Polyimide derived laser-induced graphene as adsorbent for cationic and anionic dyes

Accepted Manuscript Polyimide derived laser-induced graphene as adsorbent for cationic and anionic dyes Karthik Rathinam, Swatantra P. Singh, Yilun Li, Roni Kasher, James M. Tour, Christopher J. Arnusch PII:

S0008-6223(17)30874-6

DOI:

10.1016/j.carbon.2017.08.079

Reference:

CARBON 12328

To appear in:

Carbon

Received Date: 4 June 2017 Revised Date:

14 August 2017

Accepted Date: 31 August 2017

Please cite this article as: K. Rathinam, S.P. Singh, Y. Li, R. Kasher, J.M. Tour, C.J. Arnusch, Polyimide derived laser-induced graphene as adsorbent for cationic and anionic dyes, Carbon (2017), doi: 10.1016/j.carbon.2017.08.079. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Polyimide derived laser-induced graphene as adsorbent for cationic and anionic dyes Karthik Rathinam†¥, Swatantra P. Singh†¥, Yilun Li±, Roni Kasher ¥, James M. Tour±*, Christopher J. Arnusch¥* ¥

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Department of Desalination and Water Treatment, The Zuckerberg Institute for Water Research, The Blaustein Institutes for Desert Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990, Israel. ±

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† Authors with equal contribution

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Department of Chemistry, Department of Materials Science and NanoEngineering, SmalleyCurl Institute and NanoCarbon Center, Rice University, 6100 Main Street, Houston, Texas 77005, United States.

*Corresponding author. Tel: +972-8-656-3532. E-mail: [email protected] (Christopher J.

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Arnusch) Tel: +1-713-348-6246. E-mail: [email protected] (James M. Tour)

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Abstract: Laser-induced graphene (LIG) is fabricated on polyimide films directly by irradiation with a CO2 laser. This reagent-free method to synthesize graphene in a single step is applicable

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for many uses including water treatment technology. Here we demonstrated that LIG is an effective adsorbent for water treatment and observed removal of methylene blue (MB) and methyl orange (MO) from aqueous solutions. LIG powder was obtained by sonication of LIG

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that was scraped from polyimide films. Raman and X-ray diffraction analysis confirmed the graphene component in the material, while high-resolution scanning electron microscopy and

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atomic-force microscopy analysis indicated the presence of multilayered graphene sheets. LIG powder showed significant removal of MB and MO dyes from the aqueous solutions where hydrophobicity played an important role, but especially a high adsorption of the MB dye was seen. Adsorption of MB and MO on LIG followed a pseudo-second-order kinetic model and the

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maximum adsorption capacity (Freundlich) was 926 mg g-1 and 132 mg g-1, respectively. The adsorption process was fast and exothermic, which involved both π-π interaction and electrostatic forces as observed using Raman spectroscopy. The expedient solvent-free

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fabrication of LIG that is generated on surfaces might be an advantageous graphene-based

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adsorbent for water remediation.

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1. Introduction The use of dyes in the textile, paper, and printing industries is essential because of their broad range of color choices and their robust long-term color properties [1-3]. However, the lack

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of proper treatment of effluents that contain dyes causes water contamination, which is destructive to the environment. The presence of dyes or other recalcitrant organic compounds molecules in the environment leads to an unusable water supply, and underlines the importance

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of new treatment solutions. Numerous treatment techniques have been developed and adsorption is the most widely used technique for the removal of dyes from wastewater [4, 5]. Many

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materials have been tested as adsorbents for the removal of dyes from aqueous solutions including carbon nanotubes [6, 7], biopolymers [8], conducting polymers [9], activated carbon [10] and clay [11].

Recently, graphene and graphene-containing materials have gained much attention for a

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variety of applications [12-22] due to their fascinating physical and chemical properties, high surface area and functionalization possibilities [23]. Also, graphene derivatives have demonstrated to be efficient adsorbents for the removal of a variety of pollutants from water [16,

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17, 24-33]. Many of these studies use Hummers method for the synthesis of graphene [34]. This method uses many hazardous chemicals i.e., sulfuric acid, potassium permanganate, and NaNO3.

