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Original article
13 (
1
); 109-119
doi:
10.1016/j.arabjc.2017.02.005

Carbon nanotubes modified with 5,7-dinitro-8-quinolinol as potentially applicable tool for efficient removal of industrial wastewater pollutants

, , ,
 Department of Chemistry, Faculty of Science, Suez Canal University, Ismailia 41522, Egypt

⁎Corresponding author. Fax: +20 643230416. Enas58@yahoo.com (E.T. Abdel Salam),

Received: , Accepted: ,
© The Author(s) 2017
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The environmental pollution due to the industrial wastewater of four different areas in the Gulf of Suez, Red Sea, Egypt, was studied. Adsorption capacities toward the concerned heavy metal ions Cu(II), Zn(II), Fe(II), and Pb(II) by multiwalled carbon nanotubes (MWCNTs) and modified-MWCNTs with 5,7-dinitro-8-quinolinol were investigated. MWCNTs as well as the modified-MWCNTs were characterized using Fourier transform infrared (FTIR), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). Adsorption of the studied divalent metal ions was measured by atomic absorption spectrometry (AAS). The effects of solution conditions such as pH, shaking time, metal ion concentration, ionic strength and adsorbent dosage on the adsorption process were also examined. The obtained results showed that removals of the heavy metal ions under consideration by MWCNTs are obviously dependent on the experimental conditions. The maximum adsorption capacities as calculated applying Langmuir equation to single ion adsorption isotherms were found to be 142.8 mg/g for Cu(II), 250 mg/g for Zn(II), 111.1 mg/g for Fe(II), and 200 mg/g for Pb(II) using MWCNTs; meanwhile, the modified-MWCNTs exhibited higher values of the respective maximum adsorption capacities as 333.3 mg/g for Cu(II), 500 mg/g for Zn(II), 200 mg/g for Fe(II), and 333.3 mg/g for Pb(II). Kinetic studies were also performed and the experimental data followed a pseudo-second order model of the adsorption process. The obtained results suggest that the tested adsorption systems of MWCNTs and modified-MWCNTs have suitable affinity toward the metal ion under consideration. Both systems could act as potentially applicable tool in environmental protection.

Keywords

Multi-walled carbon nanotubes
Modified carbon nanotubes
Heavy metals
Adsorption
Kinetic studies
Langmuir isotherm
1

1 Introduction

Metal pollution resulting from massive industrialization and improper disposal of industrial wastes of heavy metals into the natural waters is one of the most important environmental concerns (Salam et al., 2012; Tofighy and Mohammadi, 2011). The presence of heavy metal ions in aqueous environments, even in small amounts, leads to bioaccumulation in higher trophic levels of the food chain. Accumulation of heavy metal ions in the organisms of the human body may create a major concern because of their toxicity and carcinogenicity to various systems such as kidney, nervous system, bones and brain (Purdom, 1980; Stafiej and Pyrzynska, 2008; Deng et al., 2010).

Many techniques and various conventional methods that used to remove heavy metal ions from water have been developed (Rao et al., 2006; Weng and Huang, 2004; Ho and McKay, 1999). Among all methods, adsorption is regarded as the most promising and widely used method (Biskup and Subotic, 2005). Therefore the search for new and more effective materials to be used as adsorbents is a continuous effort for many researchers.

Multi-walled carbon nanotubes (MWCNTs) have been proven to possess great potential as superior adsorbents because they have a large specific surface area and small hollow layered structures. MWCNTs have great potential for removing many kinds of pollutants, such as organic compounds (Li et al., 2011; Sheng et al., 2010; Abdel Salam et al., 2010; Abdel Salam and Burk, 2010; Al-Johani and Abdel Salam, 2011) as well as inorganic pollutants from water (Di et al., 2006; Lu and Liu, 2006; Lu and Chiu, 2006; Lu et al., 2006; Abdel Salam et al., 2011). Modified carbon nanotubes are better sorbents for extraction of metal ions from various samples, because functionalization not only improves dispersion in the media but also increases metal ion sorption through chemical bonding, which would be more selective than raw carbon nanotubes (Kosa et al., 2012; Wildgoose et al., 2006). Nanoscale zero-valent iron (NZVI) immobilized on carbon nanotube (CNT) composite (NZVI/CNT) was prepared, characterized and used for the sequestration of Se(IV) in water (Sheng et al., 2016). Magnetite decorated graphene oxide (MGO) magnetic composites were fabricated for the highly efficient immobilization of Eu(III) from aqueous solution (Li et al., 2014).

