Synthesis of Zeolite/Aluminum Oxide Hydrate from Coal Fly Ash: ANew Type of Adsorbent for Simultaneous Removal of Cationic andAnionic PollutantsJie 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 ofZFA has been intensively investigated to recycle the solid waste. However, problems arising from the waste alkaline solution haverarely been addressed to date. This study initiated a novel method to synthesize ZFA/Al2O3 hybrid material by introducing areaction step involving the neutralization of the waste alkaline solution with soluble Al salts into the traditional ZFA synthesisroute. When compared with ZFA, ZFA/Al2O3 was found to have a significantly higher CEC. The increases of the BET surfacearea and phosphate-immobilization capacity were even more dramatic, with the former increasing by 2−4 times and the latterincreasing by 2−3 times. The hybrid material had a significantly lower alkalinity than ZFA. The results also showed that theeffluent from the production of the hybrid material could be much more environmentally friendly.
1. INTRODUCTIONZeolites are aluminosilicate minerals with three-dimensionalframework structures containing AlO4 and SiO4 tetrahedra thatare linked to each other through the sharing of the electrons ofthe oxygen atoms, forming interconnected cages and channels.Zeolites have valuable physicochemical properties, such ascation exchange, molecular sieving, catalysis, and adsorption.Zeolites have been studied intensively and used widely in theindustrial and agricultural fields.1,2 Owing to significantachievements in this area, research on zeolites was selected asone of the top 10 breakthroughs in 2011 by the journal Science.2
Given the merit of low- or zero-cost raw materials and thepossibility of reusing solid waste, synthesis of zeolites from coalfly ash (CFA) have been extensively investigated in recentyears.3−15 Studies have shown that, because zeolites arenegatively charged, the obtained zeolites can act as adsorbentsfor removing cationic pollutants from water.16−29 However,zeolite synthesized from CFA (ZFA) is not a pure zeolite butrather is generally composed of a zeolite fraction and anonzeolite fraction. Our previous works confirmed that ZFAhas an impressive ability to simultaneously remove cationicpollutants, such as ammonium and heavy-metal ions, andoxyanionic pollutants, such as phosphate.30−33 We found that,in the simultaneous removal of ammonium and phosphate, thezeolite fraction is responsible for the adsorption of ammoniumwhereas the nonzeolite fraction containing the free orassociated oxides CaO, Al2O3, and Fe2O3 has an affinity forphosphate.30−33 Nevertheless, it must be stressed that thecontent of the oxides in ZFA for the binding of phosphate islimited, with the types and concentration levels of the oxidesdepending on the origin of the CFA. Therefore, it is impracticalto expect the actual use of all ZFAs for the efficient removal ofanionic pollutants, such as phosphate.On the other hand, despite intensive investigations since the
first successful synthesis of ZFA by Holler and Wirsching in
1985,3 problems related to the alkaline waste solution producedduring the synthesis process have drawn little attention. Thealkaline waste solution is generated in huge amounts and cancontain a number of ingredients such as Al, Si, Fe, and otheranionic or amphoteric heavy metals, which are harmful to theenvironment. Generally, the hydrothermal synthesis of zeoliteuses liquid/solid ratios as high as 3−8 L/kg, and the NaOHconcentration used in the reaction ranges from 1 to 2 M, whichmeans that the synthesis of 1 ton of ZFA would produce 3−8tons of waste alkaline solution. Following the synthesis process,the concentration of NaOH decreases to different extentsdepending on the synthetic conditions, the composition ofCFA, and so on. Although it is thought that the waste alkalinesolution could be recycled in subsequent zeolite preparationprocesses, the quality of products made in this way would getworse or unstable if the alkaline waste solution were recycleddirectly or the concentration of alkali for recycling wereinaccurately adjusted to compensate for consumption. Thesoluble components in the waste solution would graduallyaccumulate when the recycling was done, which would alsoimpact the quality of the ZFA. In addition to these limitations,ZFA shows an undesirably strong alkalinity even after repeatedwashing in distilled water. Repeated washing not onlyconsumes large amounts of water, but also discharges wastesolution with high pH. Most past investigations of ZFA focusedmainly on the synthesis and use of the zeolite itself but largelyoverlooked 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
Received: July 6, 2013Revised: September 18, 2013Accepted: September 23, 2013Published: September 23, 2013
process (Figure 1). Specifically, following the traditionalsynthesis process, the mixture of zeolite and alkaline wastesolution is further neutralized with water-soluble Al salts, andthe product is finally heated as needed to produce ZFA/Al2O3hybrid material. In this way, the obtained material is expectedto be more suitable for the simultaneous removal of cationicand anionic pollutants. This article describes the properties ofthe ZFA/Al2O3 product in terms of the adsorption of cationicand anionic pollutants. The changes in mineralogical andchemical composition during this novel synthesis process werealso analyzed. Our results indicate that, through this moreenvironmentally friendly synthesis process, a new hybridmaterial with greatly improved valuable properties can beobtained.
