Accepted Manuscript

Title: Synthesis, Characterization, and Photocatalytic Activityof N-doped Carbonaceous Material derived from Cellulose inTextile Dye Remediation

Authors: Bijay P. Chhetri, Dave Soni, Ambar Bahandur RanguMagar, Charlette M. Parnell, Hunter Wayland, FumiyaWatanabe, Ganesh Kannarpady, Alexandru S. Biris, AnindyaGhosh

PII: S2213-3437(17)30200-2DOI: http://dx.doi.org/doi:10.1016/j.jece.2017.05.010Reference: JECE 1613

To appear in:

Received date: 22-1-2017Revised date: 12-4-2017Accepted date: 4-5-2017

Please cite this article as: Bijay P.Chhetri, Dave Soni, Ambar BahandurRangu Magar, Charlette M.Parnell, Hunter Wayland, Fumiya Watanabe, GaneshKannarpady, Alexandru S.Biris, Anindya Ghosh, Synthesis, Characterization,and Photocatalytic Activity of N-doped Carbonaceous Material derived fromCellulose in Textile Dye Remediation, Journal of Environmental ChemicalEngineeringhttp://dx.doi.org/10.1016/j.jece.2017.05.010

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Graphical Abstract

2

Highlights

N-doped carbonaceous material of microcrystalline cellulose and urea was prepared.

The material was used in textile dye remediation via photocatalytic oxidation.

The material was highly efficient towards dye degradation under visible light.

The cellulose-derived material was recycled for multiple uses in dye remediation.

The mechanism of degradation and role of oxygen and radical quenchers are proposed.

3

Synthesis, Characterization, and Photocatalytic Activity of N-doped

Carbonaceous Material derived from Cellulose in Textile Dye

Remediation

Bijay P. Chhetri

1, Dave Soni

1, Ambar Bahandur Rangu Magar

1, Charlette M. Parnell

2, Hunter

Wayland1, Fumiya Watanabe

2, Ganesh Kannarpady

2, Alexandru S. Biris

2, Anindya Ghosh

1*

1Department of Chemistry, University of Arkansas at Little Rock, 2801 South University Avenue,

Little Rock, AR 72204, USA

2Center for Integrative Nanotechnology Sciences, University of Arkansas at Little Rock, 2801

South University Avenue, Little Rock, AR 72204, USA

*Corresponding author

E-mail: axghosh@ualr.edu, Phone: 501-400-4422, Fax: 501-569-8838

4

Abstract

N-doped carbonaceous materials were synthesized by pyrolysis of microcrystalline cellulose and

urea at 900C. The pyrolyzed materials were characterized via SEM, XPS, STEM/EDX, and

FT/IR. Microscopy images revealed wrinkled sheet-like morphology with stacked graphitic

layers, and elemental analyses confirmed 7.74% nitrogen content, which was evenly distributed

throughout the material. The characteristic peaks of C-N and C=N bonds in FT/IR indicated N-

atom incorporation in the material. The catalytic experiment with alcian blue 8Gx dye under

visible light showed higher degradation than in dark, which illustrated the photocatalytic nature

of the material. The pH conditions and material ratios were optimized during the photocatalytic

experiments. Various dyes were subjected to degradation photocatalytically using the

materials at pH 5.5 and recyclability studies indicated multi-cycle use of the material.

Additionally, kinetic studies of the photocatalytic degradation of indigo carmine showed that the

disappearance of the dye followed a first-order kinetics. The significance of adsorption

phenomena in the process of degradation was established by comparing the adsorption constant

(KLH = 0.898 Lmg-1

) calculated using the Langmuir-Hinshelwood model with that calculated

based on the adsorption isotherm model (KL = 0.2457 Lmg-1

). Degradation products were

characterized via various analytical techniques, including GC-MS, ion chromatography, FT/IR,

and total organic carbon analyses. Furthermore, the role of oxygen and radical quenchers were

studied and it was found that the main species responsible for dye degradation was

hydroxyl radical. N-doping of cellulose via a simple pyrolysis method has therefore been proven

to be effective in development of photocatalytic materials for pollutant mitigation.

Keywords: N-doped carbonaceous material, textile dyes, cellulose, photocatalytic activity,

visible light

5

1. Introduction

The release of chemical pollutants into the environment presents a vast array of problems

associated with public health. Among a large number of chemicals responsible for polluting

water, dyes are greatly abundant in the water system [1,2]. More than 100,000 dyes are

commercially available worldwide, with an annual production of 7 × 105 metric tons of dye

materials [3]. Azo dyes cover 50-70% of all known dyes available in market, and are frequently

used synthetic dyes for various applications [4]. Textile industries consume enormous quantities

of those dyes, with 90% used for dyeing fabrics [5]. Approximately 12-15% of the dye is wasted

during dyeing process and released as effluent into the water [6,7]. It is well known that most of

the dyes are highly toxic, mutagenic, and carcinogenic in nature. Thus, once released into the

water system, they cause undesirable effects upon the ecosystem and its inhabitants [8–10].

Many of these dyes are also very stable due to complex aromatic structure and are very difficult

to degrade or remove via conventional means, and they can be readily reduced to more

hazardous compounds such as aromatic amines, which are known to be highly carcinogenic

[11,12]. Removing these compounds from wastewater presents a new challenge in water

treatment processes.

The treatment of wastewater pollutants has attracted the attention of researchers in terms of

environmental, socio-economic, and technological potential [13]. There are several methods for

color removal from wastewater containing dyestuffs [14–17]. However, these techniques are

non-destructive, require high cost to operate, produce secondary pollutants, and have difficulty in

regenerating the adsorbent materials [18,19]. Alternatively, photocatalytic oxidation technique

appears to be an emerging technology in terms of cost, operation, and complete degradation of a

broad range of organic pollutants, including dyes [20]. Moreover, the degradation products are

6

simple molecules such as carbon dioxide, water, and mineral acids [21,22]. In this technology,

the photocatalyst is illuminated with a light of suitable wavelength, and as a result, an electron is

excited from its energy level leaving a hole in the valence band. The electrons are promoted to

the conduction band of the photocatalyst, and consequently give rise to electron-hole pairs

[23,24]. This electron-hole pair reacts with initiators such as oxygen, hydrogen peroxide, and

water to generate highly reactive hydroxyl radical species. These reactive species are powerful

oxidants and react quickly with organic pollutants adsorbed on the photocatalyst surface,

resulting in complete decomposition of toxic pollutants into harmless small compounds [25,26].

