Atomic force microscopy of humic substances: Effects of pH and ionic strength

Tapping mode atomic force microscopy was employed to determine the shape, size, and properties of the Suwannee River humic acid and a U.K. Geological Survey peat humic acid sorbed to mica. Humic acid macromolecules are shown to have a globular shape with an adsorbed height between 0.5 and 2 nm for a concentration of 10 mg L(-1), pH > 5, and I < 500 mM NaCl. Aggregation of the humic macromolecules was not systematically observed at low pH and ionic strength. Instead aggregation may have been to be related to the hydrophobicity of the sample: no aggregates were observed for the relatively hydrophilic Suwannee River humic acid between pH 3 and pH 10 (I = 5-50 mM), while aggregates of peat humic acid were observed at low pH (pH < 5) and high ionic strength (I similar to 500 mM). Calculated and measured hydrodynamic diameters determined under the same conditions were found to be systematically slightly larger than AFM heights,in good agreement with expectations given the different nature of the hive measurements.