A Light Scattering Study Of Soil Colloidal Particals Interaction

The mineral, organic and microorganism colloidal particles are the most active component, which play ubiquitous critical role in soil fertility and ecological functions. The size distribution of soil colloidal particles are mainly concentred on tens to hundreds nanos, thus possessing special interface scale effection. The interaction of various organic, inorganic and microorganism colloids will have a profoundly impact on the micro-processes and macro-phenomena occurring in soil. However, the study on microscopic effects of soil colloidal particles interactions has so far not aroused much attention due to limitation in measuring means. Conditions were discussed for application of the laser light scattering technology in the study of multi-compenent and polydispersity soil colloidal particles interactions, from the aspects of scattering angle and soil particle density in suspension, and aggregation kinetics of soil colloidal particles and structural characteristics of the aggregates thus formed with the effect of different electrolyte type and/or concentration at various pH explored. The initial conclusions as follows:(1) Light scattering technique could be still appliable to the study on the particle size distribution, aggregation kinetics and structural characteristics of the aggregates thus formed in the multi-component and polydispersion soil colloidal suspensions. The optimal measurement conditions are:the initial particle concentration ranges from 1.90 to 119 mg L^-1, while the scattering angle from 90°to 135°, but 90°is the best choice of all to determine precisely the aggregation kinetics of soil colloidal particles. In the condition that the autocorrelation function curve declines smoothly to the baseline and the scattered light intensity keeps constant with the time going on, the dynamic light scattering technique can be used to determine precisely variation of effective diameter and particle size distribution during the process of aggregation of soil colloidal particles or aggregates. While, the fractal dimension of soil aggregates can be determined through the variation of scattering exponent curve with time passing on, which is obtained from the scattered light intensity of soil colloidal particles (aggregates) at different scattering angles with the static light scattering technique.(2) The stability of both yellow earth and humus colloidal suspensions was found to be influced profoundly by pH. When the yellow earth colloidal suspensions were at pH below 6.65, rapid aggregation took place with the neutralization effect of the proton; wheras above pH12, aggregation occuring resulted from the double layer compression of K⁺. However, only when the humus colloidal suspensions were at pH below 2.0, could aggregation occurre as a result of charge neutralization around the isoelectric point. Therefore, the relatively stable state for the yellow earth soil colloidal suspensions were at pH from 6.65 to 12, while above 2.0 for humus colloidal suspensions. In this study, the average effective diameter of yellow earth and humus colloidal particles were determined to be 175±10 nm, and concentred in 100 to 300 nm for particle size distribution with dynamic light scattering technique.(3) The aggregation kinetic was found to be having some similar characteristics over electrolyte of both yellow earth colloidal suspension at pH8.0 and humus colloidal suspensions at various pH. That was, two aggregation regime, both reaction limited cluster aggregation (RLCA) and diffusion limited cluster aggregation (DLCA) were contained during the aggregation. In the RLCA regime, the growth process of soil colloidal particles transferred from linear to power-law with the increasing of electrolyte concentration, but the universal exponent growth wasn't found. However, the growth curves were found to be overlaped together and followed power-law growth at various electrolyte concentrations for DLCA. The average aggregation rate curve of soil colloidal particles over electrolyte concentrations could be marked two sections: linear increased at low electrolyte concentrations, whereas kept fluctuant around a constant at high electrolyte concentrations. A visible turning point appeared between the two sections, which could be used to determine accurately the critical flocculation concentration and aggregation kinetics of the colloidal suspensions. In this study, the critical flocculation concentration of yellow earth colloidal suspension was determined to be 1.4 mmol L^-1 on the effect of CaCl₂ at pH8.0, while 25.4 mmol L^-1 with KCl, almost 20 times higher than that of with the effect of Ca²⁺ at the same conditions. However, the critical flocculation concentration of humic colloids was found to be increased with pH on the effect of CaCl₂, which were 14.4,44.0,45.9 and 52.7 mmol L^-1 at pH3.0, pH5.0, pH6.5 and pH8.0 respectively. Furthermore, it was 427.6 mmol L^-1 at pH3.0 on the effect of KCl, about 30 times higher than that of on the effect of CaCl₂ solutions at the same pH. The ratio of critical flocculation concentration of humus at pH3.0 between K⁺ and Ca²⁺ was much higher compared to yellow earth soil colloidal suspension, indicated that the aggregation capacity of divalent and monovalent ions would exist heterogeneity when the surface electrochemical property of the colloidal particles was different.(4)In the DLCA regime, the fractal dimension of the aggregates was determined to be 1.44±0.02 for yellow earth suspensions over electrolyte solutions at pH8.0, whereas 1.80±0.05 for humic aggregates formed with the effect of CaCl₂ solutions at different pH and KCl solutions at pH3.0. However, in the RLCA regime, the fractal dimension of the aggregates exhibited increase with the decreasing of the electrolyte concentrations. The fractal dimension was measured to be about 1.53-1.65 for yellow earth suspensions, but about 1.85-1.98 for humus suspensions. The results of fractal dimension indicated that the structure of the humic aggregates formed therein was more compact compared to yellow earth aggregates with the interaction of colloidal particles.