Article pubs.acs.org/IECR
Synthesis of Zeolite/Aluminum Oxide Hydrate from Coal Fly Ash: A New Type of Adsorbent for Simultaneous Removal of Cationic and Anionic Pollutants Jie Xie, Zhe Wang, Deyi Wu,* Zhenjia Zhang, and Hainan Kong School of Environmental Science and Engineering, Shanghai Jiao Tong University, No. 800, Dongchuan Road, Shanghai 200240, China S Supporting Information *
ABSTRACT: Since the first successful synthesis of zeolite from coal fly ash (ZFA) in 1985, the preparation and application of ZFA has been intensively investigated to recycle the solid waste. However, problems arising from the waste alkaline solution have rarely been addressed to date. This study initiated a novel method to synthesize ZFA/Al2O3 hybrid material by introducing a reaction step involving the neutralization of the waste alkaline solution with soluble Al salts into the traditional ZFA synthesis route. When compared with ZFA, ZFA/Al2O3 was found to have a significantly higher CEC. The increases of the BET surface area and phosphate-immobilization capacity were even more dramatic, with the former increasing by 2−4 times and the latter increasing by 2−3 times. The hybrid material had a significantly lower alkalinity than ZFA. The results also showed that the effluent from the production of the hybrid material could be much more environmentally friendly. 1985,3 problems related to the alkaline waste solution produced during the synthesis process have drawn little attention. The alkaline waste solution is generated in huge amounts and can contain a number of ingredients such as Al, Si, Fe, and other anionic or amphoteric heavy metals, which are harmful to the environment. Generally, the hydrothermal synthesis of zeolite uses liquid/solid ratios as high as 3−8 L/kg, and the NaOH concentration used in the reaction ranges from 1 to 2 M, which means that the synthesis of 1 ton of ZFA would produce 3−8 tons of waste alkaline solution. Following the synthesis process, the concentration of NaOH decreases to different extents depending on the synthetic conditions, the composition of CFA, and so on. Although it is thought that the waste alkaline solution could be recycled in subsequent zeolite preparation processes, the quality of products made in this way would get worse or unstable if the alkaline waste solution were recycled directly or the concentration of alkali for recycling were inaccurately adjusted to compensate for consumption. The soluble components in the waste solution would gradually accumulate when the recycling was done, which would also impact the quality of the ZFA. In addition to these limitations, ZFA shows an undesirably strong alkalinity even after repeated washing in distilled water. Repeated washing not only consumes large amounts of water, but also discharges waste solution with high pH. Most past investigations of ZFA focused mainly on the synthesis and use of the zeolite itself but largely overlooked these problems. Based on an analysis of the alkaline waste solution, we developed a novel synthesis route to overcome those hurdles, by embedding one additional step of neutralization in the
1. INTRODUCTION Zeolites are aluminosilicate minerals with three-dimensional framework structures containing AlO4 and SiO4 tetrahedra that are linked to each other through the sharing of the electrons of the oxygen atoms, forming interconnected cages and channels. Zeolites have valuable physicochemical properties, such as cation exchange, molecular sieving, catalysis, and adsorption. Zeolites have been studied intensively and used widely in the industrial and agricultural fields.1,2 Owing to significant achievements in this area, research on zeolites was selected as one of the top 10 breakthroughs in 2011 by the journal Science.2 Given the merit of low- or zero-cost raw materials and the possibility of reusing solid waste, synthesis of zeolites from coal fly ash (CFA) have been extensively investigated in recent years.3−15 Studies have shown that, because zeolites are negatively charged, the obtained zeolites can act as adsorbents for removing cationic pollutants from water.16−29 However, zeolite synthesized from CFA (ZFA) is not a pure zeolite but rather is generally composed of a zeolite fraction and a nonzeolite fraction. Our previous works confirmed that ZFA has an impressive ability to simultaneously remove cationic pollutants, such as ammonium and heavy-metal ions, and oxyanionic pollutants, such as phosphate.30−33 We found that, in the simultaneous removal of ammonium and phosphate, the zeolite fraction is responsible for the adsorption of ammonium whereas the nonzeolite fraction containing the free or associated oxides CaO, Al2O3, and Fe2O3 has an affinity for phosphate.30−33 Nevertheless, it must be stressed that the content of the oxides in ZFA for the binding of phosphate is limited, with the types and concentration levels of the oxides depending on the origin of the CFA. Therefore, it is impractical to expect the actual use of all ZFAs for the efficient removal of anionic pollutants, such as phosphate. On the other hand, despite intensive investigations since the first successful synthesis of ZFA by Höller and Wirsching in © 2013 American Chemical Society
Received: Revised: Accepted: Published: 14890
July 6, 2013 September 18, 2013 September 23, 2013 September 23, 2013 dx.doi.org/10.1021/ie4021396 | Ind. Eng. Chem. Res. 2013, 52, 14890−14897
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Figure 1. (Left) Traditional process for ZFA production and (right) proposed process for zeolite/Al2O3 production.
