Laser Scattering Study Of Soil Organic/Inorganic Colloidal Particles Aggregation

Soil is a polydisperse colloidal system,which includes many different components often with diameter 1～1000 nm such as inorganic minerals,humus and microorganism etc.. 1～1000 nm is a special scale:meso-scale in the nature.The special effcct of this scale was long ignored in the study on the interaction of organic,mineral,microbial particles with this scale,though there was much study on the interfacial behavior of the soil colloid.Thus the study on the interaction with humus,mineral,microorganism was not deep.Another important reason is the absence of the research instrument about this scale.But light scattering technique,as the most important tool of colloidal and nano particles,has been widely applied to investigate the colloidal fractal aggregation of simple systems in colloid chemistry,In the natural soil,the thermodynamic conditions changing in the soil frequently,such as electrolyte composition and concentration,temperature,pH and so on,will lead soil organic and inorganic colloidal particles aggregation and dispersion alternately and spontaneously occurring.Soil colloidal aggregation and dispersion is the result of colloidal particles interaction.Analyzing the thermodynamic non-equilibrium theory and the Hierarchical model of aggregates formation,we consider that the fractal aggregation should exist widely in the process of the soil colloidal aggregation.Therefore,it is a right direction that the light scattering technology is applied to study on the aggregation kinetics of soil colloidal particles and aggregates morphology,finally to reveal the mechanism.A brookhaven multi-angle dynamic/static laser scattering instrument BI-200SM is the research grade system,for studies of both static light scattering(SLS) and dynamic light scattering(DLS).The average scattering intensity,the effective diameter and the particle size distribution of particles(or aggregates) are determined by DLS,and the aggregation kinetics is characterized by the time evolution of the aggregates diameter.The fractal dimension is determined by SLS to reflect aggregates morphology.Therefore the soil colloidal dispersion system and aggregation system wcrc investigated by DLS and SLS, the conclusions were followed(1) The light scattering technology can apply to the particle interaction in the polycomponent and polydispersion soil colloidal systems.The key of applying conditions is controlling the primary particle concentration and selecting the scattering angles.Based on that the auto correlation function(ACF) can smoothly decay to the baseline with time and the scattering light intensity keeps a constant,the primary particle concentration of the yellow earth colloid was 1.90×10^-3～0.475 g.L^-1,the scattering angles suitable for aggregation kinetics was 90°～135°,and 90°was the most.(2) The study on the primary condition and the dispersion stability of the soil colloid is the important basis of the subsequent study on the colloidal aggregation dynamic.The stability of the yellow earth and the humic colloid was respectively measured:at low electrolyte concentration(c(K⁺)＜10^-5 mmol.L^-1) and approximate pH8,the yellow earth colloid and humic colloid were all the stable polydisperse system.The poly-dispersion of the yellow earth colloid was lower than that of the humic colloid,but the stable time of the yellow earth colloid was longer.The stability of the yellow earth colloid strongly depended on pH,electrolyte types and concentration of colloidal system,but the temperature was no influence.Based on the above study,the aggregation kinetics of soil colloidal particles and the aggregates morphology were mainly investigated at different electrolyte types and concentration,pH,temperature,humic colloid,the conclusions were followed:(1) The electrolyte obviously influenced the regimes of the yellow earth colloidal aggregation.With the increasing of the electrolyte concentration,the aggregation firstly exhibited the slow reaction limited cluster aggregation(RLCA) regime,and then turned to the fast diffusion-limited cluster aggregation(DLCA) regimes.The DLCA aggregates ware loose with d_f≈1.58,and RLCA aggregates were compact with d_f≈1.45.The turning point was existed distinctly which provided a new method for measuring the critical flocculation concentration(CFC) by DLS experiment.For yellow earth colloid,the CFC of KNO₃ was 40 mmol.L^-1,the CFC of Mg(NO₃)₂ was 0.4 mmol.L^-1.the CFC ratio of the bivalent ion to the single valent ion was about 1/100,far lower than the theoretical value 1/64 of Sckulze-Hardy law.The bivalent ion Ca²⁺,Mg²⁺ distinctly promoted the fractal aggregation of the yellow earth colloidal particles,and formed looser aggregates.Although the concentration of Ca²⁺,Mg²⁺ in the soil bulk solution was low to 0.001 mmol.L^-1,the fast fractal aggregation still happened,and the CFC was lower 100 times than that of the single valent ion.(2) The more the particle primary concentration was,the shorter the aggregation finished time was,and the smaller the CFC was,and then the aggregation of the yellow earth colloidal particles was more promoted,but the aggregates were more compact.This indicated that the loose fractal aggregates could not be formed at the soil with the low water content.(3) The temperature could not change the DLCA regime.With the temperature increasing,the aggregation finished time was short,the average aggregation velocity was firstly stable,then rising,and finally falling,the fractal dimension of aggregates increased gradually,which showed that the structure of the yellow earth aggregates was more compact with the temperature increasing.(4) The mechanisms for the yellow earth colloidal aggregation were inconsistent with the pH changing.The aggregation of the yellow earth colloid was induced by the charge neutralization at the low pH,and the average aggregation velocity was far faster than that induced by the compression of the electric double layers at the high pH.(5) With the addition of the humic colloids,the soil colloidal aggregation was only the result of the interaction between the organic and inorganic colloids induced by the salt.The electrolyte concentration of the fast aggregation in the humic colloidal system was far higher than that in the yellow earth colloidal system,but the fractal dimensions of the humic aggregates were still higher than that of the yellow earth aggregates,which indicated that the structure of the humic aggregates was more compact,the structure of the humic\yellow earth aggregates was consistent with the yellow earth aggregates at the formation,but after 10 days,the humic\yellow earth aggregates were relatively stable due to the addition of the humic colloids,which revealed that the humus did not influence the structure character of the soil colloidal aggregates,but distinctively promoted the stability of the aggregates.(6) After 10 days,the fractal dimensions of the yellow earth aggregates induced by different conditions all increased,which showed the particle restructuring made the aggregates more compact.(7) The fractal dimensions of the soil colloidal aggregates at the different thermodynamic conditions were all smaller than the values from the single colloid systems,which suggested that the aggregates of the soil colloid were looser than that obtained from the simple system,due to the properties of colloidal polydispersion and polycomponentMeanwhile,the other points were concluded:(1) The fractal aggregation existed in the soil indeed,and was maybe the reason why the soil granular structure was hierarchical and porosity;(2) The necessary condition generating the soil hierarchical granular structure seemed to depend on the electrolyte types and concentration,not the organic matter(humus);(3) By adding the organic matter,the form of the soil hierarchical granular structure was only the natural result of the interaction and aggregation between organic and inorganic colloids,not of polyvalent ionic bridge bonding.