Adsorption of hazardous dyes on functionalized multiwalled carbon nanotubes in single and binary systems: Experimental study and physicochemical interpretation of the adsorption mechanism

Journal Pre-proofs Adsorption of hazardous dyes on functionalized multiwalled carbon nanotubes in single and binary systems: Experimental study and physicochemical interpretation of the adsorption mechanism Zichao Li, Lotfi Sellaoui, Dison Franco, Matias Schadeck Netto, Jordana Georgin, Guilherme Luiz Dotto, Abdullah Bajahzar, Hafedh Belmabrouk, Adrian Bonilla-Petriciolet, Qun Li PII: DOI: Reference:

S1385-8947(20)30458-7 https://doi.org/10.1016/j.cej.2020.124467 CEJ 124467

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Chemical Engineering Journal

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21 January 2020 13 February 2020 14 February 2020

Please cite this article as: Z. Li, L. Sellaoui, D. Franco, M.S. Netto, J. Georgin, G.L. Dotto, A. Bajahzar, H. Belmabrouk, A. Bonilla-Petriciolet, Q. Li, Adsorption of hazardous dyes on functionalized multiwalled carbon nanotubes in single and binary systems: Experimental study and physicochemical interpretation of the adsorption mechanism, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124467

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Adsorption of hazardous dyes on functionalized multiwalled carbon nanotubes in single and binary systems: Experimental study and physicochemical interpretation of the adsorption mechanism Zichao Li a, Lotfi Sellaouib*, Dison Francoc, Matias Schadeck Nettoc, Jordana Georginc,Guilherme Luiz Dottoc, Abdullah Bajahzard, Hafedh Belmabrouke*, Adrian Bonilla-Petricioletf, Qun Li a*

a College

of Life Sciences, College of Chemistry and Chemical Engineering, State Key

Laboratory of Bio-Fibers and Eco-Textiles, Institute of Advanced Cross-Field Science, Qingdao University, Qingdao, 266071, China b

Laboratory of Quantum and Statistical Physics, LR18ES18, Monastir University, Faculty of Sciences of Monastir, Tunisia cChemical

Engineering Department, Federal University of Santa Maria–UFSM, 1000, Roraima Avenue, 97105-900 Santa Maria, RS, Brazil

dDepartment

of Computer Science and Information, College of Science, Majmaah University, Zulfi 11932, Saudi Arabia

eDepartment

of Physics, College of Science at Al Zulfi, Majmaah University, Saudi Arabia

fInstitutoTecnológico

de Aguascalientes, Aguascalientes, 20256, México

Correspondingauthors: *Lotfi Sellaoui ([email protected]) *Hafedh Belmabrouk ([email protected]) *Qun Li ([email protected])

Abstract The crystal violet (CV) and rhodamine B (RhB) dyes were selected in this paper to study their adsorption on functionalized multiwalled carbon nanotubes (MCN). Adsorbent morphology and its corresponding physicochemical properties were characterized by applying various experimental techniques namely SEM, XRD and FTIR. Results of phenomenological modeling and the experimental adsorption data showed that the adsorption capacities of both dyes in single solutions were similar. However, synergistic and antagonistic adsorption effects were observed in binary solutions. Results indicated that the CV dye molecule was practically adsorbed in a double amount in comparison to the RhB dye molecule at most of the adsorption temperatures. Saturation adsorption capacities ranged from 0.57 to 0.86 mmol/g and from 0.75 to 0.88 mmol/g for CV and RB dyes at 298 328 K, respectively. It has been concluded that the size of dye molecule played a negligible role during the adsorption. CV and RhB dye molecules can interact with the same functional groups of the adsorbent. A competitive statistical physics model was reliable to predict the multicomponent adsorption behavior of these dyes. The adsorption orientation of both dyes was discussed using the results of the physical modeling and a possible theoretical mechanism was proposed based on the estimated adsorption energies. Finally, the effect of temperature on the adsorbent performance for both single and binary dye solutions was analyzed providing a clear understanding of the observed trends in the experimental dye adsorption capacities. Keywords: Crystal violet; rhodamine B; multiwalled carbon nanotubes; binary adsorption.

