Ion Specific Effects On The Colloidal Aggregation Of Positive And Positive-negative Charges

There are a large number of inorganic,organic and organic-inorganic colloids in the soil.The aggregation and dispersion of soil colloids directly affect the formation and stability of agglomerates,which affects the process of hydrothermal solute transport,soil loss and non-point source pollution of farmland.At present,some researchers investigated the aggregation mechanism of organic negative charge?humic acid?colloids and inorganic negative charge?montmorillonite?colloids in soil,and a series of research results has been obtained.Recent studies have shown that ion specific effects or Hofmeister effects play an important role in the aggregation of organic/inorganic negative charge colloids.However,the research on aggregation dynamics of positively charged iron oxide colloids are relatively weak,as well as positively charged colloids and organic/inorganic negatively charged mixed colloids.In this study,light scattering technique was used.Hematite and goethite were selected for positively charge colloids,and humic acid and montmorillonite were selected for the organic and inorganic negatively charge colloids.In our research,the dynamic process of the aggregation of hematite and goethite was firstly measured,and revealed the interaction mechanism between ions and iron oxide colloids in the process of aggregation and the essential causes of ion specific effects.On this basis,our further investigated the aggregation between the negatively charged humic acid or montmorillonite colloids and the positively charged goethite colloids.The kinetics of the heterogeneous aggregation of goethite-humic acid and goethite-montmorillonite were contributed to revealing the aggregation mechanism and the basis of ion specific effects.The study results was followed:?1?Through the influence of H⁺on the activation energy of variable charge colloids,we successfully established a theory of the aggregation kinetics of variable charge colloids under different pH conditions.From this theory,if the surface area of the colloids,the point of zero charge?PZC?,the temperature,and the background electrolyte concentration were given.we could obtain the relative value of the total average aggregation rate?TAA rate?of variable charge colloids.By light scattering technique,we measured the relative value of the TAA rate of hematite and goethite with the change of pH.The experimental results were consistent with the theoretical predictions,which confirmed the theoretical predictions were correct.At the same time,the study obtained the curve of the TAA rate of the aggregation of hematite and goethite colloids with pH.Based on the established theory in this study,a new method for obtaining PZC of variable charge colloids is proposed.?2?For Na⁺and K⁺systems,there is no significant difference in the critical coagulation concentration?CCC?value of the colloidal aggregation of hematite and goethite.Therefore,there is no ion-specific effect in monovalent ion system.However,for Mg²⁺,Ca²⁺and Cu²⁺systems,the CCC value sequence of ion specific effects is:Mg²⁺<Ca²⁺<<Cu²⁺.Further studies have showed that non-classical polarization of divalent cation is prone to occur,and formed-OCu⁺,-OCa⁺and-OMg⁺ligands to increase the positive charge of the colloidal surface.Therefore,that increased Electrostatic repulsive force and activation energy of between colloids,which was the cause of ion specific effects of divalent ion systems.However,a monovalent ion could not form such a ligand,so there is no ion specific effects.Whether it is monovalent or divalent cations,the CCC value of goethite colloids is larger than that of hematite colloids.Further,by measuring the zeta potential of the two colloids,it was found that the zeta potential of goethite colloids is larger than that of hematite under the same electrolyte concentration and pH conditions,so that the activation energy of the coagulation is also large.Therefore,goethite colloids had a large CCC value than hematite colloids.?3?The CCC value sequence of the ion specific effect of anion-induced hematite and goethite colloidal aggregation was:NaNO₃>Na₂SO₄>NaH₂PO₄.NO₃^-was adsorbed on the surface of two minerals by electrostatic interaction.As the concentration of NO₃^-increased,the ability for shielding the electric field is stronger,and the zeta potential value is smaller.It had been showed that SO₄^2-and H₂PO₄^-affected the electric field around the particles by two different mechanisms:one is shield the electric field on the surface by electrostatic adsorption on the surface of the two minerals;the other is through the specific adsorption.The amount of positive charge on the surface of the two colloids is reduced by two mechamisms above.Since the specific adsorption capacity of H₂PO₄^-is stronger than that of SO₄^2-,the stability of