Chemistry Africa

, Volume 2, Issue 1, pp 113–122 | Cite as

Ferrocene–Functionalized Carbon Nanotubes: An Adsorbent for Rhodamine B

  • Amal RabtiEmail author
  • Amira Hannachi
  • Hager Maghraoui-Meherzi
  • Noureddine RaouafiEmail author
Original Article


It is well-known that direct discharges of dye-contaminated wastewaters generated from various industries (i.e., textile, cosmetics and food industries,…) cause severe effects on both aquatic environment and human health. Decontamination of dye-containing wastewaters using nanomaterials-based adsorbents such as carbon nanotubes is regarded as an interesting field of investigation to control these types of pollutants. In this context, a newly prepared ferrocene-modified carbon nanotubes (amFc-MWCNTs) was applied as an adsorbent for the removal of rhodamine B (RhB) dye from aqueous solutions. The structural properties of the hybrid adsorbent were fully characterized using Raman, XPS, EDX, SEM and TEM microscopy. Adsorption isotherms and kinetics of RhB were investigated and multiple models (i.e. Langmuir, Freundlich, Hill,…) were used to fit experimental data. It was found that > 98% of RhB with initial concentration of 10 mg L−1 can be captured within 2 h when using 0.4 g L−1 of amFc-MWCNTs. The adsorption behavior of this nanomaterial fitted well with the Hill isotherm and the pseudo-second-order kinetic model. Moreover, the intra-particle diffusion was identified as the rate-limiting step of the adsorption process. After washing with acetone, regenerated amFc-MWCNTs adsorbent showed good recovery, indicating its reusability and its potential in practical applications.


Ferrocene-functionalized carbon nanotubes Adsorption Rhodamine B Hill Isotherm Kinetics 

1 Introduction

The removal of rhodamine B from wastewaters, before they returned to the natural watercourse, is primordial to provide clean and safe water, since RhB has a poisonous impact on both ecosystem and public health [1]. In fact, this dye is widely used in textile, cosmetics and food industries as a colorant, and in biotechnology as a florescent probe [2, 3], which make it easily discharged into the aquatic environment. Moreover, it is carcinogenic [4, 5], can cause risk of serious damage to the eye, and is harmful in contact with the skin and if swallowed [6].

The adsorption technique has attracted great attention for removal of dyes from wastewaters over the past years essentially due to its high efficiency and cost effectiveness [7, 8]. Several naturally occurring adsorbents such as wood derivatives (lignin [9], cellulose [10]) or clays like kaolinite and montmorillonite [11] have been used to treat dye-contaminated waters. However, they have some shortcomings that include high contact time, low adsorption capacity and difficulty of recovery. To overcome these challenges, a variety of nanomaterial-based absorbents were investigated [12, 13]. Among these nanomaterials, an increasing attention was devoted to carbon nanotubes (CNTs) for the removal of organic dyes, pesticides, phenols, aromatic amines, toxic organics and other organic contaminants [14]. CNTs possess tubular nanostructures with large surface area and excellent electrical conductivity [15], which make them attractive for water contaminants removal. However, the strong Van der Waals interactions among them result in their aggregation, which limits their adsorption efficiency. Therefore, acid-functionalized CNTs [16], carbon nanotube-cobalt ferrite nanocomposites [16], and bentonite-carbon nanotube nanocomposites [17] have been studied to overcome the above stated limitations.

Ferrocene, a sandwich organometallic, has been widely used to tune the electronic and optoelectronic structure and transport properties of carbon nanomaterials [18]. It can be attached onto the carbon nanotubes surfaces via covalent or non-covalent methods, which assure their stable dispersion and overcome their aggregation. Ferrocene-functionalized carbon nanotubes have been extensively applied in sensing and biosensing application [18, 19, 20]. However, to the best of our knowledge there have not been any works on their application in the field of dyes removal. Hence, the objective of this paper is to study the dye adsorption capacity of newly prepared ferrocene-modified CNTs (Fig. 1). Rhodamine B was chosen as a model dye. The prepared nanomaterial was fully characterized. Moreover, the rhodamine B adsorption capacity, isotherms and kinetics were investigated.
Fig. 1

