Synthesis and Characterisation of Novel Magnetic Beads as Salicylic Acid Adsorbents from an Aqueous Solution

The study focuses on the synthesis of two adsorbent forms that were prepared: magnetic nonporous hybrid beads (MNPHB), and magnetic macroporous hybrid beads (MMPHB). The salicylic acid adsorption tests on MNPHB and MMPHB were carried out at a temperature of 25 °C, pH 4, adsorbent mass of 10 mg, and an initial concentration of 10 mg l −1 . The adsorption capac ity was found to be for MNPHP and MMPHB to 9 mg g −1 and 152 mg g −1 , respectively. The adsorption kinetic was described by the pseudo-second order model and Freundlich isotherm for the MMPHB beads.


Introduction
Emerging pollutants (EPs) are contaminants that are receiving more and more scientific attention. They may cause fatal effects on humans and aquatic organisms even at very low concentrations. 1 Pharmaceuticals include antibiotics, legal and illicit drugs, etc. Nowadays, active pharmaceutical ingredients are bioaccumulating and persistent, which may cause significant consequences for ecosystems. 2 In the pharmaceutical industry, salicylic acid is used as a disinfectant antiseptic agent. 3 In the human body, acetylsalicylic acid is quickly deacetylated forming salicylic acid, which is usually found in surface waters and urban wastewater. 4 Adsorption is currently one of the most employed separation techniques for industrial water decontamination. 5,6 A wide variety of materials can be used as adsorbents including clays, biopolymers, and other minerals. 7 Hybrid, "organic-inorganic" biocomposites currently represent a major potential for research work. They are the subject of great benefits, with the properties of inorganic and organic materials at the same time. 8 Spinel ferrite adsorbents are the most suitable option for water decontamination, with high adsorption efficiency and fast kinetics. 9 Encapsulation is an economical and environmentally friendly process for immobilizing a material within a hydrogel matrix, thus retaining their adsorption properties, and facilitating their separation process from aqueous solutions. 10 The most popular anionic polysaccharide used for beads preparation is alginate. It is constituted of (1-4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues. The grafting of urea on alginate structure leads to the creation of new reactive groups that will bring additional reactivity or selectivity: these new reactive groups may increase sorp-tion capacity of the biopolymer. 11 The main objective of the present work was the synthesis of novel magnetic hybrid beads, prepared with urea grafted alginate and cobalt ferrite in order to remove salicylic acid from contaminated water. Two adsorbent forms were prepared: magnetic nonporous hybrid beads and magnetic macroporous hybrid beads. The creation of porosity on the magnetic hybrid beads is a critical factor that influences sorption properties. 11 The ionic gelation method was employed to prepare nonporous hybrid beads according to Zhao et al. 10  The CaCO 3 was removed from the beads by immersing them in a hydrochloric acid solution (0.5 M) bath for 40 min. After that, the beads were washed multiple times using distilled water and dried in an oven at 40 °C.

Characterisation of the prepared materials
The samples were characterised using Fourier Transform Infrared Spectroscopy (FTIR) by FTIR-8900 instrument (Shimadzu). The crystalline structure of the prepared beads was analysed by Rigaku SmartLab high-resolution X-ray diffractometer (XRD). The pH pzc (point of zero charge) of the two adsorbents was determined by potentiometric dosage. 16 Size, morphology, and porosity of the beads were determined by Scanning Electron Microscopy (SEM) associated with Energy Dispersive X-ray microanalysis EDX (Quanta 650, Bruker).

Adsorption studies
Salicylic acid adsorption on the hybrid beads was carried out in a thermostated cell. To 10 ml of salicylic acid aqueous solution (10 mg l −1 ), an amount of 10 mg of adsorbent was added. The pH was adjusted to the targeted value using HCl (0.1 M) and NaOH (0.1 M). The effects of pH (2 to 14), adsorbate initial concentration (10 to 100 mg l −1 ), and mass of adsorbent (10 to 120 mg) on the adsorption behaviour were studied. The residual concentration of adsorbate was determined after magnetic filtration and analysed on ultraviolet/visible spectrophotometer UV-1800 (Shimadzu) at λ max = 296 nm. The adsorption kinetics were studied by measuring the change of concentration of salicylic acid with time. The adsorption capacity, Q e (mg g −1 ), was calculated using the Eq. (1): where c 0 and c e are the initial and equilibrium concentrations of salicylic acid (mg l −1 ), respectively, V is the volume of the solution (l), and W is the weight of the adsorbent (g).

