| Numerous studies have confirmed that the soil nano/micro-scale minerals, humus and microbes play a critical role in the soil function and macroscopic effects. According to the classical theory, the interactions of particles on this scale are dominated by such as molecular force, electricity force and ionic bridging of high valence cations, hydrogen bonding and chemical bonding, hydrophilic and hydrophobic force. However, due to the particularity of rigid mineral and flexible organic particles, the interactions between mineral and humic may not be consistent with the classical theory of colloidal particles interaction. For a long term, it was generally considered that the specific ion effects origin from the difference of ion radius and ion hydration radius. However, it has been rejected with the progress of new physical chemistry in the last decade. Recently, some new findings about the the interaction between colloidal particles noted that there were strong and particular specific ion effects in the interaction of charged colloidal particles. These effects can be explained by the changes of non-valence electron energy in the ion and surface interaction rather than dispersion force, classical induced force, ion size and hydration effects. And the changes of non-valence electron energy resulting from coupling effects between quantum fluctuation of ionic outer shell electrons and the surface electric field. The strength of this force was up to104times that of the classical induction force, and could be comparable to the Coulomb force. Coulomb, dispersion and hydration effects appeared to be interwined to affect the coupling effects.Since the difference of the outer configuration electrons of ions the quantum fluctuations of the outer electrons are differet. However, because of the non-valence electrons are greatly bounded by the attrative force of the nucleus, the difference of the strength of ionic quantum fluctuations for ions are small. While the coupling effects between quantum fluctuation of ionic outer shell electrons and the electric field indicate that the strong electric field near the solid-liquid interface in the soil will rapidly enlarge this small difference, which cause the significantly different in the strength of different ions influence the interaction (coagulation and dispersion) between the colloidal particles. In the study presented here we investigated the total average aggregation rate, critical coagulation concentration, the activation energy between particles, the fractal dimension of formed aggregate and physical stability of aggregates during the colloidal particles measured by laser light scattering. It was found that there were strong specific cation or anion effects in the interation of soil organic, inorganic and organic-inorganic composites, and that the essentially origin of the specific ion effects in the process of soil colloidal particles aggregation was the ionic non-classical polarization caused by the coupling effects between quantum fluctuation of ionic outer shell electrons and the surface electric field. Moreover, we first established an approach for theoretically calculating the effective charge number of the polarized cation based on the critical coagulation concentration to quantitative characterize the absolute intensity of the non-valence electron effect. Finally clarify the mechanism of formation of soil organic-inorganic complexes. Simultaneously, the measurement method of Hamaker constant and surface potential considered the specific ion effects using laser light scattering methods were established. The main results of this study are as follows:(1) There are strong specific ion effects in whether humic acid colloids, montmorillonite colloid or montmorillonite-humic acid composite colloids aggregation process. The sequence of specific ion effects for Cu2+, Ca2+and Mg2+were Cu2+>Ca2+>Mg2+, which reflect by the formed aggregate diameter or total average aggregation rate in a given time, the critical coagulation concentration, the activation energy between particles and the structure of the formed aggregates and their stability. For example, the critical coagulation concentration of Cu2+, Ca2+and Mg2+in99%montmorillonite+1%humic acid were1.998,5.764and10.48mmol/L, respectively; the activation energy between montmorillonite particles in0.6mmol/L Cu2+, Ca2+and Mg2+systems were0.1347,1.601and2.092kT; For SO42-, Cl-and H2PO4-three kinds of anions, the specific ion effect sequence was SO42->Cl->H2PO4-, e.g. when the ionic strength was9mmol/L, the montmorillonite aggregation rate in the three anions systems were139.4,90.67and8.584nm/min, respectively.(2) It was found in humic acid colloid aggregation experiments that the scattered light intensity can be maintained stable over a wide range of electrolyte concentration (0-20mmol/L) in the Ca(NO3)2electrolyte system, but in Cu(NO3)2electrolyte system, this range of electrolyte concentration was relatively narrow, approximately0-3mmol/L; With the increase of electrolyte concentration, the corresponding spherical hydrodynamic diameter growth of the aggregates fitted a straight line relationship at low electrolyte concentrations but was better represented by a power law relationship as relatively high electrolyte concentrations, but the humic acid aggregation process was much more sensitive for the concentration change of Cu(NO3)2than Ca(NO3)2. For example, when the Cu2+concentration was1mmol/L, the average aggregation rate of humic acid particles was69.55nm/min; however, when the Ca2+concentration up to7.5mmol/L, the average aggregation rate of humic acid particles was only23.94nm/min, this rate was only0.344times as much as the former. Obviously the mean aggregation rate is much higher under the action of Cu2+than under the action of Ca2+. In addition, the strong specific ion effects between Ca2+and Cu2+is also reflected in the stability of aggregates:Generally, we use the fractal dimension to characterize the openness of the aggregates structure, the greater the fractal dimension, the more dense and tight the aggregate structure is; the smaller the fractal dimension, the more loose and open the aggregate structure is. In the Ca(NO3)2system, the higher the electrolyte concentration, the more open, the structure of the resultant aggregates, which became more open or loose in structure and smaller in fractal dimension after being set aside for50days, suggesting that the aggregation process is reversible. However, in the Cu(NO3)2system, the aggregates formed under the action of Cu2+were quite open in structure at their initial stage, but they became higher in fractal dimension and more compact in structure after being set aside for50days, suggesting that the aggregation process is irreversible. All the