Aggregation of oxidized multi-walled carbon nanotubes: Interplay of nanomaterial surface O-functional groups and solution chemistry factors.

The fast-growing production and application of carbon nanotube (CNT) materials in a variety of industrial products inevitably lead to their release to wastewater and surface water. CNT would experience oxidization in wastewater treatment plant due to the presence of large amount of disinfectants, such as H ₂ O ₂ and O ₃ , which in turn affects the environmental fates and risks of CNT. In this study, oxidized CNT materials (O-CNTs) were prepared by treating CNT with H ₂ O ₂ /UV and O ₃ (denoting as H ₂ O ₂ -CNT and O ₃ -CNT, respectively). A variety of characterizations indicated that oxygen containing groups were generated on CNT surface upon the oxidation, and the O/C ratio increased in the order of pristine CNT < H ₂ O ₂ -CNT < O ₃ -CNT. In the presence of Na ⁺ , K ⁺ and Mg ²⁺ , the O-CNTs displayed better colloidal stability than the pristine CNT, and the stability increased with the oxidation degree (indicated by O/C ratio). This could be explained by the more negative surface charge and stronger hydrophilicity of the O-CNTs. Unexpectedly, in the presence of Ca ²⁺ , the most oxidized O ₃ -CNT exhibited the poorest colloidal stability. The abundant carboxyl groups in O ₃ -CNT provided effective binding sites for cation bridging effect through Ca ²⁺ and led to stronger aggregation. Increasing pH was more favorable to disperse CNTs (both O-CNT and pristine CNT) in the presence of Na ⁺ , but much less effective in inhibiting the aggregation of O ₃ -CNT in presence of Ca ²⁺ . This could be explained by the stronger cation bridging effect due to enhanced deprotonation the -COOH groups at higher pH conditions. The calculated Hamaker constants of the CNTs decreased with the oxidation degree, implying that there was lower van der Waals force between the O-CNTs. The Derjaguin-Landau-Verwey-Overbeek (DLVO) calculation confirmed that O-CNTs had to overcome higher energy barrier and thus showed better colloidal stability than the pristine CNT in the presence of Na ⁺ .
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