Carbon Nanofibers: A New Adsorbent for Copper Removal from Wastewater

Abstract

This research describes the adsorption of Cu²⁺ onto a helical ribbon carbon nanofiber. The characterization of carbon nanofiber by zeta potential showed an isoelectronic pH of 1.9. The influence of different adsorption factors, such as stirring speed, temperature, pH, adsorbent concentration, etc., on the Cu²⁺ adsorption capacity have been evaluated. The pH has a great influence on Cu²⁺ adsorption, with the maximum adsorption capacity reached at a pH of 10. The experimental data fit well to pseudo-second order kinetic and Langmuir isotherm models (q_m = 8.80 mg·g⁻¹) at T = 298 K and pH = 4. The Cu²⁺ adsorption could be explained by the particle diffusion model. Results showed that carbon nanofiber could be successfully used for the elimination of Cu²⁺ from wastewater.

1. Introduction

Heavy metals are characterized by their high relative densities (greater than 5 g·cm⁻³) and atomic weights (between 63.5 and 200.6) [1,2,3]. Though many of these are necessary for life, higher amounts above required levels are hazardous [4].

Moreover, heavy metals are not biodegradable, being easily accumulated in living organisms, particularly in humans. These can cause serious illnesses, such as cancer, damage to the nervous system, increases in blood and respiratory pressure, kidney failure, and could become lethal at high concentrations [1,5,6,7]. For this reason, the pollution of water due to the presence of heavy metals is a major concern today world-wide. Each year large, volumes of wastewater are produced from different industrial sectors, such as mining, metallurgy and smelting, among others. The most common heavy metals present in wastewaters are nickel, zinc, lead, iron, copper, arsenic, cadmium, and uranium [8,9,10].

Among these, cooper is considered a harmful metal [9,11,12]. There are three forms of copper: Cu⁰ metal, Cu⁺ cuprous ion, and Cu²⁺ cupric ion, this last form normally appearing in the environment and considered to be the most toxic of the three [13,14]. The main contributors of copper to the environment are mining industries, printing circuits, electroplating industries, paints, fertilizers, plastics and etching, etc. [15,16,17]. Certain industries that release wastewater into the environment have reported Cu²⁺ concentrations ranging from 2.0 mg·L⁻¹ to 900 mg·L⁻¹ [18]. Therefore, the recovery of heavy metals, in general, and in particular of copper, is a subject of great social relevance due to the environmental and economic benefits of eliminating contaminates from industrial effluents.

A great number of methods can be used for the recovery or removal of heavy metals from liquid effluents [19]. The choice of one method over another is based mainly on an economic question and metal concentration [1]. Conventional methods include bioremediation [20,21], reverse osmosis [22], electrochemical treatment [23], coagulation [24], precipitation [25,26], solvent extraction [27,28,29], membrane filtration [30,31], electrodialysis [32], supercritical fluids extraction [33,34], ion exchange [35,36,37,38], and flotation [39,40].

Adsorption is one of the most popular methods for eliminating heavy metals [41]. It is a technology in which the pollutants, particularly in the case of heavy metals, can interact in a physical or chemical way with the active elements of the adsorbent. Adsorption is a technique that is easy to use and design, and it does not produce contaminants [42]. Moreover, another advantage of the adsorption process is the possibility to regenerate the adsorbent by desorption, being therefore a reversible technique, so it is considered a technology that is not aggressive with the environment. These advantages have made adsorption a leading method for wastewater treatment [13].

Different types of adsorbent have been developed, such as active carbon, clay ore, chelating materials, natural chitosan/zeolites, etc. [19,41,43,44]. In the last twenty years, the interest in nanotechnology has been growing. Materials such as fullerenes, carbon nanotubes, graphene and graphene oxide have a large number of applications in different fields, including the removal of heavy metals from wastewater [38,45]. Their unique properties, including high resistance, electrical conductivity and thermal stability, as well as having large specific surface areas, make them sustainable adsorbents for wastewater treatment, and more specifically, for the elimination of heavy metals [46,47]

This paper reports the results derived from Cu²⁺ adsorption on new helix spiral carbon nanofibers (CNF). CNF are structures composed of a helical ribbons of graphene, spiraled about and angled to the fiber axis, similar to the structure of an Archimedes screw with flights inclined to the axis [48].

