ABSTRACT

Recently, many attempts have been made to use carbon nanotubes in analytical chemistry, especially in adsorption of heavy metal ions from water. In this study, multiwalled carbon nanotubes (MWCNTs) were functionalized with tris(2- aminoethyl) amine. The functionalized nanoparticles were characterized using Fourier transform infrared (FTIR), thermal gravimetric analyzer (TGA), elemental analysis, and Raman spectroscopy. The results revealed that the functionalization reaction was successfully accomplished. Lead adsorption from water was carried out using functionalized MWCNTs and measured by flame atomic absorption spectrometry (FAAS). The effects of pH, shaking time, initial metal ion concentration, and adsorbent dosage on the adsorption process were studied via batch method. The results obtained showed that removal of lead ions strongly depended on the pH. Desorption study revealed that lead ions adsorbed on the functionalized MWCNTs could be desorbed at pH < 3 due to breakage of complexes formed on the sorbent surface. Maximum adsorption capacity of the sorbent under the optimal conditions was 43 mg/g. This favorable adsorption capacity suggests that functionalized carbon nanotubes can be applied for removal of lead from water solutions. The data obtained were fitted with the Langmuir and Freundlich isotherm adsorption models and Langmuir model showed better agreement with the experimental data.

1. Introduction

Carbon nanotubes as a new type of materials from carbon family have a coaxial cylindrical form, composed of graphene sheets within nanometer size in diameter and micrometer size in length [1,2]. These materials are classified to single-walled (SWCNTs) and multiwalled

(MWCNTs) carbon nanotubes. These different groups of nanotubes do not show the same properties. Unique physical and chemical properties such as high surface area, chemical stability, mechanical strength, low density, thermal stability, and electrical properties have reported for carbon nanotubes in different applications [3-8]. Strong adsorption capacity is the main reason for using these materials in analytical chemistry; especially in removing environmental pollutants such as phenols [9], diazinon [10], diuron [11], atrazine [12], organo phosphorus compounds [13], rhodium [14], zinc [15], chromium [16,17], nickel [18] and lead [19-21].

Lead is the most important pollutant entering human life through industrial wastewater and causes different illnesses. Due to non-degradability of lead, many attempts have done to remove this pollutant from the environment. Various methods including precipitation, liquid-liquid extraction, membrane filtration, ion exchange, and adsorption have been used for separation of lead from different waters. Among the above-mentioned methods, adsorption is a principle method for removal of heavy metals from environmental waters, because of its efficiently and simplicity. Many efforts have been focused on finding new adsorbents with higher adsorption capacity.

Carbon nanotubes can be a beneficial sorbent. However, the raw ones are rarely used for sorption of ion metals owing to van der Waals interactions among carbon atoms in graphene sheets. In contrast, 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 sorption metal ions through chemical bonding, which would be more selective than raw carbon nanotubes. Thus, functionalization by chemical groups has been considered by the researchers [22,23]. In general, some modifications on the surface of carbon nanotubes are firstly made by strong acids by introducing carboxyl, hydroxyl, and carbonyl groups on the surface of carbon nanotubes [23-25]. Further modifications are accomplished by various molecules that can be grafted through creation of amide, ester, and amine bonds. Typical molecules reported in the literature as functionalization agents are 1-(2-pyridylazo)-2-naphtol [26], thiosemicarbazide [27], 2-aminobenzothiazole [20], phenyl-iminodiaceticacid [28], ethyenediamine [29], diethylenetriamine, triethylenetetrami [30], poly(2-aminothiophenol) [31], and tris(2-aminoethyl)amine [32].

This study is motivated by the current researches in the field of adsorption of heavy metal ions using functionalized carbon nanotubes. The aim of this work was preparation of functionalized MWCNTs with tris(2-aminoethyl) amine (MWCNTs-TAA). The potential of the functionalized MWCNTs obtained was evaluated using FAAS as an adsorbent for lead. The effects of pH value, sorbent dosage, and shaking time on the adsorption process were determined. Adsorption capacity was measured via the batch method. The data obtained were simulated by Langmuir and Freundlich isotherm models.