Synthesis of amFc derivative and functionalization of MWCNTs via [1, 2]–cycloaddition

2 Experimental

2.1 Materials

Rhodamine B, sodium hydroxide PA (> 99%), hydrochloric acid (37%), ferrocenylmethanol (97%), tosyl chloride (> 99.5%), sodium azide (98%), acetone and N-Methyl-2-pyrrolidone (NMP) were purchased from Sigma-Aldrich and used without further purification. MWCNTs (content of –COOH: 3.67–4.05 wt %; purity: > 95%; outside diameter: < 8 nm; inside diameter: 2–5 nm; length: 10–30 µm) was obtained from Nanostructured & Amorphous Materials, Inc. (NanoAmor, New Mexico, USA). Milli-Q, Millipore system has served to prepare deionized water (> 18.2 MΩ cm) used for all the aqueous preparations.

2.2 Instrumentation

Transmission (TEM) and scanning (SEM) electron microscopy were conducted using high-resolution FEI Magellan 400L XHR and FEI Quanta 650 FEG Environmental scanning electron microscope, respectively. Energy-dispersive X-ray spectroscopy (EDX) was recorded using Oxford Instruments Xmax 20 mm2. X-ray photoelectron spectroscopy (XPS) analysis was performed with a Phoibos 150 analyzer (SPECS GmbH, Berlin, Germany) using monochromatic aluminum Kα X-ray (1486.74 eV) as the light source. Raman scattering measurements were carried out with a Jobin-Yvon spectrometer (T64000 model) equipped with an Nd:YAG laser (λex = 532.8 nm) and a CCD detector in a back-scattering geometry. Ultraviolet–visible (UV–vis) absorption spectra were recorded out using a Shimadzu UV-3100S spectrophotometer.

Differential pulse voltammetry (DPV) data were obtained using a Metrohm Autolab PGSTAT 302 N electrochemical workstation driven by Nova® v1.10 software, using a three-electrode set-up. Homemade screen-printed carbon and Ag/AgCl electrodes were used as the counter and reference electrodes, respectively. The working electrodes were amFc-MWCNTs hybrid either before or after adsorption of RhB prepared according to the method described elsewhere [21]. The adsorbent was collected using a ScanSpeed 1730R thermostatic centrifuge (LoboGene A/S, Denmark).

2.3 Preparation of Ferrocene–Functionalized MWCNTs

Both azidomethylferrocene (amFc) and the target hybrid material were prepared according to already reported procedure [22]. In short, ultrasound bath was used to prepare a MWCNTs suspension (1.0 g L−1) in 5 mL of NMP. After adding 150 mg of amFc, the mixture was heated at 160 °C under a gentle magnetic stirring for 24 h under N2 atmosphere. The powder was repeatedly collected by centrifugation and washed by acetone. The final product is denoted by amFc-MWCNTs.

2.4 Adsorption Experiments

Dried powder of amFc-MWCNTs (conc. 0.05–0.4 g L−1) was added to 4 mL of different RhB solutions (conc. 1–20 mg L−1) followed by magnetic stirring at 150 rpm. At predetermined time intervals, equal amounts of samples from stirred solution were taken and centrifuged at 14,000 rpm for 20 min. Then, the concentration of the RhB solution was determined by measuring absorbance changes at its absorption maximum (554 nm). The effect of pH on the adsorption efficiency was studied at initial pH ranging from 3 to 11 with a contact time of 120 min; the pH was adjusted by adding small amounts of diluted HCl or NaOH solutions.

The amounts of RhB adsorbed at equilibrium (qe, mg g−1) and the removal efficiency (R, %) by the amFc-MWCNTs adsorbent were calculated using the following equations, respectively:
$$q_{e} = \frac{{\left( {C_{0} - C_{e} } \right)V}}{m}$$
$$R = \frac{{100 \times \left( {C_{0} - C_{t} } \right)}}{{C_{0} }}$$
where C0, Ct and Ce (mg L−1) are the initial, at any time (t) and equilibrium concentration of RhB solution respectively; V (L) is the initial volume of RhB solution; and m (g) is the mass of amFc-MWCNTs.