1. Introduction With the increase in world population, and consequently the growth of products consumption, the industrial activities have shown a significant intensification during last years. Therefore, the volume of generated wastewaters has also increased. These streams could contain a significant load of pollutants such as dyes. The improper handling of these wastewaters can compromise the quality of the aqueous systems that are used as receptors for its discharge causing water pollution and environmental risks for the entire ecosystems that are exposed [1]. Dyes such as Rhodamine B (RhB) and Crystal Violet (CV) are largely used by industries especially in dyeing wool and printing printer cartridges. Both dyes are highly soluble in water (CS,RhB =15 g L-1, CS,CV = 16 g L-1) and they are visible even at very low concentrations[2–4]. Both dyes are mutagenic and carcinogenic for the human body[5,6].These dyes can be absorbed by the skin and, consequently, they can lead to health complications like respiratory or renal failure and permanent blindness[5,6]. Then, the removal of these dyes is paramount to reduce the potential of health problems to the human being, to protect the environment and to satisfy the corresponding legislations related to the presence of these pollutants. Considering these aspects, different methods for the removal/reduction of dyes in aqueous systems have been studied. Dye removal techniques include ultrafiltration, ozonation, oxidation, flocculation and adsorption [7]. Adsorption is an effective technique for dye removal due to the low operational cost, high removal efficiency especially if effective adsorbents are utilized. In particular, nano-adsorbents have been proposed as effective separation medium to reduce the content of dye in aqueous solutions[8,9]. The carbon nanotubes are solid materials that are characterized by a high surface area and porosity. These characteristics are favorable for the separation of diverse compounds in liquid phase[10,11]. The carbon nanotubes have been already used for the

removal of various pollutants such as sulfur dioxide [12], lead [13], methylene blue[14], zinc [15],

ciprofloxacin

[16],lithium

[17],

phenol

[18],crystal

violet

[18],

dopamine

derivatives[19], ibuprofen [20], uranium [21], methyl orange [22], direct blue 53 [23], direct red 23 [24] and rhodamine B [25]. There are several methods for synthesis of carbon nanotubes such as laser ablation, plasmaenhanced chemical vapor deposition, arc discharge, among others[26]. These methods could require either high temperature or high (sometimes very high) vacuum environment that can cause a difficult and expensive scale-up. On the other hand, microwave-assisted approaches have also been developed to prepare this type of adsorbents using different materials that have a high carbon amount such as wood [27–29]. In this sense, the cedar (Cederella fissilis) is a commonly extract and used wood in Brazil. During its processing, barks are generated in large amounts, thus a proper destination for this residue is needed [30].In this work, a functionalized multiwall carbon nanotube was used for the adsorption of dyes RhB and CV in single and binary solutions at different operating temperatures(i.e.,298 - 328 K). Experimental adsorption isotherms were quantified and physical models were applied to analyze and to understand the adsorption data and its corresponding adsorption mechanism. Overall, the experimental and theoretical results led to characterize the adsorption mechanism attributing new insights at molecular level for the removal of these dyes on tested adsorbent. 2. Materials and Methods 2.1 Preparation of functionalized multiwall carbon nanotube and its physicochemical characterization Cedrella fissilis bark was obtained from a local wood industry at Rio Grande do Sul State, Brazil. This material was milled using a knife mill and sieved obtaining a particle diameter of 0.20 mm. The preparation of the multiwall carbon nanotubes was done using this feedstock and applying an adaptation of the methodology described by Nie et al. [28]. It was consisted

of mixing10 g of Cedrella fissilis bark (10 g) with 5g of a catalyst (ferrocene, Fe(C5H5)2)in aTeflon vessel. This mixture was irradiated using a domestic microwave for 25 s at power of1300 W. The functionalization of the multiwall carbon nanotubes with carboxyl groups was done as follows: 1 g of multiwall carbon nanotube was added on a mixture of sulfuric acid and nitric acid (3:1 that implied 60 and 20 mL of these acids, respectively, with a concentration of 0.5 mol/L for each acid) under reflux at 80 ºC for 60 min. Then, the powder was filtered and washed several times until reaching the neutral pH, followed by a drying at 60 ºC for 24 h [22]. This adsorbent was stored for further usage and labelled as MWCNTCOOH. All reactants were of analytical grade and supplied from Sigma-Aldrich. In relation to the characterization of the adsorbent, the following techniques were employed to obtain the information of its surface chemistry: Fourier transformed infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). FTIR analysis of the adsorbent samples before and after its chemical functionalization was performed using a Shimadzu (model Prestige–21, Japan) where the spectra were recorded in the wavenumber range from 4500 to 450 cm-1.A diffractometer Rigaku (model Miniflex 300) was used to perform the XRD analysis of the adsorbent samples. SEM images of the adsorbent morphology were recorded using a VEGA3 (Czech Republic) that operated at 10 kV. Specific surface area (As) was estimated via N2 adsorption/desorption isotherms (using an equipment Micrometrics, ASAP 2020, USA) with the application of the BET method. Estimated surface area was195 ± 1.6 m2/ g with a pore volume of 0.546 cm3/ g and a mean pore diameter of 20.23 nm. 2.2 Dye adsorption assays Table 1 reports the general characteristics of the dyes used as adsorbates. A stock solution for RhB and CV were prepared using deionized water with a concentration of 1 g/L. For the adsorption assays, it was used an adsorbent dosage (i.e., ratio of mass adsorbent to adsorbate solution volume) of 0.5 g/Land the natural pH of the single and mixture solutions (i.e., 6.5 - 7)

