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Fuel 87 (2008) 2194–2200

Changes of mineralogical–chemical composition, cationexchange capacity, and phosphate immobilization capacity during

the hydrothermal conversion process of coal fly ash into zeolite

Deyi Wu *, Yanming Sui, Xuechu Chen, Shengbing He, Xinze Wang, Hainan Kong

School of Environmental Science and Engineering, Shanghai Jiao Tong University, No. 800, Dongchuan Road, Shanghai, 200240, China

Received 4 March 2007; received in revised form 29 October 2007; accepted 31 October 2007Available online 26 November 2007

Abstract

In the search for a technique to augment the nutrient removal capacity of zeolite synthesized from fly ash (ZFA), the present studyinvestigated the changes of mineralogical–chemical composition, cation exchange capacity (CEC), and phosphate immobilization capac-ity (PIC) during the synthesis process. The ZFAs were obtained as a function of temperature (40–120 oC), liquid/solid ratio (1–18 ml/g),NaOH concentration (0.5–4 mol/L) and reaction time (2–72 h). The formation of low-silica zeolites (P1, hydroxysodalite, and chabazite)and the stability of mullite were observed, causing a marked decrease in SiO2 content but roughly no change in Al2O3 content during thesynthesis process. The decrease in K2O, MgO content and the insignificant change in Fe2O3 and TiO2 content were related to the sol-ubility of the oxides while the increase in Na2O and CaO was due to the increase in CEC. A high CEC was achieved under a high tem-perature, a high liquid/solid ratio, a long reaction time, and an appropriate NaOH concentration (2 mol/L), while a maximum PIC wasachieved under relatively mild synthesis conditions instead (e.g., a reasonably short reaction time 10 h). This discrepancy was explainedby the fact that different controlling factors/components in ZFA are responsible for CEC (content and kind of zeolite) and PIC (Ca com-ponent, specific surface area, and dissociated Fe2O3 and Al2O3).� 2007 Elsevier Ltd. All rights reserved.

Keywords: Fly ash; Zeolite; Composition; Cation exchange capacity; Phosphate immobilization capacity

1. Introduction

Great amounts of coal fly ash are generated every year inthe world as a by-product of coal combustion since coal isone of the major energy sources. The amounts of fly ash pro-duced annually were: 60 million tons for USA [1], 38.5 mil-lion tons for Europe [2], and 160 million tons for China.Since fly ash has pozzolanic properties, part of the producedfly ash is currently reused in the production of building mate-rials such as concrete and cement. The reutilization rate is:25–30% for USA [1], 48% for Europe [2], and about 40%for China. However, a large proportion of fly ash is

0016-2361/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2007.10.028

* Corresponding author. Tel.: +86 21 5474 4540; fax: +86 21 5474 0825.E-mail address: dywu@sjtu.edu.cn (D. Wu).

impounded or landfilled. Therefore, it is obligatory to seekalternatives for productive reuse of fly ash.

Fly ash has many similarities in chemical compositionand physical–chemical properties with volcanic material,which is the precursor of natural zeolites. This promptedthe study on the synthesis of zeolite from fly ash (ZFA)by Holler and Wirsching [3]. Zeolites are known usefulmaterials as they contain large specific surface area and cat-ion exchange capacity (CEC) [4]. Therefore, research onthe conversion of fly ash into zeolite and application ofthe product is important as a waste management measure.

Our previous studies revealed that ZFA is a novel mate-rial for the simultaneous removal of ammonium (a cation)and phosphate (an anion) from wastewater [5–7]. Regard-ing nutrient removal from wastewater, CEC is a measureof the capacity of a material for ammonium removal, while

D. Wu et al. / Fuel 87 (2008) 2194–2200 2195

phosphate immobilization capacity (PIC) is a measure ofthe potentiality for phosphate removal [5–7]. Removal ofboth ammonium and phosphate from wastewater beforedischarge is crucial for protecting relatively stagnant waterbodies, such as lakes and estuaries, from eutrophicationthat has become an increasingly severe environmentalproblem worldwide.

Although much research work has been undertaken onthe synthesis and application of ZFA in recent years [5–16], the influence of synthesis conditions on the mineralog-ical–chemical composition, CEC and PIC of ZFA has stillnot been clarified adequately and systematically. It is pre-sumed that the study on this issue may help to understandthe process of synthesis and may provide information onhow to obtain a product that could be effectively appliedin nutrient removal from wastewater.