2.5 Desorption Experiments

For the desorption study, 0.4 mg of the amFc-MWCNTs hybrid was added to 4 mL of RhB solution (10 mg L−1) and the mixture was stirred for 120 min. The adsorbent was separated by centrifugation and the supernatant dye solution was discarded. Then, the RhB-adsorbed adsorbent was suspended in 4 mL acetone and collected by centrifugation. Before reusing the adsorbent, it was dried overnight at 60 °C. The supernatant solutions were analyzed by UV–vis spectroscopy. The cycles of adsorption–desorption processes were successively conducted four times.

3 Results and Discussion

3.1 Characterization of the Ferrocene–Functionalized MWCNTs

The ferrocene–functionalized carbon nanotube hybrid (amFc-MWCNTs) was prepared via cycloaddition between the nitrene, formed from the azidomethylferrocene decomposition at 160 °C, and the carbon–carbon double bonds from MWCNTs. The successful covalent attachment of amFc molecules to MWCNTs was assessed by Raman spectroscopy (Fig. 2a). The feature bands at around 1350–1364 cm−1 represent the disorder mode (D-band) related to sp3- hybridized carbons of the nanotubes and the band at around 1568–1587 cm−1 represent the tangential mode (G-band) assigned to the sp2 ones [23]. The relative intensity ratios of the D band to G band (ID/IG), were used to monitor purity and functionalization degree of nanotubes. ID/IG ratio increased from 0.41, for pristine MWCNTs, to 0.60 after derivatization with ferrocene derivative, suggesting increasing disorder in the nanotubes, which confirms their covalent modification. XPS measurements provided further support for the success of covalent MWCNTs sidewall functionalization (Fig. 2b, c). In fact, nitrogen and iron peaks related, respectively, to the aziridine and the ferrocene moieties, appeared at the wide scan survey of binding energy of amFc-MWCNTs. The higher resolution XPS spectrum of Fe2p area showed that it has two components at 711 eV and 724 eV, which can be assigned to Fe2p3/2 and Fe2p1/2 photoelectrons, respectively (Inset of Fig. 2b) [24]. Moreover, EDX analysis revealed the presence of a significant amount of iron (6.93%) on the MWCNTs surface after functionalization (Fig. 2d).
Fig. 2

a Raman spectra of pure MWCNTs and amFc–MWCNTs; b Wide scan survey XPS spectrum of pure MWCNTs; c Wide scan survey XPS spectrum of the amFc–MWCNTs nanomaterial, Inset: The higher resolution XPS spectrum of Fe2p area; d EDX analysis of amFc–MWCNTs

To confirm that the MWCNTs has not suffered structural damage after the functionalization step, TEM and SEM analysis were conducted. TEM images of MWCNTs before and after functionalization showed that the relatively clear and smooth MWCNTs surface became bumpy with higher roughness after ferrocene cycloaddition (Fig. 3c, d). Clearly, no evidence of significant damage of the basic structure of MWCNTs due to the functionalization was observed. Furthermore, SEM image of amFc-MWCNTs revealed the presence of numerous amFc assembling on the surface of MWCNTs bundles, which were not observed at the unmodified MWCNTs (Fig. 3e, f).
Fig. 3

TEM images of a MWCNTs and b amFc–MWCNTs; SEM micrographs of c MWCNTs and d amFc–MWCNTs

3.2 Adsorption Properties of amFc-MWCNTs

3.2.1 Effects of the Amount of amFc-MWCNTs and pH

To evaluate the adsorption capacity of amFc-MWCNTs in RhB dye removal, the hybrid adsorbent concentration varies from 0.05 to 0.4 g L−1. As shown in Fig. 4a, the intensity of the characteristic UV–vis absorption band of RhB decreased with the increase of amFc-MWCNTs concentration. In fact, > 98% of RhB was captured within 2 h when using 0.4 g L−1 of amFc-MWCNTs (Fig. 4b). The initial red-colored RhB solution turned colorless after adding the amFc-MWCNTs, which proved its adsorption ability (Inset of Fig. 4b). This efficient adsorption can be further confirmed by the electrochemical analysis performed using DPV (Fig. 4c). As it can be seen, besides the anodic peak at 252 mV related to the ferrocene moiety [22], a new peak is observed at 866 mV after the dye adsorption, which can be ascribed to the RhB oxidation [25]. The high-performance of the amFc-MWCNTs nanomaterial may be attributed to the strong π–π stacking interactions between the aromatic backbones of the dye and the large delocalized π-electron system of MWCNTs or the aromatic ferrocene moieties. Moreover, the ferrocene derivative acts as a spacer minimizing the agglomeration of MWCNTs and thus favors the RhB adsorption as confirmed by the better removal efficiency of modified CNTs hybrid compared to the unmodified MWCNTs (Fig. 4b).
Fig. 4

