Synthesis of carbon nanotubes grafted with PEG and its efficiency for the removal of phenol from industrial wastewater

Journal Pre-proof Synthesis of carbon nanotubes grafted with PEG and its efficiency for the removal of phenol from industrial wastewater Osamah A. Bin-Dahman, Tawfik A. Saleh

PII:

S2215-1532(19)30308-3

DOI:

https://doi.org/10.1016/j.enmm.2020.100286

Reference:

ENMM 100286

To appear in:

Environmental Nanotechnology, Monitoring & Management

Received Date:

18 November 2019

Revised Date:

23 January 2020

Accepted Date:

29 January 2020

Please cite this article as: Bin-Dahman OA, Saleh TA, Synthesis of carbon nanotubes grafted with PEG and its efficiency for the removal of phenol from industrial wastewater, Environmental Nanotechnology, Monitoring and amp; Management (2020), doi: https://doi.org/10.1016/j.enmm.2020.100286

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier.

Synthesis of carbon nanotubes grafted with PEG and its efficiency for the removal of phenol from industrial wastewater

Osamah A. Bin-Dahman 1*, Tawfik A. Saleh 2

2

of

Department of Chemical Engineering, College of Engineering and Petroleum, Hadhramout University, Mukalla, Hadhramout, Yemen Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia

ro

1



ur na

Highlights

lP

re

-p

*Corresponding author. E-mail address: [email protected]

A hybrid adsorbent was successfully prepared by grafting carbon nanotube (CNT) onto polyethylene glycol (PEG).



High adsorption efficiency of phenol (≈100%) was achieved by CNT/PEG

Jo

adsorbent at a contact time of 30 min.



Both the Langmuir and Freundlich isotherm models show a good fit to the adsorption experimental data.

1



The prepared hybrid material (CNT/PEG) demonstrated to be an efficient adsorbent for removal of phenol from water as well as for simultaneous removal of phenol heavy metals from industrial wastewater.

of

Abstract Carbon nanostructures have a great potential in various applications. They are considered

ro

as promising sorbents of contaminants owing to their unique physical and chemical behaviors. In this work, a hybrid adsorbent was successfully prepared by oxidizing

-p

carbon nanotube (CNT) and grafted with polyethylene glycol (PEG). The performance of the hybrid material (CNT/PEG) as an adsorbent was investigated for the phenol removal

re

in a batch system. The effects of contact time, initial solution pH, the initial concentration of phenol, and adsorbent dosage on the efficiency of phenol removal were investigated. The phenol adsorption by CNT/PEG was pH-dependent and under optimum conditions,

lP

high adsorption efficiency (≈100%) was achieved at a contact time of 30 min. The magnitude of 𝑅 2 acquired with the pseudo-second-order model is high (>0.99) which

ur na

indicates the process follows this model. Both the Langmuir and Freundlich isotherm models show a good fit to the adsorption experimental data. However, Freundlich model shows a better fit to the experimental results than the Langmuir model (𝑅 2 > 0.98). The prepared hybrid material (CNT/PEG) demonstrated to be an efficient adsorbent for removal of phenol from water as well as for simultaneous removal of phenol with

Jo

pollutants such as Cu, Hg, Cr, Fe, Co, Ni, Al and Pb from industrial wastewater. Keywords: Carbon nanotubes; polyethylene glycol functionalization; Adsorption; Phenol

1. Introduction

2

Natural sources of the accessible potable waters are threatened by organic pollutants including phenols. They are released due to various industrial events like petroleum refining, plastic, and phenolic resin productions, pharmaceutical, coal conversion, wood preservation, metal coating, and textile dyeing(Michałowicz and Duda, 2007; Hamid and Khalil-ur-Rehman 2009; Varsha et al., 2011). Phenolic compounds are highly carcinogenic and toxic pollutants (Saleh et al., 2018). Therefore, they are considered

of

noxious pollutants. The U.S. Environmental Protection Agency has determined the

between 0.5 and 1.0 ppm (El-Ashtoukhy et al., 2013).

ro

content of phenol in potable and mineral waters to 0.5 ppb and in wastewater discharges

-p

Numerous methods are used for the wastewater treatment including electrochemical

re

oxidation (Zhang et al., 2016; Pimentel et al., 2008; Polcaro et al., 2003), coagulation (Golbaz et al., 2014; Adhoum and Monser, 2004), solvent extraction (Vatai et al., 2009;

lP

Lafka et al., 2007), bioremediation (Liu et al. 2012; Luke & Burton 2001; Mella et al. 2017) and photocatalytic degradation (Teh and Mohamed, 2011; Mahvi and Maleki,

ur na

2010). However, these techniques characterized by their inherent shortcomings due to high operational costs and limited effectiveness (Radushev et al., 2008). Adsorption has been considered as one of the most attractive treatment options for phenol removal from contaminated water as a result of its simplicity, and good efficiency (Ma et al., 2013).

Jo

Various adsorbents such as activated carbon, fly ash, clay minerals, titanium oxide have been investigated for the removal of phenolic compounds from water (Saleh, 2016; AlMalack and Dauda, 2017; Wang, Gong, et al., 2019; Li et al., 2018; Hamidouche et al., 2015; Ye and Lemley 2009; Wang, Wang, et al., 2019).

