Valorization of Date Pits as an Effective Biosorbent for Remazol Brilliant Blue Adsorption from Aqueous Solution

In this work, the adsorption of Remazol Brilliant Blue (RBB) over raw date pits (RDPs) as an inexpensive adsorbent has been examined. In addition, all parameters such as the adsorbent mass, solution pH, RDP particle size, RBB initial concentration, and temperature on the adsorption of RBB inﬂuencing the adsorption procedure were studied to provide fundamental information of the adsorption equilibrium. The characterization of RDP material is investigated by X-ray diﬀraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR). Based on the calculation, the kinetic rate of the adsorption was well modeled by pseudo-second-order and Langmuir isotherm. Surface functional groups of RDP have substantially been inﬂuenced by the adsorption characteristics of RBB. The capacity of the adsorption has achieved 105mg/g and a removal eﬃciency of 90.4% at 1.5g/L RDP mass, 40mg/L initial dye concentration, pH 2, temperature of 328K, 40 µ m particle size, and contact time of 50min. The capacity of the adsorption could reach 198mg/g by increasing the ionic strength of RBB solution. Desorption tests showed that RDP adsorbent has the disadvantage of losing eﬃciency while reusing for many cycles. However, it still abundant and inexpensive. Therefore, RDP can be used as a potential low-cost bioabsorbent for the elimination of RBB from wastewater.


Introduction
Pollution refers to the deterioration of the environment by unnatural materials, causing the disappearance of several species of animals, plants, as well as the appearance of new phenomena, which has harmful effects on human health, including global warming [1][2][3]. e effects of this pollution affect not only the air and the soil, but also a large part of the water. Dyes are used in many industrial sectors, such as textiles, paper, leather, food, and cosmetic industries [4].Moreover, these industries consume huge quantities of water. Once those dyes are released, they cause significant damage to human health such as the mutagenic and carcinogenic effects [5][6][7][8] and changes in the aquatic environment [9] when they are discharged into the environment without or with insufficient treatment [10][11][12]. To reduce the impact of this pollution, several techniques have been developed and tested in the treatment of effluents loaded with dyes, namely, biological process [13], coagulation/flocculation [14], photodegradation [15][16][17][18], ozonation [19,20], oxidation [21,22], and membrane separation [23][24][25]. e adsorption technique is considered as one of the most effective methods that has been successfully adopted for removing dyes from wastewater [26][27][28][29][30][31] due to its low cost and availability. Allowing easy removal of dyes from aqueous solutions over different materials and on activated carbon in particular [32] has always been the subject of much work [33,34]. Many adsorbents have been investigated for the removal of dyes in recent years [35] such as clay [36,37], layered double hydroxides [38], metal oxides [39], goethite modified natural [40], and sediments [41,42].
e activated carbon has a high cost. Hence, the need to look for cheaper, effective, and natural available adsorbent is therefore interesting [43]. Bioadsorbent materials have been proposed as alternative adsorbents for dyes. Especially, RDPs have received considerable attention for its properties, such as low cost, natural availability, and no threat to the environment. e fruit of the date palm is composed of a fleshy pericarp and seed. Pits of date palm (seed) are a waste product of many date fruit-processing plants producing pitted dates, date powders, date syrup, date juice, chocolatecoated dates, and date confectionery [44]. In addition, the RDP are very widely distributed and abundant, which make them the promising environmental adsorbents that can be used in industrial processes [3]. Javid et al. have studied the removal of bisphenol A and nonylphenol from aqueous solutions using carbonized date pits modified with ZnO nanoparticles, and they found maximum removal efficiency under optimal conditions was 95% [45,46]. However, adsorption of RBB onto the RDP was not fully investigated [43]. erefore, the objective of this work is to investigate the physics and chemical properties of RDP bioadsorbent using multiple methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared spectroscopy (FTIR) and then to evaluate the effectiveness of using RDP as natural eco-friendly and low-cost bioadsorbent for the removal of RBB in aqueous media. Moreover, various parameters influencing the adsorption procedure of RBB adsorption on the RDP bioadsorbent, such as the adsorbent mass, solution pH, RDP particle size, and RBB initial concentration, were studied. On the other hand, the ionic strength effect using BaCl 2 on the adsorption equilibrium is important to highlight as well.