2. EXPERIMENTAL SECTION
2.1. Materials. The CFA starting materials used in thisstudy, with different compositions, particularly different CaOcontents, were obtained in China from the Wujin second powerplant (Shanghai), the Wenzhou power plant (Zhejiang), andthe Minhang power plant (Shanghai). According to specifica-tion ASTM C618, they can be classified as a Class C fly ash, aClass F fly ash, and a Class F fly ash, respectively. For ZFApreparation, approximately 150 g of fly ash was placed in abench-scale reaction vessel and mixed with 900 mL of 2.0 MNaOH solution. The slurry was boiled with reflux for 24 h withstirring at 95 °C. After the mixture was allowed to cool to roomtemperature, ZFA was recovered by centrifugation and washedwith doubly distilled water three times and with ethanol twice.Finally, ZFA products were dried in an oven at 45 °C, groundto pass through an 80-mesh sieve, and stored in airtightcontainers for later experiments. To synthesize ZFA/Al2O3,after the mixture of ZFA and waste alkaline solution wasallowed to cool to room temperature, it was neutralized withAlCl3, that is, 2 N AlCl3 solution was added dropwise (10 mL/min) to the mixture with continuous stirring. The volume ofAlCl3 added was equal to the volume of NaOH used in ZFApreparation. To guarantee a sufficient reaction of AlCl3 with thealkaline solution, stirring was maintained for another 4 h afterthe addition of AlCl3 was completed. The ZFA/Al2O3 productswere then recovered, washed, dried, and ground in a mannersimilar to that used for ZFA. In our current study, even though
drying at 45 °C led to the formation of aluminum oxidehydrate, we refer to the obtained product as ZFA/Al2O3 forconvenience.
2.2. Cation-Exchange Capacity (CEC) and Phosphate-Immobilization Capacity (PIC). Cation-exchange capacity(CEC) was determined by the ammonium acetate method.34
Phosphate-immobilization capacity (PIC) was determined by arepeated adsorption test as follows: (1) First, 40 mL ofphosphate solution with a concentration of 20 mg of P/L wasput into a preweighed centrifuge tube (W1) containing 0.2 g ofthe materials (W2). After being shaken for 24 h, the suspensionwas centrifuged, and the supernatant was poured into anothertube 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 couldbe calculated by assuming the density of the residual solution tobe 1 g/mL: V (mL) = [W3 (g) − W2 (g) − W1 (g)] × 1 mL/g.The amount of the P remaining in the residual solution thatwas not adsorbed by the sample (R) and the amount of Padsorbed 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 ofresidual solution and the amount of phosphate in the residualsolution were considered in the calculation of the subsequentequilibration step. (2) A fresh solution of the same phosphateconcentration (20 mg of P/L) was added and equilibration wasrepeated until the removal efficiency of phosphate was less than5% {removal efficiency = [(Ci − Ce)/Ci] × 100%, where Ci isthe initial phosphate concentration and Ce is the phosphateconcentration after adsorption}. The total amount of phosphateretained by the materials was thus calculated to represent themaximum immobilization capacity for phosphate (PIC).The fractionation of phosphorus for the materials saturated
with phosphate was then conducted by a sequential extractionscheme following a protocol modified from Hieltjes andLijklema.35 The fractionation scheme comprises (1) twoconsecutive extractions in 1 M NH4Cl at pH 7 (denoted asloosely bound P, calculated by subtracting R in the final step ofthe repeated adsorption test from the amount of extracted P),(2) two consecutive extractions in 0.1 M NaOH followed byextraction in 1 M NaCl [denoted as (Fe + Al)-bound P], and(3) two consecutive extractions in 0.5 M HCl [denoted as (Ca
Figure 1. (Left) Traditional process for ZFA production and (right) proposed process for zeolite/Al2O3 production.
