Removal of sodium salicylate from aqueous solution using spruce wood sawdust as an eco-friendly adsorbent

This study reports the application of Spruce wood sawdust as a low-cost and eco-friendly adsorbent for the removal of sodium salicylate from aqueous solutions. The effects of physicochemical parameters on the sodium salicylate adsorption process such as initial pH, temperature, solid/liquid ratio, initial concentration, contact time, and ionic strength were investigated. The optimum adsorption conditions were found as: equilibrium time = 3h, adsorbent dosage = 0.5 g/L, pH= 6.5, initial sodium salicylate concentration = 10 mg/L and temperature = 25°C. The kinetic study shows that the pseudo-second-order model is suitable to describe the adsorption process. The adsorption isotherm models (Langmuir, Freundlich, Temkin and generalized) were tested to understand the adsorption mechanism of sodium salicylate onto wood sawdust surface. The adsorption of sodium salicylate follows the Langmuir isotherm model. The maximum monolayer uptake capacity was found to be 99.01 mg/g. In addition, the temperature seems to have no noticeable effect on the adsorption of sodium salicylate. Finally, the thermodynamic parameters indicate that the adsorption is spontaneous (ΔG°˂ 0) and exothermic (ΔH°=-5.081 kJ/mol) in nature. Overall, Spruce wood sawdust can be used as a cost-effective and eco-friendly adsorbent for wastewater treatment applications.


Introduction
The pollution of the aquatic ecosystems is a major environmental problem and can cause severe damages to humans and biodiversity. It is well known that the industrial sectors are the major origin of water contamination. The toxicity and stability are the most common proprieties of the industrial contaminants (i.e. pharmaceuticals, heavy metals, dyes, pesticides, etc.). Recently, the pharmaceutical residues have attracted great attention as emerging pollutants in water pollution studies because of their detrimental effects at such low concentrations in aquatic environments. It is important to note that several pharmaceutical compounds like estrogens, antibiotics and anticancer have been detected in urban wastewaters 1,2 . These pharmaceutical contaminants originated from human and/or veterinary medicine. The pharmaceutical industries are the first source of this pollution in the environment. The sodium salicylate is a drug derivative used as analgesic and antipyretic as well as for the treatment of the rheumatic effects. It was qualified as a typical contaminant in wastewaters and presented high toxicity and the ability to accumulate in the ecosystem 3,4 . Therefore, it is highly recommended to treat industrial effluents before its discharge to the environment using eco-friendly wastewater treatment strategies.
Various wastewater decontamination technologies such as photocatalytic degradation [5][6][7][8] , reduction 9 , electrocoagulation 10 , and adsorption [11][12][13][14] . Among these techniques, the adsorption was considered as one of the most promising methods for the removal of pollutants from aqueous solutions because of its flexibility, low-cost, high efficiency and operational simplicity [15][16][17] . In this context, the raw agricultural by-products have been widely studied as abundant, renewable, inexpensive, biodegradable and ecofriendly materials because of their adsorption ability for removal of organic and inorganic pollutants from contaminated effluents 18 . Great interest has been paid for the application of these lingo-cellulosic biomaterials (including wood sawdust, leaves, seeds, shell, peels, husks and straws) as alternative economic adsorbents to replace the conventional activated carbons 19,20 .
Wood sawdust by-products have been proposed as an excellent candidate for cleaning up the contaminated waters because of their molecular structure containing a large amount of surface functional groups (especially hydroxyl functions) 21 . The main polysaccharides constituents of the wood sawdust are cellulose, hemicellulose and lignin 19 . This structure chemistry makes it a potential adsorbent material with a stronger affinity for various kinds of pollutants.
To the best of our knowledge, no investigation on the removal of pharmaceutical contaminants (especially sodium salicylate as a model pollutant) from aqueous solution using Spruce wood sawdust has been published so far in the available literature. Therefore, the current research work was carried out as the first study with aims to present a detailed investigation on the adsorption of sodium salicylate onto Spruce wood sawdust. The effects of various operating parameters on the adsorption process such as pH, temperature, contact time, adsorbent dose, salt effect and initial concentration were systematically evaluated. The kinetic and isotherm models were employed to describe the mass transfer mechanism at the solid-solution interface during the adsorption phenomenon. Moreover, the thermodynamic parameters were calculated to investigate the thermodynamic behavior of the sodium salicylate adsorption process.

