Research paperAdsorption of phosphate by acid-modified fly ash and palygorskite in aqueous solution: Experimental and modeling
Graphical abstract
Generalized composite SCM approach with two sites (≡ WHO, ≡ SOH) and two types of complexes can well describe phosphate adsorption data (MAC).
Introduction
Phosphorus (P) is an essential nutrient to the growth of aquatic algal and other biological organisms; however, the excessive presence of phosphate in natural water bodies, especially lakes or reservoirs, will lead to algal blooms and eventually results in degeneration of water quality. This process is well known as eutrophication (Correll, 1998). Eutrophication of the Taihu Lake (Jiangsu, China) in the summer of 2007 caused a serious water crisis to both domestic and industrial users. Removal of phosphate, using either chemical or biological technologies, from such water bodies and inflows can therefore be effective to control the eutrophication of the lakes. Phosphate can be removed from wastewater by adsorption, ion-exchange, precipitation, biological uptake, etc. (Mayer et al., 2013). Of these approaches of phosphate removal, adsorption has attracted more attention due to its simplicity, stability, and operability (Agyei et al., 2000, Das et al., 2006, Yan et al., 2007, Haghseresht et al., 2009, Zamparas et al., 2012). The adsorption of phosphate by commercial adsorbents, industrial by-products, and clay minerals has been practiced for decades. Industrial by-products (e.g., fly ash) and natural clays (e.g., palygorskite, bentonite) are getting more attractive in phosphate removal other than commercial adsorbents due to their lower cost, abundance, and excellent adsorption capabilities (Ye et al., 2006, Yan et al., 2007, Gan et al., 2009, Haghseresht et al., 2009, Zamparas et al., 2012). In fact, there are a variety of fly ash sources (e.g., coal-fired power plants) as well as clay mines (e.g., palygorskite mines) around the Taihu lake. It would be promising to solve the eutrophication problem of the Taihu lake if the abundant and cheap fly ash and/or palygorskite can be utilized as adsorbents for phosphorous removal from municipal sewages that used to be discharged into the lake without any treatments.
Fly ash is a by-product derived from the combustion of pulverized coal in power plants. The utilization of fly ash for phosphate removal from wastewaters has been studied extensively recently, and the results have indicated that fly ash is an excellent alternative adsorbent to commercial adsorbents in phosphate adsorption (Ugurlu and Salman, 1998, Agyei et al., 2000, Agyei et al., 2002, Grubb et al., 2000, Chen et al., 2007, Lu et al., 2009, Xu et al., 2010). It is reported that most of dissolved phosphate can be efficiently precipitated by the high concentration of calcium in class C fly ash (i.e., one type of fly ash generally contains > 20% CaO) (Ugurlu and Salman, 1998, Lu et al., 2009). For class F fly ash (i.e., one type of fly ash contains < 7% CaO), metal oxides in fly ash, such as Al2O3, Fe2O3, etc., can also uptake phosphate efficiently (Grubb et al., 2000, Chen et al., 2007). The phosphate adsorption capacity of class C fly ash is generally greater than that of class F fly ash due to the additional contribution from precipitation by the high content of calcium in Class C fly ash besides the adsorption of phosphate by metal oxides in both types of fly ashes (Agyei et al., 2000, Grubb et al., 2000, Chen et al., 2007, Yan et al., 2007). On the other hand, palygorskite (Pal, (Mg,Al)5(Si,Al)8O20(OH)2·8H2O) is a naturally occurring layered aluminum silicate mineral, which was widely used in catalyst, catalyst supports (Zhang et al., 2010, Zhang et al., 2014, Pushpaletha and Lalithambika, 2011), and adsorbents (Ye et al., 2006, Gan et al., 2009). Previous studies have confirmed that phosphate can be effectively removed by both natural and modified palygorskites (Ye et al., 2006, Gan et al., 2009).