was employed. Adsorption isotherms were experimentally quantified at 298, 308, 318 and 328K using dyes solutions with initial concentrations of 0.2088, 0.4175, 0.6263, 0.8350 and 1.0438 mmol/ L for RhB and CV. These experiments were performed under constant stirring using a thermostatic agitator (Marconi, model MA 093, Brazil) until reaching the equilibrium (i.e., contact time of 4 h). Every sample obtained from adsorption tests was centrifuged using a Centribio equipment (model 80-2B, Brazil) with a relative centrifugal force of1800 g for 20 min to obtain the solution sample for dye quantification. Dye concentration was measured with a spectrophotometer Shimadzu (model UV mini 1240, Japan). The adsorption capacities (qe, mmol/g) for the single and binary systems of RhB and CV were calculated by the following equation: V m

qe = (C0 - Ce)

(1)

Where C0 is the dye initial concentration (mmol/L), Ce is the dye concentration at the equilibrium (mmol/L), V is the solution volume (L) and m is the adsorbent mass (g). 3. General description of the experimental dye adsorption data and its modeling analysis Single and binary experimental adsorption data are reported in Figure 1. This figure showed that the same trend for the CV and RhB adsorption data was observed in single and binary systems in relationship with the effect of temperature. In particular, the adsorption temperature had a relevant role to remove both dyes from aqueous solution. Clearly, an increment of temperature led to an improvement of the adsorption capacities especially for dye CV. For the mono-adsorbate systems, it was noted that there was no a significant difference in the adsorption capacities besides the two dyes showed different chemical structures, see Table 1. In fact, the RhB adsorption capacity was relatively higher than that

obtained for dye CV. Regarding the binary adsorption data, the results indicated that the adsorption capacities for both dyes showed different behavior due to the presence of the coadsorbate (i.e., the presence of the other dye in the aqueous solution), see Figure 1. It was noted that the adsorption of the CV molecule was practically the double than the adsorption of dye RhB at most of operating temperatures in binary solutions. In fact, the adsorption of CV was enhanced by the presence of RhB in the solution (i.e., synergistic adsorption), while the removal of RhB was reduced in the binary solution due to the presence of CV molecules (i.e., antagonistic adsorption).These experimental findings indicated that the single and binary adsorption systems were controlled by different factors that could be associated to dye molecular parameters and operating conditions of the adsorption process. For example, the difference in adsorption capacity in binary system could be due to the adsorbate exclusion that generated an inhibition effect between both adsorbates. It could be also possible that RhB and CV dye molecules were adsorbed on two different sites of MCN generating different interactions(for example, electrostatic interactions, hydrogen bonds, Van der Walls forces or π–π interactions between the aromatic rings) [43] and, consequently, different adsorption capacities for both adsorbates. A physical-based modeling is a reliable tool to examine and explain the adsorption mechanism associated to the removal of these adsorbates at all tested temperatures. In this direction, four physical models were selected (two for single and two for binary systems) to obtain theoretical insights on the dye adsorption mechanisms. These models considered different scenarios to explain the adsorption mechanism. For instance, it was assumed that the adsorption of these dyes can be performed via the formation of one or more adsorbate layers, which is a common phenomenon in dye removal due to the possible presence of dye molecule aggregation. For the binary system, the adsorption of CV and RhB molecules can be carried out on the same MCN adsorption site or two different adsorption sites can be involved in the multicomponent removal process. Therefore, the statistical

physics models used in this paper have considered some of these scenarios for the understanding of the dye adsorption mechanisms. 3.1 Models for single dye adsorption A model that assumed a monolayer process was considered to study the adsorption of dyes CV and RhB from aqueous solution. This model considers that the formed layer of the adsorbate was associated to an adsorption energy derived from the interactions between dye molecules and functional groups of the MCN adsorbent. This single adsorption model was described by the following expression [38]:

