Highly photocatalytic activity of natural halloysite - Based material for the treatment of dyes in wastewater

Journal of Mining and Earth Sciences Vol. 62, Issue 3 (2021) 19 - 28

19

Highly photocatalytic activity of natural halloysite -

based material for the treatment of dyes in

wastewater

Son Ha Ngo^*, Nui Xuan Pham, Tuan Ngoc Tran

Faculty of Oil and Gas, Hanoi University of Mining and Geology, Vietnam

ARTICLE INFO

ABSTRACT

Article history:

In this study, the halloysite nanotube material will be fabricated from a

natural halloysite mineral and used as a support for the photocatalytic

activity phase based on TiO₂. The material is characterized by modern

physicochemical methods such as XRD, SEM, BET, UV - vis spectrum, and

EDX. Accordingly, the refined halloysite has a nanoscale with a length of

about 1.3 μm and a capillary size of about 5 nm. After the deposition of Ag

- TiO₂on the halloysite, the specific surface of the material measured by

the BET method was about 60 m²/g, and the structure of the halloysite

was intact. The band - gap energy of as - prepared materials is also

significantly improved in comparison to pure TiO₂, makes the material

capable of absorbing longer wavelengths of light in the photocatalytic

process. The Photocatalyst based on Halloysite and TiO₂showed very high

efficiency, up to more than 95% in the decomposition of typical organic

pollutant RR - 195. This result shows great potential in this novel material

in environmental treatment applications.

Received 19^thFeb. 2021

Accepted 17^thApr. 2021

Available online 30^thJune 2021

Keywords:

Ag - TiO₂,

Halloysite,

Photocatalyst,

RR - 195,

TiO₂.

about 50% of the total amount of dyes used on the

market. However, the use of dye also leads to

1. Introduction

Reactive dyes are one of the most important

technological advances of the 20^thcentury in the

field of textile. They are used more and more

because they have bright colours, rich varieties,

and high colour fastness. Currently, the number of

reactive dyes in the textile industry accounts for

environmental problems such as residual dyeing

compounds and chemical additives used in the

dyeingprocessinwastewater(Baganeetal., 2000).

These wastes are dangerous to the environment,

especially aquatic habitats, if not being treated

thoroughly (Imamura et al., 2002) due to its

solubilityinwaterandstabilityinthis environment

even with low concentrations. Therefore, the

removal of dyes has received the huge attention of

researchers.

_____________________

^*Corresponding author

E - mail: [email protected].vn

DOI: 10.46326/JMES.2021.62(3).03

20

Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

Adsorption using activated carbon has long

other noble metals, acts as

a

trap of

been used to remove various pollutants in water.

However, adsorption is only the first step to collect

pollutants and cannot treat them completely.

Therefore, many scientists have studied the

feasibility of using inexpensive, commercially

availablematerialsas dyeremovalagents byhighly

efficient processes without secondary pollutant

generation.

Recently, natural halloysite has been attracted

far -reachinginterestforeffectivewatertreatment.

This is a low - priced and abundant material with

numerous advantages such as good dispersibility,

rich in active groups, low toxicity, good biological

compatibility and availability in nature (Zhang et

al., 2016).

In the field of photocatalysis for

environmental treatment, the most popular

nanomaterial is TiO₂nanoparticles. TiO₂is a non -

toxic material which has beenappliedinthefieldof

solar energy and especially in environmental

treatment because of their strong photocatalytic

propertiesandchemicalstability(Aminetal.,2009,

Guo et al., 2009). However, TiO₂has a large band -

gap energy that requires ultraviolet irradiation to

stimulate the catalytic ability. Furthermore, the

rapid recombination of electron - hole pairs can

drastically reduce quantum efficiency. So,

narrowingtheband- gapenergyofTiO₂toincrease

the visible light absorption ability is a commonly

used method to improve photocatalytic efficiency

(Amin et al., 2009). Such methods could be the

addition of metals, metal oxides, various elements

into TiO₂lattice such as Zn, Fe, Cr, Eu, Y, Ag, Ni; or

the insertion of nonmetals such as N, C, S, F, Cl; or

simultaneously insertion of different elements into

the TiO₂crystal lattice, etc. Most of the modified

products have higher catalytic activity than the

initial TiO₂in the visible light (Cotolan et al., 2016).

