1 Introduction

Recently, the world started using nanocomposites as an alternative material to improve economic growth in all aspects of life [1]. These composites arise from the interaction of two or more substances with different chemical and physical properties. The most common type of carbon is carbon nanotubes (CNTs) that are used in the fabrication of many composites due to their distinctive properties like chemical stability, great surface area, superb mechanical properties, and good electrical and thermal conductivity [2]. Based on the layers that make up the wall of the carbon tubes, the carbon nanotubes are partitioned into two parts: single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) [3]. Because of their exceptional properties, both types have played an important role in many industrial, biological and environmental applications such as sensitizers, catalysts, adsorbents, drug carriers, and antibacterial agents. As the functionalization of the MWCNTs surface by oxidation of alcohols utilizing nitric acid and sulfuric acid, this increases the dispersion rate in the reaction solution, in addition to increasing the compatibility with the prepared composites [4]. Therefore, researchers turned towards increasing chemical and thermal stability by doping MWCNTs with several semiconductors such as ZnO, TiO2, CuO and NiO, as they are considered materials with low economic cost and non-toxic [5]. Nickel oxide (NiO) is a semiconductor material, a cubic structure and has a good bandgap value (3.6–4.0) eV. Nickel oxide exhibits excellent properties such as large surface area, high electronic mobility, good electrical properties, and efficiency at low concentrations [6]. The NiO/F-MWCNTs nanocomposite is an effective material in many medical, biological, industrial and environmental fields because of its low cost and biocompatibility and high adsorption capacity [7]. Due to the promise properties of the nanocomposite, it was applied as an effective substance in the treatment of contaminated water towards dyes and minerals. MWCNTs represent a support substance for the synthesis of nickel oxide nanopowder, preventing their accumulation and making them a hydrophilic surface [8].

The current study included the functionalization of MWCNTs using sulfuric acid and nitric acid and followed by doping with NiO nanoparticles using a simple hydrothermal method. The prepared composite was characterized using several techniques are FTIR, XRD, FESEM, HRTEM, EDX, TGA and BET. Finally, the adsorption efficiency of nanocomposite towards malachite green dye (Fig. 1) from contaminated solutions was studied. The influence of adsorbent dose, acidic function, temperature and equilibrium time on the adsorption process was also investigated (Table 1).

Fig. 1
figure 1

The structural formula of Malachite Green dye

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Table 1 A list of previous works represents the use of MWCNTs and their composites as adsorbents
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2 Experimental part

2.1 Chemicals

Pristine-MWCNTs (length ∼10–50 µm, diameter ∼8–15 nm) was obtained from Grafton VT (USA). Nitric acid, sulfuric acid and sodium hydroxide were bought from Merck Chemicals. Malachite green (MG [C6H5C(C6H4N(CH3)2)2] Cl, M.Wt 364.911 g/mol, purity 90%) was supplied from Chem-supply. Nickel nitrate Ni (NO3)2 was supplied from Sigma-Aldrich (USA). Use deionized water to prepare all solutions.

2.2 Instruments

The crystalline nature of the synthesized nanomaterials was studied using powder X-ray diffraction, XRD-6000 (Shimadzu, Japan) at a scanning rate of 0.03° s−1 by Cu Kα radiation for diffraction angle (2θ) from 20° to 80°. The oxygen-containing sorts of F-MWCNTs and nanocomposite can be established utilizing FTIR spectroscopy (Nicolet Nexus 670, Shimadzu, Japan) in the region 4000–400 cm−1 by mixing the samples with KBr discs. The surface morphology of prepared nanomaterials was observed employing field emission scanning electron microscopy FESEM (JEOL, JSM-6701F, voltages 8.0 kV, Japan) and high-resolution transmission electron microscopy HRTEM (Zeiss Libra 200 FE, Germany). The thermal stability of nanomaterials was concluded by TGA analysis using a Perkin-Elmer Diamond analyzer in an atmosphere of argon gas at the range of 0–800 °C. The specific surface area and pore size distribution were concluded employing BET and BJH analysis from the adsorption and desorption isotherms using a Quadrasorb SIMP.