Carbon-based materials have been studied over the past few decades because of their

potential applications in many areas such as supercapacitors, sensors, and catalysis [27-29].

However, further modifications of their fundamental properties are essential to meet the rapidly

increasing demand for their applications. In this context, chemical modification by doping with

heteroatoms such as nitrogen (N) is the most straightforward approach. Doping with N alters the

diverse properties of the carbon materials [27,28]. The presence of valence electrons on N brings

n-type semiconducting behavior to the existing material, significantly improving their activity

toward photocatalytic reactions [29,30].

N-doped carbonaceous materials are useful as catalysts for dye degradation, and much

attention has been devoted to the development of these materials from renewable polymers. They

have attracted attention because they potentially furnish good support materials for

photocatalytic remediation of dye pollutants from water; furthermore, researchers have proven

that cellulose-based composites are capable of treating both cationic and anionic dyes [31–33].

Cellulose, which is a type of renewable polymer, can be used for generating various types of

doped carbonaceous materials for different applications in treating of dye waste.

7

Cellulose, a major component of the plant cell wall, is a long chain polymer made up of

glucose units held together by 1,4-β-linkages. It is the most abundant, inexpensive, readily

available, and renewable polymer found in nature [34,35]. The hydroxyl groups present in

cellulose molecule are reactive and offer the opportunity for modification. The modified

cellulose can be further used to develop novel materials that have potential application in water

treatment. One such molecule used for further modification is urea, which can be used to simply

and inexpensively synthesize N-doped carbonaceous materials. Urea is a small compound which

is rich in nitrogen and readily available [36,37]. Urea itself has been pyrolyzed to synthesize N-

doped carbonaceous materials with unique graphite-like structures, which showed photocatalytic

activity toward pollutant remediation [38]. To the best of our knowledge, no study has been

performed regarding the synthesis of N-doped carbonaceous material using cellulose and urea by

pyrolysis and its photocatalytic study in organic pollutants remediation.

In this work, synthesis of N-doped carbonaceous materials was achieved by direct pyrolysis

of a cellulose and urea mixture at 900 C under an inert atmosphere. The photocatalytic nature of

the as-synthesized materials was tested for their ability to remove industrial textile dyes under

visible light conditions. Interestingly, the materials showed excellent photocatalytic activity and

achieved dye remediation from aqueous solution. Furthermore, mass ratio of cellulose and urea

mixture were optimized during material synthesis to identify the best mass ratio of material, and

pH conditions were varied to obtain the optimal pH for photocatalytic dye remediation. We

found N1 material (mass ratio of cellulose and urea = 2:1) yielded higher photocatalytic

degradation efficiency in removing dye from aqueous buffer solution at pH 5.5. The

photocatalytic treatment of various dyes demonstrated the higher degradation efficiency of the

synthesized N1 material and its capability in removing textile dyes from wastewater.

8

Furthermore, the recyclability study performed for five consecutive cycles proved the sufficient

stability of N1 material toward photocatalysis. The kinetics of the degradation of dye under

visible light was studied. It was found that the disappearance of dye followed a first-order

reaction and fit the Langmuir-Hinshelwood kinetic model. We also studied the kinetics during

dark adsorption, which can control the photodegradibility of dye. It was found that the adsorption

constant (KLH = 0.898 L·mg-1

) determined for the degradation of dye under visible light was

higher than that of dark adsorption (KL = 0.2457 L·mg-1

), which illustrated that the adsorption of

dye on the photocatalyst is essential prior to degradation. We also detected the intermediate

compounds generated during photocatalytic degradation of dye using various analytical

techniques. Results indicated that the dye degraded to small molecules and ions. Furthermore,

the photocatalytic study in the presence of oxygen and different radical quenchers demonstrated

that hydroxyl radical is the major species responsible for the dye degradation. Therefore, the

current study of the N-doped carbonaceous material derived from cellulose and urea proved to

have excellent potential towards dye remediation in wastewater treatment.

2. Experimental

2.1 General: All chemicals and solvents obtained were of analytical grade and used without

further purification. Microcrystalline cellulose and urea were purchased from Alfa Aesar (USA)

and Fischer Scientific (USA), respectively, and used as received. The average particle size of

microcrystalline cellulose was 90 µm. 1,4-benzoquinone and

bis(trimethylsilyl)trifluoroacetamide (BTSFA), were purchased from Sigma Aldrich (USA).

Solutions of KH2PO4 and K2HPO4, both 0.1 M, were used in the preparation of phosphate buffer

solution. The pH of the buffer solution was adjusted by addition of either 0.1 M HCl or 0.1 M

9

NaOH solution. The commercially available dyes were obtained and used as received. The dye

solutions were prepared by mixing the desired amount of dye in phosphate buffer solution.

[Insert Figure 1 Here]

2.2 Synthesis of N-doped carbonaceous materials: Synthesis of N-doped carbonaceous

materials were accomplished by simple solid state mixing of microcrystalline cellulose and urea.

During the synthesis, masses of cellulose and urea were changed in the ratio of 2:1, 3:8, and 1:1,

and were labelled as N1, N2, and N3, respectively. The mixture was placed in a crucible and

pyrolyzed at 900 C in a quartz tube furnace under a N2 atmosphere for 2 h and cooled. GSL-

1100X quartz tube furnace with a 30-segment temperature controller was purchased from MTI

Corporation, USA and used during sample pyrolysis. The black material was ground using a

mortar and pestle to obtain product in a fine powder form. A simple schematic for the

preparation of N-doped carbon carbonaceous material is presented in Figure 1.

2.3 Characterization: The material was characterized by scanning electron microscopy (SEM),

X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy

coupled/energy-dispersive X-ray spectroscopy (STEM/EDX), and Fourier transform infrared

spectroscopy (FT/IR). SEM images were recorded using JEOL (JSM 7000F Joel, USA) scanning

electron microscopy. XPS was performed on Thermo Scientific K-Alpha using Al Kα radiation

(1486.7 eV) with the X-ray spot size 200 μm for each sample. The base pressure in the analysis

chamber was typically 1 x 10-9

mbar. Samples were mounted to the sample mounting plate using

double-sided tape. All spectra were collected with the charge neutralization flood gun turned on.

