1 Overview

CNTs have been considered for use in environmental applications and in materials used in separation science . Such applications include, e.g., filters , membranes, and water treatment .

2 Environmental Applications

In recent work [402] a porous “nanosponge” was constructed via a CVD process that used ferrocene as the catalyst precursor and added sulfur. This “nanosponge” was then used to absorb o-dichlorobenzene and oils from a water sample; the oils absorbed were more than 100x the weight of the sponge. In similar, earlier work, B-doped CNT “nanosponges” were fabricated from CVD-deposited CNT forests that were shown to behave somewhat like surfactants but with very high oil absorption capabilities [403].

A comparative study of the water treatment capability of granular activated-C, activated-C-fiber, and CNTs [404] found that the absorption capabilities of the CNTs (including SWCNTs as well as MWCNTs) for three aromatic organic compounds tested were somewhat better than those of the activated-C materials. Other work in water treatment has shown that molecular sieves constructed from CNTs have the potential of being activated and deactivated via an external electrostatic field [405]. CNTs have also been shown to be usable in a variant of reverse osmosis methods but operating at lower pressures, lower temperatures, and higher flow rates than conventional (e.g., polycarbonate membrane based) reverse osmosis methods, although it also appears that only very specialized and very small-diameter SWCNTs function well in this application [406, 407]. In one study of desalination of water using self-supporting CNT membranes , a 99% salt rejection rate was claimed at a flux rate of water of about 12 kg/m2 h [408].

In other work, CNT filters have been demonstrated that electrochemically oxidize organic contaminants, bacteria, and viruses or remove them through simple microfiltration mechanisms [409,410,411,412,413].

A company called Seldon Water (formerly Seldon Technologies and now owned by CB Tech) markets Seldon Nanomesh-brand portable water filters for water purification and claimed to “remove bacteria, viruses, and cysts, such as Cryptosporidium and Giardia, to US EPA Drinking Water Standards (99.9999% of bacteria, 99.99% of viruses, and 99.9% of cysts) also reduce sediment, total organic carbon (TOC), chlorine, color, bad taste, and odors also removes significant levels of harmful chemical contaminants, including: Lead and Cadmium, Organophosphates (pesticides and herbicides), Disinfection byproducts, radioactive contaminants such as cesium-137” [414].

3 Applications in Separations and Related Fields

CNTs have been studied for use in solid-phase extraction, as microextraction sorbents, in chromatography, and as laser desorption/ionization substrates [415].

One of the important parameters that must be considered in the use of CNTs in separation-related applications is the point of zero charge (isoelectric point), at which CNTs’ surface has zero net charge [415]. When the pH is higher than this point, the CNT surface is negatively charged; electrostatic interactions can be established to adsorb cationic species. Conversely, when the pH decreases below the point of zero charge, protons compete with cations for the same sites on CNTs, and the subsequent neutralization of CNTs provides a decrease in the adsorption. This fact can be used, for example, to retain at a certain pH value, in metals, and to later elute them using acidic solutions, allowing for facile separation of such materials. On the other hand, it is also estimated that the CNTs’ covalent functionalization must not exceed about 10% of their surface [415, 416].

The use of CNTs in chromatographic stationary phases has been documented for the separation of the following analytes, as summarized succinctly by Herrera-Herrera et al. (type of CNT, if applicable, given in parentheses, and technique used given in brackets) [415, 417,418,419,420,421,422,423,424,425,426,427,428,429,430,431,432,433,434,435,436,437,438,439,440,441,442]: Ar and CO2 (SWCNTs) {GC-TC}; alkanes and aromatic hydrocarbons (MWCNTs functionalized with –COOH and CONH2) {GC-FID}; methanol, hexane, ethanol, carbon tetrachloride, benzene, six other alcohols, and two other esters (CNTs) {GC-FID}; benzene, toluene, dichloromethane, trichloromethane, acetonitrile, propanol, toluene, 1-butanol, m-Xylene, phenol, and naphthalene (SWCNTs functionalized with COCl, diameter < 2 nm) {GC-FID}; nine esters, nine aromatics, two alcohols, two ketones, and two alkanes (MWCNTs with NH2-R functionalization) {GC-FID}; chloro-substituted PCBs and terpenes (MWCNTs with NH2 functionalization) {HPLC-DAD}; polyaromatic hydrocarbons (PAHs), aromatics, and amines (silica-MWCNT composites, CNT diameter < 8 nm) {HPLC-UV}; one nucleobase with seven benzene derivatives (variously functionalized MWCNTs) {HPLC-UV}; 12 peptides (MWCNTs with NH2 functionalization) {HPLC-UV}; five nucleosides, a nucleobase, and four tetracycline antibiotics (silica-MWCNT composites, CNT diameter 20–40 nm) {CEC-UV}; five nucleobases, five nucleosides, eight flavonoids, and six phenolic acids (COOH-functionalized MWCNTs, diameter 20–40 nm) {CEC-UV}; five vitamins (COOH-functionalized SWCNTs, diameter about 1.2 nm) {CEC-DAD}; clenbuterol enantiomers (brominated MWCNTs, diameter 10–20 nm) {TLC}; thioamides (MWCNTs, diameter 8 nm) {micro-CEC-UV}; six nonsteroidal anti-inflammatory drugs (MWCNTs, diameter ca. 10–15 nm {CE}; and four flavonoids, four phenolic acids, and two saponins (MWCNTs, diameter 3–20 nm, and SWCNTs, diameter 0.7–1.1 nm) {CE} (Table 14.1).

Table 14.1 After ref. [415]

CNTs have also begun to be studied for applications in laser desorption/ionization (LDI) time-of-flight mass spectrometer (TOFMS). These CNT-derived materials have been used as substrates in both matrix-assisted LDI (MALDI) [207,208,209,210,211] and surface-assisted LDI (SALDI) [415, 443,444,445,446,447,448,449,450,451]. However, this incipient work is still very much in the development phase, with no conclusive results as of this writing.

4 Problems and Exercises

  1. 1.

    Briefly describe the principles and practical construction behind the following applications of CNTs: water treatment and “nanosponges.” Rate the applications using CNTs against the corresponding, more established applications using non-CNT materials (in terms of such parameters as performance, cost, size, and closeness to the market).

  2. 2.

    Enumerate and describe CNT applications in separations. Enumerate at least ten (10) chemicals whose mixtures have been separated using CNT-based materials. Define isoelectric point.

  3. 3.

    Describe the principles of the use of CNTs in mass spectrometry. What are their relative advantages and drawbacks in this application?