Effect Of Ion-interface Reaction On Phosphorus Transport

Phosphorus is a necessary nutrient for plants,and sufficient phosphorus in the soil is an important indicator of soil's high fertility.However,when excessive phosphorus transport from soil to surface water,it can cause eutrophication and water pollution.There are two main pathways for phosphorus transportation from soil to surface water:through dissolved phosphorus and through the adsorption of colloidal phosphorus,the latter is the main way for the transportation of soil phosphorus into water.Therefore,controlling soil erosion is the key to controlling transportation of farmland soil phosphorus into water bodies.As we know,the process of rainfall-induced soil erosion includes three steps:soil aggregate breakdown,formation of soil suspension,and transportation of suspension with water.Soil aggregates breakdown is the first step and the most crucial step in the process of soil erosion.Current studies have indicated the mechanism of rainfall-induced soil aggregate breakdown:when water enters soil,the soil moisture content increases,thus soil electrolyte concentration decreases;with the decrease of the electrolyte concentration,the electric field builds up immediately,the electric field produces two important repulsive forces between soil particles in aggregates:hydration force and electrostatic repulsion force,and these two forces lead to the breakdown of soil aggregates.When soil aggregates breakdown under the action of these two forces,with the help of raindrop impact and other external forces,the soil suspension is further formed,and gravity and some other forces cause soil suspension flow out,thus lead to soil erosion.Further studies indicate that the strength of the soil electric field formed after water enters the soil is closely related to the interfacial reaction of ions in the soil.Therefore,it is a crucial scientific problem to elucidate how the ion-interface reaction affects the soil erosion and the occurrence of soil phosphorus water transportation.In this paper,purple soil was chosen as the material,rainfall simulation experiments were studied,transportation of particle phosphorus and dissolved phosphorus in soil particles and the kinetics of phosphorus transportation under different electrolyte conditions were determined.The results are as follows:1.Effect of metal cation interface reaction on soil phosphorus transportationThe effects of the interface reactions of Li⁺,Na⁺and K⁺cations on soil phosphorus transportation were studied experimentally.Our study showed that,the interface reaction of cation in soil strongly influence soil phosphorous transportation in runoff,and showed specific ion effects.?1?As compared to Na⁺and Li⁺,the surface reaction of K⁺could strongly decrease particle phosphorus?PP?transportation from soil to surface water,e.g,the amounts of PP transportation for the presence of K⁺were observed just 1/69 and 1/48 smaller than that for the presence of Li⁺and Na⁺respectively.?2?The amount of dissolved phosphorus?DP?transportation for the presence of K⁺was higher than that for the presence of Na⁺or Li⁺,e.g,when the electrolyte concentration is 0.0001mol L^-1,and the presence of K⁺was found about 1.7times larger than the presence of Na⁺,thus it showed reverse specific ion effects as compared to PP transportation.?3?Increase soil electrolyte concentration could decrease both PP and DP transport from soil to surface water.The quantitative study of soil particle interaction forces indicated that,cationic non-classic polarization could give a well explanation for the observed experimental results in PP transportation.As compared to Na⁺and Li⁺,the strongest polarized K⁺strongly decreased the electric field around soil particles,thus strongly decreased the repulsive forces between adjacent soil particles in aggregate,and as the results strongly decreased PP transportations in a runoff.The observed polarizabilities of K⁺,Na⁺,and Li⁺in the soil were much higher than classic values.For example,under an electrolyte concentration of 0.0001mol L^-1,the observed non-classic polarizability of K⁺,Na⁺and Li⁺were 507,124 and 45.8?³ respectively,however the classic values for them are merely 0.814,0.139 and 0.0285?³ respectively.Secondly,for the presence of either K⁺,Na⁺or Li⁺,increase electrolyte concentration also could decrease PP and DP transportation,because it decreased the possibility of soil particles dispersing to suspension through decrease the repulsive forces among soil particles.Thirdly,as compared to Na⁺and Li⁺,the presence of K⁺seemly could increase soil phosphorous dissolution,however the physical mechanism is not clear.2.Effect of anionic interface reaction on soil phosphorus transportationExperiments were conducted to investigate the effect of interface reactions between Cl^-and NO₃^-on soil phosphorus transportation,the results showed that,the surface reaction of anion in soil also strongly influence soil phosphorous transportation in runoff,and showed specific ion effects.?1?When the cations were the same,the transportation of soil particles and particle phosphorus based on the NO₃^-interface reaction was larger than that based on the Cl^-interface reaction(except for 0.0001mol L^-1 Na⁺).For example,when the cation is Na⁺,at the electrolyte concentration of0.001mol L^-1,the cumulative transportation of soil particles under Cl^-and NO₃^-systems is 4361.4 and 5166.2g m^-2,respectively,and the cumulative transportation of particle phosphorus is 4380.2 and 5081.4mg m^-2,respectively.?2?The effect of different anionic interface reactions on the transportation of dissolved phosphorus is complicated.When the cation is Na⁺,in addition to 0.01mol L^-1 Na⁺,the amount of dissolved phosphorus transferred under the Cl^-interface reaction is much smaller than that based on the NO₃^-interface reaction,and when the cation is K⁺,in addition to0.1mol L^-1 K⁺,the situation is the opposite.However,the mechanism of soil phosphorus transportation caused by anion interface reaction is still not clear.All the results indicate that,?1?the cationic interface reaction strongly influences the phosphorus transportation in soil,and indicates that the ionic non-classical polarization in the cation-interface reaction controls the electrostatic repulsion force between soil particles in soil aggregates,thus controlling the intensity of soil phosphorus transportation.?2?the anionic interface reaction also strongly influences the phosphorus transportation in soil,and showed specific ion effects.However,the mechanism of soil phosphorus transportation caused by different anionic interface reactions is still unclear.