王林：不同改性粉煤灰对含铬废水吸附影响的研究
附录2：外文文献原文
Factors affecting the sorption of trivalent chromium by zeolite synthesized from coal ?y ash
Abstract
This research was initiated to determine the effects of different constituents and properties of zeolite synthesized from ?y ash (ZFA) on Cr(III)sorption. The uptake of Cr(III) by ZFA was in?uenced greatly by pH，increasing with the increase in pH. The pH was controlled mainly bycalcium-related components (especially CaCO3 and free CaO) and zeolite components in ZFAs. Sorption maximum of Cr(III) (Qm)，determinedby a repeated batch equilibration method，ranged from 22.29 to 99.91 mg/gforthe14ZFAs.The Qm value correlated signi?cantly with Ca-related components. The correlation coef?cients were 0.9467，0.5469，0.7521，and 0.9195 for total Ca，CaCO3，CaSO4，and f.CaO，respectively.The Qm value was also closely related to cation-exchange capacity (r = 0.6872) and speci?c surface area (r = 0.7249). Correlation coef?cientsof Qm with dissociated Fe2O3 and Al2O3 were much higher than those of total Fe and total Al contents，respectively. It was suggested that，inZFAs，zeolite and iron oxide acted as ion exchanger and adsorbent for Cr(III)，respectively，while Ca components elevated the pH of the reactionsystem and consequently promoted ion exchange and adsorption and caused the surface precipitation of chromium hydroxide.? 2008 Elsevier Inc. All rights reserved.Keywords: Zeolite; Fly ash; Trivalent chromium; Sorption; Mechanism; Correlation coef?cient; Composition
1. Introduction
Coal ?y ash is generated in great amounts every year as asolid waste produced during the combustion of coal in electric-ity/heat generation processes. It was estimated that 349 Mt of?y ash was produced worldwide in 2000 [1]. In China alone，theamount of ?y ash exceeded 160 Mt in 2004，and it is expectedto increase in future due to rapid economic growth. Currently，only part of the ?y ash is reutilized in the production of build-ing materials such as concrete and cement. For instance，thereutilization rate is 25–30% for the United States [2]，48% forEurope [3]，and about 40% for China. However，a large pro-portion of ?y ash is impounded or land?lled. In recent years，the hydrothermal synthesis of zeolite has been intensively in-vestigated as an alternative for the productive reuse of coal ?y ash [3–20] and other inorganic materials [21–23].
Zeolite synthesized from ?y ash (ZFA) has a high cation-exchange capacity (CEC) and thus has been shown to be effective in the removal of heavy metals from wastewater [3–14]
.When compared to a commercial zeolite 4A，ZFA was shown to be effective in the removal of mixed heavy metal ions (Cu2+，Cr3+，Zn2+，Co2+，and Ni2+) [5]. Similarly，by comparisonwith three commercial synthetic zeolites (4A，X，NaP1)，twopilot plant ZFAs exhibited high performances for heavy metal(Cu2+，Zn2+，Cd2+，Co2+，Ni2+，Pb2+) uptake in the puri?ca-tion of three acid mine waters [6]. In another study，the removalof heavy metals by ZFA was compared with that by a commer-cial ion exchanger，Amberlite IRC-50，and it was indicated thatthe retention capacities of ZFA for Pb2+，Zn2+，and Cr3+ byZFA are higher than those for Amberlite IRC-50 [7].
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The mechanism of heavy metal removal by ZFA has beenproposed in several previous studies. Hui et al. [5] reported thatthe mechanism of heavy metal removal by ZFA was adsorptionand ion exchange processes. Penilla et al. [9] argued that themechanism for retention of Cs+ and Cd2+ by ZFA was cationexchange and/or adsorption，while that for Pb2+ and Cr3+ wasprecipitation. Singer and Berkgaut [10] determined the adsorption isotherms for heavy metal ions on two ZFAs synthesizedfrom two Israeli coal ?y ashes at pH 5.0. This low pH was usedsince they aimed to exclude precipitation processes. They sug-gested that part of the retention might not be cation exchangebut adsorption. The adsorption was suggested not to be on thezeolite surface，but on amorphous components that remainedin the ZFAs after the synthesis process. These studies implythat，in real processes of heavy metal removal by ZFA，threemechanisms (adsorption，ion exchange，and precipitation) mayfunction individually or jointly.
Regarding the role of ZFA，Moreno et al. [6] noted that ZFAnot only acted as a sorbent for heavy metals，but also could in-crease the pH，causing metal ions to precipitate and enhancingthe ef?ciency of the decontamination process as a result. Theimportance of the pH-elevating effect of ZFA in heavy metalimmobilization in contaminated soils was suggested similarlyby Querol et al. [8]. Since the rise in pH is owing to the alkalinity of ZFA，it is presumed that other components in ZFA，e.g.，free lime，may raise the pH and boost the immobilization of heavy metals.
