Ind. Eng. Chem. Res. 2008, 47, 295-302
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Synthesis of Zeolite From Thermally Treated Sediment Deyi Wu,*,† Yikai Lu,† Hainan Kong,† Chun Ye,‡ and Xiangcan Jin‡ School of EnVironmental Science and Engineering, Shanghai Jiao Tong UniVersity, No. 800, Dongchuan Rd., Minhang District, Shanghai 200240, China, and Chinese Research Academy of EnVironmental Sciences, No. 8, Dayangfang, Beiyuan, Anwai, Chaoyang District, Beijing 100012, China
By fusion with sodium hydroxide followed by a hydrothermal reaction, a thermally treated sediment was successively converted into zeolites Na-P1, Na-X, hydroxysodalite, F Linde A, and faujasite, which exist either as a monophase or as a mixture depending on synthesis conditions. The formation of zeolite was confirmed by XRD, FT-IR, and SEM. A great increase in cation exchange capacity (CEC) and specific surface area following zeolite conversion was observed. Among the synthesis conditions, the Si/Al ratio of the starting material, the NaOH/sediment ratio, and the liquid/solid ratio were found to be the most important factors influencing the type, the crystallinity, and the CEC value of zeolite product. However, the fusion temperature (350-750 °C), the fusion time (15-240 min), and the crystallization time (2-24 h) had only very limited effects. It is concluded that thermally treated sediment, produced as a solid waste following decontamination of polluted sediment, could be recycled as a raw material for zeolite synthesis. Introduction Most persistent organic pollutants (POPs), such as dioxins and polychlorinated biphenyls (PCBs), and heavy metals entering into water bodies tend to associate with particles, which eventually deposit in sediments. The accumulated pollutants can persist over a long time. These contaminated sediments can have direct toxic effects on aquatic life and/or indirect toxic effects on human health through the bioaccumulation of toxic contaminants in the food chain. Therefore, treatment of heavily contaminated sediment from river, harbor, estuary, or other sites is of the utmost importance. Thermal treatment in a rotary kiln at temperatures typically within the range 550-650 °C is among the most promising and economical ex situ soil or sediment remediation alternatives.1 It is suitable for the removal through evaporation of all organic contaminants from soils and sediments, and a removal efficiency of >99% w/w could be achieved.1,2 Fully treated soil or sediment can be thus achieved, and flue gas treatment involves usually incineration (1000-1100 °C) and dedusting. Thermal treatment has been also designated as the best demonstrated available technology (BDAT) for treating mercury-contaminated soils and sediments.3 When the heating temperature is raised to 800-1100 °C below the melting range of the solids, heavy metals (Cu, Pb, Zn, Cd) could be additionally removed.4,5 Our previous studies revealed that, in addition to the above heavy metals, Cr6+, dioxins, and PCBs could be successfully removed and/or stabilized when the thermal treatment is conducted under an oxygen-free atmosphere (called pyrolysis).6-8 Although thermal treatment is an efficient, reliable, and widely used ex situ decontamination technology for heavily polluted sediments and soils, huge amounts of thermally treated sediments and soils are produced as a result. The treated sediments and soils are typically disposed of in a landfill, but this may no longer be appropriate due to the scarcity of land especially in urban areas. The treated soils could be returned to the site to backfill the excavation,9 but this is not an option for * To whom correspondence should be addressed. Tel.: +86-215474-4540. Fax: +86-21-5474-0825. E-mail:
[email protected]. † Shanghai Jiao Tong University. ‡ Chinese Research Academy of Environmental Sciences.