Reagents
Sawdust prepared from Spruce wood was obtained from a local company. Then, the wood sawdust was washed several times with distilled water to remove the soluble organic matter and then, was dried in an oven at 105°C for 24 h. The dried sample was then powdered using an electric grinder (SM 100 Restsch, Germany). Sieving of wood sawdust was done using a vibrating screen. The particle sizes between 250 and 500 µm were recovered for further use in adsorption experiments. The adsorbate stock solution of 1000 mg/L was prepared by dissolving an appropriate amount of sodium salicylate (C7H5NaO3) in distilled water. The solutions used in adsorption tests were obtained by successive dilution to obtain the desired concentrations.

Adsorption experiments
The adsorption experiments were conducted in 150 mL Erlenmeyer flasks containing 50 mL of adsorbate solution with a constant agitation speed of 150 rpm. The adsorption tests of sodium salicylate on the wood sawdust were performed at different experimental conditions such as initial pH (2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12), solid/liquid contact time (0-300 min), adsorbent dosage (0.1-1.5 g/L), temperature (25-50°C) and initial concentration (10-200 mg/L). The initial solution pH was adjusted to the desired values by the addition of HCl (0.1 M) or NaOH (0.1 M) solutions. The samples analysis was carried out before and after adsorption using UV-visible spectrophotometer 2300UV at the maximum absorption wavelength of 295 nm. The following equations calculate adsorption efficiency and the adsorbed amount: Adsorption efficiency: Adsorbed amount (mg/g) : where C0 (mg/L) is the initial concentration of the solution, Ce (mg/L) is the equilibrium concentration, m (g) and V (L) is the mass of the adsorbent and volume of the solution, respectively.

Adsorption kinetic study
The adsorbate/adsorbent contact time is an important factor in the adsorption process; because it determines the equilibrium adsorption capacity and the optimum contact time. The results obtained from the adsorption experiments are presented in Figure 1. The experimental data show that the adsorption of sodium salicylate on the wood sawdust increases rapidly at the beginning of the adsorption to an optimum contact time of 180 min. This adsorption behavior can be explained by the presence of a large number of vacant active sites on the wood sawdust surface at the beginning of adsorption process 22,23 . After 180 min, there is a plateau corresponding to the equilibrium adsorption of sodium salicylate on the wood sawdust. Finally, a contact time of 180 min was selected for investigation of other operating parameters which affects the adsorption process.
The experimental data of the adsorption of sodium salicylate onto wood sawdust were analyzed by applying the linearized forms of the following kinetic models: The pseudo-first-order model the equation gives 24 (3): The pseudo-second-order model 24 is expressed by the equation (4): The intraparticle diffusion model 25 is represented by the equation (5): where Qt and Qe are the adsorbed amounts (mg/g) at time t and equilibrium, respectively; and k1 (1/min), k2 (mg/g/min), and kp (mg/g/min 1/2 ) are the rate constants of pseudo-first-order, pseudo-secondorder, and intraparticle diffusion models, respectively. The values of different kinetic parameters have been generated graphically from Figure 2 and are given in Table 1. The correlation coefficient (R 2 ) value was higher for the pseudo-second-order kinetic model than that of the pseudo-first-order kinetic model. This suggests that the kinetic data were best fitted to the pseudo-second-order model. Moreover, the experimental and calculated Qe values agree very well, which further indicates that the adsorption of sodium salicylate on the wood sawdust follows the pseudo-second-order model. This indicates that the pseudo-second-order kinetic model provides a better description of the adsorption of sodium salicylate on the wood sawdust.
According to the intraparticle diffusion model, the diffusion is the rate-limiting step in the adsorption process if the (Qt vs. t 1/2 ) plot passes through the origin. From Figure 2c, it was observed that the intraparticle diffusion plot did not pass through the origin, indicating that the intraparticle diffusion mechanism was not the sole rate-limiting mechanism. Therefore, the adsorption process of sodium salicylate on the wood sawdust involves some other mass transfer mechanism like boundary layer diffusion simultaneously. A similar suggestion was reported for adsorption of phthalate esters onto zeolite/Fe3O4 26 .

