Removal Efficiency of Textile Dyes from Aqueous Solutions Using Calcined Waste of Eggshells as Eco-friendly Adsorbent: Kinetic and Thermodynamic Studies

R. Slimani,a I. El Ouahabi,b S. Benkaddour,b H. Hiyane,b M. Essoufy,a Y. Achour,c S. El Antri,b S. Lazar,b,* and M. El Haddadc aLaboratory of Spectroscopy, Molecular Modeling, Materials, Nanomaterials, Water & Environment-CERNE2D, Faculty of Sciences, Mohammed V University in Rabat, BP 1014RP, Rabat, Morocco bLaboratory of Biochemistry, Environment &Agroalimentary URAC36, Faculty of Sciences and Techniques, Hassan II University of Casablanca, BP 146, 20650, Mohammedia, Morocco cLaboratory of Analytical & Molecular Chemistry, Faculty Poly-Disciplinary of Safi, University of Cadi Ayyad, BP4162, 46000 Safi, Morocco


Introduction
Dyes are widely used in different manufacturing industries, of which the textile industry is their largest consumer, and a large producer of dye-loaded wastewater. During the dyeing process, about 10-15 % of unfixed dye is lost in the water, which comes out as a colored effluent from the industries 1 . Additionally, dyes are thermally stable compounds that are light-resistant and non-biodegradable because of their complex molecular structures. Most dyes and their degradation products cause serious damage to the ecosystem and biological life due to their toxicity 2,3 ; therefore, it is necessary to remove dyes from the wastewater to protect the eco-system and human health. There are various studies on the physicochemical strategies for removal of dyes from aqueous systems; and among the available methods, adsorption is a well-known separation technique in terms of initial cost, simplicity, design, ease of separation, and ability to treat dyes in a more concentrated form 4 .
Activated carbon is the most preferred adsorbent for dye removal because of its excellent adsorption ability. However, an extensive usage of activated carbon is restricted because of its high cost. Alternative adsorbents are required to be of low cost when minimal processing is required, abundant in nature, or derived from an industry by-product 5 . Some of the potential alternative adsorbents studied by a number of researchers include Cerastoderma lamarcki shell 6 , watermelon seeds 7 , calcined cow leather 8 , animal bones 9 , mussel shells 10 , Ensissiliqua shell 11 , and conch shells 12 .
Rhodamine B is a highly water soluble, cationic dye of the xanthene class, and widely used in biological, analytical and optical sciences. It is reported that Rhodamine B is harmful to the environment 13,14 . Alizarin Red S is an anionic dye of the anthraquinon class, which is widely used in woven fabrics, wool, and cotton textiles 15,16 . Many re-searchers have been interested in the removal of these dyes from aqueous solutions; and in fact, Ra phiahookeri fruit's epicarp removed Rhodamine B from an aqueous solution with a maximum adsorption capacity of 666.67 mg g - 1 17 . Zamouche et al. reported the adsorption of Rhodamine B onto cedar cone by studying the effect of calcination and chemical activation 18 . Adsorption of Rhodamine B has also been studied on iron-pillared bentonite, and the maximum monolayer adsorption capacity of 98.62 mg g - 1 19 was attained. Mustard husk removed Alizarin Red S with a multilayer adsorption mechanism 20 , which was also achieved on olive stone by-product giving 16 mg g -1 as adsorption capacity 21 . Roosta et al. developed an Alizarin Red S adsorption process from aqueous solution onto gold nanoparticles by the application of ultrasound, giving a high adsorption capacity of 123.4 mg g - 1 22 .
The aim of the present study was to evaluate and investigate the performance of a newly developed adsorbent, i.e., eggshell powder. This adsorbent was chosen according to its daily presence in domestic waste worldwide. It is an abundant waste, and currently not used in any way. It can be thermally activated for the rapid removal and fast adsorption of Rhodamine B and Alizarin Red S from aqueous solutions with initial dye concentration (20 mg L -1 -50 mg L -1 ), which exceeds the actual discharge limit rates. The developed adsorbent was well characterized using various analytical techniques, such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). The impact of several influential parameters, such as solution pH, initial dye concentration, contact time, adsorbent dosage, and temperature were well investigated and optimized using batch adsorption study. Moreover, kinetics and thermodynamics of adsorption of Rhodamine B and Alizarin Red S were studied and discussed.

