Wall Thickness of Industrial Multi-Walled Carbon Nanotubes Is Not a Crucial Factor for Their Degradation by Sodium Hypochlorite

Abstract

The propensity of multi-walled carbon nanotubes (MWCNTs) for biodegradation is important for their safe use in medical and technological applications. Here, we compared the oxidative degradation of two samples of industrial-grade MWCNTs—we called them MWCNT-d and MWCNT-t—upon their treatment with sodium hypochlorite (NaOCl). The MWCNTs had a similar inner diameter but they differed about 2-fold in the outer diameter. Electron microscopy combined with morphometric analysis revealed the different degradation of the two types of MWCNTs after their incubation with NaOCl—the thicker MWCNT-d were damaged more significantly than the thinner MWCNT-t. The both types of MWCNTs degraded at the inner side, but only MWCNT-d lost a significant number of the outer graphitic layers. Raman spectroscopy demonstrated that both MWCNTs had a similar high defectiveness. Using energy-dispersive X-ray spectroscopy, we have shown that the more degradable MWCNT-d contained the same level of oxygen as MWCNT-t, but more metal impurities. The obtained results suggest that the biodegradability of MWCNTs depends not only on the wall thickness but also on the defects and impurities. Thus, the biodegradability can be regulated by the synthesis conditions or the post-synthesis modifications. Such degradation flexibility may be important for both medical and industrial applications.

Diameter-Dependent Degradation of 11 Types of Carbon Nanotubes: Safety Implications

“…On the other hand, the recent study by Newman et al demonstrated 39 that the wall thickness of MWNTs is not a crucial factor for their degradation by NaClO, which is in contrast with our results. By comparing the morphological changes occurring in two different types of NaClO-treated MWNTs by TEM, they found that thicker MWNTs became degraded on the outside wall and inner sidewall, while thinner MWNTs became degraded only at the inner sidewall.…”

“…The authors suggested that the biodegradation of CNTs likely depends not only on the wall thickness but also on defects in the inner wall. 39 Our studies were performed at a higher temperature, and therefore the reaction rates were much faster than the rates reported in that study. 39 We did not find that oxidation of MWNTs with NaClO occurred only at the surface or at both the surface and the inner wall.…”

“…38 The results also suggest that the wall thickness of CNTs might not be a crucial factor for their degradation by hypochlorite, and that other factors such as impurities might also affect the degradation rate. 39 However, the above studies were limited to a comparison of two or three types of carbon nanomaterials, and the general profile of CNT biodegradation is therefore still not clear. In this study, we carried out comparative studies of 11 types of commercially available CNTs, including SWNTs, carbon nanohorns (CNHs), 40 and MWNTs produced by different manufacturers, to clarify the major factors that influence the degradation rates of CNTs.…”

A Simple Method for Removal of Carbon Nanotubes from Wastewater Using Hypochlorite

“…The biodegradation mechanism of CNTs is thought to involve oxidation by hypochlorous acid that is generated during enzymatic reaction 19–22 , and sodium hypochlorite or hypochlorous acid have since been shown to oxidize and degrade CNTs 24–28 . Interestingly, 2D graphene oxide sheets are degraded faster than 1D oxidized CNTs 29 , and the wall thickness of CNTs might be not a crucial factor for their degradation by hypochlorite 30 . However, it is still not obvious whether CNTs are completely degraded.…”

Occurrence and removal of engineered nanoparticles in drinking water treatment and wastewater treatment processes: A review

“…To study the struc-tural modifications of CNTs, a CNT dispersion solution (100 mg/L) was ultrafiltered using a 10-kDa molecular weight cutoff membrane and rinsed with water before and after NaClO treatment for 1, 3, and 24 h. Transmission electron microscopy and Raman spectroscopy were used to characterize the collected CNTs. Additionally, the study discovered that graphene oxide could be an intermediate product of single-walled CNT degradation, implying that CNT degradation may require an unzipping mechanism compared with CNT oxidation using a strong acid treatment [63]. However, the reaction rates using sodium hypochlorite were considerably higher than those using a strong acid; moreover, no obvious nanoribbon structures were discovered.…”