Effect of adsorbent dosage
The adsorbent dose was varied from 0.1 to 2.0 g/L for 3 hours and an initial concentration of 10 mg/L. Figure 3 illustrated the effect of the adsorbent dosage on the adsorbed amount and adsorption efficiency. By increasing the adsorbent dosage, a reduction in the uptake capacity was observed. This reduction of the adsorbed amount could be due to the adsorbent characteristics like the density of adsorbent. In addition, the sodium salicylate adsorption efficiency increased with increasing the wood sawdust amount.

Effect of pH
The adsorption processes can be affected by the initial pH values of the solution, due to interactions of adsorbate functional groups with adsorption sites 28,29 . The adsorption experiments were carried out to evaluate the influence of initial pH (in the pH range of 2-12) on the removal of sodium salicylate. Figure 4 represents the experimental results. The observed adsorption behavior can be explained as follows:  At acidic conditions, the adsorption of sodium salicylate on the wood sawdust was favorable. This could be explained by the electrostatic attractions between deprotonated functional groups of the adsorbate (hydroxyl and carboxyl functions) and protonated functions (positively charged) on the wood sawdust surface. The sodium salicylate was deprotonated according to the following scheme:  At alkaline medium, the decrease of the adsorption of sodium salicylate on the wood sawdust may be related to the competition between hydroxide ions and adsorbate molecules for active sites on the adsorbent surface. Similar results were obtained for adsorption of organic pollutants on the polymeric adsorbents 30,31 .
Overall, pH = 6.5 was chosen as the optimal pH for the subsequent adsorption experiments.

Effect of ionic strength
The ionic strength is one of the important factors influencing the aqueous phase equilibrium. The adsorption of organic compounds can be sensitive to changes in the ionic strength of the solution 32 . For this reason, it is important to study the ionic strength effect on the adsorption process. To investigate the impact of this parameter on the removal of sodium salicylate by wood sawdust, we introduce different masses of NaNO3 to mixtures of 50 mL of sodium salicylate solution with an initial concentration of 10 mg/L and 0.5 g/L of the wood sawdust at pH 6.5 and 25°C. The masses of interfering ions (NaNO3) used in this study are 0.5, 1.0, 1.5, 2.0 and 3.0 mg. The obtained experimental results indicate that the adsorption efficiency of sodium salicylate decreases with increasing ionic strength (concentration of NaNO3).
This suggests that the nitrate ions compete with sodium salicylate. Figure 5 shows that the adsorption capacity of wood sawdust for sodium salicylate increased with increasing initial concentration. The increase of adsorbed amount with initial concentration can be due to the increase of driving force for mass transfer from the liquid phase to the solid phase 33 . Therefore, the high initial concentration facilitates the transport of sodium salicylate from aqueous solution to the wood sawdust, which results in a high uptake capacity. In addition, a plateau was observed at a high initial concentration which could be due to the saturation of active sites on the wood sawdust surface. A similar observation was reported for adsorption of Methylene blue and Safranin-O using Fe3O4 nanoparticles 34 . In order to describe the distribution of adsorbate molecules at solid/liquid interface, the equilibrium adsorption data were analyzed using the adsorption isotherm models. The linear expressions of Langmuir, Freundlich, Temkin and Generalized models are given by the following equations (6)-(9):

