Molybdenum Disulfide and Tungsten Disulfide as Novel …
211
is important to note, though, that most of the computational results on desalination
membranes come from pure classical dynamics. Despite the widespread agreement
of these results with experiments, there is still much to advance in the chemical–
physical molecular understanding of the processes that lead to higher or lower water
fluxes, as well as the mechanisms and interactions between solution and membrane
that provide greater efficiency in solute rejection. This can be achieved by using
hybrid simulations that consider explicit charge transfer, for example.
(f) Toxicity—Along with the scalable use of MoS 2 and WS 2 nanomembranes, one
of the ultimate challenges is to understand more deeply how these structures are
going to affect the environment after release. For instance, nanomaterials may pose
a long-term threat if they are persistent and nondegradable [98]. In this regard, both
MoS 2 and WS 2 are considered chemically stable materials against environmental
stressors because of the absence of dangling bonds in the terminating S atoms [35].
Particularly the solubility of MoS 2 is low under ambient conditions, which leads to
long-term persistence in the environment. On the other hand, this scenario can differ
for extreme conditions such as high temperature or strong oxidation, where TMD
nanomaterials can be oxidized to different oxides [99]. For instance, the oxidation of
MoS 2 nanosheets has been shown to occur in aqueous solutions, leading to soluble,
low-toxic oxidation products [100]. The oxidation kinetics depend on factors such
as the pH and the crystallographic phase of the TMD.
Toxicity varies depending on preparation methods, but both MoS 2 and WS 2
nanosheets generally show high biocompatibility at concentrations up to ~100 ppm
and certain cytotoxicity at high concentrations (a few hundred ppm). For instance,
low toxicity of exfoliated, well-dispersed MoS 2 nanosheets was observed, but aggregated samples were found to induce acute lung inflammation in mice [101], raising
concerns about their size’s effects on the toxicity of these nanosheets. When it comes
to toxicity we have good reasons to believe that both MoS 2 and WS 2 nanosheets can
perform better than graphene-based nanomaterials [102], but there can be significant differences between those TMDs. Polyethylene glycol (PEG)-coated 2H-MoS 2
nanosheets have shown fast degradation and complete excretion within a month,
in marked contrast to WS 2 , which presented high levels of concentration in the
organs for months [103]. Further degradation experiments indicated that the distinctive in vivo excretion behaviors of TMDs can be attributed to their different chemical
properties. Still, the peculiarities involved in the degradation and accumulation of
each material are yet to be clarified.
6 Conclusions and Perspectives
In this chapter, we analyzed the unusual 2D film properties of the TMDs MoS 2
and WS 2 with a specific focus on their application as adsorbents and membranes
for water purification. The study of 2D materials’ transport properties, even though
inspired by the enhancement flow observed in carbon nanotubes, has a very different
211
is important to note, though, that most of the computational results on desalination
membranes come from pure classical dynamics. Despite the widespread agreement
of these results with experiments, there is still much to advance in the chemical–
physical molecular understanding of the processes that lead to higher or lower water
fluxes, as well as the mechanisms and interactions between solution and membrane
that provide greater efficiency in solute rejection. This can be achieved by using
hybrid simulations that consider explicit charge transfer, for example.
(f) Toxicity—Along with the scalable use of MoS 2 and WS 2 nanomembranes, one
of the ultimate challenges is to understand more deeply how these structures are
going to affect the environment after release. For instance, nanomaterials may pose
a long-term threat if they are persistent and nondegradable [98]. In this regard, both
MoS 2 and WS 2 are considered chemically stable materials against environmental
stressors because of the absence of dangling bonds in the terminating S atoms [35].
Particularly the solubility of MoS 2 is low under ambient conditions, which leads to
long-term persistence in the environment. On the other hand, this scenario can differ
for extreme conditions such as high temperature or strong oxidation, where TMD
nanomaterials can be oxidized to different oxides [99]. For instance, the oxidation of
MoS 2 nanosheets has been shown to occur in aqueous solutions, leading to soluble,
low-toxic oxidation products [100]. The oxidation kinetics depend on factors such
as the pH and the crystallographic phase of the TMD.
Toxicity varies depending on preparation methods, but both MoS 2 and WS 2
nanosheets generally show high biocompatibility at concentrations up to ~100 ppm
and certain cytotoxicity at high concentrations (a few hundred ppm). For instance,
low toxicity of exfoliated, well-dispersed MoS 2 nanosheets was observed, but aggregated samples were found to induce acute lung inflammation in mice [101], raising
concerns about their size’s effects on the toxicity of these nanosheets. When it comes
to toxicity we have good reasons to believe that both MoS 2 and WS 2 nanosheets can
perform better than graphene-based nanomaterials [102], but there can be significant differences between those TMDs. Polyethylene glycol (PEG)-coated 2H-MoS 2
nanosheets have shown fast degradation and complete excretion within a month,
in marked contrast to WS 2 , which presented high levels of concentration in the
organs for months [103]. Further degradation experiments indicated that the distinctive in vivo excretion behaviors of TMDs can be attributed to their different chemical
properties. Still, the peculiarities involved in the degradation and accumulation of
each material are yet to be clarified.
6 Conclusions and Perspectives
In this chapter, we analyzed the unusual 2D film properties of the TMDs MoS 2
and WS 2 with a specific focus on their application as adsorbents and membranes
for water purification. The study of 2D materials’ transport properties, even though
inspired by the enhancement flow observed in carbon nanotubes, has a very different