The typical pressure during the analysis with the flood gun on was 2 x 10-7

mbar. The collected

data were processed using the Thermo Scientific Advantage XPS software package. The binding

10

energies were calibrated according to the C1s peak at 284.8 eV from the carbon composition of

the material sample. Mixed Gaussian-Lorentzian peak shapes and a Shirley/Smart type

background subtraction were utilized in the peak analysis/fitting. Nicolet 6700 Thermo Scientific

FT/IR instrument was used and spectrum was recorded in the range of 4000–400 cm-1

.

2.4 Photocatalytic study: Photocatalytic activity of N-doped carbonaceous material was

evaluated by performing the degradation of dyes under visible light. The photodegradation was

performed using a LED lamp (75W rated power, DC voltage) with an emission between 400 and

700 nm. In this study methylene blue, alcian blue 8GX, indigo carmine, eosin yellowish,

naphthol green B, orange IV, and methyl orange were used for degradation studies. In the

photocatalytic test, 10 mg of material was added to 50 mL (3 10-5

M) of the dye solution. Prior

to light irradiation, the mixture was sonicated in dark for 5 min and kept it for another 30 min to

reach adsorption/desorption equilibrium. During photocatalysis, 5 mL aliquot of the sample was

withdrawn every 30 min and centrifuged to remove any suspended doped materials. The sample

concentration was analyzed using an ultraviolet-visible (UV-Vis) spectrometer (Cary 5000

Spectrophotometer). The data were collected over the wavelength range 400 to 800 nm. The

degradation profile was recorded by monitoring the corresponding absorption spectrum of dyes

at its maximum absorption wavelength (λmax) and the degradation efficiency was evaluated by

Eq. 1.

Degradation efficiency =

------------ (Eq. 1)

where, C0 = concentration of dye at initial time ‘t0’

Ct = concentration of dye at a given time ‘t’

11

2.5 Analyses: In the photocatalytic degradation of dye, its breakdown leads to the formation of

intermediate products and, upon complete mineralization, converts into smaller molecules such

as mineral acids, CO2, H2O, SO42-

, NO3-, etc. Therefore, an attempt to identify the formation of

any intermediate compounds from the photocatalytic degradation of indigo carmine dye was

performed using GC-MS techniques. A Shimadzu (USA), GCMS-QP2010 Ultra, gas

chromatograph (Column; length, 30 m; inner diameter, 0.25 mm; temperature range, 320/350

C) integrated directly to the mass spectrometer used as a mass selective detector was employed

to collect the spectra. During GC-MS analysis, the sample suspension was collected by stopping

the reaction at various time intervals (30, 60, and 90 min) under light illumination. The sample

suspension collected was filtered (0.45 m Millipore filter device) to remove N1 material and the

liquid portion recovered was acidified with hydrochloric acid (HCl) to pH 1.5 before extraction

with diethyl ether. The extraction was performed three times using 10 mL of diethyl ether with

20 mL of acidified liquid solution and dried over anhydrous Na2SO4. After removing diethyl

ether under reduced pressure, the resulting solid residue was dissolved in 200 μL of

bis(trimethylsilyl)trifluoroacetamide (BTSFA) and 500 μL of acetonitrile was added before

injection into GC-MS. The sample preparation method employed for GC-MS analysis has been

described in detail elsewhere [39]. The concentration of ions (SO42-

and NO3-) and the quantity

of total organic carbon (TOC) mineralized in the final product was determined using ion

chromatography and TOC analyzer, respectively (American Interplex Corp., USA). For ion

chromatography and TOC analyses, the sample suspension was collected by stopping the

reactions after 2 h of light illumination.

12

The progress of the photocatalytic reaction and any changes observed in the FT/IR

spectra of indigo carmine dye due to the formation of intermediate compounds were studied.

Spectra were obtained using Nicolet 6700 Thermo Scientific FT/IR spectrometer equipped with a

DLaTGS detector and a XT-KBr beam splitter. The sample suspensions after photocatalytic

treatment with N-doped carbonaceous material were collected after a desired time of light

illumination, centrifuged, and filtered (Millipore filter device, pore size 0.45 μm). The liquid

portion of sample was collected and evaporated slowly at 50 C in dark to minimize the effect of

external light. The solid residue obtained was dried completely and stored in the dark prior to

FT/IR analysis. The FT/IR samples were prepared by using standard KBr pellet method.

3. Results and Discussion

3.1 Characterization

SEM images of N1 material are shown in Figure 2a and 2b. Figure 2a revealed a rough,

wrinkled-like morphology with some agglomeration (red circles). It also revealed that the

carbonaceous materials exist in multi-fold graphitic layers with stacking (Figure 2b). Further

characterization of N1 material was performed by STEM/EDX, and this analysis is presented in

Figure 2c-f showing elemental mapping for carbon, nitrogen, and oxygen. It can be observed that

all three elements were evenly distributed, yet the intensity of nitrogen distribution was found to

be low compared to oxygen and carbon. We believed this could be due to the low concentration

of nitrogen in the sample.

[Insert Figure 2 here]

XPS was performed to examine the elemental composition and the chemical state of the

elements that exist within the material (Figure 3). The XPS spectrum of the survey scan revealed

13

the presence of carbon, nitrogen, and oxygen (Figure 3a) elements in N1material. In the

spectrum, peaks with binding energy values of 284.3, 399.2, and 532.0 eV are the characteristic

peaks for carbon, nitrogen, and oxygen, respectively. The nitrogen content was found to be

7.74% and from the peak area of carbon and nitrogen, the atomic ratio of nitrogen to carbon in

N1 material was found to be 8.89%.

[Insert Figure 3 here]

The high-resolution XPS spectrum of C-1s for N1 material is shown in Figure 3b. The

spectrum shows two distinct peaks at 284.7 and 285.9 eV, which are due to graphite-like sp2

C=C and sp2 C=N type bond. The appearance of other spectral peaks at 287.4 and 289.1 eV are

attributed to the formation of C-N-C and C-O type bond, respectively, confirming the nitrogen

doping within N1 material [40,41]. The high-resolution XPS spectrum of N-1s is presented in

Figure 3c. The spectrum was further deconvoluted into four different regions and provided the

type of nitrogen present from the dopant. The peak at 397.8 eV is due to pyridinic type N, 399.7

eV is indicative of pyrrolic N, and the peaks at 402.4 and 404.5 eV are due to oxidized N (N-O)

[42,43]. These results, further, illustrates nitrogen being successfully incorporation into N1

material. N atoms can replace carbon in the five or six membered aromatic rings where there is a

possibility of a condensation process taking place during the carbonization of cellulose at

temperatures above 400 C [42,44]. Figure 3d represents the high resolution XPS spectrum of O-

1s. It shows the presence of adsorbed oxygen at 530.9 eV and O-C-O at 532.5 eV [45].

