Studies on Adsorption of Fluorescein Dye from Aqueous Solutions Using Wild Herbs

The adsorption of fluorescein dye (FD) on wild herb microparticles (Juniperus (JH) and Solenostemma argel (Del) Hayne (SH)) was studied to elucidate the changes in adsorption behavior with various parameters, such as initial concentration, adsorbent dosage, pH, contact time, and temperature. It was determined that the adsorption percentage of JH for FD was as high as 85.5%, which was higher than that of SH (71.9%). The morphologies of JH and SH were analyzed using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses. The JH and SH adsorbents contained different functional groups, which were involved in the binding of the FD molecules during adsorption. The XRD patterns of JH and SH confirmed the presence of a combination of amorphous and crystalline phases in their structures. The SEM images of the surface of JH revealed the presence of deep pores ranging in size from 1.9 to 3.5 μm, while SH contained smaller pores ranging in size from 130 to 350 μm, which could help absorb large quantities of FD. The Freundlich model fitted the adsorption isotherms better than the Langmuir model. The values of the Freundlich equilibrium coefficient and separation factor ranged from 1 to 2 and from 0 to 1, respectively. The maximum adsorption capacities of JH and SH were determined to be 2.91 and 2.565 mg/g, respectively. Four kinetic models were used to analyze the experimental data, and it was determined that the pseudo-second-order kinetic model best described the adsorption process, which involved chemical adsorption and the internal diffusion. Thermodynamic parameters, including the enthalpy, entropy, and Gibbs free energy, were calculated. These parameters indicated that the adsorption of FD on JH was spontaneous and endothermic and the adsorption of FD on SH was unspontaneous and exothermic.


Introduction
Industrial organic dyes are highly toxic, and therefore, the dyeing industry, which releases 100 tons of dyes in the environment annually, is responsible for the pollution of rivers and springs [1,2]. Most of the effluents from the textile industries are often loaded with a mixture of dyes, acids, bases, and soluble solids [3]. Once these pollutants are exposed to water, their removal via adsorption becomes difficult owing to their complex molecular structures, which can withstand various environmental conditions. erefore, more studies on the treatment of aqueous solutions should be conducted to overcome this environmental problem, and methods that could be used to remove even small amounts of different types of synthetic dyes from wastewaters should be considered. e textile industries often produce liquid wastes that are loaded with mixtures of dyes, acids, bases, and soluble solids [3], making adsorption difficult. erefore, aqueous solutions treatment is one of the biggest problems we face today. Once the dyes are exposed to water, it is difficult to adsorption them, because they have a complex molecular structure, capable of tolerating various environmental conditions. Moreover, when the concentration of these dyes in water is low, the water is clear.
us, the decolorization of aqueous solutions that contain different types of synthetic dyes is important. e chemical structure of resorcinolphthalein or fluorescein dye (FD), which is one of the commercially available textile dyes, is shown in Figure 1. e molecular formula of FD, which is widely used as artificial coloring agent, is C 20 H 12 O 5 . Fluorescein dye can be obtained by heating resorcinol and phthalic anhydride on a zinc catalyst; the obtained product is a crystalline dark red powder, and its melting point ranges from 314 to 316°C. Resorcinolphthalein is also known as FD owing to its strong green fluorescein alkaline solutions even at low concentration. To date, FD has been used as dye for liquid coloring in analytical devices, in the cosmetics industries, and as aqueous detector.
While no specific method exists for the complete adsorption of dyes from aqueous solutions, some of the traditional methods for removing dyes from aqueous solutions involve biological treatment, coagulation, buoyancy, adsorption, oxidation, and excessive filtration [3], and the most important treatment of all these methods is adsorption.
Adsorption is the process where molecules from aqueous solutions adhere on the surface of the adsorbent surface via physical forces or chemical bonds. Depending on the nature of the adsorbents and the chemical structure of the dyes, adsorption occurs via electrostatic interactions or van der Waals forces.
Many natural materials, including coffee waste, have been used to adsorb various pollutants from aqueous solutions [4,5]. Large amounts of coffee beans are produced and used worldwide, and coffee waste has been used to adsorb hazardous particles from aquifers and gaseous mixtures or for the desalinization of water [6]. Coffee powder has been used as low-cost additive for the adsorption of a series of rhodamine dyes, such as rhodamine B and rhodamine 6G, from aqueous solutions [7]. Aleurites moluccanus seeds have been used for the adsorption of methylene blue dye and rhodamine B from aqueous solutions [8]. Moreover oat hulls [9], date palm fiber [10], and wood apple shells [11] have been used for the adsorption of malachite green from aqueous solutions; water hyacinth [12], date palm fiber [13], and Calligonum comosum leaf powder [14] have been used for the adsorption of crystal violet dye from aqueous solution; date palm leaf [15] has been used for the adsorption of Congo red dye from aqueous solution; teak leaf powder [16] has been studied for the adsorption of eosin yellow; and kenaf core fiber has been used for the adsorption of reactive anionic [17] and acidic dyes [18]. e aim of this study was to analyze the kinetics and thermodynamics of the adsorption of FD from aqueous solution using wild herb microparticles.