a Absorption spectra of the RhB (C0 = 10 mg L−1) at different concentrations of amFc–MWCNTs; b Removal efficiency of RhB (C0 = 10 mg L−1) at different concentrations of amFc–MWCNTs and MWCNTs, Inset: Photos of dye solution before and after using amFc–MWCNTs (0.4 g L−1); c DPV of the amFc–MWCNTs before and after adsorption of RhB in pH 7.4 PBS at a scan rate of 100 mV s−1; d Effect of pH on removal efficiency of RhB (C0 = 10 mg L−1) by amFc–MWCNTs (0.1 g L−1), The solid line is only guideline to the eye

Figure 4d shows the adsorption capacity of the amFc-MWCNTs at various pH values. The pH increases from 2 to 7 induces an increase of the removal efficiency, which can be attributed to the electrostatic repulsion decrease between the cationic form of RhB and the protonated carbon nanotubes hybrid [26, 27]. At higher pH values, the dissociation of the carboxylic acid group of both RhB and amFc-MWCNTs nanomaterial will increase, thereby increasing electrostatic repulsion between RhB and the negatively charged adsorbent, which leads to the decrease of the removal efficiency [16]. Therefore, pH value of 7 was chosen to carry all further adsorption experiments.

3.2.2 Adsorption Isotherms

The effect of varying the initial RhB concentration on its adsorption onto amFc-MWCNTs adsorbent was studied. While the adsorption capacity (qe) of the carbon nanotubes hybrid increased from 9.6 to 90.9 mg g−1 when increasing the initial RhB concentration (1–20 mg L−1), the removal percentage dropped from 97 to 16% (Fig. 5a). At a low dye concentration, a limited number of RhB molecules comes in contact with the available adsorption sites; hence, the removal efficiency is high. However, at higher concentrations, all the active sites are occupied, which led to a decrease in the adsorption percentage. Moreover, as initial RhB concentration increased, the driving force of the concentration gradient exceeded the resistance to the mass transfer of RhB molecules onto the interface of the adsorbent, which resulted in the increase of the adsorption capacity.
Fig. 5

a Effect of initial RhB concentration on its adsorption onto amFc–MWCNTs (0.1 g L−1); Equilibrium isotherm data of RhB on amFc–MWCNTs (0.1 g L−1) plotted with b the Langmuir, c the Freundlich or d the Hill equations