FT/IR analysis provides the information about the different types of functionalities present in

the N1 material (Figure S1, Supporting Information). There are several peaks were observed in

the region of 1200-1800 cm-1

. The peaks at 1260, 1384, and 1457 cm-1

are due to C-N stretching

14

frequency [46–48]. The peak at 1610 cm-1

is due to C=N stretching mode in the aromatic system

[46]. The presence of C-N and C=N bonds indicates that N atoms have been incorporated in the

structural network of the graphitic carbon. The peaks at 1450 and 1700 cm-1

are due to C=C and

C=O bonds present in the graphitic carbon aromatic ring and adsorbed carbon dioxide,

respectively [49,50]. Similarly, the peak band at 3433 cm-1

is due to the adsorbed free water

molecules and strongly bonded –OH group in the carbon substrate [50].

3.2 Photocatalytic study

3.2.1 Dye degradation under different conditions: Initially, alcian blue 8GX dye was chosen

to examine the photocatalytic activity of the N1 material. All other dyes were studied in a similar

fashion. The UV-Vis spectra of alcian blue 8GX dye degradation in the presence of N1 material

under visible light is shown in Figure S2 (Supporting information). It was clear from the data

that the absorbance of dye at 603 nm decreases with the increase in irradiation time in the

presence of a catalyst (indicated by the black arrow in Figure S2). Control experiments with the

same dye were carried out to confirm the photocatalysis. They were further used to compare the

degradation efficiency. The change of absorbance for alcian blue 8GX dye at 603 nm at different

time intervals was presented in Figure 4a.

[Insert Figure 4 here]

The dye solution, without N1 material, when irradiated with the visible light was found to

be quite stable. The experiment, when carried out in dark with N1 material in solution, only

showed 24% dye removal after 180 min. This was due to the adsorption of dye on the

carbonaceous materials. However, when the same solution with N1 material in it was irradiated

in the presence of light, approximately 59% removal of dye was achieved. From this result, it

15

was clear that dye decolorized at an appreciable rate in the presence of N1 material under visible

light. The result also indicated the viability of the material for the photocatalytic treatment of the

dyes from the aqueous solutions. A previous study on the degradation of alcian blue 8GX dye on

a semiconductor photocatalyst (ZnO and TiO2) showed pollutant removal within a short time of

light illumination. However, in their study a UV-light source was used for activating the

photocatalyst, which is not desirable. [51].

3.2.2 Effect of mass ratio of carbon and nitrogen sources in photocatalytic activity: To study

the effect of cellulose and urea, three different N-doped carbonaceous materials were synthesized

(N1, N2, and N3) and tested for the photocatalytic degradation of the alcian blue 8GX dye. The

results are presented in Figure 4b. As observed, the rate of degradation of dye is high when the

mass of cellulose is doubled compared to mass of urea (mass ratio of cellulose and urea = 2:1).

Degradation efficiency of three N-doped carbonaceous materials follow the order: N1 > N3 >

N2. The higher degradation efficiency of N1 material in the removal of dye proved the

significance of cellulose in developing doped carbonaceous materials. Therefore, we optimized

our experiment with N1 carbonaceous material for further testing with pH and other dyes.

3.2.3 Effect of pH on the photocatalytic degradation: pH is an important parameter

influencing the photocatalytic degradation of dye. pH affects the surface properties of the

photocatalyst, breakdown of dye molecules, and hydroxyl radical generation [52]. Here, we

studied the effect of pH in the degradation of alcian blue 8GX dye by N1 material in the range of

4.0 to 6.4 (Figure 5a). It was observed that the rate of photocatalytic degradation of dye was

affected when the pH of the solution was changed. Degradation of dye by N1 under visible light

16

was observed high on changing pH from 4.0 to 5.5. However, in alkaline pH the degradation rate

decreased. This phenomenon can be explained on the fact that in slightly acidic pH solution,

there were more interactions between the cationic dye and the hydroxyl radicals resulting in

higher rate of degradation of dye. At alkaline pH, these cationic dyes tend to convert into the

neutral molecules, resulting in fewer interactions between the dye.

3.2.4 Recyclability: The stability of the photocatalyst and its reusability are important

parameters in the use of these materials for organic pollutant removal. The longer life time and

repeated use of the catalyst significantly reduce the cost of the water treatment. Therefore, it is

necessary to determine whether the photocatalyst can be reused. For this reason, N1 material was

recycled for five consecutive cycles in alcian blue 8GX dye degradation. The recycling

experiment was performed in the optimized conditions with 10 mg of N1 material in 50 mL

alcian blue 8GX dye solution in phosphate buffer of pH 5.5. After the first photocatalytic

degradation of the dye in the visible light for 180 min, the material was recovered by centrifuge

and regenerated by washing with acid (1 M HCl) and hot water several times. Once dried, the

material was used to study the recyclability with a new identical batch of the alcian blue 8GX

dye solution. The efficiency of the material as a photocatalyst was determined in term of

percentage removal and compared between the cycles (Figure 5b). The results obtained

demonstrated the reusability of N1 material. The first cycle removed 60% of the dye with a slight

decrease in efficiency in the subsequent cycles. We believe this decrease is most likely due to the

repeated washing steps, which can cause new areas of the catalyst surface to become unavailable

for dye adsorption and photon absorption.

[Insert Figure 5 here]

17

3.2.5 Photocatalytic study of different dyes: The photocatalytic study using N1 material was

performed for different textile dyes such as methylene blue, methyl orange, orange IV, naphthol

green B, indigo carmine, eosin yellowish under similar conditions as mentioned before. The pH

chosen for the experiment was pH 5.5 and the concentrations were maintained at 3 × 10-5

M for

all dye solutions. Samples of the dyes were evaluated by measuring the absorbance at their

corresponding maximum absorbance wavelength. To examine the degradation profile of dyes at

different times, absorbance values were plotted against the time (Figure S3, Supporting

Information). The degradation efficiency in the removal of the dyes was calculated by using Eq.

