It is intrinsically difficult to measure the size distribution of flocculated particles. Numerous papers show that the flocs break apart when sampling, so measurements must be made in situ. But this may not always be possible. In those circumstances, it becomes important to know how much the size distribution measured ex situ, from a sample, would differ from the in situ measurement, and if the ex situ measurement can be related to the in situ measurement. de Lange et al. excellently addresses this in their pre-print ‘The Impact Of Flocculation on In Situ and Ex Situ Particle Size Measurements by Laser Diffraction’ doi: https://doi.org/10.22541/essoar.168319822.29525394/v1
A LISST-200X was mounted in a small stream for two days, and water samples were taken simultaneously with the LISST-200X measuring the in situ particle size. Water samples were spit and stored at room temperature (18-23 °C) while exposed to light and at 5 °C in the dark. After one, two, and three weeks respectively, a water sample was taken from each storage, gently agitated and the ex situ particle size distribution measured in a LISST-200X and a Malvern Mastersizer-3000.
Their data show that
• The D50 of the ex situ samples was larger than the in situ D50. Particle images showed that flocs had formed in the samples while stored.
• D50 of the ex situ samples did not change significantly with longer storage.
• It was impossible to return the ex situ samples to their original, in situ, state. They conclude that ex situ measurements are only useful for obtaining the particle size distribution of the primary particles.
Example of particle size distributions from in situ (black line) and multiple ex situ (green lines) measurements. The two vertical lines show the median diameter of the in situ and ex situ measurements, respectively.
A secondary objective of their paper was to examine the measurement time required to obtain an accurate D50. Their analysis showed that, for the dataset in question, an average time of one minute was required to obtain an accurate D50. The reason is that large particles are few and far between in situ and the sample volume of the instrument becomes important: At a given concentration, a small sample volume is statistically less likely to contain a large particle than a larger sample volume. Hence averaging is needed to observe and properly account for the larger particles.