نانوذرات سیلیکای تزیین شده با نقاط کوانتومی SnO2 برای حذف سریع رنگ کاتیونی از فاضلاب (متیلن بلو) SnO2 quantum dots decorated silica nanoparticles for fast removal of cationic dye (methylene blue) from wastewater

۱٫ Introduction Discharge of untreated color effluents from different textile industries to the environment has become a global apprehension due to its harmful impact on flora and fauna [1]. There are more than 100,000 dyes commercially available, and across the globe about 7  ۱۰۵ tonnes of dying materials are being used per annum [2]. About 10–۱۵% of used dyes are lost during the dying process and are released to environment as effluents. According to the World Bank (WB), around 17–۲۰% of industrial pollutions originate from color textile waste [3]. In general, aromatic organic dyes are stable in nature and therefore difficult to be biodegraded. Almost all of these dyes are toxic to the ecological system, and also have carcinogenic properties. Exposure to methylene blue (MB) may cause permanent injury to the eyes of human and animals [4]. In view of this, it is of immense importance to remove the dyestuff before discharging to water bodies. Various techniques such as membrane filtration, ion exchange, electrochemical, coagulation, chemical oxidation, and adsorption are used for removal of different toxic organic dyes from industrial wastewater. Among all these conventional techniques for removing dyestuff from aqueous medium, adsorption is an effective and attractive technique because of its advantages like small amount of residues, reusability of the adsorbent and simplicity of operation etc. [5,6]. Until very recent times, activated carbon was the most widely used adsorbent for removal of organic dyes, but its use is limited by its high cost, low bio-degradability, presence of macropores and requirement of a large amount of adsorbent for fast removal [3,7,8]. Considerable efforts are being made by many researchers to find appropriate replacement with high adsorption capacity, fast removal and low cost in order to effectively remove dyes from wastewaters. Nanomaterials with high surface to volume ratio is one of the most likely choices for this purpose. Moreover, for macromolecules, the particle diffusion may decrease the adsorption rate and available capacity as well [9]. Hence, it is of significant importance to fabricate a recyclable adsorbent with large surface area and small diffusion resistance for its commercial use. In view of this, much attention has been paid to fabricate different nano adsorbent materials [10]. In recent years, different inorganic oxide materials such as Fe3O4 nanoparticles [10], MnO2 nanosheets [11] and BiFeO3/a-Fe2O3 core/shell composite particles [12] have been studied as possible adsorbent for organic dyes. On the other hand, mesoporous silica nanoparticles (MSN) serve as a unique adsorbent material for different organic dyes and also serve as matrix materials for loading of different nanoparticle [3,5,13]. You et al. [14] synthesized silica hematite nanocomposite for removal of MB from aqueous solution. Shamsizadeh and co-workers [15] efficiently removed malachite green-oxalate (MG) dye using tin oxide nanoparticles loaded activated carbon. Semiconducting SnO2 quantum dots (QDs) are presently attracting consideration due to their noticeable effect in removing pollutants from wastewater [16]. In view of the available literature, SnO2 QDs loaded MSN may fulfill the requirement of nano adsorbent; which have large surface area and at the same time the presence of SnO2 QDs decreases the particle diffusion resistance [8]. To the best of our knowledge, SnO2 QDs decorated MSN (QDs-MSN) is not exploited so far as a possible adsorbent materials for removing pollutants from wastewater. In this work, cationic dye methylene blue (MB) is selected as a model dye to study the performance of QDs-MSN as an adsorbent material for water remediation. Here we present a noble approach of decorating MSN with SnO2 QDs. The effect of adsorbent dosage, contact time, initial dye concentration, salt concentration, temperature and pH of the initial solution towards adsorption capacity was investigated. Results show that QDs-MSN is an ideal aspirant for removal of positively charged organic dyes from colored wastewater. ۲٫ Experimental methods ۲٫۱٫ Materials and synthesis ۲٫۱٫۱٫ Synthesis of mesoporous silica nanoparticles (MSN) MSNs was synthesized by modified Stober’s method [17] using cetyl trimethyl ammonium bromide (CTAB, 99%, spectrochem) as surfactant material and tetraethyl orthosilicate (TEOS, 98%, Kemphasol) as precursor for SiO2. In a typical synthesis, 100 mg of CTAB was dissolved in 25 mL water–ethanol mixture (4:1) and an appropriate amount of ammonia solution (NH4OH, 25%, Merck) was added to the CTAB solution under continuous magnetic stirring. Then 1 mL of TEOS was added drop wise and the temperature of the solution was maintained at 50 C for 4 h. The obtained product was dried overnight at 60 C, after washing with ethanol and MilliQ water following centrifugation (6000 rpm for 10 min). ۲٫۱٫۲٫ Synthesis of SnO2 quantum dots (QDs) SnCl4۵H2O (98%, Sigma–Aldrich) was used as the precursor. In this method, SnCl4۵H2O and ammonium hydroxide (10 mL) were added to 40 ml of deionized water, with continuous stirring. NH4OH instantaneously reacted with SnCl4۵H2O solution and gave a white precipitation. It was then maintained at room temperature for 8 h with continuous stirring. The obtained product was centrifuged (10,000 rpm for 10 min), washed with ethanol and MilliQ water, and dried overnight at 60 C in air. ۲٫۱٫۳٫ Fabrication of SnO2 QDs/SiO2 (QDs-MSN) nanocomposite In this work, we have synthesized the QDs-MSN nanocomposite by a simple wet chemical method. In a typical synthesis batch, first 50 mg MSN was dissolved in 50 mL water–ammonia solution mixture (4:1) and sonicated for 4 min. Then 245 mg of tin precursor (SnCl4۵H2O) was added to this mixture. The final solution was maintained at room temperature under continuous stirring for 8 h. This was then centrifuged (6000 rpm for 10 min), washed with ethanol and MilliQ water, and dried overnight at 60 C in air.