The effect of natural organic compounds on the adsorption of toluene and ethylene benzene on MWCNT

Abstract

Purpose

The present study aimed to investigate the effect of humic acid (as a NOM model) on the adsorption capacity of MWCNT.

Methods

Ethyl benzene and toluene were removed from aqueous solution and adsorbed on multi walled carbon nanotubes in batch adsorption experiments at the presence of different concentrations of humic acid.

Results

The results showed that the highest adsorption of multi walled carbon nanotubes was 72 mg/g for ethyl benzene and 35 mg/g for toluene in an aqueous solution without humic acid. Langmuir isotherm model and pseudo-second-order kinetic model were the predominant models of adsorption process. Pre-loading of humic acid on MWCNT reduced the adsorption capacity of MWCNT from 14 mg/g to 8 mg/g for toluene and from 25 mg/g to 10 mg/g for ethyl benzene, when the experiments were conducted with MWCNT pre-loaded by humic acid from 0 to 30% .

Conclusions

The batch adsorption experiments showed that the presence of dissolved humic acid in the aqueous environment slightly affected the adsorption of ethyl benzene and toluene by MWCNT but, Pre-loading of humic acid on MWCNT could reduce the adsorption capacity of multi walled carbon nanotubes.

Introduction

Industrial development has led to the production of a great amounts of synthetic organic chemicals (SCOs) and volatile organic compounds (VOCs) throughout the world. One of the most important sources of these pollutants is the effluents generated by petroleum industries that can pollute soil,air and surface and ground waters. Some of these compounds including benzene, toluene, ethyl benzene, and xylene (BTEX) are so applicable in the industrial processes but pollute the environment in vast level. In addition, all compounds of BTEX can cause damage to the central nervous system. Some of these compounds are carcinogenic or suspected to be carcinogenic [1]. The US Environmental Protection Agency has established drinking water standards of 0.005 mg/l for benzene, 1 mg/l for toluene, 0.7 mg/l for ethyl benzene, and 10 mg/l for xylene isomers [2]. Removal of BTEX from aqueous solutions has been extensively studied, and different physical, chemical, and biological processes proposed for this purpose in these studies, weresuccessful in practice. Among the applied processes, it can be referred to biological remediation technologies [3], adsorption by various adsorbents [4], membrane processes and, wet air oxidation (WAO) [5]. Adsorption is a simple and effective process in the removal of organic pollutants and has been significantly cited for advantages such as high efficiency and efficacy, easy application and utilization, and extensive removal of many pollutants [6] .

One of the nanotechnology products being introduced by researchers as an adsorbent is carbon nanotubes (CNT). CNTs, particularly their polyhedral forms, were described by Igima (1991) for the first time [7]. CNTs are unique single dimension macromolecules with high thermal resistance and chemical stability [4].

CNTs have an aciform structure made through placement of graphite molecules as single tubes (lamella) that are named single walled CNT (SWCNT). If several single tubes are formed with various diameters around a vector, a multi walled CNT (MWCNT) is formed. The typical size of SWCNTs is in a range of 0.4-3 nm and for MWCNT, it is 1.4-100 nm [7]. Different studies have provided experimental evidences that nano-materials have been used to remove organic pollutants such as dioxins, 1,2-dichlorobenzene, thrihalomethanes, dyes, xylene, fluoric acid and natural organic matter, phenol crystals, poly-aromatic hydrocarbons and insecticides from aqueous solutions. Moreover, they are used as pollution detection sensors in the environment [4, 8]. In the present study, the efficiency of these adsorbents is evaluated in the removal of ethyl benzene and toluene in the presence of natural organic matters. Natural organic matters (NOM_s) are non-uniform mixture of organic compounds that are derived from plants, animals and living and dead microorganisms and their waste products. They enter to the water bodies after degradation. The NOM_s made from various materials are different in their properties. Generally, NOM_s molecules are big molecules with many functional groups such as mercapto, phenols, quinones, aldehydes, ketones, and carboxyl groups, that influence their chemical behavior [9]. The main NOM component is attributed to humic substances which are humic acid(HA), fulvic acid and humin. Humic materials are unformed, acidic, often aromatic, and hydrophobic complex. These materials are anionic polyelectrolytes with low to moderate molecular weight [10], existing as macromolecules with negative charge in natural waters. Carboxyl, hydroxyl phenolic, and hydroxyl alkilic are the most principal functional groups whose charge is firstly resulted from carboxyl and phenolic groups [11]. The effect of NOM_s on the adsorption process of organic materials is determined by two opposite effects: increasing the spatial distribution of the adsorption sites by a better distribution of MWCNT, and, decreasing the adsorption sites through mechanisms of competition or blockage of the adsorbent pores by NOM_s [12]. The later effect is important because adsorption of humic acid on many adsorbents such as pumice stone [13], graphene oxide [14], granular ferric hydroxide [14], bentonite-chitosan composite [15] is explained by other researchers. This study was carried out to show the effect of humic acid on the adsorption of toluene and ethyl benzene on MWCNT as adsorbent.