Adsorption isotherms
The following equation 33 expresses the Langmuir isotherm model: where KL is the Langmuir binding constant, Ce is the equilibrium concentration (mg/L), Qm and Qe are the adsorbed amount maximum and at equilibrium in mg/g, respectively. rL (dimensionless) is the separation factor defined as: rL =1/(1+KLC0) 35 For rL > 1 Adsorption is unfavorable For rL = 0 The adsorption is irreversible For rL = 1 Adsorption is linear For 0 < rL <1 Adsorption is favorable The Freundlich isotherm model is generally expressed as 36 : where Kf, nf are the Freundlich empirical constants.
The Temkin isotherm model is represented by equation 37 : where KT and B are the Temkin constants.
The generalized isotherm model is expressed as 38 : Ln((Qm/Qe) -1) = LnK -nbLnCe (9) where K is the saturation constant, and nb is the coordination binding constant.
The results of fitting of Langmuir, Freundlich, Temkin and Generalized equations to adsorption equilibrium data of sodium salicylate ( Figure 6) are summarized in Table 2. The analysis of isotherm models for describing the adsorption behavior of sodium salicylate on the wood sawdust was made by comparing the correlation coefficients (R 2 ). Based on these results, the Langmuir isotherm model appears most satisfactory for modeling the adsorption process. This indicates that the adsorption is a homogeneous monolayer coverage. The maximum adsorbed quantity of the sodium salicylate calculated using the Langmuir isotherm model is comparable to that obtained experimentally. The values of separation factor rL (0 < rL< 1) indicate that the adsorption of sodium salicylate on the wood sawdust is favorable, which confirm further the validity of the Langmuir model.
The maximum monolayer uptake capacity was found to be 99.01 mg/g. According to the literature, the adsorption ability of Spruce wood sawdust was compared with other materials, as shown in Table 3. It was concluded that the raw Spruce wood sawdust exhibited a functional uptake capacity for removal of sodium salicylate from aqueous solution. Nonetheless, several adsorbents exhibited an adsorption ability superior to that of our adsorbent material, but these materials are very expensive compared to Spruce wood sawdust. Overall, Spruce wood sawdust is an inexpensive, eco-friendly and efficient material for cleaning up wastewaters containing pharmaceutical compounds like sodium salicylate.

Adsorption thermodynamics
The investigation of the temperature influence on the adsorption process was carried out by varying the temperature of the solution from 25 to 50°C using a thermostatic bath, keeping all other parameters constant. Based on the experimental data, thermodynamic parameters including ΔH°, ΔS°, and ΔG° were calculated to describe thermodynamic behavior of the adsorption of sodium salicylate on wood sawdust. All parameters were calculated using the equations (10) and (11) where R is the universal gas constant (8.314 J/mol K), Kd is the distribution constant, and T is solution temperature in °K.
The distribution constant is defined by the equation (12): The plot of Ln Kd versus 1/T is presented in Figure 7. The values of thermodynamic parameters were graphically determined by linear regression analysis and are summarized in Table 4.  The negative value of the standard enthalpy ΔH° shows an exothermic character of the adsorption process. The negative values of the standard Gibbs energy ΔG° in the studied temperature range allows us to conclude that the adsorption of sodium salicylate on the wood sawdust is spontaneous in nature 43 . In addition, the ΔG° increased from -3.998 to -3.908 kJ/mol when the temperature was increased, indicating a decrease in the feasibility of adsorption at higher temperatures. The values of ΔG° range between -20 and 0 kJ/mol, which means that the adsorption of sodium salicylate onto wood sawdust is a physical adsorption type 44 . The negative value of the entropy (ΔS°) suggests a decrease in the randomness at the adsorbent/adsorbate interface during the adsorption process.

Conclusion
In the present study, the removal of sodium salicylate from aqueous solution using Spruce wood sawdust as a low-cost and eco-friendly adsorbent was systematically investigated. The effects of physicochemical parameters such as contact time, solution pH, adsorbent dosage, initial concentration and temperature on the adsorption process were evaluated. As a result, the optimal conditions required 180 min of adsorbent/solution contact using 10 mg/l of sodium salicylate and 0.5 g/L of Spruce wood sawdust at pH 6.5 and 25°C. Under these conditions, the adsorbed amount was estimated to be 13.56 milligrams of sodium salicylate per gram of Spruce wood sawdust. The pseudo-second-order model is the most suitable to describe the adsorption kinetic. Experimental data of sodium salicylate were successfully fit to the Langmuir isotherm model with a maximum monolayer uptake capacity of 99.01 mg/g. The values of thermodynamic parameters prove that the adsorption of sodium salicylate on the wood sawdust is an exothermic and spontaneous process. These results indicate that the Spruce wood sawdust is a cost-effective and ecofriendly adsorbent for the removal of sodium salicylate from aqueous solution, and it could be useful in the treatment of wastewaters.