the colloidal suspension in the H₂PO₄^-system is less than that of the SO₄^2-system,while the stability of the colloidal suspension in the NO₃^-system is the strongest.?4?In the aspect of aggregation dynamics with oppositely charged colloidal particles,this study investigated the addition of a negatively charged organic colloid,humic acid?humic acid salt?,to hydrodynamic diameter of goethite system with humic acid.It was found that hydrodynamic diameter of goethite colloids with the addition of humic acid first increased,then reached the maximum value,and then hydrodynamic diameter of goethite colloids decreased with the increase of humic acid.The critical addition amount of humic acid is 1.5 mg L^-1.The study found that the reason for this phenomenon was:First,the humic acid particles are negatively charged,and the electrostatic force between it and the goethite colloidal particles is attraction force,so with the increase of humic acid,the electrostatic attraction appears.The increase in the number of particles eventually lead to the aggregation of positive and negative charge colloids,which increased hydrodynamic diameter of the system.Secondly,when humic acid reached a critical addition amount,the amount of humic acid is equal to the number of goethite colloids.In term of the average effect,the electrostatic repulsion between the colloidal particles disappears completely.At this time,all the colloidal particles in the system are agglomerated,so hydrodynamic diameter of the system is the largest.Third,when the humic acid is further increased,these excesses The negatively charged particles cannot agglomerate,resulting in a decrease in the measured value of hydrodynamic diameter of the system.On this basis,we studied the heterogeneous aggregation kinetics of humic acid and goethite colloids when a certain amount of humic acid was added.The study found that the kinetic effects of monovalent and divalent ions on the aggregation of humic acid and goethite had an ion specific effects.For monovalent cations,the order of ion specific effects of CCC values is:LiNO₃>NaNO₃>KNO₃>CsNO₃.For divalent cations,the order of ion specific effects of CCC values is:Mg?NO₃?₂>Ca?NO₃?₂>Cu?NO₃?₂.This study indicated that the non-classical polarization of monovalent cations in the electric field generated by colloids is responsible for the ion specific effects in the aggregation of two mixed colloids.However,divalent ions can be polarized,specifically adsorbed and the cationic bridge bond in the three aspects of the ion specific effect of mixed colloids.Cu²⁺and Ca²⁺formed cation bridges on the surface of the mixed colloids,while Mg²⁺could not form bridge bonds;Cu²⁺is more capable of adsorbing on the surface of humic acid colloids than Ca²⁺.?5?This study investigated the effect of the addition of inorganic negatively charged colloid,montmorillonite,on hydrodynamic diameter of goethite.It was found that hydrodynamic diameter of goethite colloids with the addition of montmorillonite colloids first increased,then reached the maximum value,and then hydrodynamic diameter of goethite colloids decreased with the increase of montmorillonite colloids.The critical addition amount of montmorillonite is 6 mg L^-1.The study found that the reason for this phenomenon was:First,montmorillonite colloids are negatively charged,and the electrostatic force between it and the goethite colloidal particles is attraction force,so with the increase of montmorillonite colloids,the electrostatic attraction appears.The increase in the number of particles eventually lead to the aggregation of positive and negative charge colloids,which increased hydrodynamic diameter of the system.Secondly,when montmorillonite colloids reached a critical addition amount,the amount of humic acid is equal to the number of goethite colloids.In term of the average effect,the electrostatic repulsion between the colloidal particles disappears completely.At this time,all the colloidal particles in the system are agglomerated,so hydrodynamic diameter of the system is the largest.Third,when the montmorillonite colloids is further increased,these excesses The negatively charged particles cannot agglomerate,resulting in a decrease in the measured value of hydrodynamic diameter of the system.When a certain amount of montmorillonite is added,the kinetics of heterogeneous aggregation of montmorillonite and goethite colloids was further studied.The CCC values of the NaNO₃ and KNO₃ systems were 9.4 mM and 5.8 mM,respectively,and the former was1.6 times that of the latter.Therefore,the heterogeneous aggregation of goethite and montmorillonite has ion specific effects.Non-classical polarization is the source of ion specific effects.Besides,goethite played a coupling role in the heterogeneous aggregation of goethite and montmorillonite colloids,which reduced the stability of heterogeneous aggregates.