Model-fitting approach for methylene blue dye adsorption on Camelina and Sapindus seeds-derived adsorbents

Abstract

Adsorption of methylene blue dye over the adsorbent derived from Sapindus seed hull (S) and Camelina (C) is studied. Batch adsorption study on both adsorbents is compared as a function of various parameters such as time, initial concentration and temperature. Langmuir and Freundlich models were fitted to the equilibrium data of methylene blue dye adsorption. Kinetics was evaluated using pseudo-first- and second-order models. Weber Morris model helped to understand the intraparticle diffusion during adsorption of methylene blue molecules. Thermodynamic analysis on both adsorbents revealed the spontaneity of the process. The relative study illustrated a comparison in adsorption capacities among both the adsorbents. Morphology explicated the surface characteristics of both the sorbents. Fourier transform infrared spectroscopy spectrum revealed a variety of functional groups on adsorbents’ surface.

Keywords

Adsorption methylene blue waste water

Introduction

The industrial effluent from textiles, paper, plastic, leather, etc. is reported to contain dyes that pollute water resources and environment. This pollution is harmful for living species (Doğan et al., 2009; Janet et al., 2015). One such dye is methylene blue which causes numerous health hazards to human health (Hameed et al., 2007b). This has led researchers to discover a new and cheap technique for the removal of dyes from the effluents. So far, the adsorption of pollutants on adsorbents has been found to be the most attractive method due to its simplicity. Commercially available adsorbents are very expensive as they are produced from natural resources like wood or coal (Hameed et al., 2007a). Therefore, there is a need to look for some cheap raw material to produce low-cost adsorbent.

Many research works have been reported on various agricultural byproducts or low-cost adsorbents such as olive stones, peach stones, pistachio nut shells, date pits, olive pomace, Prosopis mimosaceae sawdust, Fumaria indica, diatomite, sea sand, guava leaf, Mucilaginous leaves of Diceriocaryum eriocarpum plant, eggshells, banana peel, pumpkin and rice husk in the production of low-cost adsorbents (Attia et al., 2008; Banat et al., 2007; Belhachemi and Addoun, 2011; Chowdhury and Saha, 2012; Crini, 2006; Edokpayi et al., 2015; Ghasemi and Gholami, 2015; Hameed and Daud, 2008; Iqbal and Khera, 2015; Kanyal and Bhatt, 2015; Lua et al., 2004; Nebagha et al., 2015; Ojedokun and Bello, 2016; Salman et al., 2015). In the present work, the potential of low-cost adsorbents produced from two agricultural wastes—Sapindus seeds hull (S) and Camelina (C)— in the removal of methylene blue from water is being done. A comparison of the adsorption capacity of Sapindus-derived (SAd) and Camelina-derived (CAd) with other low-cost adsorbents quoted in the literature has also been done.

Materials and methods

Preparation and characterization of the adsorbent

Sapindus seeds hulls and Camelina were dried and crushed to powder form of 100 µm particle size. A fixed amount of both the raw materials were then mixed with concentrated sulfuric acid in the ratio of 1:2 (w/w) and heated to 200℃ for 24 h. Since sulfuric acid oxidizes the organic matter adhered to agricultural wastes (Thenmozhi and Santhi, 2014) as well as breaks bonds between carbonaceous and non-carbonaceous matter (Ayyappan et al., 2005; Mohan and Singh, 2002), the chemical agent was added in slightly greater amount. The material synthesized was then washed with distilled water and was soaked in 1% NaHCO₃ solution overnight to neutralize the free acid. Both the adsorbents were again washed with distilled water till the pH reached 6–7 and then dried in an air oven at 120℃ for 4 h. SAd and CAd adsorbents were again crushed to 150 µm particle size.

The surface morphology was determined using a ZEISS EVO Series Scanning Electron Microscope EVO 50. This microscope had the resolution of 2.0 nm at 30 kV with magnification range up to 1,000,000 and an acceleration voltage up to 30 kV. The sample was mounted on a circular metallic sample holder with the help of sticky carbon tape.

The Brunauer–Emmett–Teller (BET) surface area of the adsorbents was measured by using Micromeritics ASAP 2010 apparatus. Nitrogen adsorption/desorption isotherms were measured at −196℃. Prior to gas adsorption measurements, the adsorbent was degassed at 200℃ ± 1℃ under vacuum for a period of 30 min. The total pore volume was calculated at a (p/p_o) relative pressure 0.989.