3

During the last decades, carbon nanotubes (CNTs) have been considered by many researchers as good potential for phenols adsorption from waters (Yang et al., 2008; Lin and Xing, 2008; Chen et al., 2007). CNTs have outstanding features like large surface area, porosity, and functional groups (Pan and Xing, 2008). Recently polymeric nanocomposites have been attracted great interest from researchers as promising adsorbents for water treatment applications due to their good mechanical properties, high

of

surface area, adjustable surface characteristics, and low cost (AL-Hammadi et al., 2018).

ro

Several studies have been demonstrated that incorporating nanoparticles such as silica nanoparticles and CNTs into polymers improves the efficacy of the adsorbents (Ghorai et

-p

al., 2014; Mittal et al., 2015; Turco et al., 2018). The addition of nanoparticles into a

sorbents (Kango et al., 2013).

re

polymer matrix increases the surface area of the adsorbents and offers extra active sites as

lP

In the present study, the hybrid material of multi-walled carbon nanotubes and polyethylene glycol were prepared. CNT was selected because of its excellent physical

ur na

and chemical properties while the function of PEG in the composites was introduced to further enhance the efficiency of removal. The prepared hybrid material (CNT/PEG) showed high phenol adsorption capacity and can be used as an adsorbent for phenol

Jo

removal from water.

2. Experimental 2.1 Materials

Multi-walled carbon nanotubes (MWCNTs), with a diameter of 8-15 nm, length of 10–50 m, 230 m2/g specific area, and 95 % purity, were supplied by Nanostructured and

4

Amorphous Materials Inc, USA. Polyethylene glycol (PEG) with an average molecular weight of 20000 and 98% purity was purchased from Sigma–Aldrich Company, USA. 2.2 Surface oxidation of MWCNTs MWCNTs were mixed with nitric acid (69%, AnalaR grade) (≈1g CNT/10 ml) in a 500 mL three-neck round-bottomed flask with a magnetic stir bar. A reflux condenser is equipped to the center of the flask. The reaction mixture was refluxed at 120 oC for 48

of

hours with constant agitation. Then the mixture was allowed to cool. After that, the

ro

solution of acid-modified CNTs was filtered through a filter paper of 3µm porosity and washed with deionized water until neutral pH. Finally, the product was dried in a vacuum

-p

oven for 24 hours at 60 oC and crushed.

re

2.3 Grafting of MWCNTs onto polyethylene glycol

Polyethylene glycol was melted and then mixed with the oxidized CNTs in a ratio of

lP

1:10. The solution was heated and stirred under reflux for 10 min. At the end of this period, 3 mL of sulfuric acid was added. The solution was kept on the hotplate under

ur na

reflux with stirring at 80 oC for a further 6 hours. After that, the mixture was decanted in 0.250 L of toluene and then filtered. The product was washed with deionized water to remove any traces of unreacted PEG and to remove any excess of sulfuric acid until

Jo

neutral pH. The produced CNT/PEG left in a vacuum oven to dry overnight.

5

of ro

Figure 1: Illustration of the process of the preparation of CNT grafted with PEG.

-p

2.4 Instruments for Characterization

re

The prepared material was analyzed by Fourier Transform Infrared Spectroscopy (FTIR) using Nicolet 6700 FT-IR Spectrometer from Nicolet Instrument Corporation USA. The

lP

samples of FTIR analysis were prepared by grinding potassium bromide (KBr) with a very small amount of dried material. The produced powder is compressed to form a thin

ur na

pellet. A Scanning Electron Microscope (SEM) JEOL JSM-6610 LV coupled to energydispersive X-ray spectroscopy (EDX) was employed for the characterization of the surface morphology of the material.

Jo

2.5 Analytical measurement

The stock solutions of adsorbate were prepared; 1g of phenol (Sigma-Aldrich, >99%) was dissolved in 1 L of ultra-pure deionized water (Milli-Q Ultrapure water system, Millipore). Thereafter, working solutions of desired concentrations ranging from 5mg/L to 50mg/L were prepared in clean conical flasks by diluting an appropriate volume of the stock solution with an appropriate volume of DI water. Before every experimental run, 6

the initial concentrations were measured using UV spectrophotometer at a wavelength of 271nm. For experiments requiring pH adjustment, 0.1 M HCl (Merck, Suprapure) or 0.1 M NaOH (Merck, Suprapure) solution was used and the pH was ascertained by benchtop pH meter (A0057419, Hanna). 2.6 Treatment studies

of

Batch experiments were conducted in order to investigate the removal efficiency of

ro

CNT/PEG for phenol. For each experiment, 50ml of an aqueous solution of phenol in 250ml Erlenmeyer flasks was used to study the effect of contact time (between 0 and 60

-p

min), initial solution pH (between 3 and 10), the initial concentration of phenol, and adsorbent dosage (between 5 and 100mg). A 0.45 µm filter was used to obtain a filtrate

re

after every experimental run. UV spectrophotometer at 271 nm wavelength was used to

lP

analyze the concentration of phenol. To ensure repeatability, the mean of duplicates was used in the computation of percentage removal as well as adsorption capacity as per the

ur na

following equations (Liu et al., 2019): % 𝑝ℎ𝑒𝑛𝑜𝑙 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 =

(𝐶𝑖 − 𝐶𝑓 ) 𝐶𝑖

× 100

𝐴𝑑𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 = (𝐶𝑖 − 𝐶𝑓 ) ×

(1) 𝑉

(2)

𝑚

Jo

Ci (mg/L) and Cf (mg/L) stand for initial and final concentrations whereas V (mL) and m (mg) are the volumes of the aqueous solution containing phenol and mass of adsorbent, respectively.