In this work, MWCNTs and modified-MWCNTs with 5,7-dinitro-8-hydroxyquinoline (MMWCNTs) are applied to enhance the efficiency of the removal of some divalent metal ions such as Cu(II), Zn(II), Fe(II) and Pb(II) from polluted industrial wastewater. Kinetic studies were also performed to achieve a better understanding of the adsorption process of the target heavy metal ions on MWCNTs and MMWCNTs in aqueous solution. Optimization of the adsorption behavior was also investigated in this study. Adsorption capacity was measured via the batch method where the obtained data were simulated by Langmuir isotherm model.

2

2 Experimental

2.1

2.1 Materials and methods

Multiwalled carbon nanotubes were obtained from Shenzhen Nanotech Port Co., Ltd., USA, with 15–25 nm in outer diameter, surface area of 40–600 m2/g and purity above 95% and were used as received. Solutions of Cu(II), Pb(II), Fe(II) and Zn(II) were prepared from 1000 mg/L stock solutions of metal nitrates (AR Merck) using tripled deionized water. Diluted HNO3 and NaOH were used to adjust pH of the working solutions. All other chemicals were analytical grade reagents (Sigma–Aldrich).

The modification of CNTs was performed using 5,7-dinitro-8-quinolinol by adding the modifier solution to a suspension of MWCNTs in DMSO and stirring for 4 h. The experiments were performed at room temperature. All used glassware were rinsed with 10% HNO3, to remove all impurities that may be present and to prevent further adsorption of heavy metals to the walls of the glassware, wash three times with deionized water and keep clean and dry for use.

2.2

2.2 Instruments

Metal concentrations were estimated by Thermo-Elemental SOLAAR S4 atomic absorption spectrophotometer. Model 3320 Sen Tix41-3 pH-meter fitted with combined glass-electrode was employed for measuring solution pH. Scanning electron microscope (SEM, Philips: XL30) was used for morphological analysis of the CNT. The nanostructure of the CNTs was determined by Transmission electron microscope (TEM, Philips: CM200). Surface functional groups of the prepared adsorbents were determined by Fourier transform infrared spectroscopy (FT-IR) at room temperature using Perkin – Elmer 1430 with ratio recording infrared spectrophotometer using KBr disk.

2.3

2.3 Adsorption measurements

Optimization of different adsorption parameters such as pH, ionic strength, contact time and CNTs dose was performed. Certain amounts (0.01–0.1 g) of raw and modified MWCNTs were added to 50 mL of solutions containing Zn(II), Cu(II) Fe(II) and Pb(II) ions together at room temperature, and the solution was stirred for 4 h using medium speed magnetic stirrer. The pH value of the solution was adjusted by using HNO3 and NaOH. After 4 h, the solution was filtered through 0.45 mm membrane filters and the concentration of the metal ions in the filtrate was measured using AAS. The percentage of the removed Zn(II), Cu(II) Fe(II) and Pb(II) ions from the solution was calculated.

The same experimental conditions were adopted to study the influence of the ionic strength, pH and quantity of CNTs on the heavy metal ion competitive adsorption.

All experiments were accomplished in triplicate and the mean values were reported.

2.4

2.4 Water sample sites

Water samples were collected from Suez gulf which lies along the African side of Egypt and the Sinai Peninsula of the Red Sea. Suez gulf water (SGW) and industrial wastewater (IWW) samples were used to evaluate the efficiency of the raw and modified MWCNTs for the removal of the target heavy metal ions. Water samples were collected from four locations (SGW1- SGW2- SGW3- and SGW4). The wastewater samples were collected from the industrial Ataka Electricity Company. The SGW and IWW Samples were filtered through Whatman No. 42 filter paper and kept in Teflon bottles.