2. Experimental

2.1. Materials

MWCNTs was obtained from Cheaptubes Inc. (USA) with 10 - 20 nm in outer diameter, 30 nm in length, and >95% purity. All other reagents such as concentrated HNO₃, thionyl chloride (SOCl₂), dimethyl formamide (DMF), dichloromethane (DCM), triethylamine, and tris (2-aminoethyl) amine were provided from Sigma Aldrich.

Metal stock solution (1000 mg∙L⁻¹) of pb(II) was prepared by dissolving its nitrate salt (analytical grade) in deionized water. The solution was further diluted to the required concentrations for the sorption measurements. Laboratory glassware used were repeatedly cleaned with 5% v/v HNO₃ and rinsed with deionized water.

2.2. Instruments and Apparatus

A Varian Model spectra AA.200 atomic absorption spectrometer equipped with single element hollow cathode (HCL) lamps, deuterium background correction, and airacetylene was used for pb(II) measurements. Lead HCL was used as the radiation sources at the wavelength of 217 nm. The instrumental parameter adjustments were those recommended by the manufacturer.

Thermogravimetry analysis (TGA) was conducted by a TGA-7HT from Perkin Elmer (USA) under N₂ and in the range of 25˚C - 700˚C at the heating rate of 10˚C/ min.

Infrared spectra were recorded on an EQUINOX 55 FTIR spectrometer. The pH values were adjusted with a Metrohm digital pH meter. The Raman spectra was obtained by a Raman Spectrometer; Model Almega Thermo Nicolet Dispersive Raman Spectrometer.

Elemental analysis of functionalized samples was applied using Vario EL III Analysen systeme GmbH.

2.3. Oxidized MWCNTs Preparation (MWCNT-COOH)

Oxidized MWCNTs were prepared by oxidation with 3M HNO₃. Predetermined quantities of raw MWCNTs and 250 ml HNO₃ were dispersed for 30 min in an ultrasonic bath (35 KHz, 70 W). The reaction flask equipped with a reflux condenser, a magnetic stirrer and a thermometer was mounted in the oil bath. The suspension was then stirred at 110˚C for 48 h. The mixture was cooled to the room temperature and was vacuum filtered. The mixture was then washed with deionized water until a neutral pH of the filtrate was reached. The oxidized MWCNTs were dried in a vacuum oven at 105˚C overnight.

2.4. Synthesis of MWCNTs-TAA

The 500 mg MWCNTs-COOH was suspended in 20 ml thionyl chloride and stirred at 70˚C for 24 h. Using DCM, the mixture was diluted and filtered under vacuum condition. Washing by DCM was continued until the filtrate became colorless.

The solid filtrate (MWCNTs-COCl) obtained was dried in a vacuum oven at 40˚C. In the next step, the dried MWCNTs-COCl was dispersed into 50 ml anhydrous DMF, and then 15 ml tris(2-aminoethyl) amine and 3 ml triethylamine were added. The mixture left to stir at 50˚C for 48 h. The mixture obtained was filtered through an appropriate filter. To remove the unreacted materials, the filtrate was washed by DMF until the colorless DMF was observed. Finally, the filtrate was washed by ethanol and put under vacuum oven at 80˚C for 12 h.

2.5. Batch Method Procedure

Batch adsorption experiments were performed using a portion of the sample solutions (10 mL) including pb(II) ions, transferred into a triangular flask. The pH values of the solutions were adjusted using HNO₃ or NaOH with (0.1 - 0.01) mol∙L⁻¹ concentration. Then, 10 mg of the adsorbent was added to the solution and the mixture was shaken at 150 rpm for 45 min. The mixture was filtered after centrifuge at 1500 rpm for 20 min. The concentration of pb(II) ions in the solution was determined by FAAS.

The percentage of removed pb(II) ions (R) was determined as follows [33]:

(1)

where C₀ is the initial concentration of pb(II) ions in mg∙L⁻¹, C_t is the concentration of pb(II) ions at time t in mg∙L⁻¹.

