Removal of rhodamine b and alizarin red from aqueous solutions by oxidized carbon nanotubes: kinetics and isotherm Study

Alizarin Red and Rhodamine B are widely used dyes in the textile, paper, and plastic industries. However, the disposal or release of these dyes into the environment can negatively impact on both the environment and human health. In the present study, oxidized carbon nanotubes (OCNT) were used to eliminate Alizarin Red and Rhodamine B from contaminated water in batch adsorption experiments. The adsorption outcomes suggested that OCNT has the potential to be an effective material for the removal of these dyes from contaminated water. The kinetics modeling revealed that the adsorption process of both dyes onto OCNT follows the pseudo-second-order model, indicating a chemisorption process. Moreover, the OCNT has indicated fast kinetics in which the equilibrium was achieved in 4 h. The isotherm study demonstrated that the Freundlich isotherm model best fits the experimental data, suggesting that dyes’ adsorption onto OCNT is a monolayer adsorption process. The maximum adsorption capacities were 124.7 and 614.7 mg/g for Rhodamine B and Alizarin Red, respectively. The adsorption of these dyes was found to be more efficient at pH values of 6-8. This study suggests that OCNT can be an effective adsorbent for removing Alizarin Red and Rhodamine B from contaminated water.


Introduction
Industrial operations are responsible for a huge proportion of water contaminants that damage the ecosystem, and their ever-increasing nature has contributed to the current situation of the worldwide environment [1].Industrial processes such as pulp, petrochemicals, refining, petroleum production, and paper production, battery manufacturing, pigment and paint industries, pesticide, tanneries, herbicide, fertilizer industries, as well as metallurgical and mining plants, require a large amount of chemical input and inadequate resource use of the input components results in a large number of wastes that enter the environment.Various pollutants were found in the industrial wastewaters, including dyes, phosphates heavy metal ions, sulfates, hydrocarbon and organic compounds, and other organic and inorganic contaminants [2][3][4].Water pollution caused by dyes is a significant environmental concern worldwide.Dyes are broadly utilized in industries like leather, textile, printing, and paper, leading to the discharge of huge quantities of dye-containing wastewater into water bodies.The presence of dyes in water can adversely affect aquatic ecosystems, human health, and water quality [5,6].Rhodamine B (RHB) and Alizarin Red (AZR) are two specific dyes that can contribute to water pollution.They can adversely affect fish, algae, and other aquatic organisms.The dyes can disrupt their physiological processes, impair reproduction, and ultimately lead to ecological imbalances.
Different technologies have been used to remove RHB and AZR from drinking water, including ion exchange, precipitation/coprecipitation, adsorption, and membrane filtration [7,8].Adsorption is one of the most used separation methods due to its low cost, convenience of use, and can remove low concentrations of pollutants.Carbon-based materials have emerged as promising candidates because of their superior textural characteristics, tunable structure, and excellent adsorption capabilities.Carbon-based materials, such as carbon nanotubes graphene oxide, and, activated carbon, have been extensively investigated for removing organic dyes from water sources [9,10].However, despite the extensive research conducted on carbon-based materials, a research gap still exists regarding using oxidized carbon nanotubes to remove rhodamine B and alizarin red.Oxidized carbon nanotubes possess unique surface chemistry and functional groups that can enhance the adsorption capacity and selectivity towards target pollutants [11].Previous studies have mainly focused on applying pristine carbon nanotubes or other carbon-based materials, neglecting the potential of oxidized carbon nanotubes for dye removal.Therefore, the aim of our study is to explore the effectiveness of oxidized carbon nanotubes as an adsorbent for the elimination of rhodamine B and alizarin red from water using batch mode experiments.By exploring the adsorption capacity, thermodynamics, and kinetics of these oxidized carbon nanotubes,

Materials
Oxidized carbon nanotubes (OCNT) type (Multi-Walled Carbon Nanotubes COOH) purity 95%, 1.5% ash, and 3.86% functional content were purchased from Cheap Tubes Company (Grafton, USA).Rhodamine B and Alizarine red were bought from Sinopharm Chemical Reagent Co. (Shanghai, China).Deionized (DI) water was utilized to prepare all solutions.