Preparation of RDP.
Moroccan dates were shelled, and the pits were collected and sorted in order to remove the impurities and then dried at a temperature of 110°C in the oven for 24 hours. en, the bioadsorbent was ground in a grinder and sieved in order to obtain particles of the same size with a diameter of 40, 63, 125, and 200 µm. RDP contains an approximate percentage of hemicellulose, lignin, cellulose, and carbohydrates [3].

Adsorbate.
e dye considered in this study is Remazol Brilliant Blue (RBB) analytical grade purchased for Sigma-Aldrich, it has the chemical formula C 22 H 16 N 2 Na 2 O 11 S 3 , and its maximum absorption band is located at the wavelength of 590 nm. e main problems associated with RBB dye in textile wastewaters are resistant to biodegradation, highly visible due to its bright color, even in very low concentration of dye (<1 mg/L) in the effluent, and very toxic difficult to remove by traditional methods.

Adsorption Study.
In this study, a stock solution was prepared from the RBB dye. e adsorption study was carried out by using 1 g of RDP mixed with a solution of RBB at room temperature under continuous stirring in a batch system. In order to investigate the kinetic adsorption, several samples were collected each 5 min to measure its concentration using UV-visible spectrophotometer (VR-2000) at a wavelength of 590 nm. Nevertheless, before the measure, the suspension was centrifuged to separate the natural adsorbent from the RBB liquid. During the adsorption experiment, HCl (0.5 M) and NaOH (0.5 M) from Sigma-Aldrich were used to adjust the pH solution. e RBB removal was calculated using following formula [43,47]: where C 0 and C t are the concentration of RBB at t � 0 and at t≠0, respectively. e adsorption capacity of RDP for RBB removal was obtained by applying the following equation [48]: where q e (mg/g) is the adsorption capacity at equilibrium, C 0 (mg/L) is the initial concentration of RBB, C e (mg/L) is the equilibrium concentration of RBB, V (L) is the RBB solution volume, and m (g) is the RDP mass.

Characterization
Techniques. e X-ray diffraction (X′ PERT PRO) equipped with a detector operating at 40 kV and 30 mA with Cu Kα radiation (λ �1.540598Å), infrared spectroscopy (VERTEX 70), and scanning electron microscopy (QUANTA 200) were used to identify the composition and the morphology of adsorbents materials to explore the chemical composition of RDP.

X-Ray Diffraction (XRD).
e X-ray pattern of RDP data is given in Figure 1. It can be observed that diffractogram of bioadsorbent RDP does not exhibit a horizontal basic line and displayed the presence of little diffraction peaks. e broad diffraction peak located between 20°and 25°could be ascribed to carbon species according to the native cellulose (C 6 H 12 O 6 ) and to xylane dehydrate (C 10 H 12 O 9 ·2H 2 O) [49].On the contrary, the other few small diffraction peaks may be attributed to the presence of a small amount of crystalline matter. erefore, this result indicated that the major part of the matter is amorphous.

Scanning Electron Microscopy (SEM)
. SEM analysis is made on RDP before adsorption (Figures 2(a) and 2(b)). e material has a smooth porous surface, indicating a good possibility of trapping RBB adsorption on the adsorbent surface. On the contrary, after the RBB is adsorbed into RDP biomaterials, the SEM observation shows the surface of RDP charged with RBB displaying a rough and corroded surface due to the coverage of the pores by the RBB dye molecules adsorbed in Figures 2(c) and 2(d). is result indicated that the raw date pits (RDPs) could be an efficient bioadsorbent to remove hazardous dyes in the wastewater.

Fourier-Transform Infrared Spectroscopy.
e RDP infrared spectrum studied in this work ( Journal of Chemistry atom, hydrogen, and oxygen [50][51][52]. After adsorption of RBB ( Figure 3), the functional groups present on the surfaces of RDP show band shifting for possible involvement of hydroxyl groups around the broad peak at 3400 cm −1 . e broad peak shifted to 3411 cm −1 . e initial peak at 2922 cm −1 was shifted to 2928 cm −1 and showed an alkane group was bonded to C-H stretch.
e strong band at 1622 cm −1 was shifted and corresponding to the amine group with N-H bond. erefore, the diminished peaks showed that all the functional groups are completely involved in the adsorption process of RBB over RDP [53][54][55].