3

3 Results and discussion

3.1

3.1 Characterization of CNTs

3.1.1

3.1.1 Fourier transform infrared (FTIR)

Generally, the IR spectra of the MMWCNTs contained the specific IR bands of the MWCNTs and the modifier (5,7-dinitro-8-quinolinol), which reveal the success interaction of the modifier with the MWCNTs. The O–H stretching vibration of 5,7-dinitro-8-quinolinol appear at 3438 cm−1 could be due to the intermolecular hydrogen bond (Sathyanarayana, 2004). In the IR spectrum of MWCNTs and the MMWCNTs, this band was shown as a broad absorption at 3433 cm−1 attributed to υ(OH) of the modifier overlapped with υ(OH) aqueous water. The specific IR spectra of the NO2 were appeared well for the modifier and the modified MWCNTs. The asymmetric and symmetric stretching vibrations of the NO2 group appeared at 1583 and 1548 cm−1, which have not nearly changed in both the modifier and the MMWCNTs (Krishnakumar and Ramasay, 2005).

3.1.2

3.1.2 SEM and TEM microscopy

Scanning electron microscope (SEM) imaging was used to study the morphology of the MWCNTs and MMWCNTs before and after adsorption of divalent metal ions as shown in Figs. 1 and 2. Images show that the MWCNTs were rope-like, curved and entangle around each other with inner diameters of 18–25 nm, and lengths ranging from hundreds of nanometers to micrometers.

Scanning electron microscope images of MWCNTs (a) before and (b) after adsorption of divalent metal ions.
Fig. 1 Scanning electron microscope images of MWCNTs (a) before and (b) after adsorption of divalent metal ions.
Scanning electron microscope images of the MMWCNTs with 5,7-dinitro- 8-quinolinol (a) before and (b) after removal of divalent metal ions.
Fig. 2 Scanning electron microscope images of the MMWCNTs with 5,7-dinitro- 8-quinolinol (a) before and (b) after removal of divalent metal ions.

3.2

3.2 Adsorption study

Adsorption of metal ions from aqueous solution by carbonaceous adsorbents is controlled by two different interactions. The first is electrostatic, which arises mainly from the surface charges produced on the carbon surface due to immersion in water and the ions present in the solution. The second is non-electrostatic, predominantly van der Waals in nature (Radovic et al., 2000; Lyklema, 1995). However, other factors including nature of the adsorbent and solution conditions (pH and ionic strength) can affect the adsorption process. It is known that, modification of the carbon surface with oxygen containing groups can offer not only a more hydrophilic surface structure, but also a larger number of oxygen-containing functional groups, which increases the ion-exchange capability of carbon material. Hence, the factors affecting the adsorption behavior of Zn(II), Cu(II), Fe(II) and Pb(II) from aqueous solutions are discussed. It is known that, modification of the carbon surface with oxygen containing groups can offer not only a more hydrophilic surface structure, but also a larger number of oxygen-containing functional groups, which increase the ion-exchange capability of carbon material. Hence, the factors affecting the adsorption behavior of Zn(II), Cu(II), Fe(II) and Pb(II) from aqueous solutions are discussed.

3.2.1

3.2.1 Contact time

Fig. 3 shows the effect of contact time on the adsorption efficiency of Zn(II), Cu(II), Fe(II) and Pb(II) onto MWCNTs (18–25 nm) and MMWCNTs from aqueous solutions. It indicates that the adsorption rates increase sharply in the first 15 min for Zn(II), Cu(II), Fe(II) and Pb(II) ions, and then they reach equilibrium gradually after 75 min for Pb(II) and 40 min for Zn(II), meanwhile after 30 min for Cu(II) and Fe(II). Also, Fig. 3 indicates that the adsorption process takes place rapidly on the surface of MWCNTs and MMWCNTs in the first 15 min. Accordingly the lower adsorption rate with increasing the time may be due to the longer diffusion range of metal ions into the inner cavities and interlayers of MWCNTs (Li et al., 2002).

Effect of contact time on the adsorption of metal ions (a) MWCNTs and (b) MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 3 Effect of contact time on the adsorption of metal ions (a) MWCNTs and (b) MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).

The adsorption efficiency of Zn(II) and Cu(II) after 90 min reached 56%, 40% and 67.2%, 75.5% by MWCNTs and MMWCNTs. On the other hand, adsorption of Fe(II) and Pb(II) did not show significant improvement on going from MMWCNTs to MWCNTs (76.78% and 78.81%).