7

2.7 Adsorption isotherms To design a suitable industrial adsorption system, it is crucial to know the distribution of molecules of adsorbate between the solution of adsorbate and the adsorbent as well as the interactions between both of them. Several adsorption isotherm models are applied to adsorption data to obtain the best-fit models. Application of isotherm models is based on

of

the general assumptions concerning the type of interactions between the adsorbate molecules and the adsorbent active sites, number, and nature of adsorbent layers involved

ro

in adsorbing the adsorbate. Langmuir, Freundlich, and Temkin are some of the empirical

-p

isotherm models mostly applied to single solute systems carried out in batch mode. Langmuir isotherm model (Gautam et al., 2014) is linearly expressed as: 𝑞𝑒

=𝐾

1

𝐶

+ 𝑞𝑒

𝐿 .𝑞𝑚

(3)

𝑚

re

𝐶𝑒

lP

Where KL in L/mg is the constant for sites of adsorption affinity on homogenous monolayer surface, qm in mg/g is the maximum monolayer adsorption capacity, qe in

ur na

mg/g is the amount of sorbate adsorbed, Ce in mg/L is the concentration of sorbate in solution at equilibrium. The values of both qm and KL are obtained from the slope of the line and intercept of a plot of Ce/qe versus Ce. Separation factor in Langmuir model is an important parameter given as:

Jo

1

𝑅𝐿 = (1+𝐾

(4)

𝐿 .𝐶𝑜 )

where CO (mg/L) is the initial concentration of the adsorbate. The value of R L is interpreted as follows: Value of 1 is an indication of linear adsorption, a value greater than 1 indicates an unfavorable adsorption process, between 0 and 1 means favorable

8

adsorption process while equal to 0 indicates irreversible adsorption process (Ayawei et al., 2017). The linear mathematical form of Freundlich isotherm model is: 1

𝑙𝑛 𝑞𝑒 = 𝑙𝑛 𝐾𝐹 + 𝑛 𝑙𝑛 𝐶𝑒

(5)

where KF (mg/g) represents Freundlich isotherm constant that indicates adsorption

of

capacity. The heterogeneity parameter, n, gives the description of adsorption intensity. A value of n equal to 1 indicates linear adsorption where all adsorption sites have the same

ro

sorption energies. When n is greater than 1, it implies a normal and favorable adsorption process and the higher its value, the stronger is the adsorption. Ce (mg/L) and qe (mg/g)

-p

are the adsorbate concentration in solution and amount of adsorbed adsorbate on the

𝑅𝑇 𝑏𝑇

𝑙𝑛 𝐾𝑇 +

𝑅𝑇 𝑏𝑇

𝑙𝑛 𝐶𝑒

(6)

lP

𝑞𝑒 =

re

adsorbent, respectively. Temkin isotherm model in its linear mathematical expression is:

bT (J/mol) represents Temkin isotherm constant for sorption heat, KT (L/g) is the binding

ur na

constant describing the highest binding energy while T (K) and R are temperature and gas constant, respectively. 2.8 Kinetics study

Jo

For an adsorption process, different mechanisms and factors control the rate of sorption of adsorbate on the adsorbent and these include mass transfer and chemical process. Therefore, to examine and validate the most probable mechanisms and potential ratecontrolling stages, adsorption experimental data are tested by different kinetic models. The linearized mathematical expression for the pseudo-first-order is represented as: ln ( 𝑞𝑒 − 𝑞𝑡 ) = 𝑙𝑛 𝑞𝑒 − 𝑘1 𝑡

(7) 9

qe and qt denote the amounts of phenol adsorbed in mg/g at equilibrium and at time t (min), respectively. The value of the rate constant k1 is calculated from the slope while that of parameter qe is derived from the intercept of the same graph of ln (qe−qt) against t. Pseudo-second order linear equation is applied as follows: 𝑡

=𝑘

1 2 2 𝑞𝑒

𝑡

+𝑞

(8)

𝑒

of

𝑞

where q and qe denote the amounts of phenol adsorbed (mg/g) by CNT/PEG at time t

ro

(min) and at equilibrium, respectively. Values of equilibrium rate constant k2 and

-p

parameter qe are derived from the slope and intercept of a plot of t/q against t.

3. Results and discussion

re

3.1 Characterization

lP

3.1.1 Infrared Spectroscopy Infrared Spectroscopy

In order to identify the existence of functional groups on the CNT surface, FTIR analysis

ur na

was conducted in the range of 400 to 4000 cm-1. Figure 2 shows the FTIR spectra of asreceived CNT and oxidized CNTC as well as CNT/PEG. The IR spectrum (Fig. 2a) for as-received CNT shows a bands at 3430 cm-1, that refers to the O–H stretch of the hydroxyl group attributed to the oscillation of carboxyl group, while the band at 2920 cmascribed to C–H stretching (asymmetric and symmetric mode)

of H–C=O in the

Jo

1

carboxyl group. It should be noticed that the presence of these functional groups the surface of as-received CNT can be owing to the partial oxidation of the CNT surfaces during the purification process. On the other hand, in the IR spectrum (Fig. 2b) of oxidized CNT, the new band at 1736 cm-1 is assigned to C=O stretch mode from carboxyl group (H–C=O), while the bands at 10

2361 and 2336 cm-1 is related with the O–H stretch mode from hydrogen-bonded – COOH (Davis et al., 1999). The band at 1560 cm-1 is associated to the carboxylate anion stretch mode (Ovejero et al., 2006). The presence of the bands indicates the formation of oxygen-containing sites on CNTs which confirms the success of the oxidation process. The band at 2922 cm-1 in Fig. 2c can be ascribed to the methylene group in PEG (asymmetric and symmetric stretching of C–H). The new band at a wavelength of 1085

of

cm-1 is owing to the C–O stretching of ester group. Increasing the signal strength of the

ro

band at 3427 cm-1 is associated with the O–H stretching vibrations of the hydroxyl groups

ur na

lP

re

-p

on the material.