Industrial & Engineering Chemistry Research Article
+ 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 wererecorded using a D8 ADVANCE instrument (Bruker-AXSCompany) with Cu Kα filtered radiation (30 Kv, 15 mA). Todetermine the contents of different fractions of calcium(CaCO3, CaSO4, and free CaO), analysis by XRD incombination with standard addition methods was undertakento measure the contents of CaCO3 and CaSO4, whereas freeCaO was calculated by subtracting CaCO3 and CaSO4 from thetotal CaO content, which was obtained by X-ray fluorescenceanalysis. BET surface areas were determined on a NOVA1200eapparatus (Quantachrome Company) using nitrogen adsorp-tion isotherms. Waste solution was collected and acidified foranalysis by inductively coupled plasma-atomic emission spec-troscopy (iCAP 6000 Radial, Thermo Company). The pHvalues of the materials were tested as follows: Forty milliliters of0.01 M CaCl2 was added to centrifuge tubes containing 0.2 g ofmaterial, and then the final pH after a 24-h equilibration periodwas measured using a Hach Sension+ pH meter.
3. RESULTS AND DISCUSSION3.1. Mineralogical Composition. XRD patterns of CFA,
ZFA, and ZFA/Al2O3 are given in Figure 2. It was observedthat a monomineral phase of zeolite was formed following thehydrothermal reaction of each CFA. The zeolite in Minhangand Wenzhou ZFA was identified as NaP1 (Na6A16Si10O32·12H2O), whereas Linde type A (NaA11.5Si1.5O6·5.1H2O) wasproduced in Wujin ZFA. Whereas the anhydrite (CaSO4) inWujin CFA disappeared after the hydrothermal treatment,other crystalline minerals in the CFAs were identified in theZFAs. Specifically, quartz and mullite in Minhang CFA, quartzand calcite in Wenzhou CFA, and calcite in Wujin CFA wereleft over after the synthesis process. Therefore, the ZFAs werenot pure zeolite but contained components that originatedfrom the corresponding CFAs. The peaks in the XRD patternof each ZFA were all detected for the corresponding ZFA/Al2O3, and no new peak(s) appeared upon the formation ofAl2O3 in Wenzhou ZFA/Al2O3. In contrast, peaks attributed tohydrargillite [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 inWenzhou ZFA/Al2O3, whereas it formed crystalline phases inthe other ZFA/Al2O3 products.3.2. Changes in Solid Mass and Chemical Composi-
tion. Table 1 lists the results of a mass balance, normalized to 1g of CFA, for the ZFAs and ZFA/Al2O3 hybrid materials.Following the treatment by NaOH solution at 95 °C, the massof ZFA solid product increased slightly when compared withthe mass of each raw CFA material. Treatment by AlCl3solution to neutralize the alkaline waste solution increasedthe mass of ZFA/Al2O3 solid product further, but this increasewas substantially higher for Minhang CFA than for the otherCFAs. This could be interpreted as indicating that MinhangCFA is a low-calcium fly ash whereas the calcium contents ofWenzhou 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 (SupportingInformation). Of these forms, free CaO and CaCO3 are solubleto different degrees and were partially removed by neutraliza-tion with AlCl3 solution (Table S1, Supporting Information).
