations in environmental water was constructed, and a model of natural organic matter (srnom) was selected to study the Hofmeister effect of agglomeration behavior by combining kinetic and thermodynamic research methods. The results show that srnom has significant ionic effect. Based on the effect, the ion sequence can be divided into three parts. Univalent ions can only shrink the spatial configuration of srnom and can not agglomerate it. The order is CS + & gt; Rb + & gt; K + & gt; Na + & gt; Li +. Besides Mg2 +, divalent ions can make srnom agglomerate rapidly. The order of the critical concentration of agglomerate (CCC) is cccsr & gt; CCCCA & gt; cccba. It is difficult for Mg2 + to agglomerate srnom, but at low concentration, the spatial configuration of srnom will shrink, and the effect is much better than that of monovalent ion. Therefore, the Hofmeister sequence of srnom aggregation behavior is ba & gt; CA & gt; SR & gt; gt; gt; structure shrinkage Mg & gt; CS & gt; Rb & gt; K & gt; na & gt; Li. The results of xdlvo theoretical calculation are consistent with the sequence, which shows that the interaction energy barrier between srnom particles can be used to indicate the agglomeration behavior (Fig. 1, 2). The calculation of xdlvo indicates that the energy barrier mainly comes from Lewis acid-base force, which is controlled by the interface hydrophobicity and hydration degree of srnom. Monovalent ions can only play the role of electrostatic shielding, and have little effect on Lewis acid-base force between srnom particles, which can not induce agglomeration. Ba, CA, SR can directly complexate with srnom, and the hydration energy is small, which can significantly weaken the Lewis acid-base force between srnom particles and quickly induce agglomeration. The complex ability of Mg and srnom is weak and the hydration energy is large, so although the particle size can be significantly compressed, the agglomeration can not be induced.
▲ Figure 1. Correlation analysis of (a) total interaction energy between particles and (b) equilibrium particle size and energy barrier after structure contraction of srnom in monovalent ion (source: Elsevier)
▲ Figure 2. Correlation analysis of (a) total interaction energy between particles and (b) adhesion coefficient and energy barrier of srnom in divalent ions (source: Elsevier)
From the above conclusion, the key to control the agglomeration behavior of natural organic matter lies in its interface hydrophobicity and hydration degree. In most cases, the effect of monovalent ions is not significant at the environmental concentration. Therefore, we focus on the divalent ions Ca and Mg with high abundance in the environment, and further explore the importance of the structure and source of natural organic matter. Based on the structure characterization, agglomeration kinetics and thermodynamic calculation, the agglomeration behavior of natural organic matter from different sources in Ca and Mg was studied. On the basis of the structure-activity relationship, a simple evaluation method of agglomeration behavior of organic matter was established. The natural organic matter from land has higher aromaticity, humification degree and average molecular weight, while the polar functional group and fatty carbon content of natural organic matter from water source are higher. The aggregation of organic matter from land source (cccmax = 4.88 mm) is much faster than that from water source (cccmin = 46.69 mm). In the presence of Mg, the land-based organic matter agglomerates significantly, while the source organic matter only shrinks in structure and cannot agglomerate (Fig. 3). This is mainly caused by the difference of the main force composition of the energy barrier of organic matter between land source and water source. As mentioned above, the energy barrier of source organic matter (such as srnom) is high and mainly Lewis acid-base. Due to the weak ability of Mg to weaken Lewis acid-base, the organic matter in water source only contracted in structure and could not agglomerate. However, the energy barrier of terrigenous organic matter is not high, and the electrostatic effect has a certain contribution. Mg can reduce the energy barrier of terrigenous organic matter to the extent that it is insufficient to stabilize the organic matter by weakening the electrostatic effect and Lewis acid-base force, thus resulting in agglomeration. Although the structure of terrigenous organic matter is quite different, its aggregation behavior in CA is similar. CCC of source organic matter in CA can be evaluated by simple spectroscopy index (Figure 4). The structure and ionic effect of the aggregation behavior of natural organic matter are revealed in two series of articles, and the thermodynamic basis is clarified, which provides a reference for the future study of colloidal behavior of natural organic matter.
▲ Figure 3. (a) agglomeration kinetics of organic matter in Mg2 + from land and water sources (source: Elsevier)
▲ Figure 4. Correlation fitting between critical concentration of organic matter in water source in Ca (CCC) and (a) E2 / E3, (b) fi parameters (source: Elsevier)
Reference
[1] Xu, F., Yao, Y., Alvarez, P.J.J., Li, Q., Fu,H., Yin, D., Zhu, D., Qu, X. Specific ion effects on the aggregation behaviorof aquatic natural organic matter. Journal of Colloid and Interface Science, 2019,556, 734-742.[2]Wei, P., Xu, F., Fu, H., Qu, X. Impact of originand structure on the aggregation behavior of natural organic matter.Chemosphere, 2020, 248, 125990.
Contact author
Qu Xiaolei, Professor, Nanjing University, email: xiaoleiqu@nju.edu.cn
Extended reading
[viewpoint] a new method to explore the mechanism of Carbon Stabilization in soil micro aggregates -- Tianjin University -- Yu Guanghui
[viewpoint] the effect of long-term fertilization on organic carbon composition and micro community composition in Greenhouse Soil agglomerate -- Lu Yizhong, China Agricultural University
Editor: Wang Yuqin
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