Particle Size Analysis

The dissertation below outlines the difference between toxic metallic minerals versus non-toxic plant minerals and the colloidal size and water solubility of TRC Minerals®.

Results for Particle Size Analysis of Colloidal Suspension
Dr. James F. Ranville
Environmental Geochemical Consultant
Colorado School of Mines

The TRC Minerals® sample supplied is very complex and quite unusual in its properties. I am confident in the accuracy of the methods applied to this mineral sample but due to the nature of the sample, certain ambiguities currently exist in the interpretation of the results. However I can propose several rather significant possible explanations for the results.

Two methods were applied to determine the particle size of the species present in the sample. These methods were laser light-scattering and field-flow fractionation.

Laser light-scattering is based on the fact that particles move about in solution due to random bombardment by water molecules. The smaller the particle (colloid) the more it moves. If we shine a larger beam through a suspension the particles present will scatter light. The intensity of the scattered light depends on the combined scattering of the many thousands of particles present in the region of the sample where the laser beam passes. As the particles move about in the suspension the intensity of scattering changes. If the particles are small they move about rapidly so the scattering changes rapidly. If they are larger they move about more slowly and therefore the scattering intensity changes more slowly. We can determine the size by examining how rapidly the scattering intensity is changing. The disadvantage of this technique is that the results are always dominated by the larger particles because scattering intensity increases with particle size. The result for a standard TRC Minerals® sample was:

Standard certified value	0.87 micron
Measured Value	0.92 +/- 0.02 micron
Sample measured value	0.46+/- 0.04 micron

Based on my observation of the very low scattering intensity for the sample and the known effect of a few large particles making the smaller ones I believe this result to be much too large. This result is also inconsistent with the observation that essentially no color was removed from the sample by filtration through a 0.1 micron filter.

Field-flow fractionation (FFF) is a chromatography-like method where particles and colloids are separated based on size. This separation is based on hydrodynamic forces which arise when fluid is passed through a very thin (0.25 millimeter) rectangular channel. In the type of FFF used for this sample small colloids come out of the channel before larger ones. In the analysis a small amount of sample is injected into the channel flow and this flow is then passed through a UV absorbance detector. As the colloids elute they generally are detected by their UV absorbance. A series of standards of known size or molecular weight are generally run to obtain a calibration curve which is used to convert the time it takes a colloid to come out to a size or molecular weight. The results of these analyses are plots of the detector response versus the size or molecular weight of the colloids, with size increasing along the x-axis. For most environmental samples, including solutions obtained from soils, peats, and surface and groundwaters, the UV absorbance is proportional to the amount of particles present in any given size. This TRC Minerals® sample however is not only complex but quite unusual and this relationship is not exactly true in this case.

FFF results are shown in two accompanying figures. The first figure shows the calibration curve for the polystyrene sulfonate standards used to calibrate the FFF. The molecular weights of the standards were 6,500; 16,000; and 31,000. This corresponds to a size range of roughly 0.002-0.01 microns. This calibration curve was used to determine the size of the unknown samples. The standards are plotted on the same graph as the sample, for comparison purposes.

The TRC Minerals® sample is very different in its properties from environmental samples I have previously examined. There is considerable material less than 2000 molecular weight which strongly adsorbs light present in the sample. This is not unusual for these types of samples. This is the material which gives the sample its yellow color. This is typical of dissolved organic material such as one finds in a peat bog and is consistent with a solution obtained from an organic-rich shale. Any metals present in the <2000 molecular weight size likely exist as either dissolved metals and/or metals chelated to this dissolved organic matter. A second peak of small colloids appears between 3000-6000 MW (about 0.001 microns). A somewhat more mysterious weakly negative peak (absorbance less than the baseline) occurs over the region of about 6,000 to 300,000 molecular weight (0.001 to 0.1 microns). This is quite unusual and I have never seen this in natural organic matter samples before.