The chemical compositions of CFA, ZFA, and ZFA/Al2O3are given in Table S2 (Supporting Information). The maincomponents of CFA are the oxides of Si and Al, along withvarious metallic oxides. Of them, Al2O3 and SiO2 are the mostabundant 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 canbe classified into four types. To better describe the changes inchemical composition from CFA to ZFA and from ZFA toZFA/Al2O3, the results for Wenzhou samples are graphicallypresented 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 andfrom ZFA to ZFA/Al2O3, but the extents of the decreases weregenerally small. In the three CFAs, however, the decreases were
Figure 2. XRD patterns of CFA, ZFA, and zeolite/Al2O3. L, Lindetype A; H, hydrargillite [γ-Al(OH)3]; C, calcite; A, anhydrite (CaSO4);P, NaP1; Q, quartz; M, mullite; Al, aluminum oxide hydroxide(AlOOH).
Industrial & Engineering Chemistry Research Article
more pronounced for Wenzhou CFA, as shown in Figure 3,probably because of its higher increase in moisture followingsynthesis process (Table S2, Supporting Information).The second type of change involved calcium, whose content
decreased only slightly from CFA to ZFA, but decreased greatlyfrom ZFA to ZFA/Al2O3. However, the extent of the decreasedepended on the type of CFA, specifically, on the calciumcontent of CFA. That is, a slight decrease was observed for thelow-calcium fly ash Minhang CFA, but the calcium content wasreduced almost by half for the other high-calcium CFAs. Thisdecrease in calcium content from ZFA to ZFA/Al2O3 was dueto the loss of Ca ingredients through dissolution by the acidicAlCl3 solution treatment.
Only Al was classified as the third type of change. Thecharacteristic of this type of change is that the contentdecreased slightly from CFA to ZFA, but increased from ZFAto ZFA/Al2O3. The decrease in Al content from CFA to ZFAcan be attributed to the loss of Al in waste alkaline solution(Table 4), but the dilution effect through the increase of massmight also lead to the reduction of Al content (Table 1). Onthe other hand, the increase in Al content from ZFA to ZFA/Al2O3 was clearly caused by the neutralization treatment withAlCl3. However, the Al content increased to a significantlylesser extent from ZFA to ZFA/Al2O3 in the case of MinhangCFA, 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 increasewith the synthesis treatments. The sodium content can beregarded as a measure of the number of negative charges inZFA, because sodium exists as an exchangeable cation in Na-saturated ZFAs, which were formed because of the use ofconcentrated NaOH in the synthesis process. The sodium ionson the ZFA surface were partially replaced by Al ions duringthe AlCl3 treatment, resulting in a decrease of the sodiumcontent from ZFA to ZFA/Al2O3. The increase in moisture wasanalogously due to the increase in exchangeable cation capacity(number of negative charges), which would induce a hydro-philic surface.
3.3. BET Surface Area and Cation-Exchange Capacity(CEC). The results for the BET surface areas and CECs of thematerials 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 CFAfor ZFA and ZFA/Al2O3
material mass (g)
WujinCFA 1.00ZFA 1.14ZFA/Al2O3 1.33
WenzhouCFA 1.00ZFA 1.14ZFA/Al2O3 1.34
MinhangCFA 1.00ZFA 1.05ZFA/Al2O3 1.57
Figure 3. Comparison of chemical composition among CFA, ZFA, and ZFA/Al2O3 for Wenzhou samples.