Jo

Figure 2: FTIR spectra of as-received CNT (a), oxidized CNT (b) and CNT/PEG (c). 3.1.2 Scanning Electron Microscopy The morphology and the elemental analysis of the CNT, oxidized CNT, and CNT/PEG samples were characterized using Scanning Electron Microscopy–Energy Dispersive Xray Spectroscopy (SEM/EDX). Figure 2 shows the SEM images of the CNT, oxidized CNT, and CNT/PEG with chemical structures. The elemental composition of the

11

CNT/PEG sample displayed a relatively high proportion of oxygen than CNT and oxidized CNT samples, which designates the availability of more oxygen groups on the

Jo

ur na

lP

re

-p

ro

of

CNTs due to the functionalization process.

Figure 3: SEM images and EDX spectra of CNT (a), oxidized CNT (b) and CNT/PEG (c). 12

3.2

Adsorption Experiments

3.2.1

Effect of pH

Adsorption of phenol is affected significantly by the pH since the pH of the medium governor the amount of the electrostatic charges imparted by phenol. Consequently, the adsorption rate will differ with the pH of an aqueous medium (Saleh 2015a). Phenol

of

removal by CNT/PEG was investigated within the pH range of 3 to 10, Figure 4. The removal percentage of phenol increased from 80% to about 95% as the pH was raised

ro

from 3 to 4. However, there was a fairly constant removal percentage from a pH of 4 to 9.

-p

Thereafter, a decrease in removal efficiency was noticed from pH 9 to 10 (Abdelwahab and Amin, 2013).

re

The point of zero charges (PZC) of CNT/PEG was measured to be 3.5. This means that the material surface was positively charged below 3.5 while it was negatively charged

lP

beyond 3.5. Hence, the lower removal efficiency recorded at pH of 3 can be attributed to abundant positive charges on CNT/PEG surface resulting in static repulsion forces. On

ur na

the other hand, it is most likely that there is no electrostatic interaction between the negative charge of CNT/PEG and the phenol at any pH value above 3.5 (PZC) and below 10 since phenol exists as neutral molecules below its pKa of 10. Therefore, it is possible that the removal of neutral phenol molecules within pH of 4 to 9 is due to the existence of

Jo

dispersive interactions possibly between the aromatic rings of phenols and the basal plane of CNT/PEG replenished with a high π-electron density (Smets et al., 2016). The fairly constant removal efficiency at the pH range of 4–9 can be an indication of the CNT/PEG coating stabilizing and reducing the rolling-up of the CNT in aqueous solution (Bolto and Gregory, 2007; Liu et al., 2016). Meanwhile, the fall in removal efficiency above pH of 9

13

can be attributed to the repulsive forces between phenolate anions ion and the negatively

-p

ro

of

charged surface of CNT/PEG.

Figure 4: The effect of pH on the % removal of phenol (Conditions; initial concentration

Contact Time

lP

3.2.2

re

20 ppm, at room temperature, contact time 20 min, dosage 20 mg).

The influence of contact time was studied to find the optimum adsorption time in the

ur na

uptake of phenol from the aqueous solutions. Figure 5 depicts the effect of contact time on the adsorption of phenol by CNT/PEG, which was obtained under the varying time of 1 to 60 min at three different concentrations. CNT/PEG showed fast adsorption for the removal of phenol from aqueous solution. As time passed, the spaces in the fluffy

Jo

sponge-like structure got reduced as most of the spaces were already being occupied by phenol molecules, thereby resulting in slow adsorption rate observed between 10 and 30 min. Eventually, when they were completely saturated with phenol molecules around 30 min, the equilibrium was reached. Therefore, the optimum contact time was chosen to be 30 min. The removal of phenol was very quick within the first 10 min with about 85% phenol adsorbed at the adsorbent mass of 20mg. A complete adoption was observed at 14

around 25 min for 20 mg. The outstanding initial fast adsorption rate of phenol may be attributed to the large surface areas conferred on the CNT by PEG which forms a fluffy

re

-p

ro

of

sponge-like structure that allowed the accumulation of many phenol molecules.

Figure 5: Influence of contact time as well as adsorbent dosages on the removal of

lP

phenol. (Conditions; initial concentration 20 ppm, pH 6, at room temperature, contact

ur na

time 0-60 min, dosage 10, 15 and 20 mg).

3.2.3 Effect of CNT/PEG Dosage The effect of adsorbent dosage from 5 mg to 100 mg was studied during 10 min contact time which was selected due to high efficiency at first 10 min toward the fast removal of

Jo

phenol in a short time. As observed in Figure 6, there was an increase in phenol removal as the adsorbent dosage was increasing. A significant rise from 45 to 100 % can be observed as the dosage was increased from 5 to 100 mg. The maximum removal in 10 min contact time was obtained when 80 dosage was used.

15

The increase in phenol removal efficacy with more CNT/PEG dosage can be associated with the availability of an increasing number of adsorption sites when the amount of adsorbents was increased (Anbia and Ghaffari, 2009; Pang et al., 2019). However, it can also be noticed that the removal percentage was increasing at a decreasing rate because the number of phenol molecules remaining in the solution for adsorption by further increased dosage kept decreasing. In other words, the adsorption is tending towards

ur na

lP

re

-p

ro

molecules in the solution as the dosage increased (Saleh, 2015b).

of

equilibrium between adsorbed phenol molecules by CNT/PEG and unadsorbed phenol

Figure 6: Dosage affect Conditions; contact time 10 min, pH 6, volume 30mL at room

Jo

temperature.