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Also toxic gases such as NO2 and N2O4 are released during synthesis, and wastewater containing Na+ and NO3- is generated, which sometimes complicates graphene purification [35]. These drawbacks motivated researchers to search for greener methods: Marcano et al. modified the Hummers method and excluded the use of NaNO3, which increased the yield, but used more KMnO4 and introduced H3PO4 [36]. Chen et al. excluded NaNO3 and generated less waste during synthesis and purification [35]. Recently however, Lin et al. demonstrated a one-step reagent-

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free process to print graphene on commercial polymers by a CO2 laser [37], and generated laserinduced graphene (LIG). This technique can be performed in air, without the need for solvents, and results in conformal carbon already built upon a polymer matrix, which can be advantageous

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compared to loose carbon. Another advantage includes the ability to pattern for arrays of purification systems or sensing devices. For adsorption, graphene shows a high surface area with low porosity leading to a moderate adsorption capacity [38]. Modifications can increase the

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porosity and adsorption capacity and selectivity for water contaminants, but add to the cost of the

parameters have not yet been reported.

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technology [16]. LIG is a new route to porous graphene-foam carbon upon which adsorption

LIG has been investigated for use in energy storing devices and supercapacitors [37, 3941] and recently in environmental and water treatment applications. For example, the surface properties of LIG ranging from superhydrophillic to superhydrophobic can be controlled by

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variation of fabrication conditions [42]. Also, an extremely low fouling surface property was observed, and LIG as electrodes were highly antimicrobial [43]. Therefore in the present study, to further elucidate water treatment applications, we investigated LIG as an adsorbent for

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contaminants in water and demonstrated the removal of cationic and anionic dyes from aqueous solutions. The LIG was well characterized and batch equilibrium methods were adopted to

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measure the adsorption of methylene blue and methyl orange. These dyes represent a class of common cationic and anionic organic pollutants in water. The parameters that significantly influence the adsorption capacity such as solution pH, adsorbent dosage, contact time and initial dye concentration were examined and optimized. Adsorption isotherms, kinetics and thermodynamic parameters were evaluated to explain the mechanism and feasibility of the adsorption process by LIG.

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2. Experimental 2.1. Chemicals Methylene blue and methyl orange were purchased from Sigma Aldrich (St Louis, MO).

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Sodium hydroxide and hydrochloric acid purchased from Bio-Lab (Jerusalem, Israel). Double distilled water (DDW) was used to prepare all the solutions. Technical grade acetone and ethanol were used for all the cleaning steps.

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2.2. Preparation of LIG from polyimide film

Laser induction was conducted on commercial polyimide (Kapton®, thickness 127 µm,

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McMaster-Carr, catalog No. 2271K6) film with an XLS10MWH (Universal Laser Systems) laser platform, equipped with a 10.6 µm CO2 pulsed laser (75 W). An image density of 1000 pulses inch-1 in both axes and a rastering speed of 30 cm s-1 were used for all experiments. A nozzle provided with the instrument was used to blow air towards the laser spot, while the general

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atmosphere within the laser platform was still air at ambient pressure. Laser duty cycles of 3% were used to prepare the LIG samples. LIG powder was prepared by scraping the surface layer of the LIG using a razor blade. The LIG powder was washed twice with DDW and acetone to

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remove any impurities by centrifugation (12000 × g for 30 min) and dried under vacuum. LIG powder suspensions (2 mg mL-1 in DDW water) were then bath-sonicated (D - 74224, Elma

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Singen) for 30 min to obtain a stable suspension. Then, this suspension was probe sonicated in ice bath for 120 min at high-intensity (VCX130, Sonics Vibra-cell) to achieve smaller LIG sheet sizes [44].

2.3. Characterization

High-resolution scanning electron microscopy (HR-SEM) images were taken with a FEI Quanta 400 ESEM. Transmission electron microscopy (TEM) characterizations were performed

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using a 200-kV JEOL 2100 Field Emission Gun TEM. Raman spectra were recorded with a Renishaw Raman RE01 scope with 633 nm laser. X-ray photoelectron spectroscopy (XPS) was performed on a PHI Quantera SXM scanning X-ray microprobe with 200 µm beam size and 45°

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takeoff angle, and calibrated using C 1s at 284.5 eV. The surface area of LIG was measured with a Quantachrome autosorb-3b BET surface analyzer. Atomic-force microscopy (AFM) images were captured by Cypher-ES (Asylum Research/Oxford Instruments) with an AC160TS

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(Olympus, resonance frequency of 300 kHz) and images were analyzed using Gwyddin software. X-ray diffraction (XRD) was conducted on a Rigaku D/Max ultima II with Cu Kα radiation (λ =

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1.54 Å). The crystalline sizes of LIG along c axis (Lc) and domain size in the axis (La) were calculated by using Equation 1 and Equation 2 respectively.