To obtain a higher adsorption capacity, some surface modifications (e.g., introducing or incorporating new groups, thermal treatment, acid activation, etc.) to these adsorbents are required. Haghseresht et al. (2009) found that incorporating lanthanum into bentonite can improve its adsorption capacity for phosphate in aqueous solution. Similarly, a recent study by Zamparas et al. (2012) also showed that iron-modified bentonite can uptake more phosphate than unmodified bentonite. Indeed, the other two typical modification methods, i.e., thermal treatment and acid activation have long been used to produce sorbents for certain practical applications (Bergaya et al., 2006). It is suggested that thermal treatment of palygorskite can increase its adsorption capacity for phosphate (Gan et al., 2009). Ye et al. (2006) also investigated the effects of either thermal treatment or acid activation of palygorskite on phosphate adsorption, and suggested that both modification approaches can improve phosphate adsorption efficiently. Research by Li et al. (2006b) using class F fly ashes (FA) as adsorbent for phosphate had elucidated that both thermal treatment and acid activation can significantly enhance its adsorption capacity. It was also reported that both class F and class C fly ashes modified with sulfuric acid possess a higher phosphate adsorption capacity as compared to the original fly ashes due to the dissolution of the amorphous siliceous spherical particulates that embedded active components for phosphorous adsorption (Liang et al., 2010, Xu et al., 2010).
To gain an insight into the adsorption mechanism of phosphate onto mono-component adsorbent, surface complexation modeling is generally employed to quantitatively description of phosphate adsorption data (Goldberg and Sposito, 1985, Bleam et al., 1991, Nilsson et al., 1996, He et al., 1997, Gao and Mucci, 2001, Rahnemaie et al., 2007). For multi-component or complex adsorbents, however, it still remains a great challenge to use surface complexation modeling to explore the adsorption mechanism due to the complication, even though only a few attempts using constant capacitance model (CCM) (Grubb et al., 2000) and chemical equilibrium model (Johansson and Gustafsson, 2000) has been made. To date, the only successful example of modeling the adsorption data of complex adsorbents is the generalized composite (GC) modeling approach, which was developed by Davis et al. (1998), and can successfully describe and predict the adsorption behaviors of many radionuclide and rare earth ions onto soils (Davis et al., 2004, Tertre et al., 2008). However, GC modeling approach had not been used to fit and predict phosphate adsorption on complex adsorbents yet. To the best of our knowledge, this is the first work that shows how to use the powerful GC modeling approach to describe P adsorption over complex adsorbents, gaining insights into the mechanism of phosphate adsorption.
The objective of this study was twofold: first, to examine the feasibility of using acid-modified fly ash and palygorskite (termed as MFA and MPal) as adsorbents for phosphate uptake in aqueous solution via batch experiments, and second, to gain an insight into the adsorption mechanism of both acid-modified adsorbents via the GC surface complexation modeling. The influences of pH, co-ions, and adsorbent dosage on phosphate adsorption have been studied. Equilibrium and kinetics studies were also performed and fitted with related isotherm models (i.e., Langmuir and Freundlich models) and a kinetic model (i.e., the pseudo second-order rate model). Leaching tests were also conducted to evaluate the safety of disposal or further reuse of the spent (used) adsorbents.
Section snippets
Materials
Class F fly ash (FA) was obtained from the electrostatic precipitator (ESP) ash-hopper III of No. 4 pulverized coal (lignite bituminous coal) boiler at Huaneng Changxing power plant (located in the southwest lakeshore of the Taihu lake), while the palygorskite (Pal) was originated from Xuyi County (Jiangsu, China). The chemical components of both FA and Pal were measured by X-ray fluorescence spectroscopy (XRF), and the results are listed in Table 1. All other chemicals (analytic reagent (A.R.)
Characterizations of fly ash and palygorskite
Table 1 presents the chemical composition of FA and Pal as determined by X-ray fluorescence spectrometer (XRF). The XRF data suggest that the main elementary components of FA are silica, aluminum, iron, and calcium, whereas the elementary chemistry of Pal is dominated by silica, magnesium, aluminum, and iron. Moreover, it is noted that both the calcium content (3.21 wt.%) of FA and the residual carbon content (2.44 wt.%) as determined by the loss on ignition (LOI) test are low, which implies that
Conclusions
Phosphate adsorption by acid-modified fly ash (MFA) and palygorskite (MPal) have been studied in this work. The experimental results suggest that phosphate adsorption is enhanced by acid modification, and that phosphate adsorption is pH dependent for both MFA and MPal. That is, increasing pH in the acidic range led to increased phosphate adsorption, while further increasing pH in the alkaline range resulted in a decreased phosphate uptake. Thermodynamics and kinetics results indicate that
Acknowledgments
The work was partially supported by NSFC Grant (51002080, 51310105009), open fund by Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (KFK1505), PAPD program of Jiangsu Province and the JSNSF Grant (BK20141479). Mr. L. Yang is thankful for his experimental assistance. We thank two anonymous referees and Professor Vicente Rives for their helpful comments.
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