Material and methods

Chemicals

MWCNT were purchased from Nanosany Company (Iranian Nano materials Pioneers Co., Mashhad, Iran) and were characterized using scanning electron microscope (SEM) and the X-ray diffraction (XRD) technique by manufacturer. Ethyl Benzene and toluene (GC-grade) and other chemicals that were reagent grade were purchased from Merck Inc.

Analytical methods

Toluene and ethyl benzene was measured by Purge and Trap GC-FID method (Agilent78904). The column used was HP-5 with 30 m of length and 0.320 mm of internal diameter. The initial temperature of the column was 40 °C. It increased to 120 °C after 2 min, and then to 180 °C at 10 °C/m temperature rate. Its detector temperature was 300 °C with 25 ml/min hydrogen flow, and 250 ml/min air flow. The purge and trap device was set as follows: Value Oven Temp: 150 °C, Purge Time: 11 min, Purge Temp: 30 °C, Purge Flow: 40 ml/min, Dry Purge Temp: 30 °C, Desorb Time: 2 min, Desorb Temp: 250 °C, Bake Temp: 250 °C.

Preparation of MWCNT

Purification of MWCNT was done to increase adsorption capacity by two following methods:

• A)0.3 g MWCNT was heated in 400 °C for 45 min and in 800–900 °C for 30 min under nitrogen gas. A 100 cc Round-bottom flask was used in which 25 ml of nitric acid 65% and a definite dose of heated MWCNT were poured and then reflexed for 48 h. After that, the solution was filtered and pH was set in neutral level. Then, It was put in the oven at 105 °C for 24 h to be dried [16].
• B)Another method to purify MWCNT was heating by electrical furnace at 400 °C for 4 h. Although after the test in a batch reactor, both mentioned methods had similar benzenes and toluene adsorption capacity, method B was selected because it was simpler than the first method.

Preparation of CNTs-HA complex

The CNTs-HA complex was prepared as explained in the method by Daohui Lin et al. (2012). Briefly, 5 g of MWCNT was solved in HA solution and shaken for 3 days (25C and 110 rpm) [15]. The product was centrifuged at 3500 rpm for 30 min, and the obtained solid was washed by distilled water, then, freeze-dried, ground, and at last stored as MWCNT-HA complex. The initial concentrations of HA solution were 0, 10, 50, 100, 500, 1000, and 1500 mg/l and the percent values were 0, 0.2, 1, 2, 10, 20, and 30, respectively [17]. The functional groups of MWCNT-HA complex surface were detected by AVATAR 330 Fourier transform infrared spectroscopy (FTIR) (NICOLET Co., USA).

Experimental design

The experiments in this research were conducted in three following sections in a batch system:

• S1. Adsorption of pollutants on the MWCNT under various conditions

The effect of the initial concentration of pollutants, dosage of adsorbent, initial pH and contact time as independent variables were investigated in this step.

• S2. Determination of the reciprocated and interactioneffects of MWCNT dose and dissolved HA concentration

In order to determine the simultaneous effects of MWCNT and HA, and identify their mutual interactions, response-surface method (RSM) was applied with the use of Design Expert®6.0.6,(state-Ease Inc., Minneaoolis, USA) software. In statistics, RSM explores the relationship between several depended variables and one or more independent variables. After entering the data (concentration levels of MWCNT and HA), the software presented 16 different experiments (treatments) with different concentration of MWCNT and dissolved HA using D-optimal method [18]. Each experiment in all of experimental methods was conducted in 3 replicates. Dependent variables were also measured and the results were analyzed.

S3- Pollutants adsorption on the complex of MWCNT- humic acid.