Characterization
The textural characteristics were studied from nitrogen adsorption isotherm at -196 ℃ utilizing NOVATECH LX2 analyzer, Anton Paar.Prior the N 2 testing, the sample was degassed at a temperature 300 ℃ for 6 hours.FE-SEM (Field emission scanning electron microscopy, Thermo scientific Apreo C) was utilized to explore the morphology and microstructure of the developed adsorbent.The FE-SEM was operated with an EDS (energy dispersion spectrometer).The accelerating voltage was 15 kV.The crystallinity of the hybrids was determined by XRD D8 (X-ray diffractometer, Advance, Bruker,) with a wavelength λ = 1.54056Å at generator current of 40 mA and generator voltage of 40 kV.Infrared spectra were measured between 4000 and 400 cm -1 using a Jasco FT-IR (Model: 6300) instrument.Thermogravimetric analysis (TGA) of the OCNT was achieved by Netzsch analyzer (Jupiter, STA 449 F5).

Adsorption experiments
The adsorption tests were conducted in batch mode where 50 mL of dye solution was mixed with certain amount of OCNT particles in 100 mL conical flasks using a Kuhner Lab-Shaker LS-X (Model MAZ10661LAB).To analyze the samples, they were collected and passed through a 0.45 μm PTFE membrane filter.The concentration of dyes was determined using a UV-VIS Spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) at specific wavelengths (554 nm for RHB and 421 nm for AZR).The adsorption capacity (Q), measured in mg of adsorbate per g of OCNT, and the removal efficiency (RE) expressed as a (%), were calculated using the following formulas: Where Ci and Ce are the initial and exit dye loadings (mg/L), V is the dye solution volume (L), and W is the OCNT mass (g).The adsorption trials were repeated two times, and mean numbers were presented.The effect of OCNT amount was examined using OCNT concentrations between 0.2 and 2 g/L with 200 rpm agitation.Unless mentioned otherwise, the adsorption experiments were shaken for 24 h at pH 5.6.

Adsorption modeling
To determine the isotherms of adsorption, 125 mg of OCNT were mixed with 50 mL of dye solution with loadings ranging from 50 to 200 mg/L and 50-1000 mg/L for RHB and AZR, respectively at 25 ℃, stirring speed of 200 RPM and pH 5.6 for a day.The equilibrium data were scrutinized utilizing Langmuir, Freundlich, Temkin, Jovanovic, and Redlich-Peterson (RP) isotherm models.The expressions and parameters of the applied models were presented in table 1. Table 1.Isotherm models used to explain the surfactant equilibrium data.

