Efficient Removal of Nitrogen based Industrial Pollutants by Graphene Oxide Coupled Nanotitania Composite under Visible Light Illumination

Journal of Environmental Treatment Techniques

2021, Volume 9, Issue 1, Pages: 183-191

J. Environ. Treat. Tech.

ISSN: 2309-1185

Journal web link: http://www.jett.dormaj.com

https://doi.org/10.47277/JETT/9(1)191

Efficient Removal of Nitrogen based Industrial

Pollutants by Graphene Oxide Coupled Nanotitania

Composite under Visible Light Illumination

Ryali Somasekhar , Paul Douglas Sanasi *

Research Scholar, Department of Chemistry, Jawaharlal Nehru Technological University (JNTU), Kakinada- 533003, Andhra Pradesh, India

Department of Engineering Chemistry, Andhra University College of Engineering (A), Andhra University, Visakhapatnam- 530003,

Andhra Pradesh, India

Received: 18/09/2020

Accepted: 28/10/2020

Published: 20/03/2021

Abstract

N-based industrial pollutants like Nitrobenzene (NB) and 4-nitrophenol (4-NP) were identified as hazardous among the group of industrial

chemicals by United States environmental protection agency (USPEA). Their removal from the effluents has become inevitable for the

industries as a part of wastewater treatment methodology. Thus, the selected organics have been successfully removed from their aqueous

solutions with nanotitania (anatase) composites incorporated with an optimum wt% of graphene oxide (GO) through photocatalytic

degradation. Kubelka-Munk function was applied for the composites in the UV-Vis diffuse reflectance spectral (UV-Vis DRS) studies and

the band gap has appreciably decreased with increase in wt % of GO in the composites. Compared to band gap in Degussa P25 (3.2 eV), the

same was observed as 2.60 eV in the nanotitania composite with 10 % GO. These studies were correlated with Photoluminescence (PL)

spectral analysis. The photodegradation and mineralization of Nitrogen containing industrial pollutants like nitrobenzene (NB) (95.7 % COD

loss) and 4- nitrophenol (4-NP) (97.2 % COD loss) were successfully achieved with the nanocomposite under visible light irradiation.

Keywords: Photocatalysis, Graphene oxide, Nanotitania, Nitrobenzene, 4-nitrophenol

benign techniques namely, advanced oxidation processes (AOP)

Introduction

proved as benchmark methods for the degradation as well as their

mineralization [16]. To decrease the effect of the pollutants on the

aquatic life, the researchers used metal oxide semiconducting

Industrial water effluents released by textile, pharmaceutical,

fertilizer, leather industries contains harmful organic dyestuffs.

These organic pollutants are toxic in nature due to the presence of

intense colour forming moieties in their chemical structures [1].

Nitrobenzene (NB) and 4-nitrophenol (4-NP) are two such N-

based organic pollutants, which were declared as hazardous

chemicals by United States Environmental Protection Agency,

USEPA [2-4]. NB is used in the production of dyes, explosives,

pesticides and waste waters containing more than 2 ppm of the

compound is considered as toxic by USEPA [5, 6]. High levels of

exposure to NB may cause damage to liver and kidneys in human

beings [7, 8]. Similarly, 4-NP is a common pollutant in aquatic

systems and is widely used in the synthesis of dyes, pesticides,

herbicides and pharmaceutical, which is both carcinogenic and

mutagenic as declared by the USEPA [4, 9, 10]. Presence of high

electron affinity - nitro group in NB and 4-NP makes the

compounds restricted for the detoxification. There are methods

like adsorption, photochemical reduction, ozonation, oxidation by

ozone under ultraviolet irradiation, photo-assisted fenton

oxidation, and supercritical oxidation for the remediation of these

harmful compounds. However, they were not completely

successful [11-15]. Unlike these methods, environmentally

2 2 3 2

materials such as TiO , Fe O , ZnO, Cu O, NiO, etc [17]. Due to

its high photostability and photo efficiency, nanotitania (TiO

)

and its analogue heterogeneous forms have emerged as

convincing photocatalysts for the purpose [18-20].