drying at 45 °C led to the formation of aluminum oxide hydrate, we refer to the obtained product as ZFA/Al2O3 for convenience. 2.2. Cation-Exchange Capacity (CEC) and PhosphateImmobilization Capacity (PIC). Cation-exchange capacity (CEC) was determined by the ammonium acetate method.34 Phosphate-immobilization capacity (PIC) was determined by a repeated adsorption test as follows: (1) First, 40 mL of phosphate solution with a concentration of 20 mg of P/L was put into a preweighed centrifuge tube (W1) containing 0.2 g of the materials (W2). After being shaken for 24 h, the suspension was centrifuged, and the supernatant was poured into another tube for the determination of the phosphate concentration (Ce). The tube with the residual solution was weighed again (W3), and the volume of the residual phosphate solution could be calculated by assuming the density of the residual solution to be 1 g/mL: V (mL) = [W3 (g) − W2 (g) − W1 (g)] × 1 mL/g. The amount of the P remaining in the residual solution that was not adsorbed by the sample (R) and the amount of P adsorbed by the sample (S) were calculated by the equations R (mg) = [V (mL) × C (mg/L)]/(1000 mL/L) and S (mg/g) = [(20 − Ce) (mg/L) × 0.04 (L)]/[W2 (g)]. The volume of residual solution and the amount of phosphate in the residual solution were considered in the calculation of the subsequent equilibration step. (2) A fresh solution of the same phosphate concentration (20 mg of P/L) was added and equilibration was repeated until the removal efficiency of phosphate was less than 5% {removal efficiency = [(Ci − Ce)/Ci] × 100%, where Ci is the initial phosphate concentration and Ce is the phosphate concentration after adsorption}. The total amount of phosphate retained by the materials was thus calculated to represent the maximum immobilization capacity for phosphate (PIC). The fractionation of phosphorus for the materials saturated with phosphate was then conducted by a sequential extraction scheme following a protocol modified from Hieltjes and Lijklema.35 The fractionation scheme comprises (1) two consecutive extractions in 1 M NH4Cl at pH 7 (denoted as loosely bound P, calculated by subtracting R in the final step of the repeated adsorption test from the amount of extracted P), (2) two consecutive extractions in 0.1 M NaOH followed by extraction in 1 M NaCl [denoted as (Fe + Al)-bound P], and (3) two consecutive extractions in 0.5 M HCl [denoted as (Ca
process (Figure 1). Specifically, following the traditional synthesis process, the mixture of zeolite and alkaline waste solution is further neutralized with water-soluble Al salts, and the product is finally heated as needed to produce ZFA/Al2O3 hybrid material. In this way, the obtained material is expected to be more suitable for the simultaneous removal of cationic and anionic pollutants. This article describes the properties of the ZFA/Al2O3 product in terms of the adsorption of cationic and anionic pollutants. The changes in mineralogical and chemical composition during this novel synthesis process were also analyzed. Our results indicate that, through this more environmentally friendly synthesis process, a new hybrid material with greatly improved valuable properties can be obtained.