treated sediment since deepening by excavation is favorable for water bodies such as rivers, harbors, and estuaries. Therefore, the thermally treated sediments have to be reused; otherwise it would become a “cleaned solid waste”. The cleaned solids may be incorporated into asphalt.9 The reuse of the treated sediments and soils in engineering applications, e.g., road beds, building foundation support, grading, and filling, is also suggested although the heat treatment process may alter the physical properties of the material and thus a thorough geotechnical evaluation of the treated material is necessary.9 However, the value of the material is low and the amount needed may be limited in the case of these direct ways for reuse. We consider that thermally treated sediment should have similarities in physical, chemical, and mineralogical properties with coal fly ash since both are derived from inorganic minerals (contained in sediment or coal) and both experience heat treatment. It is known that, during thermal treatment, a large part of the inorganic minerals in coal form an amorphous phase of aluminosilicate which is the most reactive in conversion into other minerals. In seeking alternatives for productive reuse, the synthesis of zeolite from coal fly ash, a byproduct generated every year in great amounts in the world, has been investigated intensively in recent years.10-21 At the same time, the use of the synthesized zeolite in the removal of heavy metals22-24 and nutrients25-27 from wastewater, as well as in gas purification as molecular sieves,28 has been evaluated. Zeolites can be applied to the treatment of nuclear wastewaters for trapping of radionuclides as well.29,30 Regarding the zeolite synthesis technology from coal fly ash, the traditional hydrothermal conversion method has been the most widely used. Recently, however, Molina and Poole12 have conducted a comparative study and have shown that the fusion method yielded a product with a high crystallinity and an excellent performance as a cation exchanger. However, detailed investigation on the fusion method is quite limited, compared with the traditional hydrothermal conversion method. Therefore, further research on the fusion method is required to promote its use and to gain more information on the synthesis process. Like coal fly ash, the thermally treated sediment may be suitable as a raw material to synthesize zeolite because sediments are constituted mainly of silica and alumina which is similar to
10.1021/ie071063u CCC: $40.75 © 2008 American Chemical Society Published on Web 12/21/2007
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zeolite and, at the same time, crystalline aluminosilicate minerals are expected to form amorphous materials after thermal treatment due to the collapse of the crystalline structure. It is considered that the conversion of thermally treated sediment to zeolite is important because (1) it can recycle the thermally treated sediment which awaits appropriate disposal, (2) use of thermally treated sediment as the raw material makes the obtained zeolite product relatively cost-effective, and (3) zeolites are useful materials made of microporous, crystalline minerals that contain large specific surface areas and cation exchange capacities and thus can be used as efficient materials to sequestrate contaminants from wastewater. The aims of the present study were (1) to probe the possibility of the conversion of thermally treated sediment into zeolite by adopting the fusion method, and (2) to examine the influence of main synthesis conditions on the formation of zeolite from the heated sediment. Experimental Section Synthesis Procedure. A sediment sample was collected from the Suzhou Creek near Wuning Road in the urban area of Shanghai, China. After being air-dried, the sample was ground to pass through a 60-mesh sieve and was then heated at 800 °C for 30 min. The thermally treated sediment had the chemical composition of SiO2 69.0%, Al2O3 11.8%, Fe2O3 5.1%, CaO 4.3%, MgO 2.3%, K2O 1.0%, and Na2O 1.0%, as determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) equipment (IRIS advantage 1000). The detailed method for chemical composition analysis was described in a previous paper.25 A portion of the heated sediment, with a Si/ Al molar ratio of 5:1, was adjusted to different Si/Al molar ratios (3:1, 2:1, and 1:1 respectively) by the addition of powdered aluminum hydroxide hydrate. The original sediment and the Si/ Al molar ratio adjusted sediments were used to synthesize zeolite. In the first, 20 g of the sediments was mixed and ground with different amounts of NaOH to obtain homogeneous mixtures with the NaOH/sediment ratio ranging from 0.8 to 2.0 g/g, which were then heated in a nickel crucible in air at 350750 °C for 15 min-4 h. Second, the fusion product was suspended in 50-400 mL of distilled water in a flask and was boiled with reflux for 2-24 h with stirring. The temperature of the solution was 95 °C during the reaction process as determined by a thermometer. At the end of the synthesis process, the solid phase was separated by centrifugation, washed with distilled water three times and with ethanol twice, and finally dried in an oven at 45 °C. The waste solution was determined for Si and Al concentrations by ICP-AES. The flowchart of the synthesis of zeolite from the heated sediment with the fusion method is depicted in Figure 1. The basic synthesis conditions are as follows: Si/Al molar ratio 5:1 (original Si/Al molar ratio of thermally treated sediment, i.e., without the addition of aluminum hydroxide hydrate); NaOH/sediment 1.2 g/g; fusion temperature 550 °C; fusion time 1 h; crystallization temperature 95 °C; crystallization time 12 h; liquid/solid ratio at crystallization stage 10 mL/g. To clarify the effect of every individual parameter on zeolite formation, all other parameters are fixed while only one target parameter is varied. However, the effect of Si/Al ratio on the synthesis of zeolite was additionally undertaken at the NaOH/ sediment ratio of 2 g/g. Experimental Method. The crystalline phase(s) in the materials was (were) identified by powder X-ray diffraction (XRD) analysis on a D8 ADVANCE X-ray diffractometer using Ni filtered Cu KR radiation (40 kV, 40 mA). The Joint
Figure 1. Flowchart showing the synthesis of zeolite from thermally treated sediment with the fusion method.
