Carbon nanotubes (CNTs) were grown (@ 900 °C) onto a quartz wool substrate using toluene as the carbon source and ferrocene as the catalyst precursor (Fig. 1a)16. The as-growth yield was generally in the range 80–90% with respect to the carbon precursor. The wool was “visually dark” throughout its length suggesting a uniform deposition density (Fig. 1b). SEM images of the as grown CNTs confirms the uniform growth over each quartz fibre (Fig. 1d); however, the length of the CNTs varies between 100–250 μm length (Fig. 1d and f) depending on the position within the reactor. The arrangement of the CNTs on the individual quartz fibres is reminiscent of two opposing brushes oriented either side of the quartz fibre (Fig. 1d): a tri-lobe structure of this type has been reported17. Closer inspection indicates that the orientation is a consequence of the inter-tube forces since the CNTs grow around the entire circumference of the quartz fiber, rather on two sides (Fig. 1f).

Figure 1 Images of the quartz wool supported-CNTs. (a) Schematic of the reaction for the growth of CNTs on to the quartz wool support showing (b) an optical image of the as grown quartz wool supported CNTs. SEM micrographs of (c) the quartz wool substrate, (d and e) the as grown quartz wool supported CNTs and (f) the supported epoxidized CNTs (SENTs). Full size image

Thermogravimetric analysis (TGA) of CNTs grown under identical conditions in the absence of the quartz wool shows a 9.83 wt.% residue above 800 °C consistent with the presence of iron catalyst impurities. Purification by wet air oxidation (WAO) and acid wash18 resulted in a reduction of the residue to 4.68 wt.%. WAO treatment on the quartz wool supported CNTs demonstrated no perceptible change in the appearance; however, comparison of the G peak and D peak intensities in the Raman spectroscopy shows a clear effect (Fig. 2). The G mode are associated with tangential displacement C-C bond stretching motions (1500–1600 cm−1 range), while the D, or disorder mode (1290–1330 cm−1 depending on the Raman excitation laser wavelength), originates from crystallinity disorders and lattice imperfections and represents the presence19 and distribution20 of sp3 carbon centres. The G’ corresponds to disorder induced carbon features arising from finite particle size distribution or lattice distortion. The non-nanotube peaks in the Raman spectra of the as grown quartz wool supported CNTs (peaks marked with * in Fig. 2) match with hematite (Fe 2 O 3 )21 and lepidocrite (γ-FeO(OH)22. The peaks associated with iron oxide (catalyst residue) are absent after WAO. In addition the G:D ratio increases from 0.86 to 1.92 after WAO. This is consistent with the SEM images after purification that shows the removal of debris (Fig. 1f).

Figure 2 Raman spectra of the quartz wool supported-CNTs. Normalized Raman intensity (633 nm) of (a) as grown quartz wool supported CNTs showing peaks (*) due to iron oxide catalyst residue, which are removed after wet air oxidation/acid wash (b). The G:D ratio decreases upon epoxidation (c) due to functionalization of the CNT side walls, while adsorption of Cd2+ does not significantly effect the spectra (d), suggesting that adsorption occurs predominantly via the epoxide oxygen, rather than the CNT sidewall. Full size image

Epoxidation of the supported CNTs is accomplished using 3-chloroperoxybenzoic acid (m-CPBA). Raman analysis after epoxidation (Fig. 2) showed a slight decrease in the G:D ratio to 1.59 from 1.92. The decrease in the ratio after the epoxidation is expected because the addition of the epoxide groups to MWNTs slightly disrupts the graphitic structure15. TGA of analogous CNTs grown in the absence of the quartz wool shows a 2.74 wt.% residue above 800 °C consistent with functionalization of the CNTs.

Un-supported epoxidized CNTs were initially tested for metal sorption by addition to solutions of an excess of the appropriate metal (Zn2+, Co2+, Ni2+, and Cd2+) salts. TGA values for the metal saturated epoxidized CNTs showed between 3.1 and 9.5 wt.% increase in the residue consistent with metal adsorption. X-ray photoelectron spectra (XPS) confirm the presence of the appropriate metal. There was only a small change in the G:D ratio after metal adsorption (1.69 versus 1.59), suggesting that the epoxide group alone is responsible for adsorbing the metal.

Quantification of metal (Cd2+, Co2+, Cu2+, Hg2+, Ni2+, and Pb2+) uptake was determined by passing a standard aqueous solution of each metal through a known mass of SENTs in a burette. The initial concentrations (C i ) for each metal (60–6000 ppm) was chosen to simulate cases of high industrial wastewater contamination (Table 1)23. The procedure was then repeated for 3 trials per metal. The residual concentration of metal (C f ) was determined, by UV-visible spectroscopy, for each aliquot after passing through the SENT from which the adsorption efficiency (Eq. 1) for each metal compound was calculated (Table 1).

$${\rm{a}}{\rm{d}}{\rm{s}}{\rm{o}}{\rm{r}}{\rm{p}}{\rm{t}}{\rm{i}}{\rm{o}}{\rm{n}}\,{\rm{e}}{\rm{f}}{\rm{f}}{\rm{i}}{\rm{c}}{\rm{i}}{\rm{e}}{\rm{n}}{\rm{c}}{\rm{y}}\,({\rm{ \% }})=[1{\textstyle \text{-}}{(C}_{{\rm{f}}}{/C}_{{\rm{i}}})]\times 100$$ (1)

Table 1 Metal concentration and adsorption efficiency. Full size table

While the adsorption efficiency is >99.4% for all metals, it appears that it is generally dependent on the initial concentration (C i ), i.e. higher C i results in lower sorption efficiency (Fig. 3a). However, it is worth noting that there is also an effect of the metal species, since Cu2+ < Co2+ < Pb2+ using similar C i . Also as seen from Fig. 3a there appears to be no dependence on the counter ion. To determine if the reason for the difference can be explained by the lower uptake being closer to the saturation, standard solutions were adsorbed through a known mass of SENT and aliquots measured (e.g. Fig. 3b) and the resulting saturation concentrations determined (Table 1). This shows that for each metal studied the adsorption efficiencies (Table 1) are far from saturation conditions, and any difference between metals is a function of the relative binding efficiency to the epoxide functionality.

Figure 3 Uptake efficiency of the quartz wool supported-CNTs. Plots of (a) SENT adsorption efficiency of different metal ions as a function of initial concentration of 50 mL sample through 0.5 g SENT, and (b) change in adsorption efficiency as a function of solution volume for Cu2+ (100 ppm) showing the saturation point as defined by the volume above which the adsorption efficiency decreases. Full size image

As noted in the introduction, it is desirable for an adsorbent to be able to be regenerated, not only to treat large volumes, but also provide a route to safely dispose or potentially recycle the heavy metals; many of which have commercial value. Firstly, washing the metal impregnated SENTs (M-SENTs) with DI water did not result in removal of any metal species (based upon XPS of the sample, and UV-visible spectroscopy of the washings); however, washing with a dilute solution (50%) of acetic acid showed removal of the metal. For example, Fig. 4 shows the XPS survey scans of Cd-SENTs before and after washing with aqueous acetic acid. Furthermore, a materials balance calculation of the atomic percentages of C and O before metal adsorption and after regeneration shows that there is a negligible loss of epoxide groups during the renewal process, ensuring recyclability. Metal adsorption after acetic acid treatment is not affected, i.e., the adsorption efficiency for a particular metal is retained.