Aggregation Kinetics of Multiwalled Carbon Nanotubes In Aquatic Systems

The initial aggregation kinetics of multiwalled carbon nanotubes (MWNTs) were examined through time-resolved dynamic light scattering. Aggregation of MWNTs is evaluated by varying solution pH and the concentration of monovalent (NaCl) and divalent (CaCl₂ and MgCl₂) salts. Suwannee River humic acid (SRHA) is used to study the effect of background natural organic matter on MWNT aggregation kinetics. MWNTs are relatively stable at solution pH and electrolyte conditions typical of aquatic environments. Increasing salt concentration and addition of divalent calcium and magnesium ions induce MWNT aggregation by suppressing electrostatic repulsion, similar to observations with aquatic colloidal particles. Increasing solution pH from acidic to basic conditions significantly reduces MWNT aggregation rate, suggesting the presence of dissociable functional groups on the MWNT carbon scaffold. The presence of humic acid in solution markedly slows MWNT aggregation rate. The enhanced MWNT stability in the presence of humic acid is attributable to steric repulsion imparted by adsorbed humic acid macromolecules.