Among the three different crystallographic forms of titania,

anatase form was proved to have superior photocatalytic activity

- +

21-23]. Its rapid electron - hole (e /h ) recombination and a wide

[

band gap of around 3.2 eV (λ ~ 380 nm) limits its application only

in the UV region [24]. Efforts like doping titania with a transition

metal, atom or ion, treating with a suitable sensitizer of higher

absorption coefficient and formation of nanocomposites with

carbonaceous materials having high surface area have been made

to enhance the photocatalytic applications in the visible region

[25]. Titanium tetrachloride was reported as the one of the best

starting material to synthesize anatase nanotitania which is

applicable for the photodegradation studies in the visible region

[

26].

Nanocomposites synthesized by exfoliation of carbon

material like graphene (GR) or graphene oxide (GO) on the

Corresponding author: Paul Douglas Sanasi,Department of Engineering Chemistry, Andhra University College of Engineering (A), Andhra

University, Visakhapatnam- 530003,Andhra Pradesh, India, ORCID:0000-0002-5839-0959; E-mail: pauldouglas12@gmail.com

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2021, Volume 9, Issue 1, Pages: 183-191

incorporating on the titania particles has emerged as an alternative

pathway in the area of photocatalysis [27]. Graphene is a single

atomic layer of graphite and possess exceptional electrical,

mechanical and thermal properties [28]. Graphene oxide is a two-

dimensional carbon material synthesized from graphite (Figure 1)

and contains sp hybridised carbon atoms that can store electrons.

The presence of oxygen containing functional groups like –

COOH, -OH will overcome the Vander Waal’s forces and

increases the inter layer spacing [29, 30]. Previously, the

synthesis of x % graphene oxide (GO) – nanotitania (NT)

composites, characterization and their photocatalytic activity

towards the degradation of some anionic and cationic organic

dyes was reported [31-34].

Unlike the degradation studies of these organic dyes, this

paper mainly presents the photodegradation and mineralisation of

NB and 4-NP (Figure 2) in the presence of these nanocomposites.

The present work also unfolds the band gap variations in the

nanocomposite on introducing GO onto the surface of NT. The

Photoluminescence (PL) studies will additionally support the

results obtained from Kubelka - Munk function curve. A Plausible

degradation mechanism and the formation of mineralization

products has been proposed.

(a)

(b)

Figure 2: Chemical Structures of NB & 4-NP

.2 Synthesis of x % Graphene oxide - Nanotitania composites

Modified Hummers method was adopted to synthesise GO [32,

5]. Nanotitania composites modified with variable wt. % of GO

were synthesized using a mixture of TiCl

:50 mL under ultrasonication conditions as shown in Figure 3.

The finally obtained photocatalysts were designated as 1 % GO,

% GO, 5 % GO and 10 % GO- nanotitania (NT) composites

4 2

and H O in the ratio of

respectively [31, 32]. In order to compare thephotocatalytic

performance of these composites towards the degradation studies,

GO free nanotitania (anatase) was also prepared separately [31].

As the aqueous solutions of these organic pollutants are

colourless in the visible region, dichromate method was used to

study the Chemical oxygen demand (COD) in these pollutants to

admit themineralization under the optimized experimental

conditions. Supporting studies like effect of pH, temperature were

also performed. Superior photocatalytic degradation and

mineralization was observed with the nanocomposite having 10

wt. % GO.

Figure 3: Facile insitu synthesis of GO – nanotitania composites

.3 Photocatalytic measurements

The photocatalytic degradation tendency of the selected

organic pollutants was evaluated with both the synthesized

nanotitania and x % GO-nanotitania composites. In 100 mL of the

diluted aqueous solutions of the pollutants, a constant weight (10

mg) of the composites was dispersed. These mixtures was kept

under magnetic stirring for about 30 minutes, so that the

adsorption/desorption equilibrium could be established in dark

conditions. After optimizing the equilibration time, the samples

were placed under 400 watt tungsten halogen lamp equipped in a

wooden breakfront along with an electric supply. Then, a known

volume of aliquots (5 mL) was extracted in regular intervals of

time (15 min), centrifuged, and the translucent solutions were

analyzed by using an UV-Vis spectrophotometer (UV-2550,

Shimadzu, wavelength range: 180-1100 nm). The percentage

degradation of the pollutant was determined from Equation 1.