There are many factors that influence heavy metal adsorption efficiency, such as initial metal concentration, temperature, sorbate amount, pH, and contact time [42,43]. Therefore, the objective of this research was to determine the effects of various experimental variables on Cu²⁺-CNF adsorption systems. Furthermore, kinetic mechanisms, adsorption isotherms models and thermodynamics parameters were investigated.

2. Materials and Methods

Carbon nanofibers donated by Antolin group S.A. were used as an adsorbent [49]. Stock metal solution was prepared from CuSO₄ (Fluka, Munich, Germany). All other reagents used in the work were analytical reagent grade.

2.1. Characterization of Carbon Nanofibers

Carbon nanofibers were examined using a Hitachi S4800 (Tokyo, Japan) scanning electron apparatus equipped with a detectors of secondary, backscattered and transmission electrons, and an Oxford Inca Energy dispersion microanalysis system. Nanomaterials were dispersed in ethanol, and drops of the suspension material were deposited on TEM grating.

The zeta potential was measured using a Zetasizer Malvern Nano ZS (Malvern Panalytical Ltd., Worcestershire, UK) at 25 °C. To study the influence of pH, aqueous suspensions were prepared in pH solutions between 1 and 13 using solutions of 0.5 M of HCl and NaOH.

2.2. Batch Adsorption Experiments

The batch adsorption experiments were carried out in a 250 mL glass reactor capable of mechanical (impeller) shaking.

Solutions (100 mL) containing Cu²⁺ were used, and weighed amounts of the adsorbent, carbon nanofibers (CNF), were added. The pH was adjusted using HCl and NaOH. The residual copper content in the aqueous solution was determined by AAS (Perkin Elmer 1100 B spectrophotometer, Waltham, MA, USA). These were filtered prior to solution analyses. The metal uptake capacity was calculated using Equation (1):

$$q_t=\left(C_0-C_{\mathrm{t}}\right)\times \frac{V}{M},$$

(1)

where q_t is the amount metals ions adsorbed (mg·Cu·g⁻¹ sorbent), while C₀ and C_t are the initial and contact time metal ion concentrations (mg·Cu·L⁻¹) in the aqueous solution, respectively. V is the volume of the solution (0.1 L), and M is the mass of the sorbent (g).

The removal efficiency was calculated using the following equation:

$$\%{\mathrm{Cu}}_{\mathrm{ad}}=\left[\left(C_0-C_{\mathrm{e}}\right)/C_{\mathrm{e}}\right]\times 100,$$

(2)

where C_e (mg·Cu·L⁻¹) is the equilibrium metal concentration in solution.

3. Results

3.1. Characterization of CNF

TEM micrographs of CNF are shown in Figure 1. The helix spiral structure is observed. The structure of CNF is a continuous cone-helix. The presence of pyrolytic amorphous carbon in the CNF is not observed [49]. CNF structural details are presented in references [48,49,50].

Table 1. CNF elemental analysis (wt %).

Sample	C	H	N	S	O
CNF	99.53	0.00	0.03	0.08	0.36

shows the elemental composition.

The isoelectric point (IEP) of CNF is obtained at a pH value of 1.9 (Figure 2). At a pH values around the pH(IEP), the sorbent surface charge is neutral. When the pH value is lower than the isoelectronic point, the sorbent surface has a positive charge, and a negative charge when the pH is greater than 1.9 [37,51]. The surface charge could affect in the adsorption properties of the adsorbent [51].

3.2. Sorption Bach Experiment

To research the efficiency of the CNF in Cu²⁺ adsorption, parameters such as contact time, stirring speed, temperature, pH, carbon nanomaterials concentration, and copper concentration have been studied.