Therefore，the retention of heavy metal ions by ZFA iscomplex and may involve adsorption and precipitation in addition to a cation-exchange process. The relative importance ofthe mechanisms may depend on the ion species，the composition and the properties of ZFA，and the experimental conditions. It should be stressed，however，that ZFA is usually not pure and contains a number of other components that originally existed in coal ?y ash. That is，part of the components in ?y ash are not incorporated into zeolite during the synthesis process，although these components may be modi?ed to different extents. The zeolite content in ZFA is typically 30–60%，depending on the synthesis conditions and the methodology adopted.
However，the previous studies dealing with the removal ofheavy metals by ZFA have been performed by using only oneor two ZFAs [4–14]. Therefore，it is dif?cult to explore therelative importance of different components of ZFA in heavymetal retention by relating the heavy metal removal perfor-mance of the ZFAs to their chemical composition and properties.
In the present study，the capacities for Cr(III) sorption of 14 ZFAs were measured and correlated with selected ZFA parameters to identify the constituents in ZFA that control Cr(III)removal. The understanding of these factors provided useful information for both the retention mechanism and the selection of a ZFA with a high Cr(III) sorption capacity. It is presumed that，in practice，the selection of a ZFA with a high Cr(III) sorption capacity would be of utmost importance to obtain a sustained Cr(III) removal in the long term.
2. Materials and methods 2.1. Materials
Fourteen coal ?y ashes of different coal origins and chemical compositions were collected from different thermal power plants in China. For ZFA preparation，approximately 25 g of ?y ash was placed in a ?ask and mixed with 150 ml of 2.0 mol/L NaOH solution. The slurry was boiled with re?ux for 48 h，with stirring. The solid phase was separated by centrifugation and washed with doubly distilled water ?ve times and with ethanol twice.
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王林：不同改性粉煤灰对含铬废水吸附影响的研究
Finally，the products were dried in an oven at 45 ?C，ground to pass through an 80-mesh sieve，and stored in airtight containers for the subsequent experiments.
2.2. Characterization of the materials
For the chemical analysis，except for silicon，the samples were digested with hydrogen ?uoride in conjunction with perchloric acid and dissolved later by hydrochloric acid. For silicon，the samples were melted with sodium hydroxide. The elemental concentrations were then measured in digestions by inductively coupled plasma atomic emission spectrometry (IRIS advantage 1000). Dissociated iron oxide (Fe2O3d) and aluminum oxide (Al2O3d) were analyzed by the dithionite–citrate–bicarbonate (DCB) extraction method [24].CaCO3 was analyzed by the modi?ed Van Slyke manometric method [25]. CaSO4 was determined by dissolution in hydrochloric acid of total sulfates followed by precipitation with barium chloride [26]. The free calcium oxide content (f.CaO) was calculated by subtracting the sum of CaCO3 and CaSO4 content from the total CaO content [27，28]. Identi?cation of the crystalline phase(s) in the materials was carried out with XRD equipment (D8 ADVANCE) using Ni-?ltered CuKα radiation (40 kV，40 mA). The CEC was determined using the ammonium acetate method [29]. The speci?c surface area was determined by N2 adsorption method (equipment model:ASAP2010). About 0.1 g of the samples was outgassed at 200 ?C under nitrogen ?ow for about 4 h prior to measurement. The nitrogen adsorption/desorption data were recorded at the liquid temperature (?196 ?C). The speci?c surface area was calculated using the Brunauer–Emmett–Teller (BET) equation.
2.3. Sorption of Cr(III) by ZFA
The sorption experiment was done using an initial Cr(III)concentration of 400 mg/L with a pH value of 3.30 and prepared from doubly distilled water and CrCl3·6H2O of analytical reagent grade. Forty ml of the aqueous solution was added to centrifuge tubes containing 0.4 g of the sample. The tubes were sealed with screw-type lids and then continuously agitated on an orbital shaker at 200 rpm for 4 h at laboratory temperature(ca. 20 ?C). A reaction time of 4 h was found to be suf?cient for Cr(III) to achieve equilibrium in preexperiments. After 4 h，the pH of the suspensions was measured with a Hach 51910 pH meter and then they were centrifuged. The Cr(III) of the supernatants was determined using a Unico spectrophotometer (Model UV-2102PCS). The Cr(III) was ?rst converted into the hexavalent form after oxidation with potassium perman-ganate at elevated temperature and under acidic conditions and then the Cr(VI) concentration was determined at 543 nm using the 1，5-dephenyl-carbazide method [30]. The ef?ciencies of Cr(III) removal were calculated from the differences between the initial and the ?nal Cr concentrations in solution. The experiments were performed in duplicate and the mean data are reported.Three representative ZFAs with low，medium，and high calcium content were chosen to evaluate the effect of pH and dosage on Cr(III) immobilization.