Committee of Powder Diffraction Standard (JCPDS) codes for XRD identification are 39-0219 for Na-P1 zeolite, 31-1271 for hydroxysodalite, 25-0619 for F Linde A, 12-0228 for faujasite, and 39-0218 for Na-X, respectively. Particle morphology was observed by scanning electron microscopy (SEM) using a JEOL JSM-7401F microscope. The Fourier transform infrared (FT-IR) spectra were recorded by a FT-IR spectrophotometer (SHIMAZU IRPrestige-21) using a KBr method. Cation exchange capacity (CEC) was determined by the ammonium acetate method.31 The results were expressed as centimoles per kilogram of solids. The specific surface area was determined on an ASAP 2010 from Micromeritics by fitting the amount of N2 adsorbed at -196 °C for the BET equation after the preliminary heating at 200 °C. The analysis of the CEC and specific surface area was done in duplicate and the mean data are reported. Results and Discussion Formation and Characterization of Zeolite from Heated Sediment. The XRD patterns of the raw and the heated sediment as well as the synthesized products are shown in Figure 2. The dominant mineral in the raw river sediment is quartz (Figure 2a). A minor amount of the clay minerals of illite and kaolinite was also detected. Upon the heat treatment at 800 °C for 30 min, however, the diffraction peaks from illite and kaolinite disappeared nearly completely from the XRD pattern of the thermally treated sediment (Figure 2a). This indicates that the layer structure of illite and kaolinite was destroyed, forming a new amorphous phase upon thermal treatment. This change in crystallinity of the minerals would boost the reactivity of the sediment inasmuch as the formed amorphous phase is more reactive in an alkaline solution like coal fly ash. The characteristic X-ray diffraction peaks of quartz remained unchanged owing to its thermal stability. Except that mullite is not formed, the XRD pattern of the heated sediment is fairly similar to coal fly ash, a byproduct produced mainly from inorganic fraction during the combustion of coal. When the heated sediment was melted with NaOH followed by crystallization of the melted product, zeolites were successfully obtained (Figure 2b-h). The yielded zeolite species included Na-P1, Na-X, hydroxysodalite, F Linde A, and faujasite, which exist either as a monophase or as a mixture of different zeolites depending on synthesis conditions. The reduction or disappearance of the intensity of quartz peaks suggested that quartz was dissolved to different extents during the synthesis process under the synthesis conditions. Along with the raw sediment and the sediment after thermal treatment, two synthesized zeolites (with codes F3 and D1 in
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Figure 2. XRD patterns of raw and heated sediments (a) and zeolite products synthesized from heated sediment showing the influence of Si/Al molar ratio: at the NaOH/sediment ratio of 2.0 g/g (b); at the NaOH/sediment ratio of 1.2 g/g (c). Influence of NaOH/sediment ratio (d); influence of fusion temperature (e); influence of fusion time (f); influence of crystallization time (g); influence of liquid/solid ratio (h). Q ) quartz; It ) illite; Kt ) kaolinite; P ) Na-P1 zeolite; HS ) hydroxysodalite; F ) faujasite; L ) F Linde A; X ) Na-X zeolite.
Figure 2b,d) were selected to investigate the formation process and the characteristics of the zeolite products. These two products were selected since a monomineral of Na-P1 (F3) and hydroxysodalite (D1) was formed without other crystalline minerals being detected. The morphologies of the heated sediment, the molten sediment, and the two synthesized zeolites under SEM are shown
in Figure 3. The particles of the sediment after heat treatment are solid blocks with an irregular shape and have either a rough or a relatively smooth surface. The size of the blocks is mostly bigger than 10 µm, but most are typically 20-30 µm in diameter (Figure 3a). The raw sediment showed very similar morphology (data not shown). However, the molten sediment is a homogeneous phase which was not separated into individual particles
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Figure 3. SEM images of heated sediment (a), fused intermediate (b), F3 zeolite (c), and D1 zeolite (d).
