Adsorption of Cu<sup>2+</sup> Ions From Aqueous Solutions Using Oxidized Multi-Walled Carbon Nanotubes

Soheil Sobhanardakani; Raziyeh Zandipak; Mehrdad Cheraghi; Raziyeh Zandipak

doi:10.17795/ajehe790

Adsorption of Cu²⁺ Ions From Aqueous Solutions Using Oxidized Multi-Walled Carbon Nanotubes

Soheil Sobhanardakani ¹ , Raziyeh Zandipak ^{2
, *} and Mehrdad Cheraghi ¹

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Abstract

Copper ion (Cu²⁺) is one of the heavy metal ions that cause environmental pollution specifically in water. Copper ion cations are not biodegradable and tend to cumulate in living organisms. Consequently, the removal of Cu²⁺ in environmental samples plays an important role in environmental pollution monitoring. The purpose of the present work was to prepare oxidized Multi-Walled Carbon Nano Tubes (MWCNTs) for removal of Cu²⁺ ions from aqueous solutions. This study was conducted under laboratory conditions. Multi-Walled Carbon Nano Tubes were oxidized and characterized by Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM) and the Brunauer, Emmett, and Teller (BET) methods. The effects of various factors, such as solution pH (3 - 9), adsorbent dose (0.006 - 0.06 g) and contact time (10 - 120 minutes) were investigated. Results showed that the suitable pH for Cu²⁺ ions removal was about 6.0, and the optimal dose was 0.03 g. Isotherm studies indicated that the Langmuir model fits the experimental data better than the Freundlich model. Maximum Cu²⁺ adsorption capacity was calculated as 200 mg g-1. The kinetics of the adsorption process was tested for the pseudo-first-order and pseudo-second-order models. The comparison among the models showed that the pseudo-second order model best described the adsorption kinetics. The results showed that oxidized MWCNTs can be used as a low cost adsorbent for the removal of Cu²⁺ ions from aqueous solutions.

1. Introduction

Heavy metals are contaminants with potential toxicity for living organisms, when present above their permissible concentrations. In developing countries where industries such as metal plating, mining, tanning, and the manufacturing of fertilizers, batteries, paper and pesticides are growing, it is common for wastewaters containing heavy cations to be expelled into the environment. These ions are a hazard to public health and the environment when discharged inappropriately (1-3). Copper is one of the most significant heavy metals, often found in different effluents (4). Conventional methods for removal of heavy metals ions from aqueous solutions principally include chemical precipitation, ion-exchange, reverse osmosis and adsorption (5, 6). Adsorption is a simple, effective, and economical method for contaminants remediation (7). Activated carbon is a commonly used adsorbent for the removal of pollutants present in ground water and wastewaters. In spite of its effectiveness in the removal of heavy metals from wastewaters, the high cost of activated carbon has restricted its widespread use (8).

Recently, Carbon Nanotubes (CNTs) have been increasingly applied for environmental protection as novel materials due to their strong adsorption affinity for heavy metals (9, 10). Carbon Nanotubes mainly include Single-Walled (SWCNTs) and Multi-Walled (MWCNTs) Carbon Nanotubes. These CNTs are considered a superior adsorbent for potential environmental remediation due to their large surface area and high reactivity (11-13). The adsorption capacity of CNTs can be improved by oxidation with KMnO₄, H₂O₂, NaOCl or HNO₃, which removes impurities, increases the surface area, and introduces oxygen-containing functional groups, thus altering adsorption characteristics (14).

The aim of the present work was to investigate the removal of Cu²⁺ ions from aqueous solutions by adsorption with oxidized MWCNTs from a kinetic and equilibrium point of view. The effects of pH, adsorbent dose and contact time on the adsorption efficiency were studied as well.

2. Materials and Methods

All chemicals used in this study were of analytical reagent grade purchased from Merck (Darmstadt, Germany). Multi-Walled Carbon Nanotubes with length of 5 – 15 mm, outer diameter of 50 – 8 0 nm, inner diameters of 5 – 10 nm, and purity of ≥ 95 were purchased from Sigma–Aldrich (Madrid, Spain). Stock solution (1000 mg L⁻¹) of Cu²⁺ were prepared by dissolving Cu (NO₃)₂.3H₂O in double-distilled water. Working standard solutions were prepared by diluting the stock solution in appropriate proportions.

The concentration of metal ions was determined by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES) (JY138 ultrace, France). A Metrohm model 713 pH-meter (Swiss) was used for pH measurements.

