Mesoporous nanocrystalline magnesium oxide for environmental remediation

Abstract

Mesoporous MgO nanocrystals are prepared through combustion route using magnesium nitrate as oxidizer and glycine as fuel. The powder has been characterized using powder X-ray diffraction (PXRD), scanning and transmission electron microscopy (SEM/TEM), surface area and porosity measurements. The PXRD pattern confirms the crystallinity and phase purity of the powder and the particle size obtained from Scherrer’s formula lies in the range 12–23 nm. The SEM result reveals that the powder is porous and agglomerated with polycrystallite nanoparticles. The pore diameter observed in TEM image is in the range 4–11 nm. The surface area of the powder is ∼107 m²/g and average pore diameter obtained from desorption is 7.8 nm. It is found that 0.15 g of as made MgO powder could remove 97% of fluoride from standard sodium fluoride solution (10 ppm) and 75% of fluoride from tube well water. In this technique 90% minimization of sludge could be achieved. The comparative studies on the fluoride removal capacity of as made, regenerated and commercial grade MgO powders are found to be 97%, 76% and 17%, respectively.

Introduction

The development of novel techniques for dehalogenation of industrial wastes containing halides has become immediate task due to their high toxicity and significant ecological hazard. Nanocrystalline alkaline earth metal oxides attract significant attention as effective chemisorbents for toxic gases, HCl, chlorinated and phosphorus containing compounds [1]. Destructive sorption takes place not only on the surface of oxide materials but also in their bulk. Although many metal oxides and metal oxide mixtures prepared and activated in the proper way may be capable of acting as adsorbents in surface chemistry. MgO, serves as a good model since it possesses a simple (NaCl) crystal structure and can be prepared with widely ranging surface areas [2]. It is important to note that the efficiency of the destructive sorption increases with a decrease in the size of the MgO crystallites. MgO works most efficiently in the destructive sorption reaction when its particle size is on a nanometer scale. The high efficiency of the nanoparticle oxides is caused not only by their high surface area but also by the high concentration of low-coordinated sites and structural defects on their surface [1]. As the particle size is scaled down to a few nanometers, the constituting atoms exhibit highly defective coordination environments. Most of the atoms have unsatisfied valencies and reside at the surface. In short, microstructural features such as small grain size, large number of interfaces and grain boundary junctions, pores, and various lattice defects that result from the chosen routes for their synthesis contribute significantly to the unique physical and chemical properties of nanomaterials [3], [4], [5]. Depending on preparation methods, MgO exhibits quite different reactivity toward adsorbed chemicals [6]. Therefore, high surface area material having the most defect sites per unit area, should be of interest as destructive adsorbent. Fine powder of MgO exhibits strong surface basicity and moderate acidity [7]. The most conventional method of synthesis of MgO is the decomposition of various magnesium salts or magnesium hydroxide [8]. The MgO product obtained by this method has relatively large and varied grain size with low surface area. Nanocrystalline MgO is usually synthesized through hydrothermal [9] and sol–gel methods [10], [11].

In continuation of our research programme on porous materials [12], [13] we have attempted to synthesize a porous nanocrystalline MgO powder with large surface area by low temperature solution combustion technique. The combustion technique takes advantages of exothermic, fast and self-sustaining chemical reaction between metal salt and a suitable organic fuel. In this technique, most of the heat required for the synthesis is supplied by the reaction itself [14]. These features make the combustion method an attractive route for the manufacture of technologically useful nanomaterials at lower cost in few minutes as compared to other synthetic routes. Besides, combustion synthesis requires relatively simple equipments and producing high purity homogeneous products with energy saving [15].

To certain extent (0.6 ppm, as per the World Health Organization) fluoride ingestion is necessary for bone and teeth developments but excessive ingestion causes a disease known as fluorosis [16]. The World Health Organization (WHO) standards and Bureau of Indian Standards (BIS): 10500 1991 [17], permit only 1.5 mg/l as a safe limit of fluoride in drinking water for human consumption. Fluorosis continues to be an endemic problem, and more and more areas are being discovered regularly that are affected by fluorosis in different parts of the world. Excess fluoride in ground water has been encounted in many countries [18], [19]. In India, as many as 200 districts spread across 17 states are suffering from high fluoride concentration. More than 6 million people are seriously affected by fluorosis and another 62 million are exposed to it [20]. Thus there is an urgent need for the development of feasible techniques that are efficient for fluoride removal.

Defluoridation of water has been carried out using various methods like coagulation and precipitation process (named as Nalgonda technique) [21], use of locally derived sample of silty clay [22], natural materials [23], soil sorbent [24] and oxide minerals [20]. The removal of excessive fluoride (F⁻) from drinking water was also attempted using ion exchange/adsorption process, in which commercially available metal oxides like activated alumina, magnesia and other materials were used as adsorbents [25]. In the present study we explore the use of combustion derived MgO powder as adsorbent for the removal of fluoride content and other ions in tube well water mainly used for drinking purpose by rural community in India. Finally, regeneration of the used MgO was also conducted using 0.1 N NaOH to evaluate whether regenerated MgO is amenable to efficient regeneration and reuse.