CHARACTERIZING THE FUNCTIONAL SIZE DISTRIBUTION OF SETTLEABLE PARTICLES IN VERY-FINE GRAINED AQUEOUS SUSPENSIONS USING A COMMONLY ACCESSIBLE SPECTROSCOPIC TECHNIQUE

Understanding the settling of particles from aqueous suspensions has been an important pursuit for nearly a century. Inquiries have been motivated by variety of questions, ranging from the fundamental physical principles of falling bodies, to the origin of sedimentary structures, to water quality and contaminant transport. Most previous work on natural particles has focused on coarse to fine sand-sized particles (2-0.2 mm). However, total suspended solids (TSS) and settleable particles naturally occur in a distribution containing a large proportion of very small particles with diameters in the fine silt to clay category (< 6 μm). Measuring the distribution of sizes in natural samples can be done with dynamic light scattering (DLS) and analytical disc centrifugation (ADC) techniques. These methods are useful for giving a composite static size distribution of a sample. Our research is focused on determining a suspension’s functional particle size distribution that defines the behavior of a suspension during a settling event. This has been done by measuring the change in light absorbance/attenuation with time, as particles settle. Light attenuation was measured at 16 λs between 360-890 nm. Particles were allowed to settle for up to 8400 minutes, while spectroscopic measurements were taken every 30 seconds. Settling velocity in water is a function of grain size, and the measured change in attenuation with time yields information about the changing distribution of sizes remaining in suspension as settling occurs. Several published settling velocity equations were applied to the experimental data. Equations were modified to accommodate grain shape information, as determined by SEM analysis. With literature models, preliminary results for NIST size standards came within 1 standard deviation (as listed by NIST) of d₅₀ and d₇₅ diameters and 4 standard deviations for d₉₀ diameters. These results already indicate the usefulness of this spectrophotometric-derived method for measuring the change in size distributions as a function of settling time and predicting the bulk size distribution of very-fine grained suspended material. Further alteration of literature formulas to better incorporate particle characterization promises to yield more accurate information about the behavior of very-fine grain suspensions.