| Soil water,as the key link between groundwater,surface water and atmospheric water,plays an important role in the growth of various crops and plants.Soil water movement has a profound influence on the occurrence of environmental problems such as soil erosion and soil erosion.Predecessors’research is mostly according to the different external conditions change,by changing or setting boundary and initial conditions to establish appropriate model to describe and explain the soil moisture movement process,rather than on the internal mechanism of soil water movement,lead to the established mathematical model is not accurate quantitative interpretation in the soil moisture movement process.Therefore,people began to explore the interaction between soil and the mechanism of soil water movement.Recent studies have shown that the interaction force between soil particles affected by soil electric field is the key to the condensation and dispersion of soil particles,which determines the stability of soil aggregates,which in turn determines the pore structure of soil,and ultimately affects the process of water movement.If the interaction force is repulsive,the aggregates are dispersed,and the soil pores are blocked by dispersed small particles,which is not conducive to soil water movement;otherwise,it is beneficial to soil water movement.At the same time,due to differences in polarization and polization-induced covalency,different metal cations have different ability to shield electric field,which leads to differences in the stability of soil aggregates,and the differences in the stability of aggregates will further lead to differences in soil pore characteristics and water transport.It is well known that PAM,as a linear macromolecular organic matter,can improve the stability of aggregates through"bridging effect".The way PAM improves the stability of aggregates is closely related to the electric field.By increasing electrolyte concentration or introducing cations with strong electric field shielding ability,the action distance and magnitude of soil electric field intensity and repulsive force can be reduced to make PAM molecular chain sufficient to form"bridging effect",thus enhancing the van der Waals attraction between particles and improving the stability of aggregates.In fact,in addition to van der Waals attraction,PAM contains part of the amide group with a small amount of positive charge,in the same sex repulsion,PAM molecules stretch,and can be attracted with negatively charged soil particles,also attractive.Obviously,these interactions can significantly improve the stability of aggregates.But on the other hand,because PAM molecules have a large amount of negative charge at the same time,and PAM molecules are almost completely straight in aqueous solution,the negative colloidal particles have a repulsion effect,reduce the stability of their aggregates,but may inhibit soil water movement.Clay minerals contained in bentonite are important components of soil,which have a large number of surface negative charges and form a strong electric field.Montmorillonite,the main mineral component of bentonite,can expand,disperse and release small particles to block pores and reduce soil moisture movement.It can be inferred that there may be complex coupling between soil electric field and PAM to affect soil water movement.Based on this,this study used indoor one dimensional vertical soil column experiment and aggregate stability,with different concentrations of Li Cl,Na Cl,KCl electrolyte solution,adjust soil electric field with different content of PAM to adjust van DE hua attraction between soil particles,the determination of the purple soil and bentonite in water infiltration rate and aggregate stability intensity,the results showed that:（1）In the water movement of bentonite,the water-ion-electric field interaction caused by water entering bentonite strongly affects the water infiltration and movement of bentonite.Under any ion concentration,the water-Li+-electric field interaction makes the bentonite almost completely lose water permeability and water conductivity.At low ion concentration≤10-3 mol/L,the"water-Na+-electric field"interaction makes the bentonite completely lose water permeability and water conductivity.Under any ion concentration,the"water-K+-electric field"interaction makes bentonite show good water permeability and water conductivity.Regardless of Na+or K+,the water movement velocity in bentonite increases with the increase of ion concentration.Under any electric field intensity（different ion types and electrolyte concentrations）,PAM always improves the water movement speed in bentonite,so the coupling between pam-electric field always shows promoting effect in the water movement of bentonite.（2）In the water movement of purple soil,when the electrolyte type and PAM content were consistent,the water movement rate increased monotonically with the increase of electrolyte concentration,and in different electrolyte systems,the water movement rate showed an ion-specific sequence of KCl>Na Cl>Li Cl.The electric field intensity decreased with the increase of electrolyte,and the soil electric field followed the sequence of Li+>Na+>K+at the same electrolyte concentration,indicating that the water movement speed of purple soil accelerated with the decrease of electric field intensity.（3）In the water movement of purple soil,there is no coupling between"bentonite and electric field",but there is an obvious coupling between"PAM and electric field",and the coupling in soil is different from that in bentonite:The coupling between PAM-electric fields promoted soil water movement in strong electric field,but inhibited it in weak electric field.The results showed that the concentration of 0.5%PAM promoted the water movement at all electrolyte concentrations in the presence of Li+/Na+.When K+is less than 10-2mol/L electrolyte,0.5%PAM content can promote water movement.When electrolyte concentration>10-2mol/L,0.5%PAM content showed inhibition effect on water movement.Due to the low polarization of Li+/Na+,the system is in strong electric field condition at all electrolyte concentrations,and the polarization of K+is correspondingly high,and only at low electrolyte concentration is in strong electric field condition.（4）In purple soil water movement,"PAM-electric field-bentonite"three very strong coupling between,"PAM-electric field-bentonite"much higher than the coupling effect between the three"PAM-electric field,the coupling effect between the"and"PAM-electric field-bentonite"between the coupling effects on soil water movement is characterized by suppression.Based on the above experimental findings,combined with the calculation of various interaction forces between particles and the analysis of the influence of these forces on the stability of soil particle condensates,we draw the following conclusions:（1）With the decrease of electric field intensity,the water movement speed in purple soil and bentonite increases,and the ion concentration affects the water movement by adjusting the bentonite electric field rather than by changing the water osmotic potential.（2）"PAM-electric field coupling effect"determines the interaction force between soil/clay particles,which determines the stability of soil aggregate/clay,and ion content of type,concentration,and PAM is respectively through affecting the electrostatic repulsion between particles and van der Waals force to influence the stability of the aggregate,ultimately affect the movement of water through the soil/clay.（3）The fundamental reason for the difference of PAM-electric field coupling effect of water movement between soil and bentonite is the huge difference of particle content>2μm between the two systems.As there are almost no large particles larger than 2μm in bentonite,the effect of PAM on water movement was stronger than that of PAM on water absorption and expansion.In soil,the opposite is true.（4）Although there is no coupling between bentonite and electric field in soil water movement,there is a very strong coupling between PAM-electric field and bentonite,and this coupling shows a strong inhibition of water movement.The basic reason is that both"PAM-bentonite"in the soil will absorb water and expand,and then strongly block the soil pore,inhibit the soil water conduction. |
PDF Full Text Request