Initial Solution pH Effect.
e pH definitely affects the adsorption of the dye. In order to determine the adsorption behavior of the RBB dye under different pH values (from 2.3 to 9.03), a series of adsorption experiments were carried out using 40 mg/L of RBB, 1 g/L RDP, particle size of 63 µm, at room temperature, and stirring at 250 rpm. Figure 4 shows that there is a variation in the RBB removal as a function of pH. Accordingly, when the pH raises from 2 to 9, the adsorption removal decreases from 86.5 to 64.8%. is is due to the neutralization of the negative charge on the surface of the adsorbents by the charged dye molecule [56]. An increased diffusion process facilitates the fixation of the dye on the active sites of the adsorbents [57]. Figure 5 shows that the pH pzc of raw RDP is 6.01 [58].

Effect of RDP Mass.
is study makes it possible to evaluate the influence of the adsorbent mass, in order to determine the optimal mass, which coincides with a better dispersion of the adsorbent particles (RDPs). Figure 6(a) below represents the variation in the adsorption capacity as a function of time and of the adsorbent mass, which varies from 0.5 to 3 g using 40 mg/L of RBB, particle size of 63 µm, pH 4, at room temperature, and stirring at 250 rpm. Figure 6(b) shows an increase in the removal with the increase in RDP mass from 63% to 82.1% when the mass of adsorbent raises from 0.5 to 3 g. Conversely, there is a decrease in the adsorption capacity from 104.2 to 58 mg/g. e increase or the decrease in the first 30 min was fast and then followed by the flat curve proving the saturation of the adsorbent. e crossing point of the removal and the adsorption capacity correspond to the optimal mass of 1.5 g (Figure 6(b)). ese results can be explained by an increase in the active sites when the masses are large. Consequently, the probability of contact between the RBB molecule and the site of the adsorbent support also increases [59].

Effect of Initial RBB Concentration.
is study makes it possible to reach the maximum values of adsorption capacity of RBB, which represents the saturation of all the    active sites available on the surface of the adsorbent. e effect of the initial RBB concentration was studied at different initial RBB concentrations varying between 10 and 60 mg/L using 1 g/L RDP, particle size of 63 µm, and solution pH 4, at room temperature and stirring at 250 rpm. According to Figure 7(a), there is a fairly rapid increase in the adsorption capacity in the area of high concentrations. e increase or the decrease in the first 30 min was fast and then followed by the flat curve proving the saturation of the adsorbent. is absorption capacity continues to decrease with the decrease in the RBB initial concentration [60,61]. In summary, the adsorption capacity of RBB on the adsorbent increases from 66.9 to 105.6 mg/g when the initial concentration of RBB increases from 10 mg/L to 60 mg/L. ese results could be explained by the existence of strong interactions between the RDP surface and the RBB. e saturation appears when the active sites are totally occupied on the adsorbent surface [62]. Plotting the adsorption capacity, and the removal as a function of the equilibrium concentrations, shows an intersection point of two curves, which corresponds to the optimal concentration which is 40 mg/L, as shown in Figure 7(b).

Effect of the Particle Size.
In order to study the effect of RDP particles sizes, a series of experiments were performed with different particles sizes from 40 to 200 µm using 40 mg/ L of RBB, 1 g/L RDP, pH 4, at room temperature, and stirring at 250 rpm. Figure 8 illustrates that decreasing particles size enhanced the adsorption capacity: the 40 µm particle size has the highest RBB removal (95%). Others mesh presented lower removal between 72% and 85.6%. Although the 200 µm size showed a slow adsorption about 49.64% at 60 min, this evolution could be explained by the link between the effective surface area of RDP particles and the adsorption efficiency in which the small particles have a large surface area exposed to adsorption and hence high adsorption [63].

Effect of Temperature.
e adsorption removal of RBB on the RDP adsorbent increases from 82.21 to 94% when the temperature rises from 298 K to 328 K using 40 mg/L of RBB, 1 g/L RDP, particle size of 63 µm, pH 4, and stirring at 250 rpm ( Figure 9). is small increase in adsorption removal indicates that the adsorption process is endothermic [64]: the system at low temperatures requires a high energy to reach equilibrium although this system at high temperatures requires less energy to reach equilibrium. e effect of temperature on the removal is in agreement with the results found by the use of a biomaterial based on RDP [60]. e slight increase in the removal as a function of temperature can be explained as follows: (i) the increase in the active sites on the RDP surface; (ii) the increasing temperature increases the mobility of RBB, inducing a swelling effect in the internal structure of RDP, which facilitated the penetration of RBB further [56].