3.2.1. Effect of the Contact Time and the Stirring Speed

Figure 3 shows the effect of the contact time on Cu²⁺ adsorption. The adsorption kinetics was investigated for better understanding of the dynamics of adsorption of metal on the CNF. The removal of Cu²⁺ between 0–30 min increased sharply, and then tended to became almost stable, which denoted accomplishment of equilibrium at 30 min. From this point, the qt values remained constant. This behavior could be due to the fact that, at the beginning, all of the active sites on the CNF surface were empty and the copper concentration high, and after 30 min, the surface active sites available diminish and the copper concentration lower, so the metal uptake remain constant [52]. The copper adsorption was a quick process.

The equilibrium time could be decreased by increasing the stirring speed, so the appropriate hydrodynamic conditions were investigated.

Table 2. Effect of the stirring speed on the Cu²⁺ adsorption onto the CNF. Experimental conditions: CNF = 0.1 g, V = 100 mL of [Cu²⁺] = 0.01 mg·L⁻¹ pH = 4, T = 298 K, θ = variable.

Stirring Speed (rpm)	qe (mg·g−1)	Adsorption (%)
250	5.12	51.2
500	7.83	78.3
1000	7.15	71.5
2000	7.37	73.7
3000	6.75	6.75

shows that an increase in the stirring speed increased qt values, reaching a maximum between 500–2000 rpm. In this stirring speed interval, the thickness of the aqueous film diffusion layer and the aqueous resistance to mass transfer are at a minimum. An increase in the stirring speed to higher than 2000 rpm decreases the Cu²⁺ adsorption. This could be due the formation of local equilibrium between the Cu²⁺ solution and the surrounding CNF and/or the agglomeration of the nanoparticles.

3.2.2. Adsorption Kinetic and Diffusion Mechanism

To understand the process of Cu²⁺ adsorption onto CNF, three kinetic adsorption models have been assessed [53]:

(a)

pseudo-first order [51]

$$\ln \left(q_{\mathrm{e}}-q_{\mathrm{t}}\right)=\ln q_{\mathrm{e}}-k_{1}t$$

(3)

(b)

pseudo-second order [54]

$$\frac{t}{q_{\mathrm{t}}}=\frac{1}{k_2{\left(q_{\mathrm{e}}\right)}^2}+\frac{1}{q_{\mathrm{e}}}t$$

(4)

(c)

Elovich model [55]

$$q_{\mathrm{t}}=\frac{1}{\mathsf{\beta }}\ln \mathsf{\alpha }\cdot \mathsf{\beta }+\frac{1}{\mathsf{\beta }}\ln t$$

(5)

where q_e and q_t (mg·g⁻¹) are the sorption capacities at equilibrium and at time t (min), respectively, k₁ is first-order adsorption rate constant (min⁻¹), and k₂ (g·mg⁻¹·min⁻¹) is the pseudo-second order constant. α is the initial adsorption rate (mg·min⁻¹) at contact time t = 0 min, and β is the extent of surface coverage and activated energy (g·mg⁻¹).

According to the calculated data, among the different adsorption reaction models, the higher correlation coefficient, R², was obtained for the pseudo-second order kinetics model (Equation (4) and

Table 3. Kinetic parameters for Cu²⁺ adsorption onto CNF.

Study	Equation	Constant	qe, β	R2
Kinetic reaction models	Pseudo-first	0.0594 min−1	qe = 21.04 mg·g−1	0.9357
	Pseudo-second	0.0574 g·mg−1·min−1	qe = 6.62 mg·g−1	0.9985
	Elovich	α = 6.18 mg·g−1·min−1	β = 8.12 g·mg−1	0.9588

). The q_e values derived from the model and the experiment are really similar: q_e (experimental) 6.51 mg·g⁻¹ [56].

