DETERMINING SIZE DISTRIBUTION OF VERY FINE PARTICLES IN ENVIRONMENTAL SAMPLES VIA A SIMPLE SETTLING EXPERIMENT: A SPECTROSCOPIC RECONNAISSANCE METHOD

Total suspended solids (TSS) and settleable particles are important constituents in aqueous environmental samples, affecting water quality directly, or indirectly as substrates for contaminant transport. Particles are very-fine, with diameters in the sub-μm to nm scale. Particle size distribution is commonly characterized by dynamic light scattering (DLS), analytical disc centrifugation (ADC), or scanning electron microscopy (SEM). While a combination of these techniques may yield acceptable results, the instrumentation required is often inaccessible, or the analysis too costly for routine reconnaissance investigations. Our research group is working on a way to determine grain-size distributions by using the change in light absorbance/attenuation with time, as particles settle in aqueous samples. Different types of samples were investigated. These included single mineral suspensions of quartz, feldspar, kaolinite, hematite, and calcite, at TSS concentrations of about 100-1000 ppm. Other samples were from groundwater wells with settleable loads of 300-6000 ppm. The majority of samples analyzed were re-suspended material recovered from point-of use water filters which sampled rural drinking water sources from various global locations. Light attenuation was measured at 16 different λs between 360-880 nm. Particles were allowed to settle for 120-240 minutes, with spectroscopic measurements taken every 30 seconds. Measurement of light attenuation vs. time (settling curves) across multiple λs provided a basis for characterizing the particle size distribution within a sample. Size distributions were quantified by application of several published settling velocity equations to the spectrophotometric data. These equations range from relatively simple power functions to complex logarithmic and polynomial functions, and allow the semi-quantitative separation of particles into brackets of approximately 1μm – 750 nm, 750 -500 nm, 500-250 nm, 250-100 nm , and <100 nm diameter. The technique is currently being tested against NIST standards, SEM analysis of particles, and dynamic imaging particle analysis (DIPA) results.