Chemicals
All chemicals used were of analytical reagent grade and used without further purification. All solutions were prepared using double distilled water. Rhodamine B and Alizarin Red S dyes were supplied by Sigma Aldrich. Some of the data on both dyes are listed in Table 1. The stock solutions of Rhodamine B and Alizarin Red S dyes (1 g L -1 ) were prepared by dissolving the required amount of dyes in distilled water.

Preparation of calcined eggshell adsorbent
Preparation of the obtained calcined eggshell (CES) absorbent was achieved by collecting eggshell waste from a restaurant. Firstly, the eggshells were washed several times with tap water and distilled water, and then left in open air for several hours, and dried for another 12 hours at 100 °C. The product was then crushed into small pieces, powdered, and calcined at 900 °C for 2 hours. The resulting material was washed with distilled water several times and dried overnight at 105 °C, In addition, the residue was chopped and the fraction of size range 75 -100 µm was collected. The powder was then dried overnight at 105 °C, and calcined at a heating rate of 2 °C min -1 to 400 °C, and maintained at this temperature for 4 hours. The powder was then cooled and stored in a dry place.

Instrumentation
The measurements of the dye concentration were performed using a molecular absorption UV-Vis spectrophotometer (BioMate6, England) using a quartz cuvette of 1.0 cm at the wavelength attributed to the maximum absorbance of each dye (see Table  1). Chemical composition of CES adsorbent was studied by X-ray fluorescence (WD-XRF). The Fourier-transform infrared spectra (FTIR) were collected by a Nicolet 5700 FT-IR spectrometer on samples prepared as KBr pellets, in a spectral range of 400 -4000 cm -1 . The polycrystalline sample of CES adsorbent was finely ground in an agate mortar with a pestle, and filled into a 0.5-mm borosilicate capillary prior to being mounted and aligned on an Empyrean Panalytical powder diffractometer, using Cu K α radiation (λ = 1.54056 Å). Three repeated measurements were collected at ambient temperature (22 °C) in the 10° < 2q < 60° range with a step size of 0.01°. Scanning electron microscopy (SEM) images were obtained with HITA-CHI-S4100 apparatus operated at 20 kV.
Adsorption-desorption isotherms of nitrogen at -196 °C were measured with an automatic adsorption device (NOVA-1000 Gas Sorption analyzer) in order to determine surface area and total pore volume. The surface area of the obtained CES adsorbent was determined by Brunauer-Emmett-Teller (BET) method.
The pH was measured by a CrisonBasic 20+ pH-meter. The pH drift method was used to measure the pH zero point charge (pH ZPC ) of CES adsorbent as described in reference 23 .

Adsorption experiments
Adsorption experiments were conducted with 50 mL of dye solution with a certain initial concentration in a beaker of 100 mL. Batch adsorption of Rhodamine B and Alizarin Red S dyes onto CES adsorbent was investigated in aqueous solutions by varying experimental conditions, such as adsorbent dosage (50 mg -200 mg), initial dye concentration (20 mg L -1 -50 mg L -1 ), pH (2 -12), and temperature (303 K -323 K). Each experiment was repeated three times to find the average.
The removal (R) percentage and the amount adsorbed q e (mg g -1 ) of each dye was calculated using the following equations: where γ 0 and γ e are the initial and the equilibrium concentrations of dye solutions (mg L -1 ), respec-tively, whereas V and m are the volume of the solution (L) and the amount of used adsorbent (g), respectively.

Desorption and regeneration experiments
The reuse of the CES was considered in consecutive sorption-desorption cycles. For the desorption experiments, previously adsorbed dyes on CES were transferred to a flask containing 100 mL of desorbing agent, such as HCl (0.1 M), NaOH (0.1 M), and distilled water. The mixture was shaken at 100 rpm using a shaker (Innova 2300, New Brunswick, USA) at room temperature for 1 hour. In the regeneration experiment, the dye-loaded adsorbent that was eluted by the desorbing agents was thoroughly washed three times with deionized water to remove any traces of desorbing agent, and then mixed again with dye-loaded wastewater for the next adsorption cycle. This procedure was used for three consecutive cycles.