Three adsorption mechanisms have been considered to evaluate the adsorption of Cu²⁺ on CNF [57,58]:

(a)

Film diffusion controlled process: The metal species diffused from the aqueous solution to the CNF surface

$$\ln \left(1-\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}\right)=-Kt$$

(6)

(b)

Particle diffusion controlled process: The ion diffused inside CNF

$$\ln \left(1-{\left(\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}\right)}^2\right)=-Kt$$

(7)

(c)

Moving boundary process

$$3-3{\left(1-\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}\right)}^{\frac{2}{3}}-2\frac{q_{\mathrm{t}}}{q_{\mathrm{e}}}=Kt$$

(8)

where K (min⁻¹) is the rate constant model, q_e and q_t (mg·g⁻¹) are the sorption capacities at equilibrium and contact time, respectively, and t (min).

The results obtained by the kinetic diffusion models show that the Cu²⁺ adsorption onto the CNF could be better explained by the particle diffusion model, where K = 0.0525 min⁻¹ (

Table 4. Adsorption mechanism kinetic parameters for Cu²⁺ adsorption onto CNF.

Study	Equation	Constant	R2
Kinetic diffusion models	Film diffusion	0.0594	0.9375
	Particle diffusion	0.0525	0.9662
	Moving boundary process	−0.0062	0.6088

).

3.2.3. pH Influence on Cu²⁺ Adsorption

As seen in Figure 4, at a low pH of two, the adsorption of Cu²⁺ onto CNF is very low. This is due to the pH value being very near the zero potential pH = 1.9, the charge in the surface being 0 (neutral), and the Cu²⁺ adsorption being nil or very low. At pH values lower than two, the potential of the surface charge of the CNF is positive, the cations in solution are repealed from the adsorbent, and it is not adsorbed. When the pH increases to between 4 and 10, the cooper adsorption increased to its maximum at pH 10 (q_e = 8.88 mg·g⁻¹). Above the isoelectronic pH, the nanomaterial surfaces have a negative charge and attract the metal ions facilitating the adsorption [18,37].

3.2.4. Effect of the Copper Concentration

The adsorption capacity depends on Cu²⁺ concentration;

Table 5. Effect of the Cu²⁺ concentration on the adsorption onto CNF. Experimental conditions: CNF = 0.1 g, V = 100 mL of [Cu²⁺] = variable, pH = 4, T = 298 K, θ = 500 rpm.

Cu2+ Concentration (g·L−1)	Recovery (%)	qe (mg·g−1)
0.005	99.00	4.95
0.01	76.67	7.67
0.02	42.25	8.45
0.04	21.92	8.77

shows an increase in copper concentration in the aqueous phase resulting in a reduction of the recovery percentage. This may be associated with a competitive effect of copper by the active position of the adsorbent.

The adsorption equilibrium data of the Cu²⁺-CNF system were checked in terms of three isotherm models: Langmuir, Freundlich, Dubinin-Radushkevich and Temkin [59]. The following equations have been used:

(a)

Langmuir [60,61]:

$$\frac{C_{\mathrm{e}}}{q_{\mathrm{e}}}=\frac{1}{q_{\mathrm{m}}b}+\frac{1}{q_{\mathrm{m}}}{C}_{\mathrm{e}}$$

(9)

$$R_{\mathrm{L}}=\frac{1}{1+b{C}_0}$$

(10)

(b)

Freundlich [62]:

$$\ln q_{\mathrm{e}}=\ln K_{\mathrm{F}}+\frac{1}{n}\ln C_{\mathrm{e}}$$

(11)

where q_e (mg·g⁻¹) is the sorption capacities at equilibrium time, C_e (mg·L⁻¹) is the metal ion concentration in the aqueous phase at equilibrium, q_m (mg·g⁻¹) is maximum sorption capacity, and b (L·mg⁻¹) is the Langmuir sorption constant. R_L is a dimensional constant separation [63]. C₀ (mg·L⁻¹) is the maximun initial copper concentration. K_F is the Freundlich constant (L·g⁻¹), and 1/n is a heterogeneity factor constant, their values must be less than one for favorable adsorption.

