Aggregation and charge reversal of humic substances in the presence of hydrophobic monovalent counter-ions: Effect of hydrophobicity of humic substances

To investigate the effect of hydrophobicity of humic substances (HSs) on their charging and aggregation, we studied the electrophoretic mobility and aggregation-dispersion of HSs in the presence of hydrophobic monovalent cations, namely, tetraphenylphosponium TPP+. The used HSs were standard Suwannee river fulvic acid (SRFA), Suwannee river humic acid (SRHA), and Leonardite humic acid (LHA) with different contents of aromatic carbons. All of the HSs in the presence of TPP+ showed charger reversal. The charge reversal pH or iso-electric point (IEP) of LHA was higher than that of SRFA and SRHA in every concentrations of TPPCl, demonstrating the strong hydrophobic interaction between HSs, especially LHA, and TPP+. We also found that the formation of large visible aggregates of all the HSs at lower pH in the presence of TPP+. Large HS aggregates were markedly formed for LHA, manifesting the existence of stronger hydrophobic attraction among LHAs with TPP+. The appearance of aggregates of all the HSs with TPP+ was confirmed by microscopic observation and the size determination by dynamic light scattering. The HS aggregates showed fractal structure. The values of fractal dimension Df of HS aggregates were 2–2.2 in quiescent conditions, indicating that the HS aggregates with TPP+ were formed via cluster-cluster aggregation with restructuring. The Df increased to 2.8–2.9 in stirring conditions, implying that the compact aggregates were formed through the continuous aggregate breakage and the regrowth between smaller aggregates and larger aggregates