A core-shell magnetic layered double hydroxide composite material for the effective decolorization of phenol red

The magnetic composite based on layered double hydroxide (Fe 3 O 4 /NiMn-LDH) was prepared by co-precipitation procedure and considered as a material to eliminate phenol red (PR) from aqueous environments. The characterization of Fe 3 O 4 /NiMn-LDH were recognized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Box-Behnken design (BBD) under response surface methodology (RSM) were applied to evaluate the effects of the process variables such as pH, adsorbent dosage, and initial PR concentration (C o ). The results indicated that a good correlation between the estimated and experimental values was found for the PR decolorization efficiency from aqueous media using Fe 3 O 4 /NiMn-LDH ( R 2 = 0.99). Furthermore, the statistical model obtained from BBD was sufficient to estimate the PR decolorization on Fe 3 O 4 /NiMn-LDH ( p < 0.0001). The optimal conditions for the PR decolorization efficiency were determined as 5.38, 24.59 mg, and 25.39 mg/L for pH, adsorbent dosage, and C o , respectively which resulted in 86.93% the PR decolorization efficiency. Finally, this work demonstrated that BBD could easefully be utilized for the optimization of the PR decolorization using Fe 3 O 4 /NiMn-LDH.


Introduction
The contamination of aquatic resources by industrial technologies is an issue of significant worry owing to rapid industrial development [1]. Dyestuffs, an industrial pollutant, are well known to affect the environmental ecosystem based on their serious health hazards and toxicity [2]. These wastes, which have wide applications in the rubber, textile, plastic, paper, food, cosmetics, pharmaceutical industries, are discharged into aqueous environments and cause great hazards for the environment [3]. Most of dyes in water bodies disrupts the esthetic nature and interferes from sunlight transmission, thus affecting the food web necessary for life. Moreover, these products lead to various disorders like lung, skin, and respiratory problems [1,3]. Organic contaminants include various class of dyes and phenol derivatives. Among them, phenol red (PR) is one of the most harmful contaminant causing severe problems such as reducing light penetration, visibility in the water bodies, adjourning the growth of microorganisms, and increasing the chemical oxygen demand (COD) [4]. Consequently, it is significant to remove these colorants from water bodies. The conventional technologies such as photocatalysis, chemical oxidation, coagulation, membrane filtration, photo degradation, and adsorption are applied to color removal from industrial effluents [4,5]. Adsorption is one of the most efficient and reliable procedure for the decolorization of colorants [6,7]. This treatment method has many advantages including high selectivity, easy handling, regeneration of adsorbent, and low cost effectiveness [6]. Adsorbent plays a significant role in separation and purification method in terms of capacity and selectivity [8]. Different materials have been utilized for the removal of organics from the water samples, however, layered double hydroxides (LDH), known anionic clays, have been promising adsorbent for wastewater treatment due to their interlayer ion exchange, high surface area, and layered structure, and thus the research focus of scientific application [9,10]. Moreover, the materials utilized in adsorption applications are commonly powders and it is quite difficult to separate the solid from the solution. To overcome such limitations, magnetic separation technology has aroused great interest. Consequently, imparting magnetic properties to the material has attracted a lot of attention to the easy and efficient separation of materials from aquatic media after adsorption [11]. Therefore, it is of great significance to prepare the LDHs with a magnetic property for the removal of organic pollutants from aqueous environments.
MJEN MANAS Journal of Engineering, Volume 10 (Issue 1) © 2022 www.journals.manas.edu.kg Response surface methodology (RSM) is one of mathematical and statistical techniques and usually utilized to optimize of a process, minimize the error of tests, and decrease the number of tests [12]. When assessing the role of variables affecting the adsorption process, the RSM can be considered as a suitable statistical technique for improving, developing, and optimizing processes [9].
The aim of this research is to the synthesis of the magnetic LDH composite based on nickel (Ni) and manganese (Mn) (Fe 3 O 4 /NiMn-LDH) and its decolorization ability for PR. The influences of variables on decolorization of PR were studied and optimized by a Box-Behnken design (BBD) combined with RSM. The adsorption mechanism of PR decolorization on the prepared adsorbent was also investigated.