OCNT characterization
Figure 1a illustrated the BET nitrogen adsorption-desorption isotherm analysis providing valuable information about the textural characteristics of OCNT.In this study, the BET analysis revealed a surface area (SA) of 302.779 m²/g, indicating a significant amount of available surface for adsorption.The high SA of OCNT was attributed to their unique tubular structure, which provides a large external SA.The pore volume of 1.74 cc/g indicated the presence of substantial pore space within the OCNT structure.The presence of pores allows for the accommodation of nitrogen molecules during the adsorption process, enhancing the overall adsorption uptake of OCNT.The calculated pore radius of 14.5053 nm suggested the presence of mesopores within the OCNT structure.Mesopores are pores with diameters typically ranging from 2 nm to 50 nm.The presence of mesopores in OCNT can offer a suitable environment for the uptake of larger molecules or compounds, allowing for the efficient removal of dyes with larger molecular sizes.Figure 1b illustrated the XRD analysis of the OCNT.The (002) plane corresponds to the basal plane of graphite, which was the plane where carbon atoms were arranged in a hexagonal lattice.The (100) plane corresponds to a plane perpendicular to the OCNT axis and passes through the center of the tube.In OCNT, the (002) plane can provide information about the degree of graphitization and the quality of the graphene-like layers in the OCNT.The (002) peak in the XRD pattern of OCNT can shift to higher or lower angles depending on the functionalization process, affecting the interlayer spacing between the graphene-like layers.On the other hand, the (100) plane can provide information about the orientation and alignment of the OCNT in a sample.In well-aligned OCNT samples, the (100) peak will be intense, indicating a high degree of alignment.However, in samples where the CNTs were randomly oriented, the (100) peak will be weaker and broader.
According to figure 2a, the FT-IR analysis of OCNT revealed several functional groups.The absorption peaks at 3300-3500 cm -1 were attributed to the O-H stretching vibrations, demonstrating the existence of hydroxyl groups (-OH) on the surface of the OCNT [12].The peaks at 2800-3000 cm -1 corresponded to C-H stretching vibrations, indicating the presence of sp3 hybridized carbon atoms bonded to hydrogen atoms.The peaks observed at 1500-1700 cm -1 were given to C=O stretching vibrations, which suggest the presence of carbonyl groups (-C=O) on the surface of OCNT.The peaks detected at 1300-1500 cm -1 corresponded to C-C stretching vibrations, indicating the existence of sp2 hybridized carboncarbon bonds.Finally, the peaks observed at 500-800 cm -1 corresponded to C-H bending vibrations [13].
The TGA utilized to explore the thermal stability and decomposition behavior of materials by measuring their weight changes as a function of temperature.The results of the TGA analysis of OCNT shown in figure 2b can provide important information about their thermal stability and composition.According to the results, when the temperature reached 485°C, the weight loss of the OCNT declined to 90%, suggesting that there was a significant mass loss below this temperature, likely due to the removal of any adsorbed or chemically bound water, as well as the degradation of any organic impurities on the OCNT's surface.When the temperature reached 600°C, the OCNT lost 60% of their weight, indicating that there was further decomposition of the OCNT, likely due to the breaking of carbon-carbon bonds in the OCNT, which leads to the release of gaseous products such as carbon monoxide and carbon dioxide.From 600°C until 800°C, the OCNT lost an additional 73% of their weight, leaving a residual weight of 27% of the total sample weight, indicating that the remaining mass was primarily composed of carbon, which could indicate the presence of residual amorphous carbon or the formation of graphitelike structures.Overall, the TGA results suggest that the OCNT has relatively good thermal stability up to 485°C but starts to decompose significantly above that temperature.Figure 3 displayed the FE-SEM images of the OCNT that provide information about their morphology, structure, surface features, and alignment.OCNT were cylindrical structures made of carbon atoms arranged in a hexagonal lattice.In an FE-SEM image, OCNT appear as thin, elongated structures with a tubular shape.The diameter of OCNT can range from a few nanometers to tens of nanometers, depending on the specific synthesis method and conditions.The surface of OCNT can also exhibit various morphologies depending on the synthesis method and any post-treatment processes.For example, OCNT can be functionalized with different chemical groups to modify their surface properties, resulting in different surface morphologies.In the FE-SEM image, one may observe the presence of different surface features such as bumps, wrinkles, and defects indicative of the morphology of the OCNT.

Effect of adsorption parameters
Figure 4a illustrated the influence of OCNT dose on the elimination of RHB and AZR.The outcomes implied that the removal effectiveness of both RHB and AZR raised as the OCNT dose increased.This could be attributed to the fact that increasing the adsorbent dose improved the SA available for adsorption, which in turn lead to an improvement in the quantity of available adsorption sites.At lower doses (0.2-0.5 g/L), the adsorption locations on the OCNT surface may not be sufficient to eliminate all dye molecules in the solution, resulting in lower removal efficiencies.However, as the dose increases to 1 g/L and beyond, the adsorption sites become more plentiful and can effectively remove a significant percentage of the dye molecules from the solution.It was worth noting that at doses of 1.5 g/L and 2 g/L, the removal effectiveness of both RHB and AZR remains nearly constant at 99.8% and 99%, respectively, indicating further increases in adsorbent dose may not significantly enhance the removal efficiency.Therefore, the optimal adsorbent dose for removing RHB and AZR using OCNT can be 1-2 g/L.The influence of time of contact on removing RHB and AZR using oxidized carbon nanotubes (OCNT) was shown in Figure 4b.The results showed that removing both dyes increases with increasing contact time.After 1 minute of contact time, the removal of RHB and AZR was 12% and 33%, respectively, which indicates a low removal efficiency.However, after 20 minutes of contact time, the removal of RHB and AZR increased to 37% and 49%, respectively.This can be attributed to the fact that OCNT requires sufficient contact time to absorb the dyes effectively.The removal efficiency of both dyes further increased to 80.8% and 89.8%, respectively, after 240 minutes of contact time, which indicates that OCNT was capable of effectively removing dyes from water.Furthermore, the findings showed that equilibrium was achieved after 600 minutes of contact time, with a removal efficiency of 96.7% and 99.8% for RHB and AZR, respectively.This suggests that a combination of surface adsorption and pore diffusion mechanisms may govern the adsorption process.At the initial stages of the adsorption process, surface adsorption may be dominant, while at later stages, pore diffusion may become the controlling factor.