Adsorption kinetics
The salicylic acid adsorption kinetics data were analysed by testing pseudo-first order and pseudo-second order kinetic models with ORIGIN 2018. The pseudo-first order model can be expressed by Eq. (2): where q e is amount of salicylic acid adsorbed at equilibrium state (mg g −1 ), q t is amount salicylic acid adsorbed at time t (mg g −1 ), and K 1 is rate constant (min −1 ) of pseudo-first order. The pseudo-second order model can be expressed by Eq. (3).
One again, q e is amount of salicylic acid adsorbed at equilibrium state (mg g −1 ) and q t the amount of salicylic acid adsorbed at time t (mg g −1 ). K 2 rate constant (g mg −1 min −1 ) of pseudo-second order. The parameters of pseudo-first and pseudo-second order kinetics are shown in Table 1.

Adsorption isotherms
The adsorption isotherm experiments were accomplished by shaking 10 ml of salicylic acid solutions of different concentrations (10-100 mg l −1 ) with 10 mg of adsorbent for 1 h, at pH 4 and 25 °C. The adsorption isotherm shows how the adsorbed molecules are distributed between the liquid and solid phases at equilibrium. The isotherms of Freundlich and Langmuir were studied to describe the salicylic acid adsorption.

Freundlich model
Freundlich isotherm is commonly used to describe the adsorption processes for the heterogeneous surface. 17 It was applied to represent the multi-layer adsorption. 18 The nonlinear and linear forms of the Freundlich model are provided by the Eqs. (4) and (5).
The amount of salicylic acid adsorbed per gram of the adsorbent at equilibrium state is marked as q e (mg g −1 ), c e is the equilibrium concentration of adsorbate (mg l −1 ), K f is Freundlich isotherm constant (mg g −1 ) that approximately indicates adsorption capacity, and n is adsorption intensity which is a function of the adsorption strength. 16

Langmuir model
This model describes the formation of a monolayer adsorbate on the external surface of the adsorbent; no further adsorption subsequently occurs. 19 It is valid for monolayer adsorption onto a surface containing a finite number of identical sites. Langmuir model is represented by Eq. (6): where c e represents the adsorbate equilibrium concentration (mg l −1 ), q e is the amount of the salicylic acid adsorbed per gram of the adsorbent at equilibrium state (mg g −1 ), q max is maximum monolayer coverage capacity (mg g −1 ), and K L is Langmuir isotherm constant (l mg −1 ).

Error analysis
Error analysis offers a rigorous tool for computing adsorption parameters via isotherm models. Several functions are used for this purpose. The sum of squares of the errors, SSE (Eq. (7)), was used in this study. 20 Calculated and experimentally determined values are marked with indices calc and exp, respectively.

Results and discussion
Several tests were carried out in order to highlight the magnetic properties of the prepared beads. The test showed that our adsorbents were attracted to the magnet, which confirmed the magnetic properties.

X-ray diffraction
In order to determine crystal phase of the magnetic biocomposites, the dried and crushed samples were analysed using XRD in the region of 2θ between 8 and 80°.

Determination of the point of zero charge
The point of zero charge pH pzc is the parameter corresponding to the pH for which the adsorbent surface has a zero charge. This method is based on the plot of Q S = f(pH) (Fig. 4), and the intersection point between the curve and the x-axis (Q S = 0) was determined by Origin. The isoelectric point was determined at pH pzc = 7.02 for MNPHB and at pH pzc = 8.5 for MMPHB. The surface is positively charged when the pH value is below pH pzc and negatively charged when it is above of pH pzc . 16

Scanning Electron Microscopy
The magnetic macroporous MMPHB beads were analysed by SEM. The porous structure is shown in Fig. 5. The prepared beads with approximately 1.854-2.501 mm diameter featured many heterogeneous macropores with medium diameters ranging from 90.52 µm to 132.6 µm. These macropores were created because of CaCO 3 dissolution in the acidic mixture (HCl) used for beads preparation with CO 2 gas generation. This subsequently generated the formation of bubbles inside the beads during gelation with calcium ions of CaCl 2 solution, then produced macroporous beads.