(Figure 3b). The formed zeolite precipitates showed morphologies very different from those of the raw, the heated, and the fused sediment (Figure 3c,d). The sizes of most of the granular aggregates formed by stacking of zeolite crystals are much smaller than the sediment particles, typically a few micrometers or even less (Figure 3c,d). The difference in the surface structures of Na-P1 zeolite and hydroxysodalite is clear. The granular aggregates of the Na-P1 zeolite have a very rough surface and are composed of fine crystals (Figure 3c), while the hydroxysodalite appear like windings wound around a center with rope (Figure 3d). The crystal structure of zeolite is constituted of a framework of [SiO4]4- and [AlO4]3- tetrahedra linked to each other at the corners by sharing their oxygens, making up a three-dimensional network with lots of voids and channels in it. The FT-IR spectra of the raw sediment, the heated sediment, and the two synthesized zeolites are shown in Figure 4. The vibrations of the aluminosilicate framework give rise to absorption bands in the range 1250-400 cm-1.13-16,32-34 The FT-IR spectra of the raw sediment and the thermally treated sediment are very similar. The most important absorption bands on the FT-IR spectra are the strong and very broad bands in the range 1180950 cm-1. These bands are generally assigned to the asymmetric internal T-O stretching vibrations of the TO4 tetrahedra (where T ) Si or Al). The bands between 475 and 420 cm-1 are attributed to the bending vibrations of T-O in the tetrahedra. The 777 cm-1 band is assigned to the symmetric stretching vibration mode of O-T-O groups.34 The bands at 692 and 527 cm-1, observed for raw sediment, are due to the Al-O vibration in an octahedron. The strength of these two bands weakened significantly upon heat treatment and completely disappeared upon the formation of zeolite as a result of the breakage of the
Figure 4. FT-IR spectra of original sediment, heated sediment, and the two synthesized zeolites obtained under different synthesis conditions (F3 and D1).
octahedra. The band at 873 cm-1 for the raw sediment could be assigned to the OH bending in the lattice structure in kaolinite and illite,34 and it disappeared upon heat treatment. Fusion with NaOH followed by crystallization altered the IR absorption
Ind. Eng. Chem. Res., Vol. 47, No. 2, 2008 299 Table 1. Changes of CEC and Specific Surface Area during the Synthesis Process of Zeolite from Heated Sediment sample
CEC (cmol/kg)
sp surf. area (m2/g)
raw sediment heated sediment F3 zeolite D1 zeolite
25.0 4.2 295.3 115.6
3.4 1.4 40.0 16.5
spectra greatly. The bands within the range 1180-950 cm-1 for the raw sediment and the heated sediment shifted to a lower wavenumber at 991 and 988 cm-1 for F3 (Na-P1 zeolite) and D1 (hydroxysodalite), respectively. It has been reported that the length of the Al-O bond is longer than that of the Si-O bond and the substitution of tetrahedral Al for Si in aluminosilicate frameworks induces a shift of the stretching vibration of the T-O bond to a lower wavenumber.13,14,34 Therefore, the shift of the T-O band to a lower wavenumber indicates the incorporation of Al in TO4 of aluminosilicate to form zeolite. The increase in intensity and broadness of the band might also suggest the increase in Al concentration in TO4 tetrahedra. The Si/Al molar ratio in zeolite structure was 1.67:1 for Na-P1 and 0.84:1 for hydroxysodalite, respectively. The bands at 731, 704, and 662 cm-1 for D1 and the bands at 739 and 669 cm-1 for F3 are assigned to the symmetric stretching vibration mode of Al-O for the Si-O-Al framework, while bands at 629 and 567 cm-1 for D1 and a broad band centered at 592 cm-1 for F3 could be ascribed to the parallel four- or six-membered doubling rings.15,16,32,33 The bands for D1 are in good agreement with the data of the low CO32-sodalite reported by Barnes et al.33 The bands due to the bending vibrations of T-O, observed at 463 and 436 cm-1 for D1 and 444 cm-1 for F3, are located at lower frequencies than the raw and the heated sediment, probably again indicating an increase of Al concentration in TO4 tetrahedra. The CEC value and the specific surface area of the initial sediment and the thermally treated sediment, as well as the two final zeolite products, are listed in Table 1. The raw sediment possesses 25.0 cmol/kg CEC which could be attributed to the permanent and variable negative charge resulting from kaolinite and illite. The thermal treatment devastated the crystal structure of the clay minerals, causing the sharp decrease in CEC value (Table 1). Similarly, coal fly ash has only a negligible CEC value to hold cations.25 As the negative charge is constantly generated by an electrical imbalance among the aluminum atom and the four oxygen atoms in the AlO4 tetrahedra, zeolite generally has a high CEC value. Since pure Na-P1 zeolite has a theoretical CEC value of 460 cmol/kg, the content of Na-P1 in F3 could be roughly estimated to be 64% from the determined CEC value of F3. However, the