(c)

Dubinin-Radushkevich [64]:

$$\ln q_{\mathrm{e}}=\ln q_{\mathrm{d}}-K\cdot {\mathsf{\epsilon }}^2$$

(12)

where:

$$\mathsf{\epsilon }=RT\ln \left(1+\frac{1}{C_e}\right)$$

(13)

$$E=\frac{1}{\sqrt{2\cdot K}}$$

(14)

where q_e are mg·g⁻¹ in equilibrium, q_d is the theoretical capacity adsorption (mg·g⁻¹), K is a constant related to the mean free energy in the adsorption (mol²·J⁻²) and E (KJ·mol⁻¹) [63]. ε is the Polanyi adsorption potential, R is the universal gas constant (8.314 J·K⁻¹·mol⁻¹), T is the temperature in K, and C_e is the metal ion concentration in the aqueous phase at equilibrium in mg·L⁻¹.

(d)

Temkin [65]:

$$q_{\mathrm{e}}=B\cdot \ln \left(K_{\mathrm{T}}+C_{\mathrm{e}}\right)$$

(15)

where q_e is the sorption capacities at equilibrium time in mol·g⁻¹, C_e is the metal ion concentration in the aqueous phase at equilibrium in mol·L⁻¹, and K_T is the isotherm equilibrium binding constant (L·g⁻¹); B is a constant related to the sorption heat in J·mol⁻¹.

According to the values of the correlation coefficient, R², the experimental results fit well to the studied isotherms,

Table 6. Calculated isotherms parameters. Experimental conditions: CNF = 0.1 g, V = 100 mL of [Cu²⁺] = variable pH = 4, T = 298 K, θ = 500 rpm.

Langmuir Model				Freundlich Model
qm	b	RL × 10−3	R2	n	KF	R2
8.80	4.44	5.6	0.9998	10.96	0.15	0.9456
Dubinin-Raduskevich				Temkin
qd	K	E	R2	KT	B	R2
8.38	9.28 × 10−9	7.34	0.9765	97.26	0.61	0.9640

. However, the best fit is obtained for the Langmuir model. The q_m value obtained from this model is similar to the experimental one, 8.77 mg·g⁻¹. The separation factor, R_L, shows that the adsorption process is favorable if the R_L value is 0 < R_L < 1, unfavorable when R_L > 1, linear when R_L = 1, and irreversible at values of R_L = 0. The Cu²⁺ adsorption on CNF is favorable with a R_L value between zero and one, the value here being 5.6 × 10⁻³. The 1/n value obtained from adjusting the data to the Freundlich isotherm is 9.12 × 10⁻², also indicating that the copper adsorption on CNF is favorable.

The energy value calculated using the Dubinin-Raduskevich model was E = 7.34 (KJ·mol⁻¹). This is lower than 8 KJ·mol⁻¹, which indicates that the mechanism of adsorption has a physical nature [63].

3.2.5. Effect of the CNF Dosage

The CNF dosage was modified in order to evaluate the effect of this parameter. It was expected that an increase in the CNF dosage will increase the adsorption percentage and diminish the adsorption capacity values.

The objective of this research was to achieve the highest possible metal recovery. The best result was obtained using 0.2 g of CNF, with 97% of the copper recovered (see

Table 7. Effect CNF concentration. Experimental conditions: CNF = variable, V = 100 mL of [Cu²⁺] = 0.01 mg·L⁻¹, pH = 4, T = 298 K, θ = 500 rpm.

CNF Concentration (g)	Recovery (%)	qe (mg·g−1)
0.05	29.06	5.81
0.08	52.10	6.51
0.1	76.67	7.67
0.2	96.69	4.83

).

3.2.6. Temperature Influence and Thermodynamic Studies

The influence of the temperature (293–333 K) on Cu²⁺ adsorption on CNF have been studied. The assay conditions were 0.01 g·L⁻¹ of Cu(II), 4 pH, 0.1 g of CNF adsorbent, and a stirring speed of 500 rpm. Figure 5 shows the variation of q_t vs. t.