The aim of the present study was to find out how toimprove the quality of ZFA in terms of nutrient removal.For this purpose, the changes of the mineralogical compo-sition, chemical composition, CEC, and PIC during thehydrothermal conversion process of coal fly ash intoZFA under a series of different synthetic conditions wereinvestigated.

2. Experimental

2.1. Materials

A sample of fly ash was supplied by the second electricpower station of Wujing, Shanghai, China. The fly ashwas passed through an 80-mesh sieve, and the zeolite con-version was studied as a function of temperature (40–120 �C), NaOH concentration (0.5–4.0 mol/L), liquid/solidratio (1–18 ml/g) and reaction time (2–72 h) in a closedsystem.

Fifteen grams of fly ash were added into NaOH solutionin a sealed Teflon reaction vessel (>100 �C) or in a sealedpolypropylene vessel (<100 �C) to give the liquid/solidratios. After being shaken, the slurry was placed in anautoclave (>100 �C) or in an air oven (<100 �C) and washeated at different temperatures for a specified time. Oncethe activation time was reached, the resulting materialwas separated into the solid phase and the waste solutionby centrifugation. The solid phase was washed three timeswith doubly distilled water and twice with ethanol. To testthe exchangeability of the Na ions on ZFA surface, thesolid phase was further washed five times with 0.5 mol/LCaCl2 solution and then washed with doubly distilled waterand ethanol until free from chloride ion (checked with 1NAgNO3). The product was dried finally in an oven at 45 �Cfor 6 h.

2.2. Characterization of the materials

The crystalline phase(s) in the samples were identified bythe powder X-ray diffraction method on a D8 ADVANCEX-ray diffractometer using a Ni filtered Cu Ka radiation

(40 kV, 40 mA). The cation exchange capacity was deter-mined using the ammonium acetate method [17]. About0.4 g of the samples was washed five times with 1.0 mol/Lammonium acetate. To remove residual ammonium thatwas not held by the sample, the solid was further washedwith doubly distilled water and ethanol until free fromchloride ion (checked with 1N AgNO3 solution). Finally,the residue was washed five times with 1.0 mol/L NaCland the released ammonium was determined by the Nesslermethod [18]. The CECs were thus calculated and expressedas meq per gram of solids. For chemical analysis except sil-icon, the samples were digested with hydrogen fluoride inconjunction with perchloric acid and dissolved later byhydrochloric acid. While for the analysis of silicon, thesamples were melted with sodium hydroxide. The elementalconcentrations were then measured in the digestions byInductively Coupled Plasma Atomic Emission Spectrome-try (IRIS advantage 1000). The dissociated iron oxide(Fe2O3d) and aluminum oxide (Al2O3d) were determinedby the dithionite–citrate–bicarbonate (DCB) extractionmethod [19]. The specific surface area was determined byN2 adsorption method (equipment model: ASAP2010).About 0.1 g of the samples was outgassed at 200 �C undervacuum for about 4 h prior to measurement. The nitrogenadsorption/desorption data were recorded at the liquidtemperature (�196 oC). The specific surface area was calcu-lated using the Brumauer–Emmett–Teller (BET) equation.For the determination of PIC value, 40 ml of phosphatesolution with a concentration of 1000 mg P/L and a pHvalue of 5.0, prepared from anhydrous KH2PO4, wereput into a centrifuge tube containing 0.5 g of fly ash orZFA. After being shaken for 24 h (a reaction time of24 h was found to be sufficient for phosphate to achieveequilibrium in preexperiments) at laboratory temperature(ca. 20 �C), the suspension was centrifuged and the super-natant was determined for phosphate concentration bythe molybdenum-blue ascorbic acid method [18] with aUnico spectrophotometer (model UV-2102PCS). Blankdetermination in the absence of solid was also conductedand amounts of phosphate immobilized (PIC) were calcu-lated from differences between the equilibrium concentra-tions with and without the solids.

