Charging, aggregation, and aggregate strength of humic substances in the presence of cationic surfactants: Effects of humic substances hydrophobicity and surfactant tail length

The binding of natural organic matters (NOM) with organic cations and resulting changes in charging and aggregation of NOMs are important in water science and technology such as the enhanced settling by flocculation and the fate control of contaminants. We measured the electrophoretic mobility and aggregate strength of humic substances (HSs), Suwannee river fulvic acid (SRFA) and Leonardite humic acid (LHA), in the presence of cationic surfactants, cetylpyridinium chloride (CPC) and dodecylpyridinium chloride (DPC), to clarify the effects of carbon content, aromaticity of HSs, and the tail length of cationic surfactants on the charging and aggregation. Both of HSs showed charge reversal in the presence of 0.1 - 0.3 mM CPC and LHA in 1 and 2 mM DPC. The isoelectric point (IEP) pH of LHA-CPC and SRFA-CPC system was higher than that of LHA-DPC system, though more DPC concentration was needed for the charge reversal of LHA in DPC. We also observed pronounced aggregation of both HSs in CPC systems around IEP pH, though LHA in higher DPC concentration showed wider range of aggregation pH including IEP pH than the range at low DPC concentration. The maximum strength of LHA aggregate was higher, around 27.6 nN in LHA-CPC system and 19.1 nN in LHA-DPC system, than that of 5.2 nN in SRFA-CPC system. This higher value of aggregate strength in LHA-CPC system than others indicates more hydrophobic interaction in LHA-CPC system. The maximum strength around IEP pH in all the systems indicates the presence of electrostatic interactions along with hydrophobic and other non-DLVO forces.