3.3

Kinetics Study

In order to have a clear idea of the mechanism of the removal, adsorption data are fitted into pseudo-first-order and pseudo-second-order kinetic models. Fig. 7a displays the fit of the adsorption data on the pseudo-first-order model. The calculated values of the rate constant k1 and parameter qe for the pseudo-first-order model are presented in Table 1. 16

The relatively low value of R2 can be an indication that the adsorption of phenol on CNT/PEG is not well represented by the pseudo-first-order model. Fig. 7b displays the fit of the adsorption data on pseudo-second-order model. Values of equilibrium rate constant k2 and parameter qe, are provided in Table 1. The resulting fitted straight line gives an indication that pseudo-second-order model well represents the adsorption. Moreover, the high value of R2 (0.9995) and the similarity between the calculated qe and experimental

of

qe further support the validity of the model for the phenol adsorption on CNT/PEG. The

ro

adsorption of phenol on CNT/PEG is most likely monolayer with chemical processes

-p

controlling the adsorption rate.

Table 1: Kinetic model parameters for the adsorption of phenol by CNT/PEG at room

re

temperature.

Pseudo first order

Pseudo second order

k1(min-1)

qe(mg/g) (cal.)

R2

k2(g/mg.min)

qe(mg/g) (cal.)

R2

20

26.7

0.115

15.47

0.9907

0.013

28.17

0.9995

15

20.62

0.12

14.97

0.9926

0.014

21.98

0.9993

14.48

0.133

15.89

0.9819

0.011

16.13

0.9965

ur na

Jo

10

lP

qe(mg/g) (exp.)

Ci(mg/L)

17

of ro -p re lP ur na

Figure 7: (a) Pseudo-first order model, (b) Pseudo-second order model, agitation speed =

Jo

150 rpm, temperature = 298 K, pH = 6).

3.4 Isotherm Models There are mathematical models employed to define experimental data of adsorption isotherms. Figure 8 shows the plots of Langmuir, Freundlich, and Temkin isotherm

18

models. The summary of the parameters of the three models is listed in Table 2. Models of Langmuir and Freundlich show a good fit to the experimental data with correlation coefficients greater than 0.97, which means that the adsorption of phenol molecules by CNT/PEG can be explained by the two models. Based on the Langmuir model (Fig. 8a); it implies that there is a strong attachment of phenol molecules onto a limited number of adsorption sites with homogenous sorption energies present on a single surface layer of

of

CNT/PEG. The maximum adsorption capacity as calculated from Langmuir isotherm was

ro

21.23 mg/g. Furthermore, the value of separation factor, RL (0.025), which is between 0 and 1, indicating favorable adsorption process of phenol on the CNT/PEG.

-p

The equilibrium data were well described by both adsorption models though the

re

Freundlich model shows a better fit to the adsorption experimental data than the Langmuir model (Fig. 8b). The Freundlich isotherm defines reversible adsorption and is

lP

not limited to the monolayer formation; it takes into account the interaction between the adsorbed molecules. Moreover, the value of 1/n is ≈ 0.323 which indicates the favorable

ur na

adsorption, confirming the remarks from the Langmuir isotherm. The performance of CNT/PEG material was compared with other materials from literature and presented in Table 3.

Jo

Table 2: Isotherm models parameters for the removal of phenol by CNT/PEG. Langmuir isotherm constants

qm

Freundlich isotherm constants

Temkin isotherm constants

(mg/g)

kL (L/mg)

R2

1/n

n

KF

R2

bT(KJ/mol)

kT(L/g)

R2

21.23

1.963

0.974

0.323

3.09

13.36

0.983

0.587

1

0.953

19

of ro -p re lP ur na Jo Figure 8: The plots of Langmuir Isotherm (a); Freundlich Isotherm (b); Temkin Isotherm (c) for phenol removal by CNT/PEG (at pH of 6, room Temperature and 20ppm initial concentration of the phenol). 20

Table 3: Comparison of phenol adsorption performance of CNT/PEG adsorbent with

Material

Adsorption capacity from (mg g-1)

% Phenol Removal

Contact Time

Initial Phenol Concentration

Activated carbon (AC)

1.348 (Langmuir Isotherm)

75%

2 hr

2 mg L-1

50mg

(Ji et al., 2013)

Carbon nanotubes (CNTs)

1.098 (Langmuir Isotherm)

62%

2 hr

2 mg L-1

50mg

(Ji et al., 2013)

Fly ash (FA)

1.007 (Langmuir Isotherm)

60%

2 hr

2 mg L-1

Carbon nanofibers (CNFs)

0.842 (Langmuir Isotherm)

41%

2 hrs

2 mg L-1

50mg

(Ji et al., 2013)

Activated carbon from sawdust

0.022 (Langmuir Isotherm)

-

3 hr

-p

other materials from literature.