Qe  f (Ce ) 

nDDm nD

C  1  1/ 2   Ce 



Qsat0 nD

C  1  1/ 2   Ce 

(2)

With the adsorption capacity at saturation given by Qsat0=nD.Dm. This adsorption model contains three parameters where nD represents the number of adsorbed dye molecules (i.e., CV or RhB) per adsorption site (NSDPAS), Dm is the density of the adsorption receptor sites andC1/2 is the equilibrium concentration at half-saturation of the monolayer formed during dye adsorption. Note that these parameters have a theoretical meaning to explain the adsorption mechanism of dyes CV and RhB. On the other hand, it is possible that two layers of adsorbed dye molecules can be formed during the adsorption process. Therefore, a double layer model was also selected to analyze the dyes adsorption. The second model indicated that the dyes adsorption on MCN was the result of the formation of two adsorbate layers. These adsorbed dye layers were associated to two adsorption energies. The first adsorption energy corresponded to the interactions between the CV and RhB dyes and functional groups of MCN, while the second energy was derived from the interactions between dye molecules. The mathematical expression of the second

model is given by [38]: nD

2nD

 Ce  C     2 e  C C Qe  f (Ce )  nD Dm  1  nD  2  2nD C  C  1  e    e   C1   C2 

(3)

Where Qsat0=2.nD.Dm is the adsorption capacity at saturation. Note that two concentrations at half-saturation were included in this model C1 and C2, which were related to both formed layers of tested dyes. 3.2 Models for binary dye adsorption Two models were chosen for modeling the binary adsorption of CV and RhB dyes. The first binary adsorption model assumed that the adsorption of dyes CV and RhB on MCN could be performed on two types of adsorption sites, which implied two different adsorption energies. In particular, it was assumed that the first adsorption site can attract only the CV dye molecule and the second adsorption site can interact with the other dye. The expression of this binary adsorption model is given by [39]:

Qe1  f (Ce) 

nD1Dm1 nD



Qsat1 nD

C1  1 C1  1 1  1  Ce  Ce 

Qe 2  f (Ce ) 

nD2 Dm 2 C  1   2   Ce 

n D2



(4)

Qsat 2 C  1   2   Ce 

n D2

(5)

where the adsorption capacity at saturation for CV and RhB dyes can be determined via the simple relationships: For CV dye: Qsat1=nD1.Dm1

For RB dye: Qsat2=nD2.Dm2 The second model assumed that both CV and RhB can be adsorbed on the same type of adsorption site of MCN surface where this adsorption site can attract both molecules. Note that the amount of adsorbed CV and RhB dye molecules per the same receptor site were represented by nD1 and nD2, respectively. In this case, the competitive adsorption mechanism can be characterized by two adsorption energies describing the interactions between both dyes and the functional groups of the adsorbent surface. The expressions of this second binary adsorption models are described as follows [40]:

Qe1 

Qe 2 

C  nD1 Dm  e1   C01  C  1   e1   C01 

n D1

C    e 2   C02 

C  nD 2 Dm  e 2   C02  C  1   e1   C01 

nD 1

nD1

nD 2

(6)

n D2

C    e 2   C02 

nD 2

(7)

where the adsorption capacity of CV and RhB at saturation can be estimated by: For CV dye: Qsat1=nD1.Dm For RB dye: Qsat2=nD2.Dm where the parameters nD1 and nD2 are the attracted numbers of CV and RhB molecules per the same adsorption site, respectively; while C01 and C02 are the concentrations at half-saturation of the layer formed by CV and RhB dye molecules. To select the best models, nonlinear regressions of the experimental data of single and binary dye adsorption were performed. Data fitting showed that both single models correlated