Materials.
We purchased FD from Sigma-Aldrich (St. Louis, MO, USA) and used distilled water to prepare FD solution.
Juniperus (JH; Figure 2), which is an evergreen trees with seedless fruit that typically grows in cold climates, is abundant in southern Saudi Arabia. e branches of JH secrete a dark colored substance that presents medical uses [19].
Solenostemma argel (Del) Hayne (SH; Figure 2), which is a simple herbaceous plant, blooms in the summer and can be found in many mountainous and desert regions of the Arabian Peninsula and North Africa [20]. Its small white flowers are grouped in tentacles and its fruit is smooth, velvety, fossilized, and dark red.

Preparation of Adsorbents.
e JH and SH adsorbents used in this study were purchased from perfumer. e herbs were first dried at 50°C for 30 min, and then they were cut into small pieces and ground to powder using a laboratory planetary ball mill (DECO-PBM-V-0.4L). e powders were sieved into particles less than 50 μm in size using an Octagon D200 digital sieve shaker and were stored in glass bottles for further use without any pretreatment.

Preparation of FD Solution.
e FD stock solution (1000 mg/L) was prepared using double distilled water. All solutions used in the experiments were prepared by diluting the stock solution to predetermined concentrations.

Methods.
e adsorption experiments were performed in flasks that contained 100 mL FD solution of predetermined concentration and different amounts of adsorbent herbs. e adsorption equilibrium was investigated for different dye concentrations that ranged between 2 and 10 mg/L. In addition, kinetics experiments were conducted using dye with the concentration of 10 mg/L and 0.5 g adsorbent at 25°C. e mixtures were shaken at 120 rpm for 16 h using a Rotaterm orbital and linear shaker. e initial concentration of dye in this study was 10 mg/ L. e effect of the pH on the adsorption of FD was investigated, and the pH was adjusted in the range of 3-12 using either 1 M NaOH or 1 M HCl. A pH meter was used to monitor the changes in pH. In addition, the adsorbentadsorbate contact time was varied between 30 and 240 min. e effect of the temperature on the adsorption process was studied in the range of 25 to 70°C for 3 h; the amount of adsorbent and the initial dye concentration used for these experiments were 0.5 g and 10 mg/L, respectively, and 10 mg/L, respectively [21]. All mixtures were filtered using 42 micron Whatman filter paper, and the dye concentrations were determined using a UV/Vis spectrophotometer at the wavelength of 450 nm. e quantity of dye adsorbed at equilibrium (q e (mg/g)) was calculated using the following equation [21]: where C 0 and C e (mg/L) are the initial concentrations of dye and the equilibrium concentration of dye at equilibrium, respectively, V (L) is the volume of the solution, and m (g) is the mass of adsorbent. e percentage of FD adsorbed from the solution (Ads FD (%)) was calculated using the following equation [21]: Four kinetic models were used to describe the behavior of adsorbent, and the Langmuir and Freundlich isotherm models were employed to study the adsorption data. e following thermodynamic parameters, enthalpy (∆H°), entropy (∆S°), and free energy (∆G°), were calculated, and the results were used to study and interpret the effect of temperature on the adsorption process. e functional groups that participated in the adsorption processes were determined using Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analysis. Scanning electron microscopy (SEM) images of the adsorbent surfaces were obtained using a JSM-6380 LA SEM instrument with the high resolution of 3.0 nm.

Characterization of Adsorbents.
e FTIR spectra of the adsorbents were used to differentiate functional groups, such as -OH (alcohols and carboxylic acids), -CH-(alkanes), -N-H (amines), and C�O (carbonyl), present in the structure of JH and SH ( Figure 3), which could form bond with FD during adsorption and therefore could play important roles in adsorption of FD [22,23]. e absorption peaks identified in the FTIR spectra of JH and SH are summarized in Table 1.
A broad hump and a sharp diffraction peak were observed in the XRD patterns of JH and SH (Figure 4), which indicated that the adsorbents consisted of combinations of amorphous and crystalline structures. e SEM images of the surface of JH and SH adsorbents are presented in Figures 5(a) and 5(b); JH contained deep pores ranging in size from 1.9 to 3.5 μm and the SH contained smaller pores ranging in size from 130 to 350 μm. is explains that the presence of these deep pores with the size of the micrometer greatly aids in adsorption of large quantities of FD molecules on the surface of adsorbents.