Adsorption of RBB over RDP under Optimum
Conditions.
e adsorption of the RBB dye solution was tested by applying the optimal conditions which are RDP mass 1.5 g/L, RBB concentration 40 mg/L, particle size of 40 µm, temperature 328 K, and the pH 2. Figure 10 illustrates the evolution of the adsorption capacity of RBB dye using raw RDP. e adsorption removal achieved very important 100% during 50 minutes with 115.4 mg/g as adsorption capacity.

Isotherms Adsorption.
For the study of adsorption isotherms, the Langmuir and Freundlich models were examined and applied to describe the adsorption process of our experimental results (Figure 11(a)). e Langmuir isotherm is one of the models which describes a monolayer adsorption. It assumes a homogeneous adsorption surface with binding sites having equal energies. e linear form of the Langmuir isotherm can be expressed as follows [65]: where K L (L/mg) is the Langmuir constant, Q max (mg/g) represents the maximum adsorption capacity under experimental conditions, and Q max and K L are determined from the plot of C e /qe as a function of C e . From the correlation factor values shown in Table 1, we conclude that the adsorption of RBB by the RDP is well represented by the Langmuir model, with a maximum adsorption capacity of 107.52 mg/g, that is to say the mechanism applied corresponds to a monolayer adsorption which  Journal of Chemistry involves identical, independent, and limited adsorption sites [66]. During the study of the Freundlich isotherm ( Figure 11(b)), the logarithmic equation used is as follows [65]: By carrying Log (q e ) as a function of C e , we obtain a line of slope 1/n and of ordinate at the origin Log (K F ), which makes it possible to determine the constant K F and the heterogeneity factor (n).
e Dubinin-Radushkevich model ( Figure 12) does not assume a homogeneous surface or constant adsorption potential, like the Langmuir model. His theory of filling the volume of micropores is based on the fact that the adsorption potential is variable and that the free enthalpy of adsorption is related to the degree of filling of the pores [67,68]. e Dubinin-Radushkevich isotherm is given by the following equation [65]:  where q mDR is the RDP adsorption capacity at equilibrium (mg/g), K DR is the Dubinin-Radushkevich constant (mol 2 / kJ 2 ), and ε is the Polanyi potential (J/mol). According to the values of R 2 (Table 2), the RDP is well represented by this model so it can be said that the adsorbent support has an average energy of adsorption less than 8 kJ/mol, which indicates that physisorption is the majority.

Kinetic Models.
e kinetics of the pseudo-first-order model and the pseudo-second-order defined, respectively, by the following equations: If the Lagergren relation is verified, by carrying Ln (q e − q t ) as a function of time (Figure 13(a)), we must obtain a line of slope k 1 . In addition, plotting t/q t as a function of time (Figure 13(b)), we must obtain a line with slope 1/q e and ordinate at the origin equal to 1/k 2 q e 2 .
It is clearly observed that the equation of the pseudofirst-order model is not linear with a correlation coefficient R 2 very lower (Table 3) so that the experimental absorption capacity is very far to that calculated by this model. So, we can deduce that the kinetic of adsorption does not follow the pseudo-first-order model [69] (Figure 13(a)). However, it can be seen from the results obtained (Figure 13(b) and Table 4), and we note that the variation in t/q t as a function of time is very linear, and the regression coefficient R 2 is satisfactory. erefore, we can conclude that the kinetics adsorption of RBB using RDP obeyed the pseudo-secondorder model [70].

Adsorption ermodynamic Studies.
e information about the adsorption thermodynamics is very crucial to provide a better understanding of the adsorption process ( Figure 14). erefore, the Van't Hoff equations were used to determine the thermodynamic parameters mainly Gibbsfree energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) of the adsorption process from the experimental data and following equations: where ΔG°is the standard free energy, kJ/mol; T is the absolute solution temperature, K; ΔH°is the standard enthalpy, kJ/mol; R is the universal gas constant, 8.314 J/mol.K; and ΔS°is the standard entropy, J/K. As shown in Table 5, the negative values of ΔG°at 298, 308, 318, and 328 indicate that adsorption spontaneity is favored at these temperatures. A similar trend has been observed at 308, 318, and 328 K for the adsorption of RBB onto RDP. e positive ΔH value 47.62 kJ/mol confirmed the endothermic nature of RBB adsorption, while the slightly ΔS value 0.048 kJ/mol·K reveals an increase in the randomness at the RBB-RDP-solution interface during the adsorption process [71].