Adsorption isotherm
Figure 5 presents the dye adsorption isotherm on OCNT, and we utilized the Langmuir, Freundlich, Temkin, Jovanovic, and RP adsorption models to match the data and gain a deeper understanding of the adsorption mechanism.Upon analyzing the results, we observed that the investigational data were most accurately described by the Freundlich models, with R-squared (R 2 ) values of 95% and 97% for RHB and AZR, respectively.These high R 2 values indicate a good fit between the Freundlich models and the obtained data for both dyes.On the other hand, the Langmuir and RP models also exhibited reasonable agreement with the experimental data, suggesting different adsorption mechanisms for removing dyes by OCNT.These outcomes offer worthy insights into the adsorption process of dyes on OCNT.The results suggest the creation of a monolayer of RHB and AZR on the surface of OCNT, potentially indicating some level of heterogeneity in the adsorption sites of OCNT.

Adsorption kinetics
Figure 6 presents the dye adsorption kinetics on OCNT, and we applied the pseudo-first order (PFO) and pseudo-second order (PSO) adsorption models to examine the experimental data and gain a more information about of the mechanism of adsorption.Upon analyzing the results, we found that the obtained data were most accurately described by the PSO model.The R 2 values obtained were 94% for RHB indicating reasonably good fits between the PSO model and the obtained data for RHB dye.The PSO model implies that the rate of adsorption was dependent on the loading of the dye and the available adsorption sites on OCNT.Moreover, these findings suggest that a chemical adsorption process might be involved in the removal of RHB by OCNT.However, for AZR, the R 2 for both kinetic models were between 0.647 and 0.766, implying that that neither of these models adequately describes the adsorption behavior of the AZR dye on the OCNT material.Further investigation and analysis may be required to understand the kinetics of the AZR dye adsorption and identify a suitable kinetic model that accurately describes the experimental data.Moreover, it indicates that the removal process of the AZR dye involves different adsorption mechanisms.This suggests that the adsorption behavior of the AZR dye on the OCNT material was more complex and cannot be solely described by PFO and PSO kinetic models.Table 2 indicates the adsorption kinetics factors of the RHB and AZR removal by OCNT.The findings indicate that the rate of adsorption was influenced by the concentration of the dyes and the accessibility of adsorption locations on OCNT surfaces.

Conclusion
The present research investigated the properties and adsorption capabilities of oxidized carbon nanotubes (OCNT) for the elimination of RHB and AZR dyes from water.The characterization of OCNT using various techniques revealed important insights into its structure and thermal stability.XRD analysis showed the graphitization and alignment of OCNT, while FT-IR analysis identified the presence of functional groups on OCNT surface.TGA analysis indicated that OCNT exhibited good thermal stability up to 485°C.FE-SEM images provided information about the morphology and surface features of OCNT, highlighting their tubular structure and various surface morphologies.The adsorption parameters were investigated, and it was found that the elimination efficiency of RHB and AZR enhanced with increasing OCNT dose.The optimal adsorbent dose for both dyes was determined to be 1-2 g/L.The removal of dyes also boosted with longer contact time, indicating the importance of sufficient contact time for effective adsorption.The equilibrium was attained after 600 minutes.Furthermore, the adsorption capacity of OCNT increased with improving original dye concentration, suggesting a higher driving force for the remediation process.However, a plateau in the adsorption uptake was attained as the available adsorption locations became saturated.Adsorption isotherm analysis demonstrated that the Freundlich model was the optimum fit to the obtained data, indicating a heterogeneous distribution of adsorption sites on OCNT surfaces.Regarding adsorption kinetics, the PSO model described the experimental data more accurately than the PFO model, suggesting that the adsorption rate was relying on the concentration of the dye and the availability of adsorption sites on OCNT surfaces.Overall, this research demonstrated the potential of OCNT as an effective material for the elimination of RHB and AZR dyes from water.The findings contribute to the understanding of OCNT properties and offer valuable insights for the design of efficient adsorption processes in water treatment applications.

Figure 4 .
Figure 4. Effect of adsorption parameters for RHB and AZR removal by OCNT; a) The impact of OCNT dose; b) Effect of contact time.