Morphology and structure of the oxidized MWCNTs was characterized by a scanning electron microscope (SEM-EDX, XL30, Philips Netherland). Fourier Transform Infrared Spectroscopy (FTIR) (4000 – 400 cm^–1) in KBr were recorded on Perkin Elmer, spectrum100, FTIR spectrometer. The specific surface area was defined by N₂ adsorption–desorption porosimetry (77 K) using a porosimeter (Bel, Japan, Inc., Tokyo).

2.1. Oxidation of Multi-Walled Carbon Nanotubes

For oxidation, 2 g of MWCNTs were placed in a 1 L round bottom flask with reflux condenser and 300 mL of concentrated nitric acid (65%) was added. The mixture was refluxed for 48 hours at 120°C, cooled to room temperature, diluted with 500 mL of double-distilled water, and vacuum-filtered through filter paper (3 mm porosity, Whatman, Maidstone, UK). Washing was repeated until the pH became neutral, followed by drying in a vacuum oven at 100°C (15).

2.2. Batch Adsorption Experiments

Batch adsorption experiments were performed by adding 0.03 g of oxidized MWCNTs to 15 mL of Cu²⁺ solution (50 – 400 mg L⁻¹) in 25 mL stoppered conical flask. The pH of the solution was adjusted to optimum values using 0.1 mol L^-1 HCl and/or 0.1 mol L^-1 NaOH. The mixture was shaken for 75 minutes and the Cu²⁺ ions-loaded oxidized MWCNTs were separated and the concentrations of the metal, which remained in the solution was determined by Inductively Coupled Plasma Spectrometry (Verian710-Es Australia) and the concentration of the Cu²⁺ ions that remained in the adsorbent phase (q_e, mg g⁻¹) was calculated using the Equation:

Equation 1.

Where C₀ and C_e are the initial and equilibrium Cu²⁺ concentration in solution, respectively (mg L⁻¹), V (L) is the volume of solution and W (g) is the weight of adsorbent. Finally, the Cu²⁺ removal efficiency was calculated by the Equation:

Equation 2.

Where C₀ and C_e (mg L⁻¹) were the initial and final Cu²⁺ concentrations.

3. Results and Discussion

Figure 1 shows the morphological structure of oxidized MWCNTs. Figure 2 shows the FTIR spectrum of oxidized MWCNTs.

Figure 1. Scanning Electron Microscope Image of Oxidized Multi-Walled Carbon Nanotubes

Figure 2. Fourier Transform Infrared Spectroscopy Spectrum of Oxidized Multi-Walled Carbon Nanotubes

The effect of initial pH on the Cu²⁺ ions removal was studied at a pH range of 3.0 – 9.0 with initial metal concentration of 50 mg L^-1 and adsorbent dose of 0.03 g. The results are shown in Figure 3. As can be seen from the figure, adsorption amount of the metal ions increased with increase of pH values up to 6.0, and then reduced gradually.

Figure 3. Effect of pH on the Removal of Cu²⁺ From Aqueous Solution by Oxidized Multi-Walled Carbon Nanotubes

C₀ = 50 mg L⁻¹, contact time = 75 minutes, dose of oxidized MWCNTs = 0.03 g, temperature = 25°C.

The effect of the adsorbent dose (varying from 0.006 to 0.06 g) on Cu²⁺ ions removal was investigated at 25°C (Figure 4). The results showed that the adsorption effectiveness increased with an increase in the adsorbent dose from 0.006 to 0.03 g. Therefore, 0.03 g was chosen as the optimum adsorbent dose for the following experiments.

Figure 4. Effect of Dose of Oxidized Multi-Walled Carbon Nanotubes on the Removal of Cu²⁺

C₀ = 50 mg L⁻¹, initial pH = 6.0, contact time = 75 minutes, temperature = 25°C

The effect of contact time was investigated in the time range of 10 - 120 minutes under pH 6.0 at 25°C. Figure 5 shows the effect of contact time on the removal of Cu²⁺ ions. The figure reveals that increased agitation time increased the uptake of Cu²⁺ and attained equilibrium after 75 minutes.

Figure 5. Effect of Contact Time on the Removal of Cu²⁺ by Oxidized Multi-Walled Carbon Nanotubes

C0 = 50 mg L−1, initial pH = 6.0, dose of oxidized MWCNTs = 0.03 g, temperature = 25°C

A kinetic study for adsorption is very important because it provides information about efficiency and the possibility of the expansion of a process. To investigate the kinetics of adsorption, three different initial concentrations of Cu²⁺ ions were chosen, 50, 70 and 100 mg L^-1. In this study, two kinetic models (pseudo-first order equation and pseudo-second order equation) were applied. Figure 6 shows the adsorption kinetic data of Cu²⁺ ions onto oxidized MWCNTs. The obtained data and the correlation coefficients, r², are given in Table 1.