The Fourier transform infrared spectroscopy (FT-IR) spectrum of adsorbents was generated using Perkin Elmer Frontier in the range of 400–4000 cm⁻¹ at room temperature. The powdered sample of adsorbent mixed with 0.3 g of KBr in 2:100 ratio and pellet—1 mm thick—was prepared by pressing the mixture in a dye at a pressure of 7–8 ton through hydraulic press. The spectrum of the pellet was recorded with a resolution of 4 cm⁻¹ and 16 scans per sample. Baseline correction was done with Spectrum 10 software.

Adsorbate: Methylene blue

The dye solution was prepared using distilled water and methylene blue (Merck), without further treatment. Distilled water was used for the preparation of solutions. Initially, a stock solution of 100 mg l⁻¹ was prepared and the working solutions were prepared by the dilution of the prepared stock solution.

Adsorption studies

The adsorption study was done in phases. Equilibrium study was conducted by taking 50 ml of four different concentrations of methylene blue solution in the range of 10–100 mg l⁻¹ at pH 6. To each solution, 100 mg of adsorbent was added. The mixture was then stirred for 360 min at 120 r/min to ensure equilibrium. The adsorption mixture was separated through centrifugation process and the supernatant was collected. The concentration of the supernatant was analyzed using ultraviolet–visible spectrophotometer (Varian Carry-100) at 668 nm. The amount of adsorption at equilibrium, q_e in mg g⁻¹, was calculated (Benadjemia et al., 2011; El Khames Saad et al., 2014) using equation (1)

q_{e} = \frac{(C_{i} - C_{e}) \times V}{m}

(1)

where m is the mass of dry adsorbent used (g), and C_i and C_e are the concentrations of methylene blue at initial and equilibrium state, respectively.

The kinetic studies of adsorption were conducted out in the similar manner as adsorption at equilibrium. However, the samples for kinetic studies of adsorption were taken at different time intervals. The effect of parameters such as pH (2, 4, 6, 8 and 10), initial concentration of methylene blue dye (10, 40, 70 and 100 mg l⁻¹) and temperature (30℃, 40℃ and 50℃) on the adsorption of dye for CAd and SAd were also studied.

Results and discussions

Scanning electron microscopy of C and S (shown in Figure 1(a) and (c), respectively) and CAd and SAd (shown in Figure 1(b) and (d), respectively), represents surface structure of the materials before and after chemical action. It is observed that C contains insignificant closed holes; however, after chemical treatment, CAd exhibits remarkable number of lengthy openings on the surface, constituting channels for the flow of fluids through them. It signifies that enormous volatiles have been removed from the base material, leaving behind the vacant space. The surface of SAd carries fewer pores mouth; yet precursor (S) highlighted complete absence of the pores in SEM image as presented in Figure 1(c). Camelina is relatively a soft material, so the chemical agent easily cleaves the bonds of carbon and non-carbonaceous substance and penetrates deep in to it. Despite the fact that BET surface area was found to be 0.683 and 0.114 m²g⁻¹ for CAd and SAd, respectively, it highlights the need for carbonization after chemical treatment for precursors. Some works have reported the similar result for low-cost adsorbents prepared from Delonix regia pods and Borassus aethiopum flower biomass (Ho et al., 2009; Nethaji et al., 2010). The use of adsorbents with BET surface area much lesser than 100 m²g⁻¹ in the removal of pollutants has also been reported (Daifullah and Girgis, 1998; Danish et al., 2011; Jassim et al., 2012; Karagöz et al., 2008; Munusamy et al., 2011; Murthy et al., 2013; Norlia et al., 2011; Singh et al., 2006).

Figure 1.

SEM images of (a) C, (b) CAd, (c) S and (d) SAd.

The FT-IR spectrum of both the precursors and adsorbents as shown in Figure 2 facilitates to understand the affinity between the adsorbent and the adsorbate. The existence of similar functional groups on the surface of CAd and SAd due to the employment of common activating agent for their production is given in Table 1. As it is apparent from Table 1, native bonds are retained by both the adsorbents with some improvement. Because dye molecules contain polar groups, these functional groups play a significant role in the new bond formation.