Industrial & Engineering Chemistry Research Article
of the ZFA/Al2O3 products had significantly greater BETsurface areas than their corresponding ZFA products. The SSAsincreased by factors of 3.54, 1.97, and 1.86 for Wujin,Wenzhou, and Minhang ZFAs, respectively, as a result of theconversion of ZFA to ZFA/Al2O3. The increase in BET surfacearea was evidently caused by the formation of aluminum oxidehydrate. Because the synthesis of ZFA/Al2O3 means theaddition of Al2O3 to the original ZFA, higher SSAs of ZFA/Al2O3 compared to the original ZFA would indicate that Al2O3has a higher SSA than ZFA. (If Al2O3 had the same SSA valueas ZFA, then the SSAs of ZFA and ZFA/Al2O3 would beidentical, and if Al2O3 had a lower SSA than ZFA, then the SSAvalue of ZFA/Al2O3 would be lower than that of the originalZFA.) The values of the BET surface areas were also expressedper unit mass of CFA, by taking into account the results of themass balance analysis in Table 1. The results are denotated asSSA(CFA). Table 2 shows that, from an initial 1 g of CFA, theBET surface area could be greatly enhanced through thesynthesis of ZFA/Al2O3 hybrid material.CEC is a measure of the potential capacity for adsorbing
cationic pollutants such as ammonium and heavy-metal ions.Because negative charge is constantly generated by an electricalimbalance between the aluminum atom and four oxygen atomsin the zeolite structure, zeolite generally has the ability toexchange cations. It is therefore not surprising that ZFAs havehigh CEC values whereas the CEC values of CFAs are very low.The CECs of the Wujin, Wenzhou, and Minhang ZFAs were117.7, 163.1, and 137.6 cmol/kg, respectively. On the otherhand, the CECs of the Wujin, Wenzhou, and Minhang ZFA/
Al2O3 materials were 133.9, 217.6, and 119.7 cmol/kg,respectively. The lower CEC of Minhang ZFA/Al2O3compared to Minhang ZFA suggests that the Al2O3 in MinhangZFA/Al2O3 has a CEC value lower than that of Minhang ZFA.A possible explanation is that the pH of Minhang ZFA/Al2O3was lower than those of Wujin and Wenzhou ZFA/Al2O3(Table 2) and, thus, Al2O3 in Minhang ZFA/Al2O3 might carryfewer negative charges.Assuming that Al2O3 carries zero charge, the production of
Al2O3 would give rise to a dilution effect for the CEC of ZFA/Al2O3. In this case, the calculated CECs were 101.3, 138.9, and92.0 cmol/kg for Wujin, Wenzhou, and Minhang ZFA/Al2O3,respectively. These calculated data were clearly lower than theexperimentally determined values, indicating that Al2O3 carriesnegative charge. In fact, it is considered that Al2O3 couldgenerate pH-dependent variable charges. Similarly to the BETsurface areas, the CECs produced per unit mass of CFA,denoted as CEC(CFA), are also listed in Table 2, based on themass balance computation. The results show that the ZFA/Al2O3 products had much greater CEC(CFA) values than thecorresponding ZFAs, suggesting that the ZFA/Al2O3 productcould more efficiently retain cationic pollutants than ZFA whenthe same amount of CFA was used in production.
3.4. PIC and Phosphorus Fractionation. The PIC valuesof the materials and the fractionations of adsorbed phosphorusare reported in Table 3. The results indicate that both the CFAsand ZFAs could remove phosphate to some extent. For boththe CFAs and ZFAs, it appears that the removal capacity isrelated to the content of CaO: specifically, the higher the
Table 2. Specific Surface Areas (SSAs), CECs, and pH Values of the Materials
material SSA (m2/g) SSA(CFA) (m2/g) CEC (cmol/kg) CEC(CFA) (cmol/kg) pH(CaCl2)
content of CaO, the higher the removal capacity for phosphate.Itskos et al.36 investigated the removal of Cr (total), Cr(VI),Cu, Ni, Pb, Zn, and Cd from wastewater using different flyashes and found that high-Ca fly ashes are more efficient inprecipitating Cd, Cu, Ni, Pb, and Zn (by forming metalhydroxide precipitates), but less efficient in precipitatinghexavalent chromium. Although both phosphate and Cr(VI)are oxyanions, high CaO content stimulated efficient removalfor phosphate but not for Cr(VI) decontamination. This can beexplained by the fact that phosphate could be precipitated byforming sparingly soluble calcium phosphate salts in high-Ca flyashes whereas calcium chromate is relatively soluble (ksp = 7.1× 10−4). From Table 3, it can also be seen that the PICs of allCFAs increased markedly after the conversion to zeolite. Thisphenomenon was previously observed for 15 CFAs by us, and itwas attributed to the increase of BET surface area anddissociated Fe2O3.
