Submicron size distributions of inorganic suspended solids in turbid waters by photon correlation spectroscopy

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Abstract
Photon correlation Spectroscopy (PCS) is a technique for determining size distributions of submicron‐sized particles. A particle diffusion coefficient, which is dependent on particle diameter, is determined from measurements of the autocorrelation function of the intensity fluctuations of light scattered from a laser beam passing through a suspension of particles. We demonstrate the applicability of the technique to particles likely to be found in runoff or turbid impoundments using soil fractions prepared to have a known size distribution. Mean particle diameters of all soil fractions determined by PCS fell within the expected range. Particle‐size distributions of natural water samples were reproducible when analyzed within minutes of collection. The effects of sample storage and pretreatment varied between water bodies and thus can be a source of uncertainty in the results. Particles from a turbid creek carying storm runoff began to coagulate within 2.5–6 hours, whereas particles from a perennially turbid impoundment remained stable for 10 days. Addition of sodium hexametaphosphate improved the reproducibility of the particle‐size distribution with prolonged storage. Applications of the technique include determination of coagulation rates and critical coagulation concentrations of electrolytes, and calculation of settling velocity distributions of submicron particles in turbid natural waters.

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