CEC value of D1 was rather low compared with F3. This can be readily explained by the formation of hydroxysodalite for D1, as the small pore size of 0.23 nm of hydroxysodalite does not permit the penetration of ammonium ion (used to determine CEC) with an ionic diameter of 0.28 nm. The specific surface areas of the raw sediment and the heated sediment were very low, and were steeply raised with the formation of the microporous zeolites. Nevertheless, the specific surface area of the zeolite products is still low when compared with other zeolites such as Na-X.13 The latter has a large pore size (0.73 nm), allowing the penetration of N2, whose diameter is about 0.364 nm. Obviously, the inner channel of hydroxysodalite is not accessible for N2; thus the value for D1 was due only to the external surface. On the other hand, NaP1 zeolite has a complex structure with two types of pore size, one with a very small diameter of about 0.3 nm and the other with a size of 0.46 nm. Accordingly, the specific surface area
of F3 determined by N2 involved the whole external surface and a part of the inner channel surface of Na-P1. The synthesis of zeolite from the raw (unheated) sediment was also attempted, and it was found that the process was much less successful than that from thermally treated sediment. The crystallinity of the obtained product, derived from the raw sediment using the same synthesis conditions as F3, is very poor. The CEC is only 38.7 cmol/kg. This difference is due mainly to the formation of amorphous materials for the thermally treated sediment which makes the conversion of the material into zeolite easier when compared to the raw sediment in which crystalline silicate minerals exist. Influence of Si/Al Ratio of Starting Material on Zeolite Formation. Si and Al are the most important components of TO4 primary building units (PBU) of zeolite structure. Different zeolite species have different Si/Al molar ratios. Therefore, it is expected that the Si/Al ratio of the starting material would strongly influence the type of the zeolite produced. The XRD patterns of the zeolite products synthesized from the heated sediment which was initially adjusted to different Si/Al molar ratios, with all other synthesis conditions being fixed, are presented in Figure 2b,c. It can be seen that, at the original Si/Al ratio of 5:1 for the starting material, Na-P1 (Si/Al ) 1.67:1) with a secondary zeolite phase of Na-X (Si/Al ) 1.23: 1) and a very small amount of hydroxysodalite (Si/Al ) 0.84: 1) (at the NaOH/solid ratio of 2.0 g/g), or a monophase of NaP1 zeolite (at the NaOH/solid ratio of 1.2 g/g) were formed. By the addition of aluminum hydroxide hydrate to reduce the Si/Al ratio of the starting material to 3:1, 2:1, and 1:1 respectively, zeolites with lower Si/Al ratios in their structure, such as faujasite (Si/Al ) 1.65:1), hydroxysodalite, and F Linde A (1.00:1) in the case of a NaOH/solid ratio of 1.2 g/g, or a monomineral of hydroxysodalite in the case of a NaOH/solid ratio of 2.0 g/g, were gradually produced. Meanwhile, the intensity of the peaks due to Na-P1 or Na-X was gradually weakened and finally disappeared at the very low Si/Al ratio of 1:1. This thus suggests that a low Si/Al ratio of the reactant drives the formation of zeolites having low Si/Al ratios, and a high Si/Al ratio of the initial reactant preferably yields zeolites with high Si/Al molar ratios. This result is in agreement with those of former researchers.35 It is therefore proposed that the adjustment of the Si/Al molar ratio before the synthesis process is a useful technique to control the type of the zeolite in a product; i.e., the Si/Al ratio should be adjusted to a high value if a zeolite product with a high Si/Al molar ratio (such as NaP1) is desired, and vice versa. Similar results have been recently reported by Inada et al.19 and Wu et al.20,21 for coal fly ash, revealing that Na-P1 formed at a high Si/Al ratio was replaced by either monomineral of hydroxysodalite or a mixture of hydroxysodalite and F Linde A when the Si/Al ratio of the starting material was lowered. Comparison of the XRD patterns in Figure 2b with those in Figure 2c shows that, at each specific Si/Al ratio of initial material, zeolites with lower Si/Al ratios were formed at the NaOH/solid ratio of 2.0 g/g when compared with the NaOH/ solid ratio of 1.2 g/g. Previous researchers19-21 have observed that, for coal fly ash, Na-P1 was obtained at the NaOH concentration equal to or lower than 3 mol/L, while hydroxysodalite was formed when the NaOH concentration exceeded 4 mol/L. For coal fly ash, it is known that the Al ingredient is more difficult to be dissolved by NaOH than the Si ingredient. Therefore, it is likely that increasing the NaOH concentration may cause a greater increase in Al dissolution than it increases Si dissolution from fly ash, for Si would be digested more easily
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Figure 5. CEC values of zeolite products synthesized under different Si/ Al molar ratios.