satisfactorily the experimental data with determination coefficients R2 close to the unity (i.e., 0.989 and 0.999). In this situation, the adjusted model parameters were analyzed with respect to the adsorption temperature and it was noted that the single adsorption mechanism can be explained via the double layer model since it contained more parameters that can provide additional information of the dyes adsorption mechanism. In addition, their trends as function of temperature were clear thus leading to a reasonable interpretation of the adsorption phenomenon. For the models of binary systems, R2 values were close to the unity ( 0.998). The model select to explain the dye binary adsorption was the corresponding to the scenario that assumed that CV and RhB molecules can be adsorbed on the same type of MCN active site. It is expected that the adsorption of CV and RhB are due to a binding with hydrogen, which can occur on the COOH and C-H groups of the material [31-33], which was consistent with the results of adsorbent characterization. The adsorption model parameters are listed in Table 2 and examples of data fitting are depicted in Appendix. 4. Results and Discussion 4.1 MWCNT-COOH characteristics FTIR spectra of MWCNT before and after its functionalization are shown in Figure 2. The absorption band around 1441 cm-1was related to the hydroxyl and carboxyl groups, while the elongation of C=C was associated to the bands at 1634 and 1554 cm-1[34]. The band located at 1383cm-1appeared after the functionalization and was attributed to H-C=O bond, which was related to the effect of sulfuric and nitric acids on the adsorbent chemistry surface[35]. This band is commonly found at multiwall carbon nanotubes functionalized with groups COOH [8,9,11].XRD patterns for samples of MWCNT and MWCNT-COOH are presented in Figure 3. The first aspect to be observed in XRD results is the peak at 2θ = 25º, which corresponds to multiwall carbon nanotubes that is also equivalent to the reflection of graphite planes (002) [36]. In the diffractogram of the sample MWCNT, the peaks relative to Fe3O4

and Fe3C(2θ = 42, 44°), intrinsically present in the carbon nanotube cavities, resulted from the preparation method used in this study[36].After the acid treatment, it was observed that the intensity of the peaks relative to these iron species decreased in the spectrum of the sample MWCNT-COOH due to their removal by acid solubilization [36,37].SEM images of MWCNT-COOH at different magnifications are given in Figure 4. It was possible to observe that MWCNT-COOH was constituted of different size particles, see Figures4a and 4b. Furthermore, the surface of these particles had a bundle of nanotubes (Figures 3c and 3d), which were distributed in a non-organized fashion. 4.2 Physical interpretations of dyes adsorption using multiwall carbon nanotube 4.2.1 Dye adsorption capacity in single and binary adsorption systems The determination of the adsorption capacity at saturation can be utilized to characterize the multicomponent adsorption effects (antagonistic and synergistic) that were observed in the experimental data of dye binary systems. The estimated values of these adsorption capacities are listed in Table 2. Overall, these adsorption capacities ranged from 0.59 to 0.88 mmol/g for CV and from 0.77 to 0.9 mmol/g for RhB in single systems at 298 - 328 K. Note that the saturation adsorption capacities of both dyes were similar at almost all adsorption temperatures although the CV and RhB dye molecules have two different sizes. At 298 K, the adsorbent showed a difference of around 20% between the saturation adsorption capacities of both dyes. The chemical structures of both dyes contained different functional groups and it could be expected that the adsorbent showed significant differences in the uptakes of these molecules. It could be also possible that the small size of the CV dye molecule could contribute to its adsorption because an adsorbate molecule with small size can move freely and rapidly on the adsorbent surface and then there are more possibilities to be attracted by the functional groups involved in the adsorption. The same magnitude order in the saturation

adsorption capacities suggested that the same type of adsorption interactions was present between the dye molecules (CV and RhB) and the functional groups of the MCN adsorbent surface. In the binary systems, the saturation adsorption capacities of CV and RhB ranged from 0.90 to 1.64 mmol/g and from 0.51 to 0.84 mmol/g at tested operating conditions. The comparison of results from single and binary adsorption tests indicated that the adsorption capacity of CV was significantly enhanced in the binary mixture, while the RhB dye adsorption decreased specially at low adsorption temperatures, see Table 2. This comparison clearly showed that in binary system there was a preference of the MCN adsorption site to bind the CV dye instead of the other dye molecule. In particular, the presence of RhB in the aqueous solution increased the adsorption of CV dye implying a synergistic adsorption. On the other hand, a decrement of the RhB dye adsorption capacity in binary systems corresponded to a competitive adsorption between both dyes present in the solution. It was also possible that the RhB dye molecule contained functional groups that favored the molecular interaction with CV thus providing additional sites for the CV dye removal. The effect of temperature on the adsorption isotherms at 298 - 328 K, with the same initial concentration of both dyes CV and RhB, is illustrated inFigure5.It was clear that the temperature improved the adsorption of CV and RhB molecules for mono- and multicomponent systems where328K was the best operating condition to remove these dyes. These results also indicated that the adsorption of both dyes was an endothermic process with tested adsorbent. This endothermic adsorption process can be associated to the fast mobility of CV and RhB dye molecules favoring its penetration into the MCN adsorbent pore and the interaction with internal functional groups [41]. 4.2.2 Understanding the dye adsorption mechanism via the parameters nD, nD1 and nD2 Parameters nD, nD1 and nD2 of the statistical physics models can be used to complete the interpretation of the adsorption mechanism of tested dyes in single and binary solutions. The