1 and the results are summarized in Table 1. As illustrated, the excellent photocatalytic activity

of the prepared material is apparent, reaching 98% degradation efficiency. Comparatively, our

photocatalytic removal of methylene blue (95%) with N1 material under visible light showed

enhanced performance than the photocatalytic degradation of the same dye performed under UV

light with nanocrystalline ZnO/cellulose photocatalyst [53]. The UV-Vis spectra show an

apparent decrease in methylene blue, indigo carmine, and eosin yellowish at 667 nm, 602 nm,

and 517 nm, respectively, at 180 min (Figure S4, Supporting Information). The decrease in

absorbance with time indicated that the material displayed efficient photocatalytic oxidation in

dye degradation.

[Insert Table 1 here]

3.3 Kinetic study with indigo carmine dye

3.3.1 First-order and second-order kinetic models: To investigate kinetics involved in the

photocatalytic dye degradation, we employed the two commonly applied kinetic models (first-

order and second-order kinetic models). The linear transform of first-order and second-order

18

kinetic models are given by Eq. 2 and Eq. 3, respectively [54].

ln(C0/Ct) = k1t ------------------ (Eq. 2)

1/Ct – 1/C0 = k2t ----------------- (Eq. 3)

where C0 and Ct are the concentrations of dye at initial time ‘t0’and a given time ‘t’, k1 and k2 are

first-order and second-order rate constants, respectively. Figure S5 (Supporting information)

shows the kinetic plots for the degradation of indigo carmine dye using these two models. It can

be seen from the Figure S5 that linear correlation coefficient (R2) values of first-order and

second-order kinetic models are 0.99 and 0.84, respectively. From this linear regression analyses,

it can be concluded that degradation of indigo carmine dye by N1 material under visible light

followed the first-order kinetic model.

3.3.2 Equilibrium dark adsorption kinetic: During the photocatalytic degradation, the

adsorption of dye on the photocatalyst surface was found to influence the rate of degradation.

The adsorbed dye molecules behave as an electron donor [55]. Upon light illumination, it injects

electrons from an excited state to the conduction band of the photocatalyst. Previous literature of

various types of organic pollutants have shown that the adsorption equilibrium constant obtained

using Langmuir-Hinshelwood kinetic model is generally different of the adsorption equilibrium

in dark [55–58]. To compare and evaluate equilibrium constants, dark adsorption kinetics of

indigo carmine dye on the photocatalyst was also performed. The Langmuir adsorption kinetic

model, which is defined by Eq. 4, has been applied to fit the experimental data.

( – ) ---------------- (Eq. 4)

Where, Qe (mg g-1

) is the adsorbed quantity, is the difference between the initial

concentration and the equilibrium concentration, is the volume, and is the mass of the

19

photocatalyst material. At low concentration range and under equilibrium conditions, the

adsorbed quantity can be calculated from Eq. 5,

----------------- (Eq. 5)

Where, Qe is the adsorbed equilibrium quantity, Ce concentration at the adsorption equilibrium,

KL is the Langmuir adsorption constant for adsorption, and Qm is the maximal absorbable dye

quantity. This relation can be rearranged as,

----------------- (Eq. 6)

A plot of Ce/Qe versus Ce is shown in Figure 6a. The values of Langmuir adsorption

constant (KL) and maximum absorbable dye quantity (Qm) calculated from intercept with Ce/Qe

axis and slope of the straight line were 0.2457 L mg-1

and 12.14 mg g-1

.

[Insert Figure 6 here]

3.3.3 Photocatalytic degradation kinetic study: The initial concentration of dye plays a major

role on the rate of degradation. It was found that higher concentration of the dye gave reduced

kinetic rate [59]. To study the effect of initial concentration on the kinetic rate, photocatalytic

degradation experiments were performed with different initial bulk concentrations of indigo

carmine and a plot of ln(Co/Ct) versus t for all the experiments is shown in Figure 6b.

From the previous research and the numerous work done with photocatalytic degradation

of most organic pollutants [55,58,60–62], it has been proposed that the kinetic involved in the

degradation rate is described by first-order kinetics by the following expression.

r = -dc/dt = kapp C --------------------- (Eq. 7)

20

Upon integrating Eq. 7, we have Eq. 8.

lnC0/Ct = kapp t -------------------(Eq. 8)

Where, kapp is the apparent rate constant (min-1

) and is affected by dye concentration. C0 is the

initial concentration of dye before light irradiation and Ct is the concentration of time at time ‘t’.

The Figure 6b showed that the photocatalytic degradation of indigo carmine dye on the

photocatalyst surface followed first-order kinetic model. The apparent rate constant (kapp) values

for different initial concentrations of dye were determined from the slope of the linear plot and

are presented in Table 2. It can be observed that the apparent rate constant values decreased with

increasing the initial concentration, which showed that the lower concentration of indigo carmine

dye better followed the first-order reaction kinetics. Additionally, the initial rate (r0) of

photocatalytic degradation rate can be determined from Eq. 9. The effect of initial concentration

of indigo carmine on the initial rate of photocatalytic degradation is presented in Figure 7a.

--------------------- (Eq. 9)

[Insert Figure 7 here]

[Insert Table 2 here]

The Figure 7a indicates the rate of degradation increased sharply with the initial

concentration of dye until it reached a saturation value, and became independent of the higher

initial concentrations. This trend followed the Langmuir-Hinshelwood adsorption model

according to the following expression (Eq. 10),

-------------------- (Eq. 10)

Where (mg L-1

min-1

) is the initial rate of degradation of dye, (min-1

) is the apparent first-

order-rate constant, kc (mg L-1

min-1

) is the constant depending on other factors influencing the

21

photocatalytic process (temperature, intensity, etc.), (L mg-1

) the adsorption equilibrium

constant of dye on the catalyst, and Co is the initial concentration of indigo carmine dye. The Eq.

10 can be rearranged in such a way to derive the linearity of the data when plotted as the

reciprocal of initial rate constant versus initial concentration.

