Specific Ion Effects Of H~+ And Alkaline Ions On Variably Charged Colloids Aggregation

The interaction of soil colloidal particles affects the formation,stabilization,and dispersion of soil aggregates,and thereby affecting the state of soil structure,the effectiveness and mobility of mineral nutrients in soil,soil moisture migration,microbial activity,soil erosion resistance as well as other macroscopic phenomena.Soil colloidal particles have a large number of negative charges on the surface,and the interactionsbetweenparticlesaregenerallydescribedbythe Derjaguin-Landan-Verwey-Overbeek（DLVO）theory.According to the DLVO theory,the stability of a colloidal suspension depends on the relative magnitude of the repulsive energy and the gravitational potential energy between the particles in the suspension.However,recent studies have shown that the classical DLVO theory fails in many systems,and the reason for the failure is that the theory ignoresspecific ion effects.Studies have shown that the reaction between the ions and the surface of the charged colloidal particles profoundly affects the interaction between the colloidal particles in the soil,which in turn affects the agglomeration kinetics of the colloidal particles.For a long time,it was thought that ion hydration was the main reason for ion-specific effects,but recent studies have found that non-classical polarization of ions in soil systems is the main reason for ion-spe1-ific effects.However,this theory does not apply to a special monovalent ion,H⁺,which has no electrons outside the core,so there is no polarization.What are the new characteristics of the specific ion effects of H⁺compared to polarizable monovalent metal ions?In order to answer this question,two kinds of materials with variable charge colloids were selected in this study:yellow soil colloid and humic acid colloid（HA）to study the Hofmeister effects of H⁺/Li⁺/Na⁺/K⁺/Cs⁺in the aggregation of colloid particles.Meanwhile,we also study the aggregation kinetics of colloidal particles in the mixed electrolyte solutions H⁺+Li⁺/Na⁺/K⁺/Cs⁺as well as the zeta potential analysis and infrared spectroscopy scanning techniques to reveal the specific ion effects and mechanisms of H⁺and other monovalent metal ions in colloidal aggregation.The results of are shown as follows:（1）The study of the aggregation kinetics of yellow soil colloids in the presence of a single soil cation indicates that there are significant differences in the aggregation induced by the three monovalent cations H⁺,K⁺,and Na⁺.A significant specific ion effect is exhibited.The Total Average Aggregation Rates（TAA rates）for colloidal agglomeration of yellow soils induced by H⁺,K⁺,Na⁺follow the sequence of H⁺>>K⁺>Na⁺.For example,the TAA rate of yellow soil colloidal aggregation in the presence of0.8 mmol L^-1 H⁺is 136.9 nm min^-1,which is far greater thanthat of K⁺and Na⁺(0 nm min^-1).Meanwhile,the TAA rates of yellow soil colloidal aggregation in the presence of10 mmol L^-1 K⁺is about 2.7 times that of Na⁺.The corresponding Critical Coagulation Concentration（CCC）corresponds to the sequence of H⁺(0.88 mmol L^-1)<<K⁺(12.40mmol L^-1)<Na⁺(23.01 mmol L^-1).At the same time,the activation energy of colloidal agglomeration of yellow soil also follows H⁺<<K⁺<Na⁺.For example,the activation energy of colloidal aggregation in the presence of 10 mmol L^-1 H⁺had been reduced to0,while the activation energy of colloidal aggregation in the presence of 10 mmol L^-1Na⁺is 0.46 kT,which is approximately 3.3 times that of K⁺.Since the surface charge density of the yellow soil colloid used in the experiment is 0.192 C m^-2,there is a strong electric field around the yellow soil colloidal particles.The aggregation of the particles depends on the influence of the above-mentioned monovalent cation on the electric field.Zeta potential measurement results show that K⁺is more strongly adsorbed on the surface of the colloid due to the higher polarizability of K⁺than Na⁺,which can more strongly shields the electric field around the yellow earth particles.For example,in the presence of 10 mmol L^-11 K⁺/Na⁺,the zeta potential of yellow soil was-29.33 mV and-37.14 mV（"-"indicates that the particles were negatively charged）.The zeta potential of yellow soil colloids in the presence of K⁺was significantly lower,resulting in the occurrence of yellow soil condensation in the presence of K⁺.Due to the covalent bond between H⁺and O on the yellow soil colloidal surface,H⁺is more capable of reducing the electric field than K⁺.For example,when the H⁺/K⁺ion concentration is 0.5 mmol L^-1,the yellow earth’s zeta The potentials are-16.65 mV and-41.45 mV,respectively,thus exhibiting the greatest degree of colloidal agglomeration in comparison with other monovalent ions.