10 g L-1

(Larous and Meniai, 2012)

Graphene

18.6 (Experimental)

63.1%

50 mg L-1

1.7 g L-1

(Li et al., 2012)

CNT/PEG

21.23 (Langmuir Isotherm)

20 mg L-1

20mg

This work

≈30 min

of

ro

50mg

(Ji et al., 2013)

ur na

≈100 %

20 mg L-1

re

lP

48 hr

Adsorbent References dosage

3.5 Proposed Mechanism of phenol Immobilization Although the adsorption is a complex process involving more than one mechanism, it can be suggested that the adsorption mechanisms occur via (i) hydrogen bond interactions,

Jo

(ii) π–π interactions, and (iii) electrostatic interactions. Based on this, the removal of phenol is suggested to comprise the steps illustrated in Figure 9. Adsorption on the surface of the CNT/PEG adsorbent which contains oxygen-containing groups can form hydrogen bonds between the oxygen and hydrogen from phenol and adsorbent. This enhances the adsorption process through phenol interaction with oxygen-containing

21

groups on the adsorbent and the adsorption thus occurs on the entire surface and brings about a uniform distribution of adsorbed phenol on the surface of the CNT/PEG adsorbent. In addition, attractions such as π–π interaction would be formed between carbonyl groups of the modified CNT and aromatic rings of the adsorbed phenol

ur na

lP

re

-p

ro

of

molecules.

Figure 9: Proposed mechanisms of interactions between phenol and the adsorbent. 3.6 Evaluating CNT/PEG for the treatment of industrial wastewaters

The reported CNT/PEG was evaluated to adsorb pollutants from an industrial

Jo

wastewater matrix with a pH of 6. CNT/PEG, 100 mg, was exposed to the wastewater (20 mL). Two experiments were designed; in the first one 20 mL of the wastewater was treated with the CNT/PEG while in the second experiment 20 mL of the wastewater was spiked with 10 ppm phenol then treated with the CNT/PEG at 298 K for 60min. The concentrations of pollutants including phenol and metal ions were 22

analyzed and listed in Table 4. As it can be seen the concentrations of the pollutants was drastically reduced indicating the high efficiency of the prepared CNT/PEG to simultaneously remove several toxic ions. The high efficiency of the prepared CNT/PEG can be explained by the presence of several chelating motifs on CNT/PEG which enables the adsorbent to trap toxic metal ions including Cu, Hg, Cr, Fe,

of

Co, Ni, Al, Pb as well as phenol. The very impressive efficiency along with simultaneously metals removal conferred the newly prepared CNT/PEG a prestigious

-p

ro

place among many adsorbents.

re

Table 4: Pollutants concentrations in industrial effluents before and after treatment with CNT/PEG at 298 K. After treatment



95

19

21

6



9



102

23

31

2



Ni

19



Al

48


9

Pb

6



Cu Hg Cr Fe

Jo

Co

9

ur na

Phenol

Original sample spiked with phenol (10,000 μg L−1) and then treated with CNT/PEG

lP

Metal

Original Wastewater sample sample −1 treated with CNT/PEG (μg L ) (μg L−1)

(DL refers to detection limits)

23

4. Conclusion A hybrid adsorbent was successfully prepared by grafting CNT onto PEG. The CNT/PEG was investigated for phenol removal as well as for simultaneous removal of phenol with pollutants such as Cu, Hg, Cr, Fe, Co, Ni, Al and Pb from industrial wastewater. High phenol adsorption efficiency (≈100%) was achieved under the conditions of initial

of

concentration 20 ppm, a dosage of 20 mg, contact time of 30 min, pH of 6 at room temperature. Phenol was also removed completely in a short time with the use of 80 mg

ro

dosage. The pseudo-second-order equation showed the best correlation for the adsorption

-p

data. Both the Langmuir and Freundlich models demonstrated a good fit to the experimental results. The maximum adsorption capacity as calculated from Langmuir

re

isotherm is 21.23 mg.g-1. The adsorption of phenol by CNT/PEG adsorbent is a complex process involving more than one mechanism. The reported results indicated that the

ur na

Author statement

lP

material is a good adsorbent for phenol removal.

Dr. Osamah A. Bin-Dahman and Dr. Tawfik A. Saleh contributed to sample preparation, interpretation of the results and wrote the manuscript.

Jo

Conflict of interst

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgement 24

Authors would like to acknowledge Hadhramout University (Mukalla, Yemen) and King Fahd University of Petroleum & Minerals (Dhahran, Saudi Arabia) for their supports.

References

of

Abdelwahab, O., Amin, N. K., 2013. Adsorption of phenol from aqueous solutions by Luffa cylindrica fibers: Kinetics, isotherm and thermodynamic studies. Egypt. J. Aquat. Res. 39, 215–223.

ro

Adhoum, N., Monser, L., 2004. Decolourization and removal of phenolic compounds from olive mill wastewater by electrocoagulation. Chem. Eng. Process. Process Intensif. 43, 1281–1287.

-p

AL-Hammadi, S. A., Al-Absi, A. A., Bin-Dahman, O. A., Saleh, T.A., 2018. Poly(trimesoyl chloride-melamine) grafted on palygorskite for simultaneous ultratrace removal of methylene blue and toxic metals. J. Environ. Manage. 226, 358– 364.

lP

re

Al-Malack, M. H., Dauda, M., 2017. Competitive adsorption of cadmium and phenol on activated carbon produced from municipal sludge. J. Environ. Chem. Eng. 5, 2718– 2729. Anbia, M., Ghaffari, A., 2009. Adsorption of phenolic compounds from aqueous solutions using carbon nanoporous adsorbent coated with polymer. Appl. Surf. Sci. 255, 9487–9492.

ur na

Ayawei, N., Ebelegi, A. N., Wankasi, D., 2017. Modelling and Interpretation of Adsorption Isotherms. J. Chem. 2017, 1–11. Bolto, B., Gregory, J., 2007. Organic polyelectrolytes in water treatment. Water Res. 41, 2301–2324.