Among them, silver is one of the most highly

efficientingredients (Guo etal., 2009;H. Donget al.,

2015). Over the years, the use of silver deposited

on semiconductors in the photocatalytic process

has much interest (for example, in the degradation

of organic pollutants, hydrogen production,

disinfection) because it can enhance the

photocatalytic activity and extend the light

absorption to the visible light region (Oros - Ruiz et

al., 2013; Grabowska et al., 2013; Shan et al., 2008;

Ansari et al., 2013; Khan et al., 2013). Silver, like

photogeneratedelectronsandasaresult,improves

the charge carrier separation (Ohtani et al., 1997).

Moreover, silver nanoparticles can absorb visible

light due to localized surface plasmon resonance

(Zhou et al., 2012), leading to new applications

such as antibacterial textiles, medical devices, food

preparation surfaces as well as air conditioning

filters and coated sanitary wares (Zielińska et al.,

2010).

Currently, the combination of TiO₂

nanoparticles with halloysite nanotubes has been

done by scientists with varied methods, and the

results are promising. A typical example of this

combination is Halloysite - TiO₂composite

photocatalytic material, which has a much higher

decomposition rate for azo Basic Blue 41 dye in

water than commercial TiO₂under UV light

(Szczepanik et al., 2017).

For that reason, the aim of this study is to

fabricate a photocatalytic material based on HNT

carrier with Ag - doped TiO₂and to evaluate the

activity of this catalyst in the photocatalytic

degradation of RR - 195, which is a persistent

pollutant present in textile wastewater.

2. Experimental

2.1. Chemicals

Raw halloysite was obtained from kaolinite

mines. Sulfuric acid (H₂SO₄, 98%); Ethanol

(C₂H₅OH); Acetic acid (CH₃COOH, 99%); Titanium

iso - propoxide (TTIP) (Ti(OC₃H₇)₄); Silver nitrate

(AgNO₃); Glucose (C₆H₁₂O₆); NH₃solution;

Hydroperoxide (H₂O₂); Reactive Red - 195

(RR195) were purchased from China; Deionized

waterandice wereproduceddirectlyatthelaband

used throughout. All the chemicals were used

without any further purification (except raw

halloysite).

2.2. Halloysite (HNT) purification

Raw halloysite was crushed and sieved to

remove particles and dried at 100⁰C, then 40 g of

the material was dissolved with 55 ml of distilled

water. After that, a solution of 2 ml of 98% sulfuric

acid was slowly added and stirred slowly at 90⁰C

for 90 minutes to obtain solution 1. This solution

was filtered and washed several times with

distilled water to remove excess H₂SO₄and was

Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

21

dried then. Next, 1.5 litters of distilled water were

added to the mixture that has just been dried in

previousstep.Themixturewasstirredfor24hours

more to acquire solution 2. Solution 2 was settled

down for 96 hours and followed by a filtration. At

this step, the upper part of solution 2 was decanted

and, the residues at the bottom of the beaker were

discarded. These separations were repeated 3

times. Finally, pure halloysite was obtained via the

centrifugation, filtration and drying.

2θ = 80⁰C. Nitrogen adsorption analyses were

carriedouton Micromeritics ASAP 2000at - 196⁰C.

All the samples were outgassed at 150⁰C under

vacuum for 1 hour prior to the measurements.

Pore size distributions were determined using the

BJH method from desorption branches of the

isotherms. EDX spectra was recorded by EDS JED -

2300 Analysis Station. UV - vis spectra was

obtained using Shimadzu UV - 3101 PC in the

ultraviolet region in the wavelength range of

200÷800 nm. Transmission electron microscopy

(TEM)wasobservedbyaJOELJEM- 1010Electron

Microsope while scanning electron microscope

was taken by Philips XL30 microscope.

2.3. Synthesis of Ag - TiO_2,TiO₂- HNT, Ag -

TiO₂/HNT

First, 3.3 ml of CH₃COOH was cooled down in

ice until completely frozen. Then, 6.6 ml of C₂H₅OH

was added and stirred for about 30 minutes to

obtaina completelytransparentsolution. TTIP(3.3

ml) was added to the solution and under magnetic

stirring for 30 minutes. Then slowly added 280 ml

of distilled H₂O to the solution, stirring for about 30

minutes, to obtain TiO₂sol. Next, 3 g of purified

halloysite was put to TiO₂sol, and stirred at 65⁰C

for24hours,thenkeptitinanautoclavefor5hours

at 180⁰C. The final solution was filtered, washed,

and dried at 70⁰C to TiO₂- HNT.

Repeat the procedure until the step of TTIP

addition. After that, 0.1 g of glucose was added

undermildstirring, andthe solutionwas heatedup

to 80⁰C for 45 minutes.

