Methyl Orange Adsorption by Fe 2 O 3 @Co-Al-Layered Double Hydroxide

: A magnetic composite consisting of iron oxide and a cobalt–aluminum-layered double hydroxide, Fe 2 O 3 @Co-Al-layered double hydroxide was prepared through linking of Fe 2 O 3 to Co-Al layered double hydroxide (LDH) by sodium acetate. Layered double hydroxides are generally described as [M 2+1–x M 3+x (OH) 2 ] [A n–x/n ·mH 2 O], where M 2+ and M 3+ are divalent and trivalent metal cations, respectively, and A is an n-valent interlayer guest anion. The composite was characterized by XRD, FTIR and UV-Vis. spectroscopy methods. Afterwards, the composite was used for methyl-orange adsorption in aqueous solution. The UV–Vis spectrum indicates that the adsorption process was satisfying. In effect, after several washings of the composite, no decrease of the adsorption capacity was observed.


Introduction
Environmental pollution is one of the three biggest social problems all over the world. Industrial wastewaters containing artificial dyes and heavy metals are potential dangers for the environment [1]. Industrial sewerage is one of the environmental pollution reasons, specially the dyes, which are vacated from various industrial processes such as paper, plastic, textile and food industries. Among the dyes in wastewater, methyl orange is a very harmful contaminant for human and aquatic animals. [2] Therefore, different methods such as physical adsorption, chemical decomposition, and microbiological decomposition are developed for the elimination of dyes and heavy metals in wastewater.
Considering low costs, with a simple design, easy handling and no sensitivity to toxic materials, the adsorption is one of the most effective methods for water purification. Usually active carbon, zeolites, and mineral nanoparticles are used for dye elimination from wastewater. Thus, it is necessary to use materials with high absorbance capacity and a wide domain for contaminants in aqueous solutions.
Layered double hydroxides (LDHs) have a favorable adsorbtion capacity of dye molecules and can compete well with other adsorbants. LDHs have shown a high capability for dye elimination by adsorption [3].
By adding magnetic property to Co-Al LDHs, the collecting of the particles becomes much easier. LDHs are named as hydrotalcite-like materials (because of the structure similarity) or named anion clays and lamellar host-guest materials which are very rare in nature [3] Most of the LDHs have a structure formula of [M(II)1−x M(III)x (OH)2] (A n− )x/n·H2O, which is similar to the hydrotalcites formula, [Mg6Al2 (OH)2] CO3·4H2O. In the LDH formula, M(II) and M(III) are di-and trivalent metals, respectively, 0.2 < x < 0.33 and A n− is the anion which is transferable between layers.

Synthesis of Fe3O4@CoAl LDH
Magnetic Fe3O4 microspheres were prepared by a modified solvothermal route [4]. Initially, 3.24 g of FeCl3·6H2O and 8.64 g of CH3COONa·3H2O were dissolved in 80 mL of ethylene glycol at 313 K to form a clear brown solution under vigorous stirring [5]. The solution was sealed in a stainless steel reactor with polytetrafluoroethylene liner. The reactor was then heated for 8 h at 473 K. After cooling down, the resultant suspension was attracted by a magnet and then washed three times at intervals with deionized water and ethanol solution. The precipitate was dried at 333 K overnight to yield a black powder of Fe3O4.
The magnetic core-shell Fe3O4@CoAl LDH nanohybrid (abbreviated as Fe3O4@LDH) was synthesized by a coprecipitation method. Typically, appropriate amounts of Co(NO3)2·6H2O and Al(NO3)3·9H2O (with the molar ratio, Co 2+ /Al 3+ = 3) were dissolved in deionized water to obtain a solution with a total metal ion concentration of 0.5 mol·L −1 . Then, a moderate amount of Fe3O4 powder (3 g·L −1 ) was ultrasonically dispersed into the mixed salt solution for 5 min. An ammonia-water solution (volume ratio of 1:4) was slowly poured into the suspension under stirring until precipitation at pH 9-10, and then a precipitate was produced. After standing at room temperature for 1 h, the suspension was filtered and then washed four times with deionized water. The filter cake was sealed at 353 K in an oven for about 24 h, and Fe3O4@LDH sol was obtained. Finally, the sol was dried at 353 K to yield a product of Fe3O4@LDH powder.

Methyl Orange Adsorption
In this case, the effect of different parameters such as methyl orange first concentration, on the absorbance mass and contact time were studied. For the isotherm adsorption survey, all the experiments were completed at room temperature and in a dark environment by adding 10 mg of Fe@Co-Al LDH to 30 mL of methyl orange solution with first concentration of 100, 250, 500, and 750, 1000 mg.L −1 . Afterwards, the solution was shaken for 120 min in 300 rpm.
The methyl orange concentrations were measured before and after adsorption by UV-vis spectroscopy in 464 nm.
The adsorption capacity is an important factor which appoints the adsorption capacity for eliminating specific amounts of contaminants. The following Equations (1) and (2) state the contaminant elimination percentage: in which C0 (mg.L −1 ) and Ce (mg. L −1 ) are the first concentration and equivalent concentration of contaminants, respectively. V (mL) introduces as solution volume, m (g) as adsorbant mass and qe (mg/g) as surface equivalent adsorption capacity [6].

Results and Discussion
The characterization of Fe@Co-Al LDH and adsorption of methyl orange was performed by the following methods. Figure 1 shows the XRD pattern which indicates that the Fe@Co-Al LDH was directly synthesized. Two sharp peaks in different 2Ө 9.98 and 20.13 were referred to (003) and (006) plates, respectively. Wide peaks at 2Ө 34.47, 39.54 and 26.28 were referred to (012), (015), (018) sheets and two peaks at 2Ө 60.67 and 61.13 were attributed to (110) and (113) sheets.

FTIR Spectrum
The FTIR spectrum of Fe@Co-Al LDH is shown in Figure 2. The widespread and intense band in the 3446 cm −1 area is due to the stretch vibrations of O−H groups present in the interlayer and water molecules which are in layers. The 1625 cm −1 is related to water bending vibrations. The sharp bands in 1379 cm −1 and 827cm −1 are related to stretch and bending vibration of the interlayer nitrate anion, respectively. However, the band seen in 1357 cm −1 is related to CO3 2− which is caused by existing CO2 in deionized water [4]. Along the adsorption on the outer surface, the interlayer NO3 − ions are replaced with methyl orange by interlayer anion exchanges, as shown in Figure 3 [2].

Scanning Electron Microscopy
The method was selected to investigate the morphology and particle size of Fe@Co-Al LDH. The result was shown in Figure 4, in which the particles have no distinct shape, but are relatively isomorph, additionally their sizes are in the range of 0.2-0.5 ηm.  Figure 5. The diagram shows that methyl orange has maximum absorbance in 464 nm and its absorbance is quenched after adsorption on Fe@Co-Al LDH. Actually, the concentration of methyl orange was 250, 500, and 1000 ppm, and the three higher curves were taken before adsorption, and the other three were taken after adsorption. The best result was for blue 500 ppm and 250 ppm of methyl orange which were totally adsorbed on Fe@Co-Al LDH.

Conclusions
A composite of an LDH based on cobalt and aluminum with Fe3O4 was prepared and applied to remove methyl orange as a toxic and dangerous dye. The results of UV-vis spectroscopy shows that the best removal is for 250 and 500 ppm methyl orange which is completely adsorbed on the composite.