Optimization of adsorption of methyl orange from aqueous solution by magnetic CoFe2O4/ZnAl-layered double hydroxide composite using response surface methodology

The CoFe2O4/ZnAl-layered double hydroxide (LDH) composite was successfully developed through a facile co-precipitation method, characterized, and applied as an effective adsorbent for the removal of methyl orange (MO) dye from aqueous solutions. The central composite design (CCD) of the response surface methodology (RSM) was employed to estimate and optimize process variables such as initial MO concentrations, solution pH, adsorbent dosage, and contact time. 98.878% adsorption efficiency was obtained at an initial concentration of 18.747 mg l−1 of MO, with an adsorbent dosage of 0.048 g, a solution pH of 2.770, and a contact time of 85.890 min. Analysis of variance (ANOVA) confirmed the significance of the predicted model (R2 = 0.9844). Kinetic and equilibrium studies indicated that the experimental data for MO adsorption were best described by pseudo-second-order kinetic and Langmuir models. The maximum monolayer adsorption capacity of the CoFe2O4/ZnAl-LDH for MO was 42.3 mg g−1.


Introduction
Dyes are widely used in several industries such as plastics, textile, paper, printing, and food industries as colorants [1]. Textile effluents contain about 15% of the total world production of dyes that is lost during the dyeing process [2]. Organic dyes from industrial effluents are commonly discharged into aqueous environments without proper treatment which caused devastating impact on human being and microorganisms. This is because of their complex aromatic molecular structure, high solubility, and low biodegradability [3,4]. One of the organic dyes often found in industrial effluents is methyl orange (MO). MO is an anionic azo dye that poses the abnormal coloration of water bodies, hinders the penetration of Sunlight, and causes depletion of dissolved oxygen in water [5,6]. Because of toxicity, carcinogenicity, or teratogenicity, it is considered as a major threats to living organisms and the surrounding environment [6,7]. Therefore, it is crucial to remove dyes from contaminated water bodies before discharging into the environment using various technologies such as photocatalytic degradation, reverse osmosis, nanofiltration, flocculation, and adsorption [8]. Among the aforementioned methods, the adsorption approach has been widely applicable due to its efficiency, economic feasibility, recycling of adsorbent, and environmental friendliness [9,10]. In recent times, adsorbents like LDH materials have gained substantial research attention by researchers.
The application of LDH materials for the adsorption of various organic dyes has been considered as a great potential for being applied in adsorption field, due to the low starting materials cost, highly positive surface charge, easy way to prepare, high efficiency and environmentally friendly nature [11]. LDHs are known as hydrotalcite-like compounds or anionic clays, widely used in the removal of organic and inorganic pollutants [12,13] , NO 3 − or Cl − ); x denoted as the molar ratio of M 2+ to total metal ions [3,9]. The presence of a high concentration of hydroxyl groups on the surface, interlayer anion mobility of its host A n− and inherent positive charges make LDHs a promising adsorbent for dye adsorption from contaminated water [14][15][16]. However, it is tedious to separate LDHs from water as it tends to disperse due to their small particle size [17]. In this regard, combining magnetic particles with LDHs has been the solution to facilitate the recovery and separation process at the end of adsorption. Among magnetic materials, spinel cobalt ferrite (CoFe 2 O 4 ), has a strong magnetic response as well as excellent chemical stability and biocompatibility [18]. Furthermore, it possesses not only high surface areas but can also bind oxy-anions [19,20]. Earlier reports demonstrate that a composite of CuAl-LDH with CoFe 2 O 4 nanoparticles can be generated [18] and the composite can remove a high level of anionic dye adsorption. These composites displayed a strong magnetic character arising from the small size of the magnetic nanoparticles in the composite. Then, the mixture was stirred for a further 3 h at 70°C in a constant temperature heating water bath. After that, a suspension was formed and the solid precipitate was collected and washed by solvents such as distilled water and ethanol several times until neutral supernatant is found. Finally, the product was dried at 100°C overnight.

Synthesis of ZnAl-LDH
ZnAl-LDH with a molar ratio of Zn to Al (3:1) was prepared by a co-precipitation method [21]. 2.5. Materials characterizations X-ray powder diffraction (XRD-7000, Drawell, China) was used to characterize the crystalline composition of the as-synthesized adsorbents. The morphology was determined by scanning electron microscope (JEOL/EO, JSM-6000 plus). Fourier transform infrared spectroscopy (FTIR, PerkinElmer) was used to identify functional groups from a wave number of 500-4000 cm −1 at a resolution of 4. The textural properties of the prepared CoFe 2 O 4 /ZnAl-LDH which include BET surface area, pore volume, and pore size distributions were determined using a Quantachrome analyzer (Nova station C, version 11.0, USA) based on the principle of adsorption/desorption of nitrogen at 77.3 K and 60/60 s (ads/des) equilibrium time.