Figure 1: Conversion of graphite to graphene oxide

Experimental

.1 Materials

All the chemicals were procured with AR grade quality (99 %

pure) from Sigma Aldrich and Merck. Nitrobenzene with

-1

molecular formula, C

(G. Mol. Wt = 123.11 g.mol ) and

NO (G. Mol. Wt

139.10 g.mol ) were chosen as the N-organic pollutants for the

-nitrophenol with molecular formula, C

6 5

-1

photocatalytic degradation study under visible light irradiation.

Double distilled water (DW) was used for the experimental

works.

C0−Cꢀ

Photocatalytic degradation % = (

) × 1ꢁꢁ

(1)

0 t

where C and C are the initial concentration and concentration of

the pollutant (mg/L) at a time interval, t (min) respectively.

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2021, Volume 9, Issue 1, Pages: 183-191

.3 Characterization

The synthesized nanotitania and x % GO-nanotitania

intensity all these PL signals have decreased steadily compared to

the bare nanotitania particles and it would lead to reduced charge

recombination [38]. The very low intense peak formation for 10

% GO-NT composite shows a significant effect GO on

nanotitania resulting in rapid transfer of photogenerated electrons

onto the GOand separating the charges effectively in the

nanotitania [39].

composites (after calcination) were characterized. The

wavelength obtained for the corresponding absorption and the

band gap energy was estimated from the UV-Vis diffuse

reflectance spectra, which was recorded using Single

monochromator UV-2600 (optional ISR-2600 Plus, λ up to 1400

nm). The change in the intensity for the respective wavelength of

the composites was recorded in the photoluminescence (PL)

spectra at room temperature on Fluorescence spectrophotometer

(

F-4600, Hitachi, Japan).

Results and Discussion

The finally obtained nanotitania and its composites have been

characterized using Powder X-Ray Diffraction (PXRD) including

their average crystallite size, Fourier transform infrared

spectroscopy (FTIR), Field emission scanning electron

microscopy coupled with electron diffraction X-Ray (FESEM-

EDAX), High resolution transmission electron microscopy and

selected area electron diffraction method (HRTEM-SAED)

instrumental techniques. These results were reported earlier [31-

4]. In order to extend the characterization of these composites,

UV-Visible diffused reflectance spectroscopy (UV-VIS DRS)

and Photoluminiscence (PL) instrumental techniques were also

included in the present studies. The former technique gave the

band gap of the nanocomposites which was calculated using

Kubelka-Munk function, and the later provided their

Luminescence behavior.

Figure 4: Band gap energy of nanotitania and x % GO – nanotitania

composites

.1 Band gap energy analysis

The band gap energy of the bare nanotitania and x % GO-

Table 1: Band gap energy values for the nanocomposites

[b]

Band gap (eV)

3.20

[a]

Photocatalyst

nanotitania composites was measured by the extrapolation of the

linear portion of the graph (Figure 4) between the modified

Kubelka-Munk function [F(R)hν] versus photon energy hν in

electron volts, eV (h is plancks constant and ν is the frequency of

the photon energy.) [26]. The band gap was reported as 3.2 eV for

pure nanotitania [26] and around 1.9 – 2.6 eV in GO [25].

In the present work,a narrow band gap was noticed in the

synthesized nanotitania (2.91 eV) compared to the commercial

Degussa P25. It may be due to its concise crystallite size (14.48

nm) [34]. Further, the band gap in the nanocomposite with 10 wt.

Degussa P25

Nanotitania (NT)

10 % GO-NT

2.91

2.60

[a] Degussa P25 –Commercial titania, NT is synthesized in the present

work. [b] Measured from Kubelka-Munk function curves.