2. EXPERIMENTAL SECTION 2.1. Materials. The CFA starting materials used in this study, with different compositions, particularly different CaO contents, were obtained in China from the Wujin second power plant (Shanghai), the Wenzhou power plant (Zhejiang), and the Minhang power plant (Shanghai). According to specification ASTM C618, they can be classified as a Class C fly ash, a Class F fly ash, and a Class F fly ash, respectively. For ZFA preparation, approximately 150 g of fly ash was placed in a bench-scale reaction vessel and mixed with 900 mL of 2.0 M NaOH solution. The slurry was boiled with reflux for 24 h with stirring at 95 °C. After the mixture was allowed to cool to room temperature, ZFA was recovered by centrifugation and washed with doubly distilled water three times and with ethanol twice. Finally, ZFA products were dried in an oven at 45 °C, ground to pass through an 80-mesh sieve, and stored in airtight containers for later experiments. To synthesize ZFA/Al2O3, after the mixture of ZFA and waste alkaline solution was allowed to cool to room temperature, it was neutralized with AlCl3, that is, 2 N AlCl3 solution was added dropwise (10 mL/ min) to the mixture with continuous stirring. The volume of AlCl3 added was equal to the volume of NaOH used in ZFA preparation. To guarantee a sufficient reaction of AlCl3 with the alkaline solution, stirring was maintained for another 4 h after the addition of AlCl3 was completed. The ZFA/Al2O3 products were then recovered, washed, dried, and ground in a manner similar to that used for ZFA. In our current study, even though 14891
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+ Mg)-bound P]. Finally, residual P was calculated as PIC − loosely bound P − (Fe + Al)-bound P − (Ca + Mg)-bound P. 2.3. Characterization. The elemental compositions of the materials were determined by X-ray fluorescence (PW2404, Philips Company). X-ray diffraction (XRD) patterns were recorded using a D8 ADVANCE instrument (Bruker-AXS Company) with Cu Kα filtered radiation (30 Kv, 15 mA). To determine the contents of different fractions of calcium (CaCO3, CaSO4, and free CaO), analysis by XRD in combination with standard addition methods was undertaken to measure the contents of CaCO3 and CaSO4, whereas free CaO was calculated by subtracting CaCO3 and CaSO4 from the total CaO content, which was obtained by X-ray fluorescence analysis. BET surface areas were determined on a NOVA1200e apparatus (Quantachrome Company) using nitrogen adsorption isotherms. Waste solution was collected and acidified for analysis by inductively coupled plasma-atomic emission spectroscopy (iCAP 6000 Radial, Thermo Company). The pH values of the materials were tested as follows: Forty milliliters of 0.01 M CaCl2 was added to centrifuge tubes containing 0.2 g of material, and then the final pH after a 24-h equilibration period was measured using a Hach Sension+ pH meter.
3. RESULTS AND DISCUSSION 3.1. Mineralogical Composition. XRD patterns of CFA, ZFA, and ZFA/Al2O3 are given in Figure 2. It was observed that a monomineral phase of zeolite was formed following the hydrothermal reaction of each CFA. The zeolite in Minhang and Wenzhou ZFA was identified as NaP1 (Na6A16Si10O32· 12H2O), whereas Linde type A (NaA11.5Si1.5O6·5.1H2O) was produced in Wujin ZFA. Whereas the anhydrite (CaSO4) in Wujin CFA disappeared after the hydrothermal treatment, other crystalline minerals in the CFAs were identified in the ZFAs. Specifically, quartz and mullite in Minhang CFA, quartz and calcite in Wenzhou CFA, and calcite in Wujin CFA were left over after the synthesis process. Therefore, the ZFAs were not pure zeolite but contained components that originated from the corresponding CFAs. The peaks in the XRD pattern of each ZFA were all detected for the corresponding ZFA/ Al2O3, and no new peak(s) appeared upon the formation of Al2O3 in Wenzhou ZFA/Al2O3. In contrast, peaks attributed to hydrargillite [γ-Al(OH)3] and aluminum oxide hydroxide (AlOOH) were detected for Wujin and Minhang ZFA/Al2O3, respectively. Hence, Al2O3 existed as an amorphous phase in Wenzhou ZFA/Al2O3, whereas it formed crystalline phases in the other ZFA/Al2O3 products. 3.2. Changes in Solid Mass and Chemical Composition. Table 1 lists the results of a mass balance, normalized to 1 g of CFA, for the ZFAs and ZFA/Al2O3 hybrid materials. Following the treatment by NaOH solution at 95 °C, the mass of ZFA solid product increased slightly when compared with the mass of each raw CFA material. Treatment by AlCl3 solution to neutralize the alkaline waste solution increased the mass of ZFA/Al2O3 solid product further, but this increase was substantially higher for Minhang CFA than for the other CFAs. This could be interpreted as indicating that Minhang CFA is a low-calcium fly ash whereas the calcium contents of Wenzhou and Wujin CFAs are much higher (Tables S1 and S2, Supporting Information). Calcium exists as free CaO, CaSO4, and CaCO3, with contents as listed in Table S1 (Supporting Information). Of these forms, free CaO and CaCO3 are soluble to different degrees and were partially removed by neutralization with AlCl3 solution (Table S1, Supporting Information).