Effect of Initial FD Concentration on Adsorption.
In this study, the initial concentration of FD, which varied from 2 to 10 mg/L, affected the percentage of FD adsorbed. When the initial FD concentration increased, the percentage of adsorbed FD decreased ( Figure 6). e lowest percentage of adsorbed FD was associated with the highest initial concentration of FD. At high FD concentrations, the competitive dispersion of FD at the pores available on the adsorbent surface increased [17]; therefore, the pores were closed and the FD molecules were prevented from migrating into the deep pores of the absorbent. Consequently, adsorption occurred only at the surface of the adsorbent [24,25].

Effect of Adsorbent Dose on Adsorption.
e effect of the adsorbent dose on the adsorption of FD on JH and SH was studied.
e percentage of adsorbed FD increased as the adsorbent doses increased (Figure 7). is was attributed to the increased availability of exchange sites or the increase in the surface area that featured large number of adsorption sites [26].

Effect of Contact Time on Adsorption.
e adsorption equilibria were analyzed for 30 to 240 min. e FD adsorption rates of both adsorbents (JH and SH) increased in time (Figure 8). e percentage of adsorbed FD increased rapidly during the first 120 min of adsorbent-adsorbate contact, and then the process slowed down for both adsorbents. is occurred because the number of active adsorption sites decreased in time [9,24].

Effect of pH on Adsorption.
Adsorption experiments were carried out at various pH levels between 3 and 12 to evaluate the effect of the pH of the FD solution on the percentage of FD adsorbed on both JH and SH. e percentage of adsorbed FD decreased slightly as the pH was increased from 3 to 5; as the pH was further increased and exceeded 7, the percentage of adsorbed FD increased for both adsorbents (Figure 9). At pH values exceeding 8, the percentage of adsorbed FD decreased as the pH was further increased.
e pH of the aqueous solution is an important parameter, because it affects the ionization of dye molecules and surface charge of adsorbents [7]. e percentage of adsorbed FD decreased as the pH was increased from 6 to 12.
at could be attributed to the adsorption of OH − ions on the surface of the adsorbents causing the surface to become negatively charged. Moreover, International Journal of Analytical Chemistry     the functional groups of the adsorbents, such as the -OH and C�O groups, could occur between these groups and the anionic FD molecules. Consequently, the percentage of adsorbed FD decreased when the pH increased from 6 to 12 [27].

Effect of High Temperature on Adsorption.
Our experimental results indicated that when the temperature increased from 25 to 70°C, the percentage of FD adsorbed on JH increased and the percentage of FD adsorbed on SH decreased. All experiments were performed at pH of 8.5 and initial FD concentration of 10 mg/L ( Figure 10). e increase in the percentage of FD adsorbed on JH with the temperature could indicate that FD reacted with the functional groups on the surface of JH [28].
Conversely, the bonds between the FD molecules and functional groups on the surface of SH were physical bonds and could be easily broken at high temperature [29][30][31][32].

Adsorption Isotherm Models
To elucidate the adsorption mechanism of FD from aqueous solutions using JH and SH as adsorbents, we used the Langmuir and Freundlich adsorption isotherm models.
e Langmuir adsorption isotherm model assumes that adsorption occurs on the surface of the adsorbent and is a monolayer process [33] and could be described as follows [29]: where C e (mg/L) is the concentration of adsorbate at equilibrium, q e is the adsorption capacity at equilibrium, q m is the maximum adsorption capacity of the adsorbate, and K L is the adsorption equilibrium constant. e relationships between C e /q e and C e for the adsorption of FD on JH and SH were linear (Figure 11), and the slopes of the plots and correlation coefficients (R 2 ) are listed in Table 2. Experimental results revealed that the Langmuir adsorption isotherm is good with the JH adsorbent and did not fit with the SH adsorbent; moreover, the maximum theoretical  International Journal of Analytical Chemistry adsorption capacities of the adsorbents are greater than the experimental ones [8].
A dimensionless physicochemical constant was applied as separation factor (R L ), which indicated the favorable nature of the adsorption process, and was calculated as follows [30]: e separation factor R L for the adsorption of FD on JH ranged from 0 to 1, which indicated the adsorption was favorable (Table 2). Typically, R L indicates whether the isotherm type is irreversible (R L � 0), favorable (0 < R L < 1), linear (R L � 1), or unfavorable (R L > 1) [30].
e Freundlich adsorption isotherm model is an experimental equation and another form of Langmuir, which can be applied to the multilayer adsorption process [33] and could be described as follows [30]: where n is the Freundlich equilibrium coefficient. e plot of the relationship between log q e and log C e is presented in Figure 11, and the slope and R 2 values are listed in Table 2. e R 2 values indicated that the Freundlich model fitted the experimental results better than the Langmuir one. e magnitude n can be used to assess the suitability of the adsorption process. When n ranged from 1 to 2, the adsorption of FD on JH and SH was somewhat difficult [30]. e obtained results revealed that the adsorption of FD on the surface of JH and SH was a multilayer process.