0.89 1

/ 2  


 =

1.84 2

/ 2 

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 =

where B1/2 (2θ) (in radian unit) is the full width and half maximum of peaks, and λ is X-ray wavelength (λ = 1.54 Å).

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2.4. Adsorption experiments

Methylene blue (MB) and methyl orange (MO) were selected as model dye pollutants for

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the adsorption study. Removal of (0 - 150 mg L-1) MB and (0 - 100 mg L-1) MO by LIG was carried out in tightly closed glass vials by adding a known amount of LIG at room temperature (298 ± 2 K) and agitated at 150 rpm. After variable contact times, the solution was centrifuged at 10000 rpm for 2 min and the supernatant was analyzed for the residual concentration of MB and MO using a UV-Vis spectrophotometer at 664 and 464 nm, respectively. Using this procedure, parameters varied included solution pH, LIG dosage, contact time and initial dye concentration.

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All the adsorption experiments were conducted at pH ~ 6.3 for MB and at pH ~ 7.4 for MO except when the pH parameter was investigated. Here, the pH of the solution was adjusted using aqueous solutions of NaOH (0.1 M) and HCl (0.1 M). Each adsorption experiment was repeated

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at least two times and the average values were reported. For the regeneration process, MB loaded LIG was suspended in 2 mL ethanol and the resulting mixture was agitated for 30 min at room

according to the following mathematical expressions: 

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temperature. The adsorption capacity (qe) and the adsorption efficiency were calculated

 =  −   × 3

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Removal % =  −  / × 100 4 where C0 and Ce are the initial and equilibrium concentration (mg L-1) of MB/MO dyes, V is the volume (L) of the MB/MO taken for the study and M is the mass of LIG (g) used for the

3. Results and Discussion

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experiment.

3.1. Characterization of LIG Powder

The morphology of LIG powder scraped and sonicated from the polyimide surface was

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investigated using SEM at low and high resolution (Fig. 1A and B, respectively). The LIG sheets

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identified in the SEM images ranged from a few nanometers to micrometers, and were further examined by AFM. Graphene sheets with a median height of 3.3 nm were observed, along with other medium or large particles (Fig. 1C, Fig S1 in supporting information), and indicated that the LIG powder contained a complex mixture of multilayer sheets. Multilayers of nano-shaped ripples of exposed edges of graphene layers are shown in the TEM image (Fig. 1D). This rippling structure could be caused by the thermal expansion caused by the laser irradiation [37]. The XPS spectrum (Fig. 1E) of the LIG shows the dominant carbon peak over oxygen and

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nitrogen. The carbon and oxygen were measured to be 95.1 % and 4.9 %, respectively. The XRD analysis of LIG showed a peak at 25.9° (2θ) for the 002 plane, and the second peak at 42.9° (2θ) corresponded to the 100 plane (Fig. 1F). The peak at 25.9° (2θ) corresponded to an

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interlayer spacing of 3.4 Å and indicated a high level of graphitization, and the peak at 42.9° (2θ) was associated with an in-plane structure. Using Eq.1 and Eq.2, the crystalline sizes Lc and La were calculated 1.6 nm and 5.8 nm, respectively. Raman spectra showed a D peak at ~ 1,350 cm, a G peak at ~ 1,580 cm-1 and a 2D peak at ~ 2,700 cm-1(Fig. 1G). These peaks are

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characteristic for graphene and the presence of the 2D peak supported the existence of the single

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layer graphene [37]. The Brunauer, Emmett and Teller (BET) surface area of LIG powders was calculated to be 98.3 m2 g-1. Taken together, the results confirmed the formation of graphene by

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3% laser power of a CO2 laser [37].