In order to study the adsorption isotherm, describing the relationship between the amounts of the adsorbed RhB (qe) and the bulk RhB concentration (Ce), Langmuir, Freundlich and Hill isotherm adsorption models were used. The Langmuir model describes the formation of a monolayer adsorption onto a surface with a finite number of identical sites [28]. Equation 3 represents its linearized form.
$$\frac{{C_{e} }}{{q_{e} }} = \frac{{C_{e} }}{{q_{maxL} }} + \frac{1}{{q_{maxL} K_{L} }}$$
where, Ce (mg L−1) is the equilibrium concentration of the dye in the solution; qe (mg g−1) denotes the amount of the dye adsorbed per unit mass of adsorbent; and qmaxL (mg g−1) and KL (L mg−1) are the maximum adsorption capacity and the Langmuir constant, respectively.
The Freundlich isotherm model is suitable for multilayer adsorption on a heterogeneous adsorbent surface [29]. It can be expressed using the following equation:
$$log q_{e} = \frac{1}{{n_{F} }}\log C_{e} + \log K_{F}$$
where, KF is the Freundlich isotherm constant; and nF is an empirical parameter representing the heterogeneity factor.
The Hill isotherm was postulated for non-ideal competitive adsorption, where the dye binding ability at one site on the homogeneous substrate may influence the binding capacity of other sites. It can be represented by [30, 31]:
$$q_{e} = q_{minH} + \frac{{q_{maxH} - q_{minH} }}{{1 + \left( {\frac{{K_{H} }}{{C_{e} }}} \right)^{{n_{H} }} }}$$
where, qminH (mg g−1) is the minimum RhB adsorption asymptote (as t → 0); qmaxH (mg g−1) is the maximum adsorption capacity; KH (mg L−1) is the dye concentration in the solution where 50% of the observed adsorption ((qmaxH − qminH)/2) occurs; and nH is the Hill cooperativity coefficient of the binding interaction.
The experimental results were fitted to the three isotherm models (Fig. 5b–d). The parameters of all isotherm equations were calculated and given in Table 1. As can be seen, the Hill model was found to be a better fit for RhB dye adsorption, which is confirmed by the high value of the coefficient of determination (R2 = 0.975) compared to those of Langmuir (R2 = 0.959) and Freundlich (R2 = 0.908) models. The Hill parameter nH was greater than unity, suggesting that a positive cooperative adsorption is the suitable mechanism for RhB adsorption onto amFc-MWCNTs hybrid [32, 33]. According to both Langmuir and Hill models, the maximum RhB adsorption capacity of amFc-MWCNTs is 98 mg g−1, which was comparable and even higher than previously reported CNTs-based adsorbents (Table 2). The nF value obtained from the Freundlich isotherm was larger than unity, indicating a strong interaction between the RhB dye and the amFc-MWCNTs nanomaterial [34].
Table 1

Isotherm constants and their correlation coefficients for RhB adsorption onto amFc–MWCNTs nanomaterial

Langmuir isotherm

QmaxL (mg g−1)

KL (L mg−1)






Freundlich isotherm


KF\(({\text{mg}}^{{(1 - 1/{\text{n}}_{{\text{F}}} )}} \,{\text{g}}^{{ - 1}} \,{\text{L}}^{{1/{\text{n}}_{{\text{F}}} }} )\)






Hill isotherm

qminH (mg g−1)

qmaxH (mg g−1)







KH (mg L−1)





Table 2

Summary of RhB maximum adsorption capacities on various carbon nanotubes–based adsorbents



Qmax (mg g−1)



pH 7.0, 24 h, 2.00 g L−1 dose, 293 K




pH 7.0, 24 h, 2.00 g L−1 dose, 293 K




pH 6.0, 80 min, 0.30 g L−1 dose, 298 K




pH 7.0, 30 min, 1.00 g L−1 dose




pH 7.0, 180 min, 0.75 g L−1 dose, 298 K




pH 9.0, 30 min, 1.00 g L−1 dose




pH 7.0, 120 min, 0.10 g L−1 dose


This work

MAPCNTs magnetic carbon nanotubes modified with NaClO

3.2.3 Kinetic Studies

The time-dependent adsorption experiments of RhB by amFc-MWCNTs were conducted in order to access to the kinetic parameters (Fig. 6a). As can be seen, the adsorption capacity increased drastically during the first 20 min and then proceeded slowly with the contact time, which is due to the gradual saturation of the adsorbent surfaces. A state of equilibrium is reached within 120 min. Pseudo first-order [37], pseudo-second-order [38], intra-particle diffusion [39] and Boyd [40] models were used to fit the results of Fig. 6a in order to examine the kinetic adsorption process (Fig. 6b–d). Their linearized form can be subsequently expressed using the following equations:
Fig. 6

a Effect of contact time on adsorption of RhB (C0 = 10 mg L−1) onto amFc–MWCNTs (0.1 g L−1); Fitting of kinetic data to b pseudo first–order, c pseudo–second order and d intra–particle diffusion kinetic models