Figure 2. XRD patterns of CFA, ZFA, and zeolite/Al2O3. L, Linde type A; H, hydrargillite [γ-Al(OH)3]; C, calcite; A, anhydrite (CaSO4); P, NaP1; Q, quartz; M, mullite; Al, aluminum oxide hydroxide (AlOOH).
The chemical compositions of CFA, ZFA, and ZFA/Al2O3 are given in Table S2 (Supporting Information). The main components of CFA are the oxides of Si and Al, along with various metallic oxides. Of them, Al2O3 and SiO2 are the most abundant components in CFA, accounting for 60.35%, 66.44%, and 86.05% for the Wujin, Wenzhou, and Minhang CFAs, respectively. According to the analytical results (Table S2, Supporting Information), changes in chemical composition can be classified into four types. To better describe the changes in chemical composition from CFA to ZFA and from ZFA to ZFA/Al2O3, the results for Wenzhou samples are graphically presented in Figure 3 as a representative. The first type of change included Si, Fe, Mg, K, and Ti, whose contents decreased gradually from CFA to ZFA and from ZFA to ZFA/Al2O3, but the extents of the decreases were generally small. In the three CFAs, however, the decreases were 14892
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Only Al was classified as the third type of change. The characteristic of this type of change is that the content decreased slightly from CFA to ZFA, but increased from ZFA to ZFA/Al2O3. The decrease in Al content from CFA to ZFA can be attributed to the loss of Al in waste alkaline solution (Table 4), but the dilution effect through the increase of mass might also lead to the reduction of Al content (Table 1). On the other hand, the increase in Al content from ZFA to ZFA/ Al2O3 was clearly caused by the neutralization treatment with AlCl3. However, the Al content increased to a significantly lesser extent from ZFA to ZFA/Al2O3 in the case of Minhang CFA, because of the high dilution effect from the mass increase (Table 1). The fourth type of change included Na and H2O, whose contents in CFA were very low but showed a dramatic increase with the synthesis treatments. The sodium content can be regarded as a measure of the number of negative charges in ZFA, because sodium exists as an exchangeable cation in Nasaturated ZFAs, which were formed because of the use of concentrated NaOH in the synthesis process. The sodium ions on the ZFA surface were partially replaced by Al ions during the AlCl3 treatment, resulting in a decrease of the sodium content from ZFA to ZFA/Al2O3. The increase in moisture was analogously due to the increase in exchangeable cation capacity (number of negative charges), which would induce a hydrophilic surface. 3.3. BET Surface Area and Cation-Exchange Capacity (CEC). The results for the BET surface areas and CECs of the materials are presented in Table 2. Specific surface area (SSA) is an important parameter in their application as adsorbents. All
Table 1. Results of Mass Balance Normalized to 1 g of CFA for ZFA and ZFA/Al2O3 material
mass (g) Wujin
CFA ZFA ZFA/Al2O3
1.00 1.14 1.33 Wenzhou
CFA ZFA ZFA/Al2O3
1.00 1.14 1.34 Minhang
CFA ZFA ZFA/Al2O3
1.00 1.05 1.57
more pronounced for Wenzhou CFA, as shown in Figure 3, probably because of its higher increase in moisture following synthesis process (Table S2, Supporting Information). The second type of change involved calcium, whose content decreased only slightly from CFA to ZFA, but decreased greatly from ZFA to ZFA/Al2O3. However, the extent of the decrease depended on the type of CFA, specifically, on the calcium content of CFA. That is, a slight decrease was observed for the low-calcium fly ash Minhang CFA, but the calcium content was reduced almost by half for the other high-calcium CFAs. This decrease in calcium content from ZFA to ZFA/Al2O3 was due to the loss of Ca ingredients through dissolution by the acidic AlCl3 solution treatment.
Figure 3. Comparison of chemical composition among CFA, ZFA, and ZFA/Al2O3 for Wenzhou samples. 14893
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Table 2. Specific Surface Areas (SSAs), CECs, and pH Values of the Materials SSA (m2/g)
material
SSA(CFA) (m2/g)
CEC (cmol/kg)
CEC(CFA) (cmol/kg)
pH(CaCl2)
