Halloysite Nanotubes: An ‘Aluminosilicate Nanosupport’ …
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be 64 times higher than pristine HNTs and showed a maximum adsorption capacity
of 102.5 mg/g at 328 K. A sequential step for reduction of Cr(VI) to Cr(III) followed
by adsorption on the positively charged surface had occurred, which enhanced the
adsorption capacity (Tian et al. 2015).
Apart from the elimination of chromium ions, several researches have stated the
removal of lead ions as well. In a study, amino groups were grafted on the surface of HNT through organosilanes treatment. This contributed a positive charge to
the surface, which enhanced the adsorption rate and showed a maximum adsorption
capacity of 12.18–46 mg/g. The wide range was obtained as a result of wide pH range
from 3 to 6. The nanocomposite also showed high reusability (Cataldo et al. 2018).
Similarly, HNT of magnetic nature were fabricated by grafting Fe 3 O 4 nanoparticles
on its surface, followed by further deposition by MnO 2 nanoparticles for the elimination of Pb (II) ions. This composite demonstrated a high adsorption capacity of
59.9 mg/g and the adsorption process were said to be spontaneous and endothermic. A major factor behind was touted to be the presence of MnO 2 nanoparticles,
which have high affinity for many heavy metals along with increased surface area,
porosity and particle size. The nanocomposite also showed high reusability even
after 5 cycles of adsorption/desorption processes (Afzali et al. 2016). Interestingly,
Pb (II) ions have also been removed through SPE techniques by employing HNT
with N-2-Pyridylsuccinamic acid grafted on it. The developed adsorbent showed an
enhancement in the adsorption rate by 67 times and the capacity of adsorption was
found to be 23.58 mg/g (He et al. 2016).
Filtration has also been utilized as an alternate method for the eradication of heavy
metals. For instance, in a study, polydopamine was attached on HNT’s surface, and
this composite was used to develop polyetherimide mixed matrix membranes for the
elimination of heavy metals such as Pb (II) and cadmium Cd (II). The membranes
showed a rejection rate of 34% and 27% for Pb (II) and Cd (II), respectively ( Hebbar
et al. 2016).
3.1.3 Pesticide Remediation and Sensing
Pesticides are chemicals that control and eliminate weeds and pests, and are widely
used across the world. However, 90% of the pesticides reach water bodies due to
agricultural runoffs and seepage (Khatri et al. 2017a, b; Khatri and Tyagi 2015).
Chronic exposure to pesticides can seriously affect biodiversity, endangered species
and destruction of habitat of birds. During the last decade, several approaches have
been established for elimination of pesticides from aqueous environment. However,
conventional techniques have the limitations of high-operational cost and need of
technical expertise. In the past few years, several HNT-based nanocomposites have
been used for the detection and removal of pesticides (Rawtani et al. 2018).
In a study, 2,4,6-trichlorophenol had been selectively recognized and removed
through HNT-based nanocomposites using SPE. Molecularly Imprinted polymers
(MMIP) were grafted and synthesized on the surface of HNTs with magnetic nanoparticles immobilized on them. The MMIPs served as sorbents for this purpose. The
135
be 64 times higher than pristine HNTs and showed a maximum adsorption capacity
of 102.5 mg/g at 328 K. A sequential step for reduction of Cr(VI) to Cr(III) followed
by adsorption on the positively charged surface had occurred, which enhanced the
adsorption capacity (Tian et al. 2015).
Apart from the elimination of chromium ions, several researches have stated the
removal of lead ions as well. In a study, amino groups were grafted on the surface of HNT through organosilanes treatment. This contributed a positive charge to
the surface, which enhanced the adsorption rate and showed a maximum adsorption
capacity of 12.18–46 mg/g. The wide range was obtained as a result of wide pH range
from 3 to 6. The nanocomposite also showed high reusability (Cataldo et al. 2018).
Similarly, HNT of magnetic nature were fabricated by grafting Fe 3 O 4 nanoparticles
on its surface, followed by further deposition by MnO 2 nanoparticles for the elimination of Pb (II) ions. This composite demonstrated a high adsorption capacity of
59.9 mg/g and the adsorption process were said to be spontaneous and endothermic. A major factor behind was touted to be the presence of MnO 2 nanoparticles,
which have high affinity for many heavy metals along with increased surface area,
porosity and particle size. The nanocomposite also showed high reusability even
after 5 cycles of adsorption/desorption processes (Afzali et al. 2016). Interestingly,
Pb (II) ions have also been removed through SPE techniques by employing HNT
with N-2-Pyridylsuccinamic acid grafted on it. The developed adsorbent showed an
enhancement in the adsorption rate by 67 times and the capacity of adsorption was
found to be 23.58 mg/g (He et al. 2016).
Filtration has also been utilized as an alternate method for the eradication of heavy
metals. For instance, in a study, polydopamine was attached on HNT’s surface, and
this composite was used to develop polyetherimide mixed matrix membranes for the
elimination of heavy metals such as Pb (II) and cadmium Cd (II). The membranes
showed a rejection rate of 34% and 27% for Pb (II) and Cd (II), respectively ( Hebbar
et al. 2016).
3.1.3 Pesticide Remediation and Sensing
Pesticides are chemicals that control and eliminate weeds and pests, and are widely
used across the world. However, 90% of the pesticides reach water bodies due to
agricultural runoffs and seepage (Khatri et al. 2017a, b; Khatri and Tyagi 2015).
Chronic exposure to pesticides can seriously affect biodiversity, endangered species
and destruction of habitat of birds. During the last decade, several approaches have
been established for elimination of pesticides from aqueous environment. However,
conventional techniques have the limitations of high-operational cost and need of
technical expertise. In the past few years, several HNT-based nanocomposites have
been used for the detection and removal of pesticides (Rawtani et al. 2018).
In a study, 2,4,6-trichlorophenol had been selectively recognized and removed
through HNT-based nanocomposites using SPE. Molecularly Imprinted polymers
(MMIP) were grafted and synthesized on the surface of HNTs with magnetic nanoparticles immobilized on them. The MMIPs served as sorbents for this purpose. The