$$Ln\left( {q_{e} - q_{t} } \right) = Ln\left( {k_{1} q_{e} } \right) - k_{1} t$$
$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} q_{e}^{2} }} + \frac{t}{{q_{e} }}$$
$$q_{t} = k_{i} t^{0.5} + C_{i}$$
$$Bt = - 0.4977 - Ln\left(1 - \frac{{q_{t} }}{{q_{e} }}\right)$$
where, qt (mg g−1) and qe (mg g−1) are the amounts of RhB adsorbed per unit mass of adsorbent at any time (t (min)) and at the equilibrium respectively; k1 (min−1) is the pseudo-first order rate constant; k2 (g mg−1 min−1) is the pseudo-second-order rate constant; ki (g mg−1 min−0.5) is the intra-particle diffusion rate constant; Ci indicates the thickness of the boundary layer; and B is equal to π2Di/r2 where Di (cm2 s−1) is the effective diffusion coefficient of the adsorbate and r (cm) is the radius of adsorbent particles assumed to be spherical.
The kinetic parameters of the fitted model are gathered in Table 3. When the pseudo-first order kinetic model was applied, a lower coefficient of determination was obtained and a great difference between the calculated adsorption capacity (425 mg g−1) and the experimental one (64 mg g−1) was registered. Therefore, the kinetic experimental data do not fit this model. On the contrary, the pseudo-second-order model gave a good fit to the experimental kinetic data, which indicates that the adsorption process is a chemisorption involving electrons sharing or exchange between RhB and the amFc-MWCNTs hybrid [1]. Besides, when using the intra-particle diffusion model, a linear plot that passes through the origin, was obtained. Thus, intra-particle diffusion is the rate-limiting step of the adsorption process. This result was further confirmed using Boyd model. In fact, plotting the calculated Bt values against time gives a straight line that passes through the origin (data not shown for brevity), indicating that the rate controlling step of the adsorption process is not a film diffusion but an intra-particle diffusion controlled mechanism.
Table 3

Kinetic parameters for the adsorption of RhB onto amFc–MWCNTs nanomaterial


Qe (mg g−1)

K1 (min−1)







qe (mg g−1)

k2 (g mg−1 min−1)*10−4






Intra–particle diffusion


ki (g mg−1 min−0.5)










3.3 Desorption and Recycling of the Adsorbent

To study the regeneration and reuse of amFc-MWCNTs adsorbent, acetone was used as the regeneration agent to perform four consecutive cycles of adsorption–desorption experiments. Figure 7 shows that the removal efficiency of RhB decreased from 69% to 56% after four cycles of adsorbent reuse. In fact, over 81% recovery was accomplished, which indicates that amFc-MWCNTs nanomaterial has a good reusability and a good potential in practical applications.
Fig. 7

Removal efficiency of RhB dye (C0 = 10 mg L−1) by regenerated amFc–MWCNTs (0.1 g L−1)

4 Conclusion

In summary, the adsorption of rhodamine B on ferrocene–functionalized carbon nanotubes was studied. Although the influence of adsorbent dosage on the uptake of rhodamine B was conducted, the effect of ferrocene loading, i.e. the degree of ferrocene functionalization on the carbon nanotubes surfaces, on the adsorption of rhodamine B was not examined. Still, it was found that amFc molecules acted as a spacer that minimizes the agglomeration of MWCNTs and thus favors the dye removal. Adsorption of the rhodamine B to amFc-MWCNTs hybrid agreed well to the Hill isotherm with a maximum adsorption capacity of 98 mg g−1. Moreover, it followed the pseudo-second-order kinetics with intra-particle diffusion as the rate-limiting step. Recycling experiments showed that the hybrid adsorbent amFc-MWCNTs could be regenerated after a simple acetone washing. Therefore, the prepared amFc-MWCNTs may be a promising adsorbent nanomaterial for the removal of aqueous organic contaminants.



The authors would like to acknowledge the financial support from the Tunisian Ministry of Higher Education and Scientific Research to our laboratory (LCAE-LR99ES15).

Compliance with ethical standards

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.


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Copyright information

© The Tunisian Chemical Society and Springer Nature Switzerland AG 2018

Authors and Affiliations

  1. 1.Faculty of Science, Department of Chemistry, Laboratory of Analytical Chemistry and Electrochemistry (LR99ES15)University of Tunis El ManarTunisTunisia

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