Jo

Chen, W., Duan, L., Zhu, D., 2007. Adsorption of Polar and Nonpolar Organic Chemicals to Carbon Nanotubes. Environ. Sci. Technol. 41, 8295–8300. Davis, W. M, Erickson, C. L, Johnston, C. T, Delfino, J. J, Porter, J. E. 1999. Quantitative Fourier Transform Infrared spectroscopic investigation humic substance functional group composition. Chemosphere. 38, 2913–2928.

El-Ashtoukhy, E. S. Z., El-Taweel, Y. A., Abdelwahab, O., Nassef, E. M., 2013. Treatment of petrochemical wastewater containing phenolic compounds by electrocoagulation using a fixed bed electrochemical reactor. Int. J. Electrochem.8, 1534–1550. 25

Gautam, R. K., Mudhoo, A. , Lofrano, G., Chattopadhyaya, M. C., 2014. Biomassderived biosorbents for metal ions sequestration: Adsorbent modification and activation methods and adsorbent regeneration. J. Environ. Chem. Eng.2, 239–259. Ghorai, S., Sarkar, A., Raoufi, M., Panda, A. B., Schönherr, H., Pal, S., 2014. Enhanced Removal of Methylene Blue and Methyl Violet Dyes from Aqueous Solution Using a Nanocomposite of Hydrolyzed Polyacrylamide Grafted Xanthan Gum and Incorporated Nanosilica. ACS Appl. Mater. Interfaces. 6, 4766–4777.

of

Golbaz, S., Jafari, A. J., Rafiee, M., Kalantary, R. R., 2014. Separate and simultaneous removal of phenol, chromium, and cyanide from aqueous solution by coagulation/precipitation: Mechanisms and theory. Chem. Eng. J.253, 251–257.

ro

Hamid, M., Khalil-ur-Rehman, 2009. Potential applications of peroxidases. Food Chem. 115, 1177–1186.

-p

Hamidouche, S., Bouras, O., Zermane, F., Cheknane, B., Houari, M., Debord J., Harel, M., Bollinger, J. C., Baudu, M., 2015. Simultaneous sorption of 4-nitrophenol and 2nitrophenol on a hybrid geocomposite based on surfactant-modified pillared-clay and activated carbon. Chem. Eng. J. 279, 964–972.

lP

re

Ji, L., Chen, W., Xu, Z., Zheng, S., Zhu, D., 2013. Graphene Nanosheets and Graphite Oxide as Promising Adsorbents for Removal of Organic Contaminants from Aqueous Solution. J. Environ. Qual. 42, 191–193. Kango, S., Kalia, S., Celli, A., Njuguna, J., Habibi, Y. and Kumar, R., 2013. Surface modification of inorganic nanoparticles for development of organic–inorganic nanocomposites—A review. Prog. Polym. Sci.38, 1232–1261.

ur na

Lafka, T. I., Sinanoglou, V., Lazos, E.S., 2007. On the extraction and antioxidant activity of phenolic compounds from winery wastes. Food Chem. 104, 1206–1214. Larous, S., Meniai, A.H., 2012. The Use of Sawdust as by Product Adsorbent of Organic Pollutant from Wastewater: Adsorption of Phenol. Energy Procedia. 18, 905–914.

Jo

Li, Y., Hu, X., Liu, X., Zhang, Y., Zhao, Q., Ning, P., Tian, S., 2018. Adsorption behavior of phenol by reversible surfactant-modified montmorillonite: Mechanism, thermodynamics, and regeneration. Chem. Eng. J. 334, 1214–1221. Li, Y., Du, Q., Liu, T., Sun, J., Jiao, Y., Xia, Y., Xia, L., Wang, Z., Zhang, W., Wang, K., Zhu, H., 2012. Equilibrium, kinetic and thermodynamic studies on the adsorption of phenol onto graphene. Mater. Res. Bull. 47, 1898–1904. Lin, D., Xing, B., 2008. Adsorption of Phenolic Compounds by Carbon Nanotubes: Role of Aromaticity and Substitution of Hydroxyl Groups. Environ. Sci. Technol. 42, 7254–7259. 26

Liu, P.R., Zhang, H.L., Wang, T., Yang, W.L., Hong, Y., Hou, Y.L., 2016. Functional graphene-based magnetic nanocomposites as magnetic flocculant for efficient harvesting of oleaginous microalgae. Algal Res. 19, 86–95. Liu, X., Ma, R., Wang, X., Ma, Y., Yang, Y., Zhuang, L., Zhang, S., Jehan, R., Chen, J., Wang, X., 2019. Graphene oxide-based materials for efficient removal of heavy metal ions from aqueous solution: A review. Environ. Pollut. 252, 62–73.

of

Liu, Y., Zeng, Z., Zeng, G., Tang, L., Pang, Y., Li, Z., Liu, C., Lei, X., Wu, M., Ren, P., Liu, Z., 2012. Immobilization of laccase on magnetic bimodal mesoporous carbon and the application in the removal of phenolic compounds. Bioresour. Technol. 115, 21–26.

ro

Luke, A. K., Burton, S. G., 2001. A novel application for Neurospora crassa: Progress from batch culture to a membrane bioreactor for the bioremediation of phenols. Enzyme Microb. Technol.29, 348–356.

-p

Ma, Y., Gao, N., Chu, W., Li, C., 2013. Removal of phenol by powdered activated carbon adsorption. Front. Environ. Sci. Eng. 7, 158–165.

re

Mahvi, A. H., Maleki, A., 2010. Photosonochemical degradation of phenol in water. Desalin. Water. Treat. 20, 197–202.

lP

Mella, B., Puchana-Rosero, M. J., Costa, D. E. S., Gutterres, M., 2017. Utilization of tannery solid waste as an alternative biosorbent for acid dyes in wastewater treatment. J. Mol. Liq. 242, 137–145.

ur na

Michałowicz, J., Duda, W., 2007. Phenols--Sources and Toxicity. Polish J. Environ. Stud. 16, 347–362. Mittal, H., Maity, A., Ray, S. S., 2015. Synthesis of co-polymer-grafted gum karaya and silica hybrid organic–inorganic hydrogel nanocomposite for the highly effective removal of methylene blue. Chem. Eng. J. 279, 166–179.