Characterization of CES adsorbent
In order to study the surface of CES adsorbent, elemental analysis of obtained support was achieved by chemical composition using XRF. The results are given in Table 2.

Effect of contact time and concentration dye on the removal efficiency onto CES adsorbent
The effect of contact time and initial dye concentration on adsorption capacity at different initial concentrations of Rhodamine B and Alizarin Red S is shown in Fig. 4. As indicated in this figure, the adsorption capacity increased with contact time, and reached equilibrium in 2 hours; however, the increase was relatively higher during the first 40 minutes. Rapid increase in adsorption during the initial stage may presumably be due to the availability of vacant active sites on the surface of each dye. The slow increase at the later stages was due to the diffusion of dyes into the pores of CES adsorbent because of occupation of external sites. The adsorption capacity increased with dye concentration at any dye-adsorbent contact time. The values of adsorption capacity for a contact time of 180 minutes are shown in Fig. 5 against the equilibrium dye concentration for different initial dye concentrations. Furthermore, Fig. 5 indicates that the equilibrium adsorption capacity increases with increasing equilibrium dye concentration. Effect of pH on the removal efficiency of both dyes onto CES adsorbent Fig. 6 illustrates results of the experiments. The removal performance of Rhodamine B increased from 18 % to 91 % when pH varied from 2 to 10. This increase can be explained by the fact that pH ZPC is equal to 10.2 (see Fig. 7), suggesting that the surface of CES adsorbent is positively charged under pH ZPC and negatively charged above pH ZPC . This result can be explained by the fact that positive charges on the surface of CES adsorbent are completely neutralized by increasing pH (basic), which makes the surface negatively charged and subsequently promotes the adsorption of Rhodamine B as a cationic dye onto CES adsorbent. Such behavior has been already described with cationic dye in a previous research paper 24 .
An opposite result was obtained with Alizarin Red S: its removal percentage decreased from 87 % to 18 % when pH increased from 2 to 10. This behavior is explained by having the surface of CES adsorbent charged positively, allowing a significant retention of a negatively charged Alizarin Red S under acidic conditions. Above pH ZPC , the capacity adsorption will be reduced since there will be a repulsion of charge between CES adsorbent and Alizarin Red S.
For the following studies, we chose a basic pH above 10 for the adsorption of Rhodamine B, and acidic pH around 2 for the adsorption of Alizarin Red S.
Effect of adsorbent dosage on the dye removal onto CES adsorbent Fig. 8 shows the variation of removal percentage and adsorption capacity compared to CES adsorbent dosage. It has been observed that R % increased for each dye (Rhodamine B or Alizarin Red S) when the CES adsorbent dosage increased from 50 mg to 200 mg. The adsorption saturation was reached at CES adsorbent dosage above 200 mg, and this amount of adsorbent was chosen for further studies. The increase in R % is due to increase in surface area and the availability of more binding sites for adsorption. The decrease in adsorption capacity with increasing adsorbent dosage might be due to interaction of adsorbent particles, like aggregation or agglomeration, which resulted with a decrease in the specific surface area.

Effect of CES adsorbent particle size on dye removal
Adsorption of Rhodamine B and Alizarin Red S onto CES adsorbent was studied using three different size fractions of adsorbent particles (75 -100 µm, 100 -250 µm, and 250 -500 µm). Fig. 9 shows the effect of particle size on the removal efficiency of Rhodamine and Alizarin Red S. As illustrated in Fig. 9, whatever the nature of the dye, the removal efficiency decreased with increasing particle size. For Rhodamine B, removal efficiency decreased from 93 % to 75 % when increasing particle size range. Furthermore, for Alizarin Red S, it decreased from 88 % to 72 %. This is because smaller particle size provides large specific surface area for adsorption, which results in higher dye removal. In this case, we chose the range 75 -100 µm particle size of CES adsorbent for further study.