（2）The aggregation kinetics of HA colloids in the presence of a single cation show that at a low concentration as 30 mmol L^-1 of H⁺,the HA colloidal particles will rapidly aggregate,and TAA rate reach 201.7 nm min^-1.For Li⁺/Na⁺/K⁺/Cs⁺,therre is no aggregation even at a high concentration of 900 mmol L^-1.These results indicate that H⁺can effectively reduce the electric field around HA,whereas Li⁺/Na⁺/K⁺/Cs⁺cannot.Infrared spectroscopy analysis showed that H⁺covalently interacts with carboxyl and hydroxyl functional groups on the surface of HA colloidal particles,neutralizing the negative charge on the surface of the colloidal particles,and greatly reducing the charge density on the surface of the HA particles,thereby reducing the electrostatic repulsion pressurebetween particles and causing the rapid agglomeration of the HA colloidal particles.However,Li⁺/Na⁺/K⁺/Cs⁺mainly shields the electric field of HA particles by compressing the double layer of colloidal particles,and it is difficult to reduce the electric field strength by reducing the surface charge density.Therefore,it is difficult for Li⁺/Na⁺/K⁺/Cs⁺to effectively initiate HA.（3）When the concentration of H⁺in the colloidal suspension increases,the colloidal aggregateion increases significantly and shows a significant specific ion effect.For example,when the concentration of H⁺in the yellow-soluted colloidal suspension increases from 10^-8 mmol L^-1 to 0.1 mmol L^-1,the TAA rates of yellow soil in 0.3 mmol L^-1 Na⁺and K⁺electrolytes rapidly increase from 0 to 20.2,26.8 nm min^-1,the activation energy rapidly decreased from infinity to 0.66,0.32 ^kT,and the corresponding CCC decreased from 23.01 and 12.40 mmol L^-1 to 1.29 and 1.20 mmol L^-1,respectively.In the HA suspension,when the H⁺concentration was 10^-8 mmol L^-1,HA did not coagulate even at a high concentration of 900 mmol in L^-1 Li⁺/Na⁺/K⁺/Cs⁺solutions.However,as the concentration of H⁺increased to 5 mmol.Li⁺/Na⁺/K⁺/Cs⁺will initiate HA agglomeration and show a significant specific ion effect.The aggregation rate of HA aggregation induced by Li⁺,Na⁺,K⁺,and Cs⁺follows Cs⁺>K⁺>Na⁺>Li⁺,the activation energy follows Cs⁺<K⁺<Na⁺<Li⁺.For example,when the electrolyte concentration is 25 mmol L^-1,the aggregation rates of HA initiated by Li⁺,Na⁺,K⁺,and Cs⁺are 5.92,19.09,41.31,and 68.43 nm min^-1,respectively;the activation energies of aggregation are 1.02,0.29,0.27,and 0.21 kT,respectively..The corresponding CCC follows the sequence of Li⁺(203.8 mmol L^-1)>Na⁺(151.7 mmol L^-1)>K⁺(72.14 mmol L^-1)>Cs⁺(56.66 mmol L^-1).When the concentration of H⁺is further increased,Li⁺,Na⁺,K⁺,and Cs⁺will initiate more rapid gelation of the colloids and exhibit a strong specific ion effect.For example,When the concentration of H⁺in HA is further increased to 10mmol L^-1,at the electrolyte concentration of Li⁺,Na⁺,K⁺,and Cs⁺is 20 mmol L^-1,the aggregation rate of HA colloidal aggregation follows Cs⁺(138.1 nm min^-1)>K⁺(102.0nm min^-1)>Na⁺(66.61 nm min^-1)>Li⁺（40.10 nm min-1）,while the corresponding CCC followed Li⁺(80.11 mmol L^-1)>Na⁺(54.42 mmol L^-1)>Cs⁺（35.61 mmol L-1）>K⁺(32.78 mmol L^-1).Li⁺/Na⁺/K⁺/Cs⁺is responsible for the difference in the electrostatic shielding effect due to the difference in their polarizability（Cs⁺>K⁺>Na⁺>Li⁺）,which in turn leads to a difference in the condensing effect,and eventually leads to the sequence of specific ion effects:H⁺>Cs⁺>K⁺>Na⁺>Li⁺.Based on the above findings,we can draw the following conclusions:（1）Whether it is yellow soil or HA,their aggregating processes show strong specific ion effects,and follow the same sequence:H⁺>Cs⁺>K⁺>Na⁺>Li⁺.（2）Whether it is yellow soil dominated by inorganic clay minerals or pure organic colloidal HA,H⁺initiate the aggregation of colloidal particles through its covalent bonding with surface functional groups,and Li⁺/Na⁺/K⁺/Cs⁺initiates the aggregation of colloidal particles mainly through their compression of the diffusion layer.（3）Since H⁺reduces the electrostatic repulsion between particles by neutralizing negative charges on the surface,and thus more effectively promotes agglomeration;the apecific ion effects of Li⁺/Na⁺/K⁺/Cs⁺are caused by the differences in their polarizability（Cs⁺>K⁺>Na⁺>Li⁺）which cause the difference in the electrostatic shielding effect,in turn,brings about the difference in the condensing effect,and eventually leads to the ion-specific sequence of H⁺>Cs⁺>K⁺>Na⁺>Li⁺.