than Al at low NaOH concentrations while Al could be only dissolved well at high NaOH concentrations. Accordingly, increasing NaOH concentration leads to a lower Si/Al ratio in the liquid phase, favoring the formation of zeolites with lower Si/Al ratios such as hydroxysodalite. This is probably true for thermally treated sediment as well. The dissolution of Si was enhanced in the case of the NaOH/sediment ratio of 2.0 g/g when compared with the NaOH/sediment ratio of 1.2 g/g, as seen from the differences in the intensities of the diffraction peaks due to quartz (Figure 2b,c). Unfortunately, we do not have the evidence at present indicating a greater increase in Al dissolution than the increase in Si dissolution at the NaOH/ sediment ratio of 2.0 g/g when compared with the ratio of 1.2 g/g. On the other hand, it is considered that a high NaOH/solid ratio (a high NaOH concentration of the solution in the subsequent crystallization step) itself may also trigger the formation of the zeolites with low Si/Al ratios. Barrer35 pointed out that a higher concentration of NaOH in the hydrothermal reaction (crystallization reaction in the case of the present study) benefits the formation of hydroxysodalite (Si/Al ) 0.84:1) rather than Na-X zeolite (Si/Al ) 1.23). As will be described later, this was confirmed in our study through the influence of liquid/ solid ratio on zeolite formation. The measurement of the concentrations of Si and Al in the waste solutions indicates that the effluents produced under all synthesis conditions, except the case where the Si/Al ratio is adjusted, contained high Si concentrations (20 200-44 975 mg/ L) but very low Al concentrations (90-194 mg/L). That is, whenever the Si/Al molar ratio of the heated sediment was not altered, the hydrothermal treatment consistently resulted in an excess Si which remained in the effluent, indicating that only a part of Si eluted from the sediment was converted to zeolite. The addition of aluminum hydroxide hydrate utilized the excess Si source for zeolite formation and reduced the Si concentration in the waste solution from 37 309 mg/L for G5 to 15 606 mg/L for G2. At the Si/Al ratio of 1:1, the Si concentration declined to be very low (213 mg/L) while an excess Al with a concentration of 1054 mg/L was found in the waste solution instead. Therefore, it is also important to adjust the Si/Al ratio of the heated sediment for adequately making use of the Si or Al source in the raw material in the practical zeolite production. The CEC values of the zeolite products synthesized under different Si/Al ratios are shown in Figure 5. At the NaOH/ sediment ratio of 1.2 g/g, the CEC value was initially enhanced with the decrease of the Si/Al ratio, and then dropped when the addition of Al resulted in a Si/Al ratio of 1:1. At the NaOH/ sediment ratio of 2.0 g/g, the CEC value of the product decreased gradually with the decline in the Si/Al ratio. Irrespective of the zeolite type, the zeolite content would increase with the addition of Al, as already discussed above based on the Si
Figure 6. CEC values of zeolite products synthesized under different liquid/ solid ratios and different crystallization times.