Adsorption kinetics study
The adsorption process of Rhodamine B and Alizarin Red S onto CES adsorbent was tested against the pseudo-first-order, pseudo-second-order, and intra-particle diffusion kinetic model.
The pseudo-first-order kinetic model was widely used to predict the sorption kinetics 25,26 and was expressed as: where, q e and q t (mg g -1 ) are the amounts adsorbed at equilibrium and at any time, respectively, t (min) is time, and k 1 (min -1 ) is the adsorption rate constant. The linear plot of log(q e -q t ) versus t gives a slope of k 1 and intercept of logq e . The values of k 1 and R 2 obtained from the plots for adsorption of Rhodamine B and Alizarin Red S dyes onto CES adsorbent are reported in Table 3. It was observed that the R 2 values obtained for the pseudo-first-order model showed no consistent trend. However, the experimental q e values did not match the calculated values obtained from the linear plots. This showed that the adsorption of Rhodamine B and Alizarin Red S dyes onto the adsorbent did not follow a pseudo-first-order kinetic model.
The pseudo-second-order equation established by Ho and McKay 27 predicts the behavior over the whole range of adsorption, and is expressed as follows: The adsorption rate constant is k 2 (g mg -1 min -1 ), and the linear plot of t/q t vs t gave 1/q e as the slope, and 2 2 e 1 k q as the intercept. As illustrated in Table 3, all the R 2 values obtained from the pseudo-second-order model (see Fig. 10) were close to one, and the agreement between the calculated and experimental q e values for Rhodamine B and Alizarin Red S dyes was quite good. This showed that the model could be applied for the entire adsorption process, and confirmed that the sorption of Rhodamine B and Alizarin Red S onto CES adsorbent followed the pseudo-second-order kinetic model. The kinetics of diffusion was also tested against the intraparticle diffusion model 28 using the following equation: where, k pi (mg g -1 min -1/2 ), the rate parameter of stage i, is obtained from the slope (straight line) of q t versus t 1/2 . C i represents the boundary layer effect, meaning that the larger the intercept, the greater the

Ta b l e 3 -Kinetic data for the adsorption of Rhodamine B and Alizarin Red S onto CES adsorbent
Dye γ 0 (mg L -1 ) q e,exp (mg g -1 )

Ta b l e 4 -Data of intraparticle diffusion model for the adsorption of dye onto CES adsorbent
Dye γ 0 (mg L -1 )  contribution of the surface sorption in the rate-controlling step. The linearity of q t versus t 1/2 confirms that the intraparticle diffusion is occurring, and the rate-limiting process is only due to the intraparticle diffusion if the plot passes through origin. The values of k pi , C i and R 2 obtained for the three regions from the plots are given in Table 4, which shows that the k p values for the three regions increase with increase in dye initial concentration. The result revealed that an increase in adsorbate concentration resulted in an increase in the driving force, which led to an increase in every dye diffusion rate. It was observed that the values of intercept increase are correlated to the increase in initial dye concentration from 20 to 50 mg L -1 , indicating the increase in the thickness of the boundary layer.

Adsorption isotherms study
In order to optimize the adsorption process, it is essential to describe the quantity adsorbed by a given amount of adsorbent. The equilibrium data was analyzed using Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms.
If the sorption takes place at specific homogeneous sites within the adsorbent, the Langmuir isotherm 29  where, γ e (mg L -1 ) is the equilibrium concentration of the adsorbent, and q e (mg g -1 ) is the amount adsorbed at equilibrium, C m (mg g -1 ) and K L (L mg -1 ) are constants related to adsorption capacity and energy of adsorption, respectively. The value of C m and K L can be determined from the slope and the intercept of the γ e /q e vs γ e linear plot. Langmuir constants K L and C m are calculated from plots and are given in Table 5.
For Rhodamine B and Alizarin Red S dyes, values of C m decreases with increase in temperature, suggesting the exothermic nature of adsorption processes.
The essential characteristic of the Langmuir isotherm can be expressed in terms of dimensionless equilibrium parameter 30 such as separation factor (R L ) used in the following equation: where γ 0 (mg L -1 ) is the initial dye concentration. The value of the parameter R L indicates the nature of the adsorption processes. R L > 1 for unfavorable adsorption, R L = 1 for linear adsorption, 0 < R L < 1 for favorable adsorption, and R L = 0 for favorable and irreversible adsorption. Fig. 11 represents all calculated values of R L versus temperature (303, 313, and 323 K) and Rhodamine B and Alizarin Red S dye concentrations (20,30,40, and 50 mg L -1 ). Thus, all values of R L were found between 0 and 1, suggesting that the adsorption of Rhodamine B and Alizarin Red S dyes onto CES adsorbent was favorable.
Freundlich isotherm is derived by assuming a heterogeneous surface with a non-uniform distribution of adsorption enthalpy over the surface. The linear form of Freundlich equation can be expressed as:

