Specific Ion Effects On Soil Colloidal Particles Aggregations

Specific ion effects, also referred to as Hofmeister effects, were discovered as long as120years ago in protein aggregation experiments. Over the past decade, researches have suggested that the specific ion effects have a profound impact on the solid/liquid interface process of nano-/micro-sized colloidal particles, which will further determine the macroscopic effects and functions of nano-/micro-sized systems. As a result, studies on the specific ion effects have been widely given rise to interests by physical, chemical and biological scientists. Furthermore, the neglect of specific ion effects has been known to be the deep reason for the failure of classic electric double layer theory and DLVO theory in explaining colloidal particles interactions.The diameters of inorganic minerals, organic matters (humus and polysaccharides) as well as microbial colloidal particles in soils all range from1to1000nm. The aggregation of soil colloidal particles would therefore also be affected by the specific ion effects, just like specific ion effects on protein aggregation. Due to the continuous changes of environmental conditions, the aggregations of soil colloidal particles in nature occur spontaneously. The microscopic processes such as the transport of soil colloidal particles and organic/inorganic ions, the formation of soil stable aggregates, the migration of water as well as the macroscopic processes such as agricultural non-point source pollution, eutrophication of water body, and soil erosion are all influenced by the aggregation of soil colloids. However, due to the lack of quantitatively characterization of specific ion effects on soil colloids aggregation, there has been few study focus on specific ion effects on soil colloids aggregation.The activation energy in an aggregation process determines the aggregation kinetics of a colloidal system, and thus activation energy is an important parameter for stability of colloidal suspension. Compared with the simple colloids, soil colloids are complex and distinct systems with polydisperse nonsperical colloidal particles. There has been none theory or methods suitable for determination of activation energy for soil colloidal particles aggregation so far, which become the key point in the present study. With the introduction of dynamic light scattering (DLS) into the study of soil colloids, the aggregation kinetics can be monitored through the variation of average effective hydrodynamic diameters of aggregates changing with aggregation time. Based on the DLS study of soil colloids aggregation kinetics, the theory and method to quantitatively characterize the specific ion effects on soil colloids aggregation were developed. Then in situ DLS measurements were performed to monitor the aggregation kinetics of montmorillonite colloids, yellow earth soil colloids, purple soil colloids as well as humus colloids in LiNO3, NaNO3, KNO3, RbNO3, CsNO3, HNO3, Mg(NO3)2, Ca(NO3)2, and Cu(NO3)2solutions with a wide range of concentrations. The results of average effective hydrodynamic diameters, total average aggregation rates, critical coagulation concentrations (CCC), and activation energies were analyzed to provide new insights into soil colloids aggregation. The main results were listed as follows:The activation energies for polydisperse nonspherical montmorillonite colloidal particles aggregation in different concentrations of LiNO3, NaNO3, KNO3, RbNO3, and CsNO3solutions were experimentally tested. Based on the determination of the aggregation rate by the DLS technique under different concentrations of alkali cation solutions, the CCC values for Li+, Na+, K+, Rb+and Cs+are obtained equal to277.0,133.0,80.3,31.7, and27.2mmol/L, respectively. the corresponding functions of activation energies vs. alkali cation concentration can be further obtained. At a given concentration of25mmol/L, the activation energies for Li+are1.2,5.7,28, and126times as much for Na+, K+, Rb+, and Cs+, respectively. The DLS measurement of activation energy provides a few advantages, such as it is a simple and sensitive measurement of the activation energy down to less than a few RT of strength, it is applicable to colloidal systems with polydisperse nonspherical as well as monodisperse spherical particles.It indicated that the activation energies can be used to quantitatively characterize specific ion effects. It can be seen that the activation energies decrease in the order of Li+>> Na+>> K+> Rb+> Cs+, which was supported by the results of effective hydrodynamic diameters, aggregation rates and CCCs. The density of the colloids significantly affect the value of activation energy for aggregation while the sequence of specific ion effects are not affected. Specifically, montmorillonite colloids with lower particle densities result in higher activation energies for the aggregation of montmorillonite colloids, while the sequences of activation energies are not affected. The dominant role of electrolyte cations during the aggregation of negatively charged montmorillonite colloidal particles was confirmed by changing NO3-into Cl-, since the avtivation energies in these two solutions are close to each other.The experimentally observed specific ion effects can be rationally interpreted by the polarization effects, which was substantially supported by the strong