Energy Dispersive X-Ray Analysis
The elemental composition of alginate/CoFe 2 O 4 and macroporous urea grafted alginate/CoFe 2 O 4 beads is shown in the EDX micrographs in Fig. 6. The reaction between hydroxyl groups OH of alginate and the amine groups of urea produced the urea grafted alginate with release of ammonia. Analysing the results of EDX, the presence of nitrogen (8.67 atom%) was noticed in the composition of the urea grafted alginate/CoFe 2 O 4 prepared beads, which confirmed the chemical modification of alginate with urea.

Study of salicylic acid adsorption on the prepared hybrid beads
The salicylic acid adsorption capacity on MNPHB and MMPHB as a time function is represented in Fig. 7. The obtained graph showed that the adsorption process had two distinct parts. The adsorption capacity increased rapidly during the first part (60 min) for the two forms of magnetic beads, until reaching values of 9.026 mg g −1 and 152.22 mg g −1 for MNPHB and MMPHB, respectively. Fig. 8 shows the adsorption capacity evolution as a function of the respective mass values of the MNPHB and    MMPHB adsorbents. In this study, several adsorbent quantities were varied from 0.01 to 0.12 mg. It was observed that the adsorption capacity was maximal at 0.01 mg for the two adsorbents. This adsorption capacity decreased as the mass increased. This phenomenon may be explained by the unsaturation of the adsorption sites.
The influence of pH on the adsorption of salicylic acid is shown in Fig. 9. Analysing the figure, there is a rise in salicylic acid adsorption capacity until pH = 4 with an adsorption capacity of 5.47 mg g −1 and 9.03 mg g −1 for MNPHB and MMPHB, respectively. Beyond this pH value, a decrease in the adsorption capacity was noticed. This result can be explained by the fact that, for this pH value, which was lower than the isoelectric point of the adsorbents (pH pzc = 7.02 and pH pzc = 8.5), the beads surface was positively charged and the salicylic acid dissociated in salicylate anions (pK a = 2.97). The rise in salicylic acid retention was probably due to the attractive interaction forces between the positively charged adsorbent and the negatively charged dissociated adsorbate. The decrease in the adsorption capacity at higher pH values may have been due to the repulsive interaction forces between the negatively charged adsorbent and the salicylate anions. The same results are endorsed by Imessaoudene et al. 24 and Liadó et al. 25 The salicylic acid concentration was varied from 10 to 100 mg l −1 . Fig. 10 shows the evolution of the adsorption capacity of MNPHB and MMPHB according to C 0 , the figure indicates that the adsorption capacity of MMPHB rapidly increased until an initial concentration value of 50 mg l −1 with an adsorption capacity of 15 mg g −1 .

Adsorption kinetics
Adsorption kinetics process is essential for wastewater treatment; it provides the necessary information on the solute retention percentage and the reaction pathways. The contact time effect on salicylic acid removal rate was monitored in this study. Analysing the results of Table 1, it was noted that the coefficients of determination (R 2 ) were higher and the SSE value lower for the pseudo-second order model than those for the pseudo-first order model. Therefore, q e experimental values failed to match well with the calculated theoretical values for pseudo-first order model. Thus, the salicylic acid sorption mechanism onto MNPHB and MMPHB followed the pseudo-second order kinetics model, which implied that the adsorption phenomenon might have occurred via a chemical process involving valence forces that exchange or share electrons.

Adsorption isotherms
The results of plotting q e = f(c e ) are shown in Figs. 11 and 12. The extracted adsorption parameters from the non-linear plots of the Freundlich and Langmuir models after salicylic acid adsorption on the MMPHB are all summarized in Table 2. It was observed that Freundlich model

Conclusions
Magnetic nonporous hybrid beads MNPHB and magnetic macroporous hybrid beads MMPHB that were prepared using urea grafted alginate and cobalt ferrite, were used to remove salicylic acid emerging pollutants from contaminated water. The adsorption tests study has demonstrated the efficiency of the MNPHB and MMPHB biocomposites in salicylic acid removal, with adsorption capacities of 9.026 mg g −1 and 152.22 mg g −1 for MNPHB and MMPHB, respectively, for an equilibrium time period of 60 min, pH 4, adsorbent mass 10 mg, initial concentration of 10 mg l −1 , and temperature of 25 °C. The higher R 2 and the lower SSE values indicated that the adsorbate onto MNPHB and MMPHB followed a pseudo-second order kinetics model. The Freundlich isotherm was more suitable for the adsorption data. MMPHB biocomposites can be considered as a new material for removal of emerging pollutants from wastewater with high efficiency.