The Specific Ion Effects On Water Movement Under The Influence Of Soil Elecrtric Field

Soil water movement in surface soil has great impacts on environment problems such as soil erosion and agricultural area-source pollution by means of controlling or affecting the transport of pollutant like soil nutrients, heavy mental, and pesticide etc. that have aroused the interests of many fields like environment and agriculture recently. Although a great amount of hydraulic models had already been developed, a frustrating fact that there was no one can precise predicts the hydraulic process of this thin soil layer. This proclaims that the problem of soil water transport is very complicated. At least, there still have two problems:first, soil aggregates stability play a pivotal role in water movement, however, a basic assumption that the water flow equation rely on is that soil aggregate is rigid or soil matrix is stability. Thus the simplified models may fail to reflect the effects bring by the aggregation or dispersion of soil aggregate. Second, it is commonly to neglect the relationship between the macroscopic progress of soil water movement and the mesoscopic progress. However, some research indicated that the soil surface electric field is crucial in mesoscopic progress that it may control the swelling or breakdown of soil aggregates that it is probably the root reason of the specific ion effects of aggregate breakdown. However, the role of soil electric field in soil water movement is not clear, therefore, the objectives of this study were to reveal and elucidate the effects of soil electric field on soil water movement. At the meanwhile, reveal that there is a strong specific ion effect on soil water movement.First, we determinated the surface properties of three soils, red earth, yellow soil and purple soil according to the combined determination method of surface properties. Second, we studied the influence of soil electric field on water movement use the column infiltration experiments with different concentration (10-5、10-3、10-2、10-1and lmol/L) NaN03solutions infiltrated through the1mm Na-saturated soil column. At last, we study the specific ion effects on soil water movement through comparing the water infiltration ratio of LiNO3, NaN03and KNO3solution of different concentrations. The results were show that:(1) The surface potential of red soil, yellow soil and purple soil were-100mV,-120mV and-125mV respectively; the surface electric field strength of these soils were2.0×108V/m、4×108V/m and3×108V/m. This reflect that the Zeta potential cannot be the substitution of surface potential and the soil electric field was used to be underestimated.(2) According to the vertical column experiments, with the increase of the concentration of NaN03solution from10-5mol/L to lmol/L, the water infiltration ratio was magnified about5times and the infiltration depth was extended from2cm to11cm. Soil water infiltration ratio decreased with the increase of soil electric field strength. At the condition of different soil electric field strength or surface electric potential, the water infiltration ratio of NaNO3(7) have the follow order:I(-97mV)> I(-150mV)>I(-206mV)>I(-264mV)≈I(-382mV). This indicated that soil electric field is the determinated factor for water movement. The water movement probably influenced by soil electric field through the next two ways:(ⅰ) the stronger the electric field is, the smaller the soil matrix potential will be, thus the velocity of the water flow in unsaturated soils will be decreased;(ⅱ) the breakdown of soil aggregates which caused by soil electric field can change the soil porosity and reduce the amount of macrospore that results in the decrease of the velocity of the water flow in saturated soils.(3) We used the vertical column experiments to study and compare the water infiltration ratio of Li/Na/K system. The results showed that the water infiltration ratio of K system was3times larger than Na system when soil electric field strength is relatively high (the electrolyte concentration is10-5mol/L), and when the soil electric field strength is relatively low, the water infiltration ratio of K and Na system was20times and13times larger than Li system respectively. When the soil electric field is same, the water infiltration ratio of different system have the next order: K> Na>>Li. Therefore, this revealed that there is a strong specific ion effects on soil water movement, and the root cause is the coupling of soil surface electric field and ionic quantum fluctuation.