Experimental design and statistical analysis
In this study, the CCD of RSM was employed to optimize the processing variables estimating the regression model equation [22,23]. RSM can be used to evaluate the effects of individual variables, the interaction of variables, and the optimum conditions for responses [24]. Four process independent variables were chosen: initial MO concentration, solution pH, adsorbent dosage, and contact time, and their effects on the response were estimated. The total number of experiments can be determined from the following equation: Where η is the response, Y i and Y j refer to variables (i and j range from 1 to k); a 0 refers to the coefficient of intercept; a j , a jj , and a ij are known to be coefficients of interaction for the variables, respectively; and e i is the error. The obtained experimental data were analyzed using Design expert 13.0 software, and the significance of the developed model was tested using analysis of variance (ANOVA). The interactions between the independent variables and their effect on the response variable were studied systematically through the response surface contour plots.

Batch adsorption studies.
The MO removal experiment was conducted in a set of each 250 ml conical flasks containing 50 ml of the required concentration of an adjusted pH of MO solution. Solution pH was varied from 2-4 by adding HCl solution. Then, the required amount of the composite was added to the solution and stirred for a particular period of time. The composite was separated from the MO solution through centrifugation (750 rpm) for 10 min. The concentration of the dye solution was then analyzed for absorbance using JASCO UV-vis spectrometer at maximum absorbance (λ max = 464 nm). For the sake of comparison, blank experiments were conducted in parallel on MO solutions, without the addition of an adsorbent. All tests were carried out in triplicate, and their mean values were used in analyzing the data.
he adsorption capacity of the materials can be calculated according to equation (4) below [26]:

= -
Where C o and C e is the initial and equilibrium concentration of MO (mg l −1 ), V is the solution volume (L), and m is the adsorbent dosage (g). All experiments were performed in triplicate, and the mean value was reported.

Adsorption isotherm analysis
In this study, Freundlich and Langmuir adsorption isotherms were used to study the adsorption mechanism [10].
The Freundlich isotherm assumes a monolayer adsorption process with heterogeneous surfaces [27] and this process further assumes that there is an interaction of adsorbed species [28]. The equation describing this model can be given as in equation (5) below [28]: Where K F is the Freundlich constant which represents the affinity of the adsorbent towards adsorbet and 1/n is an adsorption intensity that varies with the heterogeneity of the adsorbed monolayer on adsorbing surfaces. The Langmuir isotherm model assumes that the adsorption occurs on homogeneous surfaces without interaction between the adsorbed MO [29], and can be expressed according to the following equation (6) below [28].
Where b (1 mg) −1 is the Langmuir adsorption constant and q max is the maximum adsorption capacity of the adsorbent (mg g −1 ). The fitting isotherm and calculated parameters for freundlich and langmuir adsorption were presented in figure 1 and table 2. Furthermore, the favorability of Langmuir isotherm can be described by separation factor (R L ) whether it is favorable ( 0 < RL < 1 ), unfavorable ( RL > 1 ), irreversible ( RL = 0 ) or linear ( RL = 1 ) as described in the following equation (7): where Co (mg l −1 ) is the maximum initial MO concentration and b (L mg) −1 is the Langmuir equilibrium constant [30].

= +
Where, q t (mg g) −1 is the adsorption capacity at an adsorption time of t, and K 1 (1 min) −1 and K 2 (g/(mg·min)) are the rate constants of pseudo-first-order and pseudo-second-order models, respectively.  [36]. These peaks are the same as those observed in figure 2(b). Particularly, the peaks at 11.92°correspond to carbonate (CO 3

2-
) intercalated in the LDH which is resulted from Na 2 CO 3 during synthesis [37].
The diffraction peaks at a 2θ of 30°, 35.41°, 43.15°, and 62.68°corresponding, respectively, to the (220),  crystallographic density of 5.27 g·cm −3 , which is in an agreement to the theoretical density of about 5.29 g·cm −3 [39]. Details of the crystallographic planes and the corresponding (hkl) of the planes are reported elsewhere [10].