3. Results and discussion

3.1. Influence of synthesis conditions on the mineralogical

composition of ZFA

Table 1 summarizes the zeolites synthesized from the flyash and the changes of mineralogical composition underthe experimental conditions investigated. Quartz and mulliteare the two major crystalline phases in the untreated fly ash.Generally, the degree of zeolitization and the type of zeoliteobtained depended upon the synthesis conditions, but allthe three zeolites produced (P1, Na6Al6Si10O32Æ12H2O;hydroxysodalite, Na8Al6Si6O24(OH)2; chabazite, Na12Al12-Si24O72Æ40H2O) are low-silica ones. It is clear from Table 1

Table 1Synthesis conditions and the zeolite species obtained as identified by powder XRD

Experimentno.

Temperature(�C)

NaOH concentration(M)

L/S ratio(ml/g)

Reaction time(h)

XRD intensity due tozeolitesa

XRD intensity due to originalmineralsa

Roomtemperature

0 0 0 – Q(vs), M(w)

1 60 2 10 24 – Q(vs), M(w)2 80 2 10 24 – Q(vs), M(w)3 90 2 10 24 – Q(s), M(w)4 120 2 10 24 P(s), C(w) Q(w), M(w)5 120 0.5 10 24 P(m) Q(s), M(w)6 120 1 10 24 P(s) Q(s), M(w)7 120 2 10 24 P(s), C(w) Q(w), M(w)8 120 4 10 24 HS(s) Q(w), M(w)9 120 2 2.5 24 P(s) Q(m), M(w)10 120 2 5 24 P(s), C(w) Q(w), M(w)11 120 2 10 24 P(s), C(w) Q(w), M(w)12 120 2 10 2 – Q(vs), M(w)13 120 2 10 4 – Q(vs), M(w)14 120 2 10 8 C(s), P(w) Q(vs), M(w)15 120 2 10 16 C(m), P(m) Q(m), M(w)16 120 2 10 24 P(s), C(w) Q(w), M(w)

a All the products are not pure zeolite but contain some un-reacted fly ash as impurities. P, P1 zeolite; C, chabazite; HS, hydroxysodalite; Q, quartz; M,mullite (Q and M are derived from fly ash); vs, very strong; s, strong; m, medium; w, weak.

2196 D. Wu et al. / Fuel 87 (2008) 2194–2200

that there was no appreciable change of crystalline phaseunder low temperatures below 90 oC, only well-crystallizedzeolite was observed at 120 �C. From the decrease in strengthof XRD peaks, it also appears that quartz could be slowlydissolved at temperatures higher than 90 �C. Conversely,mullite remained unchanged at all synthesis conditions inthis study. The stability of mullite during hydrothermaltreatment of fly ash has also been reported by some previousresearchers [12,16]. Apparently, a high temperature facili-tated the dissolution of Si ingredient from coal fly ash andthe crystallization of zeolite phase.

The results in Table 1 show that the NaOH concentra-tion distinctively affected the type of zeolite formed andthe crystallinity of zeolites. At the concentrations equalto or lower than 1 mol/L, P1 was synthesized as a mono-mineral phase. When a 2 mol/L NaOH concentration wasadopted, chabazite was obtained as a secondary zeoliticphase. Further increase in NaOH concentration (4 mol/L)resulted in the formation of hydroxysodalite as a mono-mineral phase. The formation of hydroxysodalite at NaOHconcentrations higher than 3 mol/L was also reported byMurayama et al. [15]. Similarly, Shigemoto et al. [11]pointed out that a higher concentration of NaOH in thehydrothermal reaction benefits the formation of hydroxy-sodalite. As seen from the decrease in XRD peaks, quartzwas slowly dissolved in the alkaline medium: the higher theNaOH concentration, and the easier the dissolution ofquartz.

The liquid/solid ratio affected the type of zeolite formed(Table 1). A lower liquid/solid ratio facilitated the forma-tion of P1 as a monomineral phase, whereas the increasein the ratio led to the formation of chabazite as a secondaryzeolitic phase. Regarding the effect of reaction time on theformation of zeolite, there was no appreciable change of

crystalline phase at reaction times shorter than 4 h; onlywell-crystallized zeolite was observed at reaction timeslonger than 8 h. In addition, the main zeolite produced atthe reaction time of 8 h was chabazite, with P1 as a minorzeolite component; nevertheless longer reaction time pro-moted the formation of P1 as the major zeolite species.Therefore, it appears that chabazite is a metastable zeolitephase that later recrystallizes and is gradually replaced byP1, a more stable zeolite. The dissolution of quartzappeared not to be affected by the liquid/solid ratio, buttook place when the reaction time exceeded 8 h.