and Al concentrations in the waste solutions. The decrease in the CEC value with the decrease in Si/Al ratio is evidently attributed to the formation of hydroxysodalite, which has a very low CEC value. As will be discussed in the next section, the influence of a higher NaOH/sediment ratio involves the better mineralization (dissolution) of Si and Al from sediment by OH-, the improvement of crystallinity by Na+, and the formation of zeolite species with a lower Si/Al ratio. In the case of Figure 5, the difference in CEC between the two NaOH/sediment ratios can be interpreted by the formation of hydroxysodalite with a very low CEC value at the NaOH/sediment ratio of 2.0 g/g. Influence of Fusion Conditions on Zeolite Formation. The influence of three fusion conditions, involving NaOH/sediment ratio, fusion temperature, and fusion time, on the formation of zeolite was investigated. Figure 2d illustrates the XRD patterns of the zeolite products obtained under the NaOH/sediment ratios of 0.8, 1.2, 1.6, and 2.0 g/g, respectively. Since all other synthesis conditions are constant and only the NaOH/sediment ratio is varied, the results demonstrate the effect of the amount of NaOH relative to sediment on the zeolite formation. At the NaOH/sediment ratios e1.6 g/g, a monophase of Na-P1 zeolite was yielded, but at the NaOH/sediment ratio of 2.0 g/g, the intensity of the peaks due to Na-P1 weakened and new zeolites with lower Si/Al ratios (Na-X and hydroxysodalite) appeared instead. Shigemoto et al.10 and Molina and Poole12 investigated the effect of NaOH/ fly ash ratio on zeolite formation from coal fly ash using the fusion method, and their results are summarized in Table 2 for comparison. Similar to the present study, the increase in NaOH amount relative to the solid before fusion resulted in the production of zeolites with gradually lower Si/Al ratios. At each specific NaOH/solid ratio, nevertheless, the previous researchers obtained zeolites with lower Si/Al ratios than the present study. This can be explained by the fact that the Si/Al ratio of the fly ashes used in the former studies is lower than that of the sediment used in this study (5:1) (Table 2). It appears, therefore, that the amount of NaOH is another crucial factor regulating the zeolite type formed, in addition to the Si/Al ratio. Similar to coal fly ash, the synthesis of zeolite from the thermally treated sediment may be divided at least into the steps of (1) the dissolution of Si and Al ingredients from the heated sediment and (2) the nucleation and crystal growth of zeolite. NaOH acts primarily as a mineralizer (OH-) to motivate the dissolution of the Si and Al ingredients needed for zeolite formation. This function is supported by the fact that the peaks due to quartz weakened and finally disappeared as the NaOH/
Ind. Eng. Chem. Res., Vol. 47, No. 2, 2008 301 Table 2. Summary of Zeolites Obtained by Previous Researchers under Different NaOH/Fly Ash Ratios Using the Fusion Method NaOH/fly ash ratio data source
Si/Al
1.0
1.2
1.6
2.0
Shigemoto et al.10 Molina and Poole12
1.86 1.81
Na-X (main), Na-A (minor) Na-X
Na-X (main), Na-A (minor) Na-X
Na-X (main), HS (minor) HS (main), Na-X (minor)
HS HS
sediment ratio increased (Figure 2d). This decrease in the quartz peaks corresponds well to the increase in the concentration of Si in the effluent when the NaOH/sediment ratio is increased (the Si concentrations in effluents are 20 200, 37 309, 42 975, and 44 975 mg/L for the NaOH/sediment ratios of 0.8, 1.2, 1.6 and 2.0 g/g, respectively). The second role of NaOH in the zeolite synthesis is its effect on zeolite type crystallized. The amount of NaOH relative to sediment affects not only the dissolution of Si and Al, but also the alkalinity of the solution in the subsequent crystallization process which will influence the type of the zeolite formed as a result. The third role NaOH plays in the reaction arose from Na+ ion, which was reported to enhance the crystallization of zeolite.18,21 The CEC values for the zeolites obtained at the NaOH/ sediment ratios of 0.8, 1.2, 1.6, and 2.0 are 201.8, 279.1, 295.3, and 272.4 cmol/kg, respectively. It is clear that, up to a NaOH/ sediment ratio of 1.6 g/g, the CEC value of synthesized zeolite increased with the increase of the ratio. The CEC value falls slightly at the NaOH/sediment ratio of 2.0 g/g as a result of the formation of a small amount of hydroxysodalite (Figure 2d). The XRD patterns of the zeolite products synthesized under different fusion temperatures and fusion times are presented in Figure 2e,f. Unlike the NaOH/sediment ratio, the fusion temperature and fusion time are considered to affect the dissolution of the heated sediment only, and they would have little effect on the subsequent nucleation and crystallization of zeolite. From Figure 2e,f, it is clear that the increases in the fusion temperature from 350 to 450 °C and from 650 to 750 °C, and the increase in the fusion time from 15 to 30 min, did