Jo

Ovejero, G., Sotelo, J .L., Romero, M. D., Rodríguez, A., Ocana, M. A., Rodríguez, G., Garcia, J., 2006. Multiwalled carbon nanotubes for liquid-phase oxidation. Functionalization, characterization, and catalytic activity. Ind. Eng. Chem. Res. 45, 2206–2212. Pan, B., Xing, B., 2008. Adsorption mechanisms of organic chemicals on carbon nanotubes. Environ. Sci. Technol. 42, 9005–9013. Pang, H., Wu, Y., Wang, X., Hu, B., Wang, X., 2019. Recent Advances in Composites of Graphene and Layered Double Hydroxides for Water Remediation: A Review. Chem. Asian J.

27

Pimentel, M., Oturan, N., Dezotti, M., Oturan, M. A., 2008. Phenol degradation by advanced electrochemical oxidation process electro-Fenton using a carbon felt cathode. Appl. Catal. B Environ. 83, 140–149. Polcaro, A. M., Vacca, A., Palmas, S., Mascia, M., 2003. Electrochemical treatment of wastewater containing phenolic compounds: oxidation at boron-doped diamond electrodes. J. Appl. Electrochem. 33, 885–892.

of

Radushev, A. V., Plotnikov, A. V. & Tyryshkina, V. N., 2008. Regeneration methods of decontamination of phenol-containing waste waters. Theor. Found. Chem. Eng. 42, 781–794.

ro

Saleh, T. A., 2015a. Isotherm, kinetic, and thermodynamic studies on Hg(II) adsorption from aqueous solution by silica- multiwall carbon nanotubes. Environ. Sci. Pollut. Res. 22, 16721–16731.

-p

Saleh, T. A., 2015b. Mercury sorption by silica/carbon nanotubes and silica/activated carbon: a comparison study. J. Water Supply Res. Technol. 64, 892–903.

re

Saleh, T. A., 2016. Nanocomposite of carbon nanotubes/silica nanoparticles and their use for adsorption of Pb (II): from surface properties to sorption mechanism. Desalin. Water Treat.. 57, 10730–10744.

lP

Saleh, T. A., Adio, S. O., Asif, M., Dafalla, H., 2018. Statistical analysis of phenols adsorption on diethylenetriamine-modified activated carbon. J. Clean. Prod. 182, 960–968.

ur na

Smets, K., De Jong, M., Lupul, I., Gryglewicz, G., Schreurs, S., Carleer, R., Yperman, J., 2016. Rapeseed and Raspberry Seed Cakes as Inexpensive Raw Materials in the Production of Activated Carbon by Physical Activation: Effect of Activation Conditions on Textural and Phenol Adsorption Characteristics. Materials (Basel). 9, 565.

Jo

Teh, C. M., Mohamed, A. R., 2011. Roles of titanium dioxide and ion-doped titanium dioxide on photocatalytic degradation of organic pollutants (phenolic compounds and dyes) in aqueous solutions: A review. J. Alloys Compd. 509, 1648–1660. Turco, A., Monteduro, A., Mazzotta, E., Maruccio, G., Malitesta, C., 2018. An Innovative Porous Nanocomposite Material for the Removal of Phenolic Compounds from Aqueous Solutions. Nanomaterials. 8, 334. Varsha, Y. M., Naga Deepthi, C. H., Chenna, S., 2011. An emphasis on xenobiotic degradation in environmental clean up. J Bioremed Biodegrad. 11, 1–10.

28

Vatai, T., Škerget, M., Knez, Ž., 2009. Extraction of phenolic compounds from elder berry and different grape marc varieties using organic solvents and/or supercritical carbon dioxide. J. Food Eng. 90, 246–254. Wang, W., Gong, Q., Chen, Z., Wang, W. D., Huang, Q., Song, S., Chen, J., Wang, X., 2019. Adsorption and competition investigation of phenolic compounds on the solid-liquid interface of three-dimensional foam-like graphene oxide. Chem. Eng. J. 378, 122085.

of

Wang, W., Wang, X., Xing, J., Gong, Q., Wang, H., Wang, J., Chen, Z., Ai, Y., Wang, X., 2019. Multi-heteroatom doped graphene-like carbon nanospheres with 3D inverse opal structure: a promising bisphenol-A remediation material. Environ. Sci. Nano. 6, 809–819.

ro

Yang, K., Wu, W., Jing, Q., Zhu, L., 2008. Aqueous Adsorption of Aniline, Phenol, and their Substitutes by Multi-Walled Carbon Nanotubes. Environ. Sci. Technol. 42, 7931–7936.

re

-p

Ye, P., Lemley, A. T., 2009. Adsorption effect on the degradation of 4,6-o-dinitrocresol and p-nitrophenol in a montmorillonite clay slurry by AFT. Water Res. 43, 1303– 1312.

Jo

ur na

lP

Zhang, C., Tang, J., Peng, C., Jin, M., 2016. Degradation of perfluorinated compounds in wastewater treatment plant effluents by electrochemical oxidation with Nano-ZnO coated electrodes. J. Mol. Liq. 221, 1145–1150.

29