Amine-Functionalized MWCNTs for the Removal of Mordant Black 11 Dye

In this study, amine-functionalized MWCNTs (f-MWCNTs) have been investigated as potential material for the removal of Mordant black dye 11 (MBD 11). To evaluate the optimal condition and adsorption capability of the adsorbents (f-MWCNTs), the effect of temperature, pH, adsorbent dosage, and contact time on adsorption rate are examined. The study shows a stronger interaction between the dye and f-MWCNTs. The highest removal efficiency is observed in acidic medium (pH 2) with an initial dye concentration of 50 mg L−1, where 99% of the dye is adsorbed from the medium in 40 min using 0.05 g of f-MWCNTs. Adsorption isotherm and kinetic studies reveal that adsorption occurs by the Langmuir adsorption model and pseudo-second-order adsorption kinetics. According to the thermodynamic parameters, the adsorption exhibits endothermic and spontaneous behavior.


Introduction
One of the most common azo dyes is Mordant black 11 dye (MBD 11).In textiles, it is commonly used for dying silk, nylon, and wool, after having been treated with chromium salts.MBD 11 is a monoazo dye with sulfonate and naphthol moieties and is extremely stable and shows resistance to fading when exposed to water and sunlight due to its persistent nature and complex molecular structure.Because of these properties, this dye is used in large quantities on a commercial scale in a variety of industrial processes (Tahir et al., 2021).Effluents from these industrial processing contain MBD 11 residue in large quantities, which can damage human and aquatic life in the environment (Ashraf et al., 2006).The degradation products of the dye (for example, naphthoquinone) are also toxic, carcinogenic, and can cause skin irritation.Its long-lasting existence in drinking water reservoirs has the potential to be fatal to animals and cause severe environmental damage.Before discharging wastewater into the water reservoir, MBD 11 must be separated and removed from wastewater (El-Dars et al., 2015;Khalid & Zubair, 2018).
A variety of biological, chemical, and physical methods such as coagulation, flocculation, membrane separations, oxidation, adsorption, ion exchange, electrochemical process, advanced oxidation process, biodegradation, photocatalysis, and other processes have been employed to eradicate colorants from textile wastewater (Yaseen & Scholz, 2019).In biological treatments, a variety of microorganisms are used to discolor synthetic dyes in anaerobic or aerobic environments.In comparison to chemical and physical approaches, biological treatments are common, simple, and environmentally friendly.However, external environmental restrictions such as pH, nutrition, temperature, and are required for biological treatment (Ngulube et al., 2017).Chemical methods have the advantage of being usually efficient.However, the sludge production and deposition may raise the overall cost of the process.Such chemical technologies, in general, utilize a lot of chemical reagents and electrical energy (Zhou et al., 2015).Furthermore, the chemical residue has the potential to cause secondary emissions.Advanced oxidation processes have recently gained prominence, because of their excellent oxidizing power (Iqbal et al., 2022).These approaches are effective, but they are also expensive, and the intermediate oxides may be toxic (Zhou et al., 2019a).Membrane separation and adsorption technologies are examples of traditional physical methods.Membrane separation, which includes nanofiltration, microfiltration, ultrafiltration, and reverse osmosis, is an effective technique.The use of membrane technology in textile wastewater is not in practice as the membrane's shelf lifespan is limited and it can easily contaminate (Shamraiz et al., 2016).According to the literature, the adsorption approach is extensively used for the treatment of wastewater containing textile effluent.Adsorption technology is less expensive, simple, and effective and can be recovered after multiple reuse cycles (Awual et al., 2013;Rehman et al., 2022;Zhou et al., 2019b).
Graphene, CNTs, MOFs, and polymers based materials have been used for dyes removal and showed excellency (Au, 2020;Janjhi et al., 2022;Sajid et al., 2018) alongside other many other applications (Amu-Darko et al., 2023a;Amu-Darko et al., 2023b;Hussain et al., 2020;Okai Amu-Darko et al., 2023).However, CNTs have shown exceptional potentials as adsorbents for metal ions and a variety of organic contaminants due to their hollow, small, and layered structures with large surface area.Chemical treatment can easily be used to modify the surface to increase its adsorption potential.In comparison with other adsorbents, CNTs have unique properties that lead to their superior adsorption capacities, for example, a high aspect ratio, a fibrous shape, easily accessible large external surface area, and well-developed mesopores (Gupta et al., 2011;Inoue et al., 1998;Wang et al., 2006).CNTs have a strongly hydrophobic adsorptive surface (Rafique & Iqbal, 2011).Surface properties of CNTs can be modified by non-covalent (Sadegh et al., 2014) and covalent functionalization techniques (Bahr et al., 2001).Different surface modification techniques traditionally used for CNTs are mechanical, physicochemical, and irradiation (Chen et al., 2001;Islam et al., 2003).Covalent and non-covalent surface modification were the most commonly used physicochemical methods (Chen et al., 2001).The chemical interaction between the conjugation of hydrophilic organic molecules and the carbon atoms on the surface of CNTs is responsible for covalent functionalization (Karimi et al., 2015).This treatment can introduce various functional groups on the sidewall of nanotubes.These treatments improve CNT's adsorption capacity for wastewater toxic pollutants, which are then trapped on the surface of the nanotube (Bankole et al., 2019;Duan et al., 2020).A literature review revealed that there are only a few studies available on the adsorptive removal of MBD 11 by MWCNTs.Sobhanardakani et al. (Sobhanardakani et al., 2014) observed 98 % MBD 11 removal at pH 3 and 0.01 g MWCNTs as adsorbent.Bandari et al. (Bandari et al., 2015) utilized magnetic MWCNTs for adsorptive removal of MBD 11 and observed 99.80 % removal under optimum conditions (pH 2.11; contact time = 78 min, adsorbent dosage = 5.39 g/L).The primary objective of this research is to use amine-functionalized MWC-NTs for the adsorption of Mordant black 11 dye.Amine-functionalized multiwall carbon nanotubes (f-MWCNTs) with 4,4′-diaminodiphenyl sulfone have not yet been applied to remove the dye and other toxins.Several parameters including pH, contact time, adsorbent dosage, and initial dye concentration were discussed to determine the optimized adsorption conditions for the removal of MBD 11 from aqueous solutions.The adsorbent showed excellent dye removal efficiency.Thus, to save water bodies from dye pollutants caused by industrial effluents, aminefunctionalized carbon nanotubes with 4,4′-diaminodiphenyl sulfone can be used as efficient adsorbent.