findings provide a new way of thinking for understanding the mechanism of the formation of humic supemolecular aggregates. (3) The study found that the difference of the activation energy in various anion solutions increased with the decrease of ionic strength, i.e. the specific ion effects were stronger in low electrolyte concentrations than relative high concentrations. For instance when the ionic strength was9.5mmol/L, the difference of the activation energy of montmorillonite in Cl-and SO42-solutions was0.6716kT, and with the ionic strength rise to6.5mmol/L, this difference increased to1.334kT. Therefore, the origin of specific ion effects in the aggregation process was not dispersion force or ion hydration force. Decrease of electrolyte concentration means increasing electrostatic field, therefore, the observed specific anion effects in the experiment actually from the ion polarization in the strong electric field near interface. However, the influence of three divalent cations (Cu2+, Ca2+and Mg2+) on the activation energy is different from the monovalent cations and anions, i.e. the differences of activation energies between any two divalent cations system decreased with decreasing electrolyte concentration. For example, when the electrolyte concentrations were0.3,0.4and0.6mmol/L, the differences of activation energy between montmorillonite in Ca2+and Cu2+systems were0.5622,1.194and1.466kT, respectively. On the other hand, the specific ion effects did not appear to be caused by the differences in the hydration diameters of Cu2+, Ca2+, and Mg2+, because the radiuses of these three cations are almost the same (approximately0.42-0.45nm). Studies have shown that there were91.7%negative surface charges was neutralized by the dispersion force adsorption of Cu2+;78.7%negative surface charges was neutralized by the dispersion force adsorption of Ca2+; or34.4%negative surface charges was neutralized by the dispersion force adsorption of Mg2+if the specific ion effects come from the dispersion force. Obviously, it was impossible that there were so many dispersion force adsorbed Cu2+, Ca2+or Mg2+under such low electrolyte concentrations of0.000650～0.00799mol/l. Therefore, the ionic polarization, but not the dispersion or ionic hydration, dominated the specific ion effects. But the coupling effects of the electric field, ionic charge number and ion distribution near the surface are interwined to cause them to appear a different trend with anionic and activation of monovalent cations. But the strength of ionic polarization encountered here was up to104times that of the classical induction polarization. (4) The non-classical ion polarization will significantly enhance the ion Coulomb force near the interface. An approach for theoretically calculating the effective charge number of the polarized cation was first established based on the critical coagulation concentration in this study and sucessfully obtained the effective charge coefficient and effective charge of some cations. The effective charge coefficients of Cu2+, Ca2+, K+, Mg2+, Na+, Li+were2.45,1.96,1.92,1.41,1.16and1.05, respectively. The calculated effective charges for polarized cations near the montmorillonite surface were ZCu=4.90, ZCa=3.92, ZMg=2.82, ZK=1.92, ZNa=1.16and ZLi=1.05. It can be seen that the effective charge increased as electronic shell number increased, and the effective charge for divalent cations was greater than that for monovalent cations because divalent cations prefer to remain in the near-surface space than do monovalent cations.(5) In different mass ratio of montmorillonite-humic acid composite colloid aggregation process, the specific ion effects from strong to weak follow:100%humic acid>90%Mont+10%HA>96%Mont+4%HA>99%Mont+1%HA. As the increase of electric field intensity, the ion polarization enhanced, thereby enhancing the specific ion effects. This study also illustrates the montmorillonite-metal ion-humic acid interaction mechanism, and has been verified by infrared spectroscopy. The main impact of metal ions on the three interactions focus on the surface carboxyl groups in humic acid, the role of conformational changes of the humic acid is:HA1→HA2→HA1during the interaction process.(6) Ion polarization could greatly increase ion Coulomb attraction force on the surface, thereby compressing the diffusion double layer, increase the concentration of adsorbed counterions near the interface, thereby reducing the surface potential of the particles and the electric field near the interface. Based on this, an approach for measure the Hamaker constant of mineral was first established through the dynamic light scattering measurement. Simutaneously, a method of measuring the surface potential considering the specific ion effects was establish. The results showed that the ionic polarization will significantly reduce the surface potential of the particles. For examples, when the electrolyte concentration was50mmol/L, the measured surface potential of montmorillonite particles were-151.5mV,-100.2mV and-84.96mV in Li+, Na+and K+systems, respectively. At this time, the surface potential of montmorillonite in Li+system was1.512times as much as that of Na+,1.783times as much as that of K+. Therefore, the order of the surface potential is Li+>Na+> K+, and this order was consistent with the order of the critical coagulation concentration of the three ions.In summary, there is a strong specific ion effects during the minerals-humic acid interaction process, which has been ignored in soil academia for a long time. This study suggests that whether cationic or anionic and divalent cation or a monovalent cation, the specific ion effects in the colloidal particles aggregation process were origin from the non-classical polarization of the adsorbed counterions near the interface. The strength of ionic polarization encountered here was up to104times that of the classical induction polarization, thereby it greatly improving the ion Coulomb attraction force near the interface, so that cause the effective charge number multiplied. As the effective charge number of Ca2+and Cu2+were increase from+2to+3.92and+4.9, respectively, and that of K+increase from+1to1.92. For cations with the same valence, the more electron ion layers, the greater the increase in the effective charge number; For ions with different valence, the more ion charge is, the more increase in the effective charge number. The enhance of the Coulomb force of ions near the surface leading to a further compression of the diffusion layer and the increase of the concentration of adsorbed counterions near the interface, and finally cause the surface potential of the particles and the electric field near the interface decrease. |
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