.3 Photocatalytic Degradation Studies

.3.1 Preparation of aqueous solution of the probe molecules

-2

Standard solutions of NB (1.5 x 10 N, Colourless at low

concentration) and 4-NP (1.15 N, at pH <6.8-turbid to colourless)

were prepared by dissolving 0.15 mL (0.19 g) of pure NB liquid

and 1.6 g of pure 4-NP solid in 100 mL of DI separately. They

were kept under magnetic stirring to obtain homogenous

solutions. From these solutions, 5.2 mL of NB and 0.7 mL of 4-

NP were further dispersed in 100 mL of DI separately to obtain

GO was observed to be 2.60eV (Table 1).These observations

shows that there is shortening of band gap in the nanotitania

composite on introducing the GO on its surface [25, 36].

The band gap in the remaining GO-NT composites was

almost close to each other as observed from Figure 4. This

condition might have occurred due to close agglomeration of the

particles, but their band gap is less than that of the synthesized

nanotitania.

-5

[

8.12 x 10 ] N of NB and [7.18 x 10 ] N of 4-NP (10 ppm)

solutions respectively.

.3.2 Photocatalytic activity of the composites

The photocatalytic performance of the synthesized

.2 Photoluminescence (PL) spectral analysis

nanotitania and the nanocomposites were evaluated for the

degradation of NB and 4-NP aqueous solutions under visible light

irradiation. In the absence of either the visible light or the

photocatalyst, the tendency of degradation was found to be

negligible, indicating that the degradation in the presence of

photocatalysis condition could be phenomenal. 10 mg of each

photocatalyst was dispersed separately in different sets of the

aqueous solutions (100 mL) of the probe molecules and kept in

dark to establish adsorption - desorption equilibrium. The

The PL studies were performed to identify the efficiency of

the nanotitania particles and the x % GO-nanotitania composites

towards the charge carrier trapping, immigration and transfer, and

to analyze the recombination rate of photogenerated electron hole

pairs [37]. As depicted in Figure 5, the intensity of the synthesized

nanotitania particles was observed at 425 nm. With a gradual

increase in the wt % of GO in the nanotitania composites, the

intensity of the peaks have decreased. The emission peaks were

observed at 427, 430, 475 and 525 nm for the corresponding wt

of 1, 2, 5 and 10 % GO in the composites respectively. The

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Journal of Environmental Treatment Techniques

2021, Volume 9, Issue 1, Pages: 183-191

suspensions were then exposed to visible light irradiation and the

results were presented in Figures 6 (a) & (b).

composite. The aqueous solution of NB has exhibited a sharp

absorption peak at λmax= 260 nm. The absorbance was found to

decrease with respect to time and was almost negligible in 180

minutes. Similarly, the aqueous solution of 4-NP has shown a

minor absorption peak at λ max = 320 nm and an intense absorption

peak at λ max = 405 nm. These peaks may be attributed to

protonated and de-protonated forms of 4-NP respectively [4]. The

absorbance corresponding to these absorption maxima was found

to fall and reach a minimal in 180 minutes. These results clearly

indicates an enhanced degradation efficiency of 10 % GO – NT

composite in the degradation of organic pollutants within 180

minutes (Figure 7).

Figure 5: PL spectra of nanotitania and x % GO-nanotitania composites

It was observed that the degradation efficiency was low with

the synthesized nanotitania particles. With a gradual increase in

the composition of GO in the nanocomposites, the degradation

tendency has increased and reached a maximum efficiency with

the 10 % GO – NT for NB and 4-NP respectively. Similar

degradation efficiency was observed with 15 % GO – NT

composite in the degradation of the pollutants. It may be due to

agglomeration of the GO particles at high weight % thereby

decreasing the number of active sites on the surface of the

composites. Hence, among nanocomposites with 10 and 15 %

GO, the former with less wt % of GO was considered as better

photocatalyst for the degradation studies.