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Fig. 1. Characterization of LIG powder after 120 min sonication included SEM images of LIG powder at (A) low resolution and (B) high resolution; (C) AFM image; (D) TEM image of dispersed LIG powder; (E) XPS; (F) XRD and (G) Raman spectra of LIG powder. 9

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3.2. Adsorption of MB and MO on LIG powder The adsorption of contaminants to adsorbents can be affected by a number of parameters due to the chemical and surface properties of both the contaminant and adsorbent. The removal

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of ionic organic dyes using graphene includes electrostatic and π-π interactions although the predominant mechanism will vary depending on the contaminant [25]. The relatively low oxygen content (~5%) in the LIG powder compared to graphene oxide indicated that hydrophobic

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interactions with the contaminants could play an important role in the adsorption process. A comparison of the molecular hydrophobicity of MB and MO using the calculated octanol-water

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partition coefficient, logP was determined to be 0.09 and 0.17 respectively. These similar values suggest that the differences in adsorption were most likely due to other interactions. Thus, we performed a series of experiments that varied parameters such as the pH, adsorbent dosage, contact time and initial dye concentration. Firstly, we tested the ability of LIG to adsorb MB and

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MO dyes in aqueous solutions by varying the pH from 2.04 to 10 (Fig. 2A). The adsorption capacity of MB increased from 88 to 100%. Zeta potential analysis of LIG revealed a possible reason for the removal of MB (88%) even at low pH conditions (Fig. 2B). The zero point charge

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(pHzpc) of the LIG was measured to be pH ~ 1.9. Since the LIG surface would still contain negative charge at pH 2.04, electrostatic interaction would remain to be a factor in the binding of

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the dye at the lowest pH tested in addition to π-π interactions. The slight decrease in the MB removal at low solution pH values might be due to the competition between MB and H+ ions for the same adsorption sites [45]. On the other hand, in the case of MO, the sorption capacity increased with a decrease in solution pH due to the decreased electrostatic repulsion force between MO and the LIG surface. Also, the binding can be enhanced through π-π interactions due to the close proximity of MO to the LIG surface. Despite the negative surface charge of LIG

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in all the studied pH conditions, a significant removal of MO was nonetheless obtained at high pH and is advantageous compared to other adsorbents such as graphene oxide [31] that significantly lose the adsorption capability at high pH, and underlines the importance of the

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hydrophobic interactions of the LIG adsorbent.

Next we studied the effect of LIG concentration and competing ions on the dye adsorption. When the concentration of LIG was increased from 0.2 to 1.5 mg mL-1, the

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adsorption efficiency of both MB and MO increased from 69 to 99% and 53 to 89%, respectively (Fig. 2C). The higher dye removal at higher LIG concentrations was most likely due to the

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increased availability of the sorption sites with increased LIG amount, similar to other reported adsorbents of MB and MO dyes [17, 39, 46]. In natural water systems, inorganic ions such as Cl-, SO42-, NO3- and HCO3- are usually present and can compete with MB and MO for the same adsorption site on the LIG surface. However, the presence of these ions did not interfere with the

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adsorption efficiency of MB and MO dyes which indicated that LIG powders could be suitable for environmental applications (Fig. 2D).

The effect of contact time on the adsorption of MB and MO by LIG was investigated at

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room temperature with different initial concentrations of MB and MO dyes. The initial concentration of MB was varied from 50 - 150 mg L-1 while for MO the concentration was varied

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from 50 - 100 mg L-1. The removal of MB and MO dyes by LIG powder was very fast; the adsorption increased over time and reached equilibrium in ca. 15 min (Fig 3A and B). The adsorption kinetics was evaluated by employing three different kinetic models namely pseudofirst-order [17] pseudo-second-order [47] and intraparticle diffusion model [48]. The linear mathematical expressions (See Table 1) were used to calculate the values of the kinetic model parameters from their respective linear graphs (Table 2). Based on the correlation coefficient

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(R2) values, the adsorption process was best described by the pseudo-second-order kinetic model. Only small differences between the experimental (qe exp) and theoretical adsorption capacity (qe cal

) values obtained for the pseudo-second-order kinetic model were observed, which indicated

B

100

90

pHzpc=1.92

80

-20 -30

60

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70

MB MO

LIG

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ζ (mV)

-10

-40 -50 -60

50 2

4

6

8

10

pH

C 100

0.0

1.5

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D 100

80

60

20

0 0.0

0.2

0.4

0.6

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MB MO

40

0.8

1.0

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Removal efficiency (%)

10 0

1.2

Removal efficieny (%)

Sorption capacity (mg g-1)

A

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that the adsorption process was mainly followed by the pseudo-second-order kinetic model [26].