In the following step, a solution of AgNO₃

0.002M in excess NH₃was added into the previous

solution under hard stirring for 15 minutes to

obtain sol Ag - TiO₂. After overnight aging, the gel

was washed with acetonitrile repeatedly, vacuum -

dried at 80⁰C, and calcined in air at 550⁰C for 2 h to

produce Ag/TiO₂powders.

2.5. Measurement of photocatalytic activity

The photocatalytic activity was evaluated

through thephotocatalytic degradationof RR-195.

The efficiency of this reaction is calculated by

determining the concentration of the dye before

and after the reaction by UV - vis ultraviolet

spectroscopy. Specifically, the concentration of RR

- 195 was determined through the optical density

obtained from ultraviolet spectroscopy at 541 nm.

The efficiency of the degradation process is

calculated by the following formula:

H (%) = (C_o- C_t)/C_o

(1)

In which: C_o- initial concentration of RR - 195

(ppm); C_t- concentration of RR - 195 (ppm) after t

(mins).

All the concentrations were identified based

on calibration curves obtained by UV - vis.

3. Results and discussions

3.1. Characterization of purified HNT material

3.1.1. X - ray diffraction results

To synthesize Ag - TiO₂/HNT, the sol was

gradually cooled down to 65⁰C with the addition of

3g of purified halloysite under normal stirring for

24hours. The sol was kept at 180⁰C in an autoclave

for 5 hours. Finally, the solution was washed

several times with filtration, then dried at 70⁰C to

obtain Ag - TiO₂/HNT.

To determine the crystallographic structure of

purified halloysite material, the X - ray diffraction

spectrum was performed in the range 2θ from 5⁰C

to 80⁰C. The XRD results of halloysite material are

shown in Figure 1.

Wide - angle X - ray spectra of the purified

halloysite sample show the appearance of

characteristic peaks of halloysite nanotubes, in

which the typical reflectance angles are at 2θ at

12.20 (001); 19.90 (100); 24.840 (002); 35.020

(110); 37.980 (003) and 54.340 (211) are

2.4. Characterization

The phase structure of fabricated products

wascharacterizedbyX-raydiffraction(Bruker, D8

advance) using CuK_λ(λ= 0.15406 nm) radiation.

The data were collected in the range of 2θ = 5⁰C to

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Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

consistent with standard X - ray diffraction data of

halloysite (HNT) nanotubes (Kamble et al., 2012).

method was conducted, and the scanning electron

microscopy(SEM)imagewasobtainedasshownin

Figure 2.

The SEM image reveals that most of the

purified halloysite has the form of tubes in nano

size with porous structure and high uniformity.

3.1.2. SEM images of purified HNT

To determine the specific surface morphology

for the purified halloysite material, the SEM

Figure 1. X - ray patterns (XRD) of purified halloysite.

Figure 2. SEM images of purified Halloysite.

Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

23

Specifically, the length of the tubes is about 1.3 μm,

and diameter is about 130 nm, which is in

accordance with results published in the literature

(Rooj et al., 2010). The above results prove that

halloysite (HNT) nanotubes have beensuccessfully

achieved from the raw halloysite source in

Vietnam.

3.1.3. Nitrogen adsorption - desorption results

To determine the specific surface area and

pore volume as well as pore diameter of HNT, the

BET nitrogen adsorption - desorption isotherm at

77.3 K according was carried out for halloysite

nanotube material samples.

The curves are exhibited in Figure 3, showing

that the adsorption - desorption isothermal curve

of the HNT is of type IV and has a hysteresis curve

H3 corresponding to the mesoporous material

according to IUPAC classification. The pore size

distribution curve illustrates that the mean pore

diameter of HNT was 4.8 nm.

Figure 4. X - ray (XRD) spectrum of different

catalysts.

Typicalparameters ofHNT obtainedfromBET

measurement results:

- Specific surface area (S_BET): 28.0186 m²/g

- Total pore volume (V_pore): 0.137840 cm³/g

- Pore diameter (DBJH): 21.8489 nm.

Figure 5. Graph of [F(R)hν]²as a function of hν for

E_bgcalculation.

Figure 3. Nitrogen adsorption - desorption

isotherm at 77.3 K of HNT.

3.1.4. Characterization of Ag - TiO₂/HNT

Theresults of XRD, UV - vis solid, EDX andSEM

of Ag - TiO₂/HNT materials are exhibitedin Figures

4÷7.

X - ray spectrum of Ag - TiO₂/HNT material

(Figure 4) still confirms the existence of the

characteristic peaks of TiO₂, which proves that

there is no change in the structure of TiO₂material.

Figure 6. EDX spectrum of Ag - TiO₂/HNT.