Synthesis of magnetic composite
Fe 3 O 4 nanoparticles were synthesized by co-precipitation method in the presence of ammonia solution. Firstly, 2.2 g of ferric chloride hexahydrate (FeCl 3 ·6H 2 O) and 0.81 g of ferrous chloride tetrahydrate (FeCl 2 ·4H 2 O) were dissolved in 100 mL of deionized water. Afterwards, the mixture was stirred by adding 7 mL ammonium hydroxide (NH 4 OH) for 30 min at ambient temperature. After the reaction process, black precipitate was separated by a magnet and washed several times with deionized water and ethanol, and then dried at 90 °C for 24 h. Magnetic composite material with a Ni 2+ /Mn 3+ molar ratio of 2:1 was prepared by co-precipitation procedure. 0.5 g of Fe 3 O 4 was dispersed in 100 mL of 1:1 methanol/water solution. Then, 2.91 g of nickel nitrate hexahydrate (Ni(NO 3 ) 2 ·6H 2 O) and 1.26 g of manganese nitrate tetrahydrate (Mn(NO 3 ) 2 ·4H 2 O) were dissolved in 100 mL of deionized water by addition of an alkaline solution (0.03 M sodium carbonate (Na 2 CO 3 ) and 0.12 M sodium hydroxide (NaOH)). The obtained product was collected by a magnet and washed with deionized water and ethanol several times. Finally, Fe 3 O 4 /NiMn-LDH dried at 60 °C overnight (Scheme 1).

PR adsorption studies
A stock solution of PR was prepared by dissolving required amount of PR in deionized water and the desired dilutions were prepared using deionized water. PR adsorption tests were performed under different pH of solution (2-8), adsorbent amount (5-30 mg), and initial PR concentration (C o ) (5-45 mg/L) at ambient temperature. The contact time was a fixed at 120 min. All experiments were conducted in a 20 ml flask with 10 ml of PR solution. The mixture was shaken in orbital shaker and samples were collected by a magnet after 120 min. Then, the supernatant was centrifuged at 7000 rpm for 10 min. The remaining PR concentration was measured by a UV-vis spectrophotometer (Genesys 10S, Thermo Scientific, USA) at 559 nm after centrifugation ( Figure 1). The decolorization efficiency of PR (%) was calculated according to following equation.

Design methodology
Among design of experiment models, Box-Behnken design (BBD) is the most mainly utilized in adsorption technologies [13]. BBD was applied to examine the effects of three independent variables pH of solution (A), adsorbent amount (B, mg), and C o (C, mg/L) on the PR decolorization efficiency onto Fe 3 O 4 /NiMn-LDH. A three-factor and level (-1, 0, 1) design consisting of 17 tests were designed to optimize PR decolorization efficiency. The following equation represented the quadratic polynomial model.

Characterization results
Fourier transform infrared spectroscopy (FTIR) (Nicolet S10, Thermo Scientific, USA) was utilized to investigate changes in the functional groups of Fe 3 O 4 /NiMn-LDH with 500-4000 cm -1 scanning spectra. Figure 2 shows the FTIR spectrum of Fe 3 O 4 /NiMn-LDH. The broad and strong band at 3300 cm -1 -3600 cm -1 is originate from O-H stretching of hydroxyl group of Fe 3 O 4 /NiMn-LDH [14]. The bands at around 1635 cm -1 and 1350 cm -1 are due to the carbonate (CO 3 2-) and nitrate group (NO 3 -), respectively. The peaks at 500-700 cm -1 are assigned to the stretching vibration of Ni-O and Mn-O bonds in the material [15]. Moreover, the band at around 890 cm -1 is corresponded to Fe 3 O 4 of the material [14].