Adsorption Kinetic Models
e pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion kinetic models were used to elucidate the adsorption mechanism of FD on JH and SH and to determine the rate dominant step of the adsorption process.
e pseudo-first-order model (PFO) could be described as follows [28]: where q e and q t represent the quantity of FD adsorbed at equilibrium and at the time, t, respectively, and K 1 is the rate constant of the pseudo-first-order reaction. e pseudo-second-order model (PSO) could described as follows [28]: where K 2 is rate constant of the pseudo-second-order reaction. e intraparticle diffusion (IPD) coefficient K ipd could be calculated using the following equation [28]: where C is a constant that provides information on the thickness of the boundary layer. e liquid film diffusion model (LFD) could described as follows [28]: where F � q t /q e , K lfd is the rate constant of the liquid film diffusion reaction, t is the time, and C is a constant related to the boundary layer. e linear plots of the four kinetic models for the adsorption of FD on JH and SH are presented in Figure 12. e parameters, constants, and R 2 values for the four kinetic models are listed in Table 3. e experimental q e values of FD adsorbed did not agree with the calculated ones, and therefore the pseudo-first-order model did not fit well for the experimental data [12,22]. However, the pseudo-secondorder model fits well the kinetic data [28]. Furthermore, the results also revealed that the adsorption process involved the  exchanging or sharing of electrons between FD and JH and SH, and this was the rate dominant step of the adsorption [27]. Further, the parameters of the intraparticle diffusion and the liquid film diffusion model, indicating that they are some of the factors determining the rate in the FD adsorption process (Table 3). e process consists of two steps or more, the first step being full diffusion on the adsorbent surface, the second step being the gradual diffusion of the particles into the pores, and the third step, which is the final equilibrium step. Figure 12 shows that the linear plots of dye adsorption did not pass through the original, indicating that FD adsorbed on the JH and SH was controlled by intraparticle diffusion and more than one mechanism is involved in adsorption. ese results also indicated that JH was a better adsorbent for FD than SH.

Thermodynamics of Adsorption
e thermodynamic parameters, ∆S°, ∆H°, and ∆G°, were calculated using the following equations [34]: where K eq is the equilibrium constant, q e (mg/g) is the quantity of FD adsorbed at equilibrium, C e (mg/L) is the concentration of FD in solution at equilibrium, R is the universal gas constant (8.314 mol/K), and T (K) is the absolute temperature of the solution. e Van't Hoff plots of ln K eq vs. 1/T are illustrated in Figure 13, and ∆S°, ∆H°, and ∆G°5   Table 4. e ∆H°and ∆S°values were determined from the slopes and intercepts of the plots. e positive ∆H°and ∆S°values for the adsorption of FD on JH demonstrated the endothermic nature of the adsorption process. Conversely, the ∆H°and ∆S°values for the adsorption of FD on SH indicated that the process was exothermic and unspontaneous of the adsorption process [27,34]. e negative ΔG°values for the adsorption of FD on JH indicated that the adsorption was feasible and spontaneous. In addition, ∆G°ranged from-20 to 0 kJ/mol which indicated that the predominating adsorption mechanism was physical in nature [30,35]. e positive ∆G°value for the adsorption of FD on SH indicated that the adsorption was unspontaneous.

Conclusion
In this study, the analysis of JH and SH, which were used as adsorbents, revealed the presence of -OH (alcohols and carboxylic acids), -CH-(alkanes), -N-H (amines), and C�O (carbonyl) groups in their structures, and these groups participated in the adsorption of FD. e XRD patterns of JH and SH revealed that their structures were combinations of amorphous and crystalline phases. e SEM images of the surfaces of JH and SH revealed the presence of many deep micropores, which could adsorb large quantities of FD. e Langmuir and Freundlich isotherm models were used to describe the adsorption of FD on JH and SH. e equilibrium data fitted well the Freundlich model. e percentage of FD adsorbed from aqueous solutions on JH was 85.5%, which was higher than the percentage adsorbed on SH (71.9%). e n and R L values ranged from 1 to 2 and from 0 to 1, respectively; moreover, q m was determined to be 2.91 and 2.565 mg/g for JH and SH, respectively. e kinetics of the adsorption FD on JH and SH were examined using the pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models and was concluded that the adsorption followed the pseudo-second-order kinetic model, which involved chemical adsorption and the internal diffusion. e positive ∆H°and ∆S°values and negative ΔG°value associated with the adsorption of FD on JH confirmed that the process was endothermic and spontaneous. Conversely, the negative ∆H°and ∆S°values and positive ΔG°value associated with the adsorption of FD on SH suggested that the process was exothermic and unspontaneous.
ese results also indicated that JH was better than adsorbent for FD than SH. Further, the adsorption of dyes from aqueous solutions requires more studies.

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this article.