Hofmeister Effects On Ion Adsorption Kinetics At Yellow Earth Particle Surface

The exchange of ions at the soil/water interface is a very important physicochemical process which is not only the fundamental reason why soil has the ability to supply and maintain fertility,but also has certain self-purification and environmental capacity for inorganic/organic pollutant compounds.As well as it has a close relationship with soil erosion,formation and stabilization of soil aggregates,and macroscopic phenomena such as soil moisture infiltration and soil material migration.In the past,scholars proposed a series of ion exchange adsorption models that describe the complex soil system.Although there are many empirical or semi-empirical models for soil ion exchange adsorption,ion exchange adsorption is a complex process in which several single reactions act simultaneously.Therefore,the complex microscopic mechanisms of ion interface reactions are difficult to describe using these empirical or semi-empirical models.In recent years,some important advances have been made in the study of soil ion adsorption mechanisms,including:?1?Adsorption of ions on the surface of soil particles is the process of ion diffusion and adsorption under the strong soil electric field;?2?Weak electrostatic force adsorption and strong interactions?electrostatic and non-electrostatic?adsorption process dynamics description,etc.The above studies are only based on a constant charge surface.The surface of soil particles in nature is mostly a variable charge surface,such as various metal oxides,organic materials,and silicate minerals.Therefore,the adsorption studies of ions on the surface of variable charge provide important theoretical basis for evaluating the electrochemical properties of soil/water interface,interparticle interactions,and buffering properties of acid and alkali.In this study,we investigated the adsorption kinetics feature of several alkali metal ions on the surface of yellow earth particles,further calculated the ion diffusion distance at the solid/liquid interface and the surface electrochemical parameters of yellow earth particles,and obtained the adsorption selectivity of H⁺and alkali metal ions in the diffusion layer and double layer based on the non-classical polarization of ion kinetic model.The results show that?1?Under the conditions of stock solution and pH=4.0,the adsorption process of Li⁺,Na⁺and Cs⁺on K⁺saturation surface only had first-order kinetics but existed strong ion-specific effects.The adsorption quantity and adsorption rate were all presented as Cs⁺>Na⁺>Li⁺at the electrolyte concentration of1×10^-44 mol·L^-1 and Cs⁺>Li⁺>Na⁺at the electrolyte concentration of 1×10^-2 mol·L^-11 and1×10^-3 mol·L^-1,which indicated that the ionic specificity increased with the decrease of electrolyte concentration.Comparing the adsorption kinetics of alkali metal ions under the two conditions,it was found that the adsorption capacity of Li⁺,Na⁺,and Cs⁺at stock solution was less than that under the conditions at pH=4.0,indicating that presence of H⁺has a significant inhibitory effect on its adsorption on the surface of yellow soil particles.?2?The strong zero-order adsorption kinetics of H⁺occurs at pH=4.0,and the adsorption kinetics of H⁺in Li⁺/H⁺,Na⁺/H⁺and Cs⁺/H⁺systems at a concentration of 1×10^-2 mol·L^-1.The emergence time from the zero level to the first level turning point is 27 min,18 min and 12 min respectively,and the critical time at the concentration of 1×10^-3 mol·L^-1 is 54 min,30min and 27 min respectively,and the concentration is 1×10^-44 mol·L^-11 was 45 minutes,40 minutes,and 30 minutes,respectively.The results show that the competition strength with H⁺at different concentrations is Cs⁺>Na⁺>Li⁺,and the same ion competition ability increases with the increase of the concentration.In addition,the zero-order adsorption rate constant of H⁺is maintained at about 20 mmol·min^-1·m^-2.It can be seen that the existing of Li⁺,Na⁺,and Cs⁺only affects the strong adsorption time of H⁺and has little effect on the adsorption rate.?3?The thickness of the Stern layer decreases with increasing electrolyte concentration,and ion-specific effects follow the same pattern.Since the diffusion distance is affected by the combination of ion non-classical polarization and steric hindrance.The ion volume effect dominating in the high electrolyte concentration system results in d_Na>d_Li>d_Cs,and ion non-classical polarization dominating in the high electrolyte concentration system results in d _Li>d_Na>d_Cs.But the non-classical polarization of Cs⁺is much stronger than that of Na⁺and Li⁺,so the thickness of its Stern layer is the narrowest.?4?The calculation of surface electrochemical parameters found that in the same electrolyte type with different concentrations,the order of surface potential is?_0?Li?>?_0?Na?>?_0?Cs?,which shows that the surface potential is only affected by the polarization effect,and the volume effect has little effect on it.And the surface potential under the condition of pH=4.0 is less than that of the raw solution.Although the Stern potential?absolute value?exhibits?_S?Li?>?_S?Na?>?_S?Cs?as a whole under two conditions,the order of Stern potential is?_0?Li?>?_0?Na?>?_0?Cs?at a low concentration of 1×10^-4 mol·L^-11 due to the ion volume effect caused by the different thickness of the Stern layer,thus it also proved that ion non-classical polarization and ion volume effect control the ion-surface interaction.?5?The adsorption selectivity of metal ions in the diffusion layer is greater than that of H⁺,and the stronger the non-classical polarization,such as Cs⁺,the greater the adsorption selectivity,indicating that the ion-specific effects in the diffusion layer are mainly controlled by ion polarization effects.In the electric double layer,the adsorption selectivity of Cs⁺,Na⁺,and Li⁺ions is less than that of H⁺(except for Cs⁺/H⁺in1×10^-4 mol·L^-1),indicating that the strong adsorption ability of H⁺occurs in the Stern layer and is affected by the combined effects of polarization and volume effects.The greater selective adsorption of H⁺indicates that the volume effect of ions in the Stern layer dominates.However,in the strong electric field system of 1×10^-4 mol·L^-1,the Cs⁺adsorption is still slightly stronger than H⁺,which shows that although the steric hindrance of Cs⁺and the coordinated adsorption of H⁺,the non-classical polarization of Cs⁺strongly increases its adsorption capacity.Therefore,we speculate that ions with strong polarization may form similar coordination adsorption in strong electric field systems on the surface of yellow earth particles.According to the difference of adsorption of alkali metal ions in the adsorption kinetics on yellow soil particles under different acidity conditions,the calculation of surface electrochemical parameters under different solution conditions shows that the difference of adsorption of alkali metal ions on the surface of yellow soil particles is caused by ions.Under the action of the non-classical polarization under the action of a strong electric field and the volume effect of ions,it is analyzed from the mechanism of how the presence of H⁺inhibits the adsorption of alkali metal ions.The above theory will provide new ideas for the perfection of soil/water interface reaction theory.In this study,we found that the difference in the adsorption of alkali metal ions on the surface of yellow soil particles was caused by the combination of non-classical polarization and the volume effect of ions under the effect of strong electric fields,due to adsorption differences of alkali metal ions on adsorption kinetics of yellow soil particles under different acidity conditions and it is analyzed how the presence of H⁺inhibits the adsorption of alkali metal ions from the mechanism.The above theory will provide new ideas for the perfection of soil/water interface reaction theory.