2.3 Functionalization of MWCNTs

Covalent functionalization of the MWCNTs was carried out employing the chemical oxidation method. The acid treatment involves the mixing of sulfuric acid (H2SO4) and nitric acid (HNO3) (150/50 ml V/V) with 2 g of MWCNTs. The solution is dispersed at 40 °C for 80 min in the ultrasonic water bath. After dispersion, the suspension is separated utilizing a centrifuge at 10,000 rpm for 15 min. Afterward, the product was washed employing deionized water several times. Then, the material is dried at 60 °C for 18 h in a vacuum oven to obtain the functionalized MWCNTs (F-MWCNTs) [16].

2.4 Synthesis of NiO/F-MWCNTs nanocomposite

The NiO/F-MWCNTs nanocomposite was produced using a simple hydrothermal route. Typically, 3 g of MWCNTs was dispersed in 50 mL of deionized water by sonicator bath for 20 min. After dispersion, the F-MWCNTs solution was mixed with 0.1 M nickel nitrate Ni(NO3)2. With a continuous magnetic stirring of the solution, 1 M NaOH solution (35 mL) was added. Finally, the final solution is transferred to the sealed Teflon-lined autoclave (100 mL), closed well and heated at 150 °C for 10 h. After the hydrothermal time is completed, the suspension was cooled down to room temperature for 1 h to obtain the MWCNTs/Ni (OH)2. The resulting mixture was washed several times with 2- propanol and distilled water. Then, separated using a centrifuge at 10,000 rpm and the sample was collected and dried in a vacuum oven for 60 min. Finally, the powder is calcined at 350 °C for 10 h under aerobic conditions.

2.5 MG adsorption experiments

The adsorption experiments included, prepare the stock solution (20 mg/L) through dissolving 0.02 g MG dye powder into 100 mL distilled water. Then, mixtures of different concentrations in the range 1–20 ppm were prepared and mixing of 10 mL of all solutions with 0.01 g of NiO/F-MWCNTs nanocomposite. The solutions were shaken by placing in a thermostatic digital laboratory water bath (120 rpm) at different times until it reaches the equilibrium time by saturating the surface with dye molecules. After the equilibrium state was reached, the adsorbent particles were separated using a centrifuge (10,000 rpm) for 10 min. Then, the supernatant solutions were placed in UV–visible spectroscopy to record the dye adsorption at the wavelength of 617 nm. Depending on Eqs. (1 and 2), the quantity of adsorbate (mg/g) and the removal percentage of dye were calculated [17, 18]:

$$q_{t} = \left( {\frac{{C_{o} - C_{e} }}{m}} \right) \times V{ }$$
(1)
$$R\% = \left( {\frac{{C_{o} - C_{e} }}{{C_{o} }}} \right) \times 100{ }$$
(2)

whereas Co, Ce represents the initial concentration and the concentration of the dye after adsorption (mg/L), V represents the solution volume (mL) and m represents the adsorbent amount (g).

3 Results and discussion

3.1 Characterization of the synthesized NiO/F-MWCNTs

3.1.1 Fourier transform infrared spectroscopy (FTIR)

FTIR spectroscopy is an effective tool of wide interest in identifying the functional groups in the composition of the nanomaterials. Figure 2 displays the FTIR analysis of F-MWCNTs and NiO/F-MWCNTs nanocomposite in the wavenumber range from 400 to 4000 cm−1. The broad absorption peak at 3427 cm−1 is attributable to the stretching vibration of the hydroxyl groups (-OH). The apparent absorption peaks at 2919 and 2855 cm−1 are attributable to C-H symmetric and asymmetric stretching vibration. Also, the absorption peak at 1723 cm−1 is attributable to the presence of C=O stretching vibration belonging to the carboxyl groups resulting from the chemical oxidation process[19]. The absorption peak at 1630 cm−1 is attributable to C=C stretching vibration in the aromatic rings of the MWCNTs structure. While the absorption peaks appear at 1445, 1368, 1110 cm−1 owing to the present of the O–H bending, C–N (NO2), C–O bonds, respectively. After the synthesis of the NiO/F-MWCNTs nanocomposite, a clear decrease in the intensity of the carbonyl top was observed due to the shorthand of the carboxylic sorts (COOH) through the powder thermal treatment. Also, a new absorption peak appears at 626 cm−1 due to the vibration of the Ni–O bond. This indicates the success of the nanocomposite synthesis employing the hydrothermal technique [20].