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Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

Figure 7. SEM (a and b), TEM (c and d) images of Ag - TiO₂/HNT.

There are additional peaks which appear at 2θ =

38.1 (111) and 64.4 (220) and weaker intensity

peakat2θ=44.3(200)characterizingthepresence

of metallic silver (JCPDS 65 - 2871). The obtained

results demonstrate that the insertion of TiO₂and

Ag - TiO₂on the HNT support causes no effect on

the halloysite nanotube structure.

The band - gap energy Eg of the photocatalytic

materialsTiO₂, Ag-TiO₂,TiO₂- HNT, Ag- TiO₂/HNT

were 3.2 eV, 2.8 eV, 3.0 eV, and 2.6 eV, respectively

that were indicated in Figure 5. Those values

appear to verify that the combination of HNT and

TiO₂has adjusted the band - gap of TiO₂3.2÷3 eV.

Furthermore, the addition of Ag onto the surface of

TiO₂has drastically reduced the band - gap of TiO₂

to 2.8 eV that could consequently lead to a

significant influence on the photocatalytic

properties of TiO₂. Thereby, the excited zone of Ag

- TiO₂/HNT shifted considerably from the

ultraviolet (UV) to the visible (Vis) region, and the

material has a narrower band - gap (2.6 eV)

compared to pure TiO₂(3.2 eV)

TiO₂/HNT catalyst and no different elements

appear. Additionally, the mass percentage of the

elements are quite compatible with the theoretical

calculation used in the first step to synthesize Ag -

TiO₂/HNT.

SEM, TEM images (Figure 7) presents that the

implantationof Ag- TiO₂onto halloysite nanotubes

did not break the halloysite's tube structure. The

images show the uniform dispersion of the Ag -

TiO₂nanoparticles on halloysite (HNT) nanotubes

and the Ag nanoparticles that are tightly attached

on the surface of the TiO₂nanorods to form Ag -

TiO₂nanorods with an average size is about 10÷20

nm.

As can be describedin Figure 8andTable 1, Ag

- TiO₂/HNT has twice as much specific surface area

andsignificantlyincreasedporevolumeastheHNT

support. It could be explained that Ag - TiO₂also

has its own specific surface area and has its porous

system. As a result, the presence of Ag - TiO₂on the

inner and outer surface of halloysite nanotubes

could contribute to the number of pores

representing for the higher surface area as well as

the total pore volume. The higher pore volume of

Ag - TiO₂/HNT could guarantee the better

performance of this catalyst in terms of pollutants

Also, the X - ray energy dispersive spectrum of

Ag - TiO₂/HNT in the binding energy range of 0÷10

keV in Figure 6 points out the corresponding

signals of the elements that make up the Ag -

Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

25

adsorption. Basedontheevidence givenbyvarious

characterization techniques, it could be concluded

that Ag - TiO₂was successfully grafted on the

surface of HNT. Moreover, Ag - TiO₂particles in

nano size, which were distributed evenly on the

surface, couldbringefficientcatalytic activityto the

as - prepared nanocomposite.

photocatalytic activity. This can be elucidated by

the existence of Ag - TiO₂on the surface of

halloysite reduced the band - gap energy of TiO₂

(2.6 eV). In addition, Ag^ocan be a center of charge

transition and has high ability to trap electrons to

prevent recombination of photo - generated

electrons and holes on TiO₂surface. At the same

time, silver also has a plasmon resonance effect

resulting in the more OH radical generation and

increased photocatalytic efficiency.

Figure 8. Nitrogen adsorption - desorption

isotherm at 77.3K of Ag - TiO₂/HNT catalyst.

Figure 9. Conversion of RR - 195 in photocatalytic

degradation process using different catalysts.

Table 1. Parameters of Ag - TiO₂- HNT obtained

from BET method.

Parameter

Value

3.2.2. Impact of H₂O₂concentration on the

conversion of RR - 195

Specific surface area (S_BET- m²/ g) 57.3668

Pore diameter (D_BJH- nm)

Pore volume (V_pore- cm³/ g)

Micropore volume (cm³/ g)

10.6906

0.163701

0.000193

H₂O₂has an important impact on the

photocatalytic degradation of pollutants. The effect

of H₂O₂in RR - 195 was investigated, and the

results are presented in Figure 10.

As can be observed in Figure 10, when using 2

ml of H₂O₂with the presence of Ag - TiO₂/HNT

catalyst under UV irradiation, the conversion of RR

- 195 was 79% in a period of 360 minutes. When

increasing the amount of H₂O₂to 4ml, the

mineralization of RR - 195 after 360 minutes did

not occur faster but tended to decrease (76%). In

addition, if the amount of H₂O₂is reduced to 1ml

and 1.5 ml, the rate as well as the decomposition

efficiency RR - 195 decreased also standing at 66%

and 76%, respectively.

