Salty water desalination using carbon nanotube sheets

Salty water desalination using carbon nanotube sheets

Desalination 258 (2010) 182–186 Contents lists available at ScienceDirect Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m...

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Desalination 258 (2010) 182–186

Contents lists available at ScienceDirect

Desalination j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / d e s a l

Salty water desalination using carbon nanotube sheets Maryam Ahmadzadeh Tofighy, Toraj Mohammadi ⁎ Research Centre for Membrane Separation Processes, Faculty of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 21 January 2010 Received in revised form 3 March 2010 Accepted 5 March 2010 Available online 8 April 2010 Keywords: Carbon nanotube sheets Chemical vapor deposition Adsorption Salty water desalination

a b s t r a c t Desalination using carbon nanotube (CNT) sheets was performed. Carbon nanotube sheets were synthesized by chemical vapor deposition using cyclohexanol and ferrocene in nitrogen atmosphere at 750 °C, and oxidized with concentrated nitric acid at room temperature and then employed as adsorbent for salty water desalination (sodium chloride removal from water). Effects of adsorption time and initial salt concentration on performance of the oxidized CNT sheets were investigated. It was found out that both Langmuir and Freundlich isotherm models match the experimental data very well. The results demonstrated that the oxidized CNT sheets can be used as an effective adsorbent for salty water desalination due to their very high adsorption capacity. Also, using the oxidized CNT sheets, desalination without CNT leakage into water is economically feasible. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Desalination is a process that removes dissolved minerals from seawater, brackish water, or treated water. About 71% of the earth surface is covered by water which is in the form of the oceans, seas and the ices in the poles. However, only about 3% of water is fresh and suitable for drinking. The water of the oceans and seas is salty and not directly utilizable. Therefore, some special processes are needed to desalinate the salty water and as a result to overcome the water shortage [1–5]. There are many convectional methods that are being used to remove ions including oxidation, reduction, precipitation, membrane filtration, ion exchange and adsorption. Among the above methods, the promising process for water and waste water desalination is adsorption [6]. Carbon nanotubes (CNTs), a member in carbon family, are relatively new adsorbents that have been proven to possess great potential for removing many kinds of pollutants such as dioxin [7], ammonia [8], ozone [9] and methane [10] from air. CNTs have also been used as adsorbents to remove metal ions [11–18], fluoride [19], 1,2-dichlorobenzene [20], trihalomethanes [21] and organic pollutants [22] from water. The hexagonal arrays of carbon atoms in graphite sheets of CNTs have strong interactions with other molecules or atoms. Recently, powder of CNTs has been used as adsorbent in adsorption processes very often. In industrial scale, for example sea water desalination, when powder of CNTs is used as adsorbent, mixing of the CNTs with sea water with ultrasonic agitation is not economically and technically possible. Also, after adsorption process, it is difficult to

⁎ Corresponding author. Tel.: + 98 21 77240496; fax: +98 21 77240495. E-mail address: [email protected] (T. Mohammadi). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.03.017

completely remove powder of CNTs from sea water without centrifuging process. Also, separation of the CNTs from sea water by filtration is difficult because the filter may be quickly blocked by the CNTs. Also, the CNTs may possess some degree of toxicity and as a result their safety is controversial. Therefore, CNT sheets were synthesized as a practical adsorbent for desalination, because economical salty water desalination without CNT leakage into water is preferable. The objectives of the present study were to synthesize and oxidize the CNT sheets, and then to investigate capabilities of the oxidized CNT sheets for salty water desalination. Effects of adsorption time and initial salt concentration on performance of the oxidized CNT sheets were also investigated. Also, Langmuir and Freundlich isotherm models were applied to fit the experimental data. 2. Experimental procedure 2.1. Synthesis of the CNT sheets A schematic diagram of the experimental setup is shown in Fig. 1 [23]. The CVD system consisted of a horizontal stainless steel tube (70 cm long, 3.2 cm in diameter) was housed in a one stage cylindrical furnace. A flask (steel container) containing reagents was connected to the reactor. The reagents were prepared by dissolving ferrocene (purity ≥ 98%, B.D.H) in cyclohexanol (purity ≥ 98%, Fluka) with mass ratio of 1:10. Evaporating the reagents was performed using an oil bath. Nitrogen was used as carrier gas connected to the reactor nearby the reagent inlet. At first, the reactor was purged with nitrogen in order to eliminate oxygen from the reaction chamber. The reactor was preheated to preset temperature (at 750 °C); subsequently the flask containing the reagents was placed in the oil bath (at 220 °C) for immediate vaporization process. The obtained vapor was carried by nitrogen

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Fig. 1. Schematic diagram of the experimental CVD setup.