Figure 2.

FT-IR spectrum of (a) CAd, (b) SAd, (c) S and (d) C.

Table 1.

FT-IR spectra depicting various bonds on the surface of C, S, CAd and SAd.

C		S		CAd		SAd
Bond	Wave number (cm⁻¹)	Bond	Wave number (cm⁻¹)	Bond	Wave number (cm⁻¹)	Bond	Wave number (cm⁻¹)
C–H	948	C–O	1115	C–H	950	C–H	949
C–O	1195	C–O	1387	C–O	1194	C–O	1195
C–H	3060	C=C	1562	C–N	1250	C–N	1239
–OH	3641	N–H	3457	C–H	3023	C–H	3010
				–OH	3640	–OH	3641

C: Camelina; S: Sapindus; CAd: Camelina-derived adsorbent; SAd: Sapindus-derived adsorbent.

It is also important to examine the influence of the pH of the solution as it affects the extent of adsorption. Figure 3 represents the adsorption on CAd and SAd at different pHs (2–10). It is concluded from the data that the neutral pH favors maximum adsorption up to 12% and 99.38% for SAd and CAd, respectively. The reason behind this is that the acidic and basic nature of solution results in the protonation and deprotonation of the adsorbents’ surface charge, which was otherwise responsible for the uptake of dye molecules as already reported (Wang et al., 2008).

Figure 3.

Effect of pH on adsorption by CAd and SAd at 30℃.

Figure 4 presents a study of removal percent of dye molecules with respect to temperature which further helps in the investigation of several thermodynamic parameters like entropy, enthalpy and Gibbs free energy. As evident from the individual curve, the slope increases as the temperature of the process is increased from 30℃ to 50℃, indicating soaring recovery. This may be because the high temperature excites the dye molecules which aid in combating mass transfer resistance. Also, the boundary layer becomes thinner at elevated temperature and this enables the molecules to reach adsorption sites within the pores. A similar trend has been reported in the literature for the temperature-dependent study of methylene blue dye adsorption on natural zeolite (Han et al., 2009).

Figure 4.

Removal percentage of CAd and SAd at initial concentrations—40, 70 and 100 mg l⁻¹.

Figure 4 also indicates that lesser adsorption takes place at higher initial concentration which is in line with the result reported for PP and APP (Tiwari et al., 2014). At the initial dye concentration of 40 mg l⁻¹, CAd proved to have much higher removal capacity of 99% at 50℃ in comparison to 14.45% for SAd for the same conditions. The repulsion among the molecules could have resulted in the drop of percent removal with increase in initial concentration. The percent removal of CAd and SAd at 30℃ for 40 mg l⁻¹ is 99.39 and 12.02, respectively. Again, the huge difference in uptake of dye molecules is due to the highly porous surface of CAd in comparison to SAd.

The adsorption isotherm gives the distribution of adsorbate in solid to liquid phase and hence presents a relation among the quantity adsorbed, q_e, to the concentration in the solution at equilibrium, C_e (Tan et al., 2008). Langmuir and Freundlich isotherms are most commonly employed for the equilibrium study of adsorption (Hameed et al., 2007a).

Langmuir isotherm is used to understand the equilibrium with an important assumption of surface homogeneity (Hameed and Daud, 2008). The mathematical expression of Langmuir isotherm is presented in equation (2) (Ahmad et al., 2014)

\frac{C_{e}}{q_{e}} = \frac{1}{K_{L} q_{max}} + \frac{1}{q_{max}} C_{e}

(2)

where q_e is the amount of adsorbate adsorbed (mg g⁻¹) at equilibrium, C_e is the concentration at equilibrium (mg l⁻¹), q_max represents monolayer capacity (mg g⁻¹) and K_L corresponds to adsorption equilibrium constant (l mg⁻¹). The C_e/q_e versus C_e has a linear relationship as shown in Figure 5 and its slope and y-intercept produce q_max and K_L values, respectively, as shown in Table 2.

Figure 5.

Langmuir isotherms of methylene adsorption on CAd and SAd at different temperatures (30℃, 40℃ and 50℃).

Table 2.

Equilibrium models constants for the adsorption of methylene blue on CAd and SAd.