3.2. Influence of synthesis conditions on the chemical

composition of ZFA

Table 2 shows the chemical composition of the raw flyash and the ZFAs obtained under a series of experimentalconditions. The main components of fly ash are the oxidesof Si and Al, and various metallic oxides. The fly ash con-tains significant amounts of sulfur and unburned carbon aswell, but keeps little moisture. Comparing with the raw flyash, SiO2 content decreased with the progressive increase intemperature (40–120 �C), NaOH concentration (0.5–4.0mol/L), liquid/solid ratio (1–18 ml/g), and reaction time(2–72 h). On the other hand, the Al content remained sta-ble or even enriched slightly when compared with the rawfly ash. As a result, the Si/Al ratio dropped markedly afterthe synthesis process. The changes of the Si and Al contentsbefore and after the synthesis process indicated that a partof dissolved Si was not utilized in zeolite formation but waslost when the waste solution was subsequently discardedafter synthesis process. The analysis of Si in the waste solu-tion confirmed the presence of large amount of dissolved Siunused [9]. As described in the former section, low-silica

Table 2Chemical composition, Si/Al ratio and CEC of zeolites synthesized under different conditions

No. Temperature(oC)

Concentration(M)

L/S(ml/g)

Time(h)

SiO2

(%)Al2O3

(%)Na2O(%)

K2O(%)

CaO(%)

MgO(%)

Fe2O3

(%)TiO2

(%)SO3(%)

Moia

(%)LOIb

(%)Si/Al

CECc

1 Fly ash 51.11 15.66 0.59 1.17 8.32 0.98 9.41 0.78 7.52 0.15 4.26 2.88 0.0212 60 2 10 24 42.68 18.88 0.87 0.99 8.50 1.07 9.60 0.76 7.27 1.90 5.80 2.00 0.4683 90 2 10 24 36.31 19.73 2.44 0.54 11.81 0.74 10.04 0.79 7.21 3.87 5.95 1.63 0.8894 120 2 10 24 34.40 18.91 3.22 0.41 14.05 0.97 9.01 0.73 7.05 6.93 4.18 1.61 1.8695 120 0.5 10 24 40.24 20.41 1.65 1.05 9.79 1.01 9.47 1.10 7.60 2.33 5.28 1.74 0.5506 120 1 10 24 35.81 19.20 4.45 0.70 10.89 0.97 8.77 0.72 7.42 6.39 4.77 1.65 1.1787 120 2 10 24 34.40 18.91 3.22 0.41 14.05 0.97 9.01 0.73 7.05 6.93 4.18 1.61 1.8698 120 4 10 24 27.88 19.84 8.22 0.14 11.95 0.99 9.07 0.76 7.66 4.65 7.11 1.24 0.6269 120 2 2.5 24 35.92 17.56 3.18 0.87 11.69 0.91 8.26 1.04 7.58 8.84 4.30 1.81 1.68210 120 2 5 24 34.70 17.31 3.05 0.71 12.48 0.84 8.13 0.65 7.37 8.08 5.50 1.77 1.78311 120 2 10 24 34.40 18.91 3.22 0.41 14.05 0.97 9.01 0.73 7.05 6.93 4.18 1.61 1.86912 120 2 10 2 42.57 19.67 2.15 0.94 8.70 1.15 9.56 0.82 7.23 2.15 4.03 1.91 0.49513 120 2 10 4 36.71 20.07 2.59 0.87 9.99 1.24 10.40 0.82 7.58 3.68 4.94 1.62 0.77514 120 2 10 8 34.19 18.86 3.12 0.53 13.34 1.05 9.31 0.73 7.33 6.65 4.41 1.60 1.62315 120 2 10 16 34.10 19.78 2.35 0.57 13.52 1.02 8.60 1.05 7.56 5.80 5.57 1.52 1.89116 120 2 10 24 34.40 18.91 3.22 0.41 14.05 0.97 9.01 0.73 7.05 6.93 4.18 1.61 1.869

a Moisture.b Loss on ignition at 950 �C.c Unit: meq/g.

Fig. 1. The relationship between CEC values and CaO contents of ZFAssynthesized under different conditions.

