The Effect Of Soil Electric Field On Clay Aggregate Stability

Soil structure is an important influencing property of aggregate stability. Crumb structure could balance the four factors of water, fertilizer, gas and heat in soil. Crumb structure is the basic unit of aggregates. Aggregates bear the structure of hierarchical clustering, and are formed by cluster formation and gluing effect of different materials. Single soil particles connect with each other through ionic bond, covalent bond and hydrogen bond to form cluster; clusters are glued together with polyvalent ion, organic molecules and hyphae. Aggregate stability is influenced by soil organic content, clay mineralogy and exchangeable sodium percentage and other properties. The property of soil colloid particles determines soil aggregate stability. The colloids particles interaction is influenced by not only the DLVO forces, but also the non-DLVO forces. The move of Mecroscopic particles is governed by electrostatic repulsive force, hydration repulsive force and van der waal’s force. In the present study, the type, concentration and temperature of electrolyte concentration were used to adjust the surface electric field strength, and the aggregate stability of montmorillonite, kaolinite and their mixture was measured under the influence of different surface potential values. It was found that the fundamental reasons for aggregate breakdown are not raindrop impact, shear force of flowing water or physic-chemical dispersion, but the destructive force soil electric field. Single particles and micro-aggregates are released through the total huge repulsive force of electrostatic repulsive force and hydration repulsive force while being wetting or under rainfall. Then soil solution is formed by the disturbance of raindrop and flowing water. The nutrients carried by the released soil particles lead to soil erosion and water eutrophication. Furthermore, the Hofmeister effect on clay aggregate stability was also investigated.The main conclusions could be summarized as follows.(1) The stability of the clay aggregates is controlled by the electrostatic repulsive force, hydration repulsive force and the van der Waals’ force. The surface electric field is the fundamental reason that leads to clay aggregate breakdown, and the electric field strength determines the way in which aggregates break down. Under high surface potential values, aggregates break down in the way of explosion; under low surface potential values, aggregates break down in the way of dispersion or swelling. As for montmorillonite aggregates, the critical value for aggregates explosion is-170mV. The force range of electrostatic repulsive force, hydration repulsive force and the van der Waals’force are1000nm,15nm and50nm. Since the electrostatic repulsive force and the hydration repulsive force are both coming from the clay surface electric field, the electric field strength controls the total repulsive force. When the distance between adjacent particles is close, the hydration repulsive force could reach105atm. So the hydration repulsive force contributes much of the repulsive force of the system that breaks down the aggregates, and the electrostatic repulsive force determines the way of aggregate breakdown.(2) There was strong coupling force between the clay surface electric field and the clay mineralogy. The differences in aggregate stability between the mixture of montmorillonite and kaolinite in different mass ratio and the single clay mineral are obviously distinctive. The montmorillonite is clearly more stable than kaolinite. For the mixture, the critical surface potential value for aggregate explosion for the two ones with high kaolinite content (20%montmorillonite+80%kaolinite and the50%montmorillonite+50%kaolinite) was significantly higher that the two ones with low kaolinite content (80%montmorillonite+20%kaolinite and the100%montmorillonite); the former was210-220mV and the latter was160-170mV. Furthermore, the content of released particles for the ones with high kaolinite content (20%montmorillonite+80%kaolinite and the50%montmorillonite+50%kaolinite) were higher than those with the low kaolinite content (80%montmorillonite+20%kaolinite and the100%montmorillonite) under high surface potential values; and it was opposite for the low surface potential values, thus released particles of the latter were higher. It could be concluded that under high surface potential values, the content of released particles is controlled by both the surface electric field and the water infiltration speed; while under low surface potential values, it was the particle distribution of the aggregates themselves that determines the content of the released particles.(3) For the aggregates made up with the mixture of montmorillonite and kaolinite, the following conclusion could be predicted that the most unstable aggregates was not the ones with the highest clay (montmorillonite) content, but the silt soils predominated by sand with certain content of clay. Because the surface potential value for aggregate explosion for the silt soils is higher, naming the silt soils under high-intensity rainfall is the most erosive soil. That explains why the arid weather in the loess plateau region benefits the occurrence of soil erosion.(4) Hofmeister effect is result of the coupling of surface electric field and quantum flocculation. At the condition of fixed anions, the stability of Na+-saturated montmorillonite aggregates was weaker that the K+-saturated aggregates, and the difference was enhanced by the increasing surface potential value (decreasing electrolyte solution concentration). The polarizability of potassium is stronger that sodium. The electron cloud of the potassium is softer than the sodium, and it is more prone to change shape. The probability for it being adsorbed on the clay surface is bigger, then the screening effect of the potassium is stronger than the sodium. Under the same electrolyte solution concentration, the interaction energy of potassium in the double electric layer is1.6times of that for sodium. After taking into account of the coupling of surface electric field and quantum flocculation, the effective charge number coefficient is introduced into the system. When the electrolyte solution concentration is10-5mol/L, the surface potential of the Na+-saturated montmorillonite aggregates is1.5times of that for K+-saturated aggregates. And only when the electrolyte solution concentration is high (near1mol/L), the surface potential values approach each other. The surface potential value is the controlling factor for aggregate stability, and the surface potential value (in absolute value) is positively proportional with the content of released particles, thus with aggregate stability. The trend of the ratio of electrostatic repulsive force for Na+-saturated aggregates and K+-saturated aggregates also accords with the aggregate stability. In conclusion, Hofmeister effect is not the result of ion hydration and dispersion force, but the coupling of surface electric field and quantum flocculation.