3.2.2

3.2.2 MWCNTs dose

The effect of the MWCNTs and MMWCNTs dosage on the % of the metal ions adsorbed from aqueous solutions revealed that the removal efficiencies of metal ions increased gradually with increasing MWCNTs concentration. As shown in Fig. 4, increasing the mass of MWCNTs from 0.01 g to 0.1 g are followed by increasing the % adsorbed from 30.67%, 32.89%, 60.78% and 55.89% to 44.76%, 55.76%, 77.13% and 78.86% for Zn(II), Cu(II), Fe(II) and Pb(II) ions.

Effect of dosage (a) MWCNT and (b) MMWCNTs on the adsorption of metal ions. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 4 Effect of dosage (a) MWCNT and (b) MMWCNTs on the adsorption of metal ions. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).

Increasing the masses of MMWCNTs results in increasing the % adsorbed of the same series of ions from 60.83%, 58.9%, 67.54% and 65.56% to 67.54%, 75.99%, 77.87% and 81.33%. This may be due to the fact that increasing the adsorbent dose provided a greater surface area or more adsorption sites for the metal ions (Rengaraj and Moon, 2002). Further increasing in the amount of MWCNTs used from 0.05 g to 0.1 g had no effect on the percentage of metal ions removed (Fig. 4).

3.2.3

3.2.3 Ionic strength

The influence of the ionic strength on the adsorption of the metal ions is critical, because it can create different adsorption situations by which electrostatic interactions between the MWCNTs surface and metal ions are either attractive or repulsive. Fig. 5 shows the adsorption of Zn(II), Cu(II), Fe(II) and Pb(II) by MWCNTs and MMWCNTs using 0.001, 0.01, 0.1 and 1.0 mol/L KNO3 ionic strength. In case of MWCNTs, the adsorption % was gradually decreased from 56%, 40.98%, 76.02% and 78.1% to 0%, 5%, 48%, 67.66% and 8.39% for Zn(II), Cu(II), Fe(II) and Pb(II) with increasing the ionic strength. Using MMWCNTs, (Fig. 5(b)), the adsorption capacities also decrease with increasing ionic strength from 67.18%, 75.5%, 78.3% and 80.9% to 20.9%, 64.56%, 68.8% and 32.6% for the same series of ions.

Effect of ionic strength on metal ion adsorption onto (a) MWCNT and (b) MMWCNTs on the adsorption of metal ions. Adsorbent mass 0.03 g/50, mL; [M]2+ = 100 mg/L; contact time = 90 min; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 5 Effect of ionic strength on metal ion adsorption onto (a) MWCNT and (b) MMWCNTs on the adsorption of metal ions. Adsorbent mass 0.03 g/50, mL; [M]2+ = 100 mg/L; contact time = 90 min; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).

The decrease in adsorption efficiency by MWCNTs and MMWCNTs to the targeted metal ions may be due to:

  1. the change in the attractive nature of the electrostatic interactions between the metal ions and the MWCNTs surface at low ionic strength (0.001 and 0.1 M KNO3) to repulsive in nature on increasing the ionic strength from 0.1 to 1.0 M KNO3 and hence decreases the adsorption of metal ions,

  2. the formation of electrical double layer complexes of Zn(II), Cu(II), Fe(II) and Pb(II) ions with MWCNTs and MMWCNT favors the adsorption when the concentration of the competing salt is decreased. The accumulation of charge (such as K+) in the vicinity of the MWCNTs surface decreases heavy metal ion interaction constants. This effect created a localized potential that repels other cations, thus reducing their adsorption potential (Dastgheib and Rockstraw, 2002). Furthermore, an increase in ionic strength supplies more positive ions, which outcompete heavy metals ions for adsorption sites on the MWCNTs. Also, the influence of the ionic strength on the activity coefficients of Zn(II), Cu(II), Fe(II) and Pb(II) ions may limit their transfer to the CNTs’ surfaces (Reddad et al., 2002).

3.2.4

3.2.4 Effect of pH

Solution pH is one of the main factors that have great impacts on the adsorption process, especially for heavy metal ions, such as Zn(II), Cu(II), Fe(II) and Pb(II) as they exist in different species depending on the pH of solution. The effect of solution pH on the adsorption of ions by MWCNTs and MMWCNTs is studied in the pH 1–10 and the obtained results are illustrated in Fig. 6. It is obvious that the adsorption % for Zn(II) and Cu(II) metals ions increased gradually from 0.0% and 1.71% to 50.92% and 40.64% (MWCNTs) with increasing pH from 1 to 7. For Fe(II) and Pb(II) ions in the pH range 1–5 the adsorption % increases from 12.99% and 12.025% to 60.78% and 70.67%. In case of MMWCNTs (Fig. 6(b)), the adsorption % ions increased from 0.0%, 45.19%, 15.33% and 0% to 59.48%,74.2%, 75.67% and 79.65% for Zn(II), Cu(II), Fe(II) and Pb(II) ions (pH = 1–7).