30 As seen in Table 3, the synthesis of ZFA/Al2O3 further augmented the removal capacity for phosphate. Itis worth noting that the increase in PIC from ZFA to ZFA/Al2O3 was significant; that is, conversion of ZFA to ZFA/Al2O3improved the removal capacity for phosphate by a factor ofabout 2−3. This increase was more substantial when the PICvalue was expressed per unit mass of CFA, similarly to the BETsurface area and CEC. Moreover, it appeared that the increasein PIC through the synthesis of ZFA or ZFA/Al2O3 was muchmore pronounced in Minhang CFA with low calcium content,because the CFAs with high calcium contents already had asubstantial removal capacity for phosphate.Fractionation of phosphorus immobilized by the materials
was conducted so as to better understand the mechanism ofphosphate removal, and the results are also presented in Table3. It is shown that, in all cases, the percentage of residualphosphorus pool was low. For CFAs and ZFAs with highcalcium contents, the main P pool was either (Ca + Mg)-boundP or loosely bound P (LB P), with (Fe + Al)-bound P alsoaccounting for a considerable part of the total immobilized P. Itwas speculated that the LB P pool was principally a part of the(Ca + Mg)-bound P (probably dicalcium phosphatedehydrate), which can be relatively easily released into solution.In the case of Minhang CFA and ZFA with a low calciumcontent, however, (Fe + Al)-bound P constituted the mostsignificant form of the retained phosphorus. Compared withCFA and ZFA, the most striking characteristic of ZFA/Al2O3was that the (Fe + Al)-bound P accounted for the highestproportion of total immobilized P, at 66.96%, 81.06%, and87.22% for the Wujin, Wenzhou, and Minhang materials,respectively. Apparently, the aluminum oxide hydrate in theZFA/Al2O3 hybrids contributes greatly to phosphate removal.
The second significant fraction in total immobilized P was (Ca+ Mg)-bound P, whose pool was again related to calciumcontent (Wujin > Wenzhou > Minhang). Another strikingfeature is that LB P was very low for ZFA/Al2O3, suggestingthat the process of phosphate removal was not readilyirreversible, which would be particularly desirable in P removalpractice. The low LB P, determined by extraction with 1 MNH4Cl at pH 7, also suggests that ligand exchange (inner-sphere complex) might predominate in the P immobilizationprocess, whereas the formation of an outer-sphere complex isnot important as the immobilized phosphorus could not bereplaced by Cl− (1 M NH4Cl).
32
Therefore, the interaction between phosphate ion and oxidesurface sites in ZFA/Al2O3 by the formation of inner-spherecomplexes is predominantly important and can be expressed as
+ +
⇌ + + −
− +
−
a b
c a c
SOH(s) H PO (aq) H (aq)
S H PO (s) H O(l) ( )OH (aq)c
c
a b
43
4 2
where S refers to a metal atom (Al) in a hydroxylated oxideor on the surface of zeolite and OH refers to a reactive surfacehydroxyl; a, b, and c are stoichiometric coefficients; and c ≤ 3 isthe degree of protonation of the phosphate ion.
