The effect of ozonation on aggregation of humic substances on mica studied by atomic force microscopy

Humic substances influence adsorption and aggregation of colloids and ions in natural aquatic environment and thereby affect drinking water treatment by ozonation. In this paper the effect of ozonation on the ability of humic substances to form supramolecular structures was investigated by atomic force microscopy (AFM) surface topography. Ozonated and unozonated solutions of Suwannee River humic acid and fulvic acid were deposited onto molecularly smooth mica surfaces, left to dry and then imaged by AFM in non-contact mode. Two solutions with pH 4 and pH 7 were investigated. At each pH, significant differences in the aggregation behavior between these humic substances before and after ozonation were observed. In particular, ozonation increased the aggregate size of fulvic acid at the neutral pH. Size exclusion chromatography also revealed similar molecular weight distributions for the humic substances after ozonation. It was argued that difference in the molecular hydrophobicity or factors other than molecular weight could be responsible for the observed aggregation behavior. The outcome of this study is important for a better understanding of supramolecular structures of humic substances in ozonation, particularly those in drinking water treatment.