Synthesis of high quality zeolites from coal fly ash: Mobility of hazardous elements and environmental applications
Introduction
More than 40% of the global electric power generation relies on the burning of coal (OECD, 2011), producing a significant amount (typically, 10–15% wt) of solid waste, also known as coal combustion by-products. Currently, the annual production of coal fly ash worldwide is estimated around 780 million tonnes (Mt) (Heidrich et al., 2013). A dominant form of the coal combustion by-products is fly ash, namely, the coal ash collected from the flue gas by dust collectors in the form of glassy fine grey powder. Depending on the source of the coal, the heavy metal concentration in fly ash varies from 2.3 to 6300 ppm for arsenic, 0.02–0.36 ppm for mercury, 1.2–236 ppm for molybdenum, and 0.2–134 ppm for selenium, approximately 10 times higher than those in the original coal in most cases (Izquierdo and Querol, 2012). Therefore, fly ash is sometimes classified as a hazardous waste, and the disposal of the gigantic amount of fly ash has become a serious environmental challenge (U.S Geological Survey, 1997). Currently, most of the fly ash is disposed of in landfills, a practice that occupies valuable lands, generates airborne particulate pollutants and contaminates soils, and both surface and underground water systems. Numerous environmental pollution incidents due to fly ash have been documented, costing billions of dollars (Gottlieb et al., 2010).
Fly ash is widely used in civil engineering applications as construction materials, e.g. additives for cement and feedstock for geopolymer (Yao et al., 2015). However, the present recycling rate of fly ash is still low, only 25% on average globally (Yao et al., 2015). From the perspective of clean coal technology development, fly ash is a waste material requiring immediate attention for appropriate and effective disposal, while from the perspective of waste utilization, fly ash is a resource yet to be fully exploited. All fly ash essentially contains SiO2 and Al2O3 (in both amorphous and crystalline form) and some calcium (Ahmaruzzaman, 2010), which are compositionally similar to zeolites – a valuable material used as adsorbent or catalyst for environmental and industrial applications such as wastewater treatment, soil remediation, and gas separations (Kesraoui-Ouki et al., 1994). Therefore, converting fly ash into zeolites has received increasing attention given it has the potential to not only alleviate the problem of waste disposal but also generate high value products.
Synthesis of zeolite from coal fly ash requires digestion of the insoluble glass phase and crystalline phases such as mullite and quartz in order to transform the building blocks into aluminosilicate zeolitic frameworks by a hydrothermal treatment (Ojha et al., 2004). One-step hydrothermal process in alkaline solutions was frequently used for direct synthesis of zeolites due to the simplicity of the procedure (Elliot and Zhang, 2007, 2005). However, this method usually requires a high alkaline/fly ash ratio (∼2:1 by weight) and long reaction time (24–48 h), but incurs a low conversion (<75%) leaving a significant amount of fly ash residual in the products (Querol et al., 2002). In comparison, a two-step synthesis method, where fly ash undergoes an alkali fusion pre-treatment first, followed by hydrothermal treatment, has been reported to produce high crystalline zeolite products with a potentially lower alkaline consumption (Liu et al., 2011; Shigemoto et al., 1993). Therefore, the two-step method would be the preferred procedure for the conversion of fly ash to zeolites.
Zeolite synthesis usually requires a strong alkaline medium regardless the method used, leading to release of hazardous elements from fly ash. With the stringent environmental regulations and growing public concern about the exposure to heavy metals, many studies have been undertaken on the dissolution of various elements from fly ash (Izquierdo and Querol, 2012). Note that conversion of fly ash to zeolite not only involves a dissolution reaction but also generates a new solid product. However, the migration of hazardous elements from fly ash to zeolite products and discharged wastewater does not seem to have received sufficient attention it deserves. Furthermore, considering zeolites are known to adsorb heavy metal cations and other toxic substances (Scott et al., 2002), it is necessary to understand the loading of the elements of concern in fly ash based zeolites and their mobility during applications for wastewater treatment and soil remediation. Such knowledge will help to evaluate the environmental impacts of zeolitization of fly ash.
In this work, the conversion of a coal fly ash into type A zeolite using the two-step fusion and hydrothermal process was systematically studied, and the synthesis procedure was optimized with reduced NaOH consumption. The structure, morphology and quality of synthesized zeolites were verified using powder X-ray diffraction, scanning electron microscopy and BET surface area analysis, respectively. In addition, X-ray fluorescence, inductively coupled plasma mass spectrometry and inductively coupled plasma optical (atomic) emission spectroscopy were applied to investigate the migration of hazardous elements and leachability of those elements from zeolite products. Furthermore, an environmental application of the zeolites obtained from this study was demonstrated by removing heavy metal cations Cs+ and Sr2+ from simulated industrial wastewater under different ion concentration and temperature conditions.
Section snippets
Materials
The coal fly ash used for zeolite synthesis in this study was sourced from Datong Power Plant II of China Guodian Corporation (3720 MW) using a black coal supplied locally from Datong Coal Field, one of the largest of its kind in China with a total reserve of 35 Gt and an annual production of 70 Mt. The particle size of fly ash shows a bi-modal distribution, ranging from 0.05 to 48 μm, with peaks at 0.3 μm and 12 μm (Supportive Information Fig. S1). As shown in Table 1, this fly ash is high
Structural and morphological changes under different reaction conditions
The structures and morphologies of the raw material fly ash, the intermediates and the zeolites products were characterized by using XRD and SEM, respectively, to probe into the progression of the synthesis reaction and monitoring the type and quality of the products as they formed. Fig. 2a shows that the fly ash consisted of predominantly spherical particles ranging from submicrons to more than 10 μm in size, consistent with the particle size distribution in Supportive Information Fig. S1.
Conclusions
High quality type A zeolite was successfully synthesised from coal fly ash using a two-step, thermal fusion and hydrothermal treatment method. The optimal synthesis conditions are: fly ash and sodium hydroxide (1/1 by weight) and 2 h fusion at 600 °C followed by a hydrothermal treatment at 80 °C for 24 h, which produced a high crystalline zeolite A with typical cubic micro-morphology. Leaching tests showed noticeable mobility of toxic arsenic, selenium, lead, chromium and cadmium from fly ash
Acknowledgement
G. Li is the recipient of an Australian Research Council Discovery Early Career Research Award (DE140101824). The authors acknowledge the funding support of the Australian Research Council through the Industrial Transformation Training Centre for Liquefied Natural Gas Futures (IC150100019) and partnership with China Energy Investment Corporation Limited.
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