--------------------- (Eq. 11)

Figure 7b shows the linear relationship between the plot of 1/kapp and initial concentration

of dye (Co), which confirmed the Langmuir-Hinshelwood kinetic model for the initial rate of

degradation. The values kc and KLH calculated were 0.262 mg L-1

min-1

and 0.898 L mg-1

respectively.

3.5 Analysis of intermediate compounds: The degradation of dye under light illumination in

the presence of photocatalyst leads to the evolution of the different intermediate compounds. We

identified the intermediate compounds, formed during the N1-assisted visible light degradation

of indigo carmine. The intermediates formed during the process were analyzed by GC-MS and

identified by their ion fragmentation in the mass spectra. The most important intermediate

compounds evolved are listed in Table 3 and GC chromatogram of samples collected at different

times are provided in Figure S6 (Supporting information). Mass spectra of some acids, which

showed the plot of relative intensity against m/z were also included in the Figure S7-S8. It can

be predicted that some acids like malic, oxalic, amino-fumaric, and glycolic acids were first

formed quickly upon degradation of indigo carmine dye within 30 min of visible light

irradiation. Aromatic compounds like anthranilic acid and phenol were also detected after 30 min

of visible light illumination. These intermediate compounds upon further degradation formed

malonic, pyruvic acids which were subsequently converted into acetic and formic acids. In 90

22

min, short chain aliphatic acids of low molecular weight such as hexanoic and pentanoic acids

were detected along with formic and acetic acids. As generally observed, acetic acid requires a

longer time for mineralization [39]. The result of the end products from the complete

mineralization of acetic and formic acids will be CO2 and H2O.

[Insert Table 3 here]

The progress of the reaction on the photocatalyst N-doped carbonaceous materials was

further evaluated by FT/IR analysis. The FT/IR spectra of the samples collected at different time

intervals of visible light irradiation are illustrated in Figure S9. In the figure, the spectrum of

solid indigo carmine dye is represented by the trace (a). Several characteristic peaks can be

distinguished in the region between 1000 and 2000 cm-1

. The peaks at 1470 and 1403 cm-1

are

characteristic of aromatic ring (phenyl ring) vibration, the peak at 1633 cm-1

is due to C=O

stretching, whereas the peak at 1318 cm-1

was assigned to C-N group. A characteristic peak in

the region between 1100-1000 cm-1

was assigned to S=O stretch with the most significant peak

located at 1029 cm-1

. The peak due to the presence of C-S stretching vibration of sulfur

containing functional moieties can be found in the region 730-674 cm-1

. The peaks at 3454 and

3360 cm-1

are assigned O-H stretching from adsorbed water and N-H bond, respectively [39,62–

64]. All FT/IR spectra analyzed for samples collected after different time under visible light

irradiation is represented by traces (b)-(d), and compared with the spectra of solid indigo carmine

(trace a). With respect to the spectra of dye molecule, changes observed in the spectra of the

samples collected after variable time. This indicates the progressive degradation of adsorbed dye

molecules on the photocatalyst’s surface that took place with light irradiation. Particularly, the

absence of several peaks in the aromatic region, S=O stretching, and C-S stretching regions after

1 h of irradiation provided strong evidence that the dye first adsorbed strongly on the

23

photocatalyst surface before its degradation occurred in the chromophore region. The whole set

of bands shifted, broadened, and progressively reduced over time and almost disappeared after 2

h (trace d). The result indicated the adsorbed dye molecules on the N-doped carbonaceous

material partially or completely degrades under visible light illumination.

3.6 Formation of mineralization products: The formation of inorganic ions such as nitrate

(NO3-) and sulfate (SO4

2-) from the complete mineralization of 14 mg L

-1 (3 10

-5 M) of indigo

carmine dye was determined by ion chromatography. It was observed that both ions became

detectable and their concentrations in the final degradation solution after 2 h of illumination were

measurable. During the degradation experiment, the formation of nitrate arises from the photo-

oxidation process. The nitrate ions thus formed get desorbed and become detectable in the

solution. At the end of the experiment, the concentration of nitrate ion in solution measured was

found to be 0.14 mg L-1

. The existence of nitrate ion into the solution can be expected from the

nitrite ion oxidation which, in turn, was formed by the oxidation of ammonium ion [65]. It was

observed that the concentration of nitrate ions in the solution is twenty-six times less than the

expected value (3.72 mg L-1

). This provide additional information that either nitrogen containing

species remained adsorbed to the photocatalyst and/or the quantities of N2 or NH3 were produced

and transferred into the gaseous phase. In the case of sulfate ion concentration, its concentration

in solution was found to be 1.3 mg L-1

, which is four times less than the expected value of 5.76

mg L-1

. The observed phenomena indicate that sulfur atoms containing species are bonded

strongly to photocatalyst surface. It was also observed in the TOC analysis that 98% of the total

organic carbon was mineralized in the form of either CO2/CO after photocatalytic treatment of

indigo carmine by N1 material under visible light.

24

3.7 Role of reactive oxygen species in photocatalytic dye degradation: It has been proposed

that the light mediated degradation of organic pollutants using photocatalyst involves reactive

oxygen species such as hydroxyl radical, superoxide radical, and hydrogen peroxide. These

reactive oxygen species react with dye during photocatalytic oxidation process, which results in

complete decomposition of dye into small molecules (organic acids, CO2, H2O, and NH3 etc.)

[26]. The most important among the reactive oxygen species is the hydroxyl radical [66].

At first, we investigated possible role of oxygen in the dye degradation process. To

examine the role of oxygen, degradation of methylene blue dye was studied under excess supply

of oxygen. Comparative experiment under nitrogen-purged condition was also performed. The

dye solution (3 × 10-5

M) was subjected to visible light irradiation for 120 min with N1 material

and was continuously stirred. Prior to each experiment, the dye solution was purged with gas for

1 h with continuous purging until completion. As seen in Figure 8, the degradation efficiency

after 120 min in the oxygen-saturated condition is 96%, which is higher than that in nitrogen-

saturated condition (14%). This indicated that oxygen plays significant role in generating active

species for the photocatalytic process and most likely is responsible for delaying the electron-

hole pair recombination by trapping the electrons from the conduction band [67] during

photocatalysis.