3.5. pH of Solid Products and Dissolved Substances inWaste Solution. It is known that ZFAs have strongly alkalineproperties even after repeated washing with distilled water,which limits the usefulness of the product in many applications.It is shown in Table 2 that both CFAs and ZFAs have high pHvalues. The pH values of the ZFA/Al2O3 materials decreasedsignificantly to below 9.0. This decrease in alkalinity was clearlycaused by neutralization with the acidic AlCl3 solution. As thenormal concentration and volume of AlCl3 equaled those ofNaOH added before hydrothermal concentration, the pH ofZFA/Al2O3 was somewhat higher than expected. Thisphenomenon could be interpreted as indicating that theneutralization process in this study was different from thereaction in solution and the interaction of Al ion with ZFA(exchange of Na ion by Al ion on zeolite) would take place,which would partially consume Al ion and reduce theconcentration of AlCl3 in solution. Furthermore, the bufferingcapacity of Ca would also retard the decrease in pH. It follows,therefore, that the pH values of Wujin and Wenzhou with highCaO contents were greater than that of Minhang for allmaterials. It is presumed that the pH values of the ZFA/Al2O3materials could be reduced further to a lower level simply byincreasing the amount of AlCl3 in the neutralization process.Following the hydrothermal reaction of CFA, some dissolved
ingredients remained in the waste alkaline solution, with the
Table 4. Dissolved Ingredients Remaining in Waste Alkaline Solutiona
concentrations of Si and Al being the highest (Table 4). Giventhe occurrence of components, especially heavy metals, in thewaste solution, the environmental risk for ZFA productionwould be of great concern. Nevertheless, the results ofinductively coupled plasma analysis demonstrated that treat-ment by AlCl3 to obtain ZFA/Al2O3 greatly abated theconcentration of all ingredients in the effluent (Table 4). Theattenuation of dissolved ingredients in the waste solution canbe explained as follows: (1) For oxyanions such as Si, As,Cr(VI), and Se, the immobilization is attributed to thereduction of OH− ions in solution and the strong reaction/sorption by a large amount of formed Al2O3. (2) Foramphoteric elements such as Al, Fe, Cr(III), and Pb, theinsolubilization is due to the drop in pH. (3) For cations suchas Cd and Hg, the immobilization can be motivated byretention through ion exchange. After all, the synthesis of ZFA/Al2O3 could make the effluent much cleaner when comparedwith the production of ZFA.In addition, although we used Al2O3 to denote the product of
neutralization, it is actually a mixture dominated by aluminumoxide hydrate. That is, the mixture contains other components,especially amorphous aluminasilicates, as indicated by thereduction of the Si and Al concentrations in the effluent.
4. CONCLUSIONSThe conventional hydrothermal synthesis process of zeolitefrom coal fly ash, which produces zeolite product and wastealkaline solution, was modified by a new one-pot process tosynthesize a zeolite/Al2O3 hybrid material in our current study.Our new process employs the neutralization of the wastealkaline solution with soluble Al salt, following the conventionalprocess. The hybrid material produced in this way was found tohave encouraging properties as a chemical adsorbent.Significant increases in specific surface area, cation-exchangecapacity, and phosphate-immobilization capacity were observedfor the hybrid materials when compared with either coal fly ashor zeolite synthesized from coal fly ash. The effluent followingthe production of zeolite/Al2O3 composite was found to bemuch cleaner, in terms of both common soluble ingredients (Si,Al, Fe) and heavy metals (As, Cd, Cr, Hg, Pb, Se). The pHvalues of the hybrid materials in aqueous solution were closerto a neutral level, whereas both the fly ashes and zeolites hadundesirable alkaline pH values even after repeated washing.Compared with the traditional hydrothermal synthesis ofzeolite, the production of zeolite/Al2O3 is much moreenvironmentally friendly, and the obtained hybrid materialswere found to be more capable as adsorbents to simultaneouslytake up cation pollutants (such as ammonium) and oxyanionpollutants (such as phosphate).
■ ASSOCIATED CONTENT*S Supporting InformationChemical compositions of materials and mineralogical contentsof calcium components of materials. This material is availablefree of charge via the Internet at http://pubs.acs.org.
This research was supported by the National Key Project forWater Pollution Control (2012ZX07105-002-03).
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