3.0

4.5

6.0

7.5

9.0

10.5

pH MB

MO

80

60

40

20

0 1.4

1.6

control

Dosage (mg)

sulfate

nitrate bicarbonate chloride

Co-ions

Fig. 2. (A) Influence of solution pH on the removal of MB and MO dyes using LIG powder (dye concentration = 50 mg L-1, contact time = 15 minutes, volume = 2 mL, dosage = 1 mg mL-1 and temperature = 295 K); (B) zeta potential analysis of LIG suspension; (C) effect of LIG dosage on the removal of MB and MO dyes (pH ~ 6.3 (MB) and pH ~ 7.4 (MO), dye concentration = 50 mg L-1, contact time = 15 minutes, volume = 2 mL and temperature = 295 K) and (D) influence

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of co-existing ions on the removal of MO and MB dyes (pH ~ 7.0 (MB and MO), dye concentration = 50 mg L-1, co-ions concentration = 50 mg L-1, contact time = 15 minutes, volume = 2 mL and temperature = 295 K).

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Langmuir and Freundlich isotherm models were adopted in order to describe the adsorption process as well as maximum adsorption capacity of LIG (Table 1). Adsorption of MB and MO dyes onto LIG were conducted at three different temperatures (298, 308 and 318 K) with different concentrations and the obtained equilibrium data were fit to these two isotherm

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models. Table 3 summarizes the calculated values of Q0, kF and 1/n from the Langmuir and

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Freundlich isotherm parameters for the removal of MB and MO by LIG. The value of the dimensionless equilibrium factor (RL) was also calculated from R ( =



)*+,

, where b is the

Langmuir isotherm constant, and Co is initial concentrations of dyes. Freundlich maximum adsorption capacity (qmax) was determined from K . =

/012

+, 3/4

[49].

Models

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Table 1. Linear mathematical expression of kinetic and isotherm models

Pseudo-first-order kinetic model

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Pseudo-second-order kinetic model Intraparticle kinetic model

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Langmuir isotherm

Freundlich isotherm

Linear form

logq7 − q8  = logq7 − k :;

Plot t 2.303

t 1 t = + q 8 h q7 q8 = k ? t/ + C

log (qe-qt) vs t t/qt vs t qt vs t1/2

C7 1 C7 = B + B q7 Q b Q

Ce/qe vs Ce

1 log  = log DE + log  F

log qe vs log Ce

qe and qt - adsorption capacity (mg g-1) at equilibrium and at time t (min); kad- equilibrium rate constant; h- initial sorption rate; ki (mg g-1 min-0.5) - intraparticle diffusion rate constant; C- thickness of the boundary layer; Q0- maximum monolayer capacity (mg g-1); b- Langmuir isotherm constant (L mg-1); kF- measure of adsorption capacity (mg g-1) (L mg-1)1/n and 1/nadsorption intensity

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Table 2. Adsorption kinetic parameters for the removal of MB and MO dyes using LIG MB amount (mg L-1) -1

kad (min ) R2 qe cal (mg g-1) qe exp (mg g-1) k × 10-3 (g mg-1 min-1) R2 -1

ki (mg g min R

-0.5

)

2

50

100

150

50

75

100

0.342 0.867 105.1 97.5

0.335 0.936 200.8 189.6

0.343 0.871 288.2 277.4

0.597 0.851 89.3 86.0

0.498 0.874 105.3 100.9

0.476 0.772 119.9 113.2

4.9

4.9

5.1

9.8

12.2

42.3

0.999

0.999

0.999

0.998

0.999

0.995

11.59

14.92

15.33

6.73

6.89

12.31

0.953

0.930

0.924

0.855

0.701

0.943

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Intraparticle

Parameters

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Kinetic Models Pseudo-firstorder Pseudosecond-order