with flow rate of 400 cm3 min− 1 towards the high temperature zone of the reactor. The reactor pressure was kept constant at about 2 bar during the CVD reaction for about 3 h. Pyrolysis of the vapors took place and a CNT sheet with thickness of about 2 mm was deposited on the high temperature zone of the reactor compactly. After the CVD reaction, the furnace was switched off and the reactor was cooled down to room temperature. Finally, the as-synthesized CNT sheets were removed from the reactor wall for characterization and further processing. Scanning electron microscope (SEM, Philips: XL30) was used for analysis of the CNT sheets morphology, and nanostructure of the CNTs was determined by transmission electron microscope (TEM, Philips: CM200). 2.2. Oxidation of the CNT sheets The as-synthesized CNT sheets were immersed in concentrated nitric acid (65%, Merck) for 20 h and then washed using deionized water several times until pH of the washing water showed no change, then dried at 110 °C for 24 h. This oxidized CNT sheets were used as adsorbent for salty water desalination. The oxidized CNT sheets were as wide as 4 cm2 and as thick as 2 mm. 2.3. Adsorption procedure Batch adsorption experiments were carried out using 5000, 10,000, 20,000, 30,000 and 40,000 mg/l solutions prepared from sodium chloride (purity ≥ 99.99%, Iran) in deionized water. All the experiments were performed via soaking 100 mg of the CNT sheets in 25 ml of the sodium chloride solutions at room temperature. In all experiments, a conductometer (CRISON, GLP 32) was used to measure the salt concentration of the solutions. Water conductivity directly depends on salt concentration. This dependency (calibration curve of the conductometer) is shown in Fig. 2. The amount of adsorbed salt can be obtained using the following equation: qe =

ðC0 −Ce ÞV w

CNT with inner tube diameter of about 10 nm and outer tube diameter of about 30–40 nm). From the SEM image, the CNT sheet is regarded as an entangled CNT network. The CNT sheets have high flexibility and are not easily broken during oxidation, washing and drying processes. As shown in Fig. 4a, the oxidized CNT sheets exhibit greater sodium ion adsorption capacity than the as-synthesized CNT sheets. This can be explained by the fact that the metal ion adsorption capacity of CNTs does not directly relate to their specific surface area, pore specific volume and mean pore diameter but strongly depends upon their surface total acidity including functional groups such as carboxyls, lactones and phenols [6]. Oxidation of the CNTs can offer not only a more hydrophilic surface structure, but also a greater number of functional groups. As reported in literature different acidic functional groups such as carboxyls, lactones and phenols can be formed by nitric acid oxidation [17]. It is commonly believed that the chemical interactions between the metal ions and the surface functional groups of the CNTs are the major adsorption mechanisms [6,14,16]. Protons in the acidic functional groups of CNT sheets are exchanged with sodium ions in the salty solutions. As shown in Fig. 4b, the salty solution pH drops with adsorption of sodium ions onto the oxidized CNT sheets. This can be explained by releasing protons from the CNTs surface where sodium ions are adsorbed, consequently reducing the solution pH. As shown in Fig. 4b, the solution pH increases with adsorption of sodium ions onto the as-synthesized CNT sheets and this demonstrates that the as-synthesized CNT sheets have more basic functional groups. Oxidation of the as-synthesized CNT sheets with nitric acid reduces these basic functional groups and additionally forms the new acidic functional groups. The surface total acidity of the as-synthesized and oxidized CNT sheets were quantified by Boehm method [24] and presented in Table 1.

ð1Þ

where qe is the adsorption uptake (mg/g), C0 is the initial salt concentration (mg/l), Ce is the equilibrium salt concentration (mg/l), V is the volume of the solution (l) and w is the mass of the adsorbent (g). 3. Results and discussion 3.1. Characterization of CNT sheets Fig. 3 shows the synthesized oxidized CNT sheets: (a) an overview image and (b) a surface SEM image (inset: a TEM image of a typical

Fig. 2. Calibration curve of the conductometer.

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Fig. 3. Synthesized oxidized CNT sheets, (a) an overview image, (b) a surface SEM image (inset: TEM image of a CNT).