Fig. 2
figure 2

FTIR spectra of F-MWCNTs and NiO/F-MWCNTs nanocomposite

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3.1.2 X-rays diffraction (XRD)

Figure 3 illustrates the XRD analysis of F-MWCNTs and NiO/F-MWCNTs nanocomposite in the diffraction angle range 2θ = 20–80°. The X-ray diffractogram of F-MWCNTs exhibits a high-intensity sharp peak at 2θ = 26.06° and a weak peak at 2θ = 43.25° are assigned to the hkl planes of (002) and (100), respectively. Both peaks belong to the hexagonal graphite structure in F-MWCNTs [21]. On the other hand, the XRD pattern of the NiO/F-MWCNTs nanocomposite showed a low-intensity peak at 2θ = 26.07° corresponding to the hkl planes of (002) belong to the F-MWCNTs structure. The observed peaks at 2θ = 37.59°, 43.56°, 63.14°, and 75.57o belong to NiO crystal which is assigned to the hkl planes of (111), (200), (220) and (222), respectively [22]. The Debye-Scherr equation was used to find out the size values ​​of the prepared nanomaterials:

$$D = \frac{0.9 \lambda }{{\beta \cos \theta }}$$
(3)
Fig. 3
figure 3

XRD patterns of F-MWCNTs and NiO/F-MWCNTs nanocomposite

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where θ is the Bragg angle, D is crystalline size, β is the full width at half the maximum of the top, λ is the radiation wavelength. It has been found that the crystalline size values for the F-MWCNTs and NiO/F-MWCNTs nanocomposite are 53.33 and 25.43 nm, respectively.

3.1.3 Field emission scanning electron microscopy (FESEM) analysis

FESEM analysis is utilized to reveal the surface morphology which includes the shape, distribution and size of nanoparticles. Figure 4a and b shows FESEM images of F-MWCNTs at two different magnifications. It was observed that short tube shapes (worm-like) with slight defects were observed at the end of the tube. The images showed a decrease in the agglomeration rate of the tubes as a result of surface oxidation due to the present of the carboxylic groups, which reduces the Van der Waals forces linking the tubes [23]. While FESEM images of the NiO/F-MWCNTs nanocomposite in Fig. 4c and d showed heterogeneous spherical shapes representing NiO nanoparticles deposited on the surface of tangled tubes representing F-MWCNTs. It was observed that the distribution of NiO nanoparticles is irregular on the F-MWCNTs surface. The results show the successful attachment of NiO with F-MWCNTs using the hydrothermal method [24]. Finally, it was found that the particle size of the F-MWCNTs and NiO/F-MWCNTs nanocomposite are 37.26 and 33.14 nm, respectively. The observed sizes are almost agreed with the results of the XRD analysis.

Fig. 4
figure 4

FESEM image a, b F-MWCNTs and c, d NiO/F-MWCNTs nanocomposite

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3.1.4 Energy-dispersive X-ray spectroscopy (EDX)

EDX spectroscopy is a very popular technique utilized to recognize the primary compositions of synthesized solid nanomaterials. Figure 5 demonstrates the EDX analysis of F-MWCNTs and NiO/F-MWCNTs nanocomposite. EDX analysis of F-MWCNTs showed the presence of carbon and oxygen in weight ratios 82.81% and 18.18%, respectively. This indicates the success of the process of adding carboxylic groups to the MWCNTs surface. In addition, the absence of any additional peaks confirms the purity of the prepared MWCNTs sample. While the EDX of NiO/F-MWCNTs nanocomposite showed the existence of oxygen, nickel and carbon in weight ratios of 48.77%, 37.79% and 13.44%, respectively. This confirms the success of the decoration of the F-MWCNTs surface by NiO nanoparticles using the hydrothermal method.

Fig. 5
figure 5

EDX spectra of a F-MWCNTs and b NiO/F-MWCNTs nanocomposite

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3.1.5 High-resolution transmission electron microscopy (HRTEM) analysis

HRTEM is a common technique for identifying the morphology, structural details, particle size and crystalline defects of nanomaterials on the atomic scale [25]. Figure 6a HRTEM image of MWCNTs shows the presence of a cylindrical tube with regular edges and has an average diameter of 33.5 nm. Figure 6b HRTEM image of NiO/F-MWCNTs nanocomposite shows the presence of heterogeneous spherical shapes distributed on the surface of the nanotubes with a significant decrease in the accumulation rate of nanoparticles with an average diameter of up to 45.14 nm. This morphology is consistent with a previous report [26].