[Insert Figure 8 here]

[Insert Figure 9 here]

Next, the possible formation of main reactive oxygen species, such as superoxide (O2-˙) and

hydroxyl radical (OH˙), under the photocatalytic condition and their role in the degradation of

25

dye were studied using radical quenchers. Compounds employed for this purpose were methanol,

which is a quencher of hydroxyl radical [68], and 1,4-benzoquinone, which acts as a quencher of

superoxide radical [69]. In the presence of these radical quenchers, the photocatalytic

degradation of methylene blue with N1 material was performed and the results are presented in

Figure 9. The concentrations of radical quenchers into the solution were 2 mM and 4.8 mM,

respectively for 1,4-benzoquinone and methanol. In Figure 9a, curve A represents the

degradation profile without any quencher, and B and C are degradation curves upon addition of

methanol and 1,4-benzoquinone, respectively. Methylene blue degradation rate was not affected

by the addition of 1,4-benzoquinone, which indicates that superoxide radical is not the likely

reactive oxygen species. However, when methanol was used, the degradation of the dyes was

significantly suppressed. This result indicated that the hydroxyl radical is the main oxidative

species and played an important role in the photocatalytic bleaching of the methylene blue dye

solution. In addition, the use of methanol as hydroxyl radical quencher had a suppressing effect

on the rate constant of methylene blue degradation. The lower rate constant of the methanol

addition experiment (0.0012 min-1

) compared to those of N1 (0.0168 min-1

) and 1,4-

benzoquinone (0.0136 min-1

) indicated that hydroxyl radical was a dominant oxidative radical.

4. Conclusions: Doped carbon materials were derived from pyrolysis of inexpensive and widely

available cellulose and urea. The various analysis techniques confirmed the formation of

graphitic carbon materials with N-atoms incorporated into the graphitic carbon structure. The

photocatalytic degradation of alcian blue 8GX dye in the presence of N-doped carbonaceous

materials was investigated under visible light irradiation. The experiments showed significant

photocatalytic activity compared to controls performed in the presence of catalyst in dark

26

conditions and in absence of catalyst in light. Different N-doped carbon materials have also been

synthesized by changing the mass ratio of cellulose and urea, and the optimized ratio material

was found to be N1 (cellulose and urea = 2:1). Photodegradation was also studied under different

pH conditions for alcian blue 8GX dye and revealed that the dye degrades more slightly acidic

pH. The synthesized doped materials have also been successfully explored for the photocatalytic

degradation of different textiles dyes and proved to be highly efficient. The recyclability test

performed for alcian blue 8GX dye indicated the sufficient stability and the long-term reusability

of the photocatalyst. The kinetic study on the phtocatalytic degradation of indigo carmine dye

with different concentrations was found to followed first-order kinetics. The dependence of

initial rate of degradation with the initial concentrations showed the good agreement with the

appropriate rate constants followed the Langmuir-Hinshelwood kinetic model. The adsorption

constant (KLH) obtained using this model was found to be 0.898 L mg-1

, which was significantly

higher from that deducted from Langmuir adsorption isotherm model (KL = 0.2457 L mg-1

). The

progress of the reaction, formation of the intermediate compounds, and the fate of the

intermediate compounds formed was monitored by FT/IR and GC-MS analyses techniques. The

partial or complete disappearance of the several peaks in the spectra indicated the degradation of

indigo carmine dye adsorbed on the N1 material occurred under visible light. The formation of

several aromatic acids, phenol, and aliphatic acids from the breakdown of dye were detected by

GC-MS. Ion chromatography analysis helped to detect the evolution of nitrate and sulfate ion in

the solution and their quantity were found to be measurable. TOC analysis showed that 98% of

total organic carbon was found to be mineralized and converted into CO2/CO after photocatalytic

treatment of dye by N1 material under visible light. Thus, the superior advantages of cellulose

such as abundance, biodegradability, and low cost make this N-doped carbon material a

27

promising candidate in photocatalytic remediation of various industrial organic pollutants

including dyes and in water purification on a larger scale.

Acknowledgement

The financial support from the Center for Advanced Surface Engineering, under the National

Science Foundation (Grant No. IIA-1457888), and the Arkansas EPSCoR Program, ASSET III is

acknowledged. HW acknowledges the National Science Foundation Graduate Research

Fellowship Program Grant No. 1547889.

28

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37

Captions for Figures

Figure 1: Schematic representation for the preparation of N-doped carbonaceous material from

cellulose and urea.

Figure 2: (a-b) SEM images of nitrogen-doped carbon carbonaceous N1 material and (c-d)

STEM image and the corresponding EDX element mapping image for C, N, ad O in the N1

material. The red circles in Figure 2a indicate rough and wrinkled morphology.

Figure 3: XPS spectra of N1 material (a) survey scan, (b) XPS of C-1s, (c) XPS of N-1s, and (d)

XPS of O-1s.

Figure 4: Degradation profile of alcian blue 8GX dye with N1 material; (a) degradation in

different reactions conditions, (b) Photocatalytic degradation under different mass ratios of

cellulose and urea. Experimental conditions: Dye (3 × 10-5

M), pH 5.5, 10 mg catalyst, 25 C.

Figure 5: (a) Photocatalytic degradation of alcian blue 8GX onto N1 material under different pH

condition, (b) Recyclability of the N1 material. Experimental conditions: Dye (3 × 10-5

M), pH

5.5, 10 mg catalyst, 25 C.

Figure 6: (a) Linear plot of Ce/Qe versus Ce based on Langmuir adsorption kinetic model, (b)

Linear plots of ln(Co/Ct) versus time for photocatalytic degradation of indigo carmine at different

initial concentrations.

Figure 7: (a) Effect of initial concentration of indigo carmine dye on the initial rate of

degradation, (b) Linear plot of 1/kapp versus Co based on Langmuir-Hinshelwood adsorption

kinetic model.

Figure 8: The photocatalytic degradation of methylene blue in oxygen-and nitrogen-saturated

conditions after 120 min light irradiation with N1 material. Experimental conditions: Dye (3 ×

10-5

M), pH 5.5, 10 mg catalyst, 25 C.

Figure 9: (a) Degradation curve of methylene blue with N1 material in the presence of 1, 4-

benzoquinone and methanol under visible light, (b) Effect on the reaction rate constant of

methylene blue in the presence of these radical quenchers. Experimental conditions: Dye (3 × 10-

5 M), pH 5.5, 10 mg catalyst, 25 C.

38

Captions for Table

Table 1: Percentage degradation of different textile dyes after 180 min under visible light with

N1 material at pH 5.5. Experimental conditions: Dye (3 × 10-5

M), pH 5.5, 10 mg catalyst, 25 C.