D. Wu et al. / Fuel 87 (2008) 2194–2200 2197

zeolites were formed from the fly ash under all the synthesisconditions utilized. The loss of dissolved Si may thus beexplained by the fact that (1) formation of low-silica zeolitemade the dissolved Si ingredient excessive compared withthe dissolved Al; (2) the Si component in fly ash was prob-ably more soluble in alkaline media than Al component. (3)a part of Al, contained in mullite, was inert in alkalinemedia as shown by the XRD analysis (Table 1). Althoughno systematical investigation on the change of chemicalcomposition during the conversion process of zeolite fromcoal fly ash was conducted hitherto, two previous papers[14,15] determined the chemical composition of both rawfly ash and corresponding ZFA. The significant decreasein SiO2 content after conversion process occurred similarlyfor a fly ash from a power plant in the United States [14]and a fly ash from the Denpatsu Coal Technology in Japan[15]. The authors obtained low-silica zeolites as well (P1zeolite and hydroxysodalite).

It can thus be inferred that only reactive Al2O3 in fly ashcould contribute to the formation of zeolite. It is supposedthat the destruction of mullite prior to or during the syn-thesis process would increase the zeolite content in ZFAand adequately exploit the useful components of Si in flyash for zeolite formation. Another method to increase thecontent of zeolite would be the addition of Al-containingmaterials which could utilize the excess Si source for zeoliteformation and reduce the Si concentration in the wastesolution. The authors previous work [9] showed that theaddition of powdered aluminum hydroxide hydrateenhanced the content of zeolite in ZFA product.

An increase in moisture with the formation of zeolite isnot surprising and can be interpreted by the existence of zeo-litic water (Table 2). However, though the components of flyash are in direct contact with the alkaline solution during thesynthesis process, the amount of MgO, Fe2O3, and TiO2

remained quite stable, probably due to the insolubility ofthese oxides in NaOH medium even under heat treatmentup to 120 �C. In contrast, despite the oxide of Ca has a rela-tively higher solubility than the above metal oxides, the con-tent of CaO increased with the rise in temperature (40–120 �C), NaOH concentration (except the treatment with4 mol/L NaOH), liquid/solid ratio (1–18 ml/g), and reactiontime (2–72 h). Since all products were saturated with Ca2+ inthe present study, the exchangeable Ca2+ captured as a resultof the formation of negatively charged zeolite evidentlycaused the significant increase in the Ca content followingthe zeolite conversion. The relationship between CaO con-tent and CEC of ZFA is shown in Fig. 1. The good correla-tion between the two parameters strongly supports the viewthat high calcium content is the consequence of a high CECof the ZFA, and vise versa. It should be noted that, althoughthe Ca content is expressed by the form of CaO, Ca in ZFAexists principally as cations on zeolite surface, not oxides.

2198 D. Wu et al. / Fuel 87 (2008) 2194–2200

The decrease in K2O content by the hydrothermal treat-ment is not surprising, owing to the dissolvable property ofthe oxide of potassium in alkaline solution. However, thehydrothermal treatment in NaOH solution resulted in therise of the Na content. Following the synthesis process,the products were all initially saturated by Na+ due tothe use of NaOH in synthesis process. Yet, the Na+ wassubsequently replaced by Ca2+ in the present study. Oneof our purposes to change the Na-zeolite to Ca-zeolite isto test the exchangeability of the Na ions on zeolite surface.The higher content of Na in the Ca2+ saturated ZFA thanfly ash suggested that the exchange of Ca2+ for Na+ onzeolite surface was not completed. This implied that Na+

partially exists at places where Ca2+ could not displace.Shigemoto et al. [11] reported that a low crystallinity ofzeolite synthesized from fly ash could depress the ionexchangeability. It was thought that the existence of non-exchangeable Na+ might be associated with the low crystal-lization degree.

The relationship between the Na content (expressed asNa2O) and the CEC value is given in Fig. 2. Though thereis a tendency that the Na content increased with the CECvalue, the difference in Na content among the ZFAsobtained under different conditions was generally notgreat. But the ZFA produced using 4 mol/L of NaOH solu-tion had a distinctively high Na content.