3. Results and Discussion

3.1. Characterization of Functionalized MWCNTs

The FTIR spectra of pristine MWCNTs and MWCNTsCOOH and MWCNTs-TAA are illustrated in Figure 1. In the FTIR spectrum of MWCNTs-COOH, the peaks at 3415 and 1620 cm⁻¹ can be attributed to the stretching vibrations of O-H and C=O of carboxyl groups, respectively. In addition, the peak at 1080 cm⁻¹ is assigned to the C-O stretching vibration. Presence of these peaks suggests that oxidation of the MWCNTs have introduced COOH groups on the surface of MWCNTs. Similar results have reported by G. D. Vukovic [29,30].

In the FTIR spectrum of MWCNT-TAA a peak appears at 1580 cm⁻¹, which is originated from C-N stretching and N-H bending vibration [32]. Existence of this bond implies grafting of TAA on the MWCNT-COOH.

TGA gives useful information about the MWCNTsTAA structure because the pristine MWCNT is stable and hardly decompose below 700˚C while the weight of MWCNTs-COOH and MWCNTs-TAA decrease. Thermograms of MWCNTs-COOH and MWCNTs-TAA are demonstrated in Figure 2. As can be seen in this Figure, MWCNTs-COOH decomposition occurs gradually in the whole range of temperature. The trend of weight loss for MWCNTs-TAA is moderate and faster than that of MWCNTs-COOH up to 450˚C due to decomposition of TAA. Above 450˚C, the decomposition rate of MWCNTsTAA is slow and may be attributed to degradation of unreacted -COOH groups. Residuals of the oxidized carbon nanotube and MWCNTs-TAA at 450˚C were 6 and 9 wt%, respectively. The results of elemental analysis are embedded in thermograms. Existence of nitrogen in MWCNTs-TAA is a sign of successful functionalization of TAA on the MWCNTs surface. The percentage of N in MWCNTs-TAA is in agreement with TGA thermograms. In other words, the percentage of N shows the presence of almost 8% TAA in MWCNTs-TAA, which is approximately equal to the weight loss in the thermogram.

Raman spectroscopy is a valid method to track the carbon nanotube surface modification, which have used by researchers [32,34,35]. Figure 3 shows Raman spectra of MWCNTs-COOH and MWCNTs-TAA. As can be seen, the main peaks in the Raman spectra are D and G bonds at 1339 and 1580 cm⁻¹, respectively. D band is attributed to disordered carbon atoms of MWCNTs corresponding to sp³-hybridized and G band is related to the sp²-hybridized carbon atoms. Area ratio of the D and G bands were 1.27 for MWCNTs-COOH and 1.5 for MWCNTs-TAA. This gradual alteration is an indication for the occurrences of MWCNTs-COOH conversion to MWCNTs-TAA.

3.2. Assessment of Adsorption

3.2.1. Effect of pH

One of the most important analytical factors in adsorption of metal ions is the pH value, owing to its effect on solubility of metal ions, surface charge adsorbent and degree of protonation of binding sites of the chelating molecules on adsorbate during the reaction [36,37]. To evaluate the effect of pH on the lead adsorption, experiments were carried out with solutions of initial concentration at several pH values in the range of 3 - 11 using either NaOH (0.01 - 0.1 mol∙L⁻¹) or HNO₃ (0.01 - 0.1 mol∙L⁻¹) for pH adjustment.

Four probable forms of lead at different pH values exist that are Pb²⁺, Pb(OH)⁺, Pb(OH)₂, and Pb(OH)³⁻ [38]. At PH < 6, the dominant form of the lead is Pb²⁺ while with increase in pH value, other forms are created. It is clear that sorption initiates from pH = 3 and there is no adsorption below this pH value. This trend can be attributed to competition between H⁺ and Pb²⁺ ions for capturing the active sites. The favorite mechanism in this system is reaction between electron lone pair in nitrogen of the TAA groups and metals ion, which results in formation of metal complex. While in acidic solution (pH < 3), the active sites of TAA (electron lone pair) is protonated and inhibited from reaction with Pb²⁺.

Figure 4 shows the removal percentage versus pH. As seen in this Figure, the best removal occurred in the pH range of 5.7 - 6. By considering the precipitation constant (1.2 × 10⁻¹²) and the initial concentration (C₀ = 10 mg/L), the main reason for the best recovery in this pH range is dominant presence of Pb²⁺. In other words, above this range, there is interference between precipitation and adsorption and one cannot claim that the pb(II) removal is only accomplished by the sorbent.