values of nD, nD1 and nD are summarized in Table 2 for all adsorption systems. Note that it was expected to obtain parameter values with the same magnitude that was observed in the adsorption capacity, but a significant difference in these parameters was identified for tested dyes and operating conditions. For example, the value of nD that is related to the single adsorption of CV was 0.28 at 298 K, while this parameter for RhB dye adsorption was 1.51 at the same temperature. This difference showed that the trend of the adsorption capacity that was described in single systems was not controlled by the parameter nD although it mainly depended on nD and Dm where Qsat0=2.nD.Dm. It was obtained that all values were lower only for the adsorption of RhB at 298 K. This theoretical evidence explained that before adsorption, the CV and RhB dye molecules were separated in solution due to thermal agitation leading to an absence of the dye aggregation process. Values of parameters nD1 and nD2for binary adsorption are also listed in the Table 2. This table indicates that the number of attracted dye molecules per MCN adsorption site increased from single to binary systems for both adsorbates. On the contrary case, it was expected that the parameter nD1that was related to CV dye was only ameliorated. This observation demonstrated that the CV dye binary adsorption mechanism was partially controlled by the parameter nD1. In conclusion, the parameters nD and nD2 were not the source of the difference between the adsorption capacities from single to binary systems. Note that the calculated values of nD, nD1 and nD2 can be used to discuss the adsorption orientation of CV and RhB dye molecules at tested operating conditions. In general, three possible scenarios can be identified: a) If nD, nD1, nD2<0.5 the dye adsorption was performed via a pure parallel orientation, b) If 0.51 the dye adsorption occurred via a pure non-parallel orientation[38,39]. Adsorption of CV dye molecules was conducted via a pure parallel orientation (nD<0.5), while via a parallel and non-parallel orientation of RhB dye could be

expected at 298K. In binary dye systems, all values of this parameter were higher than the unity with the exception of the CV adsorption at 298K, see Table 2. This theoretical finding demonstrated that both dyes were adsorbed via a pure non-parallel orientation. The effect of temperature on the parameters nD, nD1 and nD2 is illustrated in Figure 6. For single adsorption systems, all values of these parameters decreased with temperature. This implied that the thermal agitation did not promote the interaction and binding of CV and RhB dye molecules in the MCN adsorption sites. For the binary systems, this parameter showed different trends with respect to the adsorption temperature for tested dyes. In particular, the increment of temperature led to an increase of the parameter nD1 for CV and a decrement of nD2for RhB. This fact corroborated the previous conclusion that indicated that the parameters nD and nD2did not play a significant role in the adsorption mechanism. The inverse trend observed for nD1 and nD2 can be explained by the competitive adsorption effect between CV and RhB dye molecules. Note that the increment of the parameter nD1of dye CV indicated that its adsorption was not significantly affected by the presence of RhB dye molecule in the binary solution. 4.2.3Interpretation of the parameter Dm in single and binary dye adsorption systems The effect of temperature on Dm in single and binary adsorption systems is reported in Figure 7.The increment of temperature caused an increase of the adsorption sites responsible to bind CV molecules in single solutions (at saturation). In this case, the CV dye density of receptor sites enhanced the adsorption. For RhB, the effect of temperature on this parameter was not clear but this random trend could be related to the type of receptor site that was responsible for RhB removal. For instance, this density increased from 318 to 328 K and it was probably due to additional adsorption sites contributed to the RhB adsorption. For the binary system, the density of receptor sites for RhB increased with temperature. This increment was mainly

due to the reduction of the bonded number of this dye molecule per MCN adsorption site, which was due to the decrease of the parameter nD2. 5. Adsorption energy and description of a possible theoretical mechanism Calculation and interpretation of the adsorption energies can complete the understanding of the adsorption mechanism in single and binary dye systems including the impact of temperature and differences on the adsorption capacities in single and binary dye solutions. The adsorption energies were estimated with the concentrations at half-saturation at tested temperatures using the following expressions:

C  E1  RT ln s   C1  C  E2  RT ln s   C2 

C  E  RT ln s  (i=1 and 2)  C0i 

(8)

(9)

(10)