slightly promote the dissolution of the Si ingredient (typically quartz). The results of the CEC value of products also show that the rise in the fusion temperature did raise the CEC value slightly (from 263.1 cmol/kg for 350 °C to 287.5 cmol/kg for 750 °C), although the increase in fusion time did not alter the CEC value of the product significantly (the CEC values for different fusion times were 279.1-282.1 cmol/kg). However, the influence of these two fusion conditions on the type and crystallinity of zeolite and the CEC value of the product was quite limited. It is suggested that, from the economic point of view, a reasonably low fusion temperature and short fusion time should be adopted for the synthesis of zeolite from thermally treated sediment. Influence of Crystallization Conditions on Zeolite Formation. A previous study by Molina and Poole12 has already confirmed that, below 95 °C, crystallization is poorly achieved for coal fly ash. This is further confirmed in the present study for the heated sediment when adopting a crystallization temperature of 75 °C. Therefore, only the influence of crystallization time and liquid/solid ratio was investigated in this study while a crystallization temperature of 95 °C was consistently used. The XRD patterns of the zeolites synthesized under the different crystallization times and the liquid/solid ratios are displayed in Figure 2g,h, while the CEC values of the synthesized products as influenced by the two crystallization conditions are shown in Figure 6. Molina and Poole12 reported that, for coal fly ash, no peaks due to zeolite were observed on XRD at a crystallization time of 30 min following the fusion process, but a maximum crystallinity of Na-X zaolite occurred
after 2 h. It is shown in the present study that the type and crystallinity of the formed zeolite, the dissolution (judged from the quartz peaks and the data of Si concentration in waste solutions) of the heated sediment, and the CEC value of the products were not considerably influenced by the crystallization time within the range from 2 to 24 h. Hence, it is suggested that, similar to the fusion temperature and the fusion time, a reasonably short crystallization time (e.g., 2 h) may be chosen during the synthesis of zeolite from thermally treated sediments from the economic point of view. The liquid/solid ratio during the crystallization period, however, strongly affected the type and crystallinity of the produced zeolite (Figure 2h) and the CEC value of the product (Figure 6). It should be noted that, as all other synthesis conditions including the NaOH/sediment ratio before fusion are constant, the liquid/solid ratio would have no effect on the dissolution of the Si and Al ingredients. That is, the Si/Al ratio in the solution prior to the crystallization of zeolite should be the same for all the liquid/solid ratios. Nevertheless, different liquid/solid ratios mean different NaOH concentrations in the crystallization step. The NaOH concentration at a low liquid/ solid ratio is high, and decreases with the increase in the ratio. As seen from Figure 2h, monomineral of hydroxysodalite was formed at the liquid/solid ratio of 2.5 mL/g, and it is replaced gradually by Na-P1 zeolite with the increase in the ratio. This fact cannot be interpreted by the dissolution behavior of Si and Al ingredients (Si/Al ratio), and it strongly supports the view that NaOH concentration in solution alone could influence the formation of different zeolites. The low CEC value achieved at low liquid/solid ratios is apparently due to the formation of hydroxysodalite. Therefore, the liquid/solid ratio plays an important role in the synthesis of zeolite from the heated sediment. Conclusion Thermally treated sediment can be converted into valuable zeolite products via fusion with sodium hydroxide followed by a hydrothermal reaction. A great increase in CEC and specific surface area was observed following the synthesis of zeolitic materials. Among the synthesis conditions, the Si/Al ratio, the NaOH/ sediment ratio, and the liquid/solid ratio influenced greatly the type and crystallinity of the formed zeolite and the CEC value of the product as a result. Contrarily, the type and properties of zeolite product were only slightly affected by the fusion temperature, the fusion time, and the crystallization time within the range investigated. It is concluded that thermally treated sediment could be recycled as a material to synthesize high value zeolite and the type and quality of the product could be controlled by selecting appropriate synthesis conditions. Acknowledgment Financial assistance from the Chinese Ministry of Science and Technology (Grant 2002AA601013) and the Committee of Science and Technology of Shanghai (Grant 04DZ12030-2) is gratefully acknowledged.
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ReceiVed for reView August 4, 2007 ReVised manuscript receiVed October 15, 2007 Accepted October 22, 2007 IE071063U