Proposed Mechanisms of RBB Adsorption
It was shown that RBB was adequately adsorbed for pH between 2 and 9, which may be due to the formation of surface hydrogen bonds between the hydroxyl groups on the raw RDP surface and the nitrogen atoms of RBB, as suggested in Figure 15. e large number and array of carboxylic and hydroxyl groups on the RDP surface implied existence of many types of RDP-RBB interaction. Moreover, in the desorption studies, the adsorption of RBB onto the raw RDP resulted in formation of an instable chemical bond between the raw RDP surface and the RBB molecules, which favored the dye molecules from being eluted from the raw RDP surface. However, higher amount of RBB molecules was eluted (∼60). e electrostatic attraction between RBB and RDP enhances the adsorption phenomenon, which leads adsorbent more suitable to adsorb the dye [72].

Effect of Ionic Strength
e ionic strength caused by the presence of salts in solution is one of the factors that controls both electrostatic and nonelectrostatic interactions between the adsorbate and the adsorbent surface [73]. In this study, NaCl and BaCl 2 (0.1 to   Figure 16 illustrates that the concentrations of 0.5 M NaCl and 0.5 M BaCl 2 are sufficient to achieve these maximums of adsorption, for example, an initial concentration of RBB 40 mg/L. As it can be observed in Figure 16, an increase in the adsorption capacity is more for BaCl 2 than NaCl, compared with the adsorption of RBB without salts. is result could be justified by the fact that BaCl 2 is a porter of more positive charges than NaCl on the surface of raw RDP [56]. Overall, the improvement of removal of RBB with increasing ionic strength can be explained by the increase in the positive charges on the surface of the adsorbent. us, it increases the electrostatic interaction between the RBB and

Journal of Chemistry
RDP surfaces. Similar observation was found [34] during the removal of RBB by cross-linked chitosan resins, using only NaCl.

Desorption, Cycles of Regeneration, and Interest of Using Raw RDP
is study aims at evaluating the adsorption rate of RBB and his desorption or the regeneration rate of the biomaterial adsorbent. is contribution gives an idea about the overall cost of the treatment process. All experiments were carried out after saturation of RDP at 1.5 g/L, with an initial solution RBB of 40 mg/L. Desorption experiments were conducted with different eluents, such as distilled water, NaOH, HCl, ethanol, and acetone. Figure 17 shows that acetone has given significant results of desorption. According to the obtained results, no interesting desorption is observed in the acidic medium. However, in the presence of NaOH, the desorption of RBB is approximately 37% successively. is behavior is related to the anionic nature of RBB, and to the ion exchange, and the functional groups content on the surface of the adsorbent. e adsorption-desorption cycles with 1 : 1 acetone water (v/v) were used as optimum solvent during the regeneration experiment. Figure 18 shows that the regeneration of RDP is possible, but not satisfactorily, due to the loss of adsorbent material performance [74]. is phenomenon is commonly explained by the loss of active sites on the surface of the adsorbent [75].

Comparison of the Treatment Efficiency with
Literature Studies e efficiency of the adsorption capacity towards different dyes, according to the literature studies, is presented in Table 6 [76][77][78] in which we have included the results of the present work and the conditions for establishing comparisons. As it can be seen in Table 6, the different biomaterials are used for the adsorption of RBB. e present work shows an important adsorption capacity, during a fast contact time of 50 min. Consequently, Moroccan RDP could be a promising bioadsorbent for the elimination of dyes in aqueous solutions.

Conclusion
RDP compared to various bioadsorbents has the potential in removing RBB from aqueous solutions. e experimental results have shown that the absorption maximum is obtained at initial RBB concentration 40 mg/L, pH 2, equilibrium contact time 50 min, temperature 328 K, particle diameter 40 µm, and RDP mass 1.5 g/L. Increasing the ionic strength of the dye solution with 0.5 M BaCl 2 enhances the adsorption capacity till 198 mg/g. Experimental data were adequately interpreted by Langmuir isotherm and pseudosecond-order kinetics.
erefore, RDP has proved effectiveness to remove RBB from solution. In addition to the advantage of its availability in large quantity in Mauritania, it presents an eco-friendly alternative to traditional processes of textile wastewater treatment even though the test of adsorption-desorption cycles demonstrates that the bioadsorbent cannot be used several times, and it is still a costeffective bioadsorbent, taking into account the high adsorption yield reached. Moreover, exploring the feasibility of using the RDP before and after thermic treatment could be an important perspective for future work.

Data Availability
All data underlying the findings of this study are fully available without restriction.

Conflicts of Interest
e authors declare that they have no conflicts of interest.   Journal of Chemistry 11