Isotherm	Parameters	CAd			SAd
Isotherm	Parameters	30℃	40℃	50℃	30℃	40℃	50℃
Langmuir	q_max (mg g⁻¹)	50	50.25	50.76	4.61	4.75	5.08
	K_L (l mg⁻¹)	3.85	7.65	19.7	0.03	0.03	0.04
	R_L	0.003	0.001	0.0005	0.25	0.22	0.20
	R²	0.991	0.992	0.993	0.999	0.998	0.999
Freundlich	K_f (mg g⁻¹)(l mg⁻¹)^1/n	31.54	37.85	47.79	0.29	0.37	0.46
	n	3.34	3.74	4.11	1.79	1.94	2.02
	1/n	0.29	0.27	0.24	0.56	0.51	0.50
	R²	0.974	0.993	0.999	0.979	0.988	0.983

CAd: Camelina-derived adsorbent; SAd: Sapindus-derived adsorbent.

It is observed that the increase in temperature from 30℃ to 50℃, favored adsorption as q_max for CAd, and SAd shows an increment of 1.52% and 10.19%, respectively. This small change in q_max with temperature is also reported for activated alumina (Singh and Pant, 2004). Several low-cost adsorbents with comparable adsorption capacity are presented in Table 3. Further, K_L values at 30℃ for CAd and SAd increase from 3.85 and 0.03 l mg⁻¹ to 19.7 and 0.04 l mg⁻¹, respectively, at 50℃.

Table 3.

Comparison of maximum adsorption capacities, q_e by several adsorbents for methylene blue dye.

Raw material	q_e (mg g⁻¹)	References
Hazelnut shell	8.82	Aygün et al. (2003)
Parthenium hysterophorus	39.68	Lata et al. (2007)
Annona Squmosa seed AC	8.525	Santhi et al. (2010)
Sunflower oil cake	15.798	Karagöz et al. (2008)
Maize cob	5.00	Kadirvelu et al. (2003)
Delonix regia pods	23.30	Ho et al. (2009)
Date pits	17.30	Banat et al. (2003)
Olive stones	16.10	Alaya et al. (2000)
Corncob	0.84	Tseng et al. (2006)
Luffa cylindrica fibers	47.00	Demir et al. (2008)
Yellow passion fruit waste	44.70	Pavan et al. (2008)
Olive pomace	42.30	Banat et al. (2007)
Wheat straw	3.82	Batzias et al. (2009)
Neem leaf powder	3.67	Bhattacharyya and Sharma (2005)
Camelina after chemical treatment	50	Present study

Separation factor, R_L, helps to identify the nature of adsorption process and is evaluated using the relation as given in equation (3) (Belhachemi and Addoun, 2011; Hameed and Daud, 2008)

R_{L} = \frac{1}{1 + K_{L} C_{o}}

(3)

where C_o is the highest initial dye concentration. R_L value between 0 and 1 indicates a favorable process of dye adsorption. In this case, CAd and SAd are found to be in the range 0.003–0.0005 and 0.25–0.2, respectively, at different set of temperatures under study.

The linear expression for Freundlich isotherm is shown in equation (4)

\log q_{e} = \log K_{f} + \frac{1}{n} {logC}_{e}

(4)

where K_f and n represent extent of adsorption and process favorability known as Freundlich constants, respectively (Gerçel et al., 2007) and are evaluated from the plot of log q_e versus log C_e (Figure 6). As it is clear from Table 2, both Freundlich constants K_f and n increase with temperature for CAd and SAd. The value of n greater than 1 represents that the dye is favorably adsorbed on CAd and SAd (Ahmad et al., 2013). For CAd and SAd, a value of 1/n is found to be in the range 0.24–0.29 and 0.50–0.56, respectively, indicating normal Langmuir isotherm at all temperatures under study (Hameed et al., 2007a). As correlation coefficient, R² close to 1 depicts best fit of the equilibrium model; hence, the present study indicates that Langmuir model is a best fit with the experimental data for the methylene blue dye adsorption on both CAd and SAd.

Figure 6.

Freundlich isotherms of methylene adsorption on CAd and SAd at different temperatures (30℃, 40℃ and 50℃).