Figure 6 (b): Degradation of 4-NP using x % GO- NT composites

Figure 6 (c): Effect of 10 % GO –NT composite on the absorbance of

.3.3 Effect of pH

The effect of pH on photocatalytic degradation of NB & 4-NP

Figure 6 (a): Degradation of NB using x% GO- NT composites

was evaluated over the range 4.0 –12.0, into solutions with 10

ppm pollutant concentration, at an optimum nanocomposite dose

Figures 6 (c) & (d) represents the study of the photocatalytic

degradation of NB & 4-NP at a wavelength range of 180 nm - 360

nm and 180 nm – 600 nm respectively with 10 % GO – NT

(

10 wt. % GO – NT = 10 mg/100 mL) and for the irradiation time

of 180 minutes. Perchloric acid, HClO and sodium hydroxide,

NaOH solutions were used for varying the pH of the solutions in

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2021, Volume 9, Issue 1, Pages: 183-191

the acidic and alkaline range respectively. The degradation

efficiency of organic molecules will be influenced by the surface

properties of the photocatalyst and pH of the solution [40]. The

surface properties of a photocatalyst can be understood from its

zero point charge (zpc) and for nanotitania it is 6.8.

nanotitania was 6.8 and the combined effect of GO and

nanotitania might have decreased the zpc of the nanocomposite

(<6.8). This condition may be due to the presence of acidic

functional groups present on GO making the zpc of the

nanocomposite more effective for degradation in the weakly

acidic range. Below and above this pH range, the degradation was

less. The presence of deactivating nitro group in NB made the

degradation tendency minimum in highly acidic/alkaline range.

In 4-NP, the hydroxyl group is a strongly activating group

substituted in para – position to the nitro group and strongly

activates the aromatic ring for attack of electrophilic hydroxyl

radical [44]. These radicals were considered as the predominant

oxidant species in the alkaline pH range [45]. As –OH group is a

para-directing group and –NO is a meta-directing group in

nature, both can influence the position of attack by hydroxyl

radical [46]. In 4-NP, both groups direct the radical to ortho –

position, and forms nitrocatechol or nitrohydroquinone

intermediates and leads to the formation of degradation products

[47]. Further, in the acidic medium, the perhydroxyl radical can

form hydrogen peroxide giving rise to the more reactive oxidant

species, hydroxyl radicals and degrades the pollutants.

In acidic medium, 4-NP exists in protonated form (λmax=320

nm) and in alkaline medium, it exists in deprotonated form

(λmax=405 nm) as shown in Figure 9 [4 , 41]. The surface of the

Figure 6 (d): Effect of 10 % GO –NT composite on the absorbance of 4-

GO – nanotitania composite may be positively charged in the

acidic medium (pH<6.8) and negatively charged in the alkaline

medium (pH >6.8).In this view, the negatively charged form of 4-

NP in its highly intense deprotonated form may adsorb strongly

on the surface of the nanocomposite in the acidic pH range.

Correspondingly, its protonated form might have associated with

the nanocomposite in the alkaline pH range. These conditions

might degrade 4-NP efficiently in the pH range of 6.5 to 7.5 as

shown in Figure 8 (b).

Figure 7: % Degradation efficiency of NB & 4-NP using x % GO-NT

composites

Its surface is positively charged in acidic range (pH<Pzpc)

and negatively charged in alkaline range (pH>Pzpc) [40]. Non –

ionizable compounds, like NB, will be better adsorbed on

uncharged catalyst surface and their degradation rate will achieve

maximum value near the zpc of the photocatalyst [41]. NB does

Figure 8 (a): Effect of pH on the degradation of NB

not have neither hydroxyl group (-OH) nor amine group (-NH ),

which can ionize at different pH conditions. The effect of pH on

the photocatalytic degradation of NB was expected to be minimal

was reported by few researchers [42]. However, in some studies,

effective degradation tendency of NB was reported in a detectable

pH range depending on the photocatalyst incorporated [43]. In the

present work, the degradation efficiency of NB was observed to

be effective in the pH range of 5.0 – 7.0 (Figure 8 (a)). The zpc of

.3.4 Effect of Temperature

The process of photocatalytic degradation is generally not

highly temperature sensitive [48]. However, its impact on the

degradation of organic pollutants needs to be estimated as a part

of industrial applications. The effect was studied for the

degradation of NB & 4-NP in the temperature range of 20°C to

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2021, Volume 9, Issue 1, Pages: 183-191

0°C with 10 mg of 10 % GO – NT composite dispersed in 10

of 10 % GO – NT composite was dispersed in 100 mL aqueous

solutions (10 ppm) of the organic pollutants and irradiated under

visible light for 180 minutes. The experimental procedure for the

determination of COD was adopted from the earlier reports [32].

