Halloysite Nanotubes: An ‘Aluminosilicate Nanosupport’ …
133
2015). Duan et al. (2012) had also employed a similar HNT-Fe 3 O 4 nanocomposite
for the removal of Methyl Violet. This nanocomposite showed a higher adsorption
capacity of 90.09 mg/g (Duan et al. 2012). The adsorption capacities of HNT-Fe 3 O 4
nanocomposites can also be increased by incorporating it with carbon-based adsorbent. In a study, the magnetic nanocomposite was incorporated with glucose originated hydrothermal carbon. Hydrothermal carbon is known to have a high adsorption capacity due to its higher number of oxygen-containing groups, which serve as
adsorption sites. This nanocomposite was further used for the adsorption of Methylene Blue (MB), and it showed an adsorption capacity of 88.42 mg/g through Langmuir Adsorption Isotherm (Jiang et al. 2014). HNT-Fe 3 O 4 nanocomposite has also
been employed to eliminate Neutral Red (NR), MB and Methyl Orange (MO). This
study also showed that the developed magnetic HNT nanocomposite was capable of
removing neutral (MB), cationic (NR) and anionic (MO) dyes (Xie et al. 2011). Similarly, HNT -Fe 3 O 4 nanocomposites have been used in the removal of naphthol green
B as well (Riahi-Madvaar et al. 2017). In addition to this, HNT-Fe 3 O 4 nanocomposites have found their way for the recognition of azo dyes as well. These nanocomposites were used to construct a magnetic hemimicelle for Solid-Phase Extraction
(SPE) adsorption of dyes. These hemimicelles offered multiple advantages such as
detection and removal of azo dyes, enhanced extraction capacity, and simplified elution of analytes. The synthesized adsorbent demonstrated an adsorption capacity of
121.95 mg/g. The hemimicelle also showed excellent Limit of Detection (LOD) of
0.042 μg/L and 0.050 μg/L in samples spiked with Methyl Red (MR) and Methyl
Orange (MO), respectively (Liu et al. 2018).
Other techniques of dye removal using guest@HNT complex include development of membranes for enhanced dye rejection and filtration. Nanofiltration films
have nanosized pores and are easy to implement. HNT implemented nanofiltration
membranes further allows enhanced hydrophilicity of the membrane because of the
hydroxyl moieties existing on the surface of HNT. Several studies have incorporated
HNT into the nanofiltration membranes and used such membranes for eliminating
harmful dyes. For instance, amine groups had been grafted on the surface of HNT
through 3-aminopropyltriethoxysilane (APTES). This composite was later incorporated in a PVDF nanofiltration membrane for the removal of Direct Red (DR). The
membrane had showed a dye rejection rate up to 94.9%, which was very high in
comparison to pure PVDF membranes. The membranes also demonstrated excellent
rejection stability and high reusability after multiple tests (Zeng et al. 2016). In yet
another approach of immobilizing amino groups on the surface of HNT for enhancing the adsorption and hydrophilicity of the membranes, HNT with immobilized
Polydopamine (PDA) have been introduced in the PVDF membranes for removal
of DR28, Direct Yellow (DY) and Direct Blue (DB) dyes. PDA contains catechol
and amino groups, which when combined with amino groups upsurge the number of
sites for adsorption on HNT. Therefore, the membrane showed a dye rejection rate
of 96.5% for DR28, while for DY, it was 85%. The membrane showed a rejection
rate of 93.7% for DB (Zeng et al. 2017).
Apart from films, several novel structures with HNT incorporated in them have
been developed as well. For instance, Polydopamine (PDA) had been immobilized
133
2015). Duan et al. (2012) had also employed a similar HNT-Fe 3 O 4 nanocomposite
for the removal of Methyl Violet. This nanocomposite showed a higher adsorption
capacity of 90.09 mg/g (Duan et al. 2012). The adsorption capacities of HNT-Fe 3 O 4
nanocomposites can also be increased by incorporating it with carbon-based adsorbent. In a study, the magnetic nanocomposite was incorporated with glucose originated hydrothermal carbon. Hydrothermal carbon is known to have a high adsorption capacity due to its higher number of oxygen-containing groups, which serve as
adsorption sites. This nanocomposite was further used for the adsorption of Methylene Blue (MB), and it showed an adsorption capacity of 88.42 mg/g through Langmuir Adsorption Isotherm (Jiang et al. 2014). HNT-Fe 3 O 4 nanocomposite has also
been employed to eliminate Neutral Red (NR), MB and Methyl Orange (MO). This
study also showed that the developed magnetic HNT nanocomposite was capable of
removing neutral (MB), cationic (NR) and anionic (MO) dyes (Xie et al. 2011). Similarly, HNT -Fe 3 O 4 nanocomposites have been used in the removal of naphthol green
B as well (Riahi-Madvaar et al. 2017). In addition to this, HNT-Fe 3 O 4 nanocomposites have found their way for the recognition of azo dyes as well. These nanocomposites were used to construct a magnetic hemimicelle for Solid-Phase Extraction
(SPE) adsorption of dyes. These hemimicelles offered multiple advantages such as
detection and removal of azo dyes, enhanced extraction capacity, and simplified elution of analytes. The synthesized adsorbent demonstrated an adsorption capacity of
121.95 mg/g. The hemimicelle also showed excellent Limit of Detection (LOD) of
0.042 μg/L and 0.050 μg/L in samples spiked with Methyl Red (MR) and Methyl
Orange (MO), respectively (Liu et al. 2018).
Other techniques of dye removal using guest@HNT complex include development of membranes for enhanced dye rejection and filtration. Nanofiltration films
have nanosized pores and are easy to implement. HNT implemented nanofiltration
membranes further allows enhanced hydrophilicity of the membrane because of the
hydroxyl moieties existing on the surface of HNT. Several studies have incorporated
HNT into the nanofiltration membranes and used such membranes for eliminating
harmful dyes. For instance, amine groups had been grafted on the surface of HNT
through 3-aminopropyltriethoxysilane (APTES). This composite was later incorporated in a PVDF nanofiltration membrane for the removal of Direct Red (DR). The
membrane had showed a dye rejection rate up to 94.9%, which was very high in
comparison to pure PVDF membranes. The membranes also demonstrated excellent
rejection stability and high reusability after multiple tests (Zeng et al. 2016). In yet
another approach of immobilizing amino groups on the surface of HNT for enhancing the adsorption and hydrophilicity of the membranes, HNT with immobilized
Polydopamine (PDA) have been introduced in the PVDF membranes for removal
of DR28, Direct Yellow (DY) and Direct Blue (DB) dyes. PDA contains catechol
and amino groups, which when combined with amino groups upsurge the number of
sites for adsorption on HNT. Therefore, the membrane showed a dye rejection rate
of 96.5% for DR28, while for DY, it was 85%. The membrane showed a rejection
rate of 93.7% for DB (Zeng et al. 2017).
Apart from films, several novel structures with HNT incorporated in them have
been developed as well. For instance, Polydopamine (PDA) had been immobilized