Effect of solution pH on the adsorption of metal ions (a) MWCNTs and (b) MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3.
Fig. 6 Effect of solution pH on the adsorption of metal ions (a) MWCNTs and (b) MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3.

The minimum adsorption observed at low pH values may be due to the fact that hydrogen ions have better adsorption than the metal ions because of their higher concentration and mobility present (Kosa et al., 2012). Consequently, the surface of the MWCNTs is predominantly covered by H+, which prevents metal ions from approaching the binding sites. This was also in agreement with the surface complex formation (SCF) theory since an increase in the pH decreases the competition for adsorption sites between protons and metal species resulting in an increase in adsorption of metal ions (Dzombak and Morel, 1990).

On increasing the pH from 5 to 10 (Fe(II) and Pb(II) ions) and from 7 to 10 (Cu(II) ions), the adsorption % decreased on both MWCNTs and MMWCNTs due to precipitation of metal hydroxides (Lv et al., 2005). On the other hand, the adsorption % of Zn(II) (pH = 1–7) increases rapidly due to the combined role of adsorption and precipitation (Deliyanni et al., 2007). The removal of zinc was potentially accomplished through the simultaneous precipitation of Zn ( OH ) 2 . d adsorption of Zn ( OH ) + . Zn ( OH ) 3 - . d Zn ( OH ) 4 2 - . Leyva et al., 2002). The decrease in the adsorption of Fe(II) (pH > 7) may be due to precipitation of metal hydroxide.

3.2.5

3.2.5 Metal ion concentration

The effect of metal ion concentration of 3, 10, 50, 100, 250 mg/L on the adsorption behavior of MWCNTs (18–25 nm) and MMWCNTs is studied and the results are shown in Fig. 7. It is obvious that with the increase in initial concentration of metal ions in aqueous solution the removal efficiency decreases. For 250 ppm, the removal efficiencies are 45.87%, 24.74%, 26.7% and 41.86% for Zn(II), Cu(II), Fe(II) and Pb(II) using MWCNTs but for the MMWCNTs the efficiencies are 50.10%, 39.0%, 30.53% and 43.62%, respectively. On the other hand, at 10 ppm, the removal efficiencies of MWCNTs are 76.40%, 56.0%, 99.1% and 97.87% for Zn(II), Cu(II), and Fe(II) and while the MMWCNT gives the values 87.30%, 66.70%, 99.40% and 99.57%, respectively.

Effect of metal ion concentration on the adsorption by (a) MWCNTs and (b) MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 7 Effect of metal ion concentration on the adsorption by (a) MWCNTs and (b) MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).

This may indicate that the adsorption interaction between the MWCNTs and metal ions is mainly of ionic nature (Elsehly et al., 2015). High concentration of metal ions limits their transfer to MWCNT surface and also may be due to saturation of the active sites of the MWCNTs with the metal ions. Increasing the removal efficiencies using the MMWCNTs may be due to the high chance of complex formation between the modifier and the metal ion.

3.3

3.3 Adsorption kinetics

To analyze the adsorption rate of the metal ions onto the MWCNTs, common kinetic models were tested under the experimental conditions namely the pseudo first-order rate equation of Lagergren (Lagergren and Vetenskapsakad, 1898) and the pseudo second-order rate equation (Ho and McKay, 2000).