An increase of the q_e value with temperature was observed, having reached a maximum at 333 K, 8 mg·g⁻¹. Therefore, the process is considered endothermic. The increase of adsorption with temperature can be due to different factors, such as a change in the pore size of the nanofibers and/or an increase of the active sites. In addition, the collisions between the metal and the surface of the nanofibers are greater, thereby contributing to a rise in the adsorption capacity [18].

The experimental data obtained at different temperatures (293–333 K) was used to calculate the thermodynamics parameters, ΔH°, ΔG°, and ΔS°, using the following equation:

$$\ln K_{\mathrm{d}}=\frac{\mathsf{\Delta }S°}{R}-\frac{\mathsf{\Delta }H°}{RT}$$

(16)

where ΔS° is the entropy of the reaction, ΔH° is the enthalpy, T is the temperature in Kelvin, and R is the universal gas constant (8.314 J·K⁻¹·mol⁻¹). K_d is defined as follows:

$$K_{\mathrm{d}}=\frac{q_{\mathrm{e}}}{C_{\mathrm{e}}}$$

(17)

where q_e, (mg·g⁻¹) and C_e (mg·L⁻¹) are the equilibrium capacity on the solid and the liquid phases, respectively. Plotting the linear fit $\ln \frac{C_{\mathrm{e}},\mathrm{CNF}}{C_{\mathrm{e}}}$ vs. $\frac{1}{T}$ to the slope could be used to calculate the ΔH° value (

Table 8. Thermodynamic parameters for Cu²⁺-CNF system.

Thermodynamic Parameters
ΔH° (KJ·mol−1)	ΔS° (J·mol−1)	ΔG° (KJ·mol−1)
26.76	100.93	293 K	313 K	333 K
		−3.07	−4.24	−7.18

).

For the entire temperature range studied, the ΔG° values were negative, this indicating that the adsorption process is spontaneous. These values are lower than 18 KJ·mol⁻¹, characteristic of a physisorption [18], and this agrees with the result obtained from R_L Langmuir parameter (see Table 6).

The positive enthalpy confirms the endothermic behavior of Cu²⁺ adsorption on the CNF. The ΔS° value is positive, which indicates an increase in the randomness in the CNF/Cu²⁺ system. In the temperature range studied, the ΔH° is lower than T·ΔS°, so the sorption process was dominated by entropic rather than enthalpic changes [51,62].

3.2.7. Cu²⁺ Elution

According to the results for Cu²⁺ adsorption on CNF, it is expected that an increase in the acid concentration changes the adsorption conditions, and Cu²⁺ could then be eluted. The effect of the acid concentration in elution was studied. For 30 min, 0.05 g of CNF load with 4.83 mg·g⁻¹ of Cu²⁺ were put in contact with 20 mL of H₂SO₄ in solutions of different concentrations at a stirring speed of 500 rpm.

An increase in the acid concentration increased the elution percentage (see

Table 9. Elution percentage.

H2SO4 Concentration	1 M	2 M	4 M
Elution recovery (%)	38.42	46.12	96.36

), reaching a percentage of around 96% with a H₂SO₄ concentration of 4 M. The kinetic elution study was performed using a solution of 2 M sulfuric acid, with equilibrium reached after 15 min. The elution kinetics was faster than the adsorption.

4. Conclusions

The results obtained in this research prove that helical carbon nanofibers are a potential adsorbent of Cu²⁺ from wastewaters. Of the parameters studied, pH has the greatest influence on copper adsorption capacity, reaching a maximum q_m value of around 8.80 mg·g⁻¹ at a pH value of 10. An increase in the temperature does not influence the adsorption capacity; however, the equilibrium is reached in less time.

The Cu²⁺ kinetics fitted a pseudo-second order (0.0574 g·mg⁻¹·min⁻¹). The Cu²⁺ adsorption is considered a particle diffusion controlled process. The experimental data adjusted to a Langmuir model, the q_m = 8.80 mg·g⁻¹, is a favorable, endothermic (ΔH° = 26.76 KJ·mol⁻¹) and physical nature adsorption. Metal elution was carried out with a H₂SO₄ 4 M solution, at 30 min, reaching an elution percentage of 96%.