Surface Modification of MWCNTs
In continuation of our previous work (Aman Qazi et al., 2020), MWCNTs (1 g) are heated under air at 500°C for 1/2 h.It was then allowed to cool (25°C).MWCNTs were ultra-sonicated in 160 mL of HCl (36 wt%) for 120 min.The resulting purified MWCNTs (p-MWC-NTs) were filtered and repeatedly washed with distilled water to neutralize their pH (Baan et al., 2008).For incorporation of the carboxylic functional groups on the surface of MWCNTs, p-MWCNTs were stirred at 70 °C in a 1:3 solution of HNO 3 (5M) and H 2 SO 4 (8M) for 6 h.Microfiltration membrane (0.4 μm) was used to vacuum filtrate freshly prepared carboxylic functionalized MWCNTs (a-MWCNTs).a-MWCNTs were rinsed with distilled water to attain pH~7 (Hamdaoui & Chiha, 2007).For the synthesis of amine-functionalized nanotubes (f-MWCNTs), 3 g of a-MWCNTs and 10 g solution of 4,4′-diaminodiphenyl sulfone in DMF solvent.Stirring under reflux was performed at 70 °C for 96 h.To get rid of excess 4,4′-diaminodiphenyl sulfone, the f-MWCNTs were washed with THF.Afterward, they were dried at 80 °C for 6 h in a vacuum oven.A diagram of f-MWCNTs is shown in Fig. 1.

Batch Adsorption Studies
From the stock solution of MBD 11 (500 ppm) different concentration solutions, i.e., 50 ppm, 40 ppm, 30 ppm, and 20 ppm were prepared by using the dilution formula, i.e., C 1 V 1 = C 2 V 2 .The effects of pH (2, 6, and 10) on f-MWCNT adsorption capacity, as well as the effect of contact time and f-MWCNT dosages (0.01-0.05 g), were investigated.A 5 ml solution was taken at a fixed time and spun at 4700 rpm for 3 min in a centrifuge to separate carbon nanotubes.Dye's concentration in the solution was determined with A UVvis spectrophotometer at λ max = 548 nm.The percentage of dye removed can be calculated by Eq. 1: C 0 (mg L −1 ) and C t (mg L −1 ) represent the original dye concentration and the dye concentration after the time (t) correspondingly.The quantity of dye absorbed (q e (mg g −1 )) was determined by Eq. 2.

