Heavy metal removal from aqueous solutions by calcium silicate powder from waste coal fly-ash
Introduction
Heavy metal pollution has become an increasingly serious environmental problem in recent decades, causing numerous diseases and disorders. An action plan for tackling soil pollution in China was released on 28th May 2017. The Action Plan for Soil Pollution Prevention and Control aims to improve soil quality, ensure safe agricultural products and a healthy living environment for people, according to the State Council, China’s cabinet. To control soil pollution by heavy metals, China has vowed to cut the discharge of major heavy metal pollutants in key industries by 10% by 2020. Finding a highly efficient remediation technology remains a bottleneck. Various processes exist for removing dissolved heavy metals from aqueous solutions, including ion-exchange, precipitation, phytoextraction, ultrafiltration, reverse osmosis, electro coagulation, electro dialysis, and adsorption. Adsorption, particularly using low-cost absorbents, has attracted a great deal of attention from the research community and industry due to its high efficiency and simplicity of operation.
Numerous studies have been reported regarding the adsorption of heavy metals by both natural and artificial adsorbents. Natural zeolites, which are abundant, low-cost adsorbents, show high adsorption capacity for heavy metal ions in water (Erdem et al., 2004). The kinetics (Kocaoba et al., 2007, Motsi et al., 2011, Panayotova and Velikov, 2002), equilibrium (Kocaoba et al., 2007), and influence of temperature and pH (Jimenez et al., 2004) have been investigated in relation to these materials. Activated carbon adsorbents are widely used in the removal of heavy metals (Adebisi et al., 2017, Demiral and Güngör, 2016, Jusoh et al., 2007, Kang et al., 2008, Tounsadi et al., 2016). Carbon nanotubes and graphene, as new carbon absorbents, have also been applied in the removal of heavy metals (Duru et al., 2016, Gu et al., 2015, Gupta et al., 2016, Ihsanullah et al., 2016, Sahraei et al., 2017). Biosorption is an alternative for the removal of heavy metals due to its low cost and eco-friendly nature (Alluri et al., 2007, Basu et al., 2017, Mudhoo et al., 2012, Tang et al., 2017, Wang and Chen, 2009). Reducing the cost of adsorbents is a critical issue for spreading the use of adsorption technologies for the removal of heavy metals.
Synthesized calcium silicates as low cost adsorbents have been used in removing heavy metals. Tobermorites were made from waste container glass using hydrothermal method and showed high removal capacities for Cd2+ and Zn2+ (Coleman et al., 2006, Coleman et al., 2014). Functionalized calcium silicate nanofibers were derived from oyster shells for removal of heavy metal ions, showing removal capacity of 203 and 256 mg g−1 for Cu(Ⅱ) and Cr (Ⅵ) cations (You et al., 2016). The cement based materials have been widely researched for stabilization or solidification of heavy metals (Chen et al., 2009, Tommaseo and Kersten, 2002). All these studies showed the potential application of calcium silicates in the removal of heavy metals.
Coal remains one of the main energy resources. Indeed, coal accounted for 62.0% of the total energy consumption in China in 2016 (Statistical Communiqué of the People’s Republic of China on the 2016 National Economic and Social Development http://www.stats.gov.cn/english/PressRelease/201702/t20170228_1467503.html). Fly-ash is one of the emission pollutants of coal combustion, with about 60 million tonnes being discharged annually in China. Dust and soluble components of fly-ash present great risks to the environment. Comprehensive utilization of fly-ash is considered of great importance by the Chinese government. Fly-ash is mainly composed of alumina and silica, with some other metal oxides. Coal resources with aluminum contents in the coal ash of over 50% are abundant in the northwest of China. Overall, the reserve of this kind of ash in China exceeds 20 billion tonnes. A demo-refinery for coal-ash-based alumina has been constructed in the Inner Mongolia Autonomous Region. The strategy of producing alumina from fly-ash has been a typical case of cyclic economy in China. This alumina production route was released as a Chinese policy in 2013. The fly-ash represents an alternative raw material for alumina due to the shortage of bauxite in China. However, the silica-based by-product constitutes a secondary pollutant and will influence the economic efficiency of the production of alumina from fly-ash if it cannot be effectively reused. Therefore, exploring novel routes for the utilization of this silica-based by-product is a promising strategy. During alumina refining, the silica in the fly-ash is removed as calcium silicate powder (CSP), a new by-product. Employment of CSP as an adsorbent to remove heavy metals in water, thereby controlling pollution with waste, should achieve a win–win situation for the economy and the environment.
Herein, we report the mechanism of Ni2+, Cu2+, Zn2+, and Co2+removal by CSP from alumina production, along with data on its performance, to pave a way for application of CSP. It has been found that the high adsorption capacity of CSP for heavy metal ions results from ion-exchange. The CSP has shown super removal capacities of the four used heavy metals. The environmental benefit for soil pollution control has been analyzed.
Section snippets
Chemicals and materials
CSP was supplied by Inner Mongolia Datang International Renewable Energy Resource Development Co., Ltd. (Hohhot, China) and was used as received. Nickel (II) nitrate and cobalt (II) chloride were purchased from TianJin JINKE Fine Chemical Research Institute (Tianjin, China). Ammonium citrate, dimethylglyoxime, ethylenediamine tetraacetic acid tetrasodium salt, hydroxylamine hydrochloride, copper (II) sulfate, zinc(II) nitrate, and sodium pyrophosphate were purchased from Xilong Scientific
Analysis of the CSP
The composition of the CSP is shown in Table 1. It was mainly composed of SiO2 and CaO, with small amounts of impurities. The particle size was about 17 μm (Fig. 1) and the crystalline phase was mainly calcium silicate hydroxide (Riversideite-9A) (shown in Fig. 2). The surface area of the CSP was 50.68 m2/g.
Adsorption isotherms
Fig. 3 shows the adsorption isotherms of the CSP for metal ions. The equilibrium adsorption increased with increasing concentration of the heavy metal ions. It should be noted that the
Environmental and economic benefits
The application of CSP in the removal of heavy metals from water will clean the water, collect heavy metals, and reduce soil pollution. We chose Inner Mongolia Datang International Recycling Resource Development (IMDIRRD) Co., Ltd. (Hohhot, China) as a case to evaluate the environmental and economic benefit of CSP adsorption as a means of removing heavy metals. IMDIRRD was founded in 2007 for the re-utilization of coal fly-ash from a coal-fired power plant. The main product of IMDIRRD is
Conclusions
CSP is a new kind of by-product from the procedure for the production of alumina from solid waste coal fly-ash. CSP has been investigated as an adsorbent for the removal of Ni2+, Cu2+, Zn2+, and Co2+ ions from simulated solutions. A high adsorption capacity of these heavy metals onto CSP was achieved, with maximum adsorptions in the range of hundreds of mg/g. The isotherms were well described by the Langmuir model. Kinetic studies have indicated that the adsorption of heavy metals follows a
Acknowledgement
The authors are thankful for the financial support by the National Key Technology R&D Program of China (2009BAB49B02).
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