Photoactivity of magnesium aluminate under solar irradiation for treatment of wastewater contaminated by methylene blue: Effect of self-combustion factors on spinel characteristics
Graphical abstract
Introduction
Recently, a wide range of investigations exhausted the advance applications of magnesium aluminate spinel (MgAl2O4) due to high melting temperature, above 2000 °C, chemical resistance in the acidic and alkali environments, the appropriate mechanical strength and optical properties [1]. A direct consequence of development in the magnesium aluminate production led to find new applications in the ceramic and refractory industries [2], catalyst support [3], catalytic reactions like steam reforming [4] and adsorption [5]. However, it was employed as photocatalyst in the degradation of organic contaminants [6].
The crystalline structure of magnesium aluminate spinel is similar to the structure of diamond [7] in which Mg atoms are located in the tetrahedral positions and Al atoms occupy the octahedral sites [8]. Different routes were suggested to create this structure such as solid state reaction by milling [9], self-heat sustained technique [10], sol–gel preparation [11,12], precipitation [13], surfactant assisted precipitation [14], co-precipitation [15], hydrothermal [16], molten salt technique [17] self-combustion [18] and microwave-assisted combustion routes [19]. However, it is rather difficult to produce MgAl2O4 with higher specific surface area by the conventional solid state preparation method.
The crystalline structure of magnesium aluminate depends on route used in the production of spinel. Nowadays, the self-combustion technique was received remarkable attention due to applicability in the fast and simple production of pure spinels. The self-combustion synthesis is an effective and economic technique for the production of nanomaterials, notably with characteristics comparable to those produced by traditional methods. This technique is based on a self-sustained reaction, occurring in the volume or layers of homogeneous system containing oxidizers like metal nitrates and fuels, not only yields nano-sized oxides but also removes the impurities from the product structure [20]. It is well known that the fuel composition is an important factor for the spinel preparation. Urea is the most popular fuel for the fabrication of ceramic materials with controllable stoichiometry. The single-phase and thermally stable MgAl2O4 powder was prepared with the microwave-assisted combustion reaction by urea [21]. The nano-crystalline magnesium aluminate spinel was also synthesized by self-combustion method in which citric and oxalic acids were used as fuels [22,23]. The nano-scale MgAl2O4 powders were fabricated via a microwave-assisted combustion by employment of urea, glycine and starch mixture. It was reported that the addition of starch markedly altered the combustion type from flame to smoldering condition, causing to dissipate the nanoparticles [24]. The physico-chemical characteristics of products are strongly affected by pH when glycine is used as a fuel [25]. By control of pH and glycine content, it is possible to achieve the nano-sized metal aluminate spinel [26]. It was shown that the gel structure is the dominant factor, affecting the nano-particle dissipation. The optimum potato-starch content for the synthesis of pure nano-particles is 40% [27].
With the industrialization of countries, the commercially consumption of organic dyes has been extensively increased in textile, paint and pigment industries, leading to critical contamination of water, ground water and soil. It was estimated that at least 10% of used dyes is discarded as wastewater, causing the skin diseases or increase cancer risk due to toxicity [28]. Therefore, it is great of importance to innovate the ecofriendly processes for removal of dyes from wastewaters below the standard levels. Although, the physical or chemical treatments like coagulation [29], biodegradation [30], electrochemical method [31], membrane filtration [32] and adsorption [33,34] are applied in the industrial scale, these techniques cannot effectively remove the organic dyes without creation secondary pollution. For example, adsorption is the most popular route because of effectiveness and simplicity but the regeneration of adsorbents is the main problem to avoid the solid waste deposition in the landfill. Recently, an extensive attention has been paid on the development of wastewater treatment by photocatalytic process due to the imbalance in the industrial water quality. Although the photocatalytic process is applied in the aquatic environment in which the dye concentration is low, the toxicity of these contaminants constitutes a potential risk for the human and leaving things even in little amount. Over the past decades, the photocatalysts like anatase, zinc, tin, cuprous and nickel oxides have found attracted interest for the removal of dyes and volatile organic compounds (VOCs) from wastewaters [35,36]. The higher photocatalytic activity and non-toxic nature of TiO2 made to widely used by researchers in the design of photoreactors [37]. On the other hand, some disadvantages such as the cost of fabrication, tendency to agglomeration and difficulty of regeneration after treatment caused to focus the investigations on the synthesis of other photocatalytic nanocomposites like magnesium aluminate to overcome the mentioned drawbacks. The MgAl2O4 nano-powder containing about 55.4% amorphous phase showed the highest photocatalytic activity for removal of methylene blue [38]. The pure magnesium aluminate nanoparticles were synthesized by combustion method using urea, oxalic and citric acid followed by the calcination at 800 °C. The produced material showed the appropriate photoactivity for removal of reactive red dye under UV or sunlight irradiations within 5 h [39]. The amorphous MgAl2O4 with the high specific surface area was obtained through the hydrothermal method. The powder calcined at 700 °C exhibited the highest photoactivity in the deletion of methylene blue [40].
Despite the extensive studies on the synthesis, application and characterization of magnesium aluminate, there is still little information about the effects of self-combustion synthesis condition on the photoactivity of spinel. In our previous studies, the influence of ignition nature and glycine content on crystalline structure of spinel has been extensively studied [25,26]. It was emphasized that the specific surface area is promoted via the gel preparation at the neutral pH and is declined by the synthesis in the acidic condition. According to the results reported by investigators, the nitrogen doping is the main factor in the enhancement of photoactivity [40] which seems to be affected by the gel preparation pH and calcination temperature. It is evident that there is a complex network interaction between the dye removal efficiency and synthesis factors. Consequently, it is great of importance to investigate the photoactivity as a function of gel preparation pH and calcination temperature, systematically based on response surface methodology and kinetic model. Undoubtedly, all of these parameters define the relationship between the removal efficiency, gel combustion nature and N doping. On the other hand, to realize the application of magnesium aluminate spinel as a potential photocatalyst in the wastewater management, the photocatalytic removal of methylene blue over MgAl2O4 was carried out under the solar irradiation which is the lack in literature.
Section snippets
Materials
All chemicals including analytical grade of magnesium nitrate, Mg(NO3)2.6H2O (9–1813446, <99.0%), and aluminum nitrate, Al(NO3)3.9H2O (2-27-7784, <98.5%), were purchased from Merck Company and used without further purification. Glycine, NH2CH2COOH (6-40-56, <99.7%), was obtained through the Merck Company and used as a fuel. Also, Methylene blue, C16H18ClN3S (MB, 319.85 g mol-1) was provided from Merck Company for preparation of dye solutions. The distilled water was employed in the all steps of
Photoactivity of prepared spinels
Fig. 1 indicates the evolution of dye removal with the solar irradiation time over the prepared spinels. The rise in efficiency or dye concentration decay show that the contaminant removal steadily increases with irradiation time took place within 160 min, depending on spinel structure. The same trend was observed when the spinels were synthesized at different pH and calcination temperature.
The maximal deletion is attained after 40 min over the spinel prepared in the acidic condition, pH: 2,
Conclusion
The results reported in this investigation clearly exhibited that the partially crystalline magnesium aluminate spinel produced by the self-combustion route is the adequate composite to apply in the photocatalytic removal of cationic dye, remained in the aqueous solutions. The comparable activity was observed respect to other commercial materials like anatase as reported in the literature. The dye removal rate was well described by a modified kinetic model, which allowed the study the effects
CRediT authorship contribution statement
Shiva Salem: Supervision, Conceptualization, Methodology, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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