Process model and statistical analysis
The BBD step executed for prediction of three independent variables including pH of solution (A), adsorbent dosage (B), and C o (C) at three levels were performed and their responses are given in Table 1 The plot of the predicted responses versus experimental ones was shown in Figure 4a. It is seen that the estimated points are scatter close to experimental data. This confirm excellent compatibility between the estimated and experimental results by RSM modelling for the PR decolorization efficiency. On the other hand, the normal % probability plot of the residuals is given in Figure 4b. As observed in Figure 4b, the residuals are normally distributed, validating the adequacy and reliability of the proposed model. www.journals.manas.edu.kg solution increased from 2 to about 5. By changing pH value of solution from 5 to 8, the decolorization percentage of PR were significantly decreased. The increase in the decolorization percentage of PR at lower pH values is attributed to interaction between formation of negatively charged PR and the positively surface property of the material [2]. In basic conditions, Fe 3 O 4 /NiMn-LDH becomes more negative, resulting the decolorization percentage of PR progressively decreased owing to the electrostatic repulsion between negatively charged PR molecule and negative material surface [4]. The effect of adsorbent dosage on the PR decolorization efficiency was studied with varying adsorbent amount 5 to 30 mg (Figure 5a,b). The PR decolorization efficiency by Fe 3 O 4 /NiMn-LDH increased significantly as increases in the adsorbent dosage from 5 to about 20-25 mg and the maximum PR decolorization efficiency was achieved at 25 mg adsorbent dosage. This can be due to the available adsorption sites with increase in the adsorbent amount leading increase in removal efficiency [1]. The influence of C o was determined by changing C o from 5 to 45 mg/L (Figure 5b). The PR decolorization efficiency increased with increases in C o of PR. Then, the PR decolorization efficiency at high dye concentrations decreased. This may due to abundant adsorption sites at first resulting increased the removal of PR. However, at further concentrations of the dye molecules, the PR decolorization efficiency reduced based on the saturation of adsorbent sites [3].

Optimization step
Numerical optimization is a technique that is used to show the areas in which the maximum response values are obtained. The optimal point is determined based on the parameters, which is considered as the criterion for the maximum PR decolorization efficiency. Solving numerical analysis for an 86.93% PR decolorization efficiency predicted it occurrence at pH = 5.38, adsorbent dosage = 24.59 mg, and C o = 25.39 mg/L.

The decolorization mechanism of PR
The decolorization mechanism are generally helpful for estimation of information about mechanism and nature of adsorption process. The decolorization mechanisms of PR molecules by Fe 3 O 4 /NiMn-LDH involved electrostatic interaction, ion exchange, hydrogen bonding and coordination with metal. Figure 6  www.journals.manas.edu.kg acidic conditions. The electrostatic interaction can be formed between the sulfonic group of PR molecule and the hydroxyl group in Fe 3 O 4 /NiMn-LDH [18]. SO 3anion of dye might be replaced by CO 3 2-anion, interlayer molecule of Fe 3 O 4 /NiMn-LDH via anion exchange [19]. H-bonding interaction can occur between the phenolic hydroxyl groups of PR molecule and the surface hydroxyls of Fe 3 O 4 /NiMn-LDH [20]. Moreover, the phenol hydroxyl groups of dye can capable of forming coordinate bonds with metal ions in Fe 3 O 4 /NiMn-LDH [21].

Conclusions
In this study, Fe 3 O 4 /NiMn-LDH with magnetic property was successfully synthesized via a co-precipitation synthesis approach. The prepared material was applied to the effective elimination of anionic dye (PR) from aqueous media. The PR decolorization experiments were performed as a function of pH, adsorbent amount, and C o using BBD based on RSM. The results of RSM showed that the model in the decolorization of PR was highly important within 99% confidence interval. The value of R 2 for the decolorization of PR indicates that the quadratic model is considerably appropriate for estimating the performance of PR decolorization on Fe 3 O 4 /NiMn-LDH. The optimum values for the maximum PR decolorization through numerical analysis were 5.38 of pH, 24.59 mg of adsorbent dosage, and 25.39 mg/L of C o . Based on the obtained conditions, the maximum PR decolorization efficiency was determined as 86.93%. The PR decolorization process on Fe 3 O 4 /NiMn-LDH indicated that the prepared material would be a promising adsorbent for the decolorization of dyes.