Fig. 6
figure 6

HRTEM image a F-MWCNTs and b NiO/F-MWCNTs nanocomposite

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3.1.6 Specific surface area and pore volume estimate

The BET is a widely used method for estimating the specific surface area (SSA) depending on the N2 gas adsorption–desorption measurements at 77 K. While the BJH method was utilized to identify the specific pore volume distribution of the solid surfaces. The BET data exhibited that the surface area value of F-MWCNTs is 332.12 m2/g, while the BJH data revealed that the pore volume value is 0.16 cm3/g. It was noticed that the adsorption isotherm of F-MWCNTs follows the type II isotherm, which confirms that the physical adsorption is obtained on porous surfaces through a monolayer covering, followed by a multilayer covering (Fig. 7). On the other hand, it was found that the specific surface area and the pore volume of ​​the NiO/F-MWCNTs nanocomposite are 412.08 m2/g and 0.13 cm3/g, respectively. In addition, the nanocomposite isotherm follows type IV, and this indicates that the sample is of a mesoporosity nature. The BJH results showed ​​that the pore diameter of the F-MWCNTs and NiO/F-MWCNTs nanocomposite are 2.4 and 2.6 nm, respectively (insert Fig. 7). The obtained pore diameter values ​​refer to the mesoporous composition of both surfaces [27].

Fig. 7
figure 7

Nitrogen adsorption and desorption isotherms and BJH plot (insert figure) of the a F-MWCNTs and b NiO/F-MWCNTs nanocomposite

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3.1.7 Thermogravimetric analysis (TGA)

The TGA analysis is a quick and facile way to determine the thermal stability of prepared nanomaterials as a result of changing their properties through losing weight with increasing temperature. Figure 8 shows TGA analysis of F-MWCNTs and NiO/F-MWCNTs nanocomposite in the range of 0–800 °C. The F-MWCNTs showed low thermal stability, resulting in a weight loss up to 99.58% in the range of 200–580 °C owing to the water elimination, functional groups and consequently the thermal decomposition of other molecules with the oxidation of carbon [21]. After decorating F-MWCNTs with NiO nanoparticles, the thermal stability is significantly increased due to the low rate of mass loss 28.32% in the range 400–520 °C is owing to the rapid pyrolysis of the residual oxygen types. In addition to the evaporation of CO2 and CO gases. The obtained thermal data confirmed that the thermal stability of the NiO/F-MWCNTs nanocomposite is higher than F-MWCNTs [28].

Fig. 8
figure 8

TGA thermograms of F-MWCNTs and NiO/F-MWCNTs nanocomposite

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3.2 Investigation of MG adsorption

3.2.1 Influence of the F-MWCNTs and NiO/F-MWCNTs nanocomposite mass

The influence of F-MWCNTs and NiO/F-MWCNTs doses at different masses (0.04–0.28 g) towards adsorption of malachite green dye was studied. Figure 9 shows the increase in the dye removal percentage with the increase in the mass of both surfaces. The NiO/F-MWCNTs nanocomposite showed a dye removal efficiency of up to 94% which is higher than the removal efficiency of F-MWCNTs (up to 88%). Therefore, all subsequent experiments were performed with NiO/F-MWCNTs.

Fig. 9
figure 9

Influence of adsorbents mass on the adsorption of MG dye

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3.2.2 Influence of contact time

The influence of contact time on the adsorption capacity of malachite green dye on NiO/F-MWCNTs nanocomposite with different times (1–140 min) at the temperature of 25 °C and initial dye concentration 10 ppm was studied. Figure 10 shows that increasing the adsorption contact time beside an increase in the adsorption capacity up to time 140 min when removal efficiency of 90.06%. After that, the adsorption became almost constant due to the saturation of the active sites with dye molecules [29].