3.3.1

3.3.1 The pseudo first-order model

The equation representing pseudo first-order rate is as follows:

(1)
Log [ ( q e - q t ) / q e ] = ( k 1 · t ) / 2.3 where k1 is the Lagergren rate constant of adsorption (min−1), and qe and qt are the amounts of adsorbed metal (mg g−1) at equilibrium and at time t. Linear plot of log (qe − qt) vs t is drawn and k1 are calculated from the slope and intercept. Results are summarized in Tables 1 and 2.
Table 1 Kinetic parameters of the metal ion adsorption by MWCNTs.
Metal ion Pseudo first-order Pseudo second-order
qe K1 R2 qe K2 R2
Zn(II) 18.15 0.037 0.964 52.63 0.0044 0.998
Cu(II) 4.6 0.012 0.951 41.667 0.013 0.998
Fe(II) 28.9 0.053 0.931 76.923 0.0032 0.998
Pb(II) 97.27 0.037 0.957 142.86 0.000123 0.911
Table 2 Kinetic parameters of the metal ion adsorption by MMWCNTs.
Metal ion Pseudo first-order Pseudo second-order
qe K1 R2 qe K2 R2
Zn(II) 31.76 0.032 0.989 66.667 0.0018 0.995
Cu(II) 49.77 0.046 0.945 83.333 0.0015 0.995
Fe(II) 37.8 0.025 0.963 83.333 0.0013 0.992
Pb(II) 87.1 0.027 0.970 125 0.00018 0.907

The obtained results show straight lines, which indicate that the pseudo-first-order Lagergren equation was appropriate for describing the adsorption of the target metal ions by MWCNTs (Fig. 8) and MMWCNTs (Fig. 9).

Pseudo-first order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 8 Pseudo-first order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Pseudo-first order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 9 Pseudo-first order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).

3.3.2

3.3.2 The pseudo-second order equation

The pseudo-second order equation has also been interpreted as a special kind of Langmuir kinetics, assuming that: (i) the adsorbate concentration is constant all the time; and (ii) the total number of binding sites depends on the amount of adsorbate that adsorbed at equilibrium. The linearized form of the pseudo-second order rate equation is given as follows:

(2)
t / q t = 1 / k 2 q e 2 + t / q e where k2 (g mg−1 min−1) is the pseudo-second order rate constant, and qe and qt are the amounts adsorbed per unit mass at equilibrium and at time t.

Linear plot of t/qt vs t was drawn and k2 values were calculated from the slope and intercept (Tables 1 and 2). The integral form of the time/adsorbed amount should be a linear function (Rudzinski and Plazinski, 2009).

By applying the pseudo-second order rate equation to the experimental data for the adsorption of Zn(II), Cu(II), Fe(II), and Pb(II) onto MWCNTs (Fig. 10) and onto MMWCNTs (Fig. 11), straight lines were obtained.

Pseudo-second order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 10 Pseudo-second order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Pseudo-second order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).
Fig. 11 Pseudo-second order plots for Fe(II), Pb(II), Cu(II) and Zn(II) adsorbed on MMWCNTs. Adsorbent mass 0.03 g/50 mL; [M]2+ = 100 mg/L; contact time = 90 min; I = 0.001 M KNO3; pH = 4 for Fe(II) and Pb(II); pH = 7 for Cu(II) and Zn(II).

From the obtained results illustrated in Figs. 8–11 and Tables 1 and 2, it is obvious that the adsorption process generally followed the pseudo second- order rate equation. All the experimental data fit with good correlation coefficient values (R2), which indicated the suitability of the pseudo second order rate equation for the description of the adsorption of metal ions from aqueous solution by MWCNTs and MMWCNTs.

3.4

3.4 Adsorption isotherms

The experimental data of Zn(II), Cu(II) Fe(II) and Pb(II) metal ion adsorption onto the MWCNTs and MMWCNTs are analyzed using Langmuir isotherm to determine the maximum metal adsorption capacities (McKay et al., 1999). The used Langmuir linearized equation is as follows:

(3)
C e / q e = C e / q m + 1 / K L q m where Ce is the equilibrium concentration of heavy metal that is remaining in solution, qe (mg/L) is the amount adsorbed at equilibrium (mg/g) and qm is the maximum adsorption capacity corresponding to complete monolayer coverage (mg/g), KL is the Langmuir model constant that is indirectly related to the energy of adsorption (L/mg). qe was calculated as follows:
(4)
q e = ( ( C o - C t ) V ) / m where Co is the initial metal ion concentration (mg/L), Ct is the final metal ion concentration after a certain period of time (mg/L), V is the initial solution volume (L) and m is the MWCNTs dose (g).