Characterization
The FTIR spectra of p-MWCNTs [Fig.2a] show C=C stretching at 1582 cm −1 and 1615 cm −1 (Peng et al., 2003).Stretching vibration of hydroxyl groups (-OH) gives a broader peak at 3433 cm −1 and 3775 cm −1 (Ramanathan et al., 2005).Peaks at 2873 cm −1 and 2941 cm −1 were due to the symmetric and asymmetric stretch of CH 2 , respectively (Ramanathan et al., 2005).A new peak in Fig. 2b (a-MWCNTs) appeared at position 1739 cm −1 which was attributed to carbonyl stretch of the carboxylic group (Bonifazi et al., 2006;Du et al., 2008;Salipira et al., 2008).A peak at 3443 cm −1 was allotted to a vibrational stretch of -OH in -COOH (Avilés et al., 2009).The stretching vibration of carboxylate anion was confirmed by a band at 1599 cm −1 (Atieh et al., 2010).FTIR spectrum of a-MWCNTs confirmed the attachment of -COOH on the surface.Figure 2c depicts the f-MWCNT spectrum.At position 1665 cm −1 , the peak resembles the stretching of the C=O (-NH-C=O) (Eren et al., 2016).A new peak around 3410 cm −1 resembles the -OH and -NH stretching vibrations (Zhao et al., 2013).The peaks at 1455 cm −1 and 1277 cm −1 were due to C-N stretching (in amide groups).N-H bending vibration may produce a peak at 1636 cm −1 , and the C-N stretching vibration of amine may cause a signal around 1358 cm −1 , respectively (Singh et al., 2015).At 1634 cm −1 and 1595 cm −1 , peaks were allotted to the deformation of N-H (primary amines) and the C=C in-plane deformation (present in benzene rings of DDS), respectively.The existence of sulfone bands is predicted by the presence of peaks at 1120 cm −1 and 1322 cm −1 which corresponds to symmetric SO 2 stretching and asymmetric SO 2 stretching, correspondingly.A peak at 1209 cm −1 shows the existence of diphenyl sulfones (Schreiber, 1949).SEM data of p-MWCNTs and f-MWCNTs are displayed in Fig. 3a and b, correspondingly.The SEM images demonstrate that the morphology of MWC-NTs after amine functionalization remains the same.This observation shows that the nanotubes are strong enough to withstand the functionalization process and remain undamaged.However, the interspaces in f-MWCNTs are lower as compared to p-MWCNTs.The high agglomeration could be attributed to the functionalization of MWCNTs.The entangled structure may be due to enhanced electrostatic interaction among the functionalized MWCNTs.The diamines molecule may cross-link adjacent MWCNTs.Similar results were also obtained by Amiri et al. (Amiri et al., 2011).

Contact Time Effect on Dye Removal
The effect of contact time and the adsorption of MBD 11 (Fig. 4) was studied using a fixed amount (0.05 g) of adsorbent (f-MWCNTs) in a fixed volume (20 ml)

Effect of the Initial pH on Dye Removal
To determine the effect of pH on adsorption rate, MBD 11 solutions with a 50 ppm concentration and pH values of 2, 6, and 10 were prepared.For the 20 ml solutions, 0.05 g of f-MWCNTs were used as an adsorbent dose.In each solution, a contact time of 40 min was taken.The pH was kept constant by adding NaOH (1 N) to keep it basic and H 2 SO 4 (1 N) to keep it acidic.The effect of initial pH on dye adsorption is displayed in Fig. 5.The figure shows that the optimum pH is 2 for the removal of MBD 11 as 99 % removal of MBD 11 is observed in an acidic medium.Dye adsorption is more at lower pH because amine groups on the surface of f-MWC-NTs may show stronger electrostatic interaction with negative groups of dye in acidic pH.As the pH drops, more amine groups become protonated, attracting the negatively charged (SO 3 -) group of MBD 11 (in solution) more effectively.However, when the pH of the solution increases beyond 7, i.e., pH>7, the % removal of dye molecules starts increasing.The % removal at pH 2 is greater than at pH 10.Therefore, both acidic and basic solutions are suitable to adsorb the MBD 11 effectively.Change in pH affects the ionization/dissociation of f-MWCNTs and dye molecules.The f-MWCNTs have both amine and residual -COOH groups.At pH<7, more amine groups attached on MWCNT surface, get protonated, and attract negatively charged dye more effectively.At pH 7, the zwitterion form of f-MWCNTs may cause agglomeration and decrease the available surface for adsorption.The aggregation of carbon nanotubes may arise due to electrostatic interaction between -N + and residual -COO -groups of f-MWCNTs.When pH increases beyond 7 the OH -, ion competes with COO -in binding with -N + (Arslan & Günay, 2017;Guo et al., 2005).This reduces the aggregation in f-MWCNTs.Consequently, an increase in the removal of MBD 11 is favored.The adsorption of MBD 11 onto f-MWCNTs may not only be due to electrostatic interaction but may also be due to hydrogen bonding between active sites of MBD 11 and amine groups existing on f-MWCNT surface.