Fig. 10
figure 10

Influence of contact time on the adsorption of MG dye

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3.2.3 Adsorption kinetics evaluation

In this section, the interaction between the MG dye and the NiO/F-MWCNTs nanocomposite and their kinetics were identified. Two kinetic models are pseudo-first-order and pseudo-second-order models, have been studied. Equation 4 shows the pseudo-first-order model [30]:

$$Ln\,(q_{e} - q_{t} ) = Ln\,q_{e} - k_{1} t$$
(4)

where k1 is the pseudo-first-order adsorption rate constant (min−1), qe and qt are equilibrium adsorption capacity (mg g−1) and the adsorption capacity at a certain time (mg g−1). Finally, the values of the pseudo-first-order constant and equilibrium adsorption capacity can be estimated from the linear relationship between ln (qe − qt) and time, since the k1 represents the slope and the qe represents the intercept. Equation 5 shows the pseudo-second-order model [31]:

$$\frac{t}{{q_{t} }} = \frac{1}{{k_{2} qe^{2} }} + \left( {\frac{1}{{q_{t} }}} \right)t$$
(5)

where k2 is the pseudo-second-order adsorption rate constant (g mg−1 min−1). The linear relationship between t/qt with time can calculate the values of the equilibrium adsorption capacity (slope) and the k2 (intercept). The kinetic constants and correlation coefficients for both models were calculated in Table 2. It was noted that the correlation coefficient for the pseudo-first-order model is R2 = 0.214, and this confirms the incompatibility of the model with the adsorption process between the dye and the NiO/F-MWCNTs nanocomposite. Figure 11 displays the linearity of the second-order model is R2 = 0.935, and this indicates that the interaction between adsorbate and adsorbent follows the pseudo-second-order model. The pseudo-second-order model suggests that the higher number of unoccupied active sites, the higher the adsorption rate. In addition, covering the adsorbent surface does not change the reaction energy [31].

Table 2 Kinetics parameters for MG dye adsorption on NiO/F-MWCNTs
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Fig. 11
figure 11

a Pseudo first-order and b pseudo-second-order kinetic models for MG dye adsorption

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3.2.4 Influence of temperature and adsorption thermodynamics

The influence of temperature on the adsorption process was investigated using various temperatures 10, 20 and 30 °C. Figure 12 shows the gradual increase in the adsorption capacity with rising solution temperature, this indicates an increase in the spread of cations of the malachite green dye on the active sites of the prepared nanocomposite surface owing to the increase in kinetic energy. The thermodynamic parameters that have been studied include Gibbs free energy (∆G°), enthalpy change (∆H°) and change in entropy (∆S°) whose equations are mentioned below:

$$\Delta G^{o} = - RT\ln K_{a}$$
(6)
$$\Delta G^{o} = \Delta H^{o} - T\Delta S^{o}$$
(7)
$$\Delta S^{o} = \frac{{\Delta H^{o} - \Delta G^{o} }}{T}$$
(8)
Fig. 12
figure 12

Influence of the temperature on the MG dye adsorption

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where R represents the universal gas constant  (8.314 J/mol K), Ka represents adsorption equilibrium constant and T represents the temperature (K). Depending on the Van’t Hoff Eq. (9), lnKa versus T−1 is plotted to estimate the ΔH° value from the slope and the ΔS° value from an intercept. The mathematical expression of the Vant Hoff equation is as shown below [32]:

$$\ln K_{a} = \frac{{\Delta S^{o} }}{R} - \frac{{\Delta H^{o} }}{RT}$$
(9)

The results obtained from the thermodynamic calculations showed that the treatment process has a negative Gibbs free energy (ΔG° = − 1.363), meaning that the interaction between the nanocomposite and the dye occurs spontaneously. Moreover, the enthalpy change value is positive (ΔH° =  + 12.31) and this suggests that the adsorption is endothermic. On the other hand, the change in entropy showed a positive value (ΔS° =  + 46.69), which indicates a high rate of randomness during the adsorption process [33].

