AFM study on the sorbed NOM and its fractions isolated from River Songhua

Natural organic matter (NOM) and its fractions from River Songhua were concentrated and fractionated with the reverse osmosis technology and XAD resin. With the purpose of exploring the adsorbed morphology of NOM and its fractions from a specific water source, the tapping-mode atomic force microscopy (TMAFM) was employed. The effect of solution properties (pH, Ca2+) on NOM microtopography was also verified. The results showed that NOM and its fractions displayed obviously diverse morphologies due to their different characteristics. The macromolecular compounds with large SUVA value were prone to adsorb on the mica surface. The particle-shaped NOM macromolecules were loosely adsorbed on the mica surface, and the molecular aggregation phenomena were proved to be ubiquitous in this experimental condition. With the increase of pH value, the surface coverage of mica decreased evidently. Spherical conformation with relatively smaller diameters was observed in the acidic condition compared with the neutral and basic condition. In the presence of 0.1 mol L−1 NaCl, aggregation phenomena were observed in all pH conditions and obvious aggregates were presented with the addition of only 5 mmol L−1 CaCl2.