The results of single metal ion adsorptions of Zn(II), Cu(II), Fe(II) and Pb(II) adsorbed by MWCNTs and MMWCNTs at 25 °C listed in Table 3 can be represented well by Langmuir model with good values of correlation coefficient (R2). This means that the adsorbates are adsorbed in such a manner that only one atomic layer of adsorbate can be adsorbed and distributed uniformly on the surface of the adsorbents. According to the Langmuir adsorption model, the saturation amounts, qm, of Zn(II), Cu(II) Fe(II) and Pb(II) adsorbed by 1 g MWCNTs are 250, 142.8, 111.1 and 200 mg/g, respectively. On the other hand, by using MMWCNTs, qm values are found to be 500, 333.3, 200 and 333.3 mg/g for Zn(II), Cu(II) Fe(II) and Pb(II), respectively.

Table 3 Langmuir isothermal adsorption models for adsorption of Zn(II), Cu(II), Fe(II) and Pb(II) by MWCNTs and MMWCNTs.
Metal ions MWCNTs MMWCNTs
qm (mg/g) KL (L/mg) R2 qm (mg/g) KL (L/mg) R2
Zn(II) 500 0.007 0.968 250 0.02 0.915
Cu(II) 333.3 0.021 0.995 142.8 0.0186 0.920
Fe(II) 200 2.5 0.999 111.1 1.8 0.999
Pb(II) 333.3 0.2 0.993 200 0.147 0.994

The experimental data for metal ions adsorption onto CNTs did not agree with Freundlich adsorption isotherm model.

3.5

3.5 Analysis of real wastewater samples

The real waste water samples are collected from four different sites in Suez Gulf, Red Sea, Egypt. To verify the applicability of the MWCNTs for the removal of heavy metals from real water sample, the samples were spiked with 50 mg/L of Cu(II), Pb(II), Fe(II) and Zn(II). Adsorption experiments using MWCNTs and MMWCNTs are then performed. The mean of removal percentage of divalent metal ions from the studied sites is listed in Table 4.

Table 4 Removal of the target heavy metal ions from spiked real sample by MWCNTs and MMWCNTs.
Metal ion % Adsorption on MWCNTs % Adsorption on MMWCNTs
Fe(II) 99.51 99.60
Pb(II) 85.82 88.04
Cu(II) 58.33 60.58
Zn(II) 7.58 11.58

The obtained results prove the effectual removal of heavy metal ions under investigation using MWCNTs as well as MMWCNTs.

3.6

3.6 Comparison with another method

From the data listed in Table 5, it is generally revealed the high efficiency of the MWCNTs and its modification for removal of the Pb(II), Cu(II) and Zn(II) ions from the working samples.

Table 5 Comparison between the removal of the target divalent metal ions by MWCNTs and MMWCNTs with another method using Zeolite as adsorbent.
Adsorbent Adsorbent dosage Metal ions Lower/Upper concentration (mg/L) pH Time qm (mg/g) Reference
Zeolite 0.3 g/50 mL Pb(II) 5–20 6 24 h 56.82
Cu(II) 44.25
Zn(II) 41.84
MWCNTs 0.03 g/50 mL Pb(II) 3–250 4 1.5 h 200 This work
Cu(II) 7 142.8
Zn(II) 7 250
MMWCNTS 0.03 g/50 mL Pb(II) 4 333.3
Cu(II) 7 333.3
Zn(II) 7 500

4

4 Conclusions

In this study the capacity of MWCNTs as well as MMWCNTs, with 5,7-dinitro-8-quinolinol as adsorbent for removal of Zn(II), Cu(II), Fe(II) and Pb(II) metal ions from water samples was evaluated. The parameters affecting the adsorption process were optimized and it was found to increase with increasing solution pH and MWCNTs mass, while decreases with increasing solution ionic strength and metal ion concentration. The effect of contact time on the adsorption efficiency showed that adsorption rates increased sharply in the first 15 min and then reached equilibrium gradually after 75 min for Pb(II) and 40 min for Zn(II), meanwhile after 30 min for Cu(II) and Fe(II). The adsorption of the metal ions was in better agreement with pseudo-second order kinetics and can be represented well by Langmuir model indicating a manner that only one atomic layer of adsorbate can be adsorbed and distributed uniformly on the surface of the adsorbents.

Acknowledgments

The authors are thankful to Faculty of science, Suez Canal University, for providing us all the laboratory facilities and chemicals. Also, we would like to thank Professor Ibrahim A. Ibrahim, Department of Chemistry, Faculty of Science, Suez Canal University, for preparing the modifier.

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