Bath adsorption

A stock solution of 2000 mg/l of the test materials was prepared in methanol and the working solution was obtained by dilution of the stock solution. Adsorption experiments were done in flasks with teflon door. A definite volume of pure water was polluted by various concentrations of toluene and ethyl benzene, then, it was shaken in a shaker incubator after adding the adsorbent and setting pH and other variables. Pollutants concentrations were measured before and after operations, and pollutants adsorption on the adsorbent was calculated using the following formula:

$$\mathrm{q}=\left({\mathrm{C}}_0–\mathrm{C}\right)/\mathrm{m}$$ 1

Where, C and C₀ are pollutants concentrations before and after adsorption operation (mg/l), m is the adsorbent dosage (g/ml) and q is adsorbed pollutants mass per unit mass of adsorbent (adsorption capacity) (mg/mg).

To calculate the effects of simultaneous evaporation, the control samples were also considered in all steps, in addition to the main samples. The isotherm models of Langmuir, Freundlich, Tempkin and kinetic models of pseudo-first-order and pseudo-second-order were used in order to analyze the adsorption process.

Results

MWCNTs characterization

Figure 1 shows the SEM image of MWCNTs. The outer diameter and the length of nanotube were 5–15 nm and 50 um, respectively. The energy dispersive X-ray spectroscopy analysis showed that this nanotube contained 97.46% of C. The detailed features of MWCNTs presented by the factory are as follows.

• Purity: > 95 wt% (carbon nanotubes) (from TGA & TEM)

• > 97 wt% (carbon content)

• Outside diameter: 20–30 nm (from HRTEM, Raman)

• Inside diameter: 5–10 nm

• Length: 10–30 um (TEM)

• SSA: > 110 m2/g (BET)

• Color: Black

• Ash: <1.5 wt% (TGA)

• Electrical conductivity: >100 s/cm

• Tap density: 0.28 g/cm3

• True density: ~2.1 g/cm3

• Multi Walled Carbon Nanotubes (MWNTs, MWCNTs) Manufacturing Method: CVD

Fig. 1.

Characteristic features SEM image of MWCNTs

Adsorption of pollutants on the MWCNT and effects of variables

Experiments were conducted with determined values of variables to study the effects of time, pH, and adsorbent dosage on the removal of ethyl benzene and toluene by MWCNT in batch reactors. Increasing time to 4 h increased the adsorption capacity of MWCNT to 37.49 mg/g for toluene and 73.14 mg/g for ethyl benzene. Adsorption became relatively fixed after 4 h. Results are shown in Fig. 2. A series of experiments were carried out with different pH. The effect of pH on the adsorption process was not significant. Therefore, the optimum pH was considered to be 7 (Fig. 2). The experiments were conducted using 5 MWCNT dosages of 0.5, 1, 1.5, 2, and 2.5 g/l with initial concentrations of 110 mg/l of toluene and ethyl benzene, respectively. Results showed that increasing adsorbent dosage increased the removal percentage of test materials (Fig. 2). Increasing of adsorbent dosages from 0.5 to 2.5 g/l increased toluene removal from 10% to 54.1% and the removal of ethyl benzene from 32.71% to 83.39%. However, increased adsorbent dosage reduced the MWCNT adsorption capacity.

Fig. 2.

Removal percentage of adsorption of ethyl benzene and toluene on MWCNT with different variables

All next experiments were conducted at pH = 7, 4 h of mixing, and in adsorbent dosage of 1.5 g/l.

Effect of initial concentration

Increasing the initial concentration of ethyl benzene and toluene from 25 to 200 mg/l reduced the toluene removal percentage from 58.33% to 20.6%, and ethyl benzene removal percentage from 88.18% to 35.84% (Fig. 2). The results showed that when MWCNT mass was constant and pollutant concentrations increased, removal percentage decreased, and adsorption capacity increased. The similar results were obtained by other researchers [8, 17].

Adsorption isotherms

5 various concentrations of 25, 200, 100, 75, and 50 mg/l of toluene and ethyl benzene were tested in order to determine the isotherm models of Langmuir, Freundlich and Tempkin [19, 20].

Langmuir isotherm: The linear relation of this isotherm is as follows:

$$\frac{C_e}{q_e}=\frac{C_e}{q_0}+\frac{1}{k_L{q}_0}$$

K_L

Langmuir constant (l / mg(

q0

maximum adsorption capacity (mg / g(

Ce

equilibrium concentration of the adsorbate (mg / l(

qe

the initial amount of the adsorbate at the time of equilibrium in mg / g.

