Heteroaggregation of multiwalled carbon nanotubes with sediments

• MWCNTs and OH-MWNTs exhibit initial first-order heteroaggregation rates with sediments.
• At high ionic strength heteroaggregation rates deviate from first-order after 5 days due to the formation of large, settleable homoaggregates.
• MWCNTs and OH-MWCNTs heteroaggregation with sediments is essentially irreversible under constant solution conditions.
• Low fractional release of OH-MWCNTs under reduced solution ionic strength indicates potential OH-Ca-NOM bridging effects.

In this study, the effect of ion type and concentration on the heteroaggregation and release of multiwalled carbon nanotubes (MWCNTs) and hydroxylated MWCNTs (OH-MWCNTs) with sediments is reported. Means of the first-order heteroaggregation rate constant (khet) for MWCNTs and OH-MWCNTs and a freshwater sediment in 0.4 mM CaCl2 (0.21 and 0.17 d−1) are significantly higher than those in 10 mM NaCl (0.096 and 0.094 d−1). All sediment-electrolyte combinations exhibit first-order heteroaggregation rates during the initial time period (≤5 days) as evidenced by the linearity of the ln concentration vs time plots, and for some low ionic strength treatments heteroaggregation is first-order over the duration of the entire experimental period. However, faster heteroaggregation rate treatments deviate from linearity at longer time periods due to the formation of large, settleable aggregates (homoaggregation). Heteroaggregation attachment efficiency (αhet) values for both MWCNTs and OH-MWCNTs increase mostly linearly with ionic strength and are greater for the sediment higher in Al hydroxide content. Both nanotube heteroaggregation with sediments and deposition on model environmental surfaces is irreversible under constant background solution conditions. When background solution ionic strength is reduced, nanotube release from sediments and a model surface is dependent on the initial solution chemistry, with nanotubes deposited from NaCl solutions released much more readily than those deposited from CaCl2 solutions. The lowest fractional release observed was for OH-MWCNTs deposited from CaCl2 solutions indicating potential OH-Ca-NOM bridging effects.

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