The surface charge depends on pH of the surrounding electrolyte. There is a pH value, called ‘point of zero charge’ (PZC), at which the net surface charge is zero. The pHPZC of the as-synthesized and oxidized CNT sheets were quantified by mass titration [25] and presented in Table 1. As observed, the pHPZC shifts to lower pH values after nitric acid oxidation. These results confirmed the data obtained from Boehm's titration. Figs. 5 and 6 show the adsorption behavior of sodium ions onto the oxidized CNT sheets at different initial salt concentrations. As observed, the adsorption of sodium ions increases slowly and reaches to a maximum uptake of 550 mg/g after about 48 h for initial salt concentration of 5000 ppm, while a maximum value of 1320 mg/g is reached for initial salt concentration of 40,000 ppm, after 6 days. This high equilibrium uptake indicates that the oxidized CNT sheets have a strong potential for practical salty water desalination applications. As shown in Fig. 6, increasing initial salt concentration increases significantly the sodium ions uptake per unit weight of the CNT sheets. Also, it can be observed that the equilibrium is reached faster at lower initial salt concentration, probably because the sorption sites adsorb available metal ions more rapidly at lower initial salt concentration [6,26]. 3.2. Adsorption isotherms The adsorption uptake is an important factor because it determines how much adsorbent is required quantitatively for enrichment of an analyte from a given solution. Fig. 7a shows the equilibrium adsorption uptake of the oxidized CNT sheets at different initial sodium ion concentrations. The experimental data for sodium ions adsorption onto the CNT sheets were analyzed using the Langmuir and Freundlich adsorption isotherm models. The Langmuir isotherm model, which is valid for

Fig. 4. Effect of adsorption time on (a) conductivity and (b) pH of the sodium chloride solution (with concentration of 10,000 ppm) for the as-synthesized and the oxidized CNT sheets.

monolayer sorption onto a surface with a finite number of identical sites and uniform adsorption energies, is given by the following equation: qe =

qmon KL Ce : 1 + KL Ce

ð2Þ

Eq. (2) can be expressed in a linear form: Ce 1 Ce = + qe qmon KL qmon

ð3Þ

where qmon is the amount of adsorption corresponding to monolayer coverage, KL is the Langmuir constant which is related to the energy of adsorption, and Ce is the equilibrium concentration of sodium ions (mg/l). As shown in Fig. 7b, the experimental data were fitted to the Langmuir equation, with a correlation coefficient value of 0.9854. The Freundlich isotherm model is a semi-empirical equation based on the adsorption occurred on a heterogeneous surface. It can be expressed by the following form: n

qe = KF Ce :

ð4Þ

Eq. (4) can be expressed in a linear form: ð5Þ

log qe = n log Ce + log Kf Table 1 Surface acidity and pHPZC of the as-synthesized and oxidized CNT sheets.

As-synthesized CNT sheets Oxidized CNT sheets

Surface acidity (mmol/g)

pHPZC

1.6 3.6

6.8 3.9

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Fig. 5. Effect of adsorption time and initial salt concentration on conductivity of the sodium chloride solution for the oxidized CNT sheets.

where qe is the adsorption uptake of sodium ions (mg/g), Ce is the equilibrium concentration of sodium ions (mg/l) and Kf and n are the Freundlich parameters related to adsorption capacity and adsorption intensity, respectively. As shown in Fig. 7c, the Freundlich model agrees well with the experimental data, with a correlation coefficient value being close to one (R2 = 0.9900). The Langmuir and Freundlich parameters are listed in Table 2. As can be observed, both Langmuir and Freundlich isotherm models match the experimental data very well. Based on the Langmuir model, the maximum sodium ions adsorption uptake onto the oxidized CNT sheets was determined as 1.4286 g/g. According to the Freundlich model, the oxidized CNT sheets can be used as an effective adsorbent for salty water desalination due to their very high adsorption capacity (Kf = 35.0764). Using the oxidized CNT sheets as adsorbent eliminates problems of agitating and removing of the CNTs with and from salty water before and after adsorption process, respectively. As a result, the oxidized CNT sheets with high adsorption capacity and high practical potential can be recommended as an effective adsorbent for salty water desalination in industrial scale.

4. Conclusion CNT sheets were synthesized by chemical vapor deposition, and oxidized with nitric acid at room temperature and then employed as a practical adsorbent for salty water desalination (sodium chloride removal from water). The results demonstrated that the adsorption

Fig. 7. (a) Effect of initial salt concentration on equilibrium adsorption uptake, (b) Langmuir and (c) Freundlich isotherm models for the oxidized CNT sheets.

capacity of the CNT sheets increases significantly after oxidation process. Both the Langmuir and Freundlich isotherm models match the experimental data very well. The results showed that the oxidized CNT sheets can be used as an effective adsorbent for salty water desalination due to their high adsorption capacity. Also, using the oxidized

Table 2 Langmuir and Freundlich parameters. Model Fig. 6. Effect of adsorption time and initial salt concentration on adsorption uptake of the sodium chloride solution for the oxidized CNT sheets.

Langmuir

Freundlich

qmon (g/g)

KL (l/mg)

R2

n

KF (mg/g)

R2

1.4286

0.0003

0.9854

0.3496

35.0764

0.9900

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