Soil contains many colloids, mainly including mineral particles such as clay and iron oxides, soil organic matter-mineral complexes, natural organic macromolecules (such as soluble humic acid), and microorganisms and their secretions. Colloids have a huge specific surface area and numerous surface reaction points. Understanding the properties of colloids in soil is very important for predicting their effects.
There is abundant evidence that the surface properties of colloidal particles can deviate significantly from those displayed by their macroscopic counterparts. To predict the chemical and physical properties of colloids in soil, they can be characterized by conventional analytical methods, such as SEM, X-ray diffraction, elemental analysis, surface area determination, total organic carbon analysis, Fourier transform infrared and nuclear magnetic resonance spectroscopy. Many advanced instrumental techniques have also been used to characterize soil nanoparticles, including X-ray photoelectron, X-ray absorption and X-ray absorption fine structure spectroscopy, and scanning transmission X-ray microscopy.
Fig.1 Colloids and colloid-related metals in soil.
The colloids in the soil are small in size and interact with each other, so the isolation yield is low. The separation mechanism of colloids is related to particle size, shape and surface area. Colloid fractionation techniques include split-thin flow fractionation(SPLIFF), flow field-flow fractionation (FIFFF), sedimentation field-flow fractionation(SdFFF), and cross-flow filtration(CFF, also called tangential flow filtration(TFF)). It has been found that colloidal particles can be successfully separated from soil by a combination of sieving, sedimentation, centrifugation and cross-flow filtration.
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