In order to understand the controlling step involved in the adsorption of methylene blue such as diffusion, adsorption and chemical reaction, various kinetic models have been applied to the experimental results. The pseudo-first-order (Kavitha and Namasivayam, 2007) expression is presented in equation (5)

\log (q_{e} - q_{t}) = \log (q_{e}) - \frac{k_{1} t}{2.303}

(5)

where q_e and q_t represent the amount of dye adsorbed at equilibrium and time t in mg g⁻¹. The first-order rate constant, k₁ (min⁻¹), is evaluated from the slope of straight line between

\log (q_{e} - q_{t})

and

\log (q_{e})

as shown in Figure 7. As shown in Table 4 for CAd, k₁ increases from 0.0168 to 0.0246 min⁻¹; however, SAd depicted a drop from 0.0355 to 0.0338 min⁻¹. A similar trend is observed in the value of q_e for both CAd and SAd as shown in Table 4. Similar values for these constants are being reported in the literature for the adsorption of methylene blue dye on coir pith carbon (Kavitha and Namasivayam, 2007).

Figure 7.

Pseudo-first-order kinetics of methylene adsorption on CAd and SAd at different temperatures (30℃, 40℃ and 50℃).

Table 4.

Kinetic models constants for adsorption of methylene blue on CAd and SAd at an initial concentration of 100 mg l⁻¹.

T (℃)	Pseudo-first-order kinetic model						Pseudo-second-order kinetic model						Intra-particle diffusion model
	k₁× 10² (min⁻¹)		q_e (mg g⁻¹)		R²		k₂ (g mg⁻¹ min⁻¹)		q₂ (mg g⁻¹)		R²		k_P (mg g⁻¹ min⁻¹)		C (mg g⁻¹)
	CAd	SAd	CAd	SAd	CAd	SAd	CAd	SAd	CAd	SAd	CAd	SAd	CAd	SAd	CAd	SAd
30	1.68	3.55	5.13	1.54	0.983	0.997	1.4 × 10⁻³	3.7 × 10⁻²	35.21	3.29	0.993	0.984	4.36	0.41	2.34	0.23
40	1.91	3.50	5.07	1.56	0.974	0.994	1.7 × 10⁻³	3.0 × 10⁻²	36.76	3.45	0.998	0.989	4.71	0.43	0.92	0.36
50	2.46	3.38	5.06	1.62	0.996	0.981	1.6 × 10⁻³	2.9 × 10⁻²	42.02	3.73	0.994	0.982	5.38	0.46	0.16	0.43

CAd: Camelina-derived adsorbent; SAd: Sapindus-derived adsorbent.

The pseudo-second-order kinetic model (Gerçel et al., 2007) is presented in equation (6)

\frac{t}{q_{t}} = \frac{1}{k_{2} q_{2}^{2}} - \frac{1}{q_{2}} t

(6)

where q₂ represents maximum adsorption in mg g⁻¹ through second-order adsorption kinetics and k₂ is the rate constant at equilibrium in g (mg min)⁻¹. Both constants—q₂ and k₂—are calculated from the plot between t/q_t versus t as shown in Figure 8 and are found to increase with temperature as shown in Table 4. Again, q₂ representing uptake tendency of adsorbent improves for CAd from 35.21 to 42.02 mg g⁻¹ and 3.29 to 3.73 mg g⁻¹ for SAd. The correlation coefficient for pseudo-second-order kinetic model is close to 1; this implies that the adsorption phenomenon is best represented by second-order mechanism. Similar results for pseudo-second-order kinetics are reported for activated carbon prepared from Euphorbia rigida (Gerçel et al., 2007).

Figure 8.

Pseudo-second-order kinetics of methylene adsorption on CAd and SAd at different temperatures (30℃, 40℃ and 50℃).

Since the abovementioned two kinetic models are inadequate to explain intraparticle diffusion, diffusion mechanism is studied using the model proposed by Weber and Morris as presented in equation (7) (Gerçel et al., 2007)

q_{t} = K_{p} t 1 / 2 + C

(7)

where K_p is the intraparticle diffusion constant (mg g⁻¹min^−0.5) and C is a constant (mg g⁻¹). The q_t versus t^0.5 shows a linear variation and the values of K_P and C are evaluated from slope and intercept on y-axis, respectively, of the graph as shown in Figure 9. The calculated values of K_P and C are shown in Table 4. The linear portion of the curves plotted at three different temperatures does not coincide with the origin, thus exhibiting the mixed role of surface adsorption and intraparticle diffusion in the rate determining step (Singh and Pant, 2004). Further, the larger intercept value is due to the boundary layer effect, as explained elsewhere (Ahmad et al., 2013; Gerçel et al., 2007). It is, therefore, concluded that CAd offers a larger boundary layer effect in comparison to SAd (Table 4), which further suggests that dye molecules had to overcome this extra resistance for adsorption.