3.3. Influence of synthesis conditions on the cation exchangecapacity of ZFA

Because negative charge is constantly generated by anelectrical imbalance among the aluminum atom and fouroxygen atoms in the zeolite structure, zeolite generallyhas the ability to exchange cations. The CEC value ofZFAs produced as a function of reaction temperature,liquid/solid ratio, NaOH concentration and reaction timeis shown in Fig. 3. The CEC value of the fly ash is verylow, suggesting that fly ash had little capability to hold pos-itively charged cations (Table 2). Within the temperature

Fig. 2. The relationship between CEC values and Na2O contents of fly ashand ZFAs synthesized under different conditions.

range adopted, the CEC value of synthesized productincreased as a function of temperature until the tempera-ture of 110 �C was reached. But this increase in CEC thenbegan to slow down. Regarding the relation of the CECvalue of ZFA with NaOH concentration, the CEC valueincreased initially with the rise in NaOH concentration.This can be interpreted by the fact that increasing NaOHconcentration facilitated zeolite formation, as seen in Table1 from either the increase in XRD intensity of P1 zeolite(0.5 and 1.0 mol/L NaOH) or the appearance of additionalXRD peaks of chabazite (2.0 mol/L NaOH). After reach-ing a maximum value at the concentration of 2 mol/L,however, the CEC then began to decline (Fig. 3). The resul-tant decrease in CEC value above the NaOH concentrationof 2 mol/L is obviously due to the formation of hydroxy-sodalite because the CEC value of hydroxysodalite as mea-sured by the ammonium acetate method is very low [2]. Thelow CEC of hydroxysodalite, when determined by theammonium acetate method, is attributed to the small poresize (0.23 nm) that does not allow the penetration ofammonium (0.28 nm). Therefore, although ammonium iswidely used to measure CEC, it seems inappropriate toapply to materials having smaller pores than the diameterof ammonium ion such as hydroxysodalite. The decreasein CEC value at NaOH/fly ash ratio higher than 1.2 (equiv-alent to a concentration of 3 mol/L) was also reported byMolina and Poole [12].

The CEC of pure synthetic NaP1 (4.7 meq/g) is higherthan that of pure synthetic chabazite (3.7 meq/g). Theliquid/solid ratio significantly affected the CEC value ofZFA. The CEC value increased greatly until the liquid/solid ratio reaching 2.5 ml/g, but then showed only slightimprovement as a function of the ratio. The increase inCEC with the increase in L/S ratio is obviously attributableto the improvement of zeolite formation, which can be seenfrom the appearance of additional XRD peaks due tochabazite (Table 1). In the case of reaction time, the CECvalue of ZFA increased sharply within the first 8 h and thenapproached a relatively constant value. The increase inCEC with the increase in reaction time can be interpretedby either the increase in zeolite content which is seen inTable 1 from the increase in XRD intensity of zeolite(within the reaction time from 2 to 8 h) or the change fromlow CEC chabazite to high CEC P1 zeolite (within reactiontime from 8 h to 24 h).

It should be noted that though there was no notableappearance of characteristic XRD peaks due to zeolite atlow temperatures (690 �C) and short reaction times(64 h) (Table 1), zeolite already formed as evidenced bythe CEC value. Molina and Poole [12] and Breck [20]reported that the formed zeolite crystals could be so smallas to be undetectable or may be present in line-broadeningof X-ray reflections when the reaction temperature is notsufficiently high.

Based on the changes of CEC value under a series ofsynthesis conditions, it was concluded that, for zeolite syn-thesis from this fly ash and for the conditions tested in this

Fig. 3. Effects of temperature (h), liquid/solid ratio (D), NaOH concentration (s) and reaction time (e) on the cation exchange capacity of zeolites (h –NaOH concentration: 2 mol/L, liquid/solid ratio: 10 ml/g, reaction time: 24 h; D – NaOH concentration: 2 mol/L, temperature: 120 �C, reaction time:24 h; s – temperature: 120 �C, liquid/solid ratio:10 ml/g, reaction time: 24 h; e – temperature: 120 �C, liquid/solid ratio:10 ml/g, NaOH concentration:2 mol/L).

D. Wu et al. / Fuel 87 (2008) 2194–2200 2199

study, the highest CEC value was achieved under a temper-ature of 110–120 �C, a reaction time of 8–16 h, a liquid/solid ratio of 2.5 ml/g, and a NaOH concentration of2 mol/L.

Fig. 4. Changes of PIC, specific surface area (SSA), and dissociated Fe2O3

and Al2O3 based on the amount of synthesized zeolite during theformation of zeolite from fly ash.