Table 2: Observed apparent rate constants values (kapp) and initial rate (ro) calculated for

different initial concentrations of indigo carmine dye.

Table 3: Main intermediates products identified during the visible light degradation of indigo

carmine using N1 material by GC-MS analysis.

39

Figure 1: Schematic representation for the preparation of N-doped carbonaceous material from

cellulose and urea.

40

Figure 2: (a-b) SEM images of nitrogen-doped carbon carbonaceous N1 material and (c-d)

STEM image and the corresponding EDX element mapping image for C, N, ad O in the N1

material. The red circles in Figure 2a indicate rough and wrinkled morphology.

41

Figure 3: XPS spectra of N1 material (a) survey scan, (b) XPS of C-1s, (c) XPS of N-1s, and (d)

XPS of O-1s.

42

Figure 4: Degradation profile of alcian blue 8GX dye with N1 material; (a) degradation in

different reactions conditions, (b) Photocatalytic degradation under different mass ratios of

cellulose and urea. Experimental conditions: Dye (3 × 10-5

M), pH 5.5, 10 mg catalyst, 25 C.

(a)

Ab

sorb

ance

Time (min)

(b)

43

Figure 5: (a) Photocatalytic degradation of alcian blue 8GX onto N1 material under different pH

condition, (b) Recyclability of the N1 material. Experimental conditions: Dye (3 × 10-5

M), pH

5.5, 10 mg catalyst, 25 C.

35

40

45

50

55

60

65

3.5 4 4.5 5 5.5 6 6.5 7

De

gra

da

tio

n %

pH

(a) (b)

44

Figure 6: (a) Linear plot of Ce/Qe versus Ce based on Langmuir adsorption kinetic model, (b)

Linear plots of ln(Co/Ct) versus time for photocatalytic degradation of indigo carmine at different

initial concentrations.

y = 0.0824x + 0.3352R² = 0.9977

0

1

2

3

4

0 10 20 30 40

Ce/Q

e(L

-1g)

Ce (mg L-1)

y = 0.0234x

R² = 0.99456

y=0.0181xR²=0.9931

y=0.0106xR²=0.97925

y=0.0137xR²=0.98526

y=0.0069xR²=0.9991

0

1

2

3

4

5

0 30 60 90 120 150 180

ln(C

o/C

t)

Time(min)

0.00002 M

0.00003 M

0.00004M

0.00005 M

0.00008 M

(a) (b)

45

Figure 7: (a) Effect of initial concentration of indigo carmine on the initial rate of degradation,

(b) Linear plot of 1/kapp versus Co based on Langmuir-Hinshelwood adsorption kinetic model.

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40

r o(m

gL

-1m

in-1

)

Co (mgL-1)

y = 3.8123x + 4.2491

R² = 0.99681

0

40

80

120

160

0 10 20 30 40

1/Kapp

Co (mg L-1)

(a) (b)

46

Figure 8: The photocatalytic degradation of methylene blue in oxygen-and nitrogen-saturated

conditions after 120 min light irradiation with N1 material. Experimental conditions: Dye (3 ×

10-5

M), pH 5.5, 10 mg catalyst, 25 C.

0

0.2

0.4

0.6

0.8

1

1.2

30 60 90 120

Ab

sorb

ance

Time (min)

nitrogen saturated

air saturated

oxygen saturated

0

20

40

60

80

100

120

nitrogen saturated

air saturated oxygen saturated

Degra

dation

%

Nitrogensaturated

Airsaturated

Oxygensaturated

Nitrogen saturated

Air saturated

Oxygen saturated

47

Figure 9: (a) Degradation curve of methylene blue with N1 material in the presence of 1, 4-

benzoquinone and methanol under visible light, (b) Effect on the reaction rate constant of

methylene blue in the presence of these radical quenchers. Experimental conditions: Dye (3 × 10-

5 M), pH 5.5, 10 mg catalyst, 25 C.

48

Graphical Abstract

49

Table 1: Percentage degradation of different textile dyes after 180 min under visible light with

N1 material at pH 5.5. Experimental conditions: Dye (3 × 10-5

M), pH 5.5, 10 mg catalyst, 25 C.

Dyes

Degradation (%)

Indigo carmine

98

Methylene blue

95

Eosin yellowish

90

Methyl orange

81

Orange IV

69

Naphthol green B

18

50

Table 2: Observed apparent rate constants values (kapp) and initial rate (ro) calculated for

different initial concentrations of indigo carmine dye.

Co (mg L-1

) kapp (min-1

) kapp 100 (min-1

) R2 ro (mg L

-1 min

-1)

9.33 0.0234 2.34 0.9945 21.83

13.99 0.0181 1.81 0.9931 25.32

18.70 0.0137 1.37 0.9853 25.62

23.32 0.0106 1.06 0.9793 24.72

37.31 0.0069 0.69 0.9992 25.37

51

Table 3: Main intermediates products identified during the visible light degradation of indigo

carmine using N1 material by GC-MS analysis.

Sample

collection

time

Intermediate

Compounds

Retention time

(min)

Molecular

weight

(g/mol)

Molecular formula

30 min Anthranilic acid 12.7 137 C6H4-(NH2)-COOH

Malic acid 8.7 134 HOOC-CHOH-CH2-COOH

Amino-fumaric acid 8.1 131 COOH-CH=C(NH2)-COOH

Oxalic acid 5.7 90 HOOC-COOH

Phenol 4.5 94 C6H5-OH

3-aminopropenoic acid 5.6 87 COOH-CH=CH-NH2

Glycolic acid 5.9 76 HOOC-CH2-COOH

Fumaric acid 5.2 116 COOH-CH=CH-COOH

60 min Pyruvic acid (2-

oxopropanoic acid)

8.4 88 CH3-CO-COOH

Malonic acid 8.6 104 COOH-CH2-COOH

90 min Hexanoic acid 2.6 116 CH3-(CH2)4-COOH

Pentanoic acid 1.2 102 CH3-(CH2)3-COOH

Acetic acid 1.6 60 CH3-COOH

Formic acid 1.4 46 H-COOH