Environmental Engineering Reference
In-Depth Information
aligned CNT membranes composed of less than 2-nm-diameter CNTs demonstrate an enhancement within three orders of mag-
nitude that are faster than the values calculated from continuum hydrodynamic models. These findings verify the fast water
permeation from the nearly frictionless graphitic wall of CNTs because the interstitial space of the arrays is filled with silicon
nitride [121]. Apart from frictionless transport, the vertically aligned CNT structure that exposes the termini of CNTs to the
outer surface also enables selective mass transport. Several experiments on pressure-driven flow have demonstrated the effec-
tiveness of vertically aligned CNT membranes in eliminating particles with sizes ranging from 1 to 10 nm. Furthermore, recent
simulation studies have revealed that the nanometer diameters of CNTs also provide a platform for ion removal from water
[122]. The ability of CNTs to allow water permeation and prevent ions from passing through is critically influenced by the CNT
diameter [123]. Raw CNTs offer a low energy barrier that enables rapid water permeation but may not adequately prevent small
ionic species such as Na
+
and Cl
−
from passing through. meanwhile, functionalized CNTs may increase the ability to gate mol-
ecule transport through their inner cavities. The attachment of a functional group may impose a charged effect and cause steric
blockage at the CNT terminals. This phenomenon reduces water flux but improves ion rejection [124]. Fornasiero et al. [125]
investigated ion transport through sub-2-nm vertically aligned CNT membranes. Negatively charged groups are introduced to
CNT terminals by plasma treatment, and the result shows that the ion rejection of membrane reaches as high as 98% when the
aqueous electrolyte solutions pass through pores. The exclusion of ions from the CNTs with negatively charged functional
groups is mainly attributed to the Donnan-type rejection mechanism, where electrostatic interactions of the charged group tend
to exclude ions of the same charge while being freely permeable to ions of opposite charge. Therefore, the combination of fast
transport and high ion rejection of vertically aligned CNT membranes may lead to efficient water desalination.
8.6
lifeCyCle assessment of Cnts
CNTs have demonstrated their valuable potential in water treatment technologies. However, concerns on the environmental impact
originating from their large-scale application prompt the need to assess the lifecycle of CNTs from the “cradle” (raw material acqui-
sition) to the “grave” (disposal or recycling) [126]. The lifecycle assessment begins with the identification of the required raw mate-
rials, including carbon feedstock (graphite or CO), metal catalyst particles (Ni, Co, and Fe), and acid reagents (HNO
3
, H
2
SO
4
, and
HCl). Carbon feedstock and metal catalysts are consumed for CNT production, whereas acid reagents are used to treat the produced
CNTs. CNTs are often produced by arc discharge, laser ablation, or CVD methods. Regardless of the production method, the energy
needed to produce CNTs is higher than that needed to produce traditional materials such as aluminum, steel, and polypropylene [127].
This high energy consumption has a significant environmental impact because energy generation is always associated with greenhouse
gas emission. The production process may also lead to the potential release of airborne pollutants such as polycyclic aromatic hydro-
carbons and volatile organic compounds [128]. In addition, subsequent treatment with acid reagent produces acidic wastewater, which
may cause water pollution if discharged into bodies of water without adequate treatment. However, among the production methods,
CVD is considered to exert the least environment impact because of its low energy consumption and increased yield of high-purity
CNTs, which reduce the need for rigorous postproduction treatment [129]. upon application, the risk of releasing CNTs into the
environment is present. Nevertheless, the exceptional abilities of CNTs for removing hazardous pollutants from water reduce its envi-
ronmental burden. Furthermore, the application of CNTs in membranes enables a low-pressure-driven separation process that reduces
energy consumption. At the end of the lifecycle, CNTs may be disposed by landfilling or incineration. Nonetheless, CNTs signifi-
cantly possess superior durability, which in turn increases their lifespan during their application. moreover, the superior durability of
CNTs also allows recovery and reuse for the processing of new materials. The reusability permits CNTs to embrace the 3R (reduce,
reuse, and recycle) concept [74]. These findings aid in reducing waste disposal and the subsequent environmental impact caused by
landfilling and incineration. The environmental impacts from the CNT lifecycle are summarized in Table 8.2.
table 8.2
environment impact analysis of Cnts
Positive impact
Negative impact
Raw material acquisition
CNT production
High-energy consumption
Release of airborne pollutants
Generation of acidic wastewater
CNT application
Promotion of energy efficient operation
Potential release of CNTs into the environment
Reduction of hazardous pollutant concentration
End of life
Superior durability that increases their lifespan
Capability to be recovered and reused