3.4. PIC of ZFAs prepared under different conditions

The PIC is an important parameter determining the suit-ability of a material for the removal of phosphate fromwastewater. In our previous studies [5–7], we reported thatZFA prepared from fly ash has not only high CEC value,but also high PIC value. The mechanism of phosphateremoval by ZFA was shown to be the precipitation ofphosphate with calcium components and/or the adsorptionof phosphate on Fe and Al components. By comparison ofthe composition and physical–chemical properties of 15ZFAs (Na-saturated) with their corresponding fly ashes,we found that the increase in PIC after zeolite synthesisprocess was principally attributed to the change in mor-phology (the increase in specific surface area as a result)and the increase in the contents of the dissociated Fe2O3

and Al2O3 [5]. It should be noted that ZFA used in thisstudy was Ca-saturated and thus increase in Ca contentcould additionally contribute to the increase in PIC, whencompared with fly ash. The mechanism of dissociatedFe2O3 and Al2O3 for the removal of phosphate can beexplained by an adsorption process through the ligandexchange between phosphate and the hydroxide groupson the surface of the hydroxylated oxides

a@SOHðSÞ þHcPOc�34 ðaqÞ þ bHþðaqÞ�@SaHbPO4ðsÞ

þ cH2OðlÞ þ ða� cÞOH�ðaqÞ

where S refers to a metal atom (Fe or Al) in a hydroxylatedoxide, OH refers to a reactive surface hydroxyl, a, b and c

are stoichiometric coefficients and c(6 3) is the degree ofprotonation of the phosphate ion.

The PIC of the ZFAs obtained at different reactiontimes is given in Fig. 4. It is shown that the PIC value rosesharply with the increase in reaction time, reached a max-imum at the reaction time of about 10 h, but then beganto decline at longer reaction times. As shown in Fig. 4,the changes of specific surface area and the contents ofthe dissociated Fe2O3 and Al2O3 with reaction time fol-lowed a rather similar pattern with PIC, i.e., a maximumvalue was reached at the reaction time of about 10 h. Thereason may be that, although the dissolution and destruc-tion of the spherical particle of the fly ash spurred theincrease in specific surface area and the contents of the dis-sociated Fe2O3 and Al2O3, further activation might resultin formation and prolonged growth of crystalline phaseswhich might abate the specific surface area and the con-tents of the dissociated Fe2O3 and Al2O3.

2200 D. Wu et al. / Fuel 87 (2008) 2194–2200

The pattern of the change of PIC value with reactiontime apparently differed from CEC, which is alreadydescribed in the former section. The discrepancy is proba-bly due to the different controlling factors/components forCEC and PIC in ZFA. The CEC value depends upon thecontent and the kind of zeolite formed, while componentsother than zeolite are crucial for phosphate removal. InZFA, they are calcium content, dissociated Fe2O3 andAl2O3 contents, and specific surface area those correlatedsignificantly with phosphate removal [5]. Therefore, aZFA obtained under relatively mild synthesis conditions(e.g., reasonably short reaction time) which would consumeless energy and less cost, may be advantageous providedthat the application of ZFA to eliminate phosphate fromwastewater is chiefly considered.

4. Conclusions

Hydrothermal treatment of a Chinese fly ash in NaOHmedia resulted in the formation of low-silica zeolites (P1,hydroxysodalite, chabazite). Due to the formation oflow-silica zeolite and the stability of mullite, the synthesisprocess caused the decrease in SiO2 content in solid phaseand Si/Al ratio with the progressive formation of zeolite asa function of temperature (40–120 �C), NaOH concentra-tion (0.5–4.0 mol/L), liquid/solid ratio (1–18 ml/g), andreaction time (2–72 h). On the other hand, the change ofthe composition of Fe2O3, MgO, TiO2 and K2O is relatedto the solubility of the oxides in alkaline solution.

In general, ZFA has the potential to simultaneouslyremove ammonium and phosphate from wastewater asshown by the high CEC and PIC values. In ZFA, theCEC was related to zeolite phase (the content and kindof zeolite), while the PIC was related to other phases (Cacomponent and dissociated Fe2O3 and Al2O3). Thus, thechange of PIC and CEC with increasing reaction timeshowed different patterns. A high CEC was achieved undera high temperature, a high liquid/solid ratio, a long reac-tion time, and an appropriate NaOH concentration(2 mol/L), while the incompleteness of the reaction for zeo-lite formation (occurred for example under a reasonablyshort reaction time) resulted in a high PIC value instead.This information is valuable since synthesis conditionscould be selected depending on which nutrient (ammoniumor phosphate) is the main contaminant to be eliminated bythe use of ZFA.