Ta b l e 5 -Isotherms data for the adsorption of Rhodamine B and Alizarin Red S dyes onto CES adsorbent at different temperatures
where, q e is the amount adsorbed (mg g -1 ), γ e is the equilibrium concentration of the adsorbate (mg L -1 ). K F (mg g -1 ) (L g -1 ) 1/n and n, the Freundlich constants, are related to adsorption capacity and adsorption intensity, respectively. Freundlich constant n is a measure of the deviation from linearity of the adsorption, and if a value for n is equal to unity, the adsorption is linear. Value of n below unity implies that adsorption process is chemical, but if a value for n is above unity, adsorption is a favorable physical process 31 . Values of K F and n, obtained from the slope and intercept of plot logq e vs. logγ e , are shown in Table  5. Values of K F decreased with the increase in temperature, showing the exothermic nature of adsorption process.
Temkin isotherm is derived by assuming that the heat of the adsorption of all molecules in the layer decreases linearly with coverage due to adsorbent-adsorbate interactions, and that the adsorption is characterized by uniform distribution of the binding energies, up to some maximum binding energy 32 where, γ e (mg L -1 ) is the concentration of the adsorbate at equilibrium, q e (mg g -1 ) is the amount of adsorbate adsorbed at equilibrium. The constant B can be expressed as follows: where, T is the temperature (K), and R is the ideal gas constant (8.314 J mol -1 K -1 ), K T and b T are the constants. A linear plot of q e versus lnγ e enables the determination of constants K T and B. The constant b T (kJ mol -1 ) is related to the adsorption enthalpy, and K T is the equilibrium binding constant (L mg -1 ) corresponding to the maximum binding energy. The experiments on the adsorption of Rhodamine B and Alizarin Red S onto CES allowed for finding all the characteristic values of K T , b T and B, as shown in Table 5. In light of the results presented, low values of b T for both dyes pointed to very weak interactions between Rhodamine B and Alizarin Red S and CES adsorbent, meaning that the adsorption of the studied dyes occurredby a physical process. The Dubinin-Radushkevich (DR) isotherm is more general than the Langmuir isotherm because it does not consider a homogeneous surface or constant adsorption potential. It is used to distinguish between physical and chemical adsorption 33 . The linear equation for this isotherm is given as follows: where: where, q S (mg g -1 ) is the maximum amount adsorbed and is considered as DR constant, B is a constant giving the mean free energy E of the sorption per molecule of sorbate, and it is calculated by the following relation: 1 Values of E less than 8 kJ mol -1 are characteristic for physisorption, and values of E between 8 and 16 kJ mol -1 are found for chemisorption. As maybe seen in Table 5, all the calculated values of E are below 8 kJ mol -1 , so it can be concluded that the adsorption of both dyes occurs by physical processes. However, the relatively low values of R 2 point to the failure of DR isotherm to fairly describe the investigated adsorption phenomena.