MO amount (mg L-1)

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The values of Q0 and kF decreased with the increase in temperature and indicated a negative influence of temperature on the adsorption of MB and MO by LIG. The value of RL and 1/n was between 0 to 1, and represented a favorable adsorption condition of MB and MO dyes onto LIG powder (Table 3) [17]. Identification of the best isotherm model was ambiguous as

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both Langmuir and Freundlich models showed almost similar R2 values (~0.98). However, chisquare (χ2) analysis effectively identified the Freundlich adsorption isotherm model to describe the adsorption process for both MB and MO more accurately (Table 3) [50]. The χ2 was

(qe − qe,m ) 2

(5)

qe,m

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χ2 = ∑

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determined using the following mathematical expression (Eq. 5):

where qe,m is the equilibrium capacity obtained by calculating from the model (mg g-1). The χ2 value will be a small number if the qe value obtained from the isotherm models is similar to experimental value, while χ2 will be a bigger number if they are different from each other.

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Table 3. Langmuir and Freundlich isotherm parameters for the removal of MB and MO dyes using LIG Langmuir

Freundlich

298

370.4

0.252

0.073

308

322.6

0.117

0.038

318

285.7

0.081

0.025

298

125.0

0.137

0.077

308

116.3

0.189

0.052

318

108.7

0.239

0.040

R2

q (mg g-1)

0.979

926.5

0.990

569.9

0.991

407

χ x 10 2

2

2.57 2.27 2.09 1.08 1.23

0.998 0.998

1.31

0.999

kF (mg g-1) (L mg-1)1/n 88.2

55.4

43.6

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RL

132.3 120

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b (L mg-1)

111.8

1/n

χ2

0.46 9 0.46 5 0.44 5 0.19 0 0.18 3 0.19 8

0.01

55.1 51.7

44.8

0.04 0.03 0.001 0.002 0.001

The maximum sorption capacity of LIG towards the removal of MB and MO was compared with that of other types of carbon-based adsorbents including graphene, activated

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MO

Q0 (mg g-1)

carbon and carbon nanotubes. Notably, LIG powder possessed a relatively high sorption capacity especially for MB (926 mg g-1) compared to many graphene-based sorbents listed in Table 4. For instance, Fe3O4@graphene nanocomposite [51], graphene-carbon nanotube hybrid [52] and

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MB

T (K)

magnetic graphene-carbon nanotube composite [53] removed MB with a maximum adsorption capacity of 45.27, 81.97 and 65.79 mg g-1, respectively. Also the adsorption capacity of LIG for

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Dyes

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max

MO (132 mg g-1) was much more than other graphene materials eg. graphene oxide (16.83 mg g1

) [31] and konjac glucomannan/graphene oxide hydrogel (93.5 mg g-1) [54]. Many adsorbents do

not efficiently remove both dyes MB and MO while LIG powder showed good adsorption performance for both MB and MO, which underline an advantage of the LIG-based adsorbent.

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R2 0.999 0.999 0.999 0.999 0.998 0.998

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140

200 150 100

50 mg L-1 100 mg L-1 150 mg L-1

50

80 60

50 mg L-1 75 mg L-1

40 20 0

0 5

10

15

20

25

0.30

0.35

C

0.25

50 mg L-1 100 mg L-1

0.20

150 mg L-1

0.05 0.00 5

10

15

20

25

30

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0

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0.15 0.10

5

10

Time (min)

D

100 mg L-1

15

20

25

30

25

30

Time (min)

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Time (min)

0

30

t/qt (min g mg-1)

0

0.30

50 mg L-1 75 mg L-1

0.25

100 mg L-1

0.20 0.15 0.10 0.05 0.00

0

5

10

15

20

Time (min)

Fig. 3. Effect of incubation time and different initial dye concentration on the sorption capacity of (A) MB and (B) MO dyes by LIG (pH ~ 6.3 for MB and pH ~ 7.4 for MO, dye concentration

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t/qt (min g mg-1)

100

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250

B

120

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A

Sorption capacity (mg g-1)

Sorption capacity (mg g-1)

300

= 50 - 150 mg L-1 for MB and 50 - 100 mg L-1 for MO, volume = 2 mL, LIG dosage = 1 mg and temperature = 295 K). Pseudo-second-order kinetic graphs obtained for (C) MB and (D) MO removal by LIG (pH ~ 6.3 for MB and pH ~ 7.4 for MO, dye concentration = 50 - 150 mg L-1 for MB and 50 - 100 mg L-1 for MO, volume = 2 mL, LIG dosage = 1 mg and temperature = 295 K).