Figure 6. A, Pseudo-First-Order Kinetic Plot and B, Pseudo-Second-Order Kinetic Plot for the Adsorption of Cu²⁺ Ions Onto Oxidized Multi-Walled Carbon Nanotubes at 25°C

Table 1. Pseudo-First Order and Pseudo-Second Order Kinetic Model Parameters for the Adsorption of Cu²⁺ ions Onto Oxidized Multi-Walled Carbon Nanotubes

C₀, mg L^-1	Pseudo-ﬁrst-Order Kinetic Model				Pseudo-Second-Order Kinetic Model
C₀, mg L^-1	q_eexp, mg g^-1	q_e1, mg g^-1	k₁, min^-1	r²	q_e2, mg g^-1	k₂, g mg^-1 min^-1	r²
50	42.34	31.15	0.053	0.918	45.04	0.002	0.997
70	57.36	79.67	0.069	0.8663	60.97	0.000	0.997
100	82.11	161.7	0.084	0.894	86.95	0.001	0.998

In order to obtain insight into the possible mechanisms for removing Cu²⁺ ions from aqueous solution onto oxidized MWCNTs, the adsorption equilibrium data were analyzed using Langmuir and Freundlich models. The fitting parameters of these two models are shown in Table 2. The Langmuir and Freundlich adsorption isotherms are presented in Figure 7.

Table 2. Isotherm Parameters of Adsorption of Cu²⁺ Ions Onto Oxidized Multi-Walled Carbon Nanotubes

	Langmuir			Freundlich
	B, L mg^-1	q_m,mg g ^-1	r²	K_f, g^{1- (1/n)} L^1/n g^-1	n	r²
Cu²⁺	0.153	200	0.996	60.94	1.32	0.986

Figure 7. A, Langmuir and B, Freundlich Isotherms for Cu²⁺ Adsorption Onto Oxidized Multi-Walled Carbon Nanotubes

Figure 1 shows the morphological structure of oxidized MWCNTs. Scanning Electron Microscope clearly suggests the crystalline tubular structure of nanotubes. Figure 2 shows the FTIR spectrum of oxidized MWCNTs, indicating that the acid treatment process introduces new functional groups. The peak at 3464.22 cm⁻¹, 1574.47 cm⁻¹ and 1711.6 cm⁻¹ are attributed to the hydroxyl groups, carboxyl groups and carbonyl groups. These functional groups provide a large number of chemical adsorption sites and thereby can increase the adsorption capacity of oxidized MWCNTs.

Specific surface areas are commonly reported as BET surface areas obtained by applying the theory of Brunauer, Emmett, and Teller (BET) to nitrogen adsorption/desorption isotherms measured at 77 K. This is a standard procedure for the determination of the specific surface area of a sample. The specific surface area of a sample is determined by physical adsorption of a gas on the surface of the solid and by measuring the amount of adsorbed gas corresponding to monomolecular layer on the surface. The data were treated according to the BET theory (16, 17).

The results of the BET method showed that the specific surface areas (SSA) of MWCNTs and oxidized MWCNTs were 115 m² g^-1 and 158 m² g^-1, respectively.

Pore size distributions of MWCNTs and oxidized MWCNTs were measured using the Barrett, Joyner and Halenda (BJH) method. The average pore diameter, and pore volume were 29 nm and 0.17 cm³ g^-1 for MWCNTs, and 36 nm and 0.24 cm³ g^-1 for oxidized MWCNTs, respectively.

The results indicated that the pore volume and average pore diameter of MWCNTs were less than oxidized MWCNTs. This can be comprehended considering the structure change of oxidized MWCNTs with nitric acid, which can easily break up the MWCNTs into smaller pieces with large amounts of defects on their surface, and open the tips and probe the holes through the MWCNTs.

The initial pH of the solution is one of the important factors controlling the adsorption of metal ions onto suspended particles by affecting the surface charge of the adsorbents as well as the degree of ionization of the adsorbate. It can be seen from Figure 3 that the removal of metal ions increases with an increase in pH of the solution to attain a maximum at pH 6.0 and thereafter it decreases when pH of the solution reaches up to 7.0. The maximum removal of Cu²⁺ at pH 6.0 was found to be 99.5%. The zero point charge (pH_ZPC) for the oxidized MWCNTs was determined around 5.0. This phenomenon appears to be due to the fact that at pH > pH_zpc the surface charge of oxidized MWCNTs becomes negative and the electrostatic interactions between the metal ions and adsorbent become stronger. Also the increase in Cu²⁺ ions removal with an increase in pH can be explained on the basis of a decrease in competition between protons and Cu²⁺ cations for the same functional groups. The decrease in adsorption at higher pH (above pH 7.0) is probably because the pH may affect the ionization degree of the adsorbate and the surface property of the adsorbent. Similar results have been found by Mariussen et al. (18).