C0

initial concentration of the adsorbent (mg/l(

In this isotherm, the R_L dimensionless constant as the separation factor of equilibrium is given by:

$$R_L=\frac{1}{1+k_L{C}_0}$$

R_L > 1 shows undesirable adsorption, RL = 1 indicates linear adsorption, R_L = 0 represents irreversible adsorption and 0 < R_L > 1 indicates the optimal adsorption [20].

Freundlich isotherm: The linear equation is as follows:

$$ln{q}_e=ln\mathit{kf}+\frac{1}{n}ln{C}_e$$

In this equation, k_f and n are Freundlich constants.

Tempkin Isotherm: The relation related to the isotherm of Tempkin is as follows.

qe BLnA_T + BLnC_e

B RT / b_T

Where A_T is the binding constant that shows the maximum binding energy (l / g), b_T is Tempkin isotherm constant, R is the universal gas constant J / mol K (314/8), T is the temperature which is set at 298 ° K, B is the constant related to heat adsorption coefficient (J / mol), and the features and constants are obtained from a linear relationship and plotting qe versus lnCe (Fig. 3).

Fig. 3.

a Langmuir, b Freundlich c Tempkin adsorption isotherms of toluene and d Langmuir, e Freundlich f Tempkin adsorption isotherms of ethyl benzene and on MWCNT

The isotherm constants the models calculated from the linier equations depicted in Fig. 3 and and the results are listed in Table 1.

Table 1.

Comparison of the coefficients isotherm parameters from ethyl benzene and toluene (25 to 200 mg/l) adsorption on MWCNT. (e: Mean of calculated net errors between experimental and modeled results)

Isotherm	ethyl benzene		toluene
	Linearity(R2)	coefficients	e	Linearity(R2)	coefficients	e
(Longmuir)		KL = 0.088 q0 = 54.94	0.035		KL = 0.04 q0 = 31.15	0.313
	0.99			0.98
(Freundlich)		Kf = 12.62 n = 2.84	0.042		Kf = 4.06 n = 2.36	0.55
	0.95			0.90
(Tempkin)		Bt = 9.91 At = 1.41	1.47		Bt = 7.89 At = 2.69	1.4
	0.97			0.96

In order to evaluate the validity of the models, differences of adsorption capacity between the experiments and models results (e) were calculated (Table 1). The calculations results showed that toluene and ethyl benzene adsorption followed Langmuir isotherm model because of high linearity (R² > 98%) and the minimum calculated net errors between experimental and modeled results (R² = 0.974 and e = 0.3 for toluene and R² = 99 and e = 0.03 for ethylene benzene). The values of R_L in both experiments of toluene ethylene benzene were between 0 and 1, showed that the adsorption process is in optimum condition.

Adsorption kinetic

Experiments were conducted with 50 mg/l of ethyl benzene and toluene and 1.5 g/l of MWCNTs in order to determine the adsorption kinetic at 5,20,40,60,80,100,120,180,240,300,360, and 420 min of the contact time. Pseudo-first-order and pseudo-second-order kinetic models were tested [20].

The linear equation of pseudo first order model is as follows:

$$ln\left(q_e-q_t\right)=ln{q}_e-\frac{{\underset{¯}{k}}_1}{2.303}t$$

Where q_e and q_t, respectively, are adsorption capacity at the time of equilibrium and time t in mg / g and k1 is the constant speed in min-1.

In this equation, the values of the parameters qe and k1 are respectively the intercept and the slope of the linear graph ln (qe-qt) versus t.

The linear equation of pseudo second-order kinetic models:

$$\frac{t}{q_t}=\frac{1}{k_2{q_e}^2}+\frac{1}{q_e}t$$

K₂

is a pseudo second-order reaction (min⁻¹).

The obtained values for both models are shown in Fig. 4. It was observed that toluene and ethyl benzene adsorption process follows the pseudo-second-order kinetic model with R² of 0.99 and the smallest mean error between experimental and calculated adsorption capacity values. The mean errors between experimental and model results were 0.006 and 0.446 for the first-order model and 0.007 and 0.0042 for the second-order model for toluene and ethyl benzenes, respectively. It was indicated that the adsorption process followed the second-order kinetic model. It meant that adsorption rate and tested materials reduction were proportional to the second power of the remained concentrations of the test materials. Fei Yu et al.(2011) reported the similar isotherm and kinetic models in a research on adsorption of toluene, ethylbenzene and m-xylene on multi-walled carbon nanotubes with oxygen contents different from aqueous solutions [17] (Table 2).

