Abstract:The aggregation behaviour of multi-walled carbon nanotubes (MWNTs) in aquatic systems was investigated in the presence of humic acid (HA). Results indicated that MWNTs could be effectively suspended in aqueous solution using the surfactant SDS with sonication. Addition of electrolytes induced MWNTs aggregation by the mechanism of compressing electric double layer, and this process was in good agreement with the classic Derjaguin-Landau-Verwey- Overbeek (DLVO) theory. The stability of MWNTs in water would be improved through the enhanced steric repulsion and hydrophily in the presence of humic acid. However, the particle size of MWNTs could be increased through the function of complexing-bridging between humic acid molecule and Ca2+, which in turn resulted in the enhancement of aggregation. The findings elucidated that the aggregation and dispersion of MWNTs in water were affected by the properties of organic matters, electrolyte species and other complex factors.