SEM analysis
The prepared magnetic CoFe 2 O 4 /ZnAl-LDH was characterized by a scanning electron microscope (SEM) to examine the morphological properties (figure 3). CoFe 2 O 4 /ZnAl-LDH composite shows uneven particle distribution, which revealed that various agglomeration of CoFe 2 O 4 particles were loaded on the surface of ZnAl-LDH [40]. Moreover, the surface morphology of CoFe 2 O 4 /ZnAl-LDH is rough. Because of the loading of CoFe 2 O 4 on ZnAl-LDH, the original layered-structure of ZnAl-LDH was changed with inhomogeneous distribution of CoFe 2 O 4 particle [10].

FTIR analysis
The FTIR spectra of the ZnAl-LDH and CoFe 2 O 4 /ZnAl-LDH composite are depicted in figure 4(a) and (b). The FTIR spectrum of the CoFe2O4/ZnAl-LDH composite illustrated in figure 4(b) was almost identical to that of the ZnAl-LDH. The strongest broad adsorption band at around 3444 cm −1 is ascribed to OH− groups as reported in another study [41]. The symmetry vibration band observed at 1640 cm−1 was attributed to the interlayer water molecule. The peak at 1376 cm −1 is showed the vibration mode of CO3 2− [15]. The lower band frequencies between 500 and 1000 cm −1 correspond to lattice vibration of metal-oxygen bonding to the layers of the LDH and CoFe 2 O 4 [41]. The vibration modes of the carbonate anions were also illustrated at 1103 cm −1  absorption peaks [37] . The presence of such vibration reveals the intercalation of carbonate ions into the interlayer space of ZnAl-LDH.

BET analysis
The typical CoFe 2 O 4 /ZnAl LDH composite presented a surface area of 499.345 m 2 g −1 , total pore volume of 0.09321 c.c./g, and pore diameter of 1.27 nm and was characterized as a microporous material. The large active surface functionalities and high surface area with high pore volume are crucial for a good adsorbent. It is revealed that the incorporation of CoFe 2 O 4 into the composite decreased the surface area, possibly by blocking available sites.

Central composite design (CCD) model and data analysis
Input variables such as MO initial concentrations, solution pH, adsorbent dosage and contact time were varied according to the design as indicated by the Design Expert Software 13. The experimental and predicted percentage removal of MO onto CoFe 2 O 4 /ZnAl-LDH adsorbent at different operating conditions were tabulated as shown below.
As suggested by the CCD, a quadratic model was chosen to represent MO removal efficiency by CoFe 2 O 4 /ZnAl-LDH composite. The second-order polynomial equation in terms of coded factors after eliminating the insignificant model terms was expressed as follows where η is the removal efficiency of MO (%), A is the concentration, B is the pH, C is the dosage of the adsorbent and D is the contact time. According to the coded quadratic equation (10), most of the parameters have negative coefficients which mean they attained their optimum value; further addition of these parameters would decrease the removal efficiency of the adsorbent. The removal efficiencies (%) of MO have been estimated by equation (3) and are presented in table 3. In general, there were agreements between the experimental and predicted values of MO removal efficiency. Furthermore, analysis of variance (ANOVA) was applied to check the statistical significance of the quadratic model (table 4). A model is said to be a good predictor of the experimental data, if Fvalue obtained is evidently greater than the tabulated F-value [30]. It was found that the regression was statistically significant at the F-value was 67.64 and the values of prob>F (<0.0001). However, values of Prob > F less than 0.0500 indicate terms are significant. The lack of fit F-value of 0.2985 indicates the lack of fit is not significant relative to the pure error. The statistical parameters, viz. R 2 , R 2 Adj and R 2 Pred were obtained as 0.9844, 0.9699 and 0.9257, respectively. 'Adeq Precision' gives the ratio of the signal to the noise. A ratio greater than 4 is desirable. Thus, the Adeq precision of 25.3891 indicates an adequate signal.

