Conductive Effects Of Yellow Earth Particles With Different Scales

The study of electrokinetic properties,electrical conductivity and ions diffusion and adsorption on electrochemistry property and process of soil surface has been paid extensive attention for half a century.Electric conductivity determination has been widely applied to the study of soil science.It can determine soil salinity,resistivity and ions diffusion coefficient,and is an indicator of soil fertility.Recently,electric conductivities are measured by many instruments,such as salinity sensor, time domain reflectometry and electromagnetic induction instrument.The researches of soil conductance were most of saline soil,not of chemical properties of the non-saline soil surface.Soil colloid is the charge carrier of soil solid phase.When imposed by external electric field to the soil suspension system,the soil particles and ions migrate.It is called conductance.As the charges of the soil particle surface adsorb the opposite charge ions from the soil solution,the soil system is electrical neutrality.In this case,it is a complicated interaction among the soil surface charges,the ions in DDL and ions in bulk solution,that affect the soil electrical conductivity.Soil conductance study may become a useful approach to reveal the interaction effects between colloidal particle surface and solution ions.In this paper,the experiment material is yellow earth.Through the determination of electric conductivities of yellow earth particles at different scales,some interesting interaction effects between surface charges and ions on conductance of variably charged soil were revealed.Firstly,the particles were separated by siphonic settlement separation method into different scales and saturated by H⁺.Secondly,we determined the electrical conductivities and pH in soil suspensions and centrifugates at different Mg(NO₃)₂ concentrations,and calculated the electrical conductivities of colloid,the total number of soil surface charges and conductive contribution rate of yellow earth particles at different scales.Lastly,a comparative analysis of these data was made.The results showed that:(1) At the same electrolyte concentration in soil colloidal suspension,the increasing rate of suspension conductivity is greater than centrifugate conductivity;with the increasing of electrolyte concentration in soil colloidal suspension,the increasing rate of proportional coefficient of centrifugate is greater than suspension.With the increasing of electrolyte concentration in soil colloidal suspension,the number of variably negative charges on soil particle surface increase,as a result,the electric conductivity of colloidal particle increase which is positive contribution to the suspension conductance.On the other hand,for the DDL is compressed and the negative charges on soil particle surface increase,the number and the concentration of ions in DDL increase and ionic activity decrease,as a result,the surface conductivity decrease which is negative contribution to the suspension conductance.But the colloidal particle conductivity and the surface conductivity are positive contribution to the suspension conductance.(2) When the pH changes little,the suspension conductivity,also changes little;the pH lower,the suspension conductivity increase faster.The pH is one of the key factors that affect the electric conductivity of variably charged soil suspension.But it is not a key factor in influence the numbers of negative charges for variably charged particle,the key factor may be the competition between H⁺ and base ions to be physically adsorbed on particle surface.As the competition decrease for H⁺,even if the pH is low,the negative surface charges may increase and the probability that H⁺ is specifically adsorbed on soil surface may decrease.(3) At the same electrolyte concentration in soil suspension,the change rate of the suspension conductivity,the centrifugate conductivity,the colloid conductivity,the total number of surface charges,is inverse ratio to yellow earth particles size.With the increasing of electrolyte concentration in soil colloidal suspension,the change rate of the suspension conductivity,the centrifugate conductivity and the colloid conductivity increase gradually;the increasing rate of the total number of surface charges increase first,then gradually decrease,and go into zero at the end. This is because soil particle surface reach it's the maximum adsorption capacity and then the colloid conductivities increase no longer.(4) The surface charges of the yellow earth particles which smaller than 0.5μm is 80 percentage of that smaller than 2μm.The yellow earth particles which smaller than 0.5μm play the main role in the contribution to the total soil electrical conductivity,and the conductive contribution rates of the 0.5～1μm and 1～2μm soil particles are comparatively small.