2.4. Maximum sorption capacity of Cr(III) on ZFA
A repeated batch equilibration method was used to measure the maximum sorption capacity (Qm) of the 14 ZFAs for Cr(III). Forty ml of the solution，with an initial Cr(III) concentration of 200 mg/L and a pH value of 3.36，was put into a preweighed centrifuge tube (W1) containing 0.4 g dry weight of sample (W2). After being shaken for 4 h，the suspension was centrifuged and the supernatant was poured into another container. The tube with the residue was weighed again (W3). The volume of the residual solution (V ) was calculated by
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assuming the density of the residual solution as 1 g/ml:V(ml) =W3(g) ?W1(g) ?W2 (g). The obtained supernatant was analyzed for Cr(III) concentration (C) and the amount of Cr(III) that remained in the residual solution (R)，as well as the amount of Cr(III) sorbed by the sample (S)，were calculated by the equations R(mg) =[V(ml) · C(mg/L)]/1000;S(mg/g) =[(200 ? C) mg/L · 0.04 L]/W2 g. The volume of the residual solution and the amount of the remaining Cr(III)were considered in the calculation of the initial volume and the initial concentration of the subsequent equilibration step.A fresh solution of the same Cr(III) concentration was added and equilibration was repeated until no further uptake by the ZFA was observed. The amount of retained Cr(III) was thus calculated to represent the maximum sorption capacity of Cr(III) by ZFA. The experiments were performed at least in duplicate，and the mean data are reported.
3. Results and discussion
3.1. Characterization of the materials
The main chemical composition of the ZFAs is given in Table 1. The ZFAs were composed mainly of SiO2 and Al2O3 components，followed by CaO and Fe2O3 components，while theMgO and K2O contents were very low. The ZFAs contained large amounts of Na2O and water as well. The high contents of these two components are due to the formation of zeolites whose negative charge is saturated chie?y by Na+ as a result of the use of concentrated NaOH solution (2 mol/L) in the zeolite conversion process，and whose water holding capacity (zeolitic water) is high.
The XRD patterns of the ZFAs are presented in Fig. 1.For all ZFAs except Nanshi and Wujin F ZFA，the monomineral of the NaP1 zeolite (Na6Al6Si10O32·12H2O) was produced. However，for the Nanshi and Wujin F ZFA，two species of zeolites (mainly hydroxysodalite，with a small amount of NaP1 as a secondary zeolite phase) were generated. These two ZFAs typically had high CaO content compared with the other ZFAs; it thus appears that a high CaO content in ?y ash might enable the formation of hydroxysodalite.
The CEC and speci?c surface area of the ZFAs are listed in Table 2 and are compared with their raw ?y ashes. The conver- sion of ?y ash into ZFA resulted in a great enhancement of CEC and speci?c surface area (Table 2). This increase is apparently owing to the formation of zeolite. The relatively low CEC value of Nanshi and Wujin F ZFA may be attributed to the formation of hydroxysodalite，whose small pore size of 0.23 nm does not permit the penetration of ammonium ions (used to determine the CEC) with an ionic diameter of 0.28 nm. It should be noted that the small pore size of hydroxysodalite would be accessible to most heavy metal ions，whose ionic diameter is small (e.g.，0.106 nm for Cr(III)). It is worthy of note that the ZFAs are not pure and contained a number of components other than zeolite (Table 1，Fig. 1).
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王林：不同改性粉煤灰对含铬废水吸附影响的研究
The comparison of the CEC value of a ZFA with that of a pure zeolite may provide a semiquantitative estimate of the zeolite content in the ZFA given that the ZFA contained only one kind of zeolite [3]. Since the monomineral of the NaP1 zeolite was formed for all ZFAs except the Nanshi and Wujin F ZFA，the zeolite content in ZFAs excluding Nanshi and Wujin F ZFA was estimated based on the CEC value of the pure NaP1 zeolite (430 cmol/kg). The zeolite content ranged from 20.5% for Datong ZFA to 50.0% for Baoshan ZFA.
3.2. Role of acid-neutralizing ability of ZFA in retention of Cr(III)
The ?rst experiment was carried out by treating a 400-mg Cr(III)/L solution with the ZFAs. The ef?ciency of Cr(III) removal and the ?nal pH reached are shown in Table 3. Although the cation-exchange capacity of the ZFAs undoubtedly contributed to the Cr(III) elimination process via ion exchange，it appears that high ef?ciency of Cr(III) uptake was related to high ?nal pH value. The initial CrCl3 solution with concentration 400 mg/L is acidic in nature (pH 3.30) (wastewaters resulting from operations employing trivalent chromium are similarly acidic)，while the ZFAs had alkaline pH values within the range from 11.21 to 12.04
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