Thermodynamics of adsorption
The values of the standard Gibbs free energy (ΔG 0 ) of adsorption for each temperature were calculated from the following equation: where, R is the universal gas constant (8.314 J mol -1 K -1 ), T is temperature, and K C is the equilibrium constant that was calculated at each temperature using the following expression: where, γ S and γ e are the equilibrium concentrations on the adsorbent and the aqueous phase, respectively. The enthalpy change (ΔH 0 ) and entropy change (ΔS 0 ) were calculated from the slope and intercept of the lnK C versus 1/T based on the Van't Hoff equation: Table 6 depicts the thermodynamic data at temperatures and initial concentrations for the adsorptive removal of Rhodamine B and Alizarin Red S from aqueous solutions onto CES adsorbent. From Table 6, the negative values of ΔG 0 indicate a spontaneous and favorable adsorption process at all studied temperatures and initial concentrations of both dyes Rhodamine B and Alizarin Red S, suggesting that the system required no energy input from outside. For a given concentration of Rhodamine B or Alizarin Red S, ΔG 0 values increased with increasing temperature. Furthermore, values of ΔG 0 increased slightly as the initial Rhodamine B or Alizarin Red S concentration in-creased from 20 to 50 mg L -1 , indicating a more favorable adsorption of both dyes at lower dye concentration. The low negative values of ΔG 0 ranging from -20 to 0 kJ mol -1 suggest that the dominant adsorption mechanism was physisorption for the removal of Rhodamine B and Alizarin Red S onto CES adsorbent from aqueous solutions. As for the Van't Hoff plot, shown in Fig. 12 coefficient (R 2 ) of the plots of lnK C versus 1/T were close to one for all initial Rhodamine B and Alizarin Red S concentrations, indicating that the estimated values of ΔH 0 and ΔS 0 were reliable. As maybe seen in Table 6, the negative values of ΔH 0 are due to the exothermic nature of the adsorption of Rhodamine B and Alizarin Red S onto CES adsorbent. Please note that similar results with negative ΔH 0 values were obtained by other studies, and were interpreted in terms of exothermic adsorption for anionic and cationic dyes 34,35 . The magnitude of enthalpy change can be used to classify the type of adsorbent-adsorbate interaction, and according to ΔH 0 values for Rhodamine B and Alizarin Red S obtained in this study, the adsorption of both dyes onto CES adsorbent was due to physical adsorption, suggesting weak interactions between CES adsorbent and chosen dye.
The negative values of ΔS 0 were very low, and pointed to increased ordering at the solid-liquid interface during the adsorption of dye onto CES adsorbent (see Table 6); the extent or ordering was larger for higher dye concentrations. The results also suggested that the enthalpy change ΔH 0 contributed more to the negative values of ΔG 0 than the corresponding entropy change ΔS 0 . This suggested that the adsorption of dyes onto CES adsorbent was an enthalpy-controlled process.

Desorption, adsorbent regeneration, and adsorbent comparison
In order to regenerate the CES adsorbent for further use, it is important to study the desorption process as well. In this case, two different desorption agents were selected: NaOH 0.1 M and HCl 0.1 M. The results of this study are shown in Fig. 13. The results show that NaOH 0.1 M is a better de-sorption agent for Rhodamine B (giving 99 % desorption), and HCl 0.1 M is a better desorption agent for Alizarin Red S (giving desorption near 98 %). As shown in Fig. 14, the removal efficiency % decreased for each new cycle after regeneration, and after a sequence of six cycles, the Rhodamine B desorption efficiency decreased from 95 % to 85 %, and that of Alizarin Red S from 94 % to 82 %. The overall loss in removal efficiency for Rhodamine B was below10 % and for Alizarin Red S below 13 %. Therefore, CES adsorbent could be used for five cycles of adsorption of Rhodamine B and Alizarin Red S.   Table 7 summarizes the adsorbents applied for the removal of Rhodamine B and Alizarin Red S from aqueous solution. According to the results of this study, CES adsorbent can be used for the removal of dies with satisfactory results in comparison to adsorbents studied earlier.

Conclusion
Removal of Rhodamine and Alizarin Red S dyes was successfully achieved using treated eggshell waste. Kinetic study showed that adsorption of the two dyes followed pseudo-second-order kinetics. Furthermore, adsorption was best described by Langmuir isotherm. In addition, thermodynamic study showed that adsorption of Rhodamine B and Alizarin Red S was feasible, spontaneous, and exothermic in nature.
As a result, it may be concluded that CES adsorbent is an effective adsorbent for the removal of Rhodamine B and Alizarin Red S from aqueous solution. Moreover, CES adsorbent is cheap and widely available. Taking into consideration that it is a waste product, its usage can resolve some waste management issues as well.