electric field originating from the montmorillonite surface. All the alkali cations have the same valence state, the activation energies for the aggregation of montmorillonite colloidal particles are not identical when the concentration is the same. Further, the activation energy differences between two alkali cation species increase sharply with decrease of electrolyte concentrations. The classical electric double layer theory, DLVO theory, and hydration of ions, surfaces and solvent molecules all give the contradiction prediction with the experimental results. The classical induction theory, although with inclusion of electric field, requires significant corrections because it predicts an opposite trend to the experimentally observed specific ion effects.Interaction forces of ions and colloidal particle surfaces show good correlation with activation energies for the aggregation of colloidal particles. Compared the interaction forces of ions and colloidal particle surfaces obtained from modified Gouy-Chapman model with the activation energy for aggregation of colloidal particles, it can be seen that the larger interaction energies correspond to lower activation energies. It has been correctly predicted that the experimental activation energies rank as Cs+<Rb+<K+<Na+<Li+, and this can be ascribed to polarization effects. The unusual aggregation kinetics in H+solutions is ascribed to the steric effect. From the experimental results, it can be seen that the aggregation rates are apparently faster in H+rather than alkali ion solutions, the value of CCC for H+(25.6mmol/L) is significantly lower than those for K+(78.9mmol/L) and Na+(139.7mmol/L), and the activation energies representing functions of ionic concentrations increase in the order of H+<<K+<Na+. The unusual ion specificity for H+has been quantitatively described by activation energies. and a nearly quantitative prediction of the activation energy ratios between H+and K+has been given by the recently proposed electrical double layer theory.The natural soil colloids (real systems present as the mixtures of several distinct minerals) are much more complicated than the montmorillonite colloid (model system), nevertheless, the activation energies reflecting specific ion effects in natural soil aggregation process rank as Li+>> Na+>> K+> Rb+> Cs+, which are resemble those of montmorillonite colloidal particles. It further indicates that specific ion effects depend on mineral type, ionic concentration and cation species. In addition to quantitatively characterizing specific ion effects for natural soil colloids, the activation energies have filled the "huge gap" between the model and real colloidal systems. The results further indicate that specific ion effects strongly affect the aggregation of humus colloids. The CCCs are determined to be343,176,11.7, and7.73mmol/L, respectively, for Na+, K+, Mg2+, and Ca2+at pH3.0. As the electrolyte concentration lower than the CCC, the activation energies are significantly different for the four systems and decrease in the order Na+> K+> Mg2+> Ca2+. At pH6.5, there is no aggregation occur in2000mmol/L Na+and K+systems. The CCCs for Mg2+and Ca2+at pH6.5are21.8and74.8mmol/L, respectively, and the activation energies decrease in the order Mg2+> Ca2+. Most importantly, decreasing the electrolyte concentration increased the difference in activation energies between two cation species with the same valence (△ENa-△EK and△EMg-△ECa), while increasing the pH increased the magnitude of (△EMg-△ECa), verifying the specific ion effects on soil colloids aggregation should be ascribe to the polarization effects.The different ion-surface reaction of Ca+and Cu2+with variably charged soil colloidal particles is further considered, and the results showed the specific ion effects of Cu2+are more dramatic. An aggregation process referred to as "Attraction-Diffusion-Limited Cluster Aggregation (ADLCA)" was observed in the Cu2+adsorption-induced aggregation of the soil particles, resulting from electrostatic attraction forces between oppositely charged particles. Based on the static light scattering, the fractal dimensions of the Cu2+specific adsorption-induced aggregation were much higher than those of the Ca2+electrostatic adsorption-induced aggregation, and the fractal dimension of the formed aggregates decrease in the order ADLCA+DLCA> ADLCA≥RLCA> DLCA. The above observations were well explained by analysis of the net interaction forces between particles in suspension,In summary, specific ion effects in soil colloidal particles aggregation can be quantitatively characterized by the activation energy, which was correlated with ionic concentrations. Therefore, the activation energy in different ionic solutions under any concentrations can be calculated. Further, the observed specific ion effects in different alkali ionic solutions can be rationally interpreted by the polarization effects whereas the unusual aggregation kinetics in H+solutions is ascribed to the steric effect. As the specific ion-surface reaction of heavy metals (Cu2+) with variably charged soil colloidal particles present, a faster aggregation process referred to as "Attraction-Diffusion-Limited Cluster Aggregation (ADLCA)" was observed. This work is of high importance and might facilitate new thoughts about environmental and ecological effects of colloidal particles in soils and environments.