Size methods soil colloids

Dawson, D.J., J.F. Ranville, B. Jackson, B.D. Honeyman, and J. Seaman. 2001. Particle size distributions of soil colloids Comparison of sizing methods and influence on transport in porous media. 9 Int. Conf. on Field Flow Fractionation, Golden CO, June 26-29, 2001. 

Another potential problem with the batch method of Zasoski and Burau (1978) is keeping the soil or colloid uniformly suspended. This could be difficult with sandy soils where the sand-sized particles could sink to the bottom of the reaction vessel, or with high organic matter soils. With the latter, humic materials could rise to the surface of the reaction vessel creating a nonuniform suspension. 

There are many other indirect techniques for determining colloidal species size or size distribution. These include sedimentation/centrifugation, conductivity, x-ray diffraction, gas and solute adsorption, ultrafiltration, viscometric, diffusiometric, and ultrasonic methods [12,13,26,69,82], Two reasons for the large number of techniques are the range of properties that can be influenced by the size of dispersed species, and the wide range of sizes that may be encountered. The grains in soils and sediments can range from colloidal size up to the size of boulders. 

We present here two procedures for applying the pipette method The first used by the U. S. Department of Agriculture (Olmstead et al, 1930), and the other described by Keen (1931). Both of these methods pertain to the determination of silt and colloids in soil suspensions, rather than to a size-distribution. However, the descriptions given indicate the manner of using the pipette. 

Size-dependent structure and properties of Earth materials impact the geological processes they participate in. This topic has not been fully explored to date. Chapters in this volume contain descriptions of the inorganic and biological processes by which nanoparticles form, information about the distribution of nanoparticles in the atmosphere, aqueous environments, and soils, discussion of the impact of size on nanoparticle structure, thermodynamics, and reaction kinetics, consideration of the nature of the smallest nanoparticles and molecular clusters, pathways for crystal growth and colloid formation, analysis of the size-dependence of phase stability and magnetic properties, and descriptions of methods for the study of nanoparticles. These questions are explored through both theoretical and experimental approaches. 

The simplest method is optical microscopy, in which visible light (photons) is used to observe a sample. It has a resolution limit around 0.25-0.5 pm, which is on the order of 2/2, where 2 is the wavelength of incident light. From a strict colloid science point of view, it lies near the upper limit of colloid particle sizes and appears to be of limited utility. However, it is of great help in the identification of minerals, because it allows observation of crystal habits (the shape and size of crystals, which are determined by their internal symmetry). With experience, many minerals can be identified in a soil sample under a microscope, even from simple inspection. A unique feature of optical microscopy is the availability of polarized light, which is handy in distinguishing minerals or even different crystal types of the same compound (Bullock et al. 1985 Cady, Wilding, and Drees 2010). 

---