| Soil organic, inorganic and microbial colloids are nanoparticles ranging from dozens to thousands nanometers. They are the material foundation of soil fertility and ecological functions. Soil colloidal particle aggregation or dispersion can profoundly influence numerous microcosmic processes and macroscopic phenomena in soil, and ions would be the indispensable influence factor of soil colloids aggregation. In the present dissertation, various soil colloids were used as research materials. The combined determination method were applied to simultaneously measure surface properties including specific surface area, surface charge number, surface charge density, electric field strength at surface and surface potential of soil colloidal particles in500-1000,200-500and<200nm size fractions, the dynamic light scattering method were used to study the impact of inorganic cations or inorganic cations/organic molecule on soil colloidal stability. Some interesting results were found:(1) For both permanently charged purple soil and variably charged yellow soil, soil properties were mainly depended on soil colloidal particles with diameter of<1000nm, and particles with diameter<200nm made the greatest contribution to the soil overall properties. Specially, nearly80%of specific surface area and85%of surface charges came from the colloidal particles, and almost half of specific surface area and surface charges of soil even came from particles with diameter<200nm. Moreover, purple soil and yellow soil exhibited significant differences in size distribution of surface properties. For a given size fraction, the purple soil had higher specific surface area, surface charges, surface potential and electrostatic field at surface than the yellow soil.Soil mineralogical composition and organic substance content had distinct influence on surface properties of soil colloids. Vermiculite, montmorillonite, illite, mica and kaolinite, which had larger specific surface area and higher surface charges mainly distributed in the colloidal particles, and the particles with diameter<200nm bore the highest percentage of expansive clay minerals, i.e. vermiculite and montmorillonite. Organic substance contents in the colloidal particles were much higher than that in courser particles. Further on, for soil particles with a given size fraction, purple soil had higher clay mineral and organic substance content than yellow soil. These can qualitatively and quantitatively explain the size distribution of surface properties of the two soil types.(2) The Hofmeister effects were found in soil colloidal particles aggregation, and they became more distinct at relatively lower cationic concentrations. The Hofmeister series followed Na+(86.1mmol/L)> K+(56.9mmol/L)> NH4+(51.4mmol/L) Mg2+(4.15mmol/L)> Ca2+(2.15mmol/L)> Cu2-(1.72mmol/L) for critical coagulation concentration (CCC) values. Hofmeister effects for total average aggregation (TAA) rates increased with the decrease of cationic concentration, for equivalent cation species, the relative position of NH4+and Cu2+within the series inverted with the increase of cationic concentration, as the electron cloud of these cations were easier to deformation.At relatively low cationic concentration (<0.1mol/L), the electric field around soil colloidal particles can profoundly affect Hofmeister effects in soil colloidal particles aggregation. Hofmeister effects increased exponentially with the electric field strength. When inclusion of relative charge coefficients β resulting from the quantum fluctuation of cation species and strong electric field, the differences of electrostatic repulsive pressure between adjacent soil particles for cation types with identical valence could qualitatively explain the changing trend of Hofmeister effects in soil colloidal particles aggregation.(3) The coupling effect of inorganic cations (Na+and Mg2+) and organic anion (DBS) can significantly influence the red soil colloidal stability. Under a given concentration of cations, the soil colloids were more reluctant to aggregate with rising DBS-concentration. For instance, at120mmol/L of Na+, as the concentration of DBS-increased from0to10mmol/L, the effective diameter of aggregates decreased from702to193nm, and the total average aggregation rates of aggregates dropped from28.6to3.36nm/min. However, under a given concentration of DBS-, as the concentration of cations increased, the thickness of diffuse double layer of colloidal particles decreased, which destroyed the colloidal stability of red soil. Moreover, caion types can also influence soil colloidal particles aggregation. For example, under10mmol/L DBS-and50mmol/L Na+or Mg2+, the ultimate effective diameter of aggregates in DBS-/Mg2+system was about10times larger than that in DBS-/Na+system, and the total average aggregation rates of aggregates in DBS-/Mg2+system was nearly60times higher than that in DBS-/Na+system.DBS-was adsorbed on soil colloidal particles surface through hydrophobic effect and electrostatic effect, thus soil colloid suspension became more stable and needed to adsorb more inorganic cations for aggregation. The enhancement of colloidal stability was attributed to the increasing electrostatic repulsive force between colloidal particles resulting from the increment of negative charge number in the particles surface, as well as to the steric hindrance of long hydrophobic chain of adsorbed DBS-between soil colloidal particles. These results can set a guide line for further researches in term of movement and transformation of other inorganic ions or heavy metal ions in soil polluted by SDBS. |
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