Equations 8 and 9 were related to the adsorption in single systems where the first and second layers were formed, while Equation 10 was used for calculations of the binary systems. Note that CS is the water solubility of the dyes. The effect of temperature on all adsorption energies is described in Figure 8.All estimated values of adsorption energies were positive revealing that the adsorption of CV and RhB in single and binary systems was endothermic. This modeling result was consistent with the effect of temperature observed for the single and binary experimental adsorption data. The low values of adsorption energies indicated that physical interactions could be involved during the formation of the adsorbate layers on MCN adsorbent surface [41]. Comparing the adsorption energies of single dye adsorption systems,

it was noted that they were relatively higher regarding the RhB dye adsorption. Note that the same behavior was observed for the adsorption capacity. The adsorption energy of CV dye increased from single to binary solutions, while the adsorption energies of RhB dye decreased. These trends were consistent with the experimental findings and the behavior of the dye adsorption capacities. Overall, the adsorption energy played a relevant role during the adsorption mechanism and it could determine the adsorbent preference for dye removal. As described, the temperature did not show a clear effect on the parameter Dm. Additionally, the adsorption capacities in single and binary systems increased and it was noted that the parameters of the attracted dye molecules per adsorption site decreased in few cases. Contrary to these findings, an increment of temperature led to an increase of the adsorption energies. This positive trend can be associated to the increment of the adsorption capacities in single and binary systems. Conclusions Single and binary adsorption of two hazardous dye pollutants on an alternative adsorbent was investigated at different operating conditions. This adsorbent showed similar adsorption capacities for the removal of CV and RhB dye molecules in mono-component aqueous solutions. In binary systems, antagonistic and synergistic adsorption effects were observed during the simultaneous removal of these dyes. Two physical models were applied to analyze the data and results suggested that the dye size did not play a significant role for the CV and RhB adsorption. It was demonstrated that there were both synergic and inhibition effects between both dyes on the adsorption sites for binary dye solutions. This implied that the presence of a second dye in solution promoted the adsorption of CV, while the removal of RhB dye molecule was affected by the other co-adsorbate. The interpretation of physical parameters explained that the adsorption energy played a relevant role in the adsorption

mechanism. Adsorption energy calculation showed that the single and binary adsorption of RhB and CV was physical-based and endothermic. Acknowledgment: The authors extend their appreciation to the deanship of Scientific Research at Majmaah University for funding this work under project number No (RGP-201912) References [1]

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1,0

CV: SS

RhB: SS

0,8

0,6

Qe (mmol/g)

Qe (mmol/g)

0,8

0,4 0,2

0,6 0,4 0,2

0,0 0,0

0,2

0,4

0,6

0,0 0,0

0,8

0,5

1,0

Ce (mmol/L)

1,5

2,0

2,5

3,0

Ce (mmol/L)

1,8 CV: BS

0,8

1,2 Qe (mmol/g)

Qe (mmol/g)

1,5

0,9 0,6

RhB: BS

0,6

0,4

0,2

0,3 0,0 0,0

0,2

0,4

0,6

0,8

0,0 0,0

1,0

0,2

0,4

Ce (mmol/L)

0,6

0,8

Ce (mmol/L)

Examples of fitting of the single and binary data

1,0 RhB: SS

0,8

0,6 0,4

T= 298 K T= 308 K T= 318 K T= 328 K

0,2 0,0 0,0

0,2

0,4 Ce (mmol/L)

0,6

0,8

Qe (mmol/g)

Qe (mmol/g)

0,8

CV: SS

0,6 T= 298 K T= 308 K T= 318 K T= 328 K

0,4 0,2 0,0 0,0

0,5

1,0

1,5 Ce (mmol/L)

2,0

2,5

3,0

1,0

1,8 CV: BS

0,8

1,2 Qe (mmol/g)

Qe (mmol/g)

1,5

0,9 0,6

T= 298 K T= 308 K T= 318 K T= 328 K

0,3 0,0 0,0

0,2

0,4

0,6

0,8

RhB: BS

0,6

0,4

T= 298 K T= 308 K T= 318 K T= 328 K

0,2

0,0 0,0

1,0

Ce (mmol/L)

0,2

0,4

0,6

0,8

1,0

Ce (mmol/L)

Figure 1: Adsorption isotherms of dyes Rhodamine B (RhB) and Crystal Violet (CV) on functionalized multiwall carbon nanotube in (SS) single and (BS) binary systems at298328K.

99

Transmittance (%)

88 77

MWCNT

66 100.8 96.0 91.2

MWCNT-COOH

86.4 4000

3000

2000

Wavelenght (cm-1)

1000

Figure 2: FTIR spectra for the multiwall carbon nanotube before and after its functionalization.