3.2.5 Adsorption isotherms

The adsorption isotherm represents the relationship between the quantity of adsorbate (qe) and the equilibrium concentration at a constant temperature [34]. Three types of adsorption isotherms have been studied, they are Langmuir, Freundlich and Temkin models. Langmuir adsorption isotherm suggests that the reaction system between the adsorbate and the adsorbent is monolayered, homogeneous and all active adsorption sites have equal activation energy. Equation 10 represents the mathematical form of the Langmuir model [35]:

$$\frac{{C_{e} }}{{q_{e} }} = \frac{{C_{e} }}{{q_{m} }} + \frac{1}{{K_{L} q_{m} }}$$
(10)

where Ce represents the equilibrium concentration of MG dye (mg L−1), qm, and qe represents the maximum adsorption capacity and the adsorption capacity at equilibrium (mg g−1), respectively. Freundlich adsorption isotherm describes the reversible, heterogeneous and multilayer adsorption, especially on the active carbon materials or organic compounds [36]. The mathematical expression of the Freundlich model is shown in Eq. (11):

$$\log q_{e} = \log K_{f} + \frac{1 }{n}logC_{e}$$
(11)

where Kf represents the adsorption constant of the Freundlich model, n represents the linear factor. Temkin adsorption isotherm suggests that the reaction temperature for all active species decreases linearly with surface coverage. Equation 12 shows the linear form of the Temkin model [37]:

$$q_{e} = BlnK_{T} + BlnC_{e}$$
(12)

where KT represents the adsorption constant of the Temkin model. Table 3 and Fig. 13 display the correlation coefficients for the three models and the adsorption isotherms data. It was found that the correlation coefficients of Langmuir and Temkin isotherms are R2 = 0.5014 and 0.879, respectively. A lower value of the R2 indicates that both isotherms are not suitable for the adsorption process. In contrast, the value of the correlation coefficient of the Freundlich isotherm model is R2 = 0.99, which indicates that the isotherm fits the obtained adsorption data. We conclude from this that the adsorption reaction is heterogeneous and multilayered.

Table 3 Langmuir, Freundlich and Temkin isotherm constants
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Fig. 13
figure 13

Adsorption isotherms a Langmuir model, b Freundlich model and c Temkin model

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3.2.6 Influence of solution pH

The influence of the pH in the range of 2–12 was studied as an influencing factor on the adsorption nature through changing surface charges according to the pH medium. Figure 14a displays the relationship between the amount of adsorbate and the acidic function. It can be seen that at low pH values, decreases in the removal percentage of the MG dye on the NiO/F-MWCNTs nanocomposite due to the ability of hydrogen ions to attach with the active surface sites which is faster than the dye molecules. At high pH values, increases of the adsorption efficiency owing to the increase of negative electrical charges (OH−) on the nanocomposite surface, and thus increases the electrostatic interaction between the positively charged MG molecules with the negatively charged adsorbent [38].

Fig. 14
figure 14

Influence of a pH medium and b NaCl weight on MG dye adsorption

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3.2.7 Influence of salt concentration

The presence of inorganic salts in the reaction solution is a significant index that changes the removal efficiency and affects the surface charge. Figure 14b displays the influence of NaCl mass on the amount of MG adsorbed by the NiO/F-MWCNTs nanocomposite. It was noticed that an increase in NaCl mass decreased adsorption capacity. This behavior is due to the competition between positively charged MG molecules and the sodium ions (Na+) to occupying the same negatively charged active sites on the NiO/F-MWCNTs surface [39]. Similar was behaviors in aqueous solution previously studied by Runping et al. [40].

4 Conclusions

In the current study, the NiO/F-MWCNTs nanocomposite was fabricated via a hydrothermal process. It was found that the synthesized nanocomposite has good properties of particle size is 25 nm, a good large surface area and high thermal stability, and this makes it an absorbent and effective material in removing hazardous substances from polluted water. It was observed that the removal efficiency of the malachite green dye on the nanocomposite was up to 94%. Most of the factors affecting the adsorption process like surfaces mass, temperature, pH, salt concentration and contact time are studied. It was found that the removal efficiency increases besides increasing the surface’s mass, the temperature and the acidic function. While adsorption kinetics models were applied, namely the pseudo-first-order model and the pseudo-second-order model, as it was established that the reaction between dye and nanocomposite obeys the pseudo-second-order model. Calculations of thermodynamic functions (ΔHo, ΔGo, ΔSo) showed that the adsorption reaction is endothermic, spontaneous and highly random. In contrast, the results of the adsorption isotherms displayed that the reaction between adsorbate and adsorbent obeys the Freundlich isotherm model (R2 = 0.990) and does not agree with the Langmuir and Temkin isotherm model. This indicates that the adsorption is heterogeneous and multilayered. Finally, the removal ability decreases with the increase in the amount of inorganic salt added to the adsorption solution.