Fig. 4.

a pseudo-first-order b pseudo-second-order kinetic models of adsorption of toluene and c pseudo-first-order d pseudo-second-order kinetic models of adsorption of ethyl benzene on MWCNT

Table 2.

The results of analysis of variance of experiments designed using D-Optimal method for toluene and ethyl benzene adsorption. (A: HA. B: MWCNT)

	parameter	Sum of squares	DF	Mean square	F value	P value	Significances
Ethyl benzene	Model	4022.8	3	1340.95	530.15	<0.0001	Sig.
	A	16.37	1	16.37	6.47	0.0258	Sig.
	B	3819.3	1	3819.35	1510.01	<0.0001	Sig.
	B2	324	1	324.02	128.1	<0.0001	Sig.
	Residue	30.35	12	2.53
	Net error	0.28	1	0.28
	Lack of fit	30.07	11	2.73	9.72	0.2456	Non-Sig.
Toluene	Model	1735.8	3	1735.81	626.3	<0.0001	Sig.
	A	19.32	1	19.32	6.98	0.0251	Sig.
	B	1661.98	1	1661.98	600	<0.0001	Sig.
	B2	17.55	1	2.77	6.34	0.027	Sig.
	Residue	32.22	12	0.14
	Net error	0.14	1	3.01
	Lack of fit	33.08	11		22.24	0.164	Non-Sig.

Determination of the reciprocated effects of variables

RSM and D-optimal methods were simultaneously used to determine the contemporaneous effect of dissolved humic acid concentration (a) and adsorbent dose (B) on the adsorption efficiency. The interaction of variables is shown by this methodology. Two variables of humic acid concentration (0–100 mg/l) and adsorbent dosage (0–2000 mg/l) were given to the software, and the software designed 16 experiments with different concentration of variables. Adsorption operations were conducted under optimum conditions resulted from the first step with initial concentration of 50 mg/l of toluene and ethyl benzene. The adsorbent concentration in all experiments showed a positive effect on the toluene and ethyl benzene removal from the aqueous solution (Fig. 5). The software also presented a multi-variable regression model for each test material. The models are as follows

$$\mathbf{\text{Toluene}}=+\mathrm{47.77464}+\left(\mathrm{0.026101}A\right)–\left(\mathrm{0.018022}B\right)+(\mathrm{2.45223}E–\mathrm{006}\left(B^{\mathbf{2}}\right)$$

$$\mathbf{\text{Ethyl Benzen}}=+\mathrm{49.01813}+\left(\mathrm{0.024025}\mathbf{A}\right)-\left(\mathrm{0.040961}\mathbf{B}\right)+\left(\mathrm{1.05381}E-\mathrm{005}{B}^{\mathbf{2}}\right)$$

Fig. 5.

The effect of humic acid and dose of MWCNT on the adsorption of toluene and ethyl benzene on MWCNT

ANOVA showed that the presented models were valid (P < 0.02) and the lack of fit of the models was not significant (P = 0.164 for toluene and P = 0.245 for ethyl benzene). The net error values (the differences between experimental and predicted values obtained by the models) were 0.28 and 0.14 for ethyl benzene and toluene, respectively. The results confirmed the validity of the presented models.

The effect of humic acid concentration (A) and adsorbent dose (B) are significant but, the F values of B (600 for toluene and 1510 for ethyl benzene) are much more than of that for A (6.9 for toluene and 6.4 for ethyl benzene) therefor it is concluded that the effect of adsorbent dose is stronger than dissolved humic acid concentration.

Adsorption by MWCNT- humic acid complex

In this step, pollutants were adsorbed on MWCNT-HA complex (pre-loaded MWCNT). The FTIR spectra of MWCNT (Fig. 6), after loading, showed some apparent characteristic bands of HA, including O–H of carboxyl group (2924–2858 cm⁻¹) and OH of phenolic group (3615 cm⁻¹) [21]. The experiments were conducted with constant concentration of ethylbenzene and toluene (50 mg/l) in neutral pH (Fig. 7). Experiments results showed that increasing HA loading percentage for preparation of MWCNTs-HA complex reduced adsorption of ethyl benzene and toluene on the adsorbent unit. Increasing of ethyl benzene and toluene on MWCNT from 0 to %30 decreased the q from 14.06 to 8.66 mg/g for toluene and from 25.22 to 10.84 mg/g for ethyl benzene.