Also, the COD studies were performed under the optimized pH

and temperature conditions as established above.

ppm of 100 mL of aqueous solutions.

Figure 8 (b): Effect of pH on the degradation of 4-NP

Under the established pHconditions and the optimized

irradiation time, the studies were performed and the results were

displayed in Figures 10 (a) & (b) respectively. It was observed

that the % degradation efficiency of NB & 4-NP was maximum

in the range of 40°C to 50°C and 50°C to 70°C respectively. This

phenomenon of increased degradation tendency with increase in

reaction temperature can be attributed to the increased collision

frequency of the pollutants in the solution with increase in

temperature [48]. However, at higher reaction temperatures, the

solubility of dissolved oxygen will be decreased and lowers the

electron withdrawal tendency from the surface of the

nanocomposite [48]. This stage leads to decrease in the

degradation efficiency and the same was observed in case of NB

Figure 10 (a): Effect of temperature on the degradation of NB

4-NP respectively.

OH₂

pH<6.8

Figure 10 (b): Effect of temperature on the degradation of 4-NP

pH>6.8

The COD content of the samples was calculated using

Equation 2 and the % loss of COD was calculated using Equation

. Table 2 represents the results and it can be observed that there

was a significant decrease in the COD of both NB & 4-NP and

the % loss of COD was noticed as 95.7 % and 97.2 % respectively.

These results were in close agreement with the % degradation

efficiency of NB & 4-NP (from figure 5 and % degradation was

calculated using equation 1). These observations indicate that the

organic compounds were efficiently degraded by the composite,

along with mineralization.

Figure 9: Protonated and Deprotonated forms of 4-nitrophenol

.3.5. Evaluation of Mineralization

In addition to the analysis of photodegradation of N- organic

pollutants, the oxygen equivalent of organic matter in the

pollutants was evaluated separately in terms of chemical oxygen

demand (COD) to investigate the mineralization tendency by

ꢂOD = [x ꢃ y] ∗ ꢁ.2 ∗ 2ꢁꢁ mg/L

(2)

2 2 7

using dichromate (K Cr O ) method [49]. For the purpose, 10 mg

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2021, Volume 9, Issue 1, Pages: 183-191

•—

where x and y are the volumes(mL) of ferrous ammonium

sulphate consumed for the blank (DI) and the sample under study

respectively. For every, 1 mL difference between the titrations 0.2

mg of oxygen will be required by 5 mL sample and 200 is the

numerical deduced as calculation factors.

TiO

(e^-CB) + GO → GO (e ) + O

→ O

(5)

(6)

(7)

(h⁺VB) + H

O →TiO

+ H + OH

•

(h⁺VB) + OH^—→ TiO

+ OH^•

[

Cꢄꢅ]ꢆ−[Cꢄꢅ]ꢇ

•—

•

(

/ OH + NB/4-NP → intermediates → CO

+ H

O + HNO

ꢂOD loss =

(3)

[

Cꢄꢅ]ꢆ

mineralization products)

(8)

where [COD]

t (min) respectively.

and [COD]

are the COD (mg/L) atinitial and time

.3.7 Evaluation of products on mineralization

The dissociation pathway for the degradation of nitrobenzene

was pictured in Figure 12 (a). On reacting with the hydroxyl

radical (obtained from the GO-NT composite under light

illumination), a transition state could be formed in which the

electrophilic hydroxyl radical substitutes on the aromatic ring.