Initial Adsorbent Dose Effect on Dye Removal
To study the initial adsorbent effect, a 50 ppm concentration of MBD 11 at pH 2 was taken in three conical flasks each.0.01, 0.025, and 0.05 g of f-MWCNTs were added to these three conical flasks, respectively.Each conical flask was stirred for 40 min.Figure 6 shows a decrease in absorbance at 548 nm with an increase in the dosage of f-MWCNT adsorbent.The inset in the graph shows that as the dose is increased, the percent removal of MBD 11 increases from 78 to 99%.This may be because the increase in dosage contributes to increased surface area and adsorptive sites.Nevertheless, the adsorption capability decreases as f-MWCNT dosage increases from 77 to 19.4 mg/g.This may be due to the unsaturation of active sites with high dosage of adsorbent (El Kassimi et al., 2023).

Initial Concentration Effect on Dye Removal
Dye solution with several initial concentrations within the range of 20-50 mg/L −1 and at pH 2 using 0.025 g dose of f-MWCNTs was stirred for 5 min contact time.Figure 7 shows that the % removal of dye declines from 74 to 60 % and q e rises from 11.7 to 23.11 mg/g.At greater initial dye concentrations, the active sites on the surface of f-MWCNTs may be surrounded with more MBD 11 molecules in the solution, which boosts the adsorbent-adsorbate interaction along with providing the essential energetic potency to overwhelm the resistance to mass transfer of MBD 11 (Sartape et al., 2017).Hence, the adsorption capacity of the adsorbent increases with an increase in the dye concentration.However, the % removal of dye reduced with escalation in the original concentration of the dye.Due to the fixed adsorbent dose, there are a fixed number of active sites; therefore, the percentage of removal decreases with increasing concentration.At low initial concentrations of MBD 11, the ratio of number of dye molecule to available active sites of adsorbent is small (even at low concentrations), and the interaction of molecules with adsorbent is high; therefore, the dye removal efficiency is high (Kumar et al., 2011).Further increase in concentration causes saturation of the adsorbent surface, and the vacant adsorption sites become fewer.As a result, most of the adsorption took place slowly.

Effect of Temperature on Dye Removal
Temperature is extremely significant in determining the extent of adsorption of MBD 11.At three different temperatures (i.e., 5, 50, and 80 °C), batch adsorption experiments were conducted at 0.05 g f-MWCNTs in 20 ml dye solution (50 ppm, 10 min stirring) and at pH 2. Figure 8 depicts the temperature's effect on the percent removal and adsorption capacity of MBD 11 on the surface of f-MWCNTs at a temperature ranging

Langmuir isotherms
Freundlich isotherm q m (mg g −1 )  This suggests an endothermic adsorption route.

Adsorption Isotherm Models
To investigate the adsorbent-adsorbate interaction, the isotherm model of Freundlich and Langmuir were applied.Langmuir's model proposed that adsorption occurs on a uniform adsorbent surface with a limited number of identical sites and is controlled by the monolayer adsorption of the adsorbate molecules.
The model presupposes consistent adsorption energies on the surface and a lack of adsorbate transmigration in the surface plane.Equation 3 expresses the linear isotherm model of Langmuir's adsorption.
C e (mg/L −1 ) and q e (mg/g) correspond to the equilibrium conc. of adsorbate and the quantity of (3) C e ∕q e = C e ∕q m + 1∕q m K L adsorbate adsorbed per unit mass of adsorbent at equilibrium correspondingly.K L (L/mg) corresponds to Langmuir isotherm constant.q m (mg/g) is the maximum monolayer coverage capacity.A dimensionless equilibrium parameter (separation factor) R L can be used to conduct further analysis of the Langmuir equation.
For optimal adsorption, the R L value should be between 1 and 0. For R L > 1 and R L = 1, adsorption is unfavorable (it denotes linear-adsorption), (4)  2 The kinetic parameter for the adsorption of MBD 11 on f-MWCNTs Pseudo-first-order kinetic model Pseudo-second-order kinetic model and R L = 0 shows irreversible adsorption.The R L value, calculated in the present investigation has been found within the range of 0 to 1.The Langmuir adsorption isotherm shows a correlation coefficient (R 2 ) of 0.99.Hence, It was found that the Langmuir adsorption isotherm model was the most appropriate to explain the removal of MBD 11 from the solution as shown in Fig. 9. Heterogeneous adsorbent surfaces and its characteristics are explained by Freundlich adsorption isotherm (Fig. 9).The empirical formula proposed by Freundlich in linear form is given as where 1/n indicates the intensity of adsorption and K f is the sorption capacity.Using 1/n value, the surface heterogeneity of site energies can be depicted, where the value of 1/n should be between 0 and 1 to be considered favorable and heterogeneous.When 1/n is equivalent to 1, the adsorption process is homogenous, and the adsorbed species do not interact.1/n > 1.46 results in unfavorable adsorption (Tseng & Wu, 2008).Normal adsorption is indicated when the 1/n value is less than one (Dada et al., 2013).In this investigation, 1/n has a value of 0.488.If the value of n is less than one, this suggests that the sorption process is favorable (Abdulrahman et al., 2008).Table 1 shows the value of n = 2.05 and R 2 = 0.843.