Fig. 6.

The FTIR spectra of a row MWCNT and b Surface-bound HA- MWCNT

Fig. 7.

Adsorption of toluene and ethyl benzene on MWCNT-HA complex

Discussion

The adsorption of ethyl benzene was more than that of toluene. The most probable reason was that toluene was more soluble than ethyl benzene (toluene: 530 mg/l, ethyl benzene: 152 mg/l) in water, therefore, ethyl benzene was easily separated from liquid phase. In a similar study on benzene, ethyl benzene, toluene, and xylene by the oxidized CNT by NaOCl, it was observed that 20–200 mg/l of BTEX was adsorbed on CNT in 4 h [8].

BTEX compounds were adsorbed on MWCNT in molecular form and therefore, physical adsorption occurred. Furthermore, it was probable that the π-π electron-donor–acceptor mechanism involving the carboxylic oxygen of MWCNT surface as the electron-donor and the aromatic ring of BTEX as the electron-acceptor was the predominant mechanisms of BTEX uptake by MWCNT [8]. Langmuir isotherm was related to the formation of a single layer at adsorption sites, and after the forming of a single layer, no layers and adsorption would occur. The q₀ shows the amount of adsorbate needed to form a single layer (mg/g), and K_L is a constant isotherm that is attributed to the energy required for adsorption. Based on the Langmuir isotherm results, the maximum adsorption capacity for toluene and ethyl benzene on MWCNT was 31.15 and 54.94 mg/g, respectively. The isotherm constants K_L, for toluene and ethyl benzene were 0.04 and 0.885 respectively. In this research, the obtained R_L for toluene and ethyl benzene was 0.19 and 0.09, respectively. Since R_L was between 0 and 1, the adsorption was completely desirable [19, 20].

The analysis of variance (ANOVA) method (with increasing both MWCNT and dissolved HA) showed that the effect of dissolved HA on the adsorption of test materials is negligible.

The reduction of q in the experiments with MWCNTs-HA complex showed that more adsorption sits were occupied by HA in higher concentrations. As a result, adsorption did not occur well. Various studies reported the effect of HA on the adsorption processes which were relatively in agreement with the findings of this research. Studies by Shujuan Zhang on the effect of natural organic materials on the adsorption of organic materials by activated carbon and CNTs (3) and the research by Xilong Wang (2011) on the effect of HA adsorption and aromatic compounds on MWCNTs are some instances [22]. Interaction mechanisms of CNTs and NOM were also well known. Hydrophobic interaction, π–π attraction, Electrostatic interaction, Hydrogen bonding, van der Waals force were the most important mechanisms. [23]. Lower adsorption capacity of MWCNT with surface-bound humic acid, in comparison with dissolved humic acid showed that the adsorption of toluene and ethyl benzene on the MWCNT was faster than that of humic acid. In the experiments with both dissolved pollutants and humic acid, the adsorption capacity was high. When MWCNT is pre-loaded by humic acid, the pores of the adsorbent are occupied with HA, and the adsorption capacity will reduce.

Conclusion

The adsorption of toluene and ethyl benzene on the MWCNT was carried out in batch experiments and the effects of contact time, adsorbent dose, initial concentration and dissolved and surface-bound humic acid were studied. Results demonstrated higher removal of ethyl benzene compared with toluene under almost similar experimental conditions is occurred. Kinetic studies showed that adsorption of toluene and ethyl benzene followed the pseudo-second-order model. Studies on Adsorption isotherms indicated that Langmuir isotherm model was well fit with experimental data. The results showed that dissolved humic acid had no significant effect on the adsorption of toluene and ethyl benzene on the MWCNT but the surface-bound humic acid decreased the adsorption capacity of MWCNT. The present study showed that the adsorption of toluene and ethyl benzene on the MWCNT was faster than that of humic acid (when both contaminants and dissolved humic acid were presented in the solution).

Abbreviations

MWCNT

Multi walled carbon nanotubes

NOM

Natural organic matters

CNT

carbon nanotubes

SWCNT

Single walled carbon nanotubes

HA

Humic acid

Compliance with ethical standards

Conflict of interest

The authors confirm no conflicts of interest associated with this publication.

Consent for publication

All authors agreed to publish this article.

Ethics approval and consent to participate.

There was no human participation in this study.