The adsorbent dose is another important parameter because it has a direct relationship with the uptake capacity of an adsorbent for a given concentration of metal at the optimism conditions. As can be observed from Figure 4, removal efficiency was increased from 35% to 99.6%, by increasing the amount of oxidized MWCNTs from 0.006 to 0.03 g. This observation can be explained by the greater number of adsorption sites made available at greater oxidized MWCNTs dosages. A similar phenomenon has also been shown in the adsorption of Cu (II) from water with amino-functionalized mesoporous Fe₃O₄ nanoparticles (19).

Contact time is one of the significant parameters for successful use of the adsorbents for practical applications. As can be seen from Figure 5, Cu²⁺ ions adsorption increases with contact time and gradually reaches equilibrium after 75 minutes. Rapid adsorption is observed within 60 minutes due to the availability of a large number of vacant sites. Subsequently, the diminishing availability of active sites and the decrease in the driving force lead to a slow adsorptive process. Similar results were observed by Zhao who investigated the effect of contact time on removal of Cu²⁺ from aqueous solution by Graphene sponge and indicated that adsorption increases with increasing contact time (20).

The prediction of adsorption rate gives crucial information for designing sustainable batch adsorption systems. Information on the kinetics of pollutant uptake is required for selecting optimum operating conditions for full-scale batch processes. In order to evaluate the kinetic mechanism that controls the adsorption process, the experimental data was analyzed using pseudo-first-order and pseudo-second-order models. The pseudo-first-order model and pseudo-second-order model Equations are presented in Equations, (3) and (4), respectively (21, 22).

Equation 3.

Equation 4.

Where q_e and q_t are the amount of Cu²⁺ ions adsorbed (mg g^-1) at equilibrium and time t (minutes); k₁ is the rate constant of pseudo-first-order (minute^-1); k₂ is the rate constant of pseudo-second-order (g mg^-1 minute^-1) for adsorption. The correlation coefficients (r²) for pseudo-second-order model are all higher than for the pseudo-first-order model (Figure 6) and the experimental data fit the pseudo-second-order model better than the pseudo-first-order model (Table 1). Also q_e value agreed with the calculated values (q_eexp), indicating a good fit of the adsorption process to this model. The results indicate that chemical adsorption might be the rate-limiting step.

Adsorption isotherms are mathematical models that describe the distribution of the adsorbate species among liquid and adsorbent, based on a set of assumptions that are mainly related to the heterogeneity/homogeneity of adsorbents, the type of coverage and possibility of interaction between the adsorbate species. The equilibrium data was fitted by conventionally and traditionally known models such as Langmuir (Equation 5) and Freundlich (Equation 6). The linear Equations were were as follows (23, 24):

Equation 5.

Equation 6.

Where ce (mg L⁻¹) is the equilibrium concentration of Cu2+ ions in solution, q_e (mg g⁻¹) is the equilibrium adsorption capacity of oxidized MWCNTs, q_m (mg g⁻¹) is the maximum adsorption capacity of oxidized MWCNTs for monolayer coverage, b (L mg⁻¹) is a constant related to the adsorption free energy, K_f (mg^1−(1/n) L^1/n g⁻¹) is a constant related to adsorption capacity, and n is an empirical parameter related to adsorption.

The correlation coefficients (r²) for Langmuir and Freundlich model were 0.996 and 0.986, respectively. The good agreement between the adsorption data and Langmuir model implies that the adsorption of Cu²⁺ on oxidized MWCNTs occurred as a single monolayer.

4. Conclusion

In summary, we have successfully prepared oxidized MWCNT for Cu²⁺ ions removal from aqueous solutions. The results demonstrated that the adsorption of Cu²⁺ on oxidized MWCNT was strongly dependent on pH and the amount of adsorption increased with an increase of adsorbent dose. The equilibrium data well fitted the Langmuir adsorption isotherms and the maximum adsorption capacity of Cu²⁺ reached 200 mg g^-1 for oxidized MWCNT indicating that adsorption occurs on a homogeneous surface by monolayer adsorption without interaction between the adsorbed ions. Also, the kinetic study showed that the pseudo-second-order model was appropriate to describe the adsorption process.

Acknowledgements

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Adsorption of Cu2+ Ions From Aqueous Solutions Using Oxidized Multi-Walled Carbon Nanotubes