Figure 9.

Intraparticle diffusion plots of methylene adsorption on CAd and SAd at different temperatures (30℃, 40℃ and 50℃).

Several thermodynamic parameters are determined from the basic relations as given in equations (8) and (9)

\ln K_{L} = \frac{Δ S \circ}{R} - \frac{Δ H \circ}{RT}

(8)

where

Δ G \circ = Δ H \circ - T Δ S \circ

(9)

In the above relations, R is the universal gas constant, T is the temperature in Kelvin and K_L is the distribution coefficient (Tan et al., 2008) and considered Langmuir constant in this study (Ahmed et al., 2011; Geçgel et al., 2012; Khare and Kumar, 2012). The values for ΔH° and ΔS° are evaluated from slope and intercept, respectively, of graph between ln K_L versus 1/T as shown in Figure 10.

Figure 10.

Thermodynamic studies of methylene adsorption on CAd and SAd at different temperatures (30℃, 40℃ and 50℃).

All the thermodynamic parameters evaluated from equations (8) and (9) are given in Table 5. The increase in process temperature results in boosting up of the negative free energy change of both solid substrates. The negative experimental values for ΔG° imply spontaneity of the process (Gerçel et al., 2007). Comparative values of SAd for ΔG° are being reported in the literature (Ahmad et al., 2013). Further, the changes in enthalpy for CAd and SAd are found to be 66.33 and 11.02 kJ mol⁻¹, respectively. The positive values of ΔS° reflect an increase in randomness at solid–solution interface as well as affinity of adsorbents for the dye molecules. In case of entropy change, CAd shows high value 229.68 J mol⁻¹K⁻¹ as compared to the value of 7.19 J mol⁻¹K⁻¹ for SAd.

Table 5.

Thermodynamic parameters for adsorption of methylene blue on CAd and SAd at initial concentration 100 mg l⁻¹.

Sample	ΔH° (kJ mol⁻¹)	ΔS° (J mol⁻¹K⁻¹)	ΔG° (kJ mol⁻¹)
Sample	ΔH° (kJ mol⁻¹)	ΔS° (J mol⁻¹K⁻¹)	30℃	40℃	50℃	E_a (kJ mol⁻¹)
CAd	66.33	229.68	−14.87	−17.55	−20.23	10.49
SAd	11.02	7.19	−2.76	−3.21	−3.66	7.17

CAd: Camelina-derived adsorbent; SAd: Sapindus-derived adsorbent.

Activation energy is calculated using the relation as presented in equation (10) (Gerçel et al., 2007)

\ln k_{2} = \ln A - \frac{E_{a}}{RT}

(10)

where k₂ is obtained from the pseudo-second-order kinetic model, A is the Arrhenius factor, R is the gas constant (8.314 J mol⁻¹K⁻¹) and T is the absolute temperature. E_a in the above equation represents activation energy for adsorption and is evaluated from the slope of the straight line between ln (k₂) versus 1/T. The E_a values as presented in Table 5 for the adsorption study of methylene blue on CAd and SAd are found to be 10.49 and 7.17 kJ mol⁻¹, respectively.

Conclusion

The hardness of raw materials—Sapindus seeds hull and Camelina—affected the surface properties of the derived adsorbents. CAd surface depicted more pore openings than SAd as illustrated by morphology which resulted in greater adsorption capacity of CAd over SAd. FT-IR spectrum revealed various functional groups that played significant role in the adsorption of methylene blue molecules. Langmuir isotherm fitted well to the equilibrium data for CAd and SAd. Experimental results fit to pseudo-second-order kinetic model for both adsorbents. Combined mechanism of intraparticle diffusion and surface adsorption was portrayed by both CAd and SAd. Thermodynamic parameters evaluated at different temperatures depicted spontaneity of the process.

This study, therefore, proves the diverse potential of chemically treated Sapindus seeds and Camelina for the adsorption of methylene blue dye from its water solution.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

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