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Thermodynamic parameters associated with the adsorption process such as standard free energy change (∆G0), standard enthalpy change (∆H0) and standard entropy change (∆S0) were evaluated in order to understand and predict the nature of the adsorption process. The values of

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∆G0, ∆H0 and ∆S0 were determined for both MB and MO dyes according to the method of Khan and Singh (Table 5) [55]. The negative values of ∆G0 and ∆H0 obtained for the present study indicated that the adsorption process was spontaneous and exothermic. The negative ∆S0 values

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revealed a decreased randomness at LIG-MB/MO interface during the adsorption [4], and the

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adsorption capacity decreased with the increase in temperature.

After adsorption of MB dyes by LIG powder, the regeneration and reuse ability of LIG was assessed by five consecutive adsorption - desorption cycles (Fig. 4A). It was clearly seen that even after 5 consecutive cycles, LIG maintained its original adsorption ability for MB. The small decrease seen in all the cycles was most likely due to handling losses of LIG powder.

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Table 4. Comparison of maximum adsorption capacity of carbon-based adsorbents for the removal of MB and MO

Kinetic model

Isotherm model

Reference

Pseudo-second-order

Langmuir

[56]

-

Pseudo-second-order

Langmuir

[57]

45.27

-

Pseudo-second-order

Langmuir

[51]

714

16.83 -

-

Langmuir Freundlich

[31] [58]

73.26

-

-

Langmuir

[59]

167.2

-

-

-

[60]

243.9

-

Pseudo-second-order

Langmuir

[61]

181.81

-

Pseudo-second-order

Langmuir

[62]

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Adsorbent

Maximum adsorption capacity (mg g-1) MB MO 153.85 17.3

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Graphene Exfoliated graphene oxide Fe3O4@graphene nanocomposite Graphene oxide Graphene oxide Magnetic graphene– Fe3O4@carbon hybrids Graphene oxide-Fe3O4 hybrid composite Graphene oxide Graphene oxide/calcium alginate

17

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-

Pseudo-second-order

Freundlich

[52]

65.79

-

Pseudo-second-order

Langmuir

[53]

250

144.9

Pseudo-second-order

476.19 Pseudo-second-order

Langmuir

[46]

Freundlich

[63]

SC

-

RI PT

81.97

51.6

Pseudo-second-order

Freundlich

[54]

578

-

Pseudo-second-order

Langmuir

[64]

581.4

934.5

Pseudo-second-order

Langmuir

[65]

128.8

89.7

Pseudo-second-order

Langmuir

[65]

38.9

49.5

Pseudo-second-order

Langmuir

[65]

84.89

67.02

Pseudo-second-order

Langmuir

[65]

TE D

M AN U

92.3

274.7

216.4

Pseudo-second-order

Langmuir

[65]

131.8

-

Pseudo-second-order

Langmuir

[66]

399

149

Pseudo-second-order

Freundlich

[6]

117.9

46

-

[6]

-

185.5

Pseudo-second-order

Langmuir and Freundlich

800

-

Pseudo-second-order

Langmuir

[68]

328.4

-

Pseudo-second-order

Langmuir

[69]

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EP

composite Graphene-carbon nanotube hybrid Magnetic graphenecarbon nanotube composite Montmorillonitepillared graphene oxide MnO2–graphene– carbon nanotube hybrid Konjac glucomannan/graphene oxide hydrogel Agar/graphene oxide composite aerogel Finger-citron-residuebased activated carbon Coconut-shell-based activated carbon Nut-shell-based activated carbon Coal-based activated carbon Wood-based activated carbon Sewage-sludge-based granular activated carbon Alkali activated multiwalled carbon nanotubes Carbon nanotubes Amino functionalized multi-walled carbon nanotubes Water soluble hyperbranched polyamine functionalized carbon nanotubes Magnetic hydroxyapatite immobilized oxidized multi-walled carbon