690

MWCNT

Intensity (counts)

460 230 0

MWCNT-COOH

1500 1000 500 0 0

25

50

75

100

2 (º)

Figure 3: XRD pattern for the multiwall carbon nanotube before and after its functionalization.

Figure 4: SEM images for the functionalized multiwall carbon nanotube (MWCNTCOOH).Magnifications:(a) × 1000, (b) × 10000, (c) × 20000 and (d) × 30000.

Dyes adsorption capacity (mmol/g)

1,6 1,4

CV: SS RhB: SS CV: BS RhB: BS

1,2 1,0 0,8 0,6 295

300

305

310

315

320

325

330

Temperature (K)

Figure 5: Variation of the adsorption capacity of dyes Rhodamine B (RhB) and Crystal Violet (CV) on functionalized multiwall carbon nanotube in single (SS) and binary (BS) systems at 298– 328K.

Parameters nD, nD1 and nD2

2,4

CV: SS RhB: SS CV: BS RhB: BS

2,1 1,8 1,5 1,2 0,9 0,6 0,3 295

300

305

310

315

320

325

330

Temperature (K)

Figure 6:Impact of temperature on the parameters nD, nD1 and nD2for the adsorption of dyes Rhodamine B (RhB) and Crystal Violet (CV) on functionalized multiwall carbon nanotube in single (SS) and binary (BS) systems.

2,4

Parameter Dm

2,1 1,8

CV: SS RhB: SS CV: BS RhB: BS

1,5 1,2 0,9 0,6 0,3 295

300

305

310

315

320

325

330

Temperature (K)

Figure 7:Impact of temperature on the parameters Dm for the adsorption of dyes Rhodamine B (RhB) and Crystal Violet (CV) on functionalized multiwall carbon nanotube in single (SS) and binary (BS) systems.

16 15

E1 (CV) E2 (CV) E1 (RhB)

SS Adsorption energy (kJ/mol)

Adsorption energy (kJ/mol)

17

E2 (RhB)

14 13 12 11 10 295

300

305

310

315

320

325

330

24

CV: BS RhB: BS

22 20 18 16 14 295 300 305 310 315 320 325 330

Temperature (K)

Temperature (K)

Figure 8:Impact of temperature on the calculated energies for the adsorption of dyes Rhodamine B (RhB) and Crystal Violet (CV) on functionalized multiwall carbon nanotube in single (SS) and binary (BS) systems.

Characteristics

RhB

CV

Cationic C28H31ClN2O3 45170 Rhodamine 610

Cationic C25H30N3Cl 42555 Gentian violet

479.02

407.99

610

590

Molecular structure

Chemical class Chemical formula Color Index Number (IC) Color Index Name Molecular weight (g mol-1) λmax (nm)

Table 1: Physicochemical properties of dyes rhodamine B and crystal violet used as adsorbates.

Single dye adsorption CV T (K) nD Dm (mmol/g) Qsat0 (mmol/g)

RhB

298 0.28 1.05

308 0.22 1.65

318 0.20 2.00

328 0.18 2.44

298 1.51 0.26

308 0.68 1.36

318 0.67 0.63

328 0.52 0.88

0.59

0.73

0.80

0.88

0.77

0.81

0.85

0.90

318 1.17

328 1.02

0.65

0.82

T (K) nDi

298 0.89

CV 308 1.74

Binary dye adsorption RhB 318 328 298 308 1.79 1.9 2.39 1.85

Dm

1.01

0.7

0.80

0.86

0.27

0.38

(mmol/g) Qsati (mmol/g)

0.90

1.22

1.43

1.64

0.61

0.68

0.76

0.84

Table 2: Adjusted parameters of the statistical physics models for the single and binary adsorption of dyes Rhodamine B (RhB) and Crystal Violet (CV) on functionalized multiwall carbon nanotube at different temperatures.

Declaration on interset statement This submission has not been published previously and not under consideration for publication elsewhere. If it will be published, it will not be published elsewhere in the same form, in English or in any other language, including electronically without the written consent of the copyright-holder. Dr Lotfi Sellaoui,

Highlights

 Adsorption of crystal violet and rhodamine B on nanotubes was theoretically and experimentally analyzed.  Surface chemistry of the functionalized multiwalled carbon nanotubes was performed.  Physicochemical interpretation of the single and binary dye adsorption mechanisms.  Dye adsorption mechanisms was characterized via statistical physics parameters.