Immediately, it may stabilize itself through formation of isomeric

nitrophenols as disproportionate products. These products break

Table 2: COD studies for NB and 4-NP

Mineralization of NB & 4-NP

(

pH=5.0 to 7.0); pH= 5.5-6.0 & 8.5 -9.0)

Time

(

min)

COD (mg/L) % COD loss % Degradation

into the mineralization products of nitrobenzene like CO , H O,

2 2

HNO [53]. The mechanistic formation of degradation products

4-NP

of 4 – nitrophenol was portrayed in Figure 12 (b). On visible light

illumination, the hydroxyl radical substitutes the deactivating

nitro group and forms an intermediate oxy radical. It undergoes

rearrangement to forms para - hydroxy phenol and hydroquinone

as intermediate products which finally dissociates into

117.5 216.3

85.3

61.8

39.2

21.6

17.3

5.4

145.8

93.6

48.1

31.7

16.3

6.8

27.3

47.8

66.6

32.4

56.9

77.7

85.6

92.6

30.4

55.1

79.8

86.0

89.8

36.3

67.4

81.7

88.9

2 2 3

mineralization products of the pollutant like CO , H O, HNO or

HNO [54].

82.1

94.7

85.4

95.7

97.2

98.7

98.8

.3.6 Plausible Mechanism for Photocatalytic Degradation

In the phenomena of photodegradation of organic pollutants,

either photocatalytic oxidation or photosensitized oxidation

mechanisms were proposed [50]. Typically, nitrobenzene and 4-

nitrophenol are having their absorption maxima in the UV and

near visible region respectively. In addition, the degradation

studies were conducted under visible light active GO –

nanotitania composites and hence in the present work, a

photocatalytic oxidation has been proposed as a possible

mechanism for the degradation of the organic pollutants. On

visible light incidence, the electrons present in the valence band

of the nanotitania excites to the conduction band (e CB) leaving a

Figure 11: Illustration of photocatalytic degradation of NB & 4-NP using

positive hole in the valence band (h VB) [50]. In the absence of

10 % GO – NT composites under visible light irradiation

GO, these e /h pairs combine vigorously and results in a very low

photocatalytic activity [50]. The d-orbital (CB) of the nanotitania

and 휋-orbital of the GO interact and forms a d-휋 overlap which

can cause synergic effect [51, 52]. In this interaction, the excited

electron (e CB) in nanotitania can be shuttled easily along the

conducting network of GO surface thereby decreasing the e

h recombination in the nanocomposites. The negative electron

subsequently reduces oxygen to superoxide ions and the positive

hole in the valence band (VB) of nanotitania oxidizes water

molecules to hydroxyl radicals. These superoxide ions and

hydroxyl radicals would oxidize both NB & 4-NP. The schematic

illustrations were shown in Figure 11 and the possible

photocatalytic reactions were shown in equations 4 to 8.

Figure 12 (a): Possible products on mineralization of NB

TiO

+ hν (visible) → TiO

(e^-CB) + TiO

(h⁺VB

)

(4)

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2021, Volume 9, Issue 1, Pages: 183-191

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0 Lixia Y, Shenglian L, Yue L, Yan X, Qing K, Qingyun C. High

Efficient Photocatalytic Degradation of p-Nitrophenol on a Unique

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Figure 12 (b): Possible products on mineralization of 4-NP

Conclusions

The photocatalytic degradation of nitrobenzene (NB) and 4-

nitrophenol (4-NP) was investigated under visible light irradiation

with GO-NT composites. With an optimum increase in % of GO

in the composites, the degradation of the N-organic pollutants was

improved and maximum degradation was achieved with

nanotitania composite having 10 % wt of GO. Further, the COD

studies performed under the established pH and temperature

conditions has confirmed the photoefficiency of the composite by

degrading 95.7 % and 97.2 % of NB and 4-NP respectively. The

results of loss of COD and degradation efficiency were compared

and a close nearness was observed in both the studies.

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Acknowledgment

The author (RSS) thank the Management of Andhra

University, Visakhaptanam for providing facilites in the Organic

Chemistry Laboratory. And also Centre for Advanced

Instrumentation, NIT-Warangal for UV-Vis DRS analysisand PL

spectral analysis.

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degradation of nitrobenzene in aqueous solution by zero-valent iron.

Chemosphere.

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789-794.

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Competing interests

The authors declare that there is no conflict of interest that

would prejudice the impartiality of this scientific work.

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Authors’ contribution

All authors of this study have a complete contribution for data

collection, data analyses and manuscript writing.

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