Kinetic Study
The adsorption kinetics of MBD 11 on f-MWCNTs are investigated with the pseudo-first-order and pseudo-second-order models as shown in Fig. 10.The pseudo-second-order equation is related to chemisorption and is represented by Eq. 6.
(5) log q e = log K f + 1∕n log C e (6) t∕q t = 1∕K 2 q 2 e + t∕q e where q t (mg/g) and K 2 represent the amounts of adsorbate adsorbed at any time t and rate constant for the pseudo-second-order reaction, respectively.The data obtained for MBD 11 adsorption onto f-MWC-NTs is well matched to the kinetics of pseudo-secondorder.Table 2 shows that its regression coefficient (R 2 ) is near to unity in comparison to pseudo-firstorder.Furthermore, the calculated value of q e (37.260 mg/g) from pseudo-second-order closely matched with experimental results (36.608 mg/g).As a result, it appears that the model of pseudo-second-order well describes the system under consideration.The presumption behind the pseudo-second-order model is that the rate-limiting sept involves chemical sorption of adsorbate on the surface adsorbent through shearing or exchange of electrons (Chowdhury et al., 2011).Table 2 contains a list of calculated kinetic parameters of pseudo-first-order and pseudo-secondorder models.

Thermodynamic Parameters
Thermodynamic parameters determine whether the process of adsorption is spontaneous or not.Entropy (ΔS 0 ), Gibbs free energy change (ΔG 0 ), and enthalpy (ΔH 0 ) were therefore measured to assess the thermodynamic feasibility of the adsorption process.The following equations were used to calculate the thermodynamic parameters: where T represents the adsorption temin Kelvin and R represents the ideal gas constant (8.314J mol −1 K −1 ).The intercept  temperatures, which confirmed the spontaneous adsorption process.A positive value for ΔH 0 confirmed the endothermic adsorption process.A positive ΔS 0 value signposted the adsorbent's affinity for MBD 11 removal and showed that randomness increases at interface of the solid solution during adsorption.

Conclusions
FTIR analysis shows successful functionalization of MWCNTs.From contact time analysis it is concluded that the adsorbate-adsorbent interaction increases at long contact time.With extended contact time, further adsorption occurs, resulting in a higher percent removal of MBD 11.The effect of pH change on the % removal of MBD 11 by f-MWCNTs showed the maximum adsorption occurs in an acidic medium.The adsorbent dosage variation on dye removal revealed that increasing the adsorbent dosage increases the % dye removal.The increased surface area exposed to dye solutions may allow for the adsorption of more dye.The adsorption of MBD 11 onto f-MWCNTs was also accurately described by Langmuir adsorption isotherm and pseudo-second-order kinetic model.

Fig. 5 a
Fig. 5 a Effect of pH on MBD 11 adsorption on the % removal and the amount dye adsorbed on f-MWCNTs (f-MWCNT dose = 0.05 g, initial MBD 11 concentration = 50 ppm, and contact

Fig. 7 Fig. 8
Fig. 7 Effect of initial dye concentration on the % removal and the amount dye adsorbed on f-MWCNTs (pH 2, f-MWCNT dose = 0.025 g, and contact time = 5 min)

Fig. 9
Fig. 9 Freundlich and Langmuir adsorption isotherm for dye adsorption from 5 to 80 °C (283.15-553.15K).The figure demonstrates that the removal of MBD 11 by f-MWCNT adsorbents was increased by raising the temperature.

Fig. 10
Fig. 10 Graph of a pseudo-first-order and b pseudo-second-order kinetics for MBD 11 adsorption

Table 3
Thermodynamic parameters for MBD 11 adsorption on f-MWCNTs Vol:. (1234567890) FundingThe authors are thankful to Researchers Supporting Project Number (RSPD2023R764) at King Saud University, Riyadh, Saudi Arabia, for financial support.Data AvailabilityThe datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material.If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.