Acknowledgement

This research has been supported by a Grant from theChinese Ministry of Science and Technology, Project No.2002AA601013.

References

[1] Grubb DG, Guimaraes MS, Valencia R. Phosphate immobiliza-tion using an acidic type F fly ash. J Hazard Mater 2000;76:217–36.

[2] Querol X, Moreno N, Umana JC, Alastuey A, Hernandez E, Lopez-Soler A. Synthesis of zeolites from coal fly ash: an overview. Int JCoal Geol 2002;50:413–23.

[3] Holler H, Wirsching U. Zeolite formation from fly ash. ForschrMiner 1985;63:21–43.

[4] Weitkanp J, Puppe L. Catalysis and zeolites: fundamentals andapplications. Germany: Springer; 1999.

[5] Chen JG, Kong HN, Wu DY, Hu ZB, Wang ZS, Wang YH. Removalof phosphate from aqueous solution by zeolite synthesized from flyash. J Colloid Interf Sci 2006;300:491–7.

[6] Wu DY, Zhang BH, Li CJ, Zhang ZJ, Kong HN. Simultaneousremoval of ammonium and phosphate from aqueous solution byzeolite synthesized from fly ash as influenced by salt treatment. JColloid Interf Sci 2006;304:300–6.

[7] Zhang BH, Wu DY, Wang C, He SB, Zhang ZJ, Kong HN.Simultaneous removal of ammonium and phosphate by zeolitesynthesized from coal fly ash as influenced by acid treatment. JEnviron Sci 2007;19:541–6.

[8] Wu DY, Kong HN, Zhao TG, Wang C, Ye C. Effects of synthesisconditions on the formation and quality of zeolite during thehydrothermal zeolitization processes of fly ash. J Inorg Mater2005;20:1153–8.

[9] Wu DY, Zhang BH, Yan L, Kong HN, Wang XZ. Effect of someadditives on the formation of zeolite from coal fly ash. Int J MinerProc 2006;80:266–72.

[10] Penilla RP, Bustos AG, Elizalde SG. Immobilization of Cs, Cd, Pband Cr by synthetic zeolites from Spanish low-calcium coal fly ash.Fuel 2006;85:823–32.

[11] Shigemoto N, Hayashi H, Miyaura K. Selective formation of Na–Xzeolite from coal fly ash by fusion with sodium hydroxide prior tohydrothermal reaction. J Mater Sci 1993;28: 4781–6.

[12] Molina A, Poole CA. Comparative study using two methods toproduce zeolites from fly ash. Miner Eng 2004;17:167–73.

[13] Hollman GG, Steenbruggen G, Janssen-JurkoviCova M. A two-stepprocess for the synthesis of zeolites from coal fly ash. Fuel1999;78:1225–30.

[14] Amrhein C, Haghnia GH, Kim TS, Mosher PA, Ggajena RC,Amanios T, et al. Synthesis and properties of zeolites from coal flyash. Environ Sci Technol 1996;30:735–42.

[15] Murayama N, Yamamoto H, Shibata J. Mechanism of zeolitesynthesis from coal fly ash by alkali hydrothermal reaction. Int JMiner Proc 2002;64:1–17.

[16] Inada M, Eguchi Y, Enoto N, Hojo J. Synthesis of zeolite from coalfly ashes with different silica–alumina composition. Fuel2005;84:299–304.

[17] Van Reeuwijk LP, editor. Procedures for soil analysis. Wagenin-gen: International Soil Reference and Information Centre (ISRIC);1992. p. 91.

[18] American Public Health Association (APHA), Standard methodsfor the examination of water and wastewater, Washington, DC;1995.

[19] Jackson ML, Lim CH. In: Klutes A, editor. Methods of soil analysis,part 1. physical and mineralogical methods. Madison: ASA andSSSA; 1986. p. 101.

[20] Breck D. Zeolite molecular sieves: structure, chemistry anduses. New York: Wiley; 1974.