This is because more •OH radicals generated

from H₂O₂promote the reaction leading to

increased decomposition rate and efficiency.

However, when the amount of H₂O₂in the solution

is too high or too low will reduce the free radicals

•OH occurs according to the equation (Szczepanik

3.2. Photocatalytic activity of Ag - TiO₂/HNT

3.2.1. Comparison of photocatalytic activity of

synthesized materials

The photocatalytic decomposition of RR - 195

was performed at room temperature with the

following conditions: the initial concentration of

RR - 195 in water was 50 ppm, then 1.5 ml of H₂O₂

was added with 0.05 g solid catalyst. Next, the

mixture was stirred for 3 hours to reach

equilibrium adsorption. Afterthat, themixture was

irradiated with UV light for the next 3 hours. The

results of the catalytic activity evaluation are

presented in Figure 9.

The results pointedoutthat theAg- TiO₂/HNT

synthesized by the direct method has the highest

26

Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

et al., 2017; Abdel Fattah et al., 2016; Natarajan et

al., 2015):

results with a relatively new photocatalytic

material system based on the halloysite natural

mineral. Specifically, purified halloysite (HNT)

nanotubes have been processed from raw

halloysite sources in kaolin mines in a specific

surface area of 28.0186 m²/g with an average

length of 1.3 μm and average diameter of 130 nm.

Ag - TiO₂/HNT photocatalytic material was then

synthesizedbythedirectmethod.Thedatashowed

that the doping of Ag significantly narrowed the

band - gap energy of pure TiO₂. Thus, the material

could be easily excited in the visible region, which

makes it more efficient in the photocatalytic

process. In addition, the specific area and pore

volume of Ag - TiO₂/HNT were significantly

improved in comparison with HNT and TiO₂. The

photocatalytic activity evaluation indicated that

the Ag - TiO₂/HNT had superior efficiency

compared to the other catalysts namely TiO₂; Ag -

TiO₂; TiO₂/HNT thanks to the presence of Ag and

the better distribution of active phase on HNT

support. As a result, Ag - TiO₂/HNT could almost

completely decomposed RR - 195 at room

temperature and neutral pH environment. This

suggests that Ag - TiO₂/HNT material could offer

extreme potential in the treatment of polluted

wastewater.

H₂O₂+ •OH  •HO₂+ H₂O

(2)

•HO₂+ •OH  O₂+ H₂O

In addition, a high amount of H₂O₂also results

in the saturation of the active sites of the catalyst,

thereby diminishing the reaction rate. For that

reason, 1.5 ml of H₂O₂was chosen to be applied to

all remaining research processes to best assess the

catalytic activity of the catalyst materials.

3.2.3. Influence of catalyst weight

The influence of the catalyst content on the

conversion of dye RR - 195 on Ag - TiO₂/HNT

catalyst was investigated. The results are shown in

Figure 11.

The results showed that after 360 minutes of

UV irradiation using 0.15 g of Ag - TiO₂/HNT, the

efficiency in RR - 195 degradation was 96%. This

valueisconsiderablyhigherthantheonesobtained

when 0.05g and 0.1g catalyst was applied, which

only reached the conversion of 66% and 90%,

respectively. This can be explained as follows:

when the amount of catalyst increases, the amount

of catalytic activity sites increases, causing the

diffusion rate of the anions RR - 195 to the active

sites on the surface, leading to an increase the

number of 96% shows the almost complete

decomposition of contaminants under normal

conditions.

Author contributions

SonHaNgo andNui XuanPhamconceivedand

planned the experiments; Tuan Ngoc Tran carried

out the experiments; Son Ha Ngo and Tuan Ngoc

Tran contributed to sample preparation. Son Ha

Ngo and Nui Xuan Pham contributed to the

interpretation of the results; Son Ha Ngo and Tuan

4. Conclusions

This study has obtained some remarkable

Figure 10. The conversion of RR - 195 when using

Ag - TiO₂/HNT at different H₂O₂concentrations.

Figure 11. Influence of catalyst weight in the

degradation process of RR - 195.

Son Ha Ngo and et al./Journal of Mining and Earth Sciences 62(3), 19 - 28

27

Ngoc Tran performed the calculations; Son Ha Ngo

took the lead in writing the manuscript. All authors

provided critical feedback and helped shape the

research, analysis and manuscript.

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Role of platinum deposits on titanium (IV)

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