18

[67]

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nanotubes Laser-induced graphene

926

132

Freundlich

Pseudo-second-order

This study

MB

298 K

5.51

308 K

7.9

318 K -∆H° (kJ mol )

9.23 49.31

-∆S° (kJ mol-1K-1)

0.18

-∆G° (kJ mol-1)

MO

8.28

8.85

9.54 10.41

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Thermodynamic parameters

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Table 5. Thermodynamic parameters for MB and MO dyes removal by LIG

0.06

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-1

3.3. Mechanism of MB and MO dyes removal by LIG

Non-covalent molecular interactions such as electrostatic attraction, π-π interaction, hydrophobic interaction, and hydrogen bonding are non-covalent forces associated with the

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binding of dyes to carbon-based adsorbents [21]. The molecular structure and physical properties of the dyes and LIG adsorbent underline that these interactions are also possible in the present case. Since all components were charged organic compounds that contained planar aromatic

EP

structures, electrostatic attraction, hydrophobic and π-π interactions were most likely the forces that contributed to the binding, and the differences in the binding of MB (positively charged) and

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MO (negatively charged) dyes to LIG powder. The contribution of hydrophobic and electrostatic interactions are discussed above in section 3.2, thus Raman analysis was performed in order to investigate the π-π interactions between the dye molecules and LIG (Fig. 4B). The LIG powder showed three distinguished D, G and 2D bands at ~1343 cm-1, ~1569 cm-1 and ~2683 cm-1, respectively. The D band corresponded to bent sp2 C-C bonds, the G band was attributed to first order scattering of E2g mode observed for C-C bond and the 2D band was attributed to second

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order zone-boundary phonons [37]. The peak attributed to the G band in the LIG powder was slightly shifted from 1569 to 1579 and 1576 cm-1 for MB and MO adsorbed LIG, respectively, which indicated π-π interactions between the dye compounds and LIG powder. Similar

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observations have been reported for other graphene-based adsorbents [25, 57]. Finally, due to the oxygen content of the LIG (4.9%), hydrogen bonding was also possible. Taken together, the plausible major interactions of MB and MO dye molecules with the LIG structure are shown in

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Fig. 4C, which illustrates that the hydrophobic force along with electrostatic and π-π interactions

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might cooperate for efficient binding of dyes.

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Fig. 4. (A) Reuse analysis of LIG for MB removal; (B) Raman spectra of LIG, MB and MO adsorbed LIG and (C) possible interactions of MB and MO dye molecules with LIG surface. 4. Conclusions

We examined the potential of LIG for the use as an absorbent and studied the removal of model contaminants MB and MO dyes from aqueous solutions. This graphene-based adsorbent is advantageous since it is produced on surfaces in a one step, reagent-free synthesis from polyimide by surface irradiation with a CO2 laser. Adsorption of MB and MO dyes was slightly 20

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influenced by solution pH, however it was not affected by inorganic ions common to natural waters. In most cases, LIG showed higher adsorption for MB than the MO. Adsorption of MB and MO by LIG followed the pseudo-second-order kinetic model and the equilibrium data were

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well described by the Freundlich isotherm model. LIG showed the maximum monolayer sorption capacity of 926 mg g-1 for MB and 132 mg g-1 for MO dyes. Thermodynamic study revealed the exothermic nature of the adsorption process. Raman analysis confirmed that π-π interactions

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were involved in the adsorption process that also consisted of electrostatic interaction between the dye molecules and LIG and accounted for adsorption differences between the dyes, while

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hydrophobic forces and hydrogen bonding rationalized general adsorption. Furthermore, LIG could be easily regenerated and reused for subsequent adsorption studies. In summary, this study underlines a potential environmental application of LIG as an efficient adsorbent for the decontamination of pollutants from water. Moreover, LIG technology provides conformal carbon

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built upon a polymer matrix with the ability to pattern, which might lead to alternative adsorption approaches in laboratory or industrial applications. Acknowledgements

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The author (K.R.) thanks Blaustein Center for Scientific Cooperation (Ben-Gurion University of the Negev) for a post-doctorate fellowship. This study was supported by United States-Israel Binational

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Science Foundation (BSF grant 2014233). References

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