Interaction effect analysis of the parameters
A 3D representation of the adsorption of MO is shown in figures 5(a)-(d). Figure 5(a) explains the 3D surface plot of MO removal efficiency as a function of initial concentration and a solution PH at the mean of contact time and adsorbent dosage. The removal efficiency was increased by decreasing the pH of the solution and obtaining the highest removal efficiency of the adsorbent at a pH of 3. The reason for this observation could be due to an increase in electrostatic attraction between the negatively charged dye molecule and the positively charged CoFe 2 O 4 /ZnAl-LDH surface [18]. On the contrary, in a basic medium, anionic MO and OH − ions compete for the positively charged binding sites on the adsorbent surface, which leads to decreased adsorption capacity. It can also be understood that the adsorbent efficiency rises with an increase in the initial MO concentration till it reaches 20 mg l −1 . The interactive effect on the CoFe 2 O 4 /ZnAl-LDH adsorption towards MO dye was significant with the p-value of 0.0631. Figure 5(b) depicts the 3D surface plot showing the combined effects of initial concentration and adsorbent dosage on the removal efficiency of MO. From the plot, it was observed that the removal efficiency increased with an increase in adsorbent dosage from 0.02 to 0.05 g/50 ml. This can be attributed to the high specific surface area of the composite and the availability of more active sites of CoFe 2 O 4 /ZnAl-LDH composite. Figure 5(c) demonstrates the interaction effect of the pH of the solution and contact time on the removal of MO via CoFe 2 O 4 /ZnAl-LDH. It is obvious that the removal percentage of MO from aqueous solution increases rapidly in the initial stages and became slower in the later stages until the attainment of equilibrium (75 min) and then decreases gradually. The probable reason could be at the initial period of adsorption, the rate of MO binding with the surface of CoFe 2 O 4 /ZnAl-LDH is great due to the availability of an abundance of adsorption sites, and then MO adsorption becomes slow. This is because of the diffusion of MO into the adsorbent pores [19]. The interaction effect has a significant effect on its MO adsorption with a p value of 0.0002 (table 4). The highest removal efficacy of MO is attained at a CoFe 2 O 4 /ZnAl-LDH dosage of 0.05g/50 ml and a contact time of 75 min.
The interactive effect of adsorbent dosage and contact time is illustrated through the 3D plots as shown in figure 5(d). The removal efficiency of the adsorbent increased with an increase in the adsorbent dosage and contact time. The interaction of adsorbent dosage and contact time has a significant effect on its MO removal with a p value of 0.0334 (table 4). Meanwhile, it was revealed by the result obtained that contact time highly interacts with other variables.

Process optimization
The optimum values of the process parameters for the maximum MO removal efficiency were determined as shown in table 5 below. The experimental (actual) removal efficiency of CoFe 2 O 4 /ZnAl-LDH at those optimal condition was in proximate to the predicted value. This proves that the adopted method to optimize process parameters for maximum adsorption of MO onto CoFe 2 O 4 /ZnAl-LDH composite is successful.  A is initial MO concentration (mg l) −1 , B is pH, C is adsorbent dose (g/50 ml), and D is contact time (min). 500 rpm for 87 min and at room temperature was taken. The non-linear fitting results from the isotherms were listed in table 6. According to the values of R 2 , the Langmuir model is more suitable than the Freundlich model to describe the adsorption of MO, indicating homogenous adsorption on magnetic CoFe 2 O 4 /ZnAl-LDH composite. Moreover, the values of R L was comprised within 0 to 1; confirm the favorability of Langmuir adsorption [42].

Adsorption kinetics
The kinetic parameters of the teo models were evaluted from the combined plots ( figure 6) and the values of the parameters are summarized in table 7.From this table, The R 2 value of the pseudo-second order model was found to be higher than obtained from the pseudo-first order model. Therefore, the pseudo-second order model precisely explains the adsorption of MO on the surface of the adsorbent, indicating that the rate-limiting step in adsorption process is probably the chemisorption [43].

Adsorption mechanism
The adsorption mechanism of MO could follow the following steps. First, Adsorption takes place at the surface of the CoFe 2 O 4 /ZnAl-LDH composite through electrostatic forces of attraction i.e., the negatively charged sulfonated group (-SO 3 -Na + ) on MO could be strongly attracted towards the positively charged CoFe 2 O 4 /ZnAl-LDH surface, followed by the interlayer CO 3 2− anion, which was consequently replaced by SO 3 − anion of MO via anion exchange [44], then the aromatic C=C and/or N=N enter into the structure of magnetic CoFe 2 O 4 /ZnAl-LDH composite.

Conclusion
Magnetic CoFe 2 O4/ZnAl-LDH composite was successfully prepared, well characterized by XRD, SEM, FTIR, and BET, and employed as an adsorbent for MO removal. Adsorption processes were performed as a function of independent process variables such as the initial MO concentration, PH, adsorbent dosage, and contact time using the RSM-CCD method. By adopting the design of experiments, statistically, it was optimized that the maximized removal (98.874%) of MO can be achieved via 18.747 mg l −1 initial MO concentration, 2.770 solution pH, 0.048 g adsorbent dosage, and 85.890 min contact time. MO adsorption on CoFe2O4/ZnAl-LDH follows the pseudo-second-order and the Langmuir monolayer models. Electrostatic attraction and anion exchange are the main adsorption mechanisms.