Partial validation of cross flow ultrafiltration by atomic force microscopy

Atomic force microscopy was used to evaluate a cross flow ultrafiltration (CFUF) system. The CFUF system was used for the size fractionation of natural colloidal material from freshwaters. Analysis of the images of bulk water, permeates, and retentates shows the primary materials observed were near-spherical structures with height dimensions up to approximately 12 nm. The number of colloidal particles (per unit area) on the mica surfaces derived from the retentates increased by a factor of 2 between a concentration factor (cf) of 1 and of 20. Colloidal densities of nanoparticles were approximately 2 orders of magnitude lower in the permeate compared to the retentate, indicating a good size fractionation. As the cf value increased from 1 to 20, the percentage of 1-nm material increased substantially in the retentates. Line transects along a surface and surface roughness values show good agreement with the above results. Results suggest